Formulating Lenses that Focus on Their Own

Researchers at the University of Wisconsin-Madison have devised some smart microlenses that change optical properties, such as focal length, on their own in response to varying environmental conditions.

The devices, either singly or in arrays, could be used for direct optical readout of sensors, in microimaging systems for medical diagnostics and biological studies, or in an optical system integrated into a lab on a chip. The microlenses are composed of oil, water, glass and, most importantly, hydrogels — jellylike polymers that respond to the environment. The hydrogels are the basis of the autonomous microlens operation, explained team leader and assistant professor of electrical and computer engineering Hongrui Jiang.

In constructing the microlenses, the scientists fabricated a hydrogel ring in a microfluidic channel system. They sandwiched the ring between a glass plate and an aperture slip, topping everything with a glass cover. They filled the bottom with water and the top with oil, after first treating the top of the aperture slip to be hydrophobic and the bottom to be hydrophilic. As a result, the water-oil interface was pinned to the aperture along a contact line.

Changing focal length

The water bulged up out of the aperture when the hydrogel expanded in response to environmental changes. When the hydrogel contracted under other environmental conditions, the water sank into the aperture. This movement changed the optical properties of the microlens, switching the focal length from divergent to convergent.

The researchers report in the Aug. 3 issue of Nature that they devised 1-mm-diameter hydrogel microlenses that respond to temperature: one with a focal length of –11.7 mm at room temperature and one with 22.8 mm at 47 °C. The transition from divergence to convergence took place at 32 °C. Another device they built reacted to changes in pH, bringing objects positioned at various distances successively into focus as the pH climbed from 2.0 to 10.0.

Hydrogels also can be sensitive to light, electric fields and antigens. These hydrogels could be combined into an array, creating an optically based sensor that could detect a number of physical, biological or chemical conditions.

One problem is that such arrays would be quite large. Another is that the response time of the sensors is now in the tens of seconds — too long for some applications. Both problems, Jiang noted, could be solved by ongoing research.

“The next immediate goal is to make the lens smaller — to the tens of microns — for a shorter response time and, ultimately, a smaller array,” he said.