Biomedical Engineering Reference
In-Depth Information
IPMNC in the electromechanical mode exhibiting increased stiffness due to redis-
tributed hydrated ions or nonlinear characteristics of electromechanical properties
of the IPMNC. These features will be discussed in detail in upcoming review papers.
2.2.2
E
LECTRICAL
P
ERFORMANCE
In order to assess the electrical properties of IPMNC, the standard AC impedance
method that can reveal the equivalent electric circuit has been adopted. A typical
measured impedance plot, provided in figure 2.14, shows the frequency dependency
of impedance of the IPMNC. It is interesting to note that the IPMNC is nearly
resistive (>50
Ω
) in the high-frequency range and fairly capacitive (>100
µ
F) in the
low-frequency range.
Based upon the preceding findings, we consider a simplified equivalent electric
circuit of the typical IPMNC such as the one shown in figure 2.15 (Shahinpoor and
Kim, 2000a). In this approach, each single unit-circuit (i) is assumed to be connected
in a series of arbitrary surface resistance (
R
ss
, of the order of a few ohms per
centimeter) in the surface. This approach is based upon the experimental observation
of the considerable surface-electrode resistance.
We assume that there are four components to each single unit circuit: the
surface-electrode resistance (
R
s
, of the order of a few tens of ohms per centimeter);
the polymer resistance (
R
p
, of the order of a few hundreds of ohms per millimeter);
the capacitance related to the ionic polymer and the double layer at the sur-
face-electrode/electrolyte interface (
C
d
, of the order of a few hundreds of micro-
farads); and an impedance (
Z
w
) due to a charge transfer resistance near the surface
electrode. For the typical IPMNC, the importance of
R
ss
relative to
R
s
may be
interpreted from
L
/
t
>> 1, where notations
L
and
t
are the length and
thickness of the electrode, respectively. It becomes a two-dimensional electrode.
In order to increase the surface conductivity, a thin layer of a highly conductive
metal (such as gold) is deposited on top of the platinum surface electrode (Sha-
hinpoor and Kim, 2000a).
Σ
R
ss
/
R
s
≈
10
3
10
2
10
1
10
0
10
1
10
2
10
3
f
(Hz)
FIGURE 2.14
The measured AC impedance spectra (magnitude) of IPMNC sample. The
moist IPMNC sample has a dimension of 5-mm width, 20-mm length, and 0.2-mm thickness.
