Aug. 4, 2006 — Without any external manipulation, a tiny new liquid lens can adjust its focal length to virtually any number.
The adaptive liquid microlens, developed by researchers at the University of Wisconsin, Madison, could lead to cheap and easy-to-make sensors, "lab-on-chip" devices used for testing biological and chemical substances in a fluid, and even "camera pills" that photograph the intestines after being swallowed.
"We don't have any electronics to control it, everything is autonomous," said the team's leader, Hongrui Jiang. Jiang and his colleagues reported their work in this week's issue of Nature.
The novel technique relies on a millimeter-sized ring made of a polymer- and water-based "hydrogel." The gel can respond to a variety of stimuli including temperature, light, chemicals, bacteria, acids and bases, and electrical fields.
The ring's response to whatever it detects is to either contract or expand.
In lab experiments, Jiang and his team tested several different hydrogel rings by placing each in a tiny reservoir of fluid sandwiched between two pieces of glass.
In each case, the ring constrained a layered droplet made from water and oil. The boundary where the two substances repelled each other served as the optical lens.
When using a temperature-sensitive hydrogel ring, the ring shrank under high temperature, pulling the drop out into a concave shape. Under low temperature, the ring swelled, squeezing the drop into a convex shape.
In another experiment, the ring contracted and expanded when exposed to liquids with different levels of acidity.
Because the hydrogel automatically reacts to the intrinsic nature of liquids, it can serve as both the sensor and a kind of artificial muscle to adjust the focal length of the droplet, giving it an autonomy similar to that found in the eyes of animals.
"The ring somewhat mimics musculature, so that's a clever attribute of the work," said L. Andrew Lyon, an associate professor at Georgia Institute of Technology in Atlanta, and an expert in microlens technology.
But the liquid nature of the entire system, he said, will significantly reduce its durability when applied to a real-world application, where gravity, vibrations, and varying temperatures could render a device inoperable.
"The challenge for making tunable lenses are robustness, ease of fabrication, and the ability to implement them into large-scale arrays easily. They haven't superseded what people have done previously in those regards," said Lyon.
Currently, Jiang's team is working to shrink the size of the ring from the millimeter scale to the micron scale. A smaller size will not only speed up the ring's response time from seconds to microseconds, it will also allow the scientists to build many lenses into an array to provide a wider field of view.
