Page 203 - Flexible Robotics in Medicine

190 Chapter 7

variable stiffness, we applied weight at a free end of the tube and observed differences in
its compliance and deformation before and after the candle wax was melted by applying a
current through the resistance wire. To ascertain that the stiffening mechanism was
compatible with the spring backbone we are employing for instrument manipulation and
actuation, we inserted the tube into spring. We attempted to actuate it with the tendon-
driven mechanism, as in our robot prototype. The spring could bend when the wax was
melted. There was a significant reduction in the flexibility of the spring backbone.

The images below show the proof of concept for a phase-change mechanism based on wax
as the medium. As can be seen in Fig. 7.17, the spring backbone without stiffening (liquid
wax) is compliant and is unable to exert tremendous forces on a piece of foam. On the
other hand, a stiffened spring (solid wax) is capable of effectively transmitting force and
can break the foam.
There is, however, a limitation with wax. We observed that the candle wax took a
considerable amount of time to melt and solidify at this scale (30 60 seconds). Hence,
even at scaled-down models, it is estimated to take still more than a few seconds for
stiffening activation/inactivation. Thus the operator must wait for the instrument to become
flexible again after stiffening it for force application. Solder, on the other hand, has a much
quicker response time and is suited for procedures requiring a frequent change in the
stiffness of the instruments. Hence, in our future work, we are exploring ways to create a
more robust sealing channel such that solder as a medium may be used for a device
requiring a more frequent change in its stiffness. It is to be noted that this is only a proof of
concept to demonstrate the phase-change process and its application to variable stiffness.
We can further improvise on this method by using a silicon rubber (polydimethylsiloxane)
etched and filled with low melting point alloy, which is biocompatible and electric leads
attached to the ends. Doing this will allow us to miniaturize the device down to the
thickness of a thin film.

Figure 7.17
Flexible and compliant backbone (left) versus rigid backbone (right).