Biomedical Engineering Reference
In-Depth Information
If the imposed electric field were dynamic and oscillatory like sinusoidal, the
color of the surfaces on the cathode side changed alternatively with the frequency
of the applied dynamic electric field. When we changed the cations to other cations,
such as sodium or calcium, then the color of the migrated cations on the cathode
side changed. The emergence of water on the cathode side was also always observ-
able. We concluded that the mechanism of electrically induced bending was due to
ionic polymer nanoscale energetics and ionic migration from one side of the canti-
lever film to the other side while carrying hydrated water or added mass water in
such hydraulic type actuation. It was observed that the ionic migration and redistri-
bution caused water, chemicals, polar fluids, or ionic liquids contained within the
macromolecular network to circulate within the materials and to transport ions and
chemicals from one point to another to cause large (>10) changes in value of a
number of properties, such as stiffness, conductivity, and material transport.
In fact, in the case of lithium cations, the color was greenish blue, which then
indicated that it was Li
+
cations migrating under the influence of the imposed electric
field and carried loose water as well as hydrated water along with it. These obser-
vations have been reported by Shahinpoor and Kim (2002j, 2004), Kim and Shahin-
poor (2003b), as well as by Leo and Cuppoletti (2004) and Hawkins et al. (2004),
and later became a routine phenomenon to be observed with hydrated samples of
IPMNCs. Figure 2.36 depicts the essential mechanism in such electrically control-
lable ionic migration accompanied by water or ionic liquid movement.
Furthermore, such ionic migration could also increase the local stiffness. We
have observed stiffness changes of more than one order of magnitude in IPMNCs
as also depicted in figure 2.1.
The migration of loaded or hydrated cations by means of an imposed local
electric field has been observed to cause deformation, stiffening, substantial changes
in local elastic modulus, substantial material transport within the material, ability to
_
+
Electrodes
+
+
+
+
+
+
+
+
+
+
+
+
+
Hydrated ion
+
Mobile cation
Fixed anion
Water
FIGURE 2.36
The cation-transport-induced actuation principle of IPMNCs. (a) Before a
voltage is applied; (b) after a voltage is applied.
