Biomedical Engineering Reference
In-Depth Information
behavior by including the frictional effects of the ion transport phenomenon. They
mostly concentrated on synthesis, modeling, and characterization of these ionic gels
in biomimetic locomotion and biomedical applications such as drug delivery systems
(DDSs) and were among the first to label them as artificial muscles. Osada (1991)
described electrochemomechanical systems based on hydrogels that contract and
dilate reversibly under electric stimuli. These were essentially thermodynamic sys-
tems capable of transforming chemical energy directly into mechanical work or,
conversely, transforming mechanical into chemical potential energy.
The isothermal conversion of chemical energy into mechanical work underlies
the motility of all living organisms and can easily be seen, for instance, in muscle,
flagella, and ciliary movement. All these biological systems are characterized by an
extremely high efficiency of energy conversion. The high conversion efficiency of
the biological systems is largely due to direct conversion of chemical energy without
unnecessary intermediate heat-producing components. Chemomechanical systems
are the only artificial systems at present that can achieve this. The chemomechanical
reaction may be used not only to generate mechanical energy on a macroscopic level
but also to transform information as a signal or receptor whereby microscopic
deformation plays an essential role in switches and sensors. Osada further classifies
synthetic polymer gels according to principle and type of reaction involved:
1.
hydrogen-ion transfer (pH muscle)
2.
ion exchange or chelation
3.
REDOX reaction (REDOX muscle)
4.
steric isomerization
5.
phase transition or order-disorder transition
6.
polymer-polymer association or aggregation
7.
electrokinetic processes
A number of chemomechanical systems such as polyelectrolyte fibers and mem-
brane have been investigated in the past. These materials expand and contract upon
changing their solubility or degree of ionization. There are also thermosensitive and
optically sensitive polymers. However, from the standpoint of robotic controls,
electroactive polyelectrolytes have been the focus of most researchers. What follows
is an explanation of materials behavior and their application as artificial muscles.
1.3.1
C
ONTRACTION
B
EHAVIOR
For a polyelectrolyte undergoing shape changes by applying DC current, the velocity
of shape change is proportional to the charge density in the gel. This may be the
first model of an electroactive artificial muscle working in an aerobic and aqueous
medium system because the gel contracts and dilates reversibly when stimulated
electrically under isothermal conditions.
The electrical control makes use of cross-linked polyelectrolyte gels. The system
is quite simple. For instance, a water-swollen polymer gel is inserted between a pair
of electrodes, which are connected to a DC source. When the electricity is turned
on, the polymer gel starts to shrink, releasing water droplets one after the other. This
