2.2. Fabrication of the Shape Memory Alloy (SMA) Actuator

In this work, an actuator was designed to fix the zebrafish in the observation region through SMA actuation. The flow chart illustrating the fabrication process of the SMA actuator is illustrated in Figure 1b. Initially, one end of a nickel–titanium (Ni–Ti) SMA guidewire (Flexinol, Dynalloy, Inc., Irvine, CA, USA) with a diameter of 0.60 mm was fixed on a 23 G hypodermic needle, and, then, the guidewire was wrapped along the needle without interspace between each wrap, forming a coil structure. After the coil reached a length of 2 mm, the guidewire was cut, and the other end was also fixed on the needle. Secondly, the coil along with the needle was heated with a hot plate at 500 °C for 10 min and then quenched in cold water. Once the SMA coil was fully cooled, the definition process of the original shape was finished. Thirdly, a small piece of borosilicate capillary tube with an inner and outer diameter of 0.55 and 1.4 mm, respectively, as well as a 24 G hypodermic needle with an outer diameter smaller than that of the SMA coil, was cut using a diamond dust rotary cutter and trimmed by sandpaper. To assemble the actuator, the SMA coil was gently stretched to an appropriate length, and its ends were glued to the hypodermic needle and borosilicate capillary tube with the help of polyurethane adhesive (Gorilla glue). Finally, the actuators were integrated into the microfluidic channel, as illustrated in the figure. The SMA coil in the actuator was designed to act as a spring that regained its preprogrammed shape and, thus, generated the force once its temperature exceeded a certain value. As depicted in Figure 1c, the borosilicate capillary part was attached to the microchannel that pushed the hypodermic needle towards the PDMS wall and compressed the wall during the SMA actuation. Once deactivated, the actuator returned back to its original position due to the elasticity of the compressed PDMS wall. In this work, the deformation of the SMA coil was controlled by applying electric currents with varying strengths.