3D and 4D Printing of Polymer Nanocomposite Materials: Processes, Applications, and Challenges [1 ed.] 0128168056, 9780128168059

Table of contents :
Front Matter
Copyright
Contributors
Preface
Acknowledgment
Introduction to 3D and 4D printing technology: State of the art and recent trends
Introduction
Designing perspective and effect of processing parameters in 3D and 4D printing
Challenging prospects
Designing challenges
Manufacturing challenges
Qualification and validation
3D printing technology
Feeding mechanisms in 3D printing
Binder jetting
Material jetting
Direct energy deposition
Powder bed fusion
Light photopolymerization
Extrusion
Sheet lamination
Classification of materials used in 3D and 4D printing
Powder materials
Wire filament materials
Printable waxes
Liquid materials
3D printer software and hardware
Evolution of 4D printing technology
Printers for 4D printing
Recent trends in 3D printing and 4D printing
Conclusions
References
3D and 4D printing of nanomaterials: Processing considerations for reliable printed nanocomposites
Introduction
Nanocomposites
Definition of nanocomposite
Benefits/advantages of nanocomposites
Additive manufacturing
AM categories
Physical phenomena in AM
Chapter organization
AM and governing physical phenomena
Printing methods commonly used in nanocomposite AM
Material extrusion
Fabrication method description
Governing physical phenomena
Vat photopolymerization
Fabrication method description
Governing physical phenomena
Powder bed fusion
Fabrication method description
Governing physical phenomena
Nanocomposites effects on processing parameters
Material properties in AM processing
Influence of nanoparticles on polymer viscosity
Effect of nanoparticles on polymer thermal properties, vitrification and crystallization
Influence of nanoparticles on interlayer adhesion
Effect of nanoparticles on polymer-light interactions
Future outlook and needs for future research
References
Polymer-based conductive composites for 3D and 4D printing of electrical circuits
Introduction
Background
The case for FDM-compatible conductive polymer composites (CPCs)
What is a four-dimensional (4D) printable material?
What is an FDM-compatible conductive polymer composite (CPC) filament?
Conductive polymer composites with carbon-based fillers
Formulation
Characterization
Microscopy
Current-voltage (I-V) measurements
Differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA)
Stress tests-Ultraviolet, electrical, thermal
Conductive polymer composites with metal-based fillers
Preparation of conductive metal filler
PCL+Cu-Ag nanowires
PVB+Ag flakes
Nylon-6/PE+Ni/Sn95Ag4Cu1 low-melting-point alloy
Performance before and after printing
PCL+Cu-Ag nanowires
PVB+Ag flakes
Nylon-6/PE+Ni/Sn95Ag4Cu1 low-melting-point alloy
Applications
2D circuit tracks
3D circuit tracks
3D chassis with 2D conductive tracks
3D circuit elements
Inductor
Capacitor
Sensors
Temperature sensor
Wearable glove with embedded flex sensor
Capacitive buttons
Challenges in printing CPCs
Summary and perspective
References
3D and 4D printing of pH-responsive and functional polymers and their composites
Introduction
Processing techniques
Stereolithography
Selective laser sintering
Inkjet and powder-inkjet printing
Extrusion 3D printing
Fused deposition modeling
Liquid deposition modeling
4D printing
Single-material 4D printing
Multiple-material 4D printing
Functional materials in 3D and 4D printing
Printing of electroactive and electromagnetically active materials
Temperature-responsive functional materials
Shape memory polymers
Temperature-responsive polymer composite hydrogels
pH-responsive polymers
Light-responsive materials
Piezoelectric functional materials
Conclusions
References
Additive manufacturing (AM) of medical devices and scaffolds for tissue engineering based on 3D and 4D printing
Introduction
Scaffolds for tissue engineering
Scaffold architecture
Mechanical properties
Biomaterials for tissue engineering and scaffold fabrication
Natural polymers
Synthetic polymers
Bioceramics
Metal-based scaffold materials
Biocomposites
Direct 3D-printing processes
Stereolithography (SLA)
Microextrusion-based 3D bioprinting
Inkjet based 3D-printing
Fused deposition modeling (FDM)
Selective laser sintering (SLS)
Indirect 3D-printing processes
4D printing for biomedical applications
Soft active shape memory polymers
Hydrogel-based 4D printing
Factors affecting 4D printing
Effect of temperature
Effect of water or solvent
Conclusions
References
Shape memory polymer blends and composites for 3D and 4D printing applications
Introduction-Historical overview
The underlying mechanism of the SME
Main trends in the use of SMP for 3DP and 4DP
Technologies of 3DP applied to SMP
Classification of SMP, blends, and composites used in 3D and 4DP
SMP and blends in SLA
SMP and blends in fused deposition modeling/fused filament fabrication
Conclusions and outlook
References
Further reading
Fabrication of 3D and 4D polymer micro- and nanostructures based on electrospinning
Introduction
Nanotechnology for advanced materials
3D polymer micro- and nanostructures
Background
Additive manufacturing
Electrospinning process
Jet initiation and elongation
Growth of bending instability and further elongation
Methods to fabricate 3D electrospun polymer micro- and nanostructures
Multilayer electrospinning
Stacking
Application of 3D collecting template
Freeze drying into shapes
Self-assembly
3D and 4D electrospinning technique
Basic principles
Apparatus
x-y-z axis motion control
3D/4D electrospinning nozzle
Solution control
High-voltage control
Ambient control
3D and 4D electrospinning process
Digital 3D model design
G-code generation
Material preparation
Printing 3D nanofibrous materials
Finishing
Characterization
Optical microscopy
Scanning electron microscopy
Surface area measurements
Processing parameters
Solution parameters
Applied voltage
Working distance
Solution flow rate
Nozzle moving speed
Temperature and humidity
Potential applications
Biomedical applications
Tissue engineering and drug development
4D nanomaterials
Energy applications
Batteries
Fuel cells
Supercapacitors
Catalysis
Filtration
Food industry
Cosmetics
Sound insulators
Summary
Challenges and future perspectives
References
Multifunctional polymer composites for 3D and 4D printing
Introduction
Multifunctional structures
Piezoelectric structures
Battery fiber structures
PV structures
Flexible electronics structures
4D structures
Conclusions
References
Graphene and graphene oxide-reinforced 3D and 4D printable composites
Introduction
A brief introduction to graphene
Emerging graphene 3DP
Feasible techniques for graphene 3DP
Direct ink writing
Additive strategies
Hydrogen bond
Electrostatic interaction
Reactive inks
Additive rheology effects
Additive-free approaches
Highly concentrated GO inks
Process assistance
Fused deposition modeling
Light-based 3DP
The properties of 3D-printed graphene-based materials
Applications of 3D-printed graphene
Energy storage applications
Solar energy
High temperature application
Sensoring
Prospect and outlook
References
Further reading
3D and 4D printing of polymer/CNTs-based conductive composites
Introduction
Traditional approaches
Solution processing of CNTs and polymer
Melting the polymer
Milling
3D printing
3D printing techniques
FDM
Stereolithography (SLA)
PolyJet
Powder bed and inkjet head 3D printing (3DP)
Selective laser sintering (SLS)
Direct write (DW)
Inkjet printing
Aerosol jet printing (AJP)
Tailoring the interface of polymer/CNTs
Applications
Electrical conductivity and transparency
Electromagnetic shielding effect
Electronic devices
Tissue engineering
4D printing
Limitations and future research
Material innovation
Polymer-CNT interfacial properties
Material homogeneity
3D equipment and printability
Conclusion
References
Further reading
Medical and biomedical applications of 3D and 4D printed polymer nanocomposites
Introduction
3D and 4D printed polymer nanocomposites for biomedical applications
3D printing technologies
Nanofillers classification
Applications
Nanocomposites for physical properties tuning of 3D printed scaffolds
Nanofibrillated cellulose (NFC)
Nanosilica/nanoclay
Ferroferric oxide (Fe3O4)
Nanohydroxyapatite (nHA)
Carbon-based nanoparticles
Nanocomposites for 3D printing of active devices
Nanocomposites for 3D printing of diagnostic and therapeutic tools
Challenges and future perspectives
Conclusion
References
Further reading
Carbon black-reinforced 3D and 4D printable conductive polymer composites
Introduction
Fabrication and characteristic
Conductive and physical mechanism
Applications of 3D and 4D printable CB composites
Strain sensor
Soft equivalent spring buffer and electrode for energy harvesting
Challenges and future perspectives
Conclusions
References
Photoactive resin formulations and composites for optical 3D and 4D printing of functional materials and devices
Introduction
Photopolymerization-based additive manufacturing (AM) technologies
Fundamentals of photopolymerization-based AM
Penetration depth, critical exposure, and cure depth
Photopolymerization kinetics
Photopolymerization kinetic models
Characterization techniques for monitoring SLA kinetics
Recoating mechanisms
Free surface
Constrained surface
Photoresin formulations
Additives
Photoinitiators
Absorbers
Composite formulations
Applications
4D printing
Sensors, actuators, and transducers
Energy applications
Biomedical applications
Conclusion
References
Hydrogels and hydrogel composites for 3D and 4D printing applications
Introduction
3D printing of hydrogels and hydrogel composites
Nozzle-based 3D printing
Inkjet printer-based 3D printing
Laser-based 3D printing
Hydrogels and hydrogel composites for 3D printing
Hydrogels derived from natural polymers
Collagen
Gelatin
Alginate
κ-Carrageenans
Gellan gum
Chitosan
Oppositely charged hydrogels
Interfacial bonding
Hydrogels from synthetic polymers
Poly (ethylene glycol)
Poly (vinyl alcohol)
Pluronics
Hydrogel composites
Double network hydrogels
Particle-reinforced hydrogels
Fiber-reinforced hydrogels
Applications of 3D printed hydrogel and hydrogel composites
Tissue engineering
Multifunctional devices
4D printing of hydrogels and hydrogel composites
Conclusion
References
Further reading
3D and 4D printing of biomaterials and biocomposites, bioinspired composites, and related transformers
Introduction
Deposition techniques for 3D printing of biomaterials
Stereolithography
Extrusion printing
Inkjet printing
Selective laser sintering/melting
Materials for 3D printing of biomaterials
Hard matter
Soft matter
Biologically derived materials
Composite materials
Applications of 3D printed biomaterials
Tissue engineering
Medicine and drug delivery
Dentistry
4D printing and its applications for biomaterials
Techniques in 4D printing
Materials and applications of biomaterials in 4D
Conclusions and future perspectives
3D printed biomaterials
4D printed biomaterials
References
3D and 4D printed polymer composites for electronic applications
Introduction
Why 3D/4D printing composites for electronic applications?
Why 3D/4D printing?
Why composites for electronic applications?
3D and 4D printable electrically conductive composites
Percolation theory in conductive composites
Processes of 3D printing of conductive composites
Extrusion of conductive composites
Inkjet printing of conductive composites
SLA of conductive composites
SLS of conductive composites
Role of conductive composites in 4D printing
Applications of 3D and 4D printable conductive composites
Printed electrode applications
Printed sensor applications
3D and 4D printable dielectric composites
Work principle of dielectric composites
Dielectric elastomer composites
Highly insulating composites
Processes of 3D printing of dielectric composites
Extrusion of dielectric composites
Inkjet printing of dielectric composites
SLA of dielectric composites
SLS of dielectric composites
Applications of 3D and 4D printable dielectric composites
RF-responsive structures
Electrical insulators
Dielectric elastomer actuators
Conclusion
References
Further reading
Fundamentals and applications of 3D and 4D printing of polymers: Challenges in polymer processing and prospec ...
Introduction
Fundamentals of 3D printing processes
VAT photopolymerization
Stereolithography
Digital light processing
Continuous liquid interface production
Multiphoton polymerization
Powder bed fusion
Selective laser sintering
Material extrusion
Fused deposition modeling
Binder jetting
Inkjet printing
Aerosol jet printing
Challenges in polymer processing
Mechanical properties
Resolution
Manufacturing speed
Multimaterial printing
Biocompatibility
Applications of 3D and 4D printing of polymers
Polymer nanocomposites in 3D and 4D printing
Fiber-reinforced polymer nanocomposites
Nanoparticle-reinforced polymer nanocomposites
Prospects of future research
Conclusions
References
Index
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D
E
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M
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Citation preview

3D AND 4D PRINTING OF POLYMER NANOCOMPOSITE MATERIALS

3D AND 4D PRINTING OF POLYMER NANOCOMPOSITE MATERIALS Processes, Applications, and Challenges Edited by

KISHOR KUMAR SADASIVUNI Center for Advanced Materials, Qatar University, Doha, Qatar

KALIM DESHMUKH New Technologies—Research Center, University of West Bohemia, Pilsen, Czech Republic

MARIAM ALALI ALMAADEED Qatar University, Doha, Qatar

Elsevier Radarweg 29, PO Box 211, 1000 AE Amsterdam, Netherlands The Boulevard, Langford Lane, Kidlington, Oxford OX5 1GB, United Kingdom 50 Hampshire Street, 5th Floor, Cambridge, MA 02139, United States © 2020 Elsevier Inc. All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Details on how to seek permission, further information about the Publisher’s permissions policies and our arrangements with organizations such as the Copyright Clearance Center and the Copyright Licensing Agency, can be found at our website: www.elsevier.com/ permissions. This book and the individual contributions contained in it are protected under copyright by the Publisher (other than as may be noted herein). Notices Knowledge and best practice in this field are constantly changing. As new research and experience broaden our understanding, changes in research methods, professional practices, or medical treatment may become necessary. Practitioners and researchers must always rely on their own experience and knowledge in evaluating and using any information, methods, compounds, or experiments described herein. In using such information or methods they should be mindful of their own safety and the safety of others, including parties for whom they have a professional responsibility. To the fullest extent of the law, neither the Publisher nor the authors, contributors, or editors, assume any liability for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions, or ideas contained in the material herein. Library of Congress Cataloging-in-Publication Data A catalog record for this book is available from the Library of Congress British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library ISBN: 978-0-12-816805-9 For information on all Elsevier publications visit our website at https://www.elsevier.com/books-and-journals

Publisher: Matthew Deans Acquisition Editor: Edward Payne Editorial Project Manager: Emma Hayes Production Project Manager: Selvaraj Raviraj Cover Designer: Matthew Limbert Typeset by SPi Global, India

Contributors

Shweta Agarwala Department of Engineering, Aarhus University, Aarhus, Denmark M. Basheer Ahamed Department of Physics, B. S. Abdur Rahman Crescent Institute of Science and Technology, Chennai, India Mariam AlAli AlMaadeed Materials Science and Technology Program, Qatar University, Doha, Qatar V. Bertana Department of Applied Science and Technology, Politecnico di Torino, Torino, Italy Ramesh Gupta Burela Mechanical Engineering Department, Shiv Nadar University, Greater Noida, India F. Catania Department of Applied Science and Technology, Politecnico di Torino, Torino, Italy Xuelong Chen School of Mechanical & Aerospace Engineering, Nanyang Technological University, Singapore, Singapore M. Cocuzza Department of Applied Science and Technology, Politecnico di Torino, Torino; CNR-IMEM, Parco Area delle Scienze, Parma, Italy Kalim Deshmukh New Technologies—Research Center, University of West Bohemia, Pilsen, Czech Republic Vishwesh Dikshit Singapore Centre for 3D Printing, School of Mechanical and Aerospace Engineering, Nanyang Technological University, Singapore, Singapore S. Ferrero Department of Applied Science and Technology, Politecnico di Torino, Torino, Italy Jonathan Kenneth Goh School of Electrical and Electronic Engineering, Singapore Polytechnic, Singapore, Singapore Kuan Eng Johnson Goh Institute of Materials Research and Engineering, Agency for Science, Technology and Research (A*STAR); Department of Physics, National University of Singapore, Singapore, Singapore

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Contributors

Guo Liang Goh Singapore Centre for 3D Printing, School of Mechanical and Aerospace Engineering, Nanyang Technological University, Singapore, Singapore Guo Dong Goh Singapore Centre for 3D Printing, School of Mechanical and Aerospace Engineering, Nanyang Technological University, Singapore, Singapore Christopher J. Hansen Department of Mechanical Engineering, University of Massachusetts Lowell, Lowell, MA, United States Lewis R. Hart Department of Chemistry, University of Reading, Reading, United Kingdom Dineshkumar Harursampath Aerospace Engineering Department, Indian Institute of Science, Bengaluru, India Wayne Hayes Department of Chemistry, University of Reading, Reading, United Kingdom Yinfeng He Faculty of Engineering, The University of Nottingham, Nottingham, United Kingdom Mohammad Talal Houkan Department of Mechanical and Industrial Engineering, Qatar University, Doha, Qatar Derek Irvine Faculty of Engineering, The University of Nottingham, Nottingham, United Kingdom Atharv Joshi Department of Materials Science & Engineering, National University of Singapore, Singapore, Singapore V.A. Kalyaev Skolkovo Institute of Science and Technology, Moscow, Russia Jagath Narayana Kamineni Mechanical Engineering Department, Shiv Nadar University, Greater Noida, India A.M. Korsunsky MBLEM, Department of Engineering Science, University of Oxford, Oxford, United Kingdom; Skolkovo Institute of Science and Technology, Moscow, Russia Toma´sˇ Kova´r´ık New Technologies—Research Center, University of West Bohemia, Pilsen, Czech Republic Toma´sˇ Krenek New Technologies—Research Center, University of West Bohemia, Pilsen, Czech Republic

Contributors

Sijun Liu Advanced Rheology Institute, Department of Polymer Science and Engineering, Shanghai Jiao Tong University, Shanghai, PR China Aqib Muzaffar Department of Physics, B. S. Abdur Rahman Crescent Institute of Science and Technology, Chennai, India Wiwat Nuansing School of Physics, Institute of Science; Center of Excellent on Advanced Functional Materials (CoE-AFM), Suranaree University of Technology, Nakhon Ratchasima, Thailand S. K. Khadheer Pasha Department of Physics, VIT-AP University, Amaravati, India C.F. Pirri Department of Applied Science and Technology, Politecnico di Torino, Torino, Italy Norbert Radacsi The School of Engineering, Institute for Materials and Processes, The University of Edinburgh, Edinburgh, United Kingdom Laura Ruiz-Cantu Faculty of Engineering, The University of Nottingham, Nottingham, United Kingdom Kishor Kumar Sadasivuni Center for Advanced Materials, Qatar University, Doha, Qatar Ehab Saleh University of Leeds, Leeds, United Kingdom A.I. Salimon Skolkovo Institute of Science and Technology; National University of Science and Technology “MISiS”, Moscow, Russia L. Scaltrito Department of Applied Science and Technology, Politecnico di Torino, Torino, Italy F.S. Senatov National University of Science and Technology “MISiS”, Moscow, Russia Sudip Kumar Sinha Department of Metallurgical Engineering, NIT Raipur, Raipur, India Fang Wang MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou, China

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Contributors

Ricky Wildman Faculty of Engineering, The University of Nottingham, Nottingham, United Kingdom Zhen Xu MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou, China Jin Xuan Department of Chemical Engineering, Loughborough University, Loughborough, United Kingdom Wai Yee Yeong Singapore Centre for 3D Printing, School of Mechanical and Aerospace Engineering, Nanyang Technological University, Singapore, Singapore Adilet Zhakeyev Department of Chemical Engineering, Loughborough University, Loughborough, United Kingdom Li Zhang State-Key Laboratory of Chemical Engineering, School of Mechanical and Power Engineering, East China University of Science and Technology, Shanghai, China Yilei Zhang Department of Mechanical Engineering, University of Canterbury, Christchurch, New Zealand Zuoxin Zhou Faculty of Engineering, The University of Nottingham, Nottingham, United Kingdom Jianxiong Zhu Department of Mechanical Engineering, Korea Advanced Institute of Science and Technology (KAIST), Daejeon, Republic of Korea

Preface

3D/4D printing or additive manufacturing (AM) is a process of joining materials to make 3D objects of complex geometries using computer software and computer-aided design (CAD). 3D/4D printing allows larger design flexibility and low cost, which enables elevated performance and helped in the transformation of the digital world. Polymers are by far the most utilized materials for 3D printing. A wide range of polymers that are used in 3D printing are thermoplastics, thermosets, elastomers, functional polymers, and composites. 4D printing has become a new and exciting branch of 3D printing. Unlike 3D printing, 4D printing allows the printed part to change its shape and function with time in response to change in external conditions such as temperature, humidity, pH, electricity, and light. Smart materials that respond to external stimuli are good candidates for 4D printing. Using this book, the reader will gain a consolidated view of the potential applications of 3D/4D printing in aerospace and architectural industry, the electronic industry as well as art and biomedical fields, depending upon the choice of worthy & suitable polymer material. 3D and 4D printing of polymer composites filled with conducting nanofillers such as carbon nanotubes (CNTs), graphene, carbon black, carbon fibers, and metal oxide nanoparticles allow the development of objects having multifunctional properties such as good electrical and thermal conductivity, mechanical strength and stiffness, etc. at relatively low cost. This book aims at providing a thorough and clear understanding of the fundamentals of 3D and 4D printing processes and the recent developments in polymer nanocomposites-based novel materials for printing applications. The common 3D printing techniques such as fused deposition modeling (FDM), selective laser sintering (SLS), selective laser melting (SLM), electron beam melting (EBM), inkjet 3D printing (3DP), stereolithography (SLA), and 3D plotting have been discussed in detail. Both 3D and 4D printing techniques have a serious impact on our society and daily life and have much more involvement in the growing technology. The current book provides a detailed discussion on the physics and chemistry behind this important area of science, which will be appealing to the researchers all over the world having interest in the development of polymer-based 3D and 4D printing materials. The content is anticipated to build on from what has been learned in an elementary (core) course on polymer processing and the development of advanced polymer nanocomposite systems for 3D as well as 4D printing and the major mechanisms involved in the device technology as well as material processing.

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Acknowledgment

This work was supported by the UREP grant # UREP23-116-2-041 from the Qatar National Research Fund (a member of Qatar Foundation). The statements made herein are solely the responsibility of the authors.

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CHAPTER 1

Introduction to 3D and 4D printing technology: State of the art and recent trends Kalim Deshmukha, Mohammad Talal Houkanb, Mariam AlAli AlMaadeedc, Kishor Kumar Sadasivunid a New Technologies—Research Center, University of West Bohemia, Pilsen, Czech Republic Department of Mechanical and Industrial Engineering, Qatar University, Doha, Qatar c Materials Science and Technology Program, Qatar University, Doha, Qatar d Center for Advanced Materials, Qatar University, Doha, Qatar b

1 Introduction Three-dimensional (3D) printing is a fabrication methodology used for printing 3D objects on the basis of controlled layer deposition of printable material until a final structure is achieved [1, 2]. Since the printing of 3D structure is achieved as a result of layered deposition, this technique is also known as additive manufacturing. 3D printing is quite opposite to subtractive manufacturing fabrication principle wherein 3D structure is created on the basis of material removal from a solid block of material. The structure in subtractive manufacturing is carved using processes like drilling, sawing, milling, broaching, etc. [3–5]. 3D printing is often termed as rapid prototyping; however, it is noteworthy that rapid prototyping involves both additive and subtractive manufacturing. These additive and subtractive manufacturing technologies are utilized on the basis of factors like choice of printable material, structural complexity, cost, and the quantity of structures. In fact, the complexity in structural geometry of an object primarily differentiates the two 3D fabrication techniques. The complex designs comprising solid and hollow parts can be fabricated by means of additive manufacturing due to simultaneous printing of hollow and solid parts of the object following a layer-wise deposition [6]. On the other hand, the deliberative choice of material for simpler designs plays a vital role in the structure confinement. The fabrication of plastic- or polymer-based objects is mainly attained using additive manufacturing, whereas the 3D structures obtained from materials like wood, metal, or rocks, etc. are attained by subtractive manufacturing [7]. Each 3D fabrication technique offers its own advantages and disadvantages. For instance, subtractive manufacturing technique is more cost effective, convenient, and useful for production of number of objects within shorter time interval. Contrary to that additive

3D and 4D Printing of Polymer Nanocomposite Materials https://doi.org/10.1016/B978-0-12-816805-9.00001-6

© 2020 Elsevier Inc. All rights reserved.

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3D and 4D printing of polymer nanocomposite materials

manufacturing is efficient, producing least waste and large energy. Hence, it uses less energy-demanding machinery [8]. In the past 30 years, 3D printing technology has been among rapidly emerging technologies. However, contemplating its enormous potential, it is heavily underutilized. Its potential in cost-effective and time-efficient printing of 3D materials presents an enticing proposition. In that regard, the new advancement in 3D printing has been the center of attention in creating objects having the capability to alter their shapes once they are removed from 3D printer [9]. This technological advancement has commenced a new era in printing and is referred to as four-dimension (4D) printing. The main objective behind 4D printing is the self-assembling property of the 3D-printed object on exposure to certain stimuli in the form of heat, pressure, moisture, chemical reactions, etc. In simple words, 4D printing can be defined as a revolutionary technology analogous to 3D printing with addition of time dimension, that is, by the inclusion of time frame to 3D printing, the concept of 4D printing evolves [10]. The addition of fourth dimension to 3D printing enables preprogramming of objects with respect to their response against various stimuli [11, 12]. 4D printing marks a futuristic approach in printing technology with an incredible potential. 4D printing provides a possibility of designing any transformable shape from a variety of materials exhibiting shape-transforming features. With 4D printing, the creation of dynamically self-assembling and transforming objects can be used in different industrial sectors for a large number of applications. The manufacturing and printing advancements, especially 3D and 4D, are the stateof-the-art technologies with great scope in various fields such as automobiles, medical implants, electronics, aerospace, and robotics [13–17]. The applicability of these manufacturing technologies in such field is due to dimensional accuracy. 3D printing technology has undergone various changes, when it comes to printable materials (polymers to metals). It has played a pivotal role in precise additive manufacturing of objects within shorter time spans. With advent of 3D and 4D printing, the additive manufacturing has gone to a next level of manufacturing with easier product development and production. Additive manufacturing in healthcare sector has been most prominent due to its versatile applicability in different healthcare departments. The process of selective laser sintering (SLS) is one of the most important and popular processes in pharmaceuticals and healthcare (dentistry and orthopedics) [18–20]. In these fields, the process parameters and variables significantly influence and control the product outcome. The outcome of the manufacturing is described on the basis of the mechanical and geometrical properties of the parts produced. The end products of the manufacturing process must inhibit the adequate strength to ensure the requirements for particular applications. In recent years, there has been an increasing trend to reduce the designing and manufacturing time leading to advancements in rapid prototyping (both 3D and 4D printing). The technological advancements have many attractive features like reduction in design time and manufacturing time, freedom from jigs and fixtures, etc. These advancements in modern

Introduction to 3D and 4D printing technology: State of the art and recent trends

times have made 3D and 4D printing technology desirable for automobile industry, biomedical engineering, jewelry, and manufacturing of intricate objects [21, 22]. Additionally, 3D and 4D printing manufacturing technologies reduces tooling and minimizes the wastages. These printing technologies have also shown great possibilities and rapid growth in the field of space and aeronautics. In 3D or 4D production of objects, lowering manufacturing costs have been critical in highly competitive environment. The main advantage of 3D and 4D printing, especially additive manufacturing, has been in lowering the cost of printable objects, cyclic time reduction, simpler supply chains, and performance improvements, thereby allowing designing freedom [23]. The latest development in printing technology has been the activities to improve the surface condition and control optimization of the printable parts. In particular, 4D printing technology is proving to be an exciting and emerging technology for creation of dynamic devices [24, 25]. 4D printing provides the option of combining smart sensing and actuating materials in order to offer novel, convenient, and versatile methods for production of custom-designed sensors, actuators, self-assembling structures, and robotics [26, 27]. This chapter provides an introductory idea about 3D and 4D printing technologies, particularly additive manufacturing with recent trends.

2 Designing perspective and effect of processing parameters in 3D and 4D printing Both 3D and 4D printing technologies expedite freedom of designing the printable objects as their prime and interesting feature. However, it is necessary to ensure the reliability and quality of the product, which can be achieved via following a set of designing rules at the preprinting stage. The rules for designing include designing based on numerical chains for additive manufacturing, especially for thin-walled metallic parts in laser manufacturing. This rule minimizes the gap between the computer-aided design (CAD) and the manufactured part. The rule consists of product optimizations at the level of part orientation, product functionalization, and manufacturing path [28, 29]. In the very first step of optimization, that is the optimization of part itself, the designation is assigned to the area selected for the product design. The second optimization (functional optimization) determines the initial geometry of parts, while the final optimization determines the manufacturing path. This rule prescribes a methodology by which generation of printing program occurs along with final computer-aided design model. The foremost designing rules in direct manufacturing provides the freedom of designing of the technical parts, which are applicable for 3D as well as 4D printing [30]. This rule benefits multiple users with accessible designs particularly in fused deposition, laser sintering, and melting methods. The geometrical freedoms availed in 3D and 4D printing provide significant improvements in functionality of series of products, thereby substitution of

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conventional parts in additive manufacturing [31, 32]. For better results, it is essential to fulfill following criteria for effective product designing in 3D and 4D printing: • Integrated designing • Lightweight materials • Individualization • Overall product efficiency These criteria determine the success of the printed object as well as provide the need for improvements economically and technologically. Economically, the cost invested in the product design and processing can be recovered by lowering manufacturing costs or by utilization of geometric freedom in product redesigning during product lifetime. The optimization of processing parameters in 3D and 4D processing has opened new window in printing technology. Likewise, the implementation of optimized laserassisted additive manufacturing to microlevel following a layer-by-layer manufacturing of nickel-based super alloys has been reported by Bi et al. [33]. The optimization in processing parameters has led to elimination of defects caused by poor weldability, which results in cracking and porosity. The micro laser-assisted additive manufacturing allowed crack-free deposition with application of minimum heat input. The grain refinement followed by heat treatment results in improvement of the tensile strength along with the required standards in aerospace industry. The use of hybrid additive manufacturing technique for fabrication of TC11 titanium sample alloy has been reported by Zhu et al. [34]. The sample fabricated by laser additive manufacturing was studied for hardness and tensile strength and the results revealed the typical zones in the fabricated sample, for example, additive manufactured zone, bonding zone, and wrought substrate zones. The sample displayed basket wave microstructure, which is the prime cause of yielding superior tensile strength. In another report, where means of additive manufacturing was used by Li et al. [35], the temperature fields in AlSi10Mg using selective laser melting sintering were used in order to study the consequences of laser power, scan speed, and cooling rate. Their results revealed that the elevation in cooling rate when laser power was increased with an increase in scan speed, there was only significant rise in cooling rate. The results provided the optimal conditions for 3D processing of the material after repetition of different conditions and combinations to produce fully dense layers with desired layer thickness in the final fabricated product.

2.1 Challenging prospects Considering the 3D and 4D printing technology, there are enormous challenges in the form of designing, manufacturing, etc., which are listed briefly in this section. 2.1.1 Designing challenges The designing aspect considering 3D and 4D printing begins with development of trained engineers, which would tender the capability to integrate the options offered by additive manufacturing [36]. To attain this, it is necessary to set a standardization of designing rules.

Introduction to 3D and 4D printing technology: State of the art and recent trends

The current designing tools do not yield the outcome of 3D and 4D to its fullest capabilities considering the potential of these technologies. Therefore it still requires a lot of work to incorporate the topological optimizations in the designing tools. 2.1.2 Manufacturing challenges After designing fixations, proper training is to be provided to future machine designers to meet the increasing dimensional needs in order to increase the manufacturing potential of the objects by using 3D and 4D printing technology [37]. In addition to that, it is essential to establish quality standards for the printable materials to ensure the product reliability and reproducibility. It is also important to develop new materials for these printing technologies. 2.1.3 Qualification and validation The conventional methods of qualification cannot be readily applied to the 3D and 4D printing technologies, as sample monitoring of the final product does not only depend on the yield mint products. To ensure better quality of the printed samples, it is necessary to develop non-destructive inspection methods to establish product quality via real-time monitoring [38]. The optimization and qualification of 3D and 4D printing methods is thus very essential to produce quality products with minimal or no defects like cracks, porosity, etc. In addition to that, the establishment of acceptable defect range needs to be developed considering the designing rules. The new printing technologies are now considered to be the means of real time production. However, to achieve the intended benefits from these technologies as per their potential, certain challenges still need to be addressed. To tackle these challenges, it requires a lot of research and development to effectively use these technologies in variety of fields and applications. These technologies, with respect to their usage, are still young and require great deal of research to widen its usage. These technologies have the tendency to impact an ineradicable imprint on the future of designing and manufacturing, thereby radically changing the printing habits in the upcoming years. The advantages and challenges of 3D additive manufacturing are summarized in Table 1 [39].

3 3D printing technology The 3D printing process involves few steps, which are initially based on CAD modeling followed by its conversion into stereolithographic file [40]. After file formation, the printable surface is sliced into the logical series of triangles, each one of which represents part of the 3D product surface model. The file on execution cuts the model into thin cross-sectional layers allowing the 3D model to be printed [41]. Fig. 1 demonstrates different types of 3D printing technologies currently available in the market whereas different research fields in which 3D and 4D printing techniques have been currently used are shown in Fig. 2.

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Table 1 Advantages and challenges of 3D additive manufacturing [39] Advantages

Challenges

• Customized products from small batches

• Production cost and speed • Changing the way and approach of using

• •

•

• • •

•

are economically attractive as compared with the traditional mass production methods Possibility to produce 3D CAD models directly meaning that tools and molds are not required. Hence, no switch over costs Can be designed in the form of digital files, which can be shared easily in order to facilitate the modification and customization of components and products Material can be saved due to the additive nature of the process. The waste materials (powder, resin), which are not used during manufacturing can be reused Possibility to achieve novel and complex structures such as lattices and free-form enclosed structures and channels Very low porosity of final products The inventory risk can be reduced by making to order with no unsold finished goods. Improving the revenue flow by paying for goods prior to their manufacturing Due to the distribution, the direct interaction between local consumer or client and producer is possible

additive manufacturing

• To remove the perception that additive

• • • • •

• • •

manufacturing is only for rapid prototyping and not for direct component and product development To develop and standardize new materials To validate the mechanical and thermal properties of existing materials and additive manufacturing technologies To develop multimaterial and multicolor systems To improve manufacturing efficiency by automation of additive manufacturing systems and process Postprocessing is required due to the stairstepping effect that arises from incrementally placing one layer on top of another for producing finishing layers To recycle the support structure materials by minimizing the need through a good build-up orientation Issues such as intellectual property rights and nonlinear collaboration with illdefined roles and responsibilities Designers and engineers with deficit skills in additive manufacturing.

3.1 Feeding mechanisms in 3D printing 3.1.1 Binder jetting The creation of 3D object by combination of powdered material by means of jet deposition of binding agent is termed as binder jetting. The materials printed by this technology are metals, polymers, and ceramics. The developers of this technology are Voxel Jet (Germany), exOne (USA), and 3D System (USA). 3.1.2 Material jetting Material jetting is the process of building parts by settling down small droplets of the filament. These small droplets are then carved as per the design instructions generated by exposing the filament drops to light UV radiations to attain a high resolution in height of

Introduction to 3D and 4D printing technology: State of the art and recent trends

Fig. 1 Different types of 3D printing technologies.

layer of about 16 μm. The materials suitable for this type of feeding mechanisms are photopolymer and wax. The technology was developed by 3D system (USA) Stratasys (USA) and LUXeXcel (Netherlands). 3.1.3 Direct energy deposition This technology uses focused thermal energy to fuse/join material onto the substrate by direct deposition for creating 3D/4D structures. The materials used for this type of printing are in the form of powders and wires. This technology was developed by Irepa Laser (France), Trumpf (Germany), NRC-IMI (Canada), and DM3D (USA).

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Ongoing research on 4D printing Space exploration 4D printed parts can be sent to space and programmed to self assemble into an object at the desired location.

2013

Construction / architecture

Medical

Materials that could be programmed to adapt and change shape in response to environment or situation. Example: Pipes that expand when demand increases.

Ongoing research on developing a nano robot built from DNA strands in the form of a clamshell basket, with double-helix’’ locks’’ that are only opened when the robot comes into contact with specific cells.

3D printing 3D printing

2015

Self assembling materials

2035 Industrial 4D painting

2045 Environmental manufacturing

Exploring materials and understanding reaction to external elements

Industry application will be explored with cost of technology more suited for industrial applications.

Holds to revamp manufacturing introducing a new field of environmental manufacturing in which ambient sources of energy, water or even light will be used as impetuses to self assemble.

Fig. 2 An overview of ongoing research on 3D and 4D printing.

3.1.4 Powder bed fusion In this technology, the 3D or 4D structures (using time dimension) are created to fuse regions of a powder bed by using thermal energy. The materials used in this technology are polymers, ceramics, and metals. This technology was developed by Matsuura Machinery (Japan), Phoenix System (France). EOS (Germany), ARCAM (Sweden), Renishaw (UK), and 3D system (US). Powder bed fusion technology uses the following processes in creation of 3D and 4D printed objects. • Direct metal laser sintering This process creates objects by melting and fusing the printing material in an inert gas chamber using a highly focused beam of laser. • Electron beam melting In this process, the products are fabricated using a high-intensity electron beam inside vacuum melting the powder placed inside it. This is followed by cooling the substance to give it a final shape.

Introduction to 3D and 4D printing technology: State of the art and recent trends

• Selective heat sintering This technique uses heating of thermal print head for layer-wise deposition of thermoplastic powder. The heating cures the thermoplastic powder to attain a 3D structure. • Selective laser melting The process uses laser to melt the metal powder forming a pool of melt inside an inert gas chamber. The melt is then rolled repeatedly in the form of layers to yield the final product. • Selective laser sintering This process is similar to the laser melting wherein a beam of laser is used to sinter the material powder. The sintered powder is then rolled in a layer-wise manner. The major point of difference among the two processes is that in laser melting the powder is heated above melting point, while as in sintering process the printable material is heated below melting point until the powdered particles fuse with each other.

3.1.5 Light photopolymerization This technique uses light in creation of 3D structures. The 3D parts are created using light to selectively cure material layers in a vat of photopolymer. The materials used in this technique are ceramics and photopolymers, and this technology was developed by Lithoz (Australia), Encision TEC (Germany), DWS Sri (Italy), and 3D system (US). This technique uses either stereolithography or digital light processing in development of 3D structures. • Stereolithography In this process, a liquid photopolymer is melted on exposure to UV radiations emitted from a laser source. The reaction of laser beam with the photopolymer causes the solidification of the resin to form 3D objects. • Digital light processing This technique projects layers of the object from the CAD image into the vat of photopolymer, which on reaction with the projection light cures and hardens the desired 3D part.

3.1.6 Extrusion This process creates 3D objects by depositing material via heated nozzle to form layer, which hardens instantly to allow deposition of next layer. The process is repeated several times until desired shape of object is attained and this process is also known as fused deposition method. Mainly polymeric materials are used in this type of printing and this technology was developed by Delta Micro factory (China), 3D systems (US), and Stratasys (US).

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3.1.7 Sheet lamination This process creates 3D structures by trimming sheets of printable material and then binding them in layers. The materials used in this process are ceramics, metallic sheets, and hybrid materials. The developers for this technology are CAM-LEM (US) and Fabrisonic (US). The technique incorporates ultrasonic additive manufacturing and laminated manufacturing. • Ultrasonic additive manufacturing This process uses the sheet lamination process to join thin metallic sheets to form objects. The metallic sheets are joined together by ultrasonic welding and CNC mill is used to trim the excess material. • Laminated object manufacturing This process is also layer-by-layer additive printing technology wherein layers are bound to each other using adhesive paper, metal, or plastic, which is least toxic. The sheets of material are cut into desired shape by a laser cutter and then glued together.

4 Classification of materials used in 3D and 4D printing The materials used in 3D and 4D printing technologies are classified as powder materials, wire filaments, printable waxes, and liquid materials. These are listed briefly in next section. Fig. 3 gives an overview of materials, which are required for 3D/4D printing.

Fig. 3 Consideration of materials required for 3D/4D printing.

Introduction to 3D and 4D printing technology: State of the art and recent trends

4.1 Powder materials The powder materials are printed by fusing, melting, or by using suitable binding materials or water color additives, whereas the powder is mixture of plaster and polymers and also wood filament. The wood filaments are composed of ground wood material with polymers like polylactic acid (PLA) or some plastic. In a report, spools of wood filament were printed using extruder nozzle of diameter 0.6 mm without defects [42].

4.2 Wire filament materials Wire filaments materials are mainly polymeric materials, and, among them, acrylonitrile butadiene styrene (ABS) is the most affordable one. The different wire filament materials with their properties are summarized in Table 2. ABS is a versatile material and can be easily sanded or hybridized with acetone. The hybridized ABS easily sticks together to yield 3D structures Table 2 Summary of wire filament–based materials Wire filament material

ABS

PLA

TPE

PVA

Extrusion temperature

Rolling bed temperature Properties

Ref.

It can be sanded, and by mixing ABS with acetone, it can be easily glued together or smoothed to a glass-like finish (petroleum) Biodegradable plastic typically made from corn or potatoes (produced from plant starch) Flexible, rubber-like materials of different varieties

215–250°C

80–110°C

Durable Strong Slightly flexible Heat resistant

[43]

170–220°C

20–55°C

Tough Strong

[44]

180–230°C

20–55°C

PVA is a special plastic that is watersoluble, Excellent film formation, High bonding power, Good barrier properties (petroleum)

160–170°C

45°C

The properties of a [45] soft rubber, making it even more flexible and elastic than soft PLA filament Used to print [46] support material

Formation

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with smooth glassy finish [43]. ABS can even bear most violent heat warping and shrinkage, which increases its manifold demands in the printing platforms. The structures printed from ABS can tolerate higher temperatures due to its higher glass transition temperatures. PLA is another polymeric material suitable for 3D/4D printing. Fig. 4 shows PLAbased different filament materials used for 3D/4D printing application. PLA is a biodegradable plastic, which can be made accessible through natural means. It is stiffer than ABS and does not require heating of printing bed while printing, but heat warping of PLA during cooling can be improved by using heated bed [44]. Thermoplastic elastomers (TPE) are a soft, flexible, and rubber-like material. These elastomers are printed using extrusion method, with rigid constructed filaments due to extensive flexibility of the materials [45]. The printing of these elastomers requires extruder to be maintained at idler pressure and its position has to be precise to avoid filament squashing. Polyvinyl alcohol (PVA) is a special type of plastic used as a supporting material in extruder printing. Due to its water solubility, it is extremely problematic to be used in high-humidity environment. Therefore, it is best suited as supporting printing material, especially with ABS [46].

4.3 Printable waxes The waxes show tendency to be 3D/4D printed using thermojet printer. The printer uses waxes composed of thermoplastics made from amides, hydrocarbons, and esters [42].

4.4 Liquid materials Liquid materials used in 3D and 4D printing use inkjet-based printer technology. The inkjet printers for such printing use UV-curable resins composed of thermosetting plastics and have different characteristics than those thermoplastics used in extrusion printing like the tensile strength and glass transition temperature [42].

5 3D printer software and hardware The printing of 3D objects through 3D printers requires special type of software as shown in Fig. 5. The use of software in physical prototype operates through CAD. The computer-aided manufacturing (CAM) unit also referred to as slider converts the CAD model into set of specific mechanical instructions for printing robot. The printer controlling software sends the instruction, which transmits the fed instructions to the printer to provide real-time interface [42]. The 3D printing of objects via filament of 3D printer takes place through series of commands called as G-code. The hardware-software interface and the flow of G-codes in the real-time printing through USB port is shown in Fig. 6. The running software interprets the G-code at a specific time interval and transmits the same to the printer for execution. The information status is sent back to the host computer through USB. The interpretation of G-code in some cases is done on host computer while sending the controls to the printer.

Introduction to 3D and 4D printing technology: State of the art and recent trends

Fig. 4 PLA-based different filament materials used for 3D/4D printing.

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Fig. 5 Software hierarchy used in 3D printing.

CAD software

Solid modeling programs Skechup Autodesk 123D Tinkercard

Sculpting modeling programs ZBrush Sculptures Mudbox

Parametric modeling programs OpenSCAD Modelbuilder Grasshopper

Polygonal modeling programs Blender 3ds max Maya Modo

Fig. 6 Hardware and software control in 3D printing.

6 Evolution of 4D printing technology 4D printing has been one of the recent approaches in printing technology. The 4D-printed objects have the ability to be transformed into desired structure with reference to time [47]. 4D printing technology has evolved to incorporate embedded wiring or conducting parts into specialized submissive components. Once 3D printing of the object is done, the printed parts are stimulated or activated using an external stimulus. This type of printing approach has great potential in the field of robotics, aerospace, furniture, and construction of buildings [48]. The other 4D printing approaches comprise incorporating composite materials having the shape morphing and capabilities of transforming into different shapes with respect to various physical mechanisms and heat

Introduction to 3D and 4D printing technology: State of the art and recent trends

activation [49]. Also, available reports clearly demonstrate the self-folding abilities of 4D printable materials on exposure to light [50]. Caputo et al. [51] adapted 3D printing route for producing sintered net-shaped part from Ni-Mn-Ga powders. The 4D parts were created by the predictable change in 3D printed parts configured as a function of time as a result of shape memory effect. Netshaped porous structures with good mechanical strength were produced by binder jetting of Ni-Mn-Ga powders followed by curing and sintering. After 3D printing, the printed parts were removed from the printer and heated to 463 K for 4 h. After 4 h, the binder was completely cured and the printed parts were removed from the print bed [51]. Fig. 7 shows several structures obtained from Ni-Mn-Ga powder using 3D printing after curing. Fig. 8 represents several sintered parts. The sintering time was 8 h for both sparkeroded and ball-milled powders. Recently, Ding et al. [52] demonstrated 4D printing of programmable 1D composite rod that can change into 3D structures very rapidly upon heating and retain the 3D configuration when cooled. The composite rods consisted of a glassy polymer and an

Fig. 7 Net-shaped 3D printed parts obtained from spark eroded Ni-Mn-Ga powders in (A) liquid nitrogen (B) liquid argon (C) and (D) Ni-Mn-Ga powders obtained by ball milling. (Reproduced with permission from Ref. M. P. Caputo, A. E. Berkowitz, A. Armstrong, P. Mullner, C. V. Solomen. 4D printing of net shaped parts made from Ni-Mn-Ga magnetic shape memory alloys. Addit. Manuf. 21 (2018) 579–588. M. P. Caputo, A. E. Berkowitz, A. Armstrong, P., Mullner, C. V. Solomen. 4D printing of net shaped parts made from Ni-Mn-Ga magnetic shape memory alloys. Add. Manuf. 21 (2018) 579-588. Copyright 2018, Elsevier Ltd.)

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Fig. 8 4D printing of sintered net-shaped parts made from ball-milled Ni-Mn-Ga powders. (Reproduced with permission from M.P. Caputo, A.E. Berkowitz, A. Armstrong, P. Mullner, C.V. Solomen, 4D printing of net shaped parts made from Ni-Mn-Ga magnetic shape memory alloys, Addit. Manuf. 21 (2018) 579–588. Copyright 2018, Elsevier Ltd.)

elastomer, which were bonded together as a result of manufacturing process. This was later programmed with a compressive stress during the printing process [52]. The detailed information was obtained about the basic deformation modes of direct 4D printed composite rods, bending and twisting, and their dependence on design parameters such as layer fraction of composites and the cross-section twist along the axis of the rod. Fig. 9 shows a 3D cubic frame that self assembles by heating 1D composite rod in such a way that each of the six cube faces represents a circular ring in the final configuration [52]. The color of the glassy polymer was used for easy visualization of each of the six cube faces. The combination of the experiments and the simulations revealed the characteristics parameters that can be used to design 1D composite rods, which can yield desired 3D structures [52]. In another study, Mulakkal et al. [53], developed a cellulose-hydrogel composite ink for additive manufacturing demonstrating the physical characteristics such as stability, swelling potential and rheology of cellulose hydrogel gel composites, and its suitability for 4D printing. The addition of carboxymethylcellulose (CMC) hydrocolloids with cellulose pulp fibers resulted in an ink with good fiber dispersion within the hydrogel. The montmorillonite (MMT) clay was added to increase the storage stability of the composite ink and to have positive effect on the extrusion process. The ink was used for 3D printing demonstrating the fabrication of complex structure, which is capable of morphing according to the designed rules, which are predetermined in response to hydration or dehydration (Fig. 10). The set up of the printer and extruder is shown in

Introduction to 3D and 4D printing technology: State of the art and recent trends

Fig. 9 Schematic representation of direct 4D printing of programmable 1D composite rods that can rapidly change into 3D structures. (Reproduced with permission from Z. Ding, O. Weeger, H.J. Qi, M.L. Dunn, 4D rods: 3D structures via programmable 1D composite rods, Mater. Des. 137 (2018) 256–265. Copyright 2018, Elsevier Ltd.)

Fig. 10 (A) 3D printer and paste extruder set up; Extruder housing is shown in insert with the nozzle for printing gel and composite materials. (B–G) Fabrication of petal architecture. (B) Generated print path from CAD model—dimensions are in mm. (C) 3D printed form. (D) Drying at room temperature initiating morphing. (E) After crosslinking to maximize and fix the 3D shape. (F) Deployed to flat configuration upon hydration. (G) drying (dehydration) recovers the 3D petal shape. Side views are shown in insert (scale bar ¼ 10 mm). (Reproduced with permission from M.C. Mulakkal, R.S. Trask, V.P. Ting, A.M. Seddan, Responsive cellulose-hydrogel composite ink for 4D printing, Mater. Des. 160 (2018) 108–118. Copyright 2018, Elsevier Ltd.)

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Fig. 10A. The 3D printing of the responsive cellulose composite in order to fabricate and actuate petal architecture is demonstrated in Fig. 10B–G. The room temperature drying of the samples maximized the crosslinking with the new configuration as shown in Fig. 10E. The flat configuration was obtained (Fig. 10F) by keeping the transformed shape specimens into water bath at room temperature. Fig. 10G demonstrates the petal architecture, which reverts back to its 3D configuration after drying [53].

7 Printers for 4D printing In conventional 3D printing of materials like ABS, PLA is optimized in accordance to the printing parameters (nozzle design and temperature), which are already fed in each 3D printer. Printing of smart materials (4D printing) with specific functionalities is a hefty process in conventional 3D printers due to material’s tendency to get clogged and agglomerated during 3D printing process. Therefore, it is necessary to develop 4D printers to specify the printing parameters. The use of printer with a coated nozzle has been reported for 4D printing of thermal polyurethane (TPU) using melt extrusion method [54]. The printer consists of heating bed for uniform heat circulation during printing arising due to higher thermal expansion coefficient of TPU and to avoid its compression in the nozzle on heating thereby clogging the nozzle. The uniform heat circulation on the heating bed also avoids flow of molten TPU over cold regions, which may lead to poor adhesion between the TPU layers and improper porosity in the printed object. In addition, in order to avoid overflow of molten TPU and for reduction of friction, the TPU printing nozzle is coated with polytetrafluoroethylene (PTFE) and is placed close to the heating device having barrel 1.2 to 1.5 times longer than the nozzles used for 3D printing of PLA and ABS. The multimaterial components’ printing is among the key features of 4D printing. The 4D printing of multimaterials allows the printed structures to have different colors, shapes, and electronic properties, which changes depending upon the response to stimuli. The multimaterial printers have the ability to print biomaterial structures and functionally graded structures by combining two or more materials within one printed structure and in that regard several printers have been developed already. Lopes et al. reported fabrication of multimaterials to print electroactive functional polymer actuators using melt extrusion method [55].

8 Recent trends in 3D printing and 4D printing 3D and 4D printing technologies will continue to evolve with the continuous research efforts in their approaches to employ new concepts of printing technology, to yield fast and cheaper product fabrication, portable machinery, and enhancement in printing definition. Fig. 11 demonstrates the applications of 4D printing in various sectors such as industrial manufacturing, robotics, consumer, military, automobile, and aerospace industry.

Introduction to 3D and 4D printing technology: State of the art and recent trends

Fig. 11 Applications of 4D printing in various sectors.

The advancements in printing technologies are mainly focused on the following directions: • Development of printing systems capable of printing new materials. • Faster printing speeds by development of new deposition means. • To attain resolutions up to micro- and nanoscale level. In this section, illustrations of some innovative printing examples are provided. The 3D printer capable of printing fully functional materials like optically transparent glass, based on innovative extrusion principle, was developed at MIT [56]. The printer comprised a special heating head and a nozzle capable of liquefying glass to produce spatial deposition in accordance to CAD design. This printer enables fabrication of real glass objects with features similar to conventionally produced glass. The printing of metallic objects is generally a difficult process as it is mainly dependent on the sintering powder bed metallic particles. In that regard, a novel extrusion method was proposed for 3D printing of metallic objects based on metal wire–feeding mechanism [57]. The process in approach is analogous to fused deposition method and requires a heating system capable of locally melting the solid metallic wire thereby producing a pool of molten metal. On the basis of energy sources, following printing technologies have emerged in recent times: • Fabrication based on electron beam freeform [58]. • Wire laser [59]. • Additive manufacturing using wire arc welding [60].

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A cost-efficient, metal 3D printing approach was recently applied to a combination of metal and powder-based systems termed as selective inhibition sintering [61]. The metallic printing by particle sintering is mainly achieved by using expensive laser or electron beam–based systems, which enables layer-wise binding to form a final metallic product. The new approach in metallic printing works on the principle of contrasting mechanism wherein printing of materials occurs in layer-by-layer spreading sequence of a sintering inhibitor over the metal particle bed. After creation of first layer, the entire metal block is sintered using high-temperature oven in which the binding takes place. The process involves single sintering or fusion of printing material with low-cost systems like ovens instead of using costly lasers. Another recent development in additive printing has been the continuous liquid interface production (CLIP), which enables 20–100 times faster printing than selective laser or polyjet printing [62, 63]. The CLIP method is based on use of oxygenpermeable, transparent window at the base of a UV-curable resin container creating dead zone to inhibit polymerization by dissolving oxygen. The polymerization takes place above the dead zone, which is continuously dictated by projection of UV images beneath the resin bath followed by pulling out the final printed object as it solidifies. This process continuously forms an interface of solid and liquid phases of the material, thereby increasing the productivity as no layer-by-layer deposition takes place. Another printing development to improve spatial resolution of printed objects is direct laser writing. This fabrication approach enables spatial resolution of mm-nm and is based on multiphoton polymerization using an ultrafast laser, which is focused inside the volume of the material to cause polymerization [64]. The movement of laser beam in accordance to CAD design enables fabrication of microdimensional model. The 4D printing technology on the other hand uses smart materials and designs to estimate changes and this smart printing has the potential to be used in various fields like bioprinting for organisms [65]. US army has already applied this technology to produce camouflage textiles, which can help soldiers in hiding in certain environment. The textiles have the ability to bend the light reflected from its surface [66]. Its bright scope in aerospace in creation of easily transportable systems cannot be neglected.

9 Conclusions The 3D and 4D printing technologies, despite being different in approach than the conventional printing technologies, can be fruitful in modern times as these technologies have been modeled to reduce manufacturing time and labor. Both these printing technologies are relatively young and still require ample amount of research to deliver and function up to their actual potential. Additionally, the evolution of 4D printing can benefit in multiple ways like reduction of time, cost, and labor, requirement for

Introduction to 3D and 4D printing technology: State of the art and recent trends

transportation, logistics, etc. 4D printing can prove to be the most consumer-friendly technology in near future because of its potential to fabricate personal designs or it provides different options on consumer goods.

Acknowledgment This publication was partially made possible by UREP grant 23-116-2-041 from Qatar National Research Fund (a member of Qatar Foundation). The statements made herein are solely the responsibility of the authors.

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CHAPTER 2

3D and 4D printing of nanomaterials: Processing considerations for reliable printed nanocomposites Christopher J. Hansen Department of Mechanical Engineering, University of Massachusetts Lowell, Lowell, MA, United States

1 Introduction This chapter, being focused on 3D printing of polymer nanocomposites, first introduces the definitions and concepts of nanocomposites and of 3D printing in order to properly establish a foundation for the reader. Due to the immense scientific (as well as public) interest in both of these topics, this defined scope is necessary to narrow the chapter contents to a tractable corpus of literature. First, nanocomposites and their benefits are defined, followed by an overview of additive manufacturing technologies.

1.1 Nanocomposites 1.1.1 Definition of nanocomposite Nanocomposites are defined as a solid material composed of two or more distinct phases, in which at least one phase has one or more dimensions of 100 nm or smaller in length [1]. While a broad range of materials, including co-polymers, gels, and porous media, could fall within this definition, this chapter will focus more on materials systems that emphasize the composite aspect of their nature. Composites often consist of a “matrix” that is defined by its interconnectivity through the material and a “functional” phase, in this case possessing at least one nanoscale dimension [2]. 1.1.2 Benefits/advantages of nanocomposites The adjective “functional” applied to this nanoscale phase hints at the benefits nanoscale dimensions impart to the nanocomposite material system. Composites are often chosen such that the secondary phase counteracts a deficiency present in the primary matrix phase. In nanocomposites, the nanoscale phase addresses the deficiency through a physical mechanism that is enhanced specifically by its nanoscale features [3]. These benefits can span properties from mechanical [4] and thermal [5], electrical [6] and optical, to

3D and 4D Printing of Polymer Nanocomposite Materials https://doi.org/10.1016/B978-0-12-816805-9.00002-8

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chemical and catalytic [7]. Nanoparticles added to polymers can enhance their specific performance with respect to stiffness, strength [8], ultimate properties, or impact properties, permeation, fire retardancy, and other desirable properties [9,10].

1.2 Additive manufacturing Additive manufacturing (AM) and 3D printing are used interchangeably in colloquial speech. Even in the title of this chapter, the terminology of 3D printing is utilized. However, in official definitions (e.g., ISO/ASTM 52900-2015(E)), the terms have distinct definitions. AM is a “process of joining materials to make parts from 3D model data, usually layer upon layer, as opposed to subtractive manufacturing and formative manufacturing methodologies” [11]. 3D Printing, on the other hand, is defined by the “fabrication of objects through the deposition of a material using a print head, nozzle, or other printer technology.” It is apparent that the 3D printing definition is more focused on retaining the defining characteristics of “printing” in its definition, such that nozzles or other printhead technologies are included. As such, this chapter will tend to use AM due to its more broad and inclusive definition though, when used, 3D printing is generally meant in an equally broad context. However, it is more instructive to describe the underlying materials and processes that define the categories of AM. 1.2.1 AM categories The ASTM Committee F42 on Additive Manufacturing technologies has developed a joint ASTM F42/ISO TC 261 AM standards document to catalogue AM according to groupings based on feedstock materials, processes and equipment, as well as finished parts [12]. The feedstock material categories include materials and their formats, such as metal, ceramic, or polymer powders, photopolymer resins, filamentary metals or polymers, and other material-format combinations. As this chapter is concerned with all nanocomposite materials, the organization will be structured instead based on the processes common to 3D printing. All 3D printing processes are categorized into one of the following seven groupings: material jetting (MJ), binder jetting (BJ), material extrusion (MatEx), sheet lamination, powder bed fusion, directed energy deposition, and vat photopolymerization. MJ is an AM process using selective deposition of droplets of material [13]. BJ is a similar process to MJ, but whereby the selective deposition is of a liquid binding agent onto a bed of powder(s) [14,15]. MatEx is an AM process where material is selective deposited through a nozzle or other orifice [16]. These three techniques are unified by their selective deposition of material(s). Powder bed fusion is an AM process in which powders are selectively fused using thermal energy [17]. Directed energy deposition fuses materials as they are being deposited with focused thermal energy [18]. Vat photopolymerization is a light-activated

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polymerization reaction to selectively cure the desired geometry [19]. These three techniques are unified in that all involve a selective exposure of energy (thermal or photonic). Sheet lamination involves bonding specific geometric regions of successively deposited sheets [20]. Depending on the implementation of the method, it may involve selective deposition of material sheets, or may use selective exposure to energy to cut material for removal of excess sheeting. In either case, this method resides in one of the two previous categories. 1.2.2 Physical phenomena in AM As with all manufacturing technologies, AM processes are guided by the physical phenomena that govern the material(s) and the geometry(ies) to be created. The dominant phenomena of interest roughly correspond to the two primary groupings listed in Section 1.2.1. Specifically, the first group united by selective deposition is dominated by the physics of material (or mass) transport of MJ, BJ, MatEx, and select sheet lamination techniques. The second group of selective energy exposure techniques is dominated by the physics of energy transport and transfer and includes powder bed fusion, directed energy deposition, vat photopolymerization, and the remaining sheet lamination techniques. The third key physics consideration, and which unites all AM techniques, is that of adhesion and bonding between and within the layers. These three primary issues will be addressed throughout this chapter.

1.3 Chapter organization The remainder of this chapter is organized into three parts. First, the physical phenomena that govern 3D printing are discussed with particular relevance to the techniques used in nanocomposite AM. Next, how these physical phenomena are affected by the presence of nanoscale materials are discussed. Finally, an extrapolation of current trends and likely future research needs is presented.

2 AM and governing physical phenomena 2.1 Printing methods commonly used in nanocomposite AM Each of the seven AM categories established by ASTM F42 (i.e., MJ, BJ, MatEx, sheet lamination, powder bed fusion, directed energy deposition, and vat photopolymerization) can be and have been used with nanoscale materials to create nanocomposites. However, several techniques are heavily represented in the literature and deserve an emphasis in discussion of their processing steps and of the governing physics. The most widely used AM techniques to create nanocomposites are MatEx, vat photopolymerization, and powder bed fusion [21]. These three AM methods are covered in detail below.

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2.2 Material extrusion 2.2.1 Fabrication method description MatEx is the most widely utilized AM method; thus, unsurprisingly, it is also the most commonly documented method in research dedicated to the additive manufacture of nanocomposite components. The generic process description for this technique is that a material is microextruded through a nozzle or other orifice onto a substrate, typically in a layer-by-layer fashion. The source material is often either in a filament format for solid materials, or housed in a reservoir for liquid- or gel-based inks. The nozzle diameter depends upon the application, but frequently ranges from 1 to 500 + μm. This orifice dimension defines the approximate size of the smallest feature dimension—both in width as well as layer thickness—though variations can be achieved by under- (
0.5 ) or overpumping (
2) the material [22]. While there may be typically be only one nozzle, multinozzle demonstrations exist that increase production throughput or allow multimaterial printing [23]. The nozzle may remain stationary with the substrate undergoing motion or, more commonly, the substrate remains stationary while the nozzle traverses its area. The part size is limited by the machine axes of motion, typically enclosing a volume of 100–1000 mm in each Cartesian dimension, though a new generation of printers (e.g., the Big Area Additive Manufacturing (BAAM) machine) is now reaching the order of 105 mm for an axial distance [24]. Techniques that fall under the MatEx category include fused filament fabrication (FFF) or fused deposition modeling, direct write assembly, direct ink writing, gel printing, paste printing, microextrusion, and robocasting. 2.2.2 Governing physical phenomena In a MatEx method, material is transported from the source location through a nozzle onto a substrate [25]. After deposition, the material then undergoes a phase transition that enables it to (re)-gain a solid-like behavior. The source material may be a solid (e.g., thermoplastic filament), a gel (e.g., viscoelastic polymer gel), or a liquid (e.g., a photocurable monomer/oligomer mixture). In each case, the source material must flow during the transport through the orifice. For liquid source materials, their natural state is to flow; by contrast, a liquid-like behavior must be induced for materials that start as solids or as viscoelastic gels. Solids are commonly heated above their melting point so as to become a liquid and enable flow. Flow of viscoelastic inks is achieved through displacement- or pressure-controlled shear of the ink, which causes yielding and subsequent flow. After the material has flowed, it is critical for the liquid state to quickly develop or recover a solid-like behavior so as to produce localized features of high fidelity. For solid materials, this solid recovery means heat transfer to the surrounding environment so as to cool below the melting temperature. Viscoelastic gels naturally recover their elastic behavior upon cessation of flow, though the timescale for this recovery must be

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sufficiently fast to retain feature dimensions. Liquids must develop their solid-like behavior, requiring the application of an external stimulus; in most cases, this stimulus is a highintensity light source that induces a rapid polymerization reaction [26]. Additional methods exist to assist in developing the elastic behavior, including thermally activated reactions and solvent evaporation [27].

2.3 Vat photopolymerization 2.3.1 Fabrication method description Vat photopolymerization was the first demonstrated 3D printing technology [28]. The generic process consists of the photopolymerization of a liquid resin that locally solidifies upon exposure to a directed energy source. The material to be patterned is restricted to liquid monomeric and/or oligomeric formulations that become reactive upon exposure to specific wavelengths of light, which then polymerize to form a solid patterned shape [29]. The technique is known for its generally fine feature sizes, with an X-Y resolution of 75 μm being typical, and a Z resolution of 25 μm is common. Original conceptions of vat photopolymerization used a UV laser or other collimated light source that is directed by means of lenses and mirrors to raster over the surface of a fluid resin bath, thereby locally polymerizing the resin to form a solid 2D layer. The part is lowered into the liquid bath by a distance equal to one layer thickness. After a wiper smooths a new layer resin across the solid surface, the process is repeated for the next layer pattern (Fig. 1). Two primary enhancements have been made in the past decade. First, the light exposure can be performed simultaneously using digital mirror displays, which project 2D light patterns onto the resin, thereby reducing the light exposure time by removing

Fig. 1 Vat photopolymerization process where exposure occurs on the top surface of a liquid bath, into which the specimen is lowered (left) and exposure through a window beneath the bath and the specimen is pulled out of the bath (right).

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the need to raster a light source [30]. The second enhancement is to shine light through an optically transparent window below the bath (Fig. 1) [31,32]. This approach eliminates the rate-limiting wiper step, thereby significantly speeding the print process. Typical build volumes for vat polymerization techniques can be 1500  750  550 mm3 for the top-down stereolithography (SLA) approach, while window-based bottom-up approach has been commercially sold up to 190  120  325 mm3. Postprocessing of vat polymerized parts typically requires removal of support materials, cleaning of residual monomeric liquid (especially in any trapped locations), and a postcure cycle to enhance final part properties. Techniques that reside within the category of vat photopolymerization include SLA, continuous light interface polymerization, digital light processing, digital mirror displaySLA, among other specific process names. 2.3.2 Governing physical phenomena In vat polymerization methods, both mass transport of the liquid resin and energy transport to initiate the polymerization reactions are key considerations [33]. The liquid resin must retain a viscosity η sufficiently low that it can flow on the timescales of changes between the layers. In traditional SLA, whereby the printed part is lowered into the bath, the wiper accelerates the process; however, after the wiper pass, the machine waits for the resin to reach a flat equilibrium to proceed. This dwell time increases linearly with the viscosity increase. In the inverted SLA approach, resin flow occurs at the bottom of the bed between the part and the optically transparent window. The elimination of the wiper and equilibration step removes the dwell time required for a flat surface, which is instead enforced by the window and the previously printed layer. However, the gap between the window and the part is often narrow at only 30–50 μm. If the printed part has fine features, the suction forces created by raising the part and a highly viscous resin filling the gap can cause distortion of the features. While an upper limit to the resin viscosity depends upon the acceptable print time and parameters, a generally accepted value is η  5 Pa-s [34]. The digital geometry to be replicated in physical form requires localized solidification of the liquid resin material. The photopolymerization reaction achieves this solidification through careful formulation to maintain this local pattern formation [35]. Resins contain a photoinitiator I that has a chemical bond that is cleaved by exposure to photons of sufficient energy hν, where h is Planck’s constant and ν is the photon wavelength. The resulting radicals initiate a free radical polymerization reaction to form a crosslinked polymer network. The polymerization continues until a termination event, either with another radical chain, or a reaction inhibitor such as oxygen. The volumetric (i.e., X-Y-Z) resolution of the printed part is dependent upon the minimum volumetric pixel (i.e., “voxel”) dimensions. These dimensions are dictated by a combination of the light source resolution and of the material formulation. The light source provides an incoming stream of photons that, when integrated over time, results in an exposure dose. The resin near the surface is exposed to the full dose, while positions

3D and 4D printing of nanomaterials

interior to the resin bath experience a lower exposure dose due to absorption by the resin. The dose at a depth z has an exponential fall-off described by Eq. (1):   z N ðzÞ ¼ N0 exp  (1) Dp where N(z) and N0 are the number of photons at depths z and at the surface, respectively, and Dp is called the penetration depth. The penetration depth is the depth at which the fraction e1 ¼ 0.37 of the photons have not been absorbed by the resin. The penetration depth is determined by the resin formulation, where the absorption is influenced by the photoinitiator concentration, the monomer absorption, and the UV absorber concentration [36]. The resin requires a minimum exposure in order to transition from a liquid to a solid material, usually at a degree of polymerization α ¼ 0.2–0.4, depending upon the average functionality of the resin monomers. The cure depth Cd can be calculated by Eq. (2):   Emax (2) Cd ¼ Dp ln Ec where Emax and Ec are the maximum exposure dose and the critical exposure dose, respectively [37]. The layer thickness cannot be larger than this depth; otherwise, the newly formed layer would not adhere to the previous layer. However, if the layer depth is too thin, substantial regions of polymerized material from subsequent layers will infill previously open regions. Likewise, the lateral resolution is related to the horizontal spread and absorption of the incident light beam. For a typical collimated light, the horizontal dose is assumed to have a Gaussian distribution, i.e., an exponential decrease from the central maximum exposure location. This horizontal dose is given by Eq. (3):  2 r N ðr Þ ¼ N0, r¼0 exp  2 (3) s where r is the radial position relative to the maximum exposure location and s is the radial beam spreading parameter. Again, the dose at a particular radial position will need to exceed the critical degree of polymerization. The resin should be formulated to minimize the spreading parameter if precise lateral dimensions are critical.

2.4 Powder bed fusion 2.4.1 Fabrication method description Powder bed fusion involves a powdered material that is fused by a directed energy source in a layer-by-layer fashion. The directed energy source, typically either a laser or an electron beam (e-beam), is rastered over the powder surface, with its speed and path

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optimized to achieve a more uniform temperature distribution and to maximize build speeds [38]. Powder exposed to the beam undergoes a fusion process. The fusion process occurs either by sintering, in which the material is elevated near to but below the melting point, or by melting, in which the material exceeds its melting point. After a layer pattern is fused, the part depth is lowered by one layer thickness, and a new layer of powder is added by a recoating roller to prepare the next layer. Powder bed fusion techniques have been used for a wide range of material families, including metals, ceramics, polymers, and multicomponent materials (e.g., alloys, composites). This flexibility has earned it a prominent space in commercial and industrial practice. The powders are frequently granules of sufficient size (
20 μm) so that gravitational forces can compete with static forces [39]. The resolution of the printed parts is competitive with other AM techniques, slightly lower than that of vat photopolymerization while slightly improved relative to MatEx. A standard X-Y resolution can be as small as 50–100 μm, while Z resolution is 50–200 μm, depending on the dictated machine parameters. Part dimensions are determined by the build volume of the machine. These can reach 700  400  600 mm3 in larger commercial machines. Techniques that reside within the category of powder bed fusion include selective laser sintering (SLS), selective laser melting (SLM), and e-beam melting. 2.4.2 Governing physical phenomena The physical phenomena dominating the powder bed fusion process are related to energy transport, with the directed energy source converted to thermal energy and locally elevating the powder temperature. The efficient absorption of incident energy, while minimizing reflection and scattering, is desirable to promote a rapid patterning process and to achieve optimal resolutions. For laser-based fusion, the laser beam is directed by lenses and mirrors to pattern the powder surface; hence, the resolution in the powder bed plane is dominated by the laser optics, the resulting spot size, and the thermal conductivity of the material [40]. The wavelength of the laser or other beam source must be efficiently absorbed so as to not heat too great a thickness of the material, thereby creating a poor Z resolution. In general, the heat transfer models are complex and must account for conduction, convection, and radiation, as well as thermal contacts between powders, reflection within powders, and phase changes. A review of typical analytical and computational models is provided in Zeng et al. [41]. Powder uniformity is desirable for uniform patterning results, and has been difficult to achieve in some nanocomposite systems [42]. The benefits of monodisperse spherical powders on thermal effect uniformity, however, impose significant porosity in the initial powder bed. Upon sintering or melting, this porosity fills with solid material and results in substantial shrinkage. Due to lateral pinning, the shrinkage is primarily restricted to the vertical directions. Part accuracy is improved by compensating for this unavoidable shrinkage via pattern and raster algorithms.

3D and 4D printing of nanomaterials

3 Nanocomposites effects on processing parameters The material properties necessary to modify and optimize nanocomposite AM correspond to the mass transport, energy transport, and adhesion considerations inherent in the seven categories of AM techniques. This section addresses the effects of nanoscale components on the material properties key to effective processing. The enhancement of properties caused by nanoadditions is extensively covered in literature [33] and is not the focus of this chapter.

3.1 Material properties in AM processing As seen in Section 2, mass transport is a ubiquitous concern all AM techniques, with global mass transport important in MatEx or vat polymerization, while local mass transport is important to the sintering or melting processes in powder bed fusion. For techniques with global mass transport, the material properties to be characterized and optimized center on liquid materials. Hence, properties defining liquids are considered: the viscosity at shear rates relevant to processing conditions, the surface tension of the liquid in contact with air and with solid material, and the volumetric change that occurs upon polymerization and/or crystallization. Energy transport considerations contribution directly to the final part resolution. Absorptivity of radiation is key for the cure depth and raster velocity. Thermal conductivity is important for lateral distribution of energy beyond the exposure area. The absolute value of the melt temperature dictates whether sintering or melt conditions will be achieved for a given energy dose. Finally, adhesion between layers—which is influenced by chemical and thermal forces—is key for strong, high-quality AM products. The bonding of one layer to a neighboring layer is tied to physical and chemical phenomena. In polymer systems, interdigitation of surface polymers will result in their physical entanglement across the bondline. Sintering involves local diffusion of atoms or molecules that cross contact boundaries, eventually erasing its presence and approaching bulk properties. In vat polymerization or MatEx, chemical reactions may occur at the interface between subsequent layers. These covalent chemical crosslinks from strong interlayer bonds that, if properly formed, are competitive strengthwise with the in-plane properties.

3.2 Influence of nanoparticles on polymer viscosity The effect on viscosity of the addition of a second phase has been studied for over hundred years at the microscale. The shear viscosity η at low volume fractions (ϕ  0.03) is defined by the Einstein equation, given by Eq. (4): η ¼ ηp ð1 + 2:5ϕÞ

(4)

where ηp is the polymer zero shear viscosity and ϕ is the volume fraction of the microscale filler [43]. An additional correction factor (6.2 ϕ2) that accounts for two-body

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Fig. 2 Viscosity as a function of volume fraction fillers for nanoscale and microscale fillers.

interactions is added for 0.03  ϕ  0.1, while the Krieger-Dougherty equation is a general empirical approach for high-volume fraction suspensions [44]. Due to the difficulties of dispersion and agglomeration in higher volume fraction nanocomposites, this chapter will restrict itself to lower volume fractions nanocomposites. The most striking difference between nanoparticles and their microscale counterparts is the dramatic increase in surface area relative to volume for nanoscale materials. For polymer-based systems (typical of MatEx techniques), polymer chains have significantly more interaction with the nanoparticle surface area. It has been observed that for lowvolume fractions of nanomaterials, the polymers interact with and are confined by the surface area, thereby reducing entanglements and resulting in a noticeable decrease in the viscosity [45]; however, at higher volume fractions, the polymer chains will begin to interact with neighboring nanoparticles and result in a gel-like network that results in a viscosity greater than would be expected by the volume fraction present (Fig. 2). The volume fraction at which these networks can form are related to the aspect ratio of the nanofiller. Carbon nanotubes (CNTs), for instance, have a very low percolation threshold, forming the network effects at such a low volume fraction that any viscosity reductions are not observed [46]. The alignment of high aspect ratio particles during MatEx flow can influence the percolation threshold, and has implications on the anisotropy of the resulting material properties [47].

3.3 Effect of nanoparticles on polymer thermal properties, vitrification and crystallization The thermal properties of polymeric systems are heavily influenced by nanoscale additions. The melt temperature, glass transition temperature, heat capacity, and thermal conductivity are all impacted and can influence AM processing conditions [48]. For SLM (a powder bed fusion technique), FFF (a MatEx technique), or other techniques with

3D and 4D printing of nanomaterials

melting transitions, the absolute value of the melt temperature is critical. The melting temperature of polymer matrices with nanoscale additives have been observed to be unaffected [49] or increase minimally (e.g., by
2–4 K) [50]. Fillers are used in systems with high rates of solidification-induced shrinkage to reduce the degree of shrinkage and warpage associated with temperature changes. This reduced shrinkage is achieved by reducing the coefficient of thermal expansion (CTE). Nanoscale materials are able to fulfill this role, in addition to the others mentioned. For instance, graphene has been shown to reduce shrinkage of an ABS polymer printed by FFF by 24% with only 4 wt% of graphene nanoplatelets (GNPs) [51]. A decrease in the CTE of up to 36% was observed for up to 5 wt% montmorillonite nanoclay content in ABS thermoplastic; this decrease in thermal expansion varied linearly with the nanoclay content [51]. Nanoscale silica has also been used up to 30 wt% silica with reduced shrinkage [52]. In techniques where polymer segmental motion is important to achieve bonding between neighboring layers (e.g., FFF [53]) or particles (e.g., SLS), nanoeffects on the glass transition temperature value influences processing conditions. The heat capacity of nanocomposites normalized by neat polymer material shows that the glass transition temperature shifts by fairly small amounts (e.g., elevated by 1–2 K) to larger amounts (6–8 K) [54]. Even for small shifts, however, the transition region is substantially larger (30–40 K) due to the slowing of segmental dynamics in the interfacial layer of polymer surrounding the nanoparticles [55]. Because the dynamics of the polymer chains have been shown to be a dominating concern in the interlayer adhesion in AM, the presence of nanoparticles will require elevated processing temperatures to overcome this slowing of the chain dynamics. Strong interactions between the polymer and nanofiller will increase Tg, while poor interactions are expected to decrease Tg (Fig. 3) [56]. A size effect is also predicted, in which smaller nanofillers (70 billion interconnected devices by 2025 [1]. The network will eventually encompass public and private infrastructure such as power grids and transportation systems, as well as personal gadgets such as home appliances and smart wearables. In particular, smart wearables present an interesting manufacturing challenge. The bodies of these gadgets need to incorporate sensors, microcontrollers, and interconnects to enable collection, processing, and transmission of biometric or environmental data for edge-computing applications. At the same time, they must be customized to the body shape, aesthetic preferences, and ergonomic comfort levels of the user. Today, two broad classes of wearable technologies—(1) clothing such as body monitoring innerwear and (2) accessories such as smart glasses and health trackers—are largely mass produced. As their production is based on high-volume-low-mix economics, tailoring designs to the body shape and aesthetic preferences of the user costs additional time and money to the user. With the smart wearables market projected to exceed 70 billion US$ by 2022 [2], a manufacturing technique yielding customized products on-demand would be essential to meet the growing demand. We believe that recent efforts made in printing three-dimensional (3D) structures with additional functionalities might offer a possible first inroad solution toward achieving high-mix-low-volume production of customized wearables. Many groups have used various 3D printing techniques to prototype novel wearable devices such as a bionic ear with a radio antenna [3], a thermotherapeutic glove with a heater and thermometer [4], as well as stretchable tactile [5,6], strain [7], and electrochemical [8,9] sensors that can be mounted on textiles. These developments have granted the additive manufacturing process an extra degree of freedom, as 3D printed objects can now be programmed by tuning the property (electrical conductivity, strain, temperature) they respond to. In this chapter, we will explore the possibilities opened by the emergence of this “fourth dimension” in 3D printing. Could customized wearables of the future be 3D printed in a user’s home? The purpose of this chapter is to shed some light on this possibility by assessing the state of the art in 3D printing of electrical circuits. To achieve this possibility, the user would have to have access to a 3D printer that can print (1) materials that provide desirable structural properties to the chassis of the wearables and (2) conductive materials for electronic components, which impart functionality (the fourth dimension) to the wearables. Moreover, as production is being carried out at a site not necessarily optimized for conventional manufacturing (home) by a nonspecialist (user), the 3D printing apparatus would have to be compact and easy to use, including set-up and maintenance.

Polymer-based conductive composites for 3D and 4D printing of electrical circuits

1.2 The case for FDM-compatible conductive polymer composites (CPCs) Based on current trends, we deposit fused deposition modeling (FDM) as a promising 3D printing paradigm for realizing the production of smart wearables on home-based 3D printers. FDM is the most widely used 3D printing technology today [10] by consumers and industry [11]. FDM printers are the cheapest 3D printers available on the market [12] and are compact enough to sit on desk tops. In practice, they can produce complex parts with distinguishable features approximately 60–100 μm apart in all three printing directions [13], which is arguably good enough for printing common objects that do not have intricate details. Only minor-to-moderate training on operation (e.g., optimizing printing parameters, build setup, chamber ambient management) and maintenance (e.g., nozzle cleaning, temperature recalibration, and platform cleaning) is needed to use FDM printers. Moreover, FDM printers are compatible with a variety of materials, including polymer composites [14]. Polymer composites are one of the two broad classes of conductive materials—the other being solvent-based inks—that have been used to 3D print electrical circuits and components. Although conductive inks have significantly greater conductivity than polymer composites, printing with them presents various challenges, which may not be easily incorporated into a user-friendly system that is readily accessible. Printing requires more overhead time due to two additional processing steps: (1) Between the printing of two layers, the deposited solvent needs to be evaporated to allow the deposited layers to be structurally sound before additional layers are printed upon them [15]; (2) After printing is completed, the printed structure needs to be cured or sintered to obtain a continuous metallic phase and therefore increase its conductivity [16]. Printing with conductive inks typically requires specially built inkjet printers such as Aerosol Jet (>US$250,000) [17] and Voxel8 (
US$10,000) [18], which are significantly more expensive than FDM printers such as Makerbot Dual (
US$2000) [19] (prices as of October 2018). Another point to appreciate is the versatility offered by performing FDM with conductive polymer composites (CPCs). CPCs allow freeform printing of freestanding 3D structures or embedded architecture, while conductive inks are typically drop-casted onto existing three-dimensional surfaces with printing in the z-axis limited to the submillimeter regime [20,21]. CPCs are also versatile in terms of the available choices of matrix and functional filler [14,22]. Their versatility is enhanced by multimaterial FDM 3D printing [23,24], which allows multiple filaments to be deposited simultaneously to print multifunctional parts [25]. Here, in the simplest case, dual-extruder 3D printers available on the market [19,26] can simultaneously extrude one conductive filament loop (providing functional electrical response) and one nonconductive thermoplastic filament loop (providing structural integrity) through their two movable nozzles. Coprinting of electronic components along with their chassis in a single-build process can potentially be extended to any number of filaments and nozzles.

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However, formulating conductive filaments for FDM with sufficiently low resistivities to achieve working circuits remains a challenge. One difficulty lies in optimizing the volume fraction of conductive filler in the filament matrix such that it is sufficiently over the conduction percolation threshold [27]—a critical value that determines dielectric to conductor transition—but not too high so as to result in high melt viscosities, which might lead to clogged nozzles and brittle filaments [28]. Another difficulty lies in obtaining a relatively homogeneous dispersion of filler in the matrix so as to achieve an uninterrupted conductive pathway with consistent conductivity in the filament [29]. These challenges prompt us to recast our hypothetical dream scenario of 4D printing customized products at home in more practical and immediate terms: what is the state of the art in 4D printing with FDM-compatible conductive polymer composites? We believe that a good place to start exploring this question is by developing a common understanding of what defines a “Four-dimensional (4D) printable material” and what constitutes an “FDM printable conductive polymer composite filament”. In Section 1.3, we introduce a set of general criteria for 4D printable materials. In Section 1.4, we contextualize these criteria to the case where electrical resistivity is the fourth dimension and a conductive polymer composite is the printable material, thus drawing a set of specific criteria for FDM printable CPCs. We believe these criteria to be useful starting points for assessing materials in terms of their practicality for 4D printing applications. After the criteria have been introduced, we report recent advances made in preparing composites with a polymer matrix incorporating carbon black and metal-based fillers to achieve low-resistivity (101 Ω m or lower) filaments. Sections 2 and 3 detail the preparation, characterization, and comparative performance of these CPCs with carbon black, carbon nanotubes, copper nanowires, low-melting-point metal alloy, and metal flakes as the conductive material. These fillers are chosen to highlight a spectrum of existing approaches relying on different formulation methods and enabling the 3D printing of different electrical circuits and components. A survey of the progress made in printing practical 2D and 3D circuits with electronics deposited on or embedded within printed structures using these different CPCs is provided in Section 4. We conclude by discussing some current challenges and opportunities for further development in Section 5.

1.3 What is a four-dimensional (4D) printable material? The following three criteria have been synthesized from the literature as well as our experience and expertise in this field. The guiding principles for these criteria are practicality and their usability. (1) Beyond the three structural dimensions, the material has at least one property that can be usefully varied in space and/or time whether by design or in response to an external stimulus. Implicit in this criterion is the requisite printability of the material by the chosen printing method. For practical applications, the variations in the fourth dimension

Polymer-based conductive composites for 3D and 4D printing of electrical circuits

offered by the material must happen in a relevant range that allows practical static or dynamic control. It is quite possible for a material to have more than one additional useful 4D property, but it makes little sense to start naming materials as 5D, 6D, and so forth. In general, we shall therefore consider 4D to include these categories. This criterion ensures that the additional degree(s) of freedom provided by the material can be demonstrated by a working prototype. The range of values every additional degree of freedom takes on will depend on the specific use case. For example, color may be the 4D property, and one may desire this to be printed in static gradation, or to change in response to temperature. Electrical conductivity, which will be discussed in this chapter in greater detail, is another possible example of such a 4D property. (2) The material’s 4D property should maintain consistent and predictable behavior before and after printing. This criterion ensures the reliability of printing with the material in order to elicit the desired useful behavior based on the print design. Consistency of material property ensures that a product can be printed reproducibly according to the intended design and functionality over many print runs, while predictability ensures that the usable variations of the properties occur within the design specification of the print. (3) The material should be compatible with a commercially available 3D printer. This is a pragmatic consideration, but a necessary one if the technology based on this material is to make economic sense for its proliferation. If our pipe dream is to have users print functional structures in their homes or at work, it follows that they must have ready access to both the material and the 3D printer. Preferably, the 3D printer should be affordable and usable by a nonspecialist.

1.4 What is an FDM-compatible conductive polymer composite (CPC) filament? Here, we instantiate the three general criteria for 4D printable materials by considering the use case of printing tuneable electronic circuits and components using CPC filaments via FDM. (1) The filament should have volume resistivity sufficient to print 3D circuits and circuit elements of practical sizes that can be powered by conventional consumer power sources (e.g., 1.5–12 V batteries). Even if a conductor’s resistance can be lowered by increasing the conducting cross-section indefinitely, this would result in the printed device having an undesirable form factor, which would be impractical. Based on the works surveyed in this chapter, resistivities for making practical circuits with FDMcompatible CPCs start from around 101 Ω m and below. Ideally, one would hope to achieve resistivities similar to that of copper (1.72  108 Ω m [30]), especially for printing circuits related to radiofrequency (RF) applications. However, such low resistivities may not be required for other classes of applications, such as conductive shields, to protect against electrostatic discharge.

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(2) The electrical properties (e.g., resistivity/conductivity, current threshold, breakdown voltage, etc.) of the material used for the filament remain consistent and predictable before and after printing. Consistency will be important to ensure circuit elements can be replicated with reproducible performance so that designing the circuit becomes a productive exercise. Predictable performance will be necessary to define the useful range of resistivity/conductivity of, for example, the printed design. (3) The filament should be compatible with an existing multimaterial FDM 3D printer. As mentioned earlier, multimaterial FDM printers would print electrical circuits and components together with their chassis in a single-build process. Compatibility includes agreement of the filament’s degradation temperature, melting point, and melt viscosity with the temperature specifications and nozzle limitations of the 3D printer, as well as the suitability of printed materials with each other and with the print bed in terms of adhesion and thermal expansivities. From Fig. 1, it is clear that the CPCs currently available do not match the performance of the best conductors, e.g., Cu, commonly used in electrical devices and systems. It is therefore unlikely that the current crop of CPCs represented in the literature would be able to replace Cu in the near term. Nevertheless, some of the CPCs reaching low resisitivities in the range of 103 to 106 Ω m may find niche applications as circuit elements (e.g., as resistor, sensor, inductor, etc.) in the near term. Exploring the practical applications that their relevant properties enable is the subject of the remainder of this chapter.

2 Conductive polymer composites with carbon-based fillers Filaments with carbon black, an amorphous form of carbon, as filler are likely the first 3D-printable CPCs to be formulated, owing to their easy availability and low cost [6]. Research groups interested in exploiting the remarkable electrical properties of

Fig. 1 Volume resistivities of the 3D printable filaments featured in Table 1. Two distinct conductivity regimes—those achieved by CPCs with carbon-based fillers and metal-based fillers, respectively—are demarcated. The gradient bar illustrates the change from low conductivity (red) to high conductivity (cyan) filaments. Gr—graphene; CB—carbon black; CNT—carbon nanotube; NW—nanowire; LMPM— low-melting-point metal.

Table 1 Evaluating existing FDM-based conductive polymer composite filaments according to the three criteria for FDM-compatible CPCs 3D printability criteriaa Resistivity (Ω m)

Loading (vol%)

(1)

(2)

(3)

Reference

ABS + graphene PCL + carbon black ABS + graphene

9.5  102
0.1 >0.1

2.75
7.6b
15b

[31] [6] [32]


0.01

3.5

✖ ✓ Not reported ✓

✓ ✓ ✓

PBT + carbon nanotubes PBT + graphene

✓

[33]


0.5

8.5

✓

✓

PLA + graphene

6  103

✓

[34]

PP + carbon black PLA + graphene Polyester + cu

>5  10–3 2  10–3 6  105

✓ ✓ ✓

[35] [36] [37,38]

PCL + Cu-Ag nanowires Nylon-6 + Ni/ Sn95Ag4Cu1 PE + Ni/ Sn95Ag4Cu1 PVB + Ag flakes

2  10–5

Not available (commercial product)
23
3.5b Not available (commercial product) 12

✖ ✓ Circuits not demonstrated Circuits not demonstrated Circuits not demonstrated Not reported

✓

[39]

✓

[40]

3  105 (before printing) 4  105 (before printing) 7  106

30

✖

Not reported ✓ ✓ Not reported Not reported ✖

35

✖

✖

✓

11

✓

✓

✓

✓ ✓ ✓ ✓

[41]

Glossary of abbreviations used: ABS, acrylonitrile butadiene styrene; PCL, polycaprolactone; PBT, polybutylene terephthalate; PLA, polylactic acid; PP, polypropylene; PE, polyethylene; PVB, polyvinyl butyral. a “✓” denotes “fulfills criterion” and “✖” denotes “does not fulfill criterion.” b Conversion from wt% to vol% was done to facilitate the comparison in this table. Generally, we assume the composite of wt% FW is composed of a fiber of density ρF FW dispersed in a matrix of density ρM according to the following relation: vol% ¼ ρF . FW + ð1FW Þ

ρM

Polymer-based conductive composites for 3D and 4D printing of electrical circuits

Matrix  filler material

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nanoscale carbon allotropes later incorporated more costly nanofillers such as functionalized graphene [31] and multiwalled carbon nanotubes [42] into polymer matrices. Several ventures—such as Black Magic 3D [34], ProtoPasta [43], and Functionalise [44]—have either been selling conductive filaments on the market or have tried to commercialize them, signaling likely demand, for instance, from the hobbyist and do-it-yourself (DIY) community. Irrespective of the form of carbon used as filler, the volume resistivity of the highest performance of these filaments seems to have a lower bound of
104 Ω m. Recent advances in the preparation and characterization of carbon-based conductive filaments have driven their volume resistivities to the order of 103 Ω m and below, making certain practical circuits and components such static dissipative devices (e.g., electromagnetic shields) and low-voltage electronics (e.g., basic battery-powered circuits with resistor and light-emitting diode) a reality. In this section, we focus on the formulation, characterization, and performance of CPCs with carbonbased fillers. We conclude this section by introducing a panel of stress tests to study the long-term usability of our PP/carbon black filament [35] for practical applications.

2.1 Formulation The formulation of filaments involves chemical and mechanical subprocesses. We outline here the general steps essential to the processing of conductive filaments with a carbonbased filler: 1. The matrix and filler starting material is added to a suitable precursor to create a dispersion at room temperature. Dichloromethane, isopropanol, and N-methylpyrrolidone (NMP) have been used as a precursor in the case of carbon black [6], carbon nanotubes [33], and functionalized graphene [31], respectively. 2. The dispersion is next mixed thoroughly to obtain a homogeneous solution of matrix and filler and to avoid phase separation between the two. Typically, mixing is carried out by vigorous stirring or with the help of a homogenizer (15,000 r.p.m) for at least one hour [31]. This step is essential to obtain a composite with a well-dispersed filler, and hence a more continuous conductive pathway. 3. The homogenized mixture is then washed and dried to obtain its powder form. 4. Finally, the composite powder is fed into an extruder for hot mix blending. The temperature of the extruder is maintained above the polymer’s glass transition temperature (typically 140–210°C) and below its degradation temperature. The composite blend is then extruded into filaments approximately 1.75 mm in diameter (commercial 3D printer filament diameter). Typically, single- [35] or twin-screw [36] extruders are used. In some cases (e.g., Ref. [35]), a simplified process may be pursued with the matrix and filler components directly fed into an extruder for hot mix blending at typical temperatures between 200 and 250°C. To achieve greater homogeneity in the

Polymer-based conductive composites for 3D and 4D printing of electrical circuits

filament composition, the filament produced in the first round is then pelletized and refed to the extruder. This process may be repeated to achieve higher levels of homogeneity, but the polymer composites can degrade after too many hot extrusion cycles. Hence, a compromise between homogeneity and polymer composite integrity is usually sought. In general, higher carbon filler loading is associated with lower resistivity or higher conductivities in the filaments produced. However, very high loading often leads also to brittle filaments, which can be difficult to feed or result in frequent filament breakage, which stalls the printing process.

2.2 Characterization A range of characterization techniques are employed to determine key morphological, electrical, thermal, and mechanical properties of the CPCs. A combination of various characterization techniques also forms part of the feedback loop in the optimization of the filament in order to achieve consistent and predictable resistivity/conductivity, i.e., satisfy Criterion 2 of Section 1.4. Electrical characterization is typically used to determine the resistivity/conductivity of the prepared filament so as to provide guidance for the kind of resistive circuit elements or conductive tracks that can be printed with the CPC, thus providing a quantitative and qualitative basis for assessing Criterion 1 of Section 1.4. Electron microscopy studies can help determine the optimum amount of filler needed to overcome the conduction percolation threshold of the polymer matrix. A homogeneous filler distribution makes the filament as a whole attain the conduction percolation limit as it increases the likelihood of uninterrupted conductive pathways being formed within the matrix. At the same time, while CPCs with high filler loading might offer increased conductivity, they also have a higher melt viscosity, which might jam the 3D printer nozzle. Studying the surface distribution of filler particles before and after printing, as well as quantifying the filler distribution using a compositional analysis method such as thermogravimetric analysis (TGA) can provide helpful feedback to direct the iterative process of formulating and testing a filament until its performance is optimized. Characterization also provides essential feedback for establishing the choice of FDM conditions compatible with the filament [31], helping the filament fulfill Criterion 3 of Section 1.4. These conditions include nozzle, print bed, and chamber ambient temperatures, which need to be optimized to enable smooth deposition while preventing degradation of the material. 2.2.1 Microscopy As the filler particles are typically smaller in size than the diffraction-limited optical microscope resolution limit of about 0.2 μm, scanning electron microscopy (SEM) or transmission electron microscopy (TEM), with higher spatial resolutions of
10 and

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0.2 nm, respectively, may be used to characterize the material morphology in the filament. Most reports use SEM instead of TEM possibly because SEM offers sufficient spatial resolution and relative ease in sample preparation. SEM is also frequently combined with energy-dispersive X-ray spectroscopy (EDS) to obtain compositional information. This allows both identification and quantification of filler particles, polymer matrix, and impurities present in the filament. More importantly, obtaining compositional information allows mapping of filler networks to identify breaks in conductive pathways, which provides a way to estimate the optimum volume fraction of filler needed to attain the conductive percolation threshold. For example, Wei et al. [31] illustrated dispersions of graphene filler in their conductive filaments by comparing SEM images of filament surfaces of different filler volume fractions (see Fig. 2). In another study, Leigh et al. [6] observed striations along the length of the printed structures from SEM images of the printed structure (see Fig. 3). These striations were attributed to rough features on the inner surface of the print nozzle. Minimizing such a possible source of disruption to the percolative pathways, for instance, by choosing a print nozzle with a smooth inner surface, can result in higher and more predictable conductivities of printed structures. SEM images of printed structures also enable the study of the interface between two printed layers, or the printed layer and the substrate. Zhang et al. [36] showed how the larger contact area offered by a broader lower layer (Fig. 4D) strengthens the cementation between that layer and the layer above. The absence of large gaps between layers in the magnified images in Fig. 4E and F indicates the strength of the interface, thus suggesting that the mechanical properties of the 3D structure are consistent. The uniform small gaps between layers also suggest that the printed structure has predictable conductivity in the z-direction. Besides SEM, scanning transmission electron microscopy (STEM) [33] and atomic force microscopy (AFM) [36] have also been used for more detailed morphological characterization of conductive polymer composite filaments. Although AFM is not an electron microscopy technique, we mention it here because of its usefulness in profiling the topography of the printed structures (see Fig. 4B and C). While these techniques provide more microscopic level information on the composites, they are generally more technically involved and costly to implement. The need for such higher resolution characterization would depend on the specific questions to be addressed for the composite, e.g., the impact of micro-/nanoscale impurities on the adhesion of the printed filament with the substrate or chassis material.

2.2.2 Current-voltage (I-V) measurements A filament’s volume resistivity, perhaps the most important quantity associated with the performance of a CPC, is determined through I-V measurements. A four-point probe

Polymer-based conductive composites for 3D and 4D printing of electrical circuits

Fig. 2 SEM images of the surfaces of (A) 2.3 wt%, (B) 3.8 wt%, and (C) 7.4 wt% G-ABS composites; (D) Illustration of graphene dispersions in polymer. A low graphene volume fraction, a sparse distribution of bright graphene sheets is seen against a dark polymer background (A). Increasing graphene loading results in a more continuous and denser graphene network (D), suitable for printable CPCs. However, further increasing the loading results in aggregations (C), which makes the filament unprintable as it is likely to cause nozzle jam. (Reproduced with permission from X. Wei, D. Li, W. Jiang, Z. Gu, X. Wang, Z. Zhang, Z. Sun, 3D printable graphene composite, Sci. Rep. 5 (2015) 11181, Springer Nature. Copyright 2015.)

measurement is typically used [35, 36]; however, if the contact resistances are low (
1%) relative to total resistance, a two-point method is acceptable [6]. The common procedure for characterizing resistivity/conductivity involves measuring resistivity/conductivity at different filler volume fractions. The volume fraction at which the resistivity/conductivity (y-axis) and filler volume fraction (x-axis) curve shows a sharp change in gradient provides an estimate of the percolation threshold.

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Fig. 3 (A) SEM image of the surface of a conductive filament before printing; (B) SEM image of the surface of the conductive filament after printing; (inset) a reduced magnification SEM image of the filament after printing. The PCL/carbon black filament showed striations along its length after printing. These striations occur as the filament contacts with microscale rough features inside the printer’s nozzle. (Reproduced with permission from S.J. Leigh, R.J. Bradley, C.P. Purssell, D.R. Billson, D.A. Hutchins, A simple, low-cost conductive composite material for 3D printing of electronic sensors, PLoS One 7 (11) (2012) e49365, PLOS. Copyright 2012.)

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Fig. 4 (A) (left) SEM image of the surface of a conductive polymer composite (scale bar: 100 μm) compared with (right) an SEM image of the surface of a printed trace (scale bar: 100 μm); (D)—(F) SEM images of a printed structure featuring the top view of a corner (D), the side view (E), and an enlarged section of the side view (F). (Reproduced with permission from D. Zhang, B. Chi, B. Li, Z. Gao, Y. Du, J. Guo, J. Wei, Fabrication of highly conductive graphene flexible circuits by 3D printing, Synth. Met. 217 (2016) 79–86, Elsevier. Copyright 2016.)

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Gnanasekaran et al. [33] used this procedure to compare the performance of their polybutylene terephthalate (PBT)/G and PBT/CNT composites (Fig. 5). Similarly, we found that our PP/carbon black composite [35] shows decreasing volume resistivity with increasing filler percentage (see Fig. 6). A percolation threshold related to
0.31 vol% of CNT and
3.3 vol% of graphene can be estimated from Fig. 5 for the PBT/CNT and PBT/G filaments, respectively, in Ref. [33]. A percolation threshold related to
5 vol% can be extracted for the PP/carbon black filament from Fig. 6 in Ref. [35]. Thus, these plots allows both groups to decide which optimum filler fraction range to work with, as composites with filler fraction below the percolation threshold were not studied further. For a more complete understanding of the filament’s performance for practical applications, measuring the resistivity of the printed structures is just as crucial as measuring the resistivity of the filament before printing. Printing is often accompanied by a loss in conductivity. One reason for this is because FDM tends to yield more well-connected conductive paths in the x-y plane (in-plane) than in the z-direction (out-of-plane), since the layer-by-layer 3D printing implies that the connectivity in the z-direction (perpendicular to the layers) will always depend on how well the layers adhere to each other. Motivated by the need to study this variation in conductivity in printed structures, some groups [6,33,40] found it useful to perform electrical characterization at different angles to the deposition direction.

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Fig. 5 A plot showing electrical conductivity (S/m) versus filler volume fraction for both CPCs in Ref. [33]. Here, a percolation threshold of
0.31 vol% of CNT and
3.3 vol% of graphene was estimated from the plot obtained and calculations based on percolation theory [45]. (Reproduced with permission from K. Gnanasekaran, T. Heijmans, S. van Bennekom, H. Woldhuis, S. Wijnia, G. de With, H. Friedrich, 3D printing of CNT- and graphene-based conductive polymer nanocomposites by fused deposition modeling, Appl. Mater. Today 9 (2017) 21–28, Elsevier. Copyright 2017.)

Polymer-based conductive composites for 3D and 4D printing of electrical circuits

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Fig. 6 A plot of volume resistivity (Ω m) versus filler weight percentage for various formulations of the CPC in Ref. [35]. Here, the resistivity changes sharply between 0 and 11wt% of filler, thus placing an upper limit of 11.3 wt% on the value of the percolation threshold. (Reproduced with permission from S.W. Kwok, K.H.H. Goh, Z.D. Tan, S.T.M. Tan, W.W. Tjiu, J.Y. Soh, Z.J.G. Ng, Y.Z. Chan, H.K. Hui, K.E.J. Goh, Electrically conductive filament for 3D-printed circuits and sensors, Appl. Mater. Today 9 (2017) 167–175, Elsevier. Copyright 2017.)

Leigh et al. [6] observed a 25% reduction in resistivity when measuring parallel to the deposition direction compared to measuring perpendicular to the deposition direction. Gnanasekaran et al. [33] studied conductivity at intermediate angles between the line joining the measuring probes and the deposition direction of the FDM print head (see Fig. 7). They reported an increase in resistance with an increase in measurement angle from 0 to 90 degrees. A difference in conductivity in the directions parallel and perpendicular to the deposition direction of the FDM print head further suggests anisotropy of the filler network and perhaps better conductivity along printed lines than across the printed lines. 2.2.3 Differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) Two thermal characterization techniques—DSC and TGA—are often used to study the melting profile and thermal stability of the conductive composite, respectively. These techniques allow us to determine the glass transition temperature Tg, the rate of cooling of the filament upon deposition, and the polymer matrix degradation temperature. By quantifying these three properties, DSC and TGA measurements provide a reference point for choosing appropriate printing parameters such as nozzle temperature, heated build chamber temperature, and print bed temperature to ensure smooth printing of filament. For instance, Sun et al. [31] obtained DSC and TGA measurements of their Acrylonitrile Butadiene Styrene (ABS)/graphene filament (Fig. 8) and calculated Tg and Tonset

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Fig. 7 Plots showing resistance against the angle between the line joining the measuring probes and the axis of deposition of the print head [33] for samples printed using PBT/CNT composites with various percolating volume fractions (A) φ ¼ 0.0062, (B) φ ¼ 0.021, (C) φ ¼ 0.036, and (D) φ ¼ 0.041. The slope of these plots changes such that resistance increases slowly for small angles (0 degree is parallel and 90 degrees perpendicular) but significantly increases above 45 degrees. This indicates anisotropy of the filler network and perhaps better conductivity along printed lines than across the printed lines. (Reproduced with permission from K. Gnanasekaran, T. Heijmans, S. van Bennekom, H. Woldhuis, S. Wijnia, G. de With, H. Friedrich, 3D printing of CNT- and graphene-based conductive polymer nanocomposites by fused deposition modeling, Appl. Mater. Today 9 (2017) 21–28, Elsevier. Copyright 2017.)

(degradation onset temperature, which is the temperature when filament loses 5% of its total weight). Knowing Tg, helped them establish an upper bound on the print bed temperature and a lower bound on the nozzle and heated build chamber temperature. On the other hand, Tonset set the upper bound for the nozzle and heated build chamber temperature and ensure that composite filaments will be softened instead of decomposed as they are extruded from the 3D printer. The nozzle and heated build chambers are often kept at different temperatures to allow a less drastic temperature change as the filament is deposited onto the print bed, thus minimizing thermal deformation during printing. Besides thermal characterization, TGA is also used to determine the amount of filler present in the composite, especially if the volume fraction is not exactly known at the time of

Polymer-based conductive composites for 3D and 4D printing of electrical circuits

Fig. 8 (A) DSC curve of ABS and G/ABS CPCs. Calculations based on these curves show that the Tg value increases slightly as the filler volume fraction is increased; (B) TGA curve of ABS and G/ABS composite. (Reproduced with permission from X. Wei, D. Li, W. Jiang, Z. Gu, X. Wang, Z. Zhang, Z. Sun, 3D printable graphene composite, Sci. Rep. 5 (2015) 11181, Springer Nature. Copyright 2015.)

preparation. Here, the composite is heated beyond the polymer degradation temperature to ensure only the filler is left behind. If the density of raw materials (filler and polymer) is known, the ratio of weight of material left behind to that of original sample can be converted into volume fraction easily. We direct the interested reader to Ref. [35] for the detailed procedure. 2.2.4 Stress tests—Ultraviolet, electrical, thermal If CPC filaments are to be used to make practical circuits for everyday applications, one can imagine these circuits being exposed to different environmental conditions such as sunlight, varying electrical loads, and a range of localized temperatures. For instance, a photovoltaic cell used for street lighting incorporating circuit elements 3D printed with the PP/carbon black filament would be exposed to all the three conditions. In order for FDM printed devices to be viable for everyday applications, we would want a reliable device that that can withstand lifetime exposure cycles comparable to a device manufactured by traditional means. We carried out ultraviolet (UV) irradiation, electrical, and thermal stress tests (Figs. 9–11, respectively) on our low-cost PP/carbon black filament [35] to assess the long-term reliability of objects printed using the filament. As we will only present the key findings here, we direct the interested reader to Ref. [35] for the detailed descriptions and discussions of these stress tests. For the PP/carbon black filament at various volume fractions, no significant change was observed prior to and following the stress test involving over 700 h of exposure to sunlight, as seen in Fig. 9. From Fig. 10, the tested filaments did not undergo drastic fluctuations in their electrical resistance even after an alternating current driven at 12 V was passed through them for 1 week. The filaments melted upon heating by an alternating current driven at voltages between 60 and 120 V, suggesting that they cannot be used for applications that require high voltages (>60 V, AC). From the thermal stress tests in Fig. 11, we observe that the more conductive filaments have lower and more consistent resistance with increasing temperature. All four CPCs

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Fig. 9 A plot showing the change in electrical resistance of PP/carbon black filaments with different filler volume fractions upon exposure to UV radiation. (Reproduced with permission from S.W. Kwok, K.H.H. Goh, Z.D. Tan, S.T.M. Tan, W.W. Tjiu, J.Y. Soh, Z.J.G. Ng, Y.Z. Chan, H.K. Hui, K.E.J. Goh, Electrically conductive filament for 3D-printed circuits and sensors, Appl. Mater. Today 9 (2017) 167–175, Elsevier. Copyright 2017.) 30 Before electrical stress After electrical stress

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Fig. 10 A bar graph comparing the electrical resistance of 3D printed wires printed with CPCs with different loadings before and after exposure to 12 V alternating current (AC) for 7 days. (Reproduced with permission from S.W. Kwok, K.H.H. Goh, Z.D. Tan, S.T.M. Tan, W.W. Tjiu, J.Y. Soh, Z.J.G. Ng, Y.Z. Chan, H.K. Hui, K.E.J. Goh, Electrically conductive filament for 3D-printed circuits and sensors, Appl. Mater. Today 9 (2017) 167–175, Elsevier. Copyright 2017.)

Polymer-based conductive composites for 3D and 4D printing of electrical circuits

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Fig. 11 Plots showing the resistance-temperature relationship of 3D printed CPC wires. In these plots, four CPCs (with a filler volume fraction of (A) 15.5%, (B) 20.4%, (C) 25.9%, and (D) 32.3%, respectively) undergo a heating (solid symbols and solid lines) and a cooling (open symbols and dashed lines) cycle. (Reproduced with permission from S.W. Kwok, K.H.H. Goh, Z.D. Tan, S.T.M. Tan, W.W. Tjiu, J.Y. Soh, Z.J.G. Ng, Y.Z. Chan, H.K. Hui, K.E.J. Goh, Electrically conductive filament for 3D-printed circuits and sensors, Appl. Mater. Today 9 (2017) 167–175, Elsevier. Copyright 2017.)

showed an increase in electrical resistance in response to an increase in temperature, i.e., a Positive Temperature Coefficient (PTC) effect.

3 Conductive polymer composites with metal-based fillers As the volume resistivity of carbon-based CPCs discussed in Section 2 does not decrease below 104 Ω m, recent studies have turned to metal-based fillers to further decrease the resistivity of 3D printable filaments. For instance, Electrifi, a copper-based filament, has a resistivity of 6.0  105 Ω m, and has also been made available on the market [37]. Advances made in formulating metal-based 3D-printable filaments reveal three approaches pertaining to the metal filler and its form—copper-silver (Cu-Ag) nanowires, Ag flakes, and tin (Sn)-based low-melting-point alloy reinforced with nickel (Ni) particles. This section reports each approach within the framework of the first two criteria laid out in Section 1.4. We focus on the preparation and performance of the three metalbased CPCs and show how well they stack up against the criteria in Section 1.4.

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3.1 Preparation of conductive metal filler One of the main challenges for thermoplastic filaments incorporating metal is achieving volume fractions that allow the required conductivity/resistivity to be obtained for the specified application without compromising printability with a standard FDM process (Criterion 3 of Section 1.4). A key priority concerns maintaining a controlled flow through the print nozzle, which minimizes the fluctuations in the printed line widths and the occurrence of nozzle jamming. 3.1.1 PCL + Cu-Ag nanowires Cruz et al. [39] successfully extruded a filament incorporating copper nanowires with high aspect ratios of about 200:1 coated with a thin film of silver (12 vol%) into polycaprolactone (PCL). The resulting PCL + Cu-Ag nanowire filament had a resistivity of 2.0  105 Ω m. The Cu-Ag nanowires were prepared using a novel low-cost (US$7.16/g), scalable multigram synthesis (4.4 g yield of 45  15 μm long nanowires in 1 h). Cu nanowires were coated with a layer of Ag to reduce oxidation. This synthesis method, detailed in Ref. [39], claims to be five times cheaper than current methods to synthesize Cu-Ag nanowires. According to this study, the aspect ratio of the nanowires (with resistivities ranging from 8  103 to 102 Ω m) has crucial impact on the CPC’s electrical properties. The percolation threshold is lower for higher aspect ratio nanowires—
5 vol% for nanowires with aspect ratio of 230 and
8 vol% for nanowires with aspect ratio of 120. This was attributed to longer and thinner nanowires being more likely to form a conductive network at low loadings. Nanowires with a higher aspect ratio also aggregate more easily if added beyond the percolation threshold as they interpenetrate and get tangled up with other nanowires. Due to the aggregation, this study found that nanowires added beyond the percolation threshold did not contribute to a further increase in conductivity. An investigation of the orientation of nanowires within the polymer matrix was not carried out; hence, the mechanism for how the high aspect ratio nanowires form the conducting percolation network at relatively low loading remains unclear. 3.1.2 PVB + Ag flakes The bottleneck of obtaining a low percolation threshold for conductive fillers with flake morphology was recently overcome by Lei et al. [41]. They produced composites with smooth flaky silver as conductive filler and polyvinyl butyral as thermoplastic matrix at a relatively low loading of 11.08 vol%. Remarkably, these filaments had a volume resistivity of 7.02  106 Ω m, the lowest-known recorded resistivity for a 3D-printable conductive polymer composite at the time of this chapter’s writing (Oct. 2018). The silver flakes were arranged face to face in conductive layers by curing. Silver powders consisting of nanometer-sized smooth flakes were first prepared by a nanofilm transition method detailed here [45]. A Conductive Silver Paste (CSP) was then

Polymer-based conductive composites for 3D and 4D printing of electrical circuits

Fig. 12 Images (A)—(C) show the three steps of the thin film transition technique—coating, curing, and peeling of cured CSPs; (D) shows an SEM image of the cross-section of CSP films. (Reproduced with permission from Z. Lei, Z. Chen, H. Peng, Y. Shen, W. Feng, Y. Liu, Z. Zhang, Y. Chen, Fabrication of highly electrical conductive composite filaments for 3D-printing circuits, J. Mater. Sci. 53 (20) (2018) 14495–14505, Springer Nature. Copyright 2018.)

prepared, coated onto Polytetrafluoroethylene (PTFE) plates, and cured in a process known as the thin film transition technique (Fig. 12). This technique produced flat flakes with an average thickness of 70 nm that were oriented parallel to the PTFE plates. The authors claimed that the flakes have a high specific surface area, which increases the probability of contact between adjacent silver flakes in the dispersion, thereby increasing the likelihood of forming a complete conductive network. Furthermore, parallel orientation of flakes is achieved due to their own gravity and due to the pressure produced by the surface tension of the wet film. Further X-ray diffraction (XRD) analysis performed on the composites showed high intensity of (111) crystal planes of silver flakes, providing evidence for their ordered orientation. Therefore, they explain that the low resistivity is possible despite low loading because of an optimal face-to-face contact between flat silver flakes, resulting in a highly connected conductive network of oriented flakes forming in the matrix without the need for excess nonoriented filler. 3.1.3 Nylon-6/PE + Ni/Sn95Ag4Cu1 low-melting-point alloy Low-Melting-Point Metal (LMPM) alloys, with their ability to be fusible at temperatures within the range of a typical FDM 3D printer, are yet another candidate for conductive fillers. Tan and Low [40] combined nickel and a tin-based LMPM alloy to achieve low resistivity filaments (3.23  105 Ω m) at high filler loading (
30 vol%) but low melt viscosity (compared to composites with nickel alone). The metal filler used was a mix of (a) micron-sized Ni powder and (b) Sn95Ag4Cu1 LMPM alloy. The LMPM alloy (melting point: 217°C) was introduced to lower the viscosity of the polymer composite melt at temperatures above the alloy’s melting point. They chose Ni particles to exploit their high electrical conductivity and corrosion resistance at room temperature. Ni particles also served to reinforce the matrix by preventing coalescence of the liquid metal and allowing LMPM alloy to be well dispersed. SEM images of such composites with Nylon (Fig. 13A) and High-density Polyethylene (HDPE) (Fig. 13B) as polymer matrix (25 vol%) show a rough fractured surface

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Fig. 13 SEM images of 25 vol% (A) nylon-6 and (B) HDPE composite; (C) EDX map of Ni (blue), Sn (red), and C (green) overlaid onto (A); (D) EDX map of Sn (red), Ni (blue), and C (green) overlaid onto (B). (Reproduced with permission from J.C. Tan, H.Y. Low, Embedded electrical tracks in 3D printed objects by fused filament fabrication of highly conductive composites, Addit. Manuf. 23 (2018) 294–302, Elsevier. Copyright 2018.)

with metallic particles distributed in the polymer matrix. Energy-dispersive X-ray spectroscopy (EDX) maps (Fig. 13C and D) show purple regions, which appear due to colocalization of red and blue signals, indicating encapsulation of Ni by Sn. The authors claim that this possibly occurs because of formation of intermetallic bonds between Ni and Sn when the LMPM alloy is in molten state. They also state that this bond formation is followed by solidification of LMPM alloy around Ni particles due to good wetting compatibility of Ni and LMPM alloy to create a well-bonded interface.

3.2 Performance before and after printing Characterization techniques similar to those employed to study carbon-based filaments in Section 2.2 are also used here to compare the performance of the metal-based filaments before and after printing. These techniques include comparison of resistivity measurements between prepared filaments and printed structures, SEM to estimate filler distribution and orientation, and electrical measurements in different directions.

Polymer-based conductive composites for 3D and 4D printing of electrical circuits

3.2.1 PCL + Cu-Ag nanowires Even though the percolation threshold was determined at around 5 vol% of 50 μm Cu-Ag nanowires, filaments containing that volume fraction were found to be nonconductive during FDM extrusion [39]. This loss in conductivity was attributed to the breakage of nanowires during extrusion, as the average length of nanowires decreased to 10  8 μm compared to their length before extrusion. When the loading was increased to 12 vol%, the filament was printable (1.0-mm nozzle, 0.2-mm-layer thickness), conductive, and stable. This was also the maximum loading at which filament was conductive without being so brittle that it could not undergo continuous extrusion. 3.2.2 PVB + Ag flakes A comparison between the volume resistivities of filaments of four different loadings is shown in Fig. 14, both before (red) and after (blue) printing [41]. The volume resistivity of the 3D printed filament is larger than the filament before printing for loadings beyond 45 wt% (7.70 vol%). The authors posited that this loss in conductivity was because of changes in morphology and silver flake distribution during FDM extrusion. Internal stresses acting on the melted composites in the narrow extrusion nozzle cause the embedded silver flakes to bend, tear, and get smashed. This results in defects in the morphology and discontinuities in the flake distribution of the 3D printed filaments, increasing their resistivity. The morphological changes accompanying FDM extrusion can be seen in Fig. 15, which shows cross-sectional SEM images of the filament before (Fig. 15A) and after (Fig. 15B) printing. Fig. 15A shows the silver flakes oriented along the length of the composites. This face-to-face contact pattern is attributed to the high viscosity and shear stresses in the melt composites.

Fig. 14 Bar graph comparing the volume resistivity (Ω cm) of CPCs with mass fraction of silver ranging from 40% to 55% both before (red) and after (blue) printing. (Reproduced with permission from Z. Lei, Z. Chen, H. Peng, Y. Shen, W. Feng, Y. Liu, Z. Zhang, Y. Chen, Fabrication of highly electrical conductive composite filaments for 3D-printing circuits, J. Mater. Sci. 53 (20) (2018) 14495–14505, Springer Nature. Copyright 2018.)

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Fig. 15 SEM image showing the cross-section of the CPC used in Ref. [41] before (A) and after (B) printing. (Reproduced with permission from Z. Lei, Z. Chen, H. Peng, Y. Shen, W. Feng, Y. Liu, Z. Zhang, Y. Chen, Fabrication of highly electrical conductive composite filaments for 3D-printing circuits, J. Mater. Sci. 53 (20) (2018) 14495–14505, Springer Nature. Copyright 2018.)

3.2.3 Nylon-6/PE + Ni/Sn95Ag4Cu1 low-melting-point alloy Electrical measurements were performed along the horizontal (X/Y), vertical (Z), or across all three (XYZ) axes on conductive tracks 3D printed using the filaments [40]. Conductivity of filament after printing decreased by one to two orders to magnitude compared to the composite before printing. The lowest reduction in conductivity (where the filament retained 11.6% of its initial conductivity) was observed for the tracks printed along the horizontal axis using the filament with highest (6.25  105 Ω m) conductivity. This is because filaments with higher conductivity contain metal particles in more well-connected conductive networks that are less likely to be broken upon printing, resulting in conductive contact between stacked printed layers. In particular, conductive tracks printed along the Z axis had the least conductivity retention (0.07%–2.4%). This is because unlike the horizontal X/Y axis, conductivity retention in the Z axis is dependent upon how well successive layers adhere to each other, which is compromised due to the introduction of voids and air gaps into 3D printed objects during FDM. The group analyzed SEM images of the cross sections of printed tracks to investigate their porosity (defined as the percentage area of voids with respect to the total cross-sectional area of the track). The calculated average volume porosity (see Ref. [40] for exact numbers) was nonuniform, indicating that there were less electrically connected pathways in the printed structures, leading to an increase in volume resistivity and a drop in filament performance.

4 Applications A variety of practical-sized circuits and circuit elements, both in two and three dimensions, have been demonstrated using the filaments discussed previously. Much of the motivation for these demonstrations comes from the need to prove their “4Dness,”

Polymer-based conductive composites for 3D and 4D printing of electrical circuits

i.e., the printed structure’s response to electrical stimuli in time and reveal the potential of 3D/4D printing for achieving table-top printed electronics. This section highlights stateof-the-art examples of 3D printed electronic components, circuits, and sensors using CPCs described in this chapter, with a focus on multimaterial 3D printing of freestanding and embedded structures. We believe that being able to apply a given filament to a use case is an important step in the production of 3D printable CPCs (Criterion 1 of Section 1.4). We’ve also analyzed the consistency and predictability of printing 2D conductive tracks on a 3D chassis to demonstrate how one could fulfill Criteria 2 of Section 1.4 in practice.

4.1 2D circuit tracks The simplest usable case involves depositing conductive tracks on different flat and flexible substrates in order to gauge the ability of a conductive filament to complete a circuit. Zhang et al. [36] printed long narrow lines of 800 μm 100 μm on a piece of A4 paper (Fig. 16A) and transparent bendable Polyimide (PI) base (Fig. 16C) using their PLA/Graphene filament. The enlarged view in Fig. 16B shows that the printed lines have a cuboid-like shape, suggesting good adhesion with the paper substrate. Fig. 16C shows that the tracks retained adhesion with PI upon being bended. The smoother surface and greater bendability of the PI substrate allows a simple light-emitting diode (LED) circuit (see Fig. 16D) to be flexible. SEM images

Fig. 16 (A) A 3D printed conductive track printed on a paper substrate. Inset: Multiple units of the conductive track printed on A4 paper; (B) zoomed-in view of the conductive track in (A); (C) A demonstration of flexible circuits where conductive tracks are printed on a polyimide substrate; (D) A demonstration of the conductive ability of the filament; (E) SEM image of the cross-section of the extruded filament; (F) zoomed-in view of the filament in (E). (Reproduced with permission from D. Zhang, B. Chi, B. Li, Z. Gao, Y. Du, J. Guo, J. Wei, Fabrication of highly conductive graphene flexible circuits by 3D printing, Synth. Met. 217 (2016) 79–86, Elsevier. Copyright 2016.)

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Fig. 17 Illustration of a printed 2D circuit and plastic solder. (A) A 2D circuit printed by laying conductive tracks on ABS and soldering ends of the tracks to a blue LED. The LED is turned on when connected to a conventional 9 V battery. (B) A break is introduced to open the circuit. (C) The CPC is used as plastic solder to close the open circuit. The blue LED turns on showing that the circuit has been closed. (Reproduced with permission from S.W. Kwok, K.H.H. Goh, Z.D. Tan, S.T.M. Tan, W.W. Tjiu, J.Y. Soh, Z.J.G. Ng, Y.Z. Chan, H.K. Hui, K.E.J. Goh, Electrically conductive filament for 3D-printed circuits and sensors, Appl. Mater. Today 9 (2017) 167–175, Elsevier. Copyright 2017.)

(Fig. 16E and F) of the filament indicate the graphene sheets present in a lamellar orientation, which might explain the origin of the conductivity and good adhesion to substrate. Our group [35] went a step further than depositing 2D conductive tracks by using our PP/Carbon black filament as plastic solder to join a blue LED to the circuit (Fig. 17A) and to repair a break in the circuit. Fig. 17B shows open circuit and Fig. 17C shows how the plastic solder is used to fuse the open ends of the printed track together to close the circuit. The soldering can be done with a commonly available soldering iron.

4.2 3D circuit tracks Besides the horizontal x-y axes, it is also possible to print conductive tracks in the vertical z-direction. Zhang et al. [36] also printed multiple layers of the 2D tracks featured in Fig. 16 to create freestanding (Fig. 18A), flexible (Fig. 18C), and stretchable (Fig. 18E) 3D tracks with a smooth surface. The filament can be extruded homogeneously with minimal damage, as seen from the uniform side length of each outer square in the spiral in Fig. 18D. A hollow pyramid was also printed (Fig. 18F) to show that the PLA/graphene composite can be used to print conductive 3D structures with complex geometries. Instead of freestanding 3D circuit tracks, which are fully conductive, many practical use cases would require conductive tracks laid inside a nonconducting chassis. To that end, we [35] created modular blocks (Fig. 19) using ABS (white) as insulator and our PP/Carbon black composite (black) as 3D circuit track. The modular block was coprinted in a single-build process. Fig. 19E and F shows the block’s modularity—a modular block with printed terminals (black) connected to a blue LED and powered by a 9 V battery is joined to another similar block via the printed pin-and-socket terminal connectors to create an extended 3D circuit. This shows that these multi-material modular 3D tracks can be used as “LEGO-like” building blocks to create more complex 3D circuits.

Polymer-based conductive composites for 3D and 4D printing of electrical circuits

Fig. 18 (A) Illustration of a printed 3D circuit; (B) zoomed-in view of the boxed area in (A); (C) demonstration of the flexibility of the printed 3D circuit;, (D) top view of the circuit shown against a ruler for scale comparison; (E) demonstration of flexibility in the z-direction by pulling one end of the printed 3D circuit vertically up while keeping the other end fixed; (F) A freestanding pyramid printed with the same CPC. (Reproduced with permission from D. Zhang, B. Chi, B. Li, Z. Gao, Y. Du, J. Guo, J. Wei, Fabrication of highly conductive graphene flexible circuits by 3D printing, Synth. Met. 217 (2016) 79–86, Elsevier. Copyright 2016.)

4.3 3D chassis with 2D conductive tracks In this example, we exploit 2D circuit printing by designing a basic cross-frame of a drone with connecting tracks embedded in the frame. The connecting tracks were printed with a consumer FDM 3D printer with a resolution of about 100 μm, using our PP/carbon black filament [35]. The conductive filament used here had a resistivity of 0.1 Ω m. The insulating portion of the crossframe is printed with a PLA filament. Fig. 20A and B show two versions of the crossframe with embedded conductive tracks printed with wires of different cross-sectional areas. Each arm of the crossframe has a pair of embedded wires, each having a length of 30 mm. The crossframe with smaller wire cross-section [A1 ¼ 0.33  0.03 mm2] in Fig. 20A has average wire resistance R1 ¼ 9  1 kΩ, whereas the larger wire cross-section [A2 ¼ 1.8  0.2 mm2] in Fig. 20B has average wire resistance R2 ¼ 1.7  0.1 kΩ. The uncertainties in the resistance mainly arise from the variations of the wire dimensions related to the resolution of the 3D printer. If the resistivity of the PP/Carbon black remains constant, we expect the wire resistance R to scale inversely with the cross-sectional area A according to Pouillet’s law: R ¼ ρL/A, where ρ is the resistivity and L is the wire length. Here, we find that A2/ A1 ¼ 5.5  1.1, and R1/R2 ¼ 5.3  0.9. Thus, the printed conductive tracks affirm, within experimental errors, that the PP/Carbon black filament can be used to print conductive circuit elements in a reproducible and predictable manner in accordance with Criterion 2

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Fig. 19 Illustration of a modular 3D circuit inside an insulating chassis (a cube with dimensions 3  3  3 cm). (A, B) Top (A) and bottom (B) view of the circuit cube shown with the location of pin and socket connectors, respectively; (C, D) Top (C) and bottom (D) schematic of the conductive track contained within the cubic chassis; (E) Working demonstration of the circuit cube where it is powered by a conventional 9-V battery and used to turn on a blue LED; (F) Demonstration of the modular extension of the circuit cube as two cubes are connected through the sockets and pins to form a longer connected 3D circuit. (Reproduced with permission from S.W. Kwok, K.H.H. Goh, Z.D. Tan, S.T.M. Tan, W.W. Tjiu, J.Y. Soh, Z.J.G. Ng, Y.Z. Chan, H.K. Hui, K.E.J. Goh, Electrically conductive filament for 3D-printed circuits and sensors, Appl. Mater. Today 9 (2017) 167–175, Elsevier. Copyright 2017.)

in Section 1.4. Fig. 20C shows the side of crossframe without the conductive tracks. Some pin holes are provided for insertion of conventional circuit components, such as diodes, LEDs, and sensors, with standard pins/leads. We further test the use of these printed tracks to power some standard circuit components that might be mounted on an arm of a crossframe to assemble the drone. Fig. 20E shows an LED mounted on one arm of the crossframe, and a small motor-propeller assembly on the opposite arm. The two terminals of both the LED and the motor are connected to the printed tracks on their mounting arm using the same PP/carbon black as solder to secure them. These components can then be powered through the connecting pins mounted at the opposite end of wires near the center of the crossframe. Although a typical 9-V battery can sufficiently power up the LED (Fig. 20D and F), up to three 9-V

Polymer-based conductive composites for 3D and 4D printing of electrical circuits

Fig. 20 Using PP/Carbon black conducting filament to print wire tracks embedded in the crossframe for a drone. Each arm has two conductive tracks with bond pads at the ends. The bond pads are used for soldering the leads of electrical components for a good connection to the track. (A) Narrower wire tracks with length ¼ 30 mm, width ¼ 1.1 mm, and thickness ¼ 0.3 mm; (B) Larger gauge wire tracks with length ¼ 30 mm, width ¼ 3.0 mm, and thickness ¼ 0.6 mm; (C) The typical under side of the crossframe in (A) or (B) showing pin holes for insertion of the leads of electrical components in order to make contact with the bond pads of the printed wire tracks; (D) A green LED mounted on one arm, and some connection pins mounted nearer to the center of the frame. The same PP/carbon black filament was used to solder all the leads to the bond pads; (E) Standard electrical components such as an LED and a motor-propeller assembly mounted on the crossframe and connected to the center pins through the printed wire tracks; (F) Powering up the LED through the printed wire tracks.

batteries connected in series are required to start the motor to spin the propeller. This shows that thicker printed tracks would be necessary if a smaller battery of lower electromotive force is desired as the power source. Alternatively, a PP/carbon black filament with lower resistivity (e.g., 0.01 Ω m) can also be used to print these tracks in order to deliver a larger current to the motor.

4.4 3D circuit elements Besides 2D and 3D conductive tracks, other basic circuit elements have also been 3D printed using CPC filaments. An example of an inductor and a capacitor is presented next.

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4.4.1 Inductor While carbon-based filaments are not conductive enough to serve as inductors, the significantly higher conductivity afforded by metal-based filaments makes 3D printing of RF circuits and wireless applications possible, as demonstrated by Flowers et al. [38]. They printed three air-core spiral inductors, each with different number of turns (5–9), using a copper-based filament. PLA served as the insulating dielectric material. Electrochemical Impedance Spectroscopy (EIS) was used to measure the relationship between inductance and the number of turns (Fig. 21B, solid line), while Wheeler’s approximation helped predict inductances according to theory (Fig. 21B, dotted line). Coil inductances between 0.5 and 3 μH were measured. To demonstrate wireless power transfer using such printed RF coils, they connected an LED to a 3D inductor with 9 turns

Fig. 21 (A) Top and side view of the spiral inductor imaged using micro Computed Tomography (CT); (B) Plot showing the relationship between inductance and the number of turns in the inductor; (C) Demonstration of wireless charging using a spiral inductor with 9 turns; (D) Plot comparing the transmitted (black) and received (blue) waveforms as measured with an oscilloscope. (Reproduced with permission from P.F. Flowers, C. Reyes, S. Ye, M.J. Kim, B.J. Wiley, 3D printing electronic components and circuits with conductive thermoplastic filament, Addit. Manuf. 18 (2017) 156–163, Elsevier. Copyright 2017.)

Polymer-based conductive composites for 3D and 4D printing of electrical circuits

and were able to power up the LED wirelessly using a commercial wireless phone charger (Fig. 21C). Fig. 21D shows that the received waveform is lower in amplitude than the transmitted waveform, suggesting that the difference in voltages between the two signals corresponds to the power transfer in the charging process. 4.4.2 Capacitor The same copper-based conductive filament used to print inductors was used to print parallel-plate capacitors, with two types of PLA—black pigmented PLA and oxidized bronze particle-filled PLA—as the dielectric. These capacitors have parallel plates that measure 10 mm  10 mm and are vertically separated by 0.2 mm (Fig. 22A). Fig 22B shows capacitance values obtained EIS measurements on printed capacitors with number of stacked capacitors ranging from 1 to 5. The capacitance is observed to have a positive linear relationship with the number of stacked layers. The capacitor with bronze-filled PLA showed a higher capacitance. They ascribed this to the incorporation of the oxide-coated metal particle filler, which increases the dielectric constant.

4.5 Sensors In addition to conductivity, some of the CPCs can also provide predictable piezoresistive, capacitive, or temperature-dependent response, opening the possibility of 3D printing sensors. The section reviews some of these sensor applications.

Fig. 22 (A) Cross-sectional view of a 3D printed parallel-plate capacitor, which uses black PLA as the separator; (B) Plot showing the positive linear relationship between capacitance and the number of stacked parallel plates for black pigmented PLA and bronze-filled PLA separators. (Reproduced with permission from P.F. Flowers, C. Reyes, S. Ye, M.J. Kim, B.J. Wiley, 3D printing electronic components and circuits with conductive thermoplastic filament, Addit. Manuf. 18 (2017) 156–163, Elsevier. Copyright 2017.)

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4.5.1 Temperature sensor Recall from Section 2.2.2 that the PP/Carbon black filament [35] showed a predictable change in electrical resistivity as a function of temperature (see Fig. 11). The positive temperature coefficient (PTC) exhibited by the material allows it to be exploited for temperature sensing. The sensor was designed and 3D printed with Universal Serial Bus (USB) connector leads as shown in Fig. 23A and was calibrated by correlating its electrical resistance in preheated water bath with the temperature of the water bath measured using a thermocouple (Fig. 23B). An Arduino interface integrated with the sensor then allows for easy readout of the temperature. Correctly calibrated, the sensor printed with PP/Carbon black showed excellent agreement (2.0  10 3 Scm 1) for an electrically stimulated tissue. This formulation on incorporation with an aqueous solution demonstrated synergistic effects in conductivity due to electron conduction of the composite and ionic conductivity of the medium [116].

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3.2 Temperature-responsive functional materials Temperature-responsive functional materials form a wide range of materials responding to cooling or heating. The SMPs and low or upper critical solution temperature polymers belong to this category of materials. 3D and 4D printing of these materials are elaborated in the subsequent section. 3.2.1 Shape memory polymers SMPs are the group of smart materials having the capability to regain their original shape after temporarily induced deformations [117]. The shape memory effect in polymers is induced by various stimulations like thermal, mechanical, light, etc.; however, the steps involved in the shape recovery are the same. Let us take as an example of thermally induced shape memory effect in the polymeric composites. The first step taking place in SMPs is the programming step in which the primary polymeric material is subjected to heating above its transition temperatures (Ttrans) {Tg or melting point (Tm)}. After completion of the first step, the material is temporarily deformed in the second step. Once the desired deformation is achieved, the material is frozen by cooling. The third and final step involves the recovery of the deformed structure into original on the application of thermodynamic stimuli, thereby releasing the accumulated stress [118]. The shape memory material can include either singular or multiple shape memory segments with one permanent element in stabilizing phase and at least one element with the ability of temporary transformation evolving in single or multiple temporary shapes [119]. When shape memory materials are used for 3D printing, it requires prime attention to safeguard the properties of the materials after printing [120]. The SMPs form important and widely explored materials for 3D printing of smart materials, which include polymers like polyurethane (PU), PCL, acrylic and epoxy-based resins [121,122]. These polymers exhibit shape morphing behavior, on the basis of which are used in the production of selfevolving dynamic structures, actuators, electronic and medical devices [118,120,122]. The 3D and 4D printing of SMPs in these applications yields better control, spatially localized elements within the structure in accordance with the required pattern and structure. The printed objects yield localized, sequential, and gradual response. The epoxy-based interconnected shape memory hinges with alternating Tg have been reported by Yu et al. [120]. 3D printing of such hybrid formulation provides accurately localized shape memory elements, thereby aligning shape memory elements in accordance with their gradually increasing Tg. The structural designing of the composite on the transformation into 3D object yielded a distinct sequential recovery. The same process is used in the 3D printing of interlocking structures [118]. Due to its low Tg (60°C) and high flexibility, PCL is popularly used in smart structure 3D printing. Estelle et al. [61] have reported 3D printing of hybrid shape memory structures using FDM method. The hybrid structure was composed of PCL and PU elastomer as core

3D and 4D printing of pH-responsive and functional polymers and their composites

and shell, respectively. The shape fixity of 3D composites showed a significant decrease with an increasing composition ratio of PCL and PU with 100% shape recovery after temporal deformations at each composition ratio. In another study, methacrylatedPCL resins were printed in 3D using SLA technology [122]. The control on tunability on the degree of crystallinity and crosslinkage in the composite was achieved on the basis of the degree of methacrylation. The 3D printing of composite leads to the formation of a shape memory thermoset polymer from crosslinked methacrylated units with 93% recovery efficiency. The 3D structure printed exhibited a high-resolution complex structure and can be used as submillimeter vascular stents and fabrication of electronic devices. As a stent, the composite due to shape memory effect shows reversible shrinking and proves less injurious during its deployment. 4D printing of glassy SMP fibers in an elastomer matrix has been reported by Ge et al. [123]. The composite comprised the elastomeric matrix without shape memory effect and with Tg of 5°C. The elastomeric matrix demonstrated a rubbery state at a modulus of 0.7 MPa at 15°C, while the Tg and modulus of glassy polymer fiber were 35°C and 3.3 MPa at the lowest temperature (15°C) of thermodynamic cycle, whereas the modulus of 13.3 MPa was observed at the highest temperature (60°C). In another report, Titbits et al. [124] used active hydrophilic expandable polymer and rigid plastic combination at various spatial arrangements for 4D printing of the composite. The 3D and 4D printed shape memory structure provides a personalization approach enabling digitization in scaling of the stents [125]. The dynamic structural changes caused by shape memory effect provide an efficient method in controlling cell morphology via mechanical stimulus. One such approach has been reported by Hendrikson et al. [126] using 3D printed shape memory PU scaffolds at Tg  32°C. The scaffold was employed for the mechanical cell stretching wherein the cells were placed on the temporary deformed scaffold and incubated at 30°C to retain their deformed shape while cell proliferation. After proliferation, the scaffold recovered to its original shape at 37°C. From their observations, the authors concluded that the mechanically stimulated shape memory cells showed better elongation in contrast to the cells seeded on controlled scaffolds. The detailed summary of 3D printed SMPs along with their applications is listed in Table 3. 3.2.2 Temperature-responsive polymer composite hydrogels In 3D and 4D printing, temperature-responsive hydrogel based polymer composites are generally considered. These composites undergo deformations in the form of coil-toglobule transitions above and below the low critical solution temperature (LCST) and upper critical solution temperature, respectively. In the mentioned critical solution temperature limits, a decrease in solubility of polymer and aggregate formation takes place due to changes of thermo-dynamical nature [131]. Thus, for the fabrication of materials displaying reversible volumetric changes in response to the temperature alterations, temperature-responsive hydrogel polymer composites present an efficient platform.

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Shape memory polymer

3D printingmethod Tg (°C)

Recovery Shape temperature fixity (°C) (%)

Epoxy

Inkjet

32, 33, 55, 57, 60, 61, 65

100

–

100

55

70

>98

>93

6.5 (helical structure) 14.2 (interlock structure) 5–20

Methacrylated- SLA polycaprolactone

Shape recovery (%)

Recovery time (s)

Poly (styreneblock-[methyl acrylate) Hybrid epoxyacrylate Poly(lauryl acrylate-cohexanediol diacrylate) PU PU-PCL hybrid

FDM

45,65,80 63–91

99–100

81.8–97.4

–

SLA

82

85

99

100

60 100

>1800 100–300as

Methacrylate copolymers PLA/ hydroxyapatite composite

SLA

15–75a

Tg + 10

80–100a 0.95

60

FDM

60

80

–

–

a

Composition dependent.

>96.3

Application

References

Structures with shape morphing

[127]

Structures with shape morphing, biomedical, and electronic devices Structures with shape morphing

[125]

Structures with shape morphing Molds

[120]

Tissue engineering Structures with shape morphing Grippers, biomedical devices Tissue engineering

[128]

[129]

[121] [61] [130] [118]

3D and 4D printing of polymer nanocomposite materials

Table 3 Summary of the 3D printed SMPs, methods of their manufacturing, their characteristics, and applications

3D and 4D printing of pH-responsive and functional polymers and their composites

The 3D printing of thermally driven actuators and structures with shape morphing use the same fabrication principle [132]. The actuator fabrication using the polymers with a critical solution temperature into shape-changing structures are generally prepared in the form of thin films. The shape morphing of composites comprises active and passive components and, for their accurate localization in the structures, it requires multistep processing and assembly. The multistep processing is reduced to a single step on the application of 3D printing, thereby yielding complex smart and hybrid structures from CAD modeling of the components. This approach has been reported by Bakarich et al. [133] in the 3D printing of thermal actuators using polymer hydrogel composite. They integrated poly (n-isopropylacrylamide) (PNIPAm) with LCST 32°C with a hydrogel network to produce a thermal effect in the form of shrinkage above LCST and expansion below LCST. The thermal and mechanical robust effects produced dual interpenetrating and crosslinked networks of aligned PNIPAm displaying reversible changes in response to critical solution temperature. The fabrication of hydrogels using 3D printing was carried out using the extrusion method in order to control the flow of water through hydrogel channels due to temperature-responsive valves. In another report, thermo-responsive PNIPAm along with polyether-based PU into 3D printed poly (2-hydroxyethylmethacrylate) (PHEMA) has been reported for shape morphing using 3D printing [134]. The composite ink was 3D printed into definite boxes, which were immersed in water to study their thermal response in order to determine the LCST effect of solvent. The printed object in the box upon hydration in cold water (20°C) below LCST displayed swelling and reopening upon immersion in hot water (60°C) to confirm its shape morphing behavior. The limitation of 3D printed thermo-responsive hydrogels with LCST is the phase transitions upon heating although they show tremendous application potential. The heating of hydrogels or hydrogel composites results in gel or composite destabilization. To avoid destabilization, the processing conditions must be optimized to negotiate the LCST effect. One such processing approach to avoid the challenge is the use of solvents other than water-like ethanol for initial 3D printing, while the other approach involves 3D printing at subambient temperatures [132]. Both these approaches negotiate the LCST effect but produce other types of limitations. Recently, an effective approach of 3D printing of thermoresponsive copolymer block was reported as an effective approach in order to avoid the limitations [135]. This approach consists of a hydrophobic triblock copolymer of polypropylene oxide and hydrophilic polyethylene oxide. The authors proposed 3D printing of concentrated polymeric solutions transforming gels post printing by the formation of micelles above critical micelle concentration (CMC). By this approach, the gelation occurs above CMC and LCST leading to the creation of physical networks and quick solution solidification. The maintenance of the solution temperature below LCST is however essential to provide smooth extrusion and loading via a nozzle. The ambient temperature

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exposure of 3D printed filaments takes place subsequent to temperature controlled extrusion. The exposure to ambient temperature results in the quick creation of stable and shape retaining gels [136]. In another report, a similar approach was used by Zhang et al. [137] in the 3D printing of triblock copolymer of poly (isopropyl glycidyl ether)-b-poly (ethylene glycol)-b-poly (isopropyl glycidyl ether) (PiPrGE-b-PEG-bPiPrGE) in the form of shear thinning temperature-responsive hydrogels. In the triblock copolymers, gelation takes place at ambient temperature post printing due to the selfassembling of polymer solution above 5°C. The composite network finds its potential applications in the biomedical field especially in cell differentiation due to high storage modulus [136]. In the 4D printing of temperature-responsive hydrogels, Jamal et al. [138] reported the shape-shifting mechanisms of hydrogels (photo patterned polyethylene glycol-based hydrogel bilayers) and rigid materials prepared using PLA technique for tissue engineering. They used bio-origami-based hydrogel to study self-bending operations. In another report, Villar et al. [139] reported bending behavior of flower-shaped network droplets into hollow spheres accompanied by osmolarity gradient of the droplets. They printed cohesive material interconnected by lipid bilayers using aqueous droplets in oil. Gladman et al. [140] reported 4D printing for fabrication of hydrogel composite ink comprising soft acrylamide matrix reinforced with the cellulose fibrils, N,N-dimethylacrylamide, Irgacure 2959 as UV photo-initiator, nanoclay, glucose oxidase, glucose, and nanofibrillated cellulose. The fabricated hydrogel ink imitates the cell wall structure of the plant cell. The composite displayed reversible shape-shifting behavior in hot and cold water when poly (N,N-dimethylacrylamide) was replaced by thermo-responsive N-isopropyl acrylamide.

3.3 pH-responsive polymers This class of smart functional materials exhibit transitions of globule-to-coil nature below or above critical pH value. This set of polymers possesses ionizable functional groups and is categorized as polybasic and polyacids [30]. The reversible transitions occurring in such polymers include pH-based protonation and deprotonation. The charging of these polymers causes electrostatic repulsion leading to stretching of polymer chains and breakdown of their globule form upon neutralization of functional groups [103]. This principle has been used by Nadgorny et al. [141] for 3D printing of pH-responsive poly (2-vinylpyridine) (P2VP). The reversible expansion and compression of quaternary and crosslinked 3D printed P2VP valve, due to dependency on pH, allow control on the flow via a column. The mechanical strength of P2VP on blending with ABS was enhanced due to superior plasticizing properties of P2VP during extrusion 3D printing [64]. In another study, Lee et al. [142] reported 3D printing of acidic collagen precursors. Their approach involves exposure of pH-responsive collagen to sodium hydrogen carbonate vapors at post

3D and 4D printing of pH-responsive and functional polymers and their composites

printing stage leading to the formation of the 3D structure by gel formation due to pH neutralization. Chen et al. [143] adopted 3D printing technique to fabricate collagen-based scaffolds. Heparin sulfate was used as a crosslinking agent and reacted with the collagen. It was observed that the internal structure of 3D printed scaffolds was porous. Also, the compression modulus and strength of scaffolds were significantly enhanced with the change in the compositions of collagen and heparin. In addition, the fabricated scaffolds demonstrated good biocompatibility when cultured with neural stem cells in vitro. In another study, Okwuosa et al. [144] recently used 3D dual extrusion FDM printing for the fabrication of the drug release capsules. The dual extrusion method provided coreshell designing using two different filaments as schematically shown in Fig. 8. The coreshell design for fabrication of gastric-resistant tablets used polyvinylpyrrolidone (PVP) as core and a methacrylic acid copolymer as shell structures. The filaments for core and shell structures were prepared using a twin-screw hot-melt extruder. The CAD modeling used in 3D printing for designing core in the shape capsule and same shaped shell with increasing thickness. The optimized thickness of shell for core protection was >0.52 mm. The active pharmaceutical drug was contained with the core, while 3D printed

Fig. 8 Diagram showing fabrication of 3D-printed shell-core enteric tablet. (A) Design of two complementary stereolithographic files using CAD software creating a core-shell structure. (B) Dual FDM 3D printer is used with two filaments; (i) Eudragit L–based filament for the enteric shell, and (ii) API, PVP-based filament core processed via HME compounder. (C) Picture shows 30% completed FDM 3D printed core-shell tablet. (Reproduced with permission from T.C. Okwuosa, B.C. Pereira, B. Arafat, M. Cieszynska, A. Isreb, M.A. Alhnan, Fabricating a shell-core delayed release tablet using dual FDM 3D printing for patient-centred therapy, Pharm. Res. 34 (2017) 427–437. Copyright 2017, Springer Publications.)

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pH-responsive polymer formulated the shell structure. The presence of Eudragit L 10055 protects the drug in the acidic gastric environment due to its limited solubility at acidic pH. The control over drug delivery is however attained at neutral physiological conditions of the intestine. The control over drug release was achieved by accurate shell thickness and 3D printing resolution. Hence, it is essential to consider parameters like the density of object, structure, and geometry while 3D printing of pH-responsive materials. For 4D printing, pH-sensitive hydrogels have been reported as self-folding devices, providing directional encapsulation and therapeutics release [145]. The 4D printed selffolding devices are composed of two differently swelling layers at different swelling ratios. Among the two swelling layers, the pH-responsive hydrogel layer comprised crosslinked poly (methyacrylic acid) showing swelling on contact with body fluids, while the nonswelling layer is composed of PHEMA. The 4D printed devices, due to the difference in swelling rate/ratio of bilayers, self-folded into the mucus and enhanced the mucoadhesion. The presence of PHEMA avoids the risk of drug leakage in the intestine, thereby acting as a diffusion barrier. The drugs encapsulated in this 4D bilayered structure for their unidirectional release were acid orange 8 and bovine serum albumin via mucosal epithelium. In 3D and 4D printing of pH-responsive materials, only a few works have been reported so far to the best of our knowledge. The 3D and 4D printing of pH-responsive polymers shows great potential for their applicability in the biomedical field due to their biodegradation and biocompatibility [144], but their applicability is restricted in case of 3D and 4D printing of pH-responsive protein hydrogel due to nonphysiological pH gelation [146]. The 3D printing methods of pH-responsive material manufacturing along with their properties and applications are listed in Table 4.

3.4 Light-responsive materials These materials demonstrate macroscopic transformation on exposure to electromagnetic irradiation [131]. The transformations occurring in such materials are cis-trans isomerization and self-assembly, which is induced by light [148,149]. The 3D printing of light-responsive Table 4 Summary of the 3D printed pH-responsive polymers, 3D printing methods used for their manufacturing, and their characteristics and applications pH-responsive polymer

3D printing method

Critical pH

P2VP/ABS

Extrusion (FDM) Extrusion (LDM) Extrusion (FDM) SLA

4.0

Collagen Eudragit L 100-55 Keratin

Applications

References

[65] [143]

5.5

In catalytic devices for flow regulators Regeneration of skin and spinal cord Controlled release of drug

3.5

Tissue engineering

[147]

7.0

[144]

3D and 4D printing of pH-responsive and functional polymers and their composites

composite comprising incorporated azobenzene dyes into photocurable resins of ethoxylated bisphenol-A methacrylates has been reported by Roppolo et al. [150] for photosoftening and hardening effects. These effects are caused due to volumetric changes, which are directly linked with cis-trans isomerization induced by light. The light-induced dynamic and macroscopic responses were used for 3D printing of smart cantilevers on a micrometer scale. In another study, a 3D printing method was developed for the fabrication of coreshell stimuli-responsive capsules for bioactive compound release by Gupta et al. [151]. The fabricated capsules comprised an aqueous core and poly (lactic-co-glycolic acid) (PLGA) as a therapeutic polymer shell. The capsules were made stimuli responsive by incorporating gold nanorods (AuNRs). A two-step 3D printing fabrication process was adopted in which the core is fabricated in the first step while the functionalizations of polymer shell using AuNRs in the second step. The release of the bioactive compound occurred due to the laser rupture of the PLGA shell caused by the photothermal effect resulting from the localized surface plasmon resonance of AuNRs [152]. The advanced manufacturing tendency of 3D printing provides control over the spatial patterns and array complexion with respect to volumetric control. The use of 3D printing technology provided the fabrication of defect-free and stable shell structure along with burst effects. The 3D printing technique used for light-responsive functional materials includes extrusion printing and SLA methods. In general, the light-responsive 3D materials are used in drug delivery, optical devices, and microcantilevers. The 4D printing of light-responsive material composed of hydroxyethyl acrylate, hydroxyethyl methacrylate, potassium 3-sulfopropyl methacrylate (PSPMA), polycaprolactone diacrylate (cross-linker), and Irgacure 819 (photoinitiator) has been reported by Huang et al. [148]. The light-induced transformation caused swelling in PSPMA, thereby generating large stress and enabling higher contrast in printing.

3.5 Piezoelectric functional materials Piezoelectric functional materials exhibit the tendency to generate electric charge due to transformations of mechanical forces like elongation or vibration into power or pressure. Conversely, when such materials are under the effect of electric field, the deformations take place due to the generation of mechanical strain. Hence, this class of materials is of greater importance for 3D and 4D fabrications of sensors, actuators, energy-generating, and other electronic devices. The strongest piezoelectric effect is observed in ceramics, but their toxicity, processing, and brittleness limit their applicability. To avoid these limitations and improve the device sensitivity, it is essential to transform the ceramics from their standard 2D structures to a high-density, complex, and folding 3D and 4D structures [153,154]. Several approaches have been put forward by many researchers to print piezoelectric devices in 3D. For instance, Chen et al. [155] developed a new 3D printing process for a piezoelectric device composed of highly concentrated BaTiO3 particles in the form of a slurry. The 3D printed structure fabricated by them showed enhanced

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density and piezoelectric response. Their approach involved photocuring at a preliminary developing step where pretreated BaTiO3 was combined with a photocurable resin and the composite was photocured by SLA light projection. The resin improved the bond strength and was removed in the postprinting stage, after which the material is sintered at high temperature to form an interconnected BaTiO3 network. The final material was fabricated for biomedical application as an ultrasonic transducer. The general approach for producing piezoelectric devices is based on the use of piezoelectric polymers especially polyvinylidene difluoride (PVDF) [156,157]. PVDF exhibits ease of processability, better flexibility and biocompatibility [158], making it an interesting material for 3D printing. Since PVDF exists in four phases (α, β, γ, δ), β-phase PVDF and its composites show the strongest piezoelectric response. Bodkhe et al. [159] recently developed novel solvent evaporation-assisted method for 3D printing of PVDF and BaTiO3 composites by creating layer by layer and self-supporting piezoelectric structures at room temperature. PVDF/BaTiO3 composites were formulated using ball milling, extrusion mixing, and sonication technique. In their approach, the authors showed the application of high printing pressures in extrusion 3D printing during evaporation of the solvent. The stretching after postprinting produces strong alignment effect on PVDF, thereby enabling multilayer fabrication of 3D contact sensor.

4 Conclusions In this chapter, 3D and 4D printing techniques have been briefly discussed. Since most of the research till this date has been done on 3D printing, 4D printing is comparatively new technique as far as technological advancements in these fascinating research fields are concerned. Hence, 4D printing has been explored to a limited extent in this chapter. A brief description of the basic 3D printing techniques with their pros and cons has been given in this chapter. The types of materials used in 4D printing, viz. single or multiple materials have been discussed. The special focus, however, is on the functional materials in 3D and 4D printing comprising electroactive and electromagnetically active materials, temperature-responsive functional materials, and pH-responsive polymers. The 3D and 4D printing of smart functional materials along with their application potential especially in the biomedical field are also highlighted in this chapter.

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Additive manufacturing (AM) of medical devices and scaffolds for tissue engineering based on 3D and 4D printing Sudip Kumar Sinha Department of Metallurgical Engineering, NIT Raipur, Raipur, India

1 Introduction In the last decade, the biomedical field has witnessed massive and sustained growth in various facets of human tissue regeneration. This process is primarily concerned with cell growth and the reconstruction of organs, and therefore tissue regeneration is an area of immense interest for scientists and academicians globally. Based on last fifty years research in the field hard tissue implants, it can be understood that organ transplantation, its substitution, and fixation are the practical alternatives for patients with injured or damaged organs. Long waiting lists for organ transplantation are a common occurrence across the world. Through June 2017, the US Department of Health and Human Services had a list of around 120,000 patients who were in dire need of various lifesaving organ transplants while only about 5200 donors offered such organs [1]. It is clearly seen that there are an increasing number of patients waiting for transplanted organs while the number of transplants that actually took place over the last few decades was somewhat constant across various age groups. Therefore, there is an extreme need to discover unconventional ways to meet the needs for this scarcity of implantable organs [1]. Three-dimensional (3D) printing is a type of additive manufacturing-based technology for the exact 3D construction of engineering components. It is extensively used in biomedical engineering. Although there are various categories of additive manufacturing techniques available, the terms 3D and 4D printing are often interchangeably used in this context for simplicity. The technology began from a liquid-based stereolithography method during the latter part of the 1980s [2]. 3D printing (popularly referred to by other names such as additive manufacturing or rapid prototyping) is primarily a layered deposition and manufacturing technique where materials are overlapped as one layer over another. During the last four decades, several 3D printing technologies have been developed based on this processing approach. The American Society for Testing and Materials 3D and 4D Printing of Polymer Nanocomposite Materials https://doi.org/10.1016/B978-0-12-816805-9.00005-3

© 2020 Elsevier Inc. All rights reserved.

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(ISO/ASTM 52900:2015) has designated additive manufacturing techniques in 50 different types, which can be further categorized by seven distinct groups: (i) jetting by binders, (ii) jetting based on various materials, (iii) extrusion of materials, (iv) vat photopolymerization, (v) powder bed fusion, (vi) energy deposition, and (vii) sheet lamination. However, in a different approach, the gross classification of 3D printing techniques is done on the basis of the raw material (or ink) deposition procedures. So far, the extrusion-based techniques are the most widely used 3D printing techniques. These include fused filament fabrication (fused deposition modeling (FDM)) and direct ink writing or direct writing (DIW). In these conventional methods, a 3D object is fabricated by deposition on a line-by-line basis and then a layer-by-layer sequence. The only distinction is that FDM melts a solid string or filament emitted through a nozzle that is heated according to the specification while the DIW method pours a viscous or semislurry ink-based solution that might be treated later. Extrusion-based techniques are advantageous because they are capable of printing a vast range of printable materials. Therefore, this technology aids in the fast and efficient fabrication of any type of customized or complex engineering component by the precise accumulation of feed materials using a solid modeling approach with the help of a digital 3D file such as a computer aided design (CAD)-based illustration or CT (computed tomography) scan imaging. The concept of tissue engineering (TE) was first posited by Langer and Vacanti in 1993 and was published in their landmark manuscript in Science [3]. They demonstrated the primary attributes and applications of 3D biodegradable scaffolds. 3D scaffolds should possess a very porous morphology with well-interconnected networks of open pores, and must have a uniform and ample pore size and their distribution for cell proliferation and penetration [4] of cells. Following this breakthrough invention, a number of manufacturing techniques were used for the creation of these porous scaffolds. However, the traditional techniques suffer major limitations as they don’t have the ability to support or have enough potential to master the scaffold architecture, pore size, and its network, it eventually forms a 3D scaffold that is incompatible and less than ideal from the ideal one. This was the motivation behind the use of 3D-printing techniques to produce tailor-made scaffolds with unique structures consisting of well-controlled and uniformly distributed pores of the desired size and shape [5–7]. The scope of application of 3D printing has enabled scientists and engineers to perform desired modifications with ease without any additional requirement of equipment or manufacturing units. The technique also has the unique advantage for manufacturers to construct devices replicating a patient’s specific anatomy (customized devices) or equipment with a very intricate internal assembly. These potential abilities have prompted enormous curiosity in 3D printing, not only in the field of medical devices but also in other applications ranging from food items to commodities humans use in their daily lives to automotive components. On the other hand, 4D printing is a much more recently investigated field that was developed by considering 3D printing as the base. It has promising applicability in

Tissue engineering based on 3D and 4D printing

advanced biomedical research. A group of researchers at the Massachusetts Institute of Technology [8] first proposed the technique in 2013. The development of a modern 4D printing device is largely based on existing 3D printing technology, a rapid growth in smart materials, and a strong foundation of mathematical modeling and subsequent design. Although similar to 3D printing, 4D printing actually adds an extra dimension of revolution in the course of time, where the as-fabricated products interact with environmental stimuli such as temperature, humidity, and light. This results in a change of the active material structure as per the requirements over time [9–12]. In spite of great improvements in the rapidly emerging and advanced 4D printing technology, its precise application in bioengineering is largely dependent on the creation of multifaceted stimuli-responsive 3D printable materials, highly sophisticated 3D printing technologies with the ability to deposit multiple active materials, and feasible computational design approaches to envisage the 3D printed entity’s performance under external stimuli. The sustained growth and development of 3D and 4D printing as an emerging technology in numerous advanced biomedical applications such as traditional biomaterials, tissues and organ (re)generation, chronic diseases, drug delivery applications, implants for hard tissue replacements, biomedical instruments, and prosthetics have motivated scientists and researchers to seek the next level of invention. The flow sheet diagram shown in Fig. 1 explains the 3D/4D printing technology in correlation with various biomaterials used and the conventional synthesis of scaffolds for tissue engineering.

Fig. 1 Schematic sketch showing relationships of additive manufacturing processes in connection to various materials used for scaffold preparation in biomedical engineering.

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2 Scaffolds for tissue engineering The concept of tissue engineering was first outlined by Langer and Vacanti, as already mentioned, in 1993. It was recognized as an interdisciplinary area that is closely associated with various streams such as biological science, materials science, engineering, and medicine [13,14]. Tissue engineering is directly related to the evolution of restorable biological substitutes, preserve, or develop tissue’s function or an entire human organ that has been severely damaged [15]. The above-mentioned requirement can be met with the use of stem cells. Typically, stem cells are distinctive types of cells that don’t transform or modify to become any cells other than the host one. They exhibit remarkable prospects to expand into various other cell types in the human body during the early stage of life and growth. Keeping this in mind, the major technique to produce these structures is to be able to securely distribute these stem cells, and construct a physically and mechanically robust and steady structure so that these stem cells can grow and be sustained. Scaffolds in human anatomy can be structurally created by the effective use of tissue engineering in combination with regenerative medicine. The scaffolds thus fabricated can provide support or substitute for organs and organ systems that have been injured due to various reasons. The fundamental approach of tissue engineering could be described as follows: In a first step, the benefactor or donor cells and growth elements are seeded on a 3D scaffold that gives initial support, along with providing the desired framework for adhering the cells, multiply and transform. The entire process is maintained and cultured in an in vitro atmosphere in a bioreactor to encourage the growth of a fresh, healthy tissue matrix. In the last stage, the biomimetic structure is transplanted into the patient. Scaffolds generally show biodegradable characteristics because after the end of the specific task, newly grown tissues can obstruct their function [16,17]. In order to develop porous scaffolds for tissue engineering, various type of biomaterials are used, provided a production technology is readily available that can work with the existing biomaterial properties. Typically, polymeric biomaterials are a popular choice to create scaffolds because these materials have the ability to supply the structural support required for the attachment of cells and finally lead to tissue development. Among the polymer materials used as scaffolds, synthetic and natural polymers have been accepted as potential alternatives, owing to their vastness, variety of properties, and biocompatibility. The first biodegradable scaffold materials for clinical applications belong to natural polymers. This arises because of their superior ability to interact with different types of cells, and the deficiency of an immune reaction. In later years, synthetic polymers came into existence because they are cost effective and they permit improved functionality over their counterpart. However, they are susceptible to toxicity or the absence of an immune response. The synthetic polymers are primarily comprised of organic materials

Tissue engineering based on 3D and 4D printing

based on poly(L-lactic acid) (PLLA), poly(glycolic acid) (PGA), poly(caprolactone) (PCL), or poly(lactic-co-glycolic) acid (PLGA) for the formation of 3D structure-based scaffold materials [18,19]. Synthetic polymers are often mixed with natural polymers to eliminate unwanted effects in correlation with cell attachment, hydrophilicity, and biodegradability. Additionally, the functionalization of the scaffold surfaces with the help of particular ligands such as protein molecules often aids in boosting cellular responses. 3D scaffolds developed from synthetic and natural biomaterials yielding nanofiberlike features, hydrogels, and sintered microparticles have been extensively investigated [20,21] over the last decade. The primary role of these highly porous 3D scaffolds is to create an environment in the vicinity of the implant to restore the damaged or missing tissue. Among the various controlling factors in a 3D scaffold, porosity plays a major role for second-generation tissue engineering, which in turn indicates the need for cell infiltration and growth along with vascularization into the 3D pore network within the as-fabricated scaffold [22]. As already mentioned, porosity plays a decisive role because the cellular networks rely on interconnected pathways when there is no engineered blood supply. The overall effect arises owing to the diffusion of O
and various nutrients and unwanted products away from the porous scaffold. All these factors play a decisive role in nutrient supply, cell migration, and proliferation to the scaffold. They also enhance the unfilled surface area required for cell-scaffold bridging and interface with nearby tissues mimicking the native extracellular matrix (ECM) environment in the structure. It is imperative to understand that when pore size is reduced, the available scaffold surface area increases. The rise in exposed scaffold surface thus increases the propensity of scaffold ligands to develop strong bonds with cells and to interrelate with. Conversely, with too small a pore size, the migration of cells within the scaffold structure becomes challenging. In addition, the surfaceto-volume ratio of a scaffold determined by pore size distribution and its networking must not be too large because it deteriorates its mechanical strength [23]. In view of the above observations, the microarchitecture of the scaffolds should precisely be designed with factors favorable to cell viability and fostering tissue ingrowth. This tradeoff between pore size distribution and scaffold properties is one of the essential models of tissue engineering and therefore needs to be taken care of during the development of novel biomaterials. An ideal 3D scaffold material should be comprised of a biocompatible, biodegradable material and its mechanical properties must match as close as possible those of the host tissue within the implant. The scaffold is not anticipated to be considered a permanent implant. In reality, they should enable the host cells to attract an extracellular matrix (ECM) and over time, it should be replaced with that of the scaffold structure. From a clinical viewpoint, it is also expected that the scaffold structure is to be simply deployed into diverse shapes and sizes to permit in situ treatment of specific defects in patient organs and tissues.

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The following list offers the desired scaffold material and structural properties: Cell and tissue compatibility and biodegradability: • The byproducts should display no toxicity once they decompose from the implanted scaffold material. • The synergistic effect of natural and synthetic scaffolds can be logically used in order to control the degradation and thereby improve the biocompatibility in scaffold tissue engineering. • The scaffold material should assist the inbuilt host cells to generate their own extracellular matrix. Bioactivity • Next-generation scaffold biomaterials should actively interact with and connect to the host tissue to stimulate in vivo mechanisms of tissue regeneration. In this way, it could enable itself to self-healing mechanism thus leading to substitute of the scaffold via regenerating tissues. • They should be osteoconductive and osteoinductive in nature. • Biochemical signals and growth regulators arising from the excitement biological tissues in correlation with cell-adhesive ligands for cell attachment and differentiation. As an example, the synthesis of hydrogels by covalent bonding or ionic cross-linking can help in protein entrapment and thus discharge them by swelling of the hydrogels.

2.1 Scaffold architecture Scaffolds containing interconnected pores create a large surface area for inbuilt vascularization, promote the formation and growth of new tissues to facilitate cell migration, and subsequently host tissue unification after implantation. The scaffold biomaterials should be modified to tailor the pore size and distribution to target tissues and cells without significantly weakening its mechanical properties and/or affecting the mechanical stability of the scaffold. Lastly, the scaffold materials must be able to degrade with time, keeping the same pace of the new extracellular matrix by forming newly formed tissue, once the implantation is done.

2.2 Mechanical properties Scaffolds with comparable compressive, elastic, and fatigue strength provide strength and stability to the host tissue. The organs and tissues mechanobiologically mimic the scaffold and therefore must maintain their structural integrity under in vivo conditions.

3 Biomaterials for tissue engineering and scaffold fabrication A vast range of materials has been explored for use in the prospective fabrication of scaffolds in tissue engineering. Typically, they can be classified into three subgroups: natural

Tissue engineering based on 3D and 4D printing

polymers, synthetic polymers, bioceramics, and composites. In the polymeric materials group, the materials are realistically fabricated by integrating with individual functional groups within its molecular backbone to determine its chemical, physical, and biological properties. Each of these various classes of biomaterials offers desired benefits and drawbacks. Therefore, it is logical to utilize composite scaffolds consisting of diverse phases, an idea that is becoming progressively popular in this field of application.

3.1 Natural polymers Research on polymer biomaterials has been a subject of interest both in academia and industry for a span of 60 years or so. Biological materials are used as scaffold biomaterials for the synthesis of natural polymers. Typical examples of natural polymer-based biomaterials include collagen, a range of proteoglycans, alginate-based substructures, and chitosan. All these substances have been tried and used in the fabrication of tissue-engineered scaffolds. Collagen is the most abundant and naturally occurring polymeric biomaterial. Some other commonly listed natural polymers investigated for this purpose include polysaccharides (chitin, chitosan, hyaluronic acid, etc.), silk fibroin, fibrin, alginate, gelatin, fibronectin, etc. They have been found to be extremely effective in tissue engineering, as they have the capacity to be restructured in in vivo conditions. Natural polymeric biomaterials have enormous potential to form scaffolds that retain the extracellular matrix composition of the host tissue. Although natural polymers demonstrate outstanding bioactivity and biodegradability for their abundant use in soft tissue engineering, their poor mechanical strength inhibits them for load-bearing applications. In addition, natural polymers also have insufficient usage where absolute support to injured or healing tissues is a requisite, owing to their fast degradation rate. Keeping these factors in mind, cross-linking of these materials is compulsory in the fabrication of long-standing tissue supports. Cross-linking is the process of attaching one polymer chain to another. Regardless of the inherent structural advantage arising from cross-linking strategies, several cross-linking elements can modify tissues by means of several methods when scaffolds are less expected to be populated by native cells. Lastly, biomaterials based on natural polymers also have the tendency to create xenogenic difficulties because the majority of these result from animal components.

3.2 Synthetic polymers Synthetic polymers, as their name suggests, are produced in the laboratory or in industrial units in the course of a chain of chemical reactions from low molecular weight organic compounds. Numerous synthetic polymers have been tried to produce scaffolds and they are grossly separated into two groups: biodegradable and nonbiodegradeable.

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Biodegradable polymers include polystyrene, poly-L-lactic acid (PLLA), polyglycolic acid (PGA), polylactide and its copolymer poly-lactic-co-glycolic acid (PLGA), polyphosphazene, polyanhydride, poly(propylene fumarate), polycaprolactone, polyurethane etc. Among the widely used nonbiodegradeable polymers are polyvinyl alcohol (PVA), polyhydroxyethymethacrylate (pHEMA), poly (N-isopropylacrylamide) (PNIPA), polymethyl methacrylate (PMMA), etc. The main benefit of this form of polymeric scaffold substitute arises from its controlled and superior physicochemical properties (e.g., porosity, degradability, and mechanical strength), lack of immunogenicity, and ease and abundance of processing owing to various artificial synthesis techniques. On the other hand, there is a possibility of denial of the artificial polymeric scaffolds from target tissues due to their inferior bioactivity. It has been found that acidic byproducts are readily formed while using synthetic polymer scaffolds during the entire degradation process, which subsequently leads to lowering the local pH. This eventually leads to its diminishing strength and results in cell-tissue necrosis.

3.3 Bioceramics Bioceramics are a special type of inorganic and nonmetallic ceramic and could be of both natural or synthetic origin. They are primarily aimed at the restoration and regeneration of affected parts arising from injury or trauma in the musculoskeletal structure as well as for periodontal irregularities. Even though soft tissue regeneration is not vastly promoted by bioceramics, there has been extensive use of these ceramic materials/scaffolds for loadbearing orthopedic applications (hip acetabular cups coatings), bone grafts/cements, and in dentistry [24]. Bioceramics are usually characterized by their high mechanical stiffness (Young’s modulus), corrosion resistance, and a hard and wear-resistant surface. Moreover, from a bone tissue point of view, they exhibit excellent osteoconductivity and biocompatibility, which arises due to the chemical and structural resemblance to the mineral phase of bone or osseous tissue. It is worth mentioning here that the mutual interactions between osteogenic cells and bioceramics are vital for bone regeneration because ceramics are identified as augmenting osteoblast differentiation and proliferation [25,26]. In spite of these numerous advantages, they suffer from severe drawbacks, including inadequate fracture toughness (leading to brittleness), low elastic properties, and exceptionally high stiffness [27]. All these actuate the limited clinical applications of these materials for tissue engineering (TE). Calcium phosphate bioceramics offer a special interest in tissue engineering practices owing to their close likeness with bone and teeth, which arises from the chemical similarity with these hard mammalian tissues. CaP-based bioceramics exhibit excellent biological performance, including osteoconductivity and bioresorbability. The material properties thus assist in integration into existing tissues by a similar process found in bone remodeling. In addition, calcium phosphate-based

Tissue engineering based on 3D and 4D printing

materials are cost effective from a fabrication point of view, and their medical grade certification is simple to achieve. Besides, the success of CaP-based bioceramics is also achieved in some extent to skin and muscle tissue replacement. However, these special categories of bioceramics suffer from poor mechanical properties such as strength, fracture toughness, and fatigue resistance, required for load-bearing applications in the biomedical field. Among the calcium phosphate-based bioceramics, the major focus has been applied to hydroxyapatite (HA), α- and β-TCP, and biphasic CaPs in the biomedical field concerning hard tissue, owing to their structural similarity with implants and bone defects [28]. HA [hexagonal, stoichiometric Ca/P ratio of 1.67 Ca10(PO4)6(OH)2] is crystalline in nature and is a very stable compound with negligible solubility among CaPs when tried in a solution below pH 4.2 [28]. Contrary to that, β-TCP is a high-temperature entity of calcium phosphate compounds that is derived by thermal breakdown at temperatures at least above 800°C. The biodegradable nature of β-TCP has often been tried in bone substitute applications in the form of granules, blocks, or in CaP-based bone cements [29]. Researchers have found that the biological absorption capability of both these CaP-based ceramics substantially differs, even if these species are analogous in terms of chemical composition. Hydroxyapatite shows slow bioresorption dynamics and hence, mostly integrates itself into the newly formed bone tissue once the implantation is done. On the contrary, β-TCP is entirely reabsorbed [30] in the bone tissue.

3.4 Metal-based scaffold materials In situations where human bones are damaged or need to be substituted, porous metallic scaffolds have been a popular material of choice, owing to their outstanding physical properties in addition to their ability to support tissue ingrowth. Out of the numerous alloys available, titanium (Ti) and tantalum (Ta)-based metals and alloys have been proven to be the best candidate material for this purpose. Medical-grade Ti alloys perform much better in bone tissue ingrowth capability when compared to stainless steel due to the 50% greater strength/weight ratio of the former. Its elastic modulus (105 GPa) is similar to that of human cortical bone (7–21 GPa). The metal/alloys also exhibit excellent corrosion resistance [31]. In addition to cast Ti alloys, porous titanium (Ti) scaffolds have also been studied as bone replacement materials. These components of biomaterials are not biodegradable and do not assimilate with biomolecules. Among the Ti-based alloys, Ti-6Al-4V in specific is broadly used in a variety of orthopedic applications because it enjoys excellent biocompatibility and improved mechanical properties over conventional stainless steel, Co-based alloys, and even pure titanium. Recently a 15-year-old boy diagnosed with cancer received a perfect implant manufactured by the German 3D printing technology giant, EOS Technology. The entire

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Fig. 2 Titanium implant fabricated by 3D printing technology for partial replacement in hip joints [32]. (Copyright 2019. Reproduced with permission from Electro optical systems (EOS) GmbH. https://www.eos. info/press/case_study/additive_manufactured_hip_implant.)

process of hip replacement, starting from the initial CAD design to the optimum implant, required only six weeks [32]. Fig. 2 shows the 3D-printed prototype structure fabricated by EOS Technology. Although several advantages have been recorded in this development, the element vanadium in the Ti-6Al-4V based alloy in isolated form still possesses cytotoxic effects, which has motivated researchers to develop novel β-Ti alloys with nontoxic elements such as Nb, Zr, and Ta [33]. On the other hand, porous metallic scaffolds are considered to be the desired material for implants in conjunction with hard tissue engineering in load-bearing applications. Their superior fatigue resistance [34] in addition to favorable compressive strength are found to be extremely effective for load-bearing applications such as the femur, vertebra, skull, and hip and knee joint replacements. These porous configurations also show similar and consistent mechanical properties to that of human bone. In addition, they assist in improving osteoblast adhesion, proliferation, and differentiation [35]. In a different architectural approach, Xue et al. fabricated Ti scaffolds with porosity ranging from 17 to 58 vol.% and with average pore sizes of 800 μm [35]. On a different note, the surface modification of Ti and its alloys has been revealed to enhance osteoconductivity, as demonstrated by Das et al. [36]. Titanium dioxide (TiO2) nanotubes have been grown onto porous Ti scaffolds via a chemical method called anodization. The overall advantage is to increase the apatite formation tendency of these scaffolds in simulated body fluid (SBF). In spite of these above-mentioned advantages, metallic scaffolds suffer from the following limitations: (i) poor biological response on the material surface or bioactivity is

Tissue engineering based on 3D and 4D printing

by far the major weakness of metallic scaffolds, (ii) biomolecules cannot be chemically integrated within the metallic scaffolds, (iii) in general, the biodegradability of metallic scaffolds is extremely negligible or not observed at all, (iv) the possibility of slow discharge of toxic metal ions/particles naturally or through corrosion or wear is another severe concern, and (v) it is not easy to hold/sustain the architecture of a porous metallic scaffold. Apart from the permanent bio-implanted metals, biodegradable metals for implant applications have shown potential for fracture fixation where entire tissue regeneration is likely. Presently, allows based on iron (Fe), magnesium (Mg), and zinc (Zn) are found to be the ones best suitable as biodegradable metals, especially for orthopedic and cardiovascular applications [37], because they exhibit excellent in vivo biocompatibility, a lesser biodegradation profile, and adequate mechanical properties to mimic bone for the period of regeneration. In principle, bioresorbable metals/alloys offer mechanical properties as compared to bioresorbable polymers such as polylactide (PLA), polyglycolide (PGA), or the polylactic-glycolic acid (PLGA) copolymer, owing to their brittleness and strength comparable to that of implants [37,38].

3.5 Biocomposites The above-mentioned single-phase scaffolds/biomaterials suffer from various practical difficulties that limit their applications as advanced biomaterials where the host tissue should be partially or fully replaced in a damaged or diseased organ. Therefore, significant research is being dedicated toward biocomposite scaffolds that consist of numerous fillers or reinforced materials in the matrix phase in the nano- and microscale as an alternative and viable solution. For example, in a particular case, fibrous-like biomimetic scaffold structures are of growing interest, both in the field of advanced soft-tissue engineering and also for state-of-the-art bone tissue engineering applications. In each of the asprepared biocomposite scaffolds, they are formed by at least one part that is not a naturally occurring component of our body. However, they suffer from the usual biomedical problems associated with biocompatibility, biodegradability, or both. Several materials have been studied for the fabrication of 3D biocomposite scaffolds for tissue engineering. The primary characteristics for the composite scaffolds include slow degradation and favorable cell biocompatibility as well as noncytotoxicity, nonantigenic, nonimmunogenic, and nonmutagenic actions. It is observed that about 30% of the presently used biomaterials are of the composite category [39]. Among the various combinations, polymers, when mixed with ceramics, result in the most popular variety of composite scaffold. Polymeric scaffolds show insufficient mechanical properties, for example, tensile strength, elastic modulus, and toughness. Inferior bioactivity is another crucial issue often found in polymer scaffolds. These attributes can be removed by controlled inclusion of ceramic material fillers, such as hydroxyapatite (HA) or tricalcium phosphate

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(α- or β-TCP). The synergistic association of softer polymeric material with stronger and biomimetic ceramic accelerates the regeneration of tissues [40]. In a list of naturally occurring polymers commonly applicable in tissue engineering dealing with hard tissues such as bone, collagen is one of the most extensively used scaffold materials. Because the human bone matrix is 90%–95% composed of elastic collagen fibers, it is a natural alternative for fabrication in a composite bone tissue scaffold. A collagen-HA-based composite scaffold has been fabricated by Villa et al. via a simple coprecipitation and freeze-casting method [41]. The composite scaffold exhibits a large extent of permeability appropriate for cell infiltration, attachment, and osteogenesis arising from its 99% interconnective pore network. Interestingly, the study found that while the in vivo testing is done for three weeks for the scaffolds implanted into a mouse calvarial defect, the defects are found to be near complete filling when compared to pure HA. On the contrary, pure HA scaffolds have not shown any positive outcomes and eventually decayed six years after being placed inside four ailing patients suffering from long bone defects [42]. Apart from collagen, some other commonly used natural polymers such as chitosan [43], chitin [44], alginate [45], and silk [46] have been successfully combined with HA for replacements in bone tissue engineering. Nevertheless, the difficulty that arises from using these natural polymeric materials is their poor mechanical properties [47]. As a result, researchers have made numerous attempts to avoid the potential weaknesses of natural polymers concerning mechanical strength by combining HA with biodegradable synthetic polymers, for instance poly (lactic acid) [48], poly (ε-caprolactone) (PCL) [49], poly (lacticco-glycolic acid) [47], and poly (D,L-lactide) [50]. Zhang et al. [51] showed the osteoconductive characteristics and cell maturation behavior of nano-HA/PCL spiral scaffolds that were synthesized with different HA/PCL weight percentages with the introduction of a controlled amount of porosity. They found that among the diverse nano-HA/PCL spiral scaffolds, the optimal HA/PCL compositional ratio is on the order of 1∶4 (weighted average) for bone tissue regeneration. Hermenean et al. [52] prepared novel 3D chitosan (CHT) scaffolds reinforced with graphene oxide (GO) to investigate osteogenic differentiation for regenerating bone tissue in critical-sized mouse calvarial defects. These composite materials show promise as implants by revealing more of the as-grown bone in the chitosan/GO-substituted defects in comparison to chitosan alone. In combination with GO, an increase in alkaline phosphatase activity equally in in vitro and in vivo trials has been observed by the addition of chitosan in these composite scaffolds.

4 Direct 3D-printing processes The 3D printing technique normally uses materials in various forms [53] under atmospheric conditions, such as fluids with the ability to solidify, flexible filaments, layered or laminated thin sheets, and small powder particles.

Tissue engineering based on 3D and 4D printing

A particular method utilizes a specific form of material to prepare the scaffold. However, if a particular material is prepared in a desired form for 3D printing, it does not ensure that the substance is 3D printable because in order to qualify for printing in the upright direction, it is also crucial to impart the bonding strength in the interlayer of the scaffold material. Consequently, the prime aspects while designing an object for 3D scaffold fabrication is based on the existing types of the material at the initial stages. Moreover, with the aim of adding to the collection of 3D printable bioscaffolds, novel and productive methods should be invented in the future to convert the present class of biomaterials into an appropriate form of feed material to be 3D printable. For instance, gelatin gel solidifies once its temperature is reduced, but working in this reduced temperature-based atmosphere does not favor the effective growth of cells. This paves the way for developing newer methods and mechanisms that involve the ease of solidification of gelatin, for instance enzymatic cross-linking [54], or a novel hybrid method for the growth of hydrogels and cells at much lower temperatures [55]. Rapid prototyping technology has evolved during the last 20–25 years, and consequently a range of diverse 3D printing and additive manufacturing techniques has been tried for medical applications. The most frequently used 3D/4D printing techniques include extrusion printing/bioprinting, stereolithography (SLA), powder deposition printing (FDM), laser-assisted printing (SLS or SLM), and direct ink writing or inkjet bioprinting (DIW, etc.). Fig. 3 shows the schematic representation of the operating principles of three major types of 3D printing techniques used in biomedical engineering. In addition, a comprehensive review stating the basic features with corresponding advantages/limitations and applications is provided in Table 1.

4.1 Stereolithography (SLA) The stereolithography (SLA) 3D printing technique is based on using photosensitive polymers (also called photopolymers) as the feedstock, thereby polymerizing the feedstock solution into a specified pattern. A UV light bulb or laser light is accurately projected onto this photopolymer and a controlled reaction is achieved by digital micromirrors. In this technique, a layer-by-layer process is adopted. Each successive layer must be entirely cured or dried before the second layer adjacent to it can be deposited to obtain the desired 3D printed object. Among the various additive manufacturing processes, stereolithography is one of the oldest techniques applied in bone tissue engineering. This sophisticated method allows very high precision of creating scaffolds for the fabrication of complex structures ranging from the micro- to the nanometer dimension. Other techniques such as extrusion-based processes deal with micrometer scales of higher orders of magnitude [68]. This technique offers the unique advantage of producing sophisticated scaffold architectures with internal complexities and exceptionally high resolution (1.2 microns) [68]. In addition, the high quality of printing, the promptness, and the cell proliferation and differentiation are

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Fig. 3 Schematic representation of three major types of 3D/4D printing technologies: (A) stereolithography (SLA), (B) fused deposition modeling (FDM), and (C) selective laser sintering (SLS).

added advantages that have made the SLA technique a well-accepted process to fabricate bone tissue engineered structures. When a 3D object is constructed by the SLA-based 3D printing technology, additional measures must be adopted to enhance the mechanical properties of the final replaceable structure as per the specific host organ requirement. Therefore, polishing and eliminating the undesirable scaffold structures that remain as an attachment must be performed precisely. The method is carefully designed by fabricating a multilayered 3D heterogeneous architecture with graded mechanical and biocompatible features by controlling the premeasured solution that is accessible to UV or other forms of light from one layer to another. It is thus possible to construct a 3D scaffold extending in both the perpendicular and horizontal directions [69,70].

Table 1 A brief review of common 3D printing techniques for biomedical applications Technique

Advantage

Disadvantage

References

Inkjet printing

• Vast range of biomaterials can be printed • Structural complexities do not require

• Potentially toxic • Low mechanical strength compared

[56]

any additional support

• High concentration of cells included in • • • • • Direct ink writing (DIW)

• • • •

the scaffold Significantly low cost Superior printing speed Creation of composition gradient is easy Multiple solution compositions can be coprinted Bioactive composites can be simultaneously printed Low viscosity material can be printed Use of hydrogels is easily realized Simplicity Multiple inks can be utilized

• • • • • • • • •

Bioplotting

• Prints viable cells • Soft tissue applications

• •

Fused deposition modeling (FDM)

• Lower cytotoxicity content than direct

•

3D printing • Relatively inexpensive

• • • •

[57]

[56]

[58]

Continued

Tissue engineering based on 3D and 4D printing

•

to SLS Time-taking process High setup cost Limited material choice As-printed cells may be affected by piezoelectric printers Continuous printing process is not feasible Vertical structures exhibit inferior activity Cell density is lower Not very suitable for complex operations Combination of thickening and thinning agents in bioink is too crucial Desired microstructure is difficult to be realized Nozzle size limitation Requires support structure for complex designs Limited material processing (often requires thermoplastics) Nonbiodegradable material used Additional support structure should be provided for shape complexity Postprocessing required Little resolution

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Technique

Advantage

Disadvantage

References

Selective laser sintering

• Scaffolds with enhanced mechanical

• It must resist shrinkage and heat

[59,60]

• • • Stereo lithography

• • • • • • • • •

Laser-assisted bioprinting (LAB)

property produced Complex structure can be incorporated with the help of a powder bed Fine and high resolution Biomaterial deposition in solid or liquid phase Very high resolution Fast fabrication Smooth surface finish Complicated internal architectures can be printed easily Can easily print components that are released in an outward direction Shear forces are excluded on print stock Printing time independent of complexity High accuracy Nozzle-free technique

• High-precision printing at ambient conditions • Single cell patterns within scaffold • Diverse bioactive materials are used. • Several different solutions printed at a time

• • • • • • • • • • • • • • •

effects. Very high temp required (up to 1400° C) Expensive and time consuming Thermal damage can happen during processing Photopolymers are generally used Expensive External assistance is required for overhang and objects with complicated design Involves photocross-linkable polymer Poor mechanical strength Limited resolution Mechanical properties that are horizontally graded are inconvenient to produce UV blue light is toxic to cells Deficiency of printing multicells Damages cell during photo curing Costly Limited height of scaffolds

[61]

[56]

3D and 4D printing of polymer nanocomposite materials

Table 1 A brief review of common 3D printing techniques for biomedical applications—cont’d

Powder fusion printing (PFP)

Extrusion printing

• A vast range of materials (metals, polymers, etc.) • Excellent mechanical strength • Complex geometries can be printed • Properties vary in vertical directions

• Precise control over printing conditions • Wide variety of materials • Can print physical and compositional gradients

• Can directly print cells and bioactive factors Vat photopolymerization

• • • • •

can be used Bioactive materials can be incorporated High resolution Cells can be incorporated Raw material base is of solid polymers High resolution

• • •

powder microstructure Horizontal property gradients are inconvenient to produce Multiple fusion steps can generate cracks Build time, material usage, and other factors could increase to provide support to overhanging components Cannot print single cell Only applicable for viscous liquids Slow speed

• • • • •

Limited materials UV source necessary Near UV blue light’s toxicity Damage to cells during photocuring Lack of multicomponent cells

• • •

• High cost • Thermal damage due to high tem-

[60]

[62,63]

[64–67]

[64–67]

perature during deposition Sheet lamination

Indirect 3D printing

• Layered laminate structure is formed • HA (hydroxyapatite), zirconia, osteoblast-like cell, human osteoprogenitor cell, and human umbilical vein endothelial cell can be formed • Shows promises for prototyping/ preproduction • Versatile materials

• Only layered laminates are formed • Requires further processing

[64–67]

• Dedicated waxes are prime requisites

[56]

for biocompatibility • Poor accuracies/resolution • Mold should be provided for casting • Fabrication time is more

Tissue engineering based on 3D and 4D printing

Directed energy deposition

• Ability to print high cell densities • Raw material base is a fluid material • Huge range of photocurable polymers

• Microfractures/voids arise from

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Earlier, this process of fabricating a layered and compositionally variable scaffold structure for a specific requirement was an uphill task for researchers and engineers. This was because the pouring solution basin must be unfilled first and then restocked or substituted with another solution in each successive layer. This dilemma has been successfully overcome by separate groups of researchers [69,71,72]. These groups of scientists have discovered a novel design approach in the SLA instrument by providing an automatic replacing unit that potentially permits layered and heterogeneous biomaterials for scaffold fabrication. However, several weaknesses have been uncovered following the application of this technique. One of the major drawbacks arises from the application of resins that are found to exhibit carcinogenic effects and could therefore be life threatening. Another common problem is that the resins are photosensitive in nature, therefore leads long term stability of the as-fabricated scaffold structure. The extremely slow production rate (1–3 cmh
1) of the 3D printed object is yet another challenging issue in SLA techniques [73]. The commonly used ultraviolet source of light for the polymerization process creates another threat because reports show that this light source damages our DNA cells and could be a potential reason for skin cancer [74,75]. To resolve this matter, visible light has been replaced as an alternative source of light in SLA-based bioprinting systems found substantial acceptance in this field. Recently a group of researchers at the University of British Columbia has successfully demonstrated a custom-made bioprinting unit comprised of a beam projector with combinations of PEGDA, GelMA, and erosin Y made photoinitiator as the injected bioink [76]. Poly(D,L-lactide)-based resin or a poly(D,L-lactide-co-3caprolactone)-based resin has been used as the raw material to fabricate porous scaffolds. The scaffold object’s mechanical properties and stability can be optimized by controlling the pore architecture and polymer compositions. A photocross-linkable PCL-based resin has been successfully fabricated by Elomaa et al. with the help of high gel-containing networks [77]. Similarly, porous scaffolds have been developed by using the resin, inhibitor, and dye. This type of as-fabricated scaffold conforms well to the targeted object to be used and demonstrates the acceptability of the resin for construction of tissue engineering scaffolds. In recent years, a considerable number of novel and promising biodegradable resins have been increasingly used. In this aspect, various materials have been developed as promising alternatives for application in the SLA process, and the list includes poly (caprolactone) [77], poly(D,L-lactide) (PDLLA) [78,79], and poly (propylene fumarate)-diethyl fumarate (PPF-DEF) [80]. In another attempt, Winder et al. [81] developed a self-modulated cranial titanium (Ti) prosthesis directly synthesized by SLA-based resin, thus making the process amenable for use on an industrial scale. In general, the SLA technique is used for tissue engineering applications related to blood vessels, cartilage, muscle-neuron coculture, etc. From futuristic point of view it is essential to develop various novel biocompatible and biodegradable photocurable polymers for successful implementation of this

Tissue engineering based on 3D and 4D printing

technique. At the same time, in order to include the list of polymeric materials, it is crucial to design and develop visible light-based STA systems in the near future.

4.2 Microextrusion-based 3D bioprinting Extrusion-based techniques have been extensively tried as an alternative method for scaffold fabrication, owing to their simplicity, diversity, and predictability. This extrusion-based method makes use of mechanical (piston or screw) or pneumatic forces to supply bioink by means of a nozzle or needles connected to cartridges loaded with ink. The subsequent micropatterning is followed by a computer-generated design. Here, filaments of the organic biomaterial are dropped layer by layer from a μ-extrusion top in 2D, and the stage or the μ-extrusion head shifts down the z axis. In the μ-extrusion printer, several cartridges can be attached to the instrument for printing of heterogeneous structures. Here, cells are mixed together with bioink. Bioink is the material that is used to envelop cells to allow an environment to extracellular matrix (ECM) and protect cells from the disturbances that are experienced by a cell in the course of printing. Inkjet printing is broadly used for 3D printing of cell-laden assembles because it can afford excellent cell viability in contrast to μ-extrusion printing. Nevertheless, the printing of viscous or sticky bioinks is reasonably difficult in most polymeric ensembles. This motivates researchers to utilize μ-extrusion printing to print glutinous bioinks. The viscosity observed in popularly used bioinks printable via μ-extrusion printing falls in the range of 30–6 107 mPas [82]. In order to meet the complexity requirements of various internal organs of our anatomy with substantially high resolution, bioinks should display a shear thinning ability to perform microextrusion via a needle. Also, rapid gelling characteristics should also be experienced to permit a layered deposition and preserve the printed profile [83,84]. On the other hand, the improvement in gelling property and the viscoelastic propensity of the bioink-based materials might contribute to the shear stress increment. This would then heavily affect the cells in the typical μ-extrusion-based printing process. All these factors contribute to the premature death of the targeted cells and damage the μ-extrusion-based cell printing practice [82]. Keeping these factors in mind, it is imperative to understand that a favorable range of bioink viscoelasticity is by far a crucial factor to estimate while fabricating through the μ-extrusion-based printing process. In this aspect, Markstedt and coworkers investigated the rheological functions and printing capability of nanocellulose-alginate-based composite bioink in conjunction with human chondrocytes. They found that the survival rate of these cells was 73% [85]. In another attempt, Kesti et al. studied a combination of poly(N-isopropylacrylamide), hyaluronan, and methacrylated hyaluronan as the prospective bioink material that is entirely derived from its rheological properties, swelling activities, printability, and prior biocompatibility results [86]. Another group of researchers [87] demonstrated the use of polypeptide-DNA hydrogel-based multilayer 3D μ-extrusion-based printing for studying its healing properties and superior mechanical strength as a new-age bioink substitute material.

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The μ-extrusion-based bioprinting tools have been successfully used to create heterogeneous scaffolds required for osteochondral regeneration. Recently, Esfahani et al. [88] applied this method of printing to produce glass-ceramic scaffolds that had adequate strength and toughness for curing of bone defects under a static load. The hexagonal interconnected porous architecture allows the formulation of a widespread surface near or within the vicinity of the contacts surrounded by printed layers and a superior load transfer potential while evaluating with other usual patterns (rectangular, zigzag, or arched). In addition, the state-of-the-art design established superior fatigue resistance, failure reliability, and flexural strength under in vivo conditions. This method of scaffold preparation for hard tissues might create opportunities for handling bone defects in orthopedics in relation to load-bearing applications as well as dental and maxillofacial replacements. In μ-extrusion-based bioprinting, the bioinks fall in both the Newtonian and non-Newtonian categories and have been vastly modified for enhanced printable viscosity. In the case of non-Newtonian fluids, the printable viscosity of the bioinks can be optimized by the strain rate variation during the entire process of printing, which also relies on its concentration and molecular weight. Besides the shape of the orifice, its size and the active temperature (in case of temperature-sensitive bioinks) also affect the strain rate during the entire printing process. Bioinks are typically controlled by shear thinning effects and the viscosity reduces with the enhancement in strain rate and acts to safeguard cells in addition to improving its resolution. The process of shear thinning restricts the entanglement of chains consisting of organic molecules arising from the sliding of chains over one another. This further helps in the smooth extrusion of viscous bioink through the opening of the nozzle. Based on this phenomenon, researchers worldwide have tried to improve the shear thinning properties of viscous bioinks used in μ-extrusion-based bioprinting [89]. While evaluating with inkjet bioprinting, μ-extrusion-based bioprinting provides denser cell growth but at the loss of speed and resolution [90]. In addition, a broad range of biomaterials can be printed with the help of this relatively cost-effective bioprinting instrument, including tissue spheroids, tissue strands, cell pellets, decellularized matrix parts, and cell-laden hydrogels. In a simple but practical alternative, many researchers have tailored the available commercial 3D printing units to print scaffold objects or have fabricated their own printing equipment internally to ease costs [91–93].

4.3 Inkjet based 3D-printing Inkjet printing is an extremely popular bioprinting technique used these days. 3D printing via inkjet is sometimes considered an alternative process for sterolithography [94]. However, the 3D bioprinting via inkjet method has found restricted applications when matched to extrusion-based processes. The prime reason behind this is the printing head moves

Tissue engineering based on 3D and 4D printing

to supply a continuous flow of the printing solution, which confines its relevance in bioprinting. Bioprinting is a type of additive manufacturing process. Moreover, it is an advanced technique to biofabricate 3D structures for functional tissues by arranging cellular components in a 3D space so as to mimic the human tissue function. This process can efficiently fabricate 3D structures as biomaterial scaffolds for tissue regeneration by embedding cells over the 3D-designed scaffold that relies on the computer-aided design model and the process having advantages as it uses bioink by encapsulating cells to prepare the 3D structure [95]. 3D inkjet printing is known for its ability to construct intricate scaffolds in various clinical sizes for biological applications. Furthermore, the developed products are mobile and easy to carry. Synthetic polymers for poly(e-caprolactone)PCL, polyglycolic acid, poly(lactic acid), etc., are mostly used as the biological ink [96]. Walczak et al. first designed and fabricated a microfluidic channel chip for capillary gel electrophoresis. The time taken to construct the biochip was around 3 h, which is the beauty of an inkjet printing process as compared to a conventional process [97]. Krivec et al. constructed a 3D printed prototype package with the adoption of a photopolymer. Additionally, they incorporated silver nanoparticle ink for radio-frequency identification to make better wireless radar signaling [98]. Kyobula et al. used a 3D thermal inkjet printing technique that is solvent-free to make a drug release carrier through natural derivatives. The architecture of the manufactured drug was designed in a honeycomb structure with a controlled cell size. The intention of the writers was to develop drug release tablets through a 3D inkjet printing technique by using beeswax in the shape of the honeycomb structure, which could then be clinically used in a solid dosage form [99]. Zhang et al. applied an inkjet (ij) 3D printing method to develop hydroxyapatite-based scaffolds along with controlled porosity in the micro/macro level. Moreover, mechanical properties in the form of the compressive strength of the 3D-printed bioceramic scaffolds with different diameters of HA nanopowders have been shown [100]. Phillippi et al. developed pattern growth factors by using inkjet printing. In their study, the authors used adult stem cells to modify the cell fate by stem cell differentiation. The biological approach has been applied to osteogenic BMP-2 by immobilizing growth factor. In regenerative medicine, it has the ultimate potential application [101]. Gao et al. developed a bioprinted scaffold and showed excellent biocompatibility. The bioink was composed of poly (ethylene glycol) and PBS, followed by mixing with human mesenchymal stem cells (hMSCs). The 3D inkjet printed polymer scaffold resulted in good cell viability with superior mechanical properties. Moreover, the construction time was reduced because of less process complexity. Hewes et al. developed microvessels with bioink comprising fibrinogen and alginate to fabricate freestanding microvessels through an inkjet 3D printer. The alginate-based solution has been used to bioprint those microvessels. With the use of the alginate template method, it was possible to load cells within the fibrin scaffold via the 3D inkjet printing method [102]. According to Cui et al. [103], there might be a chance of damaging of cells in inkjet

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3D printing process. The authors also have studied 3D printed Chinese hamster ovary (CHO) cells and evaluated cell viability. In addition to that, the authors have checked a possible number of the damaged cell membrane and was found this thermal inkjet printing method is an effective and reliable way to print mammalian cells [103]. Owczarczak et al. [104] used the application of a thermal inkjet printer to incorporate g-actin monomers into 3T3 fibroblast cells. The technique used by them does not harm cells and injected cells together with molecules. Moreover, the method exhibits superior cell viability. Inzana et al. optimized the mechanical property as well as the biocompatibility with the aid of an inkjet 3D-printing approach. The fabrication of collagen-calcium phosphates was optimized by blending a phosphoric acid binder solution with collagen. This enhanced the cell compatibility for better bone regeneration [105]. Xu et al. fabricated a 3D cellular structure in combination with fibrin and NT2 cells with a layer-by-layer approach to form a neural cell sheet, which has a wide function in tissue engineering [106].

4.4 Fused deposition modeling (FDM) FDM is one of the most widely practiced filament extrusion-based additive manufacturing processes to develop low cost complex 3D objects for bioengineering applications. This technique is also alternatively regarded as the fused filament fabrication (FFF) process and was first available for commercial used in 1991 [107]. In this technique, a thermoplastic filament or a small bead polymer is extruded from a small nozzle in a movable platform at a measured rate. An extra heater is used in the nozzle to first melt or soften the filament. A fan is provided as an extra attachment at the nozzle end for controlling the solidification and subsequent deposition rate of the as-fabricated solid 3D object in a layer-by-layer manner. The process parameters that play crucial roles in the successful fabrication of the 3D construct with predetermined pore size, porous network morphology, and internal linkages include raster diameter, gap width, the angle of the raster, etc. [108]. All these factors make FDM a simple process for fabricating 3D complex structures as compared to other traditional techniques with a much more complex approach, such as lithography and micromachining. However, FDM suffers from some critical issues such as a longer operation time and lower resolution as compared to other commonly used 3D printing techniques. Moreover, the application of a higher deposition temperature for thermoplastic materials, in a range varying from 120°C to 300°C, is not comfortable for inserting cells or drugs within the filament while preparing the 3D scaffolds. In addition, this method of 3D printing for scaffold fabrication allows only a limited range of thermoplastic polymers such as polylactic acid (PLA), polycapro-lactone (PCL), poly (lactic-co-glycolic acid) (PLGA), acrylonitrile butadiene styrene (ABS), and polylactic acid (PLA) for commercial applications. Some of these polymeric substances are frequently used in the FDM process in combination with biomaterials to produce tissue-engineered scaffolds with a low melting point.

Tissue engineering based on 3D and 4D printing

Among these, PCL is the best material of choice that has been extensively used in dental applications and wound repair because it offers high printability and biocompatibility with replaceable tissues. For the fabrication of scaffolds in bone regenerationbased applications, FDM has been found to be very effective, owing to its favorable mechanical strength and physicochemical properties that closely match human bone structure [109–111]. The mechanical and biomedical performances of natural or synthetic biopolymers are modified by doping biocompatible reinforced units such as HA and β-tricalcium phosphate (β-TCP) [112] or by simple coating [113,114]. Korpela et al. developed PCL/bioactive glass (BG)-based composite scaffolds by this method. They demonstrate superior properties equally in compressive modulus and the required biocompatibility [115]. Oladapo et al. [116] developed a new hybrid scaffold structure with a poly lactic acid (PLA) matrix supplemented with carbohydrate particles (cHA) in different proportions (PLA:cHA ¼ 10/0, 95/5, 90/10, and 80/20) for replacement in bone tissue. The hybrid composite mixed with an 80:20 proportion offers better results than the others. The pore size variation and porosity distribution network of the 3D-printed complex architectural scaffolds created by computer-aided design and subsequent layer-by-layer printing and solidification are shown in Fig. 4.

Fig. 4 Schematic illustration of bone formation by the PLA/cHA-based hybrid scaffold by the FDM technique, (A) optical images of a two-layer scaffold produced by 3D printing, (B) 3D structure showing tetragonal symmetry in successive layers, (C) radial architecture of the 3D-printed structure with different layers filled in concentric patterns, (D) SEM image of 3D-printed scaffolds with consistent 1 mm fiber spacing [116]. (Copyright 2019. Reproduced with permission from Elsevier. https://doi.org/10.1016/j.compositesb.2018.09.065.)

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4.5 Selective laser sintering (SLS) SLS is a widely used laser-assisted additive manufacturing (LAAM) technique developed by Carl Deckard in the 1980s at the University of Texas and subsequently patented in 1989. Selective laser sintering (SLS) employs a high-power CO2 laser to melt thin layers of polymer or ceramic powders to construct structures and replicas for parts from 3D CAD models, 3D digitizing system-acquired data, CT scans, and MRI scan records. The anticipated layers of deposited material are chemically bonded mutually throughout the SLS process to produce a specific shape, as depicted by the CAD-based 3D model. In SLS, the powder deposition system usually contains a set of rollers or a scraper that permits the instrument to deposit consecutive powders layer by layer with a thickness range of 20–150 μm, forming solid 3D objects. The entire fabrication operation in the SLS process is most often performed under an inert atmosphere (e.g., Ar, N2) to produce a contamination-free 3D object and also to avoid the unwanted oxidation of feed granular powder particles during the process. SLS often suffers from the deposition of layered objects with low-grade surface contours and substantial fluctuations in various dimensions (X, Y, or Z) of the as-fabricated parts. Therefore, as a secondary alternative, costly postprocessing surface treatments such as machining, heat treatment, polishing, etc. [117] are required. Like any other AM process, here also the final phase aggregate and porous microstructures of the composite scaffolds can be determined by controlling the essential processing parameters, including the laser power, the temperature of the platform, the laser scan speed, etc. [118]. Additionally, the size and shape of the powder granules are identified as deciding factors in the SLS process [119]. These parameters have shown their efficiency on the densification behavior and flowability of powder granules. The flowness of the powder particles is an important aspect because these powders must be dispersed evenly at elevated temperatures and with a thickness of 100 μm. All these factors lead to the fact that the powder particles used in SLS should possess a specific granulometry and a superior sphericity. In SLS, to fabricate the 3D structure with interlayer compositional differences, the successive base composed of the powder particles must be cleaned and a new biomaterial substance should be added. This deposition method is slow and time consuming while also contaminating the new surface layer. Heterogeneous mixing of the powder particles creates associated problems with the as-fabricated scaffolds. Some researchers [120] have revealed an alternative approach by presetting the powder layers and physically adding each deposit one at a time. Their study is successful for the synthesis of PCL- and PCL/Hap-based combinations of scaffolds with varying layers of stoichiometric HAp in order to resemble the graded configuration of the osteochondral unit. The SLS process has been successfully used to fabricate biomaterial scaffolds in various forms, including biopolymers, bioceramics, biocomposites, and even metallic components, for potential tissue regeneration applications. It is already established that biocompatibility, bioactivity, and biodegradability are essential properties for the

Tissue engineering based on 3D and 4D printing

development of tissue engineered scaffold constructs. However, the available materials that include the above properties are restricted enough for this purpose. Nevertheless, research efforts have been made for TE 3D scaffolds for biomedical applications. Some examples that have been successfully used in the SLS process to create complex structures in bone and other hard tissue engineered scaffolds include poly(ε) caprolactone (PCL) [121,122], poly-L-lactide (PLLA) [123], poly-D-lactide (PDLA) [124], polyhydroxybutyrate (PHB) [125], and poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) [123]. On the other hand, nonbiodegradable polymers such as polyetheretherketone (PEEK) [126] and ultrahigh molecular weight polyethylene (UHMWPE) [127] have been effectively transformed in sintered 3D scaffolds; these have been investigated recently. Similarly, in order to obtain the advantages of both biodegradable polymers and bioactive ceramics, for example hydroxyapatite (HA) and β-tricalcium phosphate (β-TCP), in blended TE composites, HA/PCL, HA/poly(L-lactide-co-glycolide) (PLGA), and β-TCP/PLGA [128,129] have been fabricated by the SLS technique. Researchers have implanted nano-HA/PCL and PCL-based composite scaffolds developed by the SLS process into rabbit femur defects. Their study shows excellent biocompatibility and a positive indication of curing of bone defects [122]. In a recent attempt, researchers developed an advanced version of the SLS technique with superior features, known as surface selective laser sintering (SSLS), to resist biopolymer degradation and assist the absorption of cellular entities and bioactive agents [130,131].

5 Indirect 3D-printing processes As compared to the direct additive manufacturing techniques, 3D printing of sacrificial molds and indirect 3D printing are also versatile techniques that are now becoming well accepted for scaffold fabrication in tissue engineering. These approaches of 3D scaffold architecture design and fabrication have shown great promise for structures with uniform interconnectivity, desirable pore size, and multifaceted internal/external construction using a similar (compared to direct methods) and sophisticated CAD/CAM model. In this method of scaffold fabrication, the flexibility of scaffold fabrication is enhanced toward a broad range of biomaterials ranging from polymers to ceramics. In principle, this indirect 3D fabrication technique utilizes a sacrificial mold. The mold is a predesigned dummy structure that possesses the negative framework of the target scaffold architecture. The as-prepared mold is consequently used for solidifying the scaffold of the final biomaterial. This indirect 3D-printing method is applicable for the fabrication of both polymeric and ceramic scaffolds of various biomaterials. Ceramic scaffolds includes alumina [132], HA, etc., whereas polymeric scaffolds are fabricated by poly(ε-caprolactone) (PCL), poly(lactic-co-glycolic) acid (PLGA), poly(L-lactide), collagen, and chitosanalginates [133–136]. While fabricating a polymeric scaffold, indirect 3D printing

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techniques largely utilize a solvent-based design with temporary molds. The material to be injected in the mold cavity is prepared in the form of an organic solvent. This leads to an increase in scaffold preparation time as it requires more time for mixing the proposed material in the solvent and further to solidify it within the mold by solute-solvent reactions or thermal evaporation in ambience. From this standpoint, it must be mentioned that the nature of the organic solvent and the solution composition might influence the inner construction of the scaffold, for example, the size and morphology of the interconnected pore network. Consequently, these types of 3D structural/architectural anomalies arising from scaffold fabrication can result in unwanted mechanical properties and can lead to an undesirable effect on the mechanochemical performance of the tissue engineered configuration once the implantation is done in the patient. An indirect method of printing shows a unique advantage by creating a tailored scaffold that accurately replaces the host organ that is affected. In a study [137], scientists printed a customized scaffold that exactly matched with a human mandibular condyle. By using the indirect 3D printing technique, they produced a biomimetic scaffold with an appropriate combination of PCL and chitosan. The PCL was solution-treated in chloroform and subsequently a gelatin mold prepared from 3D printing was used to pour the solution. Upon drying, the 3D-printed gelatin mold was eliminated by inserting the scaffold in dH2O at 50°C for 6 h. Once the PCL scaffold was fabricated, a coating of thin apatite layer was applied to this scaffold in order to sustain the cell growth of the bone marrow stromal cells (BMSCs). In this way, the as-fabricated PCL apatite-coated scaffolds exhibited much better proliferation characteristics of BMSCs when compared with uncoated composite scaffolds. Natural polymers such as collagen or alginate have also been used for scaffold fabrication in the indirect 3D printing method. The prime advantage of these naturally occurring superior scaffolds arises from their potential ability to develop the hMSCs for a specific time (up to 4 weeks) in vitro with substantially lower or no cytotoxicity on the other grown cells [138].

6 4D printing for biomedical applications The fast development of AM technology and advances in 3D bioprinting to build complex biocompatible structures have motivated scientists and engineers all over the world to develop similar but more effective techniques in the fourth dimension. The revolution of 4D printing technology has been highly inspired by the development of 3D-printable “smart” materials. The scope of 4D printed objects was first successfully demonstrated by the MITs Self-Assembly Lab in 2013. Skylar Tibbits, the codirector of that lab, is also regarded as the pioneer of 4D printing technology [139]. In his 2013 TED Talk (TED stands for technology, entertainment, and design), Tibbits explained

Tissue engineering based on 3D and 4D printing

the phenomenon of shape transformation within the structural design of the material by self-motivation when exposed to a programmed stimulating factor, which includes osmotic pressure, heat effect, current, ultraviolet light, or other energy sources [139–142]. Earlier, it was found that one major weakness of 3D bioprinting is that it is primarily based on the initial condition of the object that is to be 3D converted, supposing that it is stationary and inert. In contrast to that, 4D bioprinting has materialized, where a “time” factor is incorporated with 3D bioprinting as the fourth or final dimension. However, in this classification, time does not indicates the actual time to finish the job, but rather it determines that the 3D-printed biocompatible scaffold or living cellular assembly can be made to “self-transform” over time after being printed. Fig. 5 shows the statistical documentation of publication on the topic of 4D printing or 3D printing + biomedical + FDM/SLS/Inkjet from the Scopus database from 2012 to January 2019.

Fig. 5 Statistical record (from 2012 to January 2019) of publications on the topic of 4D printing or 3D printing + biomedical + FDM/SLS/Inkjet etc. from the Scopus database: (A) publications, (B) publications in different countries/regions, and (C) list of total publications of various 3D printing techniques.

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Improving the functionality and/or performance-driven capabilities of the final tissue product [143,144] has always been a bottleneck in tissue engineering research. With the implementation of 3D printing of specially designed shape memory scaffolds, an enormous opportunity has been explored in regenerative medicine by the favorable integration of 3D printing and the shape memory effect that arises from the time-dependency factor. It is worth mentioning, therefore, that mechanical stimuli (e.g. strain pattern, perfusion) have a pronounced affect on restructuring specific cells [145,146]. These cells often respond to biological stimuli and eventually control cellular activities and the efficiencies of the regenerated tissue [147,148]. However, the popularly used biochemical devices in this application are complicated and costly while their range of application is insufficient for this purpose [149]. Also, materials that exhibit selfevolving structures that execute geometric folding, curling, expansion, and some other scheduled shape changes when given a mechanical stimulus, including shape memory polymers (SMPs), are being considered for this purpose [150]. A finite number of investigations have been executed by using shape memory polymers to show the consequence of a one-time mechanical stimulus on cells in contact with 2D sheets [150,151]. In the recent past, the world has seen immense growth in 4D printing. It is worth mentioning here that the requirement for the development of various soft active 3D printable biomaterials (SAMs), sophisticated 3D printing technologies that achieve fast fabrication of multiple active materials, and advanced computational methodologies to transform rational material models to envisage the 3D printed object for its futuristic expansion in designing novel biomedical devices and bio-inspired architectures, is very much essential. As already mentioned, 4D printing adds time to 3D printing and accordingly offers some additional benefits as compared to 3D. It is capable of developing intelligent instruments that assist in transforming the profile and functions of the smart as-printed structures of the bio-inspired materials. Second, the method helps to print thin-walled construction or lattice structures, and thus has the prospect to reduce the overall time of printing and materials. A recent study confirms that self-folding can speed up the rapid prototyping of 3D objects, possibly leading to a reduction of 60%–87% of printing time and expenditure of materials [152,153]. Apart from these notable advantages, the shape-changing phenomenon triggered by various stimuli can be wisely exploited to engage less space and transportation. For instance, shape memory polymers can be converted into a horizontal surface by 4D printing for simple posttreatment operations and moving as well for storage purposes. As far as the prospective materials are concerned, 4D printing technology uses a diverse range of materials aimed at developing devices/ objects such as smart devices, metamaterials, and origami for a variety of functional applications in the aerospace, biomedical, and biomedicine fields, among others [154,155]. These range from the micro- to the macroscale dimension. It has been found that while matching with smart metals and ceramics, smart polymeric materials can readily react upon exposure to diverse stimuli with substantial deformability [156]. Therefore, until now,

Tissue engineering based on 3D and 4D printing

4D printing has primarily focused on smart active polymer-based bio-inspired materials. The following two categories of materials are extensively studied as SAMs for 4D printing: shape memory polymers (SMPS) and hydrogels. In the following section, these two groups of materials, applied in 4D printing technology, will be discussed in detail.

6.1 Soft active shape memory polymers Shape memory polymers (SMPs) are emerging smart materials that have seen major advances in the past decades. This special class of intelligent materials possesses the distinctive capability to recover into a 3D printed stable “memorized” structure once exposed to an appropriate stimulus (mostly of thermal origin, although the chemical or electromagnetic type also affects). The shape memory phenomenon can be found in our everyday life, too. For example, heat shrinkable tubes, state-of-the-art medical parts, and self-deployable components in spaceships are some of the potential areas where the technology can be adopted. The shape memory effect in these special class polymers, as found, is not intrinsic, and the overall effect arises in combination with the polymeric material’s special shape memory property as well as with mathematical models to program the deformation. Initially, a thermoresponsive shape memory polymer is allowed to transform into a temporary shape and subsequently put beyond its transition temperature (Ttrans). Once the temperature is attained, the shape memory effects are produced and, consequently, a stable memorized structure is recovered [157]. In biomedical engineering, these materials exhibit enormous possibilities concerning a variety of applications, for example negligibly invasive implants (e.g., stents), self-knotting sutures, orthodontics, and drug delivery devices [158–160]. In comparison to metal/alloy-based or ceramic-based shape memory materials, polymer-based shape memory materials have numerous benefits that exceed the two above-mentioned types of materials. These include good shape recoverability (capable of 400% stain recovery), low density, processing flexibility, control of properties (e.g., Tg, stiffness, biodegradability, and functionally grading capacity), deformation in a controlled and programmed manner, and cost effectiveness. The above-mentioned features of a typical SMP are mentioned as follows: SMPs have the following advantages over conventional materials:  Possess excellent recovery strain.  Lower density.  Cost-effective construction. However, these materials suffer from the following drawbacks as well:  Modulus of elasticity is low in comparison to other classes of materials (metals, ceramics).  Inferior strength.

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SMPs show widespread significance in the field of biomaterials and bioinspiration [161,162]. Polyurethane-based shape memory polymers achieve outstanding biocompatibility. Therefore, this can be exploited in various clinical components when placed in the human body [163,164]. Wache et al. [165] recently completed a viability study of SMPSs on sustainability on a polymer vascular stent in the drug delivery system. Significant improvements in restenosis and thrombosis have been noticed while using the SMP stent in drug delivery systems. Likewise, a polycaprolactone dimethacrylate-based shape memory tracheal stent has been fabricated by the use of an UV-LED digital light processing (DLP) printer [166]. The UV-LED light source used in this study has a wavelength of 405 nm. Stereolithography uses a laser source to fabricate the layered objects. A number of photocurable methacrylate-based copolymer networks were synthesized and printed by means of high-resolution (μm range) projection microstereolithography (PμSL) [167]. One special category of SMPs belongs to the biodegradable-based polymers, which might be fabricated from a few monomers and show a lot of promise in this field [168,169]. A typical example is the poly(ε-caprolactone) (PCL)-based biodegradable polymer, which has prospects in medical applications [170].

6.2 Hydrogel-based 4D printing Hydrogel-based 4D printing technique is primarily based on the integration of crosslinked networks of long polymer chains that bulge as a bigger volume after water or other solvent disperses into them. Upon immersion of these 3D-printed configurations in a solvent, the hydrogel structure swells at first, and thus with the generation of a strain mismatch among the two different substances, an overall shape change takes place [171–173]. Although hydrogels are easier to use, they are too soft to be transformed into rigid configurations of the tissue engineered matrix. In addition to this, the quick degradation and the overall integrity of the printed construct possesses other drawbacks of these materials. Typically, these materials are utilized in 4D printing because of their superior toughness and volume changeability when suffering environmental alterations. Some typical examples of hydrogels based on natural polymers are dextran, chitosan, and collagen while those based on synthetic polymers are poly (vinyl alcohol), poly(ethylene glycol) diacrylate (PEGDA; hydrophilic), poly(propylene glycol) dimethacrylate (PPGDMA), etc. The first applications of hydrogel-based materials in 4D printing were successfully demonstrated by Stratasys multimaterial polyjet 3D printing [174]. In this very first attempt, the researchers have practically shown that a straight strand changed into the script in “MIT” pattern in the fluidic atmosphere. The precise control of spatial distribution and the temporal arrangements of this self-assembly were recognized by the accurate printing of the hydrogel/elastomer-based smart hinges. Later, Bakarich and coworkers fabricated a hydrogel-based smart temperature-receptive valve. In order to acquire a smart valve with actuating temperature ranging from 20°C to 60°C, alginate/poly

Tissue engineering based on 3D and 4D printing

(N-isopropylacrylamide) (PNIPAAm) inks were synthesized. These special smart/intelligent valves clogged to decrease the flow rate while changing the length between 41% and 49% at 60°C. This occurs because the transition temperature of the smart valve falls in the range of 32–35°C [175]. Ikegami et al. [176] seeded human mesenchymal stem cells (hMSCs) onto a 3D bioprinted tissue assembly with a pattern framework. This enhances the transformation time of the tissue assembly from two days to two weeks, caused by matrix deposition and environmental restructuring from hMSCs. The resultant effect of the matrix deposition phenomenon from hMSCs is the formation of a highly robust grid-like pattern that could be handled easily. Overall, hydrogels loaded with bioactive multifarious materials, for example, drugs and antibodies, have shown promise as an area of promising research while they also show a great opportunity for artificial implants [177,178]. On the other hand, self-healing hydrogels can inherently and routinely cure damaged tissue and reestablish a normal state of organ functioning. These are in conjunction with other similar 4D printing technologies, and thus significantly inspire the study of self-healing hydrogels in tissue and organ regeneration applications.

7 Factors affecting 4D printing Smart or intelligent materials for biomedical applications are based on their unconventional activity to mimic the specific tissue or organ to be cured. These materials have the ability to change their biological and mechanochemical receptiveness to environmentally and naturally occurring stimuli, for instance, temperature, stress, humidity, ionic strength, pH, and electric or magnetic fields. Essentially, they are intended to be varied in a predetermined mode, and accordingly, stimulate the cells and their attachment, proliferation, and differentiation. In the following section, we will discuss the effect of two externally stimulating factors that decide the kind of smart materials to be used in the 4D-printed structure and their properties.

7.1 Effect of temperature The most frequent external stimulus to activate the 4D-printed constructs is temperature [178,179]. Thermoresponsive materials, including shape memory thermoplastics and shape memory thermosets, can fold, contract, or swell while changing their temperature. Several other categories of polymers demonstrate phase-transition temperatures, such as glass transition temperature (Tg) in thermosets or melting temperature (Tm) in thermoplastics similar to physiological temperature. This kind of behavior is found in poly (N-isopropy-lacrylamide) (PNIPAAm), which is a commonly used candidate material for drug delivery applications as well as tissue regeneration [180]. A bilayered object printed by PNIPAAm and a water-insoluble polymer poly(e-caprolactone) (PCL) exhibit the self-folding or self-unfolding characteristics once exposed to slightly elevated

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temperature; therefore, the phenomenon of yeast cell encapsulation and release is found in it [24]. In another attempt, a thermoresponsive combination involving poly(acrylic acid)-PNIPAAm and Al2O3-based ceramic powder can experience a sol-gel transition by the application of a thermal stimulus [181]. In a different study, Hendrikson et al. [182] prepared a polyurethane shape memory structure (Tg 32°C) to construct active scaffolds by the fused deposition method. A custom-made stretcher was used to print the scaffolds at 65°C, chilled down to 4°C, and released to retain a transition shape. Then, the injected cells were grown at 30°C to help in cell adhesion and proliferation. In the final step, the temperature was enhanced to 37°C and thus, a stable shape was achieved. In the entire process of dimensional recovery, the as-received cells were transformed in an elongated and aligned configuration. In general, the process involves the integration of a thermosensitive polymer with targeted cells, nutrients, or growth factors, and subsequently the whole combination is infused into the body. While being injected, the polymer experiences a phase shift owing to its temperature increment and transforms into a gel-like substance, releasing elements to form a 3D structure [183].

7.2 Effect of water or solvent Another frequent stimulus that drives the deformation (or swelling) in shape memory effects in bioprinted shape memory polymeric scaffolds is water (or solvents). Watersoluble or hydrophilic polymers typically display the SME when they come into contact with humidity. The final shape is obtained after drying. The glass transition temperature of hydrophilic SMPs may slash down to below room temperature once the water molecules are diffused into [184] it. A lower glass transition temperature (Tg) that is almost close to room temperature permits the printed objects to reveal the SME in this temperature range. In comparison to the thermally induced shape memory effect, it has been found that water-induced SME displays a slower shape recovery of active 3D printed objects. In spite of these drawbacks, including a slow reaction time, the humidity response of the SMPs for water-triggered actuation has been effectively used for various 3D printed architectures, especially in aquatic environments or at ambient temperature. For instance, Raviv et al. [185] have shown three classic examples of deformationlinear stretching, folding, and ring stretching-where water would react and develop various configurations with the help of multiple parts of the materials. In the linear stretching operation, the stiffer substance acts as a scaffold while the lively (hydrophilic) deposit creates the force for stroke. This scheme was accomplished by gathering a chain of firm disks while the center material stretches. Tibbits and other research group [186,187] have fabricated a sequence of primitives (or hinges) with a firm base of plastic material and a hydrophilic rubber (hydrogel) by the 4D printing technique that swelled while in contact with water with the use of a Connex Objet500 printer. Apart from the conventional

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fabrication of layered structures, biomimetic 4D printing of multifaceted, morphologically diverse and naturally inspired structures, for instance leaves and flowers that convert in contact to water, was executed with nanoinks [188]. During the process of extrusion of the ink out of the nozzle exit, a portion of the cellulose constituent was, as experienced in woodfiber extrusion [189], self-ordered in the hydrogel by shear stresses. This eventually resulted in swelling in a specific dimension of the extruded structure along the longitudinal direction. Zhu et al. [190] have shown a thermally effected shape memory effect for watersensitive smart active polymers based on cellulose nanowhiskers (CNWs) and thermoplastic polyurethane. Here, the reversible formation and breakage of the CNW percolation network has contributed to the shape memory effect when exposed to water.

8 Conclusions As a promising manufacturing technology of the 21st century, 3D printing, is an additive manufacturing-based process for exact 3D construction, revolutionize the world with its ability to make automated complex structures. It is a very exciting technique to develop functional devices such as laboratory chips and cell-laden scaffolds for tissue growth with respect to biomedical applications. Various efforts at developing novel biocompatible materials for bioprinting demonstrating fast cross-linking properties are fundamental requirements toward practical implementation of 3D printing technology in tissue engineering. The function and realistic potential of the 3D printing technique has not been fully utilized, owing to its lower speed and subsequent time factor for the manufacturing of 3D objects. On the other hand, 4D printing, based on the 3D printing fabrication scheme with the added time factor, has shown great interest and potential to the scientific community for practical purposes. In the future, this technique has the possibility to replace many existing methods and materials involved in developing smart, customized implantable medical devices that offer competent output for surgeons. Because it is quite possible to fabricate various medical models, the surgeon can create smart anatomies according to patient requirements any time, which has not been feasible until now. Multiresponsive biocompatible, topographical, and 4D dynamic shape-changing tissue scaffold configurations activated by diverse stimuli demand greater efforts and proficiency. Above all, the structures have to perform based on a particular application for tissue and organ regeneration matters, for example biodegradability and/or biocompatibility properties. In a nutshell, the 4D bioprinting technologies open up an excellent opportunity of an unexplored world of research with a wide range of devices of multifaceted printing, not only in tissue and organ regeneration but also in other fields in connection to scienctific understanding and technological development.

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Acknowledgment The author is grateful to Shashank Poddar, Ujjwal Chitnis, and Anish Ash for helping in the literature survey and preparing preliminary information for completing this chapter.

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CHAPTER 6

Shape memory polymer blends and composites for 3D and 4D printing applications A.I. Salimona,b, F.S. Senatovb, V.A. Kalyaeva, A.M. Korsunskya,c a Skolkovo Institute of Science and Technology, Moscow, Russia National University of Science and Technology “MISiS”, Moscow, Russia c MBLEM, Department of Engineering Science, University of Oxford, Oxford, United Kingdom b

1 Introduction—Historical overview A common trend for rapidly developing technologies and their adoption following first successful demonstrations is the concurrent emergence of multiple new applications of the fundamental underlying paradigm. Additive manufacturing at large, and in the narrower sense, 3DP of polymers gave rise to numerous variations of the core method utilizing a whole range of physical, chemical, and technological properties of printable materials. In this context, the growing range of applications exploitation of SME was anticipated and was merely a question of time. The intuitive vision of material threedimensional objects being able to change their geometric (shape, dimensions), structural (strength, stiffness), or functional properties was perhaps introduced into the public perception through 3D cinematography since 1990–2000s. In the process, the object can be thought of as becoming 4, 5, …, n-dimensional, acquiring new fascinating qualities. 2012 appears to be the first year when the use of the term “4D” first emerges in combination with the term “printing” in conference proceedings [1], although 4D effect had already been reported for traditionally 2D ink-printed barcodes through color change or fade with time. The TED (Technology, Entertainment, Design) conference in 2012 [2] is often cited as the event where 4DP of polymers was born as paradigm. On the other hand, it is believed that 3DP of polymers in which 4D effect was purposely exploited through the SME in photopolymeric ink-based composites has been first reported in a journal publication in 2013 [3]. Since then more than hundred publications appeared (according to Scopus.com) covering SME phenomena in 3D-printed polymers of different groups, polymer-based composites, metallic alloys, and polymer-paper hybrids, resulting in 4D effects [4–96]. SME in 3D-printed materials was also considered without referring to the term 4D [97–142]. The number of publications is growing steadily (as can be seen in Fig. 1), reflecting an increasing interest and the growing number of scientific groups joining this field. Significant number of review papers (11 out of the total of 94 articles on 3D and 4D Printing of Polymer Nanocomposite Materials https://doi.org/10.1016/B978-0-12-816805-9.00006-5

© 2020 Elsevier Inc. All rights reserved.

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Publication statistics 50 40 30 20 10 0 4D vs. SM 3D vs. SM 2001 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2018 3D vs. SM

4D vs. SM

Fig. 1 Publication statistics on shape memory (SM) topics in 3D and 4DP. (From Scopus.com.)

4DP [8, 22, 25, 35, 36, 49, 53, 54, 66, 85, 90]) highlights the present lack of general theory or methodology being developed at the next step after systematic summary and analysis presented in review articles. The interest in the practical applications covering the vast field of mechanical and biomedical devices is characteristic of this emerging technology still being in its early years. Its further evolution leads naturally to the stage when fundamental materials science begins to dominate the scene, providing firm base for further development and new achievements.

2 The underlying mechanism of the SME Comprehensive books were published recently on the theoretical fundamentals and applications of SMP by Jinlian Hu [143] and Hemjyoti Kalita [144], covering most of the physical mechanisms involved. Thermodynamic (Fig. 2), supramolecular (Figs. 3 and 4), and engineering (Fig. 5) aspects of the SME phenomenon were illustrated by numerous conceptual schemes, providing insights for reader. Intrinsically, SME is a specific mechanism of thermodynamically favorable process of transition from a metastable state—Temporary Shape—to a stable state—Permanent Shape. The energy barrier separating these two states can be overcome under the action of stimuli—heat, light, pH change, influence of solvent, chemical interactions, electric and magnetic fields, etc. In these processes, a certain amount of energy is absorbed from an external source that is sufficient to overcome the energy barrier. Thermodynamically favorable process must occur through the motion and conformational changes of molecules (or fragments of long molecular chains in polymers) in the space between pinning points—crosslinking sites in thermosetting resins, epoxies,

Shape memory polymer blends and composites for 3D and 4D printing applications

Shape memory effect

H B H’

A Shape

e effe chang

ct

Fig. 2 Permanent shape A and temporary shape B are separated by the energy barrier H. To make SME possible energy, the barrier H is to be overcome by a stimulus introducing additional energy to the system from external source. If the energy barrier H0 is low shape changes either gradually (e.g., viscoelastically) or instantly. (Reproduced with permission from X. Li, J. Shang, Z. Wang, Intelligent materials: a review of applications in 4D printing, Assem. Autom. 37 (2) (2017) 170–185. Emeralds. Copyright 2017.)

Unloading and heating above Tg

Cooling below Tg

Loading above Tg

Permanent shape

Fig. 3 The molecular mechanism of the SME. The zig-zag represents the coiled or folded molecular chains, and circles represent stiff crosslinking groups. (Reproduced with permission from Q. Meng, J. Hu, Y. Zhu, J. Lu, Y. Liu, Polycaprolactone-based shape memory segmented polyurethane fiber, J. Appl. Polym. Sci. 106 (4) (2007) 2515–2523, John Wiley and Sons. Copyright 2007.)

and rubbers, as demonstrated in Fig. 3 or rigid crystallites or ordered blocks in thermoplastic polymers (e.g., TPU or PLA) as illustrated in Fig. 4. The process kinetics are activated above the transition temperature, which is commonly the glass transition temperature in polymer materials. The addition of heat causing the temperature rise

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Fig. 4 The molecular mechanism of SME in SM thermoplastic polymers. (Reproduced with permission from A. Lendlein, S. Kelch, Shape-memory polymers, Angew. Chem. Int. Ed. 41 (12) (2002) 2035–2057, John Wiley and Sons. Copyright 2002.)

III

em IV ep

II

Strain

164

V

I Tl

Tg

Th

Temperature

Fig. 5 Strain-temperature phase diagram of SMP programming. (Reproduced with permission from M. Bodaghi, A.R. Damanpack, W.H. Liao, Adaptive metamaterials by functionally graded 4D printing, Mater. Des. 135 (2017) 26–36, Elsevier. Copyright 2017.)

above the glass transition point Tg is commonly considered and is intensively studied as the principal mechanism for most shape memory polymers. For engineering purposes, the scheme by Bodaghi et al. [48] (see Fig. 5) serves as a useful guide explaining a typical process route for 3D-printed shape memory polymer articles. It starts with a strain/stress-free state at a low temperature below the transition temperature, T ¼ Tl < Tg, where shape memory polymer contains a stable glassy phase (most of thermoplastic polymers are semicrystalline). It is then heated above the transition temperature range, T ¼ Th > Tg (step I). Next, the shape memory polymer in the rubbery phase is loaded

Shape memory polymer blends and composites for 3D and 4D printing applications

mechanically (strained by a deformation process) to achieve a maximum strain, εm (step II). The strain is kept fixed when the material (or a sample, such as a 3D printed article) is mechanically constrained and cooled down to the low temperature, Tl, (step III). During cooling, the rubbery phase gradually transforms to glassy phase that “memorizes” the inelastic strains. When a shape memory polymer object is finally unloaded, prestrain persists in the material, εp (step IV). When shape memory effect is activated (switched or triggered) by heating to a higher temperature, Th, the strain relaxes and the shape memory polymer sample recovers the permanent shape (step V). This is known as a free-strain recovery. If the polymer object is mechanically constrained during relaxation, recovery stresses are generated. A variety of external stimuli [144] to switch SME opens unprecedented opportunities in 4DP, which have not been fully explored at present. Considering that most publications in 4DP are devoted to the thermal triggering of the SME as the simplest stimulus, it is expected that other stimuli will be employed in the nearest future—light, pH control, use of solvents, chemical interactions, electric and magnetic fields. Possibly the very first report on FDM 3D-printed PU-Carbon black composite [131] able to switch SME via light illumination shows specific properties of TPU especially suitable for 4DP as demonstrated in Fig. 6. We view this direction of development in 4DP as the most promising for many applications since in many cases thermal heating causes many technical problems (biomedicine, artificial muscles, sensors) and undesirably

Fig. 6 The concept of photoresponsive SMP implemented in Ref. [131]. (A) Photoresponsive SM behavior of 3DP materials based on carbon black and PU. (B) 3D printed device fabricated by FDM. (C) Photoresponsive SM behavior of 3D printed device. (Reproduced with permission from H. Yang, W.R. Leow, T. Wang, J. Wang, J. Yu, K. He, D. Qi, C. Wan, X. Chen, 3D printed photoresponsive devices based on shape memory composites, Adv. Mater. 29 (33) (2017) 1701627, John Wiley and Sons. Copyright 2017.)

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complicates the design of a final technical solution. It is also obvious that light triggering is much more fast and potentially selective for different wavelengths. Water absorption as a useful stimulus is referred [84] in respect to syringe-printable hydrogels based on easily affordable natural (Dextran, Chitosan, Collagen) and synthetic polymers (polyvinyl alcohol PVA). We also see significant potential for this type of SMP, although possibilities to control shape and sizes with sufficient tolerance seem to be questionable. Biomedicine, tissue engineering, and organ phantoms are viewed as the firstchoice applications.

3 Main trends in the use of SMP for 3DP and 4DP An intrinsic feature of 3DP technologies is the fast transition from conception to embodiment. The drawbacks are well known: it is not a technology for mass production (up scaling limitations), and it is not easily applicable for all materials (at least up to now). The preparation of initial raw materials (powders for SLM/SLS, filaments for FDM and gels/suspensions/slurries for SLA or multiphoton photopolymerization lithography) becomes a separate, serious technological issue limiting the use of 3DP as it is. Thus, 3DP is still a technology for relatively low-scale production of high added value individualized products (such as, for example, biomedicine articles—implants, stents, robotics, advanced sport goods, etc.) and for rapid prototyping in science, design, testing evaluation, and high-tech industries as it was historically developing since 1990s following first applications for earlier SLS apparatuses. The use of SM polymers in 3-4DP as seen from the almost 5 years’ perspective might not be expected to revolutionize the field giving, however, some promising directions for the expansion of the technology to smart mechanical devices and, if down scaled to micro- [48, 129] and nanometer [145] dimensional level, to metamaterials. The main trends and guiding ideas as they are evolving in the use of SMP (SMP) for 3D-printed objects are structured as following: - Technologies: (a) SLA of photocurable (Laser, UV, or LED curable) ink jetted resins and polymer inks (b) Fused deposition modeling (FDM) (or fused filament fabrication (FFF)) (c) Syringe bioprinting for hydrogels and polymer inks (d) Post-treatment of laser patterning of SL 3D-printed objects (e) Mechanical extrusion. (f ) Two(multi)-photon Lithography Selective laser sintering and/or melting (SLS/SLM) or spark sintering of shape memory alloys (SMA), of course, is not a subject of this review; however, on the other hand, SMA elements are being readily hybridized with polymers to provoke SM in hybrids, e.g., by means of Joule heating with current [98].

Shape memory polymer blends and composites for 3D and 4D printing applications

- Composites: Shape memory composites (SMC) based on SMP are being developed using various reinforcements and functional polymer and nonpolymer particles like: (a) Polymer fibers. (b) Shape memory fibers (SMF); (c) Carbon nanotubes; (d) Carbon black; (e) Ferromagnetic particles; (f ) Polymer nanoparticles; (g) Hydroxyapatite particles. It is obvious that a wide range of “classic” reinforcements such as polyether, glass, cellulose, and carbon fibers as well as functionalized silica nanoparticles or graphene have not been explored leaving vast room for further composite optimization as well as the development of 3-4D-printed SM articles [146–151]. - Hybrids: The core idea of 4DP is to print a smart net-shaped 3D object that is able to perform complex, accurately controlled motions due to the programmed change of shape of its spatially distanced parts. Programming of motion can be achieved by: (1) tuning of the glass transition temperature Tg (e.g., by control over crosslinking in curable polymers) at which SME starts to unfold; (2) tuning of recovery stress and strains; (3) introduction of constraining elements; (4) variation of cross-section and shape of a structure element; (5) combination of structure elements having different Tg or recovery stress and strains in a single object. Hybridization, i.e., combination of distinctively different materials in a single article having inhomogeneous macrostructure (coating, layers, impregnated fabrics, etc.), is a principal trend in 3-4D-printed SM object. Literature reports contain examples of hybrids exploring principles (3)–(5) exemplified by: (a) combination of soft and rigid photocurable polymers. (b) combination of SMP having different thermal expansion coefficients. (c) printing of SMP over SMA surface. (d) creation of bi- and multilayered structures to implement multistage SME. (e) exploiting of anisotropy of different raster patterns in-plane or across the layers. (f ) overlapping of folded structures. (g) introduction of holes or pores. (h) introduction of wires and fibers. (i) introduction of fabrics. (j) printing of SMP over paper or polymer films. - Tuning of SM features and properties: Tuning of basic SME characteristics—glass transition temperature, recovery stress and strain—is a pivotal point in the design of 4D-printed objects. On the one hand, glass transition temperature from the design point of view sets a series of requirements for

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heating and cooling elements, which are to be positioned inside or outside of 4D-printed object or device—type, mass, and power of heaters and coolers. This, in turn, predetermines the character of the 4D-printed device—fixed or portable, cabled or equipped with batteries, sensitive to ambient medium or relatively robust, etc. On the other hand, the values of recovery stress and strain and their stability against the number of heating-cooling cycles in combination with the designed cross-section of a structure element govern the principal engineering parameters for a device—force, movements, momentum, contact area, stress and duration, accuracy and reproducibility of movements, life time, and many others. Main SME characteristics are closely structure dependent, and that they can be controlled and tuned by a number of means: (a) Selection and blending of basic polymer systems. (b) Selection and compositional tuning of cure agents and adjustment of curing procedure—temperature and time. (c) Selection and compositional tuning of UV crosslinking agent. (d) Manipulations with UV crosslinking parameters—wavelength, intensity, and duration. (e) Modification of thermal triggering through the manipulation with deformation temperature. (f ) Manipulation with crystallinity ratio through the introduction of crystallization nuclei agent—carbon nanotubes or graphene. (g) Orientation drawing during mechanical extrusion. (h) Introduction of waxes and oils adjusting molecular chain motion and sliding. (i) Introduction of metal ions altering crosslinking as a result of deformation. (j) Altering of 3DP process parameters such as layer thickness, raster angle, and extrusion temperature. - Applications: Since its first appearance on the public scene, the 4DP concept was an applicationdriven technology. Hence, reports that demonstrate smart 4D-printed solutions and discuss design paradigms are dominating over “purely” scientific research studies. It may seem that possible applications are limited only by researchers’ fantasy and curiosity. In fact, that is not the case, as the main function of 4D-printed articles and devices based on SME is mechanical, by means of using temperature change to effect mechanical motion, which in turn opens up many possibilities for further developments, when it can be used for triggering or altering of other effects or applicable phenomena— thermal, electromagnetic, optical, etc. Without claiming to be complete, a list of existing, forthcoming, and possible applications can be presented as given here: (a) Actuator for smart mechanical systems, robotics, and devices—hinges, pivots, gaskets, grippers, etc. (b) Bioinspired robots and toys.

Shape memory polymer blends and composites for 3D and 4D printing applications

(c) Origami-inspired self-folding (unfolding) robots and toys. (d) Actuated joints and grasping mechanisms. (e) Biomedical and orthopedic adaptive solutions—prostheses, self-fitting implants, stents. (f ) Elements of exoskeletons and artificial muscles, robotic hands, etc. (g) Tissue engineering and scaffolds, self-healing elements, radiopaque endovascular embolizators. (h) Metamaterials, thermorphs, auxetics, compliable cellular solids, 2D to 3D reversible surface patterns, honeycombs. (i) Drug delivery. (j) Photoresponsive devices. (k) Skin sensors. (l) Dynamic jewelry and fashionwear. (m) Deployable antennas, sunshades, and other mechanical devices. (n) Self-morphing, self-assembled, self-expanding/shrinking structures and devices.

4 Technologies of 3DP applied to SMP In order to produce an object, structure, machine, or engineering component, a tight correlation must be sought between materials, geometry, and technologies. This makes the task of design in the engineering sciences a multidisciplinary and complex challenge [152]. Historically, 3DP, as a technology, has grown fastest due to the progress in hardware embodiment of printing apparatuses and software tools. Firstly, materials were selected from the menu of readily commercially affordable solutions. In the next step, materials were adapted or even purposely formulated and synthesized to be applicable for 3DP. This appears to be a natural and rational approach, since shortening of the path between design and testing of a technical solution is one of the fundamental attributes of 3DP. 4DP, as a branch of 3DP, is not an exception, and remains largely technology driven. It is expected that the evolution of 4DP toward successful commercialization of a product will transform into a material-driven field at some point when the performance of the product becomes clearly limited by material properties. Until this happens, technology limitations remain key limiting factors in the field of 4DP. Technologies currently applied in 3DP of polymers in the development of the 4DP paradigm are overviewed in Table 1, with reference given to publications that appear to hold priority. Principal 3DP technologies applicable for polymers—SLA and FFF/FDM—are, obviously, most common for 4DP as the extension of basic options. Existing, commercially affordable, 3D printers are readily used as a rational approach to shorten the way to the desired products embodying mechanical actuation through SME evolvement. Multiphoton lithography, syringe bioprinting, and mechanical extrusion are emerging

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Table 1 Technologies for 3DP of SMP Year and reference of first publication/ number of publications

Technology

Materials

Objects

Process details

Stereolithography of photocurable resins and inks: ink jetting followed by UV photopolymerization Fused deposition modeling or fused filament fabrication

Printed active composites: Glassy SM polymer fibers Tg  35°C reinforcing soft rubbers. MA + IBoA and PLA

Laminates, coils, selffolding box

PolyJet process. Objet Connex 260

2013 [3]/34

BFB (Bits from Bytes) 3000 3D printer

2014 [109]/32

Multiphoton Lithography

PETTA polymer

Dog-bone samples for mechanical testing Architected materials

Photonic Professional GT, Nanoscribe GmbH

2017 [145]/1

Syringe bioprinting

Chitosan doped with carbon black

Mechanical extrusion

Molding and freeze drying

Selfbending soft actuators DiAPLEX Dog-bone MM-4520 with samples for carbon black mechanical testing Polyvinyl alcohol Dog-bone chemically samples for crosslinked with GA mechanical (glutaraldehyde) testing

2018 [75]/1

ME 3D printer 2018 [83]/2 (MAKERGEAR M2) 2018 [60]/1

technologies in 4DP alongside with adjacent technologies like mechanical stitching [41], gluing [54], curing inside 3D-printed sacrificial molds [19], freeze drying [60], and micropatterning [76]. An intrinsic feature of FDM/FFF, i.e., complex thermal history of a printable material during printing process, is well known from very early years of this technology. A cascade of interrelated phenomena (anisotropy of mechanical properties, hierarchy of porosity, microcracks and delamination defects, residual stresses, and postproduction

Shape memory polymer blends and composites for 3D and 4D printing applications

set) caused by thermal gradients appearing at the interfaces of layers and paths are discussed and considered as disadvantages or even obstacles in the manufacturing of highperformance ready products—parts and devices. Thermal gradients as a rule are being reduced by means of heating of printing bed heating and accelerating of printing speed that may cause, in its turn, oxidation problems. The influence of thermal history and inherited printing defects on the SME characteristics is not deeply investigated in respect to 4DP [58]. One can expect diverse effects from very positive “self-healing” [109] due to the closing of microcracks to negative secondary defect formation at thermal switching of SME. Thermal cycling around Tg can also evoke SM fatigue events, which are emphasized in rigid polymers like ABS and HIPS [58].

5 Classification of SMP, blends, and composites used in 3D and 4DP Historically, SMP were not formulated purposefully for 3-4DP—in contrast, known and proven SMP were adapted and customized for this technology. J. Hu [143] has classified SMP in a taxonomy tree. In Fig. 7, we cite this classification underlying with red lines those polymers which were demonstrated as 4D printable in publications. Due to the fact that current research in 4DP has mainly applied character 4D-printable polymers which are not equally represented in literature leaving both scientific and engineering lacunas and many unanswered questions. Popular PU-based resins, TPU, PLA, and PCL together constitute principal corpus of published works. Blends, composites, and hybrids are also being formulated using these basic polymers. Application-oriented period of 4DP technology is a necessary stage while the formulation of specific SMP for 4DP is a tendency of nearest future when fine tuning of SME characteristic will become especially relevant. H. Kalita [144] recently classified SMP and their blends particularly emphasizing possibilities to tune SME due to the combination of miscible and immiscible SMP. From the blends of miscible segmented polymers TPU, PCL, PVC, phenoxy, polybenzoxazine (PB) reported in [144] as SMP blends only combinations of TPU and PCL are referred as 4D-printable. From the blends of amorphous poly(methyl acrylate), poly(ethyl acrylate), atactic PMMA and poly(vinyl acetate),) and crystalline polymers (poly(vinylidene fluoride) (PVDF), polylactide, poly(hydroxybutyrate), poly(ethylene glycol) (PEG), polyethylene, PVC and poly(ethylene-co-vinyl acetate), which are potentially applicable in 4DP, mainly methacrylated polymers in combination with PLA are considered in literature by now. Table 2, summarizing data on covalently crosslinked glassy and semicrystalline SMP [144], contains information on 4D-printable SMP in the underlined columns.

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-

Fig. 7 Taxonomy tree of SMP by J. Hu [143]. Groups of polymers explored in 4DP are underlined with red lines.

Two main technologies of 3DP, which are applicable for polymers and their blends— SLA UV-polymerization (crosslinking curing) and FDM, predefine main classes of SMP and their blends used in 3-4DP.

5.1 SMP and blends in SLA Karger-Kocsis and K
eki [49] have recently thoroughly overviewed SM epoxies and their composites to, perhaps, the fullest extent. This class of SMP possesses a number of features especially applicable in 4DP: two-way shape memory, dual and multishape memory effects, multifunctionality, simple combination with reinforcements, including SMA, ability to form foams. PU and PCL-based methacrylated epoxies are considered as main players. The underlying idea how to govern the SME properties in polymer resins photocurable in the SLA process is to combine softer and stiffer components. Fully crosslinked (during photopolymerization) thermoset polymer has dense and rigid spatial molecular structure that leaves almost no room for molecular chain movements, and as a result such polymers have poor SME properties. Thus, in order to promote the SME in a printed part, initial resin is to be formulated allowing coexistence of two components—soft “linear chain” and hard “crosslinking” counterparts. The first is represented by relatively

Shape memory polymer blends and composites for 3D and 4D printing applications

Table 2 Main groups of covalently crosslinked SMP [144] Shape memory polymer

Rigid segment

Soft segment

Ttrans (°C)

Crosslink

Poly(vinyl alcohol)

60…65

Crosslink Poly(ethylene terephthalate))

Methacrylates Poly(ethylene glycol)

26..0.29 11…24

Epoxy 1,6Diisocyanato-2,2, 4-trimethylhexane Crosslink

Jeffamines Oligo[(rac-lactide)coglycolide

31…93 36–60

PEGDMA

56–92

Crosslink MDI/PB/1,4butanediol

Starch Poly(tetramethylene) glycol

50 70–110

Crosslink

Polycyclooctene

30…60

TDI/1,4butanediol Crosslink

PCL

30…40

Poly(ethylene)-co-1octene PCL

60…100

Covalently crosslinked glassy SMP

Poly(vinyl alcohol) glutaraldehyde Polyesters-methacrylates Poly(ethylene terephthalate)-copoly (ethylene glycol)-glycerin Epoxy-Jeffamines Copolyester-polyurethane

Methyl methacrylate-coPEGDMA Starch-glycerol PU-PB

Covalently crosslinked semicrystalline SMP

Dicumyl peroxide— polycyclooctene PU-epoxy Poly(ethylene)-co-1-octene) Alkoxysilanepoly(ε-caprolactone) Oligo[(ε-hydroxycaproate)coglycolate]dimethacrylates Natural rubber LDPE

Crosslink Crosslink

Crosslink Crosslink

Oligo [(ε-hydroxycaproate)-coglycolate]dimethacrylates Polyisoprene LDPE

40…60 18…52

0…45 60…100

4D-printable polymers are underlined.

flexible chains enabling plasticity and by this shaping at temperatures above glass transition temperature (Tg) into temporary shape. “Cross-linking” component sets the net points, which predefine thermally stable permanent shape being formed during SLA. These net points are represented by strong intermolecular bonds, predominantly covalent—as exemplified by Fig. 8. Stratasys Objet Connex family of printers has been intensively used to 3-4D print commercially affordable photo- and UV-polymerizable resins, epoxies, inks and their blends and compositions. Historically, in the initial publications [3, 5, 9], the compositions of polymer inks were not disclosed. Two principal blends for which the composition was published [14] were based on:

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Fig. 8 Underlying working principle for the formulation of SME UV (light)-curable resins for SLA 4DP. (Reproduced with permission from Y.Y.C. Choong, S. Maleksaeedi, H. Eng, J. Wei, P.-C. Su, 4D printing of high performance shape memory polymer using stereolithography, Mater. Des. 126 (2017) 219–225, Elsevier. Copyright 2016.)

- Tangoblack+, a rubbery material at room temperature, blended with polymerization agent urethaneacrylate oligomer, Exo-1,7,7-trimethylbicyclo hept-2-yl acrylate, methacrylate oligomer, polyurethane resin, and photoinitiator - Verowhite +, a rigid plastic at room temperature, blended with polymerization agent isobornyl acrylate, acrylic monomer, urethane acrylate, epoxy acrylate, acrylic monomer, acrylic oligomer, and photoinitiator This gave a number of opportunities for many scientific teams to repeat and expand the pioneering results of H. Jerry Qi’s group. Tangoblack+ and Verowhite + and similar blends like Tango+, VeroClear were later used in many 3-4DP research studies [17, 29, 31, 33, 38, 45, 54, 55, 63, 78, 83]. Commercial SMP FLX9895, RGD525, Agilus30 [83], “Clear FLGPCL02,” “Flexible FLFLGR02” [86] were blended in order to tune SM characteristics—glass transition temperature, recovery stresses and strains as a function of composition and deformation temperatures. Combination of more and less rigid SMP having different temperature ranges of SME triggering (Tg as a rule) is one of the principal approaches to generate complex change of shape and dimensions that may embody complex motion of a polymer object. Reinforcement of relatively soft polymer matrix with relatively rigid fibers, especially those also possessing SM behavior, introduces additional degree of freedom for the structuring of composites and management over properties. The idea of introducing fibers—namely,

Shape memory polymer blends and composites for 3D and 4D printing applications

fibers DM8530 (fiber 1) and DM9895 (fiber 2) manifesting SME at 57 and 38°C, respectively, into Tangoblack+ matrix [14]—was later expanded to other classes of polymers. Specific complex polymer resin blends are formulated based on benzyl methacrylate (BMA) as linear chain builder (LCB), and difunctional oligomers, poly(ethyleneglycol) dimethacrylate (PEGDMA), bisphenol A ethoxylate dimethacrylate (BPA), and di(ethyleneglycol) dimethacrylate (DEGDMA) as crosslinkers to form crosslinked network from linear polymer chains [16]. Final printable resin must contain photoabsorbers (Sudan I and Rhodamine B 0.05 and 1 wt%, respectively) and photoinitator—phenylbis(2,4,6-trimethylbenzoyl) phosphine oxide (added to into the methacrylate basic polymer blend at the concentration of 5 wt%. Methacrylated PCL blends with corresponding photoinitiator, polymerization inhibitor, and dies were used in DLP (direct laser printing—a specific version of SLA) [18, 21]. After sophisticated methacrylation of blends of PCL-diols and ε-caprolactone with isocyanatoethyl methacrylate in presence of stannous octoate catalysts, the photoinitiator 2,4,6-trimethylbenzoyl-diphenylphosphineoxide (TPO), vitamin E inhibitor, and Magenta (Toner EO2, Clariant, Germany), Yellow (Toner 3GP, Clariant, Germany), Orasol Orange G (BASF, Germany), and green (Green 201 Teal, Johnson Mattley, the Netherlands) are added to final 4D printable polymer resin. Poly(ethyleneoxide) (PEO) and poly(acrylic acid) (PAA) blends with polydopamine (PDA) Nanoparticles are patterned in thin films and then SME is activated through photoreduction and oxidation under UV or visible light [24]—see Fig. 9. We believe this research will be further elaborated since it shows significant potential for smart metamaterials and sensor technology.

Fig. 9 The concept of smart patterning of thin films via the use of 4DP paradigm. (Reproduced with permission from H. Wei, Q. Zhang, Y. Yao, L. Liu, Y. Liu, J. Leng, Direct-write fabrication of 4D active shape-changing structures based on a shape memory polymer and its nanocomposite, ACS Appl. Mater. Interfaces 9 (1) (2017) 876–883, American Chemical Society. Copyright 2017.)

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Tert-butyl acrylate (tBA) monomer being blended with di(ethyleneglycol) diacrylate (DEGDA) crosslinker (10, 20, 30,40, and 50 wt%) and with UV photoinitiator (PI) Phenylbis (2,4,6trimethylbenzoyl) phosphine oxide (BAPO) (0.5, 1,2,3, 4 and 5 wt%) [37]. A crosslinker, poly(ethylene glycol) diacrylate (PEGDA700) and another photoinitiator 2,2-dimethoxy-2-phenylacetophenon, can modify initial recipe to tune SME characteristics [42]. Lithography of hard masks on the metal foil surface may be combined with spin coating of SMP formulated as thermally cured resin formulated as blend of epoxy monomer E44 (molecular weight  450 g mol1) and curing agent poly(propylene glycol) bis (2-aminopropyl)ether (Jeffamine D230) [54]. Direct-ink-write (DIW) technology of highly stretchable semi-interpenetrating polymer network (semi-IPN) introduces a complex polymer blend and composite combination that implements a number of microstructural processes to ultimately come to SM self-healing materials for biomedical applications—artificial blood vessel tubular elements [63]. Photopolymerization in this case is one step in a longer production chain, as seen in Fig. 10. PCL (Mn ¼ 70,000–90,000) is mixed with AUD (EBECRYL 8413, Allnex) and BA with EBECRYL 8413/BA mass ratios of 1/1.5 at 70 °C, and Irgacure 819 (phenylbis (2,4,6-trimethylbenzoyl)-phosphine oxide) photoinitiator. Syringe ink printing followed by UV radiation curing is applied layer by layer. Recently, a photopolymer resin was formulated as copolymer of in-house synthesized polyurethane acrylate (PUA), diglycidyl ether diacrylate (DGEDA), and isobornyl acrylate (IBOA), where PI184 was used as free radical photoinitiator. Laser irradiation with the wavelength 355 nm was used in SL200 SLA apparatus (ZRapid Tech) to finally print net-shaped articles showing SME at about 85°C [66]. DLP version of SLA process with UV post-curing was applied to in-lab synthesized resin formulated as a blend of PCLDMA (PCL + methacrylate) and UPyMA (Ureidopyrimidinone methacrylate) with 2,4,6-trimethylbenzoyl-diphenyl-phosphinoxide as photoinitiator (TPO-L, BASF, Germany) [67]. Soybean oil epoxidized acrylate (SOEA) [15, 76] with bis(2,4,6-trimethylbenzoyl)phenylphosphineoxide (Ciba Irgacure 819) is used for biofabrication of smart biomedical scaffolds through the printing using standard 3DP platforms and in-line UV 355 nm laser curing or as combination of photopolymerization followed by laser micropatterning. In summary, the main trends to be highlighted in the development of photocurable polymer blends for SLA method of 3-4DP are: 1. SM photocurable polymers are formulated as blends or copolymers of soft long-chain matrix and rigid crosslinking agent polymers. Chemical nature of basic polymers governs glass temperature, while mass ratio between soft and rigid component predefines SM characteristics—mainly recovery stress and strain. Photopolymerization is initiated by a photoinitiator (as a rule, its content is 1–5 wt%).

Shape memory polymer blends and composites for 3D and 4D printing applications

Fig. 10 Concept of UV-assisted DIW-based 3DP of semi-IPN elastomer composites containing crystalline linear chain and crosslinked network. (A) Chemical structures of the components in the semi-IPN elastomer composite resin. (B) DIW-based 3D printer equipped with heating elements prints each layer of the filament followed by shining UV light (50 mW/cm2) to cure the resin. (C) Structure evolution of the ink during printing at 70°C and cooling down after printing. (Reproduced with permission from X. Kuang, K. Chen, C.K. Dunn, J. Wu, V.C.F. Li, H.J. Qi, 3D printing of highly stretchable, shape-memory, and self-healing elastomer toward novel 4D printing, ACS Appl. Mater. Interfaces 10 (8) (2018) 7381–7388, American Chemical Society. Copyright 2018.)

2. Main groups of photocurable SMP for SLA processing are represented by methacrylated polyurethanes PU, polycaprolactones PCL, di- and polyethyleneglycols (DEG and PEG) and tert-butyl (tB). 3. There is a shift from the use of ready commercially available polymer resins to in-lab formulated resin compositions for better control over SM characteristics.

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4. Combination of traditional SLA techniques and standard apparatuses with postprocessing or other printing techniques—ink printing, direct laser printing, and laser patterning.

5.2 SMP and blends in fused deposition modeling/fused filament fabrication FDM/FFF 3DP is a very popular technique that in last decade became an almost “household” craft in many laboratories and workshops worldwide, giving fast and facile opportunities for rapid prototyping or manufacturing of small series of customized parts and articles. A variety of commercially affordable low-priced 3D printers is supported by equivalent easy access to filaments representing many well-known customized printable polymer grades based on PLA, PU, ABS, CBS, silicone, Nylon, EVA, etc. Many of them in respect to 4DP were reviewed in 2015 [10]. Depending on particular blend formulation, polymer blends and composites based on polylactic acid (PLA) show SME in the range of 50–70°C. Being in the focus of multidisciplinary research for many applications expanding from biomedicine to biodegradable packaging, 4D printing by FDM/FFF PLA has been intensively studied in many aspects [3, 39, 43, 58, 68–72, 79, 87, 88, 109, 129, 130, 136]. Custom filaments made of blends with silver epoxy paste [39] or with graphene [68, 71, 72] for resistivity heating actuation (e.g., for deployable antennas [39]), as well as with hydroxyapatite [129] for biodegradable bone patches in reconstructive surgery can be relatively easy prepared in laboratory extruders. Similar to [153], SME during heating may have resulted in “selfhealing” of a 3D-printed PLA/HA composite by partially closing the cracks—see Fig. 11 [154]. FDM/FFF 4D-printed laminated, layered, or fiber-reinforced PLA-based composites and hybrids are exemplified by combinations with nylon/spandex fabrics [43], thermoplastic polyurethane (TPU) [69, 70], silicone elastomer matrix [79], nylon 12 and NiTi wires [87]. Great potential of PLA polymer for 4DP is related with relatively high SM characteristics—recovery ratio of over 94% and volume changes of up to 289% [88]. SME in 4D-printed PLA-based composites and hybrids is dedicatedly studied using DMA [31, 79, 88] and synchrotron radiation probing [155]. Printable thermoplastic polyurethane (TPU) is another good example of popular, easily available, commercially affordable material ready for the implementation of 4DP paradigm with tunable glass temperature in the range 30–65°C [32, 34, 41, 46, 48, 51, 69, 70, 77, 82, 85, 90, 113, 119, 131]. Desmopan® (COVESTRO Deutschland AG) is fully characterized in terms of SME properties after 3DP [32]. SMP Technologies Inc., Japan, offers a number of purposefully formulated grades of ready TPU-printable polymers having been studied in different aspects and represented in many publications [46, 48, 51, 77, 82, 85, 90]. MM3520 (SMP Technologies Inc., Japan) [34] can be FDM/FFF 3D printed and then actuated

Shape memory polymer blends and composites for 3D and 4D printing applications

T > Tg

x60

1nm

x60

1nm

T > Tg

x200 500um

x200 500um

Fig. 11 “Self-healing” of porous PLA/HA scaffold by narrowing the cracks after heating over Tg. (Reproduced with permission from F.S. Senatov, K.V. Niaza, M.Yu. Zadorozhnyy, A.V. Maksimkin, S.D. Kaloshkin, Y.Z. Estrin, Mechanical properties and shape memory effect of 3D-printed PLA-based porous scaffolds, J. Mech. Behav. Biomed. Mater. 57 (2016) 139–148, Elsevier. Copyright 2016.)

using resistivity heating due to the purposefully introduced carbon nanotubes. Stretched PU fibers were stitched to the strips of elastic rubber or fabrics in order to form waveshaped objects due to the triggering of SME [41]. DiAPLEX (PU) and Tecoflex (aliphatic thermoplastic urethane) [51] DiAPLEX MM-4520 [119] was used to print robotic gripper activated by thermal radiation from an open oven simulating curved finger mechanical grip. A specially formulated by Raytheon SM TPU after FDM 3DP has proven its excellent mechanical performance in comparison with standard Ultem 9085 polymer [113]. A group of less studied 4D-printable polymers unites diverse polymer and composite materials: - Commercially available Surlyn 9520, which is zinc-neutralized poly(ethylene)-comethacrylic acid polymer has proven its 4D printability [40]. - HIPS [58]; - ABS [58, 59]; - Polyester (PE) [39, 64]; - Nylon12 in hybrids with NiTi SMA and flexible PLA [87]; - Commercial thermoplastic elastomer SEBS with polyethylene wax and low-density polyethylene (LDPE) [123]; - polyvinylidene fluoride (PVDF) [125].

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6 Conclusions and outlook 4DP as an emerging technology is overcoming a critical stage in its development— transition from successful demonstration of capabilities to the formalization of manufacturing protocols allowing to finally create an industrial scale production flow. The interest to purposeful tuning and thorough characterization of SM properties is reflected in growing number or publications, where authors address to recovery stress–strain evolution at 4D-printed standard samples. This unimpressive routine testing must constitute healthy materials science fundamental for both further research and commercialization of fascinating self-pivoting, self-assembling, self-deploying, and selfhealing smart devices. SME in polymers that was comprehensively studied and sufficiently well understood over decades has specific features in respect to 3D-printed materials and devices. These features are viewed as following: 1. Purposeful creation of composites and hybrids to embody complex mechanical motion of ready 4D object may be relatively simple in some of 3DP methods, but it introduces interfaces where mechanical stresses are abruptly changed, causing residual stresses. Relaxation of these stresses will likely take place in the form of material damage and ultimate nonfunctionality. Moreover, they overlap with the recovery stresses generated by SME reducing the precision of mechanical motions. There are not research up to now devoted to this important issue. Softer polymers are less subjected to these effects that must give more attention to ink-jet and syringe printing techniques currently much less studied. 2. Similar to this, the most common 4DP technique—FDM—makes thermal history of printed material complex and even, perhaps, almost nonpredictable. Internal defects and residual stresses may significantly change the motion of 4D-printed object that is acceptable for impressive origami toys or soft-tissue biomedical parts but seem not to be easily tolerated in machine building, robotics, and fine mechanics or MEMS. 3. Up to now 4DP utilizes mainly thermal activation of SME as the simplest and the most common for polymers. This stimulus seems to be the least workable in engineering systems since it requires energy consuming and inertial sources of heat inside the system. Electrical, magnetic, but preferably light stimuli are highly desirable and the focus of future fundamental research in 4DP must be shifted onto respective SMP and composites.

Acknowledgment The authors thank Royal Society (Grant IEC/R2/170223) and Russian Foundation for Basic Research (Granted research project No.18-33-20132) for financial support.

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Further reading [156] Q. Meng, J. Hu, Y. Zhu, J. Lu, Y. Liu, Polycaprolactone-based shape memory segmented polyurethane fiber, J. Appl. Polym. Sci. 106 (4) (2007) 2515–2523, https://doi.org/10.1002/app.26764. [157] A. Lendlein, S. Kelch, Shape-memory polymers, Angew. Chem. Int. Ed. 41 (12) (2002) 2035–2057.

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CHAPTER 7

Fabrication of 3D and 4D polymer micro- and nanostructures based on electrospinning Norbert Radacsia, Wiwat Nuansingb,c a

The School of Engineering, Institute for Materials and Processes, The University of Edinburgh, Edinburgh, United Kingdom School of Physics, Institute of Science, Suranaree University of Technology, Nakhon Ratchasima, Thailand c Center of Excellent on Advanced Functional Materials (CoE-AFM), Suranaree University of Technology, Nakhon Ratchasima, Thailand b

1 Introduction Three-dimensional (3D) printing is revolutionizing the science and engineering of advanced materials [1]. 3D printing (also referred to as rapid prototyping or additive manufacturing technology) can easily build up 3D structures designed by a computer software, unlocking innovative complex product designs with breakthrough performance that will reshape the future of manufacturing. The “if you can imagine it, you can print it” concept opens up the door to new possibilities, which we were not able to leverage before, and has the potential to lead to novel products. However, 3D printing is hampered by two major limitations: low resolution and slow fabrication speed [2,3]. 4D printing can be defined as 3D printing objects that can change their shape over time, or in response to an environmental stimulus [4]. Such shape-changing objects can respond to changing parameters, like heat, light, moisture, pH, and can be used in soft robotic systems, smart textiles, drug delivery, or biomedical devices. This ground-breaking new technology will bring us the products of the future; however, 4D printing on the nanoscale has not been explored yet. Electrospinning, which is a highly flexible technique that can process solutions, melts, or suspensions into continuous nanofibers by using high voltage, can bring solution to 3D/4D printing on the nanoscale with rapid fabrication. 3D/4D structures on the nanoscale offers new opportunities for materials development, and this chapter introduces a novel nanoprinting technology, 3D/4D electrospinning, that combines the advantages of electrospinning and of 3D printing technology.

1.1 Nanotechnology for advanced materials Nanomaterials are revolutionary since their properties can be fundamentally different from bulk materials, leading to superconductivity, superparamagnetism, etc. [5–8]. Among these nanomaterials, one-dimensional (1D) nanofibers possess large length/diameter ratio, 3D and 4D Printing of Polymer Nanocomposite Materials https://doi.org/10.1016/B978-0-12-816805-9.00007-7

© 2020 Elsevier Inc. All rights reserved.

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leading to an extremely large surface area per unit mass. 1D nanofibers are relatively easy to produce, thus have been widely applied in many products. To this date, several different techniques, such as phase separation, template synthesis, drawing, electrospinning, etc., have been applied for the fabrication of two-dimensional (2D) flat objects (e.g., mats) by nanofibers [9,10].

1.2 3D polymer micro- and nanostructures 1D micro- and nanomaterials including nanotubes, nanowires, nanorods, and nanofibers have been heavily investigated and showed high potential applications [5,11]. Compared with 1D and 2D structures, 3D nanostructures have many advantages for applications, like batteries, fuel cells, solar cells, supercapacitors, and filtration. Besides storing and harvesting energy, other main applications of 3D micro/nanofibrous structures are regenerative medicine and tissue engineering. 3D polymer fibrous scaffolds with micro- and nanoscale features have an important role in assisting tissue regeneration such as cell adhesion, proliferation, differentiation, and migration [12,13]. Moreover, the ability to control the formation of 3D polymer fibrous microand nanostructures would open up for creating custom 3D and 4D smart materials.

2 Background 2.1 Additive manufacturing Additive manufacturing, which is widely known as three-dimensional (3D) printing, has been introduced in 1986 [14]. The novelty of this technique is that while conventional manufacturing methods that require molds, digital assembly, lithographic masks, or dies, 3D printing allows us to rapidly turn an idea in the form of a computer-aided design (CAD) into a complex real-life 3D object [1]. 3D printing is an efficient method for creating 3D objects by layer-by-layer manufacturing. Being a relatively new manufacturing process, 3D printing is continuously developing. The definition of “4D printing” is the fabrication of 3D objects that can change their shape over time in response to an environmental stimulus [15,16]. These shape-morphing systems can respond autonomously to heat, light, pH change, electricity, moisture, etc. and have potential use as soft robotic systems, smart textiles, drug delivery, or biomedical devices [4,15–17]. The most important advantage of 3D printing is that almost any geometry that can be made as a CAD model can be printed, making it possible to print objects with complex geometry. This facilitates 3D printing for potential applications in engineering, robotics, and aerospace industries for creating complex and lightweight structures [18], architectural industries for structural models [19], art fields for artifact replication or education [19], and medical fields for printing tissues and organs [20]. When an engineered CAD design is correctly paired with one or more “smart” materials, 4D printing is

Fabrication of 3D and 4D polymer micro- and nanostructures based on electrospinning

possible. Many different 3D printing platforms have been used for the fabrication of polymers and polymer-containing composites. A popular and widely used technique is the so-called fused deposition modeling (FDM), stereolithography (SLA), selective laser sintering (SLS), inkjet 3D printing, and 3D plotting. There are many other 3D printing techniques that are used only by a small number of researchers or is still in development [21]. All of these methods have their upsides as well as their limitations [21]. The FDM technique works by controlled extrusion of a solid thermoplastic filament, which is melted into a viscous liquid at the print head that is moving in the x-y-z axes and is usually heated to 170–250°C (Fig. 1). The molten plastic is extruded onto the print bed in form of layers, where they fuse together and solidify (upon cooling down). The FDM technique is the most widely applied 3D printing method, since it is very simple to use, affordable, high speed, and multimaterial capability [2]. With this method, it is also possible to have multiple extruders, enabling the manufacturing of composites. However, sometimes it suffers from nozzle clogging, and the printed objects have a mediocre resolution (50 μm in z-direction) and a relatively high degree of anisotropy [21]. The SLS technique sinters powders applying a laser beam, which heats the powder particles, which fuse together [22]. This method produces objects with good strength and similar resolution to FDM printing. However, SLS printing is very expensive and messy to use due to the powdery surface. The SLA technique uses UV curing of liquid photosensitive polymers. This method uses a UV laser that shines on a well-controlled manner inside the resin reservoir, where polymerization of the photocurable resin will occur where the laser

Fig. 1 3D printing by the fused deposition modeling (FDM) method.

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focuses, and ultimately, a 3D object is made. The main advantage of this technology is its high printing resolution (10 μm in z-direction) [21]. Additionally, since there is no nozzle needed for the SLA printing, there are no issues with clogging. However, there are several concerns for its industrial application, as the SLA printing is expensive, there are very limited types of materials that can be used, the overall printing process is toxic, and the uncured resin is a waste [21]. 3D printing includes both product and process innovation, and it continues to advance low-cost prototyping [23]. In recent years, there has been a wide range of applications for 3D printing, as 3D-printed products (composite, metal, or plastic) have been used in medical devices, aviation (as airplane parts), and personalized clothes [23]. Even ceramic materials were 3D-printed with success, which can end the issue of high tool wear, aging, or the item becoming smaller (compared with parts made by the conventional techniques) [24]. The future of 3D printing is merging it with other existing techniques [23]. There has been a lot of progress made in developing 3D printing, still, there are many challenges ahead. Table 1 shows the three major current challenges of 3D printing: (1) the variety of materials that can be printed into 3D shape is limited; (2) the resolution of the 3D printed objects is limited; (3) limited knowledge and capabilities for printing smart 4D materials. Once these challenges are solved, the rise of 3D/4D printing will lead to improving existing products, as well as to extraordinary new innovations, e.g., fabrication of new functional nanomaterials. 3D printing is already changing the supply chains and conventional manufacturing across the world, and if the challenges with the current 3D printing technology are resolved, it will boost innovation through manufacturing novel products.

2.2 Electrospinning process Electrospinning is a very popular method to process solutions, melts, or even suspensions into continuous nano- or microfibers [25]. It is the only method for bulk producing continuous long nanofibers [6]. The process uses high voltage that charges the liquid inside a metallic capillary. A typical laboratory electrospinning setup is made of four main parts (Fig. 2): a syringe pump (with a syringe inside it), a metallic nozzle, a high-voltage power supply, and a collector (which is usually conductive) [26].

Table 1 Current challenges in 3D printing Challenges in 3D printing technology

• Limited materials that can be printed into a 3D shape • Limited resolution of the 3D printed materials • Limited knowledge/capabilities for printing 4D materials

Fabrication of 3D and 4D polymer micro- and nanostructures based on electrospinning

Fig. 2 Schematic drawing of a typical electrospinning setup.

2.2.1 Jet initiation and elongation The electrostatic force produced by the high-voltage power supply is applied to a polymeric viscous solution or melt, which is dispensed through the fine nozzle orifice at a controlled rate. When the voltage is aptly increased, the surface charge density on the liquid increases at the nozzle orifice, and a jet with a specific cone shape (Taylor cone) is formed [26–28] (Fig. 3). At this point, the geometry of the formed cones is directed by the electrostatic repulsion, the liquid surface tension stability, the incoming surface ratio and gravity [28,29]. When the voltage on the nozzle is increased further (above a threshold charge density), the repulsive electrostatic forces at the Taylor cone will overcome the surface tension of liquid, and electrospinning occurs [28,29]. 2.2.2 Growth of bending instability and further elongation The solution jet that is ejected from the Taylor cone is influenced by the electric field, and the present electrical instabilities will bend and elongate the jet (Fig. 3). The drawn polymeric thread is directed to the oppositely charged or grounded collector. Solidification of the liquid solution occurs by establishing a zone that thrusts the charged molecules, allowing for evaporation to happen rapidly as the jet advances toward the collector [29], leaving only the polymer fiber on the collector mat [30]. This transition between liquid and solid phase is due to the ohmic current being transited primarily to convective flow, thus increasing its acceleration [31]. The collected fiber is in the nano/microscale diameter. It is possible to control the main characteristics of the electrospun fibers, such as nanofiber diameter, porosity, surface area, shape, etc. [32,33].

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Fig. 3 Taylor cone forms as the surface charge density increases. Above a threshold charge density, the electrostatic forces overcome the surface tension of the liquid, and electrospinning occurs.

Among several methods for producing nanofibers [34–41], electrospinning has become the most used technique for nanofiber fabrication [42]. At the moment, it is the only technique that can fabricate continuous nanofibers from a large range of polymeric and ceramic materials [25]. Furthermore, electrospinning is a process that can be operated at industrial scales [7], and already dozens of companies are applying large-scale nanofiber production. Elmarco Inc. has already developed capabilities for fabricating nanotextured sheets at pilot scale in a roll-to-roll process [43]. Since electrospinning is a versatile technique with good control of the main product parameters, research has been made into making patterned structures and controlling the shape of the electrospun products [7,44–49]. Hence, its future holds lots of potentials for applications for biosensors, biomedicine (tissue engineering, wound dressing, drug delivery, and drug development), protective clothing, cosmetics, catalysis, adsorption, gas/water filtration, fuel cells, batteries, and supercapacitors [50–59].

2.3 Methods to fabricate 3D electrospun polymer micro- and nanostructures 2.3.1 Multilayer electrospinning The thickness of the electrospun mat increases with increasing electrospinning time during the conventional process. In some cases, prolonged electrospinning can lead to a relatively high thickness; however, the thickness is limited from tens to hundreds of microns. A method for creating a 3D macrostructure is by electrospinning nanofibers onto another electrospun fiber mats [60]. Many different 3D multilayered fibrous have been produced by sequential electrospinning [61] or coelectrospinning [62]. Another

Fabrication of 3D and 4D polymer micro- and nanostructures based on electrospinning

method, controlling the electric field, is applied to the electrospinning collector to align the collected fibers and form 3D fibrous structures [63]. Pham et al. developed 3D multilayered of polycaprolactone (PCL) micro- and nanofibers with a thickness higher than 1 mm by using coelectrospinning method [63]. The proposed multilayers PCL fibrous with porosities of about 84% and 89% have potential properties because of the microfibers layer. The average pore size of microfiber with the range of 10–45 μm was dependent on its fiber diameter. Moreover, cellulose acetate 3D scaffold with porous, cellular, and dense layer has also been fabricated using electrospinning [64]. The multilayer electrospinning technique can control and adjust the fiber diameter, porosity, and composition of each layer. This advantage can improve the biological properties of the scaffold such as cell attachment, proliferation, and migration [60]. 2.3.2 Stacking The typical output of a conventional electrospinning machine is a 2D fiber mat or membrane. As the electrospinning duration increases, the membrane thickness will increase, but the build-up keeps getting slower and eventually stops, depending on the dielectric properties of the electrospun material. Stacking fiber membranes is a way of achieving macroscopic 3D electrospun composite objects. An advantage of this method is that it allows the fabrication of a thick scaffold composed of aligned fibers in a single direction, or in mixed directions. However, the different layers need to be attached to each other, which sometimes needs the addition of binder agents. Orr et al. applied a pair of ceramic magnet - copper electrode set and arranged it in parallel at 10 cm apart over a water bath to collect aligned nanofibers [65]. The water is used to rest the collected fibers on its surface. The aligned fibers were lifted off the water and the parallel electrodes were stacked together (Fig. 4). The upper layer of human skin consists of keratinocytes, while the inner layer (which is far to the skin surface) comprises fibroblast [66]. With this composition of the skin, Yang et al. [66] fabricated a separation of electrospun PCL/collagen membranes by using electrospinning directly into a liquid bath filled with cell culture media. After that, the membranes were seeded with keratinocytes and fibroblast cells (Fig. 5) [66]. A layered structure was built up by placing the keratinocyte layers at the top and the fibroblast layers at the bottom [66]. The individual layers were tightly bonded together after the layered structure was cultured for 3 days [66]. When electrospinning and deposition of the hydrogel are carried out alternately, a 3D layered construct can be formed. Xu et al. [67] used a hybrid printing device between electrospinning of PCL and inkjet printing of rabbit elastic chondrocytes, which is suspended in a fibrin-collagen hydrogel. This creates a one-step method of constructing layered scaffolds with cells seeded without the need for manual membrane transfer and cell seeding. Binding agents such as hydrogel may also be used to bond or encapsulate the layers together [66]. Another method for binding the layers is by introducing a second solution

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Nonaligned

Aligned

Syringe pump

Syringe pump

Polymer solution

Focusing cage Power + supply

+

++ Layer of electrospun fibers + + + on surface +

Focusing cage

Glass slide for sequential collection of individual layers

Power + supply

Parallel copper electrodes Sodium chloride collecting bath

Ceramic magnet

+ ++ + + + +

Polymer solution Glass slide for sequential collection of individual layers

S N S N

Distilled water

Ground

Ground

Multilayered PCL scaffolds Fig. 4 Experimental setup demonstrating how PCL scaffolds cab be stacked together to form a 3D object [65].

after the layers have been stacked up, then using thermally induced phase separation of the second solution [68]. The added advantage of this method is that it introduces porous interfaces between the fibrous layers. 3D scaffold with pressurized gas heat treatment has also been shown to be effective in fusing the fiber layers and showed much better modulus and strength compared to untreated scaffold [69]. Cold welding under high pressure has also been used to create a fusion between the fiber layers, but this method significantly reduces the porosity and air permeability through it [70]. Beachley et al. [71] seeded myoblast of several sheets of polycaprolactone before stacking them together and using fibrin gel between the layers for adhesion and encapsulation of the cells. Sintering is another method to combine the layers. Wright et al. [72] used vapor and heat sintering to fuse layers of poly(DL-lactide) and poly(L-lactide) electrospun fibers. For vapor sintering, the layers were fused by exposure to tetrahydrofuran vapor (240°C) for 10 min while heat sintering was carried out at 54°C for 30 min [72]. Sintering has the advantage of

Fabrication of 3D and 4D polymer micro- and nanostructures based on electrospinning

Fig. 5 Electrospinning directly into cell culturing media then seeding cells can result in a 3D structure. The electrospinning and cell culturing steps should be repeated until the desired cell layers are obtained. This method makes it possible to use different polymer fibers and different cell types for the different layers. (Used with permission from Sukchanta Weraporn.)

increasing the mechanical strength of the material because of the fusion between the fibers. Between the two-sintering process, heat sintering is preferred as vapor sintering requires more accurate control. Slight extension in the duration of vapor sintering condition has been shown to cause significant degradation in the fibrous structure [72]. Another method of maintaining the integrity of the membrane-stacked structure is to have cells that are first cultured on the individual membrane before putting them together for a few more days to fuse the layers. To demonstrate the viability of culturing cells on separate layers and stacking, He et al. [73] used this technique to construct a cell-based 3D cartilage scaffold. First, they placed a single sheet of gelatin/polycaprolactone nanofibrous membranes in a well of 6-well plate. Then, they seeded the nanofibrous membrane with a cell suspension containing chondrocytes and bone marrow stromal cells. After this step, another sheet of a nanofibrous membrane was stacked on top of the first sheet followed by cell seeding [73]. This was repeated until they got 10 sheets of nanofibrous membranes. The cell-scaffold construct was cultured in vitro for 1 week before implantation into mice. A great advantage of this technique is that the complex distribution of cell types in an organ can be replicated.

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2.3.3 Application of 3D collecting template Since electrospun nanofibers can be deposited onto several shapes of the collector, using a collector template is possible to fabricate 3D fibrous structures. This technique is common nowadays because of the easy controllability. The collector templates could be static or rotating. Zhang et al. used 3D mechanical collector templates based on manipulation of the electric field and electric forces for fabrication of micro- and macro- single tubes. Moreover, the shapes, sizes, structures, and patterns of the tubes are controllable [74]. Fig. 6A and B shows the tubes fabrication process with a multiple interconnected and the as-prepared crossing fibrous tubular structure. It was found that the distance between the individual 3D collector templates is an important parameter for the fabrication of tubes [74]. For example, the excessively small distance between the 3D collector may cause fiber suspension. In addition, Chen et al. fabricated 3D fabrics tube of hydroxyapatite (HAp) nanorods composited with polyvinylpyrrolidone (PVP) by changing the collectors during electrospinning process. The 3D fibrous tube can be used as the substrate for mesenchymal stem cells culture by applying a heat treatment at 600°C for 6 h. The

Fig. 6 (A) Schematic of tube fabrication process by using multiple interconnected tubular structures [74] (B) a crossing tube [74] (C) photographic image of PLA 3D nanofiber structures fabricated by rotary jet-spinning technique at 12,000 rpm rotation speed [75] (D) SEM image of fibers shown is panel [75]. ((C) Reproduced with permission from ACS Publications. Copyright 2010, American Chemical Society.)

Fabrication of 3D and 4D polymer micro- and nanostructures based on electrospinning

results demonstrated biocompatibility and cells attachment properties [76]. Moreover, Badrossamay et al. fabricated 3D fibrous structures by directly rotary jet-spinning technique [75]. Fig. 6C and D shows combination of a rotary jet spinning at the rotation speed of around 12,000 rpm and the hydrostatic pressure with centrifugal pressure. This setup can fabricate poly(lactic acid) (PLA) nanofibers with 3D structure and contained aligned nanofibers [75]. The rotating collector increases the collection of 3D nanofibrous tubular scaffolds [75]. Zhou et al. investigated the influence of electrospinning parameters on the diameter and morphology of regenerated silk fibroin in the tubular scaffold [77]. It was found that the applied voltage and solution concentration had effects on the average diameter of the fiber, while the working distance and solution flow rate were found to significantly affect the fiber uniformity. Schneider et al. [78] fabricated scaffolds of poly(lactic acid-glycolic acid) (PLGA) composited with tricalcium phosphate (TCP) by using a rotating drum, which is loaded with dry ice and covered with aluminum foil. The nanofibers scaffold has a cotton-like appearance because of the ice crystals growth at the cold surface of the rotating drum [78]. The advantages of this technique are that it is a very simple processing step, does not require the inclusion of sacrificial material, allows control of the scaffold thickness by controlling the duration of electrospinning process, and retains the entire amount of an encapsulated drug [78]. The crystals will be acting as void spacers and, once sublimated in a vacuum oven, the porosity of the resulting scaffolds will be increased up to 99.5% [79]. This ultra-porous 3D scaffold allows the creation of reinforced cellular structures, even when using large (20 μm) cells. The collector templates can be other materials such as salt particles, ice crystal, polymers (e.g., poly(ethylene oxide) (PEO)). These materials usually are composited simultaneously with the solution during the electrospinning process to increase the growth rate of the nanofibrous 3D structure. The template materials will be removed after a 3D structure thickness is reached by drying or washing process [78,80]. Kim et al. produced a 3D nanofibrous scaffold of hyaluronic acid (HA) using salt particles as a template [80]. The salt particles with a proportion of 80% and 90% to the nanofibers were simultaneously deposited during the electrospinning by an automatic vibration sieving method. Then, the HA 3D scaffolds were crosslinked with collagen by dipping the scaffold in 1-ethyl-3-(3-dimenthylaminopropyl) carbodiimide hydrochloride (EDC) solution. After that, the scaffold was leached in water to remove the salt particles. With this method, the scaffolds have irregular open pore morphology, but the thickness was reduced by about 50%. A dually interconnected pore geometry with macropores and nanopores with sizes of 50–100 nm were observed and surrounded by nanofibers. In addition, the cell culture demonstrated that collagen could enhance cell adhesion and growth [81]. 2.3.4 Freeze drying into shapes One way to form a 3D block structure of electrospun nanofibers is to put short strands into a mold, and freeze dry them. Short fibers can be achieved by chopping, grinding, or

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chemical treatment. The produced short strands of electrospun fibers can form a 3D block structure if they are bonded together. Chen et al. generated short strand nanofibers by cutting electrospun gelatin/poly(L-lactide) membrane into small pieces (1 1 cm), then dispersing them in tert-butanol [82]. They placed this into a homogenizer, and its vigorous vibration was able to break up the nanofibers into short (on average 86 μm long) strands, forming a uniform dispersion [82]. This was poured into a mold and freeze drying was applied to form a 3D scaffold made of electrospun fibers. However, since this scaffold was loose and unstable, glutaraldehyde was used to chemically crosslink the electrospun these fibers. After the crosslinking, interconnected networks were formed between the fibers, producing a stable 3D scaffold. As the size of the macropores within this 3D structure can be controlled by adjusting the freeze-drying conditions, highly porous structures can be achieved by this technique. The resultant 3D scaffold is extremely lightweight, and it has been demonstrated to be able to float on cold air above liquid nitrogen [83]. The ultra-low density of freeze-dried foam makes these structures comparable to aerogel. These foam-like scaffolds have excellent thermal insulation properties, and compression tests showed that this 3D material has one of the lowest compressive strength against density [84]. Hydrogel, gelatin, or fibrin may be used as a binding agent to keep the short-strand fibers together. Rivet et al. [85] fabricated short-strand poly(L-lactide) (PLLA) nanofibers by aligning the PLLA nanofibers on a polyvinyl alcohol (PVA) thin film as release agent, then cutting the deposited PLLA fibers into small pieces of about 1 mm width, after that removing the PLA film by dissolving the short segments in water. The dispersed chopped PLLA fibers were then mixed in agarose/methylcellulose hydrogel, which was used as a binding agent. An external binding agent is not always required. With some materials that have a relatively low melting point, the short strands can be fused together by the application of heat. Xu et al. have demonstrated this using electrospun PCL fibers [86]. Grinding and liquid nitrogen were used for making the PCL fiber mats into short strands of nanofibers. The liquid nitrogen made the fibers brittle and, after grinding, they were isolated by using a sieve [86]. The sieved fibers were then suspended in a mixture of water, ethanol, and gelatin to form a uniform dispersion [86]. Then the suspension was heated to 55°C (just 5°C below the melting point of PCL), which resulted in the agglomeration of the nanofibers, as they fused together at random contact points [86]. 2.3.5 Self-assembly The key to forming 3D/4D structures is the self-assembly process of the electrospun fibers. This process is based on the rapid solidification of the nanofibers during production, which is combined with electrostatic induction and polarization of the deposited nanofibers, resulting in a self-standing object [87,88]. Theory suggests that the start of this process is charging the top part of the deposited nanofiber mats to negative by the electric field [87,88]. These negatively charged fibers become a preferential deposition area and attract the positively charged jet ejected from the electrospinning nozzle. Because the fibers at the top of the deposited layer have a negative charge, they will repel each other, and result in a

Fabrication of 3D and 4D polymer micro- and nanostructures based on electrospinning

spongy 3D structure. To increase the polarizability of the electrospun fibers, additives can be added in the solution for inducing the repulsive forces between the nanofibers during electrospinning process. Phosphoric acid (H3PO4) has been successfully used to results in 3D nanofibrous structures of the polymer polystyrene [89,90]. The polarization and static induction effects were investigated by using electrodes charged to either negative or positive high voltage (5 kV), to test whether they attract or repel the electrospun nanofibers during the fabrication process [91]. The study concluded that both the positively and the negatively charged rod attracted the freshly deposited polystyrene fibers. Other researchers found that the fibers were attracted by the positively charged electrode but repelled by the negatively charged one [87]. The latter study did not use high voltage, which might explain the different behavior of the fibers. In addition, it was also found that the fibers structures were barely repelled or attracted by a charged rod after the experiment for 12 h. This observation could be another reason for the different results with the earlier study. In the second work, the charged rod was placed above the collector of about 1.5 cm and under the electrospinning nozzle. These tests suggest that during the electrospinning process, indeed the freshly electrospun nanofibers are instantly polarized, allowing the 3D build-up process.

3 3D and 4D electrospinning technique 3.1 Basic principles In contrast to FDM 3D printing, where high temperature is used to melt thermoplastic filaments (see Fig. 1), 3D electrospinning applies a moving nozzle connected to a high voltage and a syringe that delivers the nozzle a polymeric solution (Fig. 7). Due to rapid evaporation and/or polarization and electrostatic induction, 3D macroscopic structures can be formed into shapes by the guided nanofiber assembly process. This technique is capable of creating 3D objects with a nanometer-scale resolution [91]. Furthermore, while FDM 3D printing applies a nozzle that is located very close to the substrate, 3D electrospinning is a noncontact printing technique that is suitable for nonplanar and complex surfaces of substrates. This can reduce damage from the nozzle touching the substrate during printing [92]. These 3D electrospun structures can potentially enhance nanomaterial properties and open new doors for several new applications. The 3D structure gives rise to an unprecedented and rare combination of biological (e.g., tissue regeneration rate, cell proliferation), mechanical (e.g., stiffness, strength), and mass transport (e.g., permeability, diffusivity) properties.

3.2 Apparatus In the technical aspect, 3D electrospinning is a combination of FDM 3D printing and conventional electrospinning (see comparison result in Table 2). A simple 3D electrospinning system can be constructed using an FDM 3D printer equipped with a

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Fig. 7 Illustration of a 3D electrospinning setup.

Table 2 Details of 3D electrospinning compared to FDM 3D printer and conventional electrospinning Equipment

FDM 3D printer

Electrospinning

3D electrospinning

x-y-z motion control Filament control High-voltage nozzle Solution control High-voltage control Ambient control Digital 3D model

✓ ✓ – – – ✓ ✓

– – ✓ ✓ ✓ ✓ –

✓ – ✓ ✓ ✓ ✓ ✓

Fabrication of 3D and 4D polymer micro- and nanostructures based on electrospinning

high-voltage source of around 5–30 kV with a few milliamps currents and a solution controller. Electronic board of the FDM 3D printer has the potential to control all equipment, i.e., the applied voltage and the solution flow rate. This section details the 3D electrospinning apparatus and some important remarks. When the electrospun 3D structure is capable of changing its shape or some properties over time or upon external stimuli, the process is called 4D electrospinning. 3.2.1 x-y-z axis motion control For a conventional electrospinning setup, a stationary conducting collector such as a circular or rectangular aluminum (or copper) sheet connects to the negative terminal (or ground) of a high-voltage power supply. Electrospun nanofibers are randomly collected on the stationary collector. To precisely control the deposition, the electrospinning nozzle can be replaced by a small silicon tip (diameter of about 0.5 mm) [93–95]. Then, controlling the nozzle movement by using a programmable x-y stage [96–98]. This technique has the ability to control fibers deposition to form letters or simple patterns. 3D electrospinning setup requires a motion control system for all x, y, and z-axes in order to form 3D structures. Generally, a printing extruder of FDM 3D printer is attached to a three-axis system that allows the machine to move in three directions. Therefore, the FDM 3D printer is very useful for using in the 3D electrospinning setup. Moreover, it is simple to control the speed of nozzle movement by setting the printing speed up to 120 mm s 1. 3.2.2 3D/4D electrospinning nozzle For the simple 3D electrospinning setup, only one polymer solution is loaded into a syringe and fed through a single needle with a diameter of hundreds of micrometers. In case the 3D/4D electrospinning has to print with two different materials or more, other smaller needles are inserted. This equipment setup is called “coaxial electrospinning.” It was first demonstrated and used for the production of monodisperse capsules with diameters varying from 0.15 to 10 μm [99]. This technique has also been applied to develop a tube (hollow structures) or core-shell type nanofibers. For example, a tube is fabricated by using a polymer solution as a sheath and feeding of heavy mineral oil as a core, then removing the mineral oil by heat treatment. Besides the coaxial electrospinning, the electrospinning device’s nozzle can be applied with nitrogen gas or hot air flow in order to enhance the electrospinning process. For example, electrospinning of hyaluronic acid (hyaluronan) has been obstructed because of its high viscosity and high surface tension [100]. The blowing gas or hot air reduces the viscosity and increases the evaporation rate of the solvent. This technique introduces an additional pulling force and improves the ease of electrospinning of some polymer solutions, such as ultra-highmolecular-weight polyethylene [101].

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3.2.3 Solution control The solution flow rate is controlled by a syringe pump at a certain rate of about 1–100 mL h 1. A solution feeder can assemble using a small digital linear actuator or a stepping motor. However, the solution control part should be tested for stability under a high voltage or very high electric field conditions. Additionally, the filament control part of the FDM 3D printer can be modified for controlling of the solution flow rate, but the filament feeding rate must be adjusted to appropriated values. In case the 4D electrospinning is based on the coaxial nozzle, the setup would require separately solution control for each solution. 3.2.4 High-voltage control The high-voltage source is necessary to fabricate nanofibers 3D structures. Typically, a direct current (DC) high-voltage power supply is widely used in electrospinning system. However, alternating current (AC) high-voltage power supply has been also used for nanofibers production [102–104]. Maheshwari et al. fabricated a nanofiber pattern that depended significantly on the frequency of the AC power supply [104]. In addition, multiple strands fibers of PVP have been produced by using AC high voltage of 3.33 kV at a frequency of 50 Hz and working distances of around 2–4 cm below the needle [104]. Both DC and AC high-voltage power supply can be controlled the values manually or through the FDM 3D printer electronic board. 3.2.5 Ambient control Electrospinning process should be operated inside a closed chamber with an air environment in order to control the temperature and the relative humidity. Moreover, the chamber protects a user from a high-voltage electrical shock and volatile solvents. In case the temperature and the relative humidity must be controlled at a certain value, the 3D electrospinning system needs to include a heater, a humidifier, or a hot air blower. A vacuum chamber can be also beneficial for the control of the 3D electrospinning process. Rangkupan et al. constructed an electrospinning system, including a vacuum chamber and found that the electrical break down strength is higher than the chamber with air environment [105].

3.3 3D and 4D electrospinning process 3.3.1 Digital 3D model design The first step in 3D and 4D electrospinning is the design of a 3D model. 3D shapes can be designed or created by using a computer-aided design (CAD) software [20] or a free online 3D design software such as TinkerCAD, OnShape, and Fusion 360. The designed 3D model must be exported as an STL (stereolithography) or OBJ file format. After that, a G-code will be generated by a slicing software [91]. The G-code file can be used directly in the 3D electrospinning machine because it works based on the FDM printer.

Fabrication of 3D and 4D polymer micro- and nanostructures based on electrospinning

3.3.2 G-code generation The 3D model in STL or OBJ file format must be processed with a slicer software in order to generate a G-code file that is readable by the 3D electrospinning machine. The G-code file contains instructions to the machine and tells the nozzle path to follow, controls the working distance between the nozzle and print bed, the moving nozzle speed and the syringe pump flow rate [91]. In addition, the applied high voltage can be controlled separately or through the G-code. The pattern in which the nozzle is moving (x-y planes) can be set for any motion, as well as the print bed movement (in z-direction) and even the time of the printing can be determined [91]. 3.3.3 Material preparation Under ambient conditions, the electrospinning process produces flat (2D) mats with most solutions [25,46,106], including polystyrene solutions. According to a study by Sun et al. [87], addition of ethanol to the polystyrene solution changes the solution properties and its electrospinning results in 3D self-assembly of the fibers. However, Vong et al. found that the addition of ethanol still produced flat mats in their 3D electrospinning experiments [91]. It might be that the addition of ethanol needs to be combined with certain environmental conditions (temperature and relative humidity). Vong et al. managed to form 3D nanofibrous structures with the addition of 100 μL phosphoric acid (H3PO4) to the previously dissolve polystyrene in DMF/THF mixture (1:1) ratio [91].

3.3.4 Printing 3D nanofibrous materials When certain additives are incorporated in the solution during the electrospinning, macroscopic 3D structures can be obtained [91,106]. Until now, this process was reported only with the polymer, polystyrene, and polyvinylpyrrolidone [91,106]. The 3D build-up mechanism is not fully understood yet. Several researchers found that the mechanism might be based on the rapid solidification of the nanofibers, allowing the macrostructure to be standing on its own [87,88]. Another theory suggests that the electrostatic induction and polarization of the deposited fibers are the keys to forming 3D structures with nanofibers [87,88,91]. Fig. 8 illustrates these proposed mechanisms for the formation of 3D nanofibrous structures. There needs to be an initial nanofibrous deposited layer on the collector first. This initial layer will be polarized by the strong electric field that is present. The static induction and polarization will result in electrostatic repulsion between the forming fibers. After the rapid solidification of the nanofibrous structure, a solid, macroscopic, and “spongy” 3D object is formed. The formation of electrospun nanofibers is rapid. Within 10 min, large macroscopic shapes with a height of 3–4 cm can be generated using this technique, as shown in Figs. 9 and 10. Li et al. have produced a 17-cm-tall 3D object in just 20 min [106].

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Fig. 8 A proposed mechanism for the formation of 3D nanofibrous structures. Static induction and polarization of the electrospun nanofibers result in electrostatic repulsion between the forming fibers. After the rapid solidification of the high-surface-area object, a solid 3D nanofibrous object is formed. (Adapted from M. Vong, E. Speirs, C. Klomkliang, I. Akinwumi, W. Nuansing, N. Radacsi, Controlled three-dimensional polystyrene micro- and nano-structures fabricated by three-dimensional electrospinning, RSC Adv. 8 (2018) 15501–15512. https://doi.org/10.1039/c7ra13278f.)

Fig. 9 The evolution of the electrospun polystyrene fibers into a 3D structure over time. The 3D buildup results in a 4–5 cm tall cylinder within 10 min. (A) The 3D build-up is initiated. (B) Within 3 min the 3D structure is already observable. (C) The programmed cylinder shape is evolving. (D) Large 3D cylinder is standing in just 10 min of 3D electrospinning. (Reproduced from the study performed by M. Vong, E. Speirs, C. Klomkliang, I. Akinwumi, W. Nuansing, N. Radacsi, Controlled three-dimensional polystyrene micro- and nano-structures fabricated by three-dimensional electrospinning, RSC Adv. 8 (2018) 15501–15512. https://doi.org/10.1039/c7ra13278f.)

Fig. 10 Different 3D nanofibrous polystyrene structures electrospun by Vong et al. [91]: triangle, square, five-star polygon, and a smiley face. The structures have a height between 2 and 5 cm, and have been fabricated in a 10-min electrospinning deposition. (Adapted from M. Vong, E. Speirs, C. Klomkliang, I. Akinwumi, W. Nuansing, N. Radacsi, Controlled three-dimensional polystyrene micro- and nano-structures fabricated by three-dimensional electrospinning, RSC Adv. 8 (2018) 15501–15512. https://doi.org/10.1039/c7ra13278f.)

Fabrication of 3D and 4D polymer micro- and nanostructures based on electrospinning

3.3.5 Finishing Finishing process can be used for cleaning or improving some properties of the 3D nanofibers structure before using them. In biomedical applications, the 3D electrospun fibers should be sterilized by ultraviolet (UV) light or soaking in 70% ethanol for at least 30 min and then washed with deionized water. For improving mechanical properties, heat treatment can be used because it allows movement in the molecular chains. For example, polyacrylonitrile (PAN) electrospun nanofiber membrane with a tensile strength of 8.73 MPa was finished by using the hot-pressing technique at 220°C. The tensile strength of PAN membrane was highly increased to 63 MPa [107].

3.4 Characterization 3.4.1 Optical microscopy Physical morphology of electrospun fibers can be checked by using an optical (light) microscopy. It is simple in sample preparation and the measurement takes place under the atmospheric pressure. Although the optical microscope has some error due to the diffraction limit, it is fine enough for checking the success of the electrospinning process, or preliminary examinations of the electrospun nanofibers during the fabrication process. The optical microscope has the advantage of being much faster than other imaging techniques. 3.4.2 Scanning electron microscopy The shape, size, and porosity of the 3D/4D electrospun fiber structures can be studied with a scanning electron microscope (SEM). Prior to observation, the nonconductive polymeric structures are suggested to be coated with a thin gold or similar conductive layer by using a sputter coater, unless the electrospun fibers are conductive. 3.4.3 Surface area measurements The specific surface-area-to-volume ratio and the pores size of the 3D electrospun porous scaffold are important parameters for using them in biomedical applications, including cell adhesion, growth, and proliferation. There are several techniques that can be employed to obtain the surface area of these scaffolds. Mercury porosimetry is a technique used to compare pore size distributions of the 3D structures. However, the mercury porosimetry can provide misleading results due to the mechanical deformation of the 3D scaffold. Another technique used to evaluate the surface area is the Brunauer-Emmett-Teller (BET) surface area analysis, which is based on physical adsorption of the gas on the nanofiber surface. BET is the most frequent technique used for characterization of the specific surface area of 3D porous materials with pores diameter below 10 nm.

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3.5 Processing parameters The assembly of the polymers into 3D structures largely depends on the process parameters. So far, the key process parameters that have been identified are the (i) solution properties, (ii) applied voltage, (iii) working distance, (iv) flow rate, (v) nozzle speed, (vi) temperature, and (vii) relative humidity. Most of these parameters were investigated by Vong et al. using polystyrene as a polymer [91]. 3.5.1 Solution parameters There are four main solution parameters that play important role in the 3D assembly process [91]: the concentration, the viscosity (determined by the molecular weight and concentration of the polymer), the conductivity, and the pH. With increasing polymer concentration in the electrospinning solution, the viscosity of the solution increases. This results in increased resistance toward the thinning of the diameter of the emitted jet from the nozzle [108]. Thus, the increased solution concentration leads to a straighter and longer jet trajectory before bending starts due to the instabilities in the electric field. In this case, the deposition area is smaller than with low solution concentrations. This explains why Vong et al. found that higher concentrated polystyrene solution led to better precision of the 3D electrospun structures than lower concentrations [91]. The lower concentrations had a large deposition area, while higher concentrations led to well-defined cylinders (see Fig. 11A). The polymer concentration also determines the height of the 3D nanofibrous objects. When using lower polystyrene concentrations (7.5 and 10 wt%), Vong et al. found that the electrospun 3D objects were shorter than the ones prepared with higher polystyrene concentrations (12.5 and 15 wt%) [91]. At very low concentrations (below 7.5 wt%), the electrospun 3D structures had no distinguishable features [91]. Vong et al. suggested that there was a threshold concentration, which was needed to be reached for the polarization effect and charge induction to have enough repulsion for the fabrication of the 3D nanofibrous structure [91]. The diameter of the electrospun fiber linearly increases with the increasing polystyrene concentration, as seen in Fig. 11B [91]. The same trend has been observed in several other studies [109–111]. Furthermore, the formation of beads was observed at relatively low polystyrene concentrations [91,112,113]. Beads appear when the solution viscosity is low, leading to an intermittent jet formation [114]. 3.5.2 Applied voltage Vong et al. have also investigated how the applied potential difference influences the 3D electrospun object [91]. They tested the voltage between 6 and 20 kV, while the other electrospinning parameters were fixed. They found that under +7 kV voltage on the nozzle, the ejected jet was unstable, and solution dripping was also observed [91].

Fabrication of 3D and 4D polymer micro- and nanostructures based on electrospinning

Fig. 11 (A) 3D electrospinning polystyrene solutions from concentration between 5 and 15 wt% [91]. (B) Plot of the mean fiber diameters as a function of the polystyrene concentration, showing an increase in the fiber diameter with the increasing polystyrene concentration [91].

The 3D electrospun shapes were very similar to each other (Fig. 12) [91]. They found that the sample made at +10 kV resembled the most the designed CAD model. They also found that when applying a lower potential, more time is needed to start the 3D build-up of the 3D structures. The potential difference directly influences the electrospun fiber diameter [91]. In the case of the study done by Vong et al., the nanofiber diameter showed a nonlinear relationship, where the mean fiber diameter was first decreasing with increasing voltage, then (above +11 kV) it started to increase with the increasing potential difference [91]. 3.5.3 Working distance Vong et al. also investigated the influence of working distance between 1 and 15 cm on the 3D build-up [91]. They found that above a certain distance between the

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Fig. 12 3D electrospinning polystyrene cylinder structures as the applied voltage on the nozzle is increased from +7 to +20 kV, age inside the cylinder. (Reproduced from the study by M. Vong, E. Speirs, C. Klomkliang, I. Akinwumi, W. Nuansing, N. Radacsi, Controlled three-dimensional polystyrene micro- and nano-structures fabricated by three-dimensional electrospinning, RSC Adv. 8 (2018) 15501–15512. https://doi.org/10.1039/c7ra13278f.)

electrospinning nozzle and the collector plate (10 cm), there was no observed 3D buildup [91]. Also under a certain threshold (below 2 cm) no 3D objects were found, the nanofibers were not completely dry, and sparks could also occur [91]. However, printing precision is increased as the working distance decreases [115,116]. When Vong et al. used working distances between 3 and 9 cm, the 3D nanofibrous shapes resembled the programmed cylinder shape (see Fig. 13) [91]. The cylinders that were 3D electrospun at 3–4 cm working distances were relatively short, and the top part was not completely dry during the fabrication process [91]. When the working distance was higher (above 7 cm), the cylinder shape was distorted, and a filling of the hollow fibers was observed inside of the cylinder. It is known that as the working distance is increases, the deposition area of the electrospun nanofibers keeps increasing [117]. At a certain working distance, it is also possible to create 3D cylinders with their interior filled with nanofibers [91]. Vong et al. also found that the shape of the electrospun cylinders at higher working distances resembled an elliptic shape from a top view, instead of

Fig. 13 The influence of the working distance on the 3D build-up process and object shape, using polystyrene. (Reproduced from the study performed by M. Vong, E. Speirs, C. Klomkliang, I. Akinwumi, W. Nuansing, N. Radacsi, Controlled three-dimensional polystyrene micro- and nanostructures fabricated by three-dimensional electrospinning, RSC Adv. 8 (2018) 15501–15512. https://doi. org/10.1039/c7ra13278f.)

Fabrication of 3D and 4D polymer micro- and nanostructures based on electrospinning

Fig. 14 The correlation between the solution flow rate and the 3D structure shape. (Reproduced with permission from M. Vong, E. Speirs, C. Klomkliang, I. Akinwumi, W. Nuansing, N. Radacsi, Controlled threedimensional polystyrene micro- and nano-structures fabricated by three-dimensional electrospinning, RSC Adv. 8 (2018) 15501–15512. https://doi.org/10.1039/c7ra13278f.)

the programmed circular one [91]. Furthermore, it was found that the increase in the working distance results in the decrease of the nanofiber diameter, and the increase of the 3D shape height. The fiber diameter decreases as the jet can travel longer, resulting in increased thinning and stretching of the jet and the resulting nanofibers [118]. 3.5.4 Solution flow rate Solution flow rates between 1 and 20 mL h 1 were tested by Vong et al. [91]. They found that applying lower solution flow rates resulted in a low deposition rate onto the collector [91]. Fig. 14 shows the correlation between the flow rate and the 3D build-up. At very low solution flow rate (below 1 mL h 1) no 3D build-up was observed, while at flow rates of 3 and 4 mL h 1, they observed 3D structures with relatively low height [91]. They speculate that the low flow rate resulted in a low amount of electrospun polymer fibers, which cannot build up a large 3D structure [91]. At a high flow rate (above 7.5 mL h 1), the electrospun 3D structures did not resemble the programmed cylinder structure and were filled with nanofibers [91]. This leads to a decrease of the 3D print quality. At increased solution flow rate, there might be not enough time for complete drying of the fibers, or dripping could be observed. It is known that increased flow rates result in increased solution volume ejected from the nozzle [119]. The increased volume of the liquid solution needs more drying time. 3.5.5 Nozzle moving speed Controlling of the moving speed of the nozzle between 0.6 and 24 mm s 1 on the 3D structure was investigated by Vong et al. [91]. They found that as nozzle moving speed decreased, the final 3D object resembled less the designed cylinder [91]. With increasing nozzle speed, the inside of the cylinder was showing an increasing amount of nanofibers, and the cylinder size increased more horizontally than vertically (Fig. 15) [91]. Vong et al. concluded that at slower nozzle speeds the fiber diameters are larger, as the drying speed is restricted, yielding increased fiber diameters [91].

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Fig. 15 The correlation between the printing nozzle speed and the 3D structure shape [91]. (Reproduced from the article published by M. Vong, E. Speirs, C. Klomkliang, I. Akinwumi, W. Nuansing, N. Radacsi, Controlled three-dimensional polystyrene micro- and nano-structures fabricated by threedimensional electrospinning, RSC Adv. 8 (2018) 15501–15512. https://doi.org/10.1039/c7ra13278f.)

3.5.6 Temperature and humidity The ambient parameters in the electrospinning chamber, i.e., relative humidity and temperature affect the electrospinning process and electrospun fibers morphology. Typically, the evaporation rate of volatile solvents increases with increasing temperature. In addition, the diameter of electrospun fibers decreases or increases depending on the chemical nature of the substance [120,121]. The temperature and the humidity can be monitored by a thermo/hygrometer. For 3D electrospinning, the relative humidity inside the chamber is recommended to be controlled at below 30% in order to increase the evaporation rate of the solvent.

4 Potential applications 4.1 Biomedical applications 4.1.1 Tissue engineering and drug development Electrospinning is among the most conventional fabrication techniques applied for tissue engineering, found on the fact that electrostatic forces can be easily used to form and expand fibers out of a polymer solution [29]. In the past decade, 3D bioprinting became a very competent approach for engineered tissues, as it allows the fabrication of discrete patches and patterns, via the exact positioning of organic and inorganic biocompatible materials and living cells, in a layer-by-layer manner [122]. Up till now, bioprinted products have been limited in diameter to the microscale, causing problems to the development of microvasculature, which is of vital importance for the maturation of the multicellular-tissue construct [123]. 3D nanostructures are desirable as they resemble the native extracellular matrix, provide good attachment sites for the cells [124,125], and are capable of inducing vasculogenesis among the assorted tissue layers [126]. Several 3D nanofabrication methods led to scaffolds with low porosities, and with an unbalanced distribution of the pores, leading to structures that are not applicable for tissue engineering [60]. However, electrospinning can provide a well-controlled pore structure

Fabrication of 3D and 4D polymer micro- and nanostructures based on electrospinning

that is geometrically organized in 3D, which is optimal for tissue formation, vascularization, and nutrient dispersion [127]. 3D structures are advantageous for generating tissues for wound healing, artificial tissue/bone/ligament, and drug development [125,126]. As for the latter, drugs are usually tested in animal models before moving to human clinical trial. These are very lengthy and expensive processes. Furthermore, most drugs that work in animal models, fail in human clinical trials [128]. 3D electrospinning can potentially form realistic models of tissues and organs mimicking the corresponding organisms at a high level. 4.1.2 4D nanomaterials 4D electrospinning technology can enhance the engineering of new biomedical products, which are capable of adapting to users’ demands, biometric information [4], temperature [129,130], sweat [4], pH [130], magnetic field [131], or pressure [4]. Shape-memory materials, which can be implanted in the human body by minimally invasive surgery, have been fabricated for use as smart 3D nanostructures [132,133]. These “smart” fibrous 4D nanomaterials can change their shapes as the temperature changes [132]. These 4D nanofibrous materials have potential use for hernia repairs [133], vascular stents [133], vascular grafts [134], and bone void fillers [135]. 4D nanomaterials have great potential for drug delivery on demand and targeted drug delivery. Imagine a future where drugs can release their active ingredients on demand. There are dozens of polymers that are responsive to pH, glucose content, or heat [130]. Depending on the solution temperature, pH, or glucose content, many polymers change their shapes by self-assembly and transform into nanostructured objects with different shapes [130]. This shape-morphing property makes these materials promising for different applications, e.g., they can be potentially used for controlled drug release by carrying drug molecules inside them [130]. 4D “smart” drug nanomaterials could also fight cancer cells when they are close to the cancer tissue, as the pH of cancer cells is lower than the pH of healthy cells [136,137]. Another application could be fighting diabetes; the drug will release its active ingredient, when the glucose level in the blood is above a certain threshold, lowering the glucose level back to normal. 4D composite nanostructures could also be used as actuators, e.g., soft robotics. In order to make an actuator, the nanocomposite should be fabricated by applying two different solutions in the 3D electrospinning apparatus. These composite materials could be produced by applying either side-by-side or coaxial electrospinning. To allow its reversible movement, the composite structure should be made of two polymers: one that that is responsive to the external stimulus, and one that is stagnant, to give mechanical support and control the direction of the movement. The nanostructured actuator objects could be based on the well-studied thermoresponsive polymers, e.g., poly (N-isopropylacrylamide), which can change its shape reversibly [138,139]. This polymer could be prepared as a composite with polystyrene, a polymer that forms nano-3D structures [91].

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Another possible application for 4D nanomaterials is the fabrication of artificial nanostructured muscles. This could be achieved by using, e.g., the piezoelectric polyvinyledenedifluoride–trifluoroethylene copolymer together with C60/carbon nanotubes or polysiloxane elastomers containing trifluoropropyl groups will be electrospun with a polymer that results in 3D build-up [140]. The final 3D product could eventually change its shape, similar to the beating heart, or our lungs, as we inhale and exhale.

4.2 Energy applications 4.2.1 Batteries Lithium-ion batteries have revolutionized our lives, but remain an important challenge of modern materials science, as the need for longer lasting batteries is increasing [141]. However, in the last decade, there was no significant breakthrough in battery development. We need to progress the science in order to advance battery technology for several applications such as consumer electronics, clean energy storage system, and electric vehicles. Further breakthroughs are needed, as it is crucial to find novel solutions for advancing this technology. One possible solution is the development of nanostructured materials [142]. However, in order for these materials to be effective, a 3D porous structure is required. 3D electrospinning can fabricate highly porous 3D nanostructured anodes for Li-ion batteries [143]. These nanostructures not only enhance the energy density by the increased surface area, but mass transport of the ions and electrons [144], the lifetime and the lithium-ion diffusivity across the electrode/electrolyte interface [145]. The 3D nanostructure can also lead to improved physical interactions between the electrocatalyst and its support material [146]. Nanofibers made of PAN, polyvinyl alcohol, or phenolic resin can be simply converted into carbon nanofiber nonwovens by high-temperature sintering under nitrogen or argon, and the resulted carbon nanofiber materials can be used as advanced battery electrodes [143]. Nanofibers composited with various additive nanoparticles (e.g., graphite particles, carbon black, or carbon nanotubes) are used to enhance their electrical conductivity [147]. In recent years, the use of silicon (Si) was introduced to replace the conventional carbon (graphite) electrodes because Si has about 10 times higher theoretical capacity than the state-of-the-art carbon-based anode material [148]. Thus, Si could boost the energy density of batteries by a factor of 10. However, when Si is used in the structure, it changes volume by 320%, diminishing the opportunity for Si to be used in real-life products [145,149]. The solution to this problem is the scalable fabrication of SidC anode in a spongy 3D structure, which will allow for the large cyclic volume change, and enable the high-energy density [148,150]. Electrospinning has been successfully used for the fabrication of LiFePO4/carbon nanocomposites by mixing LiFePO4 into a PAN solution [151]. It has been observed that the composite nanomaterial had improved thermal stability, exhibited 107 times higher conductivity (after heat treatment) than that of raw LiFePO4, and the battery capacity increased with 21% [151].

Fabrication of 3D and 4D polymer micro- and nanostructures based on electrospinning

4.2.2 Fuel cells The most efficient conversion of chemical energy to electrical energy is done by fuel cells, and the conversion happens without combustion. Yet, current fuel cell technologies have not penetrated energy markets to any significant extent. Barriers to a widespread market entry include high cost per unit power output and insufficient lifetime. The current fuel cell electrodes are nano- or microstructured, good conductors for ions and electrons, highly porous, and have superior catalytic activity [142,152,153]. 3D electrospun nanostructures can further improve the conductivity at the interface due to their smaller size, the connection between the electrode, catalyst, and electrolyte is better than using micron-size or larger materials. This could lead to new improved physical interaction between the electrocatalyst and its support materials, an area where optimum design is still lacking [146]. The 3D structure will enhance further the catalytic activity, as the high surface area provides increased reaction sites at the so-called triple-phase boundary, where the catalyst, electrolyte, and gas phase meet. In the case of proton exchange membrane (PEM) fuel cells, the electrolyte is usually Nafion, a synthetic polymer that conducts protons at low temperatures (6.46); (B) simple sketch of the pH-mediated assembly mechanism of BCS and GO ink; (C) piled ink filament upon extrusion; and (D) fine detail of the printed line at the macroscopic image. (E) Optical image showing the surface of a filament after printing. (Reprinted with permission from E. Garcia-Tunon, S. Barg, J. Franco, R. Bell, S. Eslava, E. D’Elia, R.C. Maher, F. Guitian, E. Saiz, Printing in three dimensions with graphene, Adv. Mater. 27(10) (2015) 1688–1693. Copyright (2015) John Wiley and Sons.)

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Fig. 4 Schematic illustration of the fabrication process. From left to right: GNPs mix with EGB, DBP, and PVB to achieve a homogenous suspension after ultrasonication. The printable graphene ink was obtained by evaporating ethanol solvent until the suspension reached a gel-like state. The graphene ink was then extruded through a nozzle to form a 3D scaffold. (Reprinted with permission from K. Huang, J. Yang, S. Dong, Q. Feng, X. Zhang, Y. Ding, J. Hu, Anisotropy of graphene scaffolds assembled by three-dimensional printing, Carbon 130 (2018) 1–10. Copyright (2018) Elsevier.)

Fig. 5 SEM images of 3DP graphene scaffold: (A) cross-section area of a graphene scaffold and (B) magnified fracture area, (C) exterior, and (D) cross-section area of the filament as denoted in (A). (Reprinted with permission from K. Huang, J. Yang, S. Dong, Q. Feng, X. Zhang, Y. Ding, J. Hu, Anisotropy of graphene scaffolds assembled by three-dimensional printing, Carbon 130 (2018) 1–10. Copyright (2018) Elsevier).

Graphene and GO-reinforced 3D and 4D printable composites

Electrostatic interaction It has been demonstrated in many systems that electrostatic interaction is another driving force for assembling GO and graphene sheets [57,58]. With carefully selected additives, such a mechanism could also be utilized to prepare printable GO inks by electrostatic force-induced gelation. A good example is polyethylenimine (PEI), a water-soluble polyelectrolyte that can induce the rapid gelation of GO sheets due to electrostatic interaction [59]. Tubı´o et al. demonstrated the fabrication of 3D-printed rGO-Al2O3 composites with complex morphology using the formulation [60]. The GO-Al2O3 inks have shown an apparent viscosity one order of magnitude higher than the pure Al2O3 ink and a low shear stress storage/loss modulus. The final rGO-Al2O3 composites were obtained and maintained a well-defined structure after sintering the as-printed structure at 1600°C in an N2 atmosphere. Similar examples have been reported based on graphene suspension precursors [61,62]. Normally, colloidal graphene inks were developed by first creating a welldispersed suspension of graphene and a PEI aqueous mixture with the aid of surfactants and strong shear mixing, followed by the addition of an anionic polyelectrolyte such as ammonium polyacrylate (APA) to induce flocculation. The flocculated colloidal ink exhibited a good printability with an extraordinarily high solid content (60–71 wt.%). In addition to the polyelectrolyte, metal ions such as Ca2+ and Mg2+ could be used as cross-linkers to realize the gelation of GO as well [59]. The phenomenon stems from the coordination effects of metal ions with hydroxyl and carboxyl groups, and has been demonstrated to strengthen the GO fiber and paper earlier [63,64]. It is worth pointing out that the successful gelation of the GO solution that can only take place using the divalent and trivalent ions (e.g., Ca2+, Mg2+, Cu2+, Pb2+, Cr3+, Fe3+). It is because good cross-linking requires strong ionic strength and alkali metal, and alkaline-earth metal ions possess insufficiently large coordination stability constants [59]. In 2018, a group at Zhejiang University demonstrated a facile metal ion-induced gelation method to fabricate graphene aerogel microlattices with designated 3D structures [65]. The method has achieved optimal rheology of the GO ink by putting a trace of CaCl2 in the GO suspension as an efficient cross-linker. In Fig. 6, the coordination interaction between the Ca2+ and GO sheets and the printing process is schematically illustrated. It can be seen from the rheological measurements that the addition of ions significantly increases the viscosity and storage/loss modulus by approximately one order of magnitude, ensuring smooth printing. Compared to other existing strategies, this method requires a very small amount of additive (add up to 5.8 wt.% of the solid content), avoiding the property degradation caused by a heavy dose of additives. Presently, this method has been extended to fabricate a highly stretchable graphene-CNT hybrid aerogel [66].

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Fig. 6 (A) Schematic illustration of metal-ion crosslinking graphene DIW process. Following the arrows: In the GO aqueous suspension introduced a minor amount of CaCl2. The obtained GO gel is then extruded through a nozzle and deposited by a robotic hand to fabricate 3D microlattice. The microlattice is then subject to freeze drying and HI reduction to get a solid 3D rGO microlattice. (B, C) The rheology behavior of GO suspension and metal ion crosslinked GO gel (Reprinted with permission from Y. Jiang, Z. Xu, T. Huang, Y. Liu, F. Guo, J. Xi, W. Gao, C. Gao, Direct 3D printing of ultralight graphene oxide aerogel microlattices, Adv. Funct. Mater. 28(16) (2018) 1707024. Copyright (2018) John Wiley and Sons.)

Graphene and GO-reinforced 3D and 4D printable composites

Reactive inks Instead of elevating the storage modulus to an adequately high level, the reactive inks strategy could be an alternative. It involves a quick gelation or phase shift through chemical reaction upon applied stimuli after the deposition. Worsley’s group has developed a rapid gelation route of DIW 3DP with GO inks, using sol-gel chemistry [67]. The resorcinol (R) and formaldehyde (F) were used in a mole ratio of 1:2 to formulate the reaction, and sodium carbonate was introduced to catalyze the process. Previously, this sol-gel chemistry has been frequently exploited in the preparations of graphene-based macroassemblies [68,69]. The curing of the printed structure was carried out at 85°C in a sealed glass vial, followed by a thorough wash with acetone and supercritical dry treatment. The final aerogel was obtained after thermal annealing reduction and a hydrofluoric acid wash to etch away the residues. Lastly, they developed graphene composite aerogel-based supercapacitors, employing the same sol-gel chemistry [70]. Graphene nanoplatelet (GNP) was added to improve the electrical conductivity, which is essential for high-performance supercapacitors. With the incorporation of graphene, the developed ink maintained a good processability and quick gelation upon postprinting treatment, showing a versatile rheological tailoring ability. Additive rheology effects The last class of additive strategy directly exploits the viscoelastic nature of additives, usually polymers or a polymer solution, and makes graphene functional fillers to meet the printable requirements. Jakus and coworkers developed a printable graphene ink comprising graphene flakes and the biocompatible, biodegradable polylatide-co-glycolide (PLG) in graded volatility solvents [71]. The randomly oriented graphene flakes were homogenously suspended in a dissolved elastomer solution in the relatively viscous graphene-based ink. Upon extrusion, shear forces induced reorientation and alignment of the flakes along the direction of flow, facilitating the process. In addition, the rapid evaporation of the high vapor pressure solvent played a critical role in producing a self-support filament upon extrusion. The strategy permits extremely high graphene solid content (60 vol.% of solid) in the inks and a high printing speed (45 mm/s) with a nozzle diameter ranging from 100 to 1000 μm. The resulting graphene architectures are mechanically strong with high flexibility. What’s more, the excellent intrinsic properties of graphene such as the mechanical properties and electrical conductivity can be restored by part or complete removal of the polymer addition through further thermal annealing. Rocha and his team developed aqueous thermoresponsive ink for fabrication of a graphene-based electrode [72]. The printable ink was prepared by mixing the freezedried GO powders with Pluronic F127 aqueous solution, followed by homogenization and defoaming. Pluronic F127, which is a triblock nonionic copolymer, (poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO)), plays a key part

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Fig. 7 Effect of temperature on the storage/loss modulus of CMG (rGO)/Pluronic F127 and Cu/Pluronic F127 ink. (Reprinted with permission from V.G. Rocha, E. Garcia-Tunon, C. Botas, F. Markoulidis, E. Feilden, E. D’Elia, N. Ni, M. Shaffer, E. Saiz, Multimaterial 3D printing of graphene-based electrodes for electrochemical energy storage using thermoresponsive inks, ACS Appl. Mater. Interfaces 9(42) (2017) 37136–37145. Copyright (2017) ACS).

in defining the rheology of the mixture in the ink. When the temperature is below the lower critical solution temperature ( > ktd = ðMn ⦁Þ + ðMm ⦁Þ ! Mn + Mm ðdisproportionationÞ Termination (8) kin > > R⦁ + IN ! Q ðinhibitionÞ > > ; koc ðMn ⦁Þ + ðMm ⦁Þ ! fMn ⦁g + fMm ⦁g ðocclusionÞ The corresponding rates of initiation (Ri), propagation (Rp) and termination (Rt) can be expressed as follows [26, 28]: Ri ¼ ϕi Ia

(9)

Rp ¼ kp ½P⦁½M 

(10)

Rt ¼ kt ½P⦁2

(11)

where [M] is the monomer concentration and [P ⦁] is total the concentration of polymer chain radicals. The term фiIa is determined from light intensity measurements, where фi is the quantum yield of initiation and Ia is the light intensity absorbed by a photoinitiator at a depth of z [40]:  I0  Ia ¼ 1  e2:303ε½PI z (12) z where [PI] is the photoinitiator concentration, ε is the molar extinction coefficient of the photoinitiator at the wavelength emitted by a light source, and I0 is the incident light intensity at the resin surface (z ¼ 0). The rate of monomer depletion (overall rate of photopolymerization) can be expressed as [31]: 

d½M  ¼ Rp ¼ kp ½P⦁½M  ¼ kp ½P⦁½M 0 ð1  αÞ dt

(13)

where α is the monomer conversion and [M]0 is the initial monomer concentration and [M] is a monomer concentration at a given conversion, i.e., [M] ¼ [M]0 (1 α). The total concentration of polymeric radicals [P ⦁], which is the sum of concentrations of all (Mn ⦁) chains, assuming steady-state is given by the following expression [31]: d ½P⦁ ¼ Ri  Rt ¼ ϕi Ia  kt ½P⦁2 dt

(14)

To obtain kp and kt, usually, a quasi-steady-state assumption is used, which requires Ri being equal to Rt. [41] Therefore, Eq. (14) is reduced to [28]:

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Ri ¼ kt ½P⦁2

(15)

Rearranging this relationship for [P ⦁] and substituting into Eq. (13) gives [28]: rffiffiffiffiffiffiffiffi rffiffiffiffiffi d ½M  Ri ϕi Ia ¼ Rp ¼ k p ½ M   (16) 5 kp ½M  dt kt kt gives: Rearranging the above (Eq. 16) for kp/k1/2 t kp d½M =dt d½M =dt pffiffiffiffi ¼ pffiffiffiffiffiffiffiffi ¼ pffiffiffiffiffiffiffiffi kt ½M  ϕi Ia ½M 0 ð1  αÞ ϕi Ia

(17)

Under nonsteady-state conditions taking place during dark polymerization, no initiating radicals are produced [42]. Therefore, when there’s no light exposure applied (Ia ¼ 0), Eq. (14) becomes [40]: d½P⦁ ¼ kt ½P⦁2 dt

(18)

At t ¼ 0, the initial condition is given by: ½P⦁0 ¼

ðd½M =dt Þ0 kp ½M 0

(19)

Integrating Eq. (18) yields: 1 1 ¼ kt t + ½P⦁ ½P⦁0

(20)

Rearranging the Eq. (13) for [P ⦁] gives: ½P⦁ ¼

ðd½M =dt Þ kp ½M 

(21)

Substituting the values of [P ⦁] and [P ⦁]0 into Eq. (20): ½M 0 ½M  kt ¼ t+ ðd ½M =dtÞ kp ðd½M =dt Þ0

(22)

The rate of polymerization (d[M]/dt) and monomer concentration [M] can be determined experimentally via differential scanning calorimetry (DSC), which will be discussed further in the following section, while фiIa can be determined from light intensity measurements. By estimating kp/k1/2 from Eq. (17) and kt/kp from Eq. (22), the indit vidual rate constants kp and kt can also be obtained. During photopolymerization, gelation (transition from liquid to gel phase) can be observed via a rapid change in viscosity [29]. At low conversions, and hence low

Photoactive resin formulations and composites

viscosities, the reactive species (radicals and monomers) are able to diffuse freely, which allows for high propagation and termination rates [43]. As monomer conversion increases during photopolymerization, the viscosity of the resin increases, resulting in decreased rates of diffusion [41, 44]. This phenomenon results in kp and kt values changing throughout photopolymerization process. Therefore, the effect of diffusion should also be incorporated into the mechanistic kinetic model described here. Despite the complexity of the kinetics of photocuring processes in SLA-based systems, the critical conversion criterion for predicting “gelation point” can be easily implemented [33]. Experimentally obtained rate constants (kt and kp) at a point where critical conversion limit had been reached, can be employed in several mechanistic models to estimate the shape of the SLA cured part and part height [11, 28, 33–35]. These mechanistic models, however, require several assumptions and approximations, to simplify the photocuring reaction, and include a number of parameters, which need to be determined via numerical optimization approaches. Due to the complexity of cationic reactions, these models have been mainly restricted to free-radical polymerization [27]. The nature of photocuring reactions makes it challenging to obtain mechanistic kinetic models and their parameters [29]. An alternative approach employs phenomenological models, which assume that only one reaction can represent the whole photocuring process, and can be more practical for engineering purposes [31]. Bartolo [27, 29–32] developed a phenomenological kinetic model, by modifying Kamal and Sourour’s model [45], specifically for SLA processes. This model incorporates the effects of vitrification phenomena. In the vitrification process, which usually follows the aforementioned gelation step, the photocuring reaction becomes predominantly diffusion-controlled. In this step, an increase in crosslinking density and molecular weight of photopolymer resin results in a gel-to-glass transition of the photopolymer resin [4, 29, 46]. This results in the rate of reaction slowing down significantly, as it is no longer kinetically controlled (gelation step), but is instead diffusion-controlled (vitrification process) [29, 47]. Thus final degree of double bond conversion is also dependent on the vitrification step [29]. In Barotlo’s model, kinetic parameters are defined as functions of degree of conversion, temperature, light intensity, and photopolymer composition [31]: 1 0
E dα 1 C B φ I p @e RTabs Aβq αm ð1  αÞn (23) ¼ ξ dt 1 + e ðααd Þ where α is the fractional conversion, αd is the critical degree of conversion (which corresponds to onset of vitrification step), ξ is the diffusion constant, I is the light intensity, ϕ is the preexponential factor of rate constant, Tabs is the absolute temperature, R is the gas constant, E is the activation energy. Exponents p and q represent constants, while m and n are reaction orders. The overall order of the reaction is represented by a sum of n and m. Kinetic parameters: αd, ξ, E, m, and n are not constants; it was demonstrated

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experimentally that they change in a nonlinear way with a change in light intensity, photoinitiator concentration, and temperature [27, 30]. This kinetic model has been validated for various photopolymer formulations: radical, cationic, and hybrid systems [30, 31]. 3.2.2 Characterization techniques for monitoring SLA kinetics The issues related to photocure characterization are widespread, while fully cured SLA resins can be characterized the same way as the other crosslinked systems, i.e., via thermogravimetric and dynamic mechanical techniques, in reality, photopolymerized layers in SLA processes are partially cured (“green”) [23]. Analytical techniques, such as photodifferential scanning calorimetry (photo-DSC), real-time Fourier-transform infrared spectroscopy (RT-FTIR), and photorheology are usually employed to determine photopolymerization kinetics [48]. Photo-DSC was one of the first analytical techniques to be used for monitoring the kinetics of photocuring reactions [49–55]. At a given temperature, photo-DSC measures heat flux evolved during photoinduced exothermic reaction as a function of time [50]. Since free-radical photopolymerization reactions are exothermic, it can be assumed that all the heat evolution can be attributed to double bond conversion (DBC) of the monomer [55]. By monitoring heat flux produced during photocuring reaction via photo-DSC, the rate of photopolymerization (Rp) is given by [53]: Rp ¼

dH 1 dt nΔH0 M

(24)

where dH/dt is the reaction enthalpy flow measured with photo-DSC, n is the number of double bonds (C ¼C) per multifunctional monomer, ΔH0 is the theoretical heat evolved per double bond conversion, and M is the molecular weight of the monomer. Monomer conversion (α), also known as DBC, is determined from exothermic heat flow over the exposure time [55]: α¼

ΔH ðtÞ nΔH0 M

(25)

where ΔH(t) is the total heat evolved at time t. As mentioned previously, kp and kt can be obtained experimentally from photo-DSC [40]. In photo-DSC, heat release provided as a function of time represents Rp [44]. For instance, Anseth et al. obtained ratio of kp/k1/2 t as a function of Rp, which was measured using photo-DSC, by applying steady-state analysis for radical concentration during continuous light exposure [33, 54]. While nonsteady-state condition was tested by dark reaction in a flash exposure experiment, allowing for independent estimation of kt/kp ratio. By using the values obtained for kp/k1/2 and kt/kp ratios in Eqs. (17) and (22), kp and kt can be estimated independently. t

Photoactive resin formulations and composites

104

108

kt (L/mol-s)

kp (L/mol-s)

1000 100

106

104

10 1 0

(A)

0.1

0.2 0.3 Conversion

0.4

0.5

100

(B)

0

0.1

0.2 0.3 Conversion

0.4

0.5

Fig. 3 (A) kp and (B) kt as a function of DBC at 30°C and 0.6 mW cm2 light intensity for (Δ) DEGDA, (□) TrMPTrA, (O) PETeA, and (◊) DPEMHPeA [51]. (Copyright 1994. Reproduced with permission from Elsevier.)

With the help of photo-DSC, Anseth et al. determined that kt and kp decrease with an increase in monomer conversion, in both multifunctional acrylates (Fig. 3) and methacrylate systems [51]. This can be explained by diffusion limitations, which arise from an increasing network formation, as a result of an increase in monomer conversion [40]. An increase in crosslinking results in an entanglement of radicals and monomers in the network, which, in turn, inhibits diffusion of monomers to reactive sites, thereby decreasing propagation rate constant, while also inhibiting radicals from diffusing together, resulting in a decrease in termination rate constant [40]. In the case of multiacrylate systems, shown in Fig. 3, higher functionality monomers (higher molecular weights) are less reactive, due to an increase in viscosities [51]. When examining the kp/kt ratio, it remains constant for each these multiacrylate monomers, this proportionality between kt and kp further shows the significance of the diffusion-controlled reaction regime in photocuring reactions [51]. While photo-DSC provides a high degree of temperature control, it has its own limitations. Firstly, it only provides a measure of global heat flow during the photocuring reaction. Secondly, the knowledge of standard enthalpy of reaction for functional group conversion is required to determine the DBC [50, 56]. More importantly, due to photoDSC’s relatively long response time, fast photopolymerization reactions ( Ttransition [20]. In general, the shape memory behavior of 4D printed SMPs is described by several parameters, such as shape recovery (Rr), shape fixity (Rf), shape recovery stress, and shape recovery rate. Rf describes the ability of SMPs to fix the temporary (deformed) shape, while Rr demonstrates their ability to return

Photoactive resin formulations and composites

Fig. 10 (A) SLA fabricated tBA-co-DEGDA SMP, (B–C) unfolded after printing, (C–H) recovered to its permanent shape by soaking in water heated to 65°C [104]. (Copyright 2017. Reproduced with permission from Elsevier.)

to the permanent shape [109]. By optimizing SLA resin formulations, the shape memory behavior of SLA printed materials can be controlled, as was shown by Choong et al. [22, 104] who demonstrated that in an SMP formulation with the same crosslinker (diethylene glycol diacrylate, DEGDA) and monomer (tert-Butyl acrylate, tBA), the formulation with lowest DEGDA concentration provides high shape fixity values, while also providing 100% Rr after 14 thermomechanical cycles and maintaining stable Rr values (97%–99%) in subsequent cycles [104]. This was attributed to a less crosslinked network, which prevents catastrophic damage during deformation. Fig. 10 demonstrates the deformation and shape recovery process of the SLA-fabricated tBA-co-DEGDA object, where hot water above Ttransition (65 °C) acts as an external stimulus. In another study, Choong et al. [22] also compared SLA and DLP methods for fabrication of tBAco-DEGDA SMPs. An abnormal shrinkage phenomenon was discovered in DLP processed SMPs, due to the accumulation of heat from concurrent curing. Due to the prolonged exposure time, Cd values obtained in the DLP process are much larger than in SLA when the same energy density is applied [22]. As was previously mentioned, epoxy resins offer low shrinkage values, high mechanical strength, and thermal stability and are also widely used in shape memory epoxy resin formulations (SMEPs) [109]. However, due to their brittleness, SMEPs are generally limited to applications where low strains are applied [109]. As was previously mentioned, dual curing of acrylate and epoxide hybrid photopolymers results in an interpenetrating polymer network (IPN) that offers benefits of both acrylate and epoxide systems. By developing epoxy-acrylate-based hybrid photopolymer, Yu et al. fabricated shape memory composite 3D structures, with Rr and Rf values of 100% and 97%, respectively,

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after 18 consecutive shape memory cycles [109]. In this case, the SMEP resin formulation consisted of two acrylate-based, two epoxide-based, and an oxetane-based (OXT) monomers. The first examples of 4D printing were demonstrated using inkjet 3D printers (PolyJet), due to their ability for multimaterial fabrication [18, 19]. SLA-based techniques can also be employed for multimaterial fabrication, but it requires modification of a particular set-up. For instance, Ge et al. customized an SLA printer to allow for multimaterial fabrication via an automatic resin vat exchange system, to 4D print high-resolution SMPs using resin formulations comprising family of methacrylate-based co-polymer networks [21]. Different photopolymer formulations consisted of a monofunctional methacrylatebased monomer mixed with different compositions of methacrylate-based crosslinkers. When dealing with multimaterial fabrication using SLA-based techniques, it is vital to ensure a good level of adhesion at the interface between two different materials; this can be achieved by optimization of printing parameters to ensure interfacial bonding. Ge et al. sequentially printed a component made of two SMPs, with different crosslinker compositions and glass transition temperatures. The mechanical properties of this 4D printed multimaterial SMP were then tested by uniaxial stretching, with the results showing that the composite breaks at the component with a lower failure strain value, as opposed to at the interface, demonstrating a good degree of interfacial bonding. This approach was demonstrated in fabrication of multimaterial grippers made of multiples SMPs (with different cross-linkers) [21]. In 4D printing, a number of molecular switching segments have been described for a range of materials and for triggering mechanisms (stimuli) other than temperature [20]. Athermally induced transformations, triggered by stimuli other than temperature (light, pH, solvent, etc.), however, entail several practical issues, such as low Rf values (20%–55%) and slow activation times (60–120 min) [103]. Other than SMPs, one of the most common materials used in 4D printing are hydrogels, which respond to solvents by swelling. Commonly used efficient photoinitiators in SLA-based approaches are generally water-insoluble. To counteract this issue, Pawar et al. [110] modified 2,4,6-trimethylbenzoyldiphenylphosphine oxide (TPO) photoinitiator by fabricating water-dispersible nanoparticles via rapid conversion of volatile microemulsions into water-dispersible powder, which enabled SLA printing of hydrogel structures in water, without the use of any organic solvents. Han et al. [108] tailored the temperatureresponsive behavior of SLA printed Poly(N-isopropylacrylamide) (PNIPAAm) hydrogels by controlling polymerization kinetics. The influence of SLA parameters, such as resin composition, light intensity, layer thickness on temperature responsive swelling, was also considered in this study, with Cd being controlled by varying light energy doses and light absorber (Sudan I dye) concentrations. The sequential deformation of fabricated PNIPAAm hydrogel was achieved by shifting the swelling Ttransition via selective incorporation of an ionic monomer. Potential applications of these 3D printed heat-responsive hydrogels include soft robotics, microfluidics, biomedical devices,

Photoactive resin formulations and composites

flexible sensors and actuators, as well as tissue engineering. Apart from materials that are responsive to temperature and solvents, SLA-processed 4D printed materials can also be light responsive [111] and pH responsive [112, 113]. For instance, Roppolo et al. [111] used azobenzene dyes in a photopolymer formulation consisting of ethoxylated bisphenol A methacrylates, where azobenzene not only acts as a light absorber but also imparts photohardening and photosoftening effects, which can be attributed to polymer volume changes caused by light triggered cis-trans isomerization [16]. This light-responsive photopolymer was used to fabricate an array of smart microcantilevers. While the possibility of 4D printing SMPs had been demonstrated extensively in the literature, the self-healing behavior of 4D printed SMPs was only recently demonstrated by Invernizzi et al. [114] In their work, polycaprolactone methacrylate (PCLDMA) and 2-ureido-4[1H]-pyrimidinone (UPyMA) were copolymerized using a customized DLP machine to 4D print structures with thermally triggered self-healing behavior. The printing was carried out in a modified DLP machine with a customized hot plate to keep the resin at 45°C; this alongside with the use of chloroform (CHCl3) ensured diffusion of UPyMA monomers into PCLDMA. The 4D printed PCLDMA-UPyMA structures also showed great reproducibility of shape memory effect, with Rf and Rr values of 99.8% and 98.6%, respectively. Shape memory effects were persevered even after thermally triggered self-healing, as shown in Fig. 11.

Fig. 11 Shape memory effect of DLP fabricated PCLDMA-UPyMA self-healing samples—(A) cut sample; (B) repaired sample after thermal treatment at 80°C for 1 h; (C) deformed sample; (D–F) deformed sample heated at 70°C to initiate the recovery of the original shape [114]. (Copyright 2018. Reproduced with permission from Elsevier.)

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5.2 Sensors, actuators, and transducers Ceramic materials show strong piezoelectric effects; however, they are generally brittle and have limited processability (restricted to standard 2D fibers or thin films) [16]. SLA-based techniques had been employed in fabrication of high-density, complex 3D piezoelectric devices for various applications [98, 115–117]. For instance, piezoelectric materials can be processed via DLP to fabricate ultrasonic transducers [98, 116]. Chen et al. [116] developed a customized DLP machine that incorporates a sliding motion to separate printed layer and a tape-casting-based method to recoat a new layer, which allowed 3D printing of piezoelectric objects from highly viscous photopolymer piezocomposite slurry. The composite resin formulation consisted of 70 wt% of barium titanate (BaTiO3) particles dispersed in a commercial photopolymer resin. Following fabrication of green composite parts via DLP, the components were debinded and sintered, resulting in high-density components (5.7 g/cm3) [16, 116]. Measured dielectric and piezoelectric properties of sintered BaTiO3 objects indicated that DLP-fabricated piezoelectric components can be effectively used as ultrasound transducers. Researchers at Digital and Materials Technology Laboratory at the University of Warwick employed various 3D printing techniques, including SL-based methods to fabricate polymer composites for functional sensor and transducer applications. For instance, an acrylic SLA resin was loaded with 0.65Pb (Mg⅓Nb⅔)O3–0.35PbTiO3 (65PMN-35PT) to fabricate components, functionalized by postprocessing (thermal treatment), which were able to effectively detect and generate ultrasound in the MHz range [117]. Another resin formulation developed by Leigh et al. was loaded with magnetite nanoparticle and was used to fabricate a composite impeller via an SLA-based method [118]. Magnetic properties of 3D printed impeller exhibited a rotating magnetic field, with a frequency proportional to the flowrate of a liquid flowing through the sensor, which allowed for flowrate detection.

5.3 Energy applications SLA-based technologies are utilized extensively in the fabrication of structural components for energy applications [1]. As was previously mentioned, ceramic materials can be processed using lithography-based ceramic manufacturing (LCM). Scheithauer et al. [119] used a commercial Al2O3 suspension photopolymer resin (Lithalox 350, LithozGmbH, Vienna, Austria) to fabricate heat exchangers (HEXs) in a commercial LCM machine (CeraFab 7500, Lithoz-GmbH, Vienna, Austria). These structures required posttreatment steps—debinding and sintering, to produce ceramic HEXs. The freedom of design offered by LCM allowed the fabrication of complex HEX designs with a large surface-to-volume ratio (heat transfer surface of more than 3500 mm2), as well as lowpressure drops. SLA was also employed in the fabrication of complex ceramic substrates for automotive catalytic supports [120, 121], as well as alumina-based carriers for heterogeneous catalysis [122].

Photoactive resin formulations and composites

SLA-based methods can also be used to directly fabricate functional devices/ components. He et al. [123] prepared a resin formulation consisting of suspension of thermoelectric p-type Bi0.5Sb1.5Te3 (BST) alloy powders dispersed in a cationic photopolymer [124]. A commercial SLA printer was then used to fabricate thermoelectric materials. This approach presents an alternative to traditional hot pressing and spark plasma sintering methods. At higher BST loadings, Seebeck coefficient values were higher and positive, and the highest ZT value of 0.12 was obtained at 43°C in a 60 wt% BST sample, which was annealed for 6 h [123]. Inherent porosity obtained following annealing steps poses the limitation of using SLA in processing of thermoelectric materials, since it can lead to high thermal resistance values (hence an ultralow thermal conductivity of 0.2 W/mK), which is desirable in thermoelectric applications; however, the resulting high electrical resistance leads to low ZT values and hinders device performance [124]. By appropriate selection of composite materials and optimization of printing parameters, electrical conductivity can be successfully imparted to SLA printed structures. Park et al. [125] fabricated a novel 3D hierarchical electrode design for microsupercapacitors (MSC) via DLP. The photopolymer resin formulation consisted of silver nanowires (AgNW) dispersed in acrylate-based monomer and oligomer. DLP printed samples were pyrolyzed at relatively low temperatures to obtain char-silver 3D electrodes with a low electrical resistance of 40.2 Ω. The freedom of design offered by DLP allowed the realization of a novel electrode design with a high surface area and open spaces for ion diffusion for MSC applications.

5.4 Biomedical applications The application of SLA-based methods in the biomedical field had been reviewed extensively, the detailed account of which can be found in a review article by Melchels et al. [126] or a book chapter by Raman and Bashir [127]. Instead, here the focus is on reports describing the use of SLA-based 4D printing methods in the fabrication of structures for biomedical applications. Miao et al. [107] utilized customized SLA printer to fabricate scaffolds for growth of multipotent human bone marrow mesenchymal stem cells (hMSCs). The photopolymer formulation consisted of a biocompatible and renewable soybean oil epoxidized acrylate. These 4D printed scaffolds also showed a shape memory effect, where the temporary shape was fixed at 18°C and then the permanent shape was fully recovered when heated up to 37°C (human body temperature). Cytotoxicity study showed that porous scaffolds printed with soybean oil epoxidized acrylate demonstrate a significantly higher hMSC adhesion and proliferation than the conventionally used polyethylene glycol diacrylate (PEGDA). Zarek et al. [106] 4D printed a trachea stent using a commercial DLP 3D printer, where photopolymer consisted of PCL diols prepared by adding terminal methacrylate

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groups (PCLdMA). These 4D printed trachea stents display shape memory effects, where by rational selection of PCL’s molecular weight, shape memory behavior can be matched to a specific melting temperature to achieve high Rf and Rr values. These 4D printed stents were designed to be delivered in a temporary shape; once inside the body, thermal activation will recover its permanent shape, which should match the target anatomy and result in less injurious deployment. In practice, the thermal actuation of the endoluminal stent can be achieved by an infrared diode laser, resistive heating, or Curie-regulated inductive heating. Larush et al. [113] developed a resin formulation consisting of an acrylic acid monomer and a crosslinker (PEGDA) as well as photoinitiator (TPO nanoparticles) that can be printed using a DLP machine to fabricate pH-responsive hydrogels. The targeted application of 3D printed oral dosage hydrogels was to fabricate drug-loaded systems with special designs and unique drug-release characteristics, which otherwise are not possible to fabricate by conventional pharmaceutical manufacturing methods. The results showed that these hydrogels showed pH responsive swelling, with higher pH values resulting in larger swelling and faster drug release. These results show a potential for enhancing drug absorption in the intestine. In addition, 3D printed hydrogels with large surface areas and complex structures showed enhanced swelling and faster drug release.

6 Conclusion SLA-based 3D printing methods offers several advantages in the fabrication of functional composite materials. The fact that several commercial SLA and DLP printers possess an “Open Material” capability implies that any given resin formulation can be explored in a myriad of potential applications. As a result, the understanding of the fundamentals of SLA-based techniques is vital in ensuring the printability of a given photopolymer formulation. As a starting point, two optical properties of a given photopolymer resin formulation, Dp and Ec, can be obtained experimentally and used to optimize printing parameters (light source intensity, exposure time, etc.). Other important aspects, such as photopolymerization kinetic constants (kp and kt) and “gelation point” can be characterized using analytical techniques such as RT-FTIR, photo-DSC, photorheology, and recently developed “Real time-NIR/MIR-photorheology.” The optical printing parameters are not the only factors that need to be optimized, for instance, large separation forces that occur during fabrication in constrained surface SLA-based 3D printers can lead to defects and even print failures. Several strategies to minimize such forces were described in this chapter. In terms of SLA photopolymer resins, the choice of photoinitiator depends on whether the photopolymer system is free-radical, cationic, or a hybrid system. To achieve higher resolutions, improved mechanical properties, and impart additional functionalities

Photoactive resin formulations and composites

to SLA printed objects, light-absorbing materials can be incorporated into resin formulations, to reduce photopolymer’s Dp. When introducing nanoscale and microscale fillers (i.e., ceramic and metal powders) into photopolymer resins changes in viscosity as well as the light scattering effects need to be considered. At high filler loadings, the composite resin’s viscosity might be too high to be processed by SLA-based methods, however, there are several strategies that can be employed to reduce the viscosity. Different morphologies and refractive indices of filler particles can either have a reinforcing effect by promoting cross-linking and shortening “gelation time” or a detrimental effect by blocking the light, which inhibits photopolymerization and prolongs “gelation time.” Successful incorporation of filler particles can impart different functionalities to 3D printed objects. For instance, piezoelectric materials were employed to fabricate sensors, transducers, and actuators using SLA-based methods. It was also demonstrated that thermoelectric and electrically conductive materials can be processed by lithography-based 3D printing to fabricate energy conversion and storage devices. There was also a growing number of reports on 4D printed objects manufactured using SLA-based technologies that extended the advances in shape memory polymers (SMPs) and responsive hydrogels and demonstrated their potential in biomedical applications.

Acknowledgment The study is supported by The Engineering and Physical Sciences Research Council (EPSRC) in the UK under grant number EP/R012164/2 and The Royal Society under grant numbers RSG\R1\180162 and NAF\R1\180146.

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CHAPTER 14

Hydrogels and hydrogel composites for 3D and 4D printing applications Sijun Liua, Xuelong Chenb, Yilei Zhangc a

Advanced Rheology Institute, Department of Polymer Science and Engineering, Shanghai Jiao Tong University, Shanghai, PR China b School of Mechanical & Aerospace Engineering, Nanyang Technological University, Singapore, Singapore c Department of Mechanical Engineering, University of Canterbury, Christchurch, New Zealand

1 Introduction As an interdisciplinary research, tissue engineering focuses on the development of functional three-dimensional (3D) tissue with scaffolds, cells, and/or bioactive molecules in order to create biological substitutes to restore, maintain, or improve tissue functions, which could sidestep problems associated with other medical therapies for tissue damage, such as surgical reconstruction and organ transplants. With these medical therapies, countless patients’ lives have been saved and improved, but problems are also presented, such as long-term problems with surgical reconstruction, as transplant rejection and lack of donor with organ transplants. Therefore, more definitive solutions to tissue repair are highly desired, which is also the aim of tissue engineering. 3D printing, or additive manufacturing, is an advanced manufacturing process to generate 3D structures [1]. As compared to traditional processing methods, 3D printing technology is able to fabricate very complex structures with computer-aided design and manufacturing technologies. 3D printing has achieved great success in the processing of polymers [2], ceramics, and metals [3, 4]. Recently, 3D printing also demonstrates great advantages in the preparation of tissue engineering scaffolds. For example, based on the medical images obtained by computed tomography, some scaffolds with complex structures can be designed and custom-made [5]. Hydrogels possess a highly hydrated polymeric structure and can be modified to respond to various external stimuli, including temperature, light, and biological signals [6]. These unique features make hydrogels excellent materials for cell attachment and proliferation within their hydrated hydrogel networks, which offer abundant space for cell growth and facilitate the transportation of essential metabolites and nutrients to the encapsulated cells. The 3D printing of hydrogel and cells, known as biofabrication, is able to be used to fabricate living organisms. However, most hydrogels show low mechanical strength compared with native tissues such as ligament and cartilage. Therefore, improvements in the mechanical property and bioactivity of hydrogel have been a 3D and 4D Printing of Polymer Nanocomposite Materials https://doi.org/10.1016/B978-0-12-816805-9.00014-4

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challenging task for material scientists. Hydrogel composite system is one of the most suitable strategies for incorporating and combining various mechanical properties and functions, not attainable by any single hydrogel alone [7]. Conventional inorganic reinforcements are based on physical interactions with the hydrogel matrix. The physical interactions generate strong adhesion between the reinforcements and hydrogel matrix, and the enhancement of hydrogel properties is dependent on the amount, size, and shape of reinforcements. In the case of chemical modifications, the introduction of chemical groups and the covalent bonding formation at the interface induces superior interfacial bonding strength (40 ∽400 kJ/mol), which is generally higher than that of physical interaction (8∽16 kJ/mol) [8]. Thus, it is possible to provide substantial increase of mechanical strength to the hydrogel composite system, and further endow the excellent printability of hydrogel composites and expand the application of 3D printing in hydrogels. In this chapter, 3D printing techniques for hydrogel and hydrogel composites will be reviewed first. Subsequently, we will discuss in detail the structure-property relationships of different hydrogels (based on natural polymers and synthetic polymers) and hydrogel composites (double network hydrogels, particle-reinforced hydrogels, and fiberreinforced hydrogels), and the key factors of hydrogel properties (viscosity, shearthinning, thixotropy, interfacial bonding) in control of 3D printing. Additionally, we also discuss several emerging potential applications of 3D printed hydrogels and hydrogel composites, 4D printing of hydrogels and hydrogel composites. Challenges and future perspectives will also be discussed in parallel.

2 3D printing of hydrogels and hydrogel composites 3D printing is a novel technology for customized 3D structures by depositing materials layer by layer. Stereolithography is the first 3D printing technology that was introduced in 1981, followed by the fused deposition modeling (FDM) invented in 1992 [9]. 3D printing of hydrogels could be classified into three different categories, nozzle-based 3D printing, inkjet printer-based 3D printing, and laser-based 3D printing (Fig. 1) [10]. The various 3D printing approaches exhibit both advantages and limitations. The more detailed information for each technology is given as follows.

2.1 Nozzle-based 3D printing Nozzle-based 3D printing is the most popular technique to build hydrogel-based scaffolds. The sol or viscous liquids are forced and extruded out of a nozzle, syringe, or orifice and solidified onto a building stage. 3D structures are constructed layer by layer through sequential extruding materials, which follows a predesigned path constructed by computer modeling. The key to successful 3D printed structures using this method is good interlayer adhesion. Hence, various parameters of hydrogels such as solidification

3D and 4D printing of hydrogels and hydrogel composites

Fig. 1 Schematic illustration of three 3D fabrication methods, nozzle-based 3D printing, inkjet printerbased 3D printing, and laser-based 3D printing, where hydrogels and hydrogel composites were used as “ink” of 3D printer. (Reprinted with permission from J. Malda, J. Visser, F.P. Melchels, T. J€ ungst, W.E. Hennink, W.J. Dhert, J. Groll, D.W. Hutmacher, 25th anniversary article: engineering hydrogels for biofabrication, Adv. Mater. 25(36) (2013) 5011–5028, Copyright 2013 John Wiley & Sons, Inc.)

temperature, shear thinning, thixotropy, and the gelling mechanism are critical. In extrusion-based printing, either viscous or viscoelastic inks are needed initially. These printed layers are cured before next layers could be printed. Nozzle-based 3D printing can be categorized into three groups, pneumatic-, piston-, or screw-driven dispensing. Compared to the pneumatic-driven dispersing where the compression of gas volume causes delay, the piston-driven dispensing demonstrates more directly control over the hydrogels because the ink is extruded by pushing a piston. For the screw-driven dispensing, the feeding of ink is controlled by the rotation speed of screw, which allows the printing of high-viscosity inks. However, the large pressure drops at the nozzle has potential harm to cells, which is the main reason that the cells are usually 3D fabricated with high cell viability in pneumatic-driven or piston-driven dispensing systems.

2.2 Inkjet printer-based 3D printing Inkjet 3D printing could be classified into thermal inkjet printing and piezoelectric inkjet printing. A thermally induced inkjet printer utilizes a heater to heat the surrounding ink and generate vapor bubbles that expand rapidly. As a result, ink droplets will be expelled out from the printing head. A piezoelectrically induced inkjet printer uses piezoelectric actuators to apply pulses so that inks are extruded from the chamber. Inkjet 3D printing could fabricate multilayered droplets and even small 3D structures. Although the inkjet 3D printing has exhibited many advantages such as high spatial resolution (40–400 μm) and high printing speeds (5000 drops/s), it is not suitable to print complex and large constructs for biofabrication because only small droplets could be generated using this technology.

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2.3 Laser-based 3D printing Laser-based 3D printing fabricates 3D structures in a vat of photocurable hydrogels under the deposition of laser energy, in specific designed patterns [11, 12]. The mechanism is to induce radical polymerization or crosslinking of a polymer within a monomer reservoir using UV light. The exposure of UV to photocurable liquid causes a thin single layer of gel formed on the surface, which is sequentially moved upward or downward with the sample stage to allow the next layer formation on top of preformed layers. During this process, designed 3D structures are directly materialized in the liquid vat. There are many advantages in laser-based 3D printing. First of all, high-resolution droplets (50∽60 μm) can be generated easily. Secondly, high viability of cells (∽95%) could be achieved due to the lack of mechanical forces during printing. Thirdly, there is decreased chance of contamination due to the lack of direct contact with inks or the dispenser. Particularly, two-photon polymerization is able to fabricate constructs with spatial resolution of ∽100 nm [13], while stereolithography is able to print patterns with height in centimeter [14]. Recently, nondiffraction laser beams, particularly Bessel beams, were shaped and utilized for high-speed printing of high aspect ratio structures, such as vascular tubes [15, 16].

3 Hydrogels and hydrogel composites for 3D printing Due to the excellent biocompatibility and degradability, physical hydrogels based on natural polymers are commonly used for tissue engineering. However, physical hydrogels are usually weak by nature; thus, the printed filaments spread easily. In contrast, synthetic hydrogels often show poor biocompatibility and nonnatural degradation, but can be prepared with improved mechanical properties. Therefore, it is challenging to fabricate biocompatible hydrogels with improved mechanical properties and degradability through 3D printing technique. Many researchers have put in efforts to overcome this limitation using hydrogel composites. In the following sections, we mainly review 3D printability of various hydrogels, and present the latest progress on the design of hydrogels and hydrogel composites for complex 3D structures with a high-fidelity via 3D printing strategy.

3.1 Hydrogels derived from natural polymers Hydrogels fabricated with natural polymers, including proteins (gelatin and collagen) as well as polysaccharides (alginate, carrageenan, gellan gum, and chitosan) (Fig. 2), are the most appealing inks for biofabrication as the swollen networks allow for the permeability of oxygen to provide a highly hydrated environment of cell proliferation, nutrients, and watersoluble metabolites. The different gelation mechanisms deriving from various molecular structures of natural polymers produce various 3D network structures and gel properties, which determine the 3D printability of resulting hydrogels. Therefore, we will mainly discuss the gelation mechanisms of natural polymers in aqueous solution and the structure-property relationship of natural polymer hydrogels, as well as their influences on 3D printing.

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Fig. 2 Molecular structures of various natural proteins such as collagen and gelatin as well as polysaccharides such as alginate, κ-carrageenan, chitosan, and gellan gum.

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3.1.1 Collagen Collagen is the main component of extracellular matrix and has been widely used for bioapplication in the past decade. There are many forms of collagen, but the most common one is type I with triple-helical structures that are able to bundle into fibrils. Type I collagen could be used as a hydrogel for tissue engineering due to its self-assembling tendencies. It is accepted that the gelation of collagen involves three steps: (1) formation of fibril from triple helices; (2) linear growth of fibril; (3) network structure formation of linear fibrils [17, 18]. Yang and coworkers constructed 3D printed scaffolds by combining Type I collagen into sodium alginate to serve as 3D printing inks, and the resulting alginate/collagen distinctly enhanced the expression of genes, facilitated cell adhesion, and accelerated cell proliferation [19]. Recently, the gelation behavior of collagen during heating and the printability of collagen hydrogels at different temperatures was systematically investigated. As seen in Fig. 3B, a mechanically stable, highly porous, and biocompatible block was constructed by manipulating processing parameters, concentration, and crosslinker (genipin) [20]. 3.1.2 Gelatin As a derivative of collagen, gelatin retains the nature from collagen and promotes cell adhesion and proliferation. Gelatin is able to dissolve in aqueous solution to form a sol at high temperature. With decreasing temperature, it forms a gel, as shown in Fig. 4A.

Fig. 3 Construction of a porous cell-laden collagen hydrogel-based mesh structure via 3D fabrication system supplemented with temperature controllers. (Reprinted with permission from Y.B. Kim, H. Lee, G.H. Kim, Strategy to achieve highly porous/biocompatible macroscale cell blocks, using a collagen/genipin-bioink and an optimal 3D printing process, ACS Appl. Mater. Interfaces 8(47) (2016) 32230–32240. Copyright 2016 American Chemical Society.)

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Gelatin hydrogels are quite elastic, and its modulus depends on the polymer concentration and temperature. Much attention has been paid to deal with the 3D printing and biocompatibility of gelatin hydrogels. For example, gelatin/alginate hydrogels can be used as bioinks for 3D printing, and the properties of gelatin/alginate bioink and postprinting constructs can be well tuned by ratio of gelatin to alginate and ionic strength of solvent [21]. Recently, the influence of 3D printing parameters (speed, pressure, and temperature), hydrogel building block concentration, and cell density on construct architectures of gelatin scaffolds have been studied using a different photoinitiator (VA-086) rather than the conventional Irgacure 2959. The results indicated that the overall cell viability and mechanically stability of cell-laden gelatin scaffolds are strongly dependent on the pressure and needle shape as shown in Fig. 4B–E [22]. 3.1.3 Alginate Alginate is composed of consecutive (1 ! 4)-linked β-D-mannuronate (MM) blocks, consecutive α-L-guluronate (GG) blocks, alternating (1 ! 4)-linked β-D-mannuronate, α-L-guluronate (MG) blocks, and is an anionic polysaccharide. In the presence of divalent cations, alginate aqueous solution becomes solid-like gel [23, 24]. Because of the coordination interaction between G blocks and divalent cations, the connection of the G blocks of various polymer chains with divalent cations results in 3D gel

Fig. 4 (A) Gel mechanism of gelatin in aqueous solution, (B) the heating mantle was used to control the temperature of ink in the syringe, (C) the 3D printed gelatin hydrogel-based constructs, (D) the porous scaffolds in top view, and (E) cell viability within the scaffold. ((A) Adapted with permission from S.L. Kosaraju, A. Puvanenthiran, P. Lillford, Naturally crosslinked gelatin gels with modified material properties, Food Res. Int. 43(10) (2010) 2385–2389. Copyright 2010 Elsevier. (B–E) Adapted with permission from T. Billiet, E. Gevaert, T. De Schryver, M. Cornelissen, P. Dubruel, The 3D printing of gelatin methacrylamide cell-laden tissue-engineered constructs with high cell viability, Biomaterials 35(1) (2014) 49–62. Copyright 2014 Elsevier.)

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network formation as shown in Fig. 5A. Owing to the favorable biocompatibility and easy processing in 3D printing, alginate is the most frequently used hydrogel in biofabrication [25]. However, the alginate hydrogel is often mechanically weak, which leads to the collapse of 3D constructed patterns. To achieve better quality of 3D printing, there are many strategies that were used to improve the 3D printability of alginate hydrogel. For example, Li and coworkers found that the addition of more ionic crosslinker (Ca2+) and graphene oxide into alginate hydrogel is able to improve the fidelity of the printed patterns. However, the divalent cations in the junctions can be gradually dissolved into the cell culture media, which leads to the poor stability of the printed alginate hydrogel scaffolds. Jia and coworkers also found that the media of in vitro culture results in the rapid decrease in the mechanical strength of alginate hydrogels [26]. Recently, Gao and coworkers designed a coaxial bioprinting strategy to construct the hollow alginate filament as shown in Fig. 5B and C, where a CaCl2 solution and an alginate solution were extruded separately under the different flow rates. It was also found that there is no occurrence of luminal occlusion within the printed microchannel, Fig. 5D and E [27].

Fig. 5 (A) Gel mechanism of alginate in the presence of Ca2+, (B) schematic of the coaxial 3D printing setup, (C) the hollow alginate filaments fabricated via coaxial 3D printing, (D) the filament obtained by using microscope, and (E) cell culture media within the hollow alginate filaments. (Adapted with permission from Q. Gao, Y. He, J.-z. Fu, A. Liu, L. Ma, Coaxial nozzle-assisted 3D bioprinting with builtin microchannels for nutrients delivery, Biomaterials 61 (2015) 203–215. Copyright 2015 Elsevier.)

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3.1.4 κ-Carrageenans Carrageenan belongs to the hydrophilic linear sulfated galactans, and κ-carrageenan is one of carrageenans family. It is generally accepted that the gelation of κ-carrageenan can be divided into two-steps as κ-carrageenan is composed of alternating α(1-3)-D-galactose-4-sulfated and β(1-4)-3,6-anhydro-D-galactose. As you can see in Fig. 6A, κ-carrageenan disperses in aqueous solution in the form of random coils at high temperatures. By decreasing temperature, the random coils undergo a conformational transition to associate into double helices, and further results in the gelation. A 3D network is formed as a result of the aggregation of double helices. The sol-gel transition of κ-carrageenan is thermoreversible and sensitive to the presence of salts. κ-Carrageenan has demonstrated a wide range of applications. For instance, κ-carrageenan can be used to enhance the cheese texture [28], adjust the dairy viscosity [29, 30], and stabilize the toothpaste preparations [31]. κ-Carrageenan is also used to reduce the amount of polymorphic transformation in tableting, or to fabricate controlled-release delivery systems [32]. Recently, based on thermoreversible gelation nature of κ-carrageenan, Liu and coworkers successfully printed κ-carrageenan hydrogel into complicated 3D patterns as shown in Fig. 6B–D [33].

Fig. 6 (A) Gel mechanism of κ-carrageenan in aqueous solution during cooling, (B) 3D printed constructs when κ-carrageenan hydrogel is used as the ink, and (C) grids with different mesh sizes, and (D) hollow triangular prism and cube in top view and side view. (Reprinted with permission from S. Liu, L. Li Ultrastretchable and self-healing double-network hydrogel for 3D printing and strain sensor. ACS Appl. Mater. Interfaces 9(31) (2017) 26429–26437. Copyright 2017 American Chemical Society.)

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3.1.5 Gellan gum Gellan gum is composed of 1,4-a-L-rhamnose, 1,3-b-D-glucose, 1,4-b-D-glucose, 1,4-b-Dglucuronic acid and is a microbial polysaccharide [34]. It is commonly accepted that the solgel transition of gellan gum involves two-steps, (1) conformation change from random coil to double helix during cooling and (2) aggregation of the double helices. The sol-gel transition temperature and gel strength of gellan gum depend on the cation species, polymer, and cation concentration. Compared to monovalent ones (K+ and Na+), divalent cations (Mg2+ and Ca2+) are able to promote a more efficient gelation. On the other hand, gellan gum hydrogel is less pH dependent in contrast to other polysaccharide hydrogels (carrageenan, alginate, and agar) [35]. Therefore, gellan gum has been widely used in medical field and food industry. Although gellan gum hydrogel displays an excellent gelation capability and mechanical property, there are few reports for 3D printing of gellan gum hydrogel. Recently, Yu and coworkers constructed gellan gum hydrogel-based patterns using the injectable Bioprinter with a customized heating mantle as shown in Fig. 7A, where the thermistor is attached on the surface of the syringe and nozzle to prevent the solidification of gellan gum aqueous solution [36]. Recently, Lozano and coworkers designed a simple and new strategy to 3D print brain-like structures, which consisted of various layers of primary neural cells encapsulated in peptide-modified gellan gum hydrogels in Fig. 7D [37].

Fig. 7 (A) 3D printing setup with a custom heating system, (B) 3D printed scaffolds with various mesh sizes, (C) 3D printing of layered brain-like structures and its schematic diagram, and (D) the printed brain-like layered structure, in which each color represents a layer. ((A and B) Adapted with permission from I. Yu, S. Kaonis, R. Chen, A study on degradation behavior of 3D printed gellan gum scaffolds, Procedia CIRP 65 (2017) 78–83. Copyright 2017 Elsevier. (C and D) Adapted with permission from R. Lozano, L. Stevens, B.C. Thompson, K.J. Gilmore, R. Gorkin III, E.M. Stewart, Marc in het Panhuis, M. Romero-Ortega, G.G. Wallace, 3D printing of layered brain-like structures using peptide modified gellan gum substrates, Biomaterials 67 (2015) 264–273. Copyright 2015 Elsevier.)

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3.1.6 Chitosan Chitosan is a positively charged polysaccharide and composed of randomly distributed β-(1 ! 4)-linked D-glucosamine and N-acetyl-D-glucosamine. Usually, chitosan is soluble in aqueous solution of weak acid [38–40]. Chitosan is widely used in tissue engineering [41–45], drug delivery [46–50], wound dressing [51–53], and gene therapy [54, 55] because of its nontoxicity, antimicrobial activity, good biocompatibility, and biodegradability. Chitosan can form a physical hydrogel by immersing the acidic chitosan aqueous solution in the alkaline-coagulating bath [56]. The pH-dependent solubility and gelation properties provide a convenient approach for the fabrication of chitosan hydrogel. But the poor mechanical properties of chitosan hydrogel fabricated from acidic solution seriously impede its applications. Very recently, the alkali/urea aqueous solution was designed and used to dissolve chitosan through the freezing-thawing process. Subsequently, the enhanced chitosan physical hydrogel can be fabricated by removing alkali and urea [57, 58]. Chitosan hydrogel prepared from the alkali/urea aqueous solution demonstrates excellent mechanical properties compared to that from acid-dissolving method. Since the formation of chitosan hydrogel involves the neutralization of chitosan acidic solution, chitosan hydrogel is not suitable for the ink of 3D printer to construct complex patterns. By controlling rheological property of chitosan solution and solvent evaporation, the 3D printing of complex structures made of chitosan ink was reported, as shown in Fig. 8. It was found that the neutralization step has a crucial effect on the formation and mechanical properties of hydrogel [59]. 3.1.7 Oppositely charged hydrogels In general, hydrogels formed from natural polymers demonstrate favorable biocompatibility and cell attachment. Most importantly, many of them are suitable for the 3D printing to construct complex structures due to the reversible physical crosslinking or thermoresponsive gelation properties. However, natural hydrogels show poor gel strength, which makes the printed filaments spread easily. That is to say, the printed filament is unable to support itself weight, which will lead to the deformation or collapse of 3D structures. Currently, multihydrogels, printed in one complex 3D pattern, is a great strategy to multilayer construct with high fidelity and show great promise in bioapplication. Li and coworkers systematically investigate the interactions between positively charged hydrogels and negatively charged hydrogels. It was found that the interaction of two oppositely charged hydrogels is higher than that of single negatively or positively charged hydrogels in Fig. 9A, and the printed multilayered two oppositely charged hydrogel construct demonstrates a good structure integrity, as shown in Fig. 9B [60, 61]. 3.1.8 Interfacial bonding Inspired by improved interfacial interaction via two oppositely charged hydrogels, it is not hard to imagine that any components, which is able to improve the interfacial interaction between the printed layers, can be used to improve the stackability of hydrogels.

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Fig. 8 (A) Schematic of 3D printing of chitosan acidic solution and main process parameters, (B) rheological properties of chitosan acidic solution, (C) the printed chitosan scaffold with 30 layers, (D) one scaffold fabricated using the acidic mixture ink, and (E) fluorescent microscopy image of a 3D printed leaf. (Adapted with permission from Q. Wu, D. Therriault, M.-C. Heuzey, Processing and properties of chitosan inks for 3D printing of hydrogel microstructures, ACS Biomater Sci. Eng. 4(7) (2018) 2643–2652. Copyright 2018 American Chemical Society.)

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Fig. 9 (A) Molecular structures of gelatin methacrylate and κ-carrageenan, and schematic of the interaction between gelatin methacrylate and κ-carrageenan chains, (B) schematic of the 3D printing procedure, in which syringe 1 is κ-carrageenan hydrogel and syringe 2 is the gelatin methacrylate hydrogel. (Reprinted with permission from H. Li, Y.J. Tan, S. Liu, L. Li, Three-dimensional bioprinting of oppositely charged hydrogels with super strong Interface bonding, ACS Appl. Mater. Interfaces 10(13) (2018) 11164–11174. Copyright 2018 American Chemical Society.)

Recently, a 3D printable hydrogel was fabricated by introducing an interfacial bonding agent in the blend of alginate and methylcellulose [62]. The interfacial bonding agent, working as a chelating agent, was used to remove the interfacial calcium ions firstly. Subsequently, the interfacial adhesion between layers of hydrogels was promoted by immersing the printed patters in a CaCl2 bath to build the crosslinks between Ca2+ ions and alginate chains. As seen in Fig. 10, the alginate/methylcellulose hydrogel is able to fabricate various 3D structures up to 150 layers aided by the interfacial bonding agent.

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Fig. 10 (A) Schematic showing the bonding mechanism at the interface of alginate/methylcellulose hydrogel and (B) pictures of a grid structure with 50 layers, a star structure with 100 layers, and a spiral structure with 150 layers. (Reprinted with permission from H. Li, Y.J. Tan, K.F. Leong, L. Li, 3D bioprinting of highly thixotropic alginate/methylcellulose hydrogel with strong interface bonding, ACS Appl. Mater. Interfaces 9(23) (2017) 20086–20097. Copyright 2017 American Chemical Society.)

3.2 Hydrogels from synthetic polymers 3.2.1 Poly (ethylene glycol) Poly (ethylene glycol) (PEG) is one type of hydrophilic macromolecules, and PEG hydrogel usually is hard to be used as ink of 3D printer. However, hydrogels formed using PEG derivatives as crosslinker in different compositions are able to be used for 3D printing [63]. For example, by using a laser inkjet printer, the photolithographic masks were prepared by printing the desired photoactive PEG-diacrylate (PEGDA) hydrogel patterns onto transparencies [64]. This technique possesses many advantages, such as simplicity and low expense, and can be expanded to a variety of other photoactive substrates. Recently, Tseng and coworkers first 3D fabricated one construct by combining the PEGDA-dithiothreitol-borax hydrogel with a nonglucose-sensitive hydrogel. By immersing the 3D construct in the culture medium containing glucose to remove the PEGDA-dithiothreitol-borax hydrogel, a tubular channel was finally formed [65]. 3.2.2 Poly (vinyl alcohol) Poly (vinyl alcohol) (PVA) is also a hydrophilic synthetic polymer with large amount of hydroxyl groups on the PVA molecular chain. Based on the experimental strategies, many

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methods, including traditional freeze-thawing [66], physical crosslinking [67], and chemical crosslinking [68, 69], have been reported to prepare PVA hydrogels. Due to high water content, low toxicity, good mechanical properties, and good biocompatibility [70, 71], PVA hydrogel is able to be used as artificial material for 3D printing. For example, by combining alginate and hydroxyapatite (HA) into PVA solution, Bendtsen and coworkers prepared an alginate/PVA/HA hydrogel for 3D printing of mouse calvaria cells. It was found that the addition of PVA-HA significantly improved the viscosity and printability of the alginate hydrogel. At the same time, the 3D printed scaffold also displayed excellent integrity and mechanical properties even if it was incubated in cell culture media for 14 days [72]. 3.2.3 Pluronics Pluronics, PEOn-PPOm-PEOn (n and m are number-average block lengths) triblock copolymers, is another type of synthetic polymers for 3D bioprinting [73–77]. Among them, F127 (PEO100-PPO65-PEO100) demonstrates a wide application in the area of drug delivery and is regarded as the most prominent member [78]. At low copolymer concentration or temperature, PEO-PPO-PEO chains dissolve in aqueous solution in the form of unimers. With increasing temperature and copolymer concentration, the aggregation of hydrophobic PPO blocks takes place, leading to the formation of micelles. When the concentration of micelles increases to a certain value, the randomly dispersed micelles can form an ordered structure or arrange into a crystalline lattice, and the aqueous solution transforms into a solid-like gel. F127 hydrogel is suitable for 3D printing due to its advantageous gelation temperature and viscoelasticity. It has been found that there is no obvious decrease in cell viability after loading the fibroblasts into the F127 hydrogel, shown in Fig. 11 [79]. On the other hand, F127 hydrogel cannot be used for the long-term cell viability as pluronic is a synthetic polymer. In order to further improve the cell viability of F127 hydrogel, one strategy by combining the different natural polymer hydrogels into F127 hydrogel has been proposed [80].

3.3 Hydrogel composites Polymer hydrogels have been extensively used in the field of tissue engineering; however, the natural hydrogels are usually very brittle and majority of synthetic hydrogels also have the drawbacks of low toughness and limited recoverability. It is highly desirable yet challenging to fabricate high strong hydrogels to be used in load-bearing tissues and robotics. Hydrogel composite system is one of the most suitable strategies for incorporating and combining various hydrogel functions and properties, not attainable by single hydrogel alone. In the past two decades, a diverse range of reinforcements have been proposed utilizing various composite designs such as double network (DN), particle-filled, and fiber-filled hydrogel composite systems. In DN hydrogel composite system, the interpenetration of two polymer networks forms a unique microstructure, which strongly affects the resulting hydrogel’s mechanical performances. Particle or fiber-filled reinforcements are based on physical/chemical interactions with the hydrogel matrix, and these interactions generate strong adhesion between the reinforcements and hydrogel

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(C) Fig. 11 (A) Schematic of cell-laden F127 hydrogel constructs, (B) rheological property of 25wt% F127 in PBS at 0°C and 37°C, respectively, and (C) FESEM micrographs of F127 hydrogel scaffold (needle about 200 μm, fiber spacing 600 μm), inset is fluorescence micrographs of cell laden scaffolds. (Adapted with permission from E. Gioffredi, M. Boffito, S. Calzone, S.M. Giannitelli, A. Rainer, M. Trombetta, P. Mozetic, V. Chiono, Pluronic F127 hydrogel characterization and biofabrication in cellularized constructs for tissue engineering applications, Procedia CIRP 49 (2016) 125–132. Copyright 2016 Elsevier.)

matrix. On the other hand, compared with well-understood and evaluated conventional manufacturing processes, 3D printing of hydrogel composite systems is a relatively new research topic. In recent years, remarkable development in 3D printing systems has been witnessed in fabrication of hydrogel composites with improved mechanical and functional properties. Herein, we mainly introduce the strengthening and toughening mechanisms in the hydrogel composites, and then review the development on 3D printing technique of hydrogel composites. 3.3.1 Double network hydrogels Double network (DN) hydrogels, as its name implies, consist of two polymer networks with different physical properties [81–85]. Owing to the outstanding strength and

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toughness (comparable to those of rubbers), DN hydrogels were considered to be innovative materials and received increasing attentions [86, 87]. However, the permanent breaking of chemical bonds in traditional chemically crosslinked DN system usually caused a low recoverability and toughness. A conclusion was drawn based on many studies that the introduction of reversible noncovalent physical network into the original covalent structure can remarkably elevate recoverability of the final DN hydrogels as the physical crosslinking, such as ionic bonds [88] and hydrogen bonds [89–91], is reversible. More importantly, the introduction of reversible physical bonding simultaneously also endows the printability of DN hydrogels. For example, a simple “one-pot” method to fabricate thermoresponsive κ-carrageenan/PAAm DN hydrogels has been proposed, which demonstrates both excellent recoverability and self-healing capability thanks to the thermoreversible gelation of κ-carrageenan in aqueous solution [92]. By controlling the temperature of cartridge via a thermal controller, the pregel solution of κ-carrageenan/ acrylamide could be used as printing ink to construct complicated 3D structures, such as cone and dumbbell-shaped patterns. As can be seen in Fig. 12A, the printed hydrogel

Fig. 12 (A) 3D printed κ-carrageenan/PAAm hydrogels exhibit remarkable mechanical property, (B) 3D printed gellan gum/PEG hydrogel can recover to its initial shape after pressing, and (C) uniaxially stretching and recovery of 3D printed alginate/PEG hydrogel. ((A) Adapted with permission from S. Liu, L. Li Ultrastretchable and self-healing double-network hydrogel for 3D printing and strain sensor. ACS Appl. Mater. Interfaces 9(31) (2017) 26429–26437. Copyright 2017 American Chemical Society. (B) Reprinted with permission from D. Wu, Y. Yu, J. Tan, L. Huang, B. Luo, L. Lu, C. Zhou, 3D bioprinting of gellan gum and poly (ethylene glycol) diacrylate based hydrogels to produce human-scale constructs with high-fidelity, Mater. Des. 160 (2018) 486–495. Copyright 2018 Elsevier. (C) Adapted with permission from S. Hong, D. Sycks, H.F. Chan, S. Lin, G.P. Lopez, F. Guilak, K.W. Leong, X. Zhao, 3D printing of highly stretchable and tough hydrogels into complex, cellularized structures, Adv. Mater. 27(27) (2015) 4035–4040. Copyright 2015 John Wiley & Sons, Inc.)

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samples also demonstrate an excellent mechanical strength after UV curing [33]. By taking advantages of shear thinning, recovery properties of gellan gum and fast UV crosslinking of PEGDA, Wu and coworkers fabricated different DN hydrogels samples with high strength and stretchable property based on extrusion-based 3D bioprinting in Fig. 12B [93]. Through adding nanoclay to tune the rheological property of alginate/ PEG pregel solution, Zhao and coworkers created a tough alginate/PEG hydrogel with an excellent 3D printability, Fig. 12C [94]. 3.3.2 Particle-reinforced hydrogels Introducing inorganic particles into hydrogels was widely used to tune both mechanical and biological properties of pure hydrogels [95]. A particle-reinforced hydrogel composite is often formed from ex situ process where the as-prepared particles are dispersed into a hydrogel-forming liquid, as shown in Fig. 13A. In the case of microparticle-filled hydrogel, the mechanical enhancement is much lower compared to hydrogels filled by nanosized particles, but it is easier to get a uniform distribution within the hydrogel through simple mixing due to its relatively low surface-to-volume ratio. This approach allows excellent control over the quantity of incorporated particles and greatly facilitates the study of optimal experiment conditions. Printability is one of the most important criteria to consider for 3D printing of ex situ particle-filled hydrogels. It is crucial in determining the degree of accuracy and precision relative to the computed spatial and temporal design.

Fig. 13 Schematics of 3D printing of (A) ex situ and (B) in situ particle-reinforced hydrogel composites. (Reprinted with permission from T.-S. Jang, H.-D. Jung, M.H. Pan, W.T. Han, S. Chen, J. Song, 3D printing of hydrogel composite systems: recent advances in technology for tissue engineering, International Journal of Bioprinting 4 (2018) https://doi.org/10.18063/ijb.v4i1.126. Copyright 2018 Whioce Publishing Pte. Ltd.)

3D and 4D printing of hydrogels and hydrogel composites

The printability of hydrogel composites requires stimuli-dependent viscosity, which may involve changes in temperature and shear thinning to avoid nozzle clogging and to maintain the intended structure after printing. For 3D printing of particle-reinforced hydrogel composites, it is necessary that the nozzle size is bigger than the particle size. In addition, the incorporation of these additives may lead to a decrease in the accuracy of printed scaffolds due to an increase in nozzle size or even make the resulting material completely unstable. It was also reported that the addition of ceramic or metal particles often interrupts the crosslinking of hydrogels, thus decreasing the printability of materials [96]. A large amount of inorganic particle often produces a high viscosity, which may make it hard to print the particle-filled hydrogel solution in the desired way. The in situ incorporation of particles into hydrogel scaffolds during and/or after 3D printing is a more effective approach than the ex situ method for achieving uniform distribution and high loadings, as postloaded particles do not hinder the printing process, as shown in Fig. 13B. Egorov and coworkers combined in situ mineralization with 3D printing in which calcium chloride and ammonium hydrogen phosphate solutions were mixed with sodium alginate slurry and then 3D-bioplotter printing was employed to fabricate a cubic-shaped 3D composite structure. The compressive strength of composite hydrogels was gradually increased from 0.4 to 1.0 MPa with increasing precipitated CaCI2 loading up to 2.0 wt%. However, the weak bonding between filaments led to the relatively low mechanical properties of 3D printed scaffold, which is a major limitation of the in situ particle incorporation approach for particle-filled hydrogel composite systems [97]. 3.3.3 Fiber-reinforced hydrogels The presence of fiber can also improve the mechanical performance of hydrogel matrix, where the fiber loading and its distribution throughout the matrix decide the stiffness and strength of hydrogel composites. For example, using stiff cellulose fibrils as a short fiber reinforcement, the cellulose-acrylamide hydrogel composite 3D structures were constructed via the extrusion 3D printing technique [98]. The maximum concentration of cellulose fibers inside a soft acrylamide matrix is less than 0.8 wt% in order to ensure smooth, clog-free print behavior of composite ink. During the printing process, cellulose short fibers inside the composite ink undergo shearing forces due to the small nozzle size and orientate themselves along the printing direction, which can induce anisotropic mechanical properties of printed filaments such as anisotropic stiffness and swelling behaviors. As compared to short fiber-filled hydrogel composites, the long and continuous fiberfilled hydrogel composites demonstrate a greatly improved mechanical performance due to the continuous fiber-hydrogel matrix interactions as opposed to disconnected interactions in short-fiber-reinforced hydrogels. As such, the load transfer from the matrix to each fiber also becomes more continuous. However, in spite of its outstanding performance, the most challenging issue for applying this composite system to the 3D printing

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process is a practical way to achieve a uniform distribution and intended alignment of continuous fibers within the hydrogel matrix. Agrawal and coworkers fabricated continuous elastic polyurethane (PU) fibers-reinforced PU/PEG hydrogel composites. The PU polymer solution was contained in a pressure-driven syringe and mounted on the dispensing 3D printing system. The entire printing process was performed under water to form a continuous elastic microfiber rapidly through solvent exchange. And then, the prepared continuous fibers were impregnated with the PEG gel to build continuous PU fiber-PEG hydrogel composites. With 24 wt% continuous fibers, the elastic modulus of composites was two times higher than that of pure hydrogels [99]. Recently, Visser and coworkers prepared PCL-reinforced gelatin methacrylamide (GelMA) hydrogel composites (Fig. 14C) by infusing and crosslinking GelMA within a high-porosity PCL scaffold fabricated via melt-electrospinning writing technique as shown in Fig. 14A and B [100]. As compared to hydrogels or microfiber scaffolds alone, the stiffness of the PCL/GelMA hydrogel composites synergistically increased up to 54-fold, which approaches the stiffness and elasticity of articular cartilage tissue. Besides, the human chondrocytes incorporated in PCL/GelMA hydrogel composites remain viable and responsive to an in vitro physiological loading regime, as shown in Fig. 14D. Hydrogel composites combined with a continuous fiber scaffold have demonstrated an extraordinary mechanical property comparable with tendon, cartilage, and ligament. However, the previous approach requires at least a two-step fabrication process involving 3D printing of continuous fiber scaffold structure followed by immersion of the scaffold into a hydrogel precursor solution followed by crosslinking. Recently, Bakarich and coworkers developed a more advanced technique for fiber-reinforced hydrogel composites using a one-step process [101]. This hydrogel composite was printed by selectively patterning a combination of two different UV curable inks, one is alginate/acrylamide gel solution for the matrix and the other is adhesive epoxy resin for the reinforcement. For evaluating mechanical properties of fiber-reinforced hydrogel composites, a dog-bone shaped tensile strength specimen with uniaxial oriented continuous epoxy fiber was fabricated. The printed hydrogel composites showed a combination of properties in between pure hydrogel and epoxy resin, and its elastic modulus, failure strength, and failure strain were gradually increased by increasing the relative volume of epoxy fibers. A noticeable finding in this study is that there is no limitation of fiber reinforcement amount inside the hydrogel matrix. They showed extremely wide range of fiber volume fraction from 0% to 100% inside alginate/PAAm hydrogel matrix, and the bonding between the hydrogel and fiber is much stronger under the applied stress, which makes the matrix and fibers equally deformed without any interfacial slipping. The reinforced fibers experience a larger stress than hydrogel matrix. These studies have proven the feasibility of using 3D printing for fabrication of fiber-reinforced hydrogel composites, but further advancement of composite 3D printing techniques is crucial before they can be practically employed in various tissue engineering.

3D and 4D printing of hydrogels and hydrogel composites

Fig. 14 (A) PCL 3D scaffolds fabricated through melt electrospinning, (B) thin PCL fibers were stacked with a spacing of 1 mm in a 0–90 degrees orientation (scale bar, 1 mm), (C) stereomicroscopy image of a GelMA/PCL hydrogel composite (scale bars, 2 mm), (D) compressive stress-strain curves of GelMA, PCL scaffold, GelMA/PCL, and native cartilage (yellow). (Adapted with permission from J. Visser, F.P. Melchels, J.E. Jeon, E.M. Van Bussel, L.S. Kimpton, H.M. Byrne, W.J. Dhert, P.D. Dalton, D.W. Hutmacher, J. Malda, Reinforcement of hydrogels using three-dimensionally printed microfibres, Nat. Commun. 6 (2015) 6933. Copyright 2015 Springer Nature.)

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4 Applications of 3D printed hydrogel and hydrogel composites 4.1 Tissue engineering Hydrogels have found numerous applications in tissue engineering owing to their unique biomimetic properties. A variety of 3D printing techniques used to produce complex hydrogel structures have been developed recently due to its increasing popularity. For example, Markstedt and coworkers prepared the nanofibrillated cellulose/alginate bioink and used for 3D bioprinting of living soft tissue with cells. As seen in Fig. 15A–F, using MRI and CT images as blueprints, anatomically shaped cartilage structures, human ear, and sheep meniscus were successfully 3D printed. The cell culture experiments exhibited a high cell (human chondrocytes) viability of 73% and 86% after 1 and 7 days in the nanocellulose-based bioink [102]. Recently, Butcher and coworkers printed alginate/ gelatin hydrogel composites-based aortic valves by designing a dual print-head extruder-based 3D printer. By properly distributing the volume of wall part and the leaflet part onto the two printhead, a very delicate structure of aortic valve with two types of live cells was fabricated; more than 80% of both cell types remain viable after 7 days as shown in Fig. 15G–I [103].

Fig. 15 (A) 3D printed small grids in the size of 7.2  7.2 mm2 after crosslinking, the shapes of (B) the grid deformation and (C) recovery, and (D) 3D printed (E) human ear and (F) sheep meniscus, (E) side view and (F) top view of meniscus. (G) Scheme bioprinting process of aortic valve conduit via dual syringes, (H) fluorescent image of aortic valve conduit, and (I) as-printed aortic valve conduit. ((A–F) Reprinted with permission from K. Markstedt, A. Mantas, I. Tournier, H.C. Martínez Ávila, D. Ha€gg, P. Gatenholm, 3D bioprinting human chondrocytes with nanocellulose–alginate bioink for cartilage tissue engineering applications, Biomacromolecules 16(5) (2015) 1489–1496. Copyright 2015 American Chemical Society. (G–I) Adapted with permission from B. Duan, L.A. Hockaday, K.H. Kang, J.T. Butcher, 3D bioprinting of heterogeneous aortic valve conduits with alginate/gelatin hydrogels, J. Biomed. Mater. Res. A 101(5) (2013) 1255–1264. Copyright 2012 John Wiley & Sons, Inc.)

3D and 4D printing of hydrogels and hydrogel composites

4.2 Multifunctional devices The functional devices fabricated by 3D printing can simultaneously achieve complicated shapes and external stimuli triggered conformation changes. However, the scarcity of printable materials limited the development of multifunctional device in 3D printing. Hydrogels are advancing the development of such machines because of their innate flexibility and other characteristics that make them attractive to researchers. Recently, efforts have been made to integrate photonic and electrical functions into hydrogel for novel multifunctional devices, including strain sensors for monitoring the movement of human body and soft robots, through 3D printing approach. Based on the strain sensitivity of conductivity of κ-carrageenan/PAAm hydrogel, Liu and coworkers fabricated the wearable strain sensor, where κ-carrageenan/acrylamide hydrogel acts as the conductive ink of 3D printer, to detect the human body movement [33]. On the basis of 3D printing and laser cutter, Zhao and coworkers designed and fabricated hydraulic-driven hydrogel actuators with high responsive speed (97%). Biomimetic architectures have also been reported [62] by Gauvin et al. in which methacrylate functionalized

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gelatine has been synthesized to form hydrogels. The hydrogels were then crosslinked in the presence of UV light with both single-layer (2D) and multilayer (3D) structures with precise internal architectures afforded. Human umbilical vein endothelial cells were then seeded into the structure by incubation of the cell suspension. The interconnectivity of the pores within the structure gave rise to uniform cell distribution throughout the scaffolds, resulting in high cell density and homogeneous distribution. Variation of the porous architecture (hexagonal or log pile) and methacrylate functionalized gelatine concentration resulted in control of the mechanical properties of the scaffolds. Wallace and coworkers have produced discrete layers of primary neural cells encapsulated in a peptide-modified biopolymer (gellan gum-RGD) hydrogel combined with primary cortical neurons to demonstrate a new method to 3D bioprint brain-like structures [63]. The modified gellan gum hydrogel was found to have a dramatic positive effect on primary cell proliferation and network formation as demonstrated by successful encapsulation, survival, and networking of primary cortical neurons and glial cells. The constructs afforded demonstrate usefulness in applications ranging from cell behavior studies, understanding brain injuries and neurodegenerative diseases to drug testing. Connon and coworkers have recently reported [64] a collagen-based bioink as a human corneal stroma substitute. This proof-of-concept study formulated low-viscosity formulations containing Sodium alginate and methacrylate-containing type I collagen in different ratios to create scaffolds with structural integrity. Human corneal keratocytes were then encapsulated and the resulting material exhibited high cell viability after 1 day postprinting (>90%) and 1 week postprinting (83%). Synthetic and biological materials can be used in conjunction to form dual networks with biological activity. Lee et al. have confirmed [65] the feasibility of fabricating an earshaped scaffold from poly(caprolactone) and a cell-laden alginate hydrogel for use in regenerative medicine. The construct also included sacrificial poly(ethylene glycol) layers, which were removed post deposition to form a complex porous structure, similar to those found in the human body. Cartilage and fat tissue were induced to regenerate efficiently via chondrogenesis and adipogenesis as a result of chondrocytes and adipocytes, derived from adipose-derived stromal cells, which were encapsulated within the hydrogel.

3.4 Composite materials By combining material classes and printing these simultaneously, the structural complexity and functional performance of 3D printed constructs can be greatly enhanced. To achieve this, however, printing technologies require excellent degrees of spatial and compositional precision. Composites have also been reported for use in biologically relevant applications. Natural composite materials, such as bone and wood, are typically bonded by the combination of platelets or fibers, which reinforce the materials into yield complex

3D and 4D printing of biomaterials and biocomposites, bioinspired composites, and related transformers

architectures. These features result in properties that exceed the sum of their parts, often combining stiffness, low density, and high strength. Hard matter composites such as hydroxyapatite/apatite-wollastonite glass-ceramic composites have been fabricated [66] in situ by Suwanprateeb et al. with mean green strength of 1.27 MPa, which was sufficient to enable handling. Sintering at 1300°C for 3 hours gave the best-performing material (flexural modulus ¼ 34.10 GPa, flexural strength ¼ 76.82 MPa), which was correlated with a decrease in the porosity of the composite as a result of liquid-phase sintering. Bioactivity testing in simulated body fluid and in vitro toxicity studies revealed that the ceramic composite was nontoxic in addition to being bioactive, with osteoblast cells were observed to attach and attain normal morphology on the surface of the composite. Bergmann et al. have reported [67] bone substitute material comprising biodegradable β-tricalcium phosphate. By combining this material with a bioglass comprising SiO2 (45.35%), Al2O3 (0.14%), TiO2 (0.02%), CaO (24.92%), Na2O (23.10%), and P2O5 (6.21%), a granular formulation was produced for layer-by-layer AM. The powder (layer thickness ¼ 50–75 μm) was then bound using an inkjet-printed solution of orthophosphoric acid and pyrophosphoric acid, which is able to react with the β-tricalcium phosphate to form a bone cement comprising dicalcium hydrogen phosphate and dicalcium pyrophosphate. The bioresorbable material was post deposition hardened by annealing at 1000°C to yield materials with a bending strength of 14.9  3.6 MPa. Synthetic polymers are amenable to formation with fillers to enhance their physical and biological characteristics. In 2015, Wei et al. [68] demonstrated the first example of a biocompatible graphene composite material comprising up to 5.6 wt% graphene dispersed in poly(acrylonitrile-butadiene-styrene). Raman spectroscopy, UV-Vis spectroscopy, and SEM were used to confirm the presence and exfoliation of the graphene sheets within the polymer matrix, thus determining good dispersion of the filler material. The maximum electrical conductivity of the cast polymer composite was measured as 6.4  10
5 S m
1 with 3.8 wt% graphene; when 3D printed, this reduced to 2.5  10
7 S m
1 as a result of internal voids formed during the printing process, although still presents the potential for conductive polymers to be printed. Fillers, which are able to provide mechanical strength and mimic bioglasses, have also been reported by Hayes and coworkers [19]. A novel supramolecular polymer composite has been 3D inkjet printer to generate constructs in a proof-of-concept study. The biodegradable poly(caprolactone), decorated with self-assembling recognition motifs, was formulated with silica nanoparticles to form dual organic/inorganic networks. The biocompatible hybrid scaffolds were deposited to form self-supporting structures, which revealed cell attachment to demonstrate their potential use in regenerative medicine. A hybrid inkjet printing/electrospinning system for TE applications has been disclosed [69] by Xu et al. to fabricate viable cartilage tissues. Poly(caprolactone) fibers were created and deposited by an electrospinning printhead before rabbit elastic chondrocytes, suspended in a

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fibrin-collagen hydrogel, were printed concurrently in order to fabricate a five-layer tissue construct of 1 mm in thickness. The chondrocytes printed within the hybrid construct were more than 80% viability one week after printing and the cells proliferated and maintained their basic biological properties within this time. Moreover, the hybrid scaffold demonstrated enhanced mechanical properties (E ¼ 1.76 MPa) compared to conventional hydrogel constructs (E ¼ 0.41 MPa) generated using inkjet printing alone and electrospun poly(caprolactone) (E ¼ 0.77 MPa). Kotz et al. have reported [70] a nanocomposite comprising transparent fused silica glass embedded in a polymer matrix. Silica particles (c.40 nm in diameter) were dispersed in a monomeric matrix of hydroxyethylmethacrylate to afford constructs, upon irradiation with UV light and heat treatment, with a resolution of a few tens of micrometers. The printed fused silica glass was nonporous, with the optical transparency of commercial fused silica glass, while retaining a smooth surface. Indeed, by doping with metal salts, colored glasses can be created for use as optical filters and enabling the creation of arbitrary macro- and microstructures in fused silica glass. In 2014, Lewis and coworkers reported [71] an artificial network of blood vessels from a gelatine methacrylate-derived UV-curable formulation containing fibroblast cells to form a biocompatible extracellular matrix (ECM). A poly (ethylene oxide)/poly(propylene oxide) copolymer, which is able to undergo thermally reversible gelation (about 4°C), was 3D printed to form 1D, 2D, or 3D vascular networks, which were encapsulated in the ECM formulation and chilled below 4°C to liquefy and remove the polymer to yield open channels, which may find use as artificial organs. Human bone morphogenetic protein-2 (rhBMP-2)-loaded poly(lactic acid) scaffolds filled with calcium phosphate (CaP) nanoparticle have been described [15] by Maurmann et al. to produce porous scaffolds. The 3D geometry was constructed by alternating layers of rhBMP-2 loaded poly(lactic acid) and layers of CaP-filled poly(lactic acid) in order to obtain a hierarchical porous scaffold structure. Multicomponent composites have also been reported such as a poly(ε-caprolactone)/hydroxyapatite/carbon nanotube mixture reported by Gonc¸alves et al. [72] The resulting slurry possessed a viscosity between 2.5 Pa s and 7 Pa s by solvent evaporation and was deposited using a paste extrusion 3D printer with a 0.45-mm-diameter needle. Biologically derived polymer composites such as an alginate bioink that incorporates nanofibrillated cellulose has been reported by Gatenholm and coworkers [73]. The formulation takes advantage of the shear thinning and mechanical properties of the nanofibrillated cellulose combined with the rapid crosslinking capability of alginate to enable the bioprinting of living soft tissue with human nasoseptal chondrocyte cells. Anatomically shaped cartilage structures, such as a human ear, were 3D printed using magnetic resonance imagining (MRI) and computed tomography (CT) images as models. Human chondrocytes were also bioprinted in the noncytotoxic, nanocellulose-based bioink, which exhibited a cell viability of 73% and 86% after 1 and 7 days of 3D culture, respectively. 3D printed with nanoporous hydroxyapatite granulates to form scaffolds with

3D and 4D printing of biomaterials and biocomposites, bioinspired composites, and related transformers

interconnected and tailored pores, ranging from the nanometer to millimeter, which can support the reconstruction of centimeter-sized osseous defects has been reported by Fierz et al. [74] to mimic bone. Histological analysis of the scaffolds seeded with osteogenicstimulated progenitor cells confirmed the suitability of the 3D-printed scaffolds for potential clinical applications.

4 Applications of 3D printed biomaterials 4.1 Tissue engineering TE is a multidisciplinary field that applies the principles of engineering and life sciences toward the development of biological substitutes [75]. Fundamental to TE is the utilization of living cells to achieve biological substitutes for implantation into the body and/or to foster the remodeling of tissue in some other active manner. Within the field of TE, there are two main approaches that are employed; the first is the use of scaffolds as a cell support to encourage the cells to form their own matrix. The second methodology takes advantage of the scaffold as growth factor or drug-delivery device to induce and aid regeneration inside the body [76]. Key to the use of scaffolds in TE are three main characteristics that the structures should possess [77]. Primarily, the scaffold should be able to mimic the architecture of the native ECM by providing space for vascularization, new tissue formation, and nutrient transport. Additionally, the scaffold should be able to interact with the cellular component to facilitate their activities such as proliferation and differentiation. Finally, the scaffold has to provide a 3D structural support while matching the mechanical properties of native tissues/organs. Advances in TE have shown to be successful in building a number of tissues that have been used in the clinic such as skin and cornea [78]. However, constructing complex solid organs remains a major physical and biological challenge. Among other challenges are the lack of methods that can simultaneously replicate the tissue micro- and macroarchitecture and methods that can deliver multiple cell types at precise locations. AM has been utilized to try to address these limitations [79] because of its main advantages of precise control and personalized customization [80]. Recently, the term 3D bioprinting has been coined for a specialized class of 3D printing. 3D bioprinting is a layer-by-layer AM technique in which precise positioning of biological materials and living cells [40] can be achieved. In the recent years, the use of 3D bioprinting for manufacturing scaffolds has increased as a result of the benefits of being able to control pore size, shape, and distribution. In addition to this, combined with the ability of CAD manipulation and 3D medical imaging such as CT, 3D bioprinting permits the fabrication of patient-specific constructs [39]. Bone regeneration was one of the first TE applications to take advantage of the technological advances offered via 3D printing of materials [81]. The first scaffolds fabricated for this purpose were manufactured using a biodegradable polymer, namely, poly(caprolactone), and fused deposition modeling as the printing technique (Fig. 5) [80]. The precise

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Fig. 5 3D printing allows the porosity and pore size in scaffolds for systematically studying its effect on bone formation and vascular ingrowth. (Reproduced with permission from M.O. Wang, C.E. Vorwald, M.L. Dreher, E.J. Mott, M.-H. Cheng, A. Cinar, H. Mehdizadeh, S. Somo, D. Dean, E.M. Brey, J.P. Fisher, Evaluating 3D-printed biomaterials as scaffolds for vascularized bone tissue engineering, Adv. Mater. 27 (2015) 138–144. Copyright (2014) John Wiley and Sons.)

deposition control of 3D printing has facilitated the study of the effect of porosity and pore size on bone ingrowth and vascularization, allowing to create the most adequate scaffolds for host integration [82]. Many different materials have been 3D printed for manufacturing these scaffolds, among them are synthetic polyesters (i.e., PEG-PLA, PCL, etc.), natural polymers (i.e., alginate, chitosan, gelatine, etc.), ceramic composites (i.e., calcium phosphate and hydroxyapatite), and combinations of them [83]. A noteworthy example of the 3D printed combination materials is the hyperplastic “bone” scaffold, which is composed of 90 wt% hydroxyapatite and 10 wt% poly(caprolactone) or poly(lactic-co-glycolic acid) [84]. These scaffolds became rapidly vascularized and showed and osteoregenerative properties when implanted in vivo. Khalyfa et al. have also reported [85] a highly biocompatible calcium phosphate powder-binder, which has been used to produce porous medical implants such as scaffolds for cranial reconstruction (Fig. 6A). Another example of a 3D bioprinting for TE application is cardiac regeneration. Tissue-engineered heart valve conduits have been fabricated using 3D bioprinting and reported by Duan et al. [86] The trileaflet valve conduits were bioprinted using a combination of methacrylatefunctionalized hyaluronic acid and methacrylate-functionalized gelatine with encapsulated human aortic valve interstitial cells. Prevascularized stem cell patches have also been bioprinted for promoting cardiac repair [87]. In in vivo studies, these patches exhibited

3D and 4D printing of biomaterials and biocomposites, bioinspired composites, and related transformers

Fig. 6 Representation of different applications of 3D bioprinting in tissue engineering. (A) 3-D printed proof-of-concept porous cuboid structure and structurally complex cranial structure. (B) Liver on a chip manufactured with 3 different biomaterials and 2 different cell types. ((A) Reproduced with permission from A. Khalyfa, S. Vogt, J. Weisser, G. Grimm, A. Rechtenbach, W. Meyer, M. Schnabelrauch, Development of a new calcium phosphate powder-binder system for the 3D printing of patient specific implants, J. Mater. Sci. Mater. Med. 18 (2007) 909–916. (B) Reproduced from H. Lee, D.-W. Cho, One-step fabrication of an organ-on-a-chip with spatial heterogeneity using a 3D bioprinting technology, Lab Chip 16 (2016) 2618–2625 with permission from the Royal Society of Chemistry.)

enhanced cardiac functions, reduced cardiac hypertrophy, and increased migration of cells from patch to the infarct area. Liver microtissues used for drug testing and disease modeling are another example of 3D bioprinted TE constructs. Faulker-Jones et al. have reported [88] the fabrication of functional mini livers using human induced pluripotent stem cells encapsulated in an alginate hydrogel matrix and deposited with a microvalve printer. The human liver is a very complex organ, highly vascularized and composed of many different cell types. This kind of complexity cannot be easily reproduced in vitro, but using the versatility of 3D printing Lee et al. designed [89] a one-step method for

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fabricating an organ on a chip that can tackle this complexity. The artificial liver was fabricated with a microfluidic system with multiple cell types and biomaterials, which showed that hepatocytes cultured in this system had increased liver function when compared with the ones cultured with standard static methods (Fig. 6B).

4.2 Medicine and drug delivery The use of AM for medical applications and drug delivery has grown rapidly in the previous decade. [90] In the medical field, 3D printing can be used for surgical planning, medical education, and customized implant design [91]. The combination of AM and advanced medical imaging enables the creation of patient-specific implants and the reproduction of the complex architecture of tissues. [92] Combining 3D printing with advanced medical imaging, a suite of techniques including the use of CT or MRI in combination with postprocessing tools and algorithms [93], a range of medical implants have been 3D printed for a variety of applications, including heart valves [86], ears [94], articular surface [95], meniscus [96], trachea splint [97], bone [98], cranium [99], and mandible [100] to name but a few. The postprocessing tools used in advanced medical imaging allow a series of 2D images to be converted into a 3D view or model of the anatomy [101]. After the imaging data are acquired, they are then saved in DICOM (digital imaging and communications in medicine) format. The DICOM files are then manipulated using 3D postprocessing tools, which usually include thresholding, segmentation, sculpting, trimming, and smoothing tools. The contours of a segmented region of interest can be computationally transformed into a 3D triangle mesh. The mesh data then are further processed using CAD software where additional smoothing and editing is performed to finally generate a 3D STL (stereolithography) file, which is compatible with 3D printer software [102]. This combination of techniques has allowed for customdesigned and personalized implants and scaffolds to be produced quickly and effectively. Otology, or the study of the anatomy and diseases of the ear, is one of the first medical fields that explored 3D printing as a manufacturing method for implants and devices to be produced. The hearing aid industry has already transitioned its entire operations to the use of 3D printing [103]. Since the implementation of 3D printing technology in this field, more than 10 million hearing aid shells have already been manufactured using 3D printers. Currently, EnvisionTEC has nine different biocompatible polymers that have been developed specially for hearing aids, most of them strong, though, waterresistant photocrosslinked polymers [104]. 3D printing has reduced the hearing aid manufacturing process to three steps: scanning, modeling, and printing. As a result of this new technology, hundreds of thousands of custom-made products are able to be produced en masse every year [105]. The concept of personalized medicine has also driven the use of 3D printing in pharmaceutical drug delivery. Compared to the traditional pharmaceutical product

3D and 4D printing of biomaterials and biocomposites, bioinspired composites, and related transformers

manufacturing process, 3D printing offers several advantages such as specific dosage to each patient needs, reduction of materials wastage, and amenability to broad types of pharmaceutical active ingredients, including poorly water-soluble peptides and proteins [106]. Examples of 3D printed pharmaceutical formulations include guaifenesin tables [107], levofloxacin implants [108], rifampicin nanoparticles [109], and a folic acid nanosuspension [110].

4.3 Dentistry Digital dentistry and AM are rapidly transforming the dental industry and now 3D printing is used for a wide range of dental applications, including dental and orthodontics models, direct crowns and bridges, dental aligners, night guards, surgical drill guides, flexible gingiva masks, and denture bases (Fig. 7) [111]. Using AM methods for the fabrication of dental models is an approach that has increasingly been employed for surgical planning, simulation, and orthodontics. Studies have compared the models manufactured with 3D printing techniques with those made with the traditional plaster method and revealed that the 3D printed constructs are clinically acceptable in terms of accuracy and reproducibility [112]. The advantages of using AM techniques instead of the traditional method arise from the ability to use digital models to fabricate patient-specific designs, which increase the efficiency of manufacture and improved comfort for the patient.

Fig. 7 Current dental applications of 3D printing. (A) Invisalign® dental aligners, (B) bridges (E-DENT 400C&B MHF), (C) dental and orthodontic models (E-MODEL), (D) denture bases (E-DENTURE), (E) night guards (E-GUARD), and (F) surgical guides (E-GUIDE). (Images from B–F were reproduced with permission from EnvisionTEC.)

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Fabrication of dental aligners using 3D printing has completely transformed the orthodontics treatment. The company Invisalign® uses digital dentistry and SLA technology to manufacture approximately 220,000 aligners per day, and almost 8 million per year [113]. 3D printing enables each set to be personalized easily to individual patient-specific needs [114]. The material used for fabricating these aligners can include styrenic block copolymers, silicone rubbers, elastomeric alloys, thermoplastic elastomers, thermoplastic vulcanizate elastomers, polyurethane elastomers, block copolymer elastomers, polyolefin blend elastomers, and thermoplastic copolyester elastomers [115]. The main advantages of these devices over conventional aligners or braces are their improved aesthetics, increased comfort to wear, and improved oral hygiene as a result of the device being removable [116]. 3D printing has also been used for aiding periodontal repair. Rasperini et al. [117] have reported the first in-human case treatment of a large periodontal osseous defect using a 3D printed patient-specific scaffold loaded with growth factors. The scaffolds were 3D printed using selective laser sintering technologies and poly(caprolactone) containing 4 wt% hydroxyapatite. The internal part of the scaffold had a reservoir for storing a human recombinant platelet-derived growth factor. The study reported that after 12 months of in vivo implantation the scaffold remained intact; however, the growth factor release kinetics required further optimization.

5 4D printing and its applications for biomaterials 4D printing is a novel technology, which is pushing the boundaries of traditional AM techniques. Unlike conventional 3D printing that only provides the user the freedom of controlling the shape of the product, the concept of 4D printing introduces another dimension, time, into the 3D structure. Momeni et al. [118] described such a process as: “A targeted evolution of 3D printed structure, in terms of shape, property and functionality,” and therefore the final geometry or property of the 3D printed construct can vary under different situations and in different time frames. Although many definitions for 4D printing have been proposed, here the process is described as “the use of a 3D printer to print objects which could change or alter their shape after the manufacturing; such a change could be taken place due to many factors such as air, heat and other chemical reactions caused due to materials used in the manufacturing of these object[s]” as defined by Ramesh et al. [119] Khoo et al. further categorized 4D printing into single-material 4D printing and multimaterial 4D printing (Scheme 2) [120]. Both methodologies rely on smart materials that can exhibit different geometry or functional performance under a range of conditions such as pH [121], or temperature [122,123]. However, the multimaterial approach possesses increased flexibility in the number and class of material available and as a result has access to wider applications. It facilitates the user to not only use different smart materials, but also these

3D and 4D printing of biomaterials and biocomposites, bioinspired composites, and related transformers

Single material 4D printing

Smart material

Environment change

Multimaterial 4D printing Conventional material

Environment change Smart material

Scheme 2 Schematic of two routines that smart materials are used for manufacturing 4D printed smart devices.

materials in combination with a “conventional” material, to achieve designed performance under different conditions. This capability significantly increases the number of applicable materials suitable for the 4D printing process. However, as traditional materials do not possess the ability to react or translate in response to an external environmental change, such a methodology may require extra design consideration with respect to the distribution of different materials within the construct to achieve the desired performance or translation over time. As a wider pallet of materials and applications are accessible using the multimaterial approach to 4D printing, this section will focus solely on this class of 4D constructs. Furthermore, the American Society for Testing and Materials (ASTM) categorizes conventional 3D print techniques into seven distinct groups, within which material jetting and material extrusion are the two key techniques, which have found use in AM of multimaterials. It is thus not surprising that these two techniques have also been exploited in the field of 4D printing to the greatest extent. It should be noted that other techniques such as stereolithographic and selective laser sintering can also be used to fabricating multimaterial objects via 3D printing; however, these methods become increasingly complicated when switching materials during the printing process.

5.1 Techniques in 4D printing Material extrusion or extrusion printing is a technique in which a paste or molten materials are forced through a nozzle by screw or compressed air to form a filament of material, which is easily deposited and solidifies rapidly post printing. The size of the nozzle is directly related to the diameter of the extruded filament, and therefore the resolution of the objects fabricated from such a method is of a similar scale, approximately as low as one hundred of microns. By integrating independent printheads for use with multiple different materials, multimaterial structures can be produced by selectively

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Fig. 8 Schematic of the two multimaterial extrusion methods: (A) Multimaterial extrusion printer based on different printheads and independent feedstocks. (B) Multimaterial extrusion printer based on shared printhead and independent feedstock.

depositing the desired material as required (Fig. 8A). The number of materials that can be included in a single structure will also increase as more printhead and feedstocks are integrated into the printer [124, 125]. The bioprinter from Aether©, for example, has demonstrated the capability of integrating up to 10 different printheads inside one printer, thus allowing fabrication of highly complex and intricate structures. To achieve a gradient multimaterial composite with higher resolution, a slightly modified printhead is introduced (Fig. 8B), in which different materials can enter a mixing chamber before extrusion. By controlling the input volume ratio of materials A and B, a range of different composite materials can be produced by using only a few input materials [59]. Jetting techniques have been recognized as one of the most well-established technologies for fabricating multimaterial components [120]. During the material jetting or inkjet printing process, formulations are ejected from the printhead as a series of individual, of picoliter volume, droplets. Layers of material are subsequently deposited on the substrate and combined to form three-dimensional structures. By introducing multiple printheads containing different materials, the composition of the printed structure can be altered (Fig. 9A) to fabricate a variety of composite materials. As such, the composition can be manipulated at the single droplet level (30 μm, Fig. 9B), and therefore fine tuning of the functionality of the printed structure can be controlled by changing the density and distribution of different materials at targeted locations. In comparison to 3D objects printed via the extrusion method, printing using material jetting technologies allows access to structures with higher feature resolution (80 μm vs. 100–200 μm for extrusion printing). Although the resolution of the printed structures is improved, inkjet-based methodologies require strict operating and materials parameters to be observed such as

3D and 4D printing of biomaterials and biocomposites, bioinspired composites, and related transformers

Fig. 9 (A) Schematic of multimaterial jetting technique: multimaterial structure was achieved by coprint of different materials from different printheads and (B) schematic of how a range of polymer composite was formed by controlling the distribution different materials.

material viscosity (normally between 1 and 30 mPa s), and thus the applicable materials are limited when compared with material extrusion technologies. Despite the prevalence of material extrusion and material jetting techniques in 4D printing technologies, other 3D print methodologies have been utilized to produce responsive multimaterial structures. In 2006, Inamdar et al. demonstrated [126] the use of a multivat SLA system with different resins located in each vat to produce multimaterial structures (Fig. 10A). Chen and Zhang [127] have also described the use of SLA with integrated an automatic material feeding and cleaning system to sequentially fill the resin vat with different materials to achieve multimaterial structures (Fig. 10B). Both methods require cleaning steps when switching building material in order to minimize

Fig. 10 Schematic of the two multimaterial stereolithography techniques: (A) multivat stereolithography and (B) single vat with multimaterial feed stock technique.

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crosscontamination and exert control of material composition, As a result, the overall production sequence is complex and time consuming. Other fabrication methods such as laser sintering [128] or laser melting [129] with controlled distribution of powders within the build space have been trialed to form multimaterial objects. The integration of material extrusion techniques with SLA to deposit different materials [130] has also been attempted although both methodologies are not widely used as a result of fabrication limitations.

5.2 Materials and applications of biomaterials in 4D Smart hydrogels are a varied group of materials that have excellent potential in biomedical or bioengineering applications. Hydrogels are normally recognized as having similar properties to natural tissues, amenable to environment change, biocompatible, and easily modified [119]. Many of the smart hydrogel structures used for 4D printing process rely on manipulating the hydrogel geometry through controlling the swelling ratio of the structure. The swelling ratio can be controlled by introducing isotropic fibers or designing the molecular structures to achieve different swelling in response to environment stimuli such as pH, temperature, electric field [131], and induce geometry translations accordingly. Synthetic hydrogens such as those comprising polymeric materials have been widely reported. For example, Dutta and Cohn [132] have prepared a poly(ethylene oxide)-poly (propylene oxide)-poly(ethylene oxide) triblock copolymer with methacrylate functionality which underwent gelation in water. The material exhibited fast and crucially reversible swelling at different pH’s and temperatures. Using SLA, a 3D structure was successfully produced. Han et al. [133] have reported the use of poly(ethylene glycol) co-polymerized with acrylic acid to achieve a hydrogel that can exhibit reversible deformation in the presence of an electronic field. The deformation was induced by the presence of ionized carboxylic groups within the hydrogel structure that resulted in variation of the osmotic pressure in the network. Through SLA methods, 3D printed smart hydrogels structures were produced that can undergo movement by controlling the electronic field in close proximity to the object. Composites have also found significant use in 4D printing applications. Gladman et al. [134] have prepared smart hydrogels containing a soft acrylamide matrix embedded with stiff cellulose fibrils, which mimics the composition of plant cell walls. When the hydrogel swelling in water environment, the aligned fiber structure will restrict the deformation and therefore cause anisotropic swelling. By incorporating digital design and 4D print, they produced structures that can self-assemble into designed geometry in water environment. Moreover, Xiong et al. [135] have reported in their recent publication the use of hydrogel-containing magnetic particles for preparing smart hydrogels. Fe3O4 nanoparticles were formulated into an aqueous solution of poly(ethylene glycol)

3D and 4D printing of biomaterials and biocomposites, bioinspired composites, and related transformers

di-acrylate and cured by nanoscale SLA technique (two-photon) to prepare smart 4D hydrogel structures. Conformational changes within the hydrogel were subsequently induced through the application of a magnetic field. Shape memory polymers are a group of materials that has found popularity in biomedical applications. In 2002, both Lendlein and Langer [136], and El Feninat et al. [137] stated the potential of using biodegradable, elastic shape memory polymers for biomedical applications, respectively. A number of other proof-of-concept studies have demonstrated the use of shape memory polymers as catheters [138], stents [139], and surgical sutures [136]. To effectively make use of shape memory polymers in 4D printing, two elements are required, namely, these are a crosslinking point and switchable segments [140]. The crosslinking point could be either a covalent bond or physical/mechanical bond that fixes the polymer chains together, while the switchable segments may comprise a polymer moiety, which can be “activated” by passing through a thermal transition such as glass transition or melting. The printed structures (original shape) can be programmed into a temporary shape when heated above a thermal transition temperature. The transition temperatures for each material may be tuned by using polymer segments with different glass transition temperatures, for example. To reverse the process, the temporary shape can once again be heated to its transition temperature, allowing the original structure to be formed once again. Generally, materials, which are selected for shape memory 4D printing, are biopolymers featuring acrylate or methacrylate moieties. Photopolymerization is a method to quickly create a polymer network, which is constructed of covalent bonds. Moreover, it is also one of the most important chemical reactions that have been widely used in 3D print techniques such as material extrusion, material ink jetting, and SLA as described in Section 2. Biomaterials such as poly(caprolactone) and poly(ethylene glycol), which have been chemically modified to enable UV curing, are therefore strong candidates for 4D printing applications. Zarek et al. [141] demonstrated the use of a poly(caprolactone) precursor with methacrylate moieties to manufacture a self-assembled device for personalized endoluminal applications. Hot melt SLA was used to create customized tracheal stents that could expand and anchor itself after placement to prevent stent migration or stent fracture. Lo´pez-Valdeolivas et al. [123] reported a diacrylate liquid crystal polymer precursor, which exhibited a thermoresponsive and reversible shape-morphing behavior upon photocuring. It was suggested that this material could be used as an actuator or smart device in medicine applications. Furthermore, Yakacki et al. [139] demonstrated the use of bis-acrylate terminated poly(ethylene glycol) with different molecular weights to produce shape memory structures and claimed the potential application in stent manufacturing. More recently, Ge et al. [142] have described a bis-acrylate terminated poly(ethylene glycol) structure afforded through microstereolithography techniques. The resulting 3D printed material was used to manufacture a shape memory stent that can be deformed and then return to the original geometry at an elevated temperature.

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6 Conclusions and future perspectives 6.1 3D printed biomaterials Biomaterials and biofabrication have a unique set of requirements, which must be upheld in order to achieve the desired function or application for the construct. Through the use of AM, advancements have been made in many key areas such as complexity, customization, cost-effective production, and ease of access [42]. 3D bioprinting promises solutions to some of the most challenging hurdles in the field of biomedical science, including TE, dentistry, and drug-delivery systems by creating complex architectures. However, the most significant obstacles to the advancement of 3D bioprinting technologies are perhaps the lack of viable bioinks and biocompatible materials. Traditional singlecomponent formulations lack one or more of the characteristics desired in a bioink, including high structural fidelity and printability, high mechanical strength postprinting, and bioactivity and biodegradability. Increasing crosslink density of polymers, in an attempt to enhance the mechanical and rheological deficiencies of bioinks, tends to reduce the cytocompatibility of single-component formulations. Recent developments in advanced bionics avoid these tradeoffs without sacrificing cell viability. In time, 3D printing will allow the development of new biomedical implants, engineered tissues and organs and find use in controlled and personalized drug-delivery systems [143]. The flexibility offered by AM also allows for complex shapes and geometries to be engineered from multimaterials, which are not easily accessed through traditional manufacturing techniques [144]. Patient-specific needs will also be addressed through customizable designs, drug dose, and release profiles; allowing healthcare to be tailored to individual needs [145–147]. Moreover, AM will find applications in planning of surgeries, improving their efficiencies and effectiveness while reducing further procedures, which may be required to adapt implants to the patient [148]. Cost benefits may also be observed through the use of AM techniques. Prototyping of parts will become less time consuming and small productions volumes, which are typical in the biomedical industry, may become more cost effective with minimal material wastage and faster build times when compared to traditional manufacturing techniques [90]. Perhaps the most exciting innovation will be the open access of information. 3D printers are no longer confined to the laboratory and easy access to CAD files, such as via the National Institutes of Health project: 3D Print Exchange, will allow for ideas and designs to be shared among researchers and the general public. This information sharing will broaden the scope of 3D printing and allow for ideas and designs to be readily shared to further progress the field of biomaterials, ultimately finding use in modern society.

6.2 4D printed biomaterials 4D printing as an advanced manufacturing technique opens a new era in manufacturing of smart devices for biomedical or bioengineering application that has subverted people’s

3D and 4D printing of biomaterials and biocomposites, bioinspired composites, and related transformers

understanding in device design and manufacturing. It provides the user with opportunities to designing a dynamic structure that can be programmed and exhibit desired functionality in different situations. Although traditional single-material 3D printing techniques can be used for processing smart materials for such 4D print concept, the trends of 4D printing are using those techniques that can produce devices that contain more than one material such as inkjet printing or extrusion. This will provide the user the ability to either combine different smart materials or use smart materials in collaboration with conventional materials to achieve devices with more complex functionality. Although 4D printing has already shown enormous potentials in different areas, it is still in an early stage of development and requires further investigation to unlock the full potential of this technique. The development of 4D printing will require close interdisciplinary collaboration between such fields as materials chemistry and engineering. One of the key challenges for 4D printing is the availability of smart biomaterials that can be used to drive the spatial conformation of the device while exposed to a desired environment stimulus. Current research is predominantly focused on either shape memory materials or smart hydrogels, but as the research momentum increases in this interdisciplinary field, more breakthroughs are envisaged that will help expand the smart 4D printing materials database.

Acknowledgments The authors would like to acknowledge funding from the EPSRC (EP/N024818/1) support of a postdoctoral fellowships for LRH, YH, LRC, and ZZ.

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CHAPTER 16

3D and 4D printed polymer composites for electronic applications Ehab Saleh University of Leeds, Leeds, United Kingdom

1 Introduction 3D and 4D printing are multidisciplinary technologies that combine material science and manufacturing processes toward fabricating tangible structures from a computer design with predetermined functionality. The coupling of the materials and processes is key to achieve well-defined functionality. This coupling can be achieved by optimizing the materials for the processes and vice versa. A part of the material optimization is the introduction of composites for 3D and 4D printing, where composite properties can be highly tailored for the processes and the applications. The high tuneability of composite properties is key attraction to merge 3D printing processes with composite materials. Mechanical [1–3], thermal [4, 5], optical [6–8], biological, [9, 10] and electrical [11–15] properties can be tailored by varying composite matrices to match the process requirements, which enables the unprecedented freedom of design, which is associated with 3D printing, with a wide window of composite properties that can be controlled digitally. In this chapter, 3D and 4D printing of composites and their properties in relation to electronic applications will be presented. These properties will include conductive and dielectric composites with 3D and 4D processes used to fabricate them. The chapter will start by presenting the merits of 3D/4D printing in conjunction with composites for electronic applications. This will be followed by a detailed section on electrically conductive composites with theoretical explanation of the percolation theory in relation to composites and the additive manufacturing (AM) processes used to fabricate 3D and 3D structures. Following that, a section on dielectric composites will be presented explaining the concept of dielectric materials in general and the 3D and 4D processes used in depositing dielectric composites. Both sections on conductive and dielectric composites will be wrapped up with selected cutting-edge electronic applications constructed using 3D and 4D printing of composites, such as printed conductive electrodes, sensors, dielectric elastomer actuators, and highly insulating composites. To enable a comprehensive flow of the chapter considering the background of the reader, the arrangement of this chapter presents a layman introduction to concepts that 3D and 4D Printing of Polymer Nanocomposite Materials https://doi.org/10.1016/B978-0-12-816805-9.00016-8

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the reader might not be familiar with; however, detailed investigation in each topic is thoroughly presented in relation to 3D and 4D printing of composites for electronic applications.

2 Why 3D/4D printing composites for electronic applications? To answer the question on why to 3D/4D print composites for electronic applications, we need to split this into two parts (1) why 3D and 4D printing in the first place and (2) why composites for electronics? The answer to these two questions will lay down the importance of the combination of these two concepts of 3D/4D printing combined with composite science for electronic applications.

2.1 Why 3D/4D printing? To start with 3D printing, a short answer to the question of “why” is, because of the unprecedented freedom of design that 3D printing offers. The nature of 3D printing to build 3D tangible structures by adding layers upon layers of materials, hence the term AM, which is a unique advantage of 3D printing, and one of the reasons as of why this technology has been adopted and flourished. The fact that each layer is deposited sequentially means patterns within each layer can be designed with minimal restriction; hence, the overall 3D structure could contain complex inner features that are extremely challenging to manufacture using other advanced manufacturing technologies. The freedom of design is certainly unique to 3D printing but not the only reason why the technology has seen a potential. Being a digital manufacturing platform by nature is another key advantage of 3D printing shared with subtractive manufacturing and several other technologies that can translate computer designs into tangible objects digitally. Low material waste, ability to manufacture whole structures saving part assembly, and cost effectiveness in certain applications are all advantages of 3D printing that could be common with other manufacturing processes. 4D printing has a clear addition to 3D printing by adding a factor of time to 3D printed static features, in which controlled motion of the printed structures occurs in time. This feature of actuation can be induced by a chemical reaction, temperature difference, optical irradiation, or any other source of energy that initiates motion in a controlled and predetermined manner. This actuation feature of 4D printing enables various opportunities for new applications, which will be discussed in this chapter.

2.2 Why composites for electronic applications? Composites are unavoidable common materials that we see in our day-to-day interactions; therefore, the question concerning this chapter on “why composites” is specific to “electronic applications.”

3D and 4D printed polymer composites for electronic applications

Targeting material properties that don’t exist in a single material can be the simplest form of answer to the question on “why composites for electronic applications.” To elaborate on properties related to electronic applications, the following list shows descriptive examples of some of these properties: 1. Tailored electrical properties: polymers in general are considered electrically insulating materials apart from few exceptions. Metals, on the other hand, are relatively very good electrical conductors. A tailored electrical conductivity state in between the two can be very attractive for customizable resistive and electromagnetic components. This can be achieved in a composite, which contains conductive fillers in a polymer carrier. A key factor to establish Ohmic conductivity, where electrons can flow from one side to the other of the composite, is by percolating the conductive filler in which adjacent particles are touching each other to form one conductive path along the structure. Further details on the percolation theory are presented in Section 3.1. 2. Tailored electromagnetic properties: unlike electrical properties of composites, electromagnetic properties don’t require percolation in order to function in electromagnetic applications. Electromagnetic-responsive composites contain materials that interact with electric field and/or magnetic field. The two properties associated with these fields are the permittivity and the permeability of a material. In simple terms, a material that has a high susceptibility to hold an electric charge when exposed to an electric field is considered to have high permittivity, whereas a material that has a high degree of magnetization when exposed to a magnetic field is considered to have high permeability. A composite that contains materials with high permittivity or permeability will have larger effect on manipulating electromagnetic waves. These waves could be in the form of radio frequency (RF) or light. A material that has tailored electromagnetic properties is highly attractive to manipulate RF signals in applications such as metamaterials, antenna design, and waveguides. 3. Tailored mechanical properties: electrically conductive materials, metals for example, are mostly stiff with variable level of ductility but limited ability to stretch. Wearable or flexible electronic applications, for example, require electrical conductors with very specific mechanical properties to avoid rapid fatigue and short lifetime. Composites with tailored mechanical properties used in electronic applications have a large specific influence in 4D printing. For example, soft actuators and the field of dielectric elastomers are highly influenced by the manipulation of mechanical properties and the dielectric property of the material as it will be described in Section 4.1.1. 4. Tailored acoustic properties: although acoustic properties are not directly related to electronics, their importance comes from their influence on electronic applications. Acoustic properties of materials are highly related to their mechanical properties, in which stiff materials are considered better acoustic conductors, whereas softer materials damp acoustic waves making them better acoustic-absorbing materials. Composites that can manipulate acoustic waves in a controlled manner are of a high

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importance in ultrasonic applications and audio systems, which have major common grounds with electronic applications. 5. Tailored bioelectrical properties: the work on biocompatible materials, that are safe to be in contact with a living body, often requires properties that don’t exist in individual materials without forming a composite. For example, biocompatible polymers are mostly electrically insulators, which limit many functionalities that are vital in biology. Neural applications or controlled cell growth can be highly enhanced when biocompatible materials are given new electrical properties by forming bioelectrical composites. The combination of 3D/4D printing and composite materials with its high property tuneability brings an unprecedented capability to AM in which tailored materials can be fabricated to form complex structures that are otherwise extremely challenging to achieve.

3 3D and 4D printable electrically conductive composites Electrical conductivity here means the conventional Ohmic conductivity where free electrons flow through the conduction band of an atom or a molecule. This concept requires physical connection between adjacent electron carriers. In the case of conductive composites, a matrix of polymer filler is formed where the filler exhibits electrical conductivity and must form a continuous path to carry electrons throughout the composite, which introduces the theory of percolation.

3.1 Percolation theory in conductive composites Percolation theory as a concept was first introduced as a mathematical model to predict the probability of molecule branching to form larger molecules [16]. In relation to electrical conductivity of composites, the percolation theory describes the probability of conductive fillers within a polymer to form a continuous connected electron flow path(s) as shown in Fig. 1.

Fig. 1 Percolation of fillers of various shapes within a polymer matrix.

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In addition to the bulk electrical conductivity of the filler material(s), Fig. 1 was illustrated to highlight two major factors that influence the overall conductivity of a composite, which are: (1) The amount of conductive filler in relation to the total area/volume of the composite. (2) The shape of the filler, which affects the probability of adjacent filler connectivity. This means adding a filler to a polymer matrix will introduce no conductivity until the filler entities are connected to each other from one end of the connection path to the other end. This explains the percolation threshold [17] in which no conductivity is established in a composite until certain percentage of filler is reached as shown in Fig. 2. The electrical conductivity (σ) ratio represents the percentage of the electrical conductivity of the polymer-filler matrix over the electrical conductivity of the neat filler. The percolation threshold varies for different materials and is highly dependent on the shape of the filler where spherical particles, nanowires, or platelet sheets could shift the threshold to increase or decrease its areal or volumetric percentage.

3.2 Processes of 3D printing of conductive composites Several 3D printing technologies have contributed to manufacturing electrically conductive composites. In this chapter, the focus is primarily on extrusion printing, inkjet printing, stereolithography (SLA), and selective laser sintering (SLS). In common between these technologies is the filler-loading limit to reach percolation, which varies between different AM technologies. For example, in inkjet printing, this comes in the form of viscosity limitation where high solid loading leads to high viscosity, which is a barrier for most jetting technologies. The filler-loading challenge can be related to optical limitation, such as the case of SLA where high filler loading prevents light from penetrating within the

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composite layers reducing the polymerization level of the composites and the overall efficiency of the process. Details of each of these manufacturing technologies in relation to composites for electronic applications are presented in the following subsections: 3.2.1 Extrusion of conductive composites Two extrusion-based technologies have seen extensive advancements in 3D printing; these are (1) fused filament fabrication (FFF) or as it is commercially trademarked as fused deposition modeling (FDM) and (2) paste extrusion (PE) often known as direct write technology (DWT). The concept of conductive composites in FFF is to infuse thermoplastic polymers with conductive filler to form a conductive filament with minor changes to the processability requirement of the produced filament. Common polymers used in conductive composites of FFF are polylactic acid (PLA), acrylonitrile butadiene styrene (ABS), and polypropylene (PP). Carbon-derived fillers have been one of the most common fillers in FFF due to their cost effectiveness, shape and form diversity, availability, and chemical stability [12–15]. Kwok et al. showed PP-carbon black composite with a percolation threshold of around 10 wt% reaching a resistivity as low as 1–5 mΩ m at 40 wt% of carbon black [12]. Zhang et al. used reduced graphene oxide (rGO) to form PLA-rGO filaments demonstrating a conductivity of 4.76 S/cm for 6 wt% rGO in the composite matrix [18]. Marasso et al. and Foo et al. used commercial carbon-derived conductive filaments of PLA-graphite/ graphene to fabricate various electronic applications. Although these reports established electrical conductivity, they showed high resistivity when compared to bulk metals limiting their electrical conductivity to low current applications [14, 15]. Paste extrusion has been used widely to 3D print conductive composites due to the large window of viscosity enabled for extrusion compared to inkjet printing, for example. Postiglione et al. formulated a conductive paste by dissolving PLA in dichloromethane (DCM) as the carrier polymer and used various quantities of multiwalled carbon nanotubes (MWCNTs) as the conductive filler. An electrical conductivity of around 20 S/m was demonstrated for 5 wt% of MWCNT loading [19]. 3.2.2 Inkjet printing of conductive composites Inkjet printing has been utilized in fabricating composite structures by forming jettable inks of relatively low-viscosity liquid monomers mixed with various types of conductive fillers. Although inkjet printing has excellent low material waste and high printing resolution when compared to other AM technologies, the viscosity limitation of the technology is a major hurdle to the amount of solid loading that can be added to an ink. Conventional piezoelectric inkjet printing, capable of jetting liquids with viscosities below 30 MPa, depends on the surface tension, density, and nozzle size of the printheads [20]. This viscosity limit can be a challenge to the amount of filler loading that can be added to a liquid to form a composite [21–23]. Due to the nozzle size limitation in most

3D and 4D printed polymer composites for electronic applications

inkjet printing technologies, the size of the filler particles used in inkjet printing is common in the nanoscale to avoid nozzle blockage. However, nonconventional inkjet printing technologies are capable of jetting higher viscosities with a potential compromise in print resolution and material throughput [24]. Metal nanoparticles such as silver and copper nanoparticles, and carbon-derived fillers such as carbon nanotubes (CNT), carbon black, and graphene are among the most common conductive fillers used in inkjet printing of composites [21, 22]. Sangermano et al. used a photocurable ink formed of acrylic monomer, poly(ethylene glycol) diacrylate (PEGDA), as a liquid carrier with around 30 wt% of silver nanoparticles to reach percolation [22]. Eshkalak et al. and Small et al. demonstrated the use of CNT to form conductive inks in inkjet printing for various sensing applications [21, 23]. The common factor for all these composites is the relatively high electrical resistance limiting their use to mostly sensing application. The fact that conductive fillers are surrounded with low-viscosity insulating polymer means high infiltration of the polymer within the composite matrix, which limits the percolation paths within the printed structures. 3.2.3 SLA of conductive composites Being the first 3D printing technology to emerge in early 1980s, SLA has seen large advancements in material development due to the wide window of processable rheology and the simplicity of the printing mechanism [25–27]. The dependency of SLA on photocurable polymers means a limitation to filler loading due to a limitation of light penetration when high content of filler is loaded [26]. Gonzalez et al. used a polymer mixture of diacrylate and methacrylate of poly ethylene glycol (PEGDA:PEGMEMA) with 0.5 wt% CNT to reach a conductivity of 20 μS/cm with around 70% level of polymerization. Higher quantities of CNT were investigated, but the low level of polymer conversion of around 50% for 1 wt% of CNT meant lower mechanical properties and shape accuracy [26]. Scordo et al. used a conductive organic polymer filler, poly(3,4ethylenedioxythiophene) (PEDOT), with PEGDA to form a conductive composite that was processed at a ratio of 1:5 of PEDOT:PEGDA to achieve a conductivity of 0.05 S/cm. The high conductivity achieved here is due to the high filler loading, which was enabled due to the use of translucent conductive polymeric filler such as PEDOT [27]. In a different approach to form conductive composites using SLA, Fantino et al. used silver salt (silver nitrate) with PEGDA and a photoinitiator to form a resin that was polymerized using UV light-forming silver nanoparticles with the polymer as a result of photoreduction. Thermogravimetric analysis (TGA) of the composite showed a filler content of around 10 wt% was reduced, providing a resistivity value of 150 kΩ.m [28]. 3.2.4 SLS of conductive composites SLS, being a powder bed fusion technology, has different factors affecting the processability of conductive composites. For example, in photocurable polymer composites, the

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lack of light penetration due to the filler interaction with the light reduces the polymerization of the printed structures resulting in mechanically “weaker” structures, whereas in SLS the addition of filler often reduces the energy requirement to sinter the composite powder as the filler increases the laser absorption often resulting in higher temperatures generated for the same laser energy, and higher interlayer thermal conductivity resulting in higher layer fusion hence stronger structures [29]. Another major factor for powder fusion technologies is the flowability of the powders in which the introduction of fillers for the powder matrix could increase or decrease the flowability of the powder depending on the isotropy of the filler shape. Lanzl et al. used copper powder with polyamide 12 powder in SLS, and showed the effect of the filler shape on the flowability of the powders where anisotropic flakes produced around 300% thermal conductivity higher than spherical isotropic filler for the same quantities of copper content of 5 vol% [30]. Li et al. used CNT with flexible thermoplastic polyurethane (TPU) to form electrically conductive flexible 3D structures with a conductivity of 0.1 S/m for 1 wt% of CNT [31].

3.3 Role of conductive composites in 4D printing In 4D printing, the main role of printing electrically conducive composites is to print conductive electrodes to deliver an electric current to the actuating active materials. A major advantage of electrically conductive composites in 4D printing is the tuneability of the mechanical properties of the electrodes, which is highly crucial to the lifetime of 4D printed structures. Although metallic electrodes offer high electrical conductivity, their mechanical properties and fatigue behavior introduces a challenge for the actuating parts of 4D printed structures. However, the relatively high resistivity of conductive composites restricts their applications to low current loads unless a major alteration to the geometry of the structure occurs to allow for higher currents to flow as it will be described in the section on applications of printable conductive composites.

3.4 Applications of 3D and 4D printable conductive composites Two major areas of applications which make use of 3D and 4D printable conductive composites; are: (1) sensors and (2) planer electrodes. This is due to, the previously mentioned, high resistivity of conductive composites. However, to put this into an application context, the target resistivity or conductivity needs to be explained here to identify when electrical resistivity is considered too high and when it is not. Electrical resistivity (ρ) and conductivity (σ) are universal properties of materials (ρ ¼ 1/σ), whereas electrical resistance (R) and conductance (G) are not. To take electrical resistance as an example, R is geometry dependent as described in Fig. 3, where (L) is the length of the track, (W) is the width, and (T) is the thickness. As shown in Fig. 3A, the resistance of a structure can be changed by altering the dimensions of the structure, whereas the resistivity of the material is not geometry

3D and 4D printed polymer composites for electronic applications

Fig. 3 Resistance is geometry dependent where the shape of the structure determines the overall resistance.

dependent and can only be changed by changing the material formulation. A material with certain resistivity can be 3D printed to form a long thin track to make high resistive structure or can be designed to form a thin disk shape with large planer area to obtain low resistance as shown in Fig. 3B. Understanding the difference between resistivity and resistance raises a question on what determines the threshold of high and low resistance. The answer to this is application-driven based on the load used to deliver the power to. For example, if the aim is to print an electrically conductive track to power a small heater, and this heater has a resistance of 10 Ω then the printed track must have a total resistance much lower than the resistance of the load (the heater in this case). However, if the aim is to power a temperature sensor with an internal resistance of 10 kΩ, then a printed track of 10 or 100 Ω can perform well for that particular application. By understanding the concept of resistivity, the previously mentioned areas of applications of sensors and planer electrodes can be well understood. Sensors are required to draw low current signals, hence most sensors operate with high internal resistance; therefore, printing active sensing materials such as graphene or CNT within a polymer matrix to form a composite is highly plausible. Planer electrodes, on the other hand, might require high current; hence, the electrodes are designed to be thin but with large planer area to reduce the resistance of the electrodes and enable more current to flow. The following are examples of 3D and 4D printed electrodes and sensors applications: 3.4.1 Printed electrode applications Using FFF, Foo et al. printed 17-mm-diameter electrodes of 1 mm thick using graphene fused PLA filament. The electrodes were used to form a structure of a supercapacitor in addition to enabling electron flow from and to the supercapacitor as shown in Fig. 4. This highlights one important aspect in printing such materials where extra functionality can be added to conventional polymers when a composite is formulated, and the importance

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Fig. 4 Graphene-infused conductive electrodes printed using FFF to form supercapacitor [15]. (Copyright 2018. Reproduced with permission from Springer Nature. Permission evaluation was conducted, an automated certificate was produced under CC BY 4.0. File is attached (CC-BY-4.0.pdf ).)

of composites in forming flexible conductive materials, which is otherwise challenging to find in a natural single material [15]. Guo et al. used SLS to fabricate 50  50  4 mm structures of a polyamide 12 composite containing a mixture of carbon-derived fillers (carbon black and carbon fiber) to fabricate electrodes embedded in proton-exchange membrane (PEM) fuel cells. The embedment of the electrodes within the structure highlights one of the advantages of printing electrodes without the need for further assembly steps [32]. 3.4.2 Printed sensor applications Fabricating customizable sensors is a major application area in many AM technologies, and the use of conductive composites is a key enabler of this application. Strain sensors are fundamentally enabled in conductive flexible composites as the percolation of the filler is affected by the motion of the printed structure. Leigh et al. demonstrated the use of 3D printing of conducive composites to fabricate strain sensors embedded in a

3D and 4D printed polymer composites for electronic applications

glove and capacitive sensors to act as customizable buttons for human interface devices (HID) [33]. Bustillos et al. reported the use of boron nitride nanoplatelets (BNNP) within a photosensitive polymer to 3D print photosensors using SLA. The presence of around 1 wt% of BNNP had a major impact on the mechanical properties of the structure, which was investigated thoroughly by the team [34]. In another investigation, Truby et al. have embedded printed sensors made of various composites within a 4D printed fluidic elastomer actuator. The inflatable actuator was all-printed using silicone elastomers in which various sensors are printed within the structure to provide a closed-loop feedback to control the actuation behavior as shown in Fig. 5.

4 3D and 4D printable dielectric composites Dielectrics, in essence, are electrical insulators but can be polarized under the effect of an electric field. In other words, a dielectric material won’t allow conventional electrons to flow through it as in the case of electrical conductors, but when a voltage is applied across a dielectric material a charge is built up within the material due to a polarization effect. The dielectric constant of a material is the relative permittivity of that material compared to the permittivity of vacuum (ε0). This permittivity of vacuum describes the behavior of two charges in vacuum with known distance between them in relation to the force generated between the two charges. This is a measure of the electrical capacitance of two charges in vacuum over the distance between them, hence the unit of ε0 is farad per meter (F/m), and the dielectric constant of a material is the relative permittivity of that material in relation to the permittivity of vacuum. In practice, a material with high dielectric constant or high relative permittivity is a material that can hold high charge and form high capacitance. This has major implications on electromagnetic waves and radiofrequency applications where a material with high permittivity has higher tendency to manipulate electromagnetic waves; hence, various 3D printed demonstrators are investigating manipulation of electromagnetic waves by printing predetermined structures with certain dielectric properties. On the contrary, materials with low dielectric constant tend to act as good electrical insulators; hence, several investigations have studied the use of 3D printing of composites with highly insulating fillers for electrical insulation applications. In 4D printing, the field of dielectric elastomer actuators is directly affected by the dielectric properties of a material as described in the work principle section.

4.1 Work principle of dielectric composites The work principle of two of the major applications of dielectric composites is explained here. Dielectric elastomer actuators are part of the field of soft robotics where materials with relatively high dielectric constant are induced by an electric field to initiate motion. The composites of dielectric elastomer actuators use soft and rubbery polymer matrices

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Fig. 5 Soft robotic grippers with sensory feedback. (A) Images of an interaction sequence between a ball and a soft robotic gripper. (B) ΔR of each sensor as a function of time during the interaction sequence. (C) Still images show the gripper holding nothing (left), a smooth ball (middle), and a spiked ball (right). (D) ΔR for each sensor is plotted as a function of time. (E) Thermal images of a gripper holding a room temperature (RT) ball, a hot ball (60°C), and a cold ball ( 0°C). (F) ΔRcurvature and ΔRcontact plotted as a function of time. (G) Still images showing a gripper with embedded fine and deep contact sensors holding a foam ball at 97 kPa (P1), 124 kPa (P2), and 152 kPa (P3) (scale bar is 10 mm). (H) The ΔRcurvature, ΔRfine, and ΔRdeep are plotted as a function of time for the sequence shown in (G) [35]. (From R.L. Truby, et al., Soft somatosensitive actuators via embedded 3D printing, Adv. Mater. 30 (15) (2018) 1706383.)

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and often conductive fillers to gain high dielectric constant. On the other hand, highly insulating composite applications use nonconductive glass and ceramic fillers in polymer matrices to enhance their insulating properties and increase their breakdown threshold. 4.1.1 Dielectric elastomer composites The work principle of a dielectric elastomer is similar to a capacitor where an opposite polarity charge is held across it when an electric field is applied as shown in Fig. 6A. This opposite charge at each side of the elastomer generates an attraction force inward. When the dielectric material between those opposite charges is elastic, the attraction force between the charges induces motion on the structure by “squeezing” inwards with the direction of the electric field and generating a perpendicular motion toward the sides of the dielectric material as shown in Fig. 6B. This concept highlights three major factors, among others, that influence the actuation: (1) The charge that can be held across the dielectric material, where the higher the charge, the stronger the attraction force. This charge accumulation capability is represented in the dielectric constant of a material, where high dielectric constant means higher charge holding capability. (2) The elasticity of the material, where the softer the material, the higher motion displacement occurs. (3) The electric field across the material, in which higher electric field leads to stronger attraction force, which is influenced by the ability of the composite material to withstand high electric field before breaking down [36]. The balance between these three factors can be challenging because a material with high conductive filler loading has higher tendency to breakdown at high electric fields; also a material with higher solid filler loading is likely to be harder than a soft rubbery polymer with no filler, so a major development is to tune these parameters to reach an optimal balance. 4.1.2 Highly insulating composites Electrical breakdown of a substance occurs when high electric field is applied across an electrically insulating material resulting in material ionization and current flow through

Fig. 6 Actuation in dielectric elastomers occurs when an electric field is applied across a dielectric elastomer which compresses the elastomer due to opposite polarity charge attraction.

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Fig. 7 Electrical breakdown occurs as voltage increases across an insulating gap.

the material due to the excessive electric field. A simple example is dry air, which is electrically insulating in normal ambient conditions. When high voltage is applied across a small gap (i.e., high electric field), a spark occurs due to the ionization of air molecules allowing electric current to flow across the gap as shown in Fig. 7. This sudden electron flow within an electrical insulator (insulating breakdown) is a property of materials, in which higher breakdown threshold represents higher insulation capability. Polymers are generally considered electrically insulating; however, applications that use high voltages require materials with higher breakdown thresholds, glasses for example. Manufacturing polymers has flourished in the past 100 years compared to glass, which is known to mankind for many centuries yet more challenging to manipulate. To gain the best of advances in polymer manufacturing and the excellent insulating properties of glasses and ceramics, the field of polymer composites with highly insulating fillers has emerged. The ability to manufacture such materials with high freedom of design has been very attractive; hence, the tendency to use 3D printing to manufacture highly insulating composites has increased.

4.2 Processes of 3D printing of dielectric composites Dielectric composites have wide variety of applications; this is reflected in the processes that are investigated to print dielectric composites using 3D and 4D printing technologies. The following processes showcase several applications with an emphasis on the process aspects, whereas Section 16.4.3 will primarily focus on the applications.

3D and 4D printed polymer composites for electronic applications

4.2.1 Extrusion of dielectric composites Extrusion, in general, is one of the most accessible 3D printing technologies; this is due to the popularity of FFF technology and the availability of open-source hardware and software resources. Dielectric composites have been manufactured using FFF for several applications in recent years. Khatri et al. formulated a filament of thermoplastic composite that consists of ABS and barium titanate (BaTiO3) in order to 3D print high permittivity structures using FFF. The 3D printed composite has seen an increase in the relative permittivity at 200 kHz from 3.08 for neat ABS to 11.5 for the composite of 35 vol% of barium titanate [37]. Park et al. used paste extrusion to print polystyrene-derived polymer and barium titanate to achieve a dielectric constant of 9 [38], whereas Kotikian et al. used paste extrusion to fabricate liquid crystal elastomer composites, which actuates when temperature changes forming 4D printed structures. The printed actuators showed a specific work of around 30 J/kg [39]. 4.2.2 Inkjet printing of dielectric composites Inkjet printing with its capability of printing multiple materials in one structure that has been used to fabricate various composites with different fillers and polymers. Saleh et al. used iron oxide filler in Tri(propylene glycol) diacrylate (TPGDA) and a photoinitiator to formulate an electromagnetic-responsive photocurable ink. The composite ink was printed alongside neat TPGDA monomer to enable the manipulation of electromagnetic waves by means of altering the geometry of the two inks [40]. Mikolajek et al. used high dielectric constant filler of barium strontium titanate (Ba0.6Sr0.4TiO3) dispersed in poly(methyl methacrylate) (PMMA) at a ratio of 50 vol% to form an ink that can be jetted from conventional piezoelectric printheads. The composite has achieved a relative permittivity of 28 at 10 kHz [41]. 4.2.3 SLA of dielectric composites Most investigations to formulate electrically conductive composites could lead to a large alternation to the dielectric properties. However, the use of electrically conductive fillers could lead to high percolation, which is desirable for electrical conductivity but not for dielectric applications. Applications that require high dielectric constant aim at low electrical conductivity to avoid leaking the charge between the electrodes toward each other. Hence, electrically insulating materials with high dielectric constant are excellent for capacitance, elastomer actuation, and electromagnetic applications. Yang et al. used high dielectric filler material of Ag-Pb(Zr,Ti)O3 particles in methacrylated monomer to form an acrylic composite and used in SLA. A relative permittivity of 120 at 100 Hz was achieved for a resin with 18 vol% filler content. A capacitor was fabricated using the resin with a specific capacitance of around 63 F/g and current density

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of 0.5 A/g [42]. Credi et al. used magnetic iron (II,III) oxide nanopowder as a filler in ethoxylate diacrylate polymer resin to form a magnetically responsive structure using SLA 3D printing [43]. 4.2.4 SLS of dielectric composites SLS with its ability to produce complex structures without the need for support materials is used to fabricate some electronic applications related to dielectric composites. Li et al. formulated an epoxy composite with different quantities of glass fiber filler to manipulate the electrical breakdown threshold of the composite [44]. Thompson et al. used 2 wt% of carbon black with Nylon-12 to increase the IR absorption of the neat polymer and introduce high-speed sintering technology. The carbon-nylon composite has shown a dielectric constant of 3.5 at 100 Hz compared to 2.5 for neat Nylon-12 [45].

4.3 Applications of 3D and 4D printable dielectric composites The simplicity of formulating dielectric composites has widened their applications into many fields; this includes thorough investigations into their mechanical, optical, and biological properties in 3D and 4D printing. For this chapter, electronic applications are the target area of study; hence, three application categories are presented: 4.3.1 RF-responsive structures The fact that composites offer great level of tuning of electrical properties, their use in manipulating RF waves has seen high growth. This manipulation of electromagnetic waves is fundamentally geometry-dependent, which highlights the importance of using 3D printing with its unprecedented freedom of design to fabricate RF responsive composites. An excellent example of that is the work of Wu et al. who used a dual-head FFF printer to 3D print microwave devices to generate orbital angular momentum at 12–18 GHz frequency range as shown in Fig. 8 [46]. The devices were fabricated using a standard ABS polymer alongside a composite, which contains 32 vol% of barium titanate.

Fig. 8 3D printed RF devices to generate orbital angular momentum [46]. (Reproduced with permission from MDPI. MDPI open access publications are licensed under Creative Commons CC BY 4.0: https://www. mdpi.com/authors/rights.)

3D and 4D printed polymer composites for electronic applications

4.3.2 Electrical insulators The work on additively manufactured composites for electrical insulation purposes is still in its infancy despite the importance of fabricating customized structures of highly insulating composites. Li et al. used glass fiber in phenol formaldehyde resin (PF) to form a highly insulating composite using SLS [31]. A glass fiber content of 60 vol% showed an electrical breakdown voltage of 29.6 kV for 1-mm-thick samples, whereas 80 vol% of glass fiber had a breakdown threshold at 32.5kV for the same sample dimensions. Li et al. used several composites of aluminum oxide (Al2O3), strontium titanate (SrTiO3), and barium titanate (BaTiO3) in a photocurable acrylic to form composites, which were used in SLA 3D printing. Out of these three, composites with alumina filler showed the highest breakdown threshold of around 50kV/mm at volume fraction of 5%–20% of filler as shown in Fig. 9 [47]. 4.3.3 Dielectric elastomer actuators Dielectric elastomer actuators were highly enabled by the introduction of 3D printing allowing a dimension of time in the form of actuation to take place, which is, in essence, the concept of 4D printing. Soft composites with high dielectric constant are 3D printed to follow a predetermined shape, which can be altered by inducing an energy source, which is electric field for the case of electronic applications. Tibbits introduced early 4D demonstrations using short carbon fiber filler composite in an epoxy to form layers of materials with different stiffness to control their actuation behavior as shown in Fig. 10 [35, 48]. Kuhnel et al. used paste extrusion of UV-curable silicone with graphite nanoparticles filler to form a dielectric elastomer composite, which showed excellent mechanical and electrical properties for actuation purposes [49]. In two separate reports, Ambulo et al. and Kotikian et al. demonstrated the use of liquid crystal monomers to form liquid crystal

Fig. 9 Electrical breakdown threshold of ceramic filled composite of aluminum oxide (AO), strontium titanate (ST), and barium titanate (BT) [47]. (Reproduced with permission from the authors. Figure was reproduced with a permission from the author, file is attached (Zhang_20190213 ZhangGJ_Permission-Request-Form.pdf ).)

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Fig. 10 Time-lapsed illustration of 4D structure as it changes with time [48]. (Reproduced with permission from WILEY-VCH Verlag GmbH & Co. Permission was authorized by the publisher, file is attached (Tibbits_RightsLink Printable License.pdf ).)

elastomer composite actuators, which undergo up to 40% reversible contraction along the print direction when heated [39, 50].

5 Conclusion Composites and their use in 3D and 4D printing for electronics applications have been discussed in this chapter, in which the need for composites has been explained and the advantages of 3D and 4D printing have also been presented highlighting the major gains when composite materials are used in disruptive technologies such as 3D and 4D printing. From an electrical point of view, in this chapter, composites were divided into two categories: (1) electrically conducive composites and (2) dielectric composites. Each of these categories was discussed thoroughly in terms of general concepts for the layman reader, and in-depth details on the most common processes with electrically conductive and dielectric composites as well as practical applications in each category. Several examples for each of the AM processes were laid down, where some technologies matched certain applications better than others. This connection between the processes and the rigorous understanding of each process in conjunction with the material was highly emphasized. Advantages and disadvantages of the processes were embedded in each category with respect to the material, where some processes produced relatively “weaker” parts when high filler content was loaded, whereas other processes gained strength when relatively high filler content was used, as in the case of SLA and SLS, respectively. An emphasis on the applications for each category was presented. For example, 3D printed electrodes and sensors were selected for the conductive composite category with explanation on the electrical current limitation and the effect of that on the choice of applications. The dielectric composites category had several applications presented, including most common RF-responsive materials and dielectric elastomer actuators. These led to the connection of 3D printing with 4D printing and laid down the concept of 4D printing and the influence of dielectric materials on introducing 4D printing.

3D and 4D printed polymer composites for electronic applications

This chapter presented these composites, processes, and applications from an electronics point of view. Many other properties of composites were discussed elsewhere in this book; most common are the mechanical and the biological properties. This is also the case of 4D printing where the prime focus here was on electrically stimulated structures, whether these stimuli were directly driven electrically such as the case of dielectric elastomer actuators or indirectly driven such as temperature-dependent 4D structures that are electrically heated, hence indirectly stimulated by an electric current.

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[37] B. Khatri, K. Lappe, M. Habedank, T. Mueller, C. Megnin, T. Hanemann, Fused deposition modeling of ABS-barium Titanate composites: a simple route towards tailored dielectric devices, Polym. 10 (2018). [38] J.Y. Park, Y.Y. Kang, H.W. Yoon, N.K. Park, Y. Jo, S. Jeong, J.C. Won, Y.H. Kim, Viscoelastic properties of a 3D-printable high-dielectric paste with surface-modified BaTiO3, Compos. Sci. Technol. 159 (2018) 225–231. [39] A. Kotikian, R.L. Truby, J.W. Boley, T.J. White, J.A. Lewis, 3D printing of liquid crystal elastomeric actuators with spatially programed nematic order, Adv. Mater. 30 (2018) 1706164. [40] E. Saleh, P. Woolliams, B. Clarke, A. Gregory, S. Greedy, C. Smartt, R. Wildman, I. Ashcroft, R. Hague, P. Dickens, C. Tuck, 3D inkjet-printed UV-curable inks for multi-functional electromagnetic applications, Addit. Manuf. 13 (2016) 143–148. [41] M. Mikolajek, A. Friederich, C. Kohler, M. Rosen, A. Rathjen, K. Kr€ uger, J.R. Binder, Direct inkjet printing of dielectric ceramic/polymer composite thick films, Adv. Eng. Mater. 17 (2015) 1294–1301. [42] Y. Yang, Z. Chen, X. Song, B. Zhu, T. Hsiai, P.-I. Wu, R. Xiong, J. Shi, Y. Chen, Q. Zhou, K.K. Shung, Three dimensional printing of high dielectric capacitor using projection based stereolithography method, Communication 22 (2016) 414–421. [43] C. Credi, A. Fiorese, M. Tironi, R. Bernasconi, L. Magagnin, M. Levi, S. Turri, 3D printing of cantilever-type microstructures by stereolithography of ferromagnetic photopolymers, ACS Appl. Mater. Interfaces 8 (2016) 26332–26342. [44] Z. Li, W. Zhou, Y. Lei, P. Chen, C. Yan, C. Cai, H. Li, L. Li, Y. Shi, Glass fiber-reinforced phenol formaldehyde resin-based electrical insulating composites fabricated by selective laser sintering, Polymers 11 (1) (2019) 135. [45] M.J. Thompson, D. Whalley, N. Hopkinson, Investigating Dielectric Properties of Sintered Polymers for Rapid Manufacturing, 2008. [46] Y. Wu, D. Isakov, P.S. Grant, Fabrication of composite filaments with high dielectric permittivity for fused deposition 3D printing, Mater. (Basel, Switz.) 10 (2017) 1218. [47] W. Li, L. Zhang, C. Wang, L. Xiaoran, M. Xu, G.-J. Zhang, Dielectric properties and 3D printing feasibility of UV curable polymer composites, in: IEEE Int. Conf. High Volt. Eng. Appl., Athens, 2018. [48] S. Tibbits, 4D printing: multi-material shape change, Archit. Des. 84 (2014) 116–121. [49] D.T. Kuhnel, J.M. Rossiter, C.F.J. Faul, 3D printing with light: towards additive manufacturing of soft, electroactive structures, Proc. SPIE (2018). [50] C.P. Ambulo, J.J. Burroughs, J.M. Boothby, H. Kim, M.R. Shankar, T.H. Ware, Four-dimensional printing of liquid crystal elastomers, ACS Appl. Mater. Interfaces 9 (2017) 37332–37339.

Further reading [51] R.L. Truby, et al., Soft somatosensitive actuators via embedded 3D printing, Adv. Mater. 30 (15) (2018) 1706383.

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CHAPTER 17

Fundamentals and applications of 3D and 4D printing of polymers: Challenges in polymer processing and prospects of future research Kalim Deshmukha, Aqib Muzaffarb, Tomáš Ková
ríka, Tomáš K
reneka, M. Basheer Ahamedb, S. K. Khadheer Pashac a

New Technologies—Research Center, University of West Bohemia, Pilsen, Czech Republic Department of Physics, B. S. Abdur Rahman Crescent Institute of Science and Technology, Chennai, India c Department of Physics, VIT-AP University, Amaravati, India b

1 Introduction During the 1980s, three-dimensional (3D) printing technologies were introduced, to meet the demands pertaining to the highly specialized rapid prototyping (RP) and model making. Over the years, 3D printing has gone through several developmental transformations and has appeared as a resourceful platform for computer-assisted design (CAD) and RP [1]. 3D printing enables the production of tailored structures from polymers, metals, and ceramics without incorporating any requirement for molds or machining structures, which are used in conventional manufacturing. Since the conventional fabrication methods were directed by processing constraints for mass production, 3D printing due to its inherent ability enables faster manufacturing based on the CAD to produce customized objects to meet the specific requirement or application [2]. 3D printing term is often synonymously used for additive manufacturing (AM), layered manufacturing (LM), RP, 3D fabbing, and solid freeform fabrication. 3D printing produces 3D objects with high shape complexity. 3D printing starts with CAD to create a virtual structure followed by its digital slicing. The virtual structure slices and coordinates are used to guide the motors, which, in turn, control the position of 3D dispenser orifice as shown in Fig. 1. 3D printing has been popular for manufacturing of complex 3D objects by fusion of layered materials [3]. The rise in 3D printing has enhanced the RP field due to its cost effectiveness and fast conversion rates of CAD [4]. Due to its customizable characteristics, flexibility, and facile nature, 3D printing has been widely adopted in the field of ornaments, polymer textiles, sensors, supercapacitor electrodes, tissue and scaffold engineering, metamaterials, and robotics [5–11]. The concept of four-dimensional (4D) printing is ascribed to the extension of 3D printing with the addition of time dimension. The 4D 3D and 4D Printing of Polymer Nanocomposite Materials https://doi.org/10.1016/B978-0-12-816805-9.00017-X

© 2020 Elsevier Inc. All rights reserved.

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Fig. 1 Basic principle of 3D printing (A) product idea development and the transformation of digital data via CAD (B) preprocessing of CAD model into virtual layered data and later its transfer to 3D printer (C) 3D printing of final product after postprocessing. (Adapted from S.C. Ligon, R. Liska, J. Stampfl, M. Gurr, R. M€ ulhaupt, Polymers for 3D printing and customized additive manufacturing, Chem. Rev. 117 (15) (2017) 10212–10290.)

printing had been rapidly expanded and developed to enhance its impact in research and in order to introduce novel manufacturing approaches. The materials capable of changing the shape dynamically are fabricated or manufactured via multimaterial 3D printing methods, while 4D printing is an ambivalent and broadly accepted method in these cases. As a consequence of that, in recent times, 4D printing has been defined in a more detailed way specifically mentioning the constraints requiring the functionality or change in shape to be induced directly after printing [12]. 4D printing can be defined as the 3D printing of self-transforming materials when exposed to predefined stimulus like heat, light, pressure, or other energy sources [13–16]. The advent of smart materials responding to external stimulus has been extensively demonstrated for 4D printing of shape recovery polymers, sensors, and actuators [17]. There have been numerous attempts made to combine 3D printing with smart materials to manufacture 4D objects activated by external stimuli post printing. The stimuliresponsive 3D manufactured structures are dubbed with the fourth dimension of time to yield 4D printed structures [18]. The effects especially shape morphing post printing entails the key feature of 4D printing. The shape transformations in 4D printed structures can be obtained by various external stimuli to produce shrinkage, folding, or expansion of the printed structure. The shape memory polymers as smart materials are commonly

Fundamentals and applications of 3D and 4D printing of polymers

explored for self-folding and autoresponsive 4D printed objects. In general, the principal characteristics of the transformations taking place during 4D printing are governed by various principles based on the composition of the objects from smart materials or bilayer products with inhomogeneity and different properties [19]. Considering the stimuli, 4D printing can be categorized based on the parameters like solvents, humidity, temperature, pressure, pH, or light that induces transformation [20]. The considerations of properties of smart materials used for 4D printing include mechanical properties, glass transition, the rate of recovery, and swelling ratio [21]. For 3D and 4D printing of polymers, the computer-aided manufacturing (CAM) is generally performed via layer-by-layer deposition with the thickness of layers in the range of 15–500 μm [2]. The layer thickness less than 50 μm of polymeric materials is nonrecognizable by the naked eye in stair steps associated with layerwise deposition approach. When polymer layers are thicker, postpolymer processing is used to remove supporting structures and improvement of surface properties. The multidimensional printing of polymers has been achieved by conventional polymer processing techniques such as computer numeric control machining, which is despite being fast, does not yield complex structure manufacturing. In contrast to that, AM of polymeric structures although being slow enables CAD-based fabrication of complex structures, biosystems, and manufacturing of multifunctional materials. The development of manufacturing systems, which are easy to use and which exhibit fast building speeds and low cost has been among the trending research areas [4,22]. The AM of polymer-based systems has been developed from small-scale niche manufacturing processes to the limelight of greater-scale manufacturing in recent times. Although of significant progress in AM technology in the last few decades, there are still a lot of challenges that require the proper establishment to widen the range of AM of polymeric systems on a greater scale. The challenges concerned with AM are inappropriate material properties like thermomechanical characteristics, anisotropy, porosity, corrosion, long-time stability, creep, and cost effectiveness [20,23]. There exist various types of AM processes, which are categorized in different ways based on their applicability in a particular field. These AM processes are functional prototyping, rapid manufacturing, and visual prototyping depending on the initial conditions of the manufacturing materials or the physical principles concerned with the layerwise solidification [24]. The 3D and 4D printing of polymeric materials are likely to play a pivotal role in near future owing to their various advantages. The 4D printed objects like functional wires, motors, and batteries have allowed researchers to utilize micro or nanosmart devices and actuators [15,25]. 4D printing has been successfully used in fields like security, biomedical devices, optoelectronic devices, etc. [26–29]. This chapter describes the general processes concerned with 3D and 4D printing of polymeric materials along with the challenges associated with the polymer processing. The interesting prospects of future research in 3D and 4D printing of polymeric materials have also been elaborated.

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2 Fundamentals of 3D printing processes The polymeric materials come in various forms with diverse properties and are used in a wide variety of applications [30]. In modern times, the polymer industry has grown greatly and is now larger than the metal industry [31,32]. Polymers used in multidimensional printing are classified as thermosets and thermoplastics and are used in the form of powder, resin, and filament differing from each other in thermal behavior [33]. Thermosets are the nonmelting polymers and typically are in the form of viscous fluids at the start of the printing process and attain solid form after curing. The curing of thermosets occurs by mixing them with a catalyst, exposure to light, or by heat treatment. After solidification of thermosets, exposure to high temperatures may result in defects in their structural integrity. Generally, photopolymer thermosets having the ability to solidify on exposure to UV or laser light are used in the material jetting process such as stereolithography and digital light processing (DLP). The examples of 3D printed products made from thermoset polymers include bowling balls, knobs of cooking tops, etc. On the other hand, thermoplastics exhibit melting ability with repetitive solidifications and retain their inherent properties [34]. 3D printing methods like injection molding and material extrusion are used for thermoplastic printing, thereby heating solid thermoplastic to a malleable state followed by extrusion or injection molding of the polymer into a die or build platform wherein solidification takes place. Common examples of 3D printing of thermoplastics are plastic bottles and food packaging products. The processes associated with the 3D printing of polymers are illustrated in Fig. 2. These processes are discussed in the upcoming sections.

2.1 VAT photopolymerization It is an AM process in which a liquid photopolymer resin is placed in a vat followed by its selective curing using UV light to activate the polymerization. In VAT Processes in polymer 3D printing

Vat photopolymerization

Stereolithography Digital light processing (DLP) Continuous liquid interface production (CLIP) Multiphoton polymerization (MPP)

Powder bed fusion

Material extrusion

Selective laser sintering (SLS)

Fused deposition modelling (FDM)

Fig. 2 Processes used in 3D printing of polymers.

Material and binder jetting

Inkjet printing Aerosol printing

Fundamentals and applications of 3D and 4D printing of polymers

photopolymerization, the photocurable resins on exposure to laser undergo chemical reactions and become solid. The chemical reaction of photocurable resins is known as photopolymerization, which occurs in the presence of photoinitiators, additives, and a reactive oligomer or monomer. Generally, most of the photopolymers used in AM are curable in the UV by linkage of small monomer chains in the presence of photoinitiator, which acts as a catalyst to increase the reaction rate. It is essential for the polymer to exhibit sufficient crosslinkage and strength in order to avoid redissolving into monomer and to remain structurally stable under various forces. The lithography-based approaches such as stereolithography, DLP, and multiphoton polymerization (MPP) are grouped under vat photopolymerization processing. 2.1.1 Stereolithography The process of building solid structures by exposure of photopolymer with optical fibers or masks controlled by X-Y plotter was described by Kodama in 1980 [35]. Another report [36] demonstrates a similar method in which a laser beam is directed and controlled by plotter and a lab jack in Z-direction. In stereolithography, a laser beam is used to crosslink photopolymerizable polymers, thereby fabricating 3D parts by the formation of vertical layering [37]. The schematic illustration of stereolithography fabrication process is shown in Fig. 3 [38]. Stereolithography fabrication method provides high spatial resolution due to spot size focusing of the laser beam. The setup for stereolithography comprises a vat containing the liquid polymer and a building platform with elevator. The laser beam from the laser source is vertically focused on the building platform to fabricate 3D scaffolds [38,39]. In stereolithography, the exposure of laser light occurs by sequential

Fig. 3 Schematic illustration of stereolithography process [38]. (Reproduced with permission from M.C. Tanzi, S. Fare, G. Candiani, Chapter 3: Manufacturing Technologies, in: Foundations of Biomaterials Engineering, 2019, pp. 137–196. Copyright 2019, Elsevier.)

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scanning of the laser beam across the plane of the photosensitive polymer surface. The scanning speed of the laser beam determines the time duration of fabricating the slice of the 3D structure. The setup of stereolithography is provided with a pair of mirrors within the scanner (Galvanoscanner) to control the lateral position of the laser beam. The production of the 3D structure occurs in a layerwise manner. After initial layer formation, it is presented as a set of coordinates, which define the tilting angle of the mirrors within the two mirrors, thereby guiding the laser beam to the position along the plane of the photopolymer. The penetration depth of light determines the vertical resolution and curing depth of the structure, which is controlled using a suitable set of absorbers to the photopolymer resin. In addition to that, the viscosity of the polymer resin plays a vital role in layer deposition time and as such the polymer resins are generally mixed with nonreactive additives or solvents to decrease their viscosity [40]. 2.1.2 Digital light processing This process is similar to stereolithography as both techniques use light-based crosslinking of photopolymer resins and layer-wise deposition of polymer to form a 3D structure. The apparatus of stereolithography comprise of a reservoir which is filled with a photocurable polymer resin, UV light as a laser source, an electronic system to control the X-Y movement of the light beam and the fabrication platform that can move in the vertical plane as shown in Fig. 4 (top). This set up is also called as a bottom-up set-up. The surface scanning of the photocurable materials gives 2D structures via single photon absorption at the material surface. The 3D structures can be made possible via layer by layer approach in which the fabrication platform can move step by step in the Z-direction once the 2D layer is completed. The curing of subsequent layers is carried out on the top of the previous layers via irradiation from the above. Another important and emerging trend in stereolithography is the use of DLP technology. DLP is also similar to classical lithography and hence it is referred as dynamic mask photolithography. The technique uses binary patterns such as black and white images to store information of each layer in the structure. The binary patterns are then processed by means of a digital micromirror device (DMD), which serves as a dynamic mask for projecting 2D structures on the surface [41]. The schematic illustration of the working principle of DLP technique in the top-down scheme for 3D printing of polymeric resins is shown in Fig. 4 (bottom) [42]. However, it can also be used in bottom-up set-ups. In DLP, the whole layer of structure is obtained using a single exposure step and hence the production time is shorter in comparison to stereolithography. DLP is a top-down approach having a nonadhering transparent plate at the base of the vat. The polymerization of photosensitive polymeric resin takes place through irradiation from bottom and the build platform moves in the opposite direction to that of stereolithography. As such in DLP, a new layer is placed beneath the previous layer. The projection facility provided with the setup enables fast curing of layers in one go, thereby reducing

Fundamentals and applications of 3D and 4D printing of polymers

Fig. 4 Schematic illustration of the bottom up and top down stereolithography set-ups (https://s100. copyright.com/CustomerAdmin/PLF.jsp?ref¼dddd2f12-ee08-4c4c-af54-5e7afd345548). (Reproduced with permission from T. Billiet, M. Vandenhaute, J. Schelfhout, S. Van Vlierberghe, P. Dubruel, A review of trends and limitations in hydrogel-rapid prototyping for tissue engineering, Biomaterials 33 (26) (2012) 6020–6041. Copyright 2012, Elsevier.)

fabrication time. DLP provides a lateral resolution of 10–50 μm based on the pixels provided by the DMD and the patterns projected onto the building platform [42]. Contrary to that, the vertical resolution depends on the penetration depth of light into the photosensitive polymer and the depth of curing. The vertical resolution can be enhanced by the addition of light-absorbing materials like naphthol-containing dyes. 2.1.3 Continuous liquid interface production The inhibition of oxygen in vat photopolymerization leads to incomplete curing of the photopolymer resin and surface irregularities especially when polymerization takes place

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Fig. 5 Schematic representation of CLIP printing [43]. (Reproduced with permission from J.W. Stansbury, M.J. Idacavage, 3D printing with polymers: challenges among expanding options and opportunities, Dent. Mater. 32 (1) (2016) 54–64. Copyright 2016, Elsevier.)

in the air [43,44]. The presence of oxygen can cause quenching of photoexcited photoinitiators, which can lead to the creation of peroxides by combining with photocleaved photoinitiators. Avoiding oxygen inhibition will lead to efficient initiation and propagation of polymeric chains [45]. The continuous liquid interface production (CLIP) technique employs oxygen permeable film for photopolymerization at the surface close to the irradiation source and, as a consequence of that, the requirement of intermediate recoating of each layer is negotiated. Fig. 5 schematically describes CLIP printing technique [43]. CLIP process is initiated with the projection of a continuous sequence of UV light via oxygen-permeable and UV-transparent window beneath a photopolymer resin vat. In the CLIP process, there is a creation of the dead zone above the transparent window, which maintains a liquid interface below the advancing part. The curing of photopolymer takes place above the dead zone, which continuously withdraws the resin, thereby creating suction forces responsible for the renewal of reactive liquid polymeric resin. 2.1.4 Multiphoton polymerization MPP is among the most promising tools for 3D fabrication of polymeric materials due to multiphoton absorption (MPA) [46]. The MPA takes place via electronic transitions in polymeric materials driven by multiple photons (two or more) instead of a single photon. Due to the simultaneous presence of multiple photons, the absorption probability is dependent on the intensity of the light used to the power of a number of photons responsible for driving the electronic transition [47]. The printing of structure begins with focusing of a laser beam through a microscopic objective to produce intensified beam capable of driving MPA efficiently within the focal volume of the beam. The

Fundamentals and applications of 3D and 4D printing of polymers

Fig. 6 Schematic illustration of MPP setup. (Adapted from J. Torgersen, A. Ovsianikov, V. Mironov, N. Pucher, X. Qin, Z. Li, K. Cicha, T. Machacek, R. Liska, V. Jantsch, J. Stampfl, Photo-sensitive hydrogels for three-dimensional laser microfabrication in the presence of whole organisms, J. Biomed. Opt. 17 (10) (2012) 105008.)

consequences of light-driven physical or chemical transformations produce effects within a specific volume of the material [48]. The scanning of the position of the laser beam focused using the transformations creates arbitrarily complex 3D structures. In two-photon polymerization (2PP), the excitation is attained by means of chemical activation or by physical means to enable development of 3D structure with high spatial resolution. The dependence of absorption probability quadratically on intensity is the main advantage of 2PP under constrained focusing to confine the absorption within the lower volume. The 2PP has been successfully utilized in the polymerization of resins using conventional UV absorbing initiators under high laser power [2]. The schematic setup for MPP is shown in Fig. 6. The setup comprises a laser beam source emitting a beam, which initially passes through a collimator and then followed by passage through an acoustic modulator. The modulator spreads the laser beam to get diffractions of zero and first order. The output of the first order can be achieved by switching the modulator, which feeds the output by λ/2 wave plate. The polarizing beam splitter is used to adjust the laser power (intensity). The collimated laser beam is focused onto the sample holder containing photopolymer by the microscope objective. The setup is provided with a camera behind the semitransparent mirror, which allows the observation of photopolymerization on the illumination of the sample with suitable light [48].

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3D and 4D printing of polymer nanocomposite materials

2.2 Powder bed fusion Powder bed fusion technique comprises a combination of many techniques like electron beam melting (EBM), direct metal laser sintering (DMLS), selective heat sintering (SHS), selective laser sintering (SLS), and selective laser melting (SLM). In powder bed fusion, a laser source or an electron beam is used for melting and fusion of powdered material to create functional parts. The processing involves spreading the powder material over the previously prepared layers. The mechanisms involving manufacturing by powder bed fusion include a roller or a reservoir or a hopper to dispense fresh material supply. Considering the processing of polymers, the SLS technique is mainly used. Powder bed fusion sinters the powder in a layer-by-layer sequence and the addition and fusion of layers occurs by movements of the rollers and the building platform. The materials fabricated by powder bed fusion include ceramics, metals, composites, and polymers as summarized in Fig. 7 [2]. The varieties of powders suitable for fabrication via powder fusion are differentiated as polymer-based powders (including composites and pure polymers) and metal and ceramic-based powders. Generally, semicrystalline and amorphous thermoplastics, thermosets, and elastomers are used in this method. 2.2.1 Selective laser sintering SLS is the most popular RP technique used in the fabrication of highly flexible 3D structures from powdered polymers. The printed structures are of high quality and complex geometries [49]. The process uses a laser beam for sintering selected area in the powdered polymer to create solid objects in a layerwise deposition manner. The schematic design of the SLS technique is shown in Fig. 8 [2]. The process for SLS comprises powder deposition and solidification followed by movement of building platform to allow next layer deposition. These steps are repeated sequentially until the last layer of the material is

Fig. 7 Materials classification for powder bed fusion fabrication. (Adapted from S.C. Ligon, R. Liska, J. Stampfl, M. Gurr, R. M€ ulhaupt, Polymers for 3D printing and customized additive manufacturing, Chem. Rev. 117 (15) (2017) 10212–10290.)

Fundamentals and applications of 3D and 4D printing of polymers

Fig. 8 SLS setup comprising (A) building platform (vertically movable), (B) powder bed (C) laser source, (D) laser optics, (E) deposition hopper for feeding, and (F) distribution and leveling blade. (Adapted from S.C. Ligon, R. Liska, J. Stampfl, M. Gurr, R. M€ ulhaupt, Polymers for 3D printing and customized additive manufacturing, Chem. Rev. 117 (15) (2017) 10212–10290.)

deposited and sintered [2]. The solidification of the layers occurs by a laser source. The scanning of model contours also occurs by laser to deposit layers in accordance with the CAD data. The absorption of the laser beam by the powdered polymer results in the formation of soft, melted, and solidified adjacent particles [50,51]. The powder deposition among the sintered adjacent layers is carried out using a roller or blade to distribute powdered polymer particles supplied by hoppers. It is worth mentioning that during the entire process the building chamber is kept at an elevated temperature (