2.4.2. Salt and pH Sensing
Etalons with salt and pH responsivity pNIPAm-co-10% acrylic acid (AAc)) were prepared following the group’s previous fabrication procedure. After we change the solution media from deionized water (DI water) to 0.5 M sodium chloride (NaCl) solution, the color of the etalon chips changed from green to red in 260 s and could return back to green in 260s when put back to DI water (Figure 11(b)). Similarly, when the pH was changed from 6.02 to 2.69 , the color of the etalon chips changed from green to red in 620 s and could change from red to green in 620 s (Figure 11(c)). The swimming device served as a platform for different technologies and could potentially be applied in more research areas.
Conclusion
Our work first aim to present an innovative design using bistable metal strip and shape memory alloy Nitinol to fabricate a biomimetic actuator as a swimming device, which exhibited both slow moving and fast snapping capability like an octopus. We hope to inspire more research on combining different materials to achieve novel designs for more applications. Sensing application is just one of the many applications this device can achieve. Remote sampling collecting, video recording, humidity, temperature and air quality monitoring can all be potentially realized with thoughtful integration. Also, more complicated systems can be incorporated such as multiple devices working together as a team to improve efficiency. GPS and camera systems can be added to provide more flexible navigation and environment recognition capability. However, we do need to consider the long time memory loss of Nitinol wire after more repetitions, which was not exhaustedly investigated in this work. Future research should look into this and come up with an estimate time for Nitinol wire memory exhaustion. Also, how to recharge the battery when the device is far away in the field, and how to make sure we don’t lose connection when battery runs out, etc. A straightforward solution might be adding solar panels, which can be a future improvement for the device. More importantly, bio-fouling can be a big challenge for devices that are deployed and immersed in water environment for long period of time. Special protective coatings or antifouling design for the device should be investigated.
In summary, a wirelessly controlled, programmable, swimming device was developed. The properties of the Nitinol alloy, bistable metal strips were investigated to find out optimum design. Hydrogel coating was chosen to provide a stable under-water environment for the Nitinol wires, which facilitates under-water actuation. A fish fin like PDMS flipper was also designed and increased the swimming ability. The swimming device could navigate in all directions and could be controlled wirelessly and easily programmable thanks to the untethered design with a portable battery and wireless control module. We also showcased its successful integration with an etalon-based sensing platform, which can potentially inspire more innovative future applications.