Biomedical Engineering Reference
In-Depth Information
16
12
a
8
b
4
0
0.0
0.1
0.2
0.3
0.4
0.5
ε
N
FIGURE 3.60
Tensile testing results (normal stress,
ε
N
). Note that both
samples were fully hydrated when they were tested. (a) Solution recast membrane; (b) as-
received membrane.
σ
N
, vs. normal strain,
configuration, internal stresses are usually built up when transverse generative strain
is converted into bending motion, which lessens the mechanical output energy. (Note
that flexural strength is generally lower than tensile or compressive strength since
the thickness is small.)
Based upon these measurements, we can define the electromechanical coupling
factor,
k
(or thermodynamic efficiency,
E
ff
), as:
U
(
=
stored mechanical energy
electricinput energy)
)
2
m
Ek
==
(3.5)
ff
U
(
=
e
Based upon equation (3.5), a graph was constructed (see fig. 3.62) that shows
the thermodynamic efficiency of the IPMNC as a function of frequency. Note
that this graph presents the experimental results for the conventional IPMNC
and the additive (PVP)-treated improved IPMNC (particle controlled). It is of
note that the optimum efficiencies occur at near 8-10 Hz for these new IPMNCs.
The optimum values of these IPMNCs are approximately 25-30%. At low
frequencies, the water leakage out of the surface electrodes seems to cost the
efficiency significantly. However, the additive (PVP)-treated IPMNC shows a
dramatic improvement in efficiency since less water transports out of the surface
electrodes. The important sources of energy consumption for the IPMNC actu-
ation could be from
necessary mechanical energy needed to cause the positive/negative strains for
the IPMNC strip
I/V hysteresis due to the diffusional water transport within the IPMNC
thermal losses—joule heating
decomposition due to water electrolysis
water leakage out of the porous electrodes
