Sharp view with artificial eye
They are not prepared for tomorrow's technology: today's lens systems are large, rigid and vulnerable. Nature shows how it can be done better - with flexible lenses whose focal length can be changed by muscles. A new type of artificial lens is now following the same path. For use in optical microsystems of the future.
"Look me in the eyes, little one!" The invitation puts one of the most impressive marvels of evolutionary tinkering before our own electromagnetic radiation receptors. If, for fun, we compare the most modern high-tech versions of video systems with the blue, brown, green or gray specimens of natural origin, then several clear differences immediately catch the eye: biological eyes are not only much more beautiful, but also a lot smaller, lighter and more flexible.
We owe the advantage in part to the lens of the eye, which, depending on the tension of the attached muscles, breaks the incident light to a greater or lesser extent, so that a sharp image falls on the retina. Researchers would also like to be able to do something like this with their artificial systems. According to the traditional method, these use several rigid lenses that are shifted against each other to focus the image. Too many components, too many vulnerable mechanics and far too big overall. At a time when phones take pictures and computers record their users picking their noses, such clunky technology might just about do. But the future is known to be nano or at least micro for the time being. And so she urgently needs new lenses.
The salvation of the optical future may lie in its very origins. Because the light-refracting power of water droplets heralded the age of microscopy. Since water is a little "tougher" than air for light, the direction of a light beam changes when it transitions between the two media. What only causes a kink in the optics on a flat surface of water causes originally parallel rays to focus when it drips. Used skilfully, this primitive lens as a waterdrop magnifying glass magnifies fingerprints, beetle legs, and blood cells with amazing quality. Even bacteria should be recognizable under extremely small drops of water.
The water drop magnifier is currently experiencing a renaissance in a new guise at the University of Wisconsin in Madison. There, a team of engineers led by Hongrui Jiang is developing a controllable adaptive liquid microlens that essentially works according to the old principle: light falls on a curved water surface and is refracted in the process. But with a few simple tricks, it becomes an optical system that in many respects comes very close to the eye as a model.
Trick number one is to fix the drop in place and make sure the water doesn't evaporate. For this purpose, the artificial lens is placed in a micro chamber between two panes of glass. The lower part of this chamber contains water, while oil floats in the upper part. Both liquids are separated by an orifice with a hole. It is precisely at this hole that water and oil meet, and this is where the lens drop is formed. Trick number two is the controllable deformation of the water drop. This task is performed by a ring of hydrogel that encloses the water below the aperture.
As a material, you can choose from various substances that change their volume in response to environmental stimuli. For example, N-isopropylacrylamide expands at temperatures below 32 degrees Celsius. A gel ring made of this material consequently compresses the water inside the ring in a cool environment. There is only one way to avoid it: through the aperture hole - a beautifully rounded mound of water is created. As it gets warmer, the gel ring contracts again, the water uses this new space, and the lens flattens out or even acquires a negative curvature.
In addition to temperature-sensitive gels, the researchers tested a version that reacts to acidity in their experiments. However, substances that respond to light or electric fields are also conceivable. The only important thing is that they can repeatedly expand and shrink and thus increase and decrease the curvature of the duckweed via the pressure within the ring. Like the natural lens in the eye, this then breaks the light and focuses on objects at different distances. A process that still took ten or more seconds in the current attempts.
The new duckweeds are by no means the first liquid lenses whose focal lengths can be manipulated at will. But in contrast to their competition, they get by with very simple mechanisms that can also be linked to existing technologies. Even ordinary electrodes would suffice to bring the lenses into focus. And then we would get pictures from unexpectedly thin endoscopes or from mini-laboratories on biochemical chips, the results of which the naked eye could no longer read. In such a future, our eyes might not be any better in comparison - but they will definitely remain more beautiful.
