Biomedical Engineering Reference
In-Depth Information
Note that conjugated polymers are organic semiconductors and have a band gap.
They can emit light, the color of which can be tailored through the chemical structure.
They can generate a current upon absorbing light and thus can be used in photovoltaic
devices as well. The conductivity of conductive or conjugated polymers depends on
the doping level. They are usually p-doped. The dopant concentration is about
25-30% for polypyrrole. The doping level depends on the oxidation state of the
polymer, which can be electrochemically controlled. For actuator application, one
has to change the oxidation level by the application of a potential. Thus, many of
the properties of the material change, including its volume, color, mechanical prop-
erties, and hydrophobicity.
Note that conjugated conductive polymers (CCPs) have the potential to act
biomimetic and like artificial muscles due to the requisite requirements pertaining
to collapse of the internal network structures due to electronic jump between mac-
romolecular chains. Therefore, conducting conjugated polymers that are formed by
principal families of polymers, such as polyacetylenes, polypyrroles (Ppy), poly-
thiophenes, and polyanilines (PANi), has been the focus of some recent research
and development on artificial muscles. Essentially, the electronic conductivity in the
conjugated polymers is due to the ability of electrons jumping between polymeric
molecular chains. The presence of dopant agents, which modify the local density of
electrons on the electronic valence bands, causes such electronic jumps. The dopants
known as type “p” remove electrons from a valence band, leaving the molecule
positively charged or oxidized. The “n” dopants add electrons to the electronic
valence band, so the net charge of the molecule will be negative or the polymer will
be in a reduced state.
Thus, similar to ionic polymer conductor nanocomposites, such deformations in
conductive polymers are governed by oxidation-reduction (REDOX) processes.
Thus, the conjugated polymer can be oxidized (p-doped) or reduced (n-doped) by
introducing positive or negative ions, or photons. These changes are all electrochem-
ically controlled so that the neutral state, the reduced state, the oxidized state, or
any other intermediate state of the polymer can be reached to apply the appropriate
electric potential.
8.2
DEFORMATION OF CONDUCTING
OR CONJUGATED POLYMERS
There has been a tremendous number of pioneering works on this subject by Baugh-
man and coworkers from 1990 through 1996 and Otero and coworkers from 1990
through 1997 on conducting polymers as artificial muscles; by Smela, Pei, Inganäs,
and Lundström on microactuators in the form of bending bilayer strips built from
polyaniline for artificial electrochemical muscles; and De Rossi, Della-Santa and
Mazzoldi and coworkers from 1990 through 1998 on characterization and modeling
of conducting polymers for muscle-like linear actuator applications.
It should be noted that the deformational change in the CCP was first reported
by Burgmayer and Murray in 1982. Using PPy membranes, they showed that the
membrane permeability of certain ions could be changed by two orders of magnitude
