Biomedical Engineering Reference
In-Depth Information
10
3
10
2
10
1
10
0
10
1
10
2
10
3
(Hz)
f
FIGURE 7.3
The measured AC impedance characteristics of an IPMNC sample (dimension
= 5-mm width, 20-mm length, and 0.2-mm thickness).
measured impedance plot, provided in figure 7.3, shows the frequency dependency
of impedance of the IPMNC.
Overall, 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.
IPMNCs generally have a surface resistance,
R
, of about a few ohms per centimeter,
SS
near-boundary resistance,
R
, of a few tens of ohms per centimeter, and cross-
S
resistance,
R
, of a few hundreds of ohms per millimeter; typical cross capacitance,
P
C
, is a few hundreds of microfarads per millimeter.
Based upon these findings, we consider a simplified equivalent electric circuit
of the typical IPMNC such as the one shown in figure 7.4 (de Gennes et al., 2000).
In this approach, each single unit circuit (i) is assumed to be connected in a series
of arbitrary surface resistance (
g
) on the surface.
This approach is based upon the experimental observation of the considerable
surface electrode resistance (see fig. 7.4). We assume that there are four components
to each single unit circuit: the surface electrode resistance (
R
ss
R
), the polymer resis-
s
), the capacitance related to the ionic polymer and the double layer at the
surface-electrode/electrolyte interface (
tance (
R
p
) due to a
charge transfer resistance near the surface electrode. For the typical IPMNC, the
importance of
C
), and an intricate impedance (
Z
d
w
R
relative to
R
may be interpreted from
Σ
R
/
R
≈
L
/
t
>> 1, where
ss
s
ss
s
notations
are the length and thickness of the electrode. Note that the problem
now becomes a two-dimensional one; the fact that the typical value of
L
and
t
t
is ~1-10
µ
m
makes the previous assumption.
Thus, a significant overpotential is required to maintain the effective voltage
condition along the surface of the typical IPMNC. An effective technique to solve
this problem is to overlay a thin layer of a highly conductive metal (such as gold)
on top of the platinum surface electrode (de Gennes et al., 2000).
Note that figure 7.4 depicts a digitized rendition of the equivalent circuit for
IPMNC, which is a continuous material. Figure 7.5 depicts the measured surface
