Biomedical Engineering Reference
In-Depth Information
artificial muscle by a rectangular hyperbolic function, which resembles that of natural
muscles as:
(
FaVb
+
)(
+
)
=
(
F ab
+
)
(1.29)
0
where
F
= force during contraction
F
0
= force during isometric contraction
V
= velocity of contraction
a, b
= constants
The power output, which is the product of the force and velocity of contraction,
was found to be the maximum at medium contraction velocities and low contractile
forces. Maximum power-to-weight ratio measured was 5.8 mW/g as compared to
40-200 mW/g for natural muscles.
Caldwell and Taylor also found that the solvent content had a significant effect
on the elastic modulus of the material. Increasing the liquid content caused a decrease
in Young's modulus according to:
K
SC
w
G
=
1
(1.30)
3
(
−
)
1
where
S
w
= linear swelling ratio (ratio of swollen to dry polymer length) and
K
1
,
C
1
= constants.
Therefore, by controlling the percentage of the solvent content and the external
solvent concentration, these researchers could vary the compliance of the gripper.
Their gripper consisted of muscle cell chambers, wire tendons, and a scissors
mechanical gripper. The polymer muscles were bundles of 0.1-mm thick strips of
PVA-PAA that contracted with acetone and expanded with water by means of
computer-controlled hydraulic solenoid valves. They used a PD controller to control
gripper position. The cycle time for opening and closing of the gripper was under
15 sec, with measured positional accuracy of
.
In 1993, Oguro, Asaka, et al. (1993) of Osaka National Research Institute in
Japan (now called AIST) were the first to report deformation of ion-exchange
membrane polyelectrolytes when they were plated by metals and placed in an electric
field. They observed bending of strips of these polyelectrolyte membranes toward
anode electrode when placed in a weak electric field. Although the initial attempt
was to achieve better efficiencies for fuel cell membrane applications, the accidental
movement of this polymer membrane proved to be a breakthrough for the future of
polymeric biomechanical sensor and actuators.
Shahinpoor and Mojarrad (1996) have investigated characterization, modeling, and
application of chemoactive (pH-driven) and electroactive ionic polymeric gels, using
nonlinear theories and numerical simulation. Brock (1991a, 1991b, 1991c) of MIT
also has been involved with microminiature packaging of pH-activated artificial mus-
cles. Shiga and coworkers (1993) investigated deformation of small-diameter
±
2
°
