Optics

MICRO-OPTICS: Hydrogel-controlled liquid microlenses respond to stimuli

Sept. 1, 2006

Small and versatile adaptive liquid lenses have been fabricated by researchers at the University of Wisconsin-Madison; the many potential uses of these lenses stem from the fact that their focal length can respond to a number of unusual stimuli, such as pH or the presence of biological substances like antigens.

John Wallace

FIGURE 1. Seen in cross section, a hydrogel ring (tan), which wraps around a water reservoir and can expand (I, blue) or contract (II, red) in response to an external stimulus, propels a water/oil surface upward (I) or downward (II). Because the refractive indices of the two liquids are different, the result is a focusable liquid lens.

The microfluidic lenses get their optical power from a fluid interface that has oil on one side and water on the other (with different refractive indices); the edge of this interface is “pinned” by a circular interface between a hydrophilic sidewall and its hydrophobic top surface (see Fig. 1). Below the sidewall is a hydrogel ring, which contacts or expands in response to external stimuli, changing the volume of the water inside the ring, which then pushes or pulls on the oil/water surface and alters its optical power.

The solid supporting portion of the device’s structure is fabricated by polymerizing a UV-curable plastic using photolithography. In one experimental example, a temperature-sensitive hydrogel, which contracts as the temperature is raised, is placed in the device. In this case, a plastic outer ring restricts the hydrogel so that it can only expand inward; although the hydrogel absorbs water from the reservoir within, it expands by a volume greater than the water it absorbs. The liquid lens, which is about 2 mm in diameter, takes 20 to 25 seconds to change focal length from -11.7 mm at 23°C to infinity at 47°C.

A second example replaces the temperature-sensitive gel with one that responds to pH; in this case, no outer plastic ring is present, allowing the hydrogel ring to respond to the pH of the surrounding solution. The response time is about 12 seconds for a visual change in the shape of the lens meniscus.

FIGURE 2. Two adjacent liquid microlenses are driven by hydrogels that respond oppositely to changes in pH. The initial pH-12 buffer was replaced with a pH-2 buffer; images from top to bottom correspond to times of 0, 13, 30, and 56 seconds after the change in buffer. Each lens is 0.5 mm across.

Arrays respond locally

Adaptive-microlens arrays are possible; to demonstrate the concept, the researchers fabricated a pair of liquid microlenses, each with a different hydrogel-one expands under increasing pH, the other contracts under the same conditions (see Fig. 2). Arrays of stimuli-driven microlenses, whether using identical or differing hydrogels, have the scientifically interesting ability to respond to very local stimuli, thus providing a 2-D “map” of the immediate environmental conditions.

The lenses can be fabricated in diameters ranging from the micrometer to the millimeter level-in other words, at any size at which surface tension is the dominant shaping force, says Hongrui Jiang, one of the Wisconsin researchers. Micro-imaging and other optofluidic systems, as well as lab-on-a-chip technology, are applications of interest. “Biological and chemical sensing are most-immediately realizable,” he notes. “I am most excited by a micro-imaging system that incorporates these lenses.”

REFERENCE

1. L. Dong et al., Nature 442 (Aug. 3, 2006).

About the Author

John Wallace | Senior Technical Editor (1998-2022)

John Wallace was with Laser Focus World for nearly 25 years, retiring in late June 2022. He obtained a bachelor's degree in mechanical engineering and physics at Rutgers University and a master's in optical engineering at the University of Rochester. Before becoming an editor, John worked as an engineer at RCA, Exxon, Eastman Kodak, and GCA Corporation.