Tiny 'paddleboat' could ship drugs around the body

NewScientist - December 4, 2008, by Paul Marks

A MICROSCOPIC swimming machine that works like a paddle steamer could help deliver drugs inside the body and move chemicals around inside miniaturised labs. The device is the first artificial microswimmer to move without using chemical propulsion or bending itself into different shapes.



For microscale swimmers, the viscosity of water presents a much bigger barrier to motion than we are used to on everyday scales. It is like swimming through honey for a human: any forward movement during one half of a swimming stroke would be negated by an opposite backwards motion in the second half, with the result that the swimmer goes nowhere. "In a stiff fluid, what you achieve in half of your swimming cycle you undo in the next half-cycle," says Ramin Golestanian, a physicist at the University of Sheffield in the UK.

That's why bacteria like Escherichia coli use a rotating corkscrew-like tail called a flagellum to propel themselves forward. With a continuously rotating propeller rather than a backwards-forwards swimming motion, the bacteria barrel along.

Now Golestanian and Pietro Tierno at the University of Barcelona in Spain have been able to achieve a similar goal with a micromachine that swims by mimicking a paddle wheel. The researchers built their microswimmer from two beads, 1 and 3 micrometres in diameter. They coated the beads in a protein called streptavidin that binds strongly to DNA and then fastened them together with two 8-nanometre strands of DNA.

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