Biomechanical Microbot Toys Display the Movement of Biological Machines.

By stringing together self-propelled mini-games, researchers at the UvA Institute of Physics have found a key to studying the movement of microorganisms and the molecular motors inside our cells.

Hexbug Nano v2 microbeads use vibrations to propel themselves forward. By attaching several of these toys to a flexible silicone rubber string, the resulting structure is “elastic”. This means that it will return to its original shape after being deformed, while the self-propelled active components that make up it are constantly trying to push the structure in a certain direction.

Depending on the size of the chain links and whether the chains were clamped at one or both ends, the elastic chains showed a range of motion types, including self-oscillation, self-synchronization, and self-capture.

“By experimenting with these rubber strings, we discovered that there is an interaction between activity and elasticity: when activity takes over, the strings oscillate and self-synchronize,” says Corentin Coulais, head of the Laboratory of Mechanistic Materials at the University of Amsterdam.

He continues, “Mechanical self-oscillation and synchronization are key features of biological machines, and are useful features for making new types of autonomous robots. These active chains really allow us to determine the nature of these nonlinear phenomena.”

The results have been published in the journal Physical review letters.

Self-oscillating behavior of a rubber string fixed at one end, self-synchronizing behavior of two strings coupled to a rigid rod, and self-synchronizing behavior of a string fixed at both ends. Credit: Elaine Cheng

Self-oscillation, self-synchronization, and self-pickup

When the structure is self-swinging, it means that it swings back and forth on its own. In chains, the microbes may begin to bend the chain to the left. However, because the chain is attached at one end, the elastic links resist this movement, reorienting the robots so that they begin to push and bend the chain to the right. The elastic chain will again resist this movement, until the robots start moving left again.

Synchronization occurs when two flexible strings are connected at one end by a sufficiently rigid rod. By waving, the two connected strings start to sway automatically at the same frequency, like seaweed being moved by the same waves.

Finally, by taking a single elastic string and clamping its two ends, it exhibits ‘self-capturing’ behaviour. When you bend a playing card with your fingers, you can make it “pop” to bend the other way by pushing hard enough from the side. Elastic strings do this on their own, repeatedly going from bent left to bent right.

Guided play

“We started this research by playing with microbe toys. But in general, the idea was to explore materials out of equilibrium. In soft matter, energetic liquids have been studied extensively in the last 25 years, but their rigid analogues have been investigated much less. Collis says.

Next on the list is the exploration of elastic behavior on smaller scales, for example in so-called colloidal systems, which consist of tiny particles suspended in a liquid. Although they are still model systems, they are closer to a biological system because of their similar length scales and the presence of liquid. At any scale, it would also be interesting to use clever design to embed multiple self-oscillations within a single structure to elicit more complex motion patterns. With a better understanding of self-oscillations, we hope it will become possible to create new types of autonomous robots.