Results and Discussion

For this study, in order to generate devices capable of exhibiting slow and fast swimming behavior upon electrical stimulation, the power of Nitinol and bistable strips needed to be harnessed using careful device design principles. The design we converged on after many iterations is shown schematically in Figure 1. As can be seen, four pieces of pre-set short helical Nitinol wires were stretched and attached to the tip of a bistable metal strip. When a voltage was applied to the Nitinol wires, e.g., the front left (FL) piece, the FL Nitinol wire heats up and will shorten thus pulling the tip of the bistable metal strip causing it to snap and coil (trigger process). This snapping process results in lengthening/straightening of the back left (BL) Nitinol wire. The shortening/contraction of the BL Nitinol wire can further be triggered by application of a voltage/heat pulling the tip of the bistable metal strip to uncoil and return to a flat state (reset process). This reset process once again lengthens the FL Nitinol, which prepares the device for its next trigger process. Thus, by carefully (and independently) controlling the conformational state of the four pieces of Nitinol wires with voltage, the state of the bistable strip can be manipulated and controlled allowing slow and fast movement of the  device’s arms, much like the control an octopus has over its tentacles. Although, to understand how all of the pieces of the device work together to achieve the desired behavior, a detailed examination of  the device's components is required, as detailed below.
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 2. 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 3). 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 4 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.
Meanwhile, we investigated how the angle that the force was applied to the bistable metal strip, via the thread, impacted the magnitude of the trigger and reset forces. As shown in Figure 3, using the extended state of the bistable metal strip as a reference, we varied the angle of force application to trigger/reset the bistable metal strips. Three different angles were investigated for both the trigger and reset processes. As can be seen in Figure 5, larger trigger/reset angles led to smaller forces required for triggering and resetting. This could be explained from the force analyses of the process. When the angle of the applied force is large, the effective normal force, which is the projection of the vector force applied onto the tip of the metal, is likewise large. As a result, if the normal force required to trigger or reset the metal strip remains the same, larger angles lead to smaller vector force required. From the result, we concluded that we can minimize the force by maximizing the angle of force application approaching 90°. However, the angle was limited by the actual reasonable dimensions of the device. For our device design, considering the dimensional constraints and aesthetic aspects, we used 45° for triggering and 60° for resetting.          
2.1.2. Energy Output
While the above investigation revealed the impact of the taper ratio of the bistable metal strip on the trigger and rest forces, it also revealed that the taper ratio impacted the energy that can be generated as a result of the snapping process. To further understand the relationship between snapping energy output and taper ratio, we performed energy output tests. Specifically, the benefit of using bistable metal strips for device propulsion is its fast and powerful snapping behavior, which hypothesized would lead to fast movement of the device developed here. Hence, we wanted to retain good snapping performance while minimizing the magnitude of the trigger and reset forces. To accomplish this, we used a regular ping pong ball that was hung over the top side of the bistable metal strip as shown in Figure S3. By triggering the tip of the bistable metal strip, the strip snapped, coiled, and struck the center of the ping pong ball, sending it into motion. The ping pong ball then moved in a circular path, like a pendulum, and eventually reached its highest point before swinging back. Assuming no energy loss during the process, the energy output of the bistable metal strip is directly proportional to the maximum height the ping pong ball reached. Analysis of the height the ping pong ball reached as a function of the bistable metal strip taper ratio allowed us to determine the relationship between energy generation and bistable metal strip taper ratio. As shown in Figure 6, the height the ping pong ball can reach (and the energy generated) greatly decreased with larger taper ratios; i.e., larger taper ratios yielded lower snapping energy output. In fact, the 7-0 taper ratio metal strip did not have enough energy to move the ping pong ball. Since snapping energy is essential to the performance  of the swimming device, we decided to maximize the energy output by adopting a 4-3 taper ratio design for our future devices.
 
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 7(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 7(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.
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 8(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 8(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).
 
2.3. Swimming Device
2.3.1. Assembly of the Device
The device was fabricated based on the aforementioned test results. The main body of the device was composed of a bistable metal strip with a pivot point going through the strip perpendicularly. Nitinol wires were connected at both tips of the metal strip against the pivot to set the angle required for triggering and resetting. A wireless control system was designed and assembled as shown in Figure 9(a). To increase the propelling ability, polydimethylsiloxane (PDMS) sheets with rigid plastic (3M PP2950 film) strips embedded were fabricated, which were inspired by fish fins. A lithium ion polymer battery was used as a portable power source.
2.3.2. Swimming Demonstration
Swimming behavior was achieved with wireless control and multiple swimming modes (Video S5-S7). The swimming device was immersed under water with a Styrofoam box floating on water carrying all the electronics (Figure 9(b)). By controlling the individual electronic switches, slow movement, and faster snapping actions could be realized. When both arms were working together, the swimming device could swim forward. When triggered to swim slowly, we determined the swimming speed to be 1.2 cm/s (Video S8), while it was increased to 5.5 cm/s upon triggering fast swimming (Video S9) (calculation see Figure S8). While with only one arm working, the device could turn. In the slow swimming speed regime, we observed the device could turn 180° within 10 seconds (Video S10). By combining different modes of actuation, the swimming device could easily navigate a water tank by swimming straight, speeding up, changing direction as well as slowing down.
 
2.4. Sensing Application
Microgel-based etalons were constructed by sandwiching poly(N-isopropylacrylamide) (pNIPAm)-based microgels between two thin Au layers. In addition to the native thermoresponsivity of pNIPAm, further responsivity can be imparted to microgels via copolymerization. For example, pNIPAm-based microgels can collapse and swell upon heating and cooling, respectively, while also exhibiting pH-and ionic strength-dependent solvation states by incorporating acrylic acid (AAc) into the pNIPAm microgels. Such responsive microgels in etalons allows their visual color, and peaks in reflectance spectra, to shift upon application of any of these stimuli, allowing the color to be correlated to the composition of the water, and its temperature. This is due to the microgel solvation state mediating the distance between the etalon’s Au layers, which are responsible for the etalon color; by changing the Au-Au distance, the color of the device changes. We can predict the etalons’ optical properties by equation (1):