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Hydrogels get a new focus Circular hydrogels control adaptive liquid microlenses at an oil–water interface.
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HONGRUI JIANG AND LIANG DONG
Jiang says that along with shrinking The idea of using a polymer that reydrogels could give visualization the lenses, they’re also trying to build sponds to a stimulus to move an object devices a whole new focus. Hongisn’t new, “but it usually was a mechani- arrays so they can integrate the lenses rui Jiang and his colleagues at the Uniright into the lab-on-a-chip system. cal actuation of a valve or a cantilever versity of Wisconsin, Madison, have fab“You can have this complete optical debeam or simply the gel itself pushing ricated adaptive liquid microlenses that tection system right on the are formed at oil–water interchip,” he says. With most labfaces and manipulated by circuon-a-chip systems that rely on lar hydrogels. By responding to fluorescence for external detectemperature or pH, the hydrotion, a liquid lens array could gel expands or contracts, changshrink the instrumentation to a ing the lens’s shape and altering fraction of its current size. its focal length. The lenses could At the same time, the invesbe incorporated into lab-on-atigators are examining other chip or other biomedical devices hydrogels that respond to difto provide a novel imaging sysferent stimuli. “If we can make tem for detection technologies a lens array where individual (Nature 2006, 442, 551–554). lenses are responsive to differ“There aren’t any electronics ent kinds of stimuli, then we to control” the lens, Jiang says. have something that can moni“We have provided a very eletor different areas in space or gant method of incorporating multiple parameters,” Jiang the adaptive nature of the hysays. drogel into the lens itself.” A cirAlthough many lenses recular hydrogel sits like an island quire a flat, rigid structure, in the middle of a microfluidic Jiang and his colleagues have channel system, within a layer of demonstrated that they can water and oil, and is pinned in shape lenses like cylinders or place between a glass slide and From top to bottom, two different pH-sensitive microlenses build them on flexible materian aperture slip. In a hole at the respond to a change in buffer from pH 12.0 to 2.0. The images als. With this conformability, center of the hydrogel, the wawere taken over a period of 60 s. Jiang sees the potential to inter layer forms a droplet at its corporate these liquid lenses into medsomething,” says Stefan Zauscher of interface with the oil. As the hydrogel Duke University. “Here, it’s been incor- ical diagnostics. “If we can put lenses in responds to a stimulus, it either absorbs porated into an interesting device.” This an array for an imaging system in a water from or releases water into the spherical [format], we could put this in idea takes research into stimuli-responcentral droplet. The absorption or rea pill,” he says. Such a “lab-in-a-pill” sive hydrogels to a new level, adds Jianlease changes both the water droplet’s could provide a new window into medjun Cheng of the University of Illinois volume and the shape of the meniscus at icine and disease. at Urbana–Champaign. the oil–water interface. Experts note that the researchers will Jiang and colleagues are now workFor the temperature-sensitive lenses, need to work out details such as the roing to make smaller lenses. Their protoJiang and colleagues used a hydrogel bustness of the system, the optics of the type liquid lenses of ~1-mm-diam have based on N-isopropylacrylamide; for lenses, and their tolerance to impurities. relatively slow response times (tens of the pH-sensitive lenses, hydrogels were Cheng points out that a lot of steps are based on acrylic acid or 2-(dimethylami- seconds) based on diffusion in and out still needed before they can create a useof the hydrogel. Jiang anticipates that no)ethyl methacrylate. The investigators ful device for biological samples. It’s showed that a lens made with the acrylic they could make lenses as small as tens true that it takes effort to develop any of micrometers, and this should move acid hydrogel could focus on a ball and technology, but Zauscher adds that “it’s the response times to between subseca needle tip spaced 1.4 cm apart. The cool and it works.” a ond and tens of milliseconds—fast lens focused clearly on the needle at pH —Sarah Webb enough for biological applications. 4.0 and on the ball at pH 10.0. 7378
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© 2006 AMERICAN CHEMICAL SOCIETY
