Biomedical Engineering Reference
In-Depth Information
has been verified and quantified experimentally in the case of several ionic gels such
as poly(2-acrylamido-2-methyl-1-propanesulfonic) acid (PAMPS). The degree of
swelling changes as other media, such as a mixture of water and ethanol or glycerin
or other solvents, are used as compared with using pure water only.
1.3.2
M
ECHANISMS
A quantitative interpretation of the phenomena observed is that the application of
an electric field causes a pumping of largely mobile ions, partly macronetwork ions,
and surrounding hydrated water to the opposite directions until they reach the
electrode. In general, the velocity of migration and the velocity of the gel contraction
are governed by the quantity of mobile ions
q
n
and the gradient of electric field
E
,
which is the driving force pumping these ions. The force produced in the gel is
proportional to the product of
q
n
and
∆
∆
E
:
FE
n
=×
∆
(1.26)
If we ignore the effect of the elastic forces due to covalently cross-linked network
and if the gel is free to move as an aggregate of particles in the electric field, a
slightly cross-linked and highly swollen polymer eventually attains a steady state
velocity
U
.
This value is apparently proportional to the velocity of contraction of the gel
v
over a time period:
E
q
I
vU
×=
∆
×
n
=
∆
E
×
(1.27)
r
µ
where
r
is a hydrodynamic frictional coefficient, which depends on the geometry of
the gel, thickness of double layer, viscosity of the medium, and others. If the average
mobility of the medium in the gel is
, the rate of contraction may be proportional
to the product of the gradient of the electric field and current
I
.
µ
1.4
ROLE OF MICROPARTICLES IN CONTRACTION
OF GELS
In order to establish the system with minimized response time, microparticles of
sodium salt of polyacrylic acid gel were synthesized and the size change of the
particle with time under various electric fields was measured. Experiment showed
that there is a threshold voltage below which no contraction occurs. As the electric
voltage increased, the contraction increased as well. This also affected the volumetric
reduction in size of the sample. Volume reduction by a factor of 30 or 3000% was
easily achieved within less than 1 min under 6-V DC current. After removal of the
electrical current, the gel returned to its original size within 5 min.
Rate of volume change is a function of size of the particle and relative volume
change decreased with increasing size of the particle. The experimental results
