(click to enlarge) As the hydrogel ring expands and contracts, it squeezes water in the chamber,
causing it to bulge against the oil. The process changes the shape of the lens and alters its focal length.
“The lenses harness the energy around them to control themselves,” says lead researcher Hongrui Jiang, assistant professor of electrical and computer engineering. “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 muscle is made of a ring of polymer hydrogel that expands and contracts in response to environmental stimuli, such as temperature or acidity levels. The movement does not require electronics for power or control and the polymer ring is only millimeters wide, making it appropriate for building on a microscale. The lens is made from a simple drop of water. Changes in the gel ring force the water droplet to change its focal length, similar to human eye functions.
A water-oil interface forms the lens, which sits on a water-filled tube with hydrogel walls. The tube's open top is covered with a thin polymer. The researchers applied a hydrophilic surface treatment to the aperture walls and underside. They applied a hydrophobic surface treatment to the top of the aperture. Where the hydrophilic and hydrophobic edges meet, the water-oil lens is pinned in place.
When the hydrogel swells in response to a substance, the water in the tube bulges 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.
The device could someday be used to scan blood samples simply by changing the polymer gel to react to different stimuli, such as proteins or salts. It could also react to electricity and light stimuli. “The ability to respond in autonomous fashion to the local environment is new and unique,” says David Beebe, a professor of biomedical engineering who also worked on the lens.
“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.
At this point, however, researchers say much work must be done before the device is able to be truly sensitive. So far, the achievement is just a proof of principle. The lenses only react to major temperature swings of about 10º–20ºC, or a change in pH from acidic to alkaline.
Jiang says the lenses could be scaled down to the micrometer range and set into multiple arrays to quickly and easily monitor several substances in any given sample. These lab-on-a-chip applications “should come pretty soon,” Jiang says.