2.2.4. Under-Water Actuation
To generate a swimming device, it must be able to operate immersed in water at various water conditions, e.g., temperature and currents. One advantage of having water present is the quick cooling of the heated Nitinol compared to Nitinol in air. This is due to the fact that water has 4 times larger heat capacity and 25 times larger heat conductivity than air, which is beneficial for multiple consecutive actuations. However, the drawback is that water brings the Nitinol wire temperature down too quickly when the water temperature is low or when there is turbulence. Hence, we performed some actuation tests by immersing the Nitinol wire under water at room temperature and heating by application of a voltage. The results showed that a much higher voltage was required to achieve similar Nitinol contraction compared to when the Nitinol was heated in air. Also, we can clearly see water convection on the surface of the Nitinol wire indicating quick and large heat loss (Video S1). With slight turbulence in the water, the Nitinol was not able to actuate at all, even at much higher voltage due to faster heat loss (Video S2), which made the actuation process unpredictable and uncontrollable. In order to hinder this heat loss process, and to provide the Nitinol wire with a stable, controllable and predictable actuation environment, we coated the Nitinol wire with a layer of polyacrylamide (PAAm) hydrogel (Figure 9(a)). A hydrogel was chosen as the coating material because it acts as a physical barrier to hinder the heat loss, but at the same time it can still dissipate the heat since it is a water-rich gel. As seen in Figure 9(b-c), with this hydrogel coating, the actuations were successfully observed and were stable even in turbulence while the one without hydrogel coating completely failed to actuate (Video S3-S4).