MADISON – When Hongrui Jiang looked into a fly’s eye, he saw a way to make a tiny
lens so “smart” that it can adapt its focal length from minus infinity to plus
infinity – without external control.
Incorporating hydrogels that respond to physical, chemical or biological stimuli and
actuate lens function, these liquid microlenses could advance lab-on-a-chip
technologies, optical imaging, medical diagnostics and bio-optical microfluidic
systems.
Jiang, a University of Wisconsin-Madison assistant professor of electrical and
computer engineering; David Beebe, a professor of biomedical engineering; Liang
Dong, a postdoctoral researcher; and Abhiskek Agarwal, a doctoral student, describe
the technology 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, says Beebe. “The ability to respond in autonomous fashion to the local
environment is new and unique,” he says.
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, says Jiang.
That’s where hydrogels – thick, jellylike polymers – are important. Researchers can
tune a hydrogel to be responsive to just about any stimulus parameter, including
temperature and pH, says Jiang. So as the hydrogel “senses” the substance of
interest, it responds with the programmed reaction. “We use the hydrogel to provide
actuation force,” he says.
A water-oil interface forms his group’s lens, which resides atop a water-filled tube
with hydrogel walls. The tube’s open top, or aperture, is thin polymer. The
researchers applied one surface treatment to the aperture walls and underside,
rendering them hydrophilic, or water-attracting. They applied another surface
treatment to the top side of the aperture, making them hydrophobic, or
water-repelling. Where the hydrophilic and hydrophobic edges meet, the water-oil
lens is secured, or pinned, in place.
When the hydrogel swells in response to a substance, the water in the tube bulges up
and the lens becomes divergent; when the hydrogel contracts, the water in the tube
bows down and the lens becomes convergent. “The smaller the focal length, the closer
you can look,” says Jiang.
Because they enable researchers to receive optical signals, the lenses may lead to
new sensing methods, he says. 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,” says 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 says.
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,” says Jiang. “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.”
Fabricating lenses is a straightforward, inexpensive process that takes just a
couple of hours. The real advantage, however, is their autonomous function, says
Jiang. “That forms a universal platform,” he says. “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.
