Mohsen Shahinpoor

Ionic polymer-metal composites: I. Fundamentals

This paper, the first in a series of four review papers, presents a brief summary of the fundamental properties and characteristics of ionic polymeric-metal composites (IPMCs) as biomimetic sensors, actuators and artificial muscles. The forthcoming three review papers, to follow this paper, will address in detail such fundamentals and, in particular, manufacturing techniques and the electronic and electromechanical characteristics of IPMCs (part II), the phenomenological modelling of the underlying sensing and actuation mechanisms in IPMCs (part III) and the potential application areas for IPMCs (part IV). This paper is a summary of all recent findings and current state-of-the art manufacturing techniques, phenomenological laws and mechanical and electrical characteristics. A number of methodologies in developing high-force-density IPMCs are also reported.

Ionic polymer-metal composites (IPMCs) as biomimetic sensors, actuators and artificial muscles - a review

This paper presents an introduction to ionic polymer-metal composites and some mathematical modeling pertaining to them. It further discusses a number of recent findings in connection with ion-exchange polymer-metal composites (IPMCs) as biomimetic sensors and actuators. Strips of these composites can undergo large bending and flapping displacement if an electric field is imposed across their thickness. Thus, in this sense they are large motion actuators. Conversely by bending the composite strip, either quasi-statically or dynamically, a voltage is produced across the thickness of the strip. Thus, they are also large motion sensors. The output voltage can be calibrated for a standard size sensor and correlated to the applied loads or stresses. They can be manufactured and cut in any size and shape. In this paper first the sensing capability of these materials is reported. The preliminary results show the existence of a linear relationship between the output voltage and the imposed displacement for almost all cases. Furthermore, the ability of these IPMCs as large motion actuators and robotic manipulators is presented. Several muscle configurations are constructed to demonstrate the capabilities of these IPMC actuators. This paper further identifies key parameters involving the vibrational and resonance characteristics of sensors and actuators made with IPMCs. When the applied signal frequency varies, so does the displacement up to a critical frequency called the resonant frequency where maximum deformation is observed, beyond which the actuator response is diminished. A data acquisition system was used to measure the parameters involved and record the results in real time basis. Also the load characterizations of the IPMCs were measured and it was shown that these actuators exhibit good force to weight characteristics in the presence of low applied voltages. Finally reported are the cryogenic properties of these muscles for potential utilization in an outer space environment of a few Torrs and temperatures of the order of -140 degrees Celsius. These muscles are shown to work quite well in such harsh cryogenic environments and thus present a great potential as sensors and actuators that can operate at cryogenic temperatures.

Ionic polymer-metal composites: IV. Industrial and medical applications

This paper, the last in a series of four review papers to appear in this journal, presents some critical applications using ionic polymer?metal composites?(IPMCs). Industrial and biomedical applications of IPMCs are identified and presented along with brief illustration.

Ionic polymer–metal composites: II. Manufacturing techniques

This paper, the second in a series of four review papers to appear in this journal, presents a detailed description of various techniques and experimental procedures in manufacturing ionic polymer–metal composites (IPMCs) that, if fully developed, can be used as effective biomimetic sensors, actuators and artificial muscles as well as fully electroded with embedded electrodes for fuel cells. The performance of IPMCs manufactured by different manufacturing techniques are presented and compared. In particular, a number of issues such as force optimization using the Taguchi design of experiment technique, effects of different cations on electromechanical performance of IPMCs, electrode and particle size and distribution control, manufacturing cost minimization approaches, scaling and three-dimensional (3D) muscle production issues and heterogeneous composites by physical loading techniques are also reviewed and discussed.

Ionic polymer–metal composites: III. Modeling and simulation as biomimetic sensors, actuators, transducers, and artificial muscles

This paper, the third in a series of four review papers to appear in this journal, presents a number of descriptions of various modeling and simulation techniques and, briefly, the associated experimental results in connection with ionic polymer–metal composites and, in general, ionic polymer–conductor composites, as soft biomimetic distributed sensors, actuators, transducers, and artificial muscles.

The effect of surface-electrode resistance on the performance of ionic polymer-metal composite (IPMC) artificial muscles

Reported in this study are the effects of the surface-electrode resistance on the performance of ionic polymer-metal composite (IPMC) artificial muscles. The IPMC artificial muscles manufactured in this study is composed of a perfluorinated ion-exchange membrane, platinum composited by using a chemical processing technique that employs a platinum salt and appropriate reducing agents. Furthermore, the IPMC artificial muscles were optimized for producing improved forces by changing multiple process parameters including the time-dependent concentrations of the salt and reducing agents. However, the analytical results confirmed that the platinum electrode is successfully deposited on the surface of the material 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. The test results show that the lower surface-electrode resistance generates higher actuation capability in the IPMC artificial muscles. This observation is briefly discussed based on the role that the equivalent circuit for the IPMC plays and a possible electrophoretic cation-transport phenomenon under the influence of an electric field.

Ionic polymer–conductor composites as biomimetic sensors, robotic actuators and artificial muscles—a review

This paper will first cover a brief summary of the fundamental properties and characteristics of ionic polymeric-conductor composites (IPCCs) as biomimetic sensors, robotic actuators and artificial muscles. It will then address such fundamentals in more details. In particular, it will discuss the manufacturing techniques including a comparison between chemical plating and physical loading of a conductor phase, the electronic and electromechanical characteristics of IPCCs, the phenomenological modeling of the underlying sensing and actuation mechanisms in IPCCs, some preliminary constitutive modeling of such complex electroactive materials, as well as some potential industrial and medical application areas for IPCCs, respectively. It will also briefly discuss the potential of IPCCs in developing distributed biomimetic micro- and nano sensors and actuators.

Micro-Electro-Mechanics of Ionic Polymeric Gels As Electrically Controllable Artificial Muscles

Ionic polymeric gels are three-dimensional networks of cross-linked macromlecular polyelectrolytes that swell or shrink in aqueous solutions on addition of alkali or acids, respectively. Reversible dilation and contraction of the order of more than 1000 percent have been observed in our laboratory for polyacrylonitrile (PAN) fibers. Furthermore, it has been experimentally observed that swelling and shrinking of ionic gels can also be induced electrically. Thus, direct computer control of large expansions and contractions of ionic polymeric gels by means of a voltage gradient appears to be possible. These gels possess an ionic structure in the sense that they are generally composed of a number of fixed ions (polyions) pertaining to sites of various polymer cross-links and segments and mobile ions (counter ions or unbound ions) due to the presence of a solvent which is electrolytic. Electrically-induced dynamic deformation of ionic polymeric gels such as polyacrylic acid plus sodium acrylate cross-linked with bisacrylamide (PAAM), or poly(2-acrylamido-2-methylpropanesulfonic acid) or PAMPS or various combinations of chemically-doped polyacrylic acid plus polyvinyl alcohol (PAA-PVA) can be easily observed in our laboratory. Such deformations give rise to an internal molecular network structure with bound ions (polyions) and unbound or mobile ions (counterions) when submerged in an electrolytic liquid phase. In the presence of an electric field, these ionic polymeric networks undergo substantial contraction accompanied by exudation of the liquid phase contained within the network. Under these circumstances, there are generally four competing forces acting on such ionic networks: the rubber elasticity, the polymer-liquid viscous interactions due to the motion of the liquid phase, inertial effects due to the motion of the liquid through the ionic network, and the electrophoretic interactions. These forces collectively give rise to dynamic osmotic pressure and network deformation and subsequently determine the dynamic equilibrium of such charged networks. On the other hand there are situations in which a strip of such ionic polymeric gels undergoes bending in the presence of a transverse electric field with hardly any water exudation. Under these circumstances there are generally three competing forces acting on the gel polymer network: the rubber elasticity, the polymer-polymer affinity and the ion pressure. These forces collectively create the osmotic pressure which determines the equilibrium state of the gel. The competition between these forces changes the osmotic pressure and produces the volume change or deformation. Rubber elasticity tends to shrink the gel under tension and expand it under compression. Polymer-polymer affinity depends on the electrical attraction between the polymer and the solvent. Ion pressure is the force exerted by the motion of the cations or anions within the gel network. Ions enter the gel attracted by the opposite charges on the polymer chain while their random motions tend to expand the gel like an ionic (Fermionic) gas. Two mechanisms are presented for the reversible nonhomogeneous large deformations and in particular contraction/expansion with water exudation as well as bending of strips of ionic polymeric gels in the presence of an electric field. An analytical model is first presented for the dynamics of contraction of ionic polymeric gels with liquid exudation in the presence of an electrical field. The proposed model considers the dynamic balance between the internal forces during the contraction. These forces are assumed to be due to the viscous effects caused by the motion of the liquid, the inertial forces due to the motion of the liquid in and out of the network, and the electrophoretic forces due to the motion of the charged ions in the solvent as it exudes from the ionic polymeric gel network. The effects of rubber elasticity of the network as well as ion-ion interactions have been assumed negligible in this case compared with the inertial, viscous and electrophoretic effects. The governing equations, thus obtained are then solved exactly for the velocity of liquid exudation from within the network as a function of time and radial distance in cylindrical samples. The relative weight of the gel sample is then related to this velocity by an integral equation. This integral equation is then numerically solved to obtain a relationship between the amount of contraction as a function of time, electric field strength and other pertinent material and geometrical parameters. The results of the numerical simulations are compared with some experiment results on PAMPS contractile fibers and satisfactory agreements are observed. Next, the case of electrically-induced bending of strips of ionic polymeric gels is considered. Exact expressions are given relating the deformation characteristics of the gel to the electric field strength or voltage gradient, gel dimensions and other physical parameters such as the resistance and the capacitance of the gel strip. It is concluded that direct voltage control of such nonhomogeneous large deformations in ionic polymeric gels is possible.

Biomechanics of the Coracoclavicular Ligament Complex and Augmentations Used in Its Repair and Reconstruction

Augmentation is a well-accepted and common component of coracoclavicular ligament repairs and reconstructions. The purpose of this study was to examine and compare the strength, stiffness, and mode of failure of the coracoclavicular ligament complex and four different augmentation techniques in cadaveric shoulders. There was no significant difference in the mean failure load between the intact ligament complex (724.9 230.9 N) and augmentations performed with braided polydioxanone (PDS) (676.7 115.4 N) or braided polyethylene placed through (986.1 391.1 N) or around (762.7 218.2 N) the clavicle. The mean failure load for augmentations using a 6.5-mm cancellous screw through the clavicle and into a single cortex of the coracoid (390.1 253.6 N) was significantly lower than that for the intact coracoclavicular ligaments. There was no difference in mean stiffness between the intact coracoclavicular ligament complex (115.9 36.2 N/mm) and the braided polyethylene augmentations placed through (99.8 22.2 N/mm) or around (90.0 25.5 N/mm) the clavicle. Polydioxanone augmentations were significantly less stiff (27.4 3.3 N/mm) than the intact complex, while screw augmentations were significantly stiffer (250.4 88.2 N/mm). There were no significant differences in strength or stiffness of braided polyethylene reconstructions placed around or through a drill hole in the clavicle.

Biomimetic robotic propulsion using polymeric artificial muscles

Biomimetic fish-like propulsion using polyelectrolyte ion-exchange membrane metal composites as a propulsion fin for a robotic swimming structure, such as a boat swimming in water medium, was investigated. The membrane was chemically plated with platinum. The resulting membrane was cut in a strip to resemble fish-like caudal fin for propulsion. A small function generator circuit was designed and built to produce approximately /spl plusmn/2.0 V amplitude signal at desired frequency up to 50 Hz. The circuit board was mounted on a buoyant Styrofoam shaped like a boat or a tadpole. The fin was attached to the rear of the boat. By setting the signal frequency to the desired value and thereby setting the frequency of bending oscillation of the membrane, a proportional forward propulsion speed could be obtained. The speed was then measured using a high speed camera. Several theoretical hydrodynamic models were then presented to characterize speed-frequency of the forward motion using available theories on biological fish motion. The results were compared to experimental data which showed close agreement.