Microbots have made great strides in recent years as scientists and engineers work on creating cell-sized robots that can swim through the bloodstream and act as tiny medical commandos. However, such tiny automata are tricky to steer and control, so researchers led by Clemens Bechinger of the Max Planck Institute for Intelligent Systems are taking a page from nature and developing simple microswimmers that can mimic the light-seeking behavior of some bacteria.

Phototaxis is a common behavior in the animal and plant kingdoms. When we talk about being drawn like a moth to a flame, that's an example of phototaxis. But it can also act in the opposite direction, as turning on the lights in a cheap hotel room and watching the cockroaches scatter can show.


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What makes phototaxis so attractive to roboticists is that it is a very simple behavior based on light intensity. Some creatures are drawn to light, some are repelled by it, and some seek out an area of just the right intensity. What the Max Planck team wanted to do was to mimic this behavior in a way that could be applied to robots the size of bacteria as a way to not only steer, but to move them at the same time.

According to the team, simplicity was the key to creating the microswimmers. Building phototactic robots on a human scale is very easy and the first robotic "tortoises" were built by pioneering cyberneticist William Grey Walter back in the late 1940s. But for all their being "simple" life forms, bacteria use highly complex mechanisms to sense and respond to light, which the team had no hope of duplicating in a microbot, so they settled on a remarkably minimalist design.

The microswimmers don't look much like robots or bacteria. In fact, they are nothing but glass microbeads with a width of a few thousandths of a millimeter with one hemisphere covered in carbon black. This led to their being called "Janus particles" after the two-faced Roman god of beginnings and endings.

These are suspended in a solution of water and a soluble organic chemical, which under heat separates from the water. When a light is shone uniformly on the Janus particles, the black side heats up more than the other and the solution next to it breaks up, causing a lower concentration. Then, like adding a drop of fresh water to a salt solution, the higher concentrate rushes in to reestablish the balance. This acts like oars in the particle as the solution flows past, pushing the particle in the direction of its transparent side.

But the Janus particles were still a long way from being controllable. According to Bechinger, when an even light was set over the microswimmers, they'd go off in any direction. However, if the light was changed to produce a gradient of light to dark, the particles would swim toward the light. More importantly, they would steer toward it because the light would cause any particle not pointing at it to heat unevenly, which would make the solution flow faster around one side than the other and cause it to rotate.

Unfortunately, Bechinger's team found that in ordinary light, the effect was only effective over a tenth of a millimeter before the particles started to veer off in random directions. Researcher Celia Lozano found a way to navigate over longer distances through the use of a system of lasers, lenses, and mirrors that produced a saw-toothed light field made up of areas of increasing and decreasing brightness. Any particles in the areas of decreasing brightness moved farther into the darkness, but the ones in the light areas went straight for the light and maintained their course even when passing through the areas of decreaasing brightness because the area was too narrow for them to have time to reverse course. The result was controllable, stable movement.

The Janus particles are still in the laboratory phase, but their simple, easy to manufacture design holds great promise. According to the team, the swimmers can not only be steered by light, but also chemically, which means they could one day be tailored to seek out tumours in the body to deliver precise doses of chemotherapy.

The team's paper appears in the journal Nature Communications.

Source: Max Planck GesellSchaft