Researchers from Tampere University in Finland and Anhui Jianzhu University in China have made a significant breakthrough in soft robotics. Their study introduces the first toroidal, light-driven micro-robot that can move autonomously in viscous liquids, such as mucus. This innovation marks a major step forward in developing micro-robots capable of navigating complex environments, with promising applications in fields such as medicine and environmental monitoring.

Their paper, titled "Light-steerable locomotion using zero-elastic-energy modes," was recently published in Nature Materials.

A peek through an optical microscope reveals a hidden universe teeming with life. Nature has devised ingenious methods for micro-organisms to navigate their viscous environments: for example, E. coli bacteria employ corkscrew motions, cilia move in coordinated waves, and flagella rely on a whip-like beating to propel themselves forward. However, swimming at the microscale is akin to a human attempting to swim through honey, due to the overwhelming viscous forces.

Inspired by nature, scientists specializing in cutting-edge micro-robotic technologies are now on the trail of a solution. At the heart of Tampere University's pioneering research is a synthetic material known as liquid crystalline elastomer. This elastomer reacts to stimuli like lasers. When heated, it rotates on its own due to a special zero-elastic-energy mode (ZEEM), caused by the interaction of static and dynamic forces.

According to Zixuan Deng, a Doctoral Researcher at Tampere University and the first author of the study, this discovery not only represents a significant leap forward in soft robotics but also paves the way for the development of micro-robots capable of navigating complex environments.

"The implications of this research extend beyond robotics, potentially impacting fields such as medicine and environmental monitoring. For instance, this innovation could be used for drug transportation through physiological mucus and unblocking blood vessels after the miniaturization of the device," he says.

Doughnut shape simplifies control of swimming robots

For decades, scientists have been fascinated by the unique challenges of swimming at the microscale, a concept introduced by physicist Edward Purcell in 1977. He was the first to imagine the toroidal topology—a doughnut shape—for its potential to improve the navigation of microscopic organisms in environments where viscous forces are dominant and inertial forces are negligible. This is known as the Stokes regime or the low Reynolds number limit. Although it seemed promising, no such toroidal swimmer had been demonstrated.

Now, a breakthrough in toroidal design has simplified the control of swimming robots, eliminating the need for complex architectures. By using a single beam of light to trigger non-reciprocal motion, these robots leverage ZEEM to autonomously determine their movements.

"Our innovation enables three-dimensional free swimming in the Stokes regime and opens up new possibilities for exploring confined spaces, such as microfluidic environments. In addition, these toroidal robots can switch between rolling and self-propulsion modes to adapt to their environment," adds Deng.

Deng believes that future research will explore the interactions and collective dynamics of multiple tori, potentially leading to new methods of communication between these intelligent microrobots.

Culminating the development of light-driven soft robotics

This latest study represents the culmination of findings from two major research projects.

The first project, STORM-BOTS, aims to train a new generation of researchers in the field of soft robotics, with a specific focus on liquid crystal elastomers. As part of this project, Deng's doctoral dissertation research is centered on developing light-driven soft robots that can move efficiently in both air and water. His work is co-supervised by Professor Arri Priimagi and Professor Hao Zeng from Tampere University.

The second project, ONLINE, explores non-equilibrium soft actuator systems. This project aims to achieve self-sustained motion, enabling novel robotic functions such as locomotion, interaction, and communication.