Artificial Muscles Using Electroactive Polymers - Biomimetics: Biologically Inspired Technologies - page 275

reduction occur at the electrodes, inducing a considerable volume change due mainly to the
exchange of ions with an electrolyte. A sandwich of two conductive polymer electrodes (e.g.,
polypyrrole or polyaniline (PAN) doped in HCl) with an electrolyte between them forms an EAP
actuator. When a voltage is applied between the electrodes, oxidation occurs at the anode and
reduction at the cathode. Ions (e.g., H þ in PAN) migrate and diffuse between the electrolyte and the
electrodes to balance the electric charge. Addition of the ions causes swelling of the polymer and
conversely their removal results in shrinkage and as a result the sandwich bends (Figure 10.5). One
of the parameters that affects the response is the thickness of the layers; thinner layers are faster (as
fast as 40 Hz) (Madden et al., 2000, 2001) but induce lower force. Since strong shear forces act on
the electrolyte layer, attention is needed to protect the material from premature failure. In addition,
actuators made of this type of EAP materials are sensitive to cyclic operation and they tend to
fail after several tens of cycles. Conductive polymer actuators generally require voltages in the
range of 1 to 5 V, and the speed increases with the voltage having relatively high mechanical energy
densities of over 20 J/cm 3 but with low efficiencies at the level of 1% if no electrical energy is
recovered (Madden et al., 2002).
In recent years, several conductive polymers were reported, including polypyrrole, polyethyle-
nedioxythiophene, poly( p -phenylene vinylene)s, polyanilines, and polythiophenes (Anquetil et al.,
2002). Complexes between polypyrrole and sulfonated detergents offer relatively good stability in
aqueous media, but they are relatively soft compared to other conjugated polymers. Most actuators
that use conductive polymers exploit voltage-controlled swelling to induce bending. Conjugated
polymer microactuators were first fabricated at Link ¨ pings University, Sweden. Among the devices
that were demonstrated include a miniature box that can be opened and closed electrically (Smela
et al., 1995). Efforts are also being made to develop linear CP actuators where a number of groups
have used polypyrrole for this purpose. The reported actuators were measured to produce a stress of
approximately 5 MPa with moderate strains (~ 2%) (Kaneto et al., 1995). Current efforts at Eamax,
Japan, seem to suggest significant increase in the actuation capability that can be obtained using
conductive polymer EAP materials.

Figure 10.5 Conductive EAP actuator is shown bending under stimulation of 2 V, 50 mA.