Video: 鈥楾ractor beam鈥� makes light work of particles
Microscopic particles can be steered through the tiny channels of a microfluidic chip using light, US researchers have shown.
They modified a standard piece of lab equipment to direct particle traffic using light, the trick could prove vital for rapid chemical and biological analysis with handheld devices.
Microfluidic chips contain microscopic tubes that can be used to ferry cells and particles around for chemical or biological experimentation. Controlling fluids and particles at small scales is difficult, however.
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The US team, led by and Michal Lipson at Cornell University, Ithaca, US used light to control these unruly particles.
They constructed a microfluidic chip with 鈥渨aveguides鈥� built into the walls of each fluid channel. The waveguides were designed to act like leaky pipes: as laser light passes through, it does not bounce perfectly off the inside walls, and a weak electromagnetic field 鈥� called the evanescent field 鈥� leaks out.
Mystery 鈥榗ushion鈥�
Particles that flow across a waveguide鈥檚 path are captured by its electromagnetic field and pulled along in the same direction as the light inside the waveguide. The approach can even steer particles around a bend (see video, top right).
鈥淲e are able to grab things out of the flow and propel them along a new path,鈥� says Erickson. He adds that microfluidic work is inherently fiddly, because drag, friction and viscosity are much stronger at such smaller scales. 鈥淥ptics is the opposite 鈥� as you confine things to smaller spaces they get faster and more useful,鈥� Erickson adds.
So, while larger objects do not feel the force of a bombardment of photons, small particles, like the 0.5 to 3.0-micrometre polystyrene balls used in the Cornell experiments, can easily be caught and redirected using electromagnetic radiation. Cells or other small objects could be steered in the same way, the researchers say.
鈥淭he field strongly polarises the particle and draws it towards the field鈥檚 centre,鈥� says Erickson. Other effects not yet fully understood prevent the particle from physically touching the waveguide. 鈥淵ou could say it鈥檚 transported on a kind of cushion,鈥� Erickson adds.
The researchers now plan to integrate different kinds of waveguide into microfluidic chips capable of exerting stronger effects on passing objects.
Journal reference: (vol 15, p 14322)
