Miniature robots or microbots now have a new way to move—using on-ly 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 small-scale 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 light-powered body-shape changes.
By constructing the microrobot from a continuously addressable, soft active material and taking the onus of coordination 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 associated with the study.
Cross section of a liquid-crystal elastomer microrobot (false color image) whose movement, powered by light, mimics that of a ciliate protozoa. The green illuminated sections show where radial expansion—or deformation—occurs versus the relaxed state. The schematic further depicts this localized phase transition—from the nematic to the isotropic phase. Credit: Stefano Palagi, MPI-IS.
The speed with which a microrobot moves depends on the medium, the wavelength of the light pattern, and structural parameters such as its length. Using a light pattern that is swept at 2 Hz across the microbot causes it to move a distance of 100 µm with speeds of 2–3 µm/sec. The study suggests that this performance can be improved through the use of faster responding materials.
The researchers also developed a detailed theoretical model to analyze their results. An interesting observation is that the microbot exhibits two modes of motion, termed positive and negative. The motion is led by radial expansion at small wavelengths, whereas longitudinal contraction becomes important at large wavelengths (relative to body length). This causes the microbot to swim in opposite directions depending on the deformation wavelength. These distinct modes are also seen in microscopic protozoa called ciliates.
Peter Palffy-Muhoray, a professor at Kent State University and also not connected with the work, told MRS, “The authors have succeeded both in producing microrobots capable of swimming and locomotion, and in showcasing the tremendous versatility of soft photo-responsive materials. Capable of large, rapid and continuous shape changes, liquid-crystal elastomers literally dance under structured light. They are certain to play a key role in the emerging robotics–human interface.”
