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Über dieses Buch

This book is the second edition of Soft Actuators, originally published in 2014, with 12 chapters added to the first edition. The subject of this new edition is current comprehensive research and development of soft actuators, covering interdisciplinary study of materials science, mechanics, electronics, robotics, and bioscience. The book includes contemporary research of actuators based on biomaterials for their potential in future artificial muscle technology. Readers will find detailed and useful information about materials, methods of synthesis, fabrication, and measurements to study soft actuators. Additionally, the topics of materials, modeling, and applications not only promote the further research and development of soft actuators, but bring benefits for utilization and industrialization. This volume makes generous use of color figures, diagrams, and photographs that provide easy-to-understand descriptions of the mechanisms, apparatus, and motions of soft actuators. Also, in this second edition the chapters on modeling, materials design, and device design have been given a wider scope and made easier to comprehend, which will be helpful in practical applications of soft actuators. Readers of this work can acquire the newest technology and information about basic science and practical applications of flexible, lightweight, and noiseless soft actuators, which differ from conventional mechanical engines and electric motors. This new edition of Soft Actuators will inspire readers with fresh ideas and encourage their research and development, thus opening up a new field of applications for the utilization and industrialization of soft actuators.





Chapter 1. Progress and Current Status of Materials and Properties of Soft Actuators

In this chapter, brief history and current status of soft actuators made of various materials driven by different stimuli are described with typical references as milestones of the progress. The soft actuators originated from unique characteristics of cross-linked polymer gels for understanding their physical and chemical properties of dimensional changes and phase transitions induced by various environmental stimuli such as pH, salt, solvent, heat, light, and electric field. The ‘explosion’ of research and development of soft actuators in the 1990s extended over a variety of materials such as conductive polymers, elastomers, carbon nanotubes, and biomaterials, which had driven further progress in soft actuators not only from the fundamental viewpoint of basic science and materials chemistry and physics but also from the engineering viewpoint for the practical applications to light-weight, low-cost, no-noise, less-pollution, and high-efficiency micro- and macro-artificial muscles and soft robotic systems.

Hidenori Okuzaki

Chapter 2. Current Status of Applications and Markets of Soft Actuators

In this chapter, the current status of applications and markets of soft actuators will be described with reference to some published patents in the expected application fields.

Kinji Asaka, Kayo Nakamura

Materials of Soft Actuators: Thermo-Driven Soft Actuators


Chapter 3. Electromagnetic Heating

In this study, we demonstrated the surface modification of carbon microcoil (CMC) with grafting polymers onto the CMC and prepared composite material of poly(N-isopropyl acrylamide) (PNIPAM) gel as thermo-sensitive polymer gels by electromagnetic heating. The properties of the material such as content of carbon micro-coils, swelling ratio, breaking strength, and thermo-sensitivity, and its function as drug carrier were evaluated. The composite gels were shrunken with increase of temperature as well as normal PNIPAM gel and responded to electromagnetic waves with the presence of contained CMCs which absorb electromagnetic waves and generate heat. The surface temperature of composite gel was reached 43 °C within 150 s and changed its shape with squeezed water.

Takeshi Yamauchi

Chapter 4. Thermo-Responsive Nanofiber Mats Fabricated by Electrospinning

Copolymers of N-isopropylacrylamide and stearyl acrylate (PNIPA-SAX) with various SA feed ratios (X = 1–10 mol%) were synthesized and electrospun into nanofiber mats. It was found that average diameter of nanofibers electrospun at concentration of 25 % and voltage of 30 kV linearly increased from 165 nm (PNIPA) to 497 nm (PNIPA-SA10) with increasing the SA content. The PNIPA-SAX (X = 3–10 mol%) nanofiber mats were insoluble in water at 25 °C, in which inter-polymer and inter-fiber physical cross-links were formed through hydrophobic interaction of stearyl side-chains. With increasing the temperature from 25 to 40 °C the PNIPA-SA3 nanofiber mat exhibited significant volume contraction of 66 %, while that of a single nanofiber estimated by AFM measurements was found to be 37 %. The results allowed us to conclude that not only swelling-deswelling but also dissociation-association of the nanofibers via hydrophobic interactions were crucially important for the macroscopic volume changes of the nanofiber mats.

Hidenori Okuzaki

Chapter 5. Evolution of Self-Oscillating Polymer Gels as Autonomous Soft Actuators

In living systems, there are many autonomous and oscillatory phenomena to sustain life such as heart beating. We developed “self-oscillating” polymer gels that undergo spontaneous cyclic swelling-deswelling changes without any on-off switching of external stimuli, as with heart muscle. The self-oscillating gels were designed by utilizing the Belousov-Zhabotinsky (BZ) reaction, an oscillating reaction, as a chemical model of the TCA cycle. We have systematically studied these self-oscillating polymer gels since they were first reported in 1996. Potential applications of the self-oscillating polymers and gels include several kinds of functional material systems such as biomimetic actuators, mass transport systems, and functional fluids. For example, it was demonstrated that an object was autonomously transported in the tubular self-oscillating gel by the peristaltic pumping motion similar to an intestine. Further, self-oscillating polymer brush surface like cilia, vesicles, or colloidosomes undergoing cell-like autonomous shape oscillations with buckling was prepared. Besides, autonomous sol-gel oscillation and amoeba-like motion were realized utilizing well-designed block copolymer solution. In this chapter, our recent progress on the self-oscillating polymer gels is summarized.

Ryo Yoshida

Chapter 6. Polyrotaxane Actuators

With the emergence of supramolecular chemistry and modern nanotechnology, a great deal of research has been conducted on idiosyncratic classes of molecular structures held together by non-covalent mechanical interactions. Examples include the catenanes, rotaxanes, and knots, which are termed mechanically interlocked molecules. Steadily but inevitably, mechanically interlinked architectures are beginning to effectuate their promise as components of molecular machines that display a number of outstanding performances. Moreover, the consolidation of chemical, biological, and physical sciences has unlocked multitudinous and versatile techniques to implement supramolecular structures into new hybrid materials and stimuli-responsive artificial machines at the molecular level, such as actuators, muscles, shuttles, motors, pumps, valves, switches, piston-cylinders, ratchets, elevators, and so on. The chapter “Polyrotaxane Actuators” is organized into four sections. In the first section, the definition, synthesis, and properties of rotaxanes and polyrotaxanes are given. The design, synthesis, applications, and modifications of polyrotaxane-based molecular machines are described in the second section. The third and fourth sections delineate the practical and effective uses of polyrotaxanes in the fabrication of soft materials and the future prospects of polyrotaxanes as actuators, respectively.

Abu Bin Imran, Mohammad Harun-Ur-Rashid, Yukikazu Takeoka

Materials of Soft Actuators: Electro-Driven Soft Actuators


Chapter 7. Ionic Conductive Polymers

Electroactive polymers (EAPs) are attracting considerable interest due to their special characteristics, including high flexibility and low weight. Ionic conductive polymers have the potential to play a main role in the realization of smart systems for applications such as bio-inspired and autonomous robotics, medical devices, and aerospace. In this chapter, the fundamental aspects of ionic polymer actuators such as fabrication methods, evaluation methods, and recent progresses are mainly described.Especially, ionic polymer-metal composites (IPMCs) are one of the most promising EAP materials for the artificial muscle-like actuators and sensors. However, IPMC has some problems to overcome for practical uses. Among the problems, back relaxation and fabrication of IPMC are big issues. In this chapter, some challenges to solve these problems are introduced.

Kunitomo Kikuchi, Shigeki Tsuchitani

Chapter 8. Conducting Polymers

Soft actuators based on conducting polymers are discussed in terms of strain, stress and stability taking the mechanism into consideration. The actuation is generated by the insertion of anions from the electrolyte solution, which is triggered by electrochemical redox reactions. Characteristics of the actuation in polypyrrole, polyaniline, polythiophene, and poly(3,4-ethylenedioxythiophene) (PEDOT) are described. The maximum strain and stress are reported to be 39.9 % and 22 MPa, respectively, in polypyrrole actuator. However, the strain is usually less than 10 %. The stress (contraction force) originates from the elasticity of conducting polymers or Young’s modulus. Creeping under tensile loads, which is intimate issue in soft actuators, is discussed in terms of conformation change of polymer chains and shape memory effect. The actuation generated by sorption and desorption of moisture controlled with electrical heating is also introduced with the mechanism and characteristics.

Keiichi Kaneto

Chapter 9. Humidity-Sensitive Conducting Polymer Actuators

Free-standing films made of poly(3,4-ethylenedioxythiophene) doped with poly(4-styrenesulfonate) (PEDOT/PSS) were prepared by casting water dispersion of its colloidal particles. Specific surface area, water vapor sorption, and electro-active polymer actuating behavior of the resulting films were investigated by means of sorption isotherm, and electromechanical analysis. It was found that the non-porous PEDOT/PSS film, having a specific surface area of 0.13 m2/g, sorbed water vapor of 1,080 cm3(STP)/g, corresponding to 87 wt%, at relative water vapor pressure of 0.95. Upon application of 10 V, the film underwent contraction of 2.4 % in air at 50 % relative humidity (RH) which significantly increased to 4.5 % at 90 % RH. The principle lay in desorption of water vapor sorbed in the film due to Joule heating, where electric field was capable of controlling the equilibrium of water vapor sorption. The film generated contractile stress as high as 17 MPa under isometric conditions and work capacity attained 174 kJ/m3, where Young’s modulus of the film increased from 1.8 to 2.6 GPa by application of 6 V at 50 % RH. On the basis of this phenomenon, linear actuators utilizing PEDOT/PSS films were successfully developed and applied to leverage actuator and Braille cell.

Hidenori Okuzaki

Chapter 10. Carbon Nanotube/Ionic Liquid Composites

Both carbon nanotubes and ionic liquids are very attractive materials in the present scientific fields. Recently, we combine these two materials into the polymer matrix to make a conductive electrode film which expands and contracts when alternative square voltages are applied. We utilize these interesting phenomena for electroactive polymer actuators (electric-driven soft actuators). In this chapter, we introduce recent studies for electroactive polymer actuators composed of carbon nanotube/ionic liquid composites and their application potential for a thin and light Braille display as well.

Takushi Sugino, Kenji Kiyohara, Kinji Asaka

Chapter 11. Ion Gels for Ionic Polymer Actuators

Ionic polymer actuators are driven by the migration or diffusion of ions and generally exhibit significant deformation (i.e., bending) under low-voltage (<5 V) applications. However, the durability of conventional ionic polymer actuators decreases under open atmosphere owing to the evaporation of solvents, which are essential for the movement of ions, from the actuators. In order to overcome this drawback, ionic polymer actuators that can be operated under open atmosphere and even under vacuum are being developed using ionic liquids (ILs). Combining macromolecules with ILs as additives can result in highly ion-conducting polymer electrolytes (ion gels) suitable for applications in ionic polymer actuators. However, the contribution of polymeric materials to the high performance of IL-based polymer actuators is yet to be elucidated. In this chapter, IL-based polymer electrolytes comprising block copolymers and polyimides are demonstrated to enable easily processable ionic polymer actuators with high performance and durability. The displacement response is also analyzed using our proposed displacement model.

Masayoshi Watanabe, Satoru Imaizumi, Tomohiro Yasuda, Hisashi Kokubo

Chapter 12. Ionic Liquid/Polyurethane/PEDOT:PSS Composite Actuators

The transparent ionic liquid/polyurethane (IL/PU) gels were synthesized by addition reaction of polyol and diisocyanate in the presence of 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide. With increasing the IL content from 0 to 40 wt%, both ionic conductivity and electric-double-layer capacitance increased from 3.1 × 10−5 S/cm and 9.6 pF/cm2 to 8.8 × 10−5 S/cm and 277 pF/cm2, respectively, while the compression modulus slightly decreased from 0.49 to 0.44 MPa. The IL/PU/PEDOT:PSS composites were fabricated by sandwiching the IL/PU gel between two conductive polymer films made of poly(3,4-ethylenedioxythiophene) doped with poly(4-styrenesulfonate) (PEDOT:PSS) as soft and flexible electrodes. Upon application of an electric field, the IL/PU/PEDOT:PSS composite showed quick and intensive bending toward anode, where the bending displacement attained 3.8 mm at IL = 40 wt% and 2 V, corresponding to the strain of 0.32 %.

Hidenori Okuzaki

Chapter 13. Dielectric Gels

Dielectric gels of various types are recently found electrically active, and can be used for actuators. Polymer gels swollen with large amount of dielectric solvent deforms by applying dc voltage. The deformation is based on the solvent flow (or ion drag) through the polymer network. They shows contractile, bending, and crawling deformation. Advantages are very swift deformation in air, small electric current, and large strain up to over 10 % depending on the degree of crosslinks among the polymer chain. Disadvantages are low durability because of the solvent bleed-out, and relatively high voltage.Dielectric elastomers (sometime gel-like) can be good candidate when the polymer chains are flexible enough and sensitive enough to the electric field, although flexible polymer chains can not take the role of solvents. Similarity to the gel is that the electrically induced asymmetric charge distribution causes the bending deformation. Advantages of this system are low electric current, relatively swift deformation at high voltage, and good durability. Disadvantages of this system are requirement of high voltage, small strain, and basically very limited stress. For attaining large strain, very high voltages are necessary for the actuation such as over 10 kV/mm. We show the cases of polyurethane and poly(methyl methacrylate-b-n-butyl acrylate-b-methyl methacrylate) triblock copolymer.Plasticized polymer system provides another possibility, and we think at this moment the best candidate from the viewpoint of easy processing and variable possibilities. Polymers with large content of plasticizer (we call this category as “polymer gel” in stead of plasticized polymer) shows peculiar deformation such as amoeba-like creep deformation. In some cases, we investigated the gels show high power, high toughness, very low current, and variable application possibilities. The characteristics comes out from the very large dielectric constant from the cooperative interaction between the polymer and plasticizer both of which have very low dielectric constant. By applying the characteristic properties, not only the electro-mechanical function but also the electro-optical functions and mechano-electric functions are found.Through these investigations, we could conclude the dielectric gels have great possibilities as novel type of electro-active materials.

Toshihiro Hirai

Chapter 14. Dielectric Elastomers

Electroactive polymer (EAP) is a new actuation technology with exceptional performance. An especially attractive type of electroactive polymer is dielectric elastomer (DE).DE, based on the field-induced deformation of elastomeric polymers with compliant electrodes, can produce a large strain response, a fast response time, and high electromechanical efficiency. This unique performance, combined with other factors such as low cost, suggests many potential applications, a wide range of which are under investigation. Applications that effectively exploit the properties of DEs include artificial muscle actuators for robots (especially mobile and biomimetic robots on land, sea, and air); low-cost, lightweight linear actuators; inchworms, micro light scanners, and microfluidics, solid-state optical devices; diaphragm actuators for pumps, displays, and smart skins; acoustic actuators; and rotary motors. Dielectric elastomers may also be used to generate electrical power from mechanical deformation.

Seiki Chiba

Chapter 15. Piezoelectric Polymers

The study of piezoelectric polymers has advanced, and their practical application to actuator devices has progressed. In particular, the feasibility of achieving a new soft actuator using poly-l-lactic acid (PLLA) films exhibiting shear piezoelectricity, one type of lead-free bio-based piezoelectric polymer, has been investigated. The realization of an actuator made from PLLA films was previously considered to be impractical because of the small piezoelectric constant of the films. However, the newly developed PLLA multilayer film enables the realization of an actuator with a large power while retaining its softness, flexibility, and light weight. In this section, the newly developed PLLA multilayer film is shown to have potentially limitless possibilities for realizing new types of actuator.

Yoshiro Tajitsu

Chapter 16. Thermal and Electrical Actuation of Liquid Crystal Elastomers/Gels

This review describes the thermal and electrical actuation of liquid crystal elastomers (LCEs) which have the strong coupling between the macroscopic shape and LC alignment. Various types of thermal deformation such as elongation/contraction, bending, and torsion are driven by controlling the director configuration in LCEs. The periodic surface undulation is thermally induced using the helical director configuration in cholesteric elastomers. The nematic elastomers with polydomain alignment, which are prepared in the high-temperature isotropic state, undergo the realignment of local directors at modest strengths of external field, resulting in the electrically driven deformation. The cholesteric gels with helical director configuration exhibit the pronounced electro-optical effects for selective reflection coupled to electromechanical effects.

Kenji Urayama

Materials of Soft Actuators: Light-Driven Soft Actuators


Chapter 17. Spiropyran-Functionalized Hydrogels

Photoresponsive actuators composed of a hydrogel functionalized with spiropyran are described. The hydrogel exhibits drastic shrinking in rapid response to blue light irradiation in acidic aqueous systems and is examined for the application of several photoresponsive actuator systems. Rodlike hydrogel bends drastically after 1 s light irradiation, and microrelief is formed instantly on the hydrogel sheet by the micropatterned light irradiation. Based on these characteristics of the hydrogel, a photo-controllable microfluidic system is constructed with a hydrogel sheet, and the microchannels with arbitrary width, height, and pathway are formed instantly by the micropatterned light irradiation. Also independent and parallel control of microvalve array by local light irradiation is demonstrated for a similar microfluidic system combined with fixed microchannel.

Kimio Sumaru, Toshiyuki Takagi, Shinji Sugiura, Toshiyuki Kanamori

Chapter 18. Photomechanical Energy Conversion with Cross-Linked Liquid-Crystalline Polymers

Cross-linked liquid-crystalline (LC) polymers with a photochromic moiety show photoinduced deformation with change in molecular shape and alignment of photochromic compounds. Molecular-level photoisomerization of the photochromic moieties can give rise to macroscopic deformation of the materials, allowing one to convert light energy directly into mechanical work. The photomechanical effects extend the applicability of azobenzene-containing polymers towards light-driven actuators and artificial muscles. Recently, the effect of structure–property relationships and crosslinking density on the photomechanical property of photochromic polymers was investigated. Various motions based on the photoinduced deformation of the LC polymers were achieved by forming the polymer materials. This chapter summarizes the recent progress in photoinduced movements and light-driven actuation property of the LC polymers, in particular cross-linked LC polymers with a photochromic property.

Jun-ichi Mamiya

Chapter 19. Photoredox Reaction

A photoelectrochemical actuator based on poly(acrylic acid) gel loaded with TiO2 nanoparticles and copper(II) ions swells in a water-ethanol mixed solution under ultraviolet (UV) light due to photocatalytic reduction of copper(II) to copper(0) nanoparticles and oxidation of ethanol. After UV light irradiation, the hydrogel gradually shrinks again due to aerobic oxidation of copper nanoparticles to copper(II) ions. A photoelectrochemical actuator based on poly(acrylic acid) gel loaded with TiO2 nanoparticles and silver(I) ions also swells in water under UV light due to photocatalytic reduction of silver(I) to silver(0) nanoparticles and oxidation of water. The hydrogel shrinks again under visible light due to reoxidation of silver nanoparticles to silver(I) ions and reduction of oxygen molecules by the plasmon-induced charge separation. Partial swelling and shrinking of the hydrogel are also possible.

Tetsu Tatsuma

Materials of Soft Actuators: Magneto-Driven Soft Actuators


Chapter 20. Magnetic Fluid Composite Gels

Polymer gels in which magnetic fluid is immobilized have been considered as a possible candidate of useful actuators or sensors. Particularly, remote actuation is possible without any harmful damage to the body in medical actuation. This means the system can also be applied for energy harvesting system, too. In this chapter the author summarize the some characteristics of magnetid fluid (or ferrofluid) which implies super paramagnetic property. Then the immobilization of these material in polymer gels, and the structural changes induced in the magnetic gels by applying magnetic field or gradient of magnetic field. Some applications of these gels are introduced in the last section.

Toshihiro Hirai

Chapter 21. Magnetic Particle Composite Gels

Magnetic soft materials containing solid state magnetic particles demonstrate various motions and magnetorheological behavior in response to magnetic fields. When a rotational magnetic field is applied to magnetic gels containing with magnetized particles, the magnetic gels exhibit rotational motion. When a non-uniform magnetic field is applied to magnetic gels, the elongation of magnetic gels is observed. The rotational motion of magnetic gels can be applied to a fluid pump that delivers water in straight and spiral tubes. A bead of magnetic gels loaded with drugs undergoes accelerated drug release depending on the rotation rates. The elongational motion of magnetic gels can be applied to an elongation-contraction actuator or a microvalve. Under uniform magnetic fields, the magnetic gels show variable viscoelastic behavior depending on the field-strength, which is called the magnetorheological effect. The dynamic modulus of magnetic hydrogels increases by two orders of magnitude synchronized with magnetic fields. The magnetorheological effect of magnetic gels can be applied to haptic devices or intelligent dampers. Actuators and magnetorheological effects of magnetic soft materials consisting of solid state magnetic particles are described.

Tetsu Mitsumata



Chapter 22. Molecular Mechanism of Electrically Induced Volume Change of Porous Electrodes

Electroactive soft actuators that make use of the volume change of porous electrodes on applying voltage are recently drawing attention. We discuss the mechanism of the volume change of the porous electrodes on applying voltage by using the Monte Carlo simulation. We show that, when the pore size of the electrode is comparable to the size of the electrolyte ions, slight change in the molecular structure or in the external field can drastically change the thermodynamic properties in porous electrodes. In particular, the pressure exerted inside the porous electrodes on applying voltage can be two or three orders of magnitude larger than the atmospheric pressure. Those behaviors are explained by the balance between the volume exclusion interaction and the electrostatic interaction.

Kenji Kiyohara, Takushi Sugino, Kinji Asaka

Chapter 23. Computational Modeling of Mechanical Sensors Using Ionic Electroactive Polymers

This paper discusses mechanical sensors using ionic electroactive polymers (ionic EAPs). Based on the features such as low-voltage operation, large generating force, compact and lightweight, flexible and silent, and driving in air and water, ionic EAPs attract attention as soft actuator materials (artificial muscle materials) in robot engineering, MEMS, or medical field. In order to understand the actuator and sensor functions of ionic EAPs, several studies have been conducted to formulate analytical models of ionic EAPs, but most are on actuator modeling, and the simulations of mechanical sensor behaviors are based on black box modeling. Therefore, analytical models of the mechanical sensors using ionic EAPs and numerical simulations have been carried out. In other words, for mechanical sensors using ionic EAPs, detailed computational models at continuum scale have newly been formulated. The validity of the proposed models has been demonstrated by comparing the calculated results for the mechanical sensor of cantilever type with the experimental results. The proposed computational models are all considered to be extremely useful from a viewpoint of engineering as basic algorithms of computing which support the design and control of mechanical sensors using conducting polymers or IPMCs.

Yutaka Toi, Seongwon Yoo

Chapter 24. Distributed Parameter System Modeling

This chapter discusses a distributed parameter system modeling of ionic polymer-metal composite actuators based on modified Yamaue’s electro-stress diffusion coupling model. The lowest order linear time invariant state equation with the spatial variable is derived to carry out the simulation. An introductory method for simulation based on the state space model is also shown. The results of the simulation demonstrate the effectiveness of the derived model by showing the differences of the responses for the different cation species.

Kentaro Takagi, Gou Nishida, Bernhard Maschke, Kinji Asaka

Chapter 25. Control of Electro-active Polymer Actuators with Considering Characteristics Changes

Electro-active polymers (EAPs) are functional polymeric materials which respond to electrical stimuli with shape change. Since EAPs can be activated by electric field, driving equipment and control system are able to be easily implemented. Simple feedback control methods such as PID control and optimal control with identified model are effective; however, actuator characteristics of EAPs are likely to change depending on environmental conditions such as temperature and humidity or on material fatigue by iterative actuation. Therefore, feedback control methods considering the characteristics changes are desired. In this chapter, we explain two effective control methods for characteristics changes. One is a self-tuning control, which is a type of adaptive control, and another is cellular actuator control method for an integrated actuator. As example cases of the applied results, experimental results of feedback control of ionic polymer-metal composite (IPMC) actuators are demonstrated.

Norihiro Kamamichi

Chapter 26. Motion Design-A Gel Robot Approach

The main focus of this chapter is to propose methods for motion design of deformable machines, using a particular electroactive polymer gel. They are deformable like mollusk that can locomote dynamically or manipulate things softly. Such a machine has been a dream in the past but is now experimentally possible. Mechanisms consisting of the gel, hereafter called ‘gel robots’, were designed, developed, and controlled experimentally. It includes: (1) a mathematical model of the gel to be applied for design and control of distributed mechanisms, (2) gel robots driving systems, (3) control of gel robots for dynamic deformations. This chapter overviews a gel robot approach based on agent model for motion design of deformable robots utilizing electroactive polymers with simulation and experimental results.

Mihoko Otake

Chapter 27. Motion Control

Poly vinyl chloride (PVC) gel actuators show great potential because of such positive characteristics as movement in the air, large deformation, and being lightweight. We propose a configuration of a contraction type actuator and investigate its various characteristics. The contraction strain is 10–15 %, the response frequency is 3–7 Hz and the applied voltage 200–600 V. The generating stress is proportional to the number of layers, and the stress is then about 10 kPa when the actuator height is 10 mm. To use this actuator as a control element, we develop a mathematical model. Based on these results, we develop a position feedback control technique for the actuator and investigate the validity of the control method. The control law included a feed forward term to compensate for the elastic characteristic of the PVC gel actuator. The control method had good performance.

Minoru Hashimoto

Chapter 28. IPMC Actuation Mechanisms and Multi-physical Modeling

This chapter mainly introduces physical deformation theory of IPMC actuator. At first a series of comparative experiments focused on water content and polymer backbones of IPMC were designed and performed to disclose the actuation mechanisms of relaxation and slow anode deformation. Then a multi-physical model was set up which emphasized on water-related transport process and various eigen stresses. Through numerical analysis, inter-coupling between cation and water, pressure and hydration effects were investigated on the transport process. And in contrast to hydrostatic pressure, osmotic pressure and electrostatic stress and their properties with cation and water concentrations were analyzed to explain IPMC deformation evolvement with water content. Finally, model simplification was discussed for deformation prediction in engineering application.

Zicai Zhu, Hualing Chen, Longfei Chang

Chapter 29. Sensing Properties and Physical Model of Ionic Polymer

Ionic polymer shows great potential to imitate natural mechanical sensation because of ion-migration mechanism. In this chapter, sensing properties of IPMC sensor were introduced. Under a bending deformation, how ambient humidity influenced the voltage response of IPMC was investigated. And then the effect of various cations on the electrical responses of IPMC at various ambient humidities was revealed by a series of experiments. The electrical response evolvement with water content and cation type was explained thoroughly based on transport theory. Further, a multi-physical model was set up for IPMC sensor by utilizing the same equations for IPMC actuator. Numerical results showed that the model was capable to fit the voltage and current response of IPMC with various cations at different humidities well. Finally, we presented a new concept of ionic polymer senor based on deeply understanding on sensing mechanism. A 3 × 3 pressure sensor array was presented, which showed much higher voltage. It proved that ionic polymer sensor can work as a pressure sensor, not a cantilever anymore. It makes us believe that ionic polymer sensor is a promising direction and still far from well developed.

Zicai Zhu, Hualing Chen, Yanjie Wang

Chapter 30. Modeling of Dielectric Elastomer Actuator

Dielectric elastomer deforms in terms of area expansion under a voltage. The large and quick strain in dielectric elastomer features promising applications in soft robotics. This chapter introduces the characteristics of dielectric elastomer in physics as well as the performance of dielectric elastomer actuator (DEA). Mechanism of actuation is explained, and a free energy model of DEA is established for characterization. A specific nonlinear mechanical behavior, “strain-stiffening,” is highlighted and is then incorporated in the harnessing of snap-through instability. By harnessing the instability, DEA is capable of new performance in actuation which further broadens its applications in functional surface and muscle-like actuator.

Bo Li, Hualing Chen, Guimin Chen

Chapter 31. Modeling of Dielectric Gel Using Multi-physics Coupling Theory

Gel is an organic mixture of polymer network and solvents. Owing to the large amount of solvents, gel is very soft and self-compatible. This chapter introduces dielectric gel, where both the polymer network and solvents are non-conductive. The mechanism of voltage-induced deformation in dielectric gel is explained with the physics of the solvent migration. Bending and creeping actuation in dielectric gel can also be induced using the design of electrode configuration. In the theory of dielectric gel, electro-chemo-mechanical quantities are coupled in a thermodynamics model, and the constitutive relations are obtained. The actuation model is implanted into several actuator cases to characterize their specific performances. Some examples of dielectric gel in robotics are illustrated. They include an amoeba robot, an artificial lens, and a soft exoskeleton, all showing dielectric gel a good candidate in soft robotics.

Bo Li, Longfei Chang, Yanjie Wang

Chapter 32. Modeling and Control of Fishing-Line/Sewing-Thread Artificial Muscles (Twisted and Coiled Polymer Fibers, TCPFs)

This chapter introduces a study on modeling and control of a fishing-line/sewing-thread artificial muscle, which is named as a twisted and coiled polymer fiber (TCPF). The beginning part of this chapter briefly introduces outline of fishing-line artificial muscle actuators, and the following part describes modeling and control. Observing the result of system identification of a TCPF driven by Joule heating, we clarify that the response from the applied power to the temperature is particularly dominant in the system dynamics. By controlling heating wire, contraction of a fishing-line artificial muscle can be converged to a reference position. Moreover, by controlling a cooling fan, it is also possible to improve response of the expansion of an actuator.

Kentaro Takagi, Norihiro Kamamichi, Ken Masuya, Kenji Tahara, Toshihira Irisawa, Kinji Asaka



Chapter 33. Underwater Soft Robots

Two underwater soft robots using ionic polymer-metal composites (IPMCs), a ray-like robot and a quadruped robot, are introduced. For autonomous operation of the ray-like robot, miniaturized electrical devices are developed. A simple traveling wave input is employed to generate the motion of the fin. The propulsion speed of the robot is able to be controlled by the parameters of the traveling wave. In the experiment we observed that the amplitude of the fin increased toward the backward in spite of the uniform control input. This phenomenon may be the key to achieve the energy-efficient swimming of underwater robots by utilizing the elasticity of the actuator. The underwater quadruped robot is developed from a sheet of IPMC of which electrode is segmented into some parts to be controlled independently. We demonstrate the electro-discharge machining (EDM) method is useful to segment the electrode with the minimum damage to the polymer. In the experiment we found by accident that the deformation of the IPMC became gradually large by ion exchange with the copper electrode contact. Finally we show the gait of turtle is effective to control the developed quadruped robot.

Kentaro Takagi, Zhi-Wei Luo, Kinji Asaka

Chapter 34. IPMC Actuator-Based Multifunctional Underwater Microrobots

A variety of microrobots have commonly been used in the fields of biomedical engineering and underwater operations during the last few years. Due to their compact structure, low driving power, and simple control systems, microrobots can complete a variety of underwater monitoring operations, even in restricted underwater environments. Generally speaking, compact structure, multi-functionality, flexibility and precise positioning are considered incompatible characteristics for underwater microrobots. Nevertheless, we have designed several novel types of bio-inspired locomotion, using ionic polymer metal composite (IPMC) and shape memory alloy (SMA) actuators. We reviewed a number of previously developed underwater microrobot prototypes that were constructed to demonstrate the feasibility of these types of biomimetic locomotion. Based on these prototypes, we summarized the implemented techniques and available results for efficient and precise underwater locomotion. In order to combine compact structure, multi-functionality, flexibility and precise positioning, we constructed a prototype of a new lobster-like microrobot and carried out a series of experiments to evaluate its walking, rotating, floating and grasping motions. Diving/surfacing experiments were performed by electrolyzing the water around the surfaces of the actuators. Three proximity sensors were installed on the microrobot to detect an object or avoid an obstacle while walking.

Shuxiang Guo, Liwei Shi

Chapter 35. Medical Applications

The applications of soft actuators in medical field have been increasing along with its industrial applications. Soft actuators could be used in the human body or be implanted for some extended time. Because soft actuators are flexible, light weight, and deformable to complex shapes, they have advantages over traditional electric motors. Although we are yet to see extensive commercial use of soft actuators in medical field, its number of studies, patents, and scopes are expanding rapidly.Here, we review some of the new and sustained research works of soft actuators in medical field, with specific field of applications.

Tadashi Ihara, Taro Nakamura

Chapter 36. Elastomer Transducers

Dielectric elastomers, transducers that couple the deformation of a rubbery polymer film to an applied electric field, show particular promise with features such as simple fabrication in a variety of size scales, high strain and energy density, high efficiency and fast speed of response, and inherent flexibility, environmental tolerance, and ruggedness. A variety of actuator configurations has been demonstrated at various size scales including rolled “artificial muscle” actuators, framed and bending beam actuators for efficient opto-mechanical switches, and diaphragm and thickness-mode actuators for new types of motors, pumps, and valves. The performance benefits of dielectric elastomers can allow for new generations of devices in microrobotics, communications, biotechnology, aeronautics, and aerospace.Dielectric elastomer has also been shown to operate in reverse as a generator. It has several characteristics, making it potentially well suited for power takeoff systems using wave, water current, wind, human motion, etc.

Mikio Waki, Seiki Chiba

Chapter 37. Dielectric Elastomer Sensors: Development of a Stretchable Strain Sensor System

Conventionally, to measure the strain of rigid objects, such as metal objects, strain gauges are widely used. For strain measurement of flexible objects with a wide dynamic range (for example, 100% or more), strain sensors are required. A flexible strain sensor is expected to enable a variety of technologies to be realized—for example, measurement applications, human interfaces, smart wear, skin motion monitoring, and robotic skin.We have developed a stretchable strain sensor, C-STRETCH®, using compound techniques involving elastomers and elastic conductive materials. This sensor has a wide dynamic range (up to 200% elongation) and is a very soft, very thin film with high responsiveness and excellent measurement accuracy.

Hideo Otaka

Next-Generation Bio-actuators


Chapter 38. Tissue-Engineering Approach to Making Soft Actuators

Recent achievements in tissue-engineering research make it viable to construct biological tissue and organs in vitro from living cells and a scaffold substrate. This technology can be applied not only to patients for regenerating malfunctioning tissue and organs in vivo but also to other purposes such as three-dimensional tissue modeling for drug screening in vitro. The aim of this chapter is to describe bioactuators, which are actuators made of cultured skeletal muscle cells in vitro, on the basis of our investigations. Living muscles are driven by actin–myosin molecular motors through transformation of the biochemical energy of adenosine triphosphate (ATP) into mechanical energy. They have excellent characteristics of light weight, high flexibility, and remarkable efficiency for energy conversion in comparison with mechanical actuators that require electricity as a power source. Therefore, bioactuators have the potential to be flexible and highly efficient actuators on a micro- to macroscale. Tissue-engineered skeletal muscle with a native-like contractile property has been successfully constructed and adapted as a bioactuator to drive a microlever object. There are still several issues to be resolved for creation of a large and powerful bioactuator that works long term with proven reliability, and these are expected to be addressed in the near future through intensive and vigorous tissue-engineering studies.

Toshia Fujisato, Shunya Takagi, Tomohiro Nakamura, Hiroshi Tsutsui

Chapter 39. Integration of Soft Actuators Based on a Biomolecular Motor System to Develop Artificial Machines

Fabrication of soft actuators that may perform multiple tasks simultaneously, as observed for the complex natural systems, is one of the goals in biomimetics. Biomolecular motor systems are the smallest natural machine that can perform mechanical work with a high efficiency. Because of their wide range of scalability and adaptability, the biomolecular motor systems are promising candidates for developing biomimetic soft actuators. The biological power units are able to convert chemical energy obtained from hydrolysis of adenosine triphosphate (ATP) into mechanical work. By virtue of their highly efficient mechanism of power generation, they are able to form highly ordered structures in living organism, which facilitates their emergent functions. To exploit the advantages of the biomolecular motor systems, nowadays they are used as building blocks of biomimetic soft actuators or devices. In this chapter we discuss the latest applications of a classical biomolecular motor system microtubule/kinesin in designing biomimetic soft actuators and micro devices. Nowadays the microtubule/kinesin system can be reconstructed and self-assembled or integrated to complex hierarchical structures which offer emergent functions. Utilization of biomolecular motor systems can greatly advance the development of highly efficient biomimetic soft actuators which in turn would benefit soft robotics in near future.

Jakia Jannat Keya, Kentaro Kayano, Arif Md. Rashedul Kabir, Akira Kakugo

Chapter 40. Employing Cytoskeletal Treadmilling in Bio-actuators

In this chapter, we describe bio-actuators consisting of cytoskeletal proteins capable of exhibiting treadmilling. The treadmilling is realized by formation of a filamentous protein complex through the addition of unit proteins at one end and dissociation at the other end accompanying a sequence of nucleotide triphosphate hydrolysis. We have demonstrated the creation of hydrogels that autonomously oscillate owing to the treadmilling of actin or tubulin and even have the capability of being driven by walking motor proteins. These hydrogels have great potential as bio-actuators because they are easy to make on a centimeter scale.

Ryuzo Kawamura, Ken-Ichi Sano, Yoshihito Osada

Chapter 41. Construction and Functional Emergence of Bioactuated Micronanosystem and Living Machined Wet Robotics

The development of micromachining technology and microelectromechanical system (MEMS) has greatly contributed to the development of micro-nano-robots adapted to various environments. In particular, soft actuators, which are paying attention as flexible driving mechanisms, have biologically autonomous functions, so that it is possible to give a great revolution to the field of robots which are driven for heteronomy.In the system constructed by the cellular buildup method, in principle, it is possible to incorporate various biological functions into the artificial system. Since the approach from conventional robotics uses mechanical parts, biocompatibility was also low, but approaches from wet robotics can bring a solution to these problems.In this chapter, we will introduce the development and characteristics of bioactuators driven by cells and tissues that have been developed by microfabrication technology as a driving source.

Yoshiyuki Takashima, Kaoru Uesugi, Keisuke Morishima
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