Biomimetic Soft Multifunctional Miniature Aquabots

Abstract

Engineered locomotive systems are widespread on the macro scale (e.g., planes, trains, automobiles), yet miniaturized mobile systems are largely limited to the research lab. In particular, integration of specific functions such as sensing or drug delivery into a small robot is a great challenge. The ability to miniaturize parts affecting motion, such as MEMS-based components and electroactive polymers, while widely demonstrated has not led to practical advances in miniature multifunctional mobile robots. Rather complex construction, limits in the use of diverse materials, and cost (in case of the MEMS-based bot) all hinder the production of these robots. The small (micro-/millimeter scale) system within aqueous environments is particularly challenging, yet there are important potential applications for robots in such environments. To date, engineers have explored soft materials for creating components at the micro-/millimeter scales; for example, actuators such as ionic polymer metal composites, conducting polymers, and carbon nanotube actuators, and engineered devices housing hydrogel valves, filters, and lenses, as well as other soft material actuators that mimic natural movement (earthworm, myriapod, etc.). Still, the creation of miniature multifunctional robots that operate in aqueous environments has been elusive. Nature provides numerous examples of small, multifunctional and autonomous ‘‘robots’’ in the insect and marine worlds after which we aimed to model miniature polymeric aquabots. Here we present mini (microto millimeter) soft aquabots that combine multiple functionalities to perform multifunctional operations in aqueous environments, effectively simulating their natural counterparts. Despite extensive research on macroscale robotics and miniature micro-electromechanical systems, relatively little attention has been paid to the creation of miniature soft robots with diverse shapes, actuation mechanisms, and integrated functionalities. As a fabrication method, we describe a technique that takes advantage of the small volume required by amicrofluidic chamber; this will be used for photopolymerizing multifunctional minibots that possess several ‘‘organs,’’ allowing for movement, sensing/signaling, and capture/transport/release. A diverse group of aquabots mimicking three kinds of living organisms – octopus, sperm, and myriapod, each representing a different mode of locomotion – has been developed. These aquabots have the following key features: 1) diverse shape and small size (sub-millimeter feature sizes); 2) a variety of functions (e.g., actuation, sensing) integrated in a single robot; and 3) rapid, scalable fabrication both in dimension and quantity. The aquabots were fabricated out of soft materials, due to the ease of polymerization, as well as the ability to tailor the material composition towards achieving certain functionality (e.g., responsiveness to pH, temperature, or electric/ magnetic field). Sequential in situ photopolymerization within a microfluidic device allowed for precise fabrication of different components (arms, legs, body; see detailed procedure for photopolymerization in Experimental section). The bodies were constructed using a relatively inert polyethylene glycol (PEG) material, whereas ionic electroactive polymers (electroactive hydrogels) were chosen from a large number of available soft materials to serve as the actuators, thus facilitating controlled movement. Depending on the application, some aquabots contained temperature-, pH-, or chemical-sensitive polymers in addition to their electroactive hydrogel and PEG components. The gel network of polymerized actuator material was anionic, causing positively charged surfactant molecules to bind to its surface; this surfactant binding leads to an osmotic pressure difference between the gel interior and the external solution, thus inducing the contraction and curvature of the gel strip. Four parameters control electroactive hydrogelbased actuators: 1)magnitude of the applied potential; 2) angle of the applied potential; and both 3) width and 4) length of the actuator. Figure 1A illustrates the behavior of a rectangular gel actuator under changing parameters (width of post and applied voltage; see Supporting Information Fig. S1 for more complete parameter characterization). In situ photopolymerization facilitates the control of the geometric parameters and is amenable to the use of laser or multiphoton polymerization methods for improved resolution (e.g., further reduce actuator width to achieve faster and larger motion). To date, diverse electroactive hydrogels have been broadly applied to creating polymeric microactuators (see the detailed behavior of electric-sensitive hydrogel in Supporting Information). Here, we demonstrate the use of electroactive hydrogel for fabricating octopus-, sperm-, and myriapod-like aquabots that retain respective characteristic movement (Fig. 2; movies are available for octopus-like aquabot and sperm-like aquabots in Supporting Information – Videos 1 and 2, respectively). The repeated bending test under aquatic conditions was carried out to investigate the durability of the electroactive hydrogel strips (see Supporting Information for a detailed durability test conditions). The operation of the communications