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Autonomous lenses may bring microworld into focus

Artistic rendering of a smart liquid microlens

An artistic rendition of a smart liquid microlens where a stimuli-responsive hydrogel (dark circular ring) regulates the shape of the liquid lens (center). The liquid microlens autonomously adapts to local environmental stimuli (denoted by small spheres and yellow plasma rays outside the hydrogel) in microfluidics. The stimuli can be biological and chemical agents, and physical parameters (light, temperature, pH, electric field, etc.). (large image)

Hongrui  Jiang

Hongrui Jiang (large image)

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.

The technology is featured on the cover of the Aug. 3 issue of the journal Nature. Jiang, a University of Wisconsin-Madison assistant professor of electrical and computer engineering; David Beebe, a professor of biomedical engineering; postdoctoral researcher Liang Dong; and doctoral student Abhishek Agarwal describe it in the Aug. 3 issue.

David J. Beebe

David J. Beebe (large image)

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.

movie icon VIDEO 1
This movie shows that a single liquid microlens can autonomously respond to a stimulus (e.g., temperature in this movie) and change its focal length. The temperature-sensitive hydrogel triggers a change in the curvature of the liquid microlens. As the environmental temperature increases from 23°C to 47°C, the liquid microlens bows down. As the device cools down naturally, the liquid microlens grows back.

movie icon VIDEO 2
This movie shows that a liquid microlens array can mimic the function of an insect compound eye to monitor different areas in space, and has the potential to realize complex functions by leveraging different hydrogels (e.g., two different pH-responsive hydrogels, AA and DMAEMA, in this movie). In the movie, both smart liquid microlenses are integrated in one microchannel. They are exposed to the same pH buffer solution at all times. Each microlens monitors a designated area in space. By replacing initial pH-12.0 buffer with pH-2.0 buffer, the liquid microlens of the DMAEMA hydrogel-based microlens gradually bows up, while the AA hydrogel-based microlens bows down.

Optical image (top-view) of a temperature-adaptive liquid

Optical image (top-view) of a temperature-adaptive liquid microlens (large image)

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.”

Adaptive focusing of two objects (top) and corresponding optical
                        images of the shape of the liquid microlens (bottom)

Top row: adaptive focusing of two
objects — a needle tip and a small ball connected to a pillar — separated by 1.38 cm realized by a pH-adaptive liquid microlens. Bottom row: corresponding optical images of the shape of the liquid microlens. (large image)

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) partially funded the research. The researchers are patenting the technology through WARF.