Biomedical Engineering Reference
In-Depth Information
12
11
10
9
8
7
6
5 Hz
5
0.1 Hz
0.5 Hz
1 Hz
10 Hz
20 Hz
4
3
2
1
0
0.000 0.001 0.002 0.003 0.004 0.005 0.006 0.007
ε
N
FIGURE 2.18
Frequency dependency of the IPMNC in terms of the normal stress,
σ
N
, versus
ε
N
, under an imposed step voltage of 1 V. This Nafion-117 IPMNC has a
cation of Li
+
and a size of 5
the normal strain,
×
20 mm.
Note that the scan rate is 100 mV/sec. A simple behavior with a small hysteresis
can be seen. It does not show any distinct reduction or reoxidation peaks within
±
4 V,
except for a decomposition behavior at ~
±
1.5 V, where the extra current consumption
is apparently due to electrolysis.
In figure 2.18, frequency response of the IPMNC is expressed in terms of the
normal stress versus the normal strain. Its frequency dependency shows that as
frequency increases, the beam displacement decreases. However, it must be realized
that, at low frequencies (0.1-1 Hz), the effective elastic modulus of the IPMNC
cantilever strip under an imposed voltage is also rather small.
On the other hand, at high frequencies (5-20 Hz) such moduli are larger and
displacements are smaller. This is due to the fact that, at low frequencies, water and
hydrated ions have time to gush out of the surface electrodes, whereas at high
frequencies they are rather contained inside the polymer.
Therefore, the nature of water and hydrated ion transport within the IPMNC can
affect the moduli at different frequencies and presents potential application to smart
materials with a circulatory system. This obviously is a biomimetic phenomenon in
the sense that all living systems have some kind of circulatory fluid to keep them
smart and surviving. This is also of interest in a similar analogy to ionic hydraulic
actuators (Shahinpoor and Kim, 2001d).
Encapsulation by highly elastic thin membranes such as Saran
®
F-310 (Dow
Chemicals) or liquid latex has been effective in maintaining a fairly constant polar
medium for the cations' mobility and consistent performance.
The ionic polymer metal composites or ionic polymer conductor composites are
basically water-loving, living muscles. However, water can be replaced with ionic liquids
or other polar liquids, as reported in a number of publications (see Bennet and Leo, 2003).
A new family of encapsulated IPMNCs equipped with thin compliant electrodes showing
higher efficiencies and power densities in actuation have been fabricated and tested.
