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276 Biomimetics: Biologically Inspired Technologies

10.3.2.4 Carbon Nanotubes

In 1999, carbon nanotubes (CNT) with diamond-like mechanical properties emerged as formal EAP
(Baughman et al., 1999; Spinks et al., 2004). The carbon–carbon bond in nanotubes (NT) that are
suspended in an electrolyte and the change in bond length are responsible for the actuation
mechanism. A network of conjugated bonds connects all carbons and provides a path for the
flow of electrons along the bonds. The electrolyte forms an electric double layer with the nanotubes
and allows injection of large charges that affect the ionic charge balance between the NT and the
electrolyte. The more the charges injected into the bond the larger the dimension changes. Removal
of electrons causes the nanotubes to carry a net positive charge, which is spread across all the carbon
nuclei causing repulsion between adjacent carbon nuclei and increasing the C2C bond length.
Injection of electrons into the bond also causes lengthening of the bond resulting in an increase in
nanotube diameter and length. These dimension changes are translated into macroscopic movement
in the network element of entangled nanotubes and the net result is extension of the CNT.
Considering the mechanical strength and modulus of the individual CNTs and the achievable
actuator displacements, this actuator has the potential of producing higher work per cycle than
any previously reported actuator technologies and of generating much higher mechanical stress.
However, to date such energy densities and forces have not been realized in macro-scale devices.
The material consists of nanometer-size tubes and was shown to induce strains in the range of 1%
along the length. A CNT actuator can be constructed by laminating two narrow strips that are cut
from a CNT sheet using an intermediate adhesive layer, which is electronically insulated. The
resulting ‘‘cantilever device’’ is immersed in an electrolyte such as a sodium chloride solution, and
an electrical connection is made in the form of two nanotube strips. Application of about 1.0 V is
sufficient to cause bending of the actuator, and the direction depends on the polarity of the field with
a response that is approximately quadratic relationship between the strain and charge.

10.4 EAP CHARACTERIZATION

Accurate and detailed information about the properties of EAP materials is critical to designers of
related mechanisms or devices. To assess the competitive capability of EAPs, a performance matrix
that consists of comparative performance data is necessary. Such a matrix needs to show the
properties of EAP materials as compared to other classes of actuators, including piezoelectric
ceramic, shape memory alloys, hydraulic actuators, and conventional motors. Studies are currently
underway to define a unified matrix and establish effective test capabilities (Sherrit et al., 2004).
Test methods are being developed to allow measurements with minimum effect on the EAP
material. While the electromechanical properties of electronic-type EAP materials can be addressed
with some of the conventional test methods, ionic-type EAPs such as IPMC pose technical
challenges. The response of these materials suffers complexities associated with the mobility of
the cation on the microscopic level, strong dependence on the moisture content, and hysteretic
behavior. Video cameras and image processing softwares allow the study of the deformation of
IPMC strips under various mechanical loads. Simultaneously, the electrical properties and the
response to electrical activation can be measured. Nonlinear behavior has been clearly identified in
both the mechanical and electrical properties and efforts were made to model this behavior (Sherrit
et al., 2004).

10.5 APPLICATIONS OF EAP

Compared to existing actuators, EAPs have properties that potentially make them very attractive for a
wide variety of biomimetic applications. As polymers, EAP materials can be easily formed in various