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. 鈥淚n a stiff fluid, what you achieve in half of your swimming cycle you undo in the next half-cycle,鈥� says , a physicist at the University of Sheffield in the UK.
That鈥檚 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.
Advertisement
Now Golestanian and 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.
The beads are made of a magnetic material and so align themselves with any applied magnetic field. By rotating this magnetic field, the researchers set the beads spinning, and were delighted to find that the beads moved through water at about 1 micrometre per second. 鈥淚 didn鈥檛 expect to see real propulsion like [that seen] in bacteria, to tell the truth,鈥� says Tierno.
The movement occurs only when the micromachine is close to the bottom of a vessel. This is because there is a less mobile boundary layer that 鈥渟ticks鈥� to the bottom surface of the fluid container and so exerts a larger force on the rotating bead than the rest of the water (see diagram). This makes the whole thing move, just as a paddle wheel can propel a boat because water resists the paddles more than air does.
Golestanian says: 鈥淚t鈥檚 like a unicycle wheel with the smaller bead as the pedal making it go around 鈥� with the DNA as the pedal shaft.鈥�
The team believes its technology can easily be shrunk to the nanoscale 鈥� the level at which it would be useful as a drug carrier. 鈥淢icroscale and nanoscale hydrodynamics are not all that different,鈥� says Golestanian. The boundary-layer properties that the device needs to swim should be present in small blood vessels.
Tierno says the swimming beads could also shuttle reagents from one part of a miniaturised 鈥渓ab-on-a-chip鈥� to another. John Illingworth, a biologist at the University of Leeds in the UK, is impressed. 鈥淲hat they鈥檝e done is certainly tough to achieve,鈥� he says.
