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 10(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). It should be noted here that the slow swimming speed is measured in two consecutive cycles with about 1.4 s time interval between the cycles and each actuation was about 0.3 s duration in terms of voltage application with 10% power output of the battery. Changes in time interval between cycles, voltage application time duration, and battery power output can all affect the slow swimming mode speed. The parameters can be manipulated and programmed easily. The fast swimming is measured after one single snapping movement and the speed is reproducible and decided due to the clear energy transform and release path. Some other factors like temperature of the water can also affect the swimming speed of the device. However, the overall swimming speed mainly depends on the time interval between cycles. Just like the octopus propelling their tentacles, more frequent the propelling, the faster the overall swimming speed. We can program the time interval between the cycles to control the speed of the swimming device. 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):