BioEd Online

Mini-lenses made out of water droplets have been made to bulge and contract in response to temperature and pH changes in their environment. The clever trick makes for a tiny sensor whose changing focal length could one day be used to quickly and simply monitor all sorts of things, from blood samples to miniature chemical reactors.

Hongrui Jiang from the University of Wisconsin in Madison and colleagues, who publish their findings in this week's Nature¹, built their miniature lenses using millimetre-wide rings of hydrogel, a material that expands and contracts when exposed to different temperatures or acidities.

A drop of water within the ring then bulges or contracts in response, changing the focus of the lens. The whole thing is sandwiched within a glass case, with tiny channels allowing any fluid outside to leak in and affect the hydrogel ring.

Adjustable lenses have been designed before, but mainly for optical use, in things such as spectacles, so that users can switch from near to far focus (see ' Better bifocals on the horizon '). These lenses, in contrast, act as tiny sensors of the world around them.

Taking the temperature

So far the achievement is just a proof of principle. The lenses only react to fairly major swings of about 10-20 ºC or a change in pH from acidic to alkaline. And there are already well-established ways to measure temperature and pH, without the need for a novel mini-lens system.

But the team plans to develop hydrogels that react to other conditions to different proteins or salts from the body, for example.

The lenses could then be scaled down to the micro-metre range and set into thousand-strong arrays to instantly and easily monitor a slew of substances in any given sample. These lab-on-a-chip applications "should come pretty soon," Jiang says.

But it should also be possible to produce larger versions, where the focusing is visible to the naked eye. A scaled-up lens might change its focus when detecting a specific protein in a blood sample; the sample might test positive, for example, if some text placed beneath the lens in the blood comes into focus.

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