Biomedical Engineering Reference
In-Depth Information
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Introduction to Ionic
Polymers, Ionic Gels,
and Artificial Muscles
1.1
INTRODUCTION
The focus of many scientists and researchers has been to achieve efficiencies as high as
50% in direct conversion of chemical to mechanical energy as occurs in biological
muscles. In comparison, most internal combustion engines and steam turbines have about
30% efficiency at best. First, it would be useful to give a brief review of electroactive
polymers in general. For a brief description of biological muscles, see appendix A.
1.2
A BRIEF HISTORY OF ELECTROACTIVE POLYMERS
(EAPS) AND ARTIFICIAL/SYNTHETIC MUSCLES
Roentgen (1880) appears to have been the first to make an electroactive polymer.
He used a rubber band that could change its shape by being charged or discharged.
Later, Sacerdote (1899) formulated the strain response to electric field activation in
polymers. In 1925 Eguchi (1925) reported the discovery of a piezoelectric polymer
that he called an electret. He found that when molten carnauba wax, rosin, and
beeswax were solidified by cooling in the presence of a DC electric field, the resulting
material was piezoelectric. Generally, electrical excitation is only one type of stim-
ulator that can induce elastic deformation in polymers. Other activation mechanisms
include chemical (Katchalsky, 1949; Kuhn et al., 1950; Steinberg et al., 1966;
Shahinpoor, 1992, Shahinpoor et al., 1997a, 1997b; Otero et al., 1995), thermal
(Kishi et al., 1993; Tobushi et al., 1992; Li et al., 1999), pneumatic (Shahinpoor et
al., 2001), optical (van der Veen and Prins, 1971), and magnetic (Zrinyi et al., 1997).
Chemically stimulated polymers were discovered more than half a century ago
when it was shown that collagen filaments could reversibly contract or expand when
dipped in acidic or alkaline solutions, respectively (Katchalsky, 1949). This early
work pioneered the development of synthetic polymers that mimic biological muscles
(Steinberg et al., 1966; Shahinpoor et al., 1998). However, electrical stimulation has
remained the best means of EAP material actuation and sensing. Shahinpoor and
Mojarrad (1994, 1996, 1997a, 1997b, 1997c, 1997d, 2000) were among the pioneers
in making electrically active—in sensing and actuation—ionic polymer conductor
nanocomposites (IPCNCs) and ionic polymer-metal nanocomposites (IPMNCs).
Zhang and colleagues (1998) were able to observe a substantial piezoelectric activity
in PVF2-TrFE as early as 1998.
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