

Video: Magentic fields can make microscale machines do useful work Video: Magentic fields can make microscale machines do useful work

Tiny, free-roving micromachines have proved their potential for a range of uses from non-invasive surgery to mini chemical plants. But such small devices present a big hurdle to researchers: they are very tricky to power. The processes we rely on for fuel or power at normal scales just can’t be scaled down enough.

Now two new studies demonstrate what could be a solution: using magnetic fields to remotely power and control microscopic machines.

One of those machines is a micro-motor inspired by the corkscrewed spinning tail, or flagellum, used by some swimming bacteria. Built by Bradley Nelson at the Swiss Federal Institute of Technology in Zurich and international colleagues, it spins and swims like the real thing, and can propel a mass roughly the same size as real bacteria.

The artificial flagellum is driven by the changing magnetic field produced by three pairs of electromagnetic coils positioned to cover the X, Y and Z axes of 3D space, and positioned around the tank of water in which the machine moves. Its 47-micrometer-long helical tail is fashioned from a ribbon of a semiconductor material and it has a 4.5-micrometer-long magnetic “head” composed of chromium, nickel and gold.


Spinning field

By continually varying the electric current passing through each magnet pair, the team is able to generate a rotating magnetic field. The magnetic head constantly adjusts to align with the changing field, which causes the tail to spin and drives the machine forwards (see movie).

“The fastest [speed] we have achieved with the current setup is 20 micrometers per second [around four-tenths of a body length],” says Nelson. “But with some minor electronic modifications we expect over 100 micrometers per second.”

Self-propelled devices like this could be useful in biomedicine, where they could manipulate sub-cellular objects, or help in targeted drug delivery, Nelson says. “Magnetic approaches have the advantage that they don’t require changes in the chemical composition of the environment,” he adds.

Guided whirlpools

Alexey Snezhko at the Argonne National Laboratory in Illinois agrees. “Any mechanism of controlled self-propulsion at micro- and nanoscale becomes important,” he says. “Magnetic swimmers constitute one of the [most] promising directions since such swimmers could be manipulated non-invasively.”

His own team’s studies into magnetic manipulation at the microscopic scale have produced self-assembling magnetic “snakes” capable of ferrying a microscopic cargo across the surface of a liquid.

The snakes start off as individual 90-micrometer-wide nickel spheres floating on the surface of a beaker of water. Magnetic coils placed around the container causes these tiny balls to spin on the spot, generating small whirlpools that drive them across the surface. When two spheres pass sufficiently close to each other they join up due to magnetic attraction, eventually forming long snake-like strings.

Tiny tugboats

The combined effect of the spinning particles in a snake generates stronger whirlpools that act rather like engines, but because the engines are all similar in size and working in opposite directions they cancel one another out and the snake remains motionless.

However, if an object – for example a polystyrene bead – obstructs one of those whirlpool engines then the snake is forced out of balance and is driven across the surface of the water by the others. It’s a mechanism that has no analogy in nature, says Snezhko.

Tweaking the magnetic fields provides a way to move around small-scale cargo with precision. “The swimmers can easily push big particles, so potentially they can be used as a tool for targeted delivery,” he says. However, it is unlikely that the snakes could be used to deliver drugs inside the body because they only move on the surface of a liquid. They are likely to be more useful to mix chemicals and increase reaction rates, Snezhko says. They could also be used to clean up the surface of a water body, he suggests.

“The magnetic particles could be functionalised by special molecules to adsorb specific particles or bacteria, so that while swimming they can collect those specific particles.”

Journal reference: Snezhko’s paper: Physical Review Letters (upcoming publication). Nelson’s paper: Applied Physics Letters (DOI: 10.1063/1.3079655)