Microfluidic cavities can be built around swimming bacteria using photopolymerisation, say US scientists. The method makes it quicker and easier to use the bacteria's motion as a power source, they claim.

Swimming bacteria cause a current that moves a bead in the opposite direction.

Bacterial motion can be used to mix and transport fluids and microparticles in microfluidic devices, removing the need for external pumps and power sources. But until now, scientists have had to build a whole new device each time they wanted to change a small part of its structure.

Bryan Kaehr and Jason Shear from the University of Texas, US, have now developed a much quicker method of modifying devices. They filled their microfluidic device with a protein solution, then polymerised the protein to create solid structures by focusing a laser beam on to it. This can be done with the bacteria swimming around in the solution, making it 'a highly efficient strategy for optimising microscopic architectures,' says Shear.

Kaehr and Shear used a variety of Escherichia coli bacteria that swim continuously in one direction. By making a circular microfluidic cavity with an angled entrance, they forced the bacteria entering the cavity to swim round in the same direction. The motion of the bacteria's tail-like flagella sets up a flow of liquid in the opposite direction. The researchers demonstrated that this flow is able to transport a bead.

One of the key advances over previous bacterial microfluidic devices is that the bacteria are not attached to the device's walls, says Shear - they can enter and leave the cavity, and when they die are simply replaced. Ideally, the bacterial population should be kept constant, he adds, which could be achieved by incorporating an outlet port that allows excess bacteria to be flushed out.

'It's a vivid demonstration of how the work done by the proton-driven flagellar motors can be harnessed, through collective motion, to greater and greater length scales,' says Greg Huber, an expert in biological physics at the University of Connecticut Health Center, US. 'It opens up the possibility of the precise control of flow, and objects in the flow, and even the use of bacteria as actuators for external devices coupled to the macroscopic world.'

Shear says he has high hopes for the technology. 'We could imagine that bacterially powered microfluidic devices could be employed as deep-sea or extraterrestrial environmental sensors, lying dormant until an environmental cue activates the motile population, allowing a device to function under specific conditions,' he comments.

David Barden

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