Look smart with liquid lenses

3 August 2006

Miniature lenses smart enough to respond to their surroundings have been unveiled by US scientists this week. The deceptively simple devices, made from water droplets, look set to revolutionise medical gadgets.

Antenna brings the tiny technology into focus...

This story was published in Nature on 3 August 2006.

These tiny liquid lenses are only millimetres wide.

Image: Hongrui Jiang and Liang Dong

The adaptable mini-lenses change shape in response to their environment, working a bit like the human eye.
Our eyes can focus on different distances because tiny muscles within our eyeballs adjust the shape of our lenses. These man-made liquid lenses work in a similar way, but they're made up of droplets of water and are only millimetres wide.

The mini-lenses can sense their surroundings, just like a human eye.

Image: Joao Estevao A. de Freitas

The top row of images shows the lens focussing on a needle tip and a small ball. The bottom row shows the lens shape. When the lens is more curved, the top image appears sharper.

Image: Hongrui Jiang and Liang Dong

To make the lenses, scientists squeeze water droplets out through a circular hole - or aperture - into a film of oil. A bulge forms where the water meets the oil and this is where the lens takes shape. The amount this bulge curves determines the lens's light-focusing abilities.
How do they work?
Underneath the aperture, a ring of special jelly-like material called hydrogel surrounds the water droplet, acting a bit like an artificial muscle. It controls how much water can bulge through the hole - and so how much the lens curves.
When the hydrogel expands it squeezes the lens out of the aperture, increasing the amount it curves. When the hydrogel contracts it flattens the lens. As the lens changes shape its light-focusing abilities change too.

These two lenses are changing shape in opposite directions. From top to bottom, in the lens on the left the hydrogel ring is expanding, increasing the amount the lens curves. On the right, the hydrogel ring is contracting which flattens the lens.

Image: Hongrui Jiang and Liang Dong

Hongrui Jiang, electrical engineer, University of Wisconsin-Madison

Image: University of Wisconsin-Madison

'Hydrogels are part of a family of magical materials which can be tuned to respond to changes in their environment - for example temperature, light, electric field and pH. This means that you can design a smart lens to respond to any of these factors.'
Hongrui Jiang, electrical engineer, University of Wisconsin-Madison

This tiny lens is only 2 millimetres across. From top to bottom, the images show the lens changing shape at 23, 30, 37, and 47 degrees C.

Image: Hongrui Jiang and Liang Dong

Hongrui and his team have made lenses which change shape in response to temperature and pH. The lens in this image can change from convex to concave in seconds, when the temperature rises by 20 degrees C. This change in lens shape is due to the hydrogel ring contracting as the temperature rises.
What will these tiny lenses be used for?
The lenses are cheap and easy to make, they can be made in different shapes, and because they are made of water, they're harmless too. These advantages mean the lenses hold huge promise for use in medical gadgets of the future.
'We imagine that the lenses could be put onto endoscopes to look into our bodies less invasively. The lenses, because they are so small, could be wrapped around the tip of an endoscope to allow doctors to look at cells in the body from all directions.'
David Beebe, biomedical engineer, University of Wisconsin-Madison

David Beebe, biomedical engineer, University of Wisconsin-Madison

Image: University of Wisconsin-Madison

The team hope that their pioneering technique is the first step towards making a whole network of lenses, each of which could respond to different changes in its environment. This means you could have a system of lenses simultaneously sensing many different environmental factors.
This type of network would behave a bit like an insect's eye, which contains thousands of tiny sensors. The image the insect 'sees' is made up from a combination of the inputs from each sensor.

This dragonfly's eye is made up of rows and rows of tiny sensors for detecting light.

Image: David L. Green

Hongrui and his team hope to improve their lenses by speeding up the time it takes for them to change shape, and by finding other environmental factors for them to respond to.
In the future, they hope the lenses will be able to monitor all sorts of things quickly and simply, from blood samples to miniature chemical reactions.
.