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Thermo-responsive hydrogel-based circular valve 461

springs are placed precisely in the center of the mold (at 2 mm from the bottom of
the mold).

20.2.2.2 Hydrogel
In order to fabricate the hydrogel mix, two separate stock solutions were prepared. The first
stock solution comprised of acrylamide, N,N-methylene-bis-acrylamide, deionized water,

and photoinitiator [11,12]. The contents of the first stock solution were heated at 90 C for
30 minutes to create a well-dissolved solution [11,12]. The second stock solution comprised
of deionized water and sodium alginate [11,12]. The contents of the second stock solution

were heated at 90 C for 30 minutes to create a well-dissolved solution [11,12]. The two
stock solutions were then homogenized together and heated for another 30 minutes at 90 C

[11,12]. This is to ensure a wholly homogenized and well-dissolved solution. The solution
was then cooled to room temperature before use in fabrication. The concentration of the
final hydrogel mixed can be seen in Table 20.2. Recipe for acrylamide-sodium alginate
hydrogel [11,12].

20.2.2.3 Shape memory alloy
The SMA wires used in the fabrication of the tentacle were obtained from Dynalloy Inc.
(CA, United States) under the trade name Flexinol. The selection of SMA wires for the
valve is carried out based on the dimension of the wire and its transition temperature.

The SMA wires with a transition temperature of 70 C are chosen over the wires with
90 C transition temperature, although the wires with higher transition temperatures have

a higher cooling rate. The lower transition temperature is preferred to reduce the heating/
actuation time of the valve. Also, frequent exposure of the hydrogel material to high
temperatures could induce permanent damages. Two SMA wires of diameters 250 and
375 µm and a length of 18 mm were chosen. The SMA wire can be programmed to
memorize a specific conformation by fixing the wire in the desired shape in a mandrel of
afixture [14] and heated at 500 C for a short period of time. This high temperature

arranges the atoms into a compact and regular manner, causing a rigid cubic
arrangement, known as the austenite phase [15]. In this application, the SMA wire is
programmed to remember a curved conformation, with a curvature of 500 m 21 .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, 250 and 375 µm
diameter, 18 mm long SMA wires 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 Care foundto

suit the best for this application.
The SMA springs are programmed to compress when actuated. Two diameters of SMA
wires, 100 and 150 µm and a length of 3 mm were chosen. In this application, the SMA