Bio Focus: Soft microrobots propelled by structured light
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iniature robots or microbots now have a new way to move—using only light. In an article published in a recent issue of Nature Materials (doi:10.1038/ NMAT4569), a group of European researchers, led by Peer Fischer and first authored by Stefano Palagi, both from the Max Planck Institute for Intelligent Systems in Germany, explain how a soft microbot can be made to swim in a viscous medium just using patterned light. Researchers have repeatedly turned to nature for inspiration in designing smallscale robots. Worms move by sending waves of muscle contractions down their body in a process termed peristalsis, similar to how food moves through the digestive tract in humans. Centipedes move their numerous legs sequentially creating what is called metachronal motion. However, implementing these designs in a small machine has not been easy or efficient. The microbot developed in the present study is a liquid-crystal elastomer (LCE) cylinder, the largest being 170 μm in diameter and a millimeter in length. When appropriately excited using light, the LCE contracts lengthwise and expands radially. Alternating light and dark patterns over the cylinder causes the microbot to deform its body in a periodic way, creating wave-like propulsive motion. Thus the robot can be made to swim in a liquid using only lightpowered body-shape changes. By constructing the microrobot from a continuously addressable, soft active material and taking the onus of coordination
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of movement away from the microbot and putting it in the hands of an external light field that can be structured easily, the researchers have inverted the concept of microbot actuation. “It’s the first time I’ve seen people make an analogue of a microscopic swimmer. Since it’s actuated by structured light, there are no external forces or torques acting on the device, so it is a true swimmer,” says Thomas Powers, a professor at Brown University not asCross section of a liquid-crystal elastomer microrobot (false sociated with the study. color image) whose movement, powered by light, mimics that The speed with which a of a ciliate protozoa. The green illuminated sections show microrobot moves depends where radial expansion—or deformation—occurs versus the state. The schematic further depicts this localized on the medium, the wave- relaxed phase transition—from the nematic to the isotropic phase. length of the light pattern, Credit: Stefano Palagi, MPI-IS. and structural parameters such as its length. Using a light pattern that is swept at 2 Hz across the microbot causes it to These distinct modes are also seen in mimove a distance of 100 μm with speeds of croscopic protozoa called ciliates. 2–3 μm/sec. The study suggests that this Peter Palffy-Muhoray
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