One of the most exciting applications of soft robotics is the creation of flexible locomotive robots. With their compliant nature, these robots could be ideal for traversing the constrictive and unpredictable terrain commonly found during search-and-rescue situations. For years, researchers have aspired to design locomotive systems viable for this purpose. WSL has attributes that suggest it could excel in this arena; however, our team feels it has been under-investigated by the academic community.
WSL is inspired by the movement of an amoeba, whose locomotion heavily depends on pseudopods and cytoplasmic streaming. In this process, endoplasm flows into developing pseudopods causing them to grow away from the cell body. At the same time rear facing pseudopods are retracted into the cell. This motion allows these microorganisms to move effectively through their environment. In WSL, an inverting torus is often used to mimic the effects of cytoplasmic streaming. [1] As the torus inverts, the entire exterior facing membrane moves in the same direction and returns though the center.
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WSL has several advantages compared to other popular locomotive mechanisms such as two-anchor and wheeled locomotion. In two-anchor locomotion, a robot uses anisotropic friction to move forward by extending and contracting its body. In this system, the forward pushing ability is limited by the difference in the coefficient of friction in the forward and backward directions. [2] This limits their efficiency in restricted environments. Wheeled locomotion faces a similar limitation in that the friction force on the bottom half of the wheel has to be larger than the force on the top half. Most wheeled solutions simply prevent the top half from contacting the environment altogether, as a result this solution has limited utility in highly confined spaces. WSL does not face such limitations; any surface, top, bottom, or sides, can be used for forward movement. This property would give the robot a lot of potential for uses in the emergency response and medical fields; it would be well suited for traversing tight spaces such as those found in collapsed buildings or the human gastrointestinal tract. [1]
Several robots have been developed to explore the applications and viability of WSL. (A) Shows an FFT filled with faro-fluid that can be manipulated using magnetic fields. [4] In (B) an FFT was successfully driven using ionic polymer-metal composites (IPMC). [5] (C) Shows an early prototype of an SMA actuated torus. [1] While most WSL robots can be classified as soft, traditional WSL robots have been developed. (D) Shows a snake-like robot utilizing WSL actuated by traditional DC motors. [6]
