Ionic Polymer Metal Composites for Sensors and Actuators [1st ed.] 978-3-030-13727-4, 978-3-030-13728-1

Table of contents :
Front Matter ....Pages i-vi
Metal-Organic Framework Composites IPMC Sensors and Actuators (Bianca Maranescu, Aurelia Visa)....Pages 1-18
Polysaccharide-Based Ionic Polymer Metal Composite Actuators (A. Popa, A. Filimon, L. Lupa)....Pages 19-34
Conducting Polymer Based Ionic Polymer Metal Composite Actuators (David Gendron)....Pages 35-52
Role of Metal Ion Implantation on Ionic Polymer Metal Composite Membranes (Adina Maria Dobos, A. Filimon)....Pages 53-73
Study on Time-Dependent Bending Response of IPMC Actuator (Hyung-Man Kim, N. D. Vinh)....Pages 75-138
Ionic Polymer-Metal Composite Membranes Methods of Preparation (Fatma Aydin Unal, Hakan Burhan, Fatima Elmusa, Shukri Hersi, Fatih Sen)....Pages 139-148
Ionic Polymer-Metal Composite Actuators Operable in Dry Conditions (Fatma Aydin Unal, Hakan Burhan, Sumeyye Karakus, Gizem Karaelioglu, Fatih Sen)....Pages 149-159
Pressure Sensors Based on IPMC Actuator (Gokhan Topcu, Tugrul Guner, Mustafa M. Demir)....Pages 161-182
Robotic Assemblies Based on IPMC Actuators (D. Josephine Selvarani Ruth)....Pages 183-194
Design and Fabrication of Deformable Soft Gripper Using IPMC as Actuator (Srijan Bhattacharya, Bikash Bepari, Subhasis Bhaumik)....Pages 195-207

Citation preview

Engineering Materials

Inamuddin Abdullah M. Asiri Editors

Ionic Polymer Metal Composites for Sensors and Actuators

Engineering Materials

This series provides topical information on innovative, structural and functional materials and composites with applications in optical, electrical, mechanical, civil, aeronautical, medical, bio- and nano-engineering. The individual volumes are complete, comprehensive monographs covering the structure, properties, manufacturing process and applications of these materials. This multidisciplinary series is devoted to professionals, students and all those interested in the latest developments in the Materials Science field.

More information about this series at http://www.springer.com/series/4288

Inamuddin Abdullah M. Asiri •

Editors

Ionic Polymer Metal Composites for Sensors and Actuators

123

Editors Inamuddin Department of Chemistry Faculty of Science King Abdulaziz University Jeddah, Saudi Arabia

Abdullah M. Asiri Department of Chemistry Faculty of Science King Abdulaziz University Jeddah, Saudi Arabia

ISSN 1612-1317 ISSN 1868-1212 (electronic) Engineering Materials ISBN 978-3-030-13727-4 ISBN 978-3-030-13728-1 (eBook) https://doi.org/10.1007/978-3-030-13728-1 Library of Congress Control Number: 2019932625 © Springer Nature Switzerland AG 2019 This work is subject to copyright. All rights are reserved by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, expressed or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer imprint is published by the registered company Springer Nature Switzerland AG The registered company address is: Gewerbestrasse 11, 6330 Cham, Switzerland

Contents

Metal-Organic Framework Composites IPMC Sensors and Actuators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Bianca Maranescu and Aurelia Visa Polysaccharide-Based Ionic Polymer Metal Composite Actuators . . . . . A. Popa, A. Filimon and L. Lupa

1 19

Conducting Polymer Based Ionic Polymer Metal Composite Actuators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . David Gendron

35

Role of Metal Ion Implantation on Ionic Polymer Metal Composite Membranes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Adina Maria Dobos and A. Filimon

53

Study on Time-Dependent Bending Response of IPMC Actuator . . . . . . Hyung-Man Kim and N. D. Vinh

75

Ionic Polymer-Metal Composite Membranes Methods of Preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139 Fatma Aydin Unal, Hakan Burhan, Fatima Elmusa, Shukri Hersi and Fatih Sen Ionic Polymer-Metal Composite Actuators Operable in Dry Conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149 Fatma Aydin Unal, Hakan Burhan, Sumeyye Karakus, Gizem Karaelioglu and Fatih Sen Pressure Sensors Based on IPMC Actuator . . . . . . . . . . . . . . . . . . . . . . 161 Gokhan Topcu, Tugrul Guner and Mustafa M. Demir

v

vi

Contents

Robotic Assemblies Based on IPMC Actuators . . . . . . . . . . . . . . . . . . . 183 D. Josephine Selvarani Ruth Design and Fabrication of Deformable Soft Gripper Using IPMC as Actuator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 195 Srijan Bhattacharya, Bikash Bepari and Subhasis Bhaumik

Metal-Organic Framework Composites IPMC Sensors and Actuators Bianca Maranescu and Aurelia Visa

Abstract Metal-organic frameworks (MOFs), a highly studied class of complex structured porous materials, containing different types of central metal ions attached to organic linkers, are used in various applications such as catalysis, separation, absorption, photochemistry, proton conductivity, biotechnology, magnetism and sensoristic science etc. The architectural structures of MOFs provide special properties as improved thermal and mechanical stabilities, high surface areas and large pore sizes to these materials. The need for new functionalities is to take into account that the fabrication methods must be robust, scalable, friendly to environment and cost-effective. Keywords Metal-organic frameworks · Sensors · Molecular machines · Volatile organic compounds · Actuators

1 Introduction Development of metal-organic frameworks (MOFs) or coordination polymers (CPs) is one of the most active research fields among chemistry and materials research groups, due to a high diversity of applications [1–9]. MOFs now can be designed and constructed starting from a wide diversity of metal ions or metal-containing clusters, and organic ligands (Fig. 1). The variety of metal ions, organic linkers and structural styles come up with an endless number of potential combinations. International Roadmap for Devices and Systems (IRDS), a descendant of International Technology Roadmap for Semiconductors (ITRS) provides predictions about likely semiconductors and devices, with the main aim to align academic and research lab efforts with electronic devices manufacturers and equipment suppliers. The 2017 edition reports, under emerging research materials, identify opportunities as B. Maranescu · A. Visa (B) Institute of Chemistry Timi¸soara of the Romanian Academy, 300223 Timi¸soara, Romania e-mail: [email protected] © Springer Nature Switzerland AG 2019 Inamuddin and A. M. Asiri (eds.), Ionic Polymer Metal Composites for Sensors and Actuators, Engineering Materials, https://doi.org/10.1007/978-3-030-13728-1_1

1

2

B. Maranescu and A. Visa

Fig. 1 General structure of MOF

multifunctional materials/sensors, generation and energy storage, medical, as well as flexible electronics [10]. Rely on the application setting, the synthetic route is followed both to benefit from the complexity of organics moiety, to decrease the production costs and to reduce the size to molecular dimensions.

2 Design and Synthesis Since 1995, when the first synthesis was reported in the literature [11], most of the syntheses have been performed by solvothermal methods using conventional heating [12–16]. Usually, the reaction times needed were long, several days in the case of solvothermal synthesis to some weeks for diffusion methods. Therefore, it is important to find more efficient and techno-economically viable alternative synthetic techniques, that can be used successfully to scale up the production of MOFs. The newly developed approaches, such as ultrasounds [17–21], mechanochemical [22–26], microwave (MW) [27–31] and electrochemical synthesis [32–36], provide a promising alternative of time-consuming synthesis by shortening the synthesis times and increasing the production yield. By comparing with traditionally used zeolites and carbon materials, MOFs are unique with reference to their extraordinarily high porosities, tunable pores, and various functional sites. Diverse methods have been applied to detect/sense different levels of gases, anions, cations, and various pollutants in addition of developing materials for sensing, drug delivery, and heterogeneous catalysis in many contexts (Fig. 2).

Metal-Organic Framework Composites IPMC Sensors and Actuators

3

Fig. 2 MOF as actuators and sensor

3 Properties and Applications 3.1 Nano/Molecular Machines Based Actuators Very important and promising research direction is the design and building of molecular machines. In 2016 Nobel prize in chemistry was given to Jean-Pierre Sauvage, Sir James Fraser Stoddart and Ben L. Feringa for this fascinating domain [37, 38]. In order to construct nanomaterials with machine-like functions, some technologies were proposed by M. Venturi and co-workers [39]. Light-powered dynamic nanomachines which imply light energy stimulation of materials are of great interest. Furthermore, light stimuli are applied to nanomachines from small distances without physical interaction, and the energy quanta are measured by tuning the intensity, wavelength or polarization of the light. C. Barrett and co-workers [40] highlighted in their review article the most encouraging models of the organization of switches and rotors into linear, twodimensional assembly, and strong three-dimensional crystals. These dynamic systems were designed and constructed to answer to a high diversity of actuation stimuli. Actuators working on this principle of MOFs can also be used as artificial molecular machines [41, 42]. M. Garcia Garibay and his co-workers [42, 43] performed researches in the domain of pillared paddlewheel MOFs constructed by a modular approach using 9,10-bis-(pyridylethynyl)triptycene as a column and molecular rotator and dicarboxylate linkers of varying lengths and steric bulk. Using 1,4-bicyclo[2.2.2]octane dicarboxylic acid (BODCA)-MOF, a material with a high-symmetry BODCA linker in a Zn4 O cubic lattice, an ultrafast rotation in an amphidynamic crystalline metalorganic framework was performed (Fig. 3a, b). Four key architectural elements were taking into account: (1) rotor size and shape, (2) the accessibility of free volume higher than the volume of rotator, (3) the chemical nature of ligand as well as of metal and the degree of freedom for rotator to the stator and (4) the axial symmetries of rotator and stator. These kinds of molecules open new perspectives in the areas as gas storage, catalysis, and photochemistry.

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B. Maranescu and A. Visa

(a)

O

HO

(b)

OH

O

Fig. 3 Chemical structures of the molecular compact rotor bicyclo[2.2.2]octane (a) and a larger triptycene molecular rotator—9,10-bis-(pyridylethynyl)triptycene (b)

Another type of molecular machines, transport processes in living cells via bubble propulsion, were discovered by Li J. and co-workers [44]. They started with UiOtype metal-organic frameworks, namely Zr4+ -based UiO (UiO  University of Oslo) and transformed this material into self-propelled micromotors using distinct metalbased propulsion systems. The MOFs were designated for their important properties like high chemical stability and numerous types of tunability. Using a mixture of two ligands (2,2 -bipyridine-dicarboxylic acid and biphenyldicarboxylic acid in 1:3 molar ratio), UiO-67-bpy0.25 MOF was synthesized as a platform for metal-based propulsion systems. The metals used were Co2+ and Mn2+ as shown in Fig. 4. The Co2+ and Mn2+ metal-based catalytic engine sites were investigated for the transformation of H2 O2 into water and oxygen for transport of oxygen into living cells by bubble-propelled motion. Using different fuel levels starting from 5 to 15% (v/v) H2 O2 , it was observed that at a higher H2 O2 concentration in solution, the speeds of movement of UiO-67-Co(bpy)0.25 and UiO-67-Mn(bpy)0.25 particles increase giving a faster conversion of chemical energy into mechanical energy. Miniaturisation and low energy devices in our days have pushed the classical motor concept of energy conversion into mechanical power to the new challenges. Processes that resemble metabolizing and producing resources in nature have inspired researchers to produce biological motors.

Fig. 4 Metal-based propulsion systems

Metal-Organic Framework Composites IPMC Sensors and Actuators

5

Y. Ikezoe and co-workers have reported such a mechanism by incorporating biological diphenylamine (DPA) peptide into MOF structure, the movement being produced by the non-equilibrium condition from the reconfiguration of self-assembling peptides. Such bio-MOFs structures can be used for molecular transport, drug injection, or osmotic pumping devices [45]. The hybrid peptide MOF motor [Cu2 L2 ted]n , where L  1,4benzenedicarboxylate with a pore size of 0.75 nm, ted  triethylenediamine was used. The idea was to incorporate DPA peptides into MOF using a proper solvent 1,1,1,3,3,3-hexafluoro-2-propanol. After characterization of the hybrid peptide, this MOF motor was used with good results as a novel biomimetic artificial motor. The new research provided a clear understanding of the energy transduction mechanism in biological systems. The hybrid MOF DPA peptides were considered as fuel which acted as a motor for a longer time with the increasing the DPA quantity inside the pores. The mechanism of this motor is as follows: by mixing the hybrid peptide MOF motor with sodium ethylenediaminetetraacetate (Na-EDTA) the DPA peptide is released. This is happened because EDTA partially destroys the ordered structure of MOF.

3.2 pH Sensors/Actuators The pH is a usually measured parameter in numerous applications, such as biomedical diagnostics, bioprocessing, environmental and engineering-interconnected fields. Being an important symbol of water quality, pH is affected by countless factors, most of them as water pollutants. The solution’s pH is measured by different approaches such as metal-metal oxide electrodes, potentiometric and fluorescent pH sensors. Till now, only a few MOFs have been investigated as fluorescent pH sensors [46, 47]. An adequate and sensitive fluorescence sensor based on MOF containing amino groups, namely Al-MIL-101-NH2 , was constructed by Y. Lu and B. Yan [48]. The pH sensor was prepared from AlCl3 ·6H2 O and 2-aminoterephthalic acid (BDC-NH2 ) in DMF at 110 °C, via the solvothermal method. The Al-MIL-101-NH2 materials display good luminescence in aqueous solution with different pH going from 4.0 to 7.7, which can provide the possibility for pH sensing. The structural integrity of this compound in the analyzed medium is very important. The fluorescent intensities of Al-MIL-101-NH2 increase with the increase of pH, as expected. Furthermore, by incorporating Fe2 O3 nanoparticles inside of AlMIL-101-NH2 , the sensor can be easily removed from testing solutions by simply applying a magnetic field (Table 1, Fig. 5d). Another aluminium MOF Al(OH)(bpydc) known in literature as MOF-253, is the first metal-organic framework with open 2,2 -bipyridine (bpy) coordination sites [56]. Because MOF-253 is an ideal carrier for lanthanides bipyridine, it plays a double role: it can fix and sensitize lanthanide ions at the same time (Table 1, Fig. 5a).

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Table 1 MOFs as pH sensors listed by pH range domain and metal pH range

Metal

MOF

References

4–7.7

Al

Al-MIL-101-NH2

[48]

5–7.2

Al

MOF253-Eu-TTA

[46]

2–11

Zn

Zn-cpon-1

[49]

5–7.5

Eu

ITQMOF-3-Eu

[50]

7–10

Eu

[H3 O][Eu3 (HBPTC)2 (BPTC)(H2 O)2 ]-4DMA

[51]

1.7–11.3

Zr

RB-PCN

[52]

1–9 1–12

Zr

UiO-66-NH2 UiO-66-N  N-ind

[53]

5–6.8

Zr

FITC-UiO-NMOF

[54]

0–3

Zr

PCN-222

[55]

B. Yang and co-workers reported a Eu3+ ratiometric pH sensor by postsynthetically modification of the MOF-253 structure [46]. The MOF-253-Eu–TTA was prepared as follows: first Eu3+ ions were introduced into MOF-253 forming MOF-253-Eu and second the deprotonated TTA (TTA  2-thenoyltrifluoroacetone) was used to further sensitize Eu3+ ions. Two new bio-friendly zinc based MOFs (Zn-cpon and Zn-cpon-1) were synthesized by Y. Yang and co-workers using a semi-rigid 5-(4 -carboxyphenoxy) nicotinic acid (H2 cpon) with Zn(NO3 )2 ·6H2 O in 1:2 and 1:1 molar ratio, respectively. These MOFs presented a ratiometric pH-sensing dual-emission and good drug delivery behaviour (Table 1, Fig. 5b) [49]. Comparing with Zn-cpon-1, Zn-cpon has lower luminescence intensity that can be attributed to the dense structural 2D packing type and more flexible movement of ligand. In the pH interval 2–11, two trends were observed: first, with the decrease of pH values from 6.5 to 2.0, the emission intensity at 532 nm increased and the emission at 443 nm decreased nearly to zero. In the pH range of 6.5–8, the emission intensity of Zn-cpon-1 remained constant. However, when the pH increased from 8 to 11.5, the intensity of emission at 443 nm decreased with the emission intensity of the peak at 532 nm increased, which remained stable till pH 11. Zn-cpon-1 is an excellent drug delivery system, which showed the prior encapsulating behaviour to 5-fluorouracil (5-FU) comparing with 6-mercaptopurine (6-MP). This behavior is attributed to the impact of dimensions and shape matching. J. Rocha and co-workers [50] described a Eu-based MOFs ((ITQMOF-3-Eu) which was prepared under hydrothermal conditions by the reaction of H2 PhenDCA ligand and the Eu3+ salt or oxide. This MOF exhibited a linear pH-dependence across the range from 5 to 7.5 (Table 1, Fig. 5c). Additionally, X. Lu and co-workers found an effective dual-emission fluorescent metal-organic framework nanocomposites probe (denoted as RB-PCN) constructed for sensitive and detection of pH in the 1.7–11.3 interval. RB-PCN was prepared by

Metal-Organic Framework Composites IPMC Sensors and Actuators

(a)

7

(b)

5-(4'-carboxyphenoxy)nicotinic acid (H2cpon)

2,2′-bipyridine-5,5′-dicarboxylic acid (H2bpydc)

(c)

(d)

2-aminobenzene-1,4-dicarboxylate (bdc-NH2)

1,10-phenanthroline-2,9-dicarboxylic acid (H2PhenDCA)

(e)

(f)

tetrakis(4-carboxyphenyl)porphyrin

3,30,5,50-biphenyltetracarboxylic acid

(H4tcpp)

(H4BPTC)

Fig. 5 The ligands used for pH sensing

8

B. Maranescu and A. Visa

encapsulating the DBI-PEG-NH2 -functionalized Fe3 O4 into the Zr-MOFs and then further reacted with rhodamine B isothiocyanates (RBITC) [52]. D. Bradshaw and co-workers constructed a pH-sensing ability MOF namely UiO66-NH2 [Zr6 O4 (OH)4 (bdc-NH2 )6 , bdc-NH2  2-aminobenzene-1,4-dicarboxylate]. This MOF displayed fluorescence behaviour by the protonation of the pendant NH2 groups (Table 1, Fig. 5d) [53]. The prepared MOF was stable in aqueous solutions between pH 1 and 9. To increase its stability in basic conditions, a post-synthetic modification strategy was employed to incorporate an indole (ind) moiety at the amine group via a diazotization reaction with the formation of UiO-66-N  N-ind. After this diazotization, its structure remained stable up to pH 12 which is very important for enhanced sensing sensitivity of the MOF emission and its increased chemical stability (Table 1, Fig. 5d). J. Li and co-workers found an effective pH sensor having double function—fluorescent and colorimetric shifts under strongly acidic conditions, namely zirconium–porphyrin MOF (PCN-222) [Zr6 (OH)8 (tcpp)4 , H4 tcpp  tetrakis(4carboxyphenyl)porphyrin] (Table 1, Fig. 5e) [55]. In acidic medium pH 0–3, solid material PCN-222 changes its colour from purple to deep green. This change was also observed in the fluorescence spectrum of the material. Using a fluorescein isothionate (FITC)-functionalized UiO MOF, W. Lin and coworkers synthesized a nanoscale MOF useful in instantly intracellular pH sensing in living cells [54]. FITC is a useful fluorescent probe molecule which shows pH-dependent fluorescence. Its disadvantage is that FITC is rapidly ejected from live cells and because of this its usability is limited. By linking covalently FITC molecules with a nanoscale MOF framework, the FITC could be retained within the live cells. Therefore, authors used FITC to link it to a UiO-NMOF [Zr6 O4 (OH)4 (aminoTPDC)6 , amino-tpdc  2 -amino-1,1 :4,1 -terphenyl-4,4 -dicarboxylic acid]. By incorporation of FITC into UiO-NMOF, the pH sensing performance was not interferenced and the composite material F-UiO took up into human cell lung cancer H460 cell. On these cells, the material was used to follow the pH change at 5–6.8 interval. D. Sun and co-workers developed an Eu-based MOF, [H3 O][Eu3 (HBPTC)2 (BPTC)(H2 O)2 ]-4DMA or UPC-5 (H4 BPTC  3,30,5,50biphenyltetracarboxylic acid) used for pH sensing in the range of 7.5–10 (Table 1, Fig. 5f) [51]. The UPC-5 was obtained by solvothermal reaction of H4 BPTC and Eu(NO3 )3 ·6H2 O in DMA/H2 O at 90 °C for 2 days. The Eu3+ metal ions were coordinated in two ways: Eu1 coordinated by eight oxygen atoms from six carboxylate groups, and Eu2 was coordinated through nine oxygen atoms from eight COOH groups and one coordinated H2 O molecule. The relationship among luminescence intensity and the pH were linear in the pH range 7.5–10. Therefore, this material is a candidate for biological applications.

Metal-Organic Framework Composites IPMC Sensors and Actuators

9

3.3 VOC Sensors Volatile organic compounds (VOCs) are compounds that rapidly converted into vapours or gases at ambient temperature (methanol, ethanol, toluene, acetone, DMF, DMA or mixtures of solvents, aldehydes, phenols, etc.). All scents of odours are VOCs and these are coming from solvents evaporation, solids sublimation, industrial processes or other sources causing severe environmental problems. Most models for the selective identification of small solvent molecules were described based on guest type-dependent luminescent answers for MOFs, based on the shift of emission spectrum or by producing a change in the luminescent intensity [57]. Most of MOFs used as VOCs sensors are based on transitional metals such as Zn, Cd, and Cu with different ligands as presented in Table 2. J. P. Zang and co-workers used a well-known ligand 1H-Imidazo[4,5-f] [1, 10] phenanthroline (Hip) (Table 2, Fig. 6a), in reaction with Zn(NO3 )2 /Zn(ClO4 )2 under solvothermal conditions to obtain a flexible porous metal azolate framework [Zn7 (ip)12 ](OH)2 (MAF-34), with active uncoordinated imidazolate nitrogen donors on the pore surface. MAF-34 in the presence of methanol/ethanol vapour shows drastic structural and luminescent changes [58]. An innovative luminescent sensor and gas reservoir MOF, [Zn3 (OH)2 (btca)2 ]·DMF·4H2 O (1, H2 btca  benzotriazole-5-carboxylic acid) (Table 2, Fig. 6b) constructed by X. Huang and co-workers exhibited green luminescence with λmax  490 nm in common solvents. In benzene, methylbenzene, and xylene this MOF showed intense solvatochromism with a colour change from green to blue upon

Table 2 MOFs as VOC sensors listed by metal and solvent molecule recognition VOC sensor

Metal

MOF

References

Methanol/ethanol

Zn

[Zn7 (ip)12 ](OH)2 (MAF-34)

[58]

Benzene, toluene, xylene

Zn3 (OH)2 - (btca)2 ]·DMF·4H2 O

[59]

Benzene, mesytilene

NUS-1

[60]

Methanol

[Zn3 (cpoip)2 (4,4 -bpy)2 ·H2 O]

[61]

Cd(5-aip) (4,4 -bpy)*3DMA

[62]

Acetone

Cd

Acetone

Cd2 (TBA)2 (bipy)(DMA)2

[63]

p-Nitroaniline

Cd-PDA

[64]

Cu2 (mpbatb)-DUT71

[65]

Etanol

Cu

Cu4 (mpbatb)2 (dabco)3.5 -DUT72 Cu4 (mpbatb)2 (1,3-bib)0.5 -DUT73a Cu4 (mpbatb)2 (1,4-bib)0.5 -DUT73b Cu4 (mpbatb)2 (bpta)0.5 -DUT74 DMF

Cu, Eu

{Cu3 Eu2 (PBA)6 (NO3 )6 ·H2 O}n

[66]

10

B. Maranescu and A. Visa

(a)

(b)

1H-Imidazo[4,5f][1,10]phenanthroline

(d)

(c )

benzotriazole-5-carboxylic acid(H2btca)

(e)

4,4′-(2,2-diphenylethene-1,1diyl)dibenzoic acid

(g)

5-aip=5-aminoisophthalic acid

(f)

9 phenylcarbazole 3,6 dicar boxylic acid

(h)

4-(1H-tetrazol-5-yl)-benzoic acid

(j)

(i)

1,3-bib

Dabco

(k)

Hpba

H4mpbatb

(l)

(m)

1,4-bib

Fig. 6 The ligands used for VOC sensing

H3cpoip

Bpta

Metal-Organic Framework Composites IPMC Sensors and Actuators

11

saturation [60]. M. Zhang and co-workers developed a turn-on fluorescence MOF namely NUS-1 composed by Zn4 O building units and 4,4 -(2,2-diphenylethene-1,1diyl)dibenzoic acid (DPEB) as tetraphenylethene (TPE) ligand (Fig. 6c). The chemosensing research was performed by soaking NUS-1 crystals in various VOCs followed by photoluminescence tests which showed the intensity changes and peak shifts were more clearly. For NUS-1 soaked in benzene, λmax  504 nm has the largest red shift (18 nm) whereas NUS-1 soaked in mesitylene, λmax  458 nm) has the largest blue shift (28 nm) [60]. A novel luminescent transition-metal MOF, namely [Zn3 (cpoip)2 (4,4 bpy)2 ·H2 O] was synthesized by Z. Jin and co-workers shown in Fig. 6f. This fascinating molecule reveals a turn-on switching by solvent molecules, fast response, excellent selectivity and high sensitivity for methanol molecules in ethanol solution. The sensing property of this MOF can be used in alcoholic beverages; attributed to methanol concentration in the methanol-ethanol mixture, it produces a fluorescence intensity in the range of 0–0.01 [61]. A new pillar layer MOF-Cd(5-aip)L*3DMA (DMA  N,Ndimethylacetamide, 5-aip  5-aminoisophthalic acid (Fig. 6c) and L  4,4 -bipyridine were synthesized by N. H. Wang and co-workers [62]. Solvents used for luminescence study were methanol, ethanol, 1,3-propanediol, cyclohexane, tetrahydrofuran, N,N-dimethylformamide, DMA, acetone and acetonitrile. The experimental results showed that the luminescence intensity of pillared MOF was influenced by solvent, especially by acetone. Thus, acetone has a wide absorption range from 307 to 360 nm, while 1,3-propanediol displayed no absorption, therefore this molecule can be used as acetone detecting sensor. Besides, a luminescence of Cd-MOF as a sensor molecule was developed out by Y. Liu and co-workers [63]. The Cd2 (TBA)2 (bipy)(DMA)2 has been synthesized starting from Cd(NO3 )2 ·4H2 O, 4-(1H-tetrazol-5-yl)-benzoic acid (H2 TBA) (Fig. 6g) as ligand and 4,4 -bipyridine (bipy) using the solvothermal method. Sensitive luminescent phenomena were detected when compound Cd-MOF was exposed to ethanol, methanol, trichloromethane, acetone, acetonitrile, dichloromethane, 1-propanol, 2propanol. The luminescent intensity changed with diverse organic solvents. Acetone exhibited the most quenching behaviour and hence Cd2 (TBA)2 (bipy)(DMA)2 can be used as a possible luminescent probe for acetone detection. Due to high demand of sensing materials for organic molecules and also degradation of organic contaminants, new photoactive materials were synthesized by J. Wang and co-workers by mixing Cd(NO3 )2 ·4H2 O and 9-phenylcarbazole-3,6-dicarboxylic acid (H2 PDA) (Fig. 6e) as linker, in 2:1 molar ratio and dissolving in 2:1 mixture of DMA/H2 O under solvothermal conditions. The obtained material, (Cd–PDA) was investigated to observe quenching behaviour upon various aromatic compounds like benzene, toluene, nitrobenzene, aniline, o-, m- and p-nitroaniline. The fluorescence intensities remained approximately constant in case of benzene, toluene, nitrobenzene, aniline. The o-, m- and p-nitroanilines exhibited a decrease in intensity. The best result observed onto p-nitroaniline was attributed to the synergistic effects of electron transfer and hydrogen-bonding interaction. Therefore, Cd−PDA was a good chemosensor for p-nitroaniline isomers, showing 3.5 ppm ultra-sensitivity in solution [64].

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S. Kaskela and co-workers [65] synthesized a new ligand, 4,4 ,4 ,4 -(1,3phenylenebis(azanetriyl))tetrabenzoic acid (H4 mpbatb) as well as a new MOF called DUT-71 using copper(II)chloride dehydrate in solvothermal conditions using DMF and ethanol, (1:1) in oven 80 °C for 2 days. With the new MOF Cu2 (mpbatb) (further named as DUT-71) as starting material, different stabilizing N-donor ligands such as 1,4-diazabicyclo[2.2.2]octane, 1,3-bis(1H-imidazol-1-yl)benzene, 1,4bis(1H-imidazol-1-yl)benzene and 3,6-di(4-pyridyl)-1,2,4,5-tetrazine were hosted into the framework and conducted to DUT-72, DUT-73a and DUT-73b, DUT-74, respectively as illustrated in Fig. 6h–l. Also, several other DUT-MOFs were formed by post-modification of DUT71 adding different amounts of stabilizing N-donors ligands to obtain DUT-90, DUT-91 and DUT-95 MOF. The colour modification from blue to green under ethanol exposure followed by UV/Vis spectroscopy made these MOFs favourable candidates for sensing applications at ethanol concentrations ranging from 50 to 600 ppm. In the reaction of Eu(NO3 )3 ·6H2 O and Cu(NO3 )2 ·3H2 O with 4-(pyrimidin5-yl) benzoic acid (HPBA) ligand in CH3 CN at 85 °C for 3 days yielded a non-luminescent 3d-4f heterobimetallic CuEu-organic framework as blue crystals (Fig. 6m). The compound {Cu3 Eu2 (PBA)6 (NO3 )6 ·H2 O}n , named NBU-8, was investigated as solvent “turn-on” detection. The fluorescence responses of NBU-8 in various common organic solvents like acetone, dichloromethane, trichloromethane, N,N -N,N -dimethyl acetamide, 1,4-dioxane, dimethylformamide, ethyl acetate, ethanol, tetrahydrofuran methanol, hexane and acetonitrile were monitored. The luminescence intensity of NBU-8 at λmax  615 nm of the investigated solvents showed a high selective luminescence recovery towards DMF molecule. This effect made NUB-8 an excellent candidate as a sensor for DMF molecule [66].

3.4 Humidity Sensors Humidity sensors are useful to monitor parameters in medicine, fluid pipelines, food processing and storage [67, 68]. Sensing materials used in humidity sensors have their advantages and disadvantages. The ones based on polymers are having longer response times and hysteresis versus inorganic oxide and ones that show crosssensitivity and drift [69–71]. The main principle of operations for humidity sensor is optical, acoustic or electronic, the last being dominant [72]. Electronic sensing issues are periodical calibration, low sensitivity, nonlinearity and long response times [73]. Y. Gao and co-workers have reported an europium based MOF [74] that can be used both in electronic and optical detection methods with very good results in sensing humidity. Experiments have been carried out using two isostructural europiumbased MOFs, with a 3D network with highly hydrophilic open channels filled with water molecules: [Eu(H2 O)2 (mpca)2 Eu(H2 O)6M(CN)8 ]·nH2 O (herein EuM, where mpca  2-pyrazine-5-methyl-carboxylate, M  Mo, W), Fig. 7 [74]. To prove optical detection, reversibility of the hydration-dehydration process was carried out using

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Fig. 7 Ligand mpca  2-pyrazine-5-methylcarboxylate

laser-irradiated sample, proving times of the 60 s to trap water molecule, resulting in fast optical humidity sensing response [75].

3.5 Energy Conversion Device One main feature of the electric generators is energy density—a feature where macroassemblied structure can surpass other generator types. Moisture simulated electric generators might be obtained from graphene-based materials. If constructed by a twisting process as 50 μm fiber diameter [76] when exposed to moisture can deliver instant rotation up to 5000 rotation min−1 and with a peak power of 71.9 W kg−1 . If a magnet is attached to rotating fiber, it can generate electric power in copper coils. A graphene structure can be assembled out of graphene oxide films ionized with hydronium ions (H3 O+ ) to obtain a proton concentration gradient. This enables a moisture triggered voltage generation of 35 mV [77], thus surpassing fluidic-electric generators or even some piezoelectric generators [78]. Once basic generator function is obtained, power or output voltage can be improved by various microstructure assembling techniques—like 3D networks that maximize water absorption; thus, the output voltage is increased [79]. Light energy can be absorbed in MOFs by the linkers, metal cations, or guest molecules. Therefore, several studies have been performed either on synthetization or post-synthetic functionalization and guest molecules [80]. Energy transfer and light harvesting properties gathering were mentioned for zinc benzenetribenzoate framework infiltrated with triplet-scavenging organometallic compounds, thiophene and fullerene-infiltrated MOF-177. It is very important to know the Guest-MOF definition. In order to use MOF as adsorbents materials for different species, we have to understand the mechanism of adsorption. In most of the cases, the connections between guest molecules and the framework are weak which are not chemically bond. D. Allendorf and co-workers presented the following concept: the guest molecule interaction with the framework goes beyond the relatively weak physisorption with the focus on preparation of a new material with good properties. The authors discovered that infiltrating HKUST1 pores (HKUST-1—Hong Kong University of Science and Technology-(Cu3 (btc)2 ; btc  benzenetricarboxylate, Fig. 6b) with TCNQ (tetracyanoquinodimethane), form TCNQ@HKUST-1 film which induces an increase in the electronic conductivity of more than 107 orders of magnitude compared with uninfiltrated MOF.

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4 Conclusions and Future Outlook Metal-organic frameworks show many opportunities in actuators and sensors applications. Thanks to their ability to tune the molecule for different applications and their high surface area, stability and large pore volume which allowed us to improve their active behaviour by loading with various functional groups in order of achieving excellent new properties. Due to their nanoscale organization, most of the researcher groups search between the applications to use this material into real-world from sensing toxic molecules, small and volatile organic compounds, atmospheric pollutants to more deeply research area as stimuli-sensitive systems as actuators or generators. Starting from the definition of Strengths, Weaknesses, Opportunities and Threats Analysis—as a useful method for accepting strengths and weaknesses, we look to apply this for the appreciation of MOFs in all variety of applications. Acknowledgements This work was partially supported by Program no. 2, Project no. 2.3 from the Institute of Chemistry Timisoara of Romanian Academy and by a grant of the Romanian National Authority for Scientific Research, CNCS—UEFISCDI, project number PN-III-P1-1.1-TE-20162008, within PNCDI III.

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Polysaccharide-Based Ionic Polymer Metal Composite Actuators A. Popa, A. Filimon and L. Lupa

Abstract Cellulose and chitosan are naturally abundant biopolymers which can be used as ion exchange polymers in various applications. Due to their useful characteristics, a lot of research has been done on using these materials as a base for obtaining ionic polymer metal composite actuators. The present chapter discusses numerous ways of combination between polysaccharide and various electrically conductive materials such as carbon nanotubes and graphene in the presence or absence of different ionic liquids, and subsequent use of these materials to improve the actuation performance of the polysaccharide-based actuators. Though a lot of studies have been performed for obtaining optimal compositions and suitable methods in respect of polysaccharide-based ionic polymer metal composite actuators. There is still a niche to find the best composition structure and the most efficient and low-cost method of obtaining actuators in order to meet the needs of various industries. The search continues for actuators with enhanced mechanical, electrical and electroactive performance, with good durability and flexibility in processing.

1 Introduction Polysaccharides are common in nature as these are found in most living organisms. Being a natural source of food, these are present in various tissues of seeds, stems and leaves of plants, body fluids of animals, shells of crustaceans and insects. Polysaccharides are also found in the cell walls and extracellular fluids of bacteria, yeast and A. Popa Institute of Chemistry Timisoara of Romanian Academy, 24 Mihai Viteazul Blv., 300223 Timisoara, Romania A. Filimon “Petru Poni” Institute of Macromolecular Chemistry, Aleea Grigore Ghica Voda 41A, 700487 Iasi, Romania L. Lupa (B) Faculty of Industrial Chemistry and Environmental Engineering, Politehnica University Timisoara, 6 Vasile Parvan Bld, 300223 Timisoara, Romania e-mail: [email protected] © Springer Nature Switzerland AG 2019 Inamuddin and A. M. Asiri (eds.), Ionic Polymer Metal Composites for Sensors and Actuators, Engineering Materials, https://doi.org/10.1007/978-3-030-13728-1_2

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fungi. These are used as a renewable feedstock for synthesizing high-performance macromolecular materials [1]. Cellulose, an environmentally friendly material, is the most abundant resource in the world. After cellulose, chitin is the second most frequently found biopolymer in nature [2]. Cellulose is a long-chain polysaccharide made up of D-glucose ((C6 H10 O5 )n ) which is connected by β-(1-4)-bonds [3]. Chitosan, the deacetylated form of chitin is similar in structure to cellulose (Fig. 1) and is composed of D-glucosamine units joined by β-(1-4) glucosidic bonds. Many of chitosan’s properties are due to the presence of primary amine groups with pKa 6.5 [4]. At pH lower than 6.5, the amines are positively charged and chitosan is soluble. At higher pH, the amines are increasingly deprotonated, and hence chitosan becomes insoluble [4]. Because of its adsorption capacity, non-antigenicity, bioactivity, nontoxicity, film-forming ability, bacteriostatic action, and chelating activities chitosan has many applications [5, 6]. It is used in different areas such as drug delivery [7], biosensors [8] and electroactive polymers [9]. Also, there have been many studies on its propensity to enhance actuation performance [10–14]. Cellulose, due to its unlimited resources is a low-cost option with good potential for energy transformation [15, 16]. Both cellulose and chitosan are biocompatible and biodegradable polycationic polysaccharides and can be used as electro-active biopolymers with actuation performance. Ionic polymer-metal composite (IPMC) actuators have many applications i.e. artificial muscles, underwater robotics fishes, micropumps, catheter systems in humans and transducers etc. Additionally, IPMC has valuable properties such as exhibiting large tip displacement under low voltage, good flexibility, easy actuation, lightweight, and simplity of manufacturing [17]. Likewise, the property of biocompatibility is important and desirable for low voltage actuatable materials [8]. In recent years, ionic liquids (ILs) have attracted the interest of many researchers due to their favourable properties such as nonvolatility, high stability, suitable polarity, easy recyclability and reasonable ionic conductivity [18, 19]. Ionic liquids have been investigated for use as potential solvents for some polysaccharides after the first report by Swatloski and co-workers in 2002 [6, 20]. Ionic liquids have been proposed for use as green solvents to dissolve cellulose, chitosan and its derivatives [20–22]. Due to their interesting properties, ILs have been used for obtaining polymers with bio properties which can be further used for the production of polymer actuator and molecular self-assembly [23, 24].

Fig. 1 Structure of cellulose and chitosan

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The present chapter aimed to review various materials containing cellulose and chitosan which have been reported in the literature to be used in the preparation of ionic polymer metal composite (IPMC) actuators.

2 Cellulose-Based Actuators Because of easy availability, high mechanical resistance, strong interactions with water, biocompatibility, and extended chemical modification capacity, the cellulose and its derivatives have been widely used in the process of obtaining ionic polymer actuators [25–28]. Cellulose presents strong inter and intra-molecular hydrogen bonds which make its dissolution in water difficult. In order to improve its processability, ionic liquids have been used for cellulose dissolution [20, 21, 29–31].

2.1 Cellulose-Graphene Composite Actuators Ozdemir et al. have examined the possibility of using cellulose and its derivatives for obtaining a cellulose-graphene based electroactive composite [25, 28, 32]. They dissolved cellulose (microcrystalline cellulose powder—MCC) in 1 butyl-3methylimidazolium chloride [BMCl] and N,N-dimethylacetamide (DMA) [28]. In another study, has been used carboxymethylcellulose (CMC) was dissolved in 1butyl-3-methylimidazolium bromide (BMIMBr) [25]. Influence of graphene (Gr) content (wt%) upon the electroactive properties of resulted composite actuators was evaluated. Graphene, a single sheet of graphite, was found to improve the electrical and thermal conductivities as well as the mechanical properties of the polymer composite under study [28, 33, 34]. In both cases mentioned above, the obtained films were coated with gold leaf. Increasing the Gr loading, led to an increase in the initial decomposition temperature and the char residue. From the FTIR analysis, the authors observed a very week interactions between the graphene and cellulose. Also, by using a higher Gr content the tensile strength and Young’s modulus were increased. In the case of the MCC-BMCl-Gr 0.25% films the tensile strength was found to be 22.65 MPa and for CMC-BMIMBr-Gr 0.3% it was 21.15 MPa. The authors studied the electrochemical performance of the cellulose-based composites loaded with different amounts of Gr under various DC excitation voltages (up to 7 V). Increasing the amount of graphene loaded onto the studied composite up to 0.25%, led to increases in the maximum tip displacement of the actuators. The maximum tip displacement obtained under a DC voltage of 3 V was 2.2 mm for both MC-BMCl-Gr 0.25 and CMC-BMIMBr-Gr 0.25%. For a higher content of graphene use, it was necessary to use higher voltages (5 V) to obtain greater tip displacement values. The authors concluded that the graphene loading has two major effects: (1) an increase in the tip displacement due to the increase in the electrical conductivity and (2) higher response time due to a slow ion migration in the platelets structure [25, 28].

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The same research group synthesized multilayer graphene-reinforced cellulose composites using 1-ethyl-3-methylimidazolium diethylphosphonate ionic liquid with different ratio of Gr loading (0.2, 0.4 and 0.6%) [32]. The following conclusions were drawn from this study: (a) dissolving the cellulose in the ionic liquid and its reinforcing with graphene affected the chemical structure of the cellulose, (b) increasing the graphene content led to a decrease in thermal stability, while the pyrolysis residue was increased, and (c) the tensile strength as well as the Young’s modulus increased with an increase in the graphene content. However, with graphene content higher than 0.3%, all these parameters were decreased. The electrochemical performance of the actuator increased slightly as excitation voltage increased [32]. In this case, the best tip displacement was obtained for 0.2 wt% Gr-loaded sample under an excitation voltage of 3 V [32].

2.2 Cellulose-Polyethylene Glycol Composite Actuator The electrochemical behaviour of an actuator film obtained from CMC, BMIMBr ionic liquid and polyethylene glycol (PEG) was also evaluated by the Kim et al. [35]. The influence of the PEG content upon the maximum tip displacement was studied. The presence of the ionic liquid and PEG resulted in modification of thermal behaviour of the CMC. Adding of the plasticizer (PEG) into the structure of actuator formed from carboxymethylcellulose dissolved in BMIMBr ionic liquid led to an avoidance of mass loss. It was reported that the loading of PEG improved both the mechanical and electromechanical performances of CMC. Increase in the PEG content led to the formation of actuator film with a smoother surface. The higher quantity of polyethylene glycol enhanced the tensile strength of CMC-BMIMBr but decreased Young’s modulus. The electrochemical performance of the obtained actuator was studied in the range of 1–7 V DC excitation voltages. The maximum tip displacemente of 2.7 mm was obtained by the CMC-BMIMBr film loaded with 1.5 g of PEG under the DC excitation of 5 V [35].

2.3 Cellulose-Glycerol Composite Actuators Besides studying the effect of the plasticizer content upon the electromechanical and electrochemical performances of the cellulose-based actuators, Song et al. studied the influence of the plasticizing process parameters (plasticizing time and plasticizing bath temperature) [36]. For treating an IL-cellulose film, they used an aqueous glycerol solution as a plasticizer and BMIMC1 as the ionic liquid. Increasing the concentration of the plasticizer component and keeping the actuator in the plasticizing bath for a long time led to an improvement in the flexibility of the obtained actuator. It happened due to better electron transfer between the two layers of the obtained actuator (electrode and electrolyte) [36].

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2.4 Cross-Linked Cellulose Based Actuator All the above discussed ionic polymer actuators were coated with noble metals. Besides the fact that these metals are very expensive there are other associated drawbacks with the use of metal electrodes: the electrodes are easily cracked, the polymer electrolyte is easily degraded, decreased stretchability and non-reproducibility [37, 38]. Therefore, the researchers focused on obtaining of new non-metallic electrodes with enhanced actuator performance [39–41]. Wang et al. used carboxylated bacterial cellulose (CBC) to design high performance actuator. As an electrode poly(3,4ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) with good conductive properties and ionic liquid, were used [37]. The CBC-IL membrane was obtained by dissolving the CBC dispersion in 1-ethyl-3-methylimidazolium tetra fluoroborate (EMIMBF4 ). On both sides of the composite membranes, PEDOT:PSS was deposited via a dip-coating method. Using this method, the sandwiched type actuator system was obtained. By functionalization of pure BC with the studied IL and PEDOT:PSS, the ionic conductivity and the ionic exchange capacity were increased up to 22.8 times and 1.5 times, respectively. The authors have reported a bending deformation which was 8 times larger compared to the actuators obtained from pure BC with metallic electrodes. This enhanced bending deformation can be attributed to the ion migration of the dissociated ionic liquid inside CBC due to the presence of carboxylic acid groups. The obtained actuator gave good electroactive performance even at extremely low voltage and did not show back-relaxation [37]. Haldorai and Shim developed cellulose based actuators by grafting polyacrylamide (PAAm) and poly(acrylic acid) (PAA) copolymers onto CMC [42]. The success of the copolymer grafting onto CMC was confirmed by analysis data [43]. The inactive material biaxially-oriented PA-6 film was used as a support for the fabrication of the bilayer actuator. The ultraviolet radiation was used for the cross-linking of the copolymer. The authors reported that the maximum swelling achieved at an initial pH of the solution of 7.8, having an ionic strength of 0.2 M. The bilayer film showed reversible curling/un-curling actuation in water and ethanol due to the expansion and contraction of the polymer layer in these solvents [42].

2.5 Cellulose Electroactive-Paper Actuator Kim with his co-workers has evaluated the possibility of using cellulose in various forms, as an electroactive paper actuator (EAPap) [44–48]. The advantage of the EAPap actuator was that by using low actuation voltage and decreasing the power consumption a higher bending displacement can be produced [44]. Due to the lightweight, high displacement output, biodegradability, abundance, and low actuation voltage, cellulose was found most suitable for use as an EAPap. However, the paper’s sensitivity to humidity remained its disadvantage [44, 49]. Therefore, a lot of work aimed at minimizing this drawback was carried out. In one experiment, cel-

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lulose, sodium hydroxide, sodium alginate, and urea were used to obtain blend films [44]. The actuator properties and efficiency of this new EAPap were evaluated in addition to determining the maximum tip displacement and the blocked force. They produced cellulose/sodium alginate films with a thickness of 0.065 ± 0.005 mm, and used a different amount of sodium alginate in the films (0, 5 and 10%). The cellulose films were coated with gold using a physical vapour deposition. In the presence of the AC voltage, the tip displacement of the studied actuator was improved by increasing the voltage and the quantity of sodium alginate in the actuator structure. In this case, also a higher level of humidity was beneficial for obtaining an improved bending displacement. The maximum displacement of 1.8 mm was obtained for the EAPap with a content of 10% of sodium alginate. When the humidity was increased from 30 to 70%, the maximum displacement increased to 2.59 mm. In the first 2 h, the displacement of the EAPap was decreased by 20%, but subsequently, the decrease was slower. Also increasing the quantity of sodium alginate in the obtained actuator structure led to an increase in the blocked force [44]. In another study, the same group of researchers activated the wet cellulose with three different room-temperature ionic liquids. All the ionic liquids had the same cation 1-butyl-3-methyl imidazolium (BMIM) but had different anions: hexafluorophosphate (PF6 − ), chloride (Cl− ) and tetrafluoroborate (BF4 − ) [46]. The cellulose wet films were immersed for 3 days into 2% IL. It was observed that the type of anion in the IL influenced the actuator performance, as follow: BF4 − > Cl− > PF6 − . The actuators activated with BMIMBF4 exhibited the maximum bending displacement output of 3.8 mm with low power consumption (−17 mW) at a relatively low humidity level [46]. To enhance the ion migration effect in the cellulose, the cellulose films were coated with polypyrrole conducting polymer (PPy) and ionic liquids. The obtained electroactive paper exhibited improved actuation performance that lasts longer under ambient humidity and temperature conditions [45]. Kim et al. studied the influence of chitosan and various anions (Cl− , NO3 − , CF3 COO− and CH3 COO− ) upon the actuation behaviour of electroactive paper actuators [47, 48]. N,N-Dimethylacetamide (DMAc) and lithium chloride were used to dissolve the cellulose fibres in order to obtain the electroactive paper. The cellulose film was obtained by casting and immersing in water. Then the chitosan and the solutions of the used acid and glycerol are uniformly cast upon the obtained film of cellulose. In all cases, thin gold films were coated on both sides of the cellulose films. In the first study, the influence of the nature of anion and the quantity of chitosan upon the actuation behaviour of the laminated films containing both cellulose and chitosan were investigated by dc actuation tests [48]. Among the studied anions Cl− , NO3 − and CF3 COO− , Cl− presented the biggest improvement in the displacement output of the cellulose EAPap [48]. The maximum displacement of 2.3 mm was realized when HCl and 10 wt% of chitosan were used. The authors reported that an increase in the quantity of the chitosan in the laminated film led to the increase in the number of free ions per unit area, and thus the enhanced repelling forces between the anions caused higher bending displacement. The tested actuators presented a good bending displacement even at 60% relative humidity, but this decreases with time. Therefore, the humidity

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sensitivity had been reduced, but there was still a problem with durability [48]. In the case of using of acetic acid to obtain chitosan-cellulose EAPap actuators, it was observed that the maximum displacement increased with the increase in acetic acid content up to 32%. Above this point, maximum displacement was decreased. The maximum bending displacement of 2.2 mm was obtained when a molar ratio of acetic acid: chitosan of 1:3 was used with a DC voltage of 4 V, and a humidity of 80% at room temperature [48].

3 Chitosan-Based Actuators Because of unique properties [10, 14, 50–52], chitosan has been used in many composites in order to obtain advanced materials with enhanced actuation performance. A large number of chitosan-based actuators exist including a range of conductive materials: metals, carbon, graphite, graphene, carbon nanotubes and conductive polymers [53].

3.1 Chitosan-Graphene Oxide Nanocomposites Actuator The improved electro-chemo-mechanical properties of the actuator, have been demonstrated when graphene was coupled with chitosan [14, 54]. Jeon and collaborators used functionalized chitosan in combination with graphene oxide to obtain a composite with actuator behaviour. They functionalized chitosan with a sulfonated pendent group (PSC), using dipropylsulfone by controlled treatment under moderate reaction conditions [14]. The liquid electrolyte used was 1-ethyl3-methylimidazolium trifluoromethanesulfonate. The authors demonstrated that the chemical sulfonation and increasing amount of graphene oxide (GO), which led to the gradual increase in tensile modulus, tensile strength, ion exchange capacity (IEC) and the ionic conductivity of the chitosan-based nano-biopolymer. The membrane GOPSC-IL presented a higher ohmic resistance (Rs ), double-layer capacitance (CPEdl ), Warburg diffusion element (Wdif ) and adsorption capacitance (Cad ) than chitosan functionalized with IL or chitosan functionalized with an ionic liquid and sulfonated pendant group. The multiple of interactions between the ionic liquid and the functional groups of the PSC and the GO led to a decrease in the charge transfer resistance (Rct ). It was observed that the high ionic conductivity enhanced the electrochemical capacity of the obtained actuator matrix, leading to much larger bending deformation of the obtained actuator, under harmonic and step inputs. A maximum bending curvature of 5.03 m−1 was obtained [14]. A chitosan/reduced graphene oxide (RGO) nanocomposite optical actuator was developed by Muralidharan and his co-workers using the solvent casting technique [54]. They combined the efficiency of graphene as an energy transfer unit in optical polymer actuators with the biodegradable nature of chitosan. The IR radiation

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was used to study the photomechanical actuation performance of the chitosan/RGO composites. The photomechanical actuation of the composite was drastically influenced by the RGO content. Photomechanical stress and strain were increased with the increase in RGO concentration [54]. He et al. developed a new actuator having a crack-less electrode. The actuator with a good interface and high capacitance was biocompatible [9]. This ionic electro-active actuator has a sandwich type structure consisting of an ionic electrolyte layer and two graphene film layers. It was obtained by hot-pressing the graphene film and an ionic electrolyte. The ionic electrolyte layer was obtained by mixing chitosan solution, glycerol and an ionic liquid (1-butyl-3-methyl imidazole tetrafluoroborate, BMImBF4 ). The graphene film with a low sheet resistance (10 sq−1 ) was adhered well to the electrolyte membrane because of the high surface energy (37.98 mJ m−2 ) of the ionic electrolyte polymer. The authors reported a capacitance of the ionic actuator of 27.6 mF cm−2 which was attributed to the presence of the electric double layer between the polymer matrix and graphene [9].

3.2 Chitosan-Carbon Nanotubes Nanocomposites Actuator The previously mentioned methods of obtaining chitosan-based actuators required the use of expensive raw materials and have little commercial value. The supramolecular chemistry of carbon nanotubes (CNTs), their specific surface area, and the exceptional mechanical as well as electrical properties have made them the best candidate to be used in many fields, namely: preparation of anisotropic composite materials, energy storage systems, conversion devices, biosensors and tissue engineering [8, 55–57]. Moreover, the functionalization of CNTs with chitosan has played an essential role in almost all the mentioned activities [8]. In their study, Lu and Chen obtained a biocompatible composite actuator consisting of biopolymer chitosan, carbon nanotubes and an ionic liquid (1-Ethyl-3-methylimidazolium tetrafluoroborate—EMIBF4) [8]. Both the electrode membrane (formed from chitosan and multiwalled CNTs—MWCNT) and the ionic diffusion layer (EMIBF4/chitosan/glycerol) were prepared separately first through sonication and then through evaporation. The composite actuators were obtained through their different combinations. Several samples containing various wt% MWCNT (20–80 wt%) were prepared. It was observed that the conductivities of the obtained composite membranes increased with increasing MWCNT content. The composite materials containing 80% WCNT presented an average actuation displacement speed of 2 mm s−1 . The authors tested the performance of the composite membranes obtained without ionic liquid and observed that there was no detectable actuation displacement even under a 10 V electrical stimulation. The actuation performance and the specific capacitance were greatly improved by increasing the amount of the MWCNT [8]. Li et al. used the polymer chitosan and EMIBF4 as an electrolyte layer because of their compatibility with CNT and also their favourable features as a supporting polymer and mobile ion donor for electrolyte layers of electrochemical actuators [57].

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27

For the electrodes, instead of using dispersed CNTS, they used single-walled carbon nanotube (SG-SWNTs) films with hierarchal structure and improved mechanoelectrical properties. The actuator was obtained by sandwiching the electrolyte and the electrodes via a hot-press process. The obtained actuator based on SWNT films, biocompatible polymer chitosan and EMIBF4 gave improved performance: namely a superfast response (19 ms), wide available frequency range, large stress generating rate and ultra-high output power density [57]. Zhao and coworkers proposed the use of MCNT in the preparation of chitosan gel polymers (CGP) as actuators [58–60]. They studied the influence of different contents of MCNTs upon the electrochemical properties of the chitosan polymer actuator [59] and observed that increasing the MCNTs content in the structure of the chitosan-based actuator led to a decrease in charge transfer resistance as well as electrode resistance, whereas the specific capacitance was increased. The electrode film containing 80% MCNTs gave a specific capacitance of 2.02 F/g, a maximum tip displacement of 7 mm and an electrochemical energy efficiency of 111.6245 when a sweep rate of 20 mV/s and the sweep voltage of −0.5 to 0.5 V were used [59]. In another research paper, these authors investigated the influences of the thicknesses of the electrode layer and the electrolyte layer upon the electromechanical properties of the chitosan/MCNT actuator [60]. The thickness dramatically affected the electrochemical parameters (resistance and capacitance). The authors reported that increasing the thickness of the electrolyte layer and the electrode layer improved the membrane capacitance while the decreases in displacement and response was observed [60]. Instead of the fabrication method, doping technology and organics cross-linking were proposed by other scientists. Zhao et al. conducted a study on the effect of heat treat ments of the CGP actuator in order to improve its electrochemical properties [58]. Membrane actuators using chitosan and twice dispersed MCNT assembled through a hot bonding process in the viscous state was obtained. The membranes obtained were subjected to three cycles of a heat treating process. The heat-treating process involved keeping the membrane in an oven at 60 °C for 0.5 h, followed by cooling at room temperature for 1 h. It was observed that the heat treatment led to an increase in the electrical conductivity and response speed of the CGP membrane actuators. However, after the application of the third cycle of heat treating both properties degraded [58].

3.3 Cross-Linked Chitosan-Based Actuator In order to improve the mechanical and electrochemical properties of chitosanbased actuators, different methods of functionalization and reinforcing the chitosan biopolymer have been investigated [50, 61, 62]. In order to prepare a chitosanbased electroactive actuator, Altinkaya and co-workers crosslinked the chitosan  with N,N -methylenebisacrylamide (MBA) through free radical polymerization. Poly diallyldimethylammonium chloride was used as a polyelectrolyte. The film obtained was coated with gold. The produced actuator was is symbolized as ChiPM [63]. The electroactive performance of ChiPM was investigated within the range of 1–21 VDC .

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The influence of the MBA quantity on the properties and performance of the obtained actuators was thoroughly examined. The increase of MBA quantity in the structure of the (obtained) actuator resulted in a decrease of tensile strength, probably due to the reduction of mobility in the chain [64]. Also, an increase of the MBA led to a decrease in the maximum tip displacement. The film containing 0.0123 g of MBA gave a tensile strength of 21.21 MPa and a maximum tip displacement of 26.6 mm from 11 to 15 V [63]. In another paper, these authors attempted to obtained better performance of the same chitosan based-actuator. For the crosslinking of chitosan different quantities of poly diallyldimethylammonium chloride (PDAD) (the polyelectrolyte in the previous study) were used. The effect of PDAD concentration upon the electrochemical performance, mechanical, crystalographic, morphologic, viscoelastic, and thermal properties of the obtained actuator, was examined [65]. The electrolyte used was 1ethyl-3-methylimidazolium diethylphosphonate ([EMIM]DEP). The obtained films, containing different quantities of PDAD (0, 0.25, 0.50 and 0.75 mL), were wrapped with gold leaf. The greatest tip displacement values of 12.8 mm were obtained when 25 mL of PDAD was introduced into the chitosan-based films and an excitation voltage of 9 V was used, or when 50 ml of PDAD was introduced with an excitation voltage of 5 V. At higher concentrations of PDAD, the actuators performance and the workability of the obtained materials decreased considerably [65]. Therefore, the use of more than 50 mL of PDAD should be avoided. Sun and co-workers developed a gel ionic actuator consisting of chitosan polymer dissolved in dilute acid instead of ionic liquid [50]. Genipin was used as the crosslinking agent at different ratios: 1:1000, 1:800, 1:500, 1:250 and 1:150. Compared to an IL-based chitosan polymer which produces an anode deflection, the actuator produced by Sun and collaborators [50] has a cathode deflection. The authors recommended that the efficiency of ion movement inside the electrolyte layer can be modified by crosslinking reaction, without any change in the conductivity of the electrolyte layer [50]. Table 1 lists the materials, methods and actuation performance of various polysaccharide-based ionic polymer metal composite actuators.

4 Conclusions Current work on synthesizing materials with better actuation performance has been directed toward improving the performance of various polysaccharide-based ionic polymer metal composite actuators. It has been noted that in most cases ionic liquids were used as a solvent for the dissolving the polysaccharide. Few studies have been done on polysaccharide-based ionic polymer metal composite actuators without ionic liquids in their composition. It was observed that introducing graphene oxide or carbon nanotubes into the structure of the polysaccharide-based ionic polymer metal composite actuators led to the enhancement of their electromechanical and electrochemical performance. Since cellulose and chitosan are biodegradable, light weight biocompatible, polysaccharide-based ionic polymer metal composite actua-

Microcrystalline cellulose powder (CMC)

Cellulose

CMC

Cellulose

CMCBMCl-Gr 0.25

Cel-PO4 Gr0.2

CMCBMIMBrPEG

ILcellulose

EMIBF4

SGSWNT

Chitosan

Yes—1-ethyl-3methylimidazolium tetrafluoroborate (EMIMBF4 )

CBCCarboxylate bacterial PEDOT:PSS cellulose(CBC)

Yes—BMIMCl

Yes—BMIMBr

Yes—1-ethyl-3methylimidazolium diethylphosphonate ([EMIM](DEP)

Yes—1-butyl-3methylimidazolium chloride (BMIMCl)

Carboxymethylcellulose Yes—1-butyl-3methylimidazolium bromide (BMIMBr)

CMCBMIMBrGr 0.3

Used IL

Based polysaccharide

Name

Plasticization method

Plasticization method

The films were produced in the presence of DMAc

The MCC-BMCl-Gr films were produced in the presence of N,Ndimethylacetamide (DMAc)

Plasticization method. Poly ethylene glycol was used as a plasticizer

Obtaining method

Single-walled carbon nanotube (SWNT)

hot-press process

Poly(3,4Dip coating method ethylenedioxythiophenee)polystyrenesulfonic acid (PEDOT:PSS)

30% Glycerol aqueous solutions

Polyethylene glycol

Multilayer graphene (wt 20%)

Graphene nanoplatelets (wt 25%) introduced through ultrasonation

Graphene nanoplatelets (wt 30%) introduced through ultrasonication

Loaded material

No

No

No

Coated with gold leaf on each side of the film

Coated with gold leaf on each side of the film

Coated with gold leaf on each side of the film

Coated with gold leaf on each side of the film

Coated with a noble metal

1080

44.26

–

15.8

18.86

22.65

21.15

Tensile strength, MPa

Table 1 Actuation performance of various polysaccharide-based ionic polymer metal composite actuators

4

4.9

2.89

2.7

0.5

2.2

1.52

Maximum tip displacement, mm

5

2.5

5

5

3

3

3

Excitation voltage, V

(continued)

57

37

36

35

32

28

25

References

Polysaccharide-Based Ionic Polymer Metal Composite Actuators 29

Based polysaccharide

Chitosan

Chitosan powder

Chitosan

Chitosan

Name

CGP actuator

C-MCNT

ChiPM-1

ChiPM

Table 1 (continued)

Poly (diallyldimethylammonium chloride) (PDAD)

No

BMIBF4

No

Used IL

Cross-linked with N,Nmethylenebisacrylamide (MBA)

Cross-linked with N,Nmethylenebisacrylamide (MBA)

MCNT

Multi-walled carbon nanotube (MCNT)

Loaded material

Plasticization method

Plasticization method. PEG + pDAMAC

Hot-pressing

Hot bonding process

Obtaining method

Wrapped with gold leaves

Gold material

No

No

Coated with a noble metal

5

21.21

–

–

Tensile strength, MPa

12.8

4.1

7

5.44

Maximum tip displacement, mm

5

3

5

5

Excitation voltage, V

65

63

59

58

References

30 A. Popa et al.

Polysaccharide-Based Ionic Polymer Metal Composite Actuators

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tors can be used in biomedical and biomimetic systems. However, continued efforts are needed: firstly, to find less time consuming and cheaper methods for the production of polysaccharide-based ionic polymer metal composite actuators. Secondly, there is a need for work on enhancing the mechanical, electrical and electroactive performance of these actuators, while ensuring good durability and reproducibility.

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33. Stankovich, S., Dikin, D.A., Domment, G.H.B., Kohlhaas, K.M., Zimmery, E.J., Stach, E.A., Piner, R.D., Nguyen, S.B.T., Ruoff, R.S.: Graphene-based composite materials. Nature 442, 282–286 (2006) 34. Eda, G., Chhowalla, M.: Graphene-based composite thin films for electronics. Nano Lett. 9(2), 814–818 (2009). https://doi.org/10.1021/nl8035367 35. Ozdemir, O., Karakuzu, R., Sarikanat, M., Akar, E., Seki, Y., Cetin, L., Sen, I., Gurses, B.O., Yilmaz, O.C., Sever, K., Mermer, O.: Effects of PEG loading on electromechanical behavior of cellulose-based electroactive composite. Cellulose 22, 1873–1881 (2015). https://doi.org/ 10.1007/s10570-015-0581-7 36. Song, W., Yang, L., Sun, Z., Li, F., Du, S.: Study on the actuation enhancement for ionicinduced IL-cellulose based biocompatible composite actuators by glycerol plasticization treatment method. Cellulose 25(5), 2885–2889 (2018). https://doi.org/10.1007/s10570-018-1783-6 37. Wang, F., Jeon, J.H., Park, S., Kee, C.D., Kim, S.J., Oh, I.K.: Soft biomolecule actuator based on highly functionalized bacterial cellulose nano-fiber network with carboxylic acid groups. Soft Matter 12, 246–254 (2012). https://doi.org/10.1039/C5SM00707K 38. Cheedarala, R.V., Jeon, J.H., Kee, C.D., Oh, I.K.: Bio-inspired all-organic soft actuator based on a π–π stacked 3D ionic network membrane and ultra-fast solution processing. Adv. Funct. Mater. 24, 6005–6015 (2014). https://doi.org/10.1002/adfm.201401136 39. Greco, F., Zucca, A., Taccola, S., Menciassi, A., Fujie, T., Haniuda, H., Takeoka, S., Dario, P., Mattoli, V.: Ultra-thin conductive free-standing PEDOT/PSS nanofilms. Soft Matter 7, 10642–10650 (2011). https://doi.org/10.1039/C1SM06174G 40. Greco, F., Domenici, V., Desii, A., Sinibaldi, E., Zalar, B., Mazzolai, B., Mattoli, V.: Liquid single crystal elastomer/conducting polymer bilayer composite actuator: modelling and experiments. Soft Matter 9, 11405–11416 (2013). https://doi.org/10.1039/C3SM51153G 41. Okuzali, H., Tagaki, S., Hishiki, F., Tanigawa, R.: Ionic liquid/polyurethane/PEDOT:PSS composites for electro-active polymer actuators. Sens. Actuators B 194, 59–63 (2014). https:// doi.org/10.1016/j.snb.2013.12.059 42. Haldorai, Y., Shim, J.J.: Chemo-responsive bilayer actuator film: fabrication, characterization and actuator response. New J. Chem. 38, 2653–2659 (2014). https://doi.org/10.1039/ c4nj00014e 43. Seiffert, S., Oppermann, W., Saalwachter, K.: Hydrogel formation by photocrosslinking of dimethylmaleimide functionalized polyacrylamide polymer 48, 5599–5611 (2007). https:// doi.org/10.1016/j.polymer.2007.07.013 44. Kim, J., Wang, N., Chen, Y., Lee, S.K., Yun, G.Y.: Electroactive-paper actuator made with cellulose/NaOH/urea and sodium alginate. Cellulose 14, 217–223 (2007). https://doi.org/10. 1007/s10570-007-9111-6 45. Kim, J., Yun, S., Mahadeva, S.K., Yun, K., Yang, S.Y., Maniruzzaman, M.: Paper actuators made with cellulose and hybrid materials. Sensors 10, 1473–1485 (2010). https://doi.org/10. 3390/s100301473 46. Mahadeva, S.K., Yi, C., Kim, J.: Effect of room temperature ionic liquids adsorption on electromechanical behaviour of cellulose electro-active paper. Macromol. Res. 17(2), 116–120 (2009) 47. Wang, N., Chen, Y., Kim, J.: Electroactive paper actuator made with chitosan-cellulose films: effect of acetic acid. Macromol. Mater. Eng. 292, 748–753 (2007) 48. Kim, J., Wang, N., Chen, Y.: Effect of chitosan and ions on actuation behaviour of cellulosechitosan laminated films as electro-active paper actuators. Cellulose 14, 439–445 (2007) 49. Kim, J., Seo, Y.B.: Electro-active paper actuators. Smart Mater. Struct. 11, 355–360 (2002) 50. Sun, Z., Zhao, G., Song, W.: A naturally crosslinked chitosan based ionic actuator with cathode deflection phenomenon. Cellulose 24(2), 441–445 (2016). https://doi.org/10.1007/s10570016-1161-1 51. Dos Santos, D.S., Riul, A., Malmegrum, R.R.: A layer-by-layer film of chitosan in a taste sensor application. Macromol. Biosci. 3(10), 591–595 (2003) 52. Zolfagharian, A., Kouzani, A.Z., Khoo, S.Y., Nasri-Nasrabadi, B., Kaynak, A.: Development and analysis of a 3D printed hydrogel soft actuator. Sens. Actuators A 265, 94–101 (2017). https://doi.org/10.1016/j.sna.2017.08.038

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Conducting Polymer Based Ionic Polymer Metal Composite Actuators David Gendron

Abstract Organic materials that mimic the mammalian skeleton muscles are of great interest in artificial actuators for applications such as robot legs, surgical instruments and Braille displays. These ionic polymer metal composite (IPMC) actuators are compact, lightweight, silent, strong and reliable. In this regard, conjugated or conducting polymeric materials are attractive as these offer the desired properties and their actuator operations are similar to biological muscles. This chapter focuses on four types of conjugated polymers: polyaniline, polypyrrole, polythiophene and poly(3,4ethylenedioxythiophene): polystyrene sulfonate as active materials in IMPC actuators. First, their chemical or electrochemical synthesis is described. Then, their actuators characteristics and performances are discussed and compared. In sum, this chapter aims to give the reader a good overview of the pros and cons in respect of each type of materials as well as their uses in actuators. Keywords IPMC · Conducting polymers · Actuators · Polyaniline · Polypyrrole · Polythiophene · PEDOT:PSS

1 Introduction Materials and devices that have the ability to convert electrical energy directly into mechanical work have drawn great attention recently. Since the first report on the EAPs (electroactive polymers) in the mid-1970s, applications such as artificial muscle and tactile display have become of great interest for the scientific community [1, 2]. Basically, EAPs are materials that possess the ability to respond mechanically to an electrical stimulation, therefore, generating a significant change in shape and size. Thus, EAPS are usually classified into two main categories based on their working mechanism: electronic and ionic [3]. In short, electronic-activated EAPs typically require an activation voltage as coulombic forces will drive the device. For instance, electrostatic, piezoelectric or D. Gendron (B) Oleotek, 835 rue Mooney O, G6G 2J3 Thetford Mines, Canada e-mail: [email protected] © Springer Nature Switzerland AG 2019 Inamuddin and A. M. Asiri (eds.), Ionic Polymer Metal Composites for Sensors and Actuators, Engineering Materials, https://doi.org/10.1007/978-3-030-13728-1_3

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Table 1 Desired properties of actuators (artificial muscles) Large strain

Long durability (physically)

High stability (electrochemically)

High force density

Fast response time

High power/weight ratio

Lightweight

Biocompatibility

Air-stable

Insensitivity towards magnetic field

Easy to fabricate

ferroelectric materials can be activated under DC supply. On the other hand, ionic electroactive materials that allow the displacement of ions [4]. Ionic EAPs such as ionic polymer metal composites, carbon nanotubes and conductive polymers, can be activated by the application of a very low voltage, which leads to a bending movement. Such, actuators should possess the following properties as described in Table 1. In sum, IPMC actuators (ionic polymer metal composites) possess a small activation voltage, but their energy density is poor due to their need for a wet electrolyte to function properly [5]. In this chapter, we will focus our attention on a particular class of actuators: the conducting polymer-based ionic polymer metal composites also named as CP-IPMC. We will first review the basic operating principles as well as the fabrication of a typical IPMC actuator followed by types of polymers that are used in IPMC actuators and their performances.

2 Fabrication and Operating Principles Typically, an IPMC actuator is made of a polymer matrix sandwiched between two electrodes (Fig. 1). More precisely, the polymer matrix consists of an organic polymer that contains a covalently bonded fixed ionic groups [6]. The most popular polymers are based on perfluorinated alkene containing short side-chains terminated with an ionic group (SO3 − or CO2 − ) for cation exchange. The large polymer backbone determines the mechanical property of the polymer whereas the short side-chain ionic groups interact with the electrolyte (water or an organic polar solvent) and allow the passage of ions. Commercially, there are several polymeric ion exchange matrixes such as Nafion® , AciplexTM and FlemionTM etc. [7]. The fabrication of the IPMC actuator can be achieved via several methods [8]. Here are the principal ones: Metallization by soaking [4]: In this process, the ionic polymer is soaked in a solution of [Pt(NH3 )4 ]Cl2 or [Pt(NH3 )6 ]Cl4 . The platinum cations are allowed to diffuse via an ion-exchange process. Then a reducing agent such as LiBH4 or NaBH4 is introduced to metallize the polymer. It is worth mentioning that the platinum metallic particles aren’t homogeneously distributed across the polymer membrane, and the concentration of the Pt nanoparticles remains greater near the interface boundaries.

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Fig. 1 Illustration of a IPMC actuation mechanism, b electrolyte layer sandwiched between conjugated polymer films

Physical metal loading [9]: In this process, a conductive Pt powder is added into the ionic polymer. Then, the plating is completed by the incorporation of smaller secondary particles via chemical plating (with a reducing agent). Casting method [10]: This process involves dispersing, Nafion® as well as sonicating followed by a thermal treatment. The thermal treatment increases the stiffness of the Nafion® membrane by increasing the strength of the molecules interpenetrating network. The last step of this process involves heating the membrane in hydrogen peroxide. This method is quite useful to achieve a precise (or desired) thickness, which is important since the actuator performances are affected by stiffness, electric field and thickness. Hot pressing [11]: The hot-pressing method allows an easy control over the thinness of the Nafion® film as well as enabling several films to stick together. The bending stiffness, the force and the reproducibility of the films can, therefore, be enhanced and controlled. Typically, a Nafion® film is hot pressed (180 °C) followed by immersion in a [Pt(NH3 )4 ]Cl2 solution. Reacting the film with a reducing agent completes the process. Electrode position [12]: One of the main challenges in the fabrication of IPMC actuators is the adhesion between the polymer (Nafion® ) film and the electrode. Therefore, electrodeposition using a sputter coating is a useful way to achieve uniform deposition. It can be performed on dry polymer film as well as on solvated membrane. This process leads to fabricate the mechanically and chemically stable electrodes. To properly characterize the actuator, several measurements are needed in order to obtain information about their physical and electronic properties. The properties such as tensile strength, stiffness and the thermal expansion coefficient, provide information about the mechanical strength, the blocking stress, the mechanical energy density as well as the thermal compatibility of the actuator [13].

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The electrical characterization involves the four main properties such as maximum voltage, the impedance spectra, the nonlinear current and the sheet resistance. The maximum voltage is important, as it determines the limits of the safe operation of the device. The impedance spectrum provides resistance and capacitance data which are useful to calculate the electrical energy density as well as the electrical relaxation/dissipation and the equivalent electrical circuit. The nonlinear current is used to quantify the nonlinear response and driving limitations. At last, the data obtained regarding the sheet resistance are useful from the point of view of Q&A (quality assurance). The electroactive properties such as the strain, the stress and the stiffness are detrimental in characterizing the actuator performances. The electrically induces strain is used in the calculation of the blocking stress as well as the mechanical energy density. The electrically induced force (stress) provides information about the force/torque or stress-induced charge. Finally, the stress-strain curve provides information about the voltage controlled stiffness. In this chapter, we will discuss one type of IPMC in particular, the conductive polymer IPMC. Interestingly, the first actuator that was considered an artificial muscle was a conducting polymer reported by the group of Otero [14]. But, first, let’s review the fundamentals of a conjugated polymer.

3 Electronic Conducting Polymer (Conjugated Polymer) Amongst the variety of materials used in IPMC, conducting polymer (CP) is an interesting family of polymers. In this section, we will glance over the main types of conducting polymers used in IPMC actuators, namely the polypyrroles, polyanilines, polythiophenes and poly(3,4-ethylenedioxythiophene) polystyrene sulfonates. In 1977, professors MacDiarmid, Shirakawa and Heeger, reported the electronic conductivity of polacetylene [15, 16]. For this discovery, they were awarded the Nobel prize of chemistry in 2000. Conducting polymers are materials that possess the electronic properties of semiconductors while having the physical properties of more traditional polymers. An ideal conjugated polymer is constituted of a repetition of sp2 carbon atoms that confers a rigid and planar structure. In this case, the alternating single and double bonds create a conduction band and a valence band. By increasing the number of repeated units (n), a decrease in the energy between the HOMO and LUMO bands can be achieved. When sufficient repeating units n are attained, conduction and valence bands are formed [17]. In their neutral state, conjugated polymers are considered insulators according to their energy level. In this specific state, the electronic conduction is comparable to that measured in the case of conventional polymers. Upon oxidation or reduction by chemical, electrochemical or photochemical doping, a charge is introduced or removed in the conjugated polymer. If the charge in the polymer is positive (cation radical) or negative (anion radical) then the charge will be defined as polaron (positive or negative). In the case where a supplementary electron is introduced in or removed

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(a)

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(b)

S

S S

S

aromatic

S

S

quinoid

Fig. 2 Aromatic versus quinoid

from the polymer structure, there will be formation or elimination of a bipolaron (positive or negative). These modifications create new energy bands between the valence and conduction bands which are named polaronic bands. These new bands existing in the polaronic and bipolaronic state facilitate movement of charges from the valence band to the conduction band [17]. Furthermore, there are possible resonance forms in conjugated polymers. The first form is aromatic and is energetically more stable. The second form, quinoid, possesses a lower bandgap than the aromatic form (Fig. 2). It is important to mention that the electronic conductivity in a conducting polymer or conjugated polymers arises from the HOMO (highest occupied molecular orbital) or LUMO (lowest unoccupied molecular orbital) bands from the main conjugated chain. Depending on the dopant, the polymer can become p or n type [18]. In p-type, the polymer is positively charged (an electron is removed from the valence band) whereas, in n-type, an electron is added in the conducting band. Doping of the conjugated polymer can be achieved by light, chemically, or electrochemically. Indeed, conjugated polymer can be oxidized or reduced by introducing cations, anions or photons. Doping of conjugated polymer has a direct effect on the physicochemical properties, such as colours, mechanical properties, conductivity, volume, porosity, etc. [19–21].

3.1 Polyaniline (PANI) Polyaniline (PANI) is one of the most used conductive polymers in order to make actuators. PANI can be electrochemically reduced or oxidized into several forms: namely the green emeraldine salt, violet emeraldine base, blue pernigraniline salt and blue pernigraniline base (Fig. 3). In fact, depending on its form, PANI shows different colours. In order for the redox reaction to occurs, a strong acid is necessary (pH < 3). Basically, there is an addition of electrons to the nitrogen atoms during the redox reaction, leading to a change from phenyl structures to a quinoid structure upon oxidation and vice versa during reduction. In sum, the change from the phenyl structure to the quinoid generates the strain in the PANI molecule. More precisely, the polymer is more compact in the reduced state than in the oxidized state [22].

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Fig. 3 Redox cycle of PANI [21]

PANI can be prepared according to two main methods: electrochemical polymerization and chemical polymerization. In the case of the electrochemical polymerization, aniline is dissolved in an aqueous solution containing a mixture of electrolyte and an acid. Several electrochemical methods such as potentiostatic, galvanostatic or potentiodynamic experiments can be applied to generate the polyaniline [23]. Typically, a potential of 0.7–1.2 V [vs. saturated calomel electrode (SCE)] is applied. Basically, aniline is adsorbed into the partially charged electrode under acidic conditions which ultimately generates the anilinium cation. Then, the reaction of the anilinum monomer with the oxidant enables the polymerization leading to the adsorption of PANI into the electrode as the final product. For more details, the readers are directed to go through reviews focussed on the subject [24–26]. In sum, electrochemical polymerization leads to the formation of purer PANI, free from admixtures and without need of special procedures for the purification of the obtained polymer. However, it is important to point out that the quantities of materials that can be obtained via this method are rather limited. In the case of chemical polymerization, the polymerization of aniline can be performed with various oxidants, however, ammonium persulfate is the most used. Indeed, when (NH4 )2 S2 O8 is employed, polyaniline is formed in high yield (>90%) with a conductivity of 1.2 S/cm [24, 27]. In order to obtain the different polyaniline salts, the polymerization is carried out in the presence of an acid or buffer solutions. It is important to note that the chemical polymerization of aniline is the simplest way to prepare conducting polymers (such as polythiophene, polypyrrole and so on) leading

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to interesting physicochemical properties. Interestingly, it is known that polyaniline is more stable than polypyrrole which will ultimately lead to higher performances in actuators. In a report by Kim et al. PANI coated electroactive paper actuators have been described [28]. In this specific case, the paper actuators were made by electrochemical deposition of conductive polyaniline on a cellulose paper. They observed good actuation performance depending on the PANI thickness and relative humidity. Indeed, they calculated a maximum strain of 0.12% (in bending mode) at a humidity level higher than 90% at a frequency of 3 Hz. They also investigated the dopant ion effect in a bilayer and trilayer devices. Actuation performances were higher in the trilayer devices primarily due to the benefit of volume change of each PANI layer. In fact, the expansion and contraction of conductive PANI were associated with the sorption and desorption of water molecules together with the structural changes in the layer along with the ion migration effect [29]. As stated earlier, the dopant moving inside the conducting polymer matrix causes conformational change along the polymeric chains which leads to an expansion and contraction in the polymer structure (i.e. macroscopic volume change). Considering a typical conducting polymer, IPMC actuators possess a symmetric structure, thus only a uniform volume change can be obtained. This means that there is an inherent difficulty to obtain a bending movement from a free-standing conducting polymer film. Therefore, most of the conducting polymer actuators are actually bilayer and trilayer devices. To achieve the desired bending motion, an inert (to electrochemical stimuli) and flexible film needs to be added. Adhesive tapes, polymer tapes and thin film deposited layers are the materials commonly added in typical bilayer and trilayer actuators. However, bilayer and trilayer actuators suffer from one main drawback: the delamination of the added film through long working cycles. In addition, the performance of such types of actuators is greatly reduced to the increased weight added by the addition of these layers. To overcome these limitations, the group of Wang et al. has shown the possibility of achieving the bending movement by using a monolithic configuration based on a single PANI asymmetric membrane [30]. The principal advantage of this configuration resides in the fact that the electrochemical actuator can operate without the risk of delamination. Using a coagulation bath, a series of PANI membranes were prepared and assembled into asymmetric actuator configuration. A Young modulus of 1.421 GPa was measured as well as a strain at break of 7.6%. In sum, the asymmetric structure of these PANI actuators allowed high frequency bending movement up to 20 Hz. Moreover, they observed experimentally that the lifetime of these actuators was limited by a fatigue process caused by repeated bending motions. Lately, one of the main interests in the material science community was oriented towards graphene, carbon nanotubes and their composites [31]. Indeed nanostructured carbon materials have attracted great attention for their excellent electrical conductivity, mechanical properties, large specific surface areas as well as their chemical and thermal stabilities. Actuators based on carbon nanotubes were first discussed by Baughman [32]. Later Aida et al. reported an actuator system based on carbon nanotubes bucky gel showcasing excellent performance in terms of strain and a 20%

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decrease in the actuator displacement after 15 h [33]. The intrinsic properties of carbon nanotubes or graphene were expected to show improvement in terms of output stress and power density. An interesting concept was put forward by Wei et al. demonstrating a PANI-GO (graphene oxide) nanocomposite electrodes displaying excellent electrochemical stability over thousands of cycles [34]. This interesting feature was attributed to the synergy between the PANI and the graphene oxide sheets. To pursue this work, Zhang et al. investigated the use of PANI as an interlayer spacer to obtain a reduced graphene oxide/PANI nanocomposite electrode [31]. The fabrication of such electrodes was motivated by the high specific capacitance, low cost and easy fabrication procedure from such composites. Interestingly, the PANI nanoparticles were decorated onto the reduced graphene oxide by in situ chemical polymerization. Moreover, during the film preparation, the r-GO (reduced graphene oxide) sheets restacking were inhibited by the presence of PANI particles. They reported an increased in capacitance from 79 to 207 F/g for the r-GO and r-GO/PANI electrodes respectively. In terms of actuation performances, a bending strain of 0.327% was observed under a 0.5 V square wave voltage. This actuation performance was attributed to the effect of the loosely packed r-GO sheets and the redox reaction of PANI where both the ion transportation and storage capacity of the electrode were improved (Fig. 4).

3.2 Polypyrrole (PPy) Polypyrrole is one of the most studied conducting polymers in the field of IPMC actuators [35]. This is mainly due to the fact that polypyrrole (Ppy) can be readily electrodeposited onto a metal electrode leading to a highly conductive film [36]. Interestingly, PPy films can be prepared in aqueous or organic electrolytes. The PPy films possess large electrochemical strain (up to 39%) and force output (up to 49 MPa) under optimized conditions. As mentioned earlier, PPy is of particular interest in the actuator field, due to its high conductivity, stability in the oxidized state and redox properties (Fig. 5). PPy can be synthesized by using chemical oxidants or electrochemically. In terms of chemical oxidants, FeCl3 has been the most popular choice (Fig. 6). The quality of PPy (i.e. yield and conductivity) is affected by a variety of factors, which include the choice of solvents (anhydrous organic or aqueous), the nature of the oxidant (salts of metal, halogens or organic electron acceptors), the ratio of the pyrrole/oxidant, the duration and the temperature of the reaction [37]. It is worth mentioning that the optimal ratio of Fe(III) to the pyrrole monomer is 2.4 in order to achieve nearly 100% yield of polypyrrole. Reduction in polymerization time or the temperature (0–5 °C) resulted in the improvement of the conductivity of the PPy obtained. Furthermore, the solvent has a direct influence on the properties of the PPy material produced. Out of the several solvents investigated, methanol led to the formation of PPy with the highest conductivity (190 S/cm). Moreover, controlling the oxidizing potential of the reaction (i.e. FeCl3 to FeCl2 ), enhanced

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43

Fig. 4 Actuation displacement of different r-GO and PANI actuators under different voltages. Reprinted from [31] with permission from the Royal Society of Chemistry

Fig. 5 Electrochemical redox cycle for polypyrrole. A—represents anions, é—represents electrons n

N H

+ (2+y) n FeCl3

(C4H3N)ny+ nyCl + (2+y)n FeCl2 + 2n HCl

Fig. 6 Oxidative chemical polymerization of pyrrole

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the conductivity (up to 330 S/cm for the latter). In sum, PPy prepared by chemical oxidation showed an improvement in conductivity and stability (Fig. 6). However, the formation of PPy films or coating on another material is still problematic. One way to solve this issue is by the use of composite such as mixing a PPy powder with poly(ethylene oxide) to increase the overall mechanical properties. The electrochemical synthesis of polypyrrole possesses several advantages over the chemical polymerization: the generated polypyrrole film is attached to the surface of the electrode and has high conductivity as well as possessing a charge yield close to 100%. Thus, it is possible to control adequately the mass and synthesis of PPy. Practically, in order to obtain reproducible results, the synthesis of polypyrrole must be carried out carefully. In this regard, the electrodes preparation and the use of high purity chemicals are detrimental [38, 39]. Typically, the polypyrrole film grows on the immersed anode in the electrolyte solution (which was previously purged with nitrogen or argon to remove oxygen). Finally, the electrochemical polymerization of pyrrole can be carried out with electrochemical experiments: potentiostatic, galvanostatic and potentiodynamic. The mechanical and electrochemical properties of the films depend on the nature of the chosen solvent for the polymerization, pH of the electrolyte, purity as well as the concentration of the monomer and the concentration of the electrolyte. The first PPy actuators were reported nearly 30 years ago by the group of Olle Inganäs, demonstrating that a bending motion was possible from an actuator consisting of laminates of poly(3-octylthiophene)/PPy onto a polyethylene substrate [40]. The actuator was able to bend in an electrolyte solution of LiClO4 with a platinum auxiliary electrode. Moreover, they studied the effect of I2 doping on the bending motion of the actuator. They concluded that the doping enhanced the interchain interaction in the conjugated polymer causing them to shrink while doping insertion expanded the materials. Several studies investigated the response versus time of PPy with successive doping and dedoping [41]. However, to be able to successfully use PPy as an electroactuator the mechanism of the actuation need to be first understood. In this regard, Wallace et al. aimed to develop a general model to describe the actuation behaviour or PPy [42]. They observed that there was a strong influence of the cation in the electrolyte and the counter ion incorporated in the polymer matrix. More precisely, they proposed a four-step oxidation model to describe the ion movements into and out of the polymer matrix: 1. Initial oxidation of the fully reduced polymer leads to contraction due to the expulsion of the cation incorporated during reduction (Eq. 1): P P y 0 × nC − ×

n x+ n M → P P y n+ × nC − + M x+ x x

(1)

2. Continued oxidation of the polymer draws electrolyte anions into the polymer causing swelling (Eq. 2):

Conducting Polymer Based Ionic Polymer Metal Composite Actuators

P P y 0 + nC − → P P y n+ × nC −

45

(2)

3. Reversion of the process during reduction of the polymer with the anions being expelled causing contraction (Eq. 3): P P y n+ × nC − → P P y 0 + nC −

(3)

4. Cations are again incorporated into the polymer causing swelling (Eq. 4): P P y n+ × nC − +

n x+ n M → P P y 0 × nC − × M x+ x x

(4)

This model is able to explain the differences in electroactuation behaviour between two different systems (in this case PPy/PVS and PPy/pTS) tested in various electrolytes. Therefore, it is possible to achieve reproducible action beyond the initial reduction of the polymer when immobile counteranions are used and when divalent cations are present in the electrolyte. A point that was not often discussed in the literature is that the electrochemical deposition of the conductive polymer (for instance: PANI, PPy or others) requires a metal electrode [43]. In this regard, initial attempts to attach the electrosynthesized PPy to the metal layer led to eventual delamination [44]. Another approach is to use sputtering or vacuum evaporation. Lately, it has been proposed to use chemical polymerization of the same conducting material into the membrane in order to create a conductive surface. This process has the advantage to eliminate delamination issues and avoiding cost expensive vacuum deposition techniques. Hence, Temmer et al. investigated the electrochemical, as well as actuation properties of a self-standing, air, operated tri-layer PPy actuator [45]. They prepared the actuators avoiding the use of metal-containing chemical and electrochemical routes. They found that the risk of delamination was greatly reduced, while the tunable characteristics were retained. They reported that the synthesis of polypyrrole led to the high reproducibility. In terms of strain, they obtained a value of 4.4% for actuators immersed in 1M lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) in propylene carbonate where PPy was prepared in-house. In PPy actuators, the counterions and substrate play an important role. It has been found that supportive ions such as BF4 − , PF6 − , CF3 SO3 − and TFSI− are accessible and while use in synergy with a graphene substrate, the risk of delamination greatly reduced and the electroactivity is maintained. Figure 7 shows the typical output strain and stress of polypyrrole actuator doping with the chosen counterions [46]. Typically, the geometry of IPMC polypyrrole actuator consists of a flat tri-layer device. However, other geometries were also explored and let to an interesting proof of concept. In this regard, a study by Tadess et al. investigated helically interconnected PPy-metal composite tubular actuators incorporating annular cross-section [47]. The polypyrrole was deposited on the substrate by cyclic voltammetry to provide a uniform deposition. The obtained actuator gave interesting results in terms of

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Fig. 7 Output strain and stress out of polypyrrole actuator with typical counterions

maximum force, reaching up to 50 mN. The actuator profile was obtained by varying the orientation of the counter electrode position relative to the working electrode. Moreover, in order to obtain high-quality PPy film in the helically interconnect geometry, the sacrificial PLA (polylactic acid) layer was found to be detrimental. Finally, it is important to note that the maximum steady force response reached within the 60 s. In summary, performances of polypyrrole actuators were found to be dependant on synthesis conditions of polypyrrole. In this regard, a study by Aguilar-Hernandez and Potje-Kamloth reported that the conductivity of PPy varied for five orders of magnitude between the temperature of 77–300 K [48]. As stated earlier, PPy doped with TFSI showed higher strain rate compared to the typical PPY: PF6 system [49]. At last, Ding et al. observed that PPy deposited on a platinum wire core showed improved performances mainly due to (i) the improvement in conductivity (reduction of the ohmic voltage drop) and (ii) the enhanced rigidity [50].

3.3 Polythiophene (PT) Polythiophene (PT) is one of the main classes of conjugated polymers, especially in the fields of optoelectronics and photonics [51]. More precisely, polythiophenes have been used as active material in field-effect transistors (FETs), light-emitting diodes (LEDs), organic solar cells, a photodetector and chemical sensors. It was found that polythiophene coupled in the 2–5 positions showed highly improved conductive properties as well as thermal stability. However, the material was insoluble in most solvents. The introduction of alkyl flexible side chain on the thiophene unit was first prepared by chemical oxidative polymerization with FeCl3 leading to a mixture of regioisomers. Indeed, three types of coupling can be obtained which are: head-to-tail (HT), head-to-head (HH) and tail-to-tail (TT) as shown in Fig. 8. Practically, poly(alkylthiophene)s are usually prepared by three main methods, the McCullough method [52], the Rieke Method [53] and the GRIM method [54]

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47

(Fig. 9). In the McCullough method, the key step involved the regiospecificity of 2-bromo-5-bromomagnesio-3-alkylthiophene according to the conditions described in Fig. 9. The polymerization was carried out in situ by a cross-coupling reaction with Ni(dppp)Cl2 to afford P3AT with molecular weights (Mn) between 20 and 40 kDa. The Rieke method reported soon after the McCullough method and differed from the previous method in terms of employing a highly reactive “Rieke zinc” (Zn*) with 2,5-dibromo-3-alkylthiophene which gave P3AT by in situ addition of Ni(dppe)Cl2 . Molecular weights (Mn) of 24–34 kDa were obtained. An interesting advantage of the GRIM method was that the use of cryogenic temperature and highly reactive metals not required. Therefore, the preparation of rr-P3AT was quick and easy leading to easy scale-up of the process. Typical molecular weights of 20–35 kDa were obtained. In the actuator fields, the first use of poly(alkylthiophene) was reported by Kaneto et al. showcasing an actuation strain of 2% [for both poly(3-hexylthiophene) and poly(3-dodecylthiophene)] [55]. The reports on PT actuators are rather limited but they provide an interesting outlook on the conducting polymer IPMC field. In this regard, the group of Wallace et al. has explored the use of poly(3-methylthiophene) on the strain and cycle life of actuators [56]. In fact, they found that actuators based on P3MT showed improved actuation strain with increasing applied stress. They investigated the effect of the electrolyte (propylene carbonate and ionic liquid) on the actuation behaviour, and they observed that the actuation strain was lower in ionic liquid (IL) primarily due to the absence of the solvent, thus no contribution of any osmotic effects. On the other hand, the actuation strain in the propylene carbonate

Fig. 8 Regioisomers of 3-alkylpolythiophenes

Fig. 9 Typical synthesis methods for regioregular P3AT

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Fig. 10 Actuation strain of P3MT/PF6 /Pt helix samples in PC or IL. Reprinted from [56] permission from Elsevier

(PC) was very sensitive to the potential scan rate (Fig. 10). At last, they estimated the performance of the actuators based on a linear-elastic mechanical model. Polythiophenes based actuators were also investigated for their electromechanical properties. The first report by Pattavarakorn showed the effects of the amount of crosslinking agents [tetraethyl orthosilicate (TEOS) and dibutyltin dilaurate (DBTDL)], the PT particle concentration and the electric field strength [57]. The actuator system consisting of PT particle/PDMS blend films located between two copper electrodes. They observed that in the PDMS/PT blends, the bending angle increased with increasing polythiophene particle concentration up to 5 vol.%. At last, both cross-linking agents gave the same trends, the bending angle decreased slowly with increasing percentage of a cross-linking agent. However, the concentration of PT particles has little effect. Another later study by the same group explored the utility of polythiophene hydrogel incorporating chitosan and carboxymethylchitosan (PTh/CS/CMCS) [58]. Polythiophene was obtained by chemical oxidative polymerization with FeCl3 . To form the hydrogel, PT, and CMCS were mixed with CS by solution blending. A crosslinking agent, glutaraldehyde was also added to the mixture prior to casting the film. The cross-linking agent concentration, PT particle concentration and electric field strength effects were investigated. Unfortunately, the conductive PT/CS/CMCS hydrogel showed lower electroactive performance, which was due to the higher weight and rigidity of the PT based hydrogel (Fig. 11).

3.4 PEDOT:PSS The polyethylenedioxythiophene (PEDOT) poly(styrene sulfonate) (PSS) polymer as conducting material in actuators has been widely known in the field of organic electronic as it has been reported in applications such as polymer solar cells, field-

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49

Fig. 11 Bending angle under electric field strength for CS/CMCS and PTh/CS/CMCS actuators systems. Reprinted from [55] with permission from Elsevier

effect transistors, photodetectors and so on. PEDOT:PSS is transparent between 400 and 900 nm, possess a conductivity and is easy to handle. One of the first PEDOT:PSS actuators was reported by Okuzaki and coworkers. They observed that a linear expansion occurred with increasing ambient humidity [59, 60]. The strain of the film measured with a relative humidity between 20 and 90% was 3.3%. When an electric field was applied, it was observed that the film underwent a significant contraction that was increased with the increase in applied voltage becomes higher. A resulting value of 2% was reached at 35 V that is twice as much as for PPy film in similar conditions. In terms of application, PEDOT:PSS actuators have been used for micro autofocus lenses. In fact, there are various types of actuators using laminated conducting polymers (i.e. PPy and poly(vinylidene fluoride) PVDF as well as PEDOT and nitrile butadiene rubber) [49, 61, 62]. However, the main drawback is that the manufacturing process is quite complicated considering that the membranes need to be prepared by chemical polymerization or electropolymerization. In terms of electrochemical stability, PEDOT possesses the advantage to less prone to oxidation than PPy. This is explained by its chemically active position on the five-membered ring that can be modified by oxygen and deactivated [61]. An adequate adhesion between the different actuator layers and a hydrophilic treatment on PVDF was found to be effective to build PEDOT:PSS-based actuator with good mechanical strength. Addition of poly(ethylene oxide) (PEO) leads to an improvement in the bending properties (strain) and electrical conductivity. At last, it was noted that the actuators were able to operate stably for more than a million cycles. Other researchers have reported of PEDOT:PSS in tubular actuator geometries providing control over the deformation here [63].

4 Conclusion To sum up, this chapter summarizes the basic operating principles of IPMC actuators. We have discussed the fundamentals of conducting polymers (as known as a conjugated polymer). Then the four principal classes of conducting poly-

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mer IPMCs were described: polyaniline, polypyrrole, polythiophene and poly(3,4ethylenedioxythiophene) poly(styrene sulfonate). For each class, we have detailed their preparation (chemical synthesis) as well as their physicochemical properties. We have also reviewed and compared their performance when incorporated in actuators. Further investigations on conducting polymer actuators are currently ongoing and one can expect new commercial applications in the near future.

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Role of Metal Ion Implantation on Ionic Polymer Metal Composite Membranes Adina Maria Dobos and A. Filimon

Abstract Due to the increasing need from the modern technologies and industrial applications, the diversity of electroactive materials, i.e. conductors, semiconductors, dielectrics with characteristics gradual evolution, imposes the development of a new generation of materials. Based on the new materials, horizons have opened towards the design of the new electronics generation with significant impact on the future technological systems. This chapter aims to investigate various types of polymeric ionic membranes used for high-performance ionic polymer–metal composites actuators, which exhibit a good deformation stability, and efficiency. Along with the material study, the role of metal ion implantation on ionic polymer metal composite membranes is also analyzed in order to overcome some disadvantages of the ionic polymer actuators and also to improve their stability, sustainability, and performance. Additionally, this chapter takes into account of the latest actuation models and control for designing of the engineering materials based on ionic polymer metal composites (IPMCs) and their use for integrated systems in soft sensors and actuators, as well as biomedical devices for friendly human applications.

List of Abbreviations AFM CNT DC EAPs eEAPs iEAPs FCVA IPMCs K KOH

Atomic force microscopy Carbon nanotube Direct current Electroactive polymers Electronic electroactive polymers Ionic electroactive polymers Filtered cathode vacuum arc Ionic polymer-metal composites Potassium Potassium hydroxide

A. M. Dobos · A. Filimon (B) Institute of Macromolecular Chemistry, Aleea Grigore Ghica Voda 41A, 700487 Iasi, Romania e-mail: [email protected] © Springer Nature Switzerland AG 2019 Inamuddin and A. M. Asiri (eds.), Ionic Polymer Metal Composites for Sensors and Actuators, Engineering Materials, https://doi.org/10.1007/978-3-030-13728-1_4

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Li Mg Na NaBH4 OCT PDMS PPy Pt(NH3 )4 2+ TEM Ti

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Lithium Magnesium Sodium Sodium boron hydride Optical coherence tomography Polydimethylsiloxane Polypyrrole Tetraammineplatinum complex Transmission electron microscopy Titanium

1 Introduction “Smart materials” called also “intelligent or responsive materials”, are designed materials that respond to external stimuli. Based on their response to the action of electrical stimuli, these materials can change reversibly their sizes or shapes (namely “property changing/exchangers” or “energy exchanging/exchangers”) [1]. The “property exchanger materials” exhibit changes in one or more properties, such as chemical, thermal, electrical or mechanical, due to the input energy. Instead, in the case of the “energy exchanger materials”, their properties remain unchanged but their internal energy level is modified. Polymers have been used, for a long time, as structural materials for many consumer products, but these have also started to be utilized as functional materials, namely photoresist for microfabrication processes, organic light emitting diodes and more recently as dielectric electroactive polymers (EAPs) [2, 3]. In this context, due to their special physical and chemical properties [4–9], the electroactive polymers (EAPs) have been considered as an important class of materials which have gained a great interest in various scientific and industrial fields, associated with vast possibilities to be used in any application field [10–12]. Based on their operating mechanism, EAPs are generally divided as electronic (eEAPs) or ionic (iEAPs). The eEAPs are materials in which actuation is caused by the electrostatic forces between two electrodes, which can hold the induced displacement under direct current (DC) activation, and in iEAPs the actuation is caused by the ions displacement inside the polymer membrane; being necessary an electrolyte for actuation. In contrast to conventional actuators, EAP-based actuators are lightweight, flexible, economical, and noiseless [11–13]. In the last decade, efforts have been made to increase the performance of ionic polymer actuators by discovering new ionic polymers [14–17] and various electrode materials [18, 19]. Introduction of ionic liquids into actuators has generated a great potential for achieving large bending strains with a long-cycle life [14] because the ionic liquids present a high electrochemical stability and a good ionic conductivity [20]. Also, the use of a block copolymer matrix which determines microphase separation, generating periodic nanostructures, is considered an appropriate method for developing high-performance ionic polymer actuators

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[16, 21–24]. The slow response time is assigned to the exhaustion of cations and anions near the surface of the electrode under electric field action [20]. Moreover, a major impediment in the realization of durable actuators is represented by the asymmetric diffusion of cations and anions, which leads to uncontrolled relaxation behavior [25]. These inconveniences can be eliminated by immobilizing one of the ions in the polymer matrix thus forming single-ion conductors and avoiding the preoccupations for polarization. In any case, the production of high performance actuators is limited by displacement deficiencies unlike conventional polymers where the cation and anion are in motion [25, 26]. In addition, a lower value of dielectric constant corresponding to polymer chains neighbouring ion [27] determines a lower degree of ion dissociation in single ion-conducting polymers. Therefore, the different types of polymers [2, 28–37] have been used in the manufacturing of EAP-based actuators. Among these, the ionic polymer-metal composites represent the most promising electroactive materials with properties superior to conventional EAPs, used in the applications as actuators and sensors [10]. Ionic polymer metal composites (IPMCs), well-known as iEAPs, were discovered almost fifteen years ago by researchers from the United States and Japan, separately. By applying a voltage or a mechanical deformation these materials are capable of electromechanical and mechanoelectrical response. Figure 1 illustrates the deformation of an electrically stimulated IPMC. Consequently, due to their bidirectional actuation, there is a growing demand for low-voltage-driven electromechanical transducers because of their wide use in emerging areas namely robotics [38] and biomedicine [39]. A proper candidate is the ionic polymer actuator capable of large displacement under low operation voltages (few volts). Characteristics of the polymer layer (polymer nature, conductivity, hydration degree or counter ion) along with properties corresponding to the electrode and intermediate layers (type, area and thickness) influence the transduction in IPMCs. Understanding and knowledge of the layers properties are important to enhance their transduction capabilities and applications durability of IPMC. Hence, considering the vast possibilities to use the new electroactive polymer-based actuators in various fields, this chapter includes information about IPMC layers, modeling, control, and application.

Fig. 1 Illustration of ionic-polymer metal composite deforming in the response of electrical input conditions

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2 Understanding State of Ionic Polymer-Metal Composite Structure Typically, an ionic polymer-metal composite consisting of a thin semipermeable polymer membrane located between metal electrodes confers a layered structure. The electrode layer acts as a conductive pathway, intensifying the applied electric field and improving the ion transport inside the polymeric membrane. Figure 2 shows the elements that compose an IPMC as a three-layered arrangement. The procedure of the electrodes depositing on the membrane is defined as electroding technique that must satisfy the following criteria: • • • •

the metal or metal particles deposited must be close to the surface of the membrane; an increase of the area between the membrane and electrode is required; the electrical resistance of the electrode must be minimized; a uniform metal layer with chemically and mechanically good characteristics and adhesion properties on hydration be formed.

2.1 Manufacturing Technique Due to the non-conductive nature of polymers, the conventional electroplating method cannot be used directly. Therefore, different chemical and mechanical plating methods for manufacturing IPMCs, with distinguishing features, that satisfy the above-mentioned criteria, have been developed. Chemical plating methods (or electroless deposition) are considered the best processes to fabricate the IPMCs. These are based on the deposition and reduction of metallic ions from a solution onto the surface without applying an electrical potential (reducing agents are formaldehyde, hydrazine, hydroxylamine). Takenaka et al. [40] have reported a chemical deposition process, known as reductant permeation for the platinum, rhodium, iridium, ruthenium and palladium plating. In this process, the

Fig. 2 Visualization of the elements that compose an ionic polymer-metal composites

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diffusion of the cationic reducing agent through the membrane takes place on one side in order to react with an anionic salt from the other side of the membrane. Later, Millet et al. [41] have reported another method, namely impregnation–reduction, similarly with reductant permeation, to fabricate IPMCs based on Nafion membrane plated with platinum. In the impregnation–reduction process, the metal cation is embedded within the membrane and subsequently reduced by means of a reducing agent. The manufacturing steps of Nafion based IPMC actuators are exemplified in Fig. 3. 1. Treatment of film: In this step, the metallic particles and other impurities from the films are removed. The Nafion film is boiled in 1.0 N hydrochloric acid at 80 °C for 30 min. Subsequently, the film is washed with deionized water and put into a water bath to remove acid residue.

Fig. 3 Development stages in fabrication process of Nafion based IPMC actuators

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2. Ion exchange process: The ion exchange process allows the polymeric film to absorb enough complex platinum ions (Pt(NH3 )4 2+ ), which are reduced to platinum particles in order to obtain electrodes in next step. The membrane is introduced into a weakly basic environment formed by tetraammineplatinum chloridehydrate solution mixed with ammonium hydroxide. 3. Electroless platinum plating: In this step, to form the metallic electrodes on the outside surface of the polymeric film, the platinum complex ions are reduced to platinum particles. Then, the ion-exchanged film is introduced into a deionized water bath which is gradually heated; initially, at 40 °C. As a reducing agent, sodium boron hydride solution (NaBH4 ) is used, which is added in bath and temperature is increased to about 70 °C. After about 3 h, a platinum electrode of approximatively 3 µm is deposited on the surface of the film. Finally, the IPMC is cleaned with deionized water. 4. Secondary deposition: Because the surface resistance of an IPMC degrades the actuation performance [42], it is necessary to deposit more platinum metal on the surface. In this sense, steps 2 and 3 are repeated, in order to increase the number of platinum particles on the electrodes. However, a thicker layer of platinum causes an increase in beam flexural rigidity and thus decreases the actuation performance. After the secondary deposition, the thickness of the electrode grows to 6 µm and the surface resistance is reduced by 20%. Finally, the IPMC is introduced into a sodium ion solution, which has the ability to replace the positive ions with mobile sodium ions from the IPMC. The obtaining of mechanically and chemically stable electrodes is possible by applying the electroless deposition and impregnation-reduction techniques, respectively. Although these methods produce the electrodes with good performances but are rarely used for being expensive and requiring a long time [43]. The mechanical plating technique consisting of physical vapour deposition (i.e. sputter coating) has been chosen as a simple and quick alternative to the chemical plating due to the ease of application. Sputter coating is independently performed or along with other methods, like electroplating or plasma etching. Compared with the chemical deposition technique, this technique provides an easy way of electrode deposition. There are some metals such as gold, silver, platinum, nickel or titanium which can be deposited without drastically modifying the method. Data from literature [44, 45] show that tip displacement and force are enhanced by sputter coating of the IPMCs manufactured by the impregnation-reduction process. On the other hand, the sputtering process presents some inconvenience because is applied onto dry samples and the resulting product can breaks when is used. Casting solution is a process by which electroding can occur or the thickness of the polymer can be increased. Usually, for electroding, Nafion solution is blended with carbon, silver or other conducting powders. Chung and collaborators [43] have achieved electrodes by blending a Nafion solution with silver nano-powders on solution-casted Nafion membrane. The thickness of the obtained electrode layer was 15 µm and the surface resistivity was about 2 /square. In the same way, Park and coworkers [46] have used the carbon black to obtain electrodes with surface resistance higher than that

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determined by Chung and his team. In this case, the IPMC transduction was affected due to poor adhesion of the carbon particles to the polymer. To overcome this inconvenience, hot pressing or electroplating was used in combination with electroding process. However, the transducers with specific shapes or membranes with particular thickness can be obtained by direct application of the casting solution method. In the latter case, the process must be repeated for several times concomitantly with another method of hot pressing. The direct assembly is a way to obtain an electrode using dry or solvated membranes [47]. In this case, the electroding can occurs by coloring the electrode solutions, either on the membrane or on Furon before hot pressing (T  210 °C, p  20 MPa). This pathway is followed in order to enhance the adhesion of electrode layers at membrane. It is worth to mention that the methods chosen for IPMC processing can be conditioned by some electrode features (e.g. stability, quality, etc.).

2.2 Metallic Ion Implantation In order to make IPMC suitable and functional for further experimentation, some ion species are introduced into the membrane for creating the electroactive phenomenon. The ion implantation results in the acceleration of ions from a material, under the electric field action, that will bomb a target material in order to modify its properties [48]. This process can be utilized for obtaining the compatible electrodes at the surface of the soft elastomeric polymers. The target material is fixed in the direction of the electric field, so that, the plasma cloud can form in its immediate vicinity and the ions with very high velocities get into it. The technology of obtaining a conductive layer containing crystalline nanoparticles, when heavy ions penetrate the polymer up to tens of nanometers, is known as Plasma Ion Immersion Implantation [42]. Since the ions are magnetically manipulated, the macroparticles can be selected. Currently, few papers [42, 49] have reported results on various behaviours of IPMC samples regarding the displacements achieved under associated variables. Nevertheless, in the currently available documentation, based on the ion content of the IPMC samples, the graphical relationship between the displacement and the voltage applied at the different frequency conditions has been discussed. It was found that the backbone of the polymer, the content of ions (counterions) and the size of the cations (e.g. Li+ , Na+ and K+ , and/or alkyl ammonium) have a strong impact on the IPMC response (Figs. 4 and 5). The response of an IPMC with small cations (e.g. K+ ) to the action of an electric field is fast with a slight relaxation, while of those with large cations (e.g. tetra-nbutylammonium+ ) is slow without any relaxation. It is assumed that over the polymer backbone the small cations move more easily. The fast migration of the cations and associated water molecules towards the cathode determines an initial quick bending towards the anode. The leakage of water molecules, through the channels of the polymer backbone, from the cathode towards the anode causes a relaxation and the process finalized when the water equilibrium is reestablished. Comparatively, under

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Fig. 4 Variation of the displacement as a function of applied voltage for an IPMC containing potassium (K+ , a) and magnesium (Mg2+ , b) ions at a different frequency Fig. 5 Electromechanical response of tetra-n-butylammonium+ in an IPMC

the action of an electric field, the large cations move slowly. It is believed that due to their size the relaxation process is not visible. This is a consequence of the fact that the channels of the polymer backbone are blocked by ions or more water molecules are needed to establish the equilibrium of concentrated cations [49]. In literature, new methods of obtaining compliant electrodes by implanting metal ions, using low energies which varies between 1 and 5 keV have been reported [49, 50]. To achieve the conductive electrodes from PDMS films of 20–50 µm thick, the researchers have used a filtered cathode vacuum arc (FCVA) system, with a beam size of 1 cm2 , which operates in pulse mode. This device is provided with a conductive cathode (plasma manufacturer), a high-voltage trigger for arc initiation, an anode for plasma extracting, and a solenoid which acts as a filter for macroparticle. The ion implantation occurs in a vacuum chamber at a pressure below 10−5 mbar. Ion implantation also offers the possibility to create compliant electrodes of filigree type. This is possible by partial coverage of the target with a metallic mask or by photolithography directly on the target. The electrodes will form only in areas exposed to plasma and their performance (i.e. the accuracy and dimensions) being limited by the mask fabrication technology [50]. This approach is used to process small size devices consisting of dielectric elastomeric actuators created on PDMS film. For a better understanding of the advantages and disadvantages, small size devices have been created for adjusting a mirror that can be actuated by a system of tree dielectric electroactive polymer actuators. The obtained device can be used

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in optical coherence tomography (OCT) where the size of the device (few mm) and scan speed (up to 100 Hz) have important roles.

3 Modeling and Control of Ionic Polymer-Metal Composite Actuation As noted, an IPMC has a stratified structure obtained by depositing on a polymeric membrane of some metal electrodes, each layer generating the mechanoelectric and electromechanical effects. In order to understand these effects, thorough studies on the layer containing porous electrodes [43, 50, 51], intermediate layer [47, 52] and polymer layer [53, 54] have been made.

3.1 Layered Structure of Ionic Polymer-Metal Composite The transduction in IPMC depends on the ionomer nature, counterions, and hydration state of the membrane; hence, polymer layer acts as a selective ion-exchange membrane. Nafion (perfluorosulfonate) or Flemion (perfluorocarboxylate), most often presented in the literature [55, 56], determine differences in ion-exchange capabilities due to the sulfonate and carboxylate functional groups. The latter has a higher ion-exchange capacity and good mechanical strength comparatively with the sulfonate group. Nafion is a perfluorosulfonate ionomer consisting of a poly (tetrafluoroethylene) (representing the main chain) and perfluoroether side chains (terminated with sulfonated groups). While the anions are fixed at the polymeric chain, the cations can freely move after hydration. Most of the models reported in the literature [57] to explain IPMC transduction based on Nafion have considered either the electrostatic dipole interaction or ion transport, ignoring the combined action of the two mechanisms. Thus, the forces that act on ions, which migrate through the polymer network, have been considered in the ion transport mechanism. Instead, in the electrostatic mechanism, the dipoles formed as a result of the distribution of anions and cations in a cluster, and changing of the electrostatic interactions, generated by the electric field or tip displacement, have been considered. Because the major characteristic of the ionomers is charge transport, attempts to enhance the ionic conductivity, which in turn influences the IPMC deflection and force, have been made. Additionally, other properties, such as the ion-exchange capacity, elastic modulus and, low water absorption, affect the transduction capabilities of IPMC. However, for the application of the Nafion as a transducer, some issues must be overcome, e.g. the manufacturing of different types of transducers is limited because the Nafion cannot be processed by melting, during DC actuation, due to relaxation installed as a result of water

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molecules diffusion, during electromechanical and mechanoelectrical transduction, the force and charge output characteristic to an IPMC are low. Compared to Nafion, the sulfonated poly(arylene ether sulfone) indicates a higher glass transition temperature and implicitly an improved rigidity due to its aromatic chemical structure. Moreover, the sulfonated poly(arylene thioether sulfone) exhibits a good attachment of the electrode to the polymer, increasing thus the running time of the IPMC [58]. Additionally, the poly(vinylidene fluoride) subsequently modified by sulfonation [59], has a good ionic conductivity, displacement and lock forces greater than Nafion. Although the polymer layer is the most important in IPMC constitution, the electrode layer plays an important role in IPMC transduction and cannot be neglected. Thus, by modifying the electrode layer, which is much easier to modify than the ionomer or intermediate layers, the IPMC properties can be changed. In this sense, the IPMC characteristics can change drastically being affected by some properties of the electrode, such as the morphology and electrode thickness and the type of metal used, etc. For example, Shahinpoor and Kim [42] and Kim et al. [51] have studied the surface morphology of a platinum electroded IPMC by atomic force microscopy (AFM). AFM images showed that the platinum particles from porous electrode have granular forms with a peak-valley depth of about 50 nm. Furthermore, using the transmission electron microscopy (TEM), they observed the average particle size of about 47 nm. Based on these observations, they concluded that the electrode thickness and porosity are key parameters in controlling the electrode resistance. Moreover, the electrode capacity was influenced by the thickness of the intermediate layer as well as by the distance between the particles surfaces found in this layer. In addition, increasing of the surface area of the electrode, controlled in turn by the morphological aspects, increases the overall capacitance of the IPMC. Jin et al. [60] and Tiwari [61] independently, analyzed the impact of surface roughness on IPMC properties and observed as: (i) tensile modulus and capacity are enhanced because of a good electron penetration and good adherence; (ii) the roughening improves IPMC transduction; (iii) the transduction is better for gold and palladium electrodes due to finer surfaces; so the type of material used influences the electrode morphology. Kim and coworkers [62] have demonstrated through their work that the transduction of a platinum electrode, with the capacity to form structures similar to a needle, decreased as the surface roughness increased. On the other hand, Park and colleagues [63] have noticed the formation of spherical particles of different sizes in the case of gold and palladium IPMCs. Thus, the increase of the thickness and also of electrode conductivity influence the transduction in IPMC. This behaviour is caused by the charge transfer which is equal to the inverse of the electrode resistivity. By increasing the number of plating cycles and lowering the temperature, during the IPMC manufacturing process, an increase in electrode thickness can be achieved. Each reduction leads to an increase of the metallic particles density on the electrode surface, and therefore, to a decrease of the roughness and surface resistance. There-

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fore, it can be concluded that the first stage of temperature decreasing affects the thickness of the electrode, while the second stage influences the electrode density [64]. A special attention should be given to the type of metals used for electrodes, which must keep their physical and chemical properties unchanged under hydration and be resistant to corrosion, taking into account the hydration requirement of IPMCs. In this context, due to the corrosion resistance and high conductivity, the platinum [40] and gold are usually used for electroding. By adding powders of palladium [62] or silver [43], some properties of platinum electrodes can be improved. Also, the gold electrodes that are environmentally friendly, flexible and have a high electric conductivity present the possibility to be used in different applications. In the case of IPMCs based on carbon nanotubes (CNT) as electrodes [64–67], the weak dispersion within the membrane limits their applicability domains [68]. Large surface areas and specific capacitances are properties that make iridium [69] and ruthenium [70] to be used in the detriment of gold. Other metals used for electroding with similar properties to platinum and gold electroded IPMCs are: copper [71], palladium [40], silver [72], lead [72] and nickel [63, 72]. However, high corrosive nature of nickel, silver and copper limits their applicability in hydrated conditions. The importance of polymer and the electrode layers of IPMCs has been extensively reported in the literature. However, the role of the intermediate layer in IPMC traduction has not been overlooked. Thus, a particular impact on the area between the two above-mentioned layers and, implicitly, on the IPMCs capacitance has the intermediate layer [73]. In their study, Sunghee and collaborators [74] have noted that platinum particles penetrate the polymeric membrane determining an increase of soaking time of 3 h, thus reaching saturation. The researchers have concluded that saturation in the region of the separation surface of two different phases appears after fourth plating cycle, subsequent plating favours the increase of the electrode rigidity and thus limiting the IPMC tip movement during the electric field action. Akle et al. [75] have proved the occurrence of a relationship between the capacitance and strain. This relationship is the cause of some mechanical deformation arises as a result of the accumulation of charge between the electrode/polymer layers.

3.2 Actuation Mechanism of Ionic Polymer-Metal Composites An IPMC material is capable to deform on the application of an electric field. The actuation mechanism of an IPMC involves the motion of the ions and electroosmotic flow of water molecules, through hydrophilic and ionic channels, from the anode to cathode, generating a fast bending motion of IPMC [76]. Moreover, it was reported that the mobility of the ion-water clusters in the IPMC determined a shrinkage of one part and the stretching of the other part [76–79]. Thus, distribution of the charges in an IPMC membrane, under the action of electric voltage, determines changes of the

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Fig. 6 Schematic illustration of the electroactive mechanism in IPMC. Visualization of the deformations appearing at IPMC when an electric input is applied

electrostatic, osmotic, and elastic interactions both inside and outside of the clusters. This causes either stretching or relaxation of the polymer chains as a result of the changes of localized volume in the clusters and also of the water content. All these are predominant in generating, on the one hand of a rapid bending, and on the other hand, of slow relaxation. Therefore, the electroactive mechanism by which the IPMC material shows its bending action is illustrated in Fig. 6. When an electric potential difference is applied on the two surfaces of the polymer membrane layers connected to a circuit, the polarization created at both electrodes initiates the motion of the cations (the embedded inside the membrane) toward the cathode (cation-rich layer); hence the electromechanical transduction occurs. During this process, the water molecules surrounded around the cations are also carried by cations on the cathode side. The accumulation of these leads to an imbalance which expands by a sudden increase of cations and molecules concentration, while the contraction in the other side is due to the decrease in concentration. The cationrich layer determines extensive stretches in the polymer, generating a rapid bending diffusion to the cation-poor layer (positive pole, anode) [9, 79], building a curvature; this reaction happens in a fraction of a second. Diffusion of water from the multi-cation areas, under the action of the pressure from the tensioned polymer matrix, results in slow relaxation toward the cathode. The ions and water molecules diffuse to the cross section of the base polymer only when the difference of potential is removed. This fact causes a relaxation of the strip and a return to its original state. The sudden diffusion within the polymer, as a result of the imbalance installed following the electric field action, causes a reciprocal bending towards the opposite side during the relaxation. With increasing of the frequency in the electrical input this bending decreases.

3.3 Actuators Based on Ionic Polymer-Metal Composite Membranes Current researches on the IPMC development as actuators include the preparation techniques, experimental substantiations of the processes and a better understanding and control of the phenomena regarding the material deformation [80–82]. However,

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a bridge between the scientific research and technological application is necessary for the development of these functional materials. In this context, poly(ether ether sulfone) represents one of the transparent engineering thermoplastics and is characterized by excellent thermal stability, good resistance to inorganic acids and bases, and a good film formation property. It can be easily transformed into ion-conductive sulfonated poly(ether ether sulfone) by sulfonation reaction, for use as ion-exchange membrane to replace the Nafion, in the case of the fuel cells [83, 84]. Xue et al. [80] have synthesized by sulfonation a series of sulfonated poly(ether ether sulfones) with different substitution degrees and subsequently, have obtained actuators from sulfonated poly (ether ether sulfone) membranes. In their study, the electromechanical performances of new sulfonated poly(ether ether sulfone)-based IPMC actuators were evaluated as a function of the low direct current and sinusoidal voltages. Following their analyses, the authors concluded that these membranes can be useful as high-performance IPMC actuators in future. Dubois et al. [81] have developed and tested the first ion-implanted dielectric electroactive polymer microactuator, manufacturing from the ion-implanted membranes by a chip-scale process. The obtained actuator consists an ion-implanted polydimethylsiloxane (PDMS) membrane connected to a silicon chip containing a hole. The dielectric is represented by the polymeric membrane connected to a silicon chip with holes whose diameter varies between 1 and 3 mm2 . The implantation of Ti ions was performed on both parts of the polymeric membranes. In order to obtain big holes and for intense etching with reactive ions, KOH was used. After testing, the authors concluded the following: • Ti ions implanted in the polydimethylsiloxane membrane significantly decrease the surface resistivity, from an initial start value greater than 30 M/square to less than 100 k/square; • implanted ions determine modifications of the surface; AFM measurements on roughness indicate an increase in root mean square roughness from 30 nm corresponding to non-irradiated elastomer to 110 nm corresponding to the irradiated one; • there may be complex buckling modes, with many displacement peaks placed in different areas. In order to this aspect, the actuators with 1 mm2 square openings were manufactured and examined. The vertical displacements of 110 µm have been observed, as a result of the opening shape and the mode in which the membrane is fixed. Finally, they concluded that the ion-implanted dielectric electroactive polymer microactuators make a special connection of the high energy-density, leading to a high amplitude displacements. O’Brien and collaborators [82] have created smart actuators network using the gold ion implantation to make piezo-resistive electrodes with implanted gold. For designing of ion implanted electrodes, firstly, the authors have created a conceptual framework and then have realized and tested the piezo-resistors implanted with dif-

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ferent densities of gold. All evaluations have indicated these as the most promising category of resistors with a technological performance for applications as artificial muscle inverters.

4 Actuation Performance of Actuators Based on Ionic Polymer-Metal Composite Membranes in Different Applications In recent years, research focused on IPMC actuation has evolved from the fundamental study of specific features of the material towards the practical approach and development [85–88]. Based on the knowledge of the IPMCs architecture, there must be a suitable control and design for various applications implemented by an IPMC actuator. In this context, the latest works on the control approaches for IPMCs indicate that they can be achieved and integrated as sensors and control units in the intelligent systems with a significant influence on new technologies [89]. Chen et al. [90] created a multi-performant IPMC membrane actuator, which presents the ability to perform 3D kinematic moves. Compared to the existing techniques, this technique offers two major advantages as indicated below: (a) selection of an elastomer membrane such as PDMS membrane allows the obtaining of the inactive area; (b) creation of an uninterrupted link between the active areas (IPMC) and inactive (PDMS membrane). To demonstrate the capabilities of this new type of actuator, designed for the bio-inspired robot, the authors have built and tested a small size robotic manta ray. This study represents the first demonstration of a free robotic manta stimulated by an IPMC artificial muscle. This robot is made of two artificial pectoral fins which mimic the biological one. Thus, this research is also considered a starting point for the investigation of the stimulation mechanism of diverse types of rays, because the pectoral fin can be manufactured in various dimensions and shapes. To improve the ability of membrane for 3D kinematic motions, the authors intended to develop the resulting system by creating a robot body that mimics the body of a batoid ray, improving the hydrodynamics of the robot and, also to create a mathematical model to give useful data to optimizing and controlling the robot’s design. In another study, Wu [91] have created inherently conducting polymer-based membranes actuators applicable in the cochlear implant. In the development of new sensor, actuators and, controlled release devices, the effects induced by used polypyrrole (PPy) and electrodes, that represent an integrated part of the cochlear implant, were analyzed. Actuator systems were made from a PPy trilayer bending actuator, a PPy microfluidic pump and, a PPy-coated hollow fibre. Electromechanically investigations have indicated that the dopant ion from electroactive polymer influenced

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the polarity of the output voltage and also, the signal amplitude. These remarks have led to the establishment of the “stress-induced ion flux” mechanism. Analyzing the experimental data the authors concluded that: • the use of device consisting of a reservoir coated with an inherently conducting polymer membrane allows the electrochemically controlled release of the anionic drugs; • from the evaluation of the variation of electrochemical impedance as a function of the impact forces on the tip of a cochlear implant in artificial perilymph solution, it was established that the cochlear implant electrode can be used to identify impact forces. This approach allows the detection of risks during the implantation of the cochlear implant. The solution composition and orientation of the impact forces encountered are the factors that influence the response and have been thoroughly analyzed. Currently, attempts are being made to obtain suitable IMPCs for use in heart diagnosis and therapy. The ability of these polymers to replicate the function of the biological muscles, including large actuation strain, makes them ideal candidates for actuators used in vivo, as well as in cardiac simulators. As mentioned in the literature [92], the possibility of patients rejecting the heart transplant was 50%. In this context, Shahinpoor proposed a minimally invasive device which is implantable and can assist a weak heart (Fig. 7). The developed system, consisting of multiple fingers which can selectively assist the ventricles, is actuated by soft ionic polymeric artificial muscles (composed from ionic polymer-metal nanocomposites) that “thrive in the wet and saline environment of the inside of the human body and, in particular, around the myocardium of the heart” [92]. Furthermore, the material testing demonstrated that the ionic polymer-metal nanocomposites performed well under oscillatory activation, a characteristic required in order to cyclically compress the heart of billions of times [92]. The main advantages of the created device are that it not favour thrombogenesis, and not pose a risk of incompatibility of his artificial surfaces with the human organ during implantation in the body, being placed outside the heart. Sherif [93] developed another cardiac assist device, namely the artificial ventricle, that can be used both as single and biventricular support in defective hearts (Fig. 8). The designing concept is based on the replacement of the ventricles functionality instead of assisting, the pump is external to the heart. The realized system is constituted by an implantable pacemaker as a power supply and pulsatile, positivedisplacement blood pump. The last component mentioned contains a biocompatible compliance chamber and bioprosthetic valves, with electroactive polymeric contractive elements radially disposed of [93]. These contractive elements (marked with red in the small picture from Fig. 8) are circumferentially disposed at angles of 45° or 60° around the compliance chamber, so that, when becoming active they contract by effecting a helical motion, thus imitating myocardial muscle fibers [93]. The author affirmed that the pacemaker can operate as a permanent pulse generator, and also can be to automatically regulate, the pumping rate depending on the physiological needs of the patient. The device created is totally implantable, and does not require

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Fig. 7 General configuration of the heart compression device equipped with multiple fingers. Parts a and b allow better visualization by presenting an enlarged image of the multiple fingers system Fig. 8 Visualization of the cardiac assist device, artificial ventricle, implanted inside the human body. For better visualization of contractile elements, in the figure has included a small image

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the presence of wires, tubes or control cables that cross the skin, thus preventing infection. Hanson et al. [94] focused on the development of a similar cardiac assist device; “an artificial muscle wrap consisting of contractile bands”. The researchers have realized a simulator to test their model, establishing an impedance control pathway through which the device activity can be assisted, adapting to the patient’s need. Overall, the proposed applications are promising and contribute to the functionality, feasibility, and versatility of using IMPCs at various technological levels. Development of different models and improvement of the fundamental understandings represent the way to new applications.

5 Conclusions The electronic field has progressed in an impressive way with the development of materials capable of expanding under different forms within an artificial intelligence system. These new materials result in the development of smart devices that are capable of mimicking the sensory functions of the living organism e.g. the devices of diagnosis and therapy. With these devices of higher intelligence, the electronics performance concept will generate a fast-expanding in many domains, including public health, biomedicine, by meeting sustainability criteria in terms of public health and environmental impacts, ease of use, flexibility, and adaptability. More specific, the current work was aimed not only at discovering of the basic models but also to enter as deeply as possible in the field of new generation of electronic products. Although the major challenges still exist, the next generation of electronics will have a significant impact, multi-performance and multi-functional, in future technologies, due to the opening of new possibilities for application.

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Study on Time-Dependent Bending Response of IPMC Actuator Hyung-Man Kim and N. D. Vinh

Abstract Ionic polymer metal composite (IPMC) actuators have considerable potential for a wide range of applications. Although IPMC actuators are widely studied for their electromechanical properties, most studies have been conducted at the ambient conditions. The electromechanical performance of IPMC actuators at higher temperature is still far from understood. IPMCs are polymer-based soft composites that can be designed as soft actuators and sensors. IPMC actuators have several unique properties, including low density, large bending strain, low noise, high resilience, and low operation voltage; which make their application more practical compare to many of their metal- or ceramic-based counterparts. This chapter is a summary of all recent findings and current state-of-the art manufacturing techniques, phenomenological laws and mechanical and electrical characteristics. The time dependent bending characteristics of IPMC actuators has been widely studied, experimentally, and theoretically, as artificial muscles for biomedical applications, biomimetic micro-robotics, and harsh environment tools. The first one of contents presents a brief summary of the fundamental properties and characteristics of IPMC. The following addresses in detail the electronic and electromechanical characteristics of IPMCs, the phenomenological Modelling of the underlying sensing and actuation mechanisms in IPMCs and the potential application areas for IPMCs. Keywords Ionic polymer metal composite · IPMC actuator · Actuation mechanisms · Time dependent bending characteristics · Electromechanical characteristics · Modelling · Experiment · Application

H.-M. Kim (B) · N. D. Vinh Department of Electronic Telecommunication, Mechanical & Automotive Engineering and High Safety Vehicle Core Technology Research Center, INJE University, 197 Inje-Ro, Gimhae-Si, Gyeongsangnam-Do, 50834 Gimhae, Republic of Korea e-mail: [email protected] © Springer Nature Switzerland AG 2019 Inamuddin and A. M. Asiri (eds.), Ionic Polymer Metal Composites for Sensors and Actuators, Engineering Materials, https://doi.org/10.1007/978-3-030-13728-1_5

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List of Abbreviation ALE ALT BDF CNT DC DPP-4 EAC EAP GLP GUI FSI HDL IDDM IPMC IOP IU MUMPS NIDDM PAA PAMPS PDE PARDISO PVA SMA TBA

Arbitrary Lagrangian Eulerian Argon Laser Trabeculoplasty Backward Differential Formulation Carbon Nano Tube Direct Current Dipeptidyl Peptidase-4 Electro Active Ceramic Electro Active Polymer Glucagon-Like Peptide Graphic User Interface Fluid-Structure Interaction High-Density Lipid Insulin-Dependent Diabetes Mellitus Ionic Polymer Metal Composite Intraocular Pressure Insulin Unit MUltifrontal Massively Parallel Sparse direct Solver Non-Insulin-Dependent Diabetes Mellitus Poly Acrylic Acid Poly Acrylicamido Methyl Propane Sulfonate Partial Differential Equation Parallel Direct Sparse Solver Poly Vinyl Alcohol Shape Memory Alloy Tetra Butyl Ammonium

1 Introduction Smart materials have been promoted widely as a key technology that will underpin all manner of novel products with unique capabilities. Many smart materials and products are available commercially and the technologies continue to be the subject of widespread academic research. Nevertheless, there is a widely held view that existing smart materials could find far more widespread uses and the limited commercialization reflects in part the many misconceptions surrounding exactly what smart materials are and what they can do. Although widely discussed in the technical literature and also by the popular press, there is no universally accepted definition of exactly what “smart” or “intelligent” materials are. Many indicate that they are materials which sense some stimulus from the external environment and create a useful response but this would include conventional sensing materials such as Ionic Polymer-Metal Composites (IPMCs) which several authors do, indeed, categorize as “smart”. However, a more useful view is to consider the response rather than the

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material itself. Some authorities argue that there are no such things as smart materials but only materials that exhibit certain intrinsic characteristics which can be exploited in products, systems or structures that in turn exhibit “smart” behaviour. Examples of this behaviour include responses to external stimuli such as self-sensing, self-healing, self-actuating, self-diagnostic and shape changing [1–3]. IPMCs as multi-functional smart materials with actuation, energy harvesting, and sensing capabilities were first introduced in 1997–1998 by Shahinpoor, Bar-Cohen and co-workers as a member of the electroactive polymer (EAP) family based on research work supported by NASA–Jet Propulsion Laboratory (JPL) and under the leadership of Yoseph Bar-Cohen at JPL and Shahinpoor [4], director of the University of New Mexico’s Artificial Muscles Research Institute. However, the original idea of ionic polymer and polymer gel actuators goes back to the 1991–1993 time period of Bar-Cohen [5]. The IPMC actuators, sensors and artificial muscles are composed of a perfluorinated ion-exchange membrane, which is chemically composited with a noble metal such as gold, palladium, platinum, and silver. Note that IPMCs are excellent sensors that generate huge outputs in terms of millivolts, which can be employed for the sensing, transduction, and harvesting of energy from wind or ocean waves. These unique materials work perfectly well in a wet environment and thus they are excellent candidates for medical applications. These might range from endovascular steerers and stirrers to enable navigation within the human vasculature; use as deep brain stimulators or employed in flat diaphragm micropumps for precision drug delivery, glaucoma and hydrocephalus; artificial muscles for the surgical correction of ptosis (drooping eyelid syndrome); ophthalmological and vision improvement applications; artificial muscles to assist a failing heart; correction of facial paralysis and other applications in muscular dystrophy; to mediate the control of drainage or flow within the human body; and myriad additional purposes. On the industrial side, due to the fact that the IPMCs are excellent sensors and low-voltage actuators, they can be used for both sensing and simultaneous actuation in many engineering applications Two emerging visions of the future are to see IPMCs heavily utilized in atomic force microscopes as novel and dynamic probes in scanning probe microscopy, as well as robotic surgery to facilitate the conveyance of specific haptic, force, tactile and impedance feedback to surgeons. IPMCs as active substrate and micropillars may be used to monitor nano-bio and cellular dynamics in real time [4, 6]. IPMCs are one type of electro-active polymer that is gaining a lot of importance nowadays as actuator and sensor. IPMCs are active actuators that show large bending deformation with the application of low actuation voltage and it induces large bending strain, led to its consideration for various potential applications like robotics, aerospace and biomimetics [7–10]. A typical IPMC consists of a thin polyelectrolyte membrane (usually Nafion® or Flemion® ) and is chemically plated on both faces of the membrane by a noble metal such as gold and platinum. An IPMC undergoes a large bending deformation when an external electric potential is applied across the material. Conversely, a measurable electric potential is developed across the strip when it is subjected to a bend or twist. Thus IPMC can serve as both actuator and sensor. An IPMC offers advantages,

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such as compliant, lightweight, low voltage operation and capability of working in a water medium. These properties make it promising for numerous applications in biomedical, naval, robotic and microelectromechanical system (MEMS) engineering [11–15]. Based on the electrostatic interaction of ion transport, a model was presented by Nemat-Nasser and Li [16]. Further, micromechanics of IPMC material was investigated by Nemat-Nasser and proposed a hybrid model considering electrostatic, osmotic and elastic effects in the actuation process [17]. A technique to minimize dehydration loss and to increase the force density of IPMCs was proposed by Shahinpoor and Kim [18]. A linear electro-mechanical-model was developed by Newbury and Leo for IPMC transducers considering the visco-elastic property [19]. Extensive experimental studies on IPMCs with different backbone ionomers and various cation forms have been conducted by Nemat-Nasser and Wu [20]. A computational micromechanics model was proposed by Weiland and Leo, to assess the impact of uniform ion distribution on spherical clusters of IPMC ionomer [21]. Ionic polymer-metal composites, consisting of two metal electrodes and an ionconducting polymer between them are a promising class of ionic electroactive polymers. They can be utilized as sensors, actuators, or energy harvesters [22]. Generally, perfluorinated polymers, such as sulfonated (Nafion) or carboxylated (Flemion), are employed for IPMCs [23]. Perfluorinated sulfonic acid ionomeric polymers are synthesized by copolymerization of sulfonyl fluoride vinyl ether and tetrafluoroethylene [23–25]. Nafion is a perfluorinated sulfonic acid ionomer membrane that has a Teflon-like backbone and short side-chains terminated by the sulfonic acid group, with counter ions, such as H+ , Li+ , Na+ and K+ , and hydrophobic fluorocarbon and hydrophilic ionic phases. When the IPMC is bent mechanically, the hydrated cations on the compressed side of membrane move towards the stretched side of the membrane, resulting in the imbalance in the number of cations contacting each electrode, and this produces output voltages across the membrane.

2 Overview of IPMC Materials exhibiting coupled phenomena such as pH, temperature, and electropotential affecting mechanical deformation and vice versa (Shape Memory Alloys, thermoelastic materials), could be put to intelligent use in many engineering applications. Such materials are classified as ‘Smart’. IPMC strips respond to an electric potential applied across two electrodes and undergo mechanical deformation. Conversely, when the strip is bent, an electric potential is developed across the surface of the strip. Given its large bending deflection with low actuation voltage input property and the converse effect, IPMC strips show promise in engineering applications such as in actuators, sensors, and energy and force transducers. Further IPMC strips have been used in space and planetary applications like soft robotic actuators (dust wipers), biomedical applications (gastrointestinal endoscopic devices), and artificial muscles. A wider list of applications ranging from mechanisms, robotic toys and actuators, human-machine interfaces.

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2.1 Structure of IPMC An IPMC is a porous charged polymer, saturated with an electrolytic solvent and plated by two metallic electrodes. A difference in voltages between the electrodes generates mechanical deformations (actuation mechanism) which in turn yield a difference in voltages between the electrodes (sensing mechanism). The electrolytic solution comprises mobile ions and an uncharged solvent. The actuation mechanism is based on ion mobility. Because of the difference in voltages between the electrodes, mobile ions (due to their hydrophilicity) displace the uncharged solvent; the concentration gradient of the uncharged solvent causes a volumetric dilatation of the hydrophobic polymer which is not uniform along the thickness, hence a dilatation gradient. Thus, the polymer is deformed without any mechanical forces acting on it during the actuation mechanism [26]. IPMC strips are made up of an ionic polymer, like Nafion (perfluorosulfonate made by DuPont) or Flemion, which has fixed anions in the polymer network. This ionomeric polymer network is neutralized with an ionic solution with solvents like water and cations like (Li+ ) or tetrabutylammonium ions (TBA+ ). The surface is composited with a conductive medium like platinum or gold electrodes. A schematic of an IPMC strip is shown in Fig. 1. When an electric potential is applied between the two surface electrodes, the IPMC strip bends. Redistribution of the mobile ions and water molecules due to various physical processes like diffusion, electrophoretic solvent transport, and diffusion-deformation coupling gives rise to the electromechanical behaviour [11, 27].

Fig. 1 A schematic of a typical IPMC strip and its actuation principle. The IPMC strip bends toward the anode when an electric potential is applied across the surface of the strip (Reproduced from Ref. [20])

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IPMCs are generally fabricated through an electroless chemical reduction process, consisting of the diffusion, adsorption, and reduction of a metal salt onto an ionomeric membrane. Nafion and Flemion are typically used as the ionomers, and noble metals, such as platinum, gold, or silver serve as the electrode material. The electroless chemical reduction is often followed by an ion-exchange process, in which the sample is immersed in a solution to replace hydrogen ions with the select counterion species. Water is typically used as a solvent and several counterions, such as lithium, sodium, and potassium [28]. The type of polymer base (Nafion or Flemion), the mobile cations, and the type of solvent along with their composition affects the response of the IPMC strip. Under the application of a step voltage, Nafion-based IPMC strips bend towards the anode. Flemion based IPMC strips show an initial fast bending movement towards the anode side and then a slow relaxation towards the cathode side. The tip displacement response, when a step voltage is applied differs, depending on the type of free mobile cations even for a Flemion based sample. Response time depends on the dimension of the IPMC strip (length, thickness, and width), type of polymer, mobile ions, electrodes and applied electro-potential [20, 29]. The current state-of-the-art IPMC manufacturing technique incorporates small amount ( PDE Interfaces branch in the Model Wizard. In all of them, weak expressions can be added, which COMSOL Multiphysics adds to the overall equation. Adding one of these interfaces creates a PDE node for PDE modeling using a weak formulation. Also, add a Weak Form PDE on the domain level to any other PDE interface. When this interface is added, these default nodes are also added to the Model Builder: Weak Form PDE, Zero Flux (for a Weak Form PDE on the domain level only), and Initial Values. Right-click the main PDE node to add other nodes that implement other boundary conditions, for example. On the domain level, edge level, and boundary levels the same boundary conditions can be used as for the Coefficient Form PDE and General Form PDE. To implement the equations using the Weak Form PDE mode, we require the governing equation introduced before to be recast in their weak form as follows: ∫ ε(u) ˜ · T dv − ∫ u˜ · t da  0

(32)

∫ u˜ cdv ˙ − ∫ ∇ c˜ · Jdv + ∫ cn ˜ · J da  0

(33)

∂B

B

B

B

∂B

Study on Time-Dependent Bending Response of IPMC Actuator



∇ ψ˜ · ddv −

∂B

B

ψ˜ n · ddv +



˜ da  0 ψq

95

(34)

B

for all independent-time function chosen in an appropriate functional space. An alternative way of implementing Eq. 32 is to use the Solid Mechanics interfaces of the Structural Mechanics physics module and introduce the solute contribution, as a Weak Contribution mode in the IPMC domain. ˜ · (3λ + 2G)α(c − c0 )I dv  0 − ∫ ε(u)

(35)

B

See the COMSOL Multiphysics Modeling Guide for implementation details. In addition, we have used the COMSOL physics module Transport of Diluted Species interface to implement the ion transport equation and the AC/DC Module to implement the electrostatic equation (Poisson). The weak form (Eqs. 32–35) accounts for the mechanical, chemical and electric boundary conditions which depend on the specific problem under investigation.

4.5 Particle Tracing Particle tracing provides a Lagrangian description of a problem, in which the particles are treated as distinct entities instead of a continuous distribution. The particle trajectories are computed by solving ordinary differential equations using Newton’s law of motion. Newton’s law of motion requires specification of the particle mass and all forces acting on the particle. The forces acting on particles can be divided into two categories, those due to external fields and those due to interactions between particles. Forces due to external fields are typically computed from a finite element model, using the physics interfaces available in COMSOL Multiphysics. For each particle, a second-order ordinary differential equation is usually solved for each component of the position vector. This means that three ordinary differential equations are solved for each particle in 3D and two in 2D. At each time step, the forces acting on each particle are queried from the external fields at the current particle position. If particle-particle interaction forces are included in the model, then they are added to the total force. The particle position is then updated, and the process is repeated until the specified end time for the simulation is reached. Since the Particle Tracing Module uses a very general formulation for computing particle trajectories, the physics interfaces can be used to model charged particle motion in electromagnetic fields, large-scale planetary and galactic movement, and particle motion in laminar, turbulent, and multiphase fluid systems. The Particle Tracing Module is available to assist with these types of modeling problems. In this study, we used this module to predict the position of the particle released in the fluid when IPMCs cilia deform under electro potential. It is possible

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to model particle tracing with COMSOL Multiphysics provided that the impact of the particles on the flow field is negligible. First, we compute the flow field, and then, as an analysis step, calculate the motion of the particles. The motion of a particle is defined by Newton’s second law. Examples of forces acting on a particle in a fluid are the drag force, the buoyancy force, and the gravity force. The drag force represents the force that a fluid exerts on a particle due to a difference in velocity between the fluid and the particle. It includes the viscous drag, the added mass, and the Basset history term. Several empirical expressions have been suggested for the drag force. One of those is the one proposed by Coulson and Richardson [47]. That expression is valid for spherical particles for a wide range of particle Reynolds numbers. 3000 particles with 5 μm diameter released in the channel.

4.6 Meshing The Mesh nodes enable the discretization of the geometry into small units of simple shapes, referred to as “mesh elements”. A mesh is a result of building a meshing sequence. A meshing sequence corresponding to geometry consists of Meshing Operations and Attributes. The attribute nodes store properties that are used by the operation nodes when creating the mesh. Building an operation node creates or modifies the mesh on the part of the geometry defined by the operation node’s selection. Some of the operation nodes use properties defined by attribute nodes; for example, the Free Tetrahedral node reads properties from the Distribution and Size attribute nodes. For some operation nodes, we can right-click to add local attribute nodes as subnodes. Properties defined in local attribute nodes of an operation node override the corresponding properties defined in global attribute nodes (on the same selection). 2D Geometries: The mesh generator discretizes the domains into triangular or quadrilateral mesh elements. If the boundary is curved, these elements represent an approximation of the original geometry. The sides of the triangles and quadrilaterals are called mesh edges, and their corners are mesh vertices. A mesh edge must not contain mesh vertices in its interior. The boundaries defined in the geometry are discretized (approximately) into mesh edges, referred to as boundary elements (or edge elements), which must conform with the mesh elements of the adjacent domains. The geometry vertices are represented by vertex elements. 3D Geometries: The mesh generator discretizes the domains into tetrahedral, hexahedral, prism, or pyramid mesh elements whose faces, edges, and corners are called mesh faces, mesh edges, and mesh vertices, respectively. The Distribution node was used to specify the distribution of mesh elements along an edge, for example. Predefined distribution type was selected to specify properties of a predefined distribution method. It could be a geometric sequence (exponentially increasing or decreasing element size) or an arithmetic sequence (equal distance between elements); see COMSOL Manual for details. To create a mapped quadrilateral mesh (that we used for IPMC) for each domain, the mapped mesher maps a regular grid defined on a logical unit square onto each

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domain. The mapping method is based on transfinite interpolation. The settings in the Size and Distribution nodes used by a Mapped node determine the density of the logical meshes. For the mapping technique to work, the opposite sides of each logical unit square must be discretized by the same number of edge elements. By default, the relationship between the four sides of the logical unit square and the boundaries around a domain is based on a criterion related to the sharpest angle between boundaries. Next distribution option applied at the two edges of IPMC to distribute higher density of elements near the electrodes. Because of fluid-IPMC interaction in this region we need more detailed elements to solve equations and reach accurate results. There are various options predefined in size selection section from extremely coarse to extremely fine for the size of elements (see Fig. 7). In most of the cases of two-dimensional analysis, we used the finer option to mesh IPMC and fluid with the elements size in the range of 1.6–112 μm. Normally the smallest element size applied to IPMC-fluid interface and biggest to the triangular structure of fluid. For the rest of the system (fluid), the triangular element formation was chosen.

Fig. 7 Mesh generation and element size selection in COMSOL (Reproduced from Ref. [3])

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4.7 Moving Mesh The Moving Mesh interface, found under the Mathematics > Deformed Mesh branch when adding a physics interface, can be used to create models where the geometry, here represented by the mesh, changes shape due to some physical phenomena without the material being removed or added. Arbitrary Lagrangian-Eulerian (ALE) is a formulation where a Eulerian equation is transformed into an equation written with respect to a mesh, which can be moving in relation to both the Eulerian frame and the Lagrangian frame. The COMSOL Multiphysics solvers have built-in support for the necessary transformation of derivatives. The difference between the Deformed Geometry and Moving Mesh interfaces is that the former defines a deformation of the material frame relative to the geometry frame, while the latter defines a displacement of the spatial frame relative to the material frame. The Moving Mesh interface can be used to study both stationary states and time-dependent deformations where the geometry changes its shape due to the dynamics of the problem. For example, it can be used for fluid and solid (IPMC) domains deformations in fluid-structure interaction (FSI) or electrostatic domain deformations in MEMS. Simulations using moving meshes, with a boundary moving in the normal direction, can sometimes need a stabilizing term to suppress the formation of local boundary segments of high curvature. This can be of particular importance in an automatic re-meshing sequence, where the re-meshing step might amplify local curvature artefacts. In the fluid-IPMC interaction of micropump simulation, we applied moving mesh to predict the IPMC configuration under electro potential. Furthermore, this deformed configuration employed to obtain fluid velocity and pressure field. Also, it is possible to use deformed IPMC mesh for post-processing of results.

4.8 Solver The process of solving a problem in COMSOL Multiphysics is hierarchical. The Study node is the coarsest level (the top level). It contains the least amount of detail and defines a Study branch (see Fig. 8). When creating a new model, we can add any of the predefined Study and Study Step Types. At any time, we can also add studies. However, we choose to add a study, a study node is added to The Model Builder including a corresponding study step (for example, Stationary), and in some cases, additional study steps. The study step represents the next level of detail. Most study steps are used to control the form of the equations, what physics interfaces are included in the computation, and what mesh is used. A study step Settings window has a Physics and Variables Selection section where inclusion and exclusion of physics interfaces and variables can be adjusted and set. There are also Common Study Step Settings for many of the study features added to a sequence. Study steps correspond to part of a solver configuration (solver sequence), which is the next level of detail.

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Fig. 8 An example of the hierarchy under the Study node (Reproduced from Ref. [3])

There are also study steps for cluster computing, for example, which corresponds to part of the Job Configurations. Solver Configurations contain nodes that define variables to solve for, the solvers and settings, and additional sequence nodes for storing the solution, for example. The solvers also have nodes that can control the solver settings in detail. Knowing The Relationship Between Study Steps and Solver Configurations is useful to help define and edit the settings before computing a solution. Bear in mind, however, the default solver settings defined by the study usually provide a good starting point. All linear system solvers work on general sparse linear systems of the form Ax  b and use LU factorization on the matrix A to compute the solution x. In doing so, they use a preordering algorithm that permutes the columns of A to minimize the number of nonzeros in the L and U factors. Popular preordering algorithms include Minimum degree, Nested dissection, and Multisection. The MUMPS and SPOOLES solvers run distributed when running COMSOL Multiphysics in distributed mode (on clusters, for example). All linear system solvers benefit from shared memory parallelism (multicore processors, for example); however, MUMPS do so to a slightly lesser extent than PARDISO and SPOOLES. The MUMPS solver works on general systems of the form Ax = b and uses several preordering algorithms to permute the columns and thereby minimize the fill-in. MUMPS is multithreaded on platforms that support multithreading and also supports solving on distributed memory architectures through the use of MPI. The code is written in Fortran 90. For further details about MUMPS, see COMSOL Manual.

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The PARDISO Solver works on general systems of the form Ax = b. In order to improve sequentially and parallel sparse numerical factorization performance, the solver algorithms are based on a Level-3 BLAS update, and they exploit pipelining parallelism with a combination of left- looking and right-looking super node techniques. PARDISO is multithreaded on platforms that support multithreading. On distributed memory architectures, the solver settings are changed to corresponding MUMPS settings if needed. The code is written in C and Fortran. COMSOL uses the PARDISO version developed by Olaf Schenk and collaborators, which is included with Intel? MKL (Intel Math Kernel Libraries). Backward differentiation formula (BDF) is a multi-step formula based on numerical differentiation for solutions to ordinary differential equations. A BDF method of order n computes the solution using a nth-grade polynomial in terms of backward differences. The default time-dependent solver for Navier-Stokes is the BDF method with the maximum order set to two. Higher BDF orders are not stable for transport problems in general nor for Navier-Stokes in particular. BDF methods have been used for a long time and are known for their stability. However, they can have severe damping effects, especially the lower-order methods. Hence, if robustness is not an issue, a model can benefit from using the generalized-α method instead. Generalizedα is a solver which has properties similar to those of the second-order BDF solver but it is much less diffusive. Both BDF and generalized-α are per default set to automatically adjust the time step. While this works well for many models, extra efficiency and accuracy can often be gained by specifying a maximum time step. It is also often beneficial to specify an initial time step to make the solver progress smoothly in the beginning of the time series.

4.9 Boundary Conditions In general, the situation consists of an IPMC immersed in the fluid and we study the IPMC-fluid interaction (see Fig. 9 for schematics details). The bulk equations must be supplemented by boundary conditions that involve the interaction between IPMC and the fluid. Thus, the essential boundary condition for the displacement field u  0 are given at the clamped boundary surface ∂u B and null traction t  0 over the remain boundary ∂t B. For the chemical field, a zero flux boundary condition n · J  0 is specified over all the boundary to simulate ion blocking surfaces. For the IPMC actuation, an electric load ψ  0 and ψ  ψ + are prescribed on the boundary surface ∂B (the left side and the right side of IPMC). At the fluid wall, no slip boundary conditions applied (v  0) and at the interface of IPMC and fluid, fluid velocity is equal to velthe ocity of IPMC deformation (v  vw ). Also open boundary conditions prescribed for fluid. We consider a situation that is a fluid-structure interaction analysis, where the surrounding fluid flow is due to the deformation of the IPMC domain B. Accordingly, a moving-mesh Arbitrary-Lagrangian-Eulerian (ALE) mode is used to make sure that the flow domain R is deformed along with the IPMC body. Further, transient effects

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101

Fig. 9 A sketch of the general fluid-structure problem (Reproduced from Ref. [3])

are taken into account in the fluid, and the deformations in the IPMC are modeled using reference configuration, say, material frame in the plane strain application mode. Thus, the weak form of the Navier-Stokes Eq. (28), solving for the velocity field and the pressure in the spatial (deformed) moving coordinate system. Fluid and IPMC structure interaction requires simultaneous two-way coupling of shared fluid and deformable solid boundary interfaces S(t). On shared boundary interfaces (i.e., fluid-IPMC-like body S), the IPMC deformation domain provides a loading condition given by Eq. 29 to the fluid domain and a velocity constraint given by Eq. 30 to the flow. In the moving mesh, the motion of the deformed mesh is modeled using Winslow smoothing, which is the default smoothing when using the predefined Fluid-Structure Interaction coupled application modes. See the Moving Mesh Application Mode in the COMSOL Multiphysics Modeling Guide for implementation details. The boundary conditions control the displacement of the moving mesh with respect to the initial geometry. At the boundaries of the IPMC-like body B, this displacement is the same as the structural deformation. At the exterior boundaries of the flow domain S, the deformation is set to zero in all directions. In all simulations, we use two-dimensional quadratic Lagrange elements to interpolate the displacement u and the solute concentration c in each layer. We use linear elements approximation for the pressure p and approximate the velocity u by means of continuous quadratic elements, the so-called Taylor-Hood elements. For the time discretization, we employ a second-order backward differentiation formula (BDF), with the time steps controlled by the numerical solver during the computations. At each time step, the corresponding discrete system

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is solved by using the default option of COMSOL (PARDISO solver), with residual tolerance levels of 10−4 . This was verified 8 to be sufficient since similar solutions were obtained at lower tolerance levels. Computations were carried out on a twelvecore 64 GB memory Dell PowerEdge T620. Required post-processing performed to produce results.

5 Analysis of Time-Dependent Bending Response of IPMC Actuator Euler-Bernoulli approach has been followed to evaluate the bending response of the IPMC actuator based on the forward kinematic model. It is assumed that IPMC bends with linear curvature with a tip angle θ , this angle varies with input voltages as shown in Fig. 4. Large deflection assumptions have been made in this equation, as the integration is done with respect to lthe ength (curvature, s) the distance along the strip rather than with respect to X, the horizontal distance.

5.1 Kinetic Energy of IPMC Actuator Using the Euler-Bernoulli equation for large deflected IPMC, position of the IMPC actuator may be obtained as: px 

dθ ds



EI EI sin θ ; p y  (1 − cos θ ) M M

M EI

, the tip

(36)

The kinetic energy of the system is the sum of the kinetic energy of the IPMC actuator and the payload. As in this case, payload is considered as zero, the kinetic energy of the system is the kinetic energy of the IPMC actuator only. Since the velocity at any point on the strip depends on the position and time, hence, the kinetic energy of the system/strip at any time is an integral over the length of the strip. Using Eq. 36, p˙ x 

EI EI ˙ p˙ y  cos θ · θ; sin θ · θ˙ M M

(37)

Therefore, the velocity at the tip point is obtained as:  v 2  p˙ x2 + p˙ 2y 

EI M

2

 cos θ 2 + sin θ 2 θ˙ 2 

Thus, the kinetic energy of the system is obtained as:



EI M

2

θ˙ 2

(38)

Study on Time-Dependent Bending Response of IPMC Actuator

K E  K Elink 

1 2 v2 l ∫ ρ A ds mv  2 2 0

103

(39)

where m is the mass of the link, ρ is the mass density and A is the cross section of IPMC. Therefore, total kinetic energy is obtained as: K Elink 

  EI 2 2 1 θ˙ , m  ρ Al m 2 M

(40)

5.2 Potential Energy of IPMC Actuator The potential energy of the system is due to the gravity and torsional spring of IPMC i.e. P E  P E 1 + P E 2 . Where, P E 1 is the potential energy due to gravity and P E 2 is the potential energy due to torsional spring. However, the potential energy due to gravity can be neglected as the motion of the patch is restricted to two dimensional coordinates. Potential energy due to torsional spring is given by: P E  P E2 

1 K θ2 2

(41)

where K is the torsional spring constant.

5.3 Governing Equation of IPMC Actuator After getting the expression for both total potential energy and kinetic energy, the Lagrangian of the system is obtained as:   1 EI 2 2 1 θ˙ − K θ 2 L  K E − PE  m 2 M 2

(42)

Therefore, governing equation of the system is expressed as: d dt



∂L ∂ θ˙

 −

  ∂L EI 2 M·M m θ¨ + K θ ∂θ M

(43)

Incorporating the damping into the system governing equation may be written as:  M m

EI M

2

θ¨ + C θ˙ + K θ

(43)

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where C is the damping factor of IPMC. The state-space representation of the system is given by:     0 0 1 d θ θ +  − K − C M 1 2 2 2 θ˙ dt θ˙ m ( EMI ) m ( EMI ) m ( EMI )

(44)

5.4 Analytic Results of Time-Dependent Bending Response of IPMC Actuator In this section, results obtained from numerical simulation based on the fabricated Ag IPMC actuator properties and the experimental tip deflection data, as shown in Table 1. The tip deflections data are taken for 30 s for each input voltage. All programs are developed in MATLAB. A function called ‘ode45’ is used to solve the differential equation in state space. Fabricated Ag IPMC actuator has been analyzed and the time-dependent bending response has been studied based on the experimental data. Experimentally bending deflection of IPMC actuator has been observed for 30 s. Both tip position and rate of change of tip position has been evaluated for the time period to show the performance of the system. The damping factor is calculated by using the formula C  2ξ ω, ξ  0.018 where, is the damping coefficient of IPMC obtained experimentally. Figure 10 shows the bode diagram of the system transfer function of the IPMC for input voltage of 0.2, 0.6, and 1.0 V respectively, while Fig. 11 is showing the performance of the system for step disturbance applied on it. From Fig. 10 it can be observed that, as the input voltage increases from 0.2 to 1.0 V with a step of 0.2 V, its gain crossover frequency (magnitude of the system at 0 dB) and phase margin of the system shows steep changes at that frequency. All the curves showed one resonant peaks between 0.253 and 1.23 rad/s for applied voltage of 0.2–1.0 V, and their phase plots were almost identical.

Table 1 Physical properties of Ag-IPMC material (Reproduced from Ref. [45])

Property

Ag-IPMC

E Elastic modulus

0.081877 GPa

l Length

0.02 m

w Width

0.005 m

h Thickness

0.0002 m

ρ Density

2,125 kg/m3

K Torsional spring constant

1.3646 × 10−5 Nm

m Mass

5.56 × 10−5 kg

C Damping coefficient

5.7712 × 10−8 Nm/rad

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Fig. 10 Response of the IPMC for each input voltage (Reproduced from Ref. [45])

Fig. 11 Step response of the system transfer function of IPMC for each input voltage (Reproduced from Ref. [45])

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Moreover, the plot also revealed that the bending angles in response to unit driving potential at different driving amplitudes were different. Figures 10 and 11 shows the change in dynamic tip displacement for input voltages 0.2, 0.6 and 1.0 V. It is observed that the change in tip displacement is maximum just immediately after the input voltage has applied and gradually diminishes before reaching the steady-state. At steady-state condition, no bending response of the IPMC is observed. Form these figures it is also observed that due to moisture loss from the IPMC actuator the amplitude also decreases as the applied voltage is increased. This section addresses modeling and analysis of time-dependent bending response of silver coated IPMC based on experimental data. Dynamic modeling of IPMC has been done after formulating the forward kinematic model of IPMC following Euler-Bernoulli approach. It is observed that as the voltage increases, the response of the IPMC diminishes in a faster rate. Further, this concept of multi-segmented IPMC patches have potential application in micro-robotics and as well as in compliant mechanisms.

6 Analysis of IPMC Actuator for Micropump Applications This section presents four sections consisting of the results and numerical simulation obtained from COMSOL by solving the governing equations mentioned in three parts. The first section is related to simulating IPMC strip deformation under electro-potential and confirms the related equations with experimental and numerical results of available works. In the next three sections, different configurations of IPMC micropump are simulated and discussed with IPMC-fluid interaction model and employing the numerical method described in the previous parts. The first micropump is based on one IPMC diaphragms and developed for drug delivery application. The second micropump with the double IPMC diaphragm is suitable for extracting fluid in biomedical application. The last section concerns the new concept of micropump consists of IPMCs cilia attached inside the channel for application in highly miniaturized microfluid systems in order to size reduction. To investigate and analyze three mentioned type of micropumps, diaphragm deformation, fluid velocity, and pressure field results are presented and discussed.

6.1 Three-Dimensional IPMC Actuator Here, the ionic polymer metal composite is viewed as a thin, 3-D cantilever resembling the characteristics of a hydrated base polymer (Nafion) sandwiched between the two metallated layers. The modeling of the complex response of the IPMC-like body to electrical and mechanical stimuli is set within the context of the 3-D theory of linear elasticity. In solid mechanics, the chemically induced deformations of the IPMC cantilever are described in terms of volumetric distortion field induced by the redistribution of the ions, which carry with them the solvent’s molecules.

Study on Time-Dependent Bending Response of IPMC Actuator Table 2 Physical properties of Ag-IPMC material (Reproduced from Ref. [45])

Parameter

107

Value

Dimension

D

6.0 × 10−11

m2 s−1

Poisson’s modulus

ν

0.3

–

Anion concentration

c0

1073

mol m−3

Temperature

T

300

K

Gas constant

R

8.3143

J mol−1 K−1

Faraday constant

F

96,487

C mol−1

Permittivity

ε

0.0177

C2 N−1 m−2

Young’s modulus

E

220

MPa

Diffusion coefficient

A set of numerical experiments is employed through a specialized computational analysis in order to analyze the behavior of the IPMC cantilever in different circumstances. The computational analysis is employed with reference to a cantilever IPMC-like body having a length 17 mm, a width of 2 mm, and a thickness 180 μm, corresponding to the sample tested in the laboratory by Wallmersperger et al. [42] which includes Nafion-117 as the base ionomer and lithium as the mobile cations. All the parameters have been implemented as presented in Table 2. The behavior of the IPMC-like cantilever as an actuator has been numerically tested by different parameters. Crucial parameters of IPMC investigation include concentration, electro-potential, charge density, and displacement profiles. The coupled multiphysics problem has been solved through a commercial finite element code (COMSOL Multiphysics 4.4) using a fixed multigrid with a refined mesh along the thickness. The response of the system to applied voltages is obtained through an application of the upper voltage starting from zero. Figure 12 shows the elastic curves of the IPMC-like body when α  1.18 × 10−5 m3 /mol at different times. The curves in Fig. 13 represent the electric boundary layer and in Fig. 14 demonstrate charge density. Small figures attached in Figs. 12 and 13 were given from simulation work by Nardinocchi [43] and experimental work by Wallmersperger [42] to validate the mathematical model and the numerical simulation results. The electrochemical effects have been extensively investigated and Figs. 13 and 14 agree with the standard results concerning the evolution of the chemical concentration and the electric potential as well as the formation of the corresponding boundary layers. The steady mechanical solution, i.e., the value attained by the tip displacement in correspondence of the stationary solution for the mobile concentration, is depicted in Fig. 15. In the last part of this section, the active bending behaviour resulting from the applied electric field (after the 20 s) is calculated and illustrated in Fig. 16. This figure presents the initial geometry of IPMC (black and white sketch) and deformed geometry (colourful sketch). From dark blue (fixed part of IPMC strip) to

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Fig. 12 Elastic curves of IPMC cantilever corresponding to α  1.18 × 10−5 m3 /mol and 50 mV (Reproduced from Ref. [3])

Fig. 13 Electrical boundary layers of IPMC cantilever (Reproduced from Ref. [3])

red section of deformed IPMC, we can observe the exact displacement of IPMC under 50 mV electro-potential. This kind of simulation could deliver valuable information to predict deformation and critical parameters of IPMC in different dimension under a variety of electro-potential.

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Fig. 14 Chemical boundary layers of IPMC cantilever (Reproduced from Ref. [3])

Fig. 15 Tip displacement of IPMC cantilever corresponding to α  1.18 × 10−5 m3 /mol (Reproduced from Ref. [3])

6.2 IPMC Micropump In the past few years, there has been a growing interest in the development of microfluidic systems for various applications including biological and chemical analysis, lab on chip diagnostics and drug delivery. Micropump is a pivotal component in such microfluidic systems as it is essential for micro liquid handling and thus micropump has become an important research area. Drug delivery for treatment of incurable diseases is very critical. In the case of diabetes, the best-known method for delivering insulin to the patient is insulin dispenser pump. Energy consumption and size of these insulin pumps is a challenge for researchers in the last years. In order to

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Fig. 16 Deformation of IPMC cantilever (Reproduced from Ref. [3])

reduce size and energy consumption a lot of actuated and structures have been developed. One of the famous commercial insulin dispenser devices is called V-Go and produced in three different models for different types of patients (see Table 3 for operational specification). Normally Diabetes patient needs 0.01–0.015 insulin unit (IU  10 μl) per hour for each kilogram of his weight [48]. In this section, the main goal of the study of IPMC micropump is to check if the IPMC diaphragm is able to provide enough deformation to generate the necessary pressure and flow of insulin by trying different electro potentials. According to current commercial insulin dispenser device and IPMC diaphragm, we suggest an IPMC based micropump for insulin dispenser device (see Fig. 17). Then, we simulate two-dimensionally the micropump shown in Fig. 18 with mathematical equations to prove the capability of micropump. Micropump made with an 8 mm length IPMC strip and 100 μm width. Figure 18 shows the total displacement of IPMC diaphragm under 100, 200, and 300 mV electro-potential. The velocity of fluid inside the chamber under 100, 200 and 300 mV electro-potential demonstrate in Fig. 19. By increasing the applied electro-potential velocity of fluid increased. Flow rates of micropump at the outlet reach respectively 17 μl/hr, 40 μl/hr and 63 μl/hr under 100, 200 and 300 mV electro-potential. Compared to Table 4 these flow rates are sufficient for are sufficient for insulin delivery in the treatment of Diabetes. Figure 20 presents the fluid pressure inside a chamber under 100–300 mV. Results show the pressure increased with increasing applied electro-potential. In general, a mathematical model of fluid-structure interaction predicts the behaviour of IPMC and velocity of the fluid in IPMC based micropump and compare the obtained results and data in Table 3 prove the ability of IPMC micropump for application in insulin dispenser device.

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Table 3 Operational specification of different types of V-Go insulin pump (Reproduced from Ref. [48]) Operational specification Pump model

V-Go 20

V-Go 30

V-Go 40

Reservoir volume

560 μL (56 IU of insulin)

660 μL (66 IU of insulin)

760 μL (76 IU of insulin)

Basal rate

8.3 μL/hr (0.6 IU/hr)

12.5 μL/hr (1.2 IU/hr)

16.7 μL/hr (1.8 IU/hr)

Bolus increments

20 μL (2 IU) 18 Actuations

20 μL (2 IU) 18 Actuations

20 μL (2 IU) 18 Actuations

Nominal bolus volume

3600 μL (36 IU)

3600 μL (36 IU)

3600 μL (36 IU)

Basal volume (24 h)

200 μL (20 IU)

300 μL (30 IU)

400 μL (40 IU)

Accuracy

±10%

±10%

±10%

Fig. 17 a Micropump schematic includes the diaphragm, a fluid chamber, inlet, and outlet; b micropump schematic in suction mode; c micropump schematic in pump mode (Reproduced from Ref. [3]) Table 4 Properties of aqueous humor (Reproduced from Ref. [49])

Characteristic

Typical value

Radius of anterior chamber (m)

5.5 × 10−3

Total width of anterior chamber (m)

11 × 10−3

Height of anterior chamber (m) Dynamic viscosity of aqueous humour (Pa s) Density of aqueous humour

(km/m3 )

2.75 × 10−3 1.0 × 10−3 1.0 × 10−3

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Fig. 18 Total displacement of IPMC under a 100 mV (1.31 s), b 200 mV (1.1 s) and c 300 mV (0.74 s) (Reproduced from Ref. [3])

6.3 Double IPMC Micropump In recent years, treatment of Glaucoma by extracting accumulated fluid has been attracted huge attention. There are some implants patents involved with the treatment of Glaucoma which drainage the aqueous humor and reduce the intraocular pressure (IOP). One of the successful commercial implants is invented by Ahmed Mateen and its detail and dimensions are shown in Fig. 20. After study all patents and implants suggested before, we presented an IPMC micropump with a double IPMC diaphragm (with 100 μm thickness) as shown in Fig. 21. Micropump consists of a chamber, two IPMC diaphragms, inlet, outlet and IPMC microvalves. According to the patents and current implants, two diaphragms of IPMCs selected to reduce the dimension we need to generate the required flow rate. Compared to mentioned implants, dimension and energy consumption reduced with this structure. The main advantage of this IPMC implant would lie in control we have to change the flow rate, which depends on patient conditions. In current implants and surgical methods after 3 years, the path created for passing fluid start to narrowing (accumulation of tissues and particles) and aqueous humor will have difficulties to reach the implants and pump out of eye chamber. Here, by increasing electro potential applied to the implants and control the pump cycle it is possible to solve this problem and maintain the required flow rate. Due to the very low energy consumption of actuators, the maintenance cost decreases practically.

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Fig. 19 Fluid velocity inside chamber under a 100 mV (1.31 s), b 200 mV (1.1 s) and c 300 mV (0.74 s) (Reproduced from Ref. [3])

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Fig. 20 Ahmed flexible valve model FP7 (Reproduced from Ref. [3])

Fig. 21 Schematic of IPMC based micropump with a double IPMC diaphragm (Reproduced from Ref. [3])

A two-dimensional numerical simulation study has been performed using commercial software Comsol Multiphysics to simulate an IPMC based micropump. The numerical model solves the required governing equations to account for the electrosolid and fluid-solid coupling effects. In the electro-solid part, application of an electro-potential to the both IPMCs causes deflection of the diaphragms. In the fluidsolid interaction (FSI) approach, fluid flow described using Navier–Stokes equations are combined with the solid mechanics at the interface of FSI boundaries. The physics interface uses an Arbitrary Lagrangian-Eulerian (ALE) method to combine the fluid flow formulated using a Eulerian description and a spatial frame with solid mechanics formulated using a Lagrangian description and a material (reference) frame. The spatial frame also deforms with a mesh deformation that is equal to the displacements u of the solid within the solid domains. The mesh is free to move inside the fluid domains, and it adjusts to the motion of the solid walls. This geometric change

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Fig. 22 Total displacement of IPMC under 200 mV in 1 s (Reproduced from Ref. [3])

of the fluid domain is automatically accounted for in COMSOL Multiphysics by the ALE method. IPMC diaphragms are fixed at its edges and considered as a zerodisplacement boundary condition. The boundary conditions for the fluid model are no-slip at the fluid-wall interface and FSI and pressure boundary conditions at both the inlet and outlet of the micropump. The flow is modeled using single-phase and incompressible flow assumptions via the Laminar Flow Module. In the present model, numerical investigations are carried out for IPMC diaphragms with 100 μm thickness and 4 mm length and fluid properties of water are used. The employed mesh is considered with an element size in the range of 1.6–112 μm for IPMCs diaphragm and fluid. Figure 22 shows the deformation of IPMC diaphragms and the pumping mechanism of IPMC micropump with the application of 200 mV electro-potential. The result shows that the deformation is the maximum at the center and zero at the edges of IPMC diaphragms, as expected theoretically. So this deformation pushes the fluid out of the micropump chamber through the outlet. Figures 23 and 24 show simulation result for velocity and pressure field inside the pump chamber with the application of 200 mV. Both the velocity and pressure of the fluid observed at the maximum near the outlet. Figures 25, 26 and 27 show result for deformation of IPMC, velocity, and pressure inside the pump chamber with the application of 400 mV. Flow rate generated by the micropump increases with increasing the applied electro-potential. Our results show that during the pump mode, the membrane deflects into the chamber thus increasing the pressure of the fluid inside the chamber. As a result, the fluid in the chamber exits through the outlet. Figures 28, 29 and 30 demonstrate the simulation results in supply mode (suction) consist of IPMC deformation, fluid velocity and pressure contour in the pump chamber under 600 mV electro-potential. From the results, it can be observed that during the supply mode, the IPMCs diaphragms deflects out of the chamber and thus creating a negative pressure inside the chamber. Due to this, the fluid enters the chamber through the inlet. These results confirm the function of IPMC micropump. The streamline of pressure in pump chamber show the formation of high pressure in inlet and outlet.

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Fig. 23 Fluid velocity under 200 mV in 1 s (Reproduced from Ref. [3])

Fig. 24 Pressure of fluid under 200 mV in 1 s (Reproduced from Ref. [3])

Fig. 25 Total displacement of IPMC under 400 mV in 0.55 s (Reproduced from Ref. [3])

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Fig. 26 Fluid velocity under 400 mV in 0.55 s (Reproduced from Ref. [3])

Fig. 27 Pressure of fluid under 400 mV in 0.55 s (Reproduced from Ref. [3])

Fig. 28 Total displacement of IPMC under 600 mV (supply mode) in 0.4 s (Reproduced from Ref. [3])

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Fig. 29 Fluid velocity under 600 mV (supply mode) in 0.4 s (Reproduced from Ref. [3])

Fig. 30 Pressure of fluid under 600 mV (supply mode) in 0.4 s (Reproduced from Ref. [3])

Figures 31, 32 and 33 represent the velocity of the fluid under 600, 800 and 1000 mV. These results confirm that the fluid velocity is influenced considerably by applied electro-potential (Fig. 34). The maximum fluid velocity of 175 μm/s reached under 1000 mV electro-potential. The relation between electro-potential and deformation of IPMC diaphragm can be seen in Fig. 35. In Fig. 36, flow rates through the outlet of the micropump are shown under various electro potentials. Results show that by changing applied electro-potential we can reach the required flow rate of micropump. In this section, a novel low energy IPMC micropump is presented and simulated, in which the fluid is driven by deformation of two IPMC diaphragms. The flow fields generated in an IPMC micropump by deformation of diaphragms are simulated by considering the coupled electrical, mechanical and fluidic fields. The coupled model considers the deformation of IPMCs diaphragms simultaneously with the prediction of fluid flow. The simulation results show that IPMC micropump is able to generate a flow rate of 2.6 μl/min. These present results quantitatively provide the dependence of flow rates to the electro-potential. The pressure, velocity, and flow rate prediction models developed in the present study can be utilized to optimize the design of IPMC

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Fig. 31 Fluid velocity under 600 mV in 0.4 s (Reproduced from Ref. [3])

Fig. 32 Fluid velocity under 800 mV in 0.15 s (Reproduced from Ref. [3])

Fig. 33 Fluid velocity under 1000 mV in 0.11 s (Reproduced from Ref. [3])

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Fig. 34 Fluid velocity as a function of electro-potential (Reproduced from Ref. [3])

Fig. 35 The relationship between displacement and electro-potential (Reproduced from Ref. [3])

based micropumps that are being used for the treatment of Glaucoma. Considering all points mentioned about aqueous humor and its production IPMC micropump, we need to transfer the fluid at maximum 2.5 μl/min to the treatment of Glaucoma. It depends on the degree of Glaucoma we need to apply electro-potential up to 1 V. By referring to Fig. 36 for each patient condition we can choose the applied electropotential to extract the fluid and balance the IOP (Fig. 37).

6.4 Mesh Convergence In order to gauge how reasonable a finite element solution to a partial differential equation is a given mesh, a common strategy is to refine the mesh, compute the solution on the finer mesh, and use the solutions on the two meshes for a qualitative

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Fig. 36 Flow rate as a function of electro-potential (Reproduced from Ref. [3])

Fig. 37 Schematic of concept IPMCs cilia attached in the microchannel. Cilia deformation is like paddle movement in boating (Reproduced from Ref. [3])

comparison. The theory of the finite element method (FEM) makes these comparisons quantitative by estimating the convergence order of the FEM error on a sequence of progressively finer meshes obtained by uniform mesh refinement. Results obtained for different element sizes to validate the accuracy of numerical simulations. Related to study mesh convergence four element sizes from 0.6 to 8 μm employed to simulate a double IPMC micropump under 500 mV. Governing equations were a solver for three unknown variables including displacement, pressure, and velocity of the fluid. Results listed in Table 3. Conducting the convergence study in this manner shows how to quantify the convergence of FEM solutions and brings out the potential benefit of using higher order elements. As shown in Table 3 the changes in displacement of IPMC due to apply finer mesh elements are in order of 0.0003 to 0.001 μm. In addition, fluctuation of the velocity of the fluid and fluid pressure observed in the order of 4.5 μm/s and 0.05 Pa respectively. This convergence study illuminates that the numerical solution obtained from COMSOL is reasonable and confirm the accuracy of results in higher order elements.

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Due to the similarity of presented examples for applications of IPMC in micropump (fluid-IPMC interaction), this study can be extended to all numerical simulations.

6.5 IPMC Cilia Micropump IPMCs are an emerging class of electroactive polymers that are finding growing applications. A key application of IPMCs is within biomimetic miniature swimmers, as originally proposed by Shahinpoor [50] and later developed by different research groups like Guo et al. [51], Yeom et al. [52] and Shen et al. [53] In these studies, vibrating IPMCs are utilized as underwater propulsors, based on their ability to operate in wet environments with low actuation voltage. An IPMC-based tadpole robot was designed and realized by Kim et al. [54], and the thrust and swimming speed were measured as functions of the vibration frequency. The thrust generated by IPMCs with patterned electrodes was investigated by Kim et al. [55] using a fixed propulsor connected to a load cell. Jellyfish robots incorporating IPMCs have been studied by Chen et al. [56]. A curved IPMC actuator was considered and the thrust and swimming speed of the jellyfish robot was measured in his work. IPMCs were used as the tail of robotic fish of varying dimensions, and theoretical insights were proposed to shed light on the physics of propulsion by Shen et al. [53] and like Guo et al. [51]. Chen et al. [56] designed a robotic ray using two IPMC wings. Hubbard et al. [57] investigated the possibility of exploring patterned IPMCs to enable bending and twisting motions for underwater propulsion of robotic fish. Cha et al. [58] presented a comparison between IPMC-based biomimetic swimmers and other robots. As the interest in IPMC-based underwater propulsion grows, the need for understanding and predicting the hydrodynamics generated by an IPMC vibrating underwater in a viscous fluid has become more pressing. Abdelnour et al. [59] presented a two-dimensional (2D) numerical study on the flow induced by an IPMC vibrating along its fundamental mode. Numerical results presented there to demonstrate the central role of vorticity generation and shedding from the IPMC tip on thrust generation. The effect of higher modes in IPMC-based underwater propulsion was investigated by Lee et al. [60]. The main focus of these noteworthy efforts is to demonstrate the feasibility of IPMCs as a miniaturized, manoeuvrable, and wireless biomimetic underwater robotic swimmer. As we mentioned above, understanding the hydrodynamics generated by an IPMC deformation underwater is central to the design of such biomimetic swimmers. In this chapter, we propose the study and simulation of IPMC cilia submerged in water to detail the fluid kinematics and kinetics in the vicinity of an IPMC deformation along its fundamental structural mode. The first time Toonder et al. [61] suggested the application of cilia actuators in microchannel structure for manipulation of fluid (see Fig. 38). In this section, we try to prove this concept with numerical simulation. IPMCs cilia deformation simulated by IPMC-fluid interaction mathematical model

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Fig. 38 Electro potential applied to IPMC cilia under water (Reproduced from Ref. [3])

Fig. 39 Fluid velocity inside a chamber under 200 mV in 0 s (Reproduced from Ref. [3])

and results presented here confirm the ability of IPMCs cilia to manipulate and pump the fluid inside the channel. We consider an IPMC cilia of thickness 100 μm, 3 mm length and a box of water with 20 mm width, 8 mm height. The coupled governing equations in section five with some change in boundary conditions employed to predict the behaviour of IPMC cilia-fluid interaction under oscillation electro-potential. IPMC cilia excited by a sinusoidal voltage applied across its electrodes as shown in Fig. 38 to impose an oscillation deformation in IPMC cilia tip. Tip displacement of the IPMC at the driving frequency f  1 Hz reaches a maximum of 100 μm. Figures 40, 41, 42, 43 and 44 illustrate the instantaneous fluid velocity over one cycle of oscillation at f  1 Hz for 5 consecutive time instants t  0, t  T /4, t  T /2, t  3T /4 and t  T where T  1/f is the oscillation period. These figures show IPMC cilia have the ability to generate flow in fluid and it is possible to manipulate the fluid by oscillation movement of IPC cilia. Figures 39, 40, 41, 42 and 43 present the pressure field in the channel. The fluid flow is shown by the velocity vectors superimposed on the contours (Figs. 45, 46, 47 and 48). Here, the IPMC cilia integrated micropump is investigated in two different configurations. In the first case ten IPMC cilia employed in the upper and bottom side of the channel. Then 800 mV electro-potential applied to IPMC cilia. Figure 49 presents schematic of a configuration of IPMC cilia integrated micropump and the mesh detail in IPMC and the fluid that we applied for simulating IPMC cilia micropump. Fig-

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Fig. 40 Fluid velocity inside chamber under 200 mV in 0.25 s (Reproduced from Ref. [3])

Fig. 41 Fluid velocity inside a chamber under 200 mV in 0.5 s (Reproduced from Ref. [3])

Fig. 42 Fluid velocity inside a chamber under 200 mV in 0.7 s (Reproduced from Ref. [3])

Fig. 43 Fluid velocity inside a chamber under 200 mV in 1 s (Reproduced from Ref. [3])

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Fig. 44 Pressure inside chamber under 200 mV in 0 s (Reproduced from Ref. [3])

Fig. 45 The pressure inside a chamber under 200 mV in 0.25 s (Reproduced from Ref. [3])

Fig. 46 Pressure inside chamber under 200 mV in 0.5 s (Reproduced from Ref. [3])

Fig. 47 The pressure inside a chamber under 200 mV in 0.75 s (Reproduced from Ref. [3])

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Fig. 48 The pressure inside a chamber under 200 mV in 1 s (Reproduced from Ref. [3])

Fig. 49 Schematic of IPMC cilia micropump case one (Reproduced from Ref. [3])

Fig. 50 Fluid velocity inside a chamber under 800 mV in 0.1 s (Reproduced from Ref. [3])

Fig. 51 Fluid velocity inside a chamber under 800 mV in 0.2 s (Reproduced from Ref. [3])

ures 50, 51, 52, 53 and 54 illustrate fluid velocity inside the chamber of micropump under 800 mV electro-potential. Results show that IPMC cilia micropump generates a flow velocity of 100 μm/s at the outlet. In all cases of IPMC cilia micropumps, open boundary conditions applied at the right side of the channels and the outlet defined at the left side. Fluid velocity and pressure fields show fluid flow which is pumped by IPMCs cilia. Particle tracing analysis is performed with the Particle Tracing Module in COMSOL to analyze pumping performance of IPMCs cilia. Figure 55 evaluates the pumping

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Fig. 52 Fluid velocity inside a chamber under 800 mV in 0.3 s (Reproduced from Ref. [3])

Fig. 53 Fluid velocity inside a chamber under 800 mV in 0.4 s (Reproduced from Ref. [3])

Fig. 54 Fluid velocity inside a chamber under 800 mV in 0.5 s (Reproduced from Ref. [3])

performance by computing the trajectory of suspended particles due to fluid flow in the channel. IPMCs cilia deform and cause the particle to move toward the outlet. In another structure, due to the increase the IPMC cilia number attached to the channel, the space between the cilia decreased, in the result, the affected fluid area by IPMC cilia deformation joined together. The fluid velocity and pressure in the chamber are studied to investigate the effect of cilia deformation on the fluid in this structure. Figures 56, 57, 58, 59 and 60 illustrate the velocity field in the chamber. Compared to the structure before, there is no recirculation center in fluid and fluid velocity is more homogenous through the channel. At the beginning, the maximum of the velocity observed at the IPMCs tip because of highest displacement. So there is a fluid flow with the highest velocity pass through the tip of IPMCs cilia which came from inlet flow at the upper side and down the side of the channel. Because of the outlet position, the velocity of the fluid at the upper side of the channel is higher than the lower side of the channel. The fluid flow with the highest velocity moves toward the center of the channel after 0.5 s. To study the outlet location effect on fluid flow, in the last scenario of this configuration, the outlet moves toward the center. Figures 61, 62, 63, 64, 65, 66 and 67 present the change of fluid flow during 0.7 s. High-velocity route moves from near channel wall to center by passing time. Due to the outlet position, a symmetric fluid flow created near the channel wall and IPMCs cilia suck fluid equally into the channel. Compared to fewer cilia number configurations the fluid inside the channel is more homogeneous and continuous. The fix fluid region in front of each cilia

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Fig. 55 Particle trajectories inside channel under 800 mV in 0.1–0.5 s (Reproduced from Ref. [3])

Fig. 56 Fluid velocity inside a chamber under 800 mV in 0.1 s (Reproduced from Ref. [3])

Fig. 57 Fluid velocity inside a chamber under 800 mV in 0.2 s (Reproduced from Ref. [3])

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Fig. 58 Fluid velocity inside a chamber under 800 mV in 0.3 s (Reproduced from Ref. [3])

Fig. 59 Fluid velocity inside a chamber under 800 mV in 0.4 s (Reproduced from Ref. [3])

Fig. 60 Fluid velocity inside a chamber under 800 mV in 0.5 s (Reproduced from Ref. [3])

Fig. 61 Fluid velocity inside a chamber under 800 mV in 0.1 s (Reproduced from Ref. [3])

Fig. 62 Fluid velocity inside a chamber under 800 mV in 0.2 s (Reproduced from Ref. [3])

breaks by the highest number of cilia and removes the recirculation area at the center of the channel between two cilia. Hence, depending on the type of fluid flow and velocity required for the specific application, the outlet position, number of IPMCs cilia, the distance between them, configurations and applied electro potential could be different. In the second case, we investigate a smaller channel with 9 mm length and 2.5 mm height consist of ten IPMCs cilia attached to the inside wall. Figure 68 shows the

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Fig. 63 Fluid velocity inside a chamber under 800 mV in 0.3 s (Reproduced from Ref. [3])

Fig. 64 Fluid velocity inside a chamber under 800 mV in 0.4 s (Reproduced from Ref. [3])

Fig. 65 Fluid velocity inside a chamber under 800 mV in 0.5 s (Reproduced from Ref. [3])

Fig. 66 Fluid velocity inside a chamber under 800 mV in 0.6 s (Reproduced from Ref. [3])

Fig. 67 Fluid velocity inside a chamber under 800 mV in 0.7 s (Reproduced from Ref. [3])

configuration of cilia micropump, mesh detail and boundary conditions applied for simulation. The open boundary condition is applied to the right side of the channel and mapped mesh configuration with symmetric distribution option selected for IPMC and triangle for the fluid. In this case, the free space (for fluid flow) between two arrays of IPMCs cilia is less than the case one. External stimuli of 200 mV electropotential applied to IPMCs cilia. Deformation of IPMCs under the electro-potential and working mechanism of cilia micropump illustrated in Fig. 70. Deformation of all cilia to the left sucks the fluid from the inlet (open boundary) at the right side

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Fig. 68 Schematic of IPMC cilia integrated micropump case two (Reproduced from Ref. [3])

Fig. 69 The total displacement of IPMC cilia under 200 mV inside the chamber (Reproduced from Ref. [3])

Fig. 70 Fluid velocity inside a chamber under 200 mV in 0.7 s (Reproduced from Ref. [3])

of the channel and push to the outlet at the lower left side of the channel. The next four Figs. 12, 13 and 14) demonstrated the fluid velocity due to tip displacement of cilia inside the channel. Recirculation area at the center of the channel with the highest fluid velocity shown in Figs. 11 and 12. In addition, these results confirmed that IPMC cilia integrated micropump achieved a maximum fluid velocity of 7 μm/s at the outlet (Figs. 69, 71, 72 and 73). Particle tracing modeled in this case and the calculated particle trajectory due to deformation of IPMCs cilia and push the fluid into channel presented in Fig. 74.

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Fig. 71 Fluid velocity inside a chamber under 200 mV in 0.8 s (Reproduced from Ref. [3])

Fig. 72 Fluid velocity inside a chamber under 200 mV in 0.9 s (Reproduced from Ref. [3])

Fig. 73 Fluid velocity inside a chamber under 200 mV in 1.0 s (Reproduced from Ref. [3])

Particles reached the velocity of 7 μm/s under 200 mV in the outlet. In the last section fluid velocity of cilia micropump with higher electro-potential (800 mV) presented in Figs. 76, 77, 78, 79 and 80. Higher fluid velocity at the outlet without recirculation observed (Fig. 75). A disadvantage of using the micro-pump assembly and of using micropumps, in general, is that they have to be, in some way, integrated into microfluidic systems. This means that a large area must be devoted to the micropump, which results in the bigger microfluidic device. This means that the size of the microfluidic systems will increase. It would, therefore, be useful to have a microfluidic system by IPMC cilia flow controller, which is compact and cheap, and nevertheless easy to process. The beam-shaped cilia of IPMC can change shape and orientation as a response to an external electric potential.

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Fig. 74 Particle trajectories inside a chamber under 200 mV in 0.7–1 s (Reproduced from Ref. [3])

Fig. 75 Fluid velocity inside a chamber under 800 mV in 0.1 s (Reproduced from Ref. [3])

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Fig. 76 Fluid velocity inside a chamber under 800 mV in 0.2 s (Reproduced from Ref. [3])

Fig. 77 Fluid velocity inside a chamber under 800 mV in 0.3 s (Reproduced from Ref. [3])

Fig. 78 Fluid velocity inside a chamber under 800 mV in 0.4 s (Reproduced from Ref. [3])

Fig. 79 Fluid velocity inside a chamber under 800 mV in 0.5 s (Reproduced from Ref. [3])

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7 Summary In this chapter, a multi-physics mathematical model is presented and analyzed for simulation of the behaviours of ionic polymer metal composites strips immersed into the fluid, subject to an externally applied electric field. The balance law of chemoelectro-mechanics and constitutive equations of the involved fields were depicted. The formulation is capable of describing the bending deformation of the IPMC, the distributions of diffusive ionic concentrations, and electric potential in the IPMC strip. Three-dimensional numerical simulations are carried out for an IPMC strip. The numerically simulated results are in good agreements with published FEM solutions, which validates the presently developed models. With the derived coupled formulation, the IPMC behaviour in the sensor as well as in actuator applications can be described. By utilizing this model, we are able to explore some applications of IPMC, and by coupling governing equation of this model with Navier-Stokes equation, we are able to simulate IPMC-fluid interaction and IPMC micropump for the medical applications. These coupled equations explained the fluid velocity and pressure field in the exterior fluid. Furthermore, we focused on the application of IPMC as a micropump in the treatment of Diabetes and Glaucoma and analyzed the IPMC micropump as a device in the treatment procedure. The simulation was done in both cases and results show IPMC micropump can generate sufficient flow rate with low electric potential for insulin delivery in Diabetes case and removing excess aqueous humor in Glaucoma case. In the last part, we successfully simulated and proved a new concept for IPMC cilia integrated micropump. The result showed IPMC cilia attached inside the channel can deform and generate a flow rate of the fluid at very low electro-potential. Also, this structure can be employed as a micro flow controller to manipulate and control the fluid by altering the electric potential. Simulation results collected valuable data for fabricating microfluidic device by IPMC cilia. We believe that the investigation of IPMC cilia micropump in detail and deeper is necessary for future research. Acknowledgements This work was supported partly by Basic Science Research Program (No. 2015R1D1A1A02060006) and partly by Korea-Canada Cooperative Development Program (No. 2018K1A3A1A74064262) funded by the National Research Foundation of Korea (NRF).

References 1. Chu, W.-S., Lee, K.-T., Song, S.-H., Han, M.-W., Lee, J.-Y., Kim, H.-S., Kim, H.-S., Kim, M.S., Park, Y.-J.: Review of biomimetic underwater robots using smart actuators. Int. J. Precis. Eng. Man. 13(7), 1281–1292 (2012). https://doi.org/10.1007/s12541-012-0226-9 2. Frecker, M.I.: Recent advances in optimization of smart structures and actuators. J. Intell. Mater. Syst. Struct. 14(4–5), 207–216 (2003). https://doi.org/10.1177/104538903031062 3. Ranjbarzadeh, S.: Modeling, simulation, and applications of ionic polymer metal composites. Dissertation, Universidade Federal do Rio de Janeiro (2017)

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Ionic Polymer-Metal Composite Membranes Methods of Preparation Fatma Aydin Unal, Hakan Burhan, Fatima Elmusa, Shukri Hersi and Fatih Sen

Abstract The ionic polymer-metal composite membranes are generally perfluorinated membranes such as Nafion and Flemion. These composites are mostly electroactive materials and they can be used as activators and sensors. The electrochemical-mechanical properties of ionic polymer-metal composites depend on many factors, such as the nature of the solvent, the morphology of the electrodes, and also other factors. This chapter provides general information about the preparation methods of the ionic polymer-metal composite membranes. Keywords Membrane · Method · Ionic polymer · Metal composites

1 Introduction Ionic polymer-metal composites (IPMC) are perfluorinated ionic membranes such as Nafion and Flemion which promotes the electroactive polymer (EAP) materials with low voltage and high detection capability [1–7]. Besides, the membrane contains stabilized mobile cation in the polymer chains fixed by anionic groups and also water as a solvent. The most commonly used membrane is seen as Nafion and is preferred due to its excellent chemical stability, mechanical strength, and they are mostly used for ionic polymer-metal composite actuators and some sensor systems [8]. The Nafion membrane is generally known to be an ionic polymer that has ionic parts on the edges of the backbone polymer. In Nafion based ionic polymer-metal composite, SO3 − forms ionic separations on each side. For this reason, the anions are fixed to the backbone polymer of these groups while the cations are in the membrane in the F. A. Unal Faculty of Engineering, Metallurgical and Materials Engineering Department, Alanya Alaaddin Keykubat University, 07450 Alanya, Antalya, Turkey e-mail: [email protected] F. A. Unal · H. Burhan · F. Elmusa · S. Hersi · F. Sen (B) Sen Research Group, Faculty of Art and Science, Department of Biochemistry, Dumlupinar University, 43100 Kutahya, Turkey e-mail: [email protected] © Springer Nature Switzerland AG 2019 Inamuddin and A. M. Asiri (eds.), Ionic Polymer Metal Composites for Sensors and Actuators, Engineering Materials, https://doi.org/10.1007/978-3-030-13728-1_6

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free form [9]. In the wet state, the cations are encircled by water molecules. This is because of the electrostatic force that interacts between the water molecule and the oxygen atoms of the cation. The hydrated cations also form conjugation pairs with the anion groups to be neutralized in the absence of any mechanical and electrical stimulation. The reason for the use of ionic polymer-metal composite as a sensor or activator is that it can generate stresses when mechanical deformation occurs and can change shape when electrical stimulation is provided. For this purpose, some studies made in literature related to methods of preparing ionic polymer metal composite membranes are given below.

2 Ionic Polymer-Metal Composite (IPMC) Membranes Methods of Preparation A new nanostructured electrode surface design for ionic polymer-metal composites was carried out with the help of platinum nano thorn groups by V. Palmer et al. These assemblies were made by the currentless coating method [7, 9] with the help of many edges. The coatings and formed ionic polymer-metal composites are given in Table 1 [7]. Using the procedures described above, eight ionic polymer-metal composite samples were prepared from impregnation reduction methods from 1 to 8. The time intervals for the saturation reduction method are 2 h for the cleaning operation for each layer cycle, two hours for the reduction, and 3.5 h for saturation in the Pt complex for each case. After this, the number of chemical deposition processes was allowed to remain constant, and the first layer was applied so that the conductance of the electrode surface in the samples was obtained uniformly. These samples were prepared in an overnight process and were stored in a solution of lithium chloride (1 M LiCl) [7, 9]. With the help of these samples, the newly designed ionic polymer-metal com-

Table 1 Notation of IMPC actuators prepared in the study

Notation

No. of primary coatings

No. of secondary coatings

Measured flexural modulus (MPa)

Pt (1)

1

1

126

Pt (2)

2

1

137

Pt (3)

3

1

139

Pt (4)

4

1

153

Pt (5)

5

1

159

Pt (6)

6

1

174

Pt (7)

7

1

177

Pt (8)

8

1

189

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posite actuators were prepared and they provide greatly improved electromechanical performance thanks to nano thorn mounting electrodes compared to conventional Pt electrodes. Their efficiency increased 3–5 times in both travel ranges and maximum blocking force. Nanothorn assemblies increased both the efficiency of the large-area electrode and the progress of the boundary of the electrolyte interface. These newly developed nanoporous Pt electrodes provide an increase in their performance. These materials can also be used in various gas sensor applications and polymer electrolyte membrane fuel cells [7, 9, 10]. One of their applications is the actuators and they consist of an electrically active polymer layer (Nafion® 20%) which is sandwiched by two metal electrodes as stated above. These ionic polymer-metal composite membranes is very important due to the formation of the driving voltage. Its deformation mechanism and the perfluorosulfonic acid membrane structure are shown in Fig. 1a, b. Generally, cations in ionic polymer-metal composite actuators, such as Na+ , Li+ , and H+ exhibit more bending performance than the others in the literature [11]. Two types of physical methods were used to produce these types of electrodes. One of the methods is the evaporation of metals like gold on the other Nafion® membrane while the other metal like silver nanopowder is cast on the Nafion® membrane. During this process, gold evaporation was performed by the electron beam evaporator to obtain a thickness of about 300 nm. For this aim, the multi-step evaporation method was preferred in order to reduce the effect of both thermally stable metal and polymer

Fig. 1 a Structure of perfluorosulfonic acid membrane and b schematic diagram of the IPMC deformation

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Fig. 2 The processing flow of the IPMC actuator with Ag nano-powders Table 2 The electroless coatings of Ag using silver nitrate and glucose reaction

2AgNO3 + 2NaOH → Ag2 O↓ + H2 O + 2NaNO3 Ag2 O + 4NH3 + H2 O → 2Ag(NH3 )2OH 2Ag(NH3 )2OH + C6 H12 O6 → 2Ag↓ + C6 H11 O7 NH4 + 3NH3 + H2 O

membranes. For instance, in Fig. 2, to create the ionic polymer metal composite, the device is accomplished by the introduction of a new process with the addition of Ag nanopowders including Ni electro-forming steps, Ag-free coatings, embossing and casting operations. Briefly, the process is implemented as follows: 1. Using dilute Nafion® solution, dissolve silver nanopowders (Ag–Nafion® 5%, 0.2 g/ml). 2. After the preparation of Ag–Nafion® 5%, 0.2 g/ml, a 20% Nafion (R) electrode is poured into the film to provide better properties. The membrane is then cooked at 60 and 100 °C at a two-stage temperature. 3. Covering the adhesion layer and embossing it as one of the two structures. 4. Current-free silver coatings are required to eliminate the resistance. The ionic polymer-metal composite surface is cleaned with an alkaline solution. The electrolyzed coating of silver is then carried out using the silver nitrate (AgNO3 ) for the glucose reaction given in Table 2. Table 3 lists the concentrations and doses of the working temperatures by Ag electroless coating reaction. In this process, the redox reaction spontaneously occurred.

Ionic Polymer-Metal Composite Membranes Methods of Preparation Table 3 The concentrations and doses of Ag electroless coatings reaction as well as working temperatures

143

NaOH

AgNO3

NH4 OH

C6 H12 O6

Concentration (M)

0.5

0.6

15

0.1

Dose (ml)

30

25

1.2

35

Fig. 3 IPMC and IP2 C fabrication technique

5. The formation of nickel with the help of electrochemical techniques to obtain conduction in operation temperature of 70 °C [11]. Generally, Nafion® 117 was used as a base material while the electrode was metallic platinum in ionic polymer metal composites and PEDOT/PSS in IP2 Cs (by polymerization in situ) (Fig. 3). A scheme of related interactions among the composites is reported in Fig. 3. To manufacture ionic polymer-metal composites, the experiment includes roughening and cleaning of the Nafion® film, ion-exchange with tetrammineplatinum chloride (Pt(NH3 )4 Cl2 ) and double chemical reduction (respectively a primary coatings by sodium borohydride (NaBH4 ) and a secondary coatings by hydroxylamine hydrochloride, NH2 OH–HCl, and hydrazine, NH2 NH2 ) [12–14]. In particular, ionic polymer-metal composites were manufactured in presence of polyvinylpyrrolidone, as dispersing agent, (PVP10, Mw  10,000, 0.001 M, added either during the primary and the secondary coatings) and by 3 sequential primary coatings (adsorption for 20 h in Pt(NH3 )4 Cl2 solution/reduction by NaBH4 ). The formation of reduced platinum resulted in metallic grey layers deposited on both surfaces of the Nafion® membrane. Regarding fabrication of IP2 Cs, a typical film deposition experiment is based on the formation of the organic conductor electrode by polymerization starting from 3,4-ethylendioxytiophene (EDOT) and sodium polystyrene sulfonate (NaPSS, added to increase the solubility of EDOT) [15]. Upon addition of Fe(NO3 )3 ·9H2 O polymerized EDOT leading to a dark blue layer deposited on both

144 Table 4 Fabricated specimens

F. A. Unal et al.

Nafion® IP2 C

117

(1 h)

IPMC

Solvent

Specimen code

H2 O

Nafion® 117/H2 O

EG

Nafion® 117 EG

H2 O

IP2 C/1 h/H2 O

EG

IP2 C/1 h/EG

H2 O

IPMC/H2 O

EG

IMPC/EG

sides of the membrane piece. The membrane, which is rinsed with double distilled water, was performed in H2SO4 1 M (1 h), to display the interchange of Fe3+ with H3 O+ ions, and then in H2 O (1 h). To obtain related composite and IP2 C with alcohol like ethylene glycol as a solvent, water was eliminated drying the devices at 100 °C for 24 h; then, the devices were immersed overnight in a beaker containing ethylene glycol and, at last, heated to 60 °C for 1 h. All fabricated specimens are given in Table 4 [13, 16]. As references, pure Nafion® 117 membranes, immersed with the H2 O and EG, were used. The performances of ionic polymer metal composites and IP2 Cs based on Nafion® 117 are a function of the kind of the electrode and the solvent, without highlighting a significant superiority in the overall performance of one of the two technologies. Nevertheless, the lower production time and cost suggest that IP2 Cs are a possible alternative to ionic polymer-metal composites [13]. Besides, poly(vinyl alcohol-co-ethylene) [P(VA-co-E)] is a nontoxic and biocompatible polymer which has considerable chemical resistance, high mechanical strength, low fouling potential, and excellent film-forming properties. Since it could be compatible with Nafion owing to their similar structures that contain both hydrophobic and hydrophilic part, Nam et al. has chosen poly(vinyl alcohol-coethylene) as a blending partner with Nafion to fabricate a blend ion exchange membrane [13, 17]. In Nam’s study, the membrane was prepared as follows. Preparation of the Nafion casting membrane is such that 10 ml of Nafion dispersion are poured using a 2.5 × 2.5 × 0.5 cm Teflon mold and the solution is allowed to stand overnight at 30 °C. The mixture membranes, NPVAE-34 and NPVAE-55, are prepared in the solution overnight at 60 °C in Teflon mold. Table 5 shows the details of the materials after drying [13, 17]. Photographic images of prepared specimens of Nafion, NPVAE-34, and NPVAE55 membranes are also shown in Fig. 4. Nafion—the only membrane is transparent, but as P(VAco-E) blended into Nafion, it was seen that as the amount of solution in this membrane increased, the density of the Nafion increased, and thus, the transparency was disappeared. The prepared NPVAE-34 and NPVAE-55 membranes are costeffective up to 30 and 50% respectively, compared to pure Nafion (based on the current prices of the chemicals as of 2016).

Ionic Polymer-Metal Composite Membranes Methods of Preparation Table 5 Composition, drying condition, and thickness of the membranes: Nafion, NPVAE-34, and NPVAE-55

145

Specimens

The weight ratio of P(VA-co-E): Nafion

Drying condition

Thickness (μm)

Nafion

0:100

30 °C, overnight

280

NPVAE-34

34:66

60 °C, overnight

290

NPVAE-55

55:45

60 °C, overnight

350

Fig. 4 Photographic images of the prepared membranes: a Nafion, b NPVAE-34, and c NPVAE-55 (Size  2.5 × 2.5 cm)

Besides, to cast high-quality blend membranes, the chemical structure of the blend membranes was illuminated by Fourier Transform Infrared spectroscopy (FT-IR), and thermal properties were characterized by DSC (Differential Scanning Calorimetry). In addition to these techniques, ionic polymer-metal composites were also characterized by the cross-sectional morphologies by Scanning Electron Microscopy (SEM). Also, the chemical composition was characterized by Scanning Electron Microscopy, Electron Diffraction Analysis system (SEM-EDX). The electromechanical performances including displacement testing and blocking forces were also really important for ionic polymer-metal composites [12, 13]. Furthermore, in M. M. Hasani-Sadrabadi’ study, membranes were sulfonated at different amounts of dissolved N,N-dimethylacetamide (DMAc), and stirred for 24 h, dried at room temperature, and then, the solvent was removed. The obtained mixtures were stirred at 80 °C for 8 h and was concentrated. The concentrated solution was casted on a clean glass plate, and then, dried, at room temperature for one night, then, stored at 70 °C for average 9 h and at 120 °C overnight. For the sulfonated membranes and Nafion® 117, the membranes were handled in 3 wt% hydrogen peroxide for 30 min, washed and performed for 1 h in deionized water. Membranes were performed again in one molar sulfuric acid for 30 min and washed with deionized water [18]. In another study by T. Hwang and his colleagues, a novel ionic polymer-metal composite was manufactured. For this purpose, the Nafion solution was prepared at

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Table 6 Notation and compositions of prepared Nafion and Nafion/P(VA-co-E) blend membrane specimens Specimens

P(VA-co-E)/Nafion (vol.%)

Casting Temp. (°C)

Thickness (μm)

Remarks

Nafion

0

30

280

Flexible

NP30

30/70

60

290

Flexible

NP50

50/50

60

350

Rigid

30 and 50% and mixed with P(VA-co-E) solution. Membranes were prepared using solution casting method. The structures possessed by this membrane were visualized using differential scanning calorimetry and Fourier transform infrared spectroscopy. Membrane-associated ionic polymer-metal composites were formed by electrolytic plating to deposit electrodes on their surfaces. The 2-point probe of all prepared ionic polymer-metal composites had a measured electrode surface resistance of less than 2.0 /cm. Mixed Nafion membrane ionic polymer-metal composite devices exhibit similar properties as a result of a significant reduction in Nafion property [1, 19]. The composition of these materials is given in Table 6. The concentrations of both Nafion dispersion and P(VA-co-E)/DMSO solution were set to 5 wt% [1]. In another study, T. Hwang et al. worked on the preparation of the ionic polymer-metal composite based on Flemion. The gold electrode accumulation was made according to the impregnation-reduction method. In this method, a gold complex [Au (Phen) Cl2 ]+ was introduced into the Flemion membrane with K+ as counterions with ion exchange. The amount of gold complex in the exchange solution is sufficient to direct the change. Therefore, Flemion membrane with the gold complex is immersed in de-ionized water for reduction. A small amount of 5 wt% sodium sulfite solution is slowly added to the media. For the thin membrane, acid and deionized water are used for the rinsing step after 6 h of reduction. As a final step, all samples were deposited to provide ion exchange for 1 mol/L KOH solution. The obtained samples have ionic polymer-metal composite as the counter ion with K+ Flemion base [20]. In some of the works, some other composites which are related to the metal composites can also be used in many of the applications from sensors to catalysis, supercapacitors, etc. [21–36].

3 Conclusions As a conclusion, the ionic polymer-metal composite membranes are mostly used materials in activators, sensors and some other devices and they are generally perfluorinated membranes such as Nafion and Flemion. These composites are mostly electroactive materials and they have superior electrochemical-mechanical properties. These properties depend on many factors, such as the nature of the solvent, the morphology of the electrodes, and also other factors. This chapter provides general

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information about the preparation methods of the ionic polymer-metal composite membranes. As a vision of this chapter, the demand in the ionic polymer-metal composite membranes will be increased in future.

References 1. Hwang, T., Palmer, V., Nam, J., Lee, D.C., Kim, K.J.: A new ionic polymer—a metal composite based on Nafion/poly(vinyl alcohol-co-ethylene) blends. Smart Mater. Struct. 24, 105011 (2015) 2. Shahinpoor, M., Cohen, Y.B., Xue, T., Simpson, J.O., Smith, J.: Ionic polymer-metal composites (IPMC) as biomimetic sensors and actuators-artificial muscles. In: Proceedings of SPIE’s 5th Annual International Symposium on Smart Structures and Materials, 1998, San Diego, CA, pp. 3324–3327 (1998) 3. Chen, Z., Um, T., Bart-Smith, H.: Ionic polymer-metal composite artificial muscles in bioinspired engineering research: underwater propulsion. In: Berselli, G. (ed.) Smart Actuation and Sensing Systems—Recent Advances and Future Challenges, pp. 223–248 (2012) 4. Chen, Z., Um, T.I., Bart-Smith, H.: A novel fabrication of ionic polymer–metal composite membrane actuator capable of 3-dimensional kinematic motions. Sens. Actuators A 168, 131–139 (2011) 5. Kazem, B., Khawwaf, J.: Estimation bending deflection in an ionic polymer metal composite (IPMC) material using an artificial neural network model. Jordan J. Mech. Ind. Eng. 10, 123–131 (2016) 6. Bhat, N.D.: Modeling and precision control of ionic polymer metal composite. Masters Thesis, Texas A&M University (2003) 7. Palmer, M.V., Pugal, D., Kim, K.J., Leang, K.K., Asaka, K., Aabloo, A.: Nanothorn electrodes for ionic polymer-metal composite artificial muscles. Sci. Rep. 4, 6176 (2014) 8. Zamani, S., Nemat-Nasser, S.: Controlled actuation of Nafion-based ionic polymer-metal composites (IPMCs) with ethylene glycol as a solvent, Proc. SPIE; Smart Struct. Mater. 159–163 (2004) 9. Vinh, N.D., Kim, H.M.: Ocean-based electricity generating system utilizing the electrochemical conversion of wave energy by ionic polymer-metal composites. Electrochem. Commun. 75, 64–68 (2017) 10. Shahinpoor, M., Kim, K.J.: Ionic polymer-metal composites: IV. Industrial and medical applications. Smart Mater. Struct. 1, 197–214 (2005) 11. Chunga, C.K., Funga, P.K., Honga, Y.Z., Ju, M.S., Lin, C.C.K., Wu, T.C.: A novel fabrication of ionic polymer-metal composites (IPMC) actuator with silver nano-powders. Sens. Actuators B 117, 367–375 (2006) 12. Yu, M., Shen, H., Dai, Z.D.: Manufacture and performance of ionic polymer-metal composites. J. Bionic Eng. 4, 143–149 (2007) 13. Pasquale, G.D., Graziani, S., Gugliuzzo, C., Pollicino, A.: Ionic polymer-metal composites (IPMCs) and ionic polymer-polymer composites (IP2Cs): effects of electrode on mechanical, thermal and electromechanical behavior. AIMS Mater. Sci. 4, 1062–1077 (2017) 14. Bhandari, B., Lee, G.Y., Ahn, S.H.: A review on IPMC material as actuators and sensors: fabrications, characteristics, and applications. Int. J. Precis. Eng. Manuf. 13, 141–163 (2012) 15. Aabloo, A., Luca, V.D., Pasquale, G.D., Graziani, S., Gugliuzzo, C., Johanson, U., Marino, C., Pollicino, A., Puglisi, R.: A new class of ionic electroactive polymers based on green synthesis. Sens. Actuator A Phys. 249, 32–44 (2016) 16. Wang, Y., Chen, H., Wang, Y.: Casting membranes for ionic polymer-metal composite actuators. Soc. Plast. Eng. (SPE) (2013) 17. Nam, J.: Ionic polymer-metal composite actuators based on Nafion blends with functional polymers, University of Nevada (2016)

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18. Hasani-Sadrabadi, M.M., Ghaffarian, S.R., Majedi, F.S.: Preparation and characterization of novel ionic polymers to be used as artificial muscles. Homayoun Moaddelda Iran. J. Pharm. Sci. Summer 4, 217–224 (2008) 19. Tiwari, R.: Ionic polymer-metal composite mechanoelectric transduction: effect of impedance. Int. J. Smart Nano Mater. 3, 275–295 (2012) 20. Wang, J., Kimura, M., Taya, M.: The bio-inspired design of tactile sensors based on ionic polymer metal composites. Proc. ICCM B 4, 1–11 (2009) 21. Sen, B., Demirkan, B., Savk, A., Karahan Gülbay, S., Sen, F.: Trimetallic PdRuNi nanocomposites decorated on graphene oxide: a preferred catalyst for the hydrogen evolution reaction. Int. J. Hydrogen Energy 43, 17984–17992 (2018) 22. Eris, S., Da¸sdelen, Z., Yıldız, Y., Sen, F.: Nanostructured Polyaniline-rGO decorated platinum catalyst with enhanced activity and durability for Methanol oxidation. Int. J. Hydrogen Energy 43(3), 1337–1343 (2018) 23. Eris, S., Da¸sdelen, Z., Sen, F.: Enhanced electrocatalytic activity and stability of monodisperse Pt nanocomposites for direct methanol fuel cells. J. Colloid Interface Sci. 513, 767–773 (2018) 24. Sahin, ¸ B., Aygün, A., Gündüz, H., Sahin, ¸ K., Demir, E., Akocak, S., Sen, ¸ F.: Cytotoxic effects of platinum nanoparticles obtained from pomegranate extract by the green synthesis method on the MCF-7 cell line. Coll. Surf. B Biointerfaces 163, 119–124 (2018) 25. Sen, ¸ B., Akdere, E.H., Savk, ¸ A., Gültekin, E., Göksu, H., Sen, ¸ F.: A novel thiocarbamide functionalized graphene oxide supported bimetallic monodisperse Rh-Pt nanoparticles (RhPt/TC@GO NPs) for Knoevenagel condensation of aryl aldehydes together with malononitrile. Appl. Catal. B Environ. 225(5), 148–153 (2018) 26. Eris, S., Da¸sdelen, Z., Sen, F.: Investigation of electrocatalytic activity and stability of Pt@f-VC catalyst prepared by in-situ synthesis for Methanol electrooxidation. Int. J. Hydrogen Energy 43(1), 385–390 (2018) 27. Gulçin, I., Taslimi, P., Aygün, A., Sadeghian, N., Bastem, E., Kufrevioglu, O.I., Turkan, F., Sen, ¸ F.: Antidiabetic and antiparasitic potentials: inhibition effects of some natural antioxidant compounds on α-glycosidase, α-amylase and human glutathione S-transferase enzymes. Int. J. Biol. Macromol. 119, 741–746 (2018) 28. Sen, B., Demirkan, B., Levent, M., Savk, A., Sen, F.: Silica-based monodisperse PdCo nanohybrids as highly efficient and stable nanocatalyst for hydrogen evolution reaction. Int. J. Hydrogen Energy. https://doi.org/10.1016/j.ijhydene.2018.07.080 29. Koskun, Y., Savk, ¸ A., Sen, ¸ B., Sen, ¸ F.: Highly sensitive glucose sensor based on monodisperse palladium nickel/activated carbon nanocomposites. Anal. Chim. Acta 1010, 37–43 (2018) 30. Sen, ¸ B., Aygün, A., Savk, ¸ A., Akocak, S., Sen, ¸ F.: Bimetallic palladium-iridium alloy nanoparticles as highly efficient and stable catalyst for the hydrogen evolution reaction. Int. J. Hydrogen Energy. https://doi.org/10.1016/j.ijhydene.2018.07.081 31. Sen, B., Savk, A., Sen, F.: Highly efficient monodisperse nanoparticles confined in the carbon black hybrid material for hydrogen liberation. J. Colloid Interface Sci. 520, 112–118 (2018) 32. Sen, B., Kuyuldar, E., Demirkan, B., Onal-Okyay, T., Savk, A., Sen, F.: Highly efficient polymer supported monodisperse ruthenium-nickel nanocomposites for dehydrocoupling of dimethylamine borane. J. Colloid Interface Sci. 526, 480–486 (2018) 33. Günbatar, S., Aygun, A., Karata¸s, Y., Gülcan, M., Sen, ¸ F.: Carbon-nanotube-based rhodium nanoparticles as highly-active catalyst for hydrolytic dehydrogenation of dimethylamineborane at room temperature. J. Coll. Interface Sci. 530, 321–327 34. Park, K., Yoon, M., Lee, S., Choi, J., Thubrikar, M.: Effects of electrode degradation and solvent evaporation on the performance of ionic-polymer–metal composite sensors. Smart Mater. Struct. 19, 075002 (2010) 35. Park, K.: Characterization of the solvent evaporation effect on ionic polymer-metal composite sensors. J. Korean Phys. Soc. 59, 3401–3409 (2011) 36. Min, Y., Qing Song, H., Yan, D., Dong Jie, G., Jia Bo, L., Zhen Dong, D.: Force optimization of ionic polymer-metal composite actuators by an orthogonal array method. Mech. Eng. 56, 2061–2070 (2011)

Ionic Polymer-Metal Composite Actuators Operable in Dry Conditions Fatma Aydin Unal, Hakan Burhan, Sumeyye Karakus, Gizem Karaelioglu and Fatih Sen

Abstract Ionic polymer-metal composite (IPMC) have great applications, in terms of being utilized in areas, such as actuators, artificial muscles and more. These composite can also be used for robotics, biomedical and biomimetic applications. Among these applications, ionic polymer metal composite actuators are mostly preferred devices used in some applications, such as biomimetic robotics, biomedical devices, manipulation systems etc. For these types of actuators, ion-exchange polymer-metal composite (IPMC) is very attractive and active materials, and the synthesis and characterization conditions are very important parameters that should be thought. For this reason, this chapter provides information on ionic polymer metal composite actuators that can be operated under dry conditions. Keywords Ionic polymer · Metal composite · Actuators · Dry conditions · IPMC

1 Introduction The ionic polymer-metal composite has a charged structure which is positioned in the electro-active polymer [1–5]. The used polymers are very important electro-active polymers. For instance, the ionic actuator is a current-free electroactive polymer (EAP) that causes a bend to occur as a result of exposure to a tensile stress to its thickness as shown in Fig. 1 [6, 7]. The mentioned electroactive polymers (EAPs) are polymers that show a change in size or shape when electrical stimulation is applied [8]. These ionic electroactive polymers form a composite with metal on the surface which is electrically conductive, undergoing a large amount of bending deformation while sustaining large forces F. A. Unal Faculty of Engineering, Metallurgical and Materials Engineering Department, Alanya Alaaddin Keykubat University, 07450 Alanya, Antalya, Turkey F. A. Unal · H. Burhan · S. Karakus · G. Karaelioglu · F. Sen (B) Sen Research Group, Faculty of Art and Science, Department of Biochemistry, Dumlupinar University, 43100 Kutahya, Turkey e-mail: [email protected] © Springer Nature Switzerland AG 2019 Inamuddin and A. M. Asiri (eds.), Ionic Polymer Metal Composites for Sensors and Actuators, Engineering Materials, https://doi.org/10.1007/978-3-030-13728-1_7

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Fig. 1 Ionic polymer-metal composite (IPMC) actuator demonstrated for Pt/Nafion composite EAP [6]

when electrically stimulated. There are many electro-active polymers like ionic polymer metal composite (IPMCs) [9, 10]. Among these electroactive polymers, ionic polymer metal composite is one of the most promising materials. These composites are synthetic nanomaterials that exhibit deformation under an applied voltage or electric field [6, 11]. Ionic polymer-metal composite has some features and also advantageous as follows: (1) (2) (3) (4) (5) (6)

The drive voltage is low which is about 1.0–4.0 V. Very high response time [6]. The softness of the material (E  2.2 × 108 Pa). Miniaturization. Stable and durable towards bending cycles. Activation in solutions [6, 10].

When the low voltage is applied, ionic polymer metal composite indicates large deformation and low impedance. They work the best performance in ambient conditions. They can also be used as self-contained encapsulated actuators to operate in dry conditions [6, 10, 12].

2 Actuation Principle of Ionic Polymer-Metal Composite The theory for the operation of ionic polymer metal composite is attributed to the unique nature of the base polymer membrane for ionomer based ionic polymer metal composite. The ionomer is specially designed and synthesized to allow selective diffusion of mobile cations as well as solvent [13]. The basic actuation principle is illustrated in Fig. 2. This solvent stream forms an electro-osmotic pressure differentially and performs an exchange process toward the anode direction of the ionic polymer metal composite [12, 14]. Generally, ionic polymer metal composite is composed of two parts: ionic polymer membrane and the metal electrode. These membranes such as Nafion or Flemion are electrochemically plated on the surface of the membrane with metals such as gold and/or platinum [16]. When an ionic polymer metal composite is fabricated, it contains cations, anions, and water molecules inside. When a low voltage is applied to it (generally less than 5 V), hydrated mobile ions (either cations or anions) in the ion exchange membrane move towards anode or cathode, forming volumetric gradient or

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Fig. 2 Illustration of the actuation principle of IPMC. a After a voltage is applied; b before a voltage is applied [12, 15]

pressure gradient between electrodes on both sides. This gradient is responsible for the deformation of the ionic polymer metal composite [16–18]. In most of the studies in literature, Nafion was used for the ion exchange membrane [19]. An ionic polymer membrane known as Nafion is used to increase ion mobility in dry environments. Since Nafion has a negatively charged group of sulfonate, these anions were fixed after being converted into the ion exchange membrane. When the ionic polymer metal composite was submerged in water and were electrically excited, the cations in the ion exchange membrane were hydrated with water molecules and moved towards the anode. This movement of the hydrated cations causes the pressure gradient between the electrodes to bend ionic polymer metal composite. The hydrated cations moving towards the anode expand towards the anode by shrinking the cathode side and bend the cathode towards anode direction. When an alternating current (AC) is applied, the anode and the cathode are alternately changed and cause the ionic polymer metal composite to continuously bend to two opposite directions. Figure 2 shows the movements of hydrated cations and water molecules in the deforming ionic polymer metal composite membrane [19]. In literature, membranes were generally prepared as follows: The cast membrane of Nafion was formed by allowing 10 ml of the Nafion dispersion to stay at 30 °C overnight in a Teflon mold having dimensions of 2.5 × 2.5 × 0.5 cm throughout the overnight [19]. The blend membranes, NPVAE-34 and NPVAE-55, were prepared by casting the solution in a Teflon mold at 60 °C overnight with 10 mL of the mixed Nafion solution of P (VA-co-E)/DMSO (5 wt%) solution [1, 17]. For this purpose, Table 1 gives a general information about the materials after drying.

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Table 1 Composition, drying condition, and thickness of the membranes: Nafion, NPVAE-34, and NPVAE-55 [19]

Samples

Weight ratio of P(VA-co-E): Nafion

Drying condition

Thickness (μm)

Nafion

0:100

30 °C, overnight

280

NPVAE-34

34:66

60 °C, overnight

290

NPVAE-55

55:45

60 °C, overnight

350

The photographic images of Nafion, NPVAE-34, and NPVAE-55 are shown in Fig. 3. As shown in this figure, the membrane was transparent, but when P (VAcoE) was mixed with Nafion, the membrane of the mixture became opaque, and the hardness of the membranes increased as P (VA-co-E) content increased [17–19]. Besides, as shown in Table 2, the exact ratios of compounds were determined with the help of the weight ratio of PI as mg/cm2 [9, 15, 19]. Besides, literature indicated that the humidity was very effective on the performance of ionic polymer-metal composite actuators [3, 15, 20].

Fig. 3 Photographic images of the prepared membranes: a Nafion, b NPVAE-34, and c NPVAE-55 (Size  2.5 × 2.5 cm) [19] Table 2 The composition of materials used in the prepared specimens after casting [19] Sample name

Wt% ratio (PI: Nafion)

Total amount of PI (mg/cm2 )

Total amount of Nafion (mg/cm2 )

Casting condition

Thickness (μm)

Nafion

0:100

–

46.3

50 °C, 5 h

340

NPI-6

6:94

3.01

45.2

85 °C, overnight

278

NPI-12

12:88

5.42

38

285

NPI-18

18:82

7.22

31.9

290

NPI-30

30:70

11.7

27.4

70 °C, 2 days

310

PI

100:0

38.6

–

85 °C, overnight

350

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3 Bucky Gel Actuator The first dry actuator using “bucky gel” was produced via layer casting and used ionic liquid was an imidazolium solution containing C3 H4 N2 cations and BF4 anions. This gel permits reasonable bending stiffness with high bending displacement potential. The actuator consisting of this gel has a bimorph configuration with a polymersupported bucky gel electrode layer. This ensures fast and permanent activation in dry environments with low applied voltages [21–23].

4 Selection of Electrolyte The selection of electrolyte is a very important parameter in ionic polymer composite membranes. For instance, an electrolyte is required to make a carbon nanotube (CNT) based actuator. For this purpose, the ionic polymer membranes are being used to promote dry-environment activation to improve the functions of the actuator. Nafion can be used as an electrolyte permitting dry media actuation. However, at the same time, it may be limiting factor due to its poor mechanical and electrical properties. By the way, it allows the ion storage and migration. On the other hand, it does not allow a fast/responsive transition [23].

5 Effect of Humidity and Temperature The humidity and temperature level may affect the performance of the actuator. Generally, Nafion is the preferred electrolyte for dry media activation. In literature, if the humidity is high, an increase in actuator performance is detected. The performance of an actuator may be damaged in dry and cool environments because of the loss of water molecules in the Nafion membrane [23–25]. Besides, S. G. Lee et al. studied the effect of metal coating and the results show that the solvent and cation movements are limited with the increasing of the thickness of coating material [26]. The movement of the cation and solvent is very limited in the direction of thickness due to the relatively stable electric field on the electrodes. Also, the drying is decreased due to evaporation, reduced surface cracks, and low heat build-up. For this reason, the ionic polymer metal composite sprayed with metal can also provide higher actuation force, greater actuator displacement and increased durability [22]. The dry surface resistances of gold sprayed ionic polymer metal composite and normal ionic polymer metal composite (sized 5 mm × 30 mm) were measured by varying the distances between the two existing probes. Figure 4 shows the measured surface resistance of metal-coated dry ionic polymer metal composite and the ones without the metal coating. It can clearly be verified that metal-sputtered ionic polymer composite has lower surface resistivity than normal ionic polymer metal composite. Low surface

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Fig. 4 The surface resistance of dry IPMCs [22]

resistance helps to decrease heat build-up during operation. This can also decrease the solvent evaporation [27]. The dry surface resistances of metal sprayed ionic polymer metal composite and normal ionic polymer metal composite which are a size 5 mm x 30 mm were measured by varying the distances between the two existing probes [28]. Ionic polymer-metal composite has also a major potential for biomedical sensors [29–32]. First, it is flexible and lightweight; second, the structure is a thin film type. Hence, it can be used as a surface mount sensor. Third, it can be produced in a small size [17]. Finally, sustainable performance in wet environments reveals the potential to be used as an embedded biomedical sensor [33]. Besides, the output signal from the ionic polymer metal composite sensor changes constantly. Also, after the solvent is entirely dried, the ionic polymer metal composite sensor can not generate an output signal. The level of hydration of ionic polymer metal composite greatly influences the output signal. Insulator-gel-coating techniques successfully improved their physical properties, resulting in a longer operating time in the air. For this aim, non-volatile ionic liquids were used to prohibit evaporation and electrolysis of the inner solvent in an IPMC sensor and an actuator, respectively. Such approaches using ionic liquids greatly developed the performance of ionic polymer-metal composite actuators and sensors in terms of having higher input voltages and a longer service life [22, 30, 34]. For the output signal in the air, when ionic polymer metal composite is placed under air bending cycle for 5 h in the air, several output signals selected by bending angle after removal of offsets are displayed in Fig. 5b. This figure indicates the hydrated ionic polymer metal composite measured at the beginning of the bending movement. The dashed line shows the measured signal after one hour (120 bending cycles). Though the positive (top) peak voltages increase at first, then diminish continually, the output size (peak-to-peak) voltage with drying out of the inner solvent continually decreased. The steady decline in peak-to-peak voltages refers to

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Fig. 5 Output signals in a DI water and in b air [32, 36]

the decreasing amount of solvent during ionic polymer metal composite drying. The magnitude of exponential decay from the first order is continually decreased until it is fully diminished within 3 h. The output signal is decreased and the noise rises as ionic polymer metal composite cannot generate any discriminative signal after 5 h. The ionic polymer metal composite sensor only produced the available output signals in about three hours. The duration of this study varied in terms of the prepared film on the electrodes [35, 36]. Table 3 displays the deionized (DI) water and air parameter values. In this table, while the ionic polymer metal composite sensor was drying in air, the weight of the ionic polymer metal composite was decreased due to water evaporation. To measure the degree of dryness in the ionic polymer metal composite, the weight change corresponding to the working time of the ionic polymer metal composite sensor in the air was measured and the weight percentage change according to the original weight is demonstrated in Table 3. As shown there, the capacity (C) is proportional to the dielectric constant of the material between the two plates. Water has a larger dielectric constant than air. For this reason, the capacitance of the ionic polymer metal composite sensor decreases when the ionic polymer metal composite is drying in the air because of the decreased dielectric constant. The obtained results support the hypothesis. As the capacitance decreases slowly in the air, it keeps relatively constant values in the deionized water, as displayed in Table 3. The variations of ion diffusion resistance (Rc) of deionized water and air are also indicated in Table 3. The cations combine with water molecules due to the attracting forces between water molecules and cations. When the ionic polymer metal composite sensor operates in deionized water, a large number of water molecules penetrate into the polymer because of continuous crack formation on the surface electrodes. This results in a rise in resistance because of the movement of the hydrated cations. However, a continuous decline in resistance is monitored in the air due to the evaporation of

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Table 3 Parameter variations in DI water and air [32] Parameter

Condition

Time (min) Beginning

C (mF)

20

40

60

90

120

DI

3.71

4.03

4.38

4.21

3.81

Air

3.32

2.61

2.95

2.87

2.29

1.40

Rc (×10 )

DI

5.31

5.47

5.10

5.42

5.56

6.57

Air

6.11

5.49

4.62

4.01

3.38

3.11

R (K)

DI

3.54

3.73

3.83

3.98

3.30

2.61

Air

3.57

2.03

9.57

19.56

76.74

110.00

DI

13.30

13.06

13.74

12.80

10.67

9.09

Air

11.92

10.87

11.52

10.09

7.81

3.75

100

93.95

88.22

85.67

84.39

83.12

(×10−8

K C/θ B )

Weight (%) of IPMC in air

4.15

uncombined water molecules by cations, which leads to a reduction in resistance to the movement of the hydrated cations [32, 36]. Finally, the influence of solvent evaporation on an ionic polymer metal composite sensor was examined using a resistance circuit model. In deionized water, the ionic polymer metal composite sensor retained the relative constants of the parameters during the test. However, the parameters were changed while the ionic polymer metal composite was drying in air. In addition, the property of the dried ionic polymer metal composite, which had lost about 17% of its weight due to water evaporation, resembled a zero-order system. This demonstrates the potential of ionic polymer metal composite as a static sensor that can easily detect surface changes in a human body resulting from muscle movement [32–37]. Some of the other composites can also be used in many of the applications from catalysis to sensors [38–55].

6 Conclusions As a conclusion, ionic polymer metal composite (IPMC) have been using in many areas such as robotics, biomedical and biomimetic applications as actuators, artificial muscles, sensors etc. For these applications, ionic polymer metal composites are mostly preferred, very attractive and suitable materials. However, the synthesis and characterization conditions are very important parameters and by using these parameters, the ionic polymer-metal composite actuators can be operated in various applications under dry conditions.

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Pressure Sensors Based on IPMC Actuator Gokhan Topcu, Tugrul Guner and Mustafa M. Demir

Abstract Pressure sensors provide information regarding with the magnitude and distribution of force along the interface. To characterize the force as a measurand, pressure sensors convert the force into especially electrical signals. Ionic polymermetal composites have received great interest in pressure sensor technology apart from soft biomimetic actuator applications. In this chapter, we provide further insight into the IPMC materials in pressure sensor applications in terms of system design, working principle, and preparation. In addition, the current status of applications and markets of pressure sensors is described with reference to some published patents. Moreover, their historical evolution, various designs, and classification are also discussed.

1 Introduction Pressure is an expression of mechanical force applied normal to a surface per unit area. In today’s world, pressure sensors play a vital role that serves a practical application to quantify the force using either liquid or gas pressure. A typical system consists of a sensor chip, a receiver, and a converter (Fig. 1). Numerous types of sensors have been employed so far for controlling and monitoring pressure in various fields. Automotive industry can be a good example of where oil, water, and tire pressures need to be measured simultaneously. In the design of sensor chips (transducer), the phase of a measurand, magnitude, and distribution of pressure over contact area are considered as a critical parameter for manufacturing efficacious device. The advanced use of wireless technologies, semiconductor-based sensors, and industrial automation are expected to lead the market growth over the forecast period. High-performance, reliable, and cost-efficient pressure sensors are desired and this demand leads to the high investment of R&D activities. According to a recent report delivered by Grand View Research, Inc., pressure sensor market is estimated to reach approximately 12 G. Topcu · T. Guner · M. M. Demir (B) Department of Materials Science and Engineering, Izmir Institute of Technology, Izmir, Turkey e-mail: [email protected] © Springer Nature Switzerland AG 2019 Inamuddin and A. M. Asiri (eds.), Ionic Polymer Metal Composites for Sensors and Actuators, Engineering Materials, https://doi.org/10.1007/978-3-030-13728-1_8

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Fig. 1 Components of a sensor system

Fig. 2 Recent (2014–2017) and estimated (2018–2024) size of the pressure sensor market [1]

billion $ for the USA in next decade showing a rapid increase of industrial demand (Fig. 2) [1]. For a number of centuries, the measurement of pressure has attracted significant attention. The first recorded observation was performed by Galileo Galilei, an Italian inventor and scientist lived in the 17th century. Galilei has a patent for a machine to pump water from a river for the irrigation of land. During the process of the machine, it was observed that 10 m were the limit to which the water can rise in the pump. However, there was no sensible explanation for this observation. After almost one century, first, empirical pressure measurement was carried out by Evangelista Torricelli using the tube, which was one meter long and filled with mercury, hermetically closed from one end. The column of mercury invariably fell to about 760 mm

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leaving an empty space above its level. These two experimental findings were interpreted and formulized in 1648 by Blaise Pascal, who suggested “pressure” definition and its uniform distribution on an exerted surface. First attempt to measure atmospheric pressure via aneroid barometer system instead of a liquid was made by Lucien Vidie in 1844. The system was based on spring, whose extension was mechanically amplified on an indicator mechanism and patented in 1849. After a couple of decades, electrical measurement methods were started to develop using transduction strategy. In 1930, Roy W. Carlson who worked on the construction of dams, invented unbonded wire strain gauge to measure compressive strain in concrete structure [2]. The strain gauge was designed into a sealed metal casing with flanges and covered with a rubber sleeve. The setup was mostly made of multiple resistive wires whereby half of those were stretched and the other half were only strained under the influence of the measured strain. Although the device worked well for the purpose, there were a number of limitations, for instance, narrow pressure range, fragile wires, and transportation issue. Alternatively, carbon resistor and bonded-wire strain gauges were designed. These systems provided various advantages for measurement. For instance, the carbon resistor based gauge had a small size and low weight that offered simplicity for transportation, whereas bonded wire gauge was able to carry out the more precise measurement. The discovery of piezoresistivity in semiconductors led to the beginning of a new era for pressure measurement [3, 4]. In 1954, Charles S. Smith discovered the piezoresistive properties of semiconducting silicon and germanium, which could not be explained in terms of previously known mechanisms [5]. Within the following decade, the first piezoresistive sensor was developed and improved by using various designs. The resistance element was diffused into a silicon substrate. The system was not equipped with backing, which provides the silicon substrate to be directly bonded to the metal diaphragm by means of epoxy. Compared to the conventional metal wire and foil gauges, the semiconductor strain gauge had an output almost 100 times larger. Nevertheless, the use of adhesive causes creeping and hysteresis that were still disadvantages for measurement. To overcome the obstacles originating from adhesive, diffused semiconductor strain gauges were made by using photolithography. Besides surpassing of hysteresis and creeping, the design had a number of advantages such as small size, better resistivity, higher fatigue limit, and improved sensitivity. Today, this design still plays a key role in sensor technology. In the 1970s, capacitive pressure transducers were discovered, which had quartz having doughnut-like shape. Inside of the doughnut, there were a vacuum cavity, which consisted of the two capacitor plates. On the outside of the body, two thin film sensor elements were deposited [6]. The disadvantage of capacitive pressure transducers is their sensitivity to change in temperature. Therefore, the quartz body was replaced by ceramic one [7]. The transducer had upper and lower halves, which were bonded together by molded glass. The cavity between both halves is filled with a very small absolute pressure. This design was still used in today’s modern sensors.

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2 Pressure Sensors 2.1 Types of Pressure Sensors In daily life, many pressure sensor designs have been employed for various applications. Therefore, sort of these devices may be classified according to their design and working principle. They can be divided into three groups in terms of their design, which are load cells, tactile sensors, and pressure indicating films. On the other hand, the sensor systems can be classified as electromechanical, potentiometric, capacitive, or reluctive, based on the working principle. One of the commonly-used devices is load cells, the operation principle of which is based on measuring the force acting on a given surface. These can be installed on hoppers, reactors etc. to control their weight capacity. There are various load cell designs, which are based on electromechanical, pneumatic, and hydraulic systems. For the electromechanical process, the load cell perceives a change in pressure by measuring the change in resistance of a Wheatstone Bridge Circuit. The set up in the device (strain gauge load cell) works in a way that sensors are placed on each of the resistors and measure the change in resistance of each individual resistor upon a change in pressure. Piezoelectric materials are also employed in load cells. The operation principle is alike upon deformation as the strain gauge load cells. However, the voltage output is generated by deforming the basic piezoelectric material. Pneumatic type load cells appear to be a better option due to the absence of fluids that may contaminate the process if the diaphragm ruptures. In cases where the operation is in a remote location, the most applicable load cell is the hydraulic one. Hydraulic load cells having a piston that raise the hydraulic fluid as force increases. The pressure may be locally indicated or transmitted for remote indication or control. Tactile pressure sensors are used to measure dynamic changes in pressure [8, 9]. Compared to load cells, these sensors work at lower magnitudes. Pressure sensor arrays are large grids, namely tactel. Each tactel is capable of detecting specifically exerted forces. The arrays are usually formed of thin film. Tactel-based sensors provide a high resolution ‘map’ of the applied pressure. In addition to spatial resolution and force sensitivity, systems-integration issues such as wiring and signal routing are important for this kind of sensors. These are primarily used as analytical tools in the manufacturing and R&D processes by engineers and technicians and have been adapted for use in the robotic industry. Examples of such sensors available for consumers include arrays built from conductive rubber, lead zirconate titanate (PZT), poly (vinylidene fluoride) (PVDF) and its composites, and the metallic capacitive sensing elements. In a typical pressure measurement, experimental conditions may differ from theoretical assumptions. For instance, it is necessary to record a peak pressure when the interface pressure is non-uniformly distributed. In this sense, pressure-indicating films are good candidates for the examination of pressure distribution. Instead of electrical output, the films exhibit an optical signal where colourimetric sensors can be achieved. Lee et al. [10] fabricated mechanochromic polymer composite films and

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examined their optical responses upon mechanical force. Coloration of the films is based on photonic interference due to periodic dielectric structure, known as BraggSnell Law [11, 12]. The underlying sensing mechanism of the pressure indicating the photonic film is the change in interplanar distance between two nanostructure arrays. During the applying lateral strain, interplanar distance is lowered, which causes a dramatic decrease in the wavelength of reflected light. Another type of pressure indicating the film is designed by taking advantage of plasmonic nanoparticles [13]. Han et al. reported the colorimetric pressure sensors including Au nanoparticles showing plasmonic absorption in poly(vinyl pyrolidene). Due to their tunable optical absorbance (thanks to the change in color of the particles upon aggregation from red to blue), embedded-plasmonic particle aggregates disintegrate each other upon application of mechanical force. Therefore, color change directly indicates the magnitude of the applied pressure. Besides the classification based on the working mechanism and design, the sensor systems can be sorted according to the type of pressure sensitive element. As previously described, the pressure sensitive element can provide either optical or electrical signal. From an electrical point of view, they can be divided into electromechanical [14–16], potentiometric [17], capacitive [18], or reluctive [19] signals. Piezoelectric and electroactive polymer (EAP) systems are commonly used as pressure element for electromechanical systems. In a typical piezoelectric crystal-based sensor, when pressure is applied to the crystal, it twists and a small amount of electric charge is generated. In this type of pressure sensor, the change in pressure is proportional to the intensity of the electric charge. Its precision reaches up to 0.1 MPa. Potentiometric sensors work on the principle that change in pressure causes the arm to move back and forth across a potentiometer, and a resistance measurement can be read. It consists of an arm attached to the elastic pressure-sensing element. The elements do have a working range due to their mechanical limit, which restricts the applicability of the system. Potentiometric pressure sensors are efficient candidates to use as a cheap detector evaluating a coarse process. The sensor is designed to operate within a wide pressure range with a high sensitivity (~0.07 MPa). A capacitive sensor consists of a parallel plate capacitor and an electrode connected to a diaphragm, which is exposed to the pressure on one side and the reference pressure on the other side. Displacement along the diaphragm is detected by an electrode, which leads to change in capacitance. An attached circuit detects the change in capacitance, which in turn read the voltage in relationship to pressure change. When there is a change in pressure, the flexible element reacts by moving the ferromagnetic plate, which leads to a change in the magnetic flux of the circuit, i.e. something measurable. One of the conditions in which reductive pressure is employed is when the inductive sensor does not generate a precise enough measurement. Therefore, pressure range is relatively narrow and sensitivity is lower (~0.35 MPa) compared to potentiometric counterparts.

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2.2 Fabrication of Sensors Fabrication of the sensors needs to be meticulously carried out to achieve high sensitivity and low cost. Photolithography [9], drop casting [20], spin coating [21], impregnation [8], and deposition techniques (e.g. chemical or physical vapor) [22, 23] can be employed to fabricate certain parts of the system (electrode, transducer element, interconnection layer etc.) or integrate them. For instance, fabrication of SiGe based sensor, which is a typical microelectromechanical system (MEMS), can be successfully completed in 10 steps (Fig. 3) [24]. Among these steps, spin coating, chemical vapour deposition (CVD), and etching methods are used to arrange the parts of the sensor and allow for monolithic integration of MEMS above complementary metal-oxide semiconductor. For instance, Tian et al. developed a tire pressure measurement system (TPMS) by using lithographic techniques [25]. The thickness of 4-inch n-type silicon wafer as well as of Pyrex 7740#glass wafer was used to bond silicon and glass. The design of the resistances was fixed on the pressure element side according to the Wheatstone Bridge configuration, which was doped in p-Si. The surface of the silicon wafer was deposited with silicon nitride (Si3 N4 ) via low-pressure chemical vapour deposition (LPCVD) process. The device layout and cross-sectional illustration are clearly shown in Fig. 4. The fabrication of pressure indicating films can be carried out by mixing the particles with the polymer solution that provides homogeneous diffusion into matrix followed by using the simple drop-casting method to produce the films [13]. To measure the reversible change in pressure, structural coloured elastomers are promising candidates instead of plasmonic resonance based coloured films prepared by thermoplastics. The elastomers having opal or inverse opal structure are fabricated by using both bottom-up and top-down strategies. For instance, building blocks responsible from colouration can be obtained by using either Stöber or emulsion polymerization, which resulted in uniformed-size silica and polymeric beads, respectively. For integration of building blocks into the elastomer, several techniques such as capillary force induced assembly [10] and drop-casting onto a glassy film having nanoparticles in periodic nanostructure have been used [26]. Besides, lithographic techniques have also been used to produce periodic nanostructures onto the film for pressure measurement in wearable devices [27]. The fabrication process of ionic polymer metal composites (IPMC) based pressure sensors need careful selection of the base ion of electroactive polymers, which are typically manufactured from organic polymers that contain covalently bonded fixed ionic group. In general, there are two IPMC-based pressure sensor fabrication techniques, which are the initial compositing and surface electrode processes [28]. Depending on the process, the morphology of metal significantly changes. Use of former method provides metallization of the polymer by soaking ionic polymer in a salt solution to allow metal cations to diffuse via the ion-exchange process. Generally, the reduction of the cations is carried out within a proper reducing agent such as LiBH4 or NaBH4 . Heterogeneous distribution of the resulting metal particles across the polymer membrane can be considered as a disadvantage of this fabrication method [29]. Typically, metal coating layer thickness changes between 1 and 20 μm.

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Fig. 3 Schematic illustration of MEMS fabrication process [24]

Fig. 4 a Layout and b cross sectional view of MEMS based pressure sensor [25]

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For latter fabrication process, multiple reducing agents have been introduced to carry out reducing reactions according to Eq. 1 [30] with Pt as a model example. Predominantly initial platinum layer formed on the top leading to a rather smooth metallic surface compared to initial compositing method. In recent years, novel manufacturing methods including physical metal loading [31], electroless plating [32, 33], and using silver nanoparticles have been developed [34]. 2+  + 8OH− ⇒ 4Pt◦ + 16NH3 + LiBO2 + 6H2 O LiBH4 + 4 Pt(NH3 )4

(1)

3 Ionic Polymers and Metal Composites Thereof Electroactive polymers (EAPs) are organic and soft materials that offer both electrical and mechanical responses upon stimuli [35, 36]. They can change their shape and dimension as a response of applied electric field, or reversely, the polymers generate an electrical signal according to applied external mechanical force. Due to appropriate characteristics for applications as an actuator, these materials are attractive for the community of biomimicking such as fabrication of artificial muscle. Besides, electrically-driven soft materials, EAPs have a high potential for sensor applications such as haptic sensing [37], blood pressure and pulse monitoring [16], and chemical sensing [38] due to ease of fabrication and tailorable electromechanical coupling properties. Classification of EAPs is carried out according to their response mechanisms: electronic and ionic. The former type materials are activated by the internal electrical field and Coulomb forces, i.e. dielectric or liquid-crystal elastomers [39, 40], electrostrictive polymers [41], and piezoelectric polymers [42] are usually employed. For the latter one, generated an electronic pulse as a response to external mechanical force is governed by ionic interactions. Due to ionic motion at atomic/molecular level upon external mechanical or electrical force, these materials are associated with various features in terms of (i) migration and perturbation of the charge distribution, (ii) transportation into electrodes or through the electronic separator, which provide sensing and actuation applications. Ionic EAPs may be further categorized in IPMC, conducting polymers, and polymeric gels. In general, IPMCs have trilayered architecture, i.e. ionic polymer is located between thin layers of two metals as a sandwich structure (Fig. 5). Ionic polymer layer acts as polyelectrolyte part, which supplies ionic immobilization between electrodes. Generally, Nafion, Aciplex, Flemion, and Aquivion are commercial materials which have appropriate features for IPMC applications. Ionic polymers may be further categorized on the basis of chemical structure or morphology. In a chemical perspective, these materials are classified as; • • • •

Fluorinated ionic polymers Non-fluorinated ionic polymers Phosphoric acid doped ionic polymers Composite frameworks (organic-organic or organic-inorganic).

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Fig. 5 Trilayer structure of IPMC

There are several fluorinated polymers such as Nafion® , Neosepta-F® , Flemion® , and Asiplex® , which have been used commercially. The most effective and famous one is Nafion® (Fig. 6). It has a unique structure that consists of hydrophobic backbone and pendant side with the hydrophilic sulfonic terminated group. This structure provides hydrophobic and hydrophilic domains. As the water content increases, water-containing ion channels are developed [43]. Due to high cost and limited operating temperature of fluorinated ionic polymers, non-fluorinated counterparts such as poly (arylene ether)s, polyimides, poly (ether imide)s, polystyrene derivatives, and poly (ether etherketones) (PEEKs) have been developed as alternative EAPs (Fig. 7). Usually, all these non-fluorinated polymers are sulfonated to generate ionic channels via hydration. Relatively, the non-fluorinated polymers have 10 times lower ionic conductivity and efficiency compared to Nafion and their durability is shorter because of hydroxy radical initiated degradation [44, 45]. Nevertheless, these materials are still a good alternative due to their low cost and ease of fabrication. It is well-established that the high ionic conductive polymers provide several potential benefits such as improved electrode kinetics, high tolerance to impurities (i.e., organic residuals in a polymer matrix), and enhanced efficiency. For this purpose, H3 PO4 -doped poly (benziimidazole) derivatives have been potential candidates owing to their high ionic conductivity (105–108 S cm−1 ) [46], low gas permeability, and thermal/mechanical stability even anhydrous state [47, 48]. However, their durability is relatively shorter than fluorinated and non-fluorinated counterparts because of acid leaching [49]. In recent years, ionic nanocomposites have been studied to take advantages of the both phases. For instance, SiO2 nanoparticles provide better hydration at elevated temperatures, where zirconium phosphate derivative (ZrP) based nanoparticles improve ionic conductivity of ionic polymers [50, 51]. Chemical modification of the ionic polymers and their composites has been an effective way for further improvements of the response [52, 53]. Physical properties of ionic polymers depend not only on the chemical structure of material but also on their morphology from micro- to the nanoscale. Dimensionality (fiber, ultrathin film etc.), cross-link density, micro-phase separation (especially in binary systems) of ionic polymers have particular influence on the degree of ionic conductivity. For instance, one-dimensional ionic polymers that are fabricated by electrospinning technique have attractive outcomes in this sense. Electrospinning is

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Fig. 6 Chemical structure and appearance of Nafion

Fig. 7 Chemical structure of possible ionic polymer candidates for IPMC applications

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a technique that uses electrostatic forces to produce fiber morphology from solution or melt of polymers [54–56]. Conventional electrospinning system consists of the nozzle with the container and grounded conductive collector. High potential difference between these two parts provides jet, which electrostatically charged polymer subsequently aligns to a collector in fiber morphology. This process offers unique capabilities for producing novel fibers with controllable thickness, the degree of order, and pore structure. There are many polymers that can be used in electrospinning to form fibers from nano- to microscale. In literature, electrospun nanofibers and their applications have been reported using various synthetic and natural polymers. Since ionic and electrical conductions are directly affected due to scattering in disordered structures, highly ordered architectures are often desired for device fabrication. Due to an ordered arrangement of proton conducting channels compared to non-arranged polymer structure having longer channel lengths, polymer fibers have better ionic conductivity than their bulk counterparts. The measurement, which is conducted on single nanofiber of Nafion may be one of the convincing experiment to provide a deep understanding of the comparison of proton conductivities [57]. An order of magnitude higher ionic conductivity (~1.5 S/cm) has been observed along single fiber having a diameter of 400 nm. Moreover, proton conductivity has been found to be strictly related to the nanofiber diameter, i.e. fibers with diameters >2 μm presented the conductivity similar to that of bulk Nafion (~0.1 S/cm). Below the limit (15.59 ≈ 15 mm

Approx

Material of the finger

PDMS

Silicon rubber

Actuator type

IPMC

EAP

Actuator dimension

15/19 × 5 × 0.2

Actuation voltage

1–5 V

Needle length

4

Total weight of the finger

6.633 gm

DC Including IPMC

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5 Conclusion The current investigation entails the performance of a compliant PDMS based IPMC actuated three-finger gripper for gripping applications. Four different finger designs are shown in Table 1, where the final finger design with needle was chosen for the gripper design as it has the following advantages: (a) It shows maximum deflection among all the four fingers. (b) It has the needle at the end of the finger. This gripper is capable of gripping small objects of less than 1 mm in dimension. It was found that EAPs like IPMC can be harnessed with the compliant soft materials like PDMS to design complaint gripper. Simulation results reveal that the actuation of the finger-tip while grasping an object, PID control can adroitly cope up with the control strategy. After a comparative study of four different fingers, a PDMS based three-finger gripper design was selected for fabrication and experimental observation and reported in the present work. Acknowledgements The authors would like to acknowledge all the sources from whom, the information contained in this presentation were collated. Specifically, they acknowledge Indo-US Centre for Research Excellence on Fabrionics at IIEST-Shibpur and Chemistry and Biomimetics Laboratory, CSIR-CMERI Durgapur for providing financial support and encouragement.

References 1. Shahinpoor, M., Bar-Cohen, Y., Xue, T., Simpson, J.O., Smith, J.: Ionic polymer-metal composites (IPMC) as biomimetic sensors and actuators. In: Proceedings of SPIE’s 5th Annual International Symposium on Smart Structures and Materials, March 1998 2. Shahinpoor, M., Kim, K.J.: Ionic Polymer–Metal Composites: I. Fundamentals. Institute of Physics Publishing Smart Materials and Structures, pp. 819–833 (2001) 3. Bhattacharya, S., Chattaraj, R., Das, M., Patra, A., Bepari, B., Bhaumik, S.: Simultaneous parametric optimization of IPMC actuator for compliant gripper. Int. J. Precis. Eng. Manuf. 16(11), 2289–2297 (2015) 4. Bhattacharya, S., Bepari, B., Bhaumik, S.: IPMC-actuated compliant mechanism-based multifunctional multifinger microgripper. Mech. Based Design Struct. Mach. 42(3), 312–325 (2014) 5. Bhattacharya, S., Bepari, B., Bhaumik, S.: Novel approach of IPMC actuated finger for microgripping. In: 2015 International Conference on Informatics, Electronics & Vision (ICIEV), pp. 1–6. IEEE 6. Bhattacharya, S., Bepari, B., Bhaumik, S.: Soft robotic finger fabrication with PDMS and IPMC actuator for gripping. In: Sponsored SAI Computing Conference 2016, pp. 403–408. IEEE