Autonomous lenses may bring microworld into focus
hen 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.
The technology is featured on the cover of the Aug.
3 issue of the journal Nature. Jiang, an
assistant professor (also of electrical and computer engineering); Professor
David Beebe; postdoctoral researcher Liang Dong; and doctoral student
Abhishek Agarwal describe it in the journal.
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
(pictured), 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.”
