02 August 2006
NewScientist.com news service
Tom Simonite
SOMETIMES all it takes is a quick hug, and everything looks different.
Now a shape-shifting lens has been developed that alters its focal length when squeezed by an artificial muscle, rather like the lens in a human eye. The muscle, a ring of polymer gel, expands and contracts in response to environmental changes, eliminating the need for electronics to power or control the devices.
"The lenses harness the energy around them to control themselves," says lead researcher Hongrui Jiang at the University of Wisconsin-Madison, US, where the device has been developed (Nature, vol 442, p 551). "This would be useful for environments where it's not easy to use electronics and conditions are not constant." The devices could simplify medical imaging equipment and biosensors, he says.
The lenses themselves, which are around 4 millimetres in diameter, use a glass-oil-water interface (see Diagram). The artificial muscle encloses the watery side of the lens. The gel expands or contracts in response to environmental changes such as a rise in temperature, forcing the water to bulge into the film of oil. This changes the lens's shape and thus its focal length.
Different polymer gels can be used to create a lens that responds to changes in acidity, temperature, light, electric fields or even certain proteins. In tests of one temperature-responsive lens, the device was able to focus on objects around 20 millimetres away at 50°C, and on objects 50 millimetres away when the temperature changed to 35°C.
Adaptable liquid lenses are already used in devices such as cellphones, but these are controlled by an electric current, requiring the use of bulky and power-hungry electronics. "The lenses would be useful for medical imaging because in the right environment they could scan different depths autonomously," says Jiang. For example, a lens designed to respond to a particular protein could be implanted into the body. As levels of the protein fluctuated throughout the day, the lens would change its focus, giving doctors a changing view of the area under observation.
The lens could also be combined with a light source and simple light detector to build a sensor. Changes in the way the lens focused light in response to, say, acidity, would then be picked up by the detector.
"The technology could be very useful for temperature sensing in systems of biosensors," says Tracy Melvin, who works on biophotonics at the University of Southampton, UK. "The lenses are currently a bit big for a micro-device, but they would be easy to make smaller."
If they can be reduced in size, the lenses could dramatically simplify micro-sensors, she says, but adds that making micro-lenses out of responsive polymer gels might be simpler still. Polymer gel contact lenses that darken when the wearer's glucose levels fall have already been developed, but no one has yet built them at the micro-scale.
