Many bio-inspired robots have been developed so far after careful investigation of animals' locomotion. To successfully apply the locomotion of natural counterparts to robots for efficient and improved mobility, it is essential to understand their principles. Although a lot of research has studied either animals' locomotion or bio-inspired robots, there have only been a few attempts to broadly review both of them in a single article. Among the millions of animal species, this article reviewed various forms of aquatic locomotion in arthropods including relevant bio-inspired robots. Despite some previous robotics research inspired by aquatic arthropods, we found that many less-investigated or even unexplored areas are still present. Therefore, this article has been prepared to identify what types of new robotics research can be carried out after drawing inspiration from the aquatic locomotion of arthropods and to provide fruitful insights that may lead us to develop an agile and efficient aquatic robot.

Sensing ability enables a robot to be aware at its motion, or to modulate the locomotory behavior. While many sensing components have been developed for macroscale robots, such offthe-shelf sensors are hardly integrated with millimeter-to-centimeter scaled robots due to the size limitation. In this work, we propose a compliant mechanosensory composite (CMC) to fabricate a small compliant mechanism with an embedded sensing ability. For this purpose, a conductive polymer PEDOT:PSS was directly printed onto the two layers of flexural joints in a compliant mechanism. Owing to the variation of electric contact resistance upon bending, the CMC could measure the bending angle of the flexural joint. Three different sensor pattern topologies (e.g. planar, interdigitated, and serpentine) were tested, and the serpentine pattern was chosen. Also, its performance was further verified by analyzing the cyclic bending and transient response. Overall, a sparsely printed serpentine pattern with thicker line exhibited consistent response without a noticeable hysteresis. To demonstrate the applicability of the CMC, a small inchworm robot actuated by a micro servo motor was built, and its motion was successfully measured using the embedded sensor. In addition, multi degrees-of-freedoms mechanisms such as a four-bar spherical joint was fabricated to measure a three-dimensional motion. We expect the proposed CMC will enable a small robot to become sensible at its self motion, external load, and physical contacts in future.

The locomotion of water beetles has been widely studied in biology owing to their remarkable swimming skills. Inspired by the oar-like legs of water beetles, designing a robot that swims under the principle of drag-powered propulsion can lead to highly agile mobility. But its motion can easily be discontinuous and jerky due to backward motions (i.e. retraction) of the legs. Here we proposed novel hair-like appendages and consider their coordination to achieve steady and efficient swimming on the water surface. First of all, we propose several design schemes and fabrication methods of the hair-like appendages, which can passively adjust their projected area while obtaining enough thrust. The coordination between the two pairs of legs, as with water beetles in nature, were also investigated to achieve steady swimming without backward movement by varying the beating frequency and phase of the legs. To verify the functionality of the hair-like appendages and their coordinations, six different types of appendages were fabricated, and two robots (one with a single pair of legs and the other with two pairs of legs) were built. Locomotion of the robots was extensively compared through experiments, and it was found that steady swimming was achieved by properly coordinating the two pairs of legs without sacrificing their speed. Also, owing to the lower velocity fluctuation during swimming, it was shown that using two pairs of legs was more energy efficient than the robot with single pair of legs.

Certain aquatic insects rapidly traverse water by secreting surfactants that exploit the Marangoni effect, inspiring the development of many self-propulsion systems. In this research, to demonstrate a new way of delivering liquid fuel to a water surface for Marangoni propulsion, a microfluidic pump driven by the flow-imbibition by a porous medium was integrated to create a novel self-propelling robot. After triggered by a small magnet, the liquid fuel stored in a microchannel is autonomously transported to an outlet in a mechanically tunable manner. We also comprehensively analyzed the effects of various design parameters on the robot’s locomotory behavior. It was shown that the traveled distance, energy density of fuel, operation time, and motion directionality were tunable by adjusting porous media, nozzle diameter, keel-extrusion, and the distance between the nozzle and water surface. The utilization of a microfluidic device in bioinspired robot is expected to bring out new possibilities in future development of self-propulsion system.

No abstract