Biomedical Engineering Reference
In-Depth Information
as Na
+
for Li
+
are to be found in the vicinity of
the sulfonates. It must be noted that the length
of the side chains has a direct bearing on the
separation between ionic domains, where the
majority of the polar fluids resides, and the non-
polar domains.
Perfluoroionomers show an unusual combi-
nation of a nonpolar, Teflon-like backbone with
polar and ionic side branches under high-
resolution NMR. Liu and Schmidt-Rohr
[37]
obtained high-resolution nuclear magnetic res-
onance (NMR) spectra of solid perfluorinated
polymers by combining 28-kHz magic-angle
spinning (MAS) with rotation-synchronized
19F pulses. Their NMR studies enable more
detailed structural investigations of the
nanometer-scale structure and dynamics of
polytetrafluoroethylene (PTFE) or Teflon®-
®-
based ionomers. It has also been well established
that anions are tethered to the polymer back-
bone and cations (H
+
, Na
+
, Li
+
) are mobile and
solvated by polar or ionic liquids within the
nanoclusters of size 3-5 nm
[25
,
30]
. For recent
work on biopolymers/IPMC artificial muscles,
see Shahinpoor
[38]
and Tiwari
et al.
[39]
.
A large class of ion-containing polymers
exists and creates a rich source of ionic poly-
meric nanosensors and nanoactuators in nano-
composite form with conductive materials.
Certain dopants in the form of charge-transfer
agents can be used to generate positive or nega-
tive charges or pendant groups in an intrinsi-
cally conducting polymer by reduction/
oxidation (redox) chemical operations.
Ampholytic polymers (polyampholytes) that are
composed of macromolecules containing both
cationic and anionic groups are electoactive and
generate the basis for biomimetic electroactive
ionic biopolymer conductive composites such as
chitosan, intercalated with ionic polyelectrolytes
such as IPMCs.
It is worth noting that chitosan is a naturally
occurring substance in shellfish such as shrimps,
crabs, and lobsters and possesses many useful
properties such as wound healing. Note that
carboxylic ionic polymers for medical and
implantation applications because of chitosan's
amazing healing, medical, and diagnostic
properties. Chitosan/ionic polymers containing
equilibrated and conjugated ions within their
molecular networks and capable of being
chemically or electrolessly plated with a
conductive phase such as carbon, metal,
graphite, graphene, and carbon nanotube create
a novel family of multifunctional materials with
medical healing characteristics.
Shahinpoor and Schneider
[30]
have pre-
sented a larger family of multifunctional poly-
meric materials. Mac and Sun
[31]
have
discussed the multifunctional characteristics of
chitosan gels. On the IPMC side the reader is
referred to Shahinpoor
et al.
[25]
and five review
articles by Shahinpoor and Kim
[26-29]
and
Shahinpoor
[32]
. Furthermore, in Refs.
25 and
27
, methods of fabrication of several electri-
cally and chemically active ionic polymeric
muscles have been introduced and investi-
gated. Gel-based ionic polymer conductor com-
posites have also been introduced and
investigated [25, 33, 34].
As described in Ref. 30, several physical mod-
els have been developed to understand the
mechanisms of ion transport in ionic polymers
and membranes. Morphological features influ-
ence transport of ions in ionic polymers. These
features have been studied using a host of
experimental techniques, including small and
wide-angle X-ray scattering, dielectric relaxa-
tion, and a number of microscopic and spectro-
scopic studies [35, 36].
The emerging picture of the morphology of
ionic biopolymers is that of a two-phase system
made up of a polar medium containing ion
nanocluster networks surrounded by a hydro-
phobic medium. These nanoclusters, in the con-
text of perfluorinated sulfonic membranes, have
been conceptually described as containing an
interfacial region of hydrated, sulfonate-termi-
nated perfluoroether side chains surrounding a
central region of polar fluids. Counterions such
