Biomedical Engineering Reference
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electroactive polymers (EAP) that exhibit a large strain in response to electrical stimulation. For
this response, EAP have earned the moniker ''artificial muscles'' (Bar-Cohen, 2001, 2004). The
impressive advances in improving their actuation strain capability are attracting the attention of
engineers and scientists from many different disciplines. These materials are particularly attractive
to biomimetics since they can be used to mimic the movements of humans, animals, and insects for
making biologically inspired intelligent robots (Bar-Cohen and Breazeal, 2003) and other biomi-
metic mechanisms. Increasingly, engineers are able to develop EAP-actuated mechanisms that
were previously imaginable only in science fiction.
For many decades, it has been known that certain types of polymers can change shape in
response to electrical stimulation. Initially, these EAP materials were capable of producing only a
relatively small strain. Since the beginning of the 1990s, new EAP materials emerged exhibiting
large strains and leading to a great paradigm change with regard to the capability of EAP and their
potential. Generally, EAP materials can generate strains that are as high as two orders of magnitude
greater than the striction-limited, rigid, and fragile piezoelectric ceramics. Further, EAP materials
are superior to shape memory alloys (SMA) in higher response speed, lower density, and greater
resilience. They can be used to make mechanical devices without the need for traditional compon-
ents like gears, and bearings, which are responsible for the current high costs, weight, and
premature failures. The current limitations of EAP materials that include low actuation force,
mechanical energy density, and robustness constrain the practical application but improvements in
the field are expected to overcome these limitations.
In 1999, in recognition of the need for international cooperation among the developers, users,
and potential sponsors, the author initiated a related annual SPIE conference as part of the
Smart Structures and Materials Symposium (Bar-Cohen, 1999). This conference was held in
Newport Beach, California, USA and was the largest ever on this subject, marking an important
milestone and turning the spotlight onto these emerging materials and their potential. The
SPIE EAP Actuators and Devices (EAPAD) conferences are now organized annually and have
been steadily growing in number of presentations and attendees. Also, the author releases the
semiannual WW-EAP Newsletter electronically, and mentors a website that archives related
information and includes links to homepages of EAP research and development facilities world-
wide (http://eap.jpl.nasa.gov). In the past few years, in addition to the SPIE conferences, several
other conferences and special sessions within conferences focusing on EAP actuators have also
taken place.
10.2
HISTORY AND CURRENTLY AVAILABLE ACTIVE POLYMERS
The beginning of the field of EAP can be traced back to an 1880-experiment that was conducted by
Roentgen using a rubber-band with fixed end and a mass attached to the free end that was subjected
to electric field across the rubber-band (Roentgen, 1880). Sacerdote (1899) followed this experi-
ment with a formulation of the strain-response to electric field activation. Further milestone
progress was recorded in 1925 with the discovery of a piezoelectric polymer called electret when
carnauba wax, rosin, and beeswax were solidified by cooling while subjected to a DC-bias field
(Eguchi, 1925). Generally, electrets are polymer materials with aligned electrical dipole moments
equivalent to magnets, and they are deformed when subjected to voltage across them. However,
their strain and work output is generally too low to be applicable as actuators and therefore their use
has been limited to sensors.
Generally, electrical excitation is only one of the types of stimulators to cause elastic deform-
ation in polymers. Other activation mechanisms include chemical (Kuhn et al., 1950; Otero et al.,
1995), thermal (Li et al., 1999), pneumatic (Chou and Hannaford, 1994), optical (van der Veen
and Prins, 1971), and magnetic (Zrinyi et al., 1997). Polymers that are chemically stimulated
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