2.1. Bistable Metal Strip
2.1.1. Metal Strip Shape and Trigger/Reset Angle
Initial studies focused on investigating how the shape of the bistable metal strip and trigger/reset angles impacted its triggering and resetting force. Here, “trigger force” refers to the force required to cause the bistable metal strip to rapidly coil, doing work in the process. “Reset force” refers to the force required to bring the bistable metal strip back to its extended state so that it is ready to be triggered again. In terms of “shape”, we focused on changing the taper ratios of the bistable metal strips, while fixing the total length, as shown in Figure 3. To measure the forces generated from the snapping action of the bistable strip, an inelastic thread attached to the tip of the bistable metal strip was attached to a force detector, and the trigger/reset forces were measured by pulling the thread (Figure 4). When the thread was pulled, a force-time curve was recorded as shown in Figure S1, indicating how forces evolved during the trigger/reset processes. Different pulling rates were tested and we found pulling rates had no effect on the minimum force required for trigger and reset (Figure S2). A pulling rate of ~5 cm/s was chosen for these experiments due to ease of reproducibility of this rate. The minimum forces required for trigger and reset were plotted against the taper ratio. We concluded from the data in Figure 5 that bistable metal strips with larger taper ratios yielded smaller trigger and reset forces. When the metal strip had a larger taper ratio, we noticed a flatter surface at the tip, which results in easier actuation due to smaller force required to transform the tip from one curvature to the other.