Page 468 - Flexible Robotics in Medicine

462 Chapter 20

springs are lengthened to a length of 15 mm and embedded adjacent to each other in the
hydrogel material. The SMA wires are then actuated based on Joule heating when a voltage
is applied the temperature is raised, and the martensite fraction converts to the austenite
phase, reverting to the programmed configuration. Based on these factors, 100 and 150 µm
wire diameter, 3 mm long SMA springs with a martensite start and finish temperature of
42 C and 52 C and an austenite start and finish temperature of 68 C and 78 C are found to

suit the best for this application. An external DC supply is used as a power source, and all
the SMA wires and springs used in the experiments are connected using 150 µm diameter,
150 mm long copper wires.

20.2.2.4 Fabrication
In order to fabricate the SMA wire-based SMAHV, the programmed SMA wires were fixed
in the individual molds. The SMA wires are arranged in a loop structure with a curvature of
364 m 21 , as shown in Fig. 20.3A. Two different diameters of SMA wires were used in
the fabrication to test the performance of the two different SMAHVs. The two different
diameters of the SMA wires were 250 and 375 µm. After fixation, the hydrogel mix was
poured into the two sets of molds (one for each diameter of the SMA wire). The molds
were then put in the da Vinci UV curing chamber (XYZ Printing, Inc.) with UV LED
(λ 5 375 405 nm) for 2 hours. The molds are then removed from the chamber, and the
SMAHVs are removed from the molds and tested for their performance.

In order to fabricate the SMA spring-based SMAHV, the SMA springs were fixed in its
respective molds. A couple of SMA springs are extended and to a length of 15 mm and
fixed adjacent to each other in the mold. Two different SMA wire diameters of SMA spring
were used in the fabrication to test the performance of the two different SMAHVs. The two
different diameters of the SMA wires were 100 and 150 µm. After fixation, the hydrogel

Figure 20.4
Response time of TRHV to heating. (A) Experimental setup to test the response time of TRHV.
(B) Time-resolved displacement of TRHV during the heating of the water in which it is immersed.