Precision trapping on a microfluidic chip

For the past couple of decades, biophysicists have used optical traps to investigate the behavior of cellular structures at the single-molecule level with nanometer precision. But conventional optical-trapping instruments can manipulate only one biomolecule at a time. And methods for generating multiple traps require a laser beam whose power scales up with the number of traps. Cornell University researchers led by Michelle Wang have now developed an optoelectrical fluidic platform that overcomes both drawbacks. Their device, illustrated here, requires little power and can trap and manipulate potentially hundreds of molecules at a time. At its heart are two subcircuits (within the dashed lines), each of which splits laser light in a silicon waveguide (red) into two parallel arms. Because the arms join together in a loop, the counterpropagating waves interfere to form a standing wave whose antinodes serve as the optical traps in the part of the waveguide exposed to a fluid pool (blue). To independently manipulate the positions of the traps—shown suspending polystyrene beads and an array of six DNA molecules stretched between them—the researchers placed microheaters (orange) close to the waveguides. Local heating changes the optical path length and thus imparts a phase shift to the waves and moves the antinodes. Because all the optical elements producing the traps are tiny and on chip, the new platform is naturally resistant to thermal drift, environmental noise, and vibrations. (M. Soltani et al., Nat. Nanotech., in press.)