Introduction
For centuries, humans have been fascinated with studying nature and natural processes; this most certainly began out of curiosity, although in many cases led to a desire for mimicry \cite{whitesides2015bioinspiration} \cite{baytekin2018artificial} \cite{aubin2019electrolytic}. For example, primitive, early humans would move, dress, and act like animals to improve their hunting successes, while modern day mimicry is focused on the development of new synthetic materials for improving human health, and quality of life \cite{li2018platelets} \cite{dang2018biomimetic}. Of course, nature has the ability to adapt to changes in environmental conditions and added pressures via many years and generations of evolution, while humans are left primarily using rational thought, science, and engineering for adaptation. A great example of nature adapting to their environment for self-gain is the octopus. The octopus is an intelligent marine creature that has evolved the ability to change color and eject ink when in danger in order to escape and preserve its wellbeing. Octopus also exhibit high dexterity, capable of reaching \cite{cianchetti2011design}, grabbing \cite{cianchetti2015bioinspired}, swimming \cite{kazakidi2015vision}, and walking \cite{calisti2015dynamics}. Interestingly, they have the ability to swim slowly to disguise themselves as floating algae in ocean currents, while maintaining the ability to quickly swim away in a moment by contracting their bodies and whipping their tentacles. Here, we introduce a device composed Nitinol wire (a shape memory material), and bistable metal strips, to generate a device that is capable of mimicking the swimming behavior of an octopus by exhibiting the ability to swim slow and “instantaneously” fast by simply changing how the device’s components are electrically stimulated.
Nitinol, generated by alloying nickel and titanium, has many interesting uses and properties, e.g., shape memory, superelasticity, anticorrosion, and biocompatibility \cite{duerig1999overview}. As a result of these interesting properties and attributes, it has long been used for applications in aeronautical and space technologies, automobile industries, medical devices and civilian products \cite{kauffman1997story}. Of importance to this investigation is Nitinol’s shape memory properties, which we propose harnessing for this new approach to actuation. Nitinol can be “trained” to adopt a desired permanent shape when it’s fixed, heated above the austenite finish temperature (Af) and then rapidly cooled to below the martensite finish temperature (Mf) \cite{liu2007detwinning}. Below Mf, the Nitinol can be reshaped into temporary shapes and upon heating to above the austenite start temperature (As), a phase transition from martensite to austenite is initiated, and the Nitinol starts to recover its permanent shape. Importantly, when the temperature is lowered to martensite start temperature (Ms), the Nitinol then can be reshaped again. This shape memory behavior is key to our actuation device. Heating of Nitinol can be achieved by direct heating, or via resistive/Joule heating by applying a voltage to the wire that generates a subsequent current. For this study, we were interested in using Joule heating to trigger the shape memory behavior from the Nitinol.
Bistable materials are known to exist in two stable states that could differ in conformation dramatically. Very common examples of bistable materials are slap bracelets (a child’s toy) and metal measuring tapes. In these examples, an external stimulus (force) can be used to trigger the metal strip to quickly morph from one stable state (e.g., extended state) to the other (e.g., coiled state) in what is often called “snap-through” behavior \cite{ye2005bi}. This sudden and powerful actuation is intriguing if it can be harnessed and utilized – in our case for shape changing devices \cite{cazottes2008actuation} \cite{rothemund2018soft} that can be used for swimming.
In this work, we propose harnessing the power of shape memory materials for actuation (and ultimately swimming) by coupling Nitinol wire to bistable materials in a device. By combining these two classes of materials in a device, and utilizing the actuation afforded by Nitinol and the snapping power of bistable metal strips, we developed a novel actuator that mimics the slow and fast swimming behavior of an octopus. Furthermore, by controlling and moving the arms independently (via electrical stimulation), the device can navigate through space in any direction. Additionally, a wireless control system, equipped with a rechargeable battery, was designed to make the device totally autonomous, and untethered from external wires that otherwise would be needed for control. Finally, we demonstrate that we can integrate etalon-based sensing devices \cite{sorrell2011reflection} into the construct to monitor water pH and ionic strength. In this case, the etalons tethered to the swimming device change color (that can be captured with an on-board camera) as the water properties change. Successful integration of these sensing devices into the construct demonstrates the potential utility of these devices for environmental monitoring applications, as well as others.
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 2. 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.