Biomedical Engineering Reference
In-Depth Information
FIGURE 16.8
Scleral bands made of IPMC actuators conceived for corrections of ocular refractive errors.
(Adapted from Shahinpoor, M., and Kim, K.J.,
Smart Mater. Struct
., 14, 197-214, 2005. With permission.)
Within the biomedical fi eld, conjugated polymers present very interesting properties: biocompatibil-
ity, improvement of regeneration of tissues (e.g., uses in nerve guidance channels), neural communi-
cation, modulation of surface properties of neural recording electrodes, functionality as substrates
for cell cultures, possibility of being produced in biodegradable/bioerodible forms, and possibility
of being doped with biomolecules for controlled releases [46-50].
For the specifi c topic presented in this chapter, another property is, however, emphasized here:
their actuation capabilities. For this purpose, conducting polymers are used as components of an
electrochemical cell whose basic structure includes two electrodes immersed in an electrolyte. The
conducting polymer material constitutes one or both the electrodes of the cell. By applying a poten-
tial difference between them, redox reactions cause strongly anisotropic and reversible volume vari-
ations of the material [51-53], which can be used for actuation [51-74]. In particular, three effects
have been found to be responsible for dimensional and volume changes in conducting polymers:
interactions between polymer chains, variation of the chain conformation, and insertion of coun-
terions. The third effect is generally considered to be the most dominant. In fact, the commonly
accepted explanation of the observed deformations attributes the dimensional changes to the input/
output of ions (exchanged with the surrounding media) into/from the polymer sample, driven by an
applied voltage. In particular, the voltage produces a variation of the polymer oxidation state, caus-
ing the necessary modifi cation of the number of ions associated to each chain in order to maintain
the global electroneutrality.
Several actuating confi gurations have been proposed in order to exploit the electromechanical
properties of these materials. The most diffused structure is represented by the so-called unimorph
bilayer bender. This kind of actuator consists of a fi lm of active material coupled to a passive sup-
porting layer. The structure is operated within an electrochemical cell, having a liquid electrolyte
in which the device is immersed. The active polymeric layer of the actuator works as one electrode
of the cell, while a counterelectrode and a third reference electrode are separately immersed in the
electrolyte. One end of the bilayer is constrained, while the other end is free. The potential differ-
ence applied between the electrodes causes redox reactions of the conducting polymer. Since the
conducting polymer and the passive layers are mechanically interlocked, when the polymer swells/
shrinks, the passive layer that cannot modify its dimensions, transforms the conducting polymer
displacement into a bending movement of the structure [70-74]. A different type of bender can be
