The application of electroactive polymers (EAP) for mechanical actuation is discussed. A comparison is made between established actuation technologies and polymer actuators. In addition, mammalian muscle properties are compared to some of the observed polymer actuation properties. This paper attempts to analyze actuator performance metrics, as set for current technologies, to determine the feasibility of electroactive polymer actuators in various applications. In this analysis, different mechanical design approaches are reviewed for possible use in EAP actuator applications. Examples of EAP microactuators are presented.
Skeletal muscle represents a classic biological example of a structure-function relationship. This paper reviews basic muscle anatomy and demonstrates how molecular motion on the order of nm distances is converted into the macroscopic movements that are possible with skeletal muscle. Muscle anatomy provides a structural basis for understanding the basic mechanical properties of skeletal muscle -- namely, the length-tension relationship and the force-velocity relationships. The length-tension relationship illustrates that muscle force generation is extremely length dependent due to the interdigitation of the contractile filaments. The force-velocity relationship is characterized by a rapid force drop in muscle with increasing shortening velocity and a rapid rise in force when muscles are forced to lengthen. Finally, muscle architecture -- the number and arrangement of muscle fibers -- has a profound effect on the magnitude of muscle force generated and the magnitude of muscle excursion. These concepts demonstrate the elegant manner in which muscle acts as a biologically regenerating linear motor. These concepts can be used in developing artificial muscles as well as in performing surgical reconstructive procedures with various donor muscles.
Polyconjugated and electroactive materials are, when used for electrochemical devices, soft, wet and complex materials. Polymeric chains, water and inorganic ions are the main constituents, mimicking the composition of those organs constituents of the living creatures. During reverse electrochemical reactions, those electroactive polymers change, under control of the consumed charge: volume, color, stored charge, composition, porosity, etc. Those changes allow the design of devices such as artificial muscles, smart skins, electric organs, nervous interfaces, smart medical dosage, smart membranes, all of them working under electrochemical control. This multifunctionality will be reviewed.
NASA is seeking to reduce the mass, size, consumed power, and cost of the instrumentation used in its future missions. An important element of many instruments and devices is the actuation mechanism and electroactive polymers (EAP) are offering an effective alternative to current actuators. In this study, two families of EAP materials were investigated, including bending ionomers and longitudinal electrostatically driven elastomers. These materials were demonstrated to effectively actuate manipulation devices and their performance is being enhanced in this on-going study. The recent observations are reported in this paper, include the operation of the bending-EAP at conditions that exceed the harsh environment on Mars, and identify the obstacles that its properties and characteristics are posing to using them as actuators. Analysis of the electrical characteristics of the ionomer EAP showed that it is a current driven material rather than voltage driven and the conductivity distribution on the surface of the material greatly influences the bending performance. An accurate equivalent circuit modeling of the ionomer EAP performance is essential for the design of effective drive electronics. The ionomer main limitations are the fact that it needs to be moist continuously and the process of electrolysis that takes place during activation. An effective coating technique using a sprayed polymer was developed extending its operation in air from a few minutes to about four months. The coating technique effectively forms the equivalent of a skin to protect the moisture content of the ionomer. In parallel to the development of the bending EAP, the development of computer control of actuated longitudinal EAP has been pursued. An EAP driven miniature robotic arm was constructed and it is controlled by a MATLAB code to drop and lift the arm and close and open EAP fingers of a 4-finger gripper.
In the last period, the interest in the development of devices that emulate the properties of the 'par excellence' biological actuator, the human muscle, is considerably grown. The recent advances in the field of conducting polymers open new interesting prospects in this direction: from this point of view polyaniline (PANi), since it is easily produced in fiber form, represents an interesting material. In this conference we report the development of a linear actuator prototype that makes use of PANi fiber. All fabrication steps (fiber extrusion, solid polymer electrolyte preparation, compound realization) and experimental set-up for the electromechanical characterization are described. Quantitative measurements of isotonic length changes and isometric stress generation during electrochemical stimulation are reported. An overall assessment of PANi fibers actuative properties in wet and dry conditions is reported and possible future developments are proposed. Finally, continuum and lumped parameter models formulated to describe passive and active contractile properties of conducting polymer actuators are briefly outlined.
In this paper I argue that the mechanism of muscle contraction is similar to the mechanism of contraction in most artificial muscles. Artificial muscles typically contract by a phase- transition. Muscle is thought to contract by a sliding- filament mechanism in which one set of filaments is driven past another by the action of cyclically rotating cross- bridges -- much like the mechanism of rowing. However, the evidence is equally consistent with a mechanism in which the filaments themselves contract, much like the collapse of polymers during a phase-transition. Muscle contains three principal polymer types, organized neatly within a framework. There is evidence that all three can contract. It appears that the relative contributions of each filament are designed to confer strength, speed and versatility on this natural machine. The principles of natural contraction may be useful in establishing optimal design principles for artificial muscles.
We discuss possible mechanisms of doping induced volume change in conjugated polymers with reference to the case of poly(3,4- ethylenedioxythiophene) (PEDOT). This material has been studied in two forms; as well ordered paracrystalline material and in the form of a water swollen conducting hydrogel. Structural and electrochemical studies are discussed.
In recent years the use of ionic polymer metal composites such as Nafion-based platinum ionomers have emerged as electroactive polymer materials with great potential for robotics and other applications. An effective activation of these materials requires understanding of their mechanism of operation. Generally, the material needs to be maintained hydrated to assure its electromechanical activity. To allow the control of the response of the material, a study is underway to investigate the electrical response. Particular emphasis is placed on possible electromechanical reactions and deviations from linear dielectric behavior. Currently, efforts are made to determine the necessary drive characteristics of the source to allow low power operation (less than or equal to 1.0 W) of the material as an actuator.
Conducting polymers show volume changes during electrochemical doping. Their high strength make them potential candidates for being used as artificial muscles. We consider actuators based on three-layer structures consisting of a passive polymer substrate sheet, a thin metal film electrode and a thin film of conducting polymer. In this paper we describe our Three- layer model to study the performance of an actuator based on the transduction of bending to linear movements. We show calculated results for an undulator and C-block characterized, respectively, by a flat and semi-circular shape in the relaxed state. Knowing the mechanical parameters of the considered materials, we evaluate the efficiency of the composite structure in terms of the performed stroke and work. The model shows that the undulator contracts in a nonlinear way with respect to the relative expansion of the materials, whereas the C-block is approximately linear. Contractions as large as 80% and 45% are obtained with the undulator and C-block, respectively. Although the C-block performs better than the undulator in terms of linearity, the undulator is easier to design and manufacture due to its flat shape in the relaxed state.
Macroscopic electrochemomechanical devices formed by bilayers or triple layers between electroactive polymers and a non conducting, adherent and flexible poplymer, able to describe angular movements in liquid electrolytes under electrochemical control are presented. Using a polymeric electrolyte, able to conduct ions, sandwiched between two polypyrrole layers, a muscle working in air was constructed. Any of those devices work under electrochemical control of the conformational changes occurring in polymeric chains. They act simultaneously as actuators and sensors. Similitudes with natural muscles are underlined.
Presented are theoretical and experimental results on electrically controlled static and dynamic flexing and deformation of iono-elastic beams made with ionic-polymer metal composite (IPMC) artificial muscles. These composite materials have the capability of large motion sensing and actuation in a biomimetic fashion. The essence of the underlying iono-elastic response of such materials is due to Coulombic electro-dynamic charge interaction amongst a dispersed phase of metallic particles that are charged either positively or negatively, mobile phase of a cation such as hydrogen ions H+ (Protons) or Li+, Hydroxyl anions OH-, and a fixed anionic phase such as an assembly of sulfonates SO3- elastically attached to the backbone polymer network macromolecules. The mathematical model presented is analogous to classical Euler-Bernoulli's beam theory modified to accommodate a non-homoeneous distributed electrically-induced moment due to the presence of a non-homogeneous electric field in an elastic material. The presentation may be extended to materials governed by hyper- elasticity such as in rubber elasticity. Analytical solution obtained agree reasonably well with our experimental results on Cationic Polymer-Platinum Composites (CPPC) which are also reported in this paper.
An exceptionally high electrostrictive response was observed in electron irradiate polyvinelidene fluoride- trifluoroethylene [P(VDF-TrFE)] copolymer. Moreover, the transverse strain of the material can be tuned over a large range by different sample treatment conditions. For example, in films uniaxially stretched, the strain can be comparable or even larger than the longitudinal strain, while in films unstretched, the transverse strain is quite small. In addition, when the films are completely clamped mechanically in the lateral directions, the films can still generate large longitudinal strains. Due to relatively high elastic modulus of the films, high elastic strain energy densities, which are much higher than those in conventional piezo and electrostrictive ceramics and magnetostrictive materials.
All-polymer electrostrictive systems are developed. Two typical electro-active polymers, poly(vinyldiene- trifluoroethylene) P(VDF-TrFE) of high elastic modulus and polyurethane of low elastic modulus, are studied. The conducting polymers used as electrodes in the system are polypyrrole and polyaniline respectively. The compatible interface between the electrode polymer and electrostrictive polymer produces acoustic transparency of the all-polymer films. The dielectric and electromechanical properties of the system are characterized and compared with that of the electroactive polymer with gold electrodes. It is found that the dielectric loss of the system is a litter bit higher than that of the sample with gold electrodes at high frequency since the conductivity of the conductive polymer is lower than that of the gold. The electric field induced longitudinal strain response of the all-polymer system is the same as that of the electroactive polymer with gold electrodes. However, the electric field induced transverse strain response of the all-polymer system is higher than that of the electroactive polymer with gold electrodes.
This paper investigates the use of elastomeric dielectric materials with compliant electrodes as a means of actuation. When a voltage is applied to the electrodes, the elastomeric films expand in area and compress in thickness. The strain response to applied electric fields was measured for a variety of elastomers. A nonlinear high-strain Mooney-Rivlin model was used to determine the expected strain response for a given applied field pressure. Using this model, we determined that the electrostatic forces between the free charges on the electrodes are responsible for the observed response. Silicone polymers have produced the best combination of high strain and energy density, with strains exceeding 30% and energy densities up to 0.15 MJ/m3. Based on the electrostatic model, the electromechanical coupling efficiency is over 50%. This paper also reports recent progress in making highly compliant electrodes. We have shown, for example, that gold traces fabricated in a zig-zag pattern on silicone EPAM retain their conductivity when stretched up to 80% compared to 1 - 5% when fabricated as a uniform 2-dimensional electrode. Lastly, the paper presents the performance of various actuators that use EPAM materials. The technology appears to be well-suited for a variety of small-scale actuator applications.
We have designed and built piezoelectric polymer actuators in a 'bellows' configuration and have used them in a near-zero-g environment vibrations suppression apparatus. The actuator is based on poly(vinylidene fluoride) (PVDF) sheets produced by AMP and electroded to our specifications. The actuator consists of two bimorphs, each with a double-bend precurvature, glued together at their ends so that the actuator has its thickest air gap at the middle. Each bimorph consists of two sheets glued together. Each sheet is electroded completely on the outside (ground) side, and has three electrode areas on the other side. If the electrode on the middle half is positive, and on the outer two quarters are negative, the bimorph curvature and the actuator length increase; with opposite polarities they decrease. In the vibration isolation application, the box to be isolated has actuators mounted between it and its surrounding enclosure on the vibrating vehicle. Feedback control is provided to change actuator length to compensate for vehicle motions and vibrations. This feedback is provided by accelerometers and by laser diode position sensors. The inherent softness of the actuator provides good passive damping of higher frequencies. So far, a one-dimensional test of the system has been made using a mass on a 'folded pendulum' as a 'weightless' (no restoring force for small displacements) load. Also, a two- dimensional version was flown on NASA's KC-135, which provided 25-second near-zero-g intervals during parabolic flight segments. Our goal is three-dimensional isolation for space vehicle applications.
High performance piezoelectric polymers are of interest to NASA as they may be useful for a variety of sensor applications. Over the past few years research on piezoelectric polymers has led to the development of promising high temperature piezoelectric responses in some novel polyimides. In this study, a series of polyimides have been studied with systematic variations in the diamine monomers which comprise the polyimide while holding the dianhydride constant. The effect of structural changes, including variations in the nature and concentration of dipolar groups, on the remanent polarization and piezoelectric coefficient is examined. Fundamental structure-piezoelectric property insight will enable the molecular design of polymers possessing distinct improvements over state-of-the-art piezoelectric polymers including enhanced polarization, polarization stability at elevated temperatures, and improved processability.
Experiments have confirmed the feasibility of an innovative, new class of very simple, reliable, low mass, low packaging volume, and low-cost self-deployable structures for space and commercial applications. The concept called 'cold hibernated elastic memory' (CHEM) utilizes shape memory polymers (SMP) in open cellular (foam) structures. The SMP foam materials are under development by the Jet Propulsion Laboratory (JPL) and Mitsubishi Heavy Industries (MHI). The CHEM structures are described here and their major advantages are identified over other expandable/deployable structures. In preliminary proof- of-concept investigation conducted on SMP foams, all evaluation/test results were very encouraging and confirmed the basic characteristics of CHEM structures. The main objective of this program is to develop and validate the CHEM structure technology for most promising space applications. However, possible terrestrial commercial applications are also anticipated and described in this paper as well.
Activated polyacrylonitrile (PAN) fibers are known to elongate and contract when immersed in caustic and acidic solutions, respectively. The change in length for these pH activated fibers is greater than 100% and are comparable in strength to human muscle, yet need of strong acids and bases for actuation has limited the use of PAN fibers as linear actuators or artificial muscles. Increasing the conductivity of PAN by depositing platinum within the fibers has allowed for electrical activation of PAN artificial muscles when it is placed in an electrochemical cell. The electrolysis of water in such a cell produces hydrogen ions at a PAN anode, thus locally decreasing the pH and causing the PAN muscle to contract. Reversing the electric field allows the PAN muscle to elongate. A 40% change in PAN muscle length in less than 10 minutes is observed when it is placed as an electrode in a 10 mM NaCl electrolyte solution and connected to a 20 volt power supply. These initial results indicate potential in developing electrically activated PAN muscles and linear actuators, which would be much more applicable than chemically activated PAN.
The shape memory effect with multi stimuli responses of the hydrogels having the various type of long alkyl chains: poly (stearyl acrylate -co- acrylic acid) p(HA-co-AA), poly(16- acryloylhexadecanoic acid -co- acrylic acid) p(AHA-co-AA), poly(12-acryloyldodecanoic acid -co- acrylic acid) p(ADA-co- AA), and poly (stearyl acrylate -co- methyl acrylate) p(SA-co- MA) were investigated. P(SA-co-AA) gel exhibited the reversible order-disorder transition on heating at 49 degrees Celsius and on cooling at 42 degrees Celsius. The transition was derived from the melting of the hydrophobic domain formed by the long alkyl side chain. The Young's modulus decreased dramatically during the transition. Besides this gel showed the shape memory effect based on the order-disorder transition. The shape of deformed p(SA-co-AA) gel recovered very quickly (5s). In the p(SA-co-MA) gel system, the transition temperature could be varied. Moreover, p(AHA-co-AA) gel responded to not only temperature change but also change of pH and solvent composition. 10
Non-ionic polymer gel was found to bend and scrawl much faster than conventional polymer gels by applying d.c. electric field. The motion looks like to be an action of a biological muscle in a sense. The dimensions of the gel were 2 mm in thickness, and 7 X 5 mm2 in area. Both surface of the gel sheet was coated with thin gold film whose thickness is 0.1 (mu) . Bending angle reaches 180 degrees within 90 ms when electric field of 500 V/mm was applied. The current was around 0.03 mA under the field, suggesting that the heat generated in the motion be negligible. The gel was composed of chemically crosslinked poly(vinyl alcohol) (PVA) swollen with dimethyl sulfoxide (DMSO). Solvent loss from the gel in the action was negligible. DMSO orientation by an electric field was investigated by Raman spectroscopy. DMSO was found to flow or to generate a pressure gap under an electric field over 60 V/mm. It turned out that the orientation of DMSO does not directly induce the motion of a gel, but the electrically induced DMSO flow plays a critical role in the actuation. The electrically induced solvent flow implied that the solvent was enforced to flow from an anode side to a cathode side. The results are consistent to the observation of the bending or scrawling motion in which the cathode side is expanded much more than the anode side. As a conclusion, the concept of the 'electrically-induced solvent-drag' gel actuator provides the most promising way to a high power artificial muscle, a soft actuator, at this moment.
The sliding friction of various kinds of hydrogels has been studied and it was found that the frictional behaviors of the hydrogels do not conform to Amonton's law F equals (mu) W, which well describes the friction of solids. The frictional force and its dependencies of on the load are quite different depending on the chemical structures of the gels, surface properties of the opposing substrates, and the measurement condition. The gel friction is explained in terms of interfacial interaction, either attractive or repulsive, between the polymer chain and the solid surface. According to this model, the frictional is ascribed to the viscous flow of solvent at the interface in the repulsive case. In the attractive case, the force to detach the adsorbing chain from the substrate appears as friction. Surface adhesion between glass particles and gels measured by AFM showed a good correlation with the friction, which support the repulsion- adsorption model proposed by authors.
The ability of some polymeric gels to shrink and swell with changes in their environment makes them of interest in many applications such as artificial muscles and drug delivery systems. While much work has been done to study the behavior and properties of these gels, little information is available regarding the full constitutive description of the mechanical and actuation properties. This work is focused on developing constitutive descriptions of the mechanical properties of such gels, and to determine how these properties change due to changes in the environment. Since these gels can undergo finite elastic deformations, uniaxial tests do not provide sufficient property information and a combination of loading conditions must be used. A biaxial testing system has been developed to test thin sheets of these films, and includes the ability to monitor and change the environmental conditions around the specimen. Initial tests were performed on latex to determine the quality of the testing apparatus. Preliminary results on a polyacrylonitrile gel are presented.
Muscle-like actuators have been made from bilayers of crosslinked polyacrylamide and polyacrylic acid hydrogels sandwiched between electrodes. The polyacrylic acid responds to applied positive polarity field by contracting and expelling water which is taken up by the polyacrylamide layers. Previous studies have shown that the effective swelling modulus of polyacrylamide is much lower than polyacrylic acid. Hence the polyacrylamide acts as a sponge. As the polyacrylic acid layer contracts in the x, y and z directions the polyacrylamide is also pulled in on x and y, so that the whole stack becomes narrower and expands along the z- axis. Reversing the field reverses this effect with a time constant of about 1 minute for 1 mm thick layers with a thickness change of about 10%. Linear changes up to 50% have been obtained. Other gel actuators either transfer water across a sheet and so bend, or contract by expelling water. This new system shows a linear contraction and expansion without a volume change and so can be run (sealed) in a dry environment.
The development of practical actuators based on conducting polymers having a force capacity of several tens of kilograms is considered. Large, free standing polypyrrole (Ppy) films give force density well below the theoretical maximum due to the inherent electrical resistance of the film. This may be overcome by applying a metallic coating but this is best achieved by applying the conducting polymer as a coating on a metallized polymer substrate. Increased electrical resistance (and, hence, reduced actuator performance) was also observed when the film thickness was increased and when gel electrolytes were used. The limitations on actuator performance due to ion diffusion kinetics were illustrated by studies on the effects of scan rate. The most suitable design for scale-up to practical actuator devices was based on a bundled fiber arrangement where the Ppy was first coated onto platinum coated polyester fibers. This design produced the highest force density achieved to date.
In order to characterize the deformation behavior of polymer gels for actuators, spatial distribution of deformation of anionic, cationic and amphoteric gels under the electric field was measured. Amphoteric gel was found to be a promising material for inducing symmetric deformation, compared with anionic and cationic gels. It was also found that the deformation of electro-active gels was mainly attributed to interfacial phenomenon between gel-electrode. It can be concluded that by the use of as many amphoteric gel-electrode interfaces as possible will provide us with electro-active polymer gels that are fast responsive, largely deformable with symmetric deformation mode.
Thin film metal/polymer composite electrodes with electrical conductivities on the order of those of bulk metals have been formed on electroactive polymer actuator elements using a novel self-assembly technique. The electrodes exhibit good flexibility and mechanical performance.
Ionic polymer platinum composite (IPPC) artificial muscles have been the subject of research activities at AMRI (Artificial Muscle Research Institute) and have been identified as smart intelligent material. The potential for such artificial muscles is so vast that muscles of different enhanced characteristics will be required in the future to accomplish different desired tasks. However the immediate challenges are to identify, control and enhance different desired characteristics of artificial muscles (IPPC). One important milestone that may be regarded, as the most critical one is to enhance force produced by these artificial muscles. Obviously force enhancement if successful may put these artificial muscles into one-to-one competition against the available line of traditional force actuators which fall in the same category. In order to experimentally approach the process of optimizing the force output of ionic polymeric platinum composite (IPPC) artificial muscles, an orthogonal array method was used to identify potential specific manufacturing procedures. These sets of procedures will eventually be helpful to identify the different desired characteristics of manufactured artificial muscles. One manufactured artificial muscles are tested for force outputs, the best ones would then be easily traced back to manufacturing procedure and will be further enhanced up to the desired levels by further refining the underlying manufacturing procedures. The measure chosen for optimization process was basically the force generated by a specific piece of muscle of specific geometry.
A polyconjugated and electroactive material was electrogenerated as a film by flow of anodic currents through solutions containing the monomer 2,5-di-(-2-thienyl)pyrrole, SNS, an electrolyte (LiClO4) and the solvent, acentonitrile. The weight of the electropolymerized material increases linearly with the consumed charge: the electrogeneration is a faradaic process. The oxidized material is insoluble in some electrolytes but it solves by electrochemical reduction following again this electrodissolution a faradaic process. Electropolymerization and electrodissolution are not reverse processes. The polymerization involves the generation of new covalent bonds in order to create a new material, the polymer, from the monomer. The flow of an anodic current through a solution formed by electrodissolution of different films deposits again an electroactive material. This electrodeposition is the opposite process, related to the electrodissolution, meanwhile, the productivity of the electropolymerization process is 1/4 that of those processes. Electrodissolution and electrodeposition mimic similar ways of processability using inorganic metals.
Ionic polymer metal composites, a subclass of electro-active polymer actuators, offer a promising approach to the problem of manipulating small objects, such as those found in micro- electro-mechanical systems (MEMS). While other technological alternatives exist, such as piezo-electric devices, each has at least one characteristic impeding its widespread adoption. A new class of ionic polymer metal composite (IPMC) artificial muscles has been developed at the UNM Artificial Muscles Research Institute (AMRI). IPMC actuators and sensors have been designed, fabricated and successfully tested. These artificial muscles are made from ionic polymeric (polyelectrolyte) gels chemically treated with platinum. IPMCs are three-dimensional networks of cross-linked macromolecular polyelectrolytes with internal electrodes that swell, shrink, bend, and deform in an electric field. Thus, direct computer control of large expansions and contractions of ionic polymeric gel-noble metal composite muscles by means of voltage controller has been achieved. They exhibit large motion sensing and actuation capabilities, can be produced relatively inexpensively, and can be cut arbitrarily small. Since these devices require only a few volts for actuation, they represent a safe alternative to many problems. This paper describes the design of a microgripper which uses both the actuation and sensing capabilities of these artificial muscles.
In this paper, we describe a high-performance organic polymer light-emitting device (OLED) fabricated on the ITO coated plastic substrates. The device, using air-stable Al as the cathode, has a bi-layer structure consisting of a hole transporting (amine-fluorene) and an emissive (benzothiadiazole-fluorene) conjugated polymer layers prepared by the spin-coating technique. Device shows a green light emission (approximately 570 nm), and has a good brightness (greater than 400 cd/m2), acceptable emission efficiency (greater than 6.4 cd/A), and high external quantum efficiency (greater than 1.8%).
We present a new approach for achieving spatial frequency filtering in the analog domain. Our device, the Thin Film Analog Image Processor (TAIP), is a hybrid structure that combines the strengths of analog VLSI technology with the simplicity of a conducting polymer film. The TAIP consists of a silicon chip with a square array of metal pads corresponding to the image pixels, onto which a conducting polymer film is applied to create lateral interaction between pixels. Analog image data (0 - 2 Volts) is multiplexed into the array, and the image is processed with up to 72 dB of resolution. The TAIP arrays are capable of performing either high or low pass spatial frequency filtering, operations that become computationally intensive for large images in the digital domain. Multiple arrays can be combined to create tunable bandpass spatial frequency filters that are capable of extracting features from complex images. Two array formats have been fabricated, 60 X 80 and 320 X 240, each capable of 60 Hz operation with a power consumption of approximately 100 mW.
Besides the scale factor that distinguishes the various spices fundamentally biological muscles changes little between species indicating a highly optimized system, Electroactive polymer actuators offer the closest resemblance to biological muscles however beside the large actuation displacement these materials are falling short with regards to the actuation force. As improved materials emerging it is becoming necessary to address key issues such as the need for effective electromechanical modeling and guiding parameters in scaling the actuators. In this paper, we will review the scaling laws for three major actuation mechanisms that are of relevance to micro electromechanical systems: electrostatic actuation, magnetic actuation, thermal bimetallic actuation, and piezoelectric actuation.
Integration of smart materials and control circuitry with MEMS devices to form smart structures is becoming an important technology. With the development of smart microsystems, integrated design tools become a growing need. We have developed a CAD tool suite for integrated MEMS and ICs, which can help MEMS designers shorten design and validation time. This suite enables both accurate device design and systems design by providing 3D analysis tools as well as system level design tools inspired by VLSI CAD. This integrated MEMS CAD tool suite aids in implementing mixed-technology designs in multi signal and energy domains. Our MEMS CAD tools include: (1) physical design, (2) mixed-domain FEA enabling device analysis, (3) schematic design entry and system level behavioral simulation, (4) macromodel extraction, (5) placement and routing synthesis tools, and (6) layout extraction and DRC verification tools. The strength of our IC- enabled MEMS design suite is that it allows recursive simulation and verification between device level and system level design, which is very important for smart structures and sensory systems that require a lot of IC control circuitry. Examples of microtransducers design in smart structure applications are provided to demonstrate the design tools. Discussion of the merit and limitations of our tools are also included.
One of the critical problems in the design of autonomous insects is power consumption. The independent control of several legs is energetically expensive, while the energy capacity of typical electrochemical batteries is quite small. The net result is autonomous robotic insects that have extremely limited range. The authors propose an alternative approach to this problem that enables autonomous robot insects that exhibit extremely high movement efficiency, and thus are capable of long range missions. Specifically, the desired limb motion is obtained by designing a lightly damped skeletal structure and exciting the skeletal structure at an appropriate resonance. The approach is called elastodynamic locomotion. Rather than altering the open-loop dynamics of the machine, as is the case with conventional-scale machine control, the control actuator serves only as an excitation source that excites the open-loop dynamics of the skeleton structure. Since the motion of the insect limbs operate at their structural resonance, the acceleration and deceleration for each motion (i.e.: stride for a walking machine) requires no power, which results in a highly efficient machine. Since the motion of the insect limbs is determined by design and not by control, the primary focus of this work is in the design of a skeletal structure that will exhibit walking motion when vibrationally excited. This paper presents various insect designs that will generate a walking or flying motion with minimal actuation.
The need for more enhanced blood pressure (BP), pulse rate and rhythm senors has given rise to the possibility of using ionic polymer-metal composites (IPMCs) sensors. In this study we propose to use the IPMC sensors to measure systolic and diastolic BP, pulse rate and rhythm. The proposed IPMC sensors take advantage of the endo-ionic mobility within the polymer- metal composite by converting normal and shear load inputs into an induced voltage output across the thickness of the IPMC sensor. The fabricated IPMC sensors are suitable to be installed on the inner surface of a cuff and, therefore, both systolic and diastolic BP, pulse rate, and rhythm can be measured. An added benefit is the ability of measuring 'pulse rhythm' which give a more amplified look at heart irregularities which a typical pulse rate sensor is unable to show. Our data shows IPMC sensors can produce consistent and reliable BP readings, pulse rate, and rhythm. Typically, a linear relationship between applied maximum load and induced maximum voltage was obtained. This result can be easily translated into good BP reading.
The volume change that the conducting polymer polypyrrole (PPy) undergoes upon electrochemical oxidation and reduction can be used to make microactuators. We have made microactuators based on a PPy/Au bilayer. These actuators have been combined with other micromachined structures to make biomedical microdevices. Using an area of bilayers one can potentially arrest (nerve) fibers. They can also be used to close a micrometer sized cavity with a lid. In addition, we demonstrate a microrobotic arm that may be developed for the manipulation of small particles.
Miniature, lightweight, miser actuators that operate similar to biological muscles can be used to develop robotic devices with unmatched capabilities to impact many technology areas. Electroactive polymers (EAP) offer the potential to producing such actuators and their main attractive feature is their ability to induce relatively large bending or longitudinal strain. Generally, these materials produce a relatively low force and the applications that can be considered at the current state of the art are relatively limited. This reported study is concentrating on the development of effective EAPs and the resultant enabling mechanisms employing their unique characteristics. Several EAP driven mechanisms, which emulate human hand, were developed including a gripper, manipulator arm and surface wiper. The manipulator arm was made of a composite rod with an EAP actuator consisting of a scrolled rope that is activated longitudinally by an electrostatic field. A gripper was made to serve as an end effector and it consisted of multiple bending EAP fingers for grabbing and holding such objects as rocks. An EAP surface wiper was developed to operate like a human finger and to demonstrate the potential to remove dust from optical and IR windows as well as solar cells. These EAP driven devices are taking advantage of the large actuation displacement of these materials but there is need for a significantly greater actuation force capability.
A perfluorinated cation-exchange membrane plated with noble metals was found to bend with electric stimuli in water or a saline solution. The bent to anode is almost proportional to the applied voltage around 1.5 V, which is low enough to avoid electrolysis of water. The response is as quick as muscle. Composite of perfluorocarboxylic acid and electrodes gave larger displacement than that of perfluorosulfonic acid. Gold electrodes were deposited on the polymer electrolyte with large surface area by repeated plating, and showed larger displacement without gas evolution than platinum electrodes. Alkyl ammonium cation in the composite gave slower but larger displacement than alkali metal cations. The displacement of the strip of actuator in typical dimension of 0.2 mm thick and 10 mm long is more than 5 mm without gas evolution. A tubular actuator with four electrodes was fabricated with the newly developed components and bent more than 90 degree in 2 cm length to all directions.
A weakly crosslinked poly(2-acrylamido-2-methylpropanesulfonic acid) immersed in cationic surfactant (N-n-alkylpyridinium chloride) shows biomimetic chemomechanical movement under dc current. The principle of the movement is based on an electrokinetic molecular assembly reaction of the surfactant onto the polymer network. In order to analyze the diffusion and binding processes which are both of importance for understanding the alkyl size dependence of the chemomechanical behavior, kinetic studies of the surfactants binding were made systematically changing the alkyl size and concentration of the surfactants and ionic strength. It was found that the driving force of the surfactant diffusion is the electrochemical potential gradient, while the surfactant binding enhances the diffusion proces. A mathematical model for the surfactant diffusion was developed taking account of the surfactant binding process and obtained results were well explained the experimental observations.
Miniature, lightweight, miser actuators that operate similar to biological muscles can be used to develop robotic devices with unmatched capabilities and impact many technology areas. Electroactive polymers (EAP) offer the potential to producing such actuators and their main attractive feature is their ability to induce relatively large bending or longitudinal strain. Generally, these materials produce a relatively low force and the applications that can be considered at the current state of the art are relatively limited. While improved material are being developed there is a need for methods to develop longitudinal actuators that can contract similar to muscles. In addition, it is desirable to have these actuators in a fiber form that can be bundled to provide the necessary characteristics of stiffness, fracture toughness, resilience and large force actuation. To address this need efforts were made to develop both the material basis as well as the electromechanical modeling of the actuator.
The solid polymer fuel cell (SPFC) technology is one of the most promising sources of future energy. Its high power density and mild operating conditions make the SPFC technology highly attractive for stationary, portable, and automobile applications. In this paper, we briefly discuss the potential use of electro-active polymer materials for the SPFC technology. In order to realize the fast intrinsic kinetics of the cathode reaction an efficient utilization of the Pt catalyst is necessary. In this sense, we introduce a novel concept of a fabrication technique of the membrane-electrode assembly (MEA) that consists of a Pt-deposited ion exchange membrane and two current collectors. It appears that the manufacturing process of such MEAs is simple, efficient, and economical relative to the current state-of-art MEA technology that employs various particle distribution techniques. Also, it should be pointed out that the use of this new MEA fabrication technique could improve the rate density of H+ transport significantly.
In this work the effect of surface-electrode resistance on the actuation of ionic polymer-metal composites (IPMCs) artificial muscles is investigated. The as-received ion-exchange membrane (IEM) was platinum-composited by using a unique chemical processing technique that employs a platinum-salt and appropriate reducing agents. The IPMCs artificial muscles were optimized for producing maximum forces by changing multiple process parameters including time-dependent concentrations of the salt and reducing agents. The analytical results confirmed that the platinum electrode is successfully deposited on the surface of the IEM where platinum particles stay in a dense form that appears to introduce a significant level of surface- electrode resistance. In order to address this problem, a thin layer of silver (or copper) was electrochemically deposited on top of the platinum electrode to reduce the surface-electrode resistance. Actuation tests were performed for such IPMC artificial muscles under a low voltage. Tests results show that the lower surface-electrode resistance generates the higher actuation capability in the IPMCs artificial muscles. This observation is briefly discussed based on an equivalent circuit theory regarding the IPMC and a possible electrophoretic cation-transport phenomenon under the influence of an electric field.
Currently, it is difficult to measure the pressure distribution and motion within the human spine, for both in vivo and in vitro situations. This study proposed that small thin strips of ionic polymer-metal composites (IPMC) can be used as pressure transducers for the quantification of pressure distributions within the spine. Endo-ionic motion within IPMC sensors produces an induced voltage across the thickness of these sensors when a normal or shear load is applied. A materials testing system (MTS) was used to apply consistent pure compressive loads of 200 N and 350 N across the surface of an IPMC 2 X 2 cm strip. The output pressure response for the 200 N load (73 psi) was 80 mV in amplitude and for the 350 N (127 psi) it was 108 mV. Due to their small size, IPMC sensors have the benefit of being able to fit within small gaps in the spine. These fluid-filled gaps (facet joints) range in thickness from 1 - 3 mm, which is generally too small for typical transducers. The biocompatibility issue is also not of concern since the sensors themselves are biologically inert and, if necessary, can be coated with various flexible biocompatible materials.
Research was done on a biomimetic building material with the unique properties of bone. Bone, as well as other natural materials such as shell, obtains its toughness and strength as a result of utilizing optimum materials, structural form and carefully controlling the process of bone formation. The organic fibers are made first and the matrix grown around them as opposed to conventional ceramics in which any fibers are added to the matrix. The research presented focuses on creating a polymer/cement composite which mimics bone by controlling the chemical makeup and sequencing of fabrication. The rules under which bone material naturally forms, albeit at a macroscale, are followed in order to match the intimate connection between material phases of bone. The research presented here uses cement and condensation polymers. The proposed design more carefully controls the formation process by utilizing the symbiotic relationship of the two material formation reactions. The inorganic phase formation is initiated and controlled by the organic phase formation as occurs in bone formation. Like bone this new material offers great opportunity for improved mechanical and chemical bonding.
By electrochemical stimulation of conformational changes along a chain of a conjugated polymer a molecular muscle is obtained. The electrochemically stimulated conformational relaxation model allows a good theoretical description of the electrochemical chronoamperograms, voltammograms or chronocoulograms performed under influence of different chemical or physical variables. Electrochemical and structural magnitudes were integrated in the same model. Through a geometrical model three dimensional processes (change of volume) occurring in polymeric films were transformed in one dimensional change, describing bilayer effects, angles described by a bilayer muscle and angular movements as a function of the current flowing by the device. Being the current a function of both, electrochemical and structural magnitudes, angular movements can be described as a function of then.
In this paper we give a summary of the effects of electric field on the shape and motility of polymer gels. We look over the advantages and disadvantages of the different stimuli from the applicability point of view. We also present the characteristic features of shape distortion of magnetic field sensitive gels which have been developed in our laboratory. A comparative study has been performed on the efficiency of biological muscles and magnetic gels. A simplified model for ferrogels functioning as artificial muscles is also presented.
The long term operation of two electrode all polymer devices requires a balance between extending the potential limits as far as possible to give full oxidation and reduction of the CEPs, while ensuring that the potentials at which overoxidation occurs are not reached. In this work, the'true' potential (vs Ag/AgCl) at one electrode in a two-electrode cell was determined while applying a range of potential differences between the two polypyrrole electrodes. At an applied potential of plus or minus 2 V, only slight overoxidation of polypyrrole was evident at either electrode. However, for long term applications of polymer films it was shown that plus or minus 1.5 V would be more suitable. This potential difference was applied over an extended period in a series of potential steps to a two electrode device in a propylene carbonate solution and found to have no deleterious effects on the polymer electrodes. It was also shown that this potential difference was sufficient to fully oxidize and reduce PPy both in a supporting electrolyte solution and as a component of a two-electrode solid state device.