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Man-Made Microlenses May Lead to New Sensing Methods

Photonics.com
Aug 2006
MADISON, Wisc., Aug. 3, 2006 -- Man-made miniature lenses have been created that can alter their shape and light-focusing properties, like the lens of the human eye. The devices, in which the lens is squeezed or flattened by a surrounding hydrogel ring that acts as an artificial muscle, could be incorporated into microsystems for imaging, medical diagnostics, displays and light-based information processing.microlens.jpg
In this artist’s rendering of a smart liquid microlens developed by University of Wisconsin-Madison researchers, environmental stimuli (shown as yellow rays and tiny spheres) trigger a hydrogel (shown as a yellow ring edged in black) to swell or contract. As a result, water below the lens (center) either bulges or bows and the lens becomes divergent or convergent. Such smart microlenses could be incorporated into microsystems for imaging, medical diagnostics and other uses. (Image: Ryan Martinson, Silverline Studio/University of Wisconsin-Madison)
The University of Wisconsin-Madison researchers constructed their lenses from droplets of water, which bulge out through a circular aperture into a film of oil. The curvature of the bulge can be controlled by using a hydrogel ring whose walls can expand or contract, altering the volume of the reservoir of water below the aperture. The hydrogel ring comprises a soft polymer gel that changes in volume in response to various environmental stimuli, such as changes in temperature or acidity of the fluid that surrounds it.

Hongrui Jiang, assistant professor of electrical and computer engineering; David Beebe, professor of biomedical engineering; Liang Dong, postdoctoral researcher; and Abhiskek Agarwal, doctoral student, describe their work in the Aug. 3 issue of the journal Nature.

At this size -- hundreds of microns up to about a millimeter -- variable focal length lenses aren't new; however, existing microlenses require external control systems to function, said Beebe. "The ability to respond in autonomous fashion to the local environment is new and unique," he said. In a lab-on-a-chip environment, for example, a researcher might want to detect a potentially hazardous chemical or biological agent in a tiny fluid sample. Using traditional sensors on microchips is an option for this kind of work, but liquid environments often aren't kind to the electronics, Jiang said.

Because they enable researchers to receive optical signals, the lenses may lead to new sensing methods, he said. Researchers could measure light intensity, like fluorescence, or place the lenses at various points along a microfluidic channel to monitor environmental changes. "We've also thought about coupling them to electronics - that is, using electrodes to control the hydrogel," said Beebe. "Then you can think about lots of imaging applications, like locating the lenses at the ends of catheters."

Clustered in an array, the lenses also could enable researchers to take advantage of combinatorial patterns and provide them with more data, he said. The array format improves upon the natural compound eye, found in most insects and some crustaceans. This eye essentially is a sphere comprised of thousands of smaller lenses, each of which has a fixed focal length. "Since the lenses are fixed, an object has to be a certain distance away for it to be clearly seen. In some sense, our work is actually better than nature, because we can tune the focal length now so we can scan through a larger range of view field," said Jiang.

Fabricating lenses is a straightforward, inexpensive process that takes just a couple of hours. The real advantage, however, is their autonomous function, said Jiang. "That forms a universal platform. We have a single structure and we can put different kinds of hydrogels in and they can be responsive to different parameters. By looking at the outputs of these lenses, I know what's going on in that location."

Grants from the UW-Madison Graduate School, the Department of Homeland Security-funded National Center for Food Protection and Defense at the University of Minnesota, and from the Wisconsin Alumni Research Foundation (WARF) supported the research. The group is patenting the technology through WARF. For more information, visit: www.wisc.edu


GLOSSARY
eye
The organ of vision or light sensitivity.  
focus
1. The focal point. 2. To adjust the eyepiece or objective of a telescope so that the image is clearly seen by the observer. 3. To adjust the camera lens, plate, or film holder so that the image is rendered distinct. 4. To move an entire microscope body tube relative to a specimen to obtain the sharpest possible image.
lens
A transparent optical component consisting of one or more pieces of optical glass with surfaces so curved (usually spherical) that they serve to converge or diverge the transmitted rays from an object, thus forming a real or virtual image of that object.
light
Electromagnetic radiation detectable by the eye, ranging in wavelength from about 400 to 750 nm. In photonic applications light can be considered to cover the nonvisible portion of the spectrum which includes the ultraviolet and the infrared.
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