Microfluidics and optics

High-throughput assays can be performed using microfluidic devices, but optics have remained off the chip, according to a recent review on advances in optofluidics written by researchers from California Institute of Technology in Pasadena, from Stanford University and from Howard Hughes Medical Institute in Stanford, all in California.

However, researchers have begun integrating optics and microfluidics on the same chip. Such optofluidic devices are more compact, allow high-throughput optical analysis and enable optofluidics users to change optical properties simply by altering the composition of the fluid medium in the microfluidic channels.

The optofluidic chips generally consist of a top layer comprising controls for adjusting the fluid properties, a middle layer containing channels through which the fluid flows and a bottom layer consisting of the optical tools. In some cases, the medium — which can be a mixture of fluid and solid or just fluid — in the microfluidic channels replaces an optical component such as a lens or a waveguide.

Of these, fluid-only devices can be manipulated more easily. Because two immiscible fluids of equal density form a perfectly spherical meniscus, lenses with perfect curvature can be made from fluid-only devices at a much lower cost than solid-state optical-quality lenses.

Changing the medium in the microfluidic channels can alter optical properties. For example, the devices can act as sensors by evaluating the resulting change in the refractive index of a biochemical process, or they can switch off light transmission if a darker solution is injected into the device.

Scientists can manipulate particles in an optofluidic device using optical tweezers, electrical fields and fluidic flow. The authors said that users can manipulate up to 400 particles with optical tweezers alone. If they use optical tweezers, an electrical field and a photoconductor, they can control 15,000 particles in an area of 1 mm².

The reviewers were optimistic about the future of optofluidic devices, noting that they have applications in chemical weapons detection, medical diagnostics and environmental monitoring. (Nature, July 27, 2006, pp. 381-386.)