Biomimetics: Nature-Based Innovation [1 ed.] 9781439834763, 9781439834770, 9780429093708

Table of contents :

Introduction: Nature as a Source for Inspiration of Innovation. Artificial Senses and Organs-Natural Mechanisms and Biomimetic Devices. Biomimicry at the Cell-Material Interface. Multiscale Modeling of Plant Cell Wall Architecture and Tissue Mechanics for Biomimetic Applications. Biomimetic Composites. Electroactive Polymer Actuators as Artificial Muscles. Refreshable Braille Displays Actuated by Electroactive Polymers. Biological Optics. Biomimicry of the Ultimate Optical Device-The Plant. Biologically Inspired Design: A Tool for Interdisciplinary Education. Self-reproducing Machines and Manufacturing Processes. Biomimetic Products. Biomimetics for Medical Implants. Application of Biomimetics in the Design of Medical Devices. Affective Robotics: Human Motion and Behavioural Inspiration for Safe Cooperation between Humans and Humanoid Assistive Robots. Humanlike Robots-Capabilities, Potentials, and Challenges. Biomimetic Swimmer Inspired by the Manta Ray. Biomimetics and Flying Technology. The Biomimetic Process in Artistic Creation. Biomimetics-Reality, Challenges, and Outlook. Index.

Citation preview

K11566_cover.fhmx 7/6/11 1:41 PM Page 1 C

M

BIOENGINEERING

BIOMIMETICS SERIES

Bar-Cohen

Y

CM

MY

CY CMY

K

BIOMIMETICS SERIES

BIOMIMETICS Nature-Based Innovation

With contributions from leading experts, the book explores a wide range of applications of biomimetics across a variety of fields. Offering myriad examples of technologies inspired by biology, the book discusses artificial senses and organs; mimicry at the cell–materials interface; and modeling of plant cell wall architecture and tissue mechanics. It also covers biomimetic composites; artificial muscles; refreshable braille displays; biomimetic optics; and the mimicking of flying birds, insects, and marine biology. A series of chapters examines applications of biomimetics in manufacturing, products, medicine, and robotics—including medical devices, implants, and the development of human-like robots. Contributors also explore biologically inspired design as a tool for interdisciplinary education and describe the biomimetic process in artistic creation. The final chapter outlines the challenges to biomimetic-related innovation and offers a vision for the future. A follow-up to Biomimetics: Biologically Inspired Technologies (2005), this comprehensive reference methodically surveys the latest advances in this rapidly emerging field. It features an abundance of illustrations, including a 32-page full-color insert, and provides extensive references for engineers and scientists interested in delving deeper into the study of biomimetics.

K11566

BIOMIMETICS

Humans have always looked to nature’s inventions as a source of inspiration. The observation of flying birds and insects leads to innovations in aeronautics. Collision avoidance sensors mimic the whiskers of rodents. Optimization algorithms are based on survival of the fittest, the seed-picking process of pigeons, or the behavior of ant colonies. In recent years these efforts have become more intensive, with researchers seeking rules, concepts, and principles of biology to inspire new possibilities in materials, mechanisms, algorithms, and fabrication processes. A review of the current state of the art, Biomimetics: Nature-Based Innovation documents key biological solutions that provide a model for innovations in engineering and science.

EDITED BY

Yoseph Bar-Cohen an informa business

w w w. c r c p r e s s . c o m

Composite

6000 Broken Sound Parkway, NW Suite 300, Boca Raton, FL 33487 711 Third Avenue New York, NY 10017 2 Park Square, Milton Park Abingdon, Oxon OX14 4RN, UK

www.crcpress.com

BIOMIMETICS

Nature-Based Innovation

CRC Press Series in Biomimetics Series Editor: Yoseph Bar-Cohen Jet Propulsion Laboratory, California Institute of Technology

PUBLISHED TITLES: Biomimetics: Nature-Based Innovation Yoseph Bar-Cohen FORTHCOMING TITLES: Biomimetics and Ocean Organisms: An Engineering Design Perspective Anderson-Vincent-Montgomery Mechanical Circulatory Support for Heart Failure Bonde-Kormos Biomimicry: Architecture and Design Mazzoleni

BIOMIMETICS SERIES

BIOMIMETICS

Nature - Based Innovation EDITED BY

Yoseph Bar-Cohen

Boca Raton London New York

CRC Press is an imprint of the Taylor & Francis Group, an informa business

CRC Press Taylor & Francis Group 6000 Broken Sound Parkway NW, Suite 300 Boca Raton, FL 33487-2742 © 2012 by Taylor & Francis Group, LLC CRC Press is an imprint of Taylor & Francis Group, an Informa business No claim to original U.S. Government works Version Date: 20111017 International Standard Book Number-13: 978-1-4398-3477-0 (eBook - PDF) This book contains information obtained from authentic and highly regarded sources. Reasonable efforts have been made to publish reliable data and information, but the author and publisher cannot assume responsibility for the validity of all materials or the consequences of their use. The authors and publishers have attempted to trace the copyright holders of all material reproduced in this publication and apologize to copyright holders if permission to publish in this form has not been obtained. If any copyright material has not been acknowledged please write and let us know so we may rectify in any future reprint. Except as permitted under U.S. Copyright Law, no part of this book may be reprinted, reproduced, transmitted, or utilized in any form by any electronic, mechanical, or other means, now known or hereafter invented, including photocopying, microfilming, and recording, or in any information storage or retrieval system, without written permission from the publishers. For permission to photocopy or use material electronically from this work, please access www.copyright.com (http:// www.copyright.com/) or contact the Copyright Clearance Center, Inc. (CCC), 222 Rosewood Drive, Danvers, MA 01923, 978-750-8400. CCC is a not-for-profit organization that provides licenses and registration for a variety of users. For organizations that have been granted a photocopy license by the CCC, a separate system of payment has been arranged. Trademark Notice: Product or corporate names may be trademarks or registered trademarks, and are used only for identification and explanation without intent to infringe. Visit the Taylor & Francis Web site at http://www.taylorandfrancis.com and the CRC Press Web site at http://www.crcpress.com

Contents Preface ............................................................................................................................................ vii Acknowledgments .........................................................................................................................ix Editor............................................................................................................................................. xiii Contributors ...................................................................................................................................xv 1. Introduction: Nature as a Source for Inspiration of Innovation...................................1 Yoseph Bar-Cohen 2. Artificial Senses and Organs: Natural Mechanisms and Biomimetic Devices������� 35 Morgana M. Trexler and Ryan M. Deacon 3. Biomimicry at the Cell–Material Interface ..................................................................... 95 Kelsey A. Potter, Bo Gui, and Jeffrey R. Capadona 4. M  ultiscale Modeling of Plant Cell Wall Architecture and Tissue Mechanics for Biomimetic Applications�������������������������������������������������������������������������������������������� 131 Alejandro D. Rey, Damiano Pasini, and Yogesh Kumar Murugesan 5. Biomimetic Composites..................................................................................................... 169 Daniel G.T. Strange and Michelle L. Oyen 6. Electroactive Polymer Actuators as Artificial Muscles ............................................... 213 Yoseph Bar-Cohen 7. Refreshable Braille Displays Actuated by Electroactive Polymers.......................... 245 Yoseph Bar-Cohen 8. Biological Optics ................................................................................................................. 267 H. Donald Wolpert 9. Biomimicry of the Ultimate Optical Device—The Plant ........................................... 307 David W. Lee 10. Biologically Inspired Design: A Tool for Interdisciplinary Education ................... 331 Jeannette Yen, Marc J. Weissburg, Michael Helms, and Ashok K. Goel 11. Self-Reproducing Machines and Manufacturing Processes ..................................... 361 Adrian Bowyer

v

vi

Contents

12. Biomimetic Products .......................................................................................................... 377 Tom Masselter, Wilhelm Barthlott, Georg Bauer, Jürgen Bertling, Frank Cichy, Petra Ditsche-Kuru, Friederike Gallenmüller, Maik Gude, Tobias Haushahn, Michael Hermann, Henning Immink, Jan Knippers, Julian Lienhard, Rolf Luchsinger, Karin Lunz, Claus Mattheck, Markus Milwich, Nils Mölders, Christoph Neinhuis, Anke Nellesen, Simon Poppinga, Marcus Rechberger, Simon Schleicher, Clemens Schmitt, Hannes Schwager, Robin Seidel, Olga Speck, Thomas Stegmaier, Iwiza Tesari, Marc Thielen, and Thomas Speck 13. Biomimetics for Medical Implants ................................................................................. 431 Bert Müller 14. Application of Biomimetics in the Design of Medical Devices ................................445 Hande Argunsah and Brian L. Davis 15. A  ffective Robotics: Human Motion and Behavioral Inspiration for Safe Cooperation between Humans and Humanoid Assistive Robots���������������������������� 461 A.G. Pipe, R. Vaidyanathan, C. Melhuish, P. Bremner, P. Robinson, R.A.J. Clark, A. Lenz, K. Eder, N. Hawes, Z. Ghahramani, M. Fraser, M. Mirmehdi, P. Healey, and S. Skachek 16. Humanlike Robots—Capabilities, Potential, and Challenges ................................. 477 Yoseph Bar-Cohen 17. Biomimetic Swimmer Inspired by the Manta Ray ...................................................... 495 Frank E. Fish, Hossein Haj-Hariri, Alexander J. Smits, Hilary Bart-Smith, and Tetsuya Iwasaki 18. Biomimetics and Flying Technology .............................................................................. 525 Brenda M. Kulfan and Anthony J. Colozza 19. The Biomimetic Process in Artistic Creation ............................................................... 675 Adi Marom and Gad Marom 20. Biomimetics—Reality, Challenges, and Outlook ........................................................ 693 Yoseph Bar-Cohen Index ...................................................................................................................................... 715

Preface Through evolution, nature came up with effective solutions to its challenges, and they were improved over millions of years. Nature is effectively a giant laboratory where trial-and-error experiments are made, and through evolution the results are implemented, self-maintained, and continually evolving to address the changing challenges. These experiments involve all fields of science and engineering, and they led to an enormous pool of “inventions.” The evolution process ranged in scale from nano and micro (e.g., virus and bacteria) to macro and mega sizes (e.g., our life scale and the prehistoric giant animals like the dinosaurs). Although there is still uncertainly about the cause of the extinction of the giant terrestrial creatures, it can be argued that the evolution experiments with the terrestrial mega scale failed. Namely, as opposed to the large marine creatures (e.g., whales) that survived to live in our time, the extinct mega creatures can now be found only at excavation sites and natural history museums. Nature’s organisms and creatures are quite capable, but their systems are not necessarily performing optimally because the only thing that is critical to their survival as a species is to live long enough to reproduce. The “software” code that dictates the operation of living systems is archived in the species’ genes at the size of a fraction of a cell, and it is passed from generation to generation through self-replication. Humans have always made efforts to mimic nature’s “inventions,” using them as a model for innovation and problem solving to improve our lives. These efforts became more intensive in recent years, and systematic studies of nature are being made toward better understanding and applying more sophisticated capabilities. The studying and mimicking of nature as a field of science and engineering is now widely known as biomimetics. Researchers are seeking rules, concepts, and principles of biology to inspire new possibilities including materials, mechanisms, algorithms, and fabrication processes. Some of the benefits from these studies are improved structures, actuators, sensors, interfaces, control, software, drugs, defense, intelligence, and many others. A genetic algorithm is an example of a biologically inspired algorithm—it mimics the survival of the fittest, and it is widely used for optimization of mathematical functions. Researchers in biomimetic modeling for optimization have been looking at many other aspects of nature and have even developed methods that are based on the activities in ant colonies as well as the seed-picking process of pigeons. Not all the capabilities of nature are feasible to mimic using today’s technology, and one of the numerous examples includes mimicking the cell structure of biological systems. Such a futuristic capability to produce structures will allow constructing systems with fault tolerance, self-repair, self-growth, self-reproducibility, and many others, which characterize biological systems. Emerging nanotechnologies and self-repair of polymeric structures as well as others are increasingly enabling the potential of such capabilities. Further, it could some day become possible to engineer systems that perform similarly while having personal/individual characteristics but without having to be identical with tight manufacturing tolerances as needed to fabricate systems today. Namely, we may have cars that have various sizes and perform similarly and quite effectively even though they are far from being alike. Over the years, humans learned much from nature and the developed knowledge and knowhow helped surviving generations and continues to secure us a sustainable future. Mimicking nature can be done by copying the complete appearance and performance of vii

viii

Preface

specific creatures, and we can find many examples in toy stores, which are increasingly becoming filled with simplistic imitations of walking and barking dogs, swimming frogs, and others. Although we have copied or adapted many of nature’s inventions, there is still an enormous pool of capabilities that are still mysteries that need to be unraveled. In preparing this book, the editor sought to cover the subjects of biomimetics and biologically inspired inventions. Internationally leading experts authored the various chapters of this book, and the topics are addressing key areas of this field. Chapter 1 reviews examples and highlights the topics that make nature a source of inspiration and innovation. Chapter 2 reviews the topic of artificial senses and organs; Chapter 3 describes and discusses mimicking at the cell–materials interface level, whereas Chapter 4 describes the multiscale modeling of plant cell wall architecture and tissue mechanics for biomimetic applications. Making biomimetic composites and electroactive polymer (EAP) actuators to serve as artificial muscles are described in Chapters 5 and 6, whereas the use of EAP actuators to produce refreshable braille displays is covered in Chapter 7. The topics of biomimetic optics from the angles of biology and plants are described and discussed in Chapters 8 and 9, respectively. Biologically inspired design as a tool for interdisciplinary education is described in Chapter 10. The implementation of biomimetics in manufacturing, products, and medicine, including medical devices and implants, is described in Chapters 11 through 14, and the application to robotics is discussed in Chapters 15 and 16. The latter two chapters are focused on the development of humanlike robots and their functions. Mimicking marine biology is described in Chapter 17, whereas the mimicking of flying birds and insects in Chapter 18. Also, in Chapter 19 the biomimetic process as applicable to artistic creation is covered. Chapter 20 concludes this book with a summary of the reality, challenges, and outlook for the field of biomimetics and the inspired innovation and inventions. This book is intended to serve as a comprehensive reference document that methodically reviews the subject and provides tutorial resource as well as documented challenges and vision for the future direction of this field. Yoseph Bar-Cohen Pasadena, California

Acknowledgments The editor would like to express his appreciation of the help of Tony Vaidyanathan, University of Bristol, Bristol UK, for assisting whenever it was needed to review chapters for which he had difficulties identifying able and willing reviewers. Also, the editor would like to express his appreciation of the contributions of H. Don Wolpert, Bio-Optics, Los Angeles, California, USA, for helping whenever needed and particularly in seeking knowledgeable reviewers for various chapters. Moreover, the editor would like to thank those who reviewed the chapters for their very valuable comments and suggestions, and they were: Chapter 1 Akhlesh Lakhtakia, The Charles G. Binder (Endowed) Professor of Engineering Science and Mechanics, Pennsylvania State University, University Park, PA. Raúl José Martín Palma, Departamento de Física Aplicada, Universidad Autónoma de Madrid, Madrid, Spain.

Chapter 2 Joerg C. Gerlach, University of Pittsburgh, McGowan Institute for Regenerative Medicine, Pittsburgh, PA. Jacquelyn K. (Stroble) Nagel, Oregon State University, Corvallis, OR. Pramod Bonde, University of Pittsburgh Medical Center, University of Pittsburgh, Pittsburgh, PA.

Chapter 3 Lance C. Kam Columbia University, New York, NY. Heungsoo Shin, Department of Bioengineering, Hanyang University, Seoul, South Korea. X. Traci Cui, Department of Bioengineering, University of Pitsburgh, Pittsburgh, PA.

Chapter 4 Stephen C. Cowin, Department of Mechanical Engineering, The City College, New York, NY. Tom Masselter, Research interests: Biomechanic and Bionics, University of Freiburg, Freiburg, Germany. Mohan Srinivasarao, School of Polymer, Textile and Fiber Engineering, School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, GA.

Chapter 5 Francois Barthelat, McGill University, Montreal, Quebec, Canada. Po-Yu Chen, University of California, San Diego. Marc A. Meyers, University of California, San Diego. Bert Muller, University Hospital Basel, Basel, Switzerland. Julian Vincent, University of Bath, Bath, UK.

ix

x

Acknowledgments

Chapter 6 Federico Carpi, Interdepartmental Research Centre ‘E. Piaggio’, School of Engineering, University of Pisa, Pisa, Italy. Roy D. Kornbluh, Advanced Automation Technology Center, Information, Telecommunications, and Automation Division, SRI International, Menlo Park, CA. John D. Madden, Department of Electrical and Computer Engineering, University of British Columbia, Vancouver, British Columbia, Canada. Hani E. Naguib, Associate Professor and Canada Research Chair Tier II in Smart and Functional Polymers, Department of Mechanical and Industrial Engineering, Department of Materials Science and Engineering, University of Toronto, Toronto, Ontario, Canada.

Chapter 7 Qibing Pei, Department of Materials Science and Engineering, University of California, Los Angeles, Los Angeles, CA. Noel Runyan, National Braille Press (NBP) Center for Braille Innovation (CBI), Boston, MA. Also his wife Debby. Richard Heydt, Senior Research Engineer, Physical Sciences Division, SRI International, Menlo Park, CA.

Chapter 8 Justin Marshall, Queensland Brain Institute the University of Queensland, Australia. Shinya Yoshioka, Graduate School of Frontier Biosciences, Osaka University, Japan. Tom Cronin, University of Maryland Baltimore County (UMBC), Baltimore, MD.

Chapter 9 Heather Whitney, School of Biological Sciences, University of Bristol, Bristol, UK. William K. Smith, Biology Department, Wake Forest University, Winston-Salem, NC.

Chapter 10 Julie Linsey, Texas A&M University, College Station, TX. Tom McKeag, California College of the Arts and University of California, Berkeley, CA.

Chapter 11 Robert A. Freitas, Jr., Senior Research Fellow, Institute for Molecular Manufacturing, Palo Alto, CA. Nolan Holland, and his doctoral student, chemical engineering, Ali Ghoorchian, Engineering Cleveland State University, Cleveland, OH.

Chapter 12 Ingo Burgert, Department of Biomaterials, Max-Planck Institute of Colloids and Interfaces, Berlin, Germany. George Jeronimidis, University of Reading, Reading, United Kingdom. Rainer Erb, BIOKON International, Germany.

Acknowledgments

xi

Chapter 13 Michelle Oyen, University of Cambridge, Cambridge, UK. Stuart Stock, Feinberg School of Medicine, Northwestern University, Chicago, IL. Michel Dalstra, School of Dentistry, University of Aarhus, Denmark.

Chapter 14 Peter H. Niewiarowski, Department of Biology, University of Akron, Akron, OH. Steven Vogel, Biology Department, Duke University, Durham, NC. Jeffrey Dean, Cleveland State University, Cleveland, OH. Gail Perusek, NASA Glenn Research Center, Cleveland, OH.

Chapter 15 Lalit Gupta, Southern Illinois University Carbondale, Carbondale, IL. Kiju Lee and his graduate students, Ken Hornfeck and Yan Zhang, Case Western Reserve University, Cleveland, OH. Mark Witkowski, Imperial College London, London, United Kingdom. Craig H. Martell, Naval Postgraduate School, Monterey, CA.

Chapter 16 Graham Whiteley, Elumotion, Bath, United Kingdom. Aaron Parness, Robotic Hardware Systems Group, Jet Propulsion Laboratory/California Institute of Technology, Pasadena, CA.

Chapter 17 George V. Lauder, Department of Organismic and Evolutionary Biology, Harvard University, Cambridge, MA. Rajat Mittal, Department of Mechanical Engineering, Johns Hopkins University, Baltimore, MD. Mark Murray, United States Naval Academy, Annapolis, MD.

Chapter 18 Yoseph Bar-Cohen, Jet Propulsion Laboratory/California Institute of Technology, Pasadena, CA. Alison Flatau, University of Maryland, College Park, MD. Ravi Vaidyanathan, University of Bristol, Bristol, UK.

Chapter 19 Tomasz Arciszewski, George Mason University, Fairfax, Virginia. David Hanson, Hanson Robotics, Dallas, TX.

Chapter 20 Emilo P. Calius, Future Materials and Structures, Industrial Research Limited, Auckland, New Zealand. Raúl José Martín Palma, Departamento de Física Aplicada, Universidad Autónoma de Madrid, Madrid, Spain.

Editor

Yoseph Bar-Cohen is a senior research scientist and a supervisor of the Advanced Technologies Group at the Jet Propulsion Laboratory in Pasadena, California. He received his PhD in physics (1979) from the Hebrew University, Jerusalem, Israel. In his NDEAA lab (http://ndeaa.jpl.nasa.gov/), he led the development of many novel methods and mechanisms related to electromechanics that are actuated by electroactive materials as well as ultrasonic nondestructive evaluation methods. In the materials called composites, he discovered the ultrasonic wave phenomena polar backscattering (1979) and leaky Lamb waves (1983). He coedited and coauthored 7 books, coauthored more than 340 publications, made numerous presentations at national and international conferences, cochaired 42 conferences, and has 22 registered patents. He initiated the SPIE conference on artificial muscles, which he has been chairing since 1999. Bar-Cohen challenged engineers and scientists worldwide to develop a robotic arm driven by artificial muscles to wrestle with humans and win. He organized the first contest in 2005. For his contributions to the field of artificial muscles, Business Week named him in April 2003 one of five technology gurus who are “Pushing Tech’s Boundaries.” His accomplishments earned him two NASA Honor Award Medals, two SPIE Lifetime Achievement Awards, the SPIE President’s Award and many other honors and awards. Also, he is a fellow of two technical societies: ASNT and SPIE. Further details can be found at http://ndeaa.jpl.nasa.gov/nasa-nde/yosi/yosi.htm

xiii

Contributors Hande Argunsah Department of Chemical and Biomedical Engineering Cleveland State University and Department of Biomedical Engineering Cleveland Clinic Cleveland, Ohio Yoseph Bar-Cohen Jet Propulsion Laboratory/California Institute of Technology Pasadena, California Wilhelm Barthlott Nees Institute of the University of Bonn Bonn, Germany Hilary Bart-Smith Department of Mechanical and Aerospace Engineering University of Virginia Charlottesville, Virginia

Jeffrey R. Capadona Department of Biomedical Engineering Case Western Reserve University and Louis Stokes Cleveland VA Medical Center Cleveland, Ohio Frank Cichy Institute of Lightweight Structures and Polymer Technology Technical University of Dresden Dresden, Germany R.A.J. Clark The Centre for Speech Technology Research University of Edinburgh Edinburgh, United Kingdom Anthony J. Colozza QinetiQ North America/NASA Glenn Research Center Cleveland, Ohio

Georg Bauer Botanic Garden of the University of Freiburg Schanzenstrasse, Germany

Brian L. Davis Medical Device Development Center Austen BioInnovation Institute in Akron Akron, Ohio

Jürgen Bertling Fraunhofer Institute for Environmental, Safety, and Energy Technology Oberhausen, Germany

Ryan M. Deacon Applied Physics Laboratory The Johns Hopkins University Laurel, Maryland

Adrian Bowyer Mechanical Engineering Department University of Bath Bath, United Kingdom

Petra Ditsche-Kuru Nees Institute of the University of Bonn Bonn, Germany

P. Bremner Bristol Robotics Laboratory University of the West of England Bristol, United Kingdom

K. Eder Department of Computer Science and Bristol Robotics Laboratory University of Bristol Bristol, United Kingdom xv

xvi

Contributors

Frank E. Fish Department of Biology West Chester University West Chester, Pennsylvania

N. Hawes School of Computer Science University of Birmingham Birmingham, United Kingdom

M. Fraser Department of Computer Science and Bristol Robotics Laboratory University of Bristol Bristol, United Kingdom

P. Healey School of Electronic Engineering and Computer Science Queen Mary, University of London London, United Kingdom

Friederike Gallenmüller Botanic Garden of the University of Freiburg Schanzenstrasse, Germany

Michael Helms Fraunhofer Institute for Solar Energy Systems Freiburg, Germany

Z. Ghahramani Department of Engineering University of Cambridge Cambridge, United Kingdom

Michael Hermann Fraunhofer Institute for Solar Energy Systems ISE Freiburg, Germany

Ashok K. Goel School of Interactive Computing and Center for Biologically Inspired Design Georgia Institute of Technology Atlanta, Georgia

Henning Immink Nees Institute of the University of Bonn Bonn, Germany

Maik Gude Institute of Lightweight Structures and Polymer Technology Technical University of Dresden Dresden, Germany

Tetsuya Iwasaki Department of Mechanical and Aerospace Engineering University of California, Los Angeles Los Angeles, California

Bo Gui Department of Biomedical Engineering Case Western Reserve University Cleveland, Ohio

Jan Knippers Institute of Building Structures and Structural Design University of Stuttgart Stuttgart, Germany

Hossein Haj-Hariri Department of Mechanical and Aerospace Engineering University of Virginia Charlottesville, Virginia Tobias Haushahn Botanic Garden of the University of Freiburg Schanzenstrasse, Germany

Brenda M. Kulfan The Boeing Company Auburn, Washington David W. Lee Department of Biological Sciences Florida International University Miami, Florida

xvii

Contributors

A. Lenz Bristol Robotics Laboratory and Faculty of Engineering Technology University of the West of England Bristol, United Kingdom

M. Mirmehdi Department of Computer Science and Bristol Robotics Laboratory University of Bristol Bristol, United Kingdom

Julian Lienhard Institute of Building Structures and Structural Design University of Stuttgart Stuttgart, Germany

Nils Mölders Fraunhofer Institute for Environmental, Safety, and Energy Technology Oberhausen, Germany

Rolf Luchsinger Center for Synergetic Structures Empa Dübendorf, Switzerland Karin Lunz Fraunhofer Institute for Solar Energy Systems Freiburg, Germany Adi Marom Tisch School of the Arts New York University New York, New York Gad Marom The Institute of Chemistry The Hebrew University of Jerusalem Jerusalem, Israel Tom Masselter Botanic Garden of the University of Freiburg Schanzenstrasse, Germany

Bert Müller Biomaterials Science Center University of Basel Basel, Switzerland Yogesh Kumar Murugesan Department of Chemical Engineering McGill University Montreal, Quebec, Canada Christoph Neinhuis Botanic Garden of the Technical University of Dresden Dresden, Germany Anke Nellesen Fraunhofer Institute for Environmental, Safety, and Energy Technology Oberhausen, Germany Michelle L. Oyen Department of Engineering University of Cambridge Cambridge, United Kingdom

Claus Mattheck Karlsruhe Institute of Technology Karlsruhe, Germany

Damiano Pasini Department of Chemical Engineering McGill University Montreal, Quebec, Canada

C. Melhuish Bristol Robotics Laboratory University of the West of England and University of Bristol Bristol, United Kingdom

A.G. Pipe Bristol Robotics Laboratory and Faculty of Engineering Technology University of the West of England Bristol, United Kingdom

Markus Milwich Institute for Textile Technology and Processing Engineering Denkendorf Denkendorf, Germany

Simon Poppinga Botanic Garden of the University of Freiburg Schanzenstrasse, Germany

xviii

Kelsey A. Potter Department of Biomedical Engineering Case Western Reserve University Cleveland, Ohio Marcus Rechberger Fraunhofer Institute for Environmental, Safety, and Energy Technology Oberhausen, Germany Alejandro D. Rey Department of Chemical Engineering McGill University Montreal, Quebec, Canada P. Robinson Computer Laboratory University of Cambridge Cambridge, United Kingdom Simon Schleicher Institute of Building Structures and Structural Design University of Stuttgart Stuttgart, Germany Clemens Schmitt Botanic Garden of the University of Freiburg Schanzenstrasse, Germany Hannes Schwager Botanic Garden of the Technical University of Dresden Dresden, Germany Robin Seidel Botanic Garden of the University of Freiburg Schanzenstrasse, Germany S. Skachek Bristol Robotics Laboratory and Faculty of Engineering Technology University of the West of England Bristol, United Kingdom

Contributors

Alexander J. Smits Department of Mechanical and Aerospace Engineering Princeton University Princeton, New Jersey Olga Speck Botanic Garden of the University of Freiburg Schanzenstrasse, Germany Thomas Speck Botanic Garden of the University of Freiburg Schanzenstrasse, Germany Thomas Stegmaier Institute for Textile Technology and Processing Engineering Denkendorf Denkendorf, Germany Daniel G.T. Strange Department of Engineering University of Cambridge Cambridge, United Kingdom Iwiza Tesari Karlsruhe Institute of Technology Karlsruhe, Germany Marc Thielen Botanic Garden of the University of Freiburg Schanzenstrasse, Germany Morgana M. Trexler Applied Physics Laboratory The Johns Hopkins University Laurel, Maryland R. Vaidyanathan Department of Mechanical Engineering Imperial College London London, United Kingdom and U.S. Naval Postgraduate School Monterey, California

xix

Contributors

Marc J. Weissburg School of Biology and Center for Biologically-Inspired Design Georgia Institute of Technology Atlanta, Georgia H. Donald Wolpert Bio-Optics Los Angeles, California

Jeannette Yen School of Biology and Center for Biologically-Inspired Design Georgia Institute of Technology Atlanta, Georgia

1 Introduction: Nature as a Source of Inspiring Innovation Yoseph Bar-Cohen California Institute of Technology Pasadena, California

CONTENTS 1.1 Introduction ........................................................................................................................... 2 1.2 Independent Human Innovation or Bioinspiration ..........................................................7 1.2.1 Furniture and Many Animals Have Four Legs—Is It a Coincidence?............... 7 1.2.2 The Need to Feed Our Babies—Inspiring the Design of Bottles ........................7 1.2.3 Spider Web and the Possibly Inspired Human-Made Inventions ......................7 1.2.4 Flower Opening from Bud—Model for Folding and Efficient Packaging......... 9 1.2.5 Digging Capabilities in Nature and Drilling Tools ............................................ 10 1.2.6 The Honeycomb Shape—Inspiring Model for Lightweight Strong Structures .................................................................................................................. 11 1.2.7 Thorns, Spines, and Prickles—Possibly Inspired Barbwire .............................. 12 1.2.8 The Beak and the Human Fingers—Inspiring Model for Tongs ................... 14 1.2.9 Animal Fur as an Inspiring Model for Brushes .................................................. 14 1.3 Biologically Inspired Technologies and Mechanisms .................................................... 15 1.3.1 Artificial Intelligence ............................................................................................... 15 1.3.2 Artificial and Biomimetic Materials...................................................................... 15 1.3.3 Biosensors ................................................................................................................. 15 1.3.4 Vision and Colors ..................................................................................................... 16 1.3.5 Artificial Muscles ..................................................................................................... 16 1.3.6 Inchworm Motors .................................................................................................... 17 1.3.7 Pumping Mechanisms ............................................................................................ 18 1.3.8 Ground Penetration Using a Gopher .................................................................... 18 1.4 Robotics as a Beneficiary of Biomimetic Technologies................................................... 18 1.5 Nature as a Source of Innovation for Operation in Water ............................................. 21 1.6 Birds and Insects as the Source of Inspiring Flight ........................................................22 1.6.1 Birdman Flying Sport.............................................................................................. 24 1.6.2 A Birdlike Flying Device with Flapping Wings .................................................. 25 1.6.3 A Fishlike Blimp Propelled by Wagging Its Body and Tail ............................... 25 1.6.4 Morphing Aircraft Wings ....................................................................................... 26 1.6.5 Anti-G Fluid-Pressurized Pilot Suit Inspired by the Dragonfly ....................... 27 1.6.6 Soft Landing Inspired by Seed Dispersal ............................................................ 27 1.6.7 The Tumbleweed as a Model for Planetary Rover .............................................. 28 1

2

Biomimetics: Nature-Based Innovation

1.7 Summary—Challenges and Potential Development...................................................... 30 Acknowledgments ........................................................................................................................ 31 References....................................................................................................................................... 31 Web Sites.........................................................................................................................................34

1.1  Introduction Biomimetics is the field of science and engineering that seeks to understand and to use nature as a model for copying, adapting, and inspiring concepts and designs. Evolution led to effective solutions to nature’s challenges, which were improved over millions of years. Humans have always made efforts to use nature as a model for innovation and problem solving. These efforts have become more intensive in recent years; systematic studies of nature are being made toward better understanding of nature’s capabilities and for applying more sophisticated capabilities in various fields (Benyus, 1998; Vincent, 2001; Bar-Cohen, 2005). As part of the field of biomimetics, scientists are seeking rules, concepts, mechanisms, and principles of biology to inspire new engineering possibilities, including manufacturing, mechanisms, materials, processes, and algorithms. Some of the benefits that resulted are improved structures, actuators, sensors, interfaces, control, software, drugs, defense, intelligence, and many others. A genetic algorithm is an example of a biologically inspired algorithm—it mimics the survival of the fittest, and it is widely used for optimization of mathematical functions (Drezner and Drezner, 2005; Lipson, 2005). Modeling for optimization includes activities in ant colonies and pigeons’ seed-picking process. The latter is also known as the particle swarm optimization algorithm, and it is based on the statistical process of seed picking. This algorithm is very effective in evolving hardware, particularly in designing combinational electric circuits (Amaral et al., 2004). Other applications of this algorithm include routing vehicle and telecommunication networks. Nature is effectively a giant laboratory where trial and error experiments are made, and through evolution, the results are implemented, self-maintained, and continually evolved to address the changing challenges. In performing its experiments, nature involves all the fields of science and engineering, taking into account the principles of physics, chemistry, mechanical engineering, materials science, mobility, control, sensors, and many others. The process takes place on all the scale levels ranging from nano and micro (e.g., bacteria and viruses) to macro and mega (including humans, elephants, and whales). As opposed to mega-size sea creatures such as the whales, regardless of the cause, huge creatures such as the dinosaurs are extinct, suggesting that mega-scale land animals are unsustainable forms of life. When considering nature’s solutions as a model for inspiration, it is important to remember that nature’s solutions are the result of the survival of the fittest, and these solutions are not necessarily optimal for the required function. The dandelion is an example that nicely illustrates the survival of the fittest, which is a key to the natural selection process in evolution. This plant grows on lawns among the grass plants, and enormous efforts are made to remove it. Mowing is one of the key methods of cutting the tall plants, but with time, the only types of dandelions that manage to survive are the shorter ones (Figure 1.1). Effectively, all organisms need to do is to survive long enough to reproduce. Living systems archive the evolved and accumulated genetic information by coding it into the species’ genes and passing the information from generation to generation through selfreplication.

Introduction: Nature as a Source of Inspiring Innovation

3

FIGURE 1.1 The survival of the fittest is a key to nature’s selection process of evolution, and the example of the dandelion is shown in this photo, where the tall ones were eliminated from lawns and only the short ones are widely found.

The knowledge and the recognition of many of nature’s inventions have not always been adapted to human capabilities. For example, the use of camouflage as an effective defense was very well known to humans, and camouflage could be observed in many creatures that lived near human habitats. Both predators and preys conceal themselves in such a way that the predator is able to advance toward the prey with minimum probability of being seen until the desirable moment of striking. Further, preys reduce their visibility as much as possible to minimize the danger of detection by predators. An illustration of this capability is shown in Figure 1.2, in which the lizard’s colors and patterns match that of the ground and trees on which it lives, making it very difficult to notice its presence unless it moves. One may not intuitively think of plants as users of camouflage, but there are many related examples. Because it is critical that fruits will not be picked or eaten unless they are ripe, unripe fruits are mostly green, matching the color of the leaves to a great degree. Once the fruits are ripe, they change their color to a pronounced one to make them as visible as possible, becoming red, pink, yellow, or others. The photo in Figure 1.3 shows how easy it is to identify a ripe lemon fruit among an assortment of unripe and ripe ones on the tree. It is also interesting to note that when plants bloom, they make their flowers as visible as possible (beautiful colors, minimal obstructions, etc.), and for many, once the flower is pollinated and fertilized, the plant grows new branches that cover the developing fruits and conceal them till they are ripe. In Figure 1.4, an example of the Pamela fruit is shown, where on the left, a new branch is shown (having bright leaves), whereas on the right close to the middle and quite concealed, there is a small, developing fruit. In the years before the twentieth century, armies went to the battlefield with the soldiers wearing colorful clothing that were very easy to see. Only in the last century the use of camouflage became common to armies. This suggests that we should be more proactive about studying and implementing nature’s inventions, and this book seeks to highlight many of these inventions and the applications that were made and to increase the raise possibilities of mimick­i ng them. Nature’s materials and processes are far superior to the man-made ones. The body of the various biological creatures is a laboratory that processes chemicals from the surrounding

4

Biomimetics: Nature-Based Innovation

FIGURE 1.2 (See color insert.) The lizard on the trunk of a tree and on the ground with dry leaves is barely noticeable unless it moves because its body colors and patterns match the background quite well.

FIGURE 1.3 The fruits of the lemon tree stay green until they are ripe, and thus they camouflage the ripe ones.

and produces multifunctional structures, construction materials, energy, and waste (Mann, 1995; Nemat-Nasser et al., 2005). For example, the shell of various sea creatures such as the clams and others is a very strong structure (see Figure 1.5). Some of the capabilities of nature’s materials include self-healing, reconfigurability, and replication as well as balancing chemical content, pH, temperature, pressure, and other life essentials. Nature’s materials, including fur, leather, honey, wax, milk, and silk (Carlson et al., 2005), were well recognized for their superior capabilities and were used for thousands of years as sources of food, clothing, comfort, construction, and many other applications. The need to make

Introduction: Nature as a Source of Inspiring Innovation

5

FIGURE 1.4 Once the flowers are fertilized, developing fruits (on a Pamela tree) are covered by new branches to con­ ceal them.

FIGURE 1.5 The shell of sea creatures is a very strong structure. (Photographed by the author at the collection of Benny Herbst, Indio, California.)

these materials in any desired quantity led to developing methods of enhancing their production in the natural form as well as producing imitations. Generally, many man-made materials are processed using heat and pressure, and it is in contrast to the biological processing that occurs at ambient conditions. Natural materials, such as bone, collagen, or silk, are made inside the body of creatures using environmentally friendly processes with minimal waste, and the resulting strong materials are biodegradable and recyclable by nature.

6

Biomimetics: Nature-Based Innovation

In nature, structures are either made by creatures as integral part of their body, such as bones, shells, and nails, or made by them separately, including bird nests, cocoon shells, spider web, and underground tunnels (e.g., gophers, rats, and others). Such structures are amazingly resilient, multifunctional, and tough and can be enormously larger than the size of the creature that produces them. There are many examples of structures where the creatures demonstrate highly impressive engineering skills, including the spider and its web, the beaver and the habitat dams that it constructs on water streams, and the honeycomb that provides the bees a highly efficient packing configuration for laying their eggs and the nurturing material (the honey) for its offsprings (Gordon, 1976). The honeycomb structure is also made by humans, and it is widely used to create aircraft structures, benefiting from the low weight and high strength that is obtained. Benefiting from the up-to-date advances in science and technology, we are significantly more capable of studying the capabilities and functionalities of nature’s inventions. The recent discovery of the world’s largest beaver dam (850 m) was made using the Google satellite technology and is an example that such advances are helping to document nature’s inventions. This discovery was made by the Canadian-based ecologist, Jean Thie, in northern Alberta’s Wood Buffalo National Park, Canada. The area where the dam was built is not accessible from the ground, and the use of space technology enabled making this discovery. Also, biologists estimate that the dam would have taken at least 20 years to build, and it is visible in the National Aeronautics and Space Administration (NASA) satellite imagery obtained in 1990. The dam was built out of wood, stone, and mud and involved communal efforts of beavers and their construction skill. Plants also exhibit nature’s “inventions” that inspire innovation—the mimicking of the adherence of seeds to animal fur led to the invention of Velcro used in numerous applications, including clothing and electric wire strapping. Plants’ capability to distribute water evenly throughout their structure even in the case of huge trees offers an important model for imitation. Further, the ability of the root of plants to lift heavy structures (Figure 1.6) and to fracture rocks is another aspect that is a challenge for engineers of these structures, whereas it is relatively easy for the roots (Bar-Cohen, 2005).

Concrete slab

Root

FIGURE 1.6 A root of a tree is strong enough to lift a heavy concrete slab.

Introduction: Nature as a Source of Inspiring Innovation

7

1.2  Independent Human Innovation or Bioinspiration Biological creatures evolved over many millions of years before humans reached the level of intelligence that was sufficient to start making tools. As humans became domestic, they started seeking ways to minimize their dependence on luck in finding food and resources from the area that surrounded their habitat. Observing nature was part of their daily life, and nature inspired many ideas on how to acquire and handle food and other life essentials as well as how to protect themselves and their valuables. There are numerous inventions of nature that one may want to produce in a human-made version or that have already been mimicked. As our technical capabilities improve, it is becoming increasingly easier to adapt such nature’s inventions. This section covers a series of examples of nature’s capabilities and tools that are part of our everyday life that most likely have been inspired by these capabilities. 1.2.1  Furniture and Many Animals Have Four Legs—Is It a Coincidence? One may wonder if all the inventions and tools that resemble biological models and that are widely used by humans have been a product of mimicking of nature’s model or if it is just a coincident similarity. For example, it is interesting to notice that most mammals are fourlegged creatures and also most of our furniture (e.g., chairs and tables) is supported by four legs. It is hard to believe that all human-made solutions were purely independent inventions without the inspiration of nature, ignoring what was commonly seen in their habitats. 1.2.2  The Need to Feed Our Babies—Inspiring the Design of Bottles Developing an alternative to the human breast as a source of feeding our offsprings was critical for their survival, particularly in the absence of the mother (in case of her death or unavailability of milk). This life-critical need has inspired the baby bottle as a means of realistically imitating the breast as close as possible, with the nipple being soft to make the baby more receptive to nonbreast feeding. One may follow this logic and wonder if the shape of water bottles or any bottle in general has not been inspired by this model. The photos in Figure 1.7 show both a baby bottle and the general configuration of a water bottle emulating the model that was mimicked. 1.2.3  Spider Web and the Possibly Inspired Human-Made Inventions One may wonder to what extent humans were inspired by the various creatures that lived in their neighborhoods. The presence of spiders in humans’ habitats (Figure 1.8) should have had some inspiring role on making such things as wires, ropes, nets, sieves, screens, and woven fabrics (Figure 1.9). One cannot ignore the similarity of the spider web to the fishing net, the screen in screen doors, the kitchen strainer, or even our clothing. The spider web has become one of the icons of Halloween and a widely used holiday decoration. However, whereas the decoration is easily visible, the real spider web has a relatively low cross section, making it somewhat difficult to see, and one can appreciate this fact when trying to photograph a web. The web is made of very fine fibers that are barely visible, and this characteristic is critical for the spider’s ability to trap insects. To photograph a spider web (shown in Figure 1.8), the author shined light at night time (the middle photo) and took advantage of the condensed dews on the web in a foggy day (the photo on the right).

8

Biomimetics: Nature-Based Innovation

FIGURE 1.7 Baby bottles have been inspired by the breast in the effort to imitate this feeding source as close as possible. This model may have also inspired the shape of water bottles.

The spider web is a quite strong structure and can carry a relatively large amount of water droplets from fog, dew, or rain. One may equate the spider with a tiny “machine” that spits a wire that has incredible strength. The radial thread tensile strength of the spider web is 1154 MPa compared with 400 MPa in steel of similar weight (Vogel, 2003). The wire is produced from a biologically generated chemical that consists of digested bugs, and it is processed at room temperature and ambient atmospheric pressure. The web silk becomes cured just in time to avoid either gluing the spider to its own produced web or leaving it uncured and causing the spider to fall off its produced fiber. This just-in-time capability, which is also found in some snakes’ ability to produce fresh poison, is known in industry as JIT (just in time) and is widely used in manufacturing and shipping to avoid getting accumulation of unwanted products. Many of the spider web types are amazingly flat, and they are connected in many distant points to nearby objects without any apparent entanglement of the mounting fibers. Depending on the type of spider, the distance between the fibers in the web can be as large as several centimeters to as small as a fraction of a millimeter. The web is biodegradable, but it is able to sustain exposure to relatively harsh conditions of rain, various temperatures, wind, and sunlight. The spider has a good control over the quantities of the serum that is produced so that it will not run out of “supply” while producing the fiber and connecting one mounting location to another. To a great degree, the shape of the web resembles a fishing net, and one may wonder if it was not the model that inspired the early fishermen. The wires of the web may have given the early humans the idea for making fibers and possibly even producing fabrics. One may also wonder if it is not the improvements of mimicked webs that led humans to come up with the weaving of clothing. Beyond the use of nets to catch fish, insects, and even animals, humans modified the

Introduction: Nature as a Source of Inspiring Innovation

(a)

9

(b)

(c)

FIGURE 1.8 The spider is an amazing “engineer” creating large flat structures for trapping insects. The web is hard to see (a) unless a method is used to enhance its visibility, including the use of strong side light at night (b) or unless it has dew accumulation in a foggy day (c).

net to create tools such as bags, protective covers, screens, bandage, strainers, and sieving meshes (see Figure 1.9). Progress in microelectromechanical systems technology is increasingly enabling to produce fine, continuous fibers that have an enormous strength. For example, Dzenis (2004) reported the development of an electrospinning technique that produces 2-µm-diameter fibers from a polymer solution or molten form in a high electric field. These microfibers were found to be relatively uniform, and they do not require extensive purification. 1.2.4  Flower Opening from Bud—Model for Folding and Efficient Packaging Nature has developed amazing packaging techniques, and one can see its implementation, for example, in any plant bud and the opening of flowers as shown in Figure 1.10 for the plumeria or frangipani or temple tree. The design of umbrellas allows folding them to a small easy-to-carry shape and opening them once needed to provide a large surface for protection from rain may have been inspired by the bud in the closed form and the flower in the opened form. The striking similarity between the flower and the umbrella

10

Biomimetics: Nature-Based Innovation

(a)

(b)

(c)

FIGURE 1.9 Various human-made applications that may have been inspired by the spider web: (a) screen, (b) net, and (c) strainer.

in closed and opened shapes is shown quite close in Figure 1.10. Other applications that may have been inspired include covers of spacecraft as well as foldable solar cell panels and antenna. 1.2.5  Digging Capabilities in Nature and Drilling Tools Penetrating solid objects, such as the ground, rocks, and wood, has been achieved by creatures and plants as far back as they existed on earth millions of years ago. Ants, earthworms, termites, carpenter bees (see Figure 1.11), rodents, ground squirrels (Figure 1.12), woodpeckers, and many others are capable of making holes, tunnels, and burrows for their habitat and searching for food (Bar-Cohen, 2005). Also, roots of plants have amazing capabilities to penetrate rocks and hard soil as well as lift very heavy objects (Figure 1.6). This penetration capability of biological creatures has the equivalence in human-made tools that are used for drilling and excavation for a wide variety of applications, including making a hole in a wall, deep drilling in search of oil, development of new construction sites, and exploration of the subsurface of earth and other planets in the universe (BarCohen and Zacny, 2009). Since ancient times, humans have been digging the ground and solid objects for such applications as mining for resources, searching for water, burying objects, supporting columns and structures, and searching for food (including plant bulbs and roots). Advances in penetration tools were made as a result of discovering more effective fabrication materials, developing methods of processing and machining, increasing the capability to leverage forces, and driving tools with the aid of mechanical, electrical, pneumatic, and hydraulic actuators.

Introduction: Nature as a Source of Inspiring Innovation

(a)

11

(b)

(c)

FIGURE 1.10 The flowers (of the Plumeria frangipani temple tree) in the bud and open form (a) and the possible inspiration on the design of umbrellas. The photos show a folded (b) and an open (c) umbrella. (a)

(b)

FIGURE 1.11 Tunnels produced in wood by termites (a) and carpenter bees (b). (Courtesy of R. Peter Dillon, JPL.)

1.2.6  The Honeycomb Shape—Inspiring Model for Lightweight Strong Structures The honeycomb is a hexagonal cellular structure that honeybees use for their maximum amount of stable containment (including honey and larvae), and it is made of a minimum amount of material. The honeycomb structure looks very much the same in the bee’s application and in the human-made mechanical structure (Figure 1.13). In the case of the bees,

12

Biomimetics: Nature-Based Innovation

FIGURE 1.12 An opening to a ground squirrel burrow, which is part of an approximately 10-cm-diameter interconnected underground tunnel system with multiple openings. The burrow can reach as deep as 1 m below the surface, and usually its openings are made on sloped ground to avoid flooding and to provide better visibility for the squirrel. (a)

(b)

FIGURE 1.13 (a) The honeycomb is used by the honeybee to lay eggs and store honey to feed their offsprings. (b) The same structure is also used for making aircraft structures, taking advantage of the high compressive strength and low weight.

the honeycomb serves as a highly efficient packing container for their offsprings and for food storage when their eggs hatch. On the other hand, the honeycomb in aircraft structures provides highly efficient space filler structures that are light and strong. Honeycomb parts are widely used to produce control surfaces of aircraft structures (e.g., wings, elevators, tail, floor, and many other parts), taking advantage of their important properties of low weight and high compressive strength. Besides the benefits such as strength and weight, honeycomb structures are used for effective noise isolation in buildings, cars, and others. 1.2.7  Thorns, Spines, and Prickles—Possibly Inspired Barbwire One of the important defense methods that are used to protect large-area facilities, military bases, and others against potential intruders is barbwire fencing. One can see an

Introduction: Nature as a Source of Inspiring Innovation

13

amazing similarity between the barbs and the prickles on such stems as the rose plants. The prickles and the pins on barbwires are used for a similar role of causing pain to those who come in to contact with them, and photographs showing examples of both can be seen in Figures 1.14 and 1.15. Generally, thorns, spines, and prickles are found on a wide variety of plants, and they are hard structures with sharp pointy ends that plants use to protect themselves from herbivores (animals that eat plants). Plants with thorns, spines,

FIGURE 1.14 The rose stems and barbwires have hard and sharp pointy end structures that have a similar function of deterrence.

FIGURE 1.15 The Chorisia insignis white floss silk tree of the Bomax family, found in Peru and Argentina, has prickles on its branches that protect it from herbivores (animals that eat plants).

14

Biomimetics: Nature-Based Innovation

and prickles are widely used as a natural method of fencing large areas, and their imitation in the form of barbwires is an effective maintenance-free form of fencing. 1.2.8  The Beak and the Human Fingers—Inspiring Model for Tongs Another food-handling tool that may have been inspired by nature is the pair of tongs, and it has been probably inspired by the beak of birds and/or the human fingers (see Figure 1.16). The beak and the pair of tongs have quite similar structure and function. One cannot ignore the fact that on a daily basis humans have seen birds using their beak to pick up food, and it is a high likelihood that they sought ways to copy the beak. 1.2.9  Animal Fur as an Inspiring Model for Brushes The animal fur is also a biological feature that humans were widely exposed to. Besides using the fur as protective clothing, humans may have used it as a tool for brushing surfaces in their habitat. With time, there were probably efforts made to copy fur, leading to the development of the brush. Photographs showing the similarity of the fur and a brush are presented in Figure 1.17. (a)

(b)

FIGURE 1.16 The tongs (a) may have been likely inspired by the beak of birds (b) and/or the human fingers.

FIGURE 1.17 Fur most likely was the inspiring model for the development of the brush.

Introduction: Nature as a Source of Inspiring Innovation

15

1.3  Biologically Inspired Technologies and Mechanisms There are many examples of biologically inspired technologies, algorithms, and mechanisms that were developed in recent years. This section covers several important examples. 1.3.1  Artificial Intelligence Automatically controlling the operation of systems is limited if a simple software with only predetermined options is used. Increasingly, systems are being made “smart” using artificial intelligence (AI), where the control algorithms are mimicking nature (MussaIvaldi, 2000; Musallam et al., 2004). Making these systems smart involves using artificial intelligence algorithms that provide important control capabilities such as knowledge capture, representation and reasoning, reasoning under uncertainty, planning, vision, face and feature tracking, language processing, mapping and navigation, natural language processing, and machine learning (Kurzweil, 1999; Luger, 2001; Bar-Cohen and Breazeal, 2003). Generally, AI is a branch of computer science that studies the computational requirements for such tasks as perception, reasoning, and learning to allow the development of systems that perform these capabilities (Russell and Norvig, 2003; Amaral et al., 2004). The improvement of the understanding of human cognition (Hecht-Nielsen, 2005) is increasingly enabling scientists to understand the requirements for intelligence in general and allowing the development of intelligent devices, autonomous agents, and systems that cooperate with humans to enhance their abilities. AI researchers are using models that are inspired by the computational capability of the brain and explaining them in terms of high-level psychological constructs such as plans and goals. An important computer capability that would help greatly in mimicking natural intelligence is further advances in parallel processing. 1.3.2  Artificial and Biomimetic Materials Making effective structures can benefit greatly if they could be made of materials with nature’s characteristics of self-healing, self-replication, reconfigurability, chemical balance, durability, and multifunctionality. As humans, we have very much recognized the advantages of biological and botanical materials, and we are making great efforts to use them for many applications (Carlson et al., 2005). Learning how to process biologically inspired materials can make our choices greater and improve our ability to create recyclable materials that would help greatly in protecting the environment. Mimicking nature’s materials will also benefit humans in many other ways, including the development of more lifelike prosthetics as well as artificial hips, teeth, and structural support of bones (Chapters 3 through 5). 1.3.3  Biosensors Sensors are critical to systems monitoring their functions and allow proper response to the changing conditions as needed. Sensors imitate the senses in biological creatures. The latter are providing inputs to the central nervous system about the environment around and within the biological system body, and the muscles are commanded to perform actionbased analysis of the received information (Hughes, 1999). Biological sensory systems are extremely sensitive and limited only by quantum effects (Bialek, 1987; Bar-Cohen, 2005).

16

Biomimetics: Nature-Based Innovation

Sensors are widely used, and no system can be imagined to operate effectively without them. Pressure, temperature, and optical and acoustical sensors are used and are being improved continuously in terms of their sensing capability while reducing their size and consumed power. The eye is mimicked by the camera (with air rather than the fovea aqueous content), the whiskers of rodents are mimicked as collision avoidance sensors, and the acoustic detectors imitate the sonar in bats. Similar to the ability of our body to monitor the temperature and keep it within healthy acceptable limits, our homes, offices, and other enclosed areas have environment control that allows us to operate at comfortable temperature levels (Chapter 2). In addition to these sensors, the senses of smell and taste are increasingly being mimicked using artificial nose and artificial tongue, respectively. The progress in sensing smell reached the level in that, in 2004, two researchers Linda B. Buck and Richard Axel (1991) received a Nobel Prize for their important contributions to the field. The sense of smell is a chemical analyzer of airborne molecules, which determines the presence of dangerous and hazardous chemicals and allows us to enjoy good food and other pleasant odors. The development of artificial noses was explored since the mid-1980s (Bartlett and Gardner, 1999), and today there are commercially available electronic noses that are applied in environmental monitoring and quality control in fields such as food processing. The sense of taste is the other biological analyzer of chemicals—it examines and identifies dissolved molecules and ions (Craven and Gardner, 1996). Similar to the electronic nose, researchers explored the development of an electronic tongue that mimics the biological sensory capability (Krantz-Ruckler et al., 2001). E-tongues are increasingly being used to monitor food taste and quality, monitor environmental pollution, perform ­noninvasive diagnostics (patient’s breath, analysis of urine, sweat, and skin odor), and search for chemical/biological weapons, drugs, and explosives. 1.3.4  Vision and Colors Vision and colors are important tools that are used in biology for seeing and camouflaging and provide identification and many other benefits. Using vision and colors allows seeing the surrounding or being seen by others, taking advantage of the related optical properties (Land and Nilsson, 2002). Colors of the feathers of birds, fur of animals, and plants have been a source of inspiration for photonic applications (Chapters 8 and 9), including solar cell coatings, paints, and many others. Also, the compound eyes of insects (flies, butterflies, wasps, etc.) have inspired the development of wide field-of-view devices (such as cameras), endoscopes, bio-optical sensors, and so forth. By mimicking the way light is reflected from the scales on a butterfly’s wings, the company Qualcomm has developed Mirasol Displays that use reflected light and the way human beings perceive that light. Using an interferometric modulator element in a two-plate conductive system, the display uses near-zero power when the display is static, and also it has a refresh rate that is sufficiently fast for displaying videos. 1.3.5  Artificial Muscles Muscles are the actuators of biological systems, allowing all our physical movements using compliant actuation with linear behavior (Full and Meijer, 2004). The actuators that are the closest to mimic natural muscles are the electroactive polymers (EAPs), and for this characteristic, they gained the name “artificial muscles” (Bar-Cohen, 2004; Chapter 6). There are many types of EAP materials that are known today, and most of them have

17

Introduction: Nature as a Source of Inspiring Innovation

emerged in the 1990s. Unfortunately, these materials are still limited in their ability to generate sufficient forces to perform significant tasks such as lifting heavy objects. To help rapidly advance the field, the author initiated and organized in March 1999 the first annual international EAP Actuators and Devices Conference (Bar-Cohen, 1999), which is part of the SPIE’s Smart Structures and Materials Symposium. At the opening of the first conference, the author posed a challenge to the worldwide scientists and engineers to develop a robotic arm that is actuated by artificial muscles to win an arm wrestling match against a human opponent. On March 7, 2005, the author organized the first arm wrestling match with a human (a 17-year-old high school female student) as part of the EAP-in-Action Session of this Conference. The student easily won against all three arms, demonstrating the weakness of the current materials. In a future conference, once advances in developing EAP-actuated arms lead to a sufficiently high force, a professional wrestler will be invited for another human/machine wrestling match. 1.3.6  Inchworm Motors Another form of actuation that was biologically inspired is the movement of the inchworm. Mimicking the mobility mechanism of the larva or caterpillar led to the development of high-precision motors and linear actuators that are known as inchworm motors. The forces that are generated by such actuators that are available commercially can reach more than 30 N with zero backlash and high stability. As opposed to biological muscles, the piezoelectric-actuated inchworm motors have the advantage of zero-power dissipation when holding position. Inchworm mechanisms have many configurations with the basic principle of using two brakes and an extender. An example of an inchworm mechanism is shown in Figure 1.18, in which the brakes and an extender are traveling linearly on a shaft. These motors operate in cyclic steps, in which the first brake clamps onto the shaft and the extender pushes the second brake forward. Then, brake 2 clamps the shaft, brake 1 is released, and the extender retracts to move brake 1 forward. An alternative design of such a 1. Clamps brake #1 Brake #1

Extender

2. Extends and moves brake #2

3. Clamps brake

4. Retracts and moves brake #1 forward

FIGURE 1.18 Operation sequence of a typical inchworm mechanism.

Brake #2

Shaft

18

Biomimetics: Nature-Based Innovation

motor is the use of brakes and the extender inside a tube. The three key components of this motor perform a similar travel procedure as shown in Figure 1.18, in which the gripping is done onto the wall of the inner diameter of the tube through which the inchworm travels. Inchworm motors were used already in the NASA mission called Telesat, which was launched in the mid-1980s, for high-precision articulation in the range of nanometers. 1.3.7  Pumping Mechanisms Nature uses many forms of pumping that have inspired human-made mechanisms. The most common natural one is the peristaltic pump, in which liquids are squeezed in the required direction. Pumping via valves and chambers that change volume is found in human and animal hearts, in which the chambers expand and contract to make the blood flow through the veins and arteries. Just like in mechanical pumps, the flow of the blood is critically dependent on the action of the valves in the heart. Another form of pumping is the pumping of air in a tidal process using the diaphragm as our lungs allow us to breathe. 1.3.8  Ground Penetration Using a Gopher Since 1997, the author and his advanced technologies group at the Jet Propulsion Laboratory (JPL) have been working on the development of sampling techniques for potential use in future NASA in situ exploration missions. These developed mechanisms are mostly driven by piezoelectric actuators, and an example is the ultrasonic/sonic driller/corer (Bao et al., 2003; Bar-Cohen and Zacny, 2009). The general configuration of the ultrasonic/sonic driller/ corer allows penetrating subsurfaces to a depth that is not greater than the length of the bit because the other parts are larger in diameter. To reach greater depths, a deep drill that was inspired by the gopher has been developed (Bar-Cohen et al., 2005, 2007). A piezoelectric actuator vibrates and impacts the medium that is in contact, and the mechanism consists of a bit with a diameter equal to or greater than the actuator. This device, which mimics the gopher, is lowered via a cable into the produced borehole, cores the medium, and breaks and holds the core, and after being lifted to the surface, the core is removed (Figure 1.19). The process continues by lowering the device into the created borehole and is repeated until the desired depth is reached. This device, called the ultrasonic/sonic gopher, was designed analogously to the biological gopher that digs the ground. The ultrasonic/sonic gopher was developed to the level of a prototype and demonstrated at Mount Hood and in Antarctica to perform its intended function. A modified version that includes rotation and anchoring is currently being developed, and it is called the auto-gopher.

1.4  Robotics as a Beneficiary of Biomimetic Technologies Creating robots that mimic the shape and performance of biological creatures has always been a highly desirable engineering objective (Bar-Cohen and Breazeal, 2003; Bar-Cohen and Hanson, 2009; Chapters 15 and 16). The term robot refers to a biomimetic machine with humanlike features and functions that consist of electromechanical mechanisms. Also, it suggests a machine that is capable of manipulating objects and sensing its environment as well as being equipped with a certain degree of intelligence. Manipulator arms that are fixed to a single position and perform such tasks as painting and assembly are already

Introduction: Nature as a Source of Inspiring Innovation

19

FIGURE 1.19 The ultrasonic/sonic gopher, which is a biologically inspired ground penetrator.

widely used as part of many production lines. Robots that traverse terrains via the use of legs are increasingly being developed, and they are even considered for space and military applications (see examples in Figures 1.20 and 1.21). The industry is increasingly benefiting from the advances in robotics and related biologically inspired automation (Bar-Cohen, 2000; Bar-Cohen and Breazeal, 2003). Crawlers with the equivalence of legs as well as various manipulation devices are used to perform a variety of nondestructive evaluation tasks. At JPL, a multifunctional automated crawling system (MACS) was developed for rapid scanning of aircraft structures in field conditions (Figure 1.22). The MACS consists of two legs for the mobility on structures, with one of the legs designed also to rotate. This crawler performs scanning by “walking” on aircraft fuselages while adhering to the surface via suction cups and is capable of walking upside down on various structures. The mobility on structures is critically dependent on the capability of the legs to have controlled adherence, and alternative forms that can be considered include the use of magnetic wheels and electrostatic field. Using magnetic wheels, the author and his coinvestigator (Bar-Cohen and Joffe, 1997) conceived a rover that can operate on ships and submarines. Another legged robot is the JPL’s STAR (Kennedy et al., 2005) that has four legs and can perform multiple functions, including grabbing objects as well as climbing rocks with the aid of an ultrasonic/sonic anchor on each of the legs (Bar-Cohen, 1999; Badescu et al., 2004; Bar-Cohen and Sherrit, 2007). This anchoring mechanism allows the rover to “hang on” rocks by drilling into them via relatively low axial force, and when ready to move, the anchor is extracted by applying a reverse action.

20

Biomimetics: Nature-Based Innovation

FIGURE 1.20 The Big-Dog (made by Boston Dynamics), which is a mull-like-legged robot that was developed for military applications. (Photographed by the author.)

FIGURE 1.21 (See color insert.) A six-legged robot developed at JPL for potential application in future NASA exploration missions.

Alternative methods of making rovers traverse on walls have been studied using the gecko lizard as a model (Benyus, 1998). The millions of tiny, flexible hairs on the gecko’s feet apply van der Waals attraction forces that provide powerful adhesion to walls that are as smooth as glass. To mimic this capability, tapes with nanoscopic hairs are being developed for various applications. The entertainment and toy industries have greatly benefited from advances in robotics. Toys that mimic the appearance and movement of creatures such as frogs, fish, dogs, and

Introduction: Nature as a Source of Inspiring Innovation

21

FIGURE 1.22 MACS crawling on a wall using suction cups on two simulated legs.

even babies are now found in many stores. Also, higher-end robots and toys are becoming increasingly sophisticated, allowing them to walk and even converse with humans using a vocabulary at the level of hundreds of words (Chapter 16). Some robots can be operated autonomously or remotely reprogrammed to change their characteristic behavior. One of the first robots that was able to converse and react to human expressions, both facially and verbally, is the Kismet, which was developed at the Massachusetts Institute of Technology (Breazeal, 2002; Bar-Cohen and Breazeal, 2003). Today, such a capability is quite widely available in humanlike robots that are developed in China, Japan, Korea, and the United States (Bar-Cohen and Hanson, 2009; Chapter 16). As this technology evolves, it is becoming more likely that in the future, humanlike robots may be part of our daily life, operating at our homes and offices and doing work that currently is done by humans. Beside the benefits of this technology, there is a need for awareness of the potential risks that these robots may pose due to errors or even malicious intents.

1.5  Nature as a Source of Innovation for Operation in Water Inspiration for operating in water has many marine biology models. Fins have been an important invention for use by swimmers and divers, allowing them to significantly enhance their performance (Chapter 17). Although it may be arguable that fins were a biologically inspired invention, one can state that it is a common knowledge that swimming creatures have feet with membranes. Examples include geese, swans, seagulls, seals, frogs,

22

Biomimetics: Nature-Based Innovation

FIGURE 1.23 The goose, like other swimming creatures, has feet with membranes.

and so forth (see Figure 1.23). Imitating the legs of these creatures offered the inventors of fins a model that was improved to the point where it resembles the leg of the seal, and they have some similarity to frog’s legs. This similarity of the latter led to the naming of divers—frogmen, which is clearly a bioinspired name. The stability, the maneuverability, and the swimming performance of animals in underwater conditions are determined by the morphology, position, and mobility of their control surfaces (Chapter 17). For cetaceans, such as the whales, dolphins, and porpoises, the pectoral flippers are mobile hydrofoils that generate lift similar to the engineered hydrofoils. Flippers have various shapes, and they are used to perform lateral turning, diving, surfacing, and other mobility-related functions. Under a grant from the National Science Foundation, a team from West Chester University, Duke University, and the U.S. Naval Academy are studying the three-dimensional geometry and hydrodynamic performance of cetacean flippers with various morphologies (Figure 1.24). The results are expected to provide an insight into the maneuverability, drag, and lift performance at high Reynolds numbers. The improved understanding and the development of the related analytical tools will enable to enhance the design of watercrafts as well as support biomimetic applications related to hydrodynamics and aerodynamics (Fish, 2009).

1.6  Birds and Insects as the Source of Inspiring Flight Flying of birds and insects was the inspiring model for humans to develop flying machines as we know them today (Figure 1.25). The huge number of flying-capable species suggests that nature has extensively “experimented” with aerodynamics and has been very

Introduction: Nature as a Source of Inspiring Innovation

23

FIGURE 1.24 Laboratory setup for testing various flipper designs. (Courtesy of Frank E. Fish, West Chester University.)

FIGURE 1.25 The inspiration of nature and the use of aerodynamic principles led to the development of aircraft capabilities as the ones of the supersonic passenger plane, the Concorde. (Photographed by the author at the Boeing Aerospace Museum, Seattle, WA.)

successful. Birds are able to maneuver in flight with an amazing performance, and they are able to carry quite large prey that are relatively heavy compared with their body and also to fly to great distances in their seasonal migration. They can even catch prey while flying; for example, a hawk can catch a running rabbit. Also, birds are able to catch fish by diving into water while predicting the intersection with the hunted creature along their flight and diving paths. This capability to hunt while the hunter and the hunted creatures

24

Biomimetics: Nature-Based Innovation

are both moving fast (running, flying, or swimming) is increasingly the capability of military weapons. For example, tanks are capable of hitting a moving vehicle while both are moving fast. Also, missiles are used to hit enemy fighter aircraft or other missiles by tracking the moving target and adjusting the path during flight. In addition, one may attribute the hovering capability of helicopters to an inspiration from the dragonfly and the hummingbird. Although it was clear to humans that it is feasible to fly, the simple mimicking of bird and insect flight led to many failures until the principles of aerodynamics became better understood. 1.6.1  Birdman Flying Sport Flying via a wingsuit (see Figure 1.26) is a sport in which a special jumpsuit is used to turn the human body into an airfoil that creates lift. This suit, also known as the birdman or squirrel suit, is made of a fabric that uses the legs and the arms as structural elements of the airfoil and allows a person to mimic the gliding of animals such as the flying squirrel. To end the flight with a soft landing, the flying person deploys a parachute that when opened significantly slows the landing speed. The wingsuit can be flown from any point that provides sufficient altitude to glide through the air, and it includes skydiving from an aircraft as well as jumping from a helicopter, a hot air balloon, a cliff, a tall building, or an antenna. Wingsuits were used as early as the 1930s, and the sport was involved with many fatalities of the early pioneers who experimented with it. The use of the suit provides an increased free fall time by reducing the descent rate and a greater maneuverability than in free fall. Recent development includes a wingsuit with two small turbojet engines attached to the feet, as demonstrated by Visa Parviainen in 2005 (Abrams, 2006). Generally, the difference between the wingsuit and a flying squirrel is that the latter uses its tail as a rudder for slowing the flight speed in the air, whereas the flying person uses a parachute to control the landing. The flying person manipulates the body to control the lift and drag by changing the shape of the torso, arching or bending at the shoulders, hips, and knees, the tension on the fabric wings of the suit, and the angle of attack in which the wingsuit flies relative to the wind. These maneuvers allow for slowest vertical speed

FIGURE 1.26 (See color insert.) The wingsuit acts as an airfoil that creates lift, thus reducing the descent rate. The Australian husband and wife team, Glenn Singleman and Heather Swan, are shown flying their Phoenix-Fly V2 Wingsuits. (From James Freeman, BASEClimb project, www.baseclimb.com. With permission.)

Introduction: Nature as a Source of Inspiring Innovation

25

to obtain prolonged time in free fall as well as achieve maximum horizontal glide distance across the earth. 1.6.2  A Birdlike Flying Device with Flapping Wings A birdlike flying mechanism that is biologically inspired is under consideration at the Ohio Aerospace Institute for potential future NASA mission (Figure 1.27). This study has taken into account that flying on Mars involves low air pressure, making it more difficult than flying on Earth, and therefore it is necessary to operate within a very low Reynolds number regime. In addition to this restriction, one needs to account for the practical size limitations of a vehicle that can be deployed from Earth. An entomopter vehicle was proposed to achieve substantially higher lift by designing a biomimetic configuration and using circulation control techniques. The concept is based on the use of a microscale vortex at the wing’s leading edge as determined in 1994 by Charles Ellington of the University of Cambridge (Scott, 1999). Taking advantage of the lower gravity on Mars, one may be able to develop a flying machine that is a meter wide. Flexible solar cells will be used for generating power, and they will cover the complete wing. Under a study sponsored by the Defense Advanced Research Projects Agency, researchers at the Georgia Tech Research Institute have made a preliminary confirmation that this concept may be feasible for operation on Mars, allowing a vehicle to take off, fly slowly or hover, and land. 1.6.3  A Fishlike Blimp Propelled by Wagging Its Body and Tail Wagging the body and tail is the leading form of propulsion in water, allowing many marine species to reach significant swimming speeds. Inspired by this propulsion method and using helium-filled balloon design (Figure 1.28), researchers at EMPA and Duebendorf, Switzerland, in collaboration with the Institute of Mechanical Systems of ETH, Zürich, Switzerland, developed a propelled fishlike blimp (Lochmatter, 2007; Michel et al., 2007). For this purpose, these researchers used EAPs emulating muscles to operate a ­lighter-than-air

FIGURE 1.27 A flying mechanism that mimics a bird was proposed for planetary exploration missions. (Courtesy of Anthony Colozza, Ohio Aerospace Institute.)

26

Biomimetics: Nature-Based Innovation

DE actuator

Propulsion

Deactivated Activated FIGURE 1.28 A graphic view of the EAP-activated blimp propelled by wagging the body and tail just like a fish. (Courtesy of Silvain Michel, EMPA, Materials Science and Technology, Duebendorf, Switzerland.)

FIGURE 1.29 Photographic view of the EAP-activated blimp. The black strips are dielectric elastomer EAP. (Photographed by the author at the 2008 SPIE EAP Actuators and Devices Conference.)

vehicle. In the first phase, a blimp was developed, which has flapping fins actuated by EAP (Figure 1.29). This blimp used an electric motor and a propeller for flying while the actuated fins provided steering. Recently, the development of the novel bionic propelled blimp with wagging body and tail was completed and successfully demonstrated (Jordi et al., 2010; Chapter 6). For this purpose, fluid dynamics, structural mechanics, and aerodynamics in relation to this design were studied using systematic experimental tests. The commercial applications that are considered for this technology include the development of larger blimps for transportation, observation, and reconnaissance as well as stratospheric platforms. 1.6.4  Morphing Aircraft Wings During flight, birds adjust the shape of their wings to control the flying speed and gain great maneuverability. The morphing of the wing shape also allows birds to fly with the

Introduction: Nature as a Source of Inspiring Innovation

27

least amount of energy. This wing adjustment has inspired the development of morphing airplane wings as part of a project that is funded by the NASA Langley Research Center (Wlezien et al., 1998). The goal of this project is to develop a compliant, shape-changing truss understructure for wings, and they are covered with scales that can slide over each other to accommodate the in-flight shape changes. A graphic illustration of the NASA aircraft with morphing wings is shown in Figure 1.30. The use of such a wing design offers fuel conservation while enabling faster flights over longer distances. Further, the inflight airfoil shape modification allows for drag reduction and delay of the flow transition from laminar to turbulent by moving the transition point close to the wing’s trailing edge (Courchesne et al., 2010). 1.6.5  Anti-G Fluid-Pressurized Pilot Suit Inspired by the Dragonfly The dragonfly makes incredible flying maneuvers in air at relatively high speeds and with relatively high equivalent G levels surpassing any capability that any human pilot can sustain. For many years, this capability has been studied in an effort to adapt or inspire aeronautic innovations and solutions to existing pilot problems (Huang and Sun, 2007). During flight, the dragonfly adjusts the effects of high G on its body and rapidly maneuvers using liquid-filled sacs that surround its cardiac system. This method has inspired the Swiss company, Life Support Systems, to develop an anti-G suit that allows pilots to fly at high mach speeds with significantly reduced effects on their ability to stay coherent. The developed liquid-filled suit is called “Libelle,” which in German means dragonfly (http:// www.lssag.ch/website 2003 2014.html). Tests of the Libelle suit have shown promise with advantages over the pneumatic (compressed air) anti-G suits, and they are being considered by various air forces. 1.6.6  Soft Landing Inspired by Seed Dispersal Plants use various methods to disperse their seeds to great distances. The dispersal of seeds is needed for plant species to reduce the danger of crowding in the same local area

FIGURE 1.30 A graphic illustration of a NASA aircraft with morphing wings. (Courtesy of NASA Langley Research Center.)

28

Biomimetics: Nature-Based Innovation

and competing over the same resources while facing the risk that changes in environmental conditions may endanger the survival of future generations. The dispersal methods include using wind to propel their seeds, which are shaped with a highly effective aerodynamic configuration. There are various aerodynamic methods that seeds use, and an example of the seed of the tree Tipuana tipu (approximately 6.5  cm long) is shown in Figure 1.31. This seed is spun by the wind and disseminated away from the original tree. It is interesting to mention the tropical Asian climbing gourd Alsomitra macrocarpa that is a tree with a relatively large seed having a 13-cm wingspan. The flight of this seed resembles a boomerang, and it is capable of gliding in wide circles through the rain forest. One may see quite a similarity of the aerodynamic shape of these seeds and helicopter blades, and it most likely has been an inspiring design for many aerodynamic parts of aircraft and other human-made flying machines. The aerodynamic performance of seeds has inspired the consideration of potential alternatives for landing planetary vehicles (e.g., rovers and landers) on planets with atmospheres (such as Mars and Venus). Adapting seedlike designs (Guries and Nordheim, 1984; Peroni, 1994) may offer a better alternative to parachutes and possibly a better ability to steer the lander to reach a selected landing site (Thakoor, 2002). Some of the issues that are being studied include the determination of the appropriate vehicle size, mass distribution, and platform shape to assure stable autorotation and scalability from operation on Earth to performance on Mars. 1.6.7  The Tumbleweed as a Model for Planetary Rover The tumbleweed is another plant that offers an inspiring design for planetary mobility powered by wind (Wilson et al., 2008). As shown in Figure 1.32, the tumbleweed has inspired a futuristic rover that could one day be used as a vehicle for mobility on Mars for traversing great distances using minimal power levels (Figure 1.33). This biomimetic approach was investigated by scientists and engineers at the JPL and NASA Langley. Using three-dimensional dynamic modeling and simulations (Antol et al., 2003; Southard et al.,

FIGURE 1.31 Seeds of the Tipuana tipu have an aerodynamic shape for dispersal by the wind.

29

Introduction: Nature as a Source of Inspiring Innovation

(a)

(b) Nylon bag

Lexan tube Capacitor Darlington/ Iridium voltage regulator antenna board Serial transmitter

LCD End cap

Air pump Wing nuts

Motherboard & Key switch instrumentation

GPS antenna

Battery Fuse pack Iridium modem w/ GPS receiver Mounting pallet

Lexan flange Composite flange Rubber gasket

Rubber studs

(c)

FIGURE 1.32 The tumbleweed (a) offered an inspiration for a futuristic design of a Mars rover (b and c). (Part b by Jaret B. Matthews; part c by Jack Jones, both courtesy of JPL/Caltech/NASA.)

2007), it has been shown that exploration of Mars with Tumbleweed rovers is feasible. A likely mission scenario includes search for geologically interesting features using a group of rovers equipped with heterogeneous sensor packages. A tumbleweed rover can potentially travel longer distances and access areas such as valleys that were previously inaccessible. Varying the location of the mass imbalance is one of the methods currently being considered for steering the motion of windblown tumbleweed-like rover. Also, there is a consideration to use a swarm of Tumbleweed-based wind-driven Mars rovers to provide robust, autonomous exploration of rugged terrain. Such rovers can perform cooperative exploration and move using environmental gradients. Once the rover stops rolling, it is designed to perform a “turtle” mode of operation where tools are brought down from the center of the rover (see the middle drawing in Figure 1.32) and allow conducting such tasks as sampling and analysis in support of in situ exploration of various planets. Polar missions are ideal for the tumbleweed-based rover, where payload in the form of instrument packages can be used to search for water beneath ice or desert. In 2003,

30

Biomimetics: Nature-Based Innovation

(a)

(b)

(c)

(d)

(e)

(f )

FIGURE 1.33 Potential mission scenario for the tumbleweed rover: (a) serving as its own landing system; (b) Tumbleweed is blown across the surface of Mars; (c) partially deflating near an area of scientific interest; (d) deploying internal instruments; (e) sending data to a relay satellite in orbit around Mars; (f) reinflating and continuing the journey. (Courtesy of Jaret B. Matthews, JPL/Caltech/NASA.)

a prototype tumbleweed rover was tested in a long-distance travel that took place in Greenland. Equipped with an instrument payload and satellite communications, the rover traveled more than 80 miles in less than 2 days and sent data every half an hour to a ground station at the JPL.

1.7  Summary—Challenges and Potential Development Millions of years of evolution led to highly effective biological mechanisms that are appropriate for their intended tasks (Petr, 1996). The process of evolution eliminates failed mutations and leads to the extinction of specific species that cannot cope with significant changes in their ambient conditions. Imitating nature’s principles, processes, and mechanisms offers enormous potential for inventions that can significantly improve our lives and the tools we use. Although humans have always made efforts to mimic nature, in recent years the efforts have shifted to seeking and establishing a scientific basis, and the field gained the name biomimetics. Benefiting from the up-to-date advances in science and technology, we are significantly more capable of copying, being inspired by and adapting nature’s inventions. Nature is a great model for methods to address our needs and a source for inspiring algorithms, processes, mechanisms, and devices. Implementing innovation on the basis of nature can result in benefits such as improved drugs, stronger and multifunctional materials, and superior robots. This suggests that it is important to preserve nature’s solutions, and we need to assure that endangered species survive because they may harbor mechanisms that have not been well understood yet. We can learn manufacturing techniques from animals and plants, including the use of sunlight, and production of materials with no pollution, including the development of biodegradable fibers, ceramics, plastics, and

Introduction: Nature as a Source of Inspiring Innovation

31

various chemicals. One may develop extremely strong fibers that are like the spider web, and ceramics that are shatterproof like the pearl or seashells. Nature also provides a guide as far as the appropriateness of our innovations in terms of durability, performance, and compatibility. The inspiration of nature is expected to continue growing and to enable technological improvements with impacts on every aspect of our lives. However, there are many challenges to making biologically inspired mechanisms such as miniature flying devices that are small like a fly with speeds and performance of the dragonfly. Some of the inspired future capabilities may be considered science fiction in today’s terms, but as we improve our understanding of nature and develop better capabilities, this may become an engineering reality that is closer than we think.

Acknowledgments Some of the research reported in this chapter was conducted at the JPL, California Institute of Technology, under a contract with NASA. The author would like to express his appreciation of the very valuable comments and suggestions of the reviewers of this chapter. The reviewers were Akhlesh Lakhtakia, The Charles G. Binder (Endowed) Professor of Engineering Science and Mechanics, Pennsylvania State University, University Park, PA, and Raúl José Martín Palma, Departamento de Física Aplicada, Universidad Autónoma de Madrid, Madrid, Spain. Also, the author is appreciative of Heather Swan, Blue Skies, for her help in obtaining the photo in Figure 1.26.

References Abrams M., Birdmen, Batmen, and Skyflyers: Wingsuits and the Pioneers Who Flew in Them, Fell in Them, and Perfected Them, Crown Publishing Group, New York, (2006), 320 pp. Amaral J.F.M., J.L.M. Amaral, C. Santini, R. Tanscheiot, M. Vellasco, and M. Pacheco, Towards evolvable analog artificial neural networks controllers, Proceedings of the 2004 NASA/DoD Conference on Evolvable Hardware, Seattle, WA, June 24–26, 2004, pp. 46–52. Antol J., P. Calhoun, J. Flick, G. Hajos, R. Kolacinski, D. Minton, R. Owens, and J. Parker, Low Cost Mars surface exploration: The Mars Tumbleweed, NASA LaRC Report TM-2003-212411, (August 2003), 43 pp. Badescu M., X. Bao, Y. Bar-Cohen, Z. Chang, B. Kennedy, and S. Sherrit, Enhanced robotic walking mobility in geological analogues using extractable anchors, Journal of Mechanical Design, (2004), pp. 160–168. Badescu M., X. Bao, Y. Bar-Cohen, Z. Chang, B. E. Dabiri, B. Kennedy, and S. Sherrit, Adapting the ultrasonic/sonic driller/corer for walking/climbing robotic applications, Proceedings of the Industrial and Commercial Applications of Smart Structures Technologies, SPIE Smart Structures and Materials Symposium, Paper #5762-22 (2005). Bao X., Y. Bar-Cohen, Z. Chang, B.P. Dolgin, S. Sherrit, D.S. Pal, S. Du, and T. Peterson, Modeling and computer simulation of Ultrasonic/Sonic Driller/Corer (USDC), IEEE Transaction on Ultrasonics, Ferroelectrics and Frequency Control, Vol. 50, No. 9, (September 2003), pp. 1147–1160. Bar-Cohen Y., and B. Joffe, Magnetically attached multifunction maintenance rover (MAGMER), NTR, Docket 20229, Item No. 9854, (February 6, 1997).

32

Biomimetics: Nature-Based Innovation

Bar-Cohen Y. (Ed.), Proceedings of the First SPIE Electroactive Polymer Actuators and Devices (EAPAD) Conf., Smart Structures and Materials Symposium, Volume 3669, (1999), pp. 1–414. Bar-Cohen Y. (Ed.), Automation, Miniature Robotics and Sensors for Nondestructive Evaluation and Testing, Volume 4 of the Topics on NDE (TONE) Series, American Society for Nondestructive Testing, Columbus, OH, (2000), pp.1–481. Bar-Cohen Y., and C. Breazeal (Eds.), Biologically-Inspired Intelligent Robots, SPIE Press, Bellingham, Washington, Vol. PM122, (May 2003), pp. 1–393. Bar-Cohen Y., Electroactive Polymer (EAP) Actuators as Artificial Muscles—Reality, Potential and Challenges (2nd ed.), SPIE Press, Bellingham, Washington, Vol. PM136, (March 2004), pp. 1–765. Bar-Cohen Y., (Ed.), Biomimetics—Biologically Inspired Technologies, CRC Press, Boca Raton, FL, (November 2005), pp. 1–527, ISBN: 0849331633. Bar-Cohen Y., S. Sherrit, B. Dolgin, X. Bao, and S. Askins, Ultrasonic/Sonic Mechanism of Deep Drilling (USMOD), U.S. Patent No. 6,968,910, November 29, 2005. Bar-Cohen Y., S. Sherrit, X. Bao, M. Badescu, J. Aldrich, and Z. Chang, Subsurface sampler and sensors platform using the ultrasonic/sonic driller/corer (USDC), Proceedings of the SPIE Smart Structures and Materials Symposium, Paper #6529-18, San Diego, CA, March 19–22, 2007 Bar-Cohen Y., and S. Sherrit, Self-mountable and extractable ultrasonic/sonic anchor, U.S. Patent No. 7,156,189 (January 2, 2007). Bar-Cohen Y., and D. Hanson (Graphic Artist: A. Marom), The Coming Robot Revolution—Expectations and Fears about Emerging Intelligent, Humanlike Machines, Springer, New York, (2009), 174 pp. Bar-Cohen Y., and K. Zacny (Eds.), Drilling in Extreme Environments—Penetration and Sampling on Earth and Other Planets, Wiley-VCH, Hoboken, NJ, (2009), 827 pp. Bartlett P.N., and J.W. Gardner, Electronic Noses: Principles and Applications, Oxford University Press, Oxford, UK, (1999). Benyus J.M., Biomimicry: Innovation Inspired by Nature, Perennial (HarperCollins) Press, (1998), pp. 1–302. Bialek W, Physical limits to sensation and perception, Annual Review of Biophysics, Biophysics Chemistry, Vol. 16, (1987), pp. 455–478. Breazeal C.L., Designing Sociable Robots, MIT Press, Cambridge, MA, (2002), pp. 1–281. Buck L., and R. Axel, A novel multigene family may encode odorant receptors: a molecular basis for odor recognition, Cell, Vol. 65, (April 5, 1991), pp. 175–187. Carlson J., S. Ghaey, S. Moran, C. Anh Tran, and D.L. Kaplan, Biological materials in engineering mechanisms, in Y. Bar-Cohen (Ed.), Biomimetics—Biologically Inspired Technologies, CRC Press, Boca Raton, FL (2005), pp. 365–380. Courchesne S., A.V. Popov, and R.M. Botez, New aeroelastic studies for a morphing wing, Proceedings of the 48th AIAA Aerospace Sciences Meeting Including the New Horizons Forum and Aerospace Exposition, Paper #AIAA 2010-56, held in Orlando, Florida, January 4–7, 2010. Craven M.A., and J.W. Gardner, Electronic noses—Development and future prospects, Trends in Analytical Chemistry, Vol. 15, (1996), p. 486. Drezner T., and Z. Drezner, Genetic algorithms: Mimicking evolution and natural selection in optimization models, Chapter 5 in Y. Bar-Cohen, (Ed.), Biomimetics—Biologically Inspired Technologies, CRC Press, Boca Raton, FL (November 2005), pp. 157–176. Dzenis Y., Spinning continuous fibers for nanotechnology, Science, Vol. 304 (June 25, 2004), pp. 1917–1919. Fish F.E., Biomimetics: Determining engineering opportunities from nature, Proceedings of the Biomimetics and Bioinspiration Conference, SPIE Nanoscience and Engineering Symposium, Vol. 7401, 740109, (2009). Full R.J., and K. Meijir, Metrics of natural muscle function, in Y. Bar-Cohen (Ed.), Electroactive Polymer (EAP) Actuators as Artificial Muscles—Reality, Potential and Challenges (2nd ed.), SPIE Press, Bellingham, Washington, (2004), pp. 73–89. Gordon, J.E. The New Science of Strong Materials, or Why You Don’t Fall Through the Floor (2nd ed.), Pelican-Penguin, London, (1976), pp. 1–287.

Introduction: Nature as a Source of Inspiring Innovation

33

Guries R.P., and E. Nordheim, Flight characteristics and dispersal potential of maple samaras, Forest Science, Vol. 30, (1984), pp. 434–440. Hecht-Nielsen R., Mechanization of Cognition, Chapter 3 in [Bar-Cohen, 2005], pp. 57–128. Hod L., Evolutionary robotics and open ended design automation, Chapter 4 in Y. Bar-Cohen (Ed.), Biomimetics—Biologically Inspired Technologies, CRC Press, Boca Raton, FL (November 2005), pp. 157–176. Huang H., and M. Sun, Dragonfly forewing-hindwing interaction at various flight speeds and wing phasing, AIAA J, Vol. 45, (2007), pp. 508–511. Hughes H.C., Sensory Exotica: A World beyond Human Experience, MIT Press, Cambridge, MA, 1999, pp. 1–359. Jordi C., S.A. Michel, A. Bormann, C. Gebhardt, and G.M. Kovacs, Large planar dielectric elastomer actuators for fish-like propulsion of an airship, Proceedings of the SPIE EAPAD Conference, Paper 7642–72, March 9–11, 2010. Kennedy B., A. Okon, H. Aghazarian, M. Badescu, X. Bao, Y. Bar-Cohen, Z. Chang, B. Dabiri, M. Garrett, L. Magnone, and S. Sherrit, Lemur IIb: A robotic system for steep terrain access, Proceedings of the 8th International Conference on Climbing and Walking Robots, Received the Industrial Robot Highly Commended Award, (CLAWAR 2005), London, UK, Sept. 12–15, 2005. Krantz-Ruckler C., M. Stenberg, F. Winquist, and I. Lundstrom, Electronic tongues for environmental monitoring based on sensor arrays and pattern recognition: A review, Analytica Chimica Acta, Vol. 426, (2001), p. 217. Kurzweil R., The Age of Spiritual Machines: When Computers Exceed Human Intelligence, Penguin Press, (1999). Land M.F., and D.-E. Nilsson, Animal Eyes, Oxford University Press, Oxford, UK, (2002), 244 pp. Lochmatter P., Development of a shell-like electroactive polymer (EAP) actuator, PhD dissertation ETH No. 17221, (2007), pp. 1–330. Luger G.F., Artificial Intelligence: Structures and Strategies for Complex Problem Solving, Pearson Education Publishers, One Lake Street, Upper Saddle River, NJ (2001), pp. 1–856. Mann S. (Ed.), Biomimetic Materials Chemistry, Wiley-VCH Verlag GmbH & Co., Weinheim, Germany (1995), pp. 1–400. Michel S., C. Dürager, M. Zobel, and E. Fink, Electroactive polymers as a novel actuator technology for lighter-than-air vehicles, Proceedings of the SPIE Electroactive Polymer Actuators and Devices (EAPAD) 2007, Vol. 6524, Y. Bar-Cohen (Ed.), 65241Q (2007). Musallam S., B.D. Corneil, B. Greger, H. Scherberger, and R.A. Andersen, Cognitive control signals for neural prosthetics, Science, Vol. 305, (July 9, 2004), pp. 258–262. Mussa-Ivaldi S., Real brains for real robots, Nature, Vol. 408, (November 16, 2000), pp. 305–306. Nemat-Nasser S., and C. Thomas, Ionic polymer-metal composite (IPMC), in Y. Bar-Cohen (Ed.), Electroactive Polymer (EAP) Actuators as Artificial Muscles—Reality, Potential and Challenges (2nd ed.), SPIE Press, Bellingham, Washington, (2004), pp. 171–230. Peroni P.A., Seed size and dispersal potential of Acer rubrum (aceraceae) samaras produced by populations in early and late successional environments, American Journal of Botany, Vol. 81, No. 11, (1994), pp. 1428–1434. Petr V., Animal extinctions in the fossil record: A developmental paradigm, Bulletin of the Czech Geological Survey, Vol. 71, No. 4, (1996), pp. 351–365. Russell S.J., and P. Norvig, Artificial Intelligence: A Modern Approach, Pearson Education Publishers, One Lake Street, Upper Saddle River, NJ (2003), pp. 1–1132. Scott P., A bug’s lift, Scientific American, (April 1999). Southard L., T.M. Hoeg, D.W. Palmer, J. Antol, R.M. Kolacinski, and R.D. Quinn, Exploring Mars using a group of Tumbleweed rovers, Proceedings of the 2007 IEEE International Conference on Robotics and Automation, Roma, Italy, April 10–14, 2007. Thakoor S., Bio-inspired engineering of exploration systems, Journal of Space Mission Architecture, Issue 2, (Fall 2002), pp. 49–79. Vincent J.F.V., Stealing ideas from nature, in S. Pellegrino (Ed.), Deployable Structures, Springer-Verlag, Vienna, (2001), pp. 51–58.

34

Biomimetics: Nature-Based Innovation

Vogel S., Comparative Biomechanics: Life’s Physical World, Princeton University Press, Princeton, NJ, (2003). Wilson J.L., A.P. Mazzoleni, F.R. DeJarnette, J. Antol, G.A. Hajos, and C.V. Strickland, Design, analysis, and testing of mars Tumbleweed rover concepts, Journal of Spacecraft and Rockets, Vol. 45, No. 2, (March–April 2008), p. 370, DOI: 10.2514/1.31288. Wlezien R.W., G.C. Homer, A.R. McGowan, S.L. Padula, M.A. Scott, R.J. Silcox, and J.O. Simpson, The aircraft morphing program, Proceedings of the 39th Structures, Structural Dynamics, and Materials Conference and Exhibit, AIAA-98-1927, NASA-Langley Research Center, MS493, (1998).

Web Sites 15 Coolest Cases of Biomimicry, http://brainz.org/15-coolest-cases-biomimicry/ Biomimicry, http://www.biomimicry.net/ Center for Biologically Inspired Design, http://www.cbid.gatech.edu/ Flipper Project, http://darwin.wcupa.edu/~biology/fish/flipper/index.html Planetary Exploration using Biomimetics, http://www.niac.usra.edu/files/studies/abstracts/ 448Colozza.pdf The Biomimetics Network for Industrial Sustainability, http://www.extra.rdg.ac.uk/eng/BIONIS/ Tumbleweed rover: http://smart-machines.blogspot.com/2007/04/nasas-tumbleweed-inspiredrovers-for.html, http://www.lpl.arizona.edu/~rlorenz/mars_tumbleweed_magnetometer. pdf, http://www.nasa.gov/missions/earth/f_tumbleweed.html The largest dam that beavers built, http://www.cbc.ca/technology/story/2010/05/05/tech-largest-beaver-dam.html

2 Artificial Senses and Organs: Natural Mechanisms and Biomimetic Devices Morgana M. Trexler and Ryan M. Deacon The Johns Hopkins University Laurel, Maryland

CONTENTS 2.1 Senses and Sensors ............................................................................................................. 36 2.1.1 Introduction .............................................................................................................. 36 2.1.2 Chemical Sensors ..................................................................................................... 36 2.1.2.1 Electronic Nose.......................................................................................... 37 2.1.2.2 Electronic Tongue...................................................................................... 39 2.1.3 Auditory and Acoustic Sensors ............................................................................. 40 2.1.3.1 Biomimetic Echolocation.......................................................................... 40 2.1.3.2 Biomimetic Auditory and Acoustic Sensors .........................................42 2.1.4 Mechanical Sensors .................................................................................................44 2.1.4.1 Biomimetic Gyroscope .............................................................................44 2.1.4.2 Biomimetic Length and Velocity Sensors .............................................. 45 2.1.4.3 Biomimetic Tactile Sensors ...................................................................... 45 2.1.4.4 Biomimetic Flow Sensors ......................................................................... 48 2.1.5 Thermal Sensors ...................................................................................................... 51 2.1.5.1 Biomimetic Thermal Sensing and Imagery .......................................... 51 2.1.5.2 Biomimetic Thermal Anemometer ......................................................... 53 2.1.6 Electric Sensors ........................................................................................................ 53 2.1.6.1 Biomimetic Electroreception ................................................................... 53 2.1.6.2 Biomimetic Visual Tracking and Recognition ...................................... 56 2.1.7 Biological Inspiration for Potential Biomimetic Sensors .................................... 59 2.1.8 Future Prospects for Biomimetic Sensors ............................................................ 61 2.2 Artificial Organs ..................................................................................................................63 2.2.1 Introduction ..............................................................................................................63 2.2.2 Artificial Heart .........................................................................................................63 2.2.2.1 Replacement Heart Valves .......................................................................64 2.2.2.2 Ventricular Assist Devices .......................................................................64 2.2.2.3 Complete Heart Replacement Technologies ......................................... 67 2.2.3 Artificial Lung .......................................................................................................... 70 2.2.4 Artificial Kidney ...................................................................................................... 72 2.2.5 Artificial Pancreas.................................................................................................... 74 2.2.6 Artificial Liver .......................................................................................................... 76 2.2.7 Artificial Blood ......................................................................................................... 78 2.2.8 Artificial Skin ........................................................................................................... 79 35

36

Biomimetics: Nature-Based Innovation

2.2.9 Miscellaneous Issues for Artificial Organ Technology ......................................80 2.2.10 Future Prospects for Artificial Organs ................................................................. 82 Acknowledgments ........................................................................................................................ 82 References....................................................................................................................................... 82 Web Sites......................................................................................................................................... 92

2.1  Senses and Sensors 2.1.1  Introduction The sensory systems of plants and animals are highly evolved and optimized. They are characterized by efficient functionality, low-power requirements, small size and mass, and environmental robustness. The structures of biological sensors provide filtering and amplification of incoming signals, and the high sensitivity is currently unmatched by manmade sensors. Evolution has led to many highly efficient sensory solutions for applications, including tracking prey and predators, detecting odors and tastes, recognizing objects, and controlling motion. Analogous sensory systems are applicable to engineering problems for applications, including controlling robots and autonomous vehicles such that they avoid obstacles, detecting hazardous chemicals or monitoring air quality, and allowing an amputee to sense touch through a prosthetic limb. Because biological sensors are already performing so many highly specialized sensing tasks, looking to nature for inspiration for manmade sensors is a given. When designing biomimetic sensors, it is not component-level replication that is desirable, but mimicry of functionality. This can lead to manmade sensors similar in morphology to their biological counterpart, or those having completely different morphology while functionality is mimicked. Scrutinizing the elements of a biological sensor and selecting which elements and functions are critical for mimicking can in itself be a challenge. Oftentimes, a biological sensor is not fully understood or far more complex than necessary for an engineering application; however, the examples provided by nature have inspired great innovation and technological advancements thus far. As these biological systems are examined further, new understanding will enable engineers to replicate more closely the functionality that nature has already perfected. A selection of biomimetic sensor topics is presented in the following sections. The electronic nose and tongue seek to replicate the human smell and taste senses for applications including detection of dangerous chemicals or testing food quality. Acoustic sensors aim to mimic the echolocation in dolphins and bats for robotic guidance or medical imaging. Mechanoreceptors in plants and human skin are studied for development of a sense of touch for robots. The thermal detection systems used by beetles and snakes have led to advances in thermal imagery. Motion tracking of small targets can be realized by biomimicry of photoreceptors in the eyes of flies, and electrolocation used by fish can be used to guide underwater vehicles. Many exciting technological advances in sensor technology have been inspired by nature, and further advances are sure to be on the horizon as we continue to learn about biological sensing mechanisms. 2.1.2  Chemical Sensors Chemical sensors can identify and quantify specific substances with little interference from surrounding substances. The focus of much of the research on chemical sensors is

Artificial Senses and Organs

37

aimed at biomimicry of the human senses of taste and smell. The human nose and tongue both contain many nonspecific receptors that respond differently to liquid and gaseous substances. As such, biomimetic smell and taste sensors, or electronic nose and tongue, respectively, are based on arrays of nonspecific or low-specificity sensors. Trade-offs exist between specificity and sensitivity for varying sensor types and must be optimized for the intended application. Electronic noses and tongues can be applied for detection of illegal drugs, biological or medical assays, quality assurance of food and pharmaceuticals, environmental monitoring, explosive detection, and replacements for human taste and smell testers. 2.1.2.1 Electronic Nose Olfaction is stimulated when molecules of odorous substances move through air past turbinates, curved bony structures in the nose (Nagle et al., 1998). Turbinates create turbulent airflow patterns that carry volatile organic compounds (VOCs) to the mucus membrane lining the olfactory epithelium. The VOCs become trapped by the mucus and diffuse through to the sensory cells (bipolar neurons) in the epithelium. Each sensory cell has a dendrite that projects into the nasal cavity, where it is terminated by a knob covered by cilia (Fox, 2009). The cilia are covered in a plasma membrane containing receptor proteins that bind to VOC molecules. When an odorant molecule binds to a receptor protein, enzymatic reactions are triggered to depolarize the cell’s membrane from its typical rest potential of 90 mV (Nagle et al., 1998). The sensory cells then transmit signals along axons to clustered neural networks called glomeruli. Because olfactory sensory neurons can respond to multiple odorants, it is the patterned response of the glomeruli that identifies the olfactory quality. VOCs can reach the olfactory epithelium through the mouth as well as through the nostrils. Hence, although taste and smell are separate systems, there is some interaction. In its most traditional form, an electronic nose consists of a sample handler, an array of gas sensors, and a signal processor (Nagle et al., 1998). These three components operate serially to identify, to quantify, or otherwise to describe an odorant. Each sensor in the array must have different sensitivity, and the pattern of the response across all sensors must be distinct for different odorants. Different types of sensor arrays used in electronic noses are described in detail elsewhere (Nagle et al., 1998; Rock et al., 2008) and will be briefly described later in this chapter. Several types of sensors used for odor recognition are illustrated schematically in Figure 2.1. As reviewed by Moriizumi (2000), sensor arrays composed of metal oxide semiconductors, conductive polymers, and quartz crystal microbalances (QCM) have been used. These vary in sensitivity, selectivity, and environmental stability. When selecting a sensor type for an array, one will likely be faced with a trade-off between universality of detection of VOCs and sensitivity to specific VOCs. For this reason, it is ideal to design an electronic nose with a specific application in mind such that breadth of detection and specificity can be optimized as required by selecting materials most sensitive to the polarity and chemical composition of the VOCs of interest. Metal oxide (Capone et al., 2000; Sotetter et al., 2000; Chen et al., 2008; Kurup, 2008) or conductive polymer (Scorsone et al., 2006) atop metallic electrodes are commonly used in electronic nose applications (Figure 2.1a) (Nagle et al., 1998). Upon exposure to the VOC, the resistance of the metal oxide/polymer between the electrodes changes (due to temperature or bonding, respectively) in proportion to the concentration of the VOC. Metal oxide sensors can be designed to enhance response to specific odors by tailoring the material selection (tin oxide, zinc oxide, etc.) and by doping with catalytic metals such as Pt, Ir, or Pd (Moriizumi, 2000). Polymer material selection and side groups influence the selectivity

38

(a)

Biomimetics: Nature-Based Innovation

(b)

Odorant

Metal oxide/ conductive polymer

Electrodes

(c)

Odorant Electrodes

Polymer coating Quartz disc

Acoustic wave

Resistive heating

(d) Source contact

Odorant–

Source (n–Si) Gate insulator Conducting channel

Odorant

Output transducer Membrane Input transducer Piezoelectric substrate

Catalyst-coated gate (sensing layer) Porous metal gate contact Drain contact

(e)

Fluorescent dye Emission in organic polymer matrix Odorant

Insulator Drain (n+–Si) Substrate (p-Si)

Excitation

FIGURE 2.1 Types of sensors used in electronic noses. (a) Metal oxide or conductive polymer conductivity sensor, (b) piezoelectric QCM mass-change sensor, (c) piezoelectric SAW sensor, (d) MOSFET sensor, and (e) optical fiber fluorescence emission sensor. (Adapted from Nagle, H.T., Schiffman, S.S., et al., IEEE Spectrum, 35, 22–34, 1998.)

of conductive polymer sensor arrays (Moriizumi, 2000). Metal oxide electronic nose sensors vary in sensitivity from 5 to 500 ppm, and their performance is not highly sensitive to humidity (Nagle et al., 1998). In contrast, sensor arrays using conductive polymers sense between 0.1 and 100 ppm but are highly humidity sensitive. The differences in sensitivities and environmental stability of metal oxide versus conductive polymer need to be considered during sensor design. Careful material selection will allow for optimization of stability and selectivity for the intended application. If metal oxides and conductive polymers are used in combination, there is potential to increase the capabilities of electronic nose sensor arrays on the basis of resistance changes. Other sensor mechanisms used for electronic noses include piezoelectric QCMs (Figure 2.1b) (Stetter et al., 2000) and surface acoustic wave (SAW) (Figure 2.1c) devices (Nagle et al., 1998). These both act as mass-change sensors. The QCM sensors consist of a polymer-coated resonating disk that changes mass in the presence of an odorant. This results in a change of the resonant frequency of the sensor. SAW sensors detect mass change via a shift in voltage as the acoustic wave travels between two piezoelectric substrates over a polymeric membrane, which absorbs the odorant. The phase shift is determined by the mass and absorption properties of the membrane, which is dependent on the odorant molecules absorbed. A system composed of a sensing film and using a QCM sensor array combined with neural network pattern recognition was developed and found to be capable of detecting whiskey and perfume (Ide et al., 2000). Sensing characteristics of the mixed-liquid films are related to polarity and thus were determined to be dependent on mixing ratio. QCM sensor arrays in adsorptive films offer wide selectivity control via film material selection. However, these are sensitive to humidity and only detect at levels exceeding 10 ppm (Moriizumi, 2000), so they are less sensitive than the metal oxide/conductive polymer counterparts.

Artificial Senses and Organs

39

Two other sensor types used in arrays for electronic noses include metal-oxide-silicon field effect transistors (MOSFET) and optical fibers. VOCs can react with a catalytic metal, and diffusion of the reaction product through the transistor gate (Figure 2.1d) can change the conductivity of the MOSFET device (Nagle et al., 1998). Optical fiber sensors (Figure 2.1e) can be used to detect odorants by interrogating an active material containing chemically active fluorescent dyes, which will experience shifts in emission in the presence of specified VOCs (Nagle et al., 1998). Several other unique approaches have been investigated for development of an electronic nose. Liu et al. (2006) developed a light-addressable potentiometric sensor to monitor extracellular potential of neurons. A sensor based on piezoresistive microcantilevers was used for detection of chemical and explosive vapors (Pinnaduwage et al., 2007). Mascini et al. (2004) used piezoelectric sensors coupled with pentapeptides as biomimetic traps (receptors that mimicked the aryl hydrocarbon receptor binding site in living cells) to realize a quick, inexpensive, and easy analytical system for dioxins, which are highly toxic pollutants and carcinogens. Organic thin-film transistors (Liao et al., 2005) have also been used for VOC sensing. Covington et al. (2006) took a unique approach to mimicking the olfactory mucosa, the odor-sensitive elements, by creating an array of carbon black/polymer nanocomposite films. Analytical techniques including optical sensor systems, mass spectrometry, ion mobility spectrometry, gas chromatography, infrared (IR) spectroscopy, and substance-class-specific sensors have also been used for odor detection (Rock et al., 2008). When designing an electronic nose, sensor type should be selected with the application and analytical data in mind, as different sensor types are more suited for detecting different substances with varying polarities and chemical compositions. Environmental stability is also an important concern, particularly in environments where volatiles are likely to be present, and should be considered during design. 2.1.2.2 Electronic Tongue Taste buds or barrel-shaped receptors on the dorsal surface of the tongue enable gustation (Fox, 2009). Each taste bud consists of 50 to 100 epithelial cells, or taste cells, with long microvilli that extend through a pore in the taste bud to the external environment. Different tastes are produced by contact of different chemicals with the microvilli of these cells. Sweetness is produced by glucose, sucrose, and so forth; sourness by hydrogen ions of HCl, acetic acid, citric acid, and so forth; saltiness by NaCl; bitterness by quinine, ­caffeine, and MgCl2; and umami by monosodium glutamate, disodium inosinate, and disodium guanylate (Toko, 2004). When these chemicals contact the microvilli of taste cells, taste information is transduced into an electrical signal. When stimulated, taste cells become depolarized, produce action potentials, and release neurotransmitters (Fox, 2009). Neurotransmitters stimulate sensory neurons that convey information specific to only one of the taste categories. This information is transmitted by the nerves to the brain, which perceives the taste. Like the electronic nose, sensor arrays for electronic tongues range from highly specific to more universal recognition. In one electronic tongue, a piezoelectric bulk acoustic wave sensor modified with a molecularly imprinted polymer with highly selective binding was used as the recognition element (Tan et al., 2001). In contrast, an impedentiometric electronic tongue consisting of a composite sensor array and using chemometric techniques for discrimination of soluble compounds able to elicit different gustative perceptions was developed by Pioggia et al. (2007). The composite array is composed of chemosensitive

40

Biomimetics: Nature-Based Innovation

carbon nanotubes or carbon black in a doped polythiopene matrix. This electronic tongue was capable of classifying solutions representing the five basic tastes at concentrations spanning the range of human perception. Depending on the intended application of the tongue, either of these examples may be more desirable. Again the trade-off between universality and specificity must be considered. Humans perceive taste when olfactory receptors are stimulated by breathing out through the nose while chewing (Fox, 2009). Some electronic tongue and nose designs also incorporate crossover behavior. An electronic tongue developed by Toko (2000, 2004) uses electrode recognition sites composed of lipid/polymer membranes that use global selectivity to transform taste information into electrical signals with patterns that differ for the primary taste qualities. This device is also capable of detecting odors emitted by foods. A combination electric nose/electric tongue has been developed by Winquist et al. (1999). An array of gas sensors with different selectivity patterns, signal handling, and signal pattern recognition comprises the electronic nose. In contrast, the electronic tongue relies on pulsed voltammetry. The nose and the tongue can identify tastes and smells reasonably well independently, but their performance improves when combined. This was demonstrated by experiments aimed to classify different fruit samples with each sensor separately as well as in combination (Winquist et al., 1999). Just as human taste and smell work synergistically, this work implies that a combined technology would lead to enhanced sensory ability and VOC detection. 2.1.3  Auditory and Acoustic Sensors Several species of animals exhibit enhanced auditory capabilities. Elephants and whales can sense low-frequency infrasound over long distances (Taya et al., 2007). Bats, whales, and some birds and moths can produce and/or hear ultrasound, which is something humans can do only with specialized microphones. Because owls are active at night, they have a highly developed auditory system (Lewis, 2007). Owls use a facial disc, whose shape is altered using facial muscles, to guide sounds into the ear openings. Some owl species have asymmetrically set ear openings that allow them to identify if a sound is coming from above or below. This enables them to detect the approximately 0.03-µs time difference when sounds reach one ear before the other. Biomimicry of highly developed auditory capabilities such as these holds significant potential for a wide range of sensing applications including probing of remote environments or material properties, robotic guidance, medical imaging, underwater mapping, and geological surveying. 2.1.3.1 Biomimetic Echolocation Toothed whales (including dolphins, porpoises, river dolphins, orcas, and sperm whales), bats, oilbirds, swiftlets, shrews, and tenrecs use echolocation to orient themselves and find prey by actively producing sounds that reflect off objects in their environment, conveying information back to the biological system (Waters, 2007). Biosonar is used by toothed whales including the bottlenose dolphin because of favorable acoustics and limited underwater vision. Bottlenose dolphins (Tursiops spp.) exhibit bandwidth and adaptive control over the amplitude and spectral content of their biosonar signals, allowing them to explore areas ranging from estuaries to pelagic waters (Helweg et al., 2006). The echolocation signals used by these animals are broadband impulsive clicks with peak frequencies ranging from approximately 20 to 100 kHz (Au et al., 1974; Houser et al., 1999). Dolphins are able to extract echo characteristics including Doppler shifts, extended durations, irregular

Artificial Senses and Organs

41

envelopes, and modulation effects and use these to elucidate size, shape, and composition of targets (Moore, 1997). Several engineered sensors have been inspired by biological sonar technology. A binaural sonar receiver mimicking the bandwidth and directivity measured in the bottlenose dolphin (Au and Moore, 1984) has enabled dolphin-based sonar systems (Houser et al., 2003). Kuc (1996, 1997) developed an adaptive sonar system capable of changing its location and configuration in response to detected echoes. The sonar, consisting of a transmitter and two receivers, was mounted on the end of a robotic arm and was used to locate and identify objects via echolocation. The device can differentiate objects by using rotation of the receivers to maximize the echo amplitude and bandwidth. The design of a shallow to very shallow water-deployable sonar system for littoral navigation and object detection is in progress under the Biosonar Program at the Space and Naval Warfare Systems Center in San Diego, California (Houser et al., 2003; Capus et al., 2007). Inspired by the anatomy of the dolphin auditory systems, including generation, hearing, and signal processing, the system has exhibited competence in shallow and very shallow water mine countermeasures (Moore, 1997). Biomimetic sonar technology inspired by dolphins has also been implemented for unmanned underwater vehicles (Olivieri, 2002). It has been shown that sonars can extract useful information from echolocation signals of other sonars or from sounds produced by other external acoustic sources (Kuc, 2002). As such, traveling in fixed formation and emitting identifiable echolocation sounds can facilitate cooperative echolocation. The implications of this for biomimetic sonar can be extended to groups of autonomous underwater vehicles, for example, for improved navigation and obstacle avoidance. Unlike most manmade sensors, dolphin sonar functions well in the very shallow, reverberant, near-shore region of the ocean (Dobbins, 2007). The resolution, target detection, localization, and tracking abilities of dolphin sonar are not fully understood. However, the dolphin’s lower jaw has been identified as part of an echo receptor, and it is hypothesized that the regularity of the teeth acts as a sonar array. For this reason, a receiver concept based on the dolphin’s lower jaw and arrangement of teeth is being explored (Dobbins, 2007). The schematics in Figure 2.2 illustrate (a) the arrival, receipt, and propagation of echo signals through the jaw and (b) a mechanistic depiction of a row of teeth acting as an end-fire array beamformer. End-fire array beamformers are not commonly used in sonar systems, but based on observations of the dolphin’s jaw echo receptor, this biomimetic receiver applies this technology for acoustic sensing. In the end-fire array beamformer configuration, the teeth act as pressure transducers, and acoustic signals are transmitted either to the central nervous system (CNS) along the mandibular nerves or to the ear through the fat-filled channels in the jaw. When the echo arrives from a direction along the row of teeth, a propagation delay causes all signals to arrive at the CNS simultaneously and add constructively. Signals from other directions do not arrive simultaneously, resulting in a reduced response. Thus, biomimicry of this technology for application as an acoustic sensor would result in an array with directionality and increased sensitivity in the boresight direction. Echolocating bats can be divided into frequency modulated (FM) and constant frequency (CF)-FM (Carmena and Hallam, 2004). FM bats use multiharmonic chirps with varying duration and bandwidth over a frequency range up to approximately 200 kHz. Echolocation pulses of CF-FM bats are dominated by long CF signals followed by FM signals. The CF-FM bats modify the carrier frequency of their own calls, compensating for the Doppler shift when the bat, reflector, or both are moving, which is a testament to the importance of the Doppler-shift information in the received echoes (Carmena and Hallam, 2004). An active sonar system based on bat echolocation (Waters and Abulula, 2007) uses

42

Biomimetics: Nature-Based Innovation

(a)

1

Echo

τ

3

4

5

Teeth Lower jaw

(b)

2

τ

τ

τ

Propagation via jawbone or nerves? Delays

τ

Teeth

τ

τ

To CNS or ear

τ

6°

Boresight

FIGURE 2.2 (a) Schematic of the end-fire tooth array illustrating the arrival of echo signals, receiving of signals by teeth, and propagation either as nerve impulses to the CNS or as sound waves, through the jawbone, to the ear. (b) Schematic illustrating mechanism for a row of teeth to act as an end-fire array beamformer. (Adapted from Dobbins, P., Bioinspiration Biomimetics, 2, 19–29, 2007.)

ultrasonic FM signals reflected from targets to determine range and orientation of the reflector by time-of-flight delays. RoBat, a robot capable of echolocation with Doppler-shift compensation and obstacle avoidance, was developed by Carmena and Hallam (2004). This work illustrated the importance of incorporating Doppler-shift information, which is not typically exploited by commercial robotic ultrasonic range sensors but is fundamental to the bat’s target recognition and sense-and-act cycle (Carmena and Hallam, 2004). Bats must take into account movement of other objects to navigate effectively and avoid collision. The same is true for biomimetic robots, so Doppler shift compensation is an important component of echolocation that should be mimicked. The mechanism by which acoustic images of echoes appear back to each biological system and are comprehended is not fully understood. Ultrasound images are converted to visual representations for use in medical applications, but this is likely not the same technique used by bats. Although it is known that bottlenose dolphins are able to determine large amounts of information from echoes, the exact mechanism used for recognition remains somewhat unclear. Sound templates may be stored in their brains and whole objects identified by listening for a particular sound, or algorithms may be used to identify characteristics of objects on the basis of sounds (Harley et al., 2003). The lack of understanding of recognition mechanisms remains a challenge for developing biomimetic echolocation sensors and devices (Waters, 2007). Developing nonvisual representation of echolocation information is a topic for further research (Waters and Abulula, 2007). Once the sonar data are received, however, object identification remains a challenge in biomimetic sensors based on dolphin sonar. Signal processing based on binaural data and neural networks (Helweg et al., 2006) is an ongoing research area (Kuc, 2007). 2.1.3.2 Biomimetic Auditory and Acoustic Sensors Several researchers are working to develop biomimetic auditory and acoustic wave sensors inspired by hearing mechanisms of various biological species. Hubbard et al. (2009) are

43

Artificial Senses and Organs

developing biologically inspired circuitry that mimics mammalian hearing. Their technology consists of low-power microcircuitry that implements classification and direction-finding systems of very small size and small acoustic aperture. This mimics the mechanisms used by small animals such as gerbils to localize sounds although their ears are separated by as little as a centimeter. A fly ear-inspired directional microphone acoustic sensor has been used for gunshot localization (Liu et al., 2009). Based on the fly ear’s ability to amplify time delay, the gunshot localization sensor amplifies the time delay between the explosive blast and the muzzle blast acoustic events of a gunshot, leading to localization. Another source of inspiration for acoustic sensors is the human cochlea. Sound travels through the cochlea in the form of a pressure wave. The distance that these pressure waves travel decreases as the sound frequency increases (Fox, 2009). Sounds of different pitches, or frequencies, cause peak vibrations of the basilar membrane in different regions of the cochlea. The vibrations stimulate inner hair cells at those locations, which activate sensory neurons to convey action potentials to the brain. The brain then interprets action potentials from different regions of the cochlea as sounds from different pitches (Fox, 2009). Artificial cochleas based on the classic travelling wave model of a thin membrane of graded stiffness and width separating fluid-filled channels have resulted in poor frequency resolution, extremely limited frequency range, and very low sensitivity (Bell, 2006). The cochlear amplifier mechanism in not well understood, so enhancing responses by adding active electromechanical devices would be difficult. Bell proposes that the outer hair cells (OHCs) of the cochlea behave as single-port SAW resonators and suggests that biomimetic cochlear devices based on SAW behavior may be more straightforward and realizable than other approaches (Bell, 2006). As shown in Figure 2.3, the structure of the cochlea (Figure 2.3a) resembles that of a SAW device (Figure 2.3b). In the SAW device, electromagnetic ripples are generated and detected by two sets of interdigital electrodes on a piezoelectric substrate, and in the cochlea, a standing wave is proposed to form between the row of OHCs, which act as sensors and effectors (Bell, 2006). A biomimetic cochlear device could consist of a set of discrete sensors and actuators arrayed as interdigital electrodes and coupled back to each other with a time delay, yielding an audio frequency spectrum analyzer with narrow tuning, wide range, and improved sensitivity in comparison with existing devices. (a) Tectorial membrane Vestibular lip

(b)

Subtectorial space

Hensen’s stripe

Interdigital electrode

Marginal band Electrical input

Inner spiral sulcus IHC Basilar membrane

OHC Roughened and/or waxed back surface

Surface acoustic wave

Highly polished piezoelectric crystal

Detector output Absorber

Standing wave in subtectorial space

FIGURE 2.3 (a) Schematic of the cochlea showing standing wave formation between the row of OHCs, which act as sensors and effectors. (b) Schematic of a SAW device in which electromagnetic ripples are generated and detected by two sets of interdigital electrodes on a piezoelectric substrate. (Adapted from Bell, A., Bioinspiration Biomimetics, 1, 96–101, 2006.)

44

Biomimetics: Nature-Based Innovation

One can easily imagine that it would be beneficial to mimic the pitch discrimination capability of the human cochlea rather than designing for sensitivity to only a single frequency. Rahul Sarpeshkar’s laboratory at Massachusetts Institute of Technology has developed onchip radio frequency spectrum analyzers inspired by the cochlea (Mandal et al., 2009). The biomimetic cochleae are 3 mm × 1.5 mm in size and have 50 exponentially spaced output channels. In their model of cochlear operation, inductors correlate to fluid mass, capacitors to membrane stiffness, and active elements (transistors) to active OHC feedback mechanisms. As a result, the biomimetic cochleae can operate more than 70 dB of dynamic range, consume 300 mW of power, and analyze the radio spectrum from 600 MHz to 8 GHz. The ability to process signals over a range of frequencies and switch frequencies when necessary provides significantly improved capabilities over previous biomimetic cochleae. This development has potential application in ultra-wideband radio systems for adaptable communication strategies and fast, power-efficient spectral decomposition and analysis. 2.1.4  Mechanical Sensors Insects, mammalian muscles, human skin, arthropods with flexible shells, crustaceans, and fish of the Chordata phylum all make use of mechanical sensing and actuation mechanisms. These biological sensors detect material properties or mechanical energy in the form of movement, force, strain, or flow and transform the detected information into outputs usable by the animal (Stroble et al., 2009). Biomimetic mechanosensors have a wide range of engineering applications from dexterous robot manipulation and prosthetics to gyroscopes, length and velocity sensing, and flow sensing. 2.1.4.1 Biomimetic Gyroscope Flying insects use small structures, called halteres, with approximately 400 embedded sensilla that function as strain gauges to detect the Coriolis forces exerted on the insect as their wings flap opposite during flight (Wu et al., 2002). The two halteres of a fly resemble small balls at the end of thin rods and are noncoplanar, which allows detection of rotational velocities (Wu et al., 2003). Figure 2.4 shows a schematic of the halteres and the corresponding forces that act on them. For use in micro aerial vehicles, a biomimetic gyroscope mimicking both form and function of halteres has been developed by Wu et al. (2002, 2003). These biomimetic halteres use piezo-actuated vibrating structures to sense body Halteres kinematics

Primary inertial force

a v ω

Tangential Lateral Radial

+90°

Yaw

30°

r

Pitch

Angular velocity Angular acceleration

–90°

Roll

Orientation

ω ω

g

r

Coriolis force Centrifugal force

Σ

Ftotal

Angular inertial force Gravitational force

FIGURE 2.4 (a) Schematic of halteres and (b) force components acting on them. (Adapted from Wu, W.-C., Schenato, L., et al., Biomimetic sensor suite for flight control of a micromechanical flying insect: design and experimental results, IEEE International Conference on Robotics and Automation (ICRA), Pasadena, CA, 2003.)

45

Artificial Senses and Organs

rotational velocities via the Coriolis forces (Wu et al., 2003). The conceptual design of the biomimetic halteres and a photograph of the sensor can be seen in Figure 2.5. The design incorporates a flat beam with the wide face in the plane of the haltere beating, resulting in only one sensing degree of freedom orthogonal to the haltere beating plane. A four-bar mechanism driven by a piezoelectric actuator allows for high beat frequency and large stroke. These biomimetic sensors offer advantages over microelectromechanical systems (MEMS) gyroscopes, including lower power requirements, larger angular velocity detection range, and reduced oscillations from wing flapping by phase locking to the wing. 2.1.4.2 Biomimetic Length and Velocity Sensors The mammalian muscle spindle is a length and velocity sensor for kinesthetic awareness and movement control (Fox, 2009). Muscles requiring the finest degree of control, such as the hand, have the highest density of spindles (Fox, 2009). Shown schematically and conceptually in Figure 2.6a and b, respectively these spindles contain a springlike transducer in series with the intrafusal muscle, an internal actuator that is controlled by gamma motor neuron from the CNS (Hannaford et al., 2001; Jaax and Hannaford, 2004; Fox, 2009). The contractile regions adjacent to both ends of the sensory region filter strain before arrival at the sensory region. Then, the linearly elastic sensory region transduces strain into an analog potential that is encoded into an FM signal (Ia output) by nerve endings called type Ia fibers. Length information can be extracted by stimulating static gamma motor neurons, which innervate length-sensitive muscle spindle fiber types (Hannaford et al., 2001). Stimulation of dynamic gamma motor neurons, which innervate velocity-sensitive fiber types, can lead to extraction of velocity information (Hannaford et al., 2001). A biomimetic robotic length and velocity sensor that mimics the muscle spindle in terms of core functionalities (mechanical filtering, transduction, and encoding) and sensing behavior has been developed by Jaax et al. (2001) and Jaax and Hannaford (2004). Their design, shown in Figure 2.6c, has applications in motor control research and sensors for prosthetics. 2.1.4.3 Biomimetic Tactile Sensors Tactile sensors for biorobotic applications including dexterous manipulation look to the skin and sensitivity of human fingers for bioinspiration (Yang et al., 2005; Stroble et al., 2009). The skin of the hand is innervated by a variety of receptors that convey different information (a)

Strain gauge

Bond pads

(b)

Haltere

m

6m

m l Complaint structure

Fourbar

FIGURE 2.5 (a) Schematic of haltere design and (b) photograph of a biomimetic haltere on a four-bar structure. (Adapted from Wood, R.A., Han, C.J., et al., Integrated Uncooled Infrared Detector Imaging Arrays, IEEE, New York, 1992; Wu, W.-C., Schenato, L., et al., Biomimetic sensor suite for flight control of a micromechanical flying insect: design and experimental results, IEEE International Conference on Robotics and Automation (ICRA), Pasadena, CA, 2003.)

46

Biomimetics: Nature-Based Innovation

(c)

Ia output

Cables

γmn input Contractile region Ia output

γmn input

Encoder

Custom PCB

Transducer platform = Threaded nut Threaded rod Housing

Contractile region

Sensory region

Input

Linear acutator = mechanical filter

(b)

Sensory region

Encoder

Contractile region

Stop Cantilevers

Transducer

(a)

Rotary motor

FIGURE 2.6 Biological muscle spindle (a) anatomy and (b) conceptual diagrams. (Adapted from Hannaford, B., Jaax, K., et al., Auton. Robots., 11, 267–272, 2001.) and (c) biomimetic sensor. (Adapted from Jaax, K.N., and Hannaford, B., IEEE Trans. on Robot. Autom., 20, 390–398, 2004.)

about form, texture, and mechanical stimulation, among other things. Although strain gauges are commonly used in these applications, they are susceptible to electromagnetic noise and leave room for improvement in robustness and design flexibility. A well-designed biomimetic tactile sensor could potentially offer improvement in all of these areas. Additionally, implementation of effective force sensors would extend the capabilities of dexterous robots such that they could perform tasks associated with danger, such as space shuttle repair, or those requiring high sensitivity and precision, such as surgery. One important sensory capability of human skin is sensitivity to friction. The Meissner corpuscles are mechanoreceptive nerve endings in the skin that make use of the nonuniformity of human skin and allow us to perceive friction based on contact area (Fox, 2009). Contact with a surface with a high coefficient of friction results in restricted stretch of the convex sections of skin. But, when friction is low, stretch is not restricted, thus increasing contact area (Sano et al., 2006). This principle was mimicked for use in a robotic finger by creating a structure of silicone gel surrounded by silicone bellows that stretch into a planar structure upon contact (for low friction conditions) (Sano et al., 2006). This biomimetic concept for sensing friction is illustrated schematically in Figure 2.7. Combined with sensor coils, this tactile sensor provides feedback for adapting grip force. Another case of mimicry of the Meissner corpuscles is a tactile sensor developed by Yang et al. (2005), which uses carbon microcoil/polysilicone composites that compress and extend freely, enabling a quick and accurate response. As previously mentioned, strain gauges are inadequate for many mechanical sensing applications. Biomimicry of a strain sensor (campaniform sensillum) found in insects such as the blowfly is in progress by several researchers (Skordos et al., 2001; Wicaksono et al., 2004, 2005; Vincent et al., 2007). Shown schematically in Figure 2.8a, the campaniform sensillum is a 5- to 10-µm diameter (can be elliptical or nearly circular) opening in the cuticle

47

Artificial Senses and Organs

(a)

(b)

Rough surface f

Silicone bellows

Silicone gel

Smooth surface f

Silicone gel

High-friction contact

Low-friction contact

FIGURE 2.7 Schematic illustrating biomimetic detection of friction for a robotic finger. (a) Stretch of the silicone bellows is restricted when in contact with a high friction surface, whereas (b) contact with a low-friction surface allows stretch, thus increasing contact area. (Adapted from Sano, A., Kikuuwe, R., et al., A tactile sensing for humancentered robotics, 5th IEEE Conference on Sensors, 2006.) (a)

(b)

Cap

Collar

Joint membrane

Mesocuticle Endocuticle Socket

septum

Spongy Exocuticle cuticle Tubular Dendrite body

FIGURE 2.8 (See color insert.) (a) Schematic cross section of the blowfly campaniform sensillum. (b) Schematic of biomimetic MEMS-based strain sensor with membrane-in-recess structure including cap and collar. (Adapted from Wicaksono, D.H.B., Vincent, J.F.V., et al., Biomimetic strain-sensing microstructure for improved strain sensor: Fabrication results and optical characterization, J. Micromech. Microeng., 15, S72–S81, 2005.)

that is covered by membrane layers. Deformation in the cuticle is sensed using mechanical coupling, transduction, and encoding to transfer the information to the insect’s nervous system. The campaniform sensillum is highly sensitive, despite the high stiffness of the exocuticle, because of the unique membrane-in-recess microstructure. Membranes and sharp edges are known to be stress concentrators; thus, the combination of a membrane inside a blind hole amplifies the strain. Wicaksono et al. (2005) have mimicked the campaniform sensillum and created a MEMS-based sensor using a SiO2/SiN thin film over a membrane-in-recess structure, as shown schematically in Figure 2.8b. This biomimetic sensor improves upon current strain gauge technology. To address the needs in the area of force sensors for dexterous robotic manipulation, Park et al. (2007) have developed an exoskeletal force sensing robot finger. Their design consists of multiple fiber Bragg grating (FBG) sensors embedded in a polymer-based structure. FBG sensors are optical sensors that reflect light with a peak wavelength that shifts in proportion to the strain to which they are subjected. This wavelength shift provides the basis for strain sensing, with axial strain sensitivity of approximately 1.2 pm/microstrain. These sensors are characterized by high sensitivity, multiplexing capability, immunity to

48

Biomimetics: Nature-Based Innovation

electromagnetic noise, small size, resistance to harsh environments, and they are biosafe and inert. This design allowed for sensing and measurement of both contact and grasping forces because multiple FBGs were embedded. Shape deposition manufacturing was used to fabricate a three-dimensional structure. The sensorized SDM-fabricated finger was then characterized using an FBG interrogator. Forces of 0.01 N were detectable using this design. As part of an array of similar sensors, this biomimetic force sensor offers significant potential in improving robot dexterity. It should be noted that for prosthetics applications, high-sensitivity force sensors must be coupled with biofidelic mechanotransduction. Tactile feedback models reminiscent of natural signals are required to insure that the simulated activity is intuitive to the amputee so that they can react accordingly (Kim et al., 2009). 2.1.4.4 Biomimetic Flow Sensors The hair cell is a versatile mechanosensor used by birds, fish, and mammals for hearing, in insect joints for angle detection, and for hydrodynamic flow and vibration sensing in fish and insects (Bogue, 2009). In some species, these sensors act individually, whereas in others they act synergistically in an array, providing additional sensing capabilities. Mimicry of these flow sensors would enable enhanced underwater navigation, optimized flight control, and new experimental fluid mechanics studies. Hair cells have a sensory hair (cilium) or array of hairs (cilia) that interfaces with the sensed medium (Fox, 2009). On arthropods, a tactile hair consists of a hair shaft suspended in a cuticular socket by a soft joint membrane that divides the hair into the outer lever arm, which senses the incoming stimulus, and the inner lever arm, which transmits the stimulus energy to the dendrite (Albert et al., 2001). A schematic and micrographs of an arthropod filiform hair cell are shown in Figure 2.9a, b, and c, respectively. In some cases, a group of hair cells function together to achieve sensing. This is the case in a fish’s lateral line, a distributed array of flow sensing organs, shown in Figure 2.10. The lateral line array is made up of distributed sensor nodes (neuromasts) that span the length of the fish body (Fan et al., 2002). Each sensor node consists of a cluster of hair cells embedded in protective, gel-like domes. Synergistic operation of this array of hair cells allows flow imaging of the surroundings (Yang et al., 2007), enabling fish to adapt to the environment and survive by tracking prey, evading predators, and avoiding collision. These sensors are characteristically highly sensitive, flexible, and robust, making them ideal for flow sensing. (a) Hair Cuticular membrane

Bending

(b)

(c)

Neuron stimulated

Neuron 1 mm

10 µm

FIGURE 2.9 (a) Schematic of arthropod filiform hair cell. (Adapted from Yang, Y., Chen, N., et al., Artificial hair cell and artificial lateral line, in M. Razeghi and G.J. Brown (Eds.), Quantum Sensing and Nanophotonic Devices IV, Proceedings of SPIE, 6479, 1–9, 2007.) (b and c) Tactile hair on a spider leg (indicated by arrow in panel b). (Adapted from Albert, J.T., Friedrich, O.C., et al., J. Comp. Physiol. A Neuroethol. Sensory Neural Behav. Physiol., 187, 303–312, 2001.)

49

Artificial Senses and Organs

(a)

(b)

Lateral-line Water canal displacement

Epidermis

(c) Hair cilium

Fish lateral line Neuro-mast

Nerve

Nerve cells Neuro-mast

FIGURE 2.10 Schematic of fish lateral line systems: (a) location of lateral line on surface of fish; (b) enlarged view of a segment of the fish lateral line, showing distributed sensor nodes; (c) schematic of an individual neuron node, or neuromast, consisting of a cluster of individual hair cells. (Adapted from Fan, Z., Chen, J., et al., J. Micromech. Microeng., 12, 655–661, 2002.)

Artificial hair cells (AHCs) mimicking the arthropod filiform hair cell have been investigated at the Micro and Nanotechnology Laboratory at the University of Illinois at UrbanaChampagne (UIUC) (Chen et al., 2006, 2007; Yant et al., 2007). One AHC design (Figure 2.11a) consists of photolithographically patterned, carbon-impregnated polyurethane forcesensitive resistors for detecting the motion of a polyurethane cilium (Engel et al., 2005). The AHC can detect two-axis inputs because the force-sensitive resistors are arranged in a half-bridge configuration for each axis. Another UIUC AHC design (Figure 2.11b) incorporates a cilium-like polymer hair (80 µm diameter, 500 µm height) on a paddleshaped cantilever (2 µm thick, 40 µm wide, 200 µm long) with doped silicon strain sensors at its base (Yang et al., 2007). A schematic of the cantilever concept for SHCs can be seen in Figure 2.11c (Fan et al., 2002). In the presence of flow, the cilium couples a load to the cantilever and strain sensors. Deflections of approximately 2.5 microstrain or velocities as low as 0.1 mm/s in water flows can be detected with this design, and deflections up to 55° can be tolerated (Yang et al., 2007). At the University of Twente in The Netherlands, high aspect ratio SU8 sensory hairs on a silicon nitride membrane using capacitive change measurement have been developed (Figure 2.11d and e) (Dijkstra et al., 2005, Kirjnen et al., 2006). The SU8 hairs, up to 1 mm long, undergo a deflection because of flow momentum, which leads to basilar disc rotation and change in the capacitance between the nitride membrane and the substrate. Many AHC designs have inherent limitations related to material selection or fabrication capabilities. Most AHCs are produced on a brittle silicon substrate, limiting their application to only flat surfaces. The achievable aspect ratio of polymeric artificial hairs fabricated by molding or lithography is significantly less than the biological counterparts, which has implications in sensitivity. Additionally, artificial hairs typically serve to translate force or momentum to the attached piezoresistive or capacitive sensing elements rather than themselves being sensors. Liu et al. (2008) have developed a novel artificial hair receptor composed of polyvinylidene fluoride micro- or nanofibers. These artificial hairs are sensitive themselves and are mounted on a flexible substrate, and sizes and aspect ratios comparable with biological counterparts are attainable. Aligned micro- or nanofiber arrays were produced on an insulator film using thermodirect drawing. The success of the AHC sensor has led the researchers at UIUC to begin investigation of a biomimetic lateral line sensor based on an array of AHCs (Bogue, 2009). An artificial lateral line for hydrodynamic imaging using an array of sensors that employ hotwire anemometry is also under development. Thus far, these artificial lateral lines can detect flow as low as 200 µm/s. With system-level functionality, the biomimetic lateral line could potentially dynamically track and localize a moving target.

50

Biomimetics: Nature-Based Innovation

(a)

FSR Cilia 500 µm

(d)

(b)

Top view Vertical haircell

Force

Stain sensor Metal contacts 200 µm

(c)

Side view

Cantilever support/sensor

(e) SU8-Hair SiRN-membrance layer

Poly-Si

Chromium top contact

Bulk-Si bottom contact

FIGURE 2.11 Micrographs of AHC sensors based on (a) polyurethane force-sensitive resistors for detecting the motion of a polyurethane cilium. (Adapted from Engel, J., Chen, J., et al., Polyurethane rubber as a MEMS material: characterization and demonstration of an all-polymer two-axis artificial hair cell flow sensor, in Proceedings of the 18th IEEE International Conference on Micro Mechanical Systems, 2005.), (b) cilium-like polymer hair on a paddle-shaped cantilever with doped silicon strain sensors at its base. (Adapted from Yang, Y., Chen, N., et al., Artificial hair cell and artificial lateral line, in M. Razeghi and G.J. Brown (Eds.), Quantum Sensing and Nanophotonic Devices IV, Proceedings of SPIE, 6479, 1–9, 2007.), (c) high aspect ratio SU8 sensory hairs on a silicon nitride membrane using capacitive change measurement. (Adapted from Krijnen, G.J.M., Dijkstra, M., et al., MEMS based hair flow­sensors as model systems for acoustic perception studies, Nanotechnology, 17, S84–S89, 2006; Dijkstra, M., van Baar, J.J., et al., J. Micromech. Microeng. 15, S132–S138, 2005). Schematics corresponding to panels (b) and (c) are shown in panels (d) (Adapted from Fan, Z., Chen, J., et al., J. Micromech. Microeng. 12, 655–661, 2002.) 2006 and (e) (Adapted from Krijnen, G.J.M., Dijkstra, M., et al., Nanotechnology, 17, S84–S89, 2006; Dijkstra, M., van Baar, J.J., et al., J. Micromech. Microeng., 15, S132–S138, 2005.), respectively.

51

Artificial Senses and Organs

2.1.5  Thermal Sensors Detection of radiation in the mid to far infrared(IR) spectral range (about 3–12 µm) has applications in temperature monitoring, fire detection, sensing for military or surveillance purposes, and industrial process monitoring, among other things. Inspired by thermal sensing in beetles and snakes, many types of thermal sensors and imagers are in existence or under development. 2.1.5.1 Biomimetic Thermal Sensing and Imagery Forest fires typically burn between 500°C and 1000°C and emit electromagnetic radiation in the IR wavelengths of 2.2 to 4 µm, making IR reception a useful tool for detecting and localizing forest fires (Bleckmann et al., 2004). This detection is critical to beetles because the bark of burned trees is the ideal location for them to deposit their eggs (Schmitz and Schmitz, 2002; Bogue, 2007) because the larvae depend on wood for food, but cannot cope with the defense reactions of living trees (Apel, 1989). The IR receptors of pyrophilus beetles can be divided into bolometer-type receptors (used by Merimna beetles) and photomechanic receptors (used by Melanophila beetles). The Merimna atrata beetle detects IR radiation via four bolometer-type sensors on the ventrolateral sides of the abdominal sternite (see arrows in Figure 2.12a). Each of these sensors functions as a microbolometer, which measures temperature when heated by IR radiation (Bleckmann et al., 2004). A subepidermal thermosensitive neuron and its dendrites are attached in the center of each IR sensor (Schmitz et al., 2001). The IR sensors of boid (see arrows in Figure 2.12b) and crotalid snakes are similar to those of the Merimna beetle. Used to detect their prey, the thermosensory cells of these snakes are surrounded by an IR absorbing membrane (Bleckmann et al., 2004). Any change in ambient temperature results in a change of spike frequency (Hartline, 1974). In contrast to the microbolometer mechanism, the Melanophila species are equipped with paired pit organs (see Figure 2.13) that show response, in the form of antenna twitching, to IR stimuli (Evans, 1966; Hazel et al., 2001). Each pit organ contains 60 to 70 IR sensors or sensilla. A tiny sphere made of the same material as the beetle’s outer shell makes up each sensillum, which is connected by nerves to a highly sensitive mechanoreceptive sensory cell (Bogue, 2007). The outer sphere absorbs incoming IR radiation, causing the sphere to expand. This micromechanical event is measured by a mechanoreceptor, and a neuronal signal is generated (Hazel et al., 2001; Bogue, 2007). This photomechanical sensing is specific to IR emission with a peak at 3 µm, which is characteristic of the emission spectrum (a)

(b)

FIGURE 2.12 (See color insert.) Arrows indicate locations of the IR receptors of the (a) Australian fire beetle, Merimna atrata (2 cm length), and (b) green tree python, Morelia viridis (Boidae) (head 5 cm length). (Adapted from Bleckmann, H., Schmitz, H., et al., J. Comp. Physiol. A, 190, 971–981, 2004.)

52

Biomimetics: Nature-Based Innovation

50 µm

FIGURE 2.13 Micrograph of Melanophila beetle’s pit organ each with 50–100 sensilla that twitch in response to IR stimuli. (Adapted from Bleckmann, H., Schmitz, H., et al., J. Comp. Physiol. A, 190, 971–981, 2004.)

of a forest fire burning at 700°C (Bleckmann et al., 2004). Pit vipers, pythons, and boas also possess similar IR sensing pits (Bogue, 2007), possibly with a unique surface structure, allowing them to reflect unwanted wavelengths and enhance transmission of the IR wavelengths of interest (Stone et al., 2000). The photomechanical sensors of the Melanophila are phasic (respond rapidly to changes in stimulus intensity or rate), extremely sensitive, and respond with short latency. In contrast, the microbolometers of the Merimna are phasic as well as tonic (respond slowly to stimulus, conveying information during its entire duration). The differences in sensing mechanisms could possibly be explained by the species-dependent difference in needs and applications. The Melanophila use their IR sensors to detect and localize forest fires from large distances (Poulton, 1915; Linsley, 1943), whereas the Merimna IR receptors might be intended to simply enable the beetle to avoid landing on hot surfaces (Bleckmann et al., 2004). Although IR detectors and imagers were originally developed for military use, this technology has found itself useful for applications including firefighting, portable mine detection, night vision, border surveillance, law enforcement, search and rescue, and industrial process monitoring (Rogalski, 2002). The bolometer principle of IR detection has been widely used, as reviewed by Rogalski (2002). Although similar in principle to the IR detectors of the Merimna beetle, these IR sensors did not result from mimicry of nature. The first biomimetic thermal imager has been developed by the U.S. Air Force Research Laboratory’s Materials and Manufacturing Directorate at Wright-Patterson Air Force Base, Ohio. Inspired by the mechanisms used by the IR sensors of beetles, researchers have created a microbolometer composed of a capacitive polymer film impregnated with a suspension of proteins that changes resistivity in response to absorbed IR radiation (Kaieda, 2005; Bogue, 2007). Reduction in size and weight of the imaging array would allow it to be integrated into a micro air vehicle. This technology would be a significant asset for military use.

Artificial Senses and Organs

53

The photomechanical mechanism of IR detection has also inspired biomimicry. A device based on photomechanical technology consists of a solid absorber, with strong IR stretch resonances in the wavelength range of interest, directly connected to a sensitive mechanosensor (Bleckmann et al., 2004). High spectral sensitivity is inherent to this design. The Biomimetic Infrared Nanosystems project initiated at the University of California at Berkeley has progressed toward a photomechanic sensor using a cantilever bimorph structure produced via MEMS technology. To achieve a high thermal mismatch, one side of the bimorph is coated with chitin, which is found in the beetle’s pit organ and stretches in response to IR radiation. They have also created MEMS structures from chitosan, a soluble derivative of chitin (Bogue, 2007, 2009; Pisano et al., 2007). Related work at the Berkeley Sensor and Actuator Center is aimed at developing IR sensing arrays based on thin-film transistors coated with chitin and its derivatives (Bogue, 2009). The U.S. Air Force Research Laboratory is also working on photomechanical biomimetic sensors. These are based on bacterial thermoproteins, biological macromolecules that expand when excited by IR radiation (Bogue, 2009). Circular dichroic spectroscopic measurement of changes in a protein’s secondary structure is being considered to sense the thermoprotein’s expansion. The thermoproteins are sandwiched between an IR transparent substrate and a gold film. Expansion of the thermoproteins alters the angle of a reflected laser beam, and measurement of this change allows for quantification of IR radiation. 2.1.5.2 Biomimetic Thermal Anemometer Thermal anemometers are based on the change of the heat transfer coefficient from a heated surface to its surrounding environment when the velocity of the fluid around the surface changes. When one holds a wet finger in the air to detect the direction of wind flow, the thermal anemometer principle of human skin is used (Marques and De Almeida, 2006), and the upwind direction is determined by the coldest part of the finger, which is the part with the highest thermal dissipation rate. ThermalSkin, developed by Marques and De Almeida (2006), is a biomimetic thermal anemometer used to measure airflow intensity and direction around solid structures based on human skin. Designed for use as a robotic navigation device, ThermalSkin monitors the heat transfer coefficient change from a heated surface to its surrounding environment via fluctuation in the surrounding fluid velocity. The design consists of an array of small anemometers, or thermal scales, composed of a constant temperature self-heated thermoresistor, a signal conditioning circuit, and microcontroller to interface with the network. Although based on thermal detection principles, this technology could also be used for robotic detection and localization of odors, including chemical leaks or harmful substances. 2.1.6  Electric Sensors Another class of sensors existing in nature is based on detection of electric signals. Sources of biological inspiration for electric sensors include the electric fish, the common fly, the primate eye, the human eye, and the human muscle system (Stroble et al., 2009). Applications of biomimetic electric sensors include electrolocation for object detection, discrimination, localization, and target motion tracking. 2.1.6.1 Biomimetic Electroreception Many nocturnal fish, such as Gnathonemus petersii, use electrolocation to navigate through the water. Each fish has an electric organ in its tail that produces a weak electric field,

54

Biomimetics: Nature-Based Innovation

and changes in this electric field caused by nearby objects are detected by several ampullary low-frequency electroreceptors on the rest of its body (Schwarz and von der Emde, 2001; Lenau et al., 2008). This same technology is also used by some fish to detect living prey (Bleckmann et al., 2004). In addition to their ampullary receptors, Mormyriformes and Gymnotiformes possess high-frequency tuberous electroreceptors and an electric organ that they use to produce weak electric fields. These high-frequency electroreceptors are used to obtain sensory information from distortions of their own electric field. The distortions are caused by objects with impedances different from that of the surrounding water, as shown in Figure 2.14 (Bleckmann et al., 2004). This illustrates how the fish can gain information on both location and material from electric field. Electric organ discharges (EODs) vary by species from wave type (sinusoidal with frequencies up to 1700 Hz) to pulse type (brief pulses separated by pauses longer than pulse duration), and changes in pulse duration are used to create different types of social signals (Bleckmann et al., 2004). An EOD phase shift of as little as 1° leads to a behavioral response by the G. petersii. In addition to detecting objects and measuring their electrical resistance, electrical fish can recognize other properties, including capacitive properties (possessed by animate objects such as plants, fish, worms), which alter the amplitude and waveform of their EOD. Principles of active electrolocation have been applied to biomimetic sensing devices that produce electrical current pulses in a conducting medium, such as water or ionized gases, and simultaneously sense local current density. Biomimetic sensors have been designed >100 50 30 20 15 10 5 –10 –15 –20 –30 –50 5, the three-dimensional stress regimes simplifies to a biaxial stress state, assuming no shear resistance contributes to balance the turgor pressure. When this pressure is much higher than the external one, the cell wall experiences (i) a uniformly distributed tensile stress s t = pr t, namely, hoop stress, acting tangentially the circumference of the cell wall to belt up the turgor pressure; (ii) a negligible radial stress; and (iii) a longitudinal tension s l, which is half s t. The resulting hoop stiffness, developed by the cell wall inflated in tension, can increase with r/t as long as the cell wall does not become too thin to prompt perforation. If the external pressure dominates the turgor pressure as the protoplast deflates, the cell wall is free to bend and eventually buckle. Bending stresses generate tension in the cell wall with tension in the outer cell wall surface and compression at the interior side. This crude analogy of tissue cell wall with hydrostatic vessel follows many assumptions, among which the uniform linear elastic material being the most restrictive. Nevertheless, the insight that can be drawn here is that the stiffness of a fully turgid thin-walled tissue is very much affected by the symplast. In particular, the important benefit of this design is that the tissue resorts to the incompressibility of the fluid which exercises an increasing tension in the cell wall as the turgor pressure increases. In addition, the symplast content

154

Biomimetics: Nature-Based Innovation

within the tissue might vary with aging and environmental conditions, thereby leaving the chance of controlling the organ stiffness with time. 4.4.3.3 The Hybrid Scenario: Thick-Walled Pressurized Tissue In contrast to thin-cell walls, thick-wall cells, where the thickness is less than approximately one-fifth of its radius, do play a role in the mechanical response of the tissue to external forces. If open-ended, plant cells experience a two-dimensional nonlinear stress state with radial s r and tangential (hoop) s t components, which can be described by the Lame’s relations (Lai, 1977),

st ,r =

pi ri2 − po ro2 ri2 ro2 ( pi − po ) ± . ro2 − ri2 r(ro2 − ri2 )

(4.7)

If the external pressure is much lower than the turgor pressure, then the previous expressions can be approximated by setting po = 0 such that

st ,r =

pi ri2  r2  1 ± o2  . 2  r − ri  r  2 o

(4.8)

In contrast to the uniform stress distribution of thin-cell, under only turgor pressure, these stresses are nonlinear along the cell thickness with maximum values at the inside of the tissue cell wall. However, the tangential (hoop) stress is tensile and the radial stress is compressive. The approximate relations given in this and previous sections for modeling both turgid thin-walled hydrostatic tissues and thick-walled pressurized tissues are presented to provide a comprehensive description of alternative tissue scenarios. As an example, the former will be used in a later section to help interpret the tissue anatomy with respect to its mechanical properties. Nevertheless, we must emphasize that the tissue behavior is much more complex than the relations presented here induce to believe. A more realistic model for turgid tissues is that of a liquid-filled cellular foam (Warner and Edwards, 1988; Dünger et al., 1999; Georget et al., 2003), where the tissue is considered as a composite structure where several tissue co-exist (Figure 4.14) with often a nonlinear viscoelastic behavior. 4.4.4  Mechanical Behavior of Selected Tissues 4.4.4.1 Parenchyma Parenchyma cells consist exclusively of primary thin wall layers, which confer little resistance to bending stresses; however when fully turgid, the symplast of the tissue can confer a substantial stiffness to the tissue as a whole, by withstanding the fluid pressure in the same way hydrostatic vessels do (Section 4.4.3.2 ). Hence, the water content in parenchyma plays a significant role in withstanding external forces, but it is not the only factor. Load resistance depends also on how closely the cells are packed together. In densely packaged cell tissue, parenchyma is found in anatomical locations where compressive, bending and torsional shear stresses are high, such as toward the center and periphery of aerial stems and petioles of dicots leaves. The interstitial volume between cells is minimized to maximize the cell wall contact area, thereby optimizing the capacity to tessellate cells into a given space. This has an impact on the tissue stiffness, as closed cell packing restrains the

Multiscale Modeling of Plant Cell Wall Architecture and Tissue Mechanics

155

freedom of the cells to deform. Thus, bulky paremchymatous organs are stiffer than their counterpart, although they are made of the same material. To describe the effect of the presence of turgor pressure as well as the contribution of the cell wall geometry for externally applied infinitesimal deformations of a spherical or polyhedral cell, Nilsson et al. (1958) first formulated a linear relation of the apparent elastic modulus of parenchyma by assuming a linear elastic material under small deformation. Based on the analogy of the hydrostat, the elastic modulus of the whole tissue can be expressed as a function of the turgor pressure p, the cell geometry described by wall thickness t and average cell radius r, and the properties of the solid material Ec, elastic modulus, and v, Poisson ratio:

 3(7 − 5 n)  Ect   7 − 5n  E = 3 1 + p + 3   .  2   20(1 + n)   10(1 − n )  r 

(4.9)

The first term expresses the contribution of turgor pressure to the elastic modulus of the tissue; the second term is governed by the material and geometric properties of the cell wall within the tissue. Despite the limiting assumptions behind, this relation highlights the variable nature of the tissue stiffness, as the turgor pressure varies with age and habitat constraints. However, this relation fails to capture several other mechanical traits of parenchyma, which is a composite nonlinear viscoelastic structure, where the cell wall infrastructure, the material properties of the constituents, the topology of its cells, and the presence of air-filled interstices as well as water play distinct roles. In addition, also the permeability of the plasma membrane controls the cell fluid expulsion from the protoplast. Thus, the overall mechanical behavior depends on the rate of stress application, cell turgor, membrane permeability, and cell wall stiffness, besides the ductility of the middle lamella, size, shape, and number of cells. 4.4.4.2 Collenchyma Key player in plant growth and development, collenchyma is the prominent tissue of stems and leaves, where it is found as a component of vascular bundles, particularly in petiole, for example, celery. It has a relatively high volume fraction of gelatinous matrix, where the turgor pressure can influence up to 40% the failure stress; fluid evacuation from the cell is also important for the mechanical response of the tissue under applied stress. Collenchyma can be considered as a pressurized cellular solid that can deform at low stress to permit growth because of their low elastic range and high plasticity. For example, collenchyma of celery could elongate from 2% to 2.5% before breaking (Niklas, 1992a). The effect of aging on the material properties of collenchymas is revealed by an increase of the breaking strength in mature collenchyma, which shows a total strain of 17% less than the young counterparts. 4.4.4.3 Epidermis As continuous layer protecting the entire plant body, the epidermis experiences gradient stresses resulting from the differential growth occurring within the plant body. Normally at an internode, epidermis experiences biaxial tensile stresses as opposed to compressive growth stresses occurring toward the center. Epidermidis anisotropy changes with age, as the orientation of the cellulose microfibrils within cell walls changes during growth. In young epidermis, the dominant fiber orientation might be aligned more with

156

Biomimetics: Nature-Based Innovation

the longitudinal axis of the stem, whereas in young epidermis a transverse orientation of microfibrils dominate the ultrastructure, allowing growth elongation. In medium age epidermis, a hybrid microfibril orientation occurs whereby transverse, oblique, and longitudinal orientations coexist. Because of this orientation, the preferred deformation of the cells is affected. Young cell will be constrained to deform longitudinally, whereas older cell would prefer radial expansion. Thus, epidermis imposes limiting anisotropic boundary conditions to growth in size and shape of stems as well as other organs. Besides microfibril, the epidermis consists of a layer of closely packaged cell with a very high cell-to-cell contact area. In the epidermis, the middle lamella, which has high shear modulus, plays a major role in determining the bending and tensional shear responses of this tissue and in turn of the entire organ. Epidermal cell walls are not homogeneous, being the peripheral ones more rigid with up to five times as thick as the inner cell walls. The contribution of the epidermis is critical to the flexural and torsional stiffness of petioles. The bending resistance stiffness reduction of a turgid slender petiole cantilever, in which the epidermis has been removed to the extent of 5% to 10% of the total cross section, can reach up to 70%. Thus, the epidermis provides as stiff outer rind relative to the enchymatous core. 4.4.4.4 Vascular Tissue System Unlike parenchyma and collenchymas, in which each tissue essentially consists of a single cell type, the vascular tissue system is composed of a variety of structurally and functionally different cell types (Figure 4.14). Depending on their ontogenic age, certain tissues might vanish or increase their presence; thus, such variable heterogeneity in cell types confers the vascular tissue a marked anisotropy. We distinguish primary and secondary vascular tissues. Although the cell types might not change, their spatial distribution, proportion, and mechanical properties might differ significantly. One important feature often distinguished in vascular tissue is the marked axial symmetry; fibers, tracheids, and vessel members are typically elongated in the direction of the axis of organ elongation. This feature greatly contributes to the anisotropy of the vascular tissue. E increases with the orientation angle of the microfibrils with the longitudinal cell axis; thus, longer fibers that have lower microfibrillar angles will have a higher elastic modulus than tissues with shorter fibers. Secondary xylem, on the other hand, has a pronounced anisotropy because of the geometric asymmetry of this tissue. The elastic modulus of wood along the grain is two orders of magnitude higher than those measured tangentially and radially. The role of water content is still visible in wood, where a reduction of 27% can be measured for saturated wood. On the other hand, moistened wood is less prompt to fracture than its dry counterpart (Niklas, 1992b). 4.4.5  From Tissue to Composite-Shaped Organ: The Case of the Leaf Petiole This section concludes the plant multiscale analysis as outlined in Figure 4.2, by focusing on the length scales of the tissue and organ geometry. We examine the spatial arrangement of tissues, which are shaped into the specific morphology of a plant organ at a given time of its growth. As a paradigm organ, we select here the leaf petiole, as its geometric traits at the macroscale offer a remarkable example of how the petiole tissues confer—in certain instance—a peculiar grooved shape to the organ cross section (Figure 4.16). We are interested in studying the structural advantages that lead the tissues to grow into certain geometry as adaptive response to the loading experienced during growth. Such

Multiscale Modeling of Plant Cell Wall Architecture and Tissue Mechanics

(a)

(b) θ

157

(c)

(d) FIGURE 4.16  (See color insert.) Images of petiole hierarchies at different length scales: (a) SEM images of parenchyma tissue in micrometers; (b) microscopic image in millimeters; (c) cross section of Monstera petiole in centimeter length scale; (d) idealized grooved petiole with epidermis tissue and cellular parenchyma core with cells idealized as coiled-microfibril reinforced cylinders.

a structural mechanics approach has limitations, one being the neglect of the multifunctional design nature of the petiole, in which tissue anatomy and organ morphology are the result of conflicting requirements, each of them reconciled with the minimum metabolic cost. In fact, as often is the case in plants, the morphological and the anatomical features are trade-off developments at a given stage of the organ life. For example, at the anatomical tissue level, the conflict of load supporting and photosynthesis functions is reconciled by a compromise in which the stiff tissues are placed at a certain distance from the neutral axis rather than at the periphery where mechanical resistance would be maximized. As a structural analog of the leaf petiole, we consider a cantilever beam undergoing large deformations that are induced by self-weight and surface forces generated by the wind action and by the accumulation of external agents, such as ice, insect, and debris. How well this structural analog can predict the mechanical behavior of the petiole greatly depends on (i) the loading and (ii) the geometry differences between the cantilever model and the petiole. For the former, we should be aware that self-weight continuously varies in time as the organ grows and the wind action depends on the habitat in which the plant grows. For the latter, it is important to consider that the geometry of the petiole also varies as a function of the state of development and maturation of the organ; thus, the mechanical behavior of the petiole is not constant, as proved by the aging of the tissue properties that become stiffer and more resilient as the vascular bundles within them mature. Therefore, a major assumption followed here is that cross-sectional geometry, tissue arrangement, and structural properties of the petiole under investigation refer to a particular period of ontogeny. An approach used in the literature to model the mechanics of a layered composite organ is to obtain closed form expressions (Gielis, 2003; Speck and Rowe, 2003) of the contour of an organ and formulate analytic expressions of their mechanical properties as a function of the cross-sectional geometry (Pasini, 2007, 2008). Compared with other numeric methods

158

Biomimetics: Nature-Based Innovation

(Isnard et al., 2003a,b), this approach allows to predict the developmental impact that size, shape, and tissue arrangement have on the organ mechanics during organ growth as well as to visualize in morpho-property maps how tissue distributions and organ shape develop during ontogeny. A similar approach has been already applied for modeling extinct plants that are preserved anatomically (Speck and Vogellehner, 1988; Spatz et al., 1998a,b; Niklas and Speck, 2001). 4.4.5.1 The Effect of Tissue Layering We consider the leaf petiole in its primary stage of development as a composite two-layer structure with a rind and a core. The rind making up the epidermis may be assumed as an anisotropic material, whereas the core made out of parenchyma cells can be considered as a nearly isotropic elastic material with respect to the petiole that contains it. At this length scale, the influence on mechanics of the individual cell is not considered. The anatomical tissue heterogeneities can be expressed by the relative volume fraction of the constituent homogenized tissues and by their spatial distributions within the cross section. As a first albeit crude prediction of the contribution of the rind layer and the core tissue to the Young’s modulus, the classic Voigt and Reuss models can be used to predict the behavior of a composite material with n-tissue-dehydrated layers (Niklas, 1992b). These models, which represent the upper and the lower limits for the elastic modulus of a layered material, respectively, do not assess the influence of the organ geometry within which tissues are arranged. Thus, to capture the effect of tissue organization and organ cross-sectional geometry, we resort to an alternative approach on the basis of cross-sectional shape parameters and on the classic laminated theory. We assume that (i) the resulting strain throughout the tissue layers of the petiole is uniform; (ii) the strain at the tissues interface remains unchanged; and (iii) bending strain varies linearly along the constituent layers with no discontinuity, whereas the stress is continuous only within each tissue layer, but it is discontinuous at the layer transition. On the basis of these assumptions, the mechanical properties, including torsional, flexural, bending-induced shear (Amany and Pasini, 2009), and mass of a n-tissueshaped organ, can be determined. For example, the normalized effective flexural and mass properties of a multitissue petiole cantilever with shaped cross section (with at least an axis of symmetry) at the stem-stalk internode can be expressed as (Pasini, 2008, 2009)



k Ei  ET ∑ I E = y  i =1 E y Ii 0  0 s  k , ri  rT = y y Ai A∑   r0 r s i =1 0 

(4.10)

where S identifies the contribution of the petiole cross-sectional geometry, expressed through the shape parameters ψA and ψ I (in this case the dimensionless shape properties governing flexural and mass properties); the other group f(Lg) denotes the tissue contribution specified by ψAi and ψ Ii, which are dimensionless parameters describing the position of the ith layer of each homogenized tissue with material properties Ei, Young’s Modulus, and ρi, density, with respect to the petiole cross-sectional neutral axis. E0 and ρ 0 are reference tissue properties with respect to which all the other layers are normalized. For example, if we consider an elliptical petiole cross-section within which three tissues, such as epidermis, collenchyma, and parenchyma, are concentrically arranged, then Equations 4.10 and 4.11 can be plotted in Figure 4.17 to capture the multiscale

Multiscale Modeling of Plant Cell Wall Architecture and Tissue Mechanics

159

C1

1.17 F

Curve v1

Evolution e

C2 0.84 ET E0

G Curve v2

3 16

0.46

Curve v3 ρT ρ0

C3 4

1.31

FIGURE 4.17 (See color insert.) Dash-dot lines curves v1, v2, and v3 bound the domain of all possible combination of concentrically two and three tissues evolving within an elliptical cross section. Continuous lines define two-tissue evolution. The curves of the shaded subdomain describe the two and three tissue evolutions for concentrically arranged layers with a constrained volume percentage of 49% for M 2. For the limiting bimaterial configurations, F and G contain, respectively, 51% volume fraction of M1 and of M3.

effect of both tissue layers and cross-sectional geometry for the ellipse family. Here, M3 has been chosen as the reference tissue with material properties E0 = ρ 0 = 1; thus, the position of the cross-sectional C3 made up of tissue 1 only is determined by the coordinates C3 ((rT 3 /r0 )y A = (p/4);(ET 3 /E0 )y I = (3 p/16)); on the other hand, monotissue cross sections C1 and C2 are selected to yield C1((rT 1 /r0 ) y A = 1.31;(ET 1 /E0 ) y I = 1.17 ) and C 2((rT 2 /r0 )y A = 0.46 ;(ET 2 / E0 )y I = 0.84). Figure 4.17 shows the boundaries of the flexural modulus versus mass domain that can be obtained by arranging three tissues within an elliptical petiole cross section. Within them, there exist all the possible sandwich structures containing multiple layers arranged either symmetrically or asymmetrically about the envelope midplane. Dashed curves v1, v2, and v3 are included only to provide theoretical boundaries for the ideal ellipse domain with vertically scaled layers. The continuous curve describes the evolution of two concentrically arranged tissues. Different curves for two- and three-tissue petiole cross section can be plotted to observe the properties (flexural, torsional, shear) evolution that the petiole might develop during growth. As an example, Figure 4.17 illustrates curve evolution e describing the effect of adding a third tissue. For a constant thickness of the epidermis, starting from F and evolving to G, the effect of growing a third (green) layer is described by either the lower or the upper curves, each determined by a specific spatial arrangement of the two internal tissues (green and blue). 4.4.5.2 The Effect of Organ Cross-Sectional Geometry Often, the organ shape presents geometric features that cannot be approximated with conventional theoretical cross sections. For example, the geometry of the Monstera petiole cross section shown in Figure 4.16 requires a shape model that can capture shape irregularities including the top groove. To enable an accurate representation of natural forms, for

160

Biomimetics: Nature-Based Innovation

TABLE 4.1 Gielis Parameterization of Petiole Cross Section of Different Species Petiole Specimen

Graphical Sketches

Sweet gum petiole

Obtained Contour Plots

α

Gielis Parameters α( ) o

a

b

m

n1

n2

n3

0.9

1

8.3

3

4

2

42.8

f (f) = sin(f) (0 < f < 2p) 0.9

Bean petiole

45.3

8

5

15

2

f (f) = sin(f) ( p < f < 2p) 1

1.6

8

70

45

25

f (f) = sin(f) ( p < f < 2p)

42.3

Monstera deliciosa

1.1

f (f) = sin(f) ( p < f < 2p)

example, the top-grooved contour of the petiole, we resort to the Gielis parameterization of the Lame’ curves (Gielis, 2003) in polar coordinates (r, ϕ), which can be written as r = f (f)

1 n1  1 cos  4 f     a m  

n2

1 4  +  cos  f  m   b

n3

,

(4.11)

where the parameters a, b, m, n1, n2 given in Table 4.1 can be tuned to obtain natural shape contours of alternative petioles. To compute and to plot the shape properties of a petiole cross section, the domain integral of Equation 4.12 is transformed into a line integral using Green’s theorem. Then the integral is calculated through quadratic elements that represent the coordinates over the boundary. The procedure produces exact symbolic formulas for shapes enclosed by boundaries that can be represented by first- or second-order polynomials. The integrals are used to compute the geometric properties and the shape transformers of each petiole. Figure 4.18 illustrates the flexural stiffness curves of an ideal semiellipse compared with the natural hollow and solid cross-sectional shapes of the petioles reported in Table 4.1. All of them are depicted for an increasing thickness of the epidermis layer from zero up to the theoretical point where the epidermis fills completely the shape organ. If the semielliptical shape petiole is considered as reference, then it can be seen that an increasingly sharper groove of the petiole reduces the flexural stiffness of the cross section. Each curve shows that the peripheral cell of the epidermis, which consists of a layer of closely packaged cell with a very high cell-to-cell contact area, plays a critical role on the flexural stiffness of the petiole. Other similar graphs can be produced to assess the torsional compliance of the petiole and to verify whether the development of a groove into an increasingly flatter

Multiscale Modeling of Plant Cell Wall Architecture and Tissue Mechanics

161

0.8 Monstera deliciosa

0.7 0.6 0.5 ET E0

Sweetgum

0.4 0.3 Sweetgum

0.2 0.1

Bean

Bean

0

0.1

0.2

0.3 0.4

0.5 ρT ρ0

0.6

0.7

0.8

FIGURE 4.18 Flexural stiffness curves of hollow petiole cross sections with increasing thickness for ideal semielliptical shape and sweet gum, bean, and monster petiole cross section (Table 4.1). 0.8 0.7 D

0.6 0.5 ET E0

0.4 B

0.3

A

C

0.2 0.1 0

0

0.2

0.4

0.6

0.8

FIGURE 4.19 Flexural stiffness curves of two tissues composite petiole cross sections for thin and thick wall circular cells (Table 4.1).

cross section has the effect of easing its tendency to twist rather than to bend in response to given external loads (Pasini, 2008). Figure 4.19 shows the flexural properties resulting from combining the natural shape of the sweet gum cross section with the properties of a two cellular dry tissues, that is, the rind and the core, sandwich. The outer layer is the epidermis represented for an increasing

162

Biomimetics: Nature-Based Innovation

thickness, whereas the inner layer is the parenchyma cellular tissue with circular crosssectional cells. The upper two curves for hollow semiellipse and sweet gum cross section show that the epidermis provides a stiff outer rind relative to a zero density core. However, if the core tissue is assumed to be filled with cellular solids, then different flexural properties change with respect to the cell wall thickness. For example, curve CD describes the effect of a core tissue with thin cell wall thickness (r/t < 5), BD curve is for thick cell wall (r/t < 5), and curve AD is the ideal case of a solid circular cell. The curves shifting from AD to OD show that minimizing the cell wall thickness has the effect of improving the flexural stiffness. Despite the assumptions, among which the neglect of the main effect of the symplast, these morpho-mechanical charts might offer insight into how the petiole mechanical performance evolves during growth at the tissue and organ shape level. 4.4.6  Biomimetic Applications This chapter has not presented a systematic method to exploit a biological concept in engineering practice (Vincent and Mann, 2002), as other chapters of this book follow. Rather, our aim has been to provide an overview of plant biomechanics models that can be used to gain insight into plant mechanics at multiple length scale (nanometers, micrometers, and centimeters); in addition, we have used a length scale criterion to group a number of relevant biomimetic applications, not necessarily inspired by the models reported here. Far from being exhaustive, this section reports a few remarkable examples of plant-inspired technology at the tissue (micrometers) and the structural length scale (centimeters and meters). At the tissue level, first works studying the high work of fracture of wood suggested alternative bioinspired composite design exhibiting high work of fracture and low density. The biomimetic composite structure consisted of a network of cylindrical elements, each with helically wound walls of carbon fibers, replicating plant cell tissue (Gordon et al., 1980; Chaplin et al., 1983). Other examples of tissue-like inspired composite structures have been more recently proposed by Milwich et al. (2006) for the use in aeronautic and sport equipment industries. The study of the Dutch rush (Equisetum hyemale, Equisetaceae) and the giant reed (Arundo donax, Poaceae) inspired a Technical Plant Stem. The sources of inspiration from these two plant organs involved (1) the sandwich structure of the horsetail hollow stem consisting of two rings of strengthening tissues, linked by T-shaped pillars, which coat a third tissue with canals running through it; and (2) the structural gradients of the giant reed revealing densely packed fibers associated with vascular bundles in regions of highest stress to prevent breaking (Spatz and Speck, 1994; Spatz et al., 1997; Speck et al., 1998; Speck and Speck, 2003; Spatz and Emanns, 2004). Another strategy used by plants to adaptively develop through tissue growth their organs in response to external loads has been implemented into a computational tool, which can shape a mechanical component to maximize strength and minimize stress concentration with the least amount of material (Mattheck, 1991, 1998). The simulation design tool is widely used also in industry for shape optimization of mechanical components. Such a software has been reported to be successfully used to design new vehicle that are 30% lighter than conventional car, yet still crash safe. At a larger length scale, the type of plant organ, whether be a stem or leaf or the plant itself, has been a source of inspiration in several applications involving the design of structural components for automotive, aircraft, and biomedical and sport equipment applications (Milwich et al., 2006; Vincent, 2006; Bhushan, 2009). Also in architecture, the hingeless compliant mechanism displayed by the S. reginae flowers during the valvular pollination

Multiscale Modeling of Plant Cell Wall Architecture and Tissue Mechanics

163

has been identified as a concept generator for a deployable shading system of an architectural building facade (Poppinga et al., 2010, Lienhard et al., 2010). The stem structure of the bamboo inspired the design of a novel structure for a cylindrical shell with a load-bearing capacity of 124.8% higher than a traditional shell design (Ma et al., 2008). On the basis of the structural characteristics of bamboo, a cylindrical structure was designed to mimic the gradient distribution of vascular bundles and parenchyma cells. Another study of stem-like element was instrumental for the design of biomimetic tubular and fold-flap structures for cable entry systems and stent-like crossed helical fiber tubes (Masselter et al., 2008). Besides stems, other plant organs have inspired the design of mechanical components at larger length scale. For example, the geometry of unfolding leaves and their deployment patterns offered solution to the packaging of telecommunication antenna (Kresling, 1995). At the plant scale on the other hand, the concept of branching structure in plants has been used to improve the water absorption and liquid water transport property of knitted fabrics. It has been shown that the novel fabric structure improves both the air permeability moisture and the management property of the fabric for a better clothing comfort (Chen et al., 2009). The fast and large-scale movement in plants can inspire novel actuation systems. In plants, these movements are achieved through spatial variation of cell type and sizes in their tissues. For example, in grass, the upper epidermis is patched with thin-walled and large-sized cells that dehydrate and shrink much faster than the other cells, resulting in reduction of angle between the two symmetrical half of their leaf blades. This movement reduces transpiration by having two upper symmetric halves of the leaf blade covering each other and keeps the plant hydrated. On the other hand, the closure of Venus fly trap is initiated by rapid expansion of outer epidermal cells believed to be mediated through lowering the cell wall pH, which results in cell wall loosening and the fastest known cell growth in plants. In Mimosa pulvinus, the swollen part at the base of petiole (pulvini) consists of thick-walled water conducting vascular tissue surrounded by thin-walled motor cells. The leaf actuation is affected by increased turgor pressure caused by volume increase because of the K+ ion uptake by the motor cells at the upper site (extensor cells). Upon darkness, K+ channels in the extensor cells close but the motor cells in the lower site (flexor cells) remain open. This causes loss of turgor pressure that results in shrinkage of pulvini and the leaf droops. A piezoelectric actuator composed of a piezoelectric transducer with graded porosity inspired by functionally graded microstructure of bamboo had been designed. For the same electric field, this actuator will give much higher displacement comparing the conventional piezoelectric actuators (Taya, 2003).

4.5  Conclusion During ontogeny, a plant continuously faces the need of reconciling challenging and oftenconflicting physiological requirements and environment constraints. As a result, distinctive material and morphological traits arise at multiple length scale, from the molecular to the macroscopic level of each plant organ. To understand the multifunctional tradeoff solutions developed at each length scale, this chapter presented a multiscale analysis involving molecular biology, cellular anatomy, tissue organization, and structural organ morphology. This chapter shows that plant-based biomimetics offers material and structural solutions to multifunctional and multiobjective design problems that arise in engineering practice.

164

Biomimetics: Nature-Based Innovation

Acknowledgments This work was supported by the Le Fonds québécois de la recherche sur la nature et les technologies (FQNRT). Y. K. Murugesan acknowledges scholarship support from the MEDA program of the Faculty of Engineering and the Eugene Lamothe Fund from the Department of Chemical Engineering, McGill University. The authors gratefully acknowledge Stephen C. Cowin in Mechanical Engineering at the City College of New York, Dr. Tom Masselter in Plant Biomechanics Group at the Albert-Ludwigs-University Freiburg, and Mohan Srinivasarao in School of Polymer at Georgia Institute of Technology for giving their valuable comments for an earlier version of this chapter.

References Akagi, K., “Helical polyacetylene: Asymmetric polymerization in a chiral liquid-crystal field,” Chemical Reviews, Vol. 109, No. 11, (2009), pp. 5354–5401. Amany, A., and D. Pasini, “Material and shape selection for stiff beams under non-uniform flexure,” Materials and Design, Vol. 30, No. 4, (2009), pp. 1110–1117. Arzt, E., “Biological and artificial attachment devices: Lessons for materials scientists from flies and geckos,” Materials Science and Engineering C, Vol. 26, No. 8, (2006), pp. 1245–1250. Bhushan, B., “Biomimetics: Lessons from nature—An overview,” Philosophical Transactions of the Royal Society of London, A, Mathematical, Physical, and Engineering Sciences, Vol. 367, No. 1893, (2009), pp. 1445. Bidlack, J., M. Malone, and R. Benson, “Molecular structure and component integration of secondary cell walls in plants,” Proceedings of the Oklahoma Academy of Science, Vol. 72, (1992), pp. 51–56. Borno, R., J. Steinmeyer, and M. Maharbiz, “Transpiration actuation: The design, fabrication and characterization of biomimetic microactuators,” Journal of Micromechanics and Microengineering, Vol. 16, No. 11, (2006), pp. 2375–2383. Bouligand, Y., “Defects and textures in cholesteric analogues given by some biological systems,” Journal de Physique, Vol. C1, No. 3, (1975), pp. C1–331–C1–336. Bouligand, Y., S.T. Lagerwall, and B. Stebler, “Blue-phase drops on a glass interface and their decoration at the cholesteric transition,” Comptes Rendus Chimie, Vol. 11, No. 3, (2008), pp. 212–220. Brett, C., and K. Waldron, Physiology and Biochemistry of Plant Cell Walls, Unwin Hyman, London, (1990). Chaplin, R., J. Gordon, and G. Jeronimidis, Development of a Novel Fibrous Composite Material, 1983, US Patent No. 4409274, USA. Chen, Q., J. Fan, M. Sarkar, and G. Jiang, “Biomimetics of plant structure in knitted fabrics to improve the liquid water transport properties,” Textile Research Journal, Vol. 80, No. 6, (2009), pp. 568–576. Cosgrove, D., “Growth of the plant cell wall,” Nature Reviews Molecular Cell Biology, Vol. 6, No. 11, (2005), pp. 850–861. Dammström, S., L. Salmén, and P. Gatenholm, “The effect of moisture on the dynamical mechanical properties of bacterial cellulose/glucuronoxylan nanocomposites,” Polymer, Vol. 46, No. 23, (2005), pp. 10364–10371. De Focatiis, D.S.A., and S.D. Guest, “Deployable membranes designed from folding tree leaves,” Transactions of the Royal Society of London, Series A, Vol. 360, No. 1791, (2002), pp. 227–238. De Gennes, P., and J. Prost, The Physics of Liquid Crystals, Oxford University Press, Oxford, (1995).

Multiscale Modeling of Plant Cell Wall Architecture and Tissue Mechanics

165

De Luca, G., and A. Rey, “Monodomain and polydomain helicoids in chiral liquid-crystalline phases and their biological analogues,” European Physical Journal, E, Vol. 12, No. 2, (2003), pp. 291–302. Donald, A., A. Windle, and S. Hanna, Liquid Crystalline Polymers, Cambridge University Press, Cambridge, (2006). Dünger, U., H. Weber, and H. Buggisch, “A simple model for a fluid-filled open-cell foam,” Transport in Porous Media, Vol. 34, No. 1, (1999), pp. 269–284. Fratzl, P., and F. Barth, “Biomaterial systems for mechanosensing and actuation,” Nature, Vol. 462, No. 7272, (2009), pp. 442–448. Georget, D., A. Smith, and K. Waldron, “Modeling of carrot tissue as a fluid-filled foam,” Journal of Materials Science, Vol. 38, No. 9, (2003), pp. 1933–1938. Gibson, L., and M. Ashby, Cellular Solids: Structure and Properties (2nd ed.), Cambridge University Press, Cambridge, (1997). Gielis, J., “A generic geometric transformation that unifies a wide range of natural and abstract shapes,” American Journal of Botany, Vol. 90, No. 3, (2003), pp. 333–338. Giraud-Guille, M., “Twisted plywood architecture of collagen fibrils in human compact bone osteons,” Calcified Tissue International, Vol. 42, No. 3, (1988), pp. 167–180. Giraud-Guille, M., J. Castanet, F. Meunier, and Y. Bouligand, “The fibrous structure of coelacanth scales: A twisted ‘plywood,’” Tissue & Cell, Vol. 10, No. 4, (1978), pp. 671–686. Gordon, J., G. Jeronimidis, and M. Richardson, “Composites with high work of fracture [and discussion],” Philosophical Transactions of the Royal Society of London, Series A, Mathematical, Physical, and Engineering Sciences, Vol. 294, No. 1411, (1980), pp. 545–550. Gradwell, S., S. Renneckar, A. Esker, T. Heinze, P. Gatenholm, C. Vaca-Garcia, and W. Glasser, “Surface modification of cellulose fibers: Towards wood composites by biomimetics,” Comptes Rendus Biologies, Vol. 327, No. 9–10, (2004), pp. 945–953. Greil, P., “Templating approaches using natural cellular plant tissue,” MRS Bulletin, Vol. 35, No. (2010), pp. 145–149. Gupta, G., and A. Rey, “Texture rules for concentrated filled nematics,” Physical Review Letters, Vol. 95, No. 12, (2005), pp. 127802, 1–4. Hall, S., Biotemplating: Complex Structures from Natural Materials, World Scientific Publishing, Singapore (2009). Isnard, S., N. Rowe, and T. Speck, “Growth habit and mechanical architecture of the sand duneadapted climber Clematis flammula var. maritima L,” Annals of Botany, Vol. 91, No. 4, (2003a), pp. 407–417. Isnard, S., T. Speck, and N. Rowe, “Mechanical architecture and development in Clematis: Implications for canalised evolution of growth forms,” New Phytologist, Vol. 158, No. 3, (2003b), pp. 543–559. Jarvis, M., “Plant cell walls: Supramolecular assemblies,” Food Hydrocolloids, Vol. 25, No. 2, (2011), pp. 257–262. Kondo, T., “Hydrogen bonds in cellulose and cellulose derivatives,” in D. Severian (Ed.), Polysaccharides. Structural, Diversity and Functional Versatility, Marcel Dekker, Inc., New York, USA (1998), pp. 131–172. Kresling, B., “Plant ‘design’: Mechanical simulations of growth patterns and bionics,” Biomimetics, Vol. 3, No. 3, (1995), pp. 105–122. Kroon-Batenburg, L., P. Kruiskamp, J. Vliegenthart, and J. Kroon, “Estimation of the persistence length of polymers by MD simulations on small fragments in solution. Application to cellulose,” Journal of Physical Chemistry, B, Vol. 101, No. 42, (1997), pp. 8454–8459. Kutter, S., and M. Warner, “Reflectivity of cholesteric liquid crystals with spatially varying pitch,” European Physical Journal, E, Soft Matter and Biological Physics, Vol. 12, No. 4, (2003), pp. 515–521. Lai, Y., “Analysis of thick cylinder under non-axisymmetric loads,” Engineering Fracture Mechanics, Vol. 9, No. 1, (1977), pp. 95–100. Lepescheux, L., “Spatial organization of collagen in annelid cuticle: Order and defects,” Biology of the Cell/Under the Auspices of the European Cell Biology Organization, Vol. 62, No. 1, (1988), pp. 17–31.

166

Biomimetics: Nature-Based Innovation

Li, Z., “Bio-based composites that mimic the plant cell wall,” in Biological Systems, Virginia Polytechnic Institute and State University, Blacksburg, USA (2009). Lienhard, J., S. Poppinga, S. Schleicher, T. Speck, and J. Knippers, “Elastic architecture: Nature inspired pliable structures,” Design and Nature V, Transactions on Ecology and the Environment, Vol. 138, (2010), pp. 469–477. Ma, J., W. Chen, L. Zhao, and D. Zhao, “Elastic buckling of bionic cylindrical shells based on bamboo,” Journal of Bionic Engineering, Vol. 5, No. 3, (2008), pp. 231–238. Mann, S., “Self-assembly and transformation of hybrid nano-objects and nanostructures under equilibrium and non-equilibrium conditions,” Nature Materials, Vol. 8, No. 10, (2009), pp. 781–792. Masselter, T., U. Scharf, and T. Speck, “Plants and animals as concept generators for the development of biomimetic cable entry systems,” Journal of Bionic Engineering, Vol. 5, No. 2, (2008), pp. 167–173 Mattheck, C., Trees. The Mechanical Design, Springer Verlag, Heidelberg (1991). Mattheck, C., Design in Nature: Learning from Trees, Springer Verlag, Heidelberg (1998). Mazeau, K., C. Moine, P. Krausz, and V. Gloaguen, “Conformational analysis of xylan chains,” Carbohydrate Research, Vol. 340, No. 18, (2005), pp. 2752–2760. Meyers, M.A., A.M. Hodge, and R.K. Roeder, “Biological materials science and engineering: Biological materials, biomaterials, and biomimetics,” JOM, Vol. 60, No. 6, (2008), pp. 21–22. Milwich, M., T. Speck, O. Speck, T. Stegmaier, and H. Planck, “Biomimetics and technical textiles: Solving engineering problems with the help of nature’s wisdom,” American Journal of Botany, Vol. 93, No. 10, (2006), pp. 1455–1465. Morris, S., S. Hanna, and M. Miles, “The self-assembly of plant cell wall components by singlemolecule force spectroscopy and Monte Carlo modeling,” Nanotechnology, Vol. 15, No. (2004), pp. 1296–1301. Murugesan, Y., and A. Rey, “Thermodynamic model of structure and shape in rigid polymer-laden membranes,” Macromolecular Theory and Simulations, Vol. 19, No. 2–3, (2009), pp. 113–126. Neville, A., Biology of Fibrous Composites: Development beyond the Cell Membrane, Cambridge University Press, Cambridge, (1993). Niklas, K., Plant Biomechanics: An Engineering Approach to Plant Form and Function, University of Chicago Press, Chicago, IL (1992a). Niklas, K., “Voigt and Reuss models for predicting changes in Young’s modulus of dehydrating plant organs,” Annals of Botany, Vol. 70, No. 4, (1992b), pp. 347–355. Niklas, K., and T. Speck, “Evolutionary trends in safety factors against wind-induced stem failure,” American Journal of Botany, Vol. 88, No. 7, (2001), pp. 1266–1278. Nilsson, S., C. Hertz, and S. Falk, “On the relation between turgor pressure and tissue rigidity. II. Theoretical calculations on model systems,” Physiologia Plantarum, Vol. 11, No. 4, (1958), pp. 818–837. Onsager, L., “The effects of shape on the interaction of colloidal particles,” Annals of the New York Academy of Sciences, Vol. 51, No. 4, (1949), pp. 627–659. O’Sullivan, A., “Cellulose: The structure slowly unravels,” Cellulose, Vol. 4, No. 3, (1997), pp. 173–207. Pasini, D., “Shape transformers for material and shape selection of lightweight beams,” Materials and Design, Vol. 28, No. 7, (2007), pp. 2071–2079. Pasini, D., “On the biological shape of the Polygonaceae Rheum rhabarbarum petiole,” International Journal of Design and Nature and Ecodynamics, Vol. 3, No. 1, (2008), pp. 1–26. Pasini, D., “Bend stiffness of laminate microstructures containing three dissimilar materials,” International Journal of Mechanics and Materials in Design, Vol. 5, No. 2, (2009), pp. 175–193. Popa, V.I., and I. Spiridon, “Hemicelluloses: Structure and properties,” in S. Dumitriu (Ed.), Polysaccharides: Structural Diversity and Functional Versatility, Marcel Dekker, Inc., New York, USA (1998), pp. 297–312. Poppinga, S., T. Masselter, J. Lienhard, S. Schleicher, J. Knippers, and T. Speck, “Plant movements as concept generators for deployable systems in architecture,” Design and Nature V, Transactions on Ecology and the Environment, Vol. 138, (2010), pp. 403–409.

Multiscale Modeling of Plant Cell Wall Architecture and Tissue Mechanics

167

Putra, A., A. Kakugo, H. Furukawa, J. Gong, Y. Osada, T. Uemura, and M. Yamamoto, “Production of bacterial cellulose with well oriented fibril on PDMS substrate,” Polymer Journal, Vol. 40, No. 2, (2008), pp. 137–142. Reis, D., J. Roland, M. Mosiniak, D. Darzens, and B. Vian, “The sustained and warped helicoidal pattern of a xylan-cellulose composite: The stony endocarp model,” Protoplasma, Vol. 166, No. 1, (1992), pp. 21–34. Reis, D., B. Vian, H. Chanzy, and J. Roland, “Liquid crystal-type assembly of native celluloseglucuronoxylans extracted from plant cell wall,” Biology of the Cell, Vol. 73, No. 2–3, (1991), pp. 173–178. Rey, A.D., “Thermodynamics of soft anisotropic interfaces,” Journal of Chemical Physics, Vol. 120, (2004), pp. 2010–2019. Roland, J., D. Reis, B. Vian, B. Satiat-Jeunemaitre, and M. Mosiniak, “Morphogenesis of plant cell walls at the supramolecular level: Internal geometry and versatility of helicoidal expression,” Protoplasma, Vol. 140, No. 2, (1987), pp. 75–91. Roland, J., D. Reis, and B. Vian, “Liquid crystal order and turbulence in the planar twist of the growing plant cell walls,” Tissue and Cell, Vol. 24, No. 3, (1992) pp. 335–345. Rolin, C., B.U. Nielsen, and P.E. Glahn, “Pectin,” in S. Dumitriu (Ed.), Polysaccharides: Structural Diversity and Functional Versatility, Marcel Dekker, Inc., New York, (1998), pp. 377–431. Saheb, D., and J. Jog, “Natural fiber polymer composites: A review,” Advances in Polymer Technology, Vol. 18, No. 4, (1999), pp. 351–363. Sanchez, C., H. Arribart, and M. Guille, “Biomimetism and bioinspiration as tools for the design of innovative materials and systems,” Nature Materials, Vol. 4, No. 4, (2005), pp. 277–288. Shen, T., and S. Gnanakaran, “The stability of cellulose: A statistical perspective from a coarse-grained model of hydrogen-bond networks,” Biophysical Journal, Vol. 96, No. 8, (2009), pp. 3032–3040. Somerville, C., “Cellulose synthesis in higher plants,” Annual Review of Cell and Developmental Biology, Vol. 22, (2006), pp. 53–78. Spatz, H., H. Beismann, F. Brüchert, A. Emanns, and T. Speck, “Biomechanics of the giant reed Arundo donax,” Philosophical Transactions of the Royal Society of London, Series B, Biological Sciences, Vol. 352, No. 1349, (1997), pp. 1–10. Spatz, H., and A. Emanns, “The mechanical role of the endodermis in Equisetum plant stems,” American Journal of Botany, Vol. 91, No. 11, (2004), pp. 1936–1938. Spatz, H., L. Köhler, and T. Speck, “Biomechanics and functional anatomy of hollow-stemmed sphenopsids. I. Equisetum giganteum (Equisetaceae),” American Journal of Botany, Vol. 85, No. 3, (1998a), pp. 305–314. Spatz, H., N. Rowe, T. Speck, and V. Daviero, “Biomechanics of hollow stemmed sphenopsids: II. Calamites—To have or not to have secondary xylem,” Review of Palaeobotany and Palynology, Vol. 102, No. 1–2, (1998b), pp. 63–77. Spatz, H., and T. Speck, “Local buckling and other modes of failure in hollow plant stems,” Biomimetics, Vol. 2, (1994), pp. 149–173. Speck, T., and N. Rowe, “Modeling primary and secondary growth processes in plants: A summary of the methodology and new data from an early lignophyte,” Philosophical Transactions of the Royal Society of London, Series B, Biological Sciences, Vol. 358, No. 1437, (2003), pp. 1473–1485. Speck, T., and O. Speck, “Competence network biomimetics: Taking a leaf out of nature’s book,” Official Journal of the BioValley Network, Vol. 1, (2003), pp. 23–24. Speck, T., O. Speck, A. Emanns, and H. Spatz, “Biomechanics and functional anatomy of hollow stemmed sphenopsids: III. Equisetum hyemale,” Botanica Acta, Vol. 111, No. 5, (1998), pp. 366–376. Speck, T., and D. Vogellehner, “Biophysical examinations of the bending stability of various stele types and the upright axes of early vascular land plants,” Botanica Acta, Vol. 101, No. 3, (1988), pp. 262–268. Stevenson, C., D. Bennett, and D. Lechuga-Ballesteros, “Pharmaceutical liquid crystals: The relevance of partially ordered systems,” Journal of Pharmaceutical Sciences, Vol. 94, No. 9, (2005), pp. 1861–1880.

168

Biomimetics: Nature-Based Innovation

Svagan, A., M. Samir, and L. Berglund, “Biomimetic polysaccharide nanocomposites of high cellulose content and high toughness,” Biomacromolecules, Vol. 8, No. 8, (2007), pp. 2556–2563. Taya, M. “Bio-inspired design of intelligent materials,” Proceedings of SPIE, Vol. 5051, (2003) pp. 54–65. Teeri, T., H. Brumer, G. Daniel, and P. Gatenholm, “Biomimetic engineering of cellulose-based materials,” Trends in Biotechnology, Vol. 25, No. 7, (2007), pp. 299–306. Thompson, D., “How do cell walls regulate plant growth?,” Journal of Experimental Botany, Vol. 56, No. 419, (2005), pp. 2275–2285. Veytsman, B., and D. Cosgrove, “A model of cell wall expansion based on thermodynamics of polymer networks,” Biophysical Journal, Vol. 75, No. 5, (1998), pp. 2240–2250. Vian, B., J. Roland, and D. Reis, “Primary cell wall texture and its relation to surface expansion,” International Journal of Plant Sciences, Vol. 154, No. 1, (1993), pp. 1–9. Vincent, J., “Applications—Influence of biology on engineering,” Journal of Bionic Engineering, Vol. 3, No. 3, (2006), pp. 161–177. Vincent, J., and D. Mann, “Systematic technology transfer from biology to engineering,” Philosophical Transactions of the Royal Society of London, Series A, Mathematical, Physical, and Engineering Sciences, Vol. 360, (2002), pp. 159–174. Warner, M., and S. Edwards, “A scaling approach to elasticity and flow in solid foams,” Europhysics Letters, Vol. 5, (1988), pp. 623–628.

5 Biomimetic Composites Daniel G.T. Strange and Michelle L. Oyen University of Cambridge Cambridge, United Kingdom CONTENTS 5.1 Introduction ........................................................................................................................ 170 5.2 Nacre .................................................................................................................................... 173 5.2.1 Introduction ............................................................................................................ 173 5.2.2 Structure and Composition .................................................................................. 173 5.2.3 Properties ................................................................................................................ 175 5.2.3.1 Elastic Properties..................................................................................... 175 5.2.3.2 Strength .................................................................................................... 176 5.2.3.3 Toughness................................................................................................. 178 5.2.4 Biomimetic Synthesis Routes ............................................................................... 178 5.2.4.1 Macroscale Modeling ............................................................................. 178 5.2.4.2 Layer by Layer ......................................................................................... 179 5.2.4.3 Ice Templating ......................................................................................... 180 5.2.5 Comparing Nacre with Eggshell and Dental Enamel ...................................... 181 5.2.6 Summary................................................................................................................. 182 5.3 Bone ..................................................................................................................................... 183 5.3.1 Structure and Composition .................................................................................. 183 5.3.1.1 Bone Composition ................................................................................... 183 5.3.1.2 Bone Ultrastructure ................................................................................ 184 5.3.1.3 Bone Macrostructure .............................................................................. 185 5.3.2 Properties ................................................................................................................ 186 5.3.2.1 Elastic Properties..................................................................................... 186 5.3.2.2 Failure Properties .................................................................................... 188 5.3.2.3 Electrical Properties ................................................................................ 189 5.3.3 Biomimetic Synthesis Routes ............................................................................... 189 5.3.3.1 Bone Tissue Engineering ....................................................................... 190 5.3.3.2 Cell-Free Biomimetic Processing .......................................................... 190 5.3.3.3 Self-Healing ............................................................................................. 192 5.4 Soft Tissues ......................................................................................................................... 192 5.4.1 Structure and Composition .................................................................................. 193 5.4.1.1 Proteins and Polysaccharides ................................................................ 193 5.4.1.2 Fibrous Structure .................................................................................... 193 5.4.1.3 Hierarchical Structure ............................................................................ 195 5.4.2 Properties ................................................................................................................ 195 5.4.2.1 Nonlinear Elasticity ................................................................................ 195 5.4.2.2 Anisotropy ............................................................................................... 197 169

170

Biomimetics: Nature-Based Innovation

5.4.2.3 Time-Dependent Behavior and Hysteresis ......................................... 197 5.4.2.4 Lubrication ............................................................................................... 199 5.4.3 Biomimetic Synthesis Routes ............................................................................... 200 5.5 Summary and Outlook ..................................................................................................... 201 Acknowledgments ...................................................................................................................... 202 References..................................................................................................................................... 202

5.1  Introduction The field of biomimetic materials synthesis has grown quickly in recent years, in that ­scientists and engineers in nonbiological fields have increasingly been relying on natural biological materials for inspiration. There has been particular interest by materials ­scientists in the development of biomimetic materials based on nanocomposites found in nature, including nacre (seashell), eggshell, bone, tooth tissues, and hydrated soft tissues. In addition to the interest in bioinspired materials themselves, there is great interest in imitating the synthesis routes that allow for processing of ceramic-based materials at room temperature and pressure. Problems in technology are typically solved by engineers with the addition of large quantities of energy, in stark contrast to nature’s low-energy solutions (Vincent et al., 2006). For this reason, biomimetic processing routes, such as templated biomineral formation on an organic matrix, have potential uses beyond the realm of biomimicking materials. Of course, the two are intimately linked: the materials themselves and the properties they possess arise from the unique processing routes through which the materials are formed. Overall, the performance of these materials “in the wild” is a function of the structure–properties relationships they possess and the processing routes from which they arise, and these four elements (structure, processing, properties, and ­performance) form the basis of the fundamental paradigm of materials science and thus presents an ideal framework in which to examine natural materials and biomimicry. There exists a fundamental need to study the character of natural materials before any attempt is made at mimicry. Natural materials are made from key “building blocks” such as proteins, saccharides (sugars), nucleic acids (DNA and RNA), minerals, and water. A  wide variety in material character results from differences in assembly of the ­structural macromolecules such as proteins and sugars: relatively few base molecules (20 amino acids for proteins and a handful of sugar molecules for polysaccharides) are assembled into an almost unthinkable range of different proteins, starches, and protein– sugar ­composites (glycoproteins and proteoglycans). The informational “code” for putting together these many and complicated structures is the component that underlies the key difference between the small investments of energy required to produce natural materials compared with engineering materials (Vincent et al., 2006). Biological materials are also always ­composite materials. Much of the recent interest in biological materials is related to determining how these phases with grossly different properties and nanometer-scale feature sizes combine to form a macroscopic material that it is fundamentally stiff, strong, and tough and yet relatively light weight because of the organic component. The first mistake engineers may make when trying to understand biomimetic ­materials is in confusing structure with composition. Structure is particularly important in the ­context of organic–inorganic composite materials—such as nacre, bone, and eggshell—in which there is a significant mismatch in properties between the component phases. The overall

171

Biomimetic Composites

(effective) properties of the composite material do not depend solely on the properties and proportions of the component phases but indeed critically depend on how the component phases are physically arranged. This criticality of arrangement at fixed composition is simply illustrated by an ­example from elementary composite mechanics. Many material properties of a composite ­material, including the elastic modulus, can be expressed as the volume–fraction weighted ­combination of the properties of the component phases. In the simplest approximation for elastic modulus, the two components can be considered to be volume–fraction weighted springs in series or parallel. The two combinations (series and parallel) form extreme bounds on the actual composite’s behavior, and these simple bounds for twophase composites are frequently called the Voigt–Reuss bounds. The upper (isostrain, parallel springs, and Voigt) and the lower (isostress, series springs, and Reuss) bounds are as follows (Chawla, 1987):

EU = V2E2 + (1 − V2) E1



 V (1 − V2 )  EL =  2 + E1   E2

(5.1)

−1

(5.2)

where E is the elastic modulus and V is the volume fraction of each phase. By convention, E2 > E1 and V1 + V2  = 1. These bounds can be physically represented as the longitudinal (upper bound) and transverse (lower bound) moduli of oriented fiber-reinforced composites with fibers aligned in the longitudinal direction. For an organic–inorganic composite with a 50–50 composition by volume and a significant elastic modulus mismatch between the phases (Eorganic ~ 100 MPa, Einorganic ~ 100 GPa), the lower bound modulus would be approximately 200 MPa and the upper bound modulus would be approximately 50 GPa. Thus, at fixed composition, a range of composite elastic modulus values could be obtained, varying by over two orders of magnitude depending on how the strain is transferred between the phases. It is for this reason that the structure of natural materials must be considered in addition to the composition. Further, the elastic properties of the biological composite represent only one critical facet of their mechanical response. Although the ­failure strength of composites is often bounded by rule of mixtures approximations, as with the stiffness, the toughness is an entirely different matter. In toughness, the microstructure rules and the toughness of the composite can be significantly greater than the proportional sum of the toughness of the parts. Mechanism becomes critical in considerations of fracture behavior. There are three additional structural motifs that are frequently found in natural composites but absent in most engineering composites. Engineers typically attempt to make uniform materials: fibers or particles are evenly spaced and regularly arranged. Nature, on the other hand, seems to prefer local, random-appearing variation (Tai et al., 2007) at small length scales and longer-scale functional gradation, especially as an interface is approached (Cuy et al., 2002). Again, these gradients or local variations in properties can be found with or without variations in local composition, as the variability can be structural instead of compositional, as shown in dental enamel (Katz, 1971) and bone (Oyen et al., 2008) in which a range of elastic modulus values is found at fixed or nearly fixed composition. Further, although many engineering composites consist of one discrete (i.e., particle) phase embedded in a continuous matrix, most biological composites exhibit some degree of interpenetrating phase character.

172

Biomimetics: Nature-Based Innovation

Some of the differences to be found in the structure–properties relationships of biological materials, when compared with typical engineering composites, arise because of the different mechanisms of formation of these materials—thus, differences in material processing. As will be discussed in the following sections, natural materials are biologically directed (controlled by living cells) during their formation. Biological materials also use mechanisms of self-assembly in generating large-scale and complicated structures from simpler, smaller elements. Mineralized tissues often form by heterogeneous nucleation of mineral on protein, and this nucleation is in some cases analogous to epitaxial thin film formation: the crystal orientation of mineral can be oriented relative to the underlying protein (Hunter, 1996). Proteins such as collagen self-assemble into fibrils with diameters 30 times that of an individual triple helix collagen molecule. So one reason for mimicking biological materials processing routes is to capture the unique microstructures, and in turn the interesting structure–properties relationships, that result from the mechanisms of processing or formation. There is also great interest in the synthesis routes that allow for processing of ceramic materials at room temperature and pressure—problems in technology are typically solved by engineers with the addition of large quantities of energy, in stark contrast to nature’s low-energy solutions (Vincent et al., 2006). For this reason alone, harnessing natural approaches to materials synthesis could revolutionize some aspects of ceramic and composite processing and help to fulfill increasingly important requirements of energy efficient and ecologically sound engineering practices. This is potentially true even if the materials themselves are not of interest from a biomimetics sense. This encompasses the broader aspects of the concept of biomimetic materials: even if the end result is not to create a bone- or shell-like material, for example, the “tricks” used in nature to make materials may be of significant interest to the materials community. In considering the biomimicry of natural materials, we will hereafter follow the ­paradigm presented earlier for considering all aspects of natural and bioinspired ­materials in the framework of processing–structure–properties–performance, with emphasis on ­biomimetic processing routes and the structure–properties relationships intrinsic to ­biological materials. The case studies presented will be for three primary (and very different) materials classes: composite organic–inorganic materials that are mostly mineral, focused on nacre (seashell), but also including eggshell and dental enamel; composite organic–inorganic materials that are roughly half mineral by volume, including bone and dentin in the tooth; and composite organic–organic materials with a significant volume fraction of water and which could be rightly considered as hydrogels, including articular cartilage (Figure 5.1). These three categories of biological materials will be considered in turn, with an emphasis (a) Water

Organic

(b)

(c) PGs

Organic Mineral

Mineral

Collagen

Water

Water

FIGURE 5.1 Composition by weight of three natural composites: (a) nacre, (b) bone, and (c) articular cartilage.

Biomimetic Composites

173

on structure–properties relationships, processing and formation methods in nature, and a survey of biomimetics approaches that have been used to develop novel materials and processes within these materials classes.

5.2  Nacre 5.2.1  Introduction Mollusks are known for the ability to form shells that protect their soft bodies from ­predators and debris. The word mollusk is in fact derived from the Latin word mollis, meaning soft. The mollusk shells are typically composed of at least one ceramic phase reinforced by a small volume fraction (0.1%–5%) of organic material. A variety of shell microstructures have been observed ranging from homogenous rubble to organized ­plywood-like structures (Meyers et al., 2008a). Nacre, commonly known as mother of pearl, has received significant attention because of its attractive combination of stiffness, strength, and toughness. Nacre is found in the shells of several mollusks including abalone and oysters. The shells of these mollusks consist of two predominantly calcium carbonate layers. The outer layer, calcite, is particularly hard and protects the animal against penetration. However, it is also prone to brittle failure (Barthelat et al., 2007). The inner layer, nacre, is softer and dissipates energy through large inelastic deformations. It limits crack growth and preserves the integrity of the shell. This inner layer is subject to substantial tensile forces when external loads bend the shell. This is a mode in which ceramics are weak, but for which nacre copes admirably because of its large tensile strength parallel to the surface (Barthelat et al., 2007). However, when nacre’s specific stiffness, strength, and fracture toughness are compared with other natural and synthetic materials, it becomes apparent that it only has good and not exceptional mechanical properties (Table 5.1). Bone and wood both have a greater specific strength and toughness, and carbon fiber-reinforced plastic outperforms nacre on all counts. Jackson et al. (1990) compared nacre with a range of synthetic ceramic composites and found that nacre had comparable stiffness but a substantially larger toughness. Hence, although nacre might not be composed of inherently tough components, its architecture allows it to be much tougher than the sum of its parts. It is this architecture that we seek to mimic to create biomimetic composites that significantly outperform their constituent materials. Part of the allure of nacre as a target for biomimicry is also in the processing routes by which it is formed, and not just the resulting architecture and corresponding mechanical properties. 5.2.2  Structure and Composition Nacre (Figure 5.2) is a two-phase composite consisting of more than 95% aragonite by weight and up to 5% hydrated organic matrix. The organic matrix is composed of glycine and alanine rich protein and polysaccharide. It has been observed that although the volume of organic matrix present may seem insignificant, it is substantially more than is needed for the purpose of biomineralization (Jackson et al., 1988). Water accounts for only 0.2% of the total nacre mass when fully wet (Jackson et al., 1988).

174

Biomimetics: Nature-Based Innovation

TABLE 5.1 Specific Mechanical Properties of Nacre Compared with Other Materials

Specific Modulus, E/ρ (MN·m/kg)

Specific Tensile Strength, σ /ρ (kN·m/kg)

Specific Fracture Toughness, KIC/ρ (kN·m3/2/kg)

Material

Density, ρ (g/cm3)

Nacre Aragonite

2.7 2.9

25.9 34.5

63.0 34.5

1.5 0.1

Alumina

3.9

100.0

102.6

1.0

Bone

1.85

10.8

71.9

3.2

Carbon ­fiberreinforced plastic (vf = 58%) Mild steel

1.5

126.0

700.0

25.7

7.8

25.6

55.1

17.9

Nylon

1.15

2.6

87.0

2.6

Wood (ash)

0.67

23.6

173.1

13.4

Reference Jackson et al. (1988) Jackson et al. (1988), Kamat et al. (2000) Ashby and Jones (2005a) Currey et al. (2001), Huikes and van Rietbergen (2005) Ashby and Jones (2005b) Ashby and Jones (2005a) Ashby and Jones (2005a) Ashby and Jones (2005b)

Source: Adapted from Jackson A.P., J.F.V. Vincent, and R.M. Turner, “Comparison of nacre with other ceramic composites,” Journal of Materials Science, Vol. 25, No. 7, (1990), pp. 3173–3178. Note: Natural materials exhibit a large range in material properties and are often anisotropic. The values listed here are middle of the range values in the stiffest/strongest/toughest orientation. (a) Abalone shell: Nacre

(b) Mesolayers

(c) Aragonite tiles

0.1 mm

2 µm

FIGURE 5.2 (See color insert.) Hierarchical structure of abalone shell: (a) macrostructure showing calcite and nacreous layer, (b) mesostructure consisting of organic growth bands and nacre, and (c) nacre microstructure. (From Chen, P.-Y., A.Y.M. Lin, Y.-S. Lin, et al., “Structure and mechanical properties of selected biological materials,” Journal of the Mechanical Behavior of Biomedical Materials, 1, 208–226, 2008. With permission.)

Under a microscope, the aragonite crystals and organic matrix resemble bricks and ­ ortar (Figure 5.2c). The “bricks,” 0.5-µm-thick platelets of aragonite, are separated by m a 20- to 30-nm-thick organic “mortar” layer. The organic layer is further structured, and more polysaccharide is found toward the center of each layer (Meyers et al., 2008a). It has been proposed that the aragonite is found in single crystal form, as hexagonal tablets with their c-axis normal to the plane of the tablet layer (Meyers et al., 2008a). A  ­contrasting view holds that the platelets contain nanograins (Stempfle et al., 2010). The ­tablets are often modeled as perfectly flat although substantial waviness has been

Biomimetic Composites

175

observed (Barthelat et al., 2007). The tablets are perfectly conformal despite this waviness. In addition, the tablets are not smooth. Asperities with heights ranging from 10 to 30 nm, spaced 100 to 200 nm apart, have been identified with atomic force microscopy (Evans et al., 2001; Wang et al., 2001). Similarly, 50-nm-diameter mineral bridges between tablets have been reported (Meyers et al., 2008b). The precise arrangement of tablets differs depending on the species of mollusk. The shells of gastropods such as red abalone have a columnar architecture; the tablets are predominately arranged in vertical columns with some overlap (up to one-third of the surface area of each tablet; Barthelat et al., 2007). In contrast, bivalves such as oysters have nacreous layers with a sheet architecture; here, the tablets are more randomly distributed in the vertical direction, and no columns can be observed (Wang et al., 2001). Further macrostructure is observed in the nacreous layer of a mollusk shell. Regions of nacre are in some cases separated every 300 µm by organic bands approximately 8 µm thick. These organic bands mark interruptions in nacre growth but may also affect the mechanical properties of the nacreous layer (Meyers et al., 2008a). Nacre is a ceramic composite that has defined structural features ranging from nanometer length scales to the upper end of the micrometer length scale. In the next section, the mechanisms by which these features combine and interact to produce nacre’s attractive mechanical properties will be discussed. 5.2.3  Properties Nacre is renowned for many qualities, not the least its beautiful iridescent color. For this, it is frequently used in jewelry and other decorative items. The iridescence is a result of the interference of light with the brick and mortar architecture. Although there are many reasons to mimic this quality, here we will focus solely on the mechanical properties of nacre, as these are what truly set nacre apart from other materials. 5.2.3.1 Elastic Properties There have been several attempts to probe the microscopic elastic moduli of the individual phases of nacre through techniques such as nanoindentation. However, it is difficult to separate the contributions of the inorganic and organic components of the composite (Bruet et al., 2005; Barthelat et al., 2006). Instead, the elastic modulus of macroscopic single crystal aragonite, 100 GPa, is frequently used for composite models. It is even more difficult to determine a suitable modulus for the organic matrix, as no suitable macroscopic analogue exists. It has been proposed that keratin exhibits similar mechanical behavior and that a modulus of 4 GPa has been used to calculate the composite bounds on nacre. These values predict an upper bound on the elastic modulus of 95 GPa and a lower bound of 45 GPa (Jackson et al., 1988). The macroscopic elastic modulus has been measured via a variety of techniques and has been found to be between 60 and 80 MPa depending on its hydration state (Jackson et al., 1988; Wang et al., 2001). Neither of the composite bounds (Equations 1 and 2) predict the elastic modulus particularly well, although this to be expected as the structure of nacre does not resemble that of fiber-reinforced composites modeled by the Voigt–Reuss formulation. Jackson et al. (1988) applied more sophisticated composite models that took into account the transfer of shear loads between the interface of the tablets and the organic matrix. These models predicted moduli ranging from 58 to 82 GPa, depending on the hydration state of nacre and the assumptions used. This demonstrates that to a first approximation, the elastic modulus of nacre is solely a function of the elastic modulus of its composing phases.

176

Biomimetics: Nature-Based Innovation

5.2.3.2 Strength The strength of a material describes the maximum stress it can support before failure. The compressive strength of a ceramic is typically an order of magnitude greater than its tensile strength. This is because ceramics have a small fracture strength; when a ceramic is tested in tension, cracks in the material are pulled open and propagate through the material, causing failure. However, when the ceramic is tested in compression, these cracks propagate stably and failure only occurs when many cracks join together to form a crushed zone (Ashby and Jones, 2005a,b). The strength of nacre is anisotropic and varies depending on the orientation and mode (compression, shear, or tension). The compressive strength of nacre perpendicular to the aragonite layers is on the order of 540 MPa, similar to that of pure aragonite. Parallel to the layers, the compressive stress is only on the order of 235 MPa. The reduction of compressive strength parallel to the layers can be explained by the occurrence of micro buckling of the tablets and splitting of the organic interface (Meyers et al., 2008a). What is remarkable is that the tensile strength of nacre parallel to the layers is on the order of 170 MPa when dry and 140 MPa when wet. Although this tensile strength is approximately half that of the compressive strength in the same direction, it is much greater than the tensile strength of pure aragonite (Meyers et al., 2008a). When hydrated nacre is stretched in tension, inelastic strains of up to 1% can be observed before failure. At tensile stresses greater than 60 MPa, shearing of the organic interface occurs and individual aragonite tablets begin to slide across each other while the tablets themselves remain in a linear elastic regime. These deformations are only generated when nacre is hydrated and the organic matrix is soft and rubbery. When the matrix is dried, it becomes brittle (Barthelat et al., 2007). Dry nacre displays a more linear stress strain curve before failure. The inelastic deformation of hydrated nacre does not continue indefinitely at constant stress. Instead, strain hardening is observed and the stress must be increased for deformation to continue (Barthelat et al., 2007). This phenomenon is important as it implies that more material will be drawn into the deformation; less energy will be needed to initiate new sliding sites than to continue sliding old sites. This redistributes the load through the material and reduces stress at any stress concentration sites, increasing the macroscopic strength of the material. Several mechanisms have been proposed to explain the strain hardening that occurs when hydrated nacre is tested in tension (Figure 5.3). Smith et al. (1999) stretched molecules in the organic matrix with an atomic force microscope cantilever and found that the molecules were capable of very large extensions. A sawtooth pattern in the force extension curve was observed, which could be indicative of the unfolding of polymer chains and the breaking of weak bonds. However, although the unfolding of the polymer chains would certainly absorb energy, the process takes place at approximately constant stress (Barthelat et al., 2007). Hence, it is unlikely to substantially contribute to the observed hardening. Mineral bridges between the tablets may be responsible for increased resistance to deformation when tablets begin to slide (Figure 5.3c). Because of their 50-nm diameter, the mineral bridges would have strength much closer to the theoretical strength of the crystal; the size of possible flaws is much smaller (Meyers et al., 2008b). On the other hand, these bridges would fail early on in the inelastic deformation as they would have a much smaller strain to failure and could not be responsible for the hardening observed under large strains. Wang et al. (2001) and Evans et al. (2001) proposed that increased force between nanoasperities on the aragonite tablets might cause strain hardening (Figure 5.3b).

177

Biomimetic Composites

(a)

(b)

(c)

(d)

FIGURE 5.3 Proposed mechanisms for strain hardening observed in wet nacre: (a) stretching of polymer chains and breaking of weak bonds in the organic layer, (b) resistance caused by contact of nanoasperities, (c) breaking of mineral bridges, and (d) interlocking of wavy aragonite tablets. (Adapted from Meyers M.A., A.Y.-M. Lin, P.-Y. Chen, and J. Muyco, “Mechanical strength of abalone nacre: Role of the soft organic layer,” Journal of the Mechanical Behavior of Biomedical Materials, Vol. 1, No. 1, (2008a), pp. 76–85; Espinosa H.D., J.E. Rim, F. Barthelat, and M.J. Buehler, “Merger of structure and material in nacre and bone—perspectives on de novo biomimetic materials,” Progress in Materials Science, Vol. 54, No. 8, (2009), pp. 1059–1100.)

The  macroscopic force necessary for the aragonite tablets to climb over each other’s asperities would increase as more nanoasperities came into contact, even if the interface were frictionless. Mineral bridges that had fractured could also act as asperities and increase the resistance to deformation (Meyers et al., 2008b). Using finite element models, Evans et al. (2001) showed that sinusoidal asperities with a wavelength of 50 nm could be responsible for the strain hardening observed up to a tensile strain of 0.28%. Nevertheless, strain hardening has been reported all the way to the failure of nacre. Furthermore, with ­transmission electron microscopy, Barthelat et al. (2007) have observed that the actual number of asperities in contact between tablets is quite small and argues that the interaction between nanoasperities is too limited to be responsible for the observed hardening. There is another mechanism that has been proposed to explain the strain hardening behavior exhibited by nacre. The waviness of the aragonite tablets could cause tablets to lock up as they try to slide over each other, similar to a dovetail joint (Figure 5.3d). This would cause tablets that had not locked up to be recruited, initiating new sliding sites and spreading the deformation over larger volumes. The size of the overlap between tablets is substantially larger than the spacing between asperities, and hence this deformation could be expected to act over larger inelastic deformations. Barthelat et al. (2007) created a finite element model on the basis of the microscopic geometry of nacre and showed that the randomly dispersed waviness of the aragonite tablets could fully explain the hardening behavior observed in nacre. Interestingly, the model also showed that if the tablet boundaries were not random, then the gradual hardening behavior did not occur. It is likely that most, if not all, of these structural features contribute to the tensile strengthening of nacre. Still, more research needs to be done before the precise contributions of each of the mechanisms can be quantified. For the purpose of biomimicry, it is sufficient to know that these mechanisms exist and can be used to design new stronger synthetic materials.

178

Biomimetics: Nature-Based Innovation

5.2.3.3 Toughness Nacre is an exceptionally tough ceramic with a work of fracture that is up to three orders of magnitude greater than that of pure aragonite (Currey, 1977). All of the mechanisms that contribute to strain hardening also contribute to nacre’s toughness. Strain hardening implies that more energy must be used to deform the material, and hence more energy must be used to create and propagate cracks. In addition, there are several other structural mechanisms that contribute to the toughness of nacre that have not yet been discussed. Cracks often do not directly propagate directly through the nacreous layer perpendicular to the load as a result of nacre’s brick and mortar architecture. Instead, they follow the path of least resistance and often propagate through the organic layer. This has the consequence that the crack path length is significantly extended and that the overall energy used to propagate the crack in the perpendicular direction is much greater (Mayer, 2005). The organic growth bands observed in the nacreous layer are also thought to be powerful crack deflectors. The creation of cracks is also resisted by the formation of ligaments from the organic phase. The ligaments can bridge the interface between the tablets and can constrain the further growth of the crack even after the aragonite tablets have been pulled out from the area (Jackson et al., 1988). And as mentioned previously, the further deformation of these ligaments requires more energy to unravel the polymer filaments (Smith et al., 1999). 5.2.4  Biomimetic Synthesis Routes There has been substantial research into the process by which nacre is formed at ambient temperatures and pressures by mollusks, especially abalone (Meyers et al., 2008a). Briefly, the abalone begins by secreting proteins that cause the precipitation of a calcite layer. The calcite layer then undergoes a phase transition to aragonite. More protein is then deposited on the aragonite that arrests mineral formation. The process is then repeated until several layers of nacre are formed. Nonetheless, given that the base materials of nacre are in fact comparably weak when compared with man-made ceramics like alumina and that the biomineralization process is still not fully understood, most biomimicry efforts have attempted to mimic just the architecture of nacre and not its base materials. 5.2.4.1 Macroscale Modeling Some of the earliest attempts at mimicking nacre were made by applying nacre’s nanoscale architecture to the macroscale. The Foster-Miller corporation developed the light appliqué segmented tile (LAST) over a decade ago as a removable armor system. LAST consists of a layer of hexagonal tiles of alumina surrounded by thin rubber layers that trap shockwaves. The system lacks much of nacre’s complexity, and many of the strengthening mechanisms discussed previously would not apply. Nevertheless, its performance has been verified, and LAST is used as armor in air and ground vehicles including more than 1000 Humvees for the U.S. Marines (Meyers et al., 2008a). More recently, Mayer (2006) joined rectangular plates of alumina together with adhesive transfer tape in a fashion similar to a brick and mortar architecture. He tested a variety of beams made with different types and thicknesses of adhesive using a four-point bend test. Mayer (2006) found that nacre-like beams consisting of 89% alumina and 11% adhesive were nearly six times tougher than monolithic alumina beams. Furthermore, he found that nacre-like beams were twice as tough as beams with a similar composition arranged in

Biomimetic Composites

179

continuous layers. The nacre-like beams mimicked nacre’s mechanisms of crack deflection and ligament formation. Interestingly, Mayer (2006) found that using too strong an adhesive prevented crack deflection and reduced the toughness of composite. Espinosa et al. (2009) used rapid prototyping to create a polymer composite consisting of acrylonitrile butadiene styrene tablets with wavy dovetail ends and an epoxy matrix. When tensile testing was performed on the composite, it first exhibited strain softening as tablets began to slide and then exhibited some strain hardening and damage spreading after further strain. Although the material does not demonstrate any substantial advantages over conventional composites using these base materials, it does indicate that the dovetail mechanism could be applied to other synthetic composites, perhaps with more success. 5.2.4.2 Layer by Layer There has been some success mimicking nacre’s microscopic features macroscopically. However, several of nacre’s strengthening mechanisms are unlikely to scale well. For example, the ceramic–organic interface area will be substantially smaller for a composite with 5-mm-thick organic layers than for a composite with equal volume fractions but with 30-nm-thick organic layers. The work per volume needed to unravel polymer bonds at the interface might not be so significant for a macroscopic composite. Hence, many groups have attempted to create nanoscale ceramic composites with large surface area to volume ratios for the inorganic phase. One method is to use a layer-by-layer deposition process; a polymer layer is adsorbed onto a surface followed by a clay particle layer. The process is then repeated until a many layered composite is formed. One of the first attempts to use a layer-by-layer process to form a nacre-like nanocomposite (Tang et al., 2003) used negatively charged clay particles (montomorillonite) and positively charged polyelectrolytes (poly(diallyl dimethylammonium) chloride[PDDA]). The clay platelets were ionically attracted to the polyelectrolyte solute and become oriented parallel to the surface to maximize their attractive energy. Composites with more than 1000 alternating layers were formed. The clay layers were approximately 0.9 nm thick, and the PDDA layers were approximately 2.1 nm thick. The volume fraction of the ceramic phase was not reported; however, from the above thickness, it is certain that the composite was predominately polymer based, unlike nacre. During tensile testing, initial plastic deformation of the composites was observed. However, after the initial plastic strain, substantial hardening was observed. It was hypothesized that the hardening was due to the unraveling of polymer chains and breaking of ionic bonds between the polymer and the clay layers. When the composites were tested in a humid environment, the tensile strength of the composite substantially dropped. Polar water molecules shielded the ionic interactions and hence prevented the hardening mechanism from taking place. Podsiadlo et al. (2007) hypothesized that the strength of montmorillonite clay–polymer composites could be improved if a more adhesive polymer was used that formed stronger bonds with the clay surface. Dihydroxyphenylalanine is an amino acid found in mussel adhesive proteins that tether the organism to a wet surface. The group used crosslinked dihydroxyphenylalanine–Lys–polyethylene glycol polymers instead of PDDA in the organic matrix and found that the ultimate strength doubled from 100 to 200 MPa and that the toughness increased by a factor of eight. These polymer composites have a strength and stiffness that is substantially greater than what would be expected for a monolithic polymer. However, it must be remembered that the size of these films is only on the order of 2 µm and substantial advances will need to be made before bulk materials can be made in this way.

180

Biomimetics: Nature-Based Innovation

5.2.4.3 Ice Templating One of the most innovative and successful efforts at mimicking nacre used ice crystals to template the nacre structure (Deville et al., 2006). Under certain conditions, ice dendrites can be formed in a ceramic solution that expel any impurities from the ice crystals and entrap them in channels between the crystals. The ice can then be sublimed from the solution leaving a porous layered material (Figure 5.4). Layers as thin as 1 µm can be formed by controlling the speed of the freezing front. The porous scaffold can then be filled in with another material, metallic or organic, to form a bulk nacre-like composite. Like nacre, these composites have inorganic layers parallel to each other. In addition, the ceramic layers also end up with a large surface roughness because of particles that remain trapped in the edge of ice dendrites. Mineral bridges can also be observed, which span the lamellae. However, unlike nacre, the composites consist of nearly equal quantities of each phase. Still, Deville et al. (2006) were able to create alumina/Al-Si/Ti composites with a fracture toughness of 10 MPa m1/2 as opposed to the alumina toughness of 3 to 5 MPa m1/2. Extensive crack deflection in the composite was observed. Extremely stiff ceramics composites with a specific toughness and strength that match that of metallic aluminum alloys have been created using a modified form of ice templating (Munch et al., 2008). Laminar alumina scaffolds with a porosity of 66% were prepared using the ice templating process. The scaffolds were then compressed perpendicular to the lamellae and resintered to promote the formation of ceramic bridges between the lamellae. a)

Crystal growth direction

(b)

Ceramic particles (c)

Ice crystal (d)

FIGURE 5.4 (a) Principles of ice templating processes. Growing ice crystals expel ceramic particles creating a lamellar microstructure oriented parallel to the direction of the freezing front. A small fraction of ceramic particles can remain entrapped within the ice crystals leading to the formation of mineral bridges and surface roughness. Dense composites such as alumina–PMMA (b–d) can then be created by infiltrating the porous ceramic with a second phase. (Panel a: Adapted from Deville S., E. Saiz, R.K. Nalla, and A.P. Tomsia, “Freezing as a path to build complex composites,” Science, Vol. 311, (2006), pp. 515–518.) (Panels b–d: Reprinted from Launey M.E., E. Munch, D.H. Alsem, H.B. Barth, E. Saiz, A.P. Tomsia, and R.O. Ritchie, “Designing highly toughened hybrid composites through nature-inspired hierarchical complexity,” Acta Materialia, Vol. 57, No. 10, (2009), pp. 2919–2932. With permission.)

Biomimetic Composites

181

This led to a porosity of only 20% with a lamellar thickness of 5 µm. The scaffold was then infilled with poly(methyl methacrylate) (PMMA) to form a nacre-like composite. When a three-point bend test was performed, several similarities to nacre were observed. Large inelastic strains of 1% were observed. Damage was widely distributed ahead of crack formation and a multitude of tablets were observed to slide apart. Ligament formation was also observed. These nacre-like composites had a work of fracture that was more than 300 times greater than that of monolithic alumina. Although significant progress must be made before nacre-like composites can be formed that have the same performance gains as the actual material, substantial improvements to synthetic materials have already been seen with the use of nacre-like structures. The alumina-PMMA ceramic discussed above has an unprecedented level of toughness and is in fact many times tougher than nacre itself. As ceramic composites that more closely mimic the structure of nacre are formed, it is likely that the boundaries of engineering materials will be further extended. 5.2.5  Comparing Nacre with Eggshell and Dental Enamel Two other mineralized composites are worth mentioning in comparison with nacre: ­eggshell and dental enamel, both of which exhibit organic versus inorganic volume fractions along similar lines. Eggshell is 95% mineral—calcite—by weight, approximately 3.5% organic and 1.5% water (Nys et al., 2004). Dental enamel is 96% mineral—­hydroxyapatite—by weight, 3% water with the balance organic (Skinner and Jahren, 2003). Just as in nacre, eggshell and dental enamel both have mechanical functions that rely on mechanical properties and toughness in particular. Although the compositions of these three materials, nacre, eggshell, and enamel, have some similarities, the microstructures of the three materials are strikingly different (Figure 5.5). The enamel structure, in particular, is exceptionally complicated and arises from the complex activity of ameloblast cells during tissue formation. Because of the cell-directed microstructural complexity, enamel is less studied than either nacre or eggshell in the context of cell-free biomimetic synthesis. The avian eggshell has important barrier functions in protecting the developing chick while also controlling water and gas transport and providing a calcium store to aid in bone formation (Nys et al., 2004). The shell has a dynamic, rapidly evolving structure over its relatively short lifetime because of this last function, during which the inner surface (mammillary tips) of the shell are eroded substantially in the final few days of incubation to provide calcium for rapid bone formation (Karlsson and Lilja, 2008). There are structural differences between eggshells of fast- and slow-growing bird species associated with the calcium needs during development (Karlsson and Lilja, 2008). The fracture strengths of the inner and outer surfaces of the eggshell are different (Entwistle et al., 1995), consistent with differences in structure and differences in function: the outer surface must not fracture in response to external loading, whereas the inner surface must eventually rupture because of the action of the chick itself, or successful hatching will not occur. Eggshell represents one of the fastest mineralization rates in all known biomineralization processes: the approximately 300-μm-thick eggshell in hens forms in less than 24 hours (Lavelin et al., 2000). This rapid mineralization commences with heterogeneous nucleation of calcite on the bilayer shell membrane, and the resulting orientation of the crystals is influenced by the membrane proteins (Fernandez et al., 1997). Calcite crystals in avian eggshell can be up to 200 μm in length (Hunter, 1996). As in other biomineralized systems, a small amount of organic material is contained within the largely mineralized structure, and this is implicated in the fracture resistance of the shell.

182

Biomimetics: Nature-Based Innovation

(a) Organic matrix

Aragonite crystals

(c)

Cuticle Vertical crystal layer

Calcite crystals 2 µm (b)

Enamel rods

Hydroxyapatite crystals

Palisade layer Calcite grain boundaries Mammillary layer

5 µm

Organic shell membrane

50 µm

Protein-rich rod sheath FIGURE 5.5 Microstructures of nacre (a), enamel (b), and eggshell (c). Nacre consists of aragonite tablets joined together with an organic matrix. Enamel is made up of small needle hydroxyapatite crystals that form rods with keyhole-like cross sections. Eggshell contains much larger calcite crystals that grow outward from the organic shell membrane. (Adapted from Vincent J.F.V., Structural Biomaterials (Revised ed.), Princeton University Press, Princeton, NJ, (1990), ISBN 0691085587; Rodriguez-Navarro A., O. Kalin, Y. Nys and J.M. Garcia-Ruiz, “Influence of the microstructure on the shell strength of eggs laid by hens of different ages,” British Poultry Science, Vol. 43, No. 3, (2002), pp. 395–403; Habelitz S., S.J. Marshall, G.W Marshall Jr., and M. Balooch, “Mechanical properties of human dental enamel on the nanometre scale,” Archives of Oral Biology, Vol. 46, No. 2, (2001), pp. 173–183.)

In the context of biomimicry, a number of studies have endeavored to use natural eggshell membranes to initiate biomineralization in vitro (Fernandez et al., 2004; Bera and Ramachandrarao, 2007). Mineral deposits on the organic matrix ex vivo when the membrane is soaked in an aqueous solution of mineral salts. Similar approaches have been examined for remineralizing dental enamel with soaking demineralized teeth in a synthetic body fluid. Although these systems present a partial biomimetic solution, they are using a natural substrate and thus not a fully biomimetic solution, unlike some of the approaches discussed above in the context of nacre (Section 5.2.4). The reaction rates in such systems are also relatively slow compared with biomineralization in vivo. Further aspects of hydroxyapatite biomineralization, in the context of bone rather than tooth enamel, will be considered in the next section (Section 5.3) on bone. 5.2.6  Summary The three tissues discussed in this section—nacre, but also eggshell and enamel—have roughly the same base composition with a dominant mineral component and a small hydrated protein phase. The details of their structures are different, their biological functions are different, and they come from very different parts of the animal kingdom. The

Biomimetic Composites

183

unifying theme in these materials is that they have mechanical functions that require toughness beyond what would be found in a pure ceramic or mineral material, and as such the small but critical organic component is the dominant factor in ­controlling mechanical performance. The stiffness of these materials ends up being close to that of the pure ceramic, and the strength can exceed that of the pure ceramic. Further, these three materials represent nearly pure ceramic materials that have been formed at body temperature (close to room temperature) and ambient pressure, ­presenting an extremely promising environmentally friendly route to material synthesis. Because of the large mineral fraction, the density of nacre and related materials is quite large in the context of natural materials. We next consider bone, which has the same type of constituents as dental enamel but in very different proportions, thus resulting in a relatively small density and correspondingly good mechanical property to density ratios (Table 5.1), considering it is less than half inorganic.

5.3  Bone Bone has been used by humans as a material with which to form tools for at least 25,000 years (Weiner and Traub, 1992). It is stiff and strong and resists bending and buckling. It is also tough, avoiding catastrophic failure (Fratzl and Weinkamer, 2007). Bone tissue imparts mechanical support and protection to the vertebrate skeleton and also provides other important biological functions, including acting as a store for calcium and housing hematopoietic stem cells in bone marrow. The nonliving material of bone is an organic–­ inorganic composite, as was the case for nacre, eggshell, and dental enamel. However, unlike the biocomposites considered previously, the hydrated organic phase of bone comprises an almost equal proportion of the material and has a much greater effect on the overall tissue properties. Reasons for making bonelike materials are twofold: replacement bones have potential medical uses (Green et al., 2002), and bone’s complex composite behavior has attracted the attention of a broad community outside of traditional medicine (Mann and Weiner, 1999). 5.3.1  Structure and Composition 5.3.1.1 Bone Composition Bone tissue consists of a small proportion (1%–3% by volume) of cells embedded in a hydrated extracellular matrix (ECM). Cells are responsible for bone formation, maintenance, and remodeling, whereas the ECM carries the mechanical load and performs other physiologic functions. The ECM of bone is composed of three constituent phases: organic, mineral, and water. Adult compact bone contains approximately 30% organic material, 50% mineral, and 20% water by volume (Figure 5.1a) (Gong et al., 1964; Hayes, 1991). The organic phase of bone, called osteoid, is approximately 90% type I collagen and 10% noncollagenous proteins and other macromolecules (Kaplan et al., 1994). The mineral phase is composed of a highly substituted biological analog of hydroxyapatite (Ca5(PO4)3OH)). Carbonate (CO32–) is commonly substituted into the apatite lattice for OH– or PO43–, and sodium, potassium, magnesium, and zinc ions often occupy a common Ca2+ ion vacancy (Elliot, 2002).

184

Biomimetics: Nature-Based Innovation

5.3.1.2 Bone Ultrastructure If one wishes to consider bone ECM as a material, it makes sense to consider the fundamental structure at very small length scales (submicrometer) where the structure of bone is dominated by individual collagen fibers and hydroxyapatite crystals (Figure 5.6). Type I collagen is made up of tropocollagen molecules approximately 300 nm long and 1.5 nm in diameter that self-assemble into a quarter-staggered array with periodic banding every 67 nm because of regions of overlap (Figure 5.7). This regular arrangement of tropocollagen molecules forms larger collagen fibrils, which are in turn grouped to form large ­collagen fibers. Collagen is highly cross-linked with both intermolecular and intramolecular

Organ (macroscopic level)

m

Tissue level

mm

µm

Cell level

nm

Material (extracellular matrix level)

FIGURE 5.6 Four organizational levels for considering the materials science aspects of bone. (Top to bottom) Macroscopic (organ-level) bones are made of bone tissues (cortical and trabecular) at millimeter length scales. The bone materials are generated and maintained by complex interactions between three distinct cell types (osteoblasts, osteocytes, and osteoclasts), whereas the material itself is a composite of organic material (primarily collagen), mineral, and water. (From Oyen M.L., “The materials science of bone: Lessons from nature for biomimetic materials synthesis,” MRS Bulletin, Vol. 33, No. 1, (2008), pp. 49–55. With permission.)

{

0.87 nm

Polypeptide chain

{

8.6 nm

Tropocollagen molecule 300 nm

{

{

{

67 nm 35 nm

Collagen fibril

FIGURE 5.7 Hierarchical structure of collagen fibril.

Biomimetic Composites

185

c­ ross-links (Alberts et al., 2008), which makes bone collagen highly ­insoluble (Vater et al., 1979). The noncollagenous organic phase in bone is mostly composed of glycoproteins, osteopontin, and proteoglycans. The exact shape and size of the nanometer-sized apatite crystals that mineralize the underlying collagen matrix remain unclear. A variety of techniques used to examine bone mineral have quantified bone crystals over a wide range of shapes and sizes (Ziv and Weiner, 1994; Benezra Rosen et al., 2002; Hassenkam et al., 2004). The majority of bone apatite crystals appear as nanometer-scale plates 20 to 80 nm long and 4 to 5 nm thick although some needlelike structures have also been noted and debated (Bonucci, 2000; Currey, 2002). Mineral platelets fuse both laterally and longitudinally, forming long and broad sword-blade structures that seem to remain relatively thin, approximately 5 nm thick (Currey, 2002). The precise arrangement of the collagen fibrils (Figure 5.7) and hydroxyapatite crystals at this ultrastructural length scale is still poorly understood and is the subject of much current research and discussion (Currey, 2002). The initial formation of crystals between the collagen fibrils in the “Hodge–Petruska” gaps, originally proposed by Petruska and Hodge in 1964, may drive the resulting shape and size of bone mineral. Ultimately, the fibrillar collagen structure is interrupted and disrupted by the presence of mineral forming within the gaps (Landis et al., 1996). Crystals exist that are too large to fit within the fibril or hole region and have been observed, via TEM, to exist in the interfibrillar region (Katz and Li, 1973; Landis et al., 1996; Eppell et al., 2001). TEM observations have shown that the majority of bone crystals lie within and on the surface of the collagen fibrils (McKee et al., 1991; Benezra Rosen et al., 2002), where the long axis of the crystals lies parallel to the long axis of the collagen fibrils. The nonfibrillar organic matrix may “glue” mineralized collagen fibers together and may also serve as a source of sacrificial bonds, thus increasing the energy to fracture (Fantner et al., 2005). The water in bone has received relatively little attention from a mechanical perspective, especially in the context considering bone as a composite material. However, fluid-filled pore spaces occupy approximately 20% of a bone’s volume. Pores ranging in size include large millimeter-sized trabecular spaces, vascular pores including Haversian canals (≈20 μm), canaliculi and lacunae (≈0.1 μm), and nanometer-sized pore spaces or “matrix micropores” (Knothe-Tate, 2001) that exist between and within the collagen and mineral crystals. Bound water may stabilize the mineral crystallites by occupying OH– and possibly Ca2+ vacancy sites (Wilson et al., 2006) but likely plays a minor role in the mechanical behavior of the bone tissue. Porosity in bone facilitates movement of the interstitial fluid (i.e., unbound water) throughout the tissue that contributes to poroviscoelasticity. Fluid that exists in smaller pore spaces between the mineral crystals and collagen fibrils (Neuman et al., 1956) may add plasticity to bone’s mechanical response. In addition, the proteins and glycoproteins within the organic matrix interact with chemically unbound water through charged interactions. The availability of charged sites on the organic matrix has a profound effect on the stiffness of unmineralized matrix in tissues containing other forms of collagen (Eisenberg and Grodzinsky, 1985). Similarly, the interaction between water and bone’s organic matrix is subject to similar hydrostatic interactions, where the bone tissue stiffens with occupation of charged sites by polar solvents (Bembey et al., 2006). 5.3.1.3 Bone Macrostructure At longer length scales, there are several distinct levels of hierarchical organization in bone (Lakes, 1993; Weiner and Wagner, 1998; Currey, 2005). Characterization can be according to

186

Biomimetics: Nature-Based Innovation

the collagen organization as parallel fibered or dominantly irregular in woven (immature) bone (Bonucci, 2000). There are layers (lamellae) of parallel-fibered bone that organize into higher-order cylindrical structures called osteons, through which run a central neurovascular canal. At a microscopic structural level, bone is classified as cortical (compact, dense) or trabecular (spongy, cancellous). Cortical and trabecular bones have approximately the same material density, but trabecular bone is macroscopically porous with a relative density less than 0.7 (Gibson and Ashby, 1997). At the organ scale of a whole bone such as the femur (thigh bone), both types of bone are present, with the cortical bone forming a protective shell around the porous trabecular bone. Additional nonbone tissue components are present in whole bones, including the marrow, the vasculature, and the nerves. 5.3.2  Properties Historically, mechanical analysis of bone has emphasized tissue-level features such as the difference between cortical and trabecular bones. At this scale, there is a large difference between the mechanical responses of cortical and trabecular bones because of the large differences in relative density associated with the macroscale porosity of trabecular bone and a cellular solids approach is useful (Gibson and Ashby, 1997). Advances in instrumentation in the last decade have led to a large number of studies of the nanomechanical behavior of bone—thus at length scales comparable with the collagen molecules and apatite crystals—using both experimental (Balooch et al., 2005; Ebenstein and Pruit, 2006; Donnelly, 2006) and computational techniques (Jaeger et al., 2005). Consistent with their comparable physical densities (when macroscopic porosity is accounted for), it has been demonstrated using both nanoindentation and ultrasound attenuation techniques that the elastic modulus of cortical and trabecular bones is approximately the same (Turner et al., 1999). Therefore, to a first approximation, the material bone can be considered as a single entity and one for which key factors affecting material level properties are the mineral content (Currey, 2003) and local structure (Oyen et al., 2008). 5.3.2.1 Elastic Properties A survey of the literature shows that the average elastic modulus of normal bone material as measured by nanoindentation (where macroscopic porosity effects can be excluded) lies within the range of 15 to 30 GPa when the tissue is dry (Turner et al., 1999; Rho et al., 1999; Hengsberger et al., 2003; Oyen et al., 2008). Bone is significantly less stiff when fully hydrated (Bembey et al., 2006). For exploitation of a bottom–up materials synthesis based on biomimicry, it is necessary to understand the means by which bone’s structure influences its mechanical properties. Like nacre, the elastic properties of bone have been considered in a two-phase composite context, building on the work of Katz (1971). However, unlike nacre, there have been few truly successful mechanical models of bone as a composite. Part of the difficulty modeling bone as a composite is in knowing what material values to attribute to biological apatite and collagen. The elastic modulus of both single crystal hydroxyapatite and mineralogical fluoroapatite is reported in the range 100 to 150 GPa depending on test method and specimen orientation (Katz and Ukraincik, 1971; Zioupos et al., 1999; Oyen et al., 2008). Most modeling of collagen for mineralized tissues has incorporated a modulus of 1 to 1.5 GPa for collagen (Wagner and Weiner, 1992; Akiva et al., 1998; Kotha and Guzelsu, 2002; Qin and Swain, 2004) consistent with the value of 1.2 GPa proposed by Gosline et al. (2002) in a comparative analysis of elastic proteins. However, the material properties of collagen are scale dependent and vary with measurement technique,

187

Biomimetic Composites

Effective elastic modulus, E (GPa)

hydration state, and source of the collagen material. Values of modulus ­ranging from 0.4 to 38 GPa have been reported through atomistic modeling, x-ray diffraction, microelectromechanical systems, and atomic force microscope (AFM) testing (Gupta et al., 2004; Vesentini et al., 2005; Buehler, 2006; van der Rijt et al., 2006; Eppell et al., 2006; Shen et al., 2008; Buehler, 2008). More confounding for attempts at modeling bone as a composite is that its modulus increases dramatically at nearly constant mineral volume fraction (in the region of mineral volume fraction 0.35–0.5). The result is that bone modulus spans a region between the commonly used upper and lower Hashin–Shtrikman composite bounds for particulate composites (Figure 5.8) and therefore cannot be predicted on the basis of the mineral ­volume fraction alone (Katz, 1971). Furthermore, the elastic response of bone is anisotropic because of the local orientation variations of the collagen fibrils and platelike minerals at the material scale. Microstructural inhomogeneity, including the tubular osteonal structure, reinforces anisotropy at larger length scales. The combined effect is that in femoral cortical bone, the elastic modulus along the long axis is approximately 1.5 times the transverse value (Huiskes and van Rietbergen, 2005). As mentioned previously, bone also demonstrates time-dependent mechanical behavior because of the presence of a hydrated protein phase (Sasaki and Enyo, 1995) and exhibits poroelastic fluid flow (Cowin, 1999). The time-dependent mechanical response is observed as an increased apparent elastic modulus with increased strain rate, further complicating elastic modeling efforts. The time-dependent mechanical response of bone has been shown to be highly dependent on the hydration state (Sasaki and Enyo, 1995; Yamashita et al., 2001; Bembey et al., 2006). This substantial nanoscale heterogeneity has led to a variety of assumptions about how the organic and mineral phases of bone interrelate. Mineral crystals have been assumed to lie entirely within the collagen fibrils (Weiner et al., 1999; Jäger and Fratzl, 2000), outside of the fibrils to form interpenetrating phases between collagen and mineral (Pidaparti et al., 1996; Fritsch and Hellmich, 2007), both within (~25%) and outside (~75%) of the ­collagen (Sasaki et al., 2002), or predominantly outside of the collagen fibrils (Hellmich and Ulm, 2002). The Human Femoral head Osteomalacic IC Incus Mandible

100 80 60 40

Whale Fin otic Beaked rostrum

20 10 8 6 4 0.0

0.2 0.4 0.6 0.8 Mineral volume fraction, Vf

1.0

FIGURE 5.8 (See color insert.) Elastic modulus (E) versus mineral volume fraction (Vf) of PMMA-embedded bone samples: Human femoral head (“Normal”); human osteomalacic iliac crest (OM); human incus; human mandible (“Jaw”); fin whale otic bone; and dense beaked whale rostrum. The solid lines are the Voigt–Reuss bounds, and the dashed lines are the Hashin–Shtrikman bounds for particle composites. (From Oyen M.L., V.L. Ferguson, A.K. Bembey, A.J. Bushby, and A. Boyde, “Composite bounds on the elastic modulus of bone,” Journal of Biomechanics, Vol. 41, No. 11, (2008), pp. 2585–2588. With permission.)

188

Biomimetics: Nature-Based Innovation

specific relationship between the collagen fibrils and the mineral ­crystals has a tremendous influence on accurately predicting bone stiffness. Experimentally, mineral has been demonstrated to lie both within and outside of the collagen in dentin (Balooch et al., 2008), mineralized tendon (Landis et al., 1996), and bone (McKee et al., 1991), where the distribution of mineral in mature tissues is predominantly in the extrafibrillar compartment. It is interesting to note that the organic component of bone can be removed entirely without much reducing the elastic modulus (Vincent, 1990). That bone is best visualized as an interpenetrating phase composite (Clarke, 1992)—with some degree of mineral-mineral interaction allowing for a functional porous mineral framework—is apparent to some authors (Hellmich and Ulm, 2002; Hellmich et al., 2004) but largely ignored in other recent models (Jäger and Fratzl, 2000) that consider bone as a particle-reinforced composites with apatite as the particle phase. The most successful attempts to relate bone macroscopic mechanical behavior to the response of individual components at fundamental (ultrastructural) length scales has been via sophisticated multiscale computational modeling. Multiscale modeling approaches have explicitly incorporated anisotropy and computed individual stiffness tensor components (Hellmich and Ulm, 2002; Hellmich et al., 2004). However, it is unclear that these models present a true solution to the problem, as an extremely large (tens of GPa) elastic modulus value is used for the collagen phase, such that mineral reinforcement would be superfluous. With the collagen essentially as stiff as the composite bone itself, this model likely represents only a first step toward a full description of bone at multiple-length scales based on the nanometer-scale constituent phases. To a first approximation, detailed understanding of the organic matrix-mineral interactions is limited by intrinsically small length scales, and there exists a clear need for new tools and experimental approaches to examine materials at these scales to produce biomimetic composites. The ability to model—and thus predict—the mechanical properties of bonelike composites is currently limited by our lack of knowledge about mineral–matrix interactions at molecular length scales. It is unclear the extent to which continuum-based modeling is appropriate for capturing the physics of nanometer-scale phases and phase interactions, and it is likely that molecular modeling will be used for more thorough investigation of organic–inorganic nanocomposite mechanics in the future. 5.3.2.2 Failure Properties Given bone’s importance as a structural member within the body, and the relatively common medical complaint of fractured bone, much research has been conducted into the failure behavior of bone, including studies of both strength and fracture toughness. The strength of bone is influenced by strain rate, orientation, and whether the applied loading is tensile or compressive (Huiskes and van Rietbergen, 2005). Fracture behavior of bone has been reviewed recently and, as with the elastic modulus, there is no single, simple value that can be reported as the toughness (Ager et al., 2006; Taylor et al., 2007). The fracture resistance is different for crack initiation and propagation and depends on factors such as subject age and crack geometry relative to local microstructure (Ager et al., 2006). Further, bone exhibits the “R-curve” behavior, where the fracture resistance increases with increasing crack length. As the elastic modulus of bone generally increases with mineral content, the fracture toughness of bone decreases such that there is an intrinsic trade-off between stiffness and toughness for any value of mineral density (Currey, 2003). Recent nanomechanical studies have demonstrated substantial variability in the material

Biomimetic Composites

189

properties of bone at fundamental length scales (Oyen and Ko, 2007; Tai et al., 2007). Local elastic modulus variation has been implicated as potentially important in bone fracture behavior (Jaasma et al., 2002; Tai et al., 2007). Many of the toughening mechanisms that are present in nacre have also been observed in bone, including crack deflection and ligament formation (Ager et al., 2006). However, it is worth noting that in contrast to nacre, bone tissue is constantly being locally remodeled and repaired; bone macrostructures can be lighter and accept a higher risk of small cracks developing. In fact, cracks in bone are readily visible at the microscopic scale with the aid of dyes. Nonetheless, cracks larger than a few millimeters are rarely observed, possibly because the fracture resistance of bone does increase with crack length (Taylor et al., 2007). 5.3.2.3 Electrical Properties Although the mechanical properties of bone have received a great deal of attention, bone also has an interesting electromechanical function in that the bony material itself is piezoelectric: extrinsic mechanical loading results in generation of an electrical charge (Becker and Marino, 1982). This electrical signal has been identified as potentially important in bone regulation and remodeling (Becker and Marino, 1982). The piezoelectricity effect has recently been considered for the role it might play in biomineralization, with examination of the remineralization of demineralized natural bone collagen when subjected to mechanical loads (Noris-Suárez et al., 2007). 5.3.3  Biomimetic Synthesis Routes In either de novo bone formation or bone healing, human bone is produced in two steps: (1)  the deposition of primary bone and (2) the remodeling of the primary bone—both matrix and mineral—to form secondary bone (Boskey, 1998a). For exploitation of a bottom–up materials synthesis based on biomimicry, it is necessary to focus on the deposition of primary bone and thus to examine the current state of knowledge in the field of bone biomineralization. The subject is reviewed elsewhere (Boskey, 1998b; Boskey, 2003; Skinner and Jahren, 2003), but a few highlights are presented here. Primary bone is deposited in two stages, the formation of a collagen network and the subsequent mineralization of this collagen matrix. New osteoid bone becomes calcified 70% within days but full calcification beyond this takes months (Huiskes and van Rietbergen, 2005). In immature newly formed woven bone the average mineral size is smaller than in mature, fully mineralized bone (Kaplan et al., 1994). The apatite in bone deposits onto the organic matrix by heterogeneous nucleation, but it is unclear if the nucleation sites are on collagen or on noncollagenous proteins including glycoproteins such as biglycan (Boskey, 1998b; Skinner and Jahren, 2003). Negatively charged groups appear to direct mineralization as does protein phosphorylation (Boskey, 2003; Skinner and Jahren, 2003). Recent examination of natural bone by solid-state nuclear magnetic resonance has indicated hydroxyapatite-sugar interfaces as the fundamental interaction, and not hydroxyapatite protein interfaces (Jaeger et al., 2005). In natural systems, the cells are intricately linked with both positive and negative control of matrix mineralization through the production or organic ECM molecules and regulation of local ion transport (Boskey, 2003). Clearly, the design of appropriate biomimetic systems for bone hinges on the lack of certainty in intrinsic biomineralization and additional study is required.

190

Biomimetics: Nature-Based Innovation

5.3.3.1 Bone Tissue Engineering Tissue engineering is a basic paradigm of regenerative medicine in which the focus is the promotion of cellular activity to recapitulate developmental pathways and recreate tissues by biological means. Tissue engineering for bone has several prospective applications, including cases where resection of a large bone segment is required because of ­disease including malignancy. Current clinical practice for bone replacement usually involves an autologous (from self) or allogenic (from a donor) bone transplant. Neither of these approaches is perfect—in autologous transplants, the bone may not be of sufficiently high quality, especially if there is a systemic condition affecting bone quality, and there exists the potential for donor site morbidity. In allogenic transplants, there exists a risk of disease transmission, especially viral, and immune responses to the foreign matter (Green et al., 2002). This has led to substantial interest in creating artificial bonelike materials for use in the body. Much materials science and engineering effort has gone into the design and fabrication of porous scaffolds for different tissue engineering applications including bone (Karp et al., 2003; Hollister, 2005), and the reader is referred to other reviews on the subject (Orban et al., 2002; Meyer et al., 2004; Wiesmann et al., 2004; Sharma and Elisseeff, 2004). In basic tissue engineering approaches, cells are seeded onto a porous scaffold material optimized for cellular attachment, proliferation, and synthetic activity. The additional component in tissue engineering is the addition of factors intended to encourage cell ECM synthesis, such as growth factors or signaling molecules. Most tissue engineering scaffolds are designed to be resorbable, such that the newly synthesized ECM tissue gradually replaces the original scaffold. In this way, the scaffold provides initial mechanical support as well as a three-dimensional structure for promotion of cell activity. Scaffold materials include polymers (Liu and Ma, 2004), biopolymers (Anseth and Burdick, 2002; Harley et al., 2007), and porous apatite ceramics (Landi et al., 2005). 5.3.3.2 Cell-Free Biomimetic Processing In contrast to tissue engineering approaches, biomimetic materials synthesis emphasizes cell-free pathways to create bonelike materials. At least three different approaches for bonelike nanocomposites can be considered (Figure 5.9) with different degrees of molecular self-assembly (Zhang, 2003) and emphasizing different biomimetic pathways (Oyen, 2008): mixing of nanoscale organic and inorganic components; formation of selfassembled biopolymer networks from proteins and/or polysaccharides, followed by subsequent mineralization using the organic matrix as a template; and in situ coprecipitation of organic and inorganic components, potentially including self-assembling organic molecules. Simple mixing of nanoscale components is a typical engineering approach for composite formation, although component mixing does not result in the sorts of interactions between the organic and the inorganic phases that occur in natural systems (Tampieri et al., 2005a). Instead, a process mimicking biomineralization in some manner is required. There have been several attempts at this type of approach, either in which a fully formed organic matrix is mineralized (Hartgerink et al., 2001) or in which a coprecipitation approach is used to simultaneously deposit organic and inorganic phases (Chang et al., 2003; Bernhardt et al., 2008). A recent approach further tries to mimic the role of acidic noncollagenous proteins in the process of forming mineralized collagen composites (Jee et al., 2010). If polymeric

191

Biomimetic Composites

(a)

(b)

(c)

FIGURE 5.9 Cell-free biomimetic synthesis paths for templated bonelike composite materials: (a) mixing of nanoscale phases, (b) precipitation of mineral on self-organized biopolymer network, (c) coprecipitation of self-assembling organic and inorganic phases. (From Oyen M.L., “The materials science of bone: Lessons from nature for biomimetic materials synthesis,” MRS Bulletin, Vol. 33, No. 1, (2008), pp. 49–55. With permission.)

(i.e., nonprotein organic) matrices are considered, the polymer can be functionalized with cell attachment motifs such as the RGD peptide for integrin binding or other synthetic peptides with ECM functions (Shin et al., 2003). Such an approach can also be used in basic biomineralization studies (Boskey, 1998a), in which different functional domains of biomacromolecules can be included in or excluded from an organic network to ascertain their effect on apatite formation. These types of biomineralization approaches have been extended beyond the natural bone material set where collagen (Tampieri et al., 2005a; Bernhardt et al., 2008) or gelatin (Chang et al., 2003) have combined with apatite to make a bonelike material. Other organic components coupled with apatite have included alginate (Tampieri et al., 2005a,b), chitosan (Manjubala et al., 2006), silk (Li et al., 2006), and synthetic peptides (Hartgerink et al., 2001). Future studies will certainly include multicomponent organic matrices with both protein and sugar components. Relatively few studies have undertaken detailed investigation of the mechanical properties of biomimetic cell-free bonelike materials. Slight stiffness increases were detected by atomic force microscope indentation following early (calcium carbonate) mineralization of self-assembled fibrous elastin and fibronectin networks (Subburaman et al., 2006). A range of mechanical assays demonstrated that the stiffness, strength, and fracture toughness of gelatin-apatite composites were comparable with natural bone (Ko et al., 2006). Although the strength and toughness tests require reasonably large samples of material, the elastic properties were measured with nanoindentation; either AFM indentation (Subburaman et al., 2006) or nanoindentation (Ko et al., 2006) can be undertaken on reasonably small samples and thus provide great promise in exploring mechanical interactions in biomimetic nanocomposites. Given the interest in bone mechanical properties and the ways in which mechanical factors have motivated development of these biomimetic composites, it is likely that the next generation of studies incorporating biomimetic synthesis of bonelike materials will have a substantial mechanical component.

192

Biomimetics: Nature-Based Innovation

5.3.3.3 Self-Healing Bone is an active material that is both able to heal and to remodel because of local cell activity. Recent studies have focused on mimicking the self-healing and remodeling processes that occur in bone to design materials that can be used with less conservative factors of safety. Several of these studies have incorporated isolated healing agents, such as epoxy microcapsules, into their microstructure for a cell-free, nonliving approach to active microstructural control. The healing agents are released upon crack intrusion and ­polymerized (White et al., 2001). Recently, three-dimensional networks of epoxy have been incorporated into substrates that mimic microvasculature, ensuring that the self-healing supply is not locally limited and that multiple healing cycles can be achieved (Toohey et al., 2007; Hansen et al., 2009). Although these early attempts at self-healing materials have focused on passive systems, it is certainly possible that in the near future active agents will be present in the material that could coordinate a material response to local damage.

5.4  Soft Tissues In this chapter thus far, we have examined the biomimicry of hard, biomineralized composite materials. However, the majority of tissues in the animal kingdom are “soft” and do not contain substantial quantities of mineral. These tissues are composed of the polymers and elastomers of the natural world. Like synthetic polymeric materials, soft tissues have a diverse range of properties; their elastic moduli spans from the sub-megapascal (cartilage) to the gigapascal region (tendon) (Wegst and Ashby, 2004). However, in contrast to manmade materials, this range in mechanical properties is not achieved through substantial modification of chemical composition. Instead, it is primarily accomplished through the modification of structure (Fratzl, 2008). In fact, the composition of the ECMs of arteries, tendons, cartilage, and the dermal layer of skin are all relatively similar (Table 5.2). As the price of petroleum increases, there is growing pressure to develop biobased ­chemicals and structures to decrease the reliance on finite petroleum resources (Ragauskas et al., 2006). Furthermore, there are several attributes of soft tissues that are unmatched by man-made polymeric materials. Arteries undergo 35 to 40 million loading cycles each year but fatigue failure is rare (Holzapfel, 2008). Tendons are very flexible in bending but are stiff when large tensile forces are applied (Biewener, 2008). Synovial joints have extremely TABLE 5.2 Representative Values for the Composition of the ECM of Soft Tissues Water (wet weight)

Collagen (dry weight)

Elastin (dry weight)

Proteoglycan (dry weight)

Arterial wall Articular cartilage Dermis

71% 65%–80%

45%–52% 65%–75%

25%–34% –

– 20%

Cox (1975) Ventre et al. (2009a,b)

60%–70%

70%–80%

2%–4%

–

Tendon

55%–70%

60%–85%

2%

0.5%–3.5%

Richard et al. (1993), Ventre et al. (2009a,b) Kjaer (2004), Ventre et al. (2009a,b)

Tissue

Reference

Biomimetic Composites

193

small coefficients of friction that are maintained from the moment movement starts up to speeds of 0.6 m s–1 (Ateshian and Mow, 2005). These exceptional attributes are achieved with hierarchical structures that are structured differently at each level (Fratzl, 2008). As was the case with bone, there has been substantial research in creating tissuelike mimics that can augment the limited supply of organ transplants (Swartz and Griffith, 2006; Chung and Burdick, 2008; Kandel et al., 2008). However, unlike many of the mineralized materials considered thus far, there are few concrete examples of applying the concepts that can be learned from soft tissues to other engineering systems. 5.4.1  Structure and Composition 5.4.1.1 Proteins and Polysaccharides Like the mineralized tissues considered previously, soft tissues are not purely composed of cells, but also have a substantial ECM. There are three main types of structural ­polymers found in the ECM of soft tissues: proteins, polysaccharides, and hybrid polypeptide­polysaccharide chains. Proteins are polymers formed from different combinations of 20 amino acids. The amino acids are linked together by peptide bonds to form a linear chain. Ionic bonds and weak interactions cause the linear chains to fold into secondary and tertiary structures. These polypeptide structures can then combine with other polypeptides to form proteins with quaternary structure. The most common structural protein found in soft tissues is collagen. In fact, collagen has been compared with steel, as it is the basic structural component in the body (Fung, 1993). Collagen is a right-handed triple helix of polypeptides which themselves have a ­left-handed helical structure (Figure 5.7). There are more than 20 variants of collagen depending on which polypeptides combine together, although type I and type II collagens are the most abundant in soft tissues (Alberts et al., 2008). These fibril-forming collagens ­combine together in a quarter-staggered array to form highly cross-linked fibrils and fibers, as described previously for bone. Polysaccharides are long polymers of sugars. The most important structural polysaccharides found in soft tissues are glycosaminoglycans (GAGs). GAGs are long unbranched chains of repeating disaccharide units. GAGs are negatively charged and as a result are strongly hydrophilic. These hydrophilic macromolecules swell with large amounts of water creating a swelling pressure that can resist compressive forces (Ventre et al., 2009a). GAGs are often found covalently bonded to a protein backbone and in this form are known as proteoglycans (Figure 5.10). Relative to their mass, proteoglycans adopt highly extended conformations that pervade through a huge volume (Alberts et al., 2008). Hence, although they only comprise a small proportion by weight of the ECM, they can occupy most of the extracellular space and have a significant mechanical impact (Alberts et al., 2008). 5.4.1.2 Fibrous Structure Although the exact composition of a soft tissue varies with its type and location, soft ­tissues generally consist of collagen fibrils embedded in a ground substance. The ground substance is primarily made up of proteoglycans and water. However, the structure and the arrangement of collagen fibrils vary immensely depending on the tissue. Tendons, which are ropelike structures that transmit tensile forces between muscles and bone, have collagen fibrils that travel axially throughout the tendon (Benjamin et al., 1995). It is still not known whether fibrils run the full length of the tendon or whether their ends are

194

Biomimetics: Nature-Based Innovation

Glycosaminoglycan – –

n Proteoglycan

Core protein

–

– – –

GAG – – (KS, CS)

– –

H 2O Aggrecan aggregate HA

FIGURE 5.10 Structure of proteoglycans and GAGs. GAGs, such as keratan sulfate (KS) and chondroitin sulfate (CS), consist of long chains of anionic disaccharide units. Multiple GAGs can covalently attach to a protein to form a proteoglycan. The negative charge groups on the disaccharide units repel each other such that the GAG chains form an extended conformation. Some proteoglycans will further assemble with a long chain hyaluronan (HA) molecule (also a type of GAG) to form an aggregate that can be as large as micrometers in length.

a

b AF

NP FIGURE 5.11 AFM micrographs of (a) an intervertebral disc and (b) the dermal layer of skin, which clearly show their fibrous structures. Two distinct regions can be seen in the intervertebral disc micrograph: The annulus fibrosus (AF) containing circumferentially aligned collagen fibrils and the gel-like nucleus pulposus (NP). A “basket weave” of collagen fibers can be observed in the dermis micrograph. Scale bars are 1 µm. (From Graham H.K., N.W. Hodson, J.A. Hoyland, S.J. Millward-Sadler, D. Garrod, A. Scothern, C.E.M. Griffiths, R.E.B. Watson, T.R. Cox, J.T. Erier, A.W. Trafford, and M.J. Sherratt, “Tissue section AFM: In situ ultrastructural imaging of native biomolecules,” Matrix Biology, Vol. 29, No. 4, (2010), pp. 254–260. With permission.)

staggered throughout (Ker, 2007). On the other hand, collagen fibers in the dermal layer of skin are arranged in an entangled mass parallel to the skin surface and are optimized to resist biaxial stresses (Ventre et al., 2009a). Similarly, the collagen fibrils in the tunica media layer of the arterial wall are almost circumferentially oriented and so resist the pulsatile flow through the arteries (Holzapfel, 2008). In several soft tissues, collagen fibers display a characteristic crimped pattern when viewed under polarized light (Kastelic et al., 1978; Shadwick, 1999). These collagen fibrils are not stretched taught but instead are embedded in a wavy form and exhibit a “branching bundle” architecture (Figure 5.11).

195

Biomimetic Composites

5.4.1.3 Hierarchical Structure Significant variations in structure and composition can be observed within a tissue as well as between tissues. For example, arteries can be separated into three separate layers (Figure 5.12). The inner layer, the tunica intima, is composed of a layer of endothelial cells and directly faces the blood stream (Ventre et al., 2009a). The tunica media is the middle layer of the artery and is primarily responsible for the arteries’ mechanical properties under normal loading. The tunica media is composed of several concentric laminae that are each approximately 15 µm thick. Each lamina has a core consisting primarily of elastin and collagen fibers arranged in a helical pattern. These fibers are surrounded by smooth muscle cells embedded in a collagen network (Ventre et al., 2009a). The final layer is called the tunica adventitia and is collagen rich. The collagen fibers in this layer also follow a helical pattern but are more dispersed and wavy than in the tunica media. The tunica adventitia is less stiff than the tunica media in a stress free configuration but at significant levels of strain it changes into a stiff jacket-like tube (Holzapfel, 2008). Other tissues are not stratified but instead show gradual changes in microstructure and composition. Cartilage becomes more mineralized as it attaches toward bone (Gupta et al., 2005). These complex structures lead to complex behaviors that are related to their local mechanical environment. 5.4.2  Properties 5.4.2.1 Nonlinear Elasticity Soft tissues are not linearly elastic; the strain in the tissue does not increase as a constant proportion of the stress. Instead, most soft tissues are compliant at small strains but become stiffer as the strain increases (Figure 5.13) (Wren and Carter, 1998). The stress–strain curve of a soft tissue can be separated into a toe region were the tissue is very compliant, followed by a transition region, and finally a stiff region appearing nearly linear (and for which the tangent is often taken as an effective Young’s modulus). This nonlinear behavior can serve several purposes. In some tendons, the toe region can extend to approximately 4% strain (Screen et al., 2004); the modulus of tendon in the toe

Collagen fibers Tunica adventitia

Tunica media Tunica intima

Smooth muscle cells Endothelial cells

FIGURE 5.12 Schematic showing the layered structure of an artery, where the layers are the intima, the media, and the adventitia. (Adapted from Holzapfel G.A., “Collagen in arterial walls: Biomechanical aspects,” in P. Fratzl (Ed.), Collagen: Structure and Mechanics, Springer, Boston, MA, (2008), pp. 285–324.)

196

Biomimetics: Nature-Based Innovation

1.4 1.2

Tensile force, F (N)

1.0 Linear region

0.8 0.6 0.4

Toe region

0.2 0.0 0

1

2 Extension, e (mm)

3

4

FIGURE 5.13 Force–extension (F–e) plot of human amnion exhibiting characteristic strain stiffening. A transition from the toe region to the linear region can be observed as a result of fiber straightening and recruitment.

region is on the order of megapascals but increases to approximately 1.5 GPa in the linear region. Thus, the stiffness is small during physiological loading but there is increased resistance to deformation when strains are large, as in an injury scenario. The majority of tendons in limbs do not travel straight from muscle to bone but instead wrap around bony or fibrous pulleys (Benjamin et al., 1995). Nonlinear elastic behavior may protect arteries against aneurysms and “blowout”—soft tissues are extremely tough on a per-stiffness basis compared with mineralized tissues. Arteries are highly distensible and elastic and can smooth out pulsatile flow, although this leaves them at risk of an elastic instability whereby a small increment in pressure results in a large jump in radius (Shadwick, 1999). However, by increasing the modulus as a function of radius, this instability can be avoided. Nonlinear elasticity is thought to occur as a result of the following phenomena. In several soft tissues, the collagen fibers are crimped in an unloaded state. Straightening of these fibers occurs when load is applied to the tissue. In addition, fibers that are not oriented directly along the line of the load are realigned and recruited. The transition region of the stress strain curve is a result of the progressive straightening and reorientation of the fibers (Wren and Carter, 1998). Once all of the fibers have been straightened and recruited, the stress increases linearly with strain as further load directly stretches the stiff fibers (Wren and Carter, 1998). This recruitment and straightening behavior has been observed during tensile testing under polarized light (Diamant et al., 1972), and several models derived from this behavior have successfully explained experimental observations (Wren and Carter, 1998; Oyen, 2006). However, some tissues such as hyaline cartilage do not have crimped fibers but still exhibit strain stiffening. Wren and Carter (1998) modeled soft tissues as a two phase composites with semiconstrained fibers and showed that the toe region observed in cartilage could be explained solely through the reorientation of collagen fibers in response to applied load. Although much is known about the underlying mechanisms of this nonlinear elasticity, some questions still remain. Elastic fibers composed primarily of the protein elastin are present in small quantities in several tissues (Cox, 1975; Kjaer, 2004; Ventre et al., 2009b). Elastin

197

Biomimetic Composites

has a modulus on the order of megapascals, and collagen is thought to have a modulus on the order of a gigapascal. It would be expected that if collagen and elastin were in parallel then the tissue response would be dominated by the much stiffer collagen and removing the elastin phase should have very little effect on the total mechanical response (Yuan et al., 2000). However, a substantially changed mechanical response has been observed in artery, skin, and lung tissues that have been degraded in elastase, an enzyme that decomposes elastin (Dobrin and Canfield, 1984; Oxlund et al., 1988; Yuan et al., 2000; Ventre et al., 2009b). It is possible that the elastic fibers may contribute to the recovery of the collagen crimping after the tissue has been stretched (Kannus, 2000). However, more research is needed before the precise mechanisms that give rise to this behavior can be confirmed. 5.4.2.2 Anisotropy Soft tissues have a fibrous structure and as a result are anisotropic and have varying material properties in different orientations. Some tissues, such as tendons and ligaments, have a single preferred direction and can be considered to be transversely isotropic, whereas others, including cartilage, exhibit more complex behavior (Netti et al., 1996). The anisotropy of the tissue is again related to the local mechanical environment. Ligaments tend to have more dispersed collagen fibrils than tendons because they have to carry more off axis loads (Ventre et al., 2009a). Tendons that wrap around fibrous pulleys often have a greater proteoglycan content in the parts of the tendon that are in contact with the pulley to have a greater resistance to compression (Benjamin et al., 2008). The anisotropy of the tissues allows them to maximize their performance in the mechanical environment without being overengineered. 5.4.2.3 Time-Dependent Behavior and Hysteresis Soft tissues exhibit time- and history-dependent behavior because of the complex ­interactions between proteins, proteoglycans, ions, and water (Woo et al., 2005). The loading and unloading responses of tissues exhibit hysteresis and do not follow the same path (Figure 5.14a). Under cyclic loading, both reversible and irreversible rearrangement of the (a)

0.30

(b) 0.50

0.20 0.15 0.10 0.05 0.00 –0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 Extension, e (mm)

Force, F (N)

Force, F (N)

0.25

0.45 0.40 0.35 0.30 0.25

0

50

100 150 Time, t (s)

200

FIGURE 5.14 (a) Force–extension (F–e) plot of human amnion that has been loaded and unloaded. The amnion exhibits ­substantial hysteresis. (b) Force–time (F–t) plot showing the response of the human amnion to a constant 4.5 mm displacement. The stress within the tissue relaxes with time.

198

Biomimetics: Nature-Based Innovation

microstructure can be observed in several tissues including skin (Eshel and Lanir, 2001). This complex material behavior can be the result of molecular rearrangement (viscoelasticity), resistance to the flow of fluid within the tissue (poroelasticity), and changes to ­collagen fibril orientation that do not revert (plasticity). When a constant tensile load is applied to a tissue, it creeps over time and its length increases. When a tissue is held at a constant stretch, the stress within the tissue decreases with time (Figure 5.14b). Viscoelasticity allows the tissue to gradually adapt to the applied load. Rearrangements of the microstructure under cyclic loading reduce the overall peak stress in the tissue and as a result may help prevent fatigue failure (Woo et al., 2005; Zhang et al., 2007). Viscoelastic deformation may also help attenuate traveling pressure pulses in arteries (Shadwick, 1999). The quasilinear viscoelastic (QLV) model proposed by Fung (1981) is commonly used to fit the viscoelastic response of a soft tissue to better understand the underlying phenomena (DeFrate and Li, 2007). In the QLV model, the stress strain response is separated into an elastic function and a time-dependent relaxation function. This allows empirical (DeFrate and Li, 2007) and micromechanical (Oyen, 2006; Bates, 2007) formulations of the elastic response to be used to characterize the overall time-dependent stress strain response. However, some authors have questioned the ability of the QLV model to predict the overall stress response beyond the data it was fit to (DeFrate and Li, 2007) or found that it did not fit experimental data at all (Oyen et al., 2005). Poroelasticity arises from the frictional drag of interstitial fluid flow through the tissue (Mow et al., 2005). When a compressive load is applied to a tissue, the modulus is initially much larger than the eventual equilibrium modulus as the bulk of the load is supported by the fluid phase. As time progresses, the fluid flows out of the tissue until its pressure reaches equilibrium, and the load is redistributed among the solid phase. This time-dependent response may protect soft tissues from sudden shock loads. It has been observed that more than 90% of the compressive load is initially carried by the interstitial fluid in hyaline cartilage, protecting the collagen proteoglycan matrix from excessive stresses (Ateshian and Mow, 2005). Hence, it is no surprise that poroelastic behavior is particularly prevalent in knee cartilage and intervertebral disc cartilage. Moving fluid is also essential for several biological functions such as nutrient and waste product transport, especially in avascular cartilaginous tissues (Ateshian and Mow, 2005). Linear poroelastic (or biphasic) theory is commonly used to examine the compressive response of tissues such as articular cartilage (Mow et al., 2005). However, tissues tend to exhibit anisotropic, nonlinear, strain-dependent permeability, compromising the predicative ability of the theory. The ability to predict the time-dependent response of tissues is further complicated by the presence of proteoglycans and ions that alter the tissues swelling behavior by affecting the osmotic pressure (Lai et al., 1991). Triphasic models consisting of a solid phase, fluid phase, and ionic phase have been used to characterize experimental data although these models also have limitations (Lai et al., 1991; Lu et al., 2009). Although the simplest model to accurately describe experimental behavior is often best, it is probable that nonlinear elasticity, microstructural anisotropy, triphasic poroelasticity, and flow independent viscoelasticity will all need to be accounted for to fully predict experimental observations of soft tissues during compressive and tensile loading (Huang et al., 2001, 2005; DeFrate and Li, 2007; Lu et al., 2009). Time-dependent hysteresis adds to the complexity in trying to characterize the fracture resistance of soft tissues. In a notched elastic sample, hysteresis can be directly attributed to crack propagation, and the fracture resistance can be computed as the ratio of the work expended to the change in crack area. Such an analysis becomes more complicated in hysteretic materials (Oyen and Cook, 2001). The effect of time dependence can be seen as an

199

Biomimetic Composites

apparent increase in fracture resistance as a function of increased loading rate, if the time dependence is not explicitly considered in the analysis (Purslow, 1983). 5.4.2.4 Lubrication There are many interfaces in the body that undergo millions of cycles of loading each year with little or no wear. This is partially due to the small coefficients of friction that can be achieved between tissues. The cartilage in the diarthrodial joint has been found to have a coefficient of friction between 0.003 and 0.06 (Ateshian and Mow, 2005). These coefficients are much smaller than have been observed in typical engineering tribology systems (Figure 5.15). However, there is still considerable debate on the mechanism through which the diarthrodial joint achieves its excellent lubrication (Klein, 2006; Ateshian, 2009; Crockett, 2009). In man-made systems, small coefficients of friction tend to be attained using fluid film lubrication. A viscous fluid layer is pressurized between two load-bearing surfaces. The fluid layer supports the load transmitted across the bearing surfaces and minimizes direct contact. This mechanism is frequently used in automotive applications including ­crankshaft bearings (Ateshian and Mow, 2005). It has been hypothesized that fluid film lubrication may have an effect in the diarthrodial joint (Walker et al., 1970). Synovial fluid is a clear highly viscous fluid that is secreted into the cavity of the diarthrodial joint and is present between the two articular surfaces. However, movement in the diarthrodial joint is often much slower than in automotive applications and is unlikely to be fast enough to maintain a substantial fluid film thickness between the two layers during normal motion (Ateshian and Mow, 2005). In addition, synovial fluid is shear thinning and would become substantially less viscous at greater speeds (Klein, 2006).

Cam

Tappet shim Bucket tappet Valve spring

Valve

Valve guide

µ

Cam follower Boundary lubrication

Mixed lubrication

Piston rings Bearing

Engine bearings Full film lubrication

Journal

Fluid Pressure in fluid

Speed Mammalian joints

FIGURE 5.15 Friction coefficients of mammalian joints compared with the friction coefficients of internal combustion engine tribological systems. (From Neville A., A. Morina, T. Liskiewicz, and Y. Yan, “Synovial joint lubrication—does nature teach more effective engineering lubrication strategies?,” Proceedings of the Institution of Mechanical Engineers. Part C, Journal of Mechanical Engineering Science, Vol. 221, No. 10, (2007), pp. 1223–1230. With permission.)

200

Biomimetics: Nature-Based Innovation

Ateshian (2009) has suggested that interstitial fluid pressurization may be the dominant lubricating mechanism of the articular joint. He asserts that viscous shear stresses in the interstitial fluid are negligible; most of the friction force is likely to be caused by the solid phase of the articular surface rubbing against the solid phase of the opposing surface. However, because the bulk of the load is initially supported by the fluid phase in a poroelastic tissue, there is minimal contact between the solid phases and very small coefficient of friction while the fluid phase remains pressurized. Experimentally, it has been observed that the friction coefficient of cartilage against glass rises over time and that it reaches equilibrium at the same time as the poroelastic deformation (Ateshian and Mow, 2005). However, it remains to be determined whether this mechanism can be maintained for long lengths of time in vivo. Finally, it has been proposed that there may be local boundary lubrication between the two articular surfaces (Crockett, 2009). In this mechanism, many of the asperities between the surfaces are actually in contact, but a boundary layer exists on the surface with small friction. Several proteins, such as the aptly named lubrican, have been identified that may provide boundary lubrication (Swann et al., 1981). Klein (2006) has proposed a mechanism whereby brushes like polymer chains extend from each articular surface and repel each other because of osmotic pressure. As a result, it is more favorable for the brush bearing surfaces to distort into thinner denser layers rather than interpenetrate even under large loads. Although this mechanism has been demonstrated experimentally in several model surfaces, there is no microscopic evidence for brushlike chains dangling from articular surface (Klein, 2006; Chen et al., 2009). However, he argues that these proteins must exist at the articular surface as they have to pass through the boundary region on their way into the synovial fluid from the articular cartilage. 5.4.3  Biomimetic Synthesis Routes Although there has been relatively little research into the biomimicry of soft tissues for nonmedical applications, there have been several successes in the growing field of tissue engineering (Swartz and Griffith, 2006; Chung and Burdick, 2008; Kandel et al., 2008). Many groups have attempted to mimic the composition of soft tissues. Hydrogels are a popular scaffold to replace cartilage and other tissues because of their large water contents and poroelastic behavior. Some hydrogels have even been made using GAGs such as hyaluronan (Leach et al., 2003). However, the majority of hydrogels have substantially degraded mechanical properties when compared with soft tissues. These gels lack the complex network interactions that are present in soft tissues (Tanaka et al., 2005). Nonetheless, there have been several recent successes at producing hydrogels with substantially improved mechanical properties through the use of two interpenetrating polymer networks (Gong et al., 2003; Yasuda et al., 2005; Myung et al., 2007). In addition, hydrogels with friction coefficients as small as 10 –4 have been produced by restricting the gelation of the hydrogel with hydrophobic surfaces such that freely dangling polymer brushes remain on the surface (Gong et al., 2001). There have also been several attempts to mimic the nanofibrous nature of soft tissues (Nerurkar et al., 2009; Agarwal et al., 2009). Nanofibers can be produced through a technique known as electrospinning. A large electrostatic field is applied to a polymer droplet so that it is drawn to a collecting surface and deposited as an amorphous collection of nanofibers. Aligned nanofibers can be produced by using a rotating drum as the collector. A wide variety of nanofibers have been produced using this method with materials ranging from polyethylene oxide to collagen (Greiner and Wendorff, 2007). Electrospun

Biomimetic Composites

201

nanofibers can also have excellent mechanical properties; individual polyacrylonitrile nanofibers have been produced with a Young’s modulus of 26.8 GPa (Yuya et al., 2007). However, like most nanofabrication techniques, it still remains a significant challenge to produce significant thicknesses of material (Teo and Ramarkishna, 2006). To date, there has been significant interest in developing methods to fabricate soft tissuelike materials for medical use. There has also been active research into the phenomena that give rise to the mechanical behavior of soft tissues, although there are many difficulties associated with this as a result of their inherent complexity. However, it remains to be seen whether the mechanisms that give rise to the excellent mechanical performance of soft tissues will find their way into other mainstream engineering applications, in the form of long-lasting, high-performance, wear-resistant materials.

5.5  Summary and Outlook In this chapter, we have considered three classes of natural materials: composite organic– inorganic materials that are mostly mineral, focused on nacre (seashell), but also mentioning eggshell and dental enamel; composite organic–inorganic materials that are roughly half mineral by volume, including bone and dentin in the tooth; and composite organic– organic materials with a significant volume fraction of water and which could be rightly considered as hydrogels, including articular cartilage. These materials exhibit combinations of material properties that are not found in bulk engineering materials or even in typical engineering composites. There are several reasons for this. The underlying composition and structure of natural materials is fundamentally different from what is found in typical engineering materials. The inclusion of water as an integral part of the microstructure is one main difference. Another is the nanometer-sized features of proteins onto which biominerals are templated. A third is the dominance of so-called “weak bonds” holding the structures together: proteins themselves are covalently linked at only the first of four critical structural length scales (Alberts et al., 2008). The presence of proteins, even at relatively small quantities in materials such as nacre, means that there are critically observed mechanical effects that arise because of weak bond rupture. Finally, natural materials tend to be relatively inhomogeneous in structure and composition, which is the precise opposite of engineering processes aimed, for example, at homogeneously distributing reinforcing-phase particles in a composite matrix. To create materials that mimic natural materials, one or more of the above mentioned features can be imitated, using either a materials set that resembles those found in nature or more traditional engineering materials. The key is to be sure that the mechanisms at work in the natural systems are understood, such that the critical aspects can be accurately represented in the novel material. This may sound trivial, but in fact the basic structure– properties relationships of the materials discussed here are only somewhat understood. There is a significant amount of work to be done in studying the natural materials and understanding both their microstructure and all aspects of their mechanical responses. Further to imitation of the materials themselves, there is significant, potentially independent, interest in replicating the natural processes of biomineralization and self-­assembly. Although natural biological materials do have biologically active components (cells) that produce the organic portion of the matrix, the ECM organizes itself into large-scale (macroscopic) structures through a process of self-assembly. Biomineralization, the deposition

202

Biomimetics: Nature-Based Innovation

of mineral on an organic matrix, is itself a highly regulated process, not unlike protein self-assembly, in that the mineral crystallographic axes align with features in the organic matrix. All of this “automatic” materials processing is consistent with the observation that nature uses information where traditional engineering uses energy (Vincent et al., 2006). The formation of materials under ambient conditions, by a very organized process of self-assembly and directed heterogeneous nucleation, represents a processing mechanism that was not traditionally associated with processing of engineering materials or composites. In many cases, such as soft tissues and bone applications, the original motivation in biomimicry was medically oriented. Tissue engineering applications aim to recreate tissues that are diseased or damaged by combining cells with man-made replacement ECMs. The biological cells are present to produce and maintain the ECM within a living being. However, the material itself is the extracellular part, not the living biological cells. In contrast to the properties of the natural materials discussed here, cells are extremely compliant (kPa modulus range). They do not make a dominant mechanical contribution to structural materials. Some aspects of biomimicry have thus focused on the ECM material exclusively, considering cell-free self-assembly and mineralization processes on the basis of molecular recognition independent of biological cells. In this context, a soft tissue may be of interest for an engineering tribology application or a bonelike material for modern architecture. There are thus significant opportunities in expanding the natural materials concepts into unnatural applications within the engineered world. With processing techniques taking advantage of information instead of energy, novel materials produced under ambient conditions may be able to take over from energy-intensive metal and ceramic materials in certain applications. By not limiting the materials set to hydrated proteins, and mimicking natural materials on a more abstract scale, the possibilities grow even greater for a new paradigm in Twenty-First Century materials based on the principles of biomimetics.

Acknowledgments The authors express their appreciation of the valuable comments and suggestions of the reviewers of this chapter. The reviewers were Francois Barthelat, McGill University, Montreal, Quebec, Canada; Po-Yu Chen, University of California, San Diego; Marc A. Meyers, University of California, San Diego; Bert Muller, University Hospital Basel, Basel, Switzerland; and Julian Vincent, University of Bath, Bath, UK.

References Agarwal, S., J.H. Wendorff, and A. Greiner, “Progress in the field of electrospinning for tissue engineering applications,” Advanced Materials, Vol. 21, No. 32–33, (2009), pp. 3343–3351. Ager, J.W., G. Balooch, and R.O. Ritchie, “Fracture, aging, and disease in bone,” Journal of Materials Research, Vol. 21, No. 8, (2006), pp. 1878–1892. Akiva, U., H.D. Wagner, and S. Weiner, “Modelling the three-dimensional elastic constants of parallel-fibred and lamellar bone,” Journal of Materials Science, Vol. 33, (1998), pp. 1497–1509.

Biomimetic Composites

203

Alberts, B., A. Johnson, J. Lewis, M. Raff, K. Roberts, and P. Walter, Molecular Biology of the Cell (5th ed.), Garland Science, New York, NY, (2008), pp. 1–1268, ISBN 9780815341062. Anseth, K.S., and J.A. Burdick, “New directions in photopolymerizable biomaterials,” MRS Bulletin, Vol. 27, No. 2, (2002), pp. 130–136. Ashby, M.F., and D.R.H. Jones, Engineering Materials 1. An Introduction to Properties, Applications and Design, Elsevier Butterworth-Heinemann, Oxford, UK, (2005a), pp. 1–424, ISBN 0750663804. Ashby, M.F., and D. R. H. Jones, Engineering Materials 2. An Introduction to Microstructures, Processing and Design, Elsevier Butterworth-Heinemann, Oxford, UK, (2005b), pp. 1–451, ISBN 0750663812. Ateshian, G.A., “The role of interstitial fluid pressurization in articular cartilage lubrication,” Journal of Biomechanics, Vol. 42, No. 9, (2009), pp. 1163–1176. Ateshian, G.A., and V.C. Mow, “Friction, lubrication, and wear of articular cartilage and diarthrodial joints,” in V.C. Mow and R. Huiskes (Eds.), Basic Orthopaedic Biomechanics and Mechanobiology (3rd ed.), Lippincott Williams and Wilkins, Philadelphia, PA, (2005), pp. 447–494, ISBN 0781739330. Balooch, G., M. Balooch, R.K. Nalla, S. Schilling, E.H. Filvaroff, G.W. Marshall, S.J. Marshall, R.O. Ritchie, R. Derynck, and T. Alliston, “TGF-beta regulates the mechanical properties and composition of bone matrix,” Proceedings of the National Academy of Sciences of the United States of America, Vol. 102, No. 52, (2005), pp. 18813–18818. Balooch, M., S. Habelitz, J.H. Kinney, S.J. Marshall, and G.W. Marshall, “Mechanical properties of mineralized collagen fibrils as influenced by demineralization,” Journal of Structural Biology, Vol. 162, No. 3, (2008), pp. 404–410. Barthelat, F., C.-M. Li, C. Comi, and H.D. Espinosa, “Mechanical properties of nacre constituents and their impact on mechanical performance,” Journal of Materials Research, Vol. 21, No. 8, (2006), pp. 1977–1986. Barthelat, F., H. Tang, P.D. Zavattieri, C.-M. Li, and H.D. Espinosa, “On the mechanics of mother-ofpearl: A key feature in the material hierarchical structure,” Journal of the Mechanics and Physics of Solids, Vol. 55, No. 2, (2007), pp. 306–337. Bates, J.H.T., “A recruitment model of quasi-linear power-law stress adaptation in lung tissue,” Annals of Biomedical Engineering, Vol. 35, No. 7, (2007), pp. 1165–1174. Becker, R.O., and A.A. Marino, Electromagnetism and Life, State University of New York Press, Albany, NY, (1982), pp. 1–211, ISBN 0873955617. Bembey, A.K., A.J. Bushby, A. Boyde, V.L. Ferguson, and M.L. Oyen, “Hydration effects on the micro-mechanical properties of bone,” Journal of Materials Research, Vol. 21, No. 8, (2006), pp. 1962–1968. Benjamin, M., E. Kaiser, and S. Milz, “Structure–function relationships in tendons: A review,” Journal of Anatomy, Vol. 212, No. 3, (2008), pp. 211–228. Benjamin, M., S. Qin, and J.R. Ralphs, “Fibrocartilage associated with human tendons and their pulleys,” Journal of Anatomy, Vol. 187, (1995), pp. 625–633. Bera, T., and P. Ramachandrarao, “A Chicken’s Egg as a Reaction Vessel to Explore Biomineralization,” Journal of Bionic Engineering, Vol. 4, No. 3 (2007), pp. 133–141. Bernhardt, A., A. Lode, S. Boxberger, W. Pompe, and M. Gelinsky, “Mineralised collagen—an artificial, extracellular bone matrix—improves osteogenic differentiation of bone marrow stromal cells,” Journal of Materials Science. Materials in Medicine, Vol. 19, No. 1, (2008), pp. 269–275. Biewener, A.A., “Tendons and ligaments: Structure, mechanical behavior and biological function,” in P. Fratzl (Ed.), Collagen: Structure and Mechanics, Springer, Boston, MA, (2008), pp. 269–284, ISBN 9780387739052. Bonucci, E., “Basic composition and structure of bone,” in Y.H. An and R.A. Draughn (Eds.), Mechanical Testing of Bone and the Bone–Implant Interface, CRC Press, Boca Raton, FL, (2000), pp. 3–22, ISBN 0849302668. Boskey, A.L., “Will biomimetics provide new answers for old problems of calcified tissues?,” Calcified Tissue International, Vol. 63, No. 3, (1998a), pp. 179–182. Boskey, A.L., “Biomineralization: Conflicts, challenges, and opportunities,” Journal of Cellular Biochemistry. Supplement, Vol. 30–31, (1998b), pp. 83–91.

204

Biomimetics: Nature-Based Innovation

Boskey, A.L., “Biomineralization: An overview,” Connective Tissue Research, Vol. 44, Suppl. 1, (2003), pp. 5–9. Bruet, B.J.F., H.J. Qi, M.C. Boyce, R. Panas, K. Tai, L. Frick, and C. Ortiz, “Nanoscale morphology and indentation of individual nacre tablets from the gastropod mollusc Trochus niloticus,” Journal of Materials Research, Vol. 20, No. 9, (2005), pp. 2400–2419. Buehler, M.J., “Atomistic and continuum modeling of mechanical properties of collagen: Elasticity, fracture, and self-assembly,” Journal of Materials Research, Vol. 21, No. 8, (2006), pp. 1947–1961. Buehler, M.J., “Nanomechanics of collagen fibrils under varying cross-link densities: Atomistic and continuum studies,” Journal of the Mechanical Behavior of Biomedical Materials, Vol. 1, No. 1, (2008), pp. 59–67. Chang, M.C., C. Ko, and W.H. Douglas, “Preparation of hydroxyapatite-gelatin nanocomposite,” Biomaterials, Vol. 24, No. 17, (2003), pp. 2853–2862. Chawla, K.K., Composite Materials, Springer-Verlag, New York, NY, (1987), ISBN 0387984097. Chen, M., W.H. Briscoe, S.P. Armes, and J. Klein, “Lubrication at physiological pressures by polyzwitterionic brushes,” Science, Vol. 323, No. 5922, (2009), pp. 1698–1701. Cuy, J.L., A.B. Mann, K.J. Livi, M.F. Teaford, and T.P. Weihs, “Nanoindentation mapping of the mechanical properties of human molar tooth enamel,” Archives of Oral Biology, Vol. 47, No. 4, (2002), pp. 1737–1746. Chung, C. and J.A. Burdick, “Engineering cartilage tissue,” Advanced Drug Delivery Reviews, Vol. 60, No. 2, (2008), pp. 243–262. Clarke, D.R., “Interpenetrating phase composites,” Journal of the American Ceramic Society, Vol. 75, No. 4, (1992), pp. 739–758. Cowin, S.C., “Bone poroelasticity,” Journal of Biomechanics, Vol. 32, No. 3, (1999), pp. 217–238. Cox, R.H., “Arterial wall mechanics and composition and the effects of smooth muscle activation,” American Journal of Physiology, Vol. 229, No. 3, (1975), pp. 807–812. Crockett, R., “Boundary lubrication in natural articular joints,” Tribology Letters, Vol. 35, No. 2, (2009), pp. 77–84. Currey, J.D., “Mechanical properties of mother of pearl in tension,” Proceedings of the Royal Society of London. Series B, Biological Sciences, Vol. 196, (1977), pp. 443–463. Currey, J.D., Bones: Structure and Mechanics, Princeton University Press, Princeton, NJ, (2002), ISBN 0691090963. Currey, J.D., “The many adaptations of bone,” Journal of Biomechanics, Vol. 36, No. 10, (2003), pp. 1487–1495. Currey, J.D., “Materials science. Hierarchies in biomineral structures,” Science, Vol. 309, (2005), pp. 253–254. Currey, J.D., P. Zioupos, P. Davies, and A. Casinos, “Mechanical properties of nacre and highly mineralized bone,” Proceedings of the Royal Society of London. Series B, Biological Sciences, Vol. 268, (2001), pp. 107–111. DeFrate, L.E., and G. Li, “The prediction of stress-relaxation of ligaments and tendons using the quasi-linear viscoelastic mode,” Biomechanics and Modeling in Mechanobiology, Vol. 6, No. 4, (2007), pp. 245–251. Deville, S., E. Saiz, R.K. Nalla, and A.P. Tomsia, “Freezing as a path to build complex composites,” Science, Vol. 311, (2006), pp. 515–518. Diamant, J., A. Keller, E. Baer, M. Litt, and R.G. Arridge, “Collagen: Ultrastructure and its relation to mechanical properties as a function of ageing,” Proceedings of the Royal Society of London Series B, Biological Sciences, Vol. 180, No. 1060, (1972), pp. 293–315. Dobrin, P.B., and T.R. Canfield, “Elastase, collagenase, and the biaxial elastic properties of dog carotid artery,” American Journal of Physiology—Heart and Circulatory Physiology, Vol. 247, No. 1, (1984), pp. H124–H131. Donnelly, E., R.M. Williams, S.A. Downs, M.E. Dickinson, S.P. Baker, and M.C.H. van der Meulen, “Quasistatic and dynamic nanomechanical properties of cancellous bone tissue relate to collagen content and organization,” Journal of Materials Research, Vol. 21, No. 8, (2006), pp. 2106–2117.

Biomimetic Composites

205

Ebenstein, D.M., and L.A. Pruitt, “Nanoindentation of biological materials,” Nano Today, Vol. 1, No. 3, (2006), pp. 26–33. Eisenberg, S.R., and A.J. Grodzinsky, “Swelling of articular cartilage and other connective ­tissues: Electromechanochemical forces,” Journal of Orthopaedic Research, Vol. 3, No. 2, (1985), pp. 148–159. Elliot, J.C., “Calcium phosphate biominerals,” in M.J. Kohn, J. Rakovan, and J.M. Huges (Eds.), Phosphates: Geochemical, Geobiological, and Materials Importance, Mineralogical Society of America, Washington, DC, (2002), pp. 427–453, ISBN 093995060X. Entwistle, K.M., H. Silyn-Roberts, and S. Ochieng Abuodha, “The relative fracture strengths of the inner and outer surfaces of the eggshell of the domestic fowl,” Proceedings of the Royal Society of London Series B, Biological Sciences, Vol. 262, (1995), pp. 169–174. Eppell, S.J., B.N. Smith, H. Kahn, and R. Ballarini, “Nano measurements with micro-devices: Mechanical properties of hydrated collagen fibrils,” Journal of the Royal Society Interface, Vol. 3, No. 6, (2006), pp. 117–121. Eppell, S.J., W. Tong, J. Lawrence Katz, L. Kuhn, and M.J. Glimcher, “Shape and size of isolated bone mineralites measured using atomic force microscopy,” Journal of Orthopaedic Research, Vol. 19, No. 6, (2001), pp. 1027–1034. Eshel, H., and Y. Lanir, “Effects of strain level and proteoglycan depletion on preconditioning and viscoelastic responses of rat dorsal skin,” Annals of Biomedical Engineering, Vol. 29, No. 2, (2001), pp. 164–172. Espinosa, H.D., J.E. Rim, F. Barthelat, and M.J. Buehler, “Merger of structure and material in nacre and bone—perspectives on de novo biomimetic materials,” Progress in Materials Science, Vol. 54, No. 8, (2009), pp. 1059–1100. Evans, A.G., Z. Suo, R.Z. Wang, I.A. Aksay, M.Y. He, and J.W. Hutchinson, “Model for the robust mechanical behavior of nacre,” Journal of Materials Research, Vol. 16, No. 9, (2001), pp. 2476–2484. Fantner, G.E., T. Hassenkam, J.H. Kindt, J.C. Weaver, H. Birkedal, L. Pechenik, J.A. Cutroni, et al., “Sacrificial bonds and hidden length dissipate energy as mineralized fibrils separate during bone fracture,” Nature Materials, Vol. 4, No. 8, (2005), pp. 612–616. Fernandez, M.S., M. Araya, and J.L., Arias, “Eggshells are shaped by a precise spatio-temporal arrangement of sequentially deposited macromolecules,” Matrix Biology, Vol. 16, No. 1 (1997) pp. 13–20. Fernandez, M.S., K. Passalacqua, J.I. Arias, and J.L. Arias, “Partial biomimetic reconstitution of avian eggshell formation,” Journal of Structural Biology, Vol. 148, No. 1 (2004) pp. 1–10. Fratzl, P., “Collagen: Structure and mechanics, an introduction,” in P. Fratzl (Ed.), Collagen: Structure and Mechanics, Springer, Boston, MA, (2008), pp. 1–13, ISBN 9780387739052. Fratzl, P., and R. Weinkamer, “Nature’s hierarchical materials,” Progress in Materials Science, Vol. 52, No. 8, (2007), pp. 1263–1334. Fritsch, A., and C. Hellmich, “‘Universal’ microstructural patterns in cortical and trabecular, extracellular and extravascular bone materials: Micromechanics-based prediction of anisotropic ­elasticity,” Journal of Theoretical Biology, Vol. 244, No. 4, (2007), pp. 597–620. Fung, Y.C., Biomechanics: Mechanical Properties of Living Tissues, Springer-Verlag, New York, NY, (1981), ISBN 0387904727. Fung, Y.C., Biomechanics: Mechanical Properties of Living Tissues (2nd ed.), Springer-Verlag, New York, NY, (1993), ISBN 0387979476. Gibson, L.J., and M.F. Ashby, Cellular Solids: Structure and Properties (2nd ed.), Cambridge University Press, Cambridge, UK, (1997), ISBN 0521495601. Gong, J.K., J.S. Arnold, and S.H. Cohn, “Composition of trabecular and cortical bone,” Anatomical Record, Vol. 149, No. 3, (1964), pp. 325–331. Gong, J.P., T. Kurokawa, T. Narita, G. Kagata, Y. Osada, G. Nishimura, and M. Kinjo, “Synthesis of hydrogels with extremely low surface friction,” Journal of the American Chemical Society, Vol. 123, No. 23, (2001), pp. 5582–5583.

206

Biomimetics: Nature-Based Innovation

Gong, J.P., Y. Katsuyama, T. Kurokawa, and Y. Osada, “Double-network hydrogels with extremely high mechanical strength,” Advanced Materials, Vol. 15, No. 14, (2003), pp. 1155–1158. Gosline, J., M. Lillie, E. Carrington, P. Guerette, C. Ortlepp, and K. Savage, “Elastic proteins: Biological roles and mechanical properties,” Philosophical Transactions of the Royal Society of London Series B, Biological Sciences, Vol. 357, (2002), pp. 121–132. Graham, H.K., N.W. Hodson, J.A. Hoyland, S.J. Millward-Sadler, D. Garrod, A. Scothern, C.E.M. Griffiths, et al., “Tissue section AFM: In situ ultrastructural imaging of native biomolecules,” Matrix Biology, Vol. 29, No. 4, (2010), pp. 254–260. Green, D., D. Walsh, S. Mann, and R.O.C. Oreffo, “The potential of biomimesis in bone tissue engineering: Lessons from the design and synthesis of invertebrate skeletons,” Bone, Vol. 30, No. 6, (2002), pp. 810–815. Greiner, A., and J.H. Wendorff, “Electrospinning: A fascinating method for the preparation of ultrathin fibers,” Angewandte Chemie (International ed. in English), Vol. 46, No. 30, (2007), pp. 5670–5703. Gupta, H.S., P. Messmer, P. Roschger, S. Bernstorff, K. Klaushofer, and P. Fratzl, “Synchrotron diffraction study of deformation mechanisms in mineralized tendon,” Physical Review Letters, Vol. 93, No. 15, (2004), p. 158101. Gupta, H.S., S. Schratter, W. Tesch, P. Roschger, A. Berzlanovich, T. Schoeberl, K. Klaushofer, and P. Fratzl, “Two different correlations between nanoindentation modulus and mineral content in the bone-cartilage interface,” Journal of Structural Biology, Vol. 149, No. 2, (2005), pp. 138–148. Habelitz, S., S.J. Marshall, G.W. Marshall Jr., and M. Balooch, “Mechanical properties of human dental enamel on the nanometre scale,” Archives of Oral Biology, Vol. 46, No. 2, (2001), pp. 173–183. Hansen, C.J., W. Wu, K.S. Toohey, N.R. Sottos, S.R. White, and J.A. Lewis, “Self-healing materials with interpenetrating microvascular networks,” Advanced Materials, Vol. 21, No. 41, (2009), pp. 4143–4147. Harley, B.A., J.H. Leung, E.C.C.M. Silva, and L.J. Gibson, “Mechanical characterization of collagenglycosaminoglycan scaffolds,” Acta Biomaterialia, Vol. 3, No. 4, (2007), pp. 463–474. Hartgerink, J.D., E. Beniash, and S.I. Stupp, “Self-assembly and mineralization of peptide-amphiphile nanofibers,” Science, Vol. 294, (2001), pp. 1684–1688. Hassenkam, T., G.E. Fantner, J.A. Cutroni, J.C. Weaver, D.E. Morse, and P.K. Hansma, “Highresolution AFM imaging of intact and fractured trabecular bone,” Bone, Vol. 35, No. 1, (2004), pp. 4–10. Hayes, W.C., “Biomechanics of cortical and trabecular bone: implications for the assessment of fracture risk,” in V.C. Mow and W.C. Hayes (Eds.), Basic Orthopaedic Biomechanics, Raven Press, New York, (1991), pp. 93–142, ISBN 0881677965. Hellmich, C., and F.-J. Ulm, “Are mineralized tissues open crystal foams reinforced by crosslinked collagen?—some energy arguments,” Journal of Biomechanics, Vol. 35, No. 9, (2002), pp. 1199–1212. Hellmich, C., F.-J. Ulm, and L. Dormieux, “Can the diverse elastic properties of trabecular and cortical bone be attributed to only a few tissue-independent phase properties and their interactions? Arguments from a multiscale approach,” Biomechanics and Modeling in Mechanobiology, Vol. 2, No. 4, (2004), pp. 219–238. Hengsberger, S., J. Enstroem, F. Peyrin, and P. Zysset, “How is the indentation modulus of bone tissue related to its macroscopic elastic response? A validation study,” Journal of Biomechanics, Vol. 36, No. 10, (2003), pp. 1503–1509. Hollister, S.J., “Porous scaffold design for tissue engineering,” Nature Materials, Vol. 4, No. 7, (2005), pp. 518–524. Holzapfel, G.A., “Collagen in arterial walls: Biomechanical aspects,” in P. Fratzl (Ed.), Collagen: Structure and Mechanics, Springer, Boston, MA, (2008), pp. 285–324, ISBN 9780387739052. Huang, C.-Y., V.C. Mow, and G.A. Ateshian, “The role of flow-independent viscoelasticity in the biphasic tensile and compressive responses of articular cartilage,” Journal of Biomechanical Engineering, Vol. 123, No. 5, (2001), pp. 410–417.

Biomimetic Composites

207

Huang, C.-Y., A. Stankiewicz, G.A. Ateshian, and V.C. Mow, “Anisotropy, inhomogeneity, and tension-compression nonlinearity of human glenohumeral cartilage in finite deformation,” Journal of Biomechanics, Vol. 38, No. 4, (2005), pp. 799–809. Huiskes, R., and B. van Rietbergen, “Biomechanics of bone,” in V.C. Mow and R. Huiskes (Eds.), Basic Orthopaedic Biomechanics and Mechanobiology (3rd ed.), Lippincott Williams and Wilkins, Philadelphia, PA, (2005), pp. 123–180, ISBN 0781739330. Hunter, G.K., “Interfacial aspects of biomineralization,” Current Opinion in Solid State and Materials Science, Vol. 1, (1996), pp. 430–435. Jaasma, M.J., H.H. Bayraktar, G.L. Niebur, and T.M. Keaveny, “Biomechanical effects of intraspecimen variations in tissue modulus for trabecular bone,” Journal of Biomechanics, Vol. 35, No. 2, (2002), pp. 237–246. Jackson, A.P., J.F.V. Vincent, and R.M. Turner, “The mechanical design of nacre,” Proceedings of the Royal Society of London Series B, Biological Sciences, Vol. 234, (1988), pp. 415–440. Jackson, A.P., J.F.V. Vincent, and R.M. Turner, “Comparison of nacre with other ceramic composites,” Journal of Materials Science, Vol. 25, No. 7, (1990), pp. 3173–3178. Jaeger, C., N.S. Groom, E.A. Bowe, A. Horner, M.E. Davies, R.C. Murray, and M.J. Duer, “Investigation of the nature of the protein–mineral interface in bone by solid-state NMR,” Chemistry of Materials, Vol. 17, No. 12, (2005), pp. 3059–3061. Jäger, I., and P. Fratzl, “Mineralized collagen fibrils: A mechanical model with a staggered arrangement of mineral particles,” Biophysical Journal, Vol. 79, No. 4, (2000), pp. 1737–1746. Jee, S.-S., T.T. Thula, and L.B. Gower, “Development of bone-like composites via the polymer-induced liquite-precursor (PILP) process: Part 1. Influence of polymer molecular weight,” Acta Biomaterialia, Vol. 6, (2010), pp. 3676–3686. Kamat, S., X. Su, R. Ballarini, and A.H. Heuer, “Structural basis for the fracture toughness of the shell of the conch Strombus gigas,” Nature, Vol. 405, (2000), pp. 1036–1040. Kandel, R., S. Roberts, and J.P.G. Urban, “Tissue engineering and the intervertebral disc: The challenges,” European Spine Journal, Vol. 17, Suppl 4, (2008), pp. 480–491. Kannus, P., “Structure of the tendon connective tissue,” Scandinavian Journal of Medicine and Science in Sports, Vol. 10, No. 6, (2000), pp. 312–320. Kaplan, F.S., W.C. Hayes, T.M. Keaveny, A. Boskey, T.A. Einhorn, and J.P. Iannotti, in S.R. Simon (Ed.), Orthopaedic Basic Science, American Academy of Orthopaedic Surgeons, Rosemont, IL, (June 1994), p. 127, ISBN 0892030593. Karlsson, O., and C. Lilja, “Eggshell structure, mode of development and growth rate in birds,” Zoology, Vol. 111, (2008), pp. 494–502. Karp, J.M., P.D. Dalton, and M.S. Shoichet, “Scaffolds for tissue engineering,” MRS Bulletin, Vol. 28, No. 4, (2003), pp. 301–306. Kastelic, J., A. Galeski, and E. Baer, “The multicomposite structure of tendon,” Connective Tissue Research, Vol. 6, No. 1, (1978), pp. 11–23. Katz, E.P., and S.T. Li, “The intermolecular space of reconstituted collagen fibrils,” Journal of Molecular Biology, Vol. 73, No. 3, (1973), pp. 351–369. Katz, J.L., “Hard tissue as a composite material: I. Bounds on the elastic behavior,” Journal of Biomechanics, Vol. 4, No. 5, (1971), pp. 455–473. Katz, J.L., and K. Ukraincik, “On the anisotropic elastic properties of hydroxyapatite,” Journal of Biomechanics, Vol. 4, No. 3, (1971), pp. 221–227. Ker, R.F., “Mechanics of tendon, from an engineering perspective,” International Journal of Fatigue, Vol. 29, No. 6, (2007), pp. 1001–1009. Kjaer, M., “Role of extracellular matrix in adaptation of tendon and skeletal muscle to mechanical loading,” Physiological Reviews, Vol. 84, No. 2, (2004), pp. 649–698. Klein, J., “Molecular mechanisms of synovial joint lubrication,” Proceedings of the Institution of Mechanical Engineers. Part J, Journal of Engineering Tribology, Vol. 220, No. 8, (2006), pp. 691–710. Knothe-Tate, M.L., “Interstitial fluid flow,” in S.C. Cowin (Ed.), Bone Mechanics Handbook, CRC Press, New York, NY, (2001), pp. 22.1–22.29.

208

Biomimetics: Nature-Based Innovation

Ko, C.-C., M.L. Oyen, A.M. Fallgatter, J.-H. Kim, J. Fricton, and W.-S. Hu, “Mechanical properties and cytocompatibility of biomimetic hydroxyapatite-gelatin nanocomposites,” Journal of Materials Research, Vol. 21, No. 12, (2006), pp. 3090–3098. Kotha, S.P., and N. Guzelsu, “Modeling the tensile mechanical behavior of bone along the longitudinal direction,” Journal of Theoretical Biology, Vol. 219, No. 2, (2002), pp. 269–279. Lai, W.M., J.S. Hou, and V.C. Mow, “A triphasic theory for the swelling and deformation behaviors of articular cartilage,” Journal of Biomechanical Engineering, Vol. 113, No. 3, (1991), pp. 245–259. Lakes, R., “Materials with structural hierarchy,” Nature, Vol. 361, (1993), pp. 511–515. Landi, E., A. Tampieri, G. Celotti, R. Langenati, M. Sandri, and S. Sprio, “Nucleation of biomimetic apatite in synthetic body fluids: Dense and porous scaffold development,” Biomaterials, Vol. 26, No. 16, (2005), pp. 2835–2845. Landis, W.J., K.J. Hodgens, M.J. Song, J. Arena, S. Kiyonaga, M. Marko, C. Owen, and B.F. McEwen, “Mineralization of collagen may occur on fibril surfaces: Evidence from conventional and highvoltage electron microscopy and three-dimensional imaging,” Journal of Structural Biology, Vol. 117, No. 1, (1996), pp. 24–35. Launey, M.E., E. Munch, D.H. Alsem, H.B. Barth, E. Saiz, A.P. Tomsia, and R.O. Ritchie, “Designing highly toughened hybrid composites through nature-inspired hierarchical complexity,” Acta Materialia, Vol. 57, No. 10, (2009), pp. 2919–2932. Lavelin, I., N. Meiri, and M. Pines, “New insight in eggshell formation,” Poultry Science, Vol. 79, No. 7, (2000), pp. 1014–1017. Leach, J.B., K.A. Bivens, C.W. Patrick Jr., and C.E. Schmidt, “Photocrosslinked hyaluronic acid hydrogels: Natural, biodegradable tissue engineering scaffold,” Biotechnology and Bioengineering, Vol. 82, No. 5, (2003), pp. 578–589. Li, C., C. Vepari, H.-J. Jin, H.J. Kim, and D.L. Kaplan, “Electrospun silk-BMP-2 scaffolds for bone tissue engineering,” Biomaterials, Vol. 27, No. 16, (2006), pp. 3115–3124. Liu, X., and P.X. Ma, “Polymeric scaffolds for bone tissue engineering,” Annals of Biomedical Engineering, Vol. 32, No. 3, (2004), pp. 477–486. Lu, X.L., L.Q. Wan, X.E. Guo, and V.C. Mow, “A linearized formulation of triphasic mixture theory for articular cartilage, and its application to indentation analysis,” Journal of Biomechanics, Vol. 43, No. 4, (2010), pp. 673–679. Manjubala, I., S. Scheler, J. Bössert, and K.D. Jandt, “Mineralisation of chitosan scaffolds with nano-­apatite formation by double diffusion technique,” Acta Biomaterialia, Vol. 2, No. 1, (2006), pp. 75–84. Mann, S., and S. Weiner, “Biomineralization: Structural questions at all length scales: Editorial,” Journal of Structural Biology, Vol. 126, No. 3, (1999), pp. 179–181. Mayer, G., “Rigid biological systems as models for synthetic composites,” Science, Vol. 310, (2005), pp. 1144–1147. Mayer, G., “New classes of tough composite materials—Lessons from natural rigid biological systems,” Materials Science and Engineering C, Vol. 26, No. 8, (2006), pp. 1261–1268. McKee, M.D., A. Nanci, W.J. Landis, Y. Gotoh, L.C. Gerstenfeld, and M.J. Glimcher, “Effects of fixation and demineralization on the retention of bone phosphoprotein and other matrix components as evaluated by biochemical analyses and quantitative immunocytochemistry,” Journal of Bone and Mineral Research, Vol. 6, No. 9, (1991), pp. 937–945. Meyer, U., U. Joos, and H.P. Wiesmann, “Biological and biophysical principles in extracorporal bone tissue engineering. Part I,” International Journal of Oral and Maxillofacial Surgery, Vol. 33, No. 4, (2004), pp. 325–332. Meyers, M.A., A.Y.-M. Lin, P.-Y. Chen, and J. Muyco, “Mechanical strength of abalone nacre: Role of the soft organic layer,” Journal of the Mechanical Behavior of Biomedical Materials, Vol. 1, No. 1, (2008a), pp. 76–85. Meyers, M.A., P.-Y. Chen, A.Y.-M. Lin, and Y. Seki, “Biological materials: Structure and mechanical properties,” Progress in Materials Science, Vol. 53, No. 1, (2008b), pp. 1–206.

Biomimetic Composites

209

Mow, V.C., W.Y. Gu, and F.H. Chen, “Structure and function of articular cartilage and meniscus,” in V.C. Mow and R. Huiskes (Eds.), Basic Orthopaedic Biomechanics and Mechanobiology (3rd ed.), Lippincott Williams and Wilkins, Philadelphia, PA, (2005), pp. 181–258, ISBN 0781739330. Munch, E., M.E. Launey, D.H. Alsem, E. Saiz, A.P. Tomsia, and R.O. Ritchie, “Tough, bio-inspired hybrid materials,” Science, Vol. 322, (2008), pp. 1516–1520. Myung, D., W. Koh, J. Ko, Y. Hu, M. Carrasco, J. Noolandi, C.N. Ta, and C.W. Frank, “Biomimetic strain hardening in interpenetrating polymer network hydrogels,” Polymer, Vol. 48, No. 18, (2007), pp. 5376–5387. Nerurkar, N.L., B.M. Baker, S. Sen, E.E. Wible, D.M. Elliott, and R.L. Mauck, “Nanofibrous biologic laminates replicate the form and function of the Annulus fibrosus,” Nature Materials, Vol. 8, No. 12, (2009), pp. 986–692. Netti, P., A. D’Amore, D. Ronca, L. Ambrosio, and L. Nicolais, “Structure–mechanical properties relationships of natural tendons and ligaments,” Journal of Materials Science: Materials in Medicine, Vol. 7, (1996), pp. 525–530. Neuman, W.F., T.Y. Toribara, and B.J. Mulryan, “The surface chemistry of Bone: IX. Carbonate: Phosphate exchange,” Journal of the American Chemical Society, Vol. 78, No. 17, (1956), pp. 4263–4266. Neville, A., A. Morina, T. Liskiewicz, and Y. Yan, “Synovial joint lubrication—Does nature teach more effective engineering lubrication strategies?,” Proceedings of the Institution of Mechanical Engineers. Part C, Journal of Mechanical Engineering Science, Vol. 221, No. 10, (2007), pp. 1223–1230. Noris-Suárez, K., J. Lira-Olivares, A.M. Ferreira, J.L. Feijoo, N. Suárez, M.C. Hernández, and E. Barrios, “In vitro deposition of hydroxyapatite on cortical bone collagen stimulated by deformation-induced piezoelectricity,” Biomacromolecules, Vol. 8, No. 3, (2007), pp. 941–948. Nys, Y., J. Gautron, J.M. Garcia-Ruiz, and M.T. Hincke, “Avian eggshell mineralization: Biochemical and functional characterization of matrix proteins,” Comptes Rendus Palevol, Vol. 3, (2004), pp. 549–562. Orban, J.M., K.G. Marra, and J.O. Hollinger, “Composition options for tissue-engineered bone,” Tissue Engineering, Vol. 8, No. 4, (2002), pp. 529–539. Oxlund, H., J. Manschot, and A. Viidik, “The role of elastin in the mechanical properties of skin,” Journal of Biomechanics, Vol. 21, No. 3, (1988), pp. 213–218. Oyen, M., and R.F. Cook, “Technique for estimating the fracture resistance of cultured neocartilage,” Journal of Materials Science: Materials in Medicine, Vol. 12, (2001), pp. 327–332. Oyen, M.L., “A model for nonlinear viscoelastic mechanical responses of collagenous soft tissues,” in A.J. Bushby, V.L. Ferguson, C. Ko, and M.L. Oyen (Eds.), Mechanical Behavior of Biologic and Biomimetic Materials (MRS Symposium Proceedings 898E), Warrendale, PA, (2006), Paper No. 0898-L05-16. Oyen, M.L., “The materials science of bone: Lessons from nature for biomimetic materials synthesis,” MRS Bulletin, Vol. 33, No. 1, (2008), pp. 49–55. Oyen, M.L., and C.C. Ko, “Examination of local variations in viscous, elastic, and plastic indentation responses in healing bone,” Journal of Materials Science: Materials in Medicine, Vol. 18, No. 4, (2007), pp. 623–628. Oyen, M.L., V.L. Ferguson, A.K. Bembey, A.J. Bushby, and A. Boyde, “Composite bounds on the elastic modulus of bone,” Journal of Biomechanics, Vol. 41, No. 11, (2008), pp. 2585–2588. Oyen, M.L., R.F. Cook, T. Stylianopoulos, V.H. Barocas, S.E. Calvin, and D.V. Landers, “Uniaxial and biaxial mechanical behavior of human amnion,” Journal of Materials Research, Vol. 20, No. 11, (2005), pp. 2902–2909. Petruska, J.A., and A.J. Hodge, “A subunit model for the tropocollagen macromolecule,” Proceedings of the National Academy of Sciences of the United States of America, Vol. 51, No. 5, (1964), pp. 871–876. Pidaparti, R.M., A. Chandran, Y. Takano, and C.H. Turner, “Bone mineral lies mainly outside collagen fibrils: Predictions of a composite model for osteonal bone,” Journal of Biomechanics, Vol. 29, No. 7, (1996), pp. 909–916.

210

Biomimetics: Nature-Based Innovation

Podsiadlo, P., A.K. Kaushik, E.M. Arruda, A.M. Waas, B.S. Shim, J. Xu, H. Nandivada, et al., “Ultrastrong and stiff layered polymer nanocomposites,” Science, Vol. 318, (2007), pp. 80–83. Purslow, P.P., “Measurement of the fracture toughness of extensible connective tissues” Journal of Materials Science, Vol. 18, (1983), pp. 3591–3598. Qin, Q.-H., and M.V. Swain, “A micro-mechanics model of dentin mechanical properties,” Biomaterials, Vol. 25, No. 20, (2004), pp. 5081–5090. Ragauskas, A.J., C.K. Williams, B.H. Davison, G. Britovsek, J. Cairney, C.A. Eckert, W.J. Frederick, et al., “The path forward for biofuels and biomaterials,” Science, Vol. 311, No. 5760, (2006), pp. 484–489. Rho, J.-Y., M.E. Roy II, T.Y. Tsui, and G.M. Pharr, “Elastic properties of microstructural components of human bone tissue as measured by nanoindentation,” Journal of Biomedical Materials Research, Vol. 45, No. 1, (1999), pp. 48–54. Richard, S., B. Querleux, J. Bittoun, O. Jolivet, I. Idy-Peretti, O. de Lacharriere, and J.-L. Leveque, “Characterization of the skin in vivo by high resolution magnetic resonance imaging: Water behavior and age-related effects,” Journal of Investigative Dermatology, Vol. 100, No. 5, (1993), pp. 705–709. Rodriguez-Navarro, A., O. Kalin, Y. Nys, and J.M. Garcia-Ruiz, “Influence of the microstructure on the shell strength of eggs laid by hens of different ages,” British Poultry Science, Vol. 43, No. 3, (2002), pp. 395–403. Rosen V.B., L.W. Hobbs, and M. Spector, “The ultrastructure of anorganic bovine bone and selected synthetic hyroxyapatites used as bone graft substitute materials,” Biomaterials, Vol. 23, No. 3, (2002), pp. 921–928. Sasaki, N., and A. Enyo, “Viscoelastic properties of bone as a function of water content,” Journal of Biomechanics, Vol. 28, No. 7, (1995), pp. 809–815. Sasaki, N., A. Tagami, T. Goto, M. Taniguchi, M. Nakata, and K. Hikichi, “Atomic force microscopic studies on the structure of bovine femoral cortical bone at the collagen fibril-mineral level,” Journal of Materials Science: Materials in Medicine, Vol. 13, No. 3, (2002), pp. 333–337. Screen, H.R., D.A. Lee, D.L. Bader, and J.C. Shelton, “An investigation into the effects of the hierarchical structure of tendon fascicles on micromechanical properties,” Proceedings of the Institution of Mechanical Engineers. Part H, Journal of Engineering in Medicine, Vol. 218, No. 2, (2004), pp. 109–119. Shadwick, R.E., “Mechanical design in arteries,” Journal of Experimental Biology, Vol. 202, No. 23, (1999), pp. 3305–3313. Sharma, B., and J.H. Elisseeff, “Engineering structurally organized cartilage and bone tissues,” Annals of Biomedical Engineering, Vol. 32, No. 1, (2004), pp. 148–159. Shen, Z.L., M.R. Dodge, H. Kahn, R. Ballarini, and S.J. Eppell, “Stress–strain experiments on individual collagen fibrils,” Biophysical Journal, Vol. 95, No. 8, (2008), pp. 3956–3963. Shin, H., S. Jo, and A.G. Mikos, “Biomimetic materials for tissue engineering,” Biomaterials, Vol. 24, No. 24, (2003), pp. 4353–4364. Skinner, H.C., and A.H. Jahren, “Biomineralization,” Treatise on Geochemistry, Vol. 8, (2003), pp. 117–184. Smith, B.L., T.E. Schäffer, M. Viani, J.B. Thompson, N.A. Frederick, J. Kindt, A. Belcher, G.D. Stucky, D.E. Morse, and P.K. Hansma, “Molecular mechanistic origin of the toughness of natural adhesives, fibres and composites,” Nature, Vol. 399, (1999), pp. 761–763. Stempfle, Ph., O. Pantale, M. Rousseau, E. Lopez, and X. Bourrat, “Mechanical properties of the elemental nanocomponents of nacre structure,” Materials Science and Engineering C, Vol. 30, (2010), pp. 715–721. Subburaman, K., N. Pernodet, S.Y. Kwak, E. DiMasi, S. Ge, V. Zaitsev, X. Ba, N.L. Yang, and M. Rafailovich, “Templated biomineralization on self-assembled protein fibers,” Proceedings of the National Academy of Sciences of the United States of America, Vol. 103, No. 40, (2006), pp. 14672–14677. Swann, D.A., R.B. Hendren, E.L. Radin, and S.L. Sotman, “The lubricating activity of synovial fluid glycoproteins,” Arthritis and Rheumatism, Vol. 24, No. 1, (1981), pp. 22–30.

Biomimetic Composites

211

Swartz, M.A., and L.G. Griffith, “Capturing complex 3D tissue physiology in vitro,” Nature Reviews. Molecular Cell Biology, Vol. 7, No. 3, (2006), pp. 211–224. Tai, K., M. Dao, S. Suresh, A. Palazoglu, and C. Ortiz, “Nanoscale heterogeneity promotes energy dissipation in bone,” Nature Materials, Vol. 6, No. 6, (2007), pp. 454–462. Tampieri, A., G. Celotti, and E. Landi, “From biomimetic apatites to biologically inspired composites,” Analytical and Bioanalytical Chemistry, Vol. 381, No. 3, (2005a), pp. 568–576. Tampieri, A., M. Sandri, E. Landi, G. Celotti, N. Roveri, M. Mattioli-Belmonte, L. Virgili, F. Gabbanelli, and G. Biagini, “HA/Alginate hybrid composites prepared through bio-inspired nucleation,” Acta Biomaterialia, Vol. 1, No. 3, (2005b), pp. 343–351. Tanaka, Y., J. Gong, and Y. Osada, “Novel hydrogels with excellent mechanical performance,” Progress in Polymer Science, Vol. 30, No. 1, (2005), pp. 1–9. Tang, Z., N.A. Kotov, S. Magonov, and B. Ozturk, “Nanostructured artificial nacre,” Nature Materials, Vol. 2., No. 6, (2003), pp. 413–418. Taylor, D., J.G. Hazenberg, and T.C. Lee, “Living with cracks: Damage and repair in human bone,” Nature Materials, Vol. 6, No. 4, (2007), pp. 263–268. Teo, W.E., and S. Ramakrishna, “A review on electrospinning design and nanofibre assemblies,” Nanotechnology, Vol. 17, No. 14, (2006), pp. R89–R106. Toohey, K.S., N.R. Sottos, J.A. Lewis, J.S. Moore, and S.R. White, “Self-healing materials with microvascular networks,” Nature Materials, Vol. 6, No. 8, (2007), pp. 581–585. Turner, C.H., J. Rho, Y. Takano, T.Y. Tsui, and G.M. Pharr, “The elastic properties of trabecular and cortical bone tissues are similar: Results from two microscopic measurement techniques,” Journal of Biomechanics, Vol. 32, No. 4, (1999), pp. 437–441. Van Der Rijt, J.A.J., K.O. Van Der Werf, M.L. Bennink, P.J. Dijkstra, and J. Feijen, “Micromechanical testing of individual collagen fibrils,” Macromolecular Bioscience, Vol. 6, No. 9, (2006), pp. 699–702. Vater, C.A., E.D. Harris Jr., and R.C. Siegel, “Native cross-links in collagen fibrils induce resistance to human synovial collagenase,” Biochemical Journal, Vol. 181, No. 3, (1979), pp. 639–645. Ventre, M., F. Mollica, and P.A. Netti, “The effect of composition and microstructure on the viscoelastic properties of dermis,” Journal of Biomechanics, Vol. 42, No. 4, (2009), pp. 430–435. Ventre, M., P.A. Netti, F. Urciuolo, and L. Ambrosio, “Soft tissues characteristics and strategies for their replacement and regeneration,” in M. Santin (Ed.), Strategies in Regenerative Medicine, Springer, New York, NY, (2009), pp. 15–54, ISBN 9780387746593. Vesentini, S., C.F.C. Fitié, F.M. Montevecchi, and A. Redaelli, “Molecular assessment of the elastic properties of collagen-like homotrimer sequences,” Biomechanics and Modeling in Mechanobiology, Vol. 3, No. 4, (2005), pp. 224–234. Vincent, J.F.V., Structural Biomaterials (Revised ed.), Princeton University Press, Princeton, NJ, (1990), ISBN 0691085587. Vincent, J.F.V., O.A. Bogatyreva, N.R. Bogatyrev, A. Bowyer, and A.-K. Pahl, “Biomimetics: Its practice and theory,” Journal of the Royal Society Interface, Vol. 3, No. 9, (2006), pp. 471–482. Wagner, H.D., and S. Weiner, “On the relationship between the microstructure of bone and its mechanical stiffness,” Journal of Biomechanics, Vol. 25, No. 11, (1992), pp. 1311–1320. Walker, P.S., J. Sikorski, D. Dowson, M.D. Longfield, and V. Wright, “Features of the synovial fluid film in human joint lubrication,” Nature, Vol. 225, (1970), pp. 956–957. Wang, R.Z., Z. Suo, A.G. Evans, N. Yao, and I.A. Aksay, “Deformation mechanisms in nacre,” Journal of Materials Research, Vol. 16, No. 9, (2001), pp. 2486–2493. Wegst, U.G.K., and M.F. Ashby, “The mechanical efficiency of natural materials,” Philosophical Magazine, Vol. 84, No. 21, (2004), pp. 2167–2186. Weiner, S., and H.D. Wagner, “The material bone: Structure–mechanical function relations,” Annual Review of Materials Science, Vol. 28, No. 1, (1998), pp. 271–298. Weiner, S., and W. Traub, “Bone structure: From angstroms to microns,” FASEB Journal, Vol. 6, No. 3, (1992), pp. 879–885. Weiner, S., W. Traub, and H.D. Wagner, “Lamellar bone: Structure–function relations,” Journal of Structural Biology, Vol. 126, No. 3, (1999), pp. 241–255.

212

Biomimetics: Nature-Based Innovation

White, S.R., N.R. Sottos, P.H. Geubelle, J.S. Moore, M.R. Kessler, S.R. Sriram, E.N. Brown, and S. Viswanathan, “Autonomic healing of polymer composites,” Nature, Vol. 409, (2001), pp. 794–797. Wiesmann, H.P., U. Joos, and U. Meyer, “Biological and biophysical in extracorporal bone tissue engineering: Part II,” International Journal of Oral and Maxillofacial Surgery, Vol. 33, No. 6, (2004), pp. 523–530. Wilson, E.E., A. Awonusi, M.D. Morris, D.H. Kohn, M.M.J. Tecklenburg, and L.W. Beck, “Three structural roles for water in bone observed by solid-state NMR,” Biophysical Journal, Vol. 90, No. 10, (2006), pp. 3722–3731. Woo, S.L., T.Q. Lee, S.D. Abramowitch, and T.W. Gilbert, “Structure and function of ligaments and tendons,” in V.C. Mow and R. Huiskes (Eds.), Basic Orthopaedic Biomechanics and Mechanobiology (3rd ed.), Lippincott Williams and Wilkins, Philadelphia, PA, (2005), pp. 301–342, ISBN 0781739330. Wren, T.A.L., and D.R. Carter, “A microstructural model for the tensile constitutive and failure behavior of soft skeletal connective tissues,” Journal of Biomechanical Engineering, Vol. 120, (1998), pp. 55–61. Yamashita, J., B.R. Furman, H.R. Rawls, X. Wang, and C.M. Agrawal, “The use of dynamic mechanical analysis to assess the viscoelastic properties of human cortical bone,” Journal of Biomedical Materials Research, Vol. 58, No. 1, (2001), pp. 47–53. Yasuda, K., J.P. Gong, Y. Katsuyama, A. Nakayama, Y. Tanabe, E. Kondo, M. Ueno, and Y. Osada, “Biomechanical properties of high-toughness double network hydrogels,” Biomaterials, Vol. 26, No. 21, (2005), pp. 4468–4475. Yuan, H., S. Kononov, F.S.A. Cavalcante, K.R. Lutchen, E.P. Ingenito, and B. Suki, “Effects of collagenase and elastase on the mechanical properties of lung tissue strips,” Journal of Applied Physiology, Vol. 89, No. 1, (2000), pp. 3–14. Yuya, P.A., Y. Wen, J.A. Turner, Y.A. Dzenis, and Z. Li, “Determination of Young’s modulus of individual electrospun nanofibers by microcantilever vibration method,” Applied Physics Letters, Vol. 90, No. 11, (2007), p. 111909. Zhang, S., “Fabrication of novel biomaterials through molecular self-assembly,” Nature Biotechnology, Vol. 21, No. 10, (2003), pp. 1171–1178. Zhang, W., Y. Liu, and G.S. Kassab, “Viscoelasticity reduces the dynamic stresses and strains in the vessel wall: Implications for vessel fatigue,” American Journal of Physiology. Heart and Circulatory Physiology, Vol. 293, No. 4, (2007), pp. H2355–H2360. Zioupos, P., J.D. Currey, and A.J. Hamer, “The role of collagen in the declining mechanical properties of aging human cortical bone,” Journal of Biomedical Materials Research, Vol. 45, No. 2, (1999), pp. 108–116. Ziv, V., and S. Weiner, “Bone crystal sizes: A comparison of transmission electron microscopic and x-ray diffraction line width broadening techniques,” Connective Tissue Research, Vol. 30, No. 3, (1994), pp. 165–175.

6 Electroactive Polymer Actuators as Artificial Muscles Yoseph Bar-Cohen California Institute of Technology Pasadena, California CONTENTS 6.1 Introduction ........................................................................................................................ 213 6.2 The Evolution of the Development of EAP Materials................................................... 214 6.3 The Two EAP Groups ........................................................................................................ 216 6.3.1 Fabrication of EAP Materials ............................................................................... 217 6.3.2 Field-Activated EAP .............................................................................................. 217 6.3.2.1 Ferroelectric Polymers ............................................................................ 217 6.3.2.2 Dielectric Elastomer EAP ....................................................................... 218 6.3.2.3 Electrostrictive Elastomers .................................................................... 221 6.3.3 Ionic EAP.................................................................................................................222 6.3.3.1 Ionomeric Polymer-Metal Composites.................................................222 6.3.3.2 Conducting Polymers .............................................................................223 6.3.3.3 Carbon Nanotubes .................................................................................. 224 6.3.3.4 Ionic Polymer Gels ..................................................................................225 6.3.4 Summary of the EAP Groups .............................................................................. 226 6.4 Current and Potential Applications ................................................................................ 226 6.4.1 Dust Wiper for the Nanorover and Other Space Applications ....................... 226 6.4.2 Medical Applications ............................................................................................ 229 6.4.3 Biomimetic Robots ................................................................................................. 231 6.4.4 Full-Page Refreshable Braille Displays ............................................................... 232 6.4.5 Toys and Games .....................................................................................................234 6.4.6 Dielectric Elastomer Switches ..............................................................................234 6.5 The Arm-Wrestling Challenge—The Capability Indicator.......................................... 235 6.6 Challenges, Trends, and Potential Development .......................................................... 236 Acknowledgments ...................................................................................................................... 238 References..................................................................................................................................... 238 Web Sites....................................................................................................................................... 243

6.1  Introduction An actuator is the critical component that manipulates, mobilizes, or activates mechanical functions of mechanisms and systems. In most biological systems that are larger than 213

214

Biomimetics: Nature-Based Innovation

bacteria, muscles are providing this capability and they are able to lift loads as large as hundreds of kilograms with a response time of milliseconds. Biological muscles are driven by a complex mechanism that is very difficult to mimic. Electroactive polymers (EAPs), also known as artificial muscles, are human-made actuators that most closely emulate biological muscles (Bar-Cohen, 2004). Being polymers, they possess many advantages such as ease of manufacture and mass producibility, inherent lightweight property, and mechanical flexibility, making them highly attractive for numerous applications. In addition, some polymers respond to electrical, chemical, pneumatic, optical, or magnetic stimulation by a change in their shape or size, which leads to adding significant advantages in their use. Electrical activation is one of the most attractive methods of causing elastic deformation in polymers. The convenience and the practicality of stimulating EAPs and the significant improvements in response to these stimulations in recent years have made them the most preferred type among the responsive polymers (Bar-Cohen, 2004). It is interesting to note that some of the EAP materials also have the advantage of exhibiting the reverse effect of converting mechanical strain to electrical signal. This effect makes them useful for sensors and energy harvesting mechanisms. Many EAP materials are known today, and depending on their activation mechanism, they are divided by the author into two groups: field activated (originally named electronic) and ionic, and they are defined in the section “The Two EAP Groups.” EAP materials are still considered to be at their emerging stage that needs greater scientific and engineering advancements, and significant efforts are being made to turn them as actuators of choice. Bringing these materials to the level of application of daily-used products necessitates finding a niche that addresses critical needs. To maximize the actuation capability and operation durability, effective processing techniques are being developed for their fabrication, shaping, and electroding. Methods of reliably characterizing the response of EAP materials are being developed, and databases (Madden, 2010) are being established. EAP materials are widely being considered for biologically inspired, that is, biomimetic, applications (Bar-Cohen, 2005; Bar-Cohen and Hanson, 2009), including some that were once considered possible only in the realm of science fiction books and movies. Several novel applications of EAP materials as actuators have been already demonstrated, including robot fish, miniature gripper, loudspeaker, catheter steering element, haptic interface, active braille display, and dust wiper. Other applications that are considered include assistive walking, slithering robots, facial animatronic devices, and even an eyelid assister. The impressive advances in improving their actuation strain capability are attracting the attention of engineers and scientists from many different disciplines.

6.2  The Evolution of the Development of EAP Materials Roentgen (1880) is considered the first to conduct an experiment with EAP materials—he applied an electric field through the thickness of a rubber band having one end fixed and a mass attached to the free end, and the rubber band was stretched. The next important milestone in the field is the discovery of the piezoelectric polymer, called the electret, by Eguchi (1925). This material was produced when rosin (carnauba wax) and beeswax were solidified by cooling while being subjected to a direct current (DC) bias field. Electrets generate voltage when a stress is applied on them and have the reverse behavior of being deformed

Electroactive Polymer Actuators as Artificial Muscles

215

under electric field. They are insulating materials that can hold electric charges after being polarized in an electric field. Because the electrically generated strain in electrets is quite small for use as actuators, their application has been limited to sensors. It took 44 years for the next milestone in the field of EAP to be reported—Kawai (1969) observed significant piezoelectric activity in polyvinylidene fluoride (PVDF). This breakthrough was preceded by Fukada’s (1995) work on piezoelectric biopolymers. Investigations of PVDF and its copolymers have shown that some noncrystalline polymers with very large dielectric relaxations exhibit strong electromechanical activity because of the orientation of molecular dipoles (e.g., Wada, 1976; Kepler, 1978). Extensive research and development efforts related to PVDF have taken place mostly during the 1970s and 1980s wherein parallel to the efforts to improve the performance of the material, the material was considered for applications in many areas (Furukawa, 1989). The limited strain that can be produced by PVDF led to its use mostly for sensors and ultrasonic wave transducers (Bauer and Bauer, 2008). The pioneering of the development of responsive gels is attributed to Katchalsky and his coinvestigator, Israel (Katchalsky and Zwick, 1955). They were the first to report the chemomechanical activation mechanism in gel polymers, which causes shrinkage or swelling in the presence of an acidic or alkaline environment, respectively. Studies of the electrochemical activation of responsive gels started in the 1980s at Hokkaido University, Japan (Osada and Kishi, 1989). Developed polymers were demonstrated to create large strain under relatively low activation voltage (Osada, 1991). The development of PVDF as an EAP was followed by an extensive search for other ­polymer systems that exhibit significant response. Successes in discovering and developing effective new materials were reported mostly in the 1990s, and the examples include Oguro et  al. (1992), Lovinger (1983), Nalwa (1995), Baughman (1996), Zhang et  al. (1998), and Baughman et al. (1999). The most significant strain was demonstrated in the dielectric elastomer EAP materials, in which strains that exceed 100% with a relatively fast response speed (10 V/µm), which may be close to the breakdown level, and it is the result of the low dielectric constant in polymers (typically 10,000). For a CuPc-PVDF-based terpolymer composite having an elastic modulus of 750 MPa, the particulates increased the dielectric constant from a single digit to the range of 300–1000 (at 1 Hz) and under a field of 13 V/µm generated a strain of ~2% and a pressure level of 7.5 MPa. It is interesting to note that the significant increase in the dielectric constant compromised the electric leakage in these materials, and this needs to be taken into account (R.D. Kornbluh, personal e-mail communication about the electric leakage in composite ferroelectric EAPs, February 10, 2010). Photographs of such a composite ferroelectric EAP in passive and activated states are shown in Figure 6.1. 6.3.2.2 Dielectric Elastomer EAP Dielectric elastomer EAP actuators have become the leading practical material, and several companies are already working on developing methods of mass production and potential commercialization of related products. Generally, these are polymers with low elastic stiffness and high dielectric breakdown strength, which generate large strain under an electrostatic field. These EAP materials can be represented by a parallel plate capacitor as

Activated

Rest state

FIGURE 6.1 Photographs of a composite ferroelectric EAP in passive (right) and activated states (left). (Courtesy of Qiming Zhang, Penn State University.)

Electroactive Polymer Actuators as Artificial Muscles

219

Dielectric elastomer

+ Electrode + +++++++++++++++++++++++++++++++++++++++++++++++++++++++

–––––––––––––––––––––––– ––––––––– –––––––––––––––––

– Electrode

FIGURE 6.2 Under electrical activation, a dielectric elastomer film with compliant electrodes on both surfaces expands laterally while contracting along the thickness.

shown schematically in Figure 6.2, where highly compliant electrodes (such as conducting carbon grease) are used to avoid impeding the generated strain. These polymers have their dielectric constant unaffected by the electric field, with strain determined by the attraction force between the electrodes and the stiffness of the polymer. Because of the relatively small dielectric constant of polymers, the electrostrictive force is relatively low. However, the generated strain is large because soft materials are used. In a 1992–1993 study, Pelrine et al. (1993/1994) were the first to observe that dielectric elastomers sustain large strain (23% in silicone films) when subjected to a high electric field. In their reports, they suggested the use of dielectric EAP materials for actuation mechanisms. Independently, Zhenyi et al. (1994) reported in 1994 that they measured 3% strain in polyurethane when it was subjected to an electric field at the level of 20 V/µm. In the years that followed, significant levels of strain were obtained; in 1998, a level of strain of 30% was measured in silicone (Pelrine et al., 1998). Further, using preloaded acrylic elastomer, Pelrine et al. (2000) reported strains that are much higher than 100%. In recent years, many researchers started using dielectric elastomers to form EAP actuators and reported a significant improvement in their performance and their fabrication methods (Kornbluh et al., 2004; Zhongyang and Zhang, 2008; Carpi et al., 2008; R. Pelrine and R. Kornbluh, personal verbal and e-mail communication to clarify the pioneering history of the dielectric EAPs, April 2–4, 2009). To produce linear actuators, scientists at SRI International rolled two elastomer layers with carbon-based electrodes on both sides of one of the layers forming a cylindrical shape actuator (Kornbluh et al., 2004). Further modifications of their actuator design led to the development of the multifunctional electroelastomer roll. In this actuator (Figure  6.3), highly prestrained dielectric elastomers are rolled around a compression spring (Pei et al., 2003, 2004). By selectively actuating only certain regions of the electrodes around the periphery of the actuator, the actuator can be made to bend as well as elongate. Generally, obtaining high strain requires voltage levels that are close to the breakdown strength of the material at significantly shortened lifetime. Another concern when using dielectric elastomers as EAP is the need for prestrain that is released over time due to creep degrading the actuator performance. Researchers at Sungkyunkwan University, Korea (Jung et al., 2004), and at University of California, Los Angeles (UCLA) (Ha et al., 2006), developed promising methods to eliminate the need for prestrain. Ha et al. (2006) used an interpenetrating polymer network (IPN) where tension in the network is balanced by compression. To form an IPN thermally, cross-linkable liquid additives including a difunctional acrylate and a trifunctional acrylate were used.

220

Biomimetics: Nature-Based Innovation

Further, scientists at the University of Pisa, Italy, used a folded film structure to ­produce a contractile actuator (Figure 6.4) (Carpi and De Rossi, 2007). Investigators at EMPA, Switzerland, developed a method of stacking thousands of thin layers of a dielectric elastomer to form an effective actuator that generates contraction and does not require preload (Figure 6.5). Using this design, levels of 40% strain were measured in a 40-mm diameter,

FIGURE 6.3 A dielectric elastomer EAP-based multifunctional electroelastomer spring roll. Courtesy of Qibing Pei, UCLA, Roy Kornbluh, SRI International, and SPIE. (Adapted from Pei, Q., Pelrine, R.E., Stanford, S.E., Kornbluh, R.D., Rosenthal, M., Meijer, K., and Full, R.J., Multifunctional electroelastomer rolls and their application for biomimetic robots, in Y. Bar-Cohen (Ed.), Proceedings of the SPIE, Smart Structures and Materials 2002: Industrial and Commercial Applications of Smart Structures Technologies, 4698, San Diego, CA, 2002; Kornbluh, R.D., Pelrine, R.E., Pei, Q., Heydt, R., Stanford, S.E., Oh, S., and Eckerle, J., Electroelastomers: Applications of dielectric elastomer transducers for actuation, generation and smart structures, in Proceedings of the SPIE Smart Structures and Materials, 4698, San Diego, CA, 2002.) Dielectric elastomer Compliant electrode Dielectric elastomer ∆V

FIGURE 6.4 A contractile EAP actuator using a folded film structure. (Courtesy of Federico Carpi, University of Pisa, Italy.)

Electroactive Polymer Actuators as Artificial Muscles

221

FIGURE 6.5 Photographs of a multilayered dielectric elastomer in passive (left) and activated/contracted states (right). (Courtesy of Gabor Kovacs, EMPA, Duebendorf, Switzerland.)

FIGURE 6.6 A dielectric elastomer with wave-shaped film made by PolyPower, Danfoss, Denmark, is shown lifting 10 kg cylinders.

100-mm-long actuator, and they were able to generate as high as 250 N contractile force (Kovacs and Düring, 2009). Moreover, there was a recent development at PolyPower, Danfoss, Denmark, using a corrugated structure allowing for mass production of dielectric elastomer EAP actuators that are capable of lifting levels of kilograms (Figure 6.6) (Kiil and Benslimane, 2009). 6.3.2.3 Electrostrictive Elastomers An electrostrictive elastomer having a chain structure (i.e., backbone) sustains molecular (pendant group) alignment because of internal polarization when subjected to an

222

Biomimetics: Nature-Based Innovation

electric field. These polymers may consist of two components, a flexible backbone macromolecule and a grafted polymer that is made of a polarizable molecular or nanocrystalline structure. Subjecting such an electrostrictive elastomer to a large electric field was reported to ­produce approximately 4% strain and approximately 24 MPa stress (Su et al., 1999; Zhang et al., 2004). The grafted crystalline polar phase provides moieties in response to an applied ­electric field and cross-linking sites for the elastomer system. A  combination of the ­electrostrictive-grafted elastomer with a piezoelectric poly(vinylidene ­fluoride–trifluoroethylene) copolymer allows the production of various compositions of ­ferroelectric–electrostrictive molecular composite systems. Such combinations can be operated as a piezoelectric sensor as well as an electrostrictive actuator. Careful selection of the composition allows for creating and optimizing the molecular composite system with respect to its electrical, mechanical, and electromechanical properties. The photographs in Figure 6.7 are showing an activated grafted elastomer-based bimorph ­actuator on the right, whereas on the left, the EAP is in its rest state. 6.3.3  Ionic EAP In this group, there are four leading types of EAP materials. 6.3.3.1 Ionomeric Polymer–Metal Composites The IPMC is one of the most studied ionic EAP materials. In 1992, three different groups of researchers have reported independently the development of an IPMC-based EAP, and they include Oguro et al. (1992) in Japan and Shahinpoor (2000) and Sadeghipour et al. (1992) in the United States. The attractive characteristic of IPMC is the significant bending in response to a relatively low electrical voltage (Figure 6.8), where the base polymer provides channels for mobility of positive ions in a fixed network of negative ions on interconnected clusters (Nemat-Nasser and Thomas, 2004; Park et al., 2008). The response of an IPMC is ­relatively slow (15% typical, 90% max)

• Moderate strain (1 kV) and fields (~150 MV/m) • Typically requires DC–DC converters • Cycle life is unclear and may be limited by electrode fatigue and dissipation • Synthesis of typical materials involves environmentally regulated substances • Limited temperature range

• High voltages (>1 kV) and fields (~150 MV/m) • Typically requires DC–DC converters • Compliant (E ~ 1 MPa) • Prestretching mechanisms add substantial mass and volume, reducing actual work density and stress. The recent development in IPNs is helping to overcome this disadvantage

• Produces mostly monopolar actuation • Requires high voltages (~100 MV/m). Use of additives enabled reduction of the field in Ferroelectric EAP

• Not yet engineering material • Narrow temperature range of operation • No catch state (expends energy to maintain force w/o moving)

Disadvantages

Advantages and Disadvantages of the Two EAP Groups Relative to Mammalian Skeletal Muscles

TABLE 6.1

continued

• Lower voltages and fields are being achieved using new high dielectric composites • Small devices are favored for high-frequency operations • Unique combinations of high stiffness, moderate strain, and reasonable efficiency

• Potential to lower fields using high dielectric materials • Based on readily available materials

• Muscle is a three-dimensional nanofabricated system with integrated sensors, energy delivery, waste/heat removal, local energy supply, and repair mechanisms

Comments

Electroactive Polymer Actuators as Artificial Muscles 227

• High stress (100 MPa max, 5 MPa typical) • Moderate strains (~2%) • Low voltage (~2 V) • High work density (100 kJ/m3) • Stiff polymers (~1 GPa)

• High stress (200 MPa max) • Low voltage (2 V) • Very large operating temperatures

• Low voltage (1.23 V in aqueous systems • Generally, requires protection from evaporation • Low electromechanical coupling efficiency • Specifically for IPMC—does not hold strain under DC voltage • Slow response (fraction of a second)

Disadvantages

• Great potential as bulk materials approach properties of individual nanotubes

• Promising for low-voltage applications. Speed and power will improve dramatically at small scales

Comments

Source: Bar-Cohen, Y. (Ed.), Electroactive Polymer (EAP) Actuators as Artificial Muscles—Reality, Potential and Challenges (2nd ed.), SPIE Press, Bellingham, Washington, PM136, pp. 1–765, March 2004, as well as the Actuator Selection Tool Web site (Madden, J.D., Vandesteeg, N., Anquetil, P.A., Madden, P.G., Takshi, A., Pytel, R.Z., Lafontaine, S.R., Wieringa, P.A., and Hunter, I.W., Artificial muscle technology: Physical principles and naval prospects, IEEE J. Ocean Eng., 29, 706–728, July 2004); the latter input was used as a courtesy of John Madden, University of British Columbia, Canada.

• Bidirectional actuation depending on the voltage polarity • Some ionic EAPs (e.g., conducting polymers) have bistability • Requires low voltage

Advantages

Ionic polymers group

Ionic Polymers Group

Actuators

Advantages and Disadvantages of the Two EAP Groups Relative to Mammalian Skeletal Muscles

TABLE 6.1 (Continued)

228 Biomimetics: Nature-Based Innovation

Electroactive Polymer Actuators as Artificial Muscles

Actuated by 1–3 volts Electrode

229

Biased with 1–2 KV for dust repulsion EAP wiper

FIGURE 6.12 Combined schematic and photographic view of the EAP dust wiper.

allow NASA and other space agencies to conduct missions with capabilities that exist today only in the realm of science fiction. 6.4.2  Medical Applications One of the areas that are attracting a great deal of efforts to apply EAP actuators is the field of medicine. Potentially, EAP materials can be used as artificial muscles for operating medically related mechanisms and devices with biomimetic characteristics. Some of the applications that are considered include catheter steering mechanisms (Della Santa et al., 1996), vein connectors for repair after surgery, and smart prosthetics (Herr and Kornbluh, 2004). If EAP materials are developed to operate internal organs inside a human body, this technology can potentially make a tremendously positive impact on many human lives (Müller et al., 2009). EAP materials offer the potential of biocompatibility, but they need to meet the stringent safety and reliability requirements for operating inside or adjacent to the human body. At present, the dielectric elastomer EAP materials seem to be the most applicable because they generate the largest actuation forces and strains and also they have the highest demonstrated reliability. However, the required high voltages pose a potential hazard that must be addressed. Although the ionic EAP group has the advantage of very low voltage activation, the group being chemically active requires the use of an effective sealant to protect the internal organs and avoid contamination of the ionic content, which reduces the performance efficiency. Interfacing with machines used to complement or substitute our senses can enable ­important capabilities for medical applications. A number of such interfaces were ­investigated or have been considered, and the most significant work is the interfacing of machines and the human brain. A development by scientists at Duke University (Mussa-Ivaldi, 2000; Wessberg et al., 2000), Caltech, Massachusetts Institute of Technology, Brown University, and many other research institutes enabled this possibility. For this purpose, electrodes were connected to the brain of a monkey, and using brain waves, the monkey operated a robotic arm. If EAP-actuated robotic arms are developed with sufficient strength and dexterity to function as effective prosthetics, then this development by neurologists would help disabled people greatly. Using such haptic interfacing capability to control prosthetics would require feedback to allow the human user to “feel” the artificial limbs. The required feedback can be provided with the aid of tactile ­sensors combined with haptic devices (Carpi et al., 2009) and other interfacing mechanisms. Besides ­providing feedback, sensors are needed for the users to protect their prosthetics from potential damage that may be caused by heat, ­pressure, impact, and other potential ­hazards, just as with biological limbs. Another medical application is the activation of the eyelid to enable blinking. For many patients, there are no other functioning nerves nearby that can be rerouted to close the

230

Biomimetics: Nature-Based Innovation

Battery +–

EPAM

Sling

FIGURE 6.13 (See color insert.) Restoring eyelid blink using a sling-shaped structure that is actuated by dielectric elastomer EAP. (Courtesy of Travis T. Tollefson, Department of Otolaryngology-Head and Neck Surgery, University of California, Davis Medical Center, Sacramento, CA.)

eyelid. The option of transplanting muscle from the leg involves a surgery that can take approximately 10 hours. This surgical procedure has significant risks and is not always suitable for elderly or medically weak patients. Another alternative that is commonly used is the implant of a small weight that closes the eye via gravity. Although having high success rate, the blinking is slower than normal and cannot be synchronized with the opposite eye. Also, there are difficulties keeping the lid closed when lying down to sleep. The use of dielectric elastomers as actuators of the eyelid for helping patients with facial paralysis to blink was recently demonstrated by surgeons at the University of California, Davis (Senders et al., 2010). As shown in Figure 6.13, a sling-shaped structure is activated by the dielectric elastomer EAP made by SRI International. This mechanism is being developed to treat patients who sustained a stroke, accident, or combat injuries that caused loss of the ability to blink. The involuntary eye blinking is a crucial function that lubricates and cleans the eye, and it is controlled by a cranial nerve. The EAP artificial muscle actuator that was applied for eyelid blinking was developed by SRI International. The developed actuator configured in the form of a sling produces the level of force as generated by the eye muscles. A control package that is similar to the electronic pacemaker could be used to activate this mechanism. The sling is attached to the bone around the eye, and a small battery hidden in a natural hollow in the temple is used to power the actuator. The prototype of the developed mechanism has been tested successfully on cadavers and is expected to be tested on live patients in a number of years. The use of robotics in medical applications contributed significantly to reduction in mortality after surgery, faster recovery, and minimized complications. An example of existing robotics is the da Vinci surgical system that is now a standard tool in many hospitals worldwide. Unfortunately, the current systems are too large to conduct delicate surgical procedures in such organs as the brain. One may consider a minimally invasive robotic arm as a surgical device that has an octopus configuration with tentacles of multiple degrees

Electroactive Polymer Actuators as Artificial Muscles

231

of freedom, which are equipped with various tools. In developing such a device, besides using EAP as actuators, one may take advantage of the capability of electrorheological fluids to become highly viscous under electrical excitation. This property can be used to control the rigidity of flexible robotic arms as well as operating as a haptic interface (Fisch et al., 2003; Bar-Cohen, 2004). An illustration of such a futuristic concept is shown in Figure 6.14, and it is biologically inspired using the octopus tentacle structure offering capabilities that are impossible today (Bar-Cohen, 2005). 6.4.3  Biomimetic Robots Biomimetic applications can immensely expand the collection and functionality of robots, allowing the performance of tasks that are impossible with existing capabilities (Bar-Cohen and Breazeal, 2003; Bar-Cohen, 2005; Bar-Cohen and Hanson, 2009). Using EAP actuators, biologically inspired concepts can be applied with capabilities that are far superior to natural creatures because they are not constrained by evolution or survival needs. Flight is an example of the success of biologically inspired technology—aircraft are capable of flying faster and higher, carrying more weight, and operating in significantly more difficult environmental conditions than any existing flying creature on earth. One may produce such devices as artificial insects that may walk, swim, hop, crawl, and dig while being able to reconfigure themselves as needed. For an example of the use of EAP for actuation of robotic components, the author and his coinvestigators constructed a miniature robotic arm that was lifted by a rolled dielectric elastomer EAP as a linear actuator and with four IPMC-based fingers as bending actuators (Bar-Cohen, 2004). The linear actuator was used to raise and drop a graphite/epoxy rod that served as a simplistic representation of a robotic arm. As an end-effector on the arm, a gripper was developed (Figure 6.15)

FIGURE 6.14 A graphic view of a futuristic octopus-configured catheter for surgical applications.

FIGURE 6.15 Four-finger EAP gripper lifting a rock.

232

Biomimetics: Nature-Based Innovation

FIGURE 6.16 The six-legged robot, FLEX, which is a self-contained walking robot powered by EAP. (Courtesy of Roy Kornbluh, SRI International.)

using IPMC strips that act as fingers having hooks at the bottom of the strips mimicking the function of fingernails. As shown in Figure 6.15, this gripper, very similar to a human hand, grabbed rocks. Further, various robot designs were developed by scientists at SRI International, including the FLEX that is considered the first self-contained walking robot that is powered by EAP (Figure 6.16). This robot is loosely based on the gait of a cockroach, and each of its six legs used two dielectric elastomer actuators to move up and down as well as back and forth. Actuating its legs at frequencies of up to 10 Hz allowed the FLEX robot to move at speeds that are greater than 12 cm/s (Kornbluh et al., 2004). Other recently reported robotic applications include a fishlike blimp (made by EMPA, Switzerland, in collaboration with Aeroix GmbH and the Technical University of Berlin) that wags its body and tail via a dielectric elastomer EAP (Figure 6.17). This slightly pressurized helium-filled blimp is an 8-m long vehicle that is actuated by EAP just like biological muscles, using an agonist–antagonist configuration to create undulating movement. The developed EAP actuators were designed to work at various frequencies, ­activation voltages, and with a phase shift between the body movement and the tail movement. The flight of this ­fishlike blimp can be controlled with a joystick connected to a ground-based portable computer. The first test flight of the fully EAP-propelled fishlike blimp took place in July 2009 at Duebendorf, Switzerland (www.empa.ch/airship). 6.4.4  Full-Page Refreshable Braille Displays EAP materials have enormous potential for the development of full-page refreshable braille displays (Bar-Cohen, 1998; Bar-Cohen, 2004; Chapter 7). The key benefit of using these materials is the potential capability to pack many actuators in a small area without ­interferences. Pioneering the efforts, the author conceived a refreshable braille display in 1998 after he was inspired by the presence of visually impaired persons in a convention

Electroactive Polymer Actuators as Artificial Muscles

233

FIGURE 6.17 Fishlike blimp that uses a wagging body and tail for propulsion and is actuated by dielectric elastomer EAP (the black strips). (Courtesy of Silvain Michel, EMPA, Switzerland, http://www.empa.ch/plugin/template/ empa/*/72289/---/l=1#s5a).

held coincidentally at a hotel in Washington, D.C, where he stayed. His concept is based on an EAP actuator array made of a field-activated-type material (see further details in Chapter 7). The electric field in the cross section of rows and columns of electrode strips on the opposite sides of an EAP film activates individual elements in the array. Each of the crossing electrodes forms an actuation element and is mounted with a braille dot that is lowered by applying voltage across the thickness of the selected element. In 2003, other researchers started reporting the development of EAP-based refreshable braille displays. Nine different mechanisms were already reported, and their ­developers include investigators from Wollongong University along with Quantum Technologies, Sydney, Australia (Spinks and Wallace, 2009); Harbin Institute of Technology, China (J. Leng and X. Lan, personal e-mail communication, January 7, 2010); Darmstadt University of Technology, Germany (Matysek et  al., 2009); the University of Tokyo, Tokyo, and the National Institute of Advanced Industrial Science and Technology (AIST), Osaka, Japan (Kato et al., 2007); Sungkyunkwan University, South Korea (Choi et al., 2009); and Penn State University (Ren et al., 2008), SRI International (Heydt and Chhokar, 2003), Carolina State University (Di Spigna et al., 2009), and UCLA jointly with NASA Ames (Yu et al., 2009) in the United States. The developers used such EAP materials as the ­conducting polymers, dielectric elastomers, ferroelectric, IPMC, PVDF, and bistable EAPs. The focus of the efforts has been on developing miniature, actuated small pins/dots that can be packed into a small area while generating sufficient displacement and force at sufficient speed. The actuator moves levers, a rolled film over a prestrained spring, a bimorph ­configuration, a multilayered array, a diaphragm with spring-backed elements, and pressurized ­diaphragm elements taking advantage of the prestrain. To provide a forum for communicating the unique possibilities that EAP can provide to the broad topic of haptic/tactile interfaces, this topic was chosen for highlight in a special session of the SPIE 2010 EAPAD Conference, and several related demos were presented at its EAP-in-Action Session (SPIE, 2010).

234

Biomimetics: Nature-Based Innovation

Although the developed displays showed a performance that is close to the required specifications, there are still challenges that prevent them from being made as a commercial product (Chapter 7). These include the insufficient force in the case of IPMC as well as the short cycling life of the conducting polymers. Also, there are issues related to their reliable operation and limitations in mass production. Advances in developing more effective EAP materials and processing techniques may lead to practical, low-cost, compact, refreshable braille displays. 6.4.5  Toys and Games The application of EAP to actuate toys and games offers a great testbed for new capabilities and commercial products because there is no need for extensive product durability that other industries are required to meet. The use of EAP may allow quick turning of concepts to commercial products, and the success of few products can help raise enormous revenues to support follow-on development of other applications. The development of toys and games that are driven by EAP has already begun, and an example includes the efforts at the Auckland Bioengineering Institute’s Biomimetics Lab, New Zealand. Their game (Figure 6.18) involves making a ball continuously rotate on a round Plexiglas plate where the ball can wobble forward and backward, and a steady control of the plate’s action in the play allows maintaining rotation. The plate is wobbled and rotated by a dielectric elastomer EAP, and the mechanism that generates them is shown (see the three black circles at the left on the top-left part of Figure 6.18). The Plexiglas plate needs to be controlled to maintain the rotation of the ball in one direction. This game is illustrating an example of the novel possibilities that EAP actuators offer. 6.4.6  Dielectric Elastomer Switches The application of EAP as smart actuators that emulate biological muscles requires them to operate as a network, where intelligence and feedback capabilities are distributed throughout the actuator. Recently, the integration of an intrinsic sensor, control, and driver circuitry into the dielectric elastomer EAP was reported (O’Brien e al., 2010), bringing these artificial muscles closer to the natural analogs. This functionality was achieved by exploiting the piezoresistive behavior of dielectric elastomer devices and developing a

FIGURE 6.18 An EAP-actuated ball rotating game was demonstrated by the Auckland Bioengineering Institute’s Biomimetics Lab, New Zealand, at the EAP-in-Action Session of the SPIE EAPAD Conference on March 8, 2010.

Electroactive Polymer Actuators as Artificial Muscles

235

switching material based on carbon-loaded silicone grease to create dielectric elastomer switches (DES). The fundamental requirements for making a digital computation using DES were demonstrated experimentally, namely, compliant electromechanical NAND (logical “not AND”) gates and oscillator circuits. DES can be thought of as electromechanical relays, hybrid devices combining analog and digital processing. Their use is expected to reduce the need for bulky and rigid external circuitry. Also, they may provide simple distributed intelligence as required to produce soft, biologically inspired networks of actuators. Potential applications may include devices with many degrees of freedom such as robotic hearts, artificial intestines, and manipulators; wearable assistive devices; and ultimately electromechanical computers.

6.5  The Arm-Wrestling Challenge—The Capability Indicator In 1999, the author posed an arm-wrestling challenge (http://ndeaa.jpl.nasa.gov/­nasa-nde/ lommas/eap/EAP-armwrestling.htm) in an effort to promote worldwide ­development toward realizing the potential of EAP materials. The challenge consists of having an EAP-activated robotic arm win against humans in a wrestling match, and the icon of the challenge is shown in Figure 6.19. Choosing to focus on arm wrestling with humans was done to have the human muscles as a baseline for performance comparison. Success will allow using EAP materials to improve many aspects of our life including the development of effective implants and prosthetics, active clothing, and realistic biologically inspired robots as well as fabricating products with unmatched capabilities and dexterity. On March 7, 2005, the first arm-wrestling match with a human was held, and it was part of the EAP-in-Action Session of the SPIE EAPAD Conference. Three robotic arms wrestled

FIGURE 6.19 (See color insert.) Grand challenge for the development of EAP-actuated robotics.

236

Biomimetics: Nature-Based Innovation

FIGURE 6.20 An EAP-driven arm made by students of Virginia Tech and the human opponent, a 17-year-old student.

against a 17-year-old high school female student and the student won (Figure 6.20). In the following year, the second contest was held; however, rather than wrestling with a human opponent, the performance of the arms was measured and the results were compared. A measuring fixture was developed jointly by individuals from UCLA and members of the author’s group at the Jet Propulsion Laboratory. The fixture was strapped to the contest table, and the EAP-actuated arms were tested for speed and pulling force. To establish a baseline for performance comparison, the capability of the student who wrestled in 2005 was measured first, and then the three participating robotic arms were tested. Although these tests showed two orders of magnitude lower performance of the arms compared with the student, there is a need to take into account that humans also use forces that are generated by the leg muscles, and therefore the robotic arms need to be much stronger to counter these additional forces.

6.6  Challenges, Trends, and Potential Development Since the early 1990s, new EAP materials that generate large strains at the level of hundreds of percent, making them highly attractive for biomimetic and many other applications, have been developed. Their operational similarity to biological muscles, including resilience, damage tolerance, and the ability to induce large actuation strains (stretching, contracting, or bending) makes them unique compared with other electroactive materials. Using EAP materials to produce actuators involves many challenges including fundamental ones and the limited actuation efficiency. Advancing the materials to a mature state necessitates increasing their actuation force, electromechanical conversion efficiency, and operation lifetime. The field-activated (i.e., electronic) EAP materials require reduction of

Electroactive Polymer Actuators as Artificial Muscles

237

the drive voltage, which may be achieved by increasing the polymer dielectric constant, possibly using fillers, or by stacking of many thin layers as demonstrated by researchers at the university of Darmstadt and recently by scientists at Empa, Switzerland. Further, the ionic EAP materials require increasing their response speed as well as making them operational in “dry” environments over extended time. The latter may be achieved by developing effective protective coating and/or working with solvents that have near zero vapor pressure. Addressing the challenges to the use of EAP materials requires improving the understanding of their various activation mechanisms and the development of mass production techniques. Effective sensors and control algorithms are needed to address the unique requirements of producing and testing practical EAP actuators. A summary of the key challenges to EAP today is as follows: • Databases and standards—there is no well-established database and standard set of test procedures. • Efficiency—the actuation force and energy conversion efficiency are low. • Availability—EAP materials are not available yet in commercial quantities. • Robustness—there are lifetime and reliability issues. • Scalability—it is not obvious how to make very large EAP actuators and to massproduce them for low-cost consistent operation. • Competitiveness—EAP materials need to offer superior capability over alternative actuation technologies and, preferably, enable a niche application. Addressing the challenges to EAP materials requires continuing the technology development and the growth in multidisciplinary cooperation among experts from various fields including chemists, materials scientists, roboticists, computer and electronic engineers, and so forth. It is necessary to develop a comprehensive material science as well as effective ­electromechanics analytical tools and material-processing techniques. Researchers are increasingly making improvements in the various related areas including a better understanding of the operation mechanism of the various EAP material types. The processes of synthesizing, fabricating, electroding, shaping, and handling are being refined to maximize actuation capability and durability. Methods of reliably characterizing the response of these materials are being developed, and efforts are being made to establish databases with documented material properties to support engineers who are considering the use of these materials. An initiative in this area has been taken by the University of British Columbia, and it has formed a Web database for viewing, comparing, and submitting EAP properties (Madden, 2010). Applying EAP materials as actuators of manipulation and mobility, and as robotic devices involves many disciplines including materials, chemistry, electromechanics, computers, and electronics. Although the actuation forces of the existing materials requires further improvement, there has already been a series of reported successes in the development of mechanisms that are driven by EAP actuators. However, seeing EAP replace existing actuators in commercial devices and engineering mechanisms may be a difficult challenge and would require identifying niche applications where EAP materials would not need to compete with existing technologies. It is quite encouraging to see the growing number of researchers and engineers who are pursuing careers in EAP-related disciplines. Hopefully, the growth in the research and development activity will lead to making these materials becoming the actuators of choice.

238

Biomimetics: Nature-Based Innovation

Acknowledgments Some of the research reported in this chapter was conducted at the Jet Propulsion Laboratory (JPL), California Institute of Technology, under a contract with NASA. For the input to the section “Dielectric Elastomer Switches,” the author expresses his appreciation to Ben O’Brien, The Biomimetics Laboratory of the Auckland Bioengineering Institute, The University of Auckland, Auckland, New Zealand and to Emilo P. Calius, Future Materials and Structures, Industrial Research Limited, Auckland, New Zealand. Also, the author expresses his appreciation of the very valuable comments and suggestions of the reviewers of this chapter. The reviewers were Federico Carpi, University of Pisa, Interdepartmental Research Centre “E. Piaggio,” School of Engineering, Pisa, Italy; Roy D. Kornbluh, Telecommunications and Automation Division, SRI International, Menlo Park, CA; Hani E. Naguib, Department of Materials Science and Engineering, University of Toronto, Toronto, Ontario, Canada; and John D. Madden, Department of Electrical and Computer Engineering, University of British Columbia, Vancouver, BC, Canada.

References Anquetil, P.A., H.-H. Yu, P.G. Madden, J.D. Madden, T.M. Swager, and I.W. Hunter, Thiophene-based molecular actuators, in Y. Bar-Cohen (Ed.), Proceedings of SPIE 9th Annual Symposium on Smart Structures and Materials: Electroactive Polymer Actuators and Devices, SPIE Press, Bellingham, Washington, (March 2002), pp. 424–434. Bar-Cohen, Y., Active Reading Display for Blind (ARDIB), New Technology Report, Item No. 0008b, Docket 20410, (February 5, 1998). Bar-Cohen, Y. (Ed.), Proceedings of the first SPIE Electroactive Polymer Actuators and Devices (EAPAD) Conference, Smart Structures and Materials Symposium, Vol. 3669, (1999), pp. 1–414, ISBN 0-8194-3143-5. Bar-Cohen, Y. (Ed.), Electroactive Polymer (EAP) Actuators as Artificial Muscles—Reality, Potential and Challenges (2nd ed.), SPIE Press, Bellingham, Washington, Vol. PM136, (March 2004), pp. 1–765, ISBN 0-8194-5297-1. Bar-Cohen, Y. (Ed.), Biomimetics—Biologically Inspired Technologies, CRC Press, Boca Raton, FL, (November 2005), pp. 1–527, ISBN 0849331633. Bar-Cohen, Y., WorldWide Electroactive Polymer Actuators Webhub, http://eap.jpl.nasa.gov (last updated January 2010). Bar-Cohen, Y., and C. Breazeal (Eds.), Biologically-inspired Intelligent Robots, SPIE Press, Bellingham, Washington, Vol. PM122, (May 2003), pp. 1–393, ISBN 0-8194-4872-9. Bar-Cohen, Y., and D. Hanson, The Coming Robot Revolution—Expectations and Fears About Emerging Intelligent, Humanlike Machines, Springer, New York, NY, (2009), ISBN: 978-0-387-85348-2. Bar-Cohen, Y., T. Xue, and S.-S., Lih, Polymer Piezoelectric Transducers for Ultrasonic NDE, 1st International Internet Workshop on Ultrasonic NDE, Subject: Transducers, organized by R. Diederichs, UTonline J., Germany, (1996), http://www.ndt.net/article/yosi/yosi.htm. Bauer, S., and F. Bauer, Piezoelectric polymers and their applications, in W. Heywang, K. Lubitz, and W. Wersing (Eds.), Piezoelectricity Evolution and Future of a Technology Series, Springer Series in Materials Science, Vol. 114, (2008), pp. 157–177, ISBN: 978-3-540-68680-4. Baughman, R.H., Conducting polymer artificial muscles, Synthetic Metals, Vol. 78, (April 15, 1996), pp. 339–353.

Electroactive Polymer Actuators as Artificial Muscles

239

Baughman, R.H., C. Cui, A.A. Zakhidov, Z. Iqbal, J.N. Basrisci, G.M. Spinks, G.G. Wallace, et  al., Carbon nanotube actuators, Science, Vol. 284, (1999), pp. 1340–1344. Calvert, P., Electroactive polymer gels, in Chapter 5, in Bar-Cohen Y. (Ed.), Electroactive Polymer (EAP) Actuators as Artificial Muscles—Reality, Potential and Challenges (2nd ed.), SPIE Press, Bellingham, Washington, (2004), pp. 95–148. Calvert, P., Gel sensors and actuators (Special issue dedicated to EAP), Materials Research Society Bulletin, Vol. 33, No. 3, (March 2008), pp. 207–212. Carpi, F., and D. De Rossi, Contractile folded dielectric elastomer actuators, in Y. Bar-Cohen (Ed.), Proceedings of the Electroactive Polymer Actuators and Devices (EAPAD) Conference IX, Vol. 6524, SPIE Electronic Library, (2007), doi:10.1117/12.715594. Carpi, F., D. De Rossi, R. Kornbluh, R. Pelrine, and P. Sommer-Larsen, Dielectric Elastomers as Electromechanical Transducers: Fundamentals, Materials, Devices, Models and Applications of an Emerging Electroactive Polymer Technology, Elsevier, San Diego, CA, (2008). Carpi, F., G. Frediani, and D. De Rossi, Electroactive elastomeric haptic displays of organ motility and tissue compliance for medical training and surgical force feedback, IEEE Transactions on Biomedical Engineering, Vol. 56, No. 9, (2009), pp. 2327–2330. Cheng, Z., and Q. Zhang, Field-activated electroactive polymers (Special issue dedicated to EAP), Materials Research Society Bulletin, Vol. 33, No. 3, (March 2008), pp. 190–195. Choi, H.R., I.M. Koo, K. Jung, S-G. Roh, J.C. Koo, J-D. Nam, and Y.K. Lee, A braille display system for the visually disabled using polymer based soft actuator, in E. Smela and F. Carpi (Eds.), Biomedical Applications of Electroactive Polymer Actuators, John Wiley & Sons, Hoboken, NJ (2009), ISBN-10: 0-470-77305-7; ISBN-13: 978-0-470-77305-5. DOI: 10.1002/9780470744697.ch23. Della Santa, A., A. Mazzoldi, and D. De Rossi, Steerable microcatheters actuated by embedded conducting polymer structures, Journal of Intelligent Material Systems and Structures, Vol. 7, No. 3, (1996), pp. 292–301. Di Spigna, N., P. Chakraborti, P. Yang, T. Ghosh, and P. Franzon, Application of EAP materials toward a refreshable braille display, in Y. Bar-Cohen and T. Wallmersperger (Eds.), Proceedings of the SPIE Smart Structures Symposium, EAPAD Conference, Vol. 7287, (2009), 72871K. Eguchi, M., On the permanent electret, Philosophical Magazine, Vol. 49, (1925), p. 178. Fisch, A., C. Mavroidis, Y. Bar-Cohen, and J. Melli-Huber, Haptic and telepresence robotics, in Y.  ­Bar-Cohen and C. Breazeal (Eds.), Biologically-Inspired Intelligent Robots, SPIE Press, Bellingham, Washington, Vol. PM122, (May 2003), pp. 73–101, ISBN 0-8194-4872-9. Fukada, E., Piezoelectricity of wood, Journal of the Physical Society of Japan, Vol. 10, (1955), pp. 149–154. Furukawa, T., Piezoelectricity and pyroelectricity in polymers, IEEE Transactions on Electrical Insulation, Vol. 24, No. 3, (1989), pp. 375–394. Furukawa, T., and J.X. Wen, Electrostriction and piezoelectricity in ferroelectric polymers, Japanese Journal of Applied Physics, Vol. 23, (1984), pp. L677–L679. Ha, S.M., W. Yuan, Q. Pei, R. Pelrine, and S. Stanford, Interpenetrating polymer networks for high-­performance electroelastomer artificial muscles, Advanced Materials, Vol. 18, (2006), pp. 887–891. Hara, S., T. Zama, W. Takashima, and K. Kaneto, Free-standing polypyrrole actuators with response rate of 10.8% S(–1), Synthetic Metals, Vol. 149, (2005), pp. 199–201. Herr, H.M., and R.D. Kornbluh, New horizons for orthotic and prosthetic technology: Artificial muscle for ambulation, in Y. Bar-Cohen (Ed.), Proceedings of SPIE Smart Structures and Materials 2004: Electroactive Polymer Actuators and Devices (EAPAD), Vol. 5385, (2004), pp. 1–9. Heydt, R., and Chhokar, S., Refreshable braille display based on electroactive polymers, in Proceedings of the 23rd International Display Research Conference, IDRC 03, Vol. P7.5, Sponsored by the Society for Information Display, Phoenix, Arizona, (September 16–18, 2003), pp. 111–114. Jung, K., J. Nam, Y. Lee, and H. Choi, Micro-inchworm robot actuated by artificial muscle ­actuator based on dielectric elastomer, in Proceedings of the 2004 SPIE EAP Actuators and Devices (EAPAD), Paper No. 5385-47, Vol. 5385, San Diego, CA, March 14–18, 2004. Katchalsky, A., and M. Zwick, Mechanochemistry and ion exchange, Journal of Polymer Science, Vol. 16, No. 82, (1955), pp. 221–234.

240

Biomimetics: Nature-Based Innovation

Kato, Y., T. Sekitani, M. Takamiya, M. Doi, K. Asaka, T. Sakurai, and T. Someya, Sheet-type braille ­displays by integrating organic field-effect transistors and polymeric actuators, IEEE Transactions on Electron Devices, Vol. 54, No. 2, (2007), pp. 202–209. Kawai, H., Piezoelectricity of poly(viny1idene fluoride), Japanese Journal of Applied Physics, Vol. 8, (1969), pp. 975–976. Kepler, R.G., Piezoelectricity, pyroelectricity and ferroelectricity in organic materials, Annual Review of Physical Chemistry, Vol. 29, (1978), pp. 497–518. Kiil, H.-E., and M. Benslimane, Scalable industrial manufacturing of DEAP, in Proceedings of SPIE EAPAD Conference XI, Smart Structures Symposium, Vol. 7287, SPIE Electronic Library Paper No. 7287-26, (2009). Kornbluh, R.D., R.E. Pelrine, Q. Pei, R. Heydt, S.E. Stanford, S. Oh, and J. Eckerle, Electroelastomers: Applications of dielectric elastomer transducers for actuation, generation and smart structures, in Proceedings of the SPIE Smart Structures and Materials, Vol. 4698, San Diego, California, (2002). Kornbluh, R.D., R.E. Pelrine, Q. Pei, M. Rosenthal, S. Stanford, N. Bonwit, R. Heydt, H. Prahlad, and S.V. Shastri, Application of dielectric elastomer EAP actuators, in Y. Bar-Cohen (Ed.), Electroactive Polymer (EAP) Actuators as Artificial Muscles—Reality, Potential and Challenges (2nd ed.), SPIE Press, Bellingham, Washington, (2004), pp. 529–581. Kovacs, G.M., and L. Düring, Contractive tension force stack actuator based on soft dielectric EAP, in Y. Bar-Cohen (Ed.), Proceedings of the 11th SPIE Annual Electroactive Polymer Actuators and Devices (EAPAD) Conference, Paper No. 7287-8, SPIE Press, Bellingham, Washington, (March 2009). Kuhn, W., B. Hargitay, A. Katchalsky, and H. Eisenberg, Reversible dilation and contraction by changing the state of ionization of high-polymer acid networks, Nature, Vol. 165, (1955), pp. 514–516, Liu, Z., and P. Calvert, Multilayer hydrogels and muscle-like actuators, Advanced Materials, Vol. 12, No. 4, (2000), pp. 288–291. Lovinger, A., Ferroelectric polymers, Science, Vol. 220, (1983), pp. 1115–1121. Mabboux, P., and K. Gleason, F-19 NMR characterization of electron beam irradiated vinyllidene fluoride-trifluoroethylene copolymers, Journal of Fluorine Chemistry, Vol. 113, (2002), p. 27. Madden, J.D.W., Actuator Selection Tool, http://www.actuatorweb.org/ (last visited January 12, 2010). Madden, J.D.W., P.G.A. Madden, and I.W. Hunter, Characterization of polypyrrole actuators: Modeling and performance, in Y. Bar-Cohen (Ed.), Proceedings of 3th Annual SPIE Electroactive Polymer Actuators and Devices (EAPAD) Conference, SPIE Press, Bellingham, Washington, (March 2001), pp. 72–83. Madden, J.D.W., N. Vandesteeg, P.A. Anquetil, P.G. Madden, A. Takshi, R.Z. Pytel, S.R. Lafontaine, P.A. Wieringa, and I.W. Hunter, Artificial muscle technology: Physical principles and naval prospects, IEEE Journal of Oceanic Engineering, Vol. 29, No. 3, (July 2004), pp. 706–728. Matysek, M., P. Lotz, and H.F. Schlaak, Tactile display with dielectric multilayer elastomer actuators, in Y. Bar-Cohen and T. Wallmersperger (Eds.), Proceedings of the SPIE, Electroactive Polymer Actuators and Devices (EAPAD), Vol. 7287, (2009), 72871D, doi:10.1117/12.819217. Mirfakhrai, T., J. Oh, M. Kozlov, E.C.W. Fok, M. Zhang, S. Fang, R.H. Baughman, and J.D.W. Madden, Electrochemical actuation of carbon nanotube yarns, Smart Materials and Structures, Vol. 16, (2007), p. S243. Mirfakhrai, T., J. Oh, M. Kozlov, E.C.W. Fok, M. Zhang, S. Fang, R.H. Baughman, and J.D. Madden, Carbon nanotube yarns as high load actuators and sensors, Advances in Science and Technology, Vol. 61, (2008), pp. 65–74. Mottaghitalaba, V., B. Xia, G.M. Spinksa, and G.G. Wallace, Polyaniline fibres containing single walled carbon nanotubes: Enhanced performance artificial muscles, Synthetic Metals, Vol. 156, Nos. 11–13, (2006), pp. 796–803, doi:10.1016/j. Müller, B., H. Deyhle, S. Mushkolaj, and M. Wieland, The challenges in artificial muscle research to treat incontinence, Swiss Medical Weekly, Vol. 139, (2009), pp. 591–595. Mussa-Ivaldi, S., Real brains for real robots, Nature, Vol. 408, (2000), pp. 305–306. Nalwa, H.S. (Ed.), Ferroelectric Polymers—Chemistry, Physics, and Applications, Marcel Dekker, Inc., New York, NY, (1995), ISBN 0-8247-9468-0.

Electroactive Polymer Actuators as Artificial Muscles

241

Nemat-Nasser, S., and C.W. Thomas, Ionomeric polymer-metal composites, in Y. Bar-Cohen (Ed.), Electroactive Polymer (EAP) Actuators as Artificial Muscles—Reality, Potential and Challenges (2nd ed.), SPIE Press, Bellingham, Washington, (2004), pp. 171–230. O’Brien, B.M., E.P. Calius, T. Inamura, S.Q. Xie, and I.A. Anderson, Dielectric elastomer switches: Towards artificial muscles that can think, Applied Physics A: Materials Science & Processing, Vol. 100, No. 2, (2010) pp. 385–389, DOI: 10.1007/s00339-010-5857-z. Oguro, K., K. Asaka, N. Fujiwara, K. Onishi, and S. Sewa, Polymer electrolyte actuator driven by low voltage, Proceedings of the Materials Research Society (MRS) Symposium on Electroactive Polymers (EAP), Boston, Massachusetts, (November 29–December 1, 1999). Oguro, K., Y. Kawami, and H. Takenaka, Bending of an ion-conducting polymer film-electrode composite by an electric stimulus at low voltage, Journal of Micromachine Society, Vol. 5, (1992), pp. 27–30. Ohigashi, H., Electromechanical properties of polarized polyvinylidene fluoride films as studied by the piezoelectric resonance method, Japanese Journal of Applied Physics, Vol. 47, No. 3, (1976), pp. 949–955, doi:10.1063/1.322685. Osada, Y., Chemical valves and gel actuators, Advanced Materials, Vol. 3, No. 2, (1991), pp. 107–108, doi:10.1002/adma.19910030209. Osada, Y., and M. Hasebe, Electrically activated mechanochemical devices using polyelectrolyte gels, Chemistry Letters, Vol. 14, No. 9, (1985), pp. 1285–1288. Osada, Y., and R. Kishi, Reversible volume change of microparticles in an electric field, Journal of the Chemical Society, Vol. 85, No. 3, (1989), pp. 655–662. Otero, T.F., H. Grande, and J. Rodriguez, A new model for electrochemical oxidation of polypyrrole under conformational relaxation control, Journal of Electroanalytical Chemistry, Vol. 394, (1995), pp. 211–216. Otero, T.F., J. Rodriguez, E. Angulo, and C. Santamaria, Artificial muscles from bilayer structures, Synthetic Metals, Vol. 55–57, (1993), pp. 3713–3717. Park, I.-S., K. Jung, D. Kim, S.-M., and K.J. Kim, Physical principles of ionic polymer-metal composites as electroactive actuators and sensors, Special Issue dedicated to EAP, Materials Research Society Bulletin, Vol. 33, No. 3, (March 2008), pp. 190–195. Pei, Q., and O. Inganas, Electrochemical application of the bending beam method: 1. Mass transport and volume changes in polypyrrole during redox, Journal of Physical Chemistry, Vol. 96, (1992), pp. 10507–10514. Pei, Q., R. Pelrine, M. Rosenthal, S. Stanford, H. Prahlad, and R. Kornbluh, Recent progress on ­electroelastomer artificial muscles and their application for biomimetic robots, in Y. Bar-Cohen (Ed.), Proceedings of the SPIE Electroactive Polymer Actuators and Devices (EAPAD) Conference, Vol. 5385, (2004), pp. 41–50. Pei, Q., R.E. Pelrine, S.E. Stanford, R.D. Kornbluh, M. Rosenthal, K. Meijer, and R.J. Full, Multifunctional electroelastomer rolls and their application for biomimetic robots, in Y. Bar-Cohen (Ed.), Proceedings of the SPIE, Smart Structures and Materials 2002: Industrial and Commercial Applications of Smart Structures Technologies, Vol. 4698, San Diego, California, (2002). Pei, Q., M.A. Rosenthal, R. Pelrine, S. Stanford, and R.D. Kornbluh, Multifunctional electro­elastomer roll actuators and their application for biomimetic walking robots, in Y. Bar-Cohen (Ed.), Proceedings of the SPIE Electroactive Polymer Actuators and Devices (EAPAD), Smart Structures and Materials, Vol. 5051, (2003), pp. 281–290. Pelrine, R., R. Kornbluh, and J.P. Joseph, Electrostriction of polymer dielectrics with compliant ­electrodes as a means of actuation, Sensors and Actuators A, Physical, Vol. 64, (1998), pp. 77–85. Pelrine, R., R. Kornbluh, Q. Pei, and J. Joseph, High-speed electrically actuated elastomers with strain greater than 100%, Science, 287, No. 5454, (2000), pp. 836–839. Pelrine, R.E., R.D. Kornbluh, Q. Pei, S.E. Stanford, S. Oh, J. Eckerle, R.J. Full, M. Rosenthal, and K. Meijer, Dielectric elastomer artificial muscle actuators: Toward biomimetic motion, in Y. BarCohen (Ed.), Proceedings of the SPIE, Electroactive Polymer Actuators and Devices (EAPAD) Conference, Vol. 4695, San Diego, California, (2002), pp. 126–137. ISSN 0-8194-4443-X, ISSN 0277-786X.

242

Biomimetics: Nature-Based Innovation

Pelrine, R., and J. Joseph, FY 1992 Final Report on Artificial Muscles for Small Robots, ITAD-3393-FR93-063, SRI International, Menlo Park, California, submitted to Micro Machine Center, MBR99 Bldg. 6F 67, Kanda-sakumagashi, Chiyoda-ku, Tokyo, 101-0026 Japan (1993). Pelrine, R., and J. Joseph, FY 1993 Final Report on Artificial Muscles for Small Robots, ITAD-4570-FR94-076, SRI International, Menlo Park, California, submitted to Micro Machine Center, MBR99 Bldg. 6F 67, Kanda-sakumagashi, Chiyoda-ku, Tokyo, 101-0026 Japan (1994). Qu, L., Q. Peng, L. Dai, G.M. Spinks, G.G. Wallace, and R.H. Baughman, Carbon nanotube electroactive polymers: Opportunities and challenges, Special Issue dedicated to EAP, Materials Research Society Bulletin, Vol. 33, No. 3, (2008), pp. 215–234. Rasmussen, L., C.J. Erickson, and L.D. Meixler, The development of electrically driven mechanochemical actuators that act as artificial muscle, in Proceedings of the SPIE Electroactive Polymers and Devises (EAPAD), Vol. 7287, (2009), pp. 7287E1–72871E-13. Ren, K., S. Liu, M. Lin, Y. Wang, and Q.M. Zhang, A compact electroactive polymer actuator suitable for refreshable braille display, Sensors and Actuators A, Physical, Vol. 143, No. 2, (2008), pp. 335–342, Roentgen, W.C., About the changes in shape and volume of dielectrics caused by electricity, in G.  Wiedemann (Ed.), Annual Physics and Chemistry Series, Vol. 11, John Ambrosius Barth Publisher, Leipzig, German, (1880), pp. 771–786, (In German). Sadeghipour, K., R. Salomon, and S. Neogi, Development of a novel electrochemically active membrane and ‘smart’ material based vibration sensor/damper, Smart Materials and Structures, Vol. 1, No. 1, (1992), pp. 172–179. Sansiñena, J.M., and V. Olazabal, Conductive polymers, in Y. Bar-Cohen (Ed.), Electroactive Polymer (EAP) Actuators as Artificial Muscles—Reality, Potential and Challenges (2nd ed.), SPIE Press, Bellingham, Washington, (2004), pp. 231–259. Senders, C.W., T.T. Tollefson, S. Curtiss, A. Wong-Foy, and H. Prahlad, Force requirements for artificial muscle to create an eyelid blink with eyelid sling, Archive of Facial Plastic Surgery, Vol. 12, No. 1, (2010), pp. 30–36. Sessler, G.M., Piezoelectricity in polyvinylidenefluoride, Journal of the Acoustical Society of America, Vol. 70, No. 6, (1981), pp. 1596–1608. Shahinpoor, M., Elastically-activated artificial muscles made with liquid crystal elastomers, in Y. ­Bar-Cohen (Ed.), Proceedings of the SPIE 7th Annual International Symposium on Smart Structures and Materials, EAPAD Conference, Vol. 3987, (2000), pp. 187–192, ISBN 0-8194-3605-4. Smela, E., Conjugated polymer actuators, Special Issue dedicated to EAP, Materials Research Society Bulletin, Vol. 33, No. 3, (2008), pp. 197–204. SPIE, EAP-in-Action Program, March 8, 2010, San Diego, CA, http://spie.org//app/program/ index.cfm?fuseaction=secategorydetail&catid=306&event_id=894245&export_id=x12536&ID =x37799&redir=x37799.xml (last visited January 14, 2010). Spinks, G.M., G.G. Wallace, R.H. Baughman, and L. Dai, Carbon nanotube actuators: Synthesis, ­properties and performance, in Y. Bar-Cohen (Ed.), Electroactive Polymer (EAP) Actuators as Artificial Muscles—Reality, Potential and Challenges (2nd ed.), SPIE Press, Bellingham, Washington, (2004), pp. 261–295. Spinks, G.M., and G.G. Wallace, Actuated pins for braille displays, Chapter 13, in E. Smela and F. Carpi (Eds.), Biomedical Applications of Electroactive Polymer Actuators, John Wiley & Sons, Hoboken, NJ (2009), ISBN-10: 0-470-77305-7. DOI: 10.1002/9780470744697.ch13 Su, J., J.S. Harrison, T. St. Clair, Y. Bar-Cohen, and S. Leary, Electrostrictive graft elastomers and applications, in MRS Symposium Proceedings, Vol. 600, Warrendale, PA, (1999), pp. 131–136. Takashima, W., T. Uesugi, M. Fukui, M. Kaneko, and K. Kaneto, Mechanochemoelectrical effect of polyaniline film, Synthetic Metals, Vol. 85, (1997), pp. 1395–1396. Tasaka, S., and S. Miyata, The origin of piezoelectricity in poly(vinylidene fluoride), Ferroelectrics, Vol. 32, (1981), pp. 17–23, doi:10.1080/00150198108238668. Wada, Y., Piezoelectricity and pyroelectricity of polymers, Japanese Journal of Applied Physics, Vol. 15, (1976), pp. 2041–2057.

Electroactive Polymer Actuators as Artificial Muscles

243

Wessberg, J., C.R. Stambaugh, J.D. Kralik, P.D. Beck, M. Lauback, J.C. Chapin, J. Kim, S.J. Biggs, M.A. Srinivasan, and M.A. Nicolelis, Real-time prediction of hard trajectory by ensembles of cortical neurons in primates, Nature, Vol. 408, (2000), pp. 361–365. Wu, Y., G. Alici, J.D.W. Madden, G.M. Spinks, and G.G. Wallace, Soft mechanical sensors through reverse actuation in polypyrrole, Advanced Functional Materials, Vol. 17, (2007), pp. 3216–3222. Wu, Y., G. Alici, G.M. Spinks, and G.G. Wallace, Fast trilayer polypyrrole bending actuators for high speed applications, Synthetic Metals, Vol. 156, (2006), pp. 1017–1022. Yu, Z., W. Yuan, P. Brochu, B. Chen, Z. Liu, and Q. Pei, Large-strain, rigid-to-rigid deformation of bistable electroactive polymers, Applied Physics Letters, Vol. 95, No. 19, (2009), p. 192904, doi:10.1063/1.3263729. Zama, T., S. Hara, W. Takashima, and K. Kaneto, Comparison of conducting polymer actuators based on polypyrrole doped with BF4(–), PF6(–), CF3SO3–, and CLO4, Bulletin of the Chemical Society of Japan, Vol. 78, (2005), pp. 506–511. Zhang, Q.M., V. Bharti, and X. Zhao, Giant electrostriction and relaxor ferroelectric behavior in electron-irradiated poly(vinylidene fluoride-trifluorethylene) copolymer, Science, Vol. 280, (1998), pp. 2101–2104. Zhang, Q., C. Huang, and F. Xia, Electric field activated EAP, in Y. Bar-Cohen (Ed.), Electroactive Polymer (EAP) Actuators as Artificial Muscles—Reality, Potential and Challenges (2nd ed.), SPIE Press, Bellingham, Washington, (2004), pp. 95–148. Zhenyi, M., J.I. Scheinbeim, J.W. Lee, and B.A. Newman, High field electrostrictive response of polymers, Journal of Polymer Science B, Polymer Physics, Vol. 32, No. 16, (1994), pp. 2721–2731. Zhongyang, C., and Q. Zhang, Field-activated electroactive polymers, Special Issue dedicated to EAP, Materials Research Society Bulletin, Vol. 33, No. 3, (March 2008), pp. 183–187.

Web Sites WW-EAP Webhub, http://eap.jpl.nasa.gov Books and proceedings, http://ndeaa.jpl.nasa.gov/nasa-nde/yosi/yosi-books.htm WW-EAP Newsletter, http://ndeaa.jpl.nasa.gov/nasa-nde/lommas/eap/WW-EAP-Newsletter.html EAP Conferences, http://ndeaa.jpl.nasa.gov/nasa-nde/lommas/eap/eap-conferences.htm Armwrestling Challenge, http://ndeaa.jpl.nasa.gov/nasa-nde/lommas/eap/EAP-armwrestling.htm Information about the process of making the leading EAP materials, http://ndeaa.jpl.nasa.gov/ nasa-nde/lommas/eap/EAP-recipe.htm Sources of obtaining EAP materials, http://ndeaa.jpl.nasa.gov/nasa-nde/lommas/eap/­EAPmaterial-n-products.htm Using EAP to blink, http://archfaci.ama-assn.org/cgi/content/short/12/1/30?home, http://www. scientificamerican.com/podcast/episode.cfm?id=artificial-muscle-brings-back-blink-10-01-19

7 Refreshable Braille Displays Actuated by Electroactive Polymers Yoseph Bar-Cohen California Institute of Technology Pasadena, California CONTENTS 7.1 Introduction—Historical Perspective of Braille ............................................................ 245 7.2 Commercial and Nonelectroactive-Polymer-Actuated RBDs ..................................... 247 7.3 Refreshable Tactile Braille Display Using Electroactive Polymer ............................... 251 7.4 EAP-Based RBDs ................................................................................................................ 251 7.4.1 Ionic EAP................................................................................................................. 252 7.4.1.1 Ionic Polymer Metal Composite ............................................................ 252 7.4.1.2 Conducting Polymers ............................................................................. 253 7.4.2 Field-Activated EAP ..............................................................................................254 7.4.2.1 Roll Actuator Made of Terpolymer Ferroelectric EAP ......................254 7.4.2.2 Dielectric Elastomer Actuator ............................................................... 255 7.4.3 Bistable Electroactive Polymer ............................................................................. 259 7.4.4 Braille Printer Using Refreshable Shape-Memory Polymer Paper ................. 259 7.5 Conclusions......................................................................................................................... 260 Acknowledgments ...................................................................................................................... 263 References..................................................................................................................................... 263 Web Sites....................................................................................................................................... 265

7.1  Introduction—Historical Perspective of Braille Blindness is an impairment that hampers the ability to benefit from most of those things that seeing allows. The World Health Organization estimates that globally there are about 314 million people who are visually impaired, of whom 45 million are blind. Extensive efforts are made to eliminate blindness by direct medical attention and by developing tools that assist the blind to live an independent, normal life as much as possible. On the medical side of these efforts, a global initiative known as “Vision 2020: The Right to Sight” was established as a partnership between the World Health Organization and the International Agency for the Prevention of Blindness. This initiative was launched in 1999 with the goals of eliminating avoidable blindness by the year 2020 and preventing the projected doubling of avoidable visual impairment between 1990 and 2020. The ultimate goal of this initiative is to integrate a sustainable, comprehensive, high-quality, equitable eye care system into strengthened national healthcare systems (Dixon, 2000). Over the years, many tools have been developed to assist blind persons to live an independent, normal life. A white cane with a red tip is used while walking for avoiding obstacles 245

246

Biomimetics: Nature-Based Innovation

and extending the user’s range of touch. The user swings the cane in a low sweeping motion across the walking path to detect obstacles. Guide dogs are used by a small number of blind people to assist in mobility; where the dog is trained to walk through a desired path while guiding the blind person. Government actions in various countries have made public facilities and transportation more accessible to blind people. Tactile paving (widely used in Japan and in the United States) and audible traffic signals are making it easier and safer for visually impaired pedestrians to safely cross streets independently. Combined with governments’ mandate, the right-of-way that is given to users of white canes or guide dogs further enhances the ability of persons with visual impairments to walk safely in public areas. For visually impaired people, reading is one of the issues that constrain their activity. The writing code that is well known as braille carries the name of the Frenchman, Louis Braille, who developed it in 1825 when he was only 16 years old. He developed this code as a modification of the night writing communication method based on raised dots that was developed by Capt. Charles Barbier. Barbier was a French army officer who developed his code in response to Napoleon’s demand. Napoleon sought to provide the French soldiers at the battlefield with the ability to communicate silently at night without light. The complexity of learning Barbier’s code led to its rejection by the soldiers of the French military. Following the rejection, Barbier presented his invented code at the museum of industrial products where students of the Royal Institute for Blind Youth of Paris were also exhibiting their system of reading and writing. These students’ interest in his code led him to agreeing with their high school principal to teach his system to a selected number of students, which included Louis Braille. The latter took the 12-dot code of Barbier and reduced it to 6 dots per character and arranged them in a rectangular shape along two columns of three dots each (see Figure 7.1, left). In 1829, he used his code to publish the first book in braille, and he entitled it Method of Writing Words, Music, and Plain Songs by Means of Dots, for Use by the Blind and Arranged for Them. This modification made the code much easier for the fingertips to sense through touch and enabled significantly faster reading. This arrangement of six dots allows 64 combinations that include the blank character with no raised dots. Particular permutations are described by naming the positions where the dots are raised. The dot positions are universally numbered 1–3 from top to bottom on the left and 4–6 from top to bottom on the right. Like visible text, braille text is written along horizontal lines that are separated by a space so that one can differentiate the braille text in vertically adjacent lines. Punctuation, numbers, and uppercase letters can all be represented, although some require multiple cell representations. The extension of the braille code to eight dots (see Figure 7.1, right) with two rows of four dots increased the possible combinations to 256, representing all possible ASCII printable characters in a single cell. When braille is adapted to languages that do not use the Latin alphabet, the cells are assigned according to how they are transliterated into the Latin alphabet while the writing

Prefix all uppercase

EAP

EAP

FIGURE 7.1 EAP in uppercase expressed in braille code in the six- and eight-dot systems. (Based on input from Runyan, N., National Braille Press, Center for Braille Innovation, Boston, MA, personal e-mail communication with the author, May 2009.)

Refreshable Braille Displays Actuated by Electroactive Polymers

247

order (as in Hebrew, Arabic, and Chinese) is disregarded. There are three standard grades of braille: Grade 1: The beginners’ level of simple characters without contractions. Grade 2: Braille consists of a complex system of customs, styles, and practices. This grade consists of many shorthand words and was introduced as a spacesaving alternative to Grade 1 braille. Grade 3: Braille is a system that includes many additional multiple cell contractions, and it is almost like shorthand writing. This grade is used mostly by individuals for their own personal convenience, and it is not used in formal publications. As an alternative to the three grades of braille, Computer Braille is often used. It is a code that has a direct one-for-one mapping between braille and print characters and represents numbers and upper- or lowercase letters without multiple cell codes. The dimensions of a standard paper braille page are 28 cm × 30 cm (11 in. × 11.5 in.), and it has room for 25 lines of up to 43 characters each. The dot diameter in a braille cell is in the range 1.27–1.65 mm, and the height of the raised dots is approximately 0.5 mm. These standard dimensions serve the developers of refreshable braille displays (RBDs) as guiding specifications. However, there are additional requirements that need to be met in the active form of the braille code. These include the desired force that is needed to create a reasonable reading sensation, which is at least 0.15 N for each refreshable braille dot (Kato et al., 2007; Matysek et al., 2009). Table 7.1 summarizes the typical parameters that are used as well as the desired values, which provide a guideline for the development of such displays. The minute dimensions of the dot size, height, and spacing pose a great challenge to the developers of RBDs. This challenge to developing suitable RBDs and the high cost of current RBDs are reasons that the visually impaired population has not yet been able to fully benefit from digital communication, Internet-related capabilities, enormous information archives, and educational, employment, and recreational opportunities.

7.2  Commercial and Nonelectroactive-Polymer-Actuated RBDs As opposed to visual perception of conventional text, reading with braille systems involves tactile perception and developing the required skill to be able to read. An average adult braille reader may read at a rate of 80–100 words per minute. It usually takes at least 12–18 months to reach such a reading speed. The limited availability of braille training, reading devices, and reading materials in braille is claimed to have a major influence on the lack of braille literacy (Runyan and Nassimbene, 1974; Dixon, 2000). Competition from speech synthesizers and related audio technologies used by the blind is another factor inhibiting the increase of braille literacy. The development of RBDs has been quite limited because of the technological challenges that are associated with the dense packing of actuated pins to represent the characters. The small size and height of the dots, the close spacing between them, and the distance between the cells pose challenges to producing a full-screen refreshable display. An example of one braille cell of eight piezoelectric lever-actuated dots

248

Biomimetics: Nature-Based Innovation

TABLE 7.1 Specifications for Braille Dots Parameter

Nominal

Dot diameter Dot height (assuming no depression force from user’s finger) Dot height uniformity for adjacent dots Minimum displacement below the reading surface of an unraised, hemispherical dot top Dot spacing Distance between centers of perpendicularly adjacent dots in the same cell Distance between centers of corresponding dots in horizontally adjacent cells Height of six-dot braille cell lines Height of eight-dot braille cell lines Cell/device height Timing Setup time Dot cycling rates

Typical

1.5 ±0.1 mm 0.5 mm (–0.02 + 0.1 mm)

1.50 mm 0.50 mm

±0.05 mm 0.025 mm

±0.05 mm

2.5 mm (–0.2 + 0.1 mm)

2.45 mm

6.35 mm (–0.25 + 0.15 mm)

6.42 mm

10.75 mm (–0.75 + 0.4 mm) 13.25 mm (–0.1 + 0.5 mm)  0

m > 0

Biomimetic Swimmer Inspired by the Manta Ray

509

While some of this work was going on for general pin-jointed structures, researchers were beginning to focus in on tensegrity structures, which are class IV structures with the constraint that the cables must not go slack. Initial efforts in tensegrity research were trying to answer the form-finding problem from a static approach which consisted of questions about the prestress states of a tensegrity structure and the stability of those states. Connelly and Whiteley discussed the prestress stability of tensegrity structures from an energy formulation (Connelly and Whiteley, 1996) and later Connelly (1998) described how the form finding question could be cast into group theory. The order of the mechanisms of a statically and kinematically indeterminate structure, critical to the rigidity and stability of the structure was discussed by Vassart et al. (2000). Vassart and Motro (1999) also introduced Schek’s (1974) force density method to tensegrity form finding giving a numerical approach to finding many unique configurations. In contrast to the numerical approach, Sultan et al. (2001) derived some analytical solutions for cylindrically symmetric tensegrities. Masic et al. (2005) then later introduced an algebraic approach to tensegrity form finding that included shape constraints and took advantage of structural symmetries to calculate the initial configuration of a tensegrity. At the end of the twentieth century, research began to shift away from form finding to the load-bearing capabilities of tensegrities. Kebiche et al. (1999) developed a numerical method utilizing the global tangent stiffness matrix of a tensegrity structure that had a geometrically nonlinear correction term. It was shown that tensegrity structures rigidify under most loading conditions. The geometrically nonlinear analysis was further expanded to include material nonlinearities (Ben Kahla and Kebiche, 2000). A finite element approach was developed by Murakami (2001) where he showed that the prestress and mechanism modes characterize the response of a tensegrity structure. Then, Williamson et al. (2003) derived the necessary and sufficient condition for tensegrity equilibria and showed some analytical examples. Quirant et al. (2003) analyzed a plate structure under a distributed load and showed the sensitivity of the force densities to member length manufacturing errors. Sultan et al. (2000) showed an application of a tensegrity as the active base for a flight simulator. Sultan and Skelton (2004) developed another tensegrity application with a force and torque sensor. Recently, Schenk et al. (2007) showed a strategy for introducing finite mechanisms into tensegrity structures by using zero rest length springs, which is useful for low energy deformations of active structures. In the past 5 years, some optimization of the mechanical performance (e.g., target stiffness or minimal mass) for tensegrity structures have been developed (deJager and Skelton, 2004; Masic, 2004; Masic et al., 2006; de Oliveira and Chan, 2006). The dynamics of tensegrity systems have also been extensively studied over the past 10 years. Kahla et al. (2000) studied the nonlinear dynamics of a tensegrity beam by using a Lagrangian formulation of the equations of motion. Murakami (2001) developed the full nonlinear equations of motion with a finite element approach for cylindrically symmetric tensegrity structures. Skelton et al. (2001) derived the dynamics for cylindrically symmetric structures that give explicit analytical equations by circumventing the need to invert the mass matrix. Sultan et al. (2002) derives the analytical linearized dynamics for a specialized structure. Oliveira (2006) and Skelton (2006) present two general formulations for the dynamics of tensegrity systems. One formulation uses a classic vector differential equation, while the other formulation uses a matrix differential equation. With more research going into the dynamics of the tensegrity structures, there was a natural extension of research into the control of active tensegrity structures. One type of active structure—deployable structures—has been a niche for tensegrities due to their high strength-to-mass and cable elements. Many studies have researched deployable

510

Biomimetics: Nature-Based Innovation

tensegrities (Knight, 2000; Tibert and Pellegrino, 2002; Sultan and Skelton, 2003; Masic and Skelton, 2005). Smaili et al. (2004) and Motro (2003) introduced the concept of routing a cable throughout a tensegrity in the context of deploying the structure; however the concept was only described for a particular configuration and routing path. No model was presented nor was the concept generalized. Other types of active structures have also been studied. Some were developed to improve the vibrational response of a tensegrity (Djouadi et al., 1998; Kanchanasaratool and Williamson, 2002; Aversend et al., 2005) and were thus designed for small displacements. A few other studies have worked on large deformation active structures where the geometric nonlinearities need to be included (Pinaud et al., 2003; Masic and Skelton, 2005; Arsenault and Gosselin, 2006a,b). These studies either are purely theoretical work with strategies that are difficult to physically implement or the activation strategy is only reasonable for structures with small numbers of active elements. Activation strategies that are easy to physically implement, when there are many active members, are lacking. Recently, optimization techniques have been applied to active structures. Fest et al. (2003) have studied the use of a stochastic search algorithm to determine how actuators in a tensegrity need to be activated to keep a plane of three nodal level when loads are applied. Aldrich et al. (2003) described a simultaneous control/structure optimization to find the minimum-time reconfiguration of the system given a set of actuators with saturation. It is currently proposed to utilize the unique actuation properties of tensegrity structures for the development of an artificial batoid pectoral fin. They offer some appealing advantages over other more conventional approaches: • They can undergo large complex 3-D shape changes yet maintain high out-ofplane stiffness (able to create both curved span-wise deformation and a chordwise traveling wave). • They have low power consumption for activation due to structural efficiency (high strength-to-weight), energy-efficient deformation modes, and actuator clustering (to be discussed). • High strength-to-mass ratios (stretch dominated structures). • The cable set of members allows for actuators to be moved out of the flapping wings, lowering the inertia of the structure for more power efficiency and higher bandwidth of operation. • Low mechanical wear in dynamical applications. Moored and Bart-Smith (2007) have analyzed the design of a tensegrity structure that is required to perform large amplitude deformation. Their original focus was on the analysis and optimization of tensegrity structures with embedded actuation. In this case, actuators replace cable elements. Using a modified version of the virtual work method, they studied the actuation response of tensegrity beams and plates. The nonlinear algebraic equilibrium equations were derived and solved to find the overall topology of the structure due to actuation of an individual or collection of actuators. In this study, the quasistatic actuation response of a candidate tensegrity structure was explored to understand its potential to replicate the high amplitude motions of a batoid pectoral fin. Figure 17.12 shows the downward deflection of a tensegrity beam that consists of periodic four-strut prismatic unit cells. This type of structure—based on bar-to-bar connections—is classified as a Type 2 structure. Figure 17.13 shows a wing configuration with a planform shape of a cownose ray. This example has 19 four-strut cells with bar-to-bar connections—consists

511

Biomimetic Swimmer Inspired by the Manta Ray

50

z axis (mm)

0 –50

–100 –150 –200 –250 –300

0

100

200

300 x axis (mm)

400

500

600

FIGURE 17.12 61% downward deflection of a seven cell beam due to 20% contraction of the span-wise bottom cables. (From Moored, K., and Bart-Smith, H., The analysis of tensegrity structures for the design of a morphing wing, J. Appl. Mech., 74, 668–676, 2007. With permission.) Side view

Perspective view

300 300 200 z axis (mm)

z axis (mm)

200 100 0

0 –100 –200

–100

–300

–200 –300

100

0

100 200 300 x axis (mm)

400

500

0

–200

–400 y axis (mm)

0

500 300400 200 100 x axis (mm)

FIGURE 17.13 63% downward and 60% upward deflection of a 19 cell cownose ray shaped wing due to 20% contraction of the bottom span-wise cables and 20% contraction of the top cables, respectively. (From Moored, K., and ­Bart-Smith,  H., The analysis of tensegrity structures for the design of a morphing wing, J. Appl. Mech., 74, 668–676, 2007. With permission.)

of 279 members and is classified as a Type 4 tensegrity structure. In this example, the bottom cables are contracted by 20% causing a 63% downward deflection and the top cables are also actuated by 20% causing a 60% upward deflection. These results clearly demonstrate the potential for these structures to achieve the manta ray’s kinematics observed in nature. Prior to the work carried out by Moored and Bart-Smith (2009), the state of the art in active tensegrity structure research was to allow every cable member in the structure to be an actuator. However, embedded actuation is problematic for several reasons. Many actuators are needed to achieve a complex deformation, which adds significant mass to

512

Biomimetics: Nature-Based Innovation

the dynamic system. Additionally, the size of the actuator limits the design of the overall structure. As mentioned above, it is possible to design an active tensegrity structure where the actuator can be removed from the structure. This type of actuation is referred to as clustered actuation and can either be classified as cable-routed clustered actuation or strutrouted clustered actuation (Figure 17.14). Both cable- and strut-routed actuation reduce the number of actuators in a tensegrity structure, migrate the actuators out of the active fin structure, and relieve element size constraints. New governing equilibrium equations have been derived and the impact of clustered actuation on the prestress states and mechanisms of a tensegrity structure has been evaluated (details can be found in Moored and Bart-Smith, 2007, 2009). The clustered tangent stiffness matrix has also been derived and the influence of clustered actuation on the stability of a tensegrity structure determined. Moreover, the actuation response and loading response have been investigated in both cable-routed and strut-routed active tensegrity structures. An example of the actuation response of a cable-routed actuating tensegrity structure can be found in Figure 17.15. These analysis tools enable researchers to design and fabricate simple active tensegrity structures that can undergo complex deformations. Experimental verification of tensegrity structures is a critical aspect in determining their suitability for flapping wings. In order to evaluate the mechanics models, it is necessary to develop a methodology for constructing active tensegrity structures. Moreover, these physical models must be built and tested to establish their feasibility in the development of a bioinspired pectoral fin that can achieve kinematic deformations approximating that of a biological specimen. This is currently a focus for researchers. 17.4.2  Control Most animal locomotion such as walking, swimming, crawling, and flying, are realized by effective coupling of rhythmic body movements with the surrounding environment. For manta ray swimming, for example, the body acts as a “mechanical rectifier” when interacting with the environment (fluid), converting the local oscillatory wing motion into the global forward velocity. Biologists have found evidence (Brown, 1911; Delcomyn, 1980) that such rhythmic body movements are controlled by certain neuronal elements called central pattern generators (CPGs). The CPG receives sensory feedback signals from the body and

(a)

(b)

(c)

FIGURE 17.14 (See color insert.) Schematic of different actuation strategies. (a) Embedded actuation. (b) Cable-routed actuation. (c) Strut-routed actuation.

513

Biomimetic Swimmer Inspired by the Manta Ray

10

10 6

z axis (cm)

z axis (cm)

8 4 2 0 –2 –4 –6 0

5

10

15

20

8 6 4 2 0 –2 –4 –6 10

5

y axis (cm)

x axis (cm)

Active cluster Passive cable Pinned node

0 –5

0

5

15 10 x axis (cm)

20

FIGURE 17.15 A class 1 tensegrity beam composed of three unit cells with four cable-routed clusters. There are about 84 prestress modes for the classic structure, which is reduced to 1 global prestress mode for the clustered scenario. The top two clusters are contracted by 10% while the bottom two clusters are expanded by 10% to bend the structure in the span-wise direction. (From Moored, K.W., and Bart-Smith, H., Investigation of clustered actuation in tensegrity structures, Int. J. Solids Struct., 46, 3272–3281, 2009. With permission.)

Brain

Environment

CPG

Body Velocity

Sensory signal FIGURE 17.16 (See color insert.) Animal locomotion mechanism.

a high level (nondescriptive) command from the brain whose decision is made based on the environmental information (Figure 17.16). CPGs have been extensively studied for a wide variety of vertebrates and invertebrates, and their mathematical models have been developed and validated through carefully designed experiments (Cohen et al., 1988; Koch and Segev, 1989; Friesen and Friesen, 1994; Orlovsky et al., 1994). The cell membrane potentials of the neurons within a CPG oscillate at a certain frequency with specific phase relations, generating a pattern for the muscle activation. For instance, the body waves traveling from head to tail in the swimming motion of lampreys or leeches are generated by CPGs formed by weakly coupled segmental oscillators in a chain (Cohen et al., 1992; Williams, 1992; Zheng et al., 2007). With sensory feedback, the CPG modifies its oscillation pattern to conform to the biomechanical and environmental constraints (Cohen, 1992; Ekeberg, 1993; Hatsopoulos, 1996; Cang and Friesen, 2000; Iwasaki and Zheng, 2006).

514

Biomimetics: Nature-Based Innovation

The state of the art control theories cannot achieve natural motion: For the design of vehicles (or robotic systems) inspired by animal locomotion, a focus has been placed on mimicking the body movements, but not on the profound control mechanism underlying the motion. The standard control approach so far (e.g. in robotics (Spong and Vidyasagar, 1989), mechatronics (Ortega et al., 1998), and linear control (Zhou et al., 1996)) has been motion planning followed by feedback regulations. A limitation of this two-step approach is the difficulty in cooperating with the environment due to the fact that the trajectory generator is often outside of the feedback loop. Forcing the body to follow a prescribed motion can disturb the environment (e.g., fluid flow), leading to less efficient and potentially “noisy” locomotion (submitting unnatural acoustic signature). CPGs generate natural motion exploiting resonance: On the other hand, CPGs placed in the sensory feedback loop integrate the motion planning and feedback regulation into one step so that appropriate patterns are adaptively generated in response to environmental changes. Animals seem to utilize mechanical resonance to achieve efficient locomotion (Kugler and Turvey, 1987; Hatsopoulos, 1996). For instance, walking frequency scales with the square root of the reciprocal of the body height (Pennycuick, 1975; Alexander and Jayes, 1983; Holt et al., 1990). The wing beat frequency of some insects and birds scales with the inertia raised to the power close to −0.5 (Sotavalta, 1954), and roughly with the inverse of the wing length (Greenewalt, 1975). The CPG is capable of detecting the resonance and generating a gait that is natural for the given body biomechanics and environmental dynamics. Swimming animals can potentially lower the energy expended for locomotion by using appropriately tuned, elastic springs (Pabst, 1996). Thus, adopting CPGs as the basic control architecture would be essential for successful design of biologically inspired vehicles. Mechanisms of resonance entrainment have recently been revealed: While entrainment to resonance appears prevailing in rhythmic animal movements, there are only a few results in the literature on formal analysis of the phenomena (Hatsopoulos, 1996; Williams 1998; Verdaasdonk et al., 2006; Simoni and DeWeerth, 2007; Verdaasdonk et al., 2007). Recent studies of a co-PI (Iwasaki and Zheng, 2006; Futakata and Iwasaki, 2007) have shown that there are two basic mechanisms for entrainment of a CPG to mechanical resonance: positive rate feedback and negative integral feedback. In the former (latter) case, the CPG entrains to the resonance frequency lower (higher) than the intrinsic frequency of the CPG. The analytical result in Futakata and Iwasaki (2007) also indicated that the oscillation frequency of the coupled system is closer to the resonance frequency if the intrinsic CPG frequency is further away from resonance. Hence, if a CPG has been optimized over generations to achieve efficient locomotion exploiting a biomechanical resonance, then its intrinsic frequency should be away from the resonance frequency. In fact, studies of certain animals have revealed that the intrinsic frequency of a CPG is different from the frequency of rhythmic body movements during locomotion. For instance, undulation frequencies in swimming leeches and lampreys are typically larger than the intrinsic CPG frequency by a factor of two or more (Wallén and Willliams, 1984; Yu et al., 1999; Friesen and Cang, 2001). Bliss et al. (2006, 2008) have demonstrated the potential for the neural-based control of an active tensegrity structure. The overriding objective of this work is to uncover the fundamental control mechanism for generating natural body movements in manta ray swimming, and to establish a theory for designing CPG-based feedback controllers that achieve optimally efficient, stealthy, undulatory, or flapping wing propulsion, maneuvering, and station keeping of underwater vehicles. Initial focus has been on using a Reciprocal Inhibition Oscillator (RIO) as the CPG to control the actuation of two cables within a tensegrity beam to achieve entrainment of the first mode.

Biomimetic Swimmer Inspired by the Manta Ray

515

17.5  Prospectus Our approach to the BAUV fin propulsor is to develop a propulsor that mimics the biological principles and kinematics of the batoid rays. These animals have been chosen because of their large pectoral fins, which are capable of both undulation and flapping, and their potential high efficiency propulsion, maneuvering, and stealth capabilities. The ability to both flap and undulate provides the capability for various gaits enabling speed or agility as needed. This large surface area distributes the propulsive loads, which at the same time are responsible for dipolar acoustic radiation. The distribution of the dipoles has the capability to increase the order of the effective pole and thus reduce the noise signature of the BAUV. Furthermore, the undulatory nature of the propulsion will give rise to further potential cancellation of the far-field noise signature due to phase cancellation. Use of the manta ray design as a model for a BAUV presents a number of challenges. These animals swim by oscillation of broad fins that are seamlessly integrated into the body. The fins can simultaneously operate for propulsion, maneuverability, and stability, while engineered vehicles have components that deal with each of these parameters separately. Activation and control of the complex movements of the fins will be important in operation of a batoid BAUV for operation in various aquatic environments. Stability due to external perturbations is necessary to function in nature. The central portion of the body of a batoid is relatively stiff which can simplify construction of a BAUV. A rigid hull permits control systems, actuators, and payload to be placed onboard in a sealed compartment. The use of tensegrity structures to actuate the flapping motion of the wings can further simplify construction of the BAUV. Control of this motion can use circuitry based on CPGs. By appropriately controlling the kinematics of the wings to maintain the Strouhal number within the optimal range, the BAUV can generate large amounts of thrust with high efficiency. As much as the construction of a BAUV based on myliobatoids, such as the manta, can benefit ocean exploration, such a biomimetic robot can act as a test bed for both biologists and engineers. To date, there is not a complete understanding of the flow field that develops around the various morphological designs of the pectoral fins of batoid fishes, and how the flow field contributes to efficiency and maneuverability. Additionally, the design and fabrication of an artificial wing that can produce the maneuverability, efficiency, and propulsion of these creatures has yet to be achieved.

Acknowledgments Appreciation is expressed to Andrew Bloch and Elizabeth Barchi for assistance in preparing the manuscript. This research was conducted under a grant by the Office of Naval Research, Multi-University Research Initiative (MURI) program (Robert Brizzolaria, Program Officer; Contract No. N00014–08-1–0642). HBS would also like to acknowledge the David and Lucille Packard Foundation and the National Science Foundation (Contract No. CMS-0384884). Also, the authors would like to express their appreciation of the very valuable comments and suggestions of the reviewers of this chapter. The reviewers were George V. Lauder,

516

Biomimetics: Nature-Based Innovation

Department of Organismic and Evolutionary Biology, Harvard University, Cambridge, MA; Rajat Mittal, Department of Mechanical Engineering, Johns Hopkins University, Baltimore, MD; and Mark Murray, United States Naval Academy, Annapolis, MD.

References Abbott I.H., and A.E. von Doenhoff, Theory of Wing Sections, Dover, New York, (1949), pp. 1–693. Aldrich J., R. Skelton, and K. Kreutz-Delgado, “Control synthesis for a class of light and agile robotic tensegrity structures,” American Control Conference Proceedings, (2003). Alexander R.M., and A.S. Jayes, “A dynamic similarity hypothesis for the gaits of quadrupedal mammals,” Journal of Zoology, Vol. 201, (1983), pp. 135–152. Anderson J.M., and N.K. Chhabra, “Maneuvering and stability performance of a robotic tuna,” Integrative and Comparative Biology, Vol. 42, No. 1, (2001), pp. 118–126. Anderson J.M., K. Streitlien, D.S. Barrett, and M.S. Triantafyllou, “Oscillating foils of high propulsive efficiency,” Journal of Fluid Mechanics, Vol. 360, (1998), pp. 41–72. Anton M., A. Punning, A. Aabloo, M. Listak, and M. Kruusmaa, “Towards a biomimetic EAP robot,” Proceedings of Towards Autonomous Robotic Systems (TAROS), (2004), pp. 1–7. Arena P., L. Fortuna, M. Frasca, and G. Sicurella, “An adaptive, self-organizing dynamical system for hierarchical control of bio-inspired locomotion,” IEEE Transactions on Systems, Man and Cybernetics, Part B, Vol. 34, No. 4, (2004), pp. 1823–1837. Arsenault M., and C. Gosselin, “Kinematic, static, and dynamic analysis of a spatial three-­degreeof-freedom tensegrity mechanism,” Journal of Mechanical Design, Vol. 128, (2006a), pp. 1061–1069. Arsenault M., and C. Gosselin, “Kinematic, static and dynamic analysis of a planar 2-DOF tensegrity mechanism,” Mechanism and Machine Theory, Vol. 41, No. 9, (2006b), pp. 1072–1089. Averseng J., J.F. Dube, B. Crosnier, and R. Motro, “Active control of a tensegrity plane grid,” Decision and Control, 2005 European Control Conference. CDC-ECC’05. 44th IEEE Conference (2005), pp. 6830–6834. Ayers J., N. Rulkov, D. Knudsen, Y.B. Kim, A. Volkovskii, and A. Selverston, “Controlling underwater robots with electronic nervous systems,” Proceedings of The Third International Symposium on Aero Aqua Bio-mechanisms, Okinawa, Japan, (2006). Ayers J., C. Wilbur, and C. Olcott, “Lamprey robots,” Proceedings of The International Symposium on Aqua-Biomechanism, (2000). Bandyopadhyay P.R., “Trends in biorobotic autonomous undersea vehicles,” IEEE Journal of Oceanic Engineering, Vol. 30, No. 1, (2005), pp. 109–139. Bannasch R., “Hydrodynamics of penguins—An experimental approach,” In P. Dann, I. Norman, and P. Reilly (Eds.), The Penguins: Ecology and Management, Surrey Beatty and Sons, Norton, NSW, Australia, (1995), pp. 141–176. Barrett D., M. Grosenbaugh, and M. Triantafyllou, “The optimal control of a flexible hull robotic undersea vehicle propelled by an oscillating foil,” Proceedings of the Symposium on Autonomous Underwater Vehicle Technology (AUV), (1996), pp. 1–9. Barrett D.S., M.S. Triantafyllou, D.K.P. Yue, M.A. Grosenbaugh, and M.J. Wolfgang, “Drag reduction in fish-like locomotion,” Journal of Fluid Mechanics, Vol. 392, (1999), pp. 183–212. Ben Kahla, N., and K. Kebiche, “Nonlinear elastoplastic analysis of tensegrity systems,” Engineering Structures, Vol. 22, No. 11, (2000), pp. 1552–1566. Bliss T., H. Bart-Smith, and T. Iwasaki, “Development of a morphing structure with the incorporation of central pattern generators,” Vol. 6173, 13th SPIE Annual International Symposium on Smart Structures and Materials, San Diego, CA, USA, (Feb. 26–March 2, 2006).

Biomimetic Swimmer Inspired by the Manta Ray

517

Bliss T., H. Bart-Smith, and T. Iwasaki, Response entertainment of tensegrity structures via CPG control. Automatica, In Review, (2011). Bliss T., T. Iwasaki, and H. Bart-Smith, “CPG control of a tensegrity morphing structure for biomimetic applications,” Advances in Science and Technology, Vol. 58, (2008), pp. 137–142. Borgen M.G., G.N. Washington, and G.L. Kinzel, “Design and evolution of a piezoelectrically actuated miniature swimming vehicle,” IEEE/ASME Transactions on Mechatronics, Vol. 8, No. 1, (2003), pp. 66–76. Bose N, “Performance of chordwise flexible oscillating propulsors using a time-domain panel method,” International Shipbuilding Progress, Vol. 42, (1995), pp. 281–294. Bose N., and J. Lien, “Propulsion of a fin whale (Balaenoptera physalus): Why the fin whale is a fast swimmer,” Proceedings of the Royal Society of London B, Vol. 237, (1989), pp. 175–200. Bose N., J. Lien, and J. Ahia, “Measurements of the bodies and flukes of several cetacean species,” Proceedings of the Royal Society of London B, Vol. 242, (1990), pp. 163–173. Breder C.M., Jr., “The locomotion of fishes,” Zoologica, Vol. 4, (1926), pp. 159–297. Brown, T.G., “The intrinsic factors in the act of progression in the mammal,” Proceedings of the Royal Society, Vol. 84, (1911), pp. 308–319. Bucholdt H., M. Davies, and M. Hussey, “The analysis of cable nets,” IMA Journal of Applied Mathematics, Vol. 4, No. 4, (1968), pp. 339–358. Buchholz J.H.J., and A.J. Smits, “On the evolution of the wake structure produced by a low aspect ratio pitching panel,” Journal of Fluid Mechanics, Vol. 546, (2006), pp. 433–443. Buchholz J.H.J., and A.J. Smits, “The wake structure and thrust performance of a rigid low-aspectratio pitching panel,” Journal of Fluid Mechanics, Vol. 603, (2008), pp. 331–365. Calladine C., “Buckminster Fuller's tensegrity structures and Clerk Maxwell's rules for the construction of stiff frames,” International Journal of Solids and Structures, Vol. 14, No. 2, (1978), pp. 161–172. Calladine C., “Modal stiffness of a pretensioned cable net,” International Journal of Solids and Structures, Vol. 18, No. 7, (1982), pp. 829–846. Cang J., and W.O. Friesen, “Sensory modification of leech swimming: rhythmic activity of ventral stretch receptors can change intersegmental phase relationships,” Journal of Neuroscience, Vol. 20, (2000), pp. 7822–7829. Childress S., Mechanics of Swimming and Flying. Cambridge Studies in Mathematical Biology. Cambridge University Press, (1981). Clark R.P., and A.J. Smits, “Thrust production and wake structure of a batoid-inspired oscillating fin,” Journal of Fluid Mechanics, Vol. 562, (2006), pp. 415–429. Cohen A.H., “The role of heterarchical control in the evolution of central pattern generator,” Brain Behavior and Evolution, Vol. 40, No. 23, (1992), pp. 112–124. Cohen A.H., G.B. Ermentrout, T. Kiemel, N. Kopell, K.A. Sigvardt, and T.L. Williams, “Modelling of intersegmental coordi¬nation in the lamprey central pattern generator for locomotion,” Trends in Neuroscience, Vol. 15, No. 11, (1992), pp. 434–438. Cohen A.H., S. Rossignol, and S. Grillner, Neural Control of Rhythmic Movements in Vertebrates, Wiley, New York, (1988). Colgate J.E., and K.M. Lynch, “Mechanics and control of swimming: a review,” IEEE Journal of Oceanic Engineering, Vol. 29, No, 3, (2004), pp. 660–673. Combes S.A., and T.L. Daniel, “Shape, flapping and flexion: wing and fin design for forward flight,” Journal of Experimental Biology, Vol. 204, No. 12, (2001), pp. 2073–2085. Compagno L.J.V., “Systematics and body form,“ Sharks, Skates, and Rays—The Biology of Elasmobranch Fishes, W.C. Hamlette (Ed.), The Johns Hopkins University Press, Baltimore, MD, (1999) pp. 1–42. Connelly R., and A. Back, “Mathematics and tensegrity,” American Scientist, Vol. 86, No. 2, (1998), pp. 142–151. Connelly R., and W. Whiteley, “Second-order rigidity and prestress stability for tensegrity frameworks,” SIAM Journal on Discrete Mathematics, Vol. 9, (1996), pp. 453–491.

518

Biomimetics: Nature-Based Innovation

Coughlin D.J., L. Valdes, and L.C. Rome, “Muscle length changes during swimming in scup: sonomicrometry verifies the anatomical high-speed cine technique,” Journal of Experimental Biology, Vol. 199, (1996), pp. 459–463. Crespi A., A. Badertscher, A. Guignard, and A.J. Ijspeert, “An amphibious robot capable of snake and lamprey-like locomotion,” Proceedings of the 35th International Symposium on Robotics (ISR), (2004). Daniel T., “Forward flapping flight from flexible fins,” Canadian Journal of Zoology, Vol. 66, (1988), pp. 630–638. de Jager B, and R.E. Skelton, “Symbolic stiffness optimization of planar tensegrity structures,” Journal of Intelligent Material Systems and Structures, Vol. 14, No. 3, (2004), pp. 181–193. Delcomyn F., “Neural basis of rhythmic behavior in animals,” Science, Vol. 210, (1980), pp. 492–498. de Oliveira M., “Dynamics of systems with rods, decision and control,” 45th IEEE Conference, (2006), pp. 2326–2331. de Oliveira M., and R. Chan, “Minimum mass design of tensegrity towers and plates, decision and control,” 45th IEEE Conference, (2006), pp. 2314–2319. Deacon K., P. Last, J.E. McCosker, L. Taylor, T.C. Tricas, and T.I. Walker, Sharks & Rays, Fog City Press, San Francisco, CA, (1997), pp. 1–288. Djouadi S., R. Motro, J.S. Pons, and B. Crosnier, “Active control of tensegrity systems,” Journal of Aerospace Engineering, Vol. 11, No. 2, (1998), pp. 37–44. Dong H., R. Mittal, M. Bozkurttas, and F. Najjar, “Wake structure and performance of finite aspect ratio flapping foils,” AIAA Paper 2005–0081, (2005). Douady C.J., M. Dosay, M.S. Shivji, and M.J. Stanhope, “Molecular phylogenetic evidence refuting the hypothesis of Batoidea (rays and skates) as derived sharks,” Molecular Phylogenetic and Evolution, Vol. 26, (2003), pp. 215–221. Ekeberg O., “A combined neuronal and mechanical model of fish swimming,” Biological Cybernetics, Vol. 69, No. 5/6, (1993), pp. 363–374. Fauci, L.J., and C.S. Peskin, “A computational model of aquatic animal locomotion,” Journal of Comparative Physiology, Vol. 77, (1988), pp. 85–108. Fest E., K. Shea, B. Domer, and I.F.C. Smith, “ASCE, adjustable tensegrity structures,” Journal of Structural Engineering, Vol. 129, (2003), pp. 515. Fish F.E., “Balancing requirements for stability and maneuverability in cetaceans,” Integrative and Comparative Biology, Vol. 42, (2002), pp. 85–93. Fish F.E., and J.M. Battle, “Hydrodynamic design of the humpback whale flipper,” Journal of Morphology, Vol. 225, (1995), pp. 51–60. Fish F.E., and A.J. Nicastro, “ Aquatic turning performance by the whirligig beetle: constraints on maneuverability by a rigid biological system,” Journal of Experimental Biology, Vol. 206, (2003), pp. 1649–1656. Fish F.E., and G.V. Lauder, “Passive and active flow control by swimming fishes and mammals,” Annual Review of Fluid Mechanics, Vol. 38, (2006), pp. 193–224. Fish F.E., G.V. Lauder, R. Mittal, A.H. Techet, M.S. Triantafyllou, J.A. Walker, and P.W. Webb. “Conceptual design for the construction of a biorobotic AUV based on biological hydrodynamics,” Proceedings of the 13th International Symposium on Unmanned Untethered Submersible Technology, Durham, NH, 2003. Friesen W.O., “Reciprocal inhibition: A mechanism underlying oscillatory animal movements,” Neurosci. and Biobehav. Rev., Vol. 18, No. 4, (1994), pp. 547–553. Friesen W.O., and J. Cang, “Sensory and central mechanisms control of intersegmental coordination,” Current Opinion in Neurobiology, Vol. 11, (2001), pp. 678–683. Friesen W.O., and J.A. Friesen, NeuroDynamix: Computer Models for Neurophysiology, Oxford University Press, Oxford, (1994). Fuller R., Tensile-Integrity Structures, U.S.P.a.T. Office, Editor. 1962: Washington DC, USA. (November 13, 1962). Futakata Y., and T. Iwasaki, “Biological control mechanisms underlying entrainment to mechanical resonance,” IEEE Conference Decision and Control, (2007).

Biomimetic Swimmer Inspired by the Manta Ray

519

Garner L.J., L.N. Wilson, D.C. Lagoudas, and O.K. Rediniotis, “Development of a shape memory alloy actuated biomimetic vehicle,” Smart Materials and Structures, Vol. 9, No. 5, (2000), pp. 673–683. Green M.A., and A.J. Smits, “Unsteady flowfields generated by flapping trapezoidal foils.” In preparation, (2010). Green M.A., and A.J. Smits, “Unsteady forces on flapping rigid foils,” Journal of Fluid Mechanics, Vol. 615, (2008), pp. 211–220. Green M.A., C.W. Rowley, and A.J. Smits, “Using hyperbolic Lagrangian coherent structures to investigate vortices in bio-inspired fluid flows,” Chaos, Vol. 20, No. 1, (2010). Greenewalt C.H., “The flight of birds,” Transactions of the American Philosophical Society, Vol. 66, (1975), pp. 21–23. Guglielmini L., P. Blondeaux, and G. Vittori, “A simple model of propulsive oscillating foils,” Oceanic Engineering, Vol. 31, (2004), pp. 883–899. Hanaor A., “Prestressed pin-jointed structures: flexibility analysis and prestress design,” Computers & Structures, Vol. 28, No. 6, (1988), pp. 757–769. Hatsopoulos N.G., “Coupling the neural and physical dynamics in rhythmic movements,” Neural Computation, Vol. 8, No. 3, (1996), pp. 567–581. Heathcote S., Z. Wang, and I. Gursul, “Effect of spanwise flexibility on flapping wing propulsion,” Proceedings of the 36th AIAA Fluid Dynamics Conference and Exhibit, (2006). Heine C., Mechanics of flapping fin locomotion in the cownose ray, Rhinoptera bonasus (Elasmobranchii: Myliobatidae), Biology, Duke University, Durham, NC, (1992). Holt K.G., J. Hamill, and R.O. Andres, “The force-driven harmonic oscillator as a model for human locomotion,” Human Movement Science, Vol. 9, (1990), pp. 55–68. Iwasaki, T., “Analysis and synthesis of central pattern generators via multivariable harmonic balance,” American Control Conference, (2006), 4106–4111. Iwasaki T., and B. Liu, “Feedback control with central pattern generator for decentralized coordination of prototype mechanical rectifier,” Proceedings of the American Control Conference, Vol. 4, (2004), pp. 3059–3064. Iwasaki T., and M. Zheng, “Sensory feedback mechanism underlying entrainment of central pattern generator to mechanical resonance,” Biological Cybernetics, Vol. 94, No. 4, (2006), pp. 245–261. Jones K.D., S.J. Duggan, and M.F. Platzer, “Flapping-wing propulsion for a micro air vehicle,” AIAA Paper, Vol. 126, (2001), p. 39. Kahla N., B. Moussa, and J. Pons, “Nonlinear dynamic analysis of tensegrity systems,” Journal of the International Association for Shell and Spatial Structures, Vol. 41, No. 1, (2000), pp. 49–58. Kanchanasaratool N., and D. Williamson, “Modelling and control of class NSP tensegrity structures,” International Journal of Control, Vol. 75, No. 2, (2002), pp. 123–139. Kato N., “Median and paired fin controllers for biomimetic marine vehicles,” Applied Mechanics Reviews, Vol. 58, (2005), pp. 238–252. Katz J., and D. Weihs, “Hydrodynamic propulsion by large amplitude oscillation of an airfoil with chordwise flexibility,” Journal of Fluid Mechanics, Vol. 88, (1978), pp. 485–497. Katz J. and D. Weihs, “Large amplitude unsteady motion of a flexible slender propulsor,” Journal of Fluid Mechanics, Vol. 90, (1979), pp. 713–723. Kebiche K., M.N. Kazi-Aoual, and R. Motro, “Geometrical non-linear analysis of tensegrity systems,” Engineering Structures, Vol. 21, No. 9, (1999), pp. 864–876,. Kenner H., Geodesic Math and How to Use It, University of California Press, Berkeley, (1976). Klausewitz W., “Der lokomotionsmodus der Flugelrochen (Myliobatoidei),” Zoologischer Anzeiger, Vol. 173, (1964), pp. 111–120. Knight, B., “Deployable antenna kinematics using tensegrity structure design,” Ph.D. thesis, University of Florida, Gainesville, FL (2000). Koch C., and I. Segev, Methods in Neuronal Modeling: From Synapses to Networks, MIT Press, Cambridge, MA (1989). Kugler P.N., and M.T. Turvey, Information, Natural Law, and the Self-Assembly of Rhythmic Movement, Lawrence Erlbaum, Hillsdale, NJ (1987).

520

Biomimetics: Nature-Based Innovation

Larrabee E.E. “The screw propeller,” Scientific American, Vol. 243, (1980), pp. 134–148. Lauder G.V., and E.G. Drucker, “Forces, fishes, and fluids: Hydrodynamic mechanisms of aquatic locomotion,” Physiology, Vol. 17, (2002), pp. 235–240. Lauder G.V., and E.G. Drucker, “Morphology and experimental hydrodynamics of fish fin control surfaces,” IEEE Journal of Ocean Engineering, Vol. 29, (2004), pp. 556–571. Lewin G.C., and H. Haj-Hariri, “Modelling thrust generation by heaving airfoils in viscous flows,” Journal of Fluid Mechanics, Vol. 492, (2003), pp. 339–362. Lewin G.C., and H. Haj-Hariri, “Reduced-order modeling of a heaving airfoil,” AIAA, Vol. 43, (2005), pp. 270–283. Liao J.C., D.N. Beal, G.V. Lauder, and M.S. Triantafyllou, “The Karman gait: Novel body kinematics of rainbow trout swimming in a vortex street,” Journal of Experimental Biology, Vol. 206, No. 6, (2003), pp. 1059–1073. Licht S., V. Polidoro, M. Flores, F.S. Hover, and M.S. Triantafyllou, “Design and projected performance of a flapping foil AUV,” IEEE Journal of Oceanic Engineering, Vol. 29, No. 3, (2004), pp. 786–794. Liem K.F., and Summers, A.P., “Muscular system: gross anatomy and functional morphology of muscles,“ Sharks, Skates, and Rays—The Biology of Elasmobranch Fishes, Hamlett W.C. (Ed.), Johns Hopkins University Press, Baltimore, MD, (1999), pp. 93–114. Lighthill M.J., “Note on the swimming of slender fish,” Journal of Fluid Mechanics, Vol. 9, (1960), pp. 305–317. Lighthill M.J., “Aquatic animal propulsion of high hydromechanical efficiency,” Journal of Fluid Mechanics, Vol. 44, (1970), pp. 265–301. Lighthill M.J., Mathematical Biofluiddynamics, Philadelphia: Society for Industrial and Applied Mathematics, (1975). Lindsey C.C. “Form, function, and locomotory habits in fish,” In Fish Physiology—Locomotion, Vol. 7, W.S. Hoar and DJ Randall (Eds.), Academic Press, New York, NY, (1978), pp. 1–100. Listak M., G. Martin, D. Pugal, A. Aabloo, and M. Kruusmaa, “Design of a semiautonomous biomimetic underwater vehicle for environmental monitoring,” Computational Intelligence in Robotics and Automation, Espoo, Finland, (2005). Liu H., “Computational biological fluid dynamics: Digitizing and visualizing animal swimming and flying,” Integrative and Comparative Biology, Vol. 42, No. 5, (2002), pp. 1050–1059. Liu P., and Bose N. “Propulsive performance of three naturally occurring oscillating propeller planforms,” Ocean Engineering, Vol. 20, (1993), pp. 57–75. Low K.H., and A. Willy, “Biomimetic motion planning of an undulating robotic fish fin,” Journal of Vibration and Control, Vol. 12, (2006), pp. 1337–1359. Marchaj C.A., Aero-Hydrodynamics of Sailing, International Marine Publications, Camden, Maine. Masic M., “Design, optimization, and control of tensegrity structures,” Dissertation, Ph.D dissertation, University of California, San Diego (2004). Masic M., and R. Skelton, “Path planning and open-loop shape control of modular tensegrity structures,” Journal of Guidance, Control and Dynamics, Vol. 28, (2005), pp. 421–430. Masic M., R.E. Skelton, and P.E. Gill, “Algebraic tensegrity form-finding,” International Journal of Solids and Structures, Vol. 42, No. 16–17, (2005), pp. 4833–4858. Masic M., R. Skelton, and P. Gill, “Optimization of tensegrity structures,” International Journal of Solids and Structures, Vol. 43, No. 16, (2006), pp. 4687–4703. Maxwell J., The Scientific Papers of James Clerk Maxwell, Dover Publications, New York, (2003). Mojarrad M. “AUV biomimetic propulsion,” Oceans 2000 MTS/IEEE Conference and Exhibition, Providence, RI, (2000). Moored, K., and H. Bart-Smith, “The analysis of tensegrity structures for the design of a morphing wing,” Journal of Applied Mechanics, Vol. 74, (2007), pp. 668–676. Moored, K.W., and H. Bart-Smith, “Investigation of clustered actuation in tensegrity structures,” International Journal of Solids and Structures, Vol. 46, No. 17, (2009), pp. 3272–3281. Morel Y., and A. Leonessa, “Adaptive nonlinear tracking control of an underactuated nonminimum phase model of a marine vehicle using ultimate boundedness,” Proceedings of 42nd IEEE Conference on Decision and Control, (2003).

Biomimetic Swimmer Inspired by the Manta Ray

521

Motro R., Tensegrity: Structural Systems for the Future, 1st Ed., Kogan Page Science, London, (2003). Murakami H., “Static and dynamic analyses of tensegrity structures. Part II. Quasi-static analysis,” International Journal of Solids and Structures, Vol. 38, No. 20, (2001), pp. 3615–3629. Murakami H., “Static and dynamic analyses of tensegrity structures. Part 1. Nonlinear equations of motion,” International Journal of Solids and Structures, Vol. 38, No. 20, (2001), pp. 3599–3613. Orlovsky G.N., T.G. Deliagina, and S. Grillner, Neuronal Control of Locomotion: From Mollusc to Man, Oxford University Press, Oxford, (1994). Ortega R., A. Loria, P. Nicklasson, and H. SiraRamirez, Passivity-Based Control of Euler Lagrange Systems, Springer-Verlag, London, (1998). Pabst D., “Springs in swimming animals,” American Zoologist, Vol. 36, No. 6, (1996), pp. 723–735. Palmisano J., R. Ramamurti, K.J. Lu, J. Cohen, W. Sandberg and B. Ratna, “Design of a biomimetic controlled-curvature Robotic pectoral fin,” IEEE International Conference on Robotics and Automation, pp. 966–973, 2007. Parkes E., Braced Frameworks, Pergamon Press, (1974). Pederzani J., and H. Haj-Hariri, “Numerical analysis of heaving flexible airfoils in a viscous flow,” AIAA, Vol. 44, (2006a), pp. 2773–2779. Pederzani J., and H. Haj-Hariri, “A numerical method for the analysis of flexible bodies in unsteady viscous flows,” International Journal of Numerical Methods in Engineering, Vol. 68, (2006b), pp. 1096–1112. Pellegrino S., “Analysis of prestressed mechanisms,” International Journal of Solids and Structures, Vol. 26, No. 12, (1990), pp. 1329–1350. Pellegrino S., “Structural computations with the singular value decomposition of the equilibrium matrix,” International Journal of Solids and Structures, Vol. 30, No. 21, (1993), pp. 3025–3035. Pellegrino S., and C. Calladine, “Matrix analysis of statically and kinemetically indeterminate frameworks,” International Journal of Solids and Structures, Vol. 22, (1986), pp. 409–428. Pennycuick C., “On the running of the gnu (Connochaetes taurinus) and other animals,” Journal of Experimental Biology, Vol. 63, (1975), pp. 775–799. Perlmutter A., Guide to Marine Fishes, New York University Press, New York, NY (1961), pp. 1–431. Pinaud, J.P., M. Masic, and R. Skelton, “Path planning for the deployment of tensegrity structures,” SPIE 10th Annual International Symposium on Smart Structures and Materials, San Diego, CA, (2003). Prempraneerach P., F.S. Hover, and M.S. Triantafyllou, “The effect of chordwise flexibility on the thrust and efficiency of a flapping foil,” Proceedings of the Thirteenth International Symposium on Unmanned Untethered Submersible Technology—Special Session on Bio-Engineering Research Related to Autonomous Underwater Vehicles, Autonomous Undersea Systems Institute, Lee, New Hampshire, (2003). Pugh A., An Introduction to Tensegrity, University of California Press, Berkeley, (1976). Punning A., M. Anton, M. Kruusmaa, and A. Aabloo, “A biologically inspired ray-like underwater robot with electroactive polymer pectoral fins,” Proceedings of the International IEEE Conference on Mechatronics and Robotics (MechRob’04), Vol. 2, pp. 241–245, 2004. Quirant, J., M.N. Kazi-Aoual, and R. Motro, “Designing tensegrity systems: The case of a double layer grid,” Engineering Structures, Vol. 25, No. 9, (2003), pp. 1121–1130. Rayner J.M.V., “A vortex theory of animal flight. Part 1. The vortex wake of a hovering animal,” Journal of Fluid Mechanics, Vol. 91, (1979a), pp. 697–730. Rayner J.M.V.,”A vortex theory of animal flight. Part 2. The forward flight of birds,” Journal of Fluid Mechanics, Vol. 91, (1979b), pp. 731–763. Rohr J.J., and F.E. Fish, “Strouhal numbers and optimization of swimming by odontocete cetaceans,” Journal of Experimental Biology, Vol. 207, (2004), pp. 1633–1642. Rome L.C., and A.A. Sosnicki, “Myofilament overlap in swimming carp. II. Sarcomere length changes during swimming,” American Journal of Physiology: Cell Physiology, Vol. 260, (1991), pp. C289–C296.

522

Biomimetics: Nature-Based Innovation

Rome L.C., I.H. Choi, G. Lutz, and A.A. Sosnicki, “The influence of temperature on muscle function in fast swimming scup. I. Shortening velocity and muscle recruitment during swimming,” Journal of Experimental Biology, Vol. 163, (1992), pp. 259–279. Rome L.C., R.P. Funke, R. McN. Alexander, G. Lutz, H. Aldridge, F. Scott, and M. Freadman, “Why animals have different muscle fibre types?,” Nature, Vol. 335, (1988), pp. 824–827. Rosenberger L.J., “Pectoral fin locomotion in Batoid fishes—Undulation versus oscillation,” Journal of Experimental Biology, Vol. 204, (2001), pp. 379–394. Rosenburger L.J., and M.W. Westneat, “Functional morphology of undulating pectoral fin locomotion in the stingray Taeniura Lymma (chondrichthyes: Dastyatidae),” Journal of Experimental Biology, Vol. 202, (1999), pp. 3523–3539. Schaefer J.T., and A.P. Summers, “Batoid wing skeletal structure: Novel morphologies, mechanical implications, and phylogenetic patterns,” Journal of Morphology, Vol. 264, (2005), pp. 298–313. Schek H.J., “The force density method for form finding and computation of general networks,” Computer Methods in Applied Mechanics and Engineering, Vol. 3, (1974), pp. 115–134. Schenk, M., S. Guest, and J. Herder, “Zero stiffness tensegrity structures,” International Journal of Solids and Structures, Vol. 44, No. 20, (2007), pp. 6569–6583. Sfakiotakis M., D.M. Lane, and J.B.C. Davies, “Review of fish swimming modes for aquatic locomotion,” IEEE Journal of Oceanic Engineering, Vol. 24, No. 2, (1999), pp. 237–252. Shaw W.C., “Sea animals and torpedoes,” NOTS TP 2299 (1959), U.S. Naval Ordinance Test Station, China Lake, CA. Simoni M.F., and S.P. DeWeerth, “Sensory feedback in a half-center oscillator model,” IEEE Transactions Biomedical Engineering, Vol. 45, No. 2, (2007), pp. 193–204. Skelton R., “Dynamics of tensegrity systems: Compact forms, decision and control,” 45th IEEE conference on Decision and Control, (2006), pp. 2276–2281. Skelton R.E., J.P. Pinaud, and D.L. Mingori, “Dynamics of the shell class of tensegrity structures,” Journal of the Franklin Institute-Engineering and Applied Mathematics, Vol. 338, No. 2–3, (2001), pp. 255–320. Smaili A., R. Motro, and V. Raducanu, “New concepts for deployable tensegrity systems,” IASS Symposium: Shell and Spatial Structures from models to realization, 20–24 September 2004, Montpellier, France, (2004). Snelson K., “Continuous tension, discontinuous compression structures,” U.S.P.a.T. Office, Editor, Washington DC, USA, (1965). Sotavalta O., “The effect of wing inertia on the wing stroke frequency of moths, dragonflies, and cockroach,” Annales Entomological Fennici, Vol. 20, (1954), pp. 93–100. Spong M.W., and M. Vidyasagar, Robot Dynamics and Control, Wiley, New York, (1989). Sultan C., M. Corless, and R.E. Skelton, “Tensegrity flight simulator,” Journal of Guidance Control and Dynamics, Vol. 23, No. 6, (2000), pp. 1055–1064. Sultan C., M. Corless, and R.E. Skelton, “The prestressability problem of tensegrity structures: Some analytical solutions,” International Journal of Solids and Structures, Vol. 38, No. 30–31, (2001), pp. 5223–5252. Sultan, C., M. Corless, and R.E. Skelton, “Linear dynamics of tensegrity structures,” Engineering Structures, Vol. 24, No. 6, (2002), pp. 671–685. Sultan C., and R. Skelton, “Deployment of tensegrity structures,” International Journal of Solids and Structures, Vol. 40, No. 18, (2003), pp. 4637–4657. Sultan C., and R. Skelton, “A force and torque tensegrity sensor,” Sensors and Actuators A-Physical, Vol. 112, No. 2–3, (2004), pp. 220–231. Tarnai T., “Simultaneous static and kinematic indeterminacy of space trusses with cyclic symmetry,” ´International Journal of Solids and Structures, Vol. 16, (1980), pp. 347–359. Taylor G.I., “Analysis of the swimming of microscopic organisms,” Proceedings of the Royal Society, Vol. 209, (1951), pp. 447–461. Tibert A.G., and S. Pellegrino, “Deployable tensegrity reflectors for small satellites,” Journal of Spacecraft and Rockets, Vol. 39, No. 5, (2002), pp. 701–709.

Biomimetic Swimmer Inspired by the Manta Ray

523

Timoshenko S., and D. Young, Theory of Structures, McGraw-Hill, New York, (1965). Triantafyllou G.S., and M.S. Triantafyllou, “An efficient swimming machine,” Scientific American, Vol. 272, (1995), pp. 40–48. Triantafyllou G.S., M.S. Triantafyllou, and M.A. Grosenbaugh, “Optimal thrust development in oscillating foils with application to fish propulsion,” Journal of Fluids and Structures, Vol. 7, (1993), pp. 205–224. Triantafyllou, M.S., G.S. Triantafyllou, and R. Gopalkrishnan, “Wake mechanics for thrust generation in oscillating foils,” Physics of Fluids A, Vol. 3, (1991), pp. 2835–2837. Triantafyllou M.S., G.S. Triantafyllou, and D.K.P. Yue, “Hydrodynamics of fishlike swimming,” Annual Review Fluid Mechanics, Vol. 32, (2000), pp. 33–53. Vassart N., R. Laporte, and R. Motro, “Determination of mechanisms order for kinematically and statically indetermined systems,” International Journal of Solids and Structures, Vol. 37, No. 28, (2000), pp. 3807–3839. Vassart, N., and R. Motro, “Multiparametered form finding method: application to tensegrity systems,” International Journal of Space Structures, Vol. 14, No. 2, (1999), pp. 147–154. Verdaasdonk B.W., H.F. Koopman, and F.C. Van der Helm, “Energy efficient and robust rhythmic limb movement by central pattern generators,” Neural Networks, Vol. 19, No. 4, (2006), pp. 388–400. Verdaasdonk B.W., H.F. Koopman, and F.C. Van der Helm, “Resonance tuning in a neuro-musculoskeletal model of the forearm,” Biological Cybernetics, Vol. 96, No. 2, (2007), pp. 165–180. Vogel S., Life in Moving Fluids, Princeton University Press, Princeton, (1994), pp. 1–467. von Ellenrieder K.D., K. Parker, and J. Soria, “Flow structures behind a heaving and pitching finitespan wing,” Journal of Fluid Mechanics, Vol. 490, (2003), pp. 129–138. Wallén P., and T.L. Williams, “Fictive locomotion in the lamprey spinal cord in vitro compared with swimming in the intact and spinal animal,” Journal of Physiology, Vol. 347, (1984), pp. 225–239. Wang Z.J., “Vortex shedding and frequency selection in flapping flight,” Journal of Fluid Mechanics, Vol. 410 (2000), pp. 323–341. Webb P.W., “Hydrodynamics and energetics of fish propulsion,” Bulletin of the Fisheries Research Board of Canada, Vol. 190, (1975), pp. 1–158. Weihs D., “Effect of swimming path curvature on the energetics of fish,” Fisheries Bulletin U.S., Vol. 79, (1981), pp. 171–176. Williams T.L., “Phase coupling by synaptic spread in chains of coupled neuronal oscillators,” Science, Vol. 258, (1992). pp. 662–665. Williamson M.M., “Neural control of rhythmic arm movements,” Neural Networks, Vol. 11, (1998), pp. 1379–1394. Williamson D., R. Skelton, and J. Han, “Equilibrium conditions of a tensegrity structure,” International Journal of Solids and Structures, Vol. 40, No. 23, (2003), pp. 6347–6367. Wu T.Y., “Hydrodynamics of swimming propulsion. Part 1. Swimming of a two-dimensional flexible plate at variable forward speeds in an inviscid fluid,” Journal of Fluid Mechanics, Vol. 46, (1971a), pp. 337–355. Wu T.Y., “Hydrodynamics of swimming propulsion. Part 2. Some optimum shape problems,” Journal of Fluid Mechanics, Vol. 46 (1971b), pp. 521–544. Yu J., M. Tan, S. Wang, and E. Chen, “Development of a biomimetic robotic fish and its control algorithm,” IEEE Transaction on Systems, Man and Cybernetics (Part B), Vol. 34, No. 4, (2004), pp. 1798–1810. Yu J., Y. Hu, R. Fan, L. Wang, and J. Huo, “Mechanical design and motion control of a biomimetic robotic dolphin,” Advanced Robotics, Vol. 21, No. 3, (2007), pp. 499–513. Yu X., B. Nguyen, and W.O. Friesen, “Sensory feedback can coordinate the swimming activity of the leech,” Journal of Neuroscience, Vol. 19, No. 11, (1999), pp. 4634–4643. Zheng M., W.O. Friesen, and T. Iwasaki, “Systems level modeling of neuronal circuits for leech swimming,” Journal of Computational Neuroscience, Vol. 22, No. 1, (2007), pp. 21–38. Zhou K., J. Doyle, and K. Glover, Robust and Optimal Control, Prentice Hall, Upper Saddle River, New Jersey, (1996).

18 Biomimetics and Flying Technology Brenda M. Kulfan The Boeing Company Auburn, Washington

Anthony J. Colozza QinetiQ North America/NASA Glenn Research Center Cleveland, Ohio

CONTENTS 18.1 Introduction ........................................................................................................................ 527 18.2 Nature of Evolution ........................................................................................................... 527 18.2.1 Evolution of the Understanding of Evolution .................................................... 528 18.2.2 The Process of Evolution ....................................................................................... 530 18.2.3 Coevolution Symbiotic Relations ......................................................................... 530 18.2.3.1 Mutualism ................................................................................................ 531 18.2.3.2 Predation .................................................................................................. 532 18.2.3.3 Competition ............................................................................................. 532 18.3 Evolution of Biological Flight ...........................................................................................534 18.3.1 Four Convergent Solutions ...................................................................................534 18.3.2 Probable Steps in Evolution of Avian Flight ...................................................... 536 18.3.3 System of System Adaptations in the Evolution of Birds ................................. 537 18.3.4 Special Noise Reducing Features of an Owl ...................................................... 538 18.4 Achieving Flight by Man ..................................................................................................540 18.4.1 Evolution of Understanding ................................................................................. 541 18.4.2 Biological Inspiration ............................................................................................ 547 18.4.2.1 Flapping Wings ....................................................................................... 547 18.4.2.2 Aeroplanes ...............................................................................................548 18.4.2.3 Innovation from Seeds ........................................................................... 551 18.4.3 Chanute’s Ten Critical Elements .......................................................................... 554 18.4.4 Langley’s Aerodrome ............................................................................................ 558 18.4.5 The Wright Brothers—Achieving the Impossible Dream................................ 559 18.5 Evolution of Modern Aircraft........................................................................................... 563 18.5.1 Technical Advancements on Many Fronts ......................................................... 566 18.5.2 Coevolution in Technical Flight ........................................................................... 568 18.5.2.1 Commercial Aircraft Coevolutionary Mutualism ............................. 568 18.5.2.2 Commercial Aircraft Competition Coevolutionary Developments .......................................................................................... 568 18.5.2.3 Military Aircraft Competition Coevolutionary Development ......... 570 18.5.2.4 Military Aircraft Predator/Prey Coevolutionary Development ...... 571 18.5.3 Contrasting Biological and Technical Flight Evolutionary Drivers ............... 572 525

526

Biomimetics: Nature-Based Innovation

18.6 Aircraft Future Technology Needs and Opportunities ............................................. 574 18.6.1 Aircraft Technology Needs and Development Trends .................................. 575 18.6.1.1 Aerodynamic Efficiency Technology Development Options ....... 578 18.6.1.2 Propulsion System Technology Development Trends ................... 579 18.6.2 Aircraft Technology Development Options .................................................... 582 18.7 Biomimetics in Past, Present, and Future Aircraft...................................................... 583 18.8 Impact of Size on Nature’s Designs............................................................................... 584 18.8.1 Dimensional Analysis and Similarity for Insight into Nature ..................... 584 18.8.1.1 What Determines How Small Is Small and How Big Is Big? ....... 585 18.8.1.2 How High Can a Flea Jump? ............................................................. 585 18.8.2 Physiological Mass-Based Comparisons .......................................................... 588 18.8.2.1 Size Effects on Nature’s Flyers .......................................................... 591 18.8.3 Elastic Similarity .................................................................................................. 593 18.9 Size Effects on Airflow Characteristics ........................................................................ 595 18.10 Biological Related Approaches for Technical Innovation .......................................... 598 18.10.1 Two Approaches for Biological Research and Technology Application .. 599 18.11 Bionics—Nature’s Concepts as Inspiration for Similar Applications ...................... 602 18.11.1 Flying Cucumber and Flying Wings ............................................................ 602 18.11.2 Albatross, Gliders, and the U2 ....................................................................... 603 18.11.3 Controlling the Flight of Birds and Planes .................................................. 603 18.11.4 The Inspiration for the Helicopter: The Dragonfly? ...................................604 18.11.5 Bumblebees and the F-35 (JSF) .......................................................................604 18.11.6 Hummingbirds and Tilt Wings ..................................................................... 606 18.11.7 Spinning Seeds and Gyrocopters .................................................................. 606 18.11.8 Spiroid Wing Tips ............................................................................................608 18.11.9 Formation Flight and Ground Effect ............................................................608 18.11.10 Variable Wing Sweep ...................................................................................... 610 18.12 Biomimicry—Nature’s Concepts as Inspiration for Dissimilar Applications ........ 611 18.12.1 Classic Example—Velcro and the Burdock Seed ........................................ 612 18.12.2 Countercurrent Heat Exchanger .................................................................... 612 18.12.3 Concept Vehicle Mercedes-Benz Bionic Car ................................................ 613 18.12.4 Bionic Propeller ................................................................................................ 614 18.12.5 Owl Wings and Jet Noise ................................................................................ 615 18.12.6 Fruit Flies and Folded Wings ......................................................................... 616 18.12.7 Nautilus and Jet Engines ................................................................................ 617 18.12.8 Bats, Echolocation, Sonar, Radar and Lidar, and Clear Air Turbulence .. 619 18.12.9 Shark’s Skin and Riblets.................................................................................. 622 18.13 Neo-Bionics—Nature-Related Computational Processes for Design   Innovations����������������������������������������������������������������������������������������������������������������������� 623 18.13.1 Biological Optimization Techniques ............................................................. 623 18.13.2 Comparison of Biological “Optimization” Techniques—Traveling Salesman Problem............................................................................................ 624 18.13.3 Genetic Algorithm Wing Planform Optimization...................................... 625 18.13.4 Genetic Algorithm Wing Tip Optimization ................................................ 626 18.13.5 Evolutional Structural Optimization ............................................................ 627 18.14 Cybernetics—Reverse Engineering of Nature for Design Innovations................... 629 18.14.1 Bird Swarms and Group Dynamics .............................................................. 629 18.14.2 Distributed Feet and Mobile Robots ............................................................. 631 18.14.3 Insects and Optical Flow ................................................................................ 631

Biomimetics and Flying Technology

527

18.14.4 Nature’s Passive Flow Control Concepts ...................................................... 632 18.14.5 Grasshopper Knees and Jumping Robots .................................................... 632 18.15 Pseudo-Mimicry—Innovations Confirmed by Nature Designs and Solutions......634 18.15.1 Microraptor gui and Tandem Wings...............................................................634 18.15.2 Sea Gull and Parasol Wings ...........................................................................634 18.15.3 Wasp Nests, Damselfly Wings, Vulture Bones, and Structural Design Concepts ............................................................................................................ 636 18.15.4 Nature’s Wheels and Rotary Tracks .............................................................. 637 18.15.5 Bacterial Rotary Engines and Turbine Engines........................................... 637 18.16 Biologically Inspired Aircraft Concepts .......................................................................640 18.16.1 Insect Flight ...................................................................................................... 641 18.16.1.1 An Entomopter for Flight on Mars ............................................. 644 18.16.2 Bird, Mammal, and Dinosaur Flight ............................................................. 647 18.16.2.1 IPMC-Based Wing Design.............................................................651 18.16.2.2 The Solid State Aircraft Concept ....................................................654 18.16.2.3 SSA Mission Capabilities .................................................................656 18.16.2.4 Application of the SSA to Planetary Exploration .........................660 18.17 Ideas Are Everywhere ..................................................................................................... 666 18.17.1 Motivations for Inspiration ............................................................................ 666 18.17.2 What Do You See? ............................................................................................ 667 18.18 Parting Thoughts ............................................................................................................. 668 18.18.1 Thoughts on Ideas............................................................................................ 669 18.18.2 Thoughts on Action ......................................................................................... 669 Acknowledgments ...................................................................................................................... 669 References..................................................................................................................................... 670

18.1  Introduction In this chapter we will attempt to get a glimpse of insights and observations of some fascinating aspects of birds, insects, and flying seeds, of inspired aerodynamic concepts, as well as visions of past, present, and future aircraft developments. We will explore the fascinations of nature, the struggle to fly, and the ultimate successes of our flying machines. Man most certainly has always been fascinated and inspired by the dream of flying by observing birds soaring in the sky and flittering from tree to tree. Otto Lilienthal, the great German pioneering aviator, said “With each advent of spring, when the air is alive with innumerable happy creatures—then a certain desire takes possession of man. He longs to soar upward and to glide, free as the bird, over smiling fields, leafy woods and mirror like lakes, and so enjoy the varying landscape as fully as only a bird can do.” Many of man’s flying or gliding inventions have indeed been inspired by nature’s creations. The British aviation pioneer Sir Hiram Maxim once wrote: “Man is essentially a land animal and it is quite possible if Nature had not placed before him numerous examples of birds and insects that are able to fly, he would never have thought of attempting it himself.”

18.2  Nature of Evolution The evolution of man’s achievement of flight has been driven and continues to be driven by forces and factors similar to the forces and stimulus of the evolution of nature. An

528

Biomimetics: Nature-Based Innovation

examination of the processes and mechanisms of evolution of nature will provide interesting insights into the evolutionary development of past and present aircraft. 18.2.1  Evolution of the Understanding of Evolution Some of the key events in the evolution of the theory and understanding of evolution are shown in Figure 18.1. The grandfather of Charles Darwin, Erasmus Darwin, published his book Zoonomia or the Laws of Organic Life (1794) which was a two-volume medical work dealing with pathology, anatomy, psychology, and the functioning of the body. This book presented his early thoughts on the origin of life. Erasmus Darwin wrote that warm-blooded creatures developed from “one living filament” and acquired new parts “in response to stimuli” and that all improvements were inherited by successive generations. Erasmus, however, did not discuss the stimulus nor did he discuss the mechanism of evolution. The book incorporated early ideas relating to the theory of evolution that were later more fully developed by his grandson, Charles Darwin. Jean-Baptiste Lamarck (1809) published his beliefs on the origin of life in Philosophie Zoologique. Lamarck incorporated two unique ideas into his theory of evolution that in his day were generally accepted as true. Lamarck believed that individuals develop characteristics that are useful in response to specific needs and these characteristics were retained by usage. He also believed that individuals lose characteristics that they do not require (or use). Subsequent generations were thought to inherit all acquired new traits. Lamarck, for example, believed that the giraffe obtained their long neck and extended forelegs because a tower of giraffes was stretching to reach the leaves high up in acacia Erasmus Darwin

1795

Jean-Baptiste Lamarck

Charles Darwin

1809

• Philosophie

Zoologique • Traits developed by need • Retained by use • Lost by disuse • Acquired traits are inherited. • Zoonomia • Warm-blooded from one living filament • acquire new parts in response to stimuli, • “Improvements” being inherited by successive generations.”

1858

Gregor Mendel

1865

Thomas Morgan

James Watson & Francis Crick

1910-1915

• Origin of Species • Genes carried on • Laws of DNA • Descent with modification chromosomes inheritance • Natural selection • Mechanical basis • Factors do not • Small random changes blend—inherited of heredity • Millions of years discretely from • Modern science of • Pangensis inheritance genetics

the parents

aa

1969

1953

Evolution can occur real time

AA Aa

Aa

Aa

aa Aa AA

• Identified how genetic

information is passed from one generation to the next • Lead to fields of molecular biology and genetic engineering

Concept of genetic switch

Evolution works by:

• Changing genes • Modifying the way genes

are turned on and off

FIGURE 18.1 Evolution of the concept of evolution.

Biomimetics and Flying Technology

529

trees. This resulted in ultimately the longer necks and forelegs. These beliefs of Lamarck were ultimately disproved. As will be shown later, much of the evolution of modern aircraft is driven by specific needs and is therefore somewhat Lamarckian. Charles Darwin published the Origin of Species in 1859 in which he defined that the process of “natural selection,” (which is commonly called the survival of the fittest) occurring over millions of years, has resulted in all the species of life. This is generally considered to be one of the greatest scientific discoveries of all time (Quammen, 2009). It was once stated that “never has so much knowledge been based on so little facts.” Charles Darwin also proposed a complicated theory for the mechanism of heredity called pangenesis. In the pangenesis theory, gemmules containing hereditary information from every part of the body coalesce in the gonads and are incorporated into the reproductive cells. Gemmules were assumed to be shed by the organs of the body and carried in the bloodstream to the reproductive organs where they accumulated in the germ cells or gametes. This theory also provided a possible mechanism for the inheritance of acquired characteristics, as proposed by Jean-Baptiste Lamarck. Charles Darwin never used the word evolution instead he used the phrase “descent with modification.” He also said “I have called this principle, by which each slight variation, if useful, is preserved, by the term Natural Selection.” Gregor Mendel (1865) defined the statistical laws of inheritance and is considered the father of modern genetics. The notion of a gene has evolved with the science of genetics, which began when Mendel noticed that biological variations are inherited from parent organisms as specific, discrete traits. The biological entity responsible for defining traits was termed a gene. Prior to Mendel’s work, the dominant theory of heredity was one of blending inheritance, which proposes that the traits of the parents blend or mix in a smooth, continuous gradient in the offspring. Thomas Morgan (1912c) identified that genes carried on chromosomes defined the mechanical basis of heredity. He is generally considered to be the father of the modern science of genetics. Mice and humans (indeed, most or all mammals including dogs, cats, rabbits, monkeys, and apes) have approximately 24,000 genes. Many of the same genes control similar functions for the different species. Over 99% of all mouse genes have a direct counterpart (or “homologous gene”) in humans. The biological basis for inheritance remained unknown until DNA was identified as the genetic material in the 1940s. DNA not only confirmed the reality of evolution, it also showed at the most basic level how it reshapes living things. All organisms have many genes corresponding to many different biological traits. In cells, a gene is a portion of DNA that contains both “coding” sequences that determine what the gene does, and “noncoding” sequences that determine when the gene is active (expressed). James Watson and Francis Crick in 1953 defined the double helix structure of DNA. This discovery unlocked the mystery of how genetic information is passed from one generation to the next (Ridley, 2009). Research studies by Peter and Rosemary Grant (circ, 1970) demonstrated that natural selection can cause evolutionary change in real time rather than only over thousands of years as Darwin had believed. More recent studies have shown evolution works not just by changing genes, but by modifying the way those genes are turned on and off by a “genetic switch.” It therefore appears that the primary source for evolution turns out not to be gene changes but changes in the regulation of genes that control development. For example, a giraffe does not have special genes to make a long neck. Its neck growing genes are the same as those for a mouse; they are just switched on for a longer time. The giraffe, like the mouse, has only seven bones in its neck. However, the bones in the neck of a giraffe can be up to one foot long.

530

Biomimetics: Nature-Based Innovation

18.2.2  The Process of Evolution The current understanding of the evolution of life indicates that four conditions must be met in order for a species to exist (French, 1994): • • • •

It must work mechanically and chemically. It must be capable of being coded. It must be able to survive at all stages of growth. Its design must be able to be evolved through a series of forms from some other existing form, each form being viable in its own niche of the niche-wise progression.

The process of evolution can be described as a gradual, unrelenting improvement of living systems in response to local environmental conditions. The catalyst for evolution is scarcity or limited supply. This tends to occur as the numerical growth of a species ultimately exceeds the local resources (Murray-Smith, 2004). This results in a struggle for existence. This competition together with natural selection of a favorable inheritable trait results in an evolutionary change in a species. The process of evolution, in contrast to man’s technology developments, does not “design” by working toward specific goals or objectives. Evolution blindly cobbles together myriad random experiments over thousands of generations resulting in wonderfully elegant organisms whose goal is to stay alive long enough to produce the next generation, which then launches the next round of random experiments. The nature of the evolutionary processes of biological systems is fundamentally limiting since every new feature must develop from an existing feature. Consequently, there is no chance of making the sudden great revolutionary technology developments that are so common in the history of technology (Aqtash, 2008). 18.2.3  Coevolution Symbiotic Relations Coevolution is one of the most powerful driving forces in evolution. Coevolution involves the joint evolution of two or more species as a consequence of their ecological interaction. Each species in a coevolutionary relationship exerts strong selective pressures on the other, thereby affecting each other’s evolution. The close, prolonged association between two or more different organisms of different species that may, but does not necessarily, benefit each member is called symbiosis. The major types of coevolutionary relationships include:

1. Mutualism, which is a cooperative coevolutionary symbiotic relationship in which both species benefit 2. Predation, is a competitive coevolutionary relationship between a predator and its prey 3. Amensalism, is a symbiotic coevolutionary relationship between organisms in which one species is harmed or inhibited and the other species is unaffected 4. Competition—The simultaneous demand by two or more organisms for limited environmental resources, such as food, water, or living space

531

Biomimetics and Flying Technology

18.2.3.1 Mutualism Examples of two types of mutualism relationships are shown in Figure 18.2. Bumblebees as well as hummingbirds both obtain nourishment from flowers and various trees and in the process they spread pollen to other trees and flowers. This is an example of a service–resource relationship. An example of a service–service symbiosis is the relationship between clownfish that dwell among the stinging tentacles of sea anemones without being harmed. The anemone therefore protects clownfish from larger predators. In return, the clownfish cleans the tentacles of the anemone and also protects the anemone from the butterfly fish which otherwise would destroy them. Aposematism, which is most commonly known in the context of warning coloration, describes antipredator adaptations that serve as a warning signal of potential danger or discomfort associated with the potential prey to its potential predators. It is one form of nature’s, “advertising” signals, with many others such as the bright colors of flowers which lure pollinators. The warning signal may take the form of conspicuous colors, sounds, odors, or other perceivable characteristics. Aposematic signals are beneficial for both the predator and prey, who both avoid potential harm. As shown in Figure 18.3, aposematic evolutionary developments can occur in many species of mammals, insects, reptiles, and fish. The bright colors of the yellow-winged darter dragonfly warn birds and other predators of its noxious taste. The contrasting black and white colors of the skunk warn other animals and humans of its noxious smell. The bright colors of the cuttlefish and coral snake warn of their toxicity. Figure 18.4 shows additional examples of aposematic mutualistic developments in nature. The yellow jacket and the bumble bee both have highly visible yellow and black colorings to warn predators of their painful stings. The color patterns that each has evolved are very similar. This is an example of Mullerian mimicry and benefits both species by reducing the educational “cost” for each species to “teach” potential predators of their painful stings. Mullerian mimicry occurs when aposematic species evolve to resemble each other, presumably to reduce predator sampling rates.

Clownfish and anemones Hummingbird

White tailed bumblebee

FIGURE 18.2 (See color insert.) Examples of mutualism coevolutionary relationships.

Yellow-winged darter dragonfly warns of its noxious taste

Skunk warns of its noxious smell

Flamboyant cuttlefish colors warn of toxicity

FIGURE 18.3 (See color insert.) Examples of mutualism aposematic type of signals.

Coral snake colors warn of toxicity

532

Biomimetics: Nature-Based Innovation

Mullerian mimicry Yellow jacket

Batesian mimicry

Bumblebee

Hoverflies Wasp beetle (Harmless) FIGURE 18.4 (See color insert.) Mimicry in nature → mutualism.

The harmless and palatable hoverflies and wasp beetles have evolved very similar color patterns to exploit the aposematic protection proved by the color patterns of the yellow jacket and the bumblebee. These are examples of Batesian mimicry. Batesian mimics imitate other species to enjoy the protection of an attribute they do not share, aposematism in this case. 18.2.3.2 Predation In nature, there is a strong evolutionary pressure for animals to blend into their environment or conceal their shape; for prey animals to avoid predators and for predators to be able to sneak up on prey. Natural camouflage is one method that animals use. There are a number of methods of doing so. One method is for the animal to blend in with its surroundings, while another method is for the animal to disguise itself as something uninteresting or something dangerous. Examples of predator and prey camouflage patterns that have evolved are shown in Figure 18.5. Coevolutionary developments of bats and moths are shown in Figure 18.6. Moths are a favorite food source of bats. Bats have developed echolocation as a means to locate and capture moths. Moths in turn have subsequently developed soft coverings to absorb the bat chirps and eliminate to some degree the benefits of echolocation. The bats then developed new chirp frequencies to be able to locate moths even with the soft coverings. The moths responded by developing enhanced stealth characteristics together with jamming techniques with their own bug chirps and new evasive maneuvers. Bats subsequently developed new elaborate flight paths and the ability to turn their chirps on and off, to confuse the moths. The arms race continues. 18.2.3.3 Competition Competition occurs when two species each require a resource that is in short supply, so that the availability of the resource to one species is negatively influenced by the presence of the other species. It is a “−/−” interaction. Competition can occur between individuals that are members of the same species. This is called intraspecific competition. Examples of

533

Biomimetics and Flying Technology

Mackerel tabby (predator)

Flounder (predator)

Orange oak leaf butterfly (prey)

Ghost mantis (predator)

FIGURE 18.5 (See color insert.) Camouflage → coevolutionary predation examples.

Echolocation 1

2 Soft covering to absorb bat chirps

3 New chirp frequencies to detect soft covering 5 • Elaborate flight paths • Turn echolocation off/on

4 • Enhanced shealth • Jamming technique

(Bug chirps)

• Evasive maneuvers

And the arms race goes on FIGURE 18.6 Coevolutionary developments of bats and moths.

intraspecific competition shown in Figure 18.7 include bull elk battling for breeding rights and sea anemones battling for growing space. Interspecific competition occurs between members of different species. Two examples of interspecific competition are shown in Figure 18.8. These include the lion and leopard competing for the same prey, and the water buffalo and the rhinoceros competing for the same waterhole. There is an important evolutionary principle in nature called the competitive exclusion principle (Hardin, 1960) that states that: “no two species with similar requirements can long occupy the same niche (coexist).” The niche of a species includes all aspects of its habitat, how it makes a living, and the physical environment in which it is found. One of the species will either: • Cease to exist • Move from the niche of the stronger species • Change its requirements within the original niche It will be shown further in this chapter, that the competitive exclusion principle applies equally well to the development and success of both commercial and military aircraft.

534

Biomimetics: Nature-Based Innovation

Battling bull elk

Battling sea anemones

FIGURE 18.7

Examples of intraspecific competition.

FIGURE 18.8 Examples of interspecific competition.

18.3  Evolution of Biological Flight The evolution of flight in animals and in man’s flying vehicles as shown in Figure 18.9 has steadily progressed over a period of time of approximately 330 millions of years starting with the first winged insects (Dalton, 1999). Pterosaurs followed nearly 140 millions of years later. The earliest birdlike animals first appeared 150 millions ago followed by early flowering plants, many of which have exploited flightlike mechanisms to disperse their seeds. Modern insect orders, modern bird orders, and bats made their appearance about 55 to 75 millions of years ago. This corresponds to the Cretaceous/Tertiary Mass Extinction Event that occurred approximately 65 millions of years ago. Successful powered flight by man has existed over a minuscule period of slightly more than 100 years. Flight is one of the most demanding adaptations found in nature because of the physical challenges of moving in air under the persistent influence of gravity. Therefore flyers in nature have been subjected to strong selection for optimum morphology. 18.3.1  Four Convergent Solutions Nature has evolved four convergent solutions for the challenge of flight. These include birds, bats, pterosaurs, and insects, all of whose wing structures are shown in Figure 18.10. Bats, birds, and pterodactyl wings not only perform the same function of providing the means of flight, but they are also examples of homologous structures since the limbs of all these organisms contain many of the same sets of bones. These have been passed down to all these different animals from a common ancestor. These wings are also homologous to the human arm and hand. They all contain a sternum, clavicle, scapula, humerus, ulna, radius, and digits.

535

Biomimetics and Flying Technology

Origin of species Man

Geological time, M yr 0 Today Tertiary

Modern mammals Bats Modern bird orders Modern insect orders

50

Cretaceous/tertiary mass extinction event Bat size pterosaur

100

Cretaceous

150

Early flowering plants Early birds

200

Pterosaurs Early mammals

Jurassic Triassic

Oldest Insect Fossil

Flight events

Early dinosaurs Mammal-like-reptiles

250

Permian

300

First winged insects

Carboniferous

Early reptiles

350 Devonian

Early insects

400

FIGURE 18.9 Evolution of flight. (Bat size pterosaur: Chuang Zhao; Oldest insect fossil: Jacob Benner, Tufts University.)

Pterosaur 4

Human arm

2 3 45

Clavicle

Radius Ulna

Scapula Humerus

Bat

2

4

Bird Analogous structure Convergent evolution (Similar function)

FIGURE 18.10 Nature’s convergent wing designs.

1

Sternum

Radius

Clavicle

Sternum Scapula Ulna Humerus

Sternum 3

Ulna

Radius Scapula

Humerus Clavicle

5

1

3 2

Alula 2 3

1 Radius Ulna Humerus Scapula Coracoid

Insect

Clavicle Sternum

Homologous structure Similar anatomy

536

Biomimetics: Nature-Based Innovation

The wings of insects are analogous structures relative to the other previously discussed wings because of the fundamental differences in their internal anatomy even though they perform the similar function of flight. The design of an insect wing at first glance would appear to be very complex. The basic design is however rather simple and extremely elegant. The essential material of all insect wings is the same: a thin membrane, which is supported by blood-filled veins around the margin and within. The wings of the dragonfly are strengthened by a number of longitudinal veins that have cross connections that form closed cells in the membrane. The patterns that result from the fusions and cross connection of the wing veins are often diagnostic for different evolutionary lineages and can be used for identification of the family level in many orders of insects. 18.3.2  Probable Steps in Evolution of Avian Flight Although winged insects were the first creatures to fly, our subsequent discussions will primarily focus on the developments of the flying capabilities of birds. Flight capability opened up new sources of food, made escape from earthbound predators easier, and increased the safety of living and breeding quarters. Impenetrable barriers such as mountains, oceans, or rivers became easily navigable. Ultimately, flight made it possible to follow favorable climates and changing food sources by means of seasonal migrations. Probable steps in the evolution of flight in birds are shown in Figure 18.11. The initial steps in the evolution of flight were fundamentally driven by the needs of survival. These needs included safety from predators for both the individual and their offspring and more effective means of foraging. The early flight capabilities included parachuting

Locomotion

Goal

Adaptations

Nature’s “Technology” Achievements

1. Climbing [running}

Foraging and avoiding predators

Climbing agility [running agility]

2. Parachuting (steep gliding); [hang-gliding]

More effective foraging and avoiding predators

Larger forearm surface (propatagium, feathers)

3. Gliding

Optimal foraging, movements between foraging areas

Larger wings, lower wing loading

4. Gliding with some maneuverability

Ability to determine direction of the glide

Neuromuscular control, wing coordination, wing camber

5. Slight flapping flight

Movements for stability, maneuvers in turning and landing

Higher aspect ratio, lower wing loading

6. Flapping flight with some maneuverability

Better flight performance, commuting

Better neuromuscular control, more sophisticated wing features, camber for slow flight

Functionality

7. Flapping flight with some maneuverability, soaring

Aerial prey-capturing, hovering etc. Soaring/Migration

Highly sophisticated wing features, slots, keeled stemum, musculoskeletal system like that of modern birds

Higher, faster, farther

FIGURE 18.11 Probable steps in the evolution of avian flight capability.

Survival

Efficiency

Biomimetics and Flying Technology

537

and rudimentary gliding abilities. The enabling anatomical changes included developing the fundamental structure of the wing, including the incredible concept of the feather. The next series of steps in the march of evolution of flight provided more efficient gliding flight with some maneuverability and the initiation of slight flapping flight. These provided the ability to determine the direction of the glide and stabilizing movements for turning and landing. The anatomical changes included development of neuromuscular control for coordination of wing movements and more effective wing geometry shapes. Subsequently, evolutionary developments provided increased flight functionality and better flapping flight performance with some degree of maneuverability. The associated anatomical developments included better neuromuscular control and more sophisticated wing aerodynamic characteristics. The next category of evolutionary steps for avian flight provided the ability to fly “higher, faster, and farther” by providing flapping flight with high maneuverability soaring and hovering capabilities.

18.3.3 System of System Adaptations in the Evolution of Birds Nature’s flyers as well as man’s air vehicles can be described fundamentally as systems of systems. Many of the major subsystems such as aerodynamic, structures, flight controls, “propulsion,” “mechanical,” “navigational,” “fuel,” “air conditioning,” and “safety/ security” systems inherent in man’s flight vehicles have parallels within Nature’s flyers. There exists strong synergism between all of the functionally interdependent component subsystems. Prior developments in one subsystem are often both enabling and necessary for subsequent advancements in other subsystems. Consequently “technology” advancements in nature’s flyers and in aircraft include by necessity multiple sequenced and serendipitous developments. The “survival of the fittest” selection process for nature implies that each subsystem is in itself an optimum solution within the confines of the overall optimized system. The unending process of evolution is molded by local environmental effects and the demands of coevolution including both responsive developments and those that provide competitive advantages. Consequently nature has produced many unique acceptable flying designs as evident in the approximately 8000 bird species, 1000 species of bats, and 350,000 species of flying insects. Birds are dramatically different from all other living creatures. Feathers, toothless beaks, hollow bones, perching feet, wishbones, deep breast bones, and stumplike tailbones are only part of the combination of skeletal features that no other living animal has in common with them. Figure 18.12 shows various systems of systems developments in the evolution of birds. The developments or adaptations that are shown in the figure are grouped according to: • Weight-reducing adaptations • Power-increasing adaptations • Aerodynamic adaptations. The images (Jones, 2009) in Figure 18.13 show a kestrel, which is a small hawk, hovering on upflow air currents on the windward side of a steep hill. Even though the air currents are moderately gusty, the kestrel is able to keep its eyes and head position remarkably fixed in space allowing continual and total focus on its potential prey.

538

Weight-Reducing Developments

Biomimetics: Nature-Based Innovation

Power-Increasing Developments

Aerodynamic Developments

Thin, hollow bones

Warm-blooded

Light feathers Elimination of teeth and heavy jaws

Heat-conserving plumage Energy rich diet

Development of forelimb as a wing Wing integrated feather design

Development of the beak Gizzard for grinding food Elimination of bladder Extensive bone fusion Local region adaptions

Branching air sacs (Supercharger) Rapid circulation Large 4 chamber heart High blood sugar levels Breathing synchronized with wing beats High rate of metabolism

Infinitely variable morphing wing Streamline bodies and wing airfoil shapes Flow control adaptations

“Self healing” design philosophy

Control device (e.g., tail, tip feathers, etc.) Flight critical stability system

FIGURE 18.12 System of system adaptations in the evolution of birds.

1

2

3

4

5

6

7

8

FIGURE 18.13 Hovering kestrel highly developed structural and neuromuscular control. (Picture Credit: Gareth Jones, betacygni@Youtube, Used with permission.)

This is an excellent example of effectiveness of a bird’s evolutionary structural and neuromuscular stability and control developments. In the vernacular of an aerodynamicist, the kestrel hovering flight demonstrates a highly sophisticated flight critical dynamic morphing coupled aeroservoelastic control system. The kestrel also provides a powerful message for success: “Keep your eye upon your goal.” 18.3.4  Special Noise Reducing Features of an Owl The special noise reducing features of an owl are another example of evolutionary systems integration in nature. Much of the owl’s hunting strategy depends on stealth and surprise. Owls as shown in Figure 18.14 have many evolutionary adaptations that aid them in rightfully achieving the reputation as nature’s master of stealth. The owl has a large wing area and “tip fin” feathers for flying and gliding slowly. The wings are covered by velvety feathers to reduce mechanical noise due to fluttering,

539

Biomimetics and Flying Technology

Combs on leading primaries Generate vortices to increase lift for slow speed flight Large wing area and “Tip fins”

Feathers dull colors Almost invisible under certain conditions

For flying and gliding very slow

Soft, serrated wing trailing edge Velvety feather surfaces reduce • Mechanical noise (flutering & rubbing) • Airflow noise (high frequency)

Diffuses and reduces high frequency noise

FIGURE 18.14 Noise reducing features of an owl. Owl hearing range 100Hz–20 kHz

Lower limit of prey hearing range

Owl bi-aural hearing range 3–6 kHz

Sound intensity SPL—sound pressure

Typical spectrum of sound generated by most birds [qualitative only]

Owl noise spectrum

Mouse squeaks and leaf rattles

level

2

Sound frequency kHz

10

FIGURE 18.15 The “silent” flight of owls.

flapping, and rubbing. This surface also tends to reduce high frequency airflow noise. The soft serrated wing trailing edge muffle the owl’s wing beats, allowing its flight to be practically silent. The combs on the leading edge of the primary feathers generate vortices to increase lift for slow speed flight. The dull colors of the feathers on the lower surface make the owl almost invisible under certain conditions. The silent flight characteristics of the owl as shown in Figure 18.15 (McMasters, 2003) have two objectives. The obvious first objective is to avoid detection by the potential prey. The second and equally important objective is to enable the owl to detect the quiet noise levels such as mouse squeaks and leaf rattles. Owls as shown in Figure 18.16 have an intense and highly effective detection system for locating their prey. Their unique specialty is nighttime hunting.

540

Biomimetics: Nature-Based Innovation

FIGURE 18.16 Integrated detection system.

Owls have spectacular binocular vision allowing them to pinpoint prey and see in low light. The eyes of great horned owls are nearly as large as those of humans and are immobile within their circular bone sockets. Instead of turning their eyes, they turn their heads a full 270 degrees in order to see in other directions without moving its entire body. The very large eyes are packed with light sensitive rods and are about 100 times more sensitive in low light than ours. They can see an object a mile away at night lit by a candle. Owls are very near sighted and are unable to see within a couple of inches. Consequently owls have sensitive filoplumes on the feet and beak which act as feelers. An owl’s hearing is as good—if not better—than its vision; they have better depth perception and better perception of sound elevation (up–down direction) than humans. This is due to owl ears not being placed in the same position on either side of their head: the right ear is typically set higher in the skull and at a slightly different angle. By tilting or turning its head until the sound is the same in each ear, an owl can pinpoint both the horizontal and vertical direction of a sound. The facial disc helps to funnel the sound of prey to their ears. I have seen a video of an owl sitting up high in a tree in winter when the ground was covered by 4 to 5 in of snow. The owl suddenly swept down and captured a mouse that was traveling below the snow nearly 20 ft from the tree. The owl located the mouse moving below the snow by its incredible directional hearing capability.

18.4  Achieving Flight by Man Flight by man is a scientific achievement whose inspiration, origin, and ultimate successes were all founded on biomimetics. It is well documented (Videler, 2005; Pettigrew, 1874; Lilienthal, 1889; Chanute, 1894; Moedebeck, 1903) that the more successful of the earlier pioneers of manned flight were inspired by nature’s flying creatures and objects and were all well versed in the then current understanding of flight mechanics. “Of all animal movements, flight is indisputably the finest. It may be regarded as the poetry of motion. The fact that a creature as heavy, bulk for bulk, as many solid substances, can by the unaided

541

Biomimetics and Flying Technology

Insects

Birds

Proto-reptile

Mammals

Bats

Pterosaurs Extinct Anemophilous seeds Mesozoic era

Paleozoic era 345

248

Million years ago

Cenozoic era 65

Biological evolutionary design optimizations

FIGURE 18.17

Common view of the evolution of flight.

movements of its wings urge itself through the air with a speed little short of a cannonball, fills the mind with wonder” (Pettigrew, 1874). The concept that the inspiration of nature’s flyers leads to experimentation, then the realization of manned flight and ultimately leading to the proliferation of flight capabilities through many airplane concepts is often the traditional view of the history of flight. However, a more realistic view of the history of flight is shown in Figure 18.17. McMasters said “Nature’s evolutionary processes and man’s technology development are all bound together by the underlying requirements that each must obey the same fundamental laws of physics, chemistry—and economics.” However the evolutionary processes of biological flight are significantly different than the evolutionary processes of technical flight although as will be shown later. Aircraft evolutionary developments have been driven by coevolutionary symbiotic relationships similar to those of nature. An expanded view of the evolution of flight is shown in Figure 18.18 to highlight the fact that man’s desires, thoughts, and efforts to fly have occurred not just over the past “century of flight,” but over a period of hundreds of years. As it will be shown, this evolutionary period was highlighted by the enhancement of our knowledge of flight dynamics (KFD), our understanding of flight dynamics (UFD) plus the sequential developments of critical and necessary supporting technologies. 18.4.1  Evolution of Understanding The initial aerodynamic and flight dynamic tools that were available to the early pioneers of flight are shown in Figure 18.19. These include the information, ideas, and interpretations that they gleaned from observations of birds soaring, bats flying, and hummingbirds hovering. We call this visual flight dynamics (VFD). Otto Lilienthal (circa 1890) who is

542

Biomimetics: Nature-Based Innovation

Insects Increase of

• Knowledge (KFD) • Understanding (UFD)

Birds

Plus Mammals

Proto-reptile

Development of • Enabling tools • Critical technology

Bats

Pterosaurs

Extinct

Anemophilous seeds

Paleozoic era 345

248

Mesozoic era Million years ago

Cenozoic era 65

Biological evolutionary design optimizations

Hundreds of years

FIGURE 18.18 Expanded view of the evolution of flight.

“VFD”

(a)

Visual Flight Dynamics (Birds, seeds & bugs)

“KFD” Knowledge of Flight Dynamics

(b)

“UFD” Understanding Flight Dynamics

Early flight concepts & attempts

(c)

FIGURE 18.19 Early aerodynamic tools. (Courtesy of: (a) Kulfan, B.M.; (b) Johansson, C., Wolf, M., and Hedenstrom, A., Animal Flight Lab, Lund University, Sweden; and (c) Cupix.)

considered by many to be the pioneering father of flight stated “In order to discover the principles which facilitate flight, and to eventually enable man to fly, we must take the bird for our model.” This visual information formed the basis of their KFD from which they formalized their UFD. It should be noted that knowledge and understanding are not the same, nor is all knowledge absolute, accurate, or even factual. The longing to fly like a bird that man has long endured is typified by the passionate words of Otto Lilienthal (1889): “The observation of nature constantly revives the conviction that flight cannot and will not be denied to man forever.”

543

Biomimetics and Flying Technology

It is interesting to examine the details of the state of knowledge of the physics of bird flight as recorded in books that were published in the time period of the Wright Brothers initial flights (Pettigrew, 1874; Lilienthal, 1889; Chanute, 1894; Moedebeck, 1903). Man’s concepts of the nature and the physics of avian flight gradually developed from endless hours of observing the flight of birds over centuries of time. This source of “technical” information is called VFD. Early observations such as shown in Figure 18.19 formed the basis of the evolving KFD and the understanding, UFD (not necessarily correct) of flight dynamics that ultimately led to man’s initial attempts to fly, applied fluid dynamics (AFD). One of the earliest recorded pictures of the observed nature of flight is shown in Figure 18.20. This is a cave painting from about 11,000 years ago, of what appears to be a bird landing. The picture suggests that the artist had a rather accurate understanding of the use of wings during landing, including what appears to be the extended alulae that are nature’s form of a high lift device which behave similar to wing leading slats (Videler, 2005). Figure 18.21 shows three ancient models that appear to resemble a bird, a bat, and a delta wing aircraft. The Saqqara bird has been dated to approximately 200 BC. It appears to be a bird-shaped artifact made of sycamore wood that was discovered during the 1898 excavation of the Pa-di-Imen tomb in Saqqara, Egypt. The function of the Saqqara bird is unknown but there has been a great deal of speculation regarding its significance. Some think the Saqqara bird may be a ceremonial object; others think it may have been a toy.

Cormorant landing Alula?

FIGURE 18.20 Earliest known cave painting of a bird (9000 BC) and a double-crested cormorant.

Saqqara bird (Egyptian tomb) ~ 200 BC

FIGURE 18.21 Ceremonial objects, bird, or airplane concepts?

Bat, Moche culture 100 A.D.

Pre-Colombian model ~ 300 to 500 AD

544

Biomimetics: Nature-Based Innovation

Some have also suggested that the Saqqara bird may represent evidence of knowledge of the principles of aviation. However, the theory that the Saqqara bird is a model of a flying machine is not accepted by mainstream Egyptologists since no ancient Egyptian aircraft have ever been found. The Moche culture bat from the 100 AD time period is considered to most likely to have had some religious significance in Mesoamerican mythology. The pre-Columbian golden sculptures from around 500 to 800 AD were originally thought to be zoomorphic (representing animals). Others have thought that the sculptures provide evidence of knowledge of artificial flight since there is a rather strong resemblance to an aft-tail delta wing aircraft. Da Vinci’s well-known sketches of bird flight from about 1500 are shown in Figure 18.22. These sketches also show his interpretation of the characteristics of the flow around the bird as well as how a bird was able to control its flight path. It is generally believed, however, that these sketches were based more on his understanding of the physics of flight than on any particular flight observation. The sketches are from da Vinci’s Codex on the Flight of Birds that were not discovered until hundreds of years after da Vinci’s death. The text in that Codex as in all of da Vinci’s notes was written in a reversed mirror image style. The sketches in the figure have been converted to “regular” images. Originally, it was largely believed that this style of writing was adapted to protect his thoughts, ideas, and inventions. More recently, it has been accepted that da Vinci used this style as an efficient means to avoid smearing the ink when writing since he was left handed. Leonardo’s study of flight and innovative inventions had no significant direct impact on early flying machines, simply because his work in the field was little known before it was published in the early twentieth century. Earlier publications of his manuscripts either omitted his work on aeronautics altogether, or did not take it seriously. Hureau de Villeneuve’s article in L’Aéronaute in 1874 was the first publication that properly established Leonardo as the first to scientifically investigate flight. The sketches by Borelli shown in Figure 18.23, illustrate his concept of the manner by which birds fly as he stated in his masterpiece De Motu Animalium that appeared after his death in about 1680. “Birds fly by beating the air with their wings. They jump as it were through the air just as a person can jump on the ground—Wing beats compress the air and the air bounces back.”

FIGURE 18.22 Da Vinci’s explanation of the mechanics of bird flight (c1500).

Biomimetics and Flying Technology

545

FIGURE 18.23 Early interpretations of the nature and mechanisms of avian flight.

According to Borelli’s belief, as a bird beats its wing, it compresses the air below the wing. The wing is at a slight angle of attack and therefore as the air is compressed, the bird is moved forward with each beat and bounces through the air much the same as a runner bounces across the ground. Borelli’s correct understanding that the tail moved up and down to provide pitch control differed from the previously accepted belief advocated by Aristotle, that the tail acted as a rudder. Borelli also stated that birds change their horizontal direction by beating the left and right wings at different speeds similarly the way that a “rower alters course by pulling harder on one oar than the other.” The explanation of the mechanics of avian bird flight defined by Borelli was accepted as being scientifically correct for nearly 200 years. Pettigrew in his book Animal Locomotion or Walking, Swimming, and Flying, With a Dissertation on Aeronautics, which was published in 1874, wrote: “With regard to the production of flight by the flapping of wings,—De Motu Animalium of Borelli, published as far back as 1680, i.e. nearly two centuries ago. Indeed it will not be too much to affirm, that to this distinguished physiologist and mathematician belongs almost all the knowledge we possessed of (flapping) wings up till 1865.” A very significant supporting technology development that provided valuable insight into the nature of flight was the chronophotographic gun that Etienne-Jules Marey (Braun, 1992) perfected in 1882. With this instrument, Marey was capable of taking 12 consecutive frames a second, and the most interesting fact is that all the frames were recorded on the same picture as shown in Figure 18.24. With this instrument, it was then possible to observe the intricate motions of a bird or insect in flight. Marey’s photographs were an early form of VFD. The pictures of the sea gull in flight were the first ever images that captured the motion of a flying bird. These pictures were taken in 1886. Marey also made movies at high speed (60 frames per second). He is widely considered to be a pioneer of photography and an influential pioneer of the history of cinema. During the 1860s, Marey focused on the study of flight, first of insects and then birds. His aim was to understand how a wing interacted with the air to cause the animal to move. He also devised some ingenious apparatuses such as a corset which allowed a bird to fly around a circular track while recording the movements of its thorax and wings.

546

Biomimetics: Nature-Based Innovation

Étienne-Jules Marey photographic “gun”

Étienne-Jules Marey, “Flight of Gull”, 1886

FIGURE 18.24 Marey’s “photographic gun”; capable of taking twelve exposures in 1 s (1882).

Wing of a Stork

at downstroke

at upstroke

FIGURE 18.25 Lilienthal’s bird flight concepts. (From Lilienthal, O., Birdflight as the Basis of Aviation, Markowski International Publishers, Hummelstown, PA, 2000.)

Otto Lilienthal published his results, working in conjunction with his brother, after long years of quiet scientific study and experiment, in 1889 (Lilienthal, 1889). This book contained the “discovery” of the driving forward of arched surfaces against the wind. Lilienthal said “The problems why a flying bird does not drop to the ground, how it is sustained in the air by an invisible force, may be considered fully solved so far as the nature of this supporting force is concerned.” Figure 18.25 contains his sketches of the shapes of various bird wing geometries, plus a sketch of the details of a stork’s wing. It is obvious that the Lilienthal brothers had obtained a rather thorough understanding of the flight features of a bird.

Biomimetics and Flying Technology

547

Five methods of bird flight were distinguished (Pettigrew, 1874) in the period in which the Wright brothers entered the scene.









1. The first method was called “rowing flight” (corresponding to the modern-day vernacular of flapping flight), was formulated by a combination of chronograph measurements together with a series of photographs. The photographs were obtained simultaneously from three directions showing the movements of the wings at various sequential moments. (The early concept of flight related the mechanics of flapping flight to the motion of rowing a boat. This is substantially different than the actual mechanics of flight.) 2. The second method called gliding flight was defined as “rowing flight interrupted by the passive flights—the gliding. During gliding the flapping of the wings is halted, and the flight is sustained by the kinetic energy generated during the rowing flight.” (This definition corresponds to our present day concept of bounding or intermittent flight.) 3. The third method was called soaring. “During soaring, the bird remains over a point on the ground without flapping its wings; soaring is rendered possible by upward currents of air, forming over wooded land and on rugged rocks. The activity of the muscles is confined, in this case, to feeble balancing turns of the stretched wings about the body longitudinal axis.” 4. The fourth method was called sailing. “Sailing is seen frequently with sea gulls following ships or progressive waves. This movement is caused by the wind, reflected upwards after striking the sails or crests of the waves, holding the bird at a constant height and at a constant distance away from the sail or the wave crest, as the case may be. The difference between sailing and soaring is, that the animal not only remains at a constant height, but in the former case also is driven forwards.” 5. The fifth method was called circling, “explanation of the circling of birds is attended with especially great difficulties.” Apparently the physics of thermals caused by local uneven heating of landmasses was unknown at that time. These thermals result from a central ring of revolving air with a core of rising colder air. The birds circle to remain in the core of rising air and then glide between other thermals.

It is interesting that there was no specific mention of hovering, either as a form of active flight in the case of hummingbirds rapidly beating their wings, or as a form of passive flight as in the case of the kestrel riding on rising upward currents over a steep hill as previously shown in Figure 18.13. 18.4.2  Biological Inspiration The earliest concepts and attempts at flying were all based on attempts to directly emulate the flight of birds. Leonardo da Vinci said that “a bird is an instrument working according to a mathematical law. It lies within the power of man to make this instrument with all its motions.” 18.4.2.1 Flapping Wings Da Vinci, like many of the early pioneers of flight that followed, based on their observations of birds, believed that in order to fly, man would need a pair of flapping wings. These

548

Biomimetics: Nature-Based Innovation

Leonardo da Vinci flapping wing glider (circa 1490)

F. von Drieberg’s Dildaleon (c1845)

Meerwein’s flying apparatus (1781)

Edward Frost’s ornithopter of willow, silk, and feathers (1902)

FIGURE 18.26 Early flapping wing concepts.

devices became known as ornithopters, Figure 18.26. Da Vinci’s ornithopter concept was developed in the 1486 to 1490 time period. However there is no evidence that he actually built or tested such a concept. Da Vinci’s imagination was filled to capacity with ideas for flying machines. The concept shown in the figure is a glider equipped with flappable wings. This open-shelled model, was fitted with seats and gears for the pilot. Karl Meerwein was the first to estimate the size of a wing surface necessary to support the weight of a man using as a basis, the weight and corresponding wing area of ducks. Taking the wild duck as his model, he found that a man, weighing 200 lb with the machine, would require a surface of 126 sq. ft. His apparatus consisted of two light wooden frames covered with calico. The pilot was fastened in a horizontal position in the middle, with a balancing rod in front of him, which worked the strokes of the wings when pressed by the body. Meerwein apparently made one unsuccessful flight attempt in 1789. Friedrich von Drieberg was the first to acknowledge that man has the greatest power in the muscles of the leg, and must use these for the movements of flight. Up until this time it was commonly assumed that the wings must necessarily be moved with the arms. Von Drieberg’s concept consisted of a batlike flying apparatus in which flight was to be obtained by flapping the wing by treading with the feet, while lying horizontally. Edward Frost constructed an ornithopter made of willow, silk, and feathers supported on a wooden frame. When his ornithopter was suspended from a tree it was said that it would rise slightly with every beat of the wings. The entire contraption was much too heavy to ever fly. He built his last ornithcopter in 1904, a year after the Wright brothers’ first powered flight. Frost later became the president of the Royal Aeronautical Society. 18.4.2.2 Aeroplanes The earliest known idea for flight with fixed wing geometry as in today’s airplanes was the 1799 aircraft design by George Cayley which was sketched on a small coin. This concept

549

Biomimetics and Flying Technology

George Cayley’s 1799 aircraft design

Henson’s aerostat (Patent 1843)

a b c

Le Bris Albatross, 1868

f

b

e

d a

c

Alphonse Penaud’s powered planophore, (1871)

FIGURE 18.27 Early fixed wing flyers.

is shown in the sketch on the left side of Figure 18.27. Cayley’s design had fixed wings for lift, a movable tail for control, and rows of “flappers” beneath the wings for thrust. Cayley is regarded as the first one who recognized the advantages of separating the tasks of lift and thrust production and assigning them to different subsystems of an aircraft. Today, more than 200 hundred years later, such a separation of functions is still considered one of the key principles of aircraft design (Ackroyd, 2002). It is only at hypersonic speeds that for some designs the propulsion system and the aerodynamic surfaces have become somewhat indistinguishable since the lower surface of the vehicle acts as the inlet for the engines. Inherent stability, which is the tendency of an aircraft to return to straight and level flight when the controls are released by the pilot, was first discovered by Sir George Cayley, but not fully understood until it was later theorized by Alphonse Pénaud. A slight wing dihedral (upward bend) of an aircraft wing will create inherent stability. Most aircraft are designed with this in mind and are said to be “inherently stable.” High-performance and highly maneuverable aircraft, such as fighter planes and aerobatic aircraft, often have little or no inherent stability and when the pilot releases the controls, the aircraft may bank or pitch in one direction or another. These aircraft take much more skill and concentration to fly safely, while the most sophisticated aircraft are computer controlled. Most civilian aircraft are designed to provide a high amount of inherent stability. Henson’s steam powered fixed wing aircraft was the first patented airplane concept (1843). Alphonse Pénaud in 1871 built a planophore, a 20-in-long monoplane with a pusher propeller powered by a rubber band. It flew 131 ft in 11 s, becoming the first flight of an inherently stable aircraft. In 1874, Bishop Milton Wright bought one of Pénaud’s toy helicopters. He took it home to his boys, Orville and Wilbur. Right there, Pénaud ultimately changed the course of history since playing with this toy led to the Wright brothers’

550

Biomimetics: Nature-Based Innovation

developing their desire to fly. While many of the early glider concepts were dangerous exercises of futility, others begin to add to the accumulation of knowledge of the critical elements for successful flight. Le Bris built a glider shown in the lower left of Figure 18.27, which was inspired by the shape of the albatross. The glider consisted of a wood frame and was covered in cloth. The pilot (Le Bris) sat inside what looked like in a canoe, and used levers to operate the movements of the wings and tail. This invention, which he patented in 1857, was the first flight control concept. In 1856, he briefly “flew” the glider, which was put on top of a cart and then was attached to a horse that ran against the wind. At this point, the Artificial Albatross was released from the cart and began to rise into the air. The Albatross glider became the first ever to be photographed, albeit on the ground, by Nadar in 1868. Figure 18.28 shows the pioneering aviation giants from three countries. Otto Lilienthal from Germany conducted the first extensive series of engineering type of fixed wing glider experiments. Over the period of 1891 to 1896, he conducted over 2000 gliding flights before he perished when his glider stalled and crashed from an elevation of 50 ft. Alexander Graham Bell, after observing one of Lilienthal’s gliding flight wrote: “Lilienthal boldly launched himself into the air in an apparatus of his own construction, having wings like a bird and a tail for a rudder. Without any motor, he ran down hill against the wind. Then, upon jumping into the air, he found himself supported by his apparatus, and glided downhill at an elevation of a few feet from the ground, landing safely at a considerable distance from his point of departure. This exhibition of gliding flight fairly startled the world.”

Living a dream

Otto Lilienthal’s flight (c1896)

Chanute’s multiple winged machine (1896) FIGURE 18.28 Early fixed wing flyers—the aeronautics giants.

Pilcher’s “Pilcher Hawk” (1896)

Chanute–Herring biplane (1897)

Biomimetics and Flying Technology

551

FIGURE 18.29 Early inspiration from a bat.

Perry Sinclair Pilcher, the English aviator, built and tested a number of glider designs between 1895 and 1899 when after a structural failure; he was killed in the collapse of his last glider. His experiments provided a series of important results: • • • •

Too much wing dihedral reduced stability in side winds. Too low center of gravity makes the apparatus very difficult to control. A flying machine can safely be raised by towing it against the wind like a kite. Light wheels at the front are convenient to move the machine about and to absorb shocks in landing.

Chanute, after experimenting with various monowing gliders, started to experiment with various multiwing concepts such as those shown in Figure 18.28. The concepts were initially based on kite designs that exhibited stable flight characteristics. He ultimately ended up with the Chanute–Herring biplane concept shown in the right side of the figure. Chanute in time became a mentor to the Wright brothers. The wing planform geometry, which the Wright brothers chose for their gliders and also for the Wright flyer, was very similar to Chanute’s double-decker biplane wing. The imaginations of the early aircraft designers were almost unlimited in scope. These early aviation pioneers studied the flight characteristics of every conceivable type of flying animal—birds, insects, bats, flying fish, even flying foxes. Figure 18.29 shows the Avion III, which was designed in 1897 and modeled after the geometry of a bat. The Avion III was a primitive steam-powered aircraft built by Clément Ader between 1892 and 1897, financed by the French War Office. This aircraft retained the same basic batlike configuration of an earlier aircraft, the Éole. The Avion III was equipped with two engines driving two propellers. The propellers actually had a featherlike structure. The Avion was equipped with a small rudder as means of directional control. Trials of the aircraft began at the Satory army base near Versailles on October 12, 1897, with the aircraft taxiing along a circular track. The first actual flight was attempted on October 14, 1897. The flight ended almost immediately in a crash without ever leaving the ground. 18.4.2.3 Innovation from Seeds In addition to birds, insects and pterosaurs, nature’s “fliers” also include the seeds of wind pollinating plants that evolved to provide their parent species one of the most remarkable and effective of all seed dispersal methods, riding the wind and air currents of the world (Chen, 1984). Some common examples are the milkweed seed, which may be considered

552

Biomimetics: Nature-Based Innovation

a direct natural antecedent of the parachute, and the maple seed, a natural prototype of the autogyro. The gliding seed of the Java palm tree, Zanonia macrocarpa is of considerable historical interest, because it demonstrated to aviation pioneers the feasibility of constructing a stable tailless airplane. McMasters stated “Plants mastered the art and science of aviation long before Orville and Wilbur Wright propelled their frail craft into the air” (Chen, 1984). This is evident in the concepts that nature has developed to enable seeds to navigate to suitable soil. If a tree dropped its seeds straight down, the seedlings would have to try to grow in the shade of the parent tree and would soon choke each other out. Seeds need to be carried away from its parent tree or plant. Nature has accomplished this in a variety of ways. The most interesting aerodynamic example is the winged Zanonia macrocarpa seed shown in Figure 18.30. This kidney-shaped seed comes from a large vine of the cucumber family. It grows in the dense, moist jungles of Indonesia and has adapted its reproductive processes to regions in which there are little or no wind to distribute the seeds. The parasitic vine climbs 150-ft trees, and near the top, the Zanonia seed develops with two curved wings, transparent, gleaming, and very elastic. The seed—a kidney-shaped planform when released—begins its glide, rising on thermals from the jungle heat, and finally landing at a considerable distance from its point of departure. One professor described the Zanonia glider in this way: “Circling widely, and gracefully rocking to and fro, the seed sinks slowly, almost unwillingly, to the earth. It needs only a breath of wind to make it rival the butterflies in flight.” The Zanonia seed can perform amazingly long glides, during which it demonstrates basic inherent stability. Flights of up to 6 km from the vine have been recorded. The aerodynamic features of the Zanonia macrocarpa seed include (Alexander, 2002): • • • • • • •

Swept wing and forward CG for longitudinal stability (reduce pitch-up tendency) Swept wing and reflexed trailing edge to avoid pitch-up Reflexed trailing edge to provide quicker stall recovery Drooped leading edge for higher CLmax Dihedral for roll and yaw stability Large aspect ratio = 3~4 with a lift/drag ratio of 3 to 4 Optimum center of gravity location for lowest rate of descent or highest duration of flight

A number of the early experimenters with tailless aircraft were inspired by the Zanonia’s flying qualities. Igo Etrich adapted the principles he gleaned from his Zanonia macrocarpa Nature’s flying seed Reflexed trailing edge

‘Etrich’s leaf,’ 1906

Wing “wash-out” Dihedral

Swept leading edge Cambered airfoil section Forward CG location Metric 1

2

3

4

5

6

7

8

9 10 11 12 13

FIGURE 18.30 Early innovation from a seed.

Rumpler ‘Taube’ (dove), c1910.

553

Biomimetics and Flying Technology

observation of the Zanonia seed to his Leaf design in 1906. Rumpler developed his famous dove “utilizing a planform based on the Zanonia to which he added the ‘tail of a dove’” (Neustadt, 1909). Da Vinci utilized the concept of the spinning seed, shown in Figure 18.31 to formulate his idea of the “air screw” which is considered to be the forefather of the autogyro, the helicopter, as well as the propeller. Naturalist Christian de Launoy and his mechanic Bienvenu, about whom very little is known, developed a coaxial model of a simple helicopter powered by the tension in a bow. “When the bow has been bent by winding the cord, and the axle placed in the desired direction of height—say vertically, for instance—the machine is released,” the pair told the French Academy of Sciences in 1784. “The unbending bow rotates rapidly, the upper wings one way and the lower wings the other way, these wings being arranged so that the horizontal percussions of the air neutralize each other, and the vertical percussions combine to raise the machine. It therefore rises and falls back afterward from its own weight.” This concept was also the first counter-rotating propeller design. Large numbers of plants such as the dandelion use nature’s version of the parachute to disperse their seeds as shown in Figure 18.32. The very light seeds of the plant are attached to relatively large fluffy plumes that are released from the plant with a slight breeze. The high drag of the plumes results in very low sinking rates, allowing the seeds to be blown and dispersed far from the plant (Alexander, 2002). The first known written account of a parachute concept is contained in da Vinci’s notebooks (c l495). The parachute concept that he conceived consisted of a cloth material pulled tightly over a rigid pyramidal structure. Da Vinci never made nor tested his device. The first recorded successful test of such a parachute was made in 1595 in Venice by the inventor Fausto Veranzio who had examined da Vinci’s rough sketches of a parachute, and

Da Vinci’s helical air screw (1490) Christian de Launoy counter-rotating bow (1784)

George Cayley’s “aerial carriage,” April 1843 FIGURE 18.31 Spinning seeds, air screws, and helicopters.

John Greenough’s Aerobat (1879)

554

Biomimetics: Nature-Based Innovation

Circa 140 MYA Parachute (circa 1900)

DaVinci (circa 1480–1483) Parawing (circa 1960)

Fausto Veranzio, 1595 Parafoil (circa 1970)

FIGURE 18.32 Drifting seeds and parachutes.

Fausto set out to implement a parachute of his own. Twenty years later, he implemented his design and tested the parachute by jumping from a tower in Venice. In World War I and World War II, the classic parachute was widely used. During the early space projects, Rogallo developed a single membrane flexible wing, known as the parawing (Meyer, 1985). Large parawings were designed for recovery of reentry vehicles. The parawing parachute was designed for maximum lift as opposed to the maximum drag of conventional parachutes. The parafoil was invented in the middle 1960s by Domina Jalbert, a kite maker. The parafoil or ram-air parachute is a deformable airfoil that maintains its profile by trapping air between two rectangular shaped membranes, sewn together at the trailing edge and sides, but open at the leading edge. Several ribs are sewn to the inside of the upper and lower surfaces, maintaining an airfoil cross section in the span-wise direction. 18.4.3  Chanute’s Ten Critical Elements When the Wright brothers set as a goal the development of the first powered aircraft, they initiated their process by a search for all available knowledge of prior attempts to fly. Wilbur wrote a letter to the Smithsonian Institution requesting information and publications about aeronautics. Drawing on the work of Sir George Cayley, Chanute, Lilienthal, Leonardo da Vinci, and Langley, they began their flight experiments that year. They also built a strong networking relationship with Chanute who, “believing that the surest method is first to study past failures—made an investigation of the records of all the experiments, which had been tried during the last two or three hundred years, in the endeavor to imitate the birds. This resulted in a number of technical articles which swelled

Biomimetics and Flying Technology

555

into a book, in which the attempt was made to eliminate the causes of each failure; for up to that time there had been nothing but failures.” Mr. Chanute in 1897 had published what may be considered to have been the state-ofthe-art understanding of ten critical elements that he considered to be essential to achieve successful powered flight (Chanute, 1894; Moedebeck, 1903). Chanute stated that the first six were well in hand and understood.









1. “The Supporting Power and Resistance of Air” (ability to predict lift and drag) • “We are now enabled to calculate with some confidence the support (lift) which may be obtained by gliding at any given speed upon the air, and the power required to overcome the resistance (i.e. drag).” (Based on empirical formulae of Duchemin and Langley’s experiments.) • “More encouraging (lift) coefficients for concave surfaces have been obtained by Lilienthal in his experiments.” 2. “The Motor, Its Character and Its Energy” • “For the first time the realization of a sufficiently light motor for a dynamic flying machine seems to be within sight.” • “It now seems probable that this will be accomplished with a petroleum engine.” 3. “The Instrument for Obtaining Propulsion” • “All sorts of contrivances have been proposed; reaction jets of steam or of compressed air, the explosion of gunpowder or even nitro-glycerin, feathering paddle wheels of varied design, oscillating fins acting like the tails of fishes, flapping elastic wings like the pinions of birds, and the rotating screw.” • “Mr. Maxim and Professor Langley have made many experiments to determine the best form, speed and pitch of the screw (propeller) to obtain thrust from the air, and have materially improved that instrument” • “The Screw (propeller) seems likely to be the best device.” 4. “The Form and Kind of the Apparatus” • “Almost numberless projects have been advanced, but they can all be classified under three heads. −− Wings to sustain and propel. (Ornithopters) −− Rotating screws to lift and propel, (Helicopters) −− Aeroplanes, to consist of fixed surfaces driven by some kind of propelling instrument.” • “The first two have been the first to be proposed and experimented with. They have many warm advocates at the present time,” • “Practical experiments made within the last five years seem to indicate that success will first be achieved with aeroplanes,” 5. “The Extent of the Sustaining Surfaces” (i.e., what is the required wing area?) • “The problem, relating to the extent of surface required to support the weight of a man, has caused in the past active controversy and gathering of data.”

556







Biomimetics: Nature-Based Innovation

• “It was perceived that in consequence of the law inherent to solids, the surfaces will increase as the squares, and the weights as the cubes of the homologous dimensions; it might well be that the additional relative weight due to the greater leverage should make it impossible to compass any larger flying machine than existing birds.” • “The experiments of Lilienthal, demonstrated that a man’s weight can be well sustained, at 22 to 25 miles an hour, by an apparatus with an area/weight ratio ~ 1.25” • “This apparatus need not weigh more than from 23 to 36 pounds, without motor or propeller, so that if the latter weigh some 60 pounds more,—carrying a man of about 150 pounds, upon sustaining surfaces of approximately 200 square feet in area. (W/S ~ 1.25)” 6. “The Material and Texture of the Apparatus” • “The sixth question relates to the material to be selected for the framing of the machine, for the motor, and to the texture of the sustaining surfaces. Nature accomplishes her purposes with bone, flesh and feathers, but man has at his command metals, fuel and textile fabrics.” • “For a beginning wooden frames covered with textile fabrics will answer for a beginning.” 7. “The Maintenance of the Equilibrium” • “The seventh problem relates to the stability of the apparatus in the air, and especially in a wind.” • “This equilibrium must be maintained at all angles of incidence and under all conditions of flight.” • “Until automatic equilibrium is secured, and safety is ensured thereby, under all circumstances, it will be exceedingly dangerous to proceed to apply a motor and a propeller.” • “Man will have to work out this problem thoroughly, if he is ever to make his way safely upon the air.” 8. “The Guidance in Any Desired Direction” (control capability) • “The eighth problem relates to the steering. It has been generally supposed that this would be best effected by horizontal and vertical rudders” • “The experiments of Lilienthal, have shown that slight changes in the position of the center of gravity are immediate and effective.” • “It might be preferable to provide moving mechanism within the apparatus itself, to shift the surfaces so as to bring back the varying center of pressure over a fixed center of gravity, and that in such case the operator need not move at all, except for the purpose of steering.” • “Two forms of apparatus have been evolved, each equipped with a different device, which are now believed to be materially safer than any heretofore produced.” • “This problem cannot be said to be fully worked out, but it is not that a good deal of experimenting will be required, and that such experiments will be fraught with danger.”

Biomimetics and Flying Technology

557



9. “The Starting Up Under all Conditions” (take-off capability) • “A really adequate practical flying machine will have to possess the power of starting into the air under all conditions” • “Three principal methods have been experimented with: −− Acquiring speed and momentum using such appliances as railway tracks −− Utilizing the force of the wing. −− The reaction of rotating screws—this will eventually supersede the two others.” • “This problem is as yet unsolved” 10. “The Alighting Safely Anywhere” (landing capability) • “Alighting safely anywhere is of vital consequence and is also an unsolved problem.” • “The best method proposed involves the selection of a smooth soft piece of ground and the alighting thereon at an acute angle.” (Smooth runway) • “It would be preferable—to imitate the maneuver of the bird who stops his headway by opening his wings wide, tilting back his body back and obtaining the utmost—retardation from the air before alighting upon the ground.” • “It would be preferable to utilize the reaction of a rotating screw to diminish the forward motion.” (It is interesting to note that the recommendations proposed by Chanute for “alighting safely” are quite similar to present day landing configurations and procedures including high lift systems with extended leading and trailing edge flaps, spoilers, and reverse thrust.)

Chanute went on to say, “These last two problems—the rising and alighting safely, without special preparation of the ground—seem very difficult and are probably the last of which will be worked out.” The general common “expert” belief at the time when the Wright brothers started the pursuit of their dream was that powered manned flight was not possible. This is evident by the quotes below of key scientific experts of the day. • Distinguished scientist (1895):—An “artificial flying machine is impossible for three reasons: 1. Nature, with her utmost effort, had failed to produce a flying animal of more than fifty pounds in weight. 2. That the animal machine was far more effective than any that man may hope to make. 3. That the weight of any artificial flying machine could not be less, including fuel and engineer, than 300 or 400 pounds.” • Lord Kelvin, Royal Society President (1895)—“Heavier-than-air flying machines are impossible.” • Simon Newcomb (1835–1909), astronomer, head of the U.S. Naval Observatory— “no possible combination of known substances, known forms of machinery, and

558

Biomimetics: Nature-Based Innovation

known forms of force, can be united in a practical machine by which man shall fly long distances through the air . . .” • Widely attributed to George W. Melville, chief engineer of the U.S. Navy, 1900—“If God had intended that man should fly, he would have given him wings.” • Simon Newcomb, Canadian-born U.S. astronomer, 1902—“Flight by machines heavier than air is unpractical and insignificant, if not utterly impossible.” • Even Wilbur Wright had his moments of doubt: . . . in 1901 “I said to my brother Orville that man would not fly for fifty years. Two years later we ourselves made flights.” These comments typify the current state of the “expert” technical knowledge, understanding, and opinions about the impossibility of ever achieving powered flight by man. In spite of this prevailing negative mental environment, the Wright brothers believed so strongly in their dream that in a short period of three years, they achieved it. Their accomplishment provides a vivid example of two powerful concepts: • “Believe you can, or believe you can’t; either way you will be right.” • “If the dream is big enough the facts don’t count.” 18.4.4 Langley’s Aerodrome Langley in the United States with a rather substantial financial backing of the government was also striving to be the first to achieve powered flight. His approach was fundamentally different than the Wright brothers in that he focused on building large models to test his ideas and once successful he then planned to build a larger man-carrying version. Langley built a large model of an “aerodrome” driven through the air by a steam engine under the action of its own propellers (Figure 18.33). After a series of unsuccessful tests beginning in 1894, Langley’s unmanned steam-driven model “number 5” made a successful 90-s flight of over half a mile about 25 miles an hour at a height of 80 to 100 ft

FIGURE 18.33 Langley’s Aerodrome No. 5 in Flight, May 6, 1896. (Photograph by Alexander Graham Bell.)

Biomimetics and Flying Technology

559

FIGURE 18.34 Failure of Langley’s manned aerodrome, October 7, 1903.

on May 6, 1896. Alexander Bell was a witness of the memorable experiments made by Langley on the 6th of May, 1896, with his large-sized model, which had a wing span of about 14 ft. After observing the flight of the large model, Alexander Bell stated “No one who witnessed the extraordinary spectacle of a steam engine flying with wings in the air, like a great soaring bird; could doubt for one moment the practicability of mechanical flight.” In November model “number 6” flew more than 5000 feet. Both aircraft were launched by catapult from a houseboat on the Potomac River. The full-scale Aerodrome, which was financed by the U.S. War Department and piloted by Langley’s chief assistant Charles M. Manley, was also launched the same way on October 7 and December 8, 1903. On both attempts, the Aerodrome failed to fly and crashed into the Potomac River seconds after launch as shown in Figure 18.34. Manley was pulled unhurt from the water each time. Nine days after the December 8 failure, the Wright brothers flew into history with their four successful flights near Kitty Hawk, North Carolina. 18.4.5 The Wright Brothers—Achieving the Impossible Dream The Wright brothers launched their systematic experimental studies and technology development studies utilizing the ten critical issues and design guidelines defined by Chanute. Although the initial focus of the Wright brothers’ technology development efforts was focused on solving critical elements 7 and 8 (stability and control), there were many unresolved issues associated with elements 1 through 6. When the Wrights started their quest to fly, they looked to the great experts on the subject—the birds for guidance. In a letter to the Smithsonian, Wilbur wrote that birds “are the most perfectly trained gymnasts in the world and are especially well fitted for their work, and it may be that man will never equal them.” Wilbur became particularly interested in observing the turkey vulture, or buzzard. In a letter to Octave Chanute, he noted that when one wing tip is twisted upward and the other downward, the buzzard “becomes an animated windmill and instantly begins to turn.” The Wright Brothers developed their wing warping concept to produce a lateral turning effect similar to what they had observed in nature. Wing warping consists of the twisting motion of the wing tips of an aircraft’s opposite directions to increase the lift on one wing tip and reduce the lift near the opposite wing tip to a produce lateral control mechanism. The brothers successfully tested this method of control successfully in a kite that they built in 1899 using ropes that they pulled on from the ground as shown in Figure 18.35.

560

Biomimetics: Nature-Based Innovation

(a)

4

3

3

2

4

3

6

2 7

10 s

6 1.

6 2

6

1 3

3

5

2

5

3

10

4 2.

7

2

5

8 9

12

17 13 1

14 6 8 13

7

1

12

9

14

11

3.

(b)

How the wings were warped Left

Right

Cradle Cables attached to cradle – sliding cradle to left of machine pulls trailing edge of right wing downward Cable (not attached to cradle) is moved automatically by downward movement of right wing FIGURE 18.35 Wright brothers’ wing warp concept for lateral control.

In 1900, 1901, and 1902, Wilbur and Orville Wright achieved a considerable advance over all previous flying results. A timeline leading to the success of the Wright Flyer is shown in Figure 18.36. The Wright brothers conducted their first glider experiments in 1900. The process that they followed with this glider and all subsequent gliders was to first fly the glider as a kite to make sure that the glider behaved as expected. They would then perform their gliding flights. The Wright brothers’ gliders were the first true gliders since they were bold enough to be the first to place a man prone and riding on a gliding machine, instead of hanging below the glider as had been the technique used by previous gliding experimenters. These earlier gliders were actually hang gliders since the pilot hung below the apparatus. Following their first glider in 1900, they discarded the fixed horizontal tail, and substituted for it a hinged horizontal canard at the front, which could easily be operated by the

561

Biomimetics and Flying Technology

1903 Built the first lightweight engine

Orville Wright

Wilbur Wright

1901 Wright wind tunnel.

1900

1902

1903

1901 • Developed propeller theory • Built their own propellers FIGURE 18.36 Wright brothers’ Flyer development.

pilot while under way. They believed that the pilot should constantly balance and guide the machine by the action of the canard, steering to the right or left by warping one wing or the other using light control strings leading to his hands. The control of the machine by the canard in front was found to be even better than had been hoped, and the landings were safely made. The flight performances of both the 1900 and 1901 gliders were much worse than they had predicted using the established methods mentioned in Chanute’s critical element 1. It was following the discouraging results of 1901 that Wilbur wrote: “When we left Kitty Hawk at the end of 1901, we doubted that we would ever resume our experiments. Although we had broken the record for distance in gliding, and although Mr. Chanute, who was present at that time, assured us that our results were better than had ever before been attained, yet when we looked at the time and money which we had expended, and considered the progress made and the distance yet to go, we considered our experiments a failure. At this time I made the prediction that men would sometime fly, but that it would not be within our lifetime.” However, rather than abandon their quest and their dream, they designed and built one of the first wind tunnels in the world in addition to innovative lift force and drag force balances. They subsequently conducted over 200 careful and meticulous wind tunnel experiments in which they tested various airfoil shapes and planform geometries. Their results validated that the existing and widely accepted force prediction methods were indeed significantly in error. Using the results of their gliding experiences of 1900 and 1901 together with their extensive wind tunnel results, they designed and built their 1902 glider. The successful experiments with the 1902 glider proved that the apparatus could be controlled so well that the Wright brothers determined that it would be safe to proceed with the construction of a full flying machine equipped with a motor and propeller. The 1903 Wright Flyer had the same basic structure as the 1902 glider, but it was larger and sturdier to accommodate the weight of the motor, the transmission system, and two

562

Biomimetics: Nature-Based Innovation

rear-mounted propellers. It had a wingspan of just over 40 ft and weighed just over 600 lb. The control mechanisms of the flyer included: • A forward canard surface with a movable elevator that controlled up-and-down motion • A rear rudder that controlled side-to-side motion • The wing-warping system that controlled the roll of the craft The pilot, lying prone in a cradle on the bottom wing, could operate the front elevator with a lever. He could operate the rear rudder and the wing-warping system simultaneously by shifting his hips in the cradle. They then faced another seemingly daunting obstacle; the necessary lightweight engine did not exist. Consequently they designed and built the first lightweight aluminum engine. They also designed and built their own propeller. In the process, they developed the first propeller theory. In 1903, and on the 17th of December of that year, after many trials and modifications, the Wright brothers achieved their incredible dream of the first powered airplane flight as well as a prominent place in history forever. The Wright Flyer, as all modern aircraft and also nature’s flyers, consisted of a system of systems as shown in Figure 18.37. The aerodynamic systems included the wing and tails lift and moment producing surfaces. The stability and control systems included the front elevators for pitch control, the

Rudders (Yaw control)

Lift producing surfaces

Thrust producting propeller

Moment producing surfaces

Light weight motor Lift & moment producing surfaces Elevators (pitch control)

Wing structure

Warping wing tips (roll control)

Chain drive (propellers)

Aerodynamic systems Stability & control systems Propulsion systems Structural systems Mechanical systems Flight control systems

Manual “control system” Fuselage body (truss structure)

FIGURE 18.37 (See color insert.) Wright brothers’ Flyer system of systems.

Landing gear (skids)

563

Biomimetics and Flying Technology

High lift

Aerodynamics

Stability & control

Propulsion Weights

Avionics

Structures

Configuration design

Systems

FIGURE 18.38 Many players of multidisciplinary aircraft design.

rudders for yaw control, and the warping wing tips for roll control. The propulsion systems included the motor and thrust producing propellers. The structural systems included the entire structural design of the wing, tails, and other supporting members. The mechanical systems included the fuel systems, the chain drive for the propellers, and the landing gear skids. The flight control system was a manual control system. The accomplishments of the Wright brothers are even more remarkable considering that without any aviation-related education, they fulfilled all the technical roles shown in Figure 18.38. These are in addition to flight test, manufacturing, purchasing, the legal department, and the sales department.

18.5  Evolution of Modern Aircraft Following the Wright brothers’ successful powered flights, numerous aviators from around the world also attempted to build their own powered flying machines. Most were unsuccessful but many indeed did achieve powered flight. Figure 18.39 shows examples of a variety of powered “aeroplanes” from the January 1909 issue of FLIGHT, The Journal of the Aero Club of the United Kingdom. The development of the great varieties of early airplanes was accompanied by an equally large variety of aircraft engine concepts as shown in Figure 18.40. The Wright brothers continued to improve their own flying machines by producing improved versions in the following years. Although early fights were headline events, the possibility of commercial aviation was very slow to catch on with the general public, most of whom were afraid to ride in the new flying machines. Improvements in aircraft designs also were slow. Wilbur and Orville Wright had been sending letters to the U.S. War Department offering to sell them a flying machine. Initially the Army officials had

564

Biomimetics: Nature-Based Innovation

The Voisin-Delagrange biplane

Bleriot no. 9 monoplane

Obre Aeroplane, “came to grief in its early trials”

Goupy triplane

FIGURE 18.39 Examples of the different types of aeroplanes in 1909.

8-cyl. Fiat Aero engine

8-cyl. Renault Aero engine

10-cyl. 40.h.p R.E.P. engine

4-cyl. Dutheil-Chalmer Aero engine

FIGURE 18.40 Early aircraft engines.

refused to entertain the “folly” of flying machines. However, in 1907, the Signal Corps released the specifications for an air machine with the following design criteria: • • • •

Fly faster than 40 miles an hour Remain aloft for at least one hour Carry a two-person crew Be transported in a mule-driven wagon

Figure 18.41 shows the Wright 1908 Model A Military Flyer arriving at Fort Myer, Virginia according to design specifications aboard a wagon, and attracting the attention of children and adults.

565

Biomimetics and Flying Technology

FIGURE 18.41 The Wright 1908 model: a military flyer. 2

1

Takeoff for 20 yard flight

En route between shed and Farnborough trial grounds 4 3

In flight before the accident

Airplane after collapse and crash

FIGURE 18.42 The Wright 1908 Model A Military Flyer.

The British were also developing their own military aircraft in 1909 apparently to meet similar design specifications. Figure 18.42 shows the first but unsuccessful flight of their military aircraft (Flight 1909). With the advent of World War I, the military value of aircraft was quickly recognized and production increased dramatically to meet the soaring demand for airplanes from governments on both sides of the Atlantic. Some of the most significant developments included more powerful engines that enabled aircraft to reach speeds of up to 130 miles per hour, more than twice the speed of prewar aircraft. Man’s initial attempts to fly used nature’s flyers as models as shown in Figures 18.27 to 18.31. Subsequent aircraft as shown in Figures 18.39, 18.41, and 18.42 radically departed from nature’s graceful and efficient designs. These designers restricted by limited knowledge

566

Biomimetics: Nature-Based Innovation

and rather primitive technologies were singly focused on the achievement of flight and not the efficiency, grace, and beauty achieved by nature’s flyers. 18.5.1  Technical Advancements on Many Fronts The development of our flying vehicle’s dreams, visions, attempts, and ultimate achievement was enabled by the progressive synergistic developments in aerodynamic concepts and tools, and other critical technologies developments as shown in Figure 18.43 (Kulfan, 2001). The evolutionary events leading to manned flight occurred over thousands of years. For as long as man has harbored the desire to fly, many attempts were made to emulate the flight of birds initially by strapping on some apparatus that had some resemblance to a bird and then leaping off a tower or other high prominent location. This type of event was repeated over and over, but seldom ever more than once by the same person. These manned flight experiments which we will define as Real Fluid Dynamics, RFD, have occurred over thousands of years. Most of these early experiments failed, many of which suffered serious consequences. Lilienthal’s and Chanute’s numerous glider experiments represented the first extensive scientific type of exploratory studies of manned flight. The fundamentals of fluid dynamics, FFD, equations were developed over a period of approximately a thousand years before the Wright brothers. These include all the contributions by da Vinci, Euler, Bernoulli, Navier–Stokes as well as boundary layer theory, the concept of circulation and linear theory formulations. However, the ability to utilize Abbas Ibn Firnas, 875

750

Avion III, 1897

Mach 1.0

mph 500

Lilienthal Glider, 1891

Vincent de Groof Ornithopter, 1874

Le Bris: Albatross, 1868

Dreams & Myths

–1000

–100

Abbas Ibn Firnas Tower Jump

Aristotle

Bernoulli Da Vinci: Euler

Robins 1st Whirling Arm

727

Wright flyer 0 1900

Lilienthal Whirling Arm

LeBris Albatross Lilienthal Glider

Boundary Layer

737

757

st 1 WT

1950

2000

RFD Cilculation Linear NACA Area Supercritical Theory Airfoils Rule Thin Wing Airfoil FFD/TFD LaRC8ft NACA 1 UWAL BTWT WB-WT BSWT NTF EFD

Wright Bros Wright Bros

BTWT Upgrade ETW

Engine Development Stability & Control

Babbage Analytic Engine Computers and CFD

Univac IBM 7090

FIGURE 18.43 History of commercial air travel.

767 777

B-377

250

Lilienthal Chanute Avion III Glider

Navier-Stokes

Cayley Whirling Arm

Edme Mariotte WT Concept Abacus

–10

G. Cayley 1st Glider

747

B-707

German B-247 dove B-40 B-80

Evolving Technology Extensive Utiliztation –10000

B-47

CRAY-1

567

Biomimetics and Flying Technology

these formulations awaited the development of the digital computer and consequently, had rather little significance in enabling early powered flight. Computational fluid dynamics (CFD) did not come into existence until about 1960. The impact of CFD, since that time, has become incredibly significant. The earliest form of experimental fluid dynamics (EFD) activities utilized whirling arm mechanisms. These did provide some basic understanding of lift and drag forces. The concept of the wind tunnel was conceived about 200 years before the Wright brothers. The first wind tunnel however, was actually built only a few years before 1900. The Wright brothers built one of the first wind tunnels. Their experiments provided valuable information and extensive test data that were used to support their flight experiments and ultimately were critical elements in their success. Two of the supporting critical enabling technologies included the emergence of a fundamental understanding of aircraft stability and the development and flight validation of simple but effective control mechanisms. In addition, the Wright brothers built the first lightweight aluminum engine which they used for the Wright Flyer of 1903. This development process leading to the first manned flight clearly illustrates a typical feature of the development of new technologies and concepts. This is the requirement to advance on many disciplinary or technical fronts. Following the pioneering developments of the Wright Brothers, the demand soon arose for greater flight capabilities. This was achieved through the further development of aerodynamic tools together along with developments in other technologies that led to a dramatic increase in the performance and operational characteristics of aircraft. Man’s knowledge and understanding of the nature of biomechanics of flight have evolved through the systematic development and utilization of the engineering tools and processes, as well as critical synergistic and enabling developments in many other technical disciplines. A measure of technology advancement with time is shown by the reduction of aircraft parasite nonlift dependent drag with time as shown in Figure 18.44 0.0300

CDP Based on airplane wetted area Biplanes

0.0200 CD

P

Retractable gear

Monoplane 0.0100

Jet engine

0 1900

1910

1920

1930 Year

FIGURE 18.44 History of aircraft parasite drag reduction.

1940

1950

1960

1970

Tubulent flow skin friction

568

Biomimetics: Nature-Based Innovation

(Kulfan, 1980). Parasite drag, CDP, is composed of viscous-related skin friction, separation, and excrescence drag. The major reductions in parasite drag have occurred with revolutionary changes in the shapes of the airplane. These include changing from biplanes to propeller monoplanes with retractable landing gear to swept wing jet aircraft. However, it is also seen that the steady evolutionary improvements have also been significant. 18.5.2  Coevolution in Technical Flight The development or “evolution” of aircraft technology has many apparent parallels with the evolutionary processes of nature. Coevolutionary developments as in nature have been critical enabling elements for aircraft. • “Competition” between different manufactures to produce the “best” overall aircraft favored by the customers and users. • Predator versus prey developments of military aircraft of the various countries including offensive capabilities and detection and evasion systems. • “Mutualism” between aircraft operations and airports. • Mutualistic developments between airframe manufacturers, relative to safety, security, environmental concerns, and airspace management. • Special interiors for airlines are an example of a commensalism symbiotic coevolutionary relationship between the manufacturer and the airlines. The evolution of technology is somewhat related to the Lamarckian theory of evolution which assumed that new traits were acquired by need and usage and these new traits are passed on to subsequent generations as long as they were used. New technology developments are driven by need and these developments will indeed be passed on to subsequent generations as long as the technology is used. 18.5.2.1 Commercial Aircraft Coevolutionary Mutualism An example of mutualistic coevolutionary developments between aircraft, airports, and resorts are shown in Figure 18.45. The demand for increased commercial air flights has led to developments in airports. The developments in airports have consequently increased utilization by commercial airplanes. The increasing number of airport-related operating restrictions has led to technology improvements in airplanes. Similarly, improvements in resorts entice people to desire to travel to them. The resulting increased demand for air travel has led to more and improved airplanes. 18.5.2.2 Commercial Aircraft Competition Coevolutionary Developments The competition between competing aircraft has been a strong evolutionary driving force in the technical growth of both commercial and military aircraft. The competition includes both intraspecific competition between like species of aircraft and interspecific competition between different species of aircraft. Figure 18.46 shows an example of commercial aircraft intraspecific competition. Boeing introduced the B-247 all metal advanced commercial aircraft in 1933. This was a major technology development including an all-metal semi-monocoque fully cantilevered wing, wing flaps, retractable landing gear, control-surface trim tabs, and an autopilot. The B-247 was faster than the premier U.S. fighter aircraft. The payload however was only ten passengers.

569

Biomimetics and Flying Technology

Resorts

Aircraft Boeing 777

Airports Houari Boumedienne Airport, Algiers, Algeria

FIGURE 18.45 Commercial aircraft mutualistic relations. Douglas DC-3 (1935) Boeing B-247 (1933)

Douglas DC-2 (1934)

FIGURE 18.46 (See color insert.) Commercial aircraft intraspecific competition developments.

Douglas Aircraft Company responded in 1934 with the DC-2 with similar technical advancements to the B-247; however the DC-2 could carry 14 passengers and thus captured the market from the B-247. Consequently only 75 B-247s were built while 156 DC-2s were built. Douglas then introduced the DC-3 in 1935. The speed and range of the DC-3 revolutionized air transport in the 1930s and 1940s. The airplane carried 28 passengers and had a lasting impact on the airline industry and World War II. It is considered to be one of the most significant transport aircraft ever made. 10,928 DC-3s were built (many for the military). It is seen that the previously discussed competitive exclusion principle is a success and survival factor for aircraft as well as for nature. No two species of similar requirements can long occupy the same niche (coexist). Figure 18.47 contains examples of both intraspecific and interspecific competition for commercial aircraft. The Douglas DC-7 was a transport aircraft built by the Douglas Aircraft Company from 1953 to 1958. It was the last major piston engine powered transport made by Douglas, coming just a few years before the advent of jet aircraft. 348 were produced. In order to once again be a leading competitor in the commercial aircraft business, Boeing took the bold step of starting to plan a pure-jet airliner as early as 1949. Boeing’s military arm had gained extensive experience with large, long-range jets through the B-47 Stratojet (first flight 1947) and the B-52 Stratofortress (1952). Boeing ushered in the jet age with the introduction of the B-707 in 1957. Boeing delivered a total of 1032 Boeing 707s, which dominated passenger air transport in the 1960s and

570

Biomimetics: Nature-Based Innovation

DC-7 (1953)

• Unswept wings • Propeller driven engines • 226 DC-7s were built

Interspecific

Interspecific

B-707 (1957)

DC-8 (1958) • Swept wings • Jet engines

• 556 DC-8s were built Intraspecific

• Ushered in the Jet Age. • Established Boeing as one of the largest makers of passenger aircraft • 1032 B-707s were built

FIGURE 18.47 (See color insert.) Commercial aircraft competition coevolution.

remained common through the 1970s. The B-707 established Boeing as one of the largest makers of passenger aircraft. This is an example of interspecific competition between propeller driven, straight wing aircraft, and jet powered swept wing aircraft. As an example of intraspecific competition, Douglas responded to the B-707 competition with the introduction of their own swept wing jet aircraft the DC-8 which captured some of the market from Boeing by delivering 556 aircraft. 18.5.2.3 Military Aircraft Competition Coevolutionary Development Figure 18.48 shows an example of competition between military aircraft. The competition to build the U.S. Joint Strike Fighter was between the Boeing X-32B Demonstrator and the Lockheed Martin X-35A Demonstrator. The Lockheed Martin Configuration was declared the winner and consequently the winners were awarded the contract to build the F-35 Joint Strike Fighter. Figure 18.49 shows an example of competition between the military aircraft of the Allies and those of their enemies. The Mitsubishi A6M Zero was a long range fighter aircraft operated by the Imperial Japanese Navy Air Service from 1940 to 1945. The A6M was usually referred to by the Allies as the “Zero.” When it was introduced early in World War II, the Zero was the best carrierbased fighter in the world, combining excellent maneuverability and very long range. In early combat operations, the Zero gained a legendary reputation as a “dogfighter,” gaining the outstanding kill ratio of 12 to 1 and provided the Japanese with control of the sky in the Pacific. The Hellcat was the successor to the Grumman F4F Wildcat. While the Wildcat was a capable fighter, early air battles revealed the Japanese Zero was more maneuverable and possessed a better rate of climb than the Wildcat. The Wildcat did have some advantages over the Zero. Wildcats were able to absorb a tremendous amount of damage compared to the Zero, and had better armament. The Wildcat was also much faster in a dive than the Zero. The Hellcat was the first U.S. Navy fighter for which the design took into account lessons from combat with the Japanese Zero. The advantages of the Wildcat carried over into

571

Biomimetics and Flying Technology

Boeing X-32B JSF Demonstrator

Lockheed Martin X-35A Demonstrator

F-35 Joint Strike Fighter

FIGURE 18.48 (See color insert.) Joint Strike Fighter competition. Japan Zero

US Hellcat F6F-3

FIGURE 18.49 World War II fighter aircraft coevolutionary competition for superiority in the Pacific.

the Hellcat and, combined with other improvements, created a fighter that outclassed the Zero almost completely. On average, the Hellcat flew 55 mph faster than the Zero; at about 20,000 ft it was 70 mph faster. At altitudes in excess of 10,000 feet, it had a comparable rate of climb. At all altitudes, due to its heavier weight and greater power, it could out-dive the Zero. Its armament, power, and range gave the Hellcat great versatility. It was the major U.S. Navy fighter type involved in the Battle of the Philippine Sea, where so many Japanese aircraft were shot down that Navy aircrews nicknamed the battle “The Great Marianas Turkey Shoot.” The Hellcat proved to be the most successful aircraft in naval history, destroying 5163 aircraft while in service with the U.S. Navy and U.S. Marine Corps in the Pacific. The F6F accounted for 75% of all aerial victories recorded by the U.S. Navy in the Pacific. The aircraft performed well against the best Japanese opponents with a 13:1 kill ratio against the Mitsubishi A6M. The effect of the Hellcat was to effectively reduce the effectiveness of the Japanese Zero to zero. 18.5.2.4 Military Aircraft Predator/Prey Coevolutionary Development Figure 18.50 shows an example of predator/prey mutualistic evolution of military aircraft and air defense systems. The initial use of military airplanes was for observation

572

Biomimetics: Nature-Based Innovation

Troup movement

Observation and dropping “things” 1

2 B-17

3 • Faster and higher airplanes • Increased resolution cameras • Night time bomber flights

Lockheed U-2

SR-71 Blackbird

4 • Radar development • Interceptor fighters • Anti-aircraft searchlights • Anti-aircraft/flak

5 • Very high altitude • Long endurance • High resolution camera 7

• Acoustic locator • Anti-aircraft guns

6 • Advanced radar systems • surface to air missiles

• Long range • Supersonic speeds • Initial stealth technology

And the arms race goes on FIGURE 18.50 Coevolutionary competition: Military aircraft and air defense systems.

of enemy troops locations, fortifications, strength, and movements. One of the earliest observation airplanes was the German Taube whose unique wing design was based on Zanonia ­macrocarpa seeds. The Taube design provided for very stable flight which was very desirable for early observations. Occasionally the pilots would also drop bricks, stones, or grenades on the enemy troops. The translucent wings made it difficult for ground-based observers to detect a Taube at an altitude above 400 m. The French called it “the Invisible Aircraft,” and it is sometimes also referred to as the “world’s first stealth plane.” The first hostile engagement was an Italian Taube in 1911 in Libya, using pistols and 2 kg bombs and thus the observation plane led to the development of the bomber. The opposition ground forces responded to opposition observation “bombers” by shooting at the relatively easy to hit targets and thereby becoming the first air defense system. The air defense system essentially consists of two critical elements including the ability to detect hostile aircraft and the use of weapons to destroy them. As shown in the figure, both elements continued to evolve relative to the increasing capabilities of the opposition aircraft. 18.5.3  Contrasting Biological and Technical Flight Evolutionary Drivers Figure 18.51 illustrates the paradigm shift in commercial aircraft design drivers from the earliest aircraft to the commercial aircraft of today (Kulfan, 2001). The designs of early propeller commercial aircraft were innovation driven with the primary goal of increasing aircraft performance, that is, to fly higher, faster, and farther, as well as to become larger.

573

Biomimetics and Flying Technology

Cruising speed kts 600

Efficiency Versatile, quiet, economical

550 500

B707, DC-8

450

XB-47

400 350 300 250 200 150

, ter Fas r er, gh rthe i H Fa

100 B-247 50 0 1930

DC-3

1940

B747-100

B747-300

B747-400

Survival Better, cheaper greener

Functionality Big, far, efficient

DC-4

DC-7C DC-6B Lockheed 749

1950

Design drivers and technology focus • Innovation driven • Technology focus: – Innovation – Performance

1960

1970

2000 2010 2020 • Customer & society driven • Supplier options vs risk • Technology focus – Growth – Economics (CAROC) • Customer driven – Range • Supplier options • Manufacturer driven – Noise • Technology focus – Process, (Cost to build) • Customer input – Growth • Technology focus – Fuel efficiency/emissions – Economics – Growth – Travel affordability/time – Range – Economics – Safety/security – Range – Noise – Air traffic management Year

1980

1990

FIGURE 18.51 Commercial aircraft design drivers and technology focus.

The introduction of jet-powered swept wing aircraft provided revolutionary increases in aircraft size, cruise speed, cruise altitude, and range. The cruise speeds and altitudes have remained essentially constant for subsonic jet aircraft. The early jet aircraft designs were determined by the manufacturer with inputs from the airline customers. Technology advancements were focused on increased functionality including aircraft growth capabilities, operating economics, and range. The designs of the third generation of commercial aircraft were increasingly driven by customer inputs with supplier options, and the areas of technology development were focused on various efficiencies including family growth concepts, improved economics, increased range, and aircraft noise reduction. The new generations of commercial aircraft designs will be both customer and society driven. The manufacturer will provide design options determined by strategic assessments of economic risks and opportunities. The technology focus areas will include in addition to those of previous aircraft, more efficient manufacturing processes, fuel efficiency and reduced emissions, travel time and affordability, safety and security, and improvements in the ATM system. Because of the highly competitive nature of today’s market place, one of the primary goals of new aircraft is that of basic company survival. Unless a new airplane is the best offering to the airlines, lack of sales could force a company out of business. If we compare the technology drivers of nature (Figure 18.9) with the technology drivers for commercial aircraft (Figure 18.51), we arrive at the observation shown in Figure 18.52. The evolutionary driver for nature’s flyers was initially survival, and then gradually progressed through stages of efficiency, functionally, and ultimately higher, faster, and farther. The technical driver for manned commercial flight, however, appears to have progressed in the opposite order. This might not be biologically accurate, but it is an interesting observation.

574

Biomimetics: Nature-Based Innovation

FEW Survival

Higher, faster, farther Functionality

Efficiency

Efficiency

Functionality

Survival

Higher, faster, farther

Birds

Many

Aircraft

FIGURE 18.52 (See color insert.) Why did flight evolve?—Contrasting technology development drivers.

18.6  Aircraft Future Technology Needs and Opportunities “The rapid journey from the first tentative flights to the modern airliner is a testament to the restless search for technological improvement that has long characterized the aircraft business” (European Aeronautics, 2001). This rapid journey is evident in dramatic change in aircraft configurations as shown in Figure 18.53. Forty six years after the feeble but historically significant flight of the Wright brothers, the sleek, swept wing jet powered B47 flew. Forty six years later, the B777 and the F-18E made their debut in the commercial transport and military arenas respectively. “A generation ago, ‘Higher, Farther, Faster’ were the imperatives for any vision of the future for air transport. Now they are ‘More Affordable, Safer, Cleaner and Quieter,’— The key to securing these objectives is investment in research and technology.” The high degree of design sophistication of the modern aircraft can be seen by the areas defined by CFD and multidisciplinary design and optimization techniques in the B787 as shown in Figure 18.54 (Johnson, 2003), as well as by the advanced design and operating features of modern military aircraft as shown in Figure 18.55. Aerodynamics together with structural and manufacturing considerations largely define the exterior contours of airplanes. The technological progress in aeronautics is often associated with size and shape of the aircraft. It can be argued with some degree of validity that the vast majority of existing aircraft as shown in Figure 18.56 were established approximately 65 to 70 years ago (Kulfan 2009). Ingo Rechenberg said that he finds it “very remarkable that after 100 years develop­ment a modern aircraft still looks like a bird: a spindle body, the wing in front and the elevator behind. That’s the solution of biological evolution and that’s still the basic concept of a modern aircraft.” However, this certainly does not mean that man has exhausted all ­possible aircraft-related innovative or creative technology developments.

575

Biomimetics and Flying Technology

B 777 Wright Flyer

46 yrs 46 yrs

B 47

46 yrs F-18E

FIGURE 18.53 The rapid pace of aircraft development. Wing

Wind-tunnel corrections

Planform Vertical tail and design aft body design Aeroelastics

APU inlet and ducting

Wing-tip design controls Reynolds-number corrections High-speed wing Flutter

High-lift wing design Control-surface failure analysis

design Vortex generators Icing

Cab design Interior air quality

Wing-body Air-data ECS inlet fairing design system design Cabin location Inlet design Exhaust noise Buffet Inlet certification boundary system design Engine/airframe Engine-bay thermal Thrust-reverser integration analysis Design for FOD Community noise design nacelle design prevention

APU and propulsion fire suppression Avionics cooling Design for stability & control

FIGURE 18.54 CFD contributions to the 787. V-22 Osprey X-22 Raptor

X-35 Joint Strike Fighter

FIGURE 18.55 Aircraft multimission advanced technology concepts.

18.6.1  Aircraft Technology Needs and Development Trends The needs for continued technical innovation and development for aircraft far exceeds just the external shape or operating procedures. The needs for technical innovation permeate every element of the systems of systems that make up modern aircraft.

576

Biomimetics: Nature-Based Innovation

Messerschmitt (Lippisch) variable sweep wing patent (1941)

Lippisch delta wing supersonic fighter (1944)

Junkers swept forward wing bomber testbed (1944)

X-21A Laminar flow control airplane (1960)

Blohm and Voss (Vogt) oblique wing fighter concept (1944)

Swept wing Busemann (1935) R.T. Jones (USA) Northrup flying wing

FIGURE 18.56 Most innovative aerodynamic concepts were established 50 to 65 years ago.

Future potential ???

Turbofans Turbojets Sweptwings

Technology advance “Evolutionary” development

“Revolutionary” breakthrough

Turboprops Reciprocating engines Unswept wings Time FIGURE 18.57

General nature of technology advancements for commercial aircraft.

The general historic trend in transport aircraft technology (Kulfan, 1980) is shown in Figure 18.57. This consists of steady periods of technical evolution that reflect the process of design refinements plus technical innovations, with occasional sudden dramatic improvements that indicate the appearance of some major technical innovation such as the cantilever wing, pressurized cabins, jet engines, swept wings, or all composite structures. It is therefore convenient to distinguish between two types of technical progress: • “Revolutionary”—a development process characterized by significant rather sudden advancements. These are typically due to such things as: development and

577

Biomimetics and Flying Technology

utilization of new concepts, new technologies, or innovative use of technological advances in related technical disciplines. • “Evolutionary”—a development process in which a particular class of airplanes is progressively advanced typically due to improved understanding, refined processes, and perhaps a series of minor technology breakthroughs. Figure 18.58 illustrates the general improvement in aerodynamic cruise efficiency for transport type of aircraft with time. The aerodynamic cruise efficiency is commonly defined as the product of Mach number times the lift to drag ratio, (M L/D). Both evolutionary and revolutionary advances have made significant contributions to the aerodynamic cruise efficiency. The revolutionary advancements were due mainly to cruise speed advances that occurred as the fundamental class of airplanes changed from “classic” aircraft to “swept wing” aircraft to “supersonic” aircraft. The revolutionary advancement occurring in the late 1950s was the introduction of the jet engine along with the swept wing aircraft. In the early 1970s, the next revolutionary advancement was the introduction of the slender wing, jet engine supersonic transport. Within each class of airplanes, significant improvements were achieved by means of continuing evolutionary advances. The combined evolutionary advances, in fact, exceed the revolutionary advances. It is clear that an effective technology program must identify and also actively plan for both types of technology developments. Some of the general critical areas for which technology developments are in need for commercial air transportation that are especially related to the design of an aircraft include: • Factors that affect the costs to own, to operate, and to maintain a commercial aircraft. • Aircraft community noise reduction. This includes noise sources from the propulsion system and airframe related noise. • Design and manufacturing processes, (cost to build). • Emissions/fuel efficiency. M(L/D)

* Supersonic configurations with M(L/D) scaled to mach = 2.4

• Supersonic aircraft • Slender wing • Integrated designs TCA proj • Supersonic jets Ref H NL TCA NL • Speeds > 1,400 mph 25% • Non-linear design optimization *US SST Ref H lin *TU-144

24 22 20 18

*Concorde

16

747-100

14

707-320

12 10 8 6 4 2 0 1930

707-120

SVC10

767

747-400

777

• Swept wing aircraft • Tailored designs • Subsonic jets • 550 to 580 mph

TU-114

DC-7B Brit 300 CL-44 B377 DC-7C L1649A LO40 DC-6B DC-6 DC-4 C-4

• Classic aircraft • Unswept wings • Isolated component designs • Propellers • 180 to 350 mph

DC-3

1940

1950

1960

1970 Year

1980

1990

FIGURE 18.58 Commercial aircraft aerodynamic efficiency advancements.

2000

2010

578

Biomimetics: Nature-Based Innovation

The primary technical disciplines that are most significant in achieving the aforementioned technology development needs include aerodynamics, propulsion systems, structures, and materials. These have been areas and remain areas of continual developments. One of the major contributions to the operating costs of an airplane is the total fuel cost. Thus, reducing the amount of fuel usage is extremely important. The two primary factors contributing to the fuel usage for a specific design include the aerodynamic efficiency expressed as the lift to drag ratio, L/D, and the fuel used efficiency expressed as the thrust specific fuel consumption, SFC. For L/D, bigger is better, and for SFC, smaller is better. 18.6.1.1 Aerodynamic Efficiency Technology Development Options Figure 18.59 shows the impact of a 1% reduction in drag on the number of gallons of fuel usage per year for various subsonic transport airplanes (Anderson, 2002). Also shown in the figure is the design sensitivity of an advanced supersonic transport aircraft to 1% drag improvement. Excess drag means wasted fuel, increased operating costs, increased emissions, and potentially heavier airplanes. The cruise aerodynamic efficiency for a subsonic aircraft can be related to a parameter which is the ratio of the wing span, b, divided by the square root of the total effective wetted area, AwetEFF of the airplane. Using this parameter, we can define the ideal potential, (L/D)IP, lift-to-drag ratio for a planar transport aircraft having only fully turbulent flow friction drag and lift related drag corresponding to an optimum lift distribution for the planar wing span (Kulfan, 1980) as:



b  L   = 19.34 D IP AwetEFF

(18.1)

As shown in Figure 18.60, the L/D cruise values of existing commercial aircraft correspond to about 75% to 80% of the corresponding ideal potential values. The additional drag items include compressibility drag, non-optimum induced drag and trim drag, profile drag related to the thickness of the airplane, excrescence drag, and other miscellaneous drag items. Subsonic transport airplane 1% Drag in terms of gallons per year per airplane • 737 ≈ 15,000 • 727 ≈ 30,000 • 757 ≈ 25,000 • 767 ≈ 30,000 • 777 ≈ 70,000 • 747 ≈ 100,000

Advanced supersonic transport airplane 1 % Design improvement in supersonic (L/D)max ~ 1.5 < Mach ≤ 2.4

• ~ 1 Drag count (CD = 0.001) • Reduces airplane gross weight by 10,400 pounds • Saves 7,500 pounds of fuel (1,100 gallons) for design mission • Is equivalent to a 2,400 pound reduction in structural weight Transonic climb/Acceleration: ~ 0.95 < Mach < 1.5 • 1 Count drag reduction reduces airplane gross weight by 1,000 pounds

Subsonic climb/cruise: ~ 0.40 < Mach < 0.95

• 1 Count drag reduction reduces airplane gross weight by 1,500 pounds Also

1 Drag count of unexpected supersonic drag in a final design results in a range loss of ~ 50 nmi

FIGURE 18.59 Effect of 1% drag on subsonic and supersonic transport airplanes.

579

Biomimetics and Flying Technology

Revolutionary improvements to increase span, or span efficiency • Innovative planforms • Innovative wing tip concepts • Larger span wing concepts • Load alleviation • Etc.

30

(L/D)IP 25 20 15 10

Revolutionary improvements to reduce CD0 • Innovative designs • Laminar flow control • Turbulent viscous drag reductions • Etc.

TUR

E BUL

N

RPL T AI

ANE

P UM

XIM

MA

N OTE

TIA

D

L L/

Evolutionary improvements

Existing subsonic commercial airplanes

• Reduce profile drag • Eliminate compressibility drag • Optimum lift distribution • Multi-point optimization • Minimum trim drag • Excrescence drag reduction • Etc.

5 0

b

AWET _EFF FIGURE 18.60 Options for improvements in aerodynamic cruise efficiency, L/D.

The figure also shows options for evolutionary aerodynamic improvements to approach the ideal potential limits. Options for revolutionary aerodynamics innovations to increase the ideal potential aerodynamics limits are also shown. These include reductions in nonlift dependent drag, CD0, and effective span increases to reduce the induced drag. 18.6.1.2 Propulsion System Technology Development Trends The relative fuel usage for commercial jet aircraft as shown in Figure 18.61 has decreased approximately by 70% since their first introduction. This dramatic improvement is largely due to propulsion system efficiency improvements associated with revolutionary engine cycle developments. However as shown in Figure 18.62, the total fuel consumption has increased due to dramatic growth in air travel and the number of commercial airplanes. The engine cycle improvements have also resulted in reductions of all of the emissions components per engine as shown in Figure 18.63. The total amount of emissions has increased because of the aforementioned increase in total aircraft flights. As shown in Figure 18.64, aircraft-related noise is a major annoyance to local neighborhoods surrounding the airports. This impacts the utilization of the airplanes by the imposition of flight time restrictions and reducing the number of flights into and out of various airports. In some instances, certain classes of airplanes may not be allowed to even use specific airports. Concerns about potential noise can prohibit the development of new airports and expansion of existing airports. The restricted usage of classes of airplanes has impact on airline profits as well as on the aircraft manufacturer by reducing the potential sales for airplanes having significant operational restrictions.

580

Biomimetics: Nature-Based Innovation

100

75

Relative fuel use per seat–km, % 50

707247 ~7 Im 0% pr ov em

727100

Fuel use for a 1600-km trip on various airplanes

727200

747-200B 767-

en t

747-

757200

777200

25

0 1955 1960

1965

1970

1975

1980 1985

1990

1995

2000

Initial service date, year FIGURE 18.61 Commercial jet aircraft fuel burn efficiency trend. Gallons consumed (in billions)

Seat miles per gallon of fuel 60 58 56 24% 54 52 50 48 46 44 0

19 8 19 1 82 19 8 19 3 8 19 4 8 19 5 86 19 8 19 7 8 19 8 8 19 9 9 19 0 9 19 1 92 19 9 19 3 9 19 4 95 19 9 19 6 9 19 7 9 19 8 99 20 0 20 0 0 20 1 0 20 2 0 20 3 0 20 4 0 20 5 0 20 6 0 20 7 08

20 18 16 14 12 10 8 6 4 2 0

Seat miles per gallon of fuel Total gallons of fuel consumed

Year FIGURE 18.62 Total fuel consumption and fuel efficiency of U.S. airplanes have increased. (Courtesy of U.S. Department of Transportation.)

The primary noise contributors include jet noise, inlet noise, and airframe noise. As shown in Figure 18.64, significant jet noise reductions have been achieved largely due to higher by-pass ratio turbofans. With the 787 Dreamliner, Boeing has introduced new technologies to create better environmental performance ranging from fuel use and emissions, to community noise. These technologies include: • Reduced Fuel Use  our key technologies contribute to a 20 percent improvement in fuel use for the 787 F Dreamliner as compared to today’s similarly sized airplane. New engines, increased use of lightweight composite materials, more efficient systems applications, and modern aerodynamics each contribute to the 787’s overall performance improvements.

581

Biomimetics and Flying Technology

500 Exhaust Carbon dioxide Nitrogen oxides Water vapor Sulfate Soot

400

ICAO standards, %

Jet engine Fuel Combustion

Air

300 Hydrocarbons Carbon monoxide Oxides of nitrogen Smoke

200

100 0

Pre–1981

1981–91

1991–Present

Year of engine certification FIGURE 18.63 (See color insert.) Jet engine emissions have been reduced.

• Major annoyance to local neighborhoods • Flight time restrictions • Reduces number of flights • Flight delays and cancellations • Eliminates use of some airports • Prohibits new airports or existing airport expansions • Reduces aircraft utilization and airline profits • Restricts growth around airports • Source of numerous law suits and costly settlements

120

Noise Level (EPNDB) 1500 ft Sideline B-52 707-100

Turbojet 720 1st Generation Turbofan

110 707-300B

100

90 1950

727-100

737-100 747-100 727-100 747-100

2nd Generation Turbofan 747-300 747-400 767-200 767-300 737-200 777 737-500 737-500

707-200

1960

1970 1980 1990 Initial service date, year

2000

2100

FIGURE 18.64 Commercial jet aircraft noise level reduction trends and remaining challenges.

• Cut Emissions  he 787 has being designed to ensure it will be significantly better than today’s T requirements—more than 30% better than today’s 767 class of airplanes—and it will be better than the future, more stringent regulations being incorporated by the Committee on Aviation Environmental Protection (CAEP). • Quieter Takeoffs and Landings  oeing has worked to reduce the sound footprint—the distance across which disB turbing noise is heard. The 787 Dreamliner uses a number of new technologies— most importantly, acoustically treated engine inlets and chevrons, the distinctive serrated edges at the back of the engine, and other special treatments for the engines

582

Biomimetics: Nature-Based Innovation

and engine casings—to ensure that all sound greater than 85 decibels (about the level of loud traffic heard from the side of the road) never leaves the airport boundaries. • Point-to-Point Travel Enabled  he mission capability of the 787 Dreamliner also provides an environmental T advantage, allowing airlines to offer more direct flights connecting midsized cities. Connecting people more directly to their destinations offers a number of environmental benefits. A more direct route uses less fuel, which means fewer emissions. Likewise, fewer takeoffs and landings reduce the total noise footprint. Removing pass-through traffic keeps airports and airways clearer. • Manufacturing Technologies Mean Less Waste  ecause the 787 is made primarily of carbon-fiber composite material, which is B trimmed like cloth, manufacturing processes produce less scrap material and waste. 18.6.2  Aircraft Technology Development Options Where do we go from here in the next 100 (or even 20) years? There are at least three possibilities (McMasters, 2003b), as shown in Figure 18.65 all of which are likely to occur: • Keep running harder and harder (i.e., doing what we have been doing) to develop innovation solutions for the needs of today’s classes of aircraft. • Schedule a breakthrough (e.g., a possible Sonic Cruiser II via large reductions in sonic boom intensity and “aerospace plane” technology) or an invention (e.g., economically and logistically viable alternatives to fossil fuel propulsion schemes for transport aircraft). • Start a whole new game—one in which the gap between the possible and the achieved is once again very large, for example, the whole range of possibilities for

Continue to develop innovative solutions for today’s classes of aircraft

Expand the range of the “possible” to new concepts and/or expanded flight regimes

Start a whole new game — where the gap between “possible” and “actual” is again very large, e.g., micro air vehicles All of the above! FIGURE 18.65 Aircraft future technology needs and opportunities.

Biomimetics and Flying Technology

583

uninhabited (combat) air vehicles (UAV/UCAV) type vehicles, which represent a complete fusion of traditional and emergent aerospace vehicle technology with “information and communications technology.” A general characteristic of technology development is the need to advance the technology on all fronts. Consequently, each and every component technology or systems development becomes a critical element in the overall innovative development processes. The view of an aircraft as an integrated system of systems implies that a technology development in one system no matter how seemingly small or unrelated can become an important element in the overall system development. Consequently, one should be open beyond their immediate technical discipline to any concept that offers potential improvements for the system as a whole.

18.7  Biomimetics in Past, Present, and Future Aircraft How do technology advances happen? Sometimes all it takes is intuition or a flash of brilliance. But usually, the engineers and designers who were most successful in furthering the technology of flight used a systematic approach. “Simply, they first recognized that something needed to be done. Then, the engineer or designer would propose ways to accomplish this by observing what seemed to work in nature and/or through documented technical knowledge” (Rumerman). Why seek inspiration from nature? All birds fully and elegantly embody a number of items that have been the subject of much research and development in aviation in recent decades. These items include mission adaptive wings of extreme sophistication, an advanced high-lift system, an active flight critical control system, a self-repairing/self-reproducing composite structure, and fully integrated system architecture. There are many other technical areas of potential importance that are well demonstrated in natural flying devices such as the various uses of vortices for flow control, and the problems and benefits of controlled large-scale unsteady separated aerodynamic flows. There are numerous other reasons why it is wise to seek inspiration from nature: • Nature has conducted myriads of experiments over hundreds of millions of years. • Nature provides the ultimate potential reward for design improvements—the possibility of continued existence. • Nature abides by the same laws of physics, chemistry, and molecular interactions. • Same source of basic materials and similar operating conditions. • There remain millions of nature’s inventions we have yet to discover. “Living organisms are examples of design strictly for function, the product of blind evolutionary forces rather than conscious thought, yet far excelling the products of engineering. When a designer looks at nature he sees familiar principles of design being followed, often in surprising and elegant ways. Sometimes, as in the case of flight, he is inspired

584

Biomimetics: Nature-Based Innovation

to invention: more commonly, he discovers his ideas embodied in some animal or plant” (French, 1994). Robert J. Full, of the University of California, Department of Integrated Biology, has suggested “Do NOT Mimic Nature—Be INSPIRED by BIOLOGY, and use these novel principles with the best engineering solutions to make something better than nature.” There is obviously much to learn from the masters of flight. The focus will not be on a single technical discipline, but rather on all systems and technical disciplines involved in the complete airplane infrastructure and perhaps most importantly on tools, concepts, and processes for technical innovation.

18.8  Impact of Size on Nature’s Designs Size can have a significant effect on nature’s designs and performance characteristics. In seeking inspiration from nature’s designs, it is very important to determine if the desired biological features and/or performance characteristics are critically size dependent. This is especially true if the size of the intended technical application differs in size by orders of magnitude from the biological “model.” 18.8.1  Dimensional Analysis and Similarity for Insight into Nature A good approach to gain a fundamental understanding of the impact of size on nature’s designs is through the application dimensional analyses and simple similarity principles. A fundamental understanding of nature and also of physics is an essential element for identifying and/or conceiving innovative technology or design concepts. Figure 18.66 contains the dimensions of various physical quantities in the mass–length– time system, MLT. The physical quantities are also shown in a physiological system (Astrand, Dimensions Quantity Length Mass Time Cross Section Area Surface Area Volume Velocity Frequency Acceleration Force Impulse Energy Power

Physical

Physiological

Physiological Mass Based

L M t L2

L L3 L L2

M⅓ M M⅓ M⅔

L2 L3 Lt–1 t–1 Lt–1 LMt–2 LMt–1 L2Mt–2 L2Mt–3

L2 L3 L0 L–1 L–1 L2 L3 L3 L2

M⅔ M M0 M–⅓ M–⅓ M⅔ M M M⅔

FIGURE 18.66 Dimensional analysis, similarity, and insight into nature.

• Smallest and biggest birds • “Eat like a bird → A little or a lot? • Why do little birds with big eyes sing early in the morning? • Why no small mammals in the sea? • How tall can a tree be? • Why can a whale be so big? • If a flea was as big as a man, could it really jump over the space needle? • How is a flea like a compound bow? • Why do mosquitoes “come out” at night? • Can an insect fly faster than a commercial jet aircraft?

Biomimetics and Flying Technology

585

1977; Georgian, 1964) based on a characteristic length, L. The physiological dimensions are useful for exploring the effects of size changes on characteristic shapes and performance limits. The appropriate selection of a characteristic length can be rather ambiguous. Most often, therefore, the body mass of an organism is used as the reference index for the correlation of morphological and physiological characteristics, especially when attempting to compare similar but different creatures. Huxley’s allometric equation (Y = aMb) is often used to mathematically describe the variation of various morphological and physiological characteristics with mass. This is a very simple and at the same time, the most versatile mathematical expression for intra- or interspecies comparisons. The exponents (b) for the various allometric equations can be predicted for all biological variables definable in terms of the MLT system of physics (M = mass, L = length, T = time). The exponents can often be estimated by means of dimensional analysis using appropriate similarity criteria such as: geometric similarity, mechanical or dynamic similarity, kinematic or biological similarity; or elastic similarity. The scaling coefficient “a” is generally determined by statistical analyses of existing appropriate data sets. Using the mass-based physiological dimensions, we can identify simple relations that can provide answers to interesting questions about nature such as shown in Figure 18.66. 18.8.1.1 What Determines How Small Is Small and How Big Is Big? Birds are warm-blooded animals and must maintain essentially a constant internal temperature. Heat loss for an animal is proportional to the surface area. The heat-generating capability is proportional to the mass of the animal. Using the physiological mass-based relations shown in Figure 18.66, the relative heat loss to heat generation ratio is therefore proportional to (mass)−1/3. Smaller animals therefore, have an increasingly difficult task to maintain a constant internal body temperature. This limits the smallest size for a bird which is the male bee hummingbird that lives in Cuba. It weighs 0.056 oz and is about 2.75 in in length. The bill and tail account for half of its length. The smallest bat is the bumble bat which weighs less than a penny and when full grown is about 0.433 in in length. The whitetoothed pygmy shrew is the smallest known mammal by mass, weighing only about .05 oz and is about 1.43 in long. Because of their tremendous metabolic requirements, the tiny hummingbirds, bats, and mammals must eat a large amount of food equivalent to the average human consuming an entire refrigerator full of food. Hummingbirds eat roughly twice to three times their own body weight in flower nectar and tiny insects each day. Consequently, if someone says “you eat like a bird,” it should not be taken as a compliment. Similarly, because of temperature loss, there are no small mammals in the sea. The heat loss is even greater because of the cold water in the oceans. Little birds, as previously mentioned, require a large of nourishment daily. The square/cube law can also be used to explain the limit to the greatest height of a tree. The bending strength of a tree is proportional to its cross-section area. The mass of the tree is proportional to the cube of its linear dimension. Consequently tall trees experience greater stresses during a wind storm. This ultimately limits the height of a tree. Whales do not have to structurally support their own weight because of the buoyancy effect in the water. Therefore whales can be many times larger than the largest land mammal. 18.8.1.2 How High Can a Flea Jump? Fleas can normally jump about 100 to 200 times their own height. If a flea was as tall as a man, could it really jump over the space needle as shown in Figure 18.67?

586

Biomimetics: Nature-Based Innovation

FIGURE 18.67 Flea and the space needle.

The answer for this question can be obtained by using the above physiological massbased parameters. The maximum height of a jumping object is related to the lift-off velocity. For a standing high jump, the lift-off velocity, ∆V, times the mass equals the jumping impulse. Impulse as shown in Equation 18.1 varies directly with mass. Therefore as shown in Equation 18.2, the lift-off velocity is independent of the size of mass of the jumper. “—neither flea nor grasshopper is a better but worse jumper than a horse or a man.” (Thompson, 1942). Actual differences in jumping heights are due to the physiological and anatomical differences in the jumpers.

Impulse = M * ∆V → ∆V ≈ M0

(18.2)

The bar chart shown in Figure 18.68 shows that the froghopper, locust, and man can each achieve a maximum standing high jump of about the same height even though the mass of these jumpers varies by nearly a factor of a million. The jumping height of the man is defined by how high the center of gravity moves from liftoff to the peak height. The click beetle jumps less than the others since it jumps while lying on its back. A small creature is actually handicapped by its size. The ballistic coefficient (BC) of a body, which is a measure of its ability to overcome drag in flight, is proportional to mass divided by area and therefore varies with M1/3. Therefore, a small creature is affected more by drag during the jump trajectory. The jumping impulse is actually achieved by a finite acceleration, ∆A acting for a finite time ∆T.

Impulse = ∆A • ∆T

(18.3)

As shown in the Figure 18.66, acceleration varies with M−1/3 and the time varies with M1/3.

587

Biomimetics and Flying Technology

The required jumping acceleration is shown in Figure 18.69 as a function of the mass of the jumper. The black triangle corresponds to 175 lb (80,000 g) man with a jumping acceleration of about 1.75 g. The black line is an extrapolation to small jumper weights using the allometric relation M−1/3. This relation predicts that for a flea size creature, the required jumping acceleration would be about 300 g to 500 g. This estimate is in agreement with the experimental data shown in the figure (Burrows, 2006). Consequently for a very small creature such as a flea, the necessary acceleration becomes extremely large and well beyond the capability provided by the lever/muscle jumping system utilized by man. Standing high jump inches

30 Height

25

Froghopper

Man

Locust

20 15 10

Click beetle

5 0

0.013

0.04 3 Weight in grams

81600

Height = Upward movement of CG from lift off to peak FIGURE 18.68 Fleas, froghoppers, and man jumping heights. Jumping acceleration (Gs) vs jumper weight

1000

Acceleration, Gs

1.75 (MMAN/M)–1/3 100

10

1 0.001

Froghopper SPRINGS Grasshopper SPRINGS + LEVERS

0.01

0.1

1

Flea Froghoppers Man FIGURE 18.69 Jumping accelerations of fleas, froghoppers, and man.

Cricket LEVERS

10 100 Weight grams

1000

10000 100000

588

Biomimetics: Nature-Based Innovation

Compound bow draw weight lbs

Stored potential energy

Holding weight

Draw length FIGURE 18.70 Loading curve of a compound bow. (From Heitler, W.J., School of Biology, University of St. Andrews, Scotland, UK. With permission.)

The muscles of a flea are simply not powerful enough to do this. Consequently, the actual jumping performance of a flea would be very poor if it were not for the specialized design of its legs provided by nature. A flea jumps by releasing energy that it has stored in its elastic “springs.” These springs are loaded relatively slowly (about 50 ms) by means of muscles which convert chemical energy into potential energy stored in the elastic springs called resilin. Resilin is nature’s almost perfect elastic material. The time of the loading process is on the order of 1 ms. The flea stores the energy with an “over the top dead center” mechanism so that the muscles that load the jumping “springs” can relax and will not have to work to retain the stored energy (Burrows, 2006). When released, the stored potential energy is suddenly converted into impulsive kinetic energy that results in the jump. This is very similar to the mechanism of a catapult or that of a compound bow shown in Figure 18.70. The archer converts chemical energy in his muscles to potential energy elastic energy in the limbs of the bow, by drawing the bowstring back. The stored potential energy is indicated by the gray area in the figure. By proper design of the cams on the bow, the force required to hold the string back at its design condition can be very small or even zero force. Releasing the bowstring converts the stored potential energy into the kinetic energy of the arrow. In a similar manner, the flea can convert its stored potential energy into kinetic jumping energy. Heitler and his colleagues at the School of Biology at the University of St. Andrews, Scotland have conducted extensive studies (Heitler, 2009) to understand the jumping mechanisms of the grasshopper, which include a combination of the spring mechanism of the flea and the leverage system of a man. Figure 18.71 contains a sequence of pictures of a jumping grasshopper. The contraction/spring loading process followed by an instantaneous release are also shown in the figure. The initial flexion, initial compression, and extensor load the resilin flexor. Relaxation of the flexor releases the legs powering the jump. 18.8.2  Physiological Mass-Based Comparisons Figure 18.72 shows some comparative measurements for a number of birds and insects (Thompson, 1942). The physiological mass-based dimensions are also shown for the various

589

Biomimetics and Flying Technology

Initial flexion

Initial compression

Flexor-extensor co-contraction

Trigger relaxation of flexor

And off it goes

Bending spring

FIGURE 18.71 Jumping mechanism of a grasshopper. (From Heitler, W.J., How Grasshoppers Jump, School of Biology, University of St. Andrews, Scotland, UK, http://www.st-andrews.ac.uk/~wjh/jumping/, 2009. With permission.)

Stork Gull Pigeon Sparrow Bee Fly Physiological Dimensions

Weight gm.

Length of Wing m.

Beats per Sec.

Wing Tip Speed m/s

Force of Wing Beat gm.

Specific Force F/W

3500 1000 350 30 0.07 0.01

0.91 0.60 0.30 0.11 0.01 0.007

2 3 6 13 200 190

5.7 5.7 5.7 4.5 6.3 4.2

1480 640 160 13 0.2 0.04

0.40 0.667 0.50 0.40 3.50 4.00

≈M

≈ M⅓

≈ M-⅓

M0

M

M0

FIGURE 18.72 Size effects on nature’s flyers. (From Heitler, W.J., School of Biology, University of St. Andrews, Scotland, UK. With permission.)

quantities in the lower row of the figure. The physiological mass based dimensions can be used to predict the size related effects on the performance of birds and insects. The wing span is seen to vary approximately with the scaling factor M1/3. Periodic events repeat themselves after a time T. For biological periodic motions, the time scale is proportional to the length or M1/3. Wing beat frequency with units of 1/sec has physiological dimensions proportional to M−1/3. The wing beat frequency as predicted increases greatly for small birds and insects. The wing tip speed, which is equal to the product of wing semi-span times the rotation frequency as shown by the physiological mass parameter, is essentially constant in all birds and insects. The force that a bird can exert is proportional to its weight. The specific force which is the ratio of the force to weight is therefore independent of the size of birds. The same is true for insects. The thrust-to-weight ratio for birds is however less than one. This means that birds, with the exception of the hummingbird cannot just lift off from a surface. They must either squat and then jump to get airborne, propel themselves from some high object, or run along the ground or water to gain enough speed to begin to fly. Insects, on the other hand, have thrust-to-weight ratios much larger than one and can therefore fly directly off a surface. The sequence of pictures in Figure 18.73, show a crow first crouching and then leaping up to get airborne as it starts

590

Biomimetics: Nature-Based Innovation

to flap its wings. A bird can also lift off by simply facing into a strong wind and then begin flapping its wings. J.B. Pettigrew in his book, Animal Locomotion or Walking, Swimming, and Flying, With a Dissertation on Aeronautics, which was published in 1874, stated: “All birds which do not, like the swallow and humming-birds, drop from a height, raise themselves at first by a vigorous leap, in which they incline their bodies in an upward direction, the height thus attained enabling them to extend and depress their wings without injury to the feathers. By a few sweeping strokes delivered downwards and forwards, in which the wings are made nearly to meet above and below the body, they lever themselves upwards and forwards, and in a surprisingly short time acquire that degree of momentum which greatly assists them in their future career.” Figure 18.74 shows the other avian take-off procedures. Let us now develop a relationship between the cruising speed and the weight of insects, birds and airplanes. Using the definition of lift coefficient as derived from dimensional analysis, wing loading (weight/wing area) is proportional to velocity squared: W/S ~ V2. From the previously discussed similarity relations, wing loading is proportional to weight to the 1/3 power: W/S ~ W1/3. Consequently, this implies that the cruising speed varies with weight to the 1/6 power: V ~ W1/6. Figure 18.75 shows a correlation of the flight speeds of tiny insects through massive transport airplanes with this simple velocity versus weight relation. There are 12 orders of magnitudes of weight variation (kilograms) from the tiny insects to the large transport aircraft. There are slightly more than two orders of cruise speed variations (meters/ seconds). Cruising speeds of some of nature’s flyers are compared with various wind conditions in Figure 18.76. The cruise speed must exceed the wind speed in order for any flyer to make any progress. The cruise speed for mosquitoes and gnats is in the order of 4 to 7 mph. Consequently, they will only fly in light breeze conditions. Since these light breeze conditions generally occur in the evening, “mosquitoes only come out at night.” 1

2

3

4

5

6

FIGURE 18.73 Jumping liftoff of a crow. (From Heitler, W.J., How Grasshoppers Jump, School of Biology, University of St. Andrews, Scotland, UK, http://www.st-andrews.ac.uk/~wjh/jumping/, 2009. With permission.) Plunging takeoff

FIGURE 18.74 Avian take-off procedures.

Wind lift takeoff

Running takeoff — on land

Running takeoff — in water

591

Biomimetics and Flying Technology

Flight velocity m/s 1×103 rp

Velocity m/s

100

Ru by thr oa te

In

se

10

ho

us

ct

V ≈ W 1/6 dh

Bi

um

mi

s

rd

Ca

s

na

da

go

ng

bir d

e fl

1 1×10–6

y da ms el

os

e

0.01

1 Weight kg

es

a je

t3

Pip M u W te s an w de an rin ga lba

fly

1×10–4

lan

Le

100

7 74 B7 75 B-

Ai

1

et

W arr

ior

tro

ss

1×104

1×106

FIGURE 18.75 Cruise speeds for insects, birds, and airplanes. Nature’s Cruise Speeds, mph 1-3 4-7 8-12 13-18 19-24 25-31 32-38 39-46 47-54

Butterflies Gnats, midges, danselflies, mosquitoes Human powered aircraft, flies, dragonflies Bees, wasps, beetles, hummingbirds, swallows Sparrows, thrushes, finches, owls, buzzards Blackbirds, crows Gulls, falcons Ducks, geese Swans, coots

FIGURE 18.76 Insects and birds cruise speeds.

It is interesting to convert the data of Figure 18.75 into relative velocity which we will define as: relative velocity = velocity / physiological length = velocity / (weight)1/3  ≈ (weight)−1/6. The results are shown in Figure 18.77. On a relative basis, a small insects’ cruise speed is approximately 100 times faster than that of a modern commercial aircraft. 18.8.2.1 Size Effects on Nature’s Flyers We can use similarity relations to establish the approximate largest size for a flying bird (McMasters, 2003a). As shown in Figure 18.78, there is a strong correlation between flight muscle mass (and thus power available) and total mass of most birds. The power required to fly is proportional to cruise drag times velocity. The drag is proportional to velocity squared times area and therefore varies directly with mass. The product of drag times velocity therefore varies as M7/6. In Figure 18.79 the power available and the power required values for a pigeon are extrapolated using the above relationships. If the available power is less than that required

592

Biomimetics: Nature-Based Innovation

1×103

Relative velocity

Insects 100

≈Weight–1/6

Birds

10

1 1×10–6

Airplanes

1×10–4

0.01

1 Weight kg

100

1×104

1×106

FIGURE 18.77 Relative cruise velocity versus weight. Stork Vulture Heron Hawks

1.0

Flight muscle mass — MFM (kg)

Birds of prey Passerine birds Wading and web-footed birds

0.1

Gull

Kingfisher Pigeon

Pelican Cormorant Owl

Flight muscle mass (MFM) = 0.25 M

0.01 Hummingbird

0.001 0.001

0.01

0.1

1.0

10

Total Mass — M (kg) FIGURE 18.78 Avian flight muscle (power available) versus total mass.

for flapping flight at a particular speed, then flight is simply not possible. If it exceeds the power required, then the excess power can be used for other demanding tasks such as maneuvering or climbing flight. Using the known data for a pigeon as an anchor, we can project the curves to the point where power available exactly equals the power required. The results indicate that the maximum mass of a flying bird is about 20 kg (44 lb). This is consistent with that of a South African turkey, the kori bustard (McMasters, 2005), which is barely able to fly. Shyy (1999) presents an extensive discussion on geometric similarity and scaling along with a number of correlations relative to mass including: • Wing span • Wing area • Wing loading, ratio of weight to wing area

593

Biomimetics and Flying Technology

Power (watts)

le ~

ab vail er a

Pow

ed uir

a ~m

s

mas 7/6

ss

Kori bustard (44lb)

req er w Po

Pigeon 0.1

1

Mass kg

10

20

100

FIGURE 18.79

Power requirements for steady, level flight.

• Aspect ratio • Wing beat frequency • Various upper and lower limits 18.8.3  Elastic Similarity The concept of elastic similarity is based on the supposition that animals of various body sizes should be “elastically similar” (MacMahon, 1973) rather than geometrically similar. Elastic similarity assumes that nature uniformly optimizes its designs so the elastic deformation of an animal under its own weight should be similar at all body sizes. This implies that natural selection results in all animals, no matter what their size, having a similar probability of elastic failure of the structures supporting them. So, if a mouse has a 0.01% probability of sustaining a fracture of its limb bones, natural selection should favor elephants having the same probability of fracture. Euler buckling is a common mode of elastic failure. If a rod is loaded at both ends with a force, F, the rod will buckle by bending laterally under the load. The rod may support that force as long as the force does not exceed some critical force, (Fcrit). When this happens, the rod can no longer support the load. This mode of failure is called Euler buckling. It is similar to the mode of failure experienced in most long bones of vertebrates. Euler buckling can also be analyzed by the length of a rod needed to support a given force, F. In this case, once the length of the rod (l), exceeds a critical length, (lcr), it will buckle. The critical length has a particular relationship to the rod’s diameter, d as:

lcr3 ∝ d 2 or lcr ∝ d

2

3



(18.4)

The weight of a body is closely proportional to its volume, which is related to

Wb ∝ ld 2

(18.5)

594

Biomimetics: Nature-Based Innovation

From Equations 4 and 5, we obtain the basic elastic relations: l ∝ W 1/ 4



and d ∝ W 3 / 8

(18.6)

Using the relations in equation, and by adding the physiological elastic length and massbased parameters to Figure 18.66, we obtain Figure 18.80. Figure 18.81 shows how the shapes of a geometrically similar cylinder and an elastically similar cylinder change with an increase in weight. It is seen that the elastic similar shape becomes stockier as the weight is increased. Suppose a 40-lb gazelle, a graceful and swift little creature with long, thin legs and great speed to escape danger, is to become large, Physiological Geometric Similarity Quantity

Elastic Similarity

Physical

Length Based

Mass Based

Length Based

Mass Based

L L t t-1 M L3 L2 2 L /M M L2

L L L L-1 L3 L3 L2 L-1 L3 L2

M⅓ M⅓ M⅓ M  -⅓ M M M⅔ M-⅓ M M⅔

L D L L-1 LD2 LD2 LD D-1 LD2 D2

M¼ M⅜ M¼ M-¼ M M M⅝ M-⅜ M M¾

ML-2

L

M⅓

L

M¼

Length Diameter Time Frequency Mass Volume Surface area Relative heat loss Weight force Cross section area Compressive stress Critical buckling height

Approaches hcrit as size increases

Reduced Increased

Less as size is reduced Increased

hcrit Not affected by size increase

FIGURE 18.80 Elastic and geometric mass-based similarity parameters.

4000 lb Rhinoceros

Slenderness ration l/r

4000 lb gazelle

rity

im i

simila

sti cs

Elastic

lar ity

Geometric similarity

Ela

20 l/r 18 16 14 12 10 8 6 4 2 0 10

Geometric similarity

100

FIGURE 18.81 Elastic similarity examples.

Weight

1000

10000

40 lb gazelle

595

Biomimetics and Flying Technology

for example, 100 times as large. If its growth was governed by geometric similarity, its long thin legs would break under its own weight (Haldane, 1927). Nature’s solution to the increased weight included shorter and thicker legs like those of the rhinoceros who has no need to run at all, as shown in Figure 18.85. Nature’s solution is consistent with the concept of elastic similarity.

18.9  Size Effects on Airflow Characteristics Size or scale also affects the nature of the flow characteristics in which nature’s and man’s flyers exist. The characteristics or state of a fluid flow is characterized by a single parameter, the Reynolds number, Rel, which is the Reynolds number based on length is equal to: Re l =



rUl m

(18.7)

where: ρ = Density of the fluid U = Relative velocity of the flyer on the fluid l = Characteristic length µ = Coefficient of viscosity The Reynolds number is often interpreted as the ratio of the inertia forces to the viscous forces. Flows with large Reynolds numbers are dominated by inertia forces whereas flows governed by small Reynolds numbers are dominated by viscous forces. Figure 18.82 shows how the drags of a sphere and of a cylinder vary by Reynolds number as well as the nature of the flow over the surfaces. We can develop a relationship between physiological mass and Reynolds number using the previously developed relationships: U ∝ M 1/ 6 and l ∝ M 1/ 3. Using Equation 18.7 we obtain. Re l ∝ M 1/ 6 M 1/ 3 ∝ M



(18.8)

Re b, the beneficial value of flying in ground effects is negligible, but becomes very significant by flying very close to the surface. Figure 18.98 also contains an airplane designed to fly in ground effect. 18.11.10  Variable Wing Sweep The peregrine falcon is commonly known as the “duck hawk.” The peregrine falcon is often stated to be the fastest animal on the planet in its hunting dive, the stoop, which involves soaring to a great height and then diving steeply at speeds commonly said to be over 320 km/h (200 mph), and hitting one wing of its prey so as not to harm itself on impact. The peregrine as shown in Figure 18.99 increases the sweep of its wings during the stoop as the dive speed increases. The concept of variable wing sweep was tested with the NASA X-5 experimental flight vehicle in 1952. The X-5 was developed as an early demonstrator of flight capability with variable sweep. The X-5 was based upon the design of a Messerschmitt P. 1101 airplane discovered in Germany at the end of World War II, although the P. 1101 could vary its sweep

611

Biomimetics and Flying Technology

Normal flight

Start of dive

NASA X-5 (1952)

General Dynamics F-111 (1964)

Dive Boeing SST (1962) Stoop

FIGURE 18.99

Examples of variable wing sweep.

only on the ground. The X-5 could fly only at subsonic speeds, but it did demonstrate the ability to vary the wing sweep from 20° to 60° in flight. The concept of variable wing sweep was studied extensively by Boeing in its supersonic transport development program in about 1962. In this application, the use of variable wing sweep provided the capability to optimize the aerodynamic performance across the entire flight profile. The maximum sweep greatly reduced the wave drag at supersonic cruise conditions. The wing was unswept for takeoff and landing conditions. Following takeoff, the wing sweep continued to sweep for subsonic cruise, through transonic acceleration to supersonic cruise. The General Dynamics F-111 is a military application of variable sweep.

18.12 Biomimicry—Nature’s Concepts as Inspiration for Dissimilar Applications Biomimicry can be defined as a science that studies nature’s models and then imitates or takes inspirations from these designs and processes to solve human problems (Benyus, 1997). Phil Gates (1995) stated “Many of our best inventions are copied from other living things. We have discovered only a tiny fraction of the vast number of living organisms that share our planet. Somewhere, among the millions of organisms that remain undiscovered, there are natural inventions that could improve our lives.” Biomimetics is an interdisciplinary subject which combines engineering science, architecture, and mathematics. The basic principle is to make nature’s problem solutions usable for man. The reason for this is very simple: Nature, through billions of years of trial and error, has produced effective solutions to innumerable complex real-world problems and has done a very good job. “Any engineer must inevitably have respect for the excellence of the design that can be seen in biological systems” (Murray-Smith, 2004),

612

Biomimetics: Nature-Based Innovation

18.12.1  Classic Example—Velcro and the Burdock Seed The creation of Velcro is a classic example of biomimicry. As the story goes, after taking his dog for a walk one day in the early 1940s, George de Mestral, a Swiss inventor, became curious about the seeds of the burdock plant that had attached themselves to his clothes and to the dog’s fur. Following the irritation of having to painstakingly remove the burs from his socks, he became curious about the attachment mechanism between the burrs and his socks. Under a microscope, he discovered that the end of each burr had a hook, the socks and small loops that the hooks had attached to. He looked closely at the hook-and-loop system that the seeds use to hitchhike on passing animals aiding seed dispersal, and he realized that the same approach could be used to join other things together. The result was Velcro. The burdock plant is a group of biennial thistles. The prickly heads of these plants as shown in Figure 18.100 consists of tiny hooks. These hooks are noted for easily catching on to fur and clothing, thus providing the burdock plants an excellent mechanism for seed dispersal. The design that de Mestral developed as shown in the figure, emulates the hooks of the burdock thistle and the loops similar to those of wool. Velcro fasteners have found rather widespread usage even within the aircraft industry such as: • • • • • •

Lightweight, rustproof fasteners that do not rattle A standard component in jet planes since the 1960s Used on aircraft ranging from small pipers to the space shuttle Pallet tidy strap for pallet control/identification Fire retardant hook and loop fastener Velcro is an example of an “irritation” being a source of invention

Many have probably experienced walking through the woods and then returning with our socks full of burrs, a situation which most would categorize as a definite irritation. For most people, after the painstaking removal of the burrs, the situation is soon forgotten. De Mestral asked the question “why,” proceeded to answer the question, and then found a way to exploit the answer. This is a good model for anyone to follow as a source for innovation. 18.12.2  Countercurrent Heat Exchanger Curiosity concerning how a duck (or seagull) could stand or swim in very cold water without its legs freezing led to the discovery that nature has developed in the spindly legs of a (a)

Burdock

(b)

Hooks on burdock

(c)

Velcro hooks

(d)

FIGURE 18.100 (See color insert.) The burdock plant and Velcro. (a) Epukas; (b) Petham; (c), (d), Olivepaxel.

Velcro loops

613

Biomimetics and Flying Technology

Warmth retained in veins

33⁰

Warm arterial blood

Heat transfer

27⁰ 18⁰ 9⁰

35⁰ C 30⁰

Heat transfer Heat transfer

20⁰

Heat loss (W) 16 12 8 4

10⁰

Small heat loss

Metabolic heat production

0 –12 –6

Heat loss from feet 0 6 12 18 24 Air or water temperature (C)

30

FIGURE 18.101 (See color insert.) Nature’s countercurrent heat exchanger.

duck, a very efficient countercurrent heat exchange system (Ritchison). Birds living in cold environments must conserve body heat in order to avoid hypothermia. However, blood flowing from the body core to the legs and feet carries heat that can be readily lost through the skin. To prevent such a loss, birds have evolved a countercurrent heat exchanger system in their legs and feet as shown in Figure 18.101. The blood vessels in the legs include arteries carrying warm blood down the legs to the feet, lie in close proximity to the veins carrying cold blood back from the feet. The cold blood in the veins gradually reduces the temperature of the blood in the arteries as it flows toward the feet. By the time the blood in the arteries reaches the feet, it is nearly at ambient temperature which results in very little heat loss. In addition, the warm blood in the arteries heats up the blood returning from the feet in the veins so effectively that the return blood flow re-enters the body of the seagull at essentially the internal body temperature. Consequently, even when a duck is standing in ice cold water, there is hardly any heat loss. The principle of countercurrent heat exchange is so effective and ingenious that it has also been adapted in human engineering projects to avoid energy waste, for example, by ensuring good ventilation of buildings while avoiding the loss of heat to the environment on a cold winter’s day. 18.12.3  Concept Vehicle Mercedes-Benz Bionic Car Another example of a nature inspired innovative design is the concept vehicle MercedesBenz bionic car. Engineers, designers, and biologists at Mercedes-Benz worked hand in hand to develop the innovative concept shown in Figure 18.102. The design was based on a sea dweller from tropical latitudes, Ostracion cubicus, which is more commonly known as the boxfish. The rectangular anatomy of the boxfish is practically identical to the cross section of a car body. The fish is an excellent swimmer, having extremely good aerodynamic characteristics and can move with a seemingly minimal amount of effort. Wind tunnel tests of a one-fourth scale model of the Mercedes-Benz bionic car yielded surprisingly very low drag. The boxfish is also a marvelous natural structural concept and is able to withstand high pressures as a result of its outer skin structural design consisting of tiny interlinked hexagonal bone plates which provide maximum strength with minimal weight and effectively protect the animal from injury. It can survive unscathed following collisions with corals or other sea dwellers. In consultation with bionics experts, the automotive researchers developed a computer-assisted process

614

Biomimetics: Nature-Based Innovation

Ostracion cubicus - boxfish

*

*

*

Cr

0.00 0.20 0.40 0.60 0.80 1.00 1.20

FIGURE 18.102 Concept vehicle Mercedes-Benz bionic car. (Copyright Daimler AG. With permission.)

for transferring the growth principle used by nature to automobile engineering. It is based on the SKO method (soft kill option). Computer simulations were used to configure body and suspension components in such a way that the material in areas subject to lower loads was reduced, and in certain instances, even eliminated (“killed”) completely, while highly stressed areas were specifically reinforced. This bionic SKO process enabled an optimal component geometry to be identified that meets the balanced requirements of lightweight construction, safety, and durability. The boxfish is a prime example of the ingenious inventions developed by nature over millions of years of evolution. The basic principle of such evolutionary developments is that nothing is superfluous and each part has a purpose and often several at once. 18.12.4  Bionic Propeller Bannasch (2007) of EvoLogics R&D Lab Bionics and his colleagues extended their concept of the spiroid wing tip for reducing wing induced drag to develop a novel shape for propellers, which has been patented worldwide as the Bionic Loop Propeller. The inspiration for the bionic propeller as shown in Figure 18.103 was the split primaries of small birds with fast beating wings. The tip vortices of conventional propellers wrap around one another as the propeller rotates and thereby produce a center core of strong vorticity. Compared to regular propellers the bionic propeller sheds a continuous sheet of vorticity without the center core vortices. Test results are shown in Figure 18.104 for a conventional propeller wind turbine design and a comparable bionic wind turbine design. Relative to the conventional wind turbine, the bionic propeller wind turbine resulted in increased power output from 20% to 50% and reduced the noise emission by 50%. Some applications for the bionic propeller are shown in Figure 18.105.

615

Biomimetics and Flying Technology

FIGURE 18.103 Inspiration for the bionic propeller. (Courtesy of Rydolf Bannasch, EvoLogics GmbH.) • Continuous sheet of vorticity • No core vortices Electric power

+ 20%

50 0

26 24 22 20

+ 30%

rison

+ 50%

150 100

Noise emission (at 8,3m/s)

28

rel.db (A)

500 450 400 350 300 250 200

on

18 16

–1/2 (!)

14 12

7

8

9

10

11 12 13 14 Wind speed (m/s)

15

16

17

10 500

600

700 800 900 1000 Rotation speed (U/min)

1100

1200

FIGURE 18.104 Bionic loop propeller. (Courtesy of Rydolf Bannasch, EvoLogics GmbH.) 19% More thrust for same power input

Wind turbine

Pusher propeller

Ship propeller

FIGURE 18.105 Some applications for the bionic propeller. (Courtesy of Rydolf Bannasch, EvoLogics GmbH.)

18.12.5  Owl Wings and Jet Noise In Section 18.3.5, it was mentioned that the serrated wing trailing edge feather arrangement of the owl diffuses and reduces high-frequency noise. This serrated edge concept has been applied to aircraft engines. Figure 18.106 shows the technology development stages for a low noise serrated trailing edge nozzle concept that has been shown to be very effective in reducing high-frequency jet noise design (McMasters, 2003a). Figure 18.107 is a picture of the Boeing 787 taken during taxi at Paine Field in Everett, Washington just before the first flight of this new revolutionary airplane. The serrated trailing edges of the new advanced technology engines are quite apparent.

616

Biomimetics: Nature-Based Innovation

Biological inspiration? 100

Basic research Applied research

50 N

Technology development

0 –50

–100 –100 –50

0

50

CFD evaluations

Scale model acoustic tests Full scale flight test validation

FIGURE 18.106 Acoustic noise reduction.

FIGURE 18.107 (See color insert.) Boeing 787 during taxi for the first flight.

18.12.6  Fruit Flies and Folded Wings Fruit flies and many other flying insects fold their wings when they land to protect them. The folding wing idea has also been applied to both aircraft as well as cruise missiles as shown in Figure 18.108. A folding wing is a design feature of an aircraft to save space on the airfield, and is typical of naval aircraft that operate from the limited deck space of aircraft carriers. The folding allows the aircraft to occupy less space in a confined hangar. The Short brothers patented folding wing mechanisms for ship-borne aircraft, the first patent being granted in 1913. The wings were hinged so that they folded back horizontally alongside the fuselage and were usually held in place by latches projecting sideways from the rear of the fuselage. The Short SB.6 Seamew was a British aircraft designed in 1951 as a lightweight antisubmarine platform and featured large, broad-chord power-folding wings. The Short Seamew was selected to fulfill the Admiralty Specification for a simple, lightweight antisubmarine aircraft capable of unassisted operation from any of the Royal Navy’s aircraft carriers in

617

Biomimetics and Flying Technology

Fruit fly

First Short folder in naval service

SB.6 Seamew

Wings stored

Wings extended

KEPD-350 Cruise Missile FIGURE 18.108 Folding wing flyers.

all but the worst of conditions. Although specifically designed for naval operations, the Seamew was also intended for land-based use by the RAF. Folding wings have also been used on some air launched cruise missiles. The wings are folded when mounted under the airplane’s wing and are deployed when the missiles are released. 18.12.7  Nautilus and Jet Engines Jet propulsion is motion produced by expelling a jet of fluid (e.g., air or water) in the opposite direction to the direction of motion. By conservation of momentum, the moving body is propelled in the opposite direction to the jet. A number of animals, including cephalopods, shown in Figure 18.109 have evolved jet propulsion mechanisms. Jet propulsion in cephalopods is produced by water being exhaled through a siphon, which typically narrows to a small opening to produce the maximum exhalent velocity. In order to swim, the nautilus draws water into the living chamber through its hyponome; the water is then expelled out through its hyponome with increased momentum and thereby produces its jet propulsion. A more complex animal using jet propulsion is the squid. The main body mass is enclosed in a muscular mantle (outer covering) that has a swimming fin along each side. These fins, unlike those in other marine organisms, are not the main source of locomotion in most species. The mantle when expanded fills with water. When these muscles contract, water is expelled through a single siphon in a fast, strong jet, and the squid is propelled in the opposite direction. The squid can control its direction by rotating (moving) the siphon. Some squid are able to reach speeds high enough to shoot them out of the water and onto the decks of passing ships! The propulsion principles of the cephalopods have been mimicked in the design of the jet engine. Both the nautilus and the jet engine have three basic systems that include the inlet system, the internal components, and the exhaust system. The internal components compress and add energy to the inlet air which is then expelled through the exhaust system with increased momentum that provides the propulsive thrust. In the jet engine, the

618

Biomimetics: Nature-Based Innovation

Chambered nautilus

Squid

Octopus

Hyponome FIGURE 18.109 (See color insert.) Cephalopods: nature’s jet propulsion experts.

Intake

Air inlet

Compression

Combustion chambers

Combustion

Exhaust

Turbine

FIGURE 18.110 Modern turbojet engine.

primary energy source is the chemical energy of the fuel. For the nautilus, the energy source is the potential energy of its compressive muscles. In the jet engine, as shown in Figure 18.110, the inlet and exhaust systems are separate systems whereas in the nautilus, the hyponome serves as both the inlet and exhaust systems. The earliest jet engines were hybrid designs in which an external power source, such as a regular combustion engine, first compressed the air, which was then mixed with fuel and burned for jet thrust. One such system, called a thermojet was used in Henri Coandă’s Coandă-1910 aircraft which was the first jet-propelled aircraft ever built, with the first flight in December 1910. Turbojets are the oldest kind of present-day general-purpose jet engines. Some of the earliest jet powered aircraft are shown in Figure 18.111. Two engineers, Frank Whittle in the United Kingdom and Hans von Ohain in Germany, developed the concept independently into practical engines during the late 1930s, although credit for the first turbojet is generally given to Whittle who was first to conceive, first to formally describe, first to patent, and first to build a working engine. Von Ohain, in comparison, was first to get a turbojet into the air, which was the Heinkel He 178 in 1939. The W2/700 was the first British production jet engine, powering early models of the Gloster Meteor in 1941.

619

Biomimetics and Flying Technology

The coandă—1910

Heinkel He 178 — 1939

W2/700 — first Gloster E.28/39 — 1941 British production jet engine

FIGURE 18.111 Early jet powered aircraft.

18.12.8  Bats, Echolocation, Sonar, Radar and Lidar, and Clear Air Turbulence Echolocation, also called biosonar, is the biological sonar used by several animals such as shrews, most bats, and most cetaceans including whales, porpoises, and dolphins. Figure 18.112 is a conceptual illustration of echolocation. Echolocation utilizes pressure waves or sound waves. The speed of sound in air at sea level is about 343 m/s and in water is about 1500 m/s. Echolocation is much more complicated and certainly more amazing than illustrated in the figure since the transmitter, the target, and the receiver are typically in constant motion with irregular speeds and directions. Echolocation uses sounds made by the animal. The range to the target is determined from the time delay between the animal’s own sound emission and any echoes that return from the environment. The relative intensity of sound received at each ear provides information about the horizontal angle (azimuth) from which the reflected sound waves arrive. Echolocating animals have two ears positioned slightly apart. The echoes returning to the two ears arrive at different times and at different loudness levels, depending on the position of the object generating the echoes. The animals use this difference in time and the loudness of the echo to perceive the location (direction and distance) of the object. With echolocation, the bat or other animal can determine not only where the object is going but also how big another animal is, what kind of animal it is, and other features. Early applications of echolocation known as “acoustic location” are shown in Figure 18.113. These were actually passive echolocation concepts in that the “receiver” listened for noise being made by the target. Acoustic location in the air was used from mid-World War I to the early years of World War II for the passive detection of aircraft by picking up the noise of the engines. It was rendered obsolete before and during World War II by the introduction of radar, which was far more effective. Sonar, which is an acronym for “sound navigation and ranging,” is a direction application of nature’s echolocation technique that uses sound propagation in water used most commonly in submarines to navigate, communicate with, or detect other vessels. Two types of technology share the name “sonar.” Passive sonar is essentially listening for the sound made by vessels, and active sonar is emitting pulses of sounds and listening for echoes. The fundamental concept for sonar as shown in Figure 18.114 is essentially the same as echolocation. The first recorded use of passive sonar by humans was by Leonardo da Vinci in about 1490. He used a tube inserted into the water to detect vessels by placing an ear to the tube.

620

Biomimetics: Nature-Based Innovation

Call

Echo

Right ear Left ear FIGURE 18.112 (See color insert.) Ultrasound signals emitted by a bat, and the echo from a nearby object. (a)

(b)

FIGURE 18.113 Early applications of passive echolocation—Acoustic location. (a) Mayer’s 1880 topophone, and (b) the 1898 experiments of J.M.Bacon. Reflected wave

Sender/ receiver

Object Original wave Distance r

FIGURE 18.114 Principle of active sonar.

Radar is an acronym for “RAdio Detection And Ranging.” Radar was the next extension by man of the concept of echolocation. Radar, however, is an object detection system that uses electromagnetic waves to identify the range, altitude, direction, or speed of both moving and fixed objects such as aircraft, ships, motor vehicles, weather formations, and terrain. A radar system as shown in Figure 18.115 has a transmitter that emits radio waves. When they come into contact with an object they are scattered in all directions. The signal is thus

621

Biomimetics and Flying Technology

partly reflected back and it has a slight change of wavelength (and thus frequency) if the target is moving. The receiver is usually, but not always, in the same location as the transmitter. Although the signal returned is usually very weak, the signal can be amplified through use of electronic techniques in the receiver and in the antenna configuration. This enables radar to detect objects at ranges where other emissions, such as sound or visible light, would be too weak to detect. In aviation, aircraft are equipped with radar devices that warn of obstacles in or approaching their path and give accurate altitude readings. They can land in fog at airports equipped with radar-assisted ground-controlled approach (GCA) systems, in which the plane’s flight is observed on radar screens while operators radio landing directions to the pilot. LIDAR is the acronym for “LIght Detection And Ranging” and uses the same principle as RADAR. The lidar instrument transmits light out to a target. The transmitted light interacts with and is changed by the target. Some of this light is reflected/scattered back to the instrument where it is analyzed. The change in the properties of the light enables some property of the target to be determined. The time for the light to travel out to the target and back to the lidar is used to determine the range to the target. The fundamental concept of lidar is shown in Figure 18.116.

Return signal from house

Resulting antenna return High energy output pulse

Pulse strength

Radar pulse sent from aircraft Return signal from tree

0

2

4

Return from house Return from tree

6 8 10 12 14 16 18 Time (Distance)

FIGURE 18.115 Principles of radar. Pulse transmitted

Energy scattered off of naturallyoccuring moisture or aerosols Beam Updraft Downdraft

Relative airspeed

Doppler turbulence sensor

Turbulence

Distance or time ahead of aircraft

Radar beam spread 3.5⁰ Lidar “Pencil” beam width 10-20 cm

Lidar pulse envelope (50-100 meters) Radar pulse envelope (~300 meters)

Relative wind induces a Doppler frequency shift in the backscattered light; this frequency shift is detected by the sensor

FIGURE 18.116 General principle of Doppler radar/lidar turbulence measurement.

622

Biomimetics: Nature-Based Innovation

The primary difference between lidar and radar is that lidar uses much shorter wavelengths of the electromagnetic spectrum, typically in the ultraviolet, visible, or near infrared range. In general it is possible to image a feature or object only about the same size as the wavelength, or larger. Consequently, lidar is highly sensitive to cloud particles and has many applications in atmospheric research and meteorology. Turbulence is a major hazard for aviation. For the aviation transportation industry as a whole, the total cost is estimated to exceed $100 million per year. A whole class of turbulence, representing about 40% of turbulence accidents is designated as clear air turbulence (CAT) and cannot be detected by any existing airborne equipment, including state-of-theart weather radar. Lidar systems are being developed and undergoing evaluation testing. The results to date have demonstrated the overall feasibility of detecting CAT; however, the current systems do not have enough laser energy for adequate range of detection. The research goals related to aircraft are to develop an effective system that will be able to detect all classes of air turbulence in sufficient time for safety preparations, and/or avoidance. 18.12.9  Shark’s Skin and Riblets Sharks have been around for over 400 million years, which is quite a time to perfect its swimming skills and techniques that make them one of the most efficient predators on earth. The slippery streamlined shape helps to minimize pressure drag, and a specialized skin layer (dermal denticles) minimizes skin friction drag. Riblets are the embodiment of artificial sharkskin—a great example of biomimicry. The development of riblets to reduce turbulent skin friction came in part from the study of shark scales or dermal denticles. Riblets are stream-wise microgrooves that act to break up span-wise vortices, and reduce the surface shear stress and momentum loss. The test data indicate that turbulent flow skin friction drag reductions on the order of 8% have been achieved as shown in Figure 18.117. Riblets have been used on airliners, racing yachts, and most recently swimsuits. 0 S –2 Drag reduction %

α = 60⁰ NASA-Riblets

–4 –6

S

h-0.5s α = 30⁰ DLR-Riblets

–8 –10

0

4

FIGURE 18.117 Turbulent flow drag reduction with riblets.

8

12 16 us + S =

20

24

28

Biomimetics and Flying Technology

623

18.13 Neo-Bionics—Nature-Related Computational Processes for Design Innovations Neo-bionics can be defined as innovation by “computational inspiration.” Neo-bionics utilizes biological evolutionary or “optimization” processes found in nature as the computational strategy for a computer-aided design “optimization” with engineering constraints. Some of the familiar biologically inspired optimization techniques include • Genetic algorithms (GA) • • • • • •

Particle swarm optimization (PSO) Ant colony optimization (AC) Simulated annealing (SA) Evolutionary structural optimization (ESO) Bidirectional evolutionary structural optimization (BESO) Soft kill option (SKO)

18.13.1  Biological Optimization Techniques Nature has been described as the “great optimizer.” This is most certainly an appropriate expression as far as the superiority of its optimization skills is concerned (Ifju, 2006). A number of biologically inspired optimization techniques have been developed that emulate many of nature’s processes. These can be described as iterative global search techniques for a “best solution” using design of experiment methods based on nature-related adaptation processes to define specific combinations of values for the design variables to evaluate in a systematic approach to “get better.” Typically, each “optimization” begins with evaluations of a random set of specified parameter values to evaluate, followed by the nature related “rules” to define subsequent “next case” analyses to systematically update the set of potential parameter values and progressively move toward the desired “best” solution. The selection of the set of design parameters plus specified ranges of allowable values for each of the parameters, define the “design space” within which the selected naturerelated design of experiments (DOE) process can hopefully operate to find a best solution. Because of the random nature of the initial selections of the design parameter values plus the typically independently defined parameter limits, the “design space” is most often highly irregular and seeded with many unacceptable design possibilities. Nevertheless, numerous applications of these processes have led to very effective “best solutions.” The “best solution” is usually defined as the conditions for the best value of the merit functions after a specified number of solution evaluations or if subsequent solutions fail to improve the figure(s) of merit. These techniques in general do not fit the definition of pure mathematical optimization since there is no formal mathematical process to identify an optimum solution other than the apparent convergence of subsequent merit function evaluations. In addition, the various optimization techniques are not equally effective for every class of problem. A genetic algorithm (GA) uses techniques inspired by evolutionary biology such as inheritance, mutation, selection, and crossover (also called recombination) to define subsequent combinations of design parameter values to evaluate in the search for a best solution.

624

Biomimetics: Nature-Based Innovation

GA has been utilized in such varied fields as bioinformatics, phylogenetics, computational science, engineering, economics, chemistry, manufacturing, mathematics, physics, and other fields. Particle swarm optimization (PSO) is a population-based stochastic optimization technique that was inspired by the social behavior of bird flocking or fish schooling. PSO shares many similarities with evolutionary computation techniques such as GA. However, unlike GA, PSO has no evolution operators such as crossover and mutation. In PSO, the potential solutions, called particles, fly through the problem space by following the current optimum particles. Compared to GA, the advantages of PSO are that it is easy to implement and there are few parameters to adjust. PSO has also been successfully applied in many areas: function optimization, artificial neural network training, fuzzy system control, and other areas where GA could also be applied. Ant colony optimization (AC) is based on the foraging behaviors of ants. Simulated annealing (SA) is based on the process of the formation of crystals. Real-world systems such as the flocking of birds, formation of crystals, and the foraging behaviors of ants are examples of group behavior resulting from the collective interactions between many self-directed individuals, or agents. One fascinating aspect of phenomena such as these is that very complex system behaviors and patterns can emerge from agents interacting with one another according to a relatively simple set of rules and often unaware of the consequences of their actions in the overall scheme of things. Evolutionary structural optimization (ESO), bidirectional evolutionary structural optimization (BESO), and soft kill option (SKO) are structural optimization techniques based on nature’s techniques on effective utilization of structural materials. Examples will be discussed in greater detail later in this chapter. 18.13.2 Comparison of Biological “Optimization” Techniques—Traveling Salesman Problem Figures 18.118 and 18.119 were created from systematic studies made using the VisualBots for Excel educational type tool (Waite) to illustrate the general nature of some of the biological optimization methods when applied to “traveling salesman problems” which involve determining the shortest-distance continuous route between a set of destinations. Figure 18.118 shows the number of iterations for each of the design parameter sets that were required to achieve the optimum solution for ten repeated optimizations for each of three biological optimization methods. These include AC, GA, and SA. Traveling salesman problem

The solution

GA shortest route=377.0

Finding the solution No of iterations to optimum(=377.064) Case Ant col GA SA 1 190 185 4039 2 57 206 4194 3 143 163 [405] 4 190 207 4903 5 201 294 4284 6 185 160 4666 7 136 193 4852 8 83 248 5275 159 9 247 4571 10 146 225 4052 Mean

FIGURE 18.118 Comparison of biological optimization methods solutions.

149

213

4537

625

Biomimetics and Flying Technology

3000

Pseudo convergence

G/shortest route=56.7

Route distance

2500 2000 1500 1000 500 0

AC 1

10

GA 100 1000 10000 Iteration number

SA

100000

FIGURE 18.119 Another traveling salesman problem.

The above results show that the various optimization methods were not equally effective in determining the best solution and that the number of iterations to convergence is often random. Figure 18.119 shows the rate of convergence for these three optimization methods when applied to another traveling salesman problem for which the optimum solution is once again known. In this example it is seen that the number of iterations necessary to achieve the known optimum solution varies by orders of magnitudes depending on the optimization algorithm. The SA method never actually “found” the optimum solution. The solutions also indicate regions of pseudo-convergence, which could have been interpreted as finding an optimum if the optimum solution had not been known. 18.13.3  Genetic Algorithm Wing Planform Optimization Figure 18.120 illustrates how unexpected fundamental concepts can arise from robust configuration optimization using GA. In this example (Kroo, 1996, 2004), an evolutionary optimization algorithm was used to find the wing geometry that produced minimum total drag, yet fit inside a geometric constraint box of fixed height and span. The wing was described as a collection of variable length linearly tapered and twisted elements, whose aerodynamic characteristics were computed using a vortex lattice analysis. A random population of initially simple designs is shown at the top left side of Figure  18.120. The optimizer quickly discovered that span reduces drag, and after only five to six generations (with population size 500), and found the minimum induced drag for a roughly planar wing. As the span limiting constraint became active, the optimizer “discovered” winglets, adding vertical elements at the wing tips to further reduce vortex drag. Finally, after about 100 generations, the system found an advantage to adding horizontal tip extensions to the winglets, forming a “C” shape at the tip that lies within the upper geometric box constraint. This “C” wing design concept was investigated further and found to exhibit useful structural and control features in addition to the reduced vortex drag at fixed span. The concept was subsequently patented and is being studied for application to new aircraft concepts at Boeing, NASA, and elsewhere.

626

Biomimetics: Nature-Based Innovation

FIGURE 18.120 Neo-bionic innovative configuration development. (From Dr. Ilan Kroo, Stanford University. With permission.)

Generation 0

8 6

9

12

15

18

21

24

27

7

27 Generation GA variations of a birdlike wing tip

11%

GA optimum (27 generations)

6 L/D

3

5 4

Base wing

3 2

0.0

0.2

0.4

0.6

CL

0.8

1.0

1.2

1.4

FIGURE 18.121 GA evolutionary optimization of slotted wing tips in the wind tunnel. (Courtesy of Rydolf Bannasch, EvoLogics GmbH.)

18.13.4  Genetic Algorithm Wing Tip Optimization Another example of a neo-bionic design study (Bannasch, 2007) is shown in Figure 18.121. In this example a GA was used in conjunction with a wind tunnel test program to determine optimum orientations of a set of five wing tip segments. During the optimization, the GA would specify the orientation of each of the segments that were then subsequently

627

Biomimetics and Flying Technology

Peaches

Nature’s designs Apple

Cherries

FIGURE 18.122 Optimal shape for an object hanging in air under its own weight. (From Mike Xie, George Washington University Center for Biomimetics and Bioinspired Engineering. With permission.)

tested. After about 27 generations, an improvement of approximately 11% in lift-to-drag ratio, L/D, was achieved. 18.13.5  Evolutional Structural Optimization The ESO method (Xie, 2005) is based on the simple concept of gradually removing underutilized material from a structure so that the resulting shape evolves toward an optimum. The ESO method proves to be capable of solving size, shape, and topology structural optimization for static, dynamic, stability, and heat transfer problems or combinations of these. The traditional ESO method removes material from a structure based on von Mises stress or strain energy of each element. For certain construction materials, such as concrete and fabric, they are only suitable for sustaining compressive or tensile stress. The ESO method has been extended to the design of tension-only or compression-only structures. The validity of the ESO method depends, to a large extent, on the assumptions that the structural modification (evolution) at each step is small and the mesh for the finite element analysis is dense. If too much material is removed in one step, the ESO method is unable to restore the elements, which might have been prematurely deleted at earlier iterations. Consequently, in order to make the ESO method more robust, a bidirectional ESO (BESO) method has been developed that includes both the adding and removing of material during the optimization process. Figure 18.122 shows the results of an ESO study to determine the optimal shape for an object hanging in the air under its own weight. An initial square model is shown on the left. Before the ESO procedure is applied, two slots have been cut at the top in the initial model to create a stalk. The top end of the stalk is fixed. The only loading on this object is the gravity. By removing least stressed material from the surface, we obtain shapes with uniform stress on the surface. The results are similar to the shape of certain fruits such as apples and cherries. Figure 18.123 shows the use of the BESO algorithm in the design of an optimized bench design. The top layer of the bench is defined as a nondesign domain. The initial design has four support legs. By adding and removing material simultaneously, BESO finds the

628

Biomimetics: Nature-Based Innovation

BESO optimized stool FIGURE 18.123 BESO optimized stool design. (From Mike Xie, George Washington University Center for Biomimetics and Bioinspired Engineering. With permission.)

optimum solution shown in the figure. The figure also shows the initial four-leg support system and three intermediate geometries leading to the optimum stool design. The SKO is a relatively new method for structure optimization that was developed to transfer the growth principle used by nature to engineering structural design. In the book Design in Nature: Learning from Trees (Mattheck, 1998), Mattheck introduced what he called the Principle of Constant Stresses derived from analogies observed in the growth of trees (Quint, 2001). He found that the trees adjust their growth in a fashion such that the stresses on the surface are equally distributed. Stress peaks that occur will be reduced by a stress proportional to the growth in that area. He also observed that in nature, all unnecessary material is avoided and that material decays where it is no longer needed. Based on these bionic observations, he introduced what he called the SKO. By varying Young’s modulus in a structure, the elements that carry more of the load are rewarded by increasing Young’s modulus and simulating material growth in the area. Decreasing their respective Young’s modulus punishes the elements at lower stress states. By this, the “lazy” elements increasingly withdraw themselves from carrying the load and once they do not contribute significantly, they are purged from the set of elements. This process enables an optimal component geometry to be identified that meets the requirements of lightweight construction. The SKO method has become an integral part of Daimler Chrysler’s vehicle development engineering processes (Reuters, 2008). In the case of a car door, it has been reported that the honeycomb-design method increases stiffness by up to 40%, while the weight is reduced by around 30%, based on calculations using the SKO method. If the entire body shell structure is configured according to the SKO method, its weight would be reduced by around 30%—while retaining its exemplary stability, crash safety, and handling dynamics. The SKO method has since been used for producing components such as the engine support arms that are fitted on some rural-service buses. The bone-plate skeleton of the previously discussed boxfish demonstrates how nature is able to achieve an optimal structural design. The hexagonal structural scales of the boxfish obey the principle of maximum strength for the least weight. It should be noted that the aforementioned structural optimization methods have no proof that they will achieve an optimal design, but experience has shown that the application of these straightforward methods will result in lighter and durable structures. The weight-saving efficient structural design concepts obtainable by these evolution-based design methodologies clearly show that bionics can make contributions to greater fuel economy and operational economics for both the automotive and aerospace industries through the development of lightweight and efficient structural designs.

629

Biomimetics and Flying Technology

18.14  Cybernetics—Reverse Engineering of Nature for Design Innovations Cybernetics will be defined as the science of reverse engineering of nature using analytical tools and experimental methodologies to examine nature in great detail to gain an understanding of nature’s designs, functions, and operational procedures and thereby enable bionic or biomimicry innovations. John McMasters stated “Engineers, working closely with those from a range of scientific disciplines (e.g. zoology, botany, paleontology, neurophysiology, geology, and particularly ecology), have much to contribute to increasing our understanding of flight in nature and engineering in general.” Figure 18.124 illustrates a systematic approach to integrate results of independent and/ or coordinated studies using the various tools of the engineer or scientist to develop the knowledge database defining the physics and mechanics of insect flight. The understanding formulated from the knowledge database in this example can then be applied to the development of an artificial flying insect. 18.14.1  Bird Swarms and Group Dynamics Many have observed at various times the magnificent aerobatic displays of large flocks of birds such as shown in Figure 18.125. These displays appear to be ordered patterns of chaotic undirected motion, often without an apparent purpose or global objective. Similar types of swarming motion are displayed by insects as well as by schools of fish. CFD

BFD

UFD

10.0 9.0 8.0

Wing section

7.0 6.0

L/D

Clap A

4.0 3.0 2.0 1.0

0.0 0 10

Tau Emerald inflight

100 Rec

Rel

Fling D

B

E

100

C

F

Microtransmitter

RFD

VFD

AFD CL

Airframe Actuator

CD

RFD

Wing hinge Airfoil

FIGURE 18.124

Integrated approach to reverse engineering of nature.

Transmission

630

Biomimetics: Nature-Based Innovation

(a)

(b)

FIGURE 18.125 Bird swarms and group dynamics. (a) Starling swarm sequence. (From Winter, D. With permission.) (b) Predator/ Flock simulation with boids particle system. (Courtesy of Ennio Fioramonti.)

A swarm of bees or a flock of birds can be assumed to consist of N number of agents. These autonomous agents are in some way cooperating to achieve a global objective. This global objective can include better foraging, constructing shelter, or serving as a defense mechanism. The apparent collective intelligence of a swarm emerges from actions of the individual agents. The actions of these agents are governed by local rules of interactions of the N agents. A kind of “self organization” emerges in these systems (Trivedi, 2008). The individual (but autonomous) agent does not follow directives from a central authority or work according to some global plan. For example, a bird in a flock only adjusts its movements to coordinate with the movements of its flock mates or more precisely the members that are its neighbors. It simply tries to stay close to its neighbors, but avoids collisions with them. Each bird does not take commands from any leader bird since there is no lead bird. Any bird can fly anywhere in the swarm, either in the middle or the front or the back of the swarm. Swarm behavior gives the birds some distinct advantages like protection from predators, and searching for food. Craig Reynolds, a computer graphics researcher, in 1986 created a deceptively simple steering program called boids (Reynolds, 1987). The boid model has in its implementation, simple rules to explain and predict the motion of a flock of birds. Each boid observes the following rules. Later, a fourth rule was added: a boid should move laterally away from any boid that blocks its view.

1. Boids try to fly toward the center of mass of neighboring boids. 2. Boids try to keep a small distance away from other objects (including other boids). 3. Boids try to match velocity with near boids.

This simple model as shown in the lower part of Figure 18.125 appears to accurately predict the motion of the flock and the agents within the flock. Swarm intelligence as predicted by the boid model provides a basis which makes it possible to explore collective (or distributed) problem solving without centralized control. A team of robots that could coordinate its actions like a flock of birds could offer significant advantages over a solitary robot. Spread out over a large area, a group could function as powerful mobile sensor net, gathering information about what is out there. If the group encountered something unexpected, it could adjust and respond quickly, even if the robots in the group weren’t technically very sophisticated.

631

Biomimetics and Flying Technology

Cockroach

Biped coconut octopus

Biped algae octopus

Application “distributed foot concept”

Boston Dynamics FIGURE 18.126 Nature’s distributed foot. (Courtesy of Robert Full, University of California, Berkeley, Department of Integrative Biology.)

18.14.2  Distributed Feet and Mobile Robots Robert Full of the University of California, Berkeley, Department of Integrative Biology and his colleagues have conducted a number of clever experiments to determine the performance characteristics of nature’s foot designs. In some of their experiments, they observed grass spiders and cockroaches run across a mesh with 99% of the contact area removed. Neither insect slowed down when crossing the mesh. Upon further investigation, they determined that the “foot” or that part of the leg that contacts the ground is distributed along the whole leg as shown in the left picture in Figure 18.126. This is true for spiders or cockroaches. Crissy Huffard, Robert Full, and Farnis Barneka (Huffard, 2005) reported the first scientific documentation of underwater “bipedal” locomotion of any animal. These are the middle pictures in Figure 18.126 and include a bipedal octopus disguised as a rolling coconut and another bipedal octopus that disguises itself as floating algae by walking on two legs and holds its other arms up in the air. The distributed foot designs make it possible to move over obstacles as though they are not even present. It has been postulated that the two-armed walking behavior allows the octopus to slowly walk away from a predator while preserving its existing camouflage. The distributed foot concept is an integral part of the Robot Hexapod, RHex, developed by Full, in collaboration with Daniel Kodistchek of the University of Michigan, Martin Buehler at Canada’s McGill University and Boston Dynamics. RHex has self-correcting reflexes—“preflexes,” that act like springs and shock absorbers that help it overcome obstacles. RHex climbs over rock fields, mud, sand, vegetation, railroad tracks, up steep slopes, and stairways. RHex has a sealed body, making it fully operational in wet weather, in muddy and swampy conditions, and it can swim on the surface or dive underwater. 18.14.3  Insects and Optical Flow Insects such as the dragonfly utilize optic flow in order to navigate in and around obstacles as shown in Figure 18.127. The term “optic flow” refers to a visual phenomenon that you experience every day. Essentially, optic flow is the apparent visual motion that you experience as you move through the world. Suppose you are sitting in a moving car or a train, and are looking out the window. You see trees, the ground, buildings, and so on, appear to move backwards. This motion is optic flow. Optic flow can indicate to an insect how close it is to different objects and how quickly it is approaching them. There are clear mathematical relationships between the magnitude of the optic flow and where the object is in relation to the observer. Engineer and inventor Geoffrey Barrows has developed innovative optic-flow sensors to allow both aerial and ground vehicles to travel autonomously, by using the same techniques living creatures such as flying insects do to gauge their altitude and proximity to obstacles in their path.

632

Biomimetics: Nature-Based Innovation

Speed control through dense environments

Slow down in tight spaces

Avoiding obstacles

Saccade away from obstracles

FIGURE 18.127 Dragonfly and optic flow. (From Geoffrey Barrows, Centeye Corp. With permission). 1.0 cL

Brown skua

Wing with artificial covering feathers

Covert feathers Minimum pressure

Crow

Lift coefficient

0.8 0.6

Standard wing

0.4 Re=130000 0.2

Movement of the separation Action of covert feathers point to the region of as passive flow separation mimimum pressure control

0 00

100

200

300

α

400

Angle of incidence

FIGURE 18.128 Covert feathers as flow control “eddy flaps.”

18.14.4  Nature’s Passive Flow Control Concepts Figure 18.128 shows lifted covert feathers on a brown skua and also on a crow at landing conditions. It has been hypothesized that these coverts are passively lifted to prevent the forward movement of the separated flow that develops initially near the trailing edge as a bird increases its attitude to slow down and land. Experimental studies (Patone, 2008) of a passively lifting “eddy flap” as shown in the figure indicate that the effect was to prevent sudden drop in lift generation during stall as well as an increase in CLmax. Measured pressure distributions indicate that the eddy flaps restrict the separation eddy to the aft part of an airfoil. High-performance glider airplanes use flexible self-actuating on the upper suction side of the wing which act as “back flow barriers” as observed in nature (Liebe, 1979). The first successful experimental flight tests were performed on a Me 109 in 1939. Forty years later, Bechert (2006) and Rechenberg (1995) conducted more detailed and broad investigations. The aerodynamic concept of the leading edge slat on an airfoil or wing performs the same function as the alula that exists on the wings of some birds. Both concepts, which are shown in Figure 18.129, help to restore or retain attached flow around the leading edge and thereby increase the maximum achievable lift coefficients, CLmax. Birds as well as such equipped aircraft use their respective leading edge devices to provide lower landing speeds. It has been reported that birds with alula dramatically increase their takeoff and landing capabilities (Alvarez, 2001). 18.14.5  Grasshopper Knees and Jumping Robots Jumping can be a very efficient mode of locomotion for small robots to overcome large obstacles and travel in natural, rough terrain. Dario Floreano and his colleagues at the

633

Biomimetics and Flying Technology

Slat Separation delayed: higher stall angle

Slat

CL

CL,max slat

Airfoil with slat

CL,max normal

Alula

Unslotted airfoil Increased stall angle with slat Angle of attack, α

Alula

Alula

Northern gannet

FIGURE 18.129 Alula—Nature’s leading edge slats. (Courtesy of NASA.)

Standing high jump records (inches)

5cm

Infrared receiver body and battery Main leg foot

1=6ms

1=12ms

1=18ms

1=24ms

60 55 50 45 40 35 30 25 20 15 10 5 0

Froghopper Man

EPFL 7g Robot

FIGURE 18.130 Tiny 7 g jumping mechanism prototype.

Laboratory of Intelligent Systems, Swiss Federal Institute of Technology have developed a novel 5 cm, 7 g jumping robot that can jump obstacles ten times its own height (Kovac, 2008), as shown in Figure 18.130. It employs elastic elements in a four bar linkage leg system to allow for very powerful jumps and adjustment of jumping force, take-off angle, and force profile during the acceleration phase. This jumping mechanism is very similar to the jumping mechanism of the grasshopper. This is an example of being inspired by nature and then surpassing nature’s capability as seen in the standing high jump records bar chart.

634

Biomimetics: Nature-Based Innovation

18.15 Pseudo-Mimicry—Innovations Confirmed by Nature Designs and Solutions Pseudo-mimicry relates to technology developments or innovative concepts that are not directly inspired by nature, having similar but unrelated functions. Sometimes, we do not copy nature, but we rediscover our own inventions in a similar but unrelated concept of nature. We will extend this definition of pseudo-mimicry to include designs that may have similar functions but were not directly influenced by an awareness of nature’s similar design. Since nature’s designs have been refined over periods of millions of years, finding a design or concept in nature that is similar to one of our creations tends to suggest that we are probably on the right track. 18.15.1  Microraptor gui and Tandem Wings The Proteus shown in Figure 18.131 is a twin turbofan high-altitude multimission aircraft powered by Williams International FJ44-2E engines. It is designed to carry payloads in the 2000-lb class to altitudes above 60,000 ft and remain on station up to 14 h. Heavier payloads can be carried for shorter missions. It is intended for piloted as well as for UAV missions. Missions for Proteus include telecommunications, reconnaissance, atmospheric research, commercial imaging, and space launch. The Proteus is designed with long wings and a low wing loading needed for efficient high-altitude loiter. It excels in stability and low noise. It is capable of dynamic maneuvers needed to operate in adverse conditions. The crisp, short takeoff and landing use the unique “three-mains” landing gear design intended to increase crosswind and wet runway capability without the use of spoilers. The shape of the Proteus1 is very similar but certainly unrelated to the Microraptor gui shown in the figure. This chicken-sized microraptor, which lived in the early Cretaceous period some 140 million years ago, had long flight feathers on its forelimbs and feet, and relied on its biplane-like wing configuration to swoop from tree to tree. 18.15.2  Sea Gull and Parasol Wings Figure 18.132 shows a high degree of similarity between a sea gull in flight and a supersonic double parasol wing fighter. However, the flow physics and flight characteristics are (a)

(b)

77 cm (~30 in.) FIGURE 18.131 (a) Microraptor gui, northern China, 125 million years ago (From Xu, X. et al., Nature, 421, 335–340, 2003.); (b) Rutan “Proteus.” (From NASA.)

635

Biomimetics and Flying Technology

FIGURE 18.132 Comparison of sea gull and double parasol wing fighter.

Planform tailored to capture “+” Nacelle pressures Interference lift area Bow shock

Cp > 0

Inboard area

“Near parabolic” reflection surface

Cp = 0 Nacelle CL Cp < 0 C L

Optimized wing + body and tails

Nacelles positioned for • Wave cancellation • Interference lift

FIGURE 18.133 Aerodynamic features of the double parasol wing concept.

totally unrelated. The double parasol wing configuration was developed to exploit the use of hypersonic favorable aerodynamic interference concepts for the design of a supersonic aircraft (Kulfan, 1990). At supersonic speeds, the formation of shock waves is inevitable. The fundamental aerodynamic features inherent in this configuration included shaping the nacelles to create a large region of positive pressures (+Cp) surrounding the nacelle. The wing planform shown in Figure 18.133 was shaped and positioned above to capture the positive pressure field of the nacelles, which produces a significant amount of interference lift on the wings. The wing planform was formed (front view) into a nearly parabolic shape to maximize the interference lift. The nacelles were also positioned below the wing to develop drag reducing wave cancellation on the nacelles. The body shape and area distribution and tail planforms were further shaped and located to minimize drag. The resulting configuration achieved a cruise lift-to-drag ratio that was about 20% higher than a more conventional design.

636

Biomimetics: Nature-Based Innovation

18.15.3 Wasp Nests, Damselfly Wings, Vulture Bones, and Structural Design Concepts Figures 18.134 and 18.135 show a number of structural and material concepts similar to those found in nature. The structural design of the metacarpal bone from a vulture’s wing looks very similar to the Warren truss design and has been used for both the wing and body structural designs, such as in the Antoinette monoplane in approximately 1909 (Figure 18.134). The skin-stringer design of the damselfly wing and the honeycomb design of a wasp’s nest (Figure 18.135) are used in the design of modern aircraft. The use of temperature-resistant strong impact materials such as in the abalone shell has been used for hypersonic vehicles such as the space shuttle. The basic structural materials used by nature include composite and elastic materials whereas the primary structural materials for artificial flight include metals and composites. The current basic design philosophy for commercial aircraft includes failsafe, inspection, detection, repair, and replace. Nature’s design philosophy includes safe-life, self-repair (healing), self-inspection and cleaning (preening), and regeneration (molting).

Vulture’s wing metacarpal bone (circa 60 MYA) Warren truss (1848)

Wing and body structure of Antoinette monoplane, (circa 1909)

FIGURE 18.134 Warren truss and vulture’s wing.

Skin stringer type structure

Honeycomb structure

Damselfly wing

FIGURE 18.135 Other similar structural concepts and materials from nature.

637

Biomimetics and Flying Technology

1

2

3

4

5

6

7

8

FIGURE 18.136 Example of wing morphing during landing.

Current aircraft designs have limited morphing capability such as flaps, slats, ailerons, spoilers, elevators, rudders, and folding gear. Nature’s designs have essentially unlimited morphing capability as shown in the crow landing sequence shown in Figure 18.136. 18.15.4  Nature’s Wheels and Rotary Tracks Wheels do not appear to play a significant role in the locomotion of biological systems. This lack of biological “wheels” has been a frequent topic of semiserious debate among biologists. A common question is: “Why didn’t nature invent the wheel?” The obvious answer is “Because there were no roads.” Rotating locomotion incurs mechanical disadvantages in certain environments and situations, which may help to explain why multicellular life has not evolved wheels for locomotion. Although wheels are more energy efficient than other means of locomotion when traveling over hard, level terrain (such as paved roads), wheels have several distinct disadvantages that stem largely from the fact that many natural environments are ill-suited to the use of wheels. As shown in Figure 18.137, some organisms do use rolling as a means of locomotion. However, the entire organism rotates itself. A species of caterpillar known as the mother-ofpearl moth, curls into a ring and rolls away when threatened. The salamander Hydromantes platycephalus also curls up and rolls downhill to escape danger. The tumbleweed Corispermum hyssopifolium uses passive rolling, powered by wind, to distribute its seeds (Figure 18.138). The dung beetle uses rolling to transport the feces on which it feeds. It is appropriate then, to say that nature did invent the wheel; it just forgot the axle. A species of mantis shrimp, the stomatopod crustacean Nannosquilla decemspinosa, if washed onto a sandy beach by a wave, will immediately roll back to the water by means of consecutive backward somersaults, effectively forming a wheel with its entire body, as shown in Figure 18.139. In a sense, it can be said that nature also invented the continuous track used on many of our specialized vehicles. 18.15.5  Bacterial Rotary Engines and Turbine Engines Nature created an incredible self-assembly high-speed reverse direction rotary motor with a diameter of 30 nm shown in Figure 18.140. Mobility of bacteria, such as Salmonella

638

Biomimetics: Nature-Based Innovation

Larvae of the caterpillar Pleuroptya ruralis, (the mother-of-pearl moth) The salamander Hydromantes platycephalus

FIGURE 18.137 Nature did invent the wheel.

Dung beetle

Sagebrush

FIGURE 18.138 More of nature’s rollers.

Stomatopod shrimp Snow cat

B36 Track wheel

FIGURE 18.139 Nature invented the continuous track wheel.

Hook (universal joint) Outer membrane

{

Stator Studs C ring

Filament (propeller) L ring P ring Bushing

Propeller-like motion

{

Passive Inner (plasma) part in motion membrane S ring Rotor M ring Flagellum

{

Basal body

Back and forth beating

Cilia

FIGURE 18.140 Nature’s rotary engine (circa BsYA). (Courtesy of Namba, K., Protonic NanoMachine Group, Osaka University.)

639

Biomimetics and Flying Technology

and E. coli, with a body size of 1~2 microns, is driven by rapid rotation of a helical propeller by a tiny little motor at its base. This organelle is called the flagellum, made of a rotary motor and a thin helical filament that grows up to about 15 microns. It rotates at around 20,000 rpm, at energy consumption of only around 10−16 W and with energy conversion efficiency close to 100%. The motor switches its direction every few seconds to change the swimming direction of the cells for bacteria to seek better environments. Keiichi Namba, Graduate School of Frontier Biosciences, Osaka University and his colleagues are conducting cutting-edge research to reveal the mechanism of this highly efficient flagellum motor that is far beyond the capabilities of artificial motors. The left picture shows two bacteria cells with their flagella extending behind. The second picture shows the design of the flagella self-assembly rotary motor. The motor as in similar artificial motors includes a stator, rotor, and bushings. Cilia use a similar motor design except the bacterial flagellum acts as a rotary propeller in contrast to the cilium, which acts more like a shape-changing oar. The bacterial motor is nature’s version of the jetprop or propfan. Both have a similar internal turbine-driven high-speed radial motors and a unique external “instrument for obtaining propulsion” as shown in Figure 18.141. The differences in the design of the “instruments for obtaining propulsion” is necessitated by the different Reynolds numbers regime that each must operate within. The propfan operates in the high Reynolds number regime and the bacterial motor operates in the very low Reynolds regime. As shown in Figure 18.82, the fundamental flow characteristics are significantly different in these two worlds. The propfan operates at high Reynolds numbers where inertia effects are much more significant than viscous effects. The inertial effects of the propeller rotation imparts momentum to the flow passing through the propeller, which in turn provides the thrust. At a very low Reynolds number, inertial plays no role whatsoever. At a low Reynolds number, everything reverses just fine. Time, in fact, makes no difference. For a configuration changing quickly or slowly, the pattern of motion is exactly the same (Purcell, 1997). Consequently, a rotating propeller would simply stir the local fluid. Nature has engineered a solution for this type of flow: the use of cilia and flagella (Edd, 2003). At a very low Reynolds number, a helix that translates along its axis under an external force but without an external torque will necessarily rotate. By the linearity of the Stokes equations, the same helix that is caused to rotate due to an external torque will necessarily translate. This is the physics that underlies the mechanism of flagellar propulsion employed by many microorganisms (Purcell, 1997).

Jet-prop (propfan)

“The instrument for obtaining propulsion”

Turbine Turbine FIGURE 18.141 Nature’s propfan.

640

Biomimetics: Nature-Based Innovation

18.16  Biologically Inspired Aircraft Concepts The bird is a machine that operates according to mathematical law. Leonardo da Vinci

Flapping wing flight has fascinated humanity for centuries, from Leonardo da Vinci’s efforts to design and build flapping flight machines to the present. Such flying machines are called ornithopters. Birds and machines are subject to the same rules, which we call the aerodynamic laws of nature. Even though nature is far ahead in many areas, technology evolves much faster than plants and animals. We’ll fly as birds do, and then we’ll do it better than birds. New technologies and developments of materials, power systems, and active controls will usher in such new types of aircraft. Using electroactive materials such as ionic polymeric artificial muscles (Chapter 6), wings can be designed to move with a motion similar to birds, bats, or insects. It is because these materials can deform and mimic the movement of muscles that an accurate representation of the motion of a flapping wing can be reproduced. Early failed attempts at flapping wing flight in the 1800s convinced many people that humans could not fly by flapping wings. However, all it really proved was that they didn’t yet have the technology to succeed at this difficult task. Flapping flight is nature’s means of producing creatures that can fly. This type of flying has been perfected over the millennia and has been adapted by numerous types and sizes of creatures, as discussed in the previous sections. The most striking variation in the creatures that can fly is in their size. Sizes range from tiny insects, which are less than a millimeter across, to the giant flying dinosaurs that had wingspans of many meters as shown in Figure 18.142. The key to mimicking flapping flight is in accurately reproducing the motion of the wing. For a bird, bat, or the large dinosaurs, this wing motion has evolved to produce lift and thrust throughout the flap cycle while minimizing drag, thereby conserving energy. The wing motion of these creatures is very complex. It consists of bending, twisting, and deformation in the wing shape (chord-wise to adjust wing chamber). Precisely mimicking this motion has been a very difficult task. These creatures have evolved to being able to produce this complex motion through a combination of joints, muscles, and in the case of birds, feathers that enable them to produce the wing shape and motion that enables efficient flapping flight.

FIGURE 18.142 Range of sizes in flying creatures.

641

Biomimetics and Flying Technology

Self-Propelled Organisms

Rel No.

Large whale swimming at 10 m/s Tuna swimming at 10 m/s Duck flying at 20 m/s

3×108 3×107 3×105

Flow Characteristics

High Reynolds Numbers >>1

Large dragonfly at 7 m/s

3×104

Copepod at burst speed 0.2 m/s Thrips – Smallest flying insect Invertebrate larva at 1 mm/s

3×102 10 3×10-1

Inertial forces dominate Time dependency is important Flow separation May be turbulent Not reversible Non-linear Large momentum (vortices in wakes) Coasting

Very Low Reynolds Numbers