Page 473 - Flexible Robotics in Medicine

Thermo-responsive hydrogel-based circular valve 467

Figure 20.8
Actuation of the wire-based SMAHV. (A) Time-resolved displacement of the SMAHV with 250 µm
diameter SMA wire. (B) Time-resolved displacement of the SMAHV with 375 µm diameter SMA wire.

in a relaxed state with a large area of the lumen. Actuation of the SMA spring transforms it to
austenite phase, which contracts the SMA springs, causing a contraction in the lumen, resulting
in the reduction in the area of the lumen. This method of actuation produces an ununiform
change in the circumference of the lumen, resulting in an elliptical lumen.

20.3.2.1 Shape memory alloy wire-based shape memory alloy actuated hydrogel valve
The SMA wires in the SMAHV are actuated by applying current using an external DC
supply through copper wires. An input current of 0.5 2.0 A was applied to the SMA wire
in the SMAHV, and the displacement while decreasing the area of the lumen is video-
recorded and analyzed using Tracker 5.0 (Douglas Brownr). Each cycle of the SMAHV is
actuated for a period of 8 seconds, with an actuation phase of 5 seconds and a relaxation
phase of 3 seconds. Applying an input current of 0.5 1.0 A to both 250 and 375 µm wire
diameter SMA wire in the SMAHV produced a displacement of 0.1 0.3 mm, which is very
low and renders the valve unsuitable for CBI process due to its negligible change in the
lumen area. Applying an input current of 2.0 A and above to both 250 and 375 µm diameter
SMA wire in the SMAHV required longer cooling time for the SMA wire and the SMAHV
was unable to return to its initial position in the 3 seconds relaxation period. Due to these
drawbacks, an input current of 1.5 A is applied to both 250 and 375 µm diameter SMA wire
in the SMAHV, to determine the displacement of the SMA wire-based SMAHV.

The time-resolved displacement of the SMAHV with 250 µm diameter SMA wire when
applied with 1.5 A input current for three cycles is shown in Fig. 20.8A. It is observed that the
SMAHV reached a maximum displacement of 1 mm for all three cycles of actuation.
Although the actuation phase comprises 5 seconds, the SMAHV reached the maximum
displacement at 2 seconds and maintained the maximum displacement through the next
3 seconds of the actuation phase. The SMAHV returned to the initial position during the