2.2. Nitinol Wire

2.2.1. Phase Transition Temperature

We first investigated the phase transition process using differential scanning calorimetry (DSC), as shown in Figure S4. From the DSC, we observed the phase transition to be ~60 ℃. We also performed a water bath test on the Nitinol (Figure S5(a)). Specifically, the Nitinol was set into a helical structure (contracted state) in a furnace and subsequently stretched into an extended state. Then, the extended Nitinol wire was immersed into a beaker of water for ten seconds at various temperatures and taken out for length determination. From the data (Figure Figure S5(b)), we observed a sharp length change at ~60 ℃, which was consistent with the DSC test.

2.2.2 Actuation Force Output at Different Voltages

The forces the Nitinol wire can exert upon temperature-induced contraction/shortening determines its ability to apply force to the bistable metal strip for triggering and resetting. Hence, we investigated the force output of the Nitinol wire when stimulated to contract upon application of a voltage, which induced a temperature change above the phase transition temperature. For these experiments, the Nitinol wire was connected to a force sensor, and was not allowed to contract upon stimulation (Figure S6(a)). Different voltages were applied until the force reached 4.2 N, which was the minimum triggering force for the 4-3 taper ratio bistable metal strip; the 4-3 taper ratio was  determined to have optimal performance (see above). Once the mentioned force was reached, the applied voltage was removed, and the Nitinol wire was allowed to cool. As can be seen in Figure S6(b), higher voltages applied to the Nitinol decreased the time required for the actuation force of 4.2 N to be reached, which indicated faster actuation. Although, the higher applied voltage led to an increase in the time required for the Nitinol to cool after removing the applied voltage. This could be explained considering the Joule heating effect. Assuming Ohm’s Law applies, and the resistance of the Nitinol doesn’t change during the process, when the voltage was higher, the current was higher, thus more heat was produced in a unit time, allowing the Nitinol to reach its phase transition temperature relatively fast. Although, this excess heat needs to be dissipated from the Nitinol wire to allow for its reversibility (Figure S6(c)). More detailed discussion and calculations are shown in Figure S7.

2.2.3. Actuation Durability

Durability, such as the number of times the Nitinol can contract/extend upon stimulation before failure, is an important parameter to study for the devices being generated here. Here, two sets of consecutive tests were performed using the same setup as in the previous actuation force test (Figure S6(a)). The voltage was applied for 10 s and removed to allow the Nitinol wire to cool. From Figure 8(a), we observed an ~8.3% force decrease within the first 4 actuations, followed by a relative stabilization of the force decrease, i.e., an additional 14.8% decrease was observed after the subsequent 82 cycles.  Then the same piece of Nitinol was used to determine its ability to reach the requisite 4.2 N force needed to trigger the 4-3 taper ratio bistable metal strip From Figure 8(b), we can see the Nitinol wire was able to reach 4.2 N after 222 repeats with no observable failure. Combining two consecutive tests, the Nitinol wire could at least endure 300 cycles of successful actuation. The exact number of cycles to reach failure was not investigated, but could be easily determined in the future.