Biomedical Engineering Reference
In-Depth Information
were discovered over half-a-century ago when collagen filaments were demonstrated to revers-
ibly contract or expand upon dipping into acidic or alkaline aqueous solutions, respectively
(Katchalsky, 1949). Even though relatively little has since been done to apply such ''chemo-
mechanical'' as practical actuators, this early work pioneered the development of synthetic poly-
mers that mimic biological muscles (Steinberg et al., 1966). The convenience and practicality
of electrical stimulation, and technology progress led to a growing interest in EAP materials.
Following the 1969 observation of a substantial piezoelectric activity in polyvinylidene fluoride
(PVDF) (Bar-Cohen et al., 1996; Zhang et al., 1998), investigators started to examine other
polymer systems, and a series of effective materials have emerged. The largest progress in EAP
materials development has been reported in the last 15 years (Bar-Cohen, 2001, 2004) where
materials that can create linear strains that can reach up to 380% have been developed (Pelrine
et al., 2000; Kornbluh et al., 2004).
Generally, EAP can be divided into two major groups based on their activation mechanism:
ionic (involving mobility or diffusion of ions) and electronic (driven by electric field or Maxwell
Forces) (Bar-Cohen, 2001, 2004). A summary of the advantaged and disadvantages of these two
group of materials are listed in Table 10.1. The electronic polymers (electrostrictive, electrostatic,
piezoelectric, and ferroelectric) are driven by Maxwell Forces and can be made to hold the induced
displacement under activation of a DC voltage, allowing them to be considered for robotic
applications. Also, these materials have a greater mechanical energy density and they can
be operated in air with no major constraints. However, they require a high activation field
(
>
100-V/mm) close to the breakdown level. In contrast, ionic EAP materials (gels, polymer-
metal composites, conductive polymers, and carbon nanotubes) are driven by diffusion of ions
and they require an electrolyte for the actuation mechanism. Their major advantage is the
TABLE 10.1
A Summary of the Advantages and Disadvantages of the Two Basic EAP Groups
EAP type
Advantages
Disadvantages
Electronic EAP
.
Can operate for a long time in room
conditions
.
Requires high voltages (~100 MV/m).
Recent development allowed for (~20
MV/m) in the Ferroelectric EAP
.
Exhibits rapid response (milliseconds)
.
Can hold strain under DC activation
.
Independent of the voltage polarity, it
produces mostly monopolar actuation
due to associated electrostriction
effect
.
Induces relatively large actuation
forces
.
Exhibits high mechanical energy
density
Ionic EAP
.
Natural bi-directional actuation that
depends on the voltage polarity
.
Requires using an electrolyte
.
Requires encapsulation or protective
layer in order to operate in open air
conditions
.
Low electromechanical coupling
efficiency
.
Electrolysis occurs in aqueous
systems at more than 1.23 V
.
Except for CPs and NTs, ionic EAPs
do not hold strain under DC voltage
.
Slow response (fraction of a second)
.
Bending EAPs induce a relatively low
actuation force
.
Except for CPs, it is difficult to produce
a consistent material (particularly
IPMC)
.
Requires low voltage
.
Some ionic EAP like conducting
polymers have a unique capability of
bi-stability
.
High currents require rare earth
electrodes such as gold or platinum
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