Tiny Submersibles Navigate Honey

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.

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 micro-robots.