Biomedical Engineering Reference
In-Depth Information
Figure 10.2 (See color insert following page 302)
Two-DOF Spring Roll. (Courtesy of SRI International, Menlo
Park, CA, U.S.A.)
EAP. A variety of other actuator configurations are also possible (Kornbluh et al., 2004). Dielectric
elastomer EAP actuators require large electric fields (~100 V/mm) and can induce significant levels
of strain (10 to about 380%). Overall, the associated voltages are close to the breakdown strength of
the material, and a safety factor that lowers the potential is used. Moreover, the relatively small
breakdown strength of air (2 to 3 V/mm) presents an additional challenge for packaging the
actuator. Another concern that is associated with the use of dielectric elastomer EAP is that the
actuator requires prestraining the elastomer film and over time the prestrain is released due to creep
degrading the actuator performance. Research at Sungkyunkwan University, Korea, shows promise
with regard to actuators that do not require prestrain (Jung et al., 2004). Recently, an SRI
International research team designed a multi-functional electroelastomer roll (MER) in which
highly prestrained dielectric elastomer EAP films were rolled around a compression spring to
form an actuator (Pei et al., 2002, 2004). By selectively actuating only certain regions of electrodes
around the periphery of the actuator, the actuator can be made to bend as well as elongate. An
example of this MER actuator is shown in Figure 10.2 and it represents advancement in making
practical EAP-based actuators with a standard configuration.
10.3.1.2
Ferroelectric Polymers
Piezoelectricity was discovered in 1880 by Pierre and Paul-Jacques Curie, who found that when
certain types of crystals are compressed (e.g., quartz, tourmaline, and Rochelle salt), along certain
axes, a voltage is produced on the surface of the crystal. The year afterward, they observed the
reverse effect that upon the application of an electric voltage these crystals sustain an elongation.
Piezoelectricity is found only in noncentro-symmetric materials and the phenomenon is called
ferroelectricity when a nonconducting crystal or dielectric material exhibits spontaneous electric
polarization. There are also polymers with ferroelectric behavior and the most widely exploited one
is the poly(vinylidene fluoride), which is known as PVDF or PVF2, and its copolymers (Bar-Cohen
et al., 1996). These polymers are partly crystalline, with an inactive amorphous phase, having a
Young's modulus near 1 to 10 GPa. This relatively high elastic modulus offers a relatively high
mechanical energy density. A large applied electric field (~200 MV/m) can induce electrostrictive
(nonlinear) strains of nearly 2% (Zhang et al., 2004). Unfortunately, this level of field is danger-
ously close to dielectric breakdown, and the dielectric hysteresis (loss, heating) is very large. In
1998, Zhang and his coinvestigators introduced defects into the crystalline structure using electron
radiation to increase the dielectric constant of the copolymer P(VDF-TrFE). As a result, electro-
strictive strains as large as 5% were demonstrated at low frequency drive fields having amplitudes
of about 150 V/mm. Furthermore, the polymer has a high elastic modulus (~1 GPa), and the field-
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