Light-Driven Micro-Robot Designed to Swim Autonomously in Viscous Liquids Could Be Used for Unblocking Blood Vessels

Image: The liquid crystalline elastomeric toroidal submarines can start swimming in honey when laser beams are directed at them (Photo courtesy of Hao Zeng/Tampere University)

A glimpse through an optical microscope unveils a hidden universe full of life. Microorganisms have evolved ingenious ways to navigate their thick, viscous environments. For instance, E. coli bacteria rotate like corkscrews, cilia move in synchronized waves, and flagella use a whip-like motion to propel forward. Swimming at the microscale, however, is like trying to swim through honey due to the dominance of viscous forces. Since 1977, when physicist Edward Purcell first introduced the concept, scientists have been intrigued by the unique challenges of microscale swimming. Purcell imagined a toroidal shape—a doughnut-like form—as potentially enhancing how microscopic organisms navigate environments where viscous forces overwhelm and inertial forces are negligible, a condition known as the Stokes regime or low Reynolds number limit. Despite its promise, no toroidal swimmer has been demonstrated until now. In a groundbreaking achievement, scientists have now unveiled the first toroidal, light-driven micro-robot capable of autonomous movement in viscous fluids, such as mucus. This marks a significant advancement in the development of micro-robots designed to navigate complex environments, with potential applications in medicine.

The core of this pioneering research, conducted by scientists from Tampere University (Tampere, Finland) and Anhui Jianzhu University (Hefei, China), involves a synthetic material called liquid crystalline elastomer, which responds to stimuli such as laser light. When heated, this material rotates on its own due to a unique zero-elastic-energy mode (ZEEM), created by the interaction of static and dynamic forces. This innovation in toroidal design simplifies the control of swimming robots by eliminating the need for complicated architectures. Using a single beam of light to trigger non-reciprocal motion, these micro-robots harness ZEEM to autonomously control their movements. This allows for three-dimensional swimming in the Stokes regime, facilitating the exploration of confined spaces such as microfluidic environments. Additionally, these toroidal robots can switch between rolling and self-propulsion modes to better adapt to different surroundings.

The researchers emphasize that this discovery is a major leap forward in the field of soft robotics and opens the door to developing micro-robots capable of navigating complex environments. Future studies will likely focus on interactions and collective dynamics among multiple toroidal robots, potentially leading to new methods of communication between these intelligent micro-robots. The findings, recently published in Nature Materials, are the culmination of two major research projects. The first, STORM-BOTS, aims to train a new generation of researchers in soft robotics with a focus on liquid crystal elastomers. This project seeks to develop light-driven soft robots that move efficiently in both air and water. The second project, ONLINE, investigates non-equilibrium soft actuator systems to enable self-sustained motion, facilitating new robotic functions such as locomotion, interaction, and communication.