Light spots sort particles

Many teams of scientists are working to make components that can be used on labs-on-a-chip, which are poised to bring biological, chemical and medical lab tests to handheld devices in the field.

Researchers from the University of St. Andrews in Scotland have found a cheap, simple way to sort microscopic particles by size and by refractive index. A material's refractive index has to do with how much it bends light.

The researchers were able to sort a mix of similar-sized silica spheres and polymer spheres using the device because the two materials have different refractive indexes. They also sorted protein microcapsules, which can be used to deliver drugs, by size.

The system uses focused laser spots no larger than 30 microns in diameter and inexpensive aspherical lenses to form a three-dimensional light pattern. At the microscale a stream of light can affect the kinetic motion of a particle, much like wind moves a windmill. As different types of materials move through the lattice, they are slowed to different degrees by the bright spots of the pattern.

The system can be adjusted in real-time and has a sorting efficiency that ranges from 96 to nearly 100 percent, according to the researchers.

The method could be used practically in two to five years, according to the researchers. The work appeared in the November 27, 2003 issue of Nature.