Swimming micromachines, capable of self-assembling to perform a specific task and then disassembling once that task has been completed, have been the subject of a significant amount of research worldwide.
However, it has so far proven difficult to control individual microbots effectively when they are operating within a large group or swarm, meaning they cannot yet be used for precision tasks such as drug delivery or in microfluidics applications.
To tackle this problem, Dr Tom Montenegro-Johnson, a mathematical biologist at Birmingham University, looked at the way certain bacteria such as E.coli change their shape in order to propel themselves through their environment.
“These bacteria swim by a run-tumble method,” said Montenegro-Johnson. “This means they have a ballistic trajectory for a certain amount of time called a run, where they swim in a straight line, and then they unbundle all of their flagella, and this causes a random re-orientation, and then they swim in a straight line again,” he said.
To recreate this motion strategy, Montenegro-Johnson is developing swimming microbots in the form of flexible filaments made from a shape memory polymer.
Both ends of each filament are coated in platinum. When placed in hydrogen peroxide, the platinum catalyses its reduction into water and oxygen, causing a flow at the surface of the filament.
If the filament is straight, this flow should act as a pump, if it is bent in a U-shape, the filament will translate, or move a certain distance, and if it is bent into an S-shape, it should rotate.
By targeting ultrasound at different points within the fluid, Montenegro-Johnson plans to heat up individual microbots, triggering the shape memory response and causing them to switch between these pre-programmed shapes.
To control the swimmers, Montenegro-Johnson is planning to use a focused ultrasound keyboard developed by Ultrahaptics, a spin out from Bristol University. “They have created something called mid-air touch sensation, which consists of lots of ultrasound emitters that can focus onto an 8mm point in space, which they can then move about 16,000 times a second,” he said.
In this way the microbots should be able to navigate complex environments using a series of straight runs and on-the-spot re-orientations, in the same way as bacteria.
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