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
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
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.)
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