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Biological Materials in Engineering Mechanisms 375

Electroactive polymers often exhibit the closest performance resemblance to biological muscle.
They are considered to be electrically hard but mechanically soft materials. There are many types of
actuators which fall into this category with varying characteristics and properties. Ionic polymer–
metal composites are capable of large deformations in response to low applied voltage (Jung et al.,
2003). These structures are composed of an ion exchange polymer such as a perfluorinated ion
membrane chemically deposited with a metal such as gold or platinum. The metal ions are
dispersed throughout the hydrophilic regions of the polymer. Bending is under the control of
diffusion and Coulomb forces, occurring as a result of mobile ions migrating within the polymer
network due to the application of an electric field. These structures do not function in a dry
environment, necessitating a protective coating to retain moisture for function. Electrolysis occurs
at high voltages, which result in degradation of the material and the release of heat and gases,
restricting allowable voltage. Force generation is proportional to the thickness of the material. Ionic
polymer–metal composites have a fast response time, are tough, and can achieve large actuation
strain. They are light, compact, soft, and can be miniaturized. Conducting polymers such as
polypyrrole have been evaluated as artificial muscles (Otero et al., 1996). Volume changes are
induced by the movement of charge compensating ions to or from the polymer layers during
oxidation and reduction reactions. During oxidation electrons are extracted from the polymer
chains, resulting in the rearrangement of bonds and storage of positive charges. Conformational
movements of the chains in order to maintain electroneutrality generate free volume, which are
occupied by counterions and water molecules from the solution, resulting in swelling of the film.
During reduction, these series of events are reversed, promoting shrinkage of the film. A polymeric
triple layer muscle made of polypyrrole has demonstrated smooth, uniform movement comparable
to that of natural muscle. In addition to reproducible actuation, the rate and direction of movement
can be controlled by applied current conditions. Polymer actuators exhibit large strain and high
strength, are lightweight, and can operate at room or physiological temperatures. Actuation of
conducting polymers can be achieved at low voltages. Development of conjugated polymer
actuators containing a polymer electrolyte, which functions as the electron source and sink,
between polypyrrole layers has allowed for ‘‘dry’’ actuator function outside of an aqueous solution.
Carbon single-walled nanotube sheets exhibit properties such as mechanical flexibility, high
toughness, and electrochemical behavior that have potential for development as artificial muscles
(Baughman et al., 1999). Unlike conducting polymers in which actuation is based on
electrochemical dopant intercalation, carbon nantotube actuation results from quantum chemical
and double-layer electrostatic effects. Dimensional changes occur in covalently bonded directions
caused by a double charge injection. Applied voltage change injects an electronic charge into the
carbon nanotube sheet that is compensated at the nanotube electrolyte interface by electrolyte
ions, the double-layer. Carbon nanotube (CNT) actuator sheets are nanoscale actuators that are
organized into nanotube arrays in a manner similar to natural muscle. CNTs exhibit high work
density and good work capacity per cycle. Compared to natural muscle, CNT-actuators are
stronger and more durable. Activation only requires low voltages and the material is capable
of operation at high temperatures. The material is light, shows fast response, has a long life cycle,
and is capable of large displacements. Like most electroactive polymers, actuation requires an
electrolyte solution.
Other examples include vanadium oxides which can be used in various redox-dependent
applications such as the conversion of electrical energy into mechanical energy (Gu et al., 2003).
Also, hydrogels are an intermediate between a liquid and solid state, consisting of a polymer
network and interstitial fluid. Gel actuators made of materials such as poly(vinyl alcohol), poly-
acrylonitrile, polyacid acrylic, and polyacrylamide can undergo large volume changes in response
to stimuli such as pH, temperature, or electricity. Bending action can be induced by controlled
swelling. Conformational changes are dependent on the diffusion of solvent through the gel and as a
result of the amorphous nature of many gels, response time can be slow. The size of the gel and
distance for fluid flow are other factors that can affect response time. Hydrogels also tend to lack