28 April 2006
Artificial Eye Borrows From Nature
by Kate Melville
Using dragonflies and houseflies as models, bioengineers at the University of California, Berkeley, have created a series of artificial compound eyes. The key benefit provided by a compound-type eye is the wide field of vision it makes possible. Writing in Science, the researchers say the eyes could be used in surveillance, motion detection, environmental sensing and image-guided surgery.
The researchers, led by Luke Lee, used some novel techniques to create the new eyes. Lee explained that the eyes are the first hemispherical, three-dimensional optical systems to integrate microlens arrays - thousands of tiny lenses packed side by side - with self-aligned, self-written waveguides. These waveguides are the light-conducting channels that themselves have been created by beams of light. The lens units on the surface of the eye are packed together in the same hexagonal, honeycomb pattern as in an insect's compound eye.
While an insect's ommatidia (individual sensory unit) end in a photoreceptor cell that transmits a light signal to the creature's optic nerve, Lee plans to couple his ommatidia with CCD photodiodes, the light-capturing units used in digital cameras. He indicated they could also be linked to spectroscopes for chemical detection and analysis.
To create the artificial eye, the team made a mold by first creating a flat array of tiny, domed lenses arranged in the hexagonal honeycomb pattern. On top of this, they applied a thin slab of an elastic polymer called polydimethylsiloxane (PDMS), creating a concave pattern of the lenses in the polymer. By affixing the PDMS membrane over the opening of a vacuum chamber and applying negative air pressure, they pulled it into the dome shapes they needed, controlling its form by using different pressures. They then had a hemisphere-shaped cup pocked with some 8,700 indentations: a compound-eye mold that could be used over and over again using soft lithography technology.
To make the eye itself, they used an epoxy resin that cures into a hardened form when exposed to ultraviolet light. The mold with the resin was baked at a low temperature just long enough to slightly harden the material. But before it was completely set, the resin was turned out so the waveguides could be created in the still-soft resin. When struck by a beam of light, each of the eye's elements acts as a lens, focusing the light and sending it into the material below. Like a welder's torch burning a hole into metal, over time the focused light beams etched holes in the resin creating the tiny channels that the researchers call self-written waveguides. "The lenses and waveguides are the most important part of the system," Lee said. "People have said that it would be totally impossible to create them with an angle, but now that we've done it, we're ready to integrate imaging or chemical sensing into the eyes."
The end result is that the waveguides pierce the resin at angles that head toward the center of the dome, just like the converging ommatidia of an insect eye. Because the microlenses create the waveguides, each microlens is perfectly aligned with its waveguide. The self-alignment, self-writing processes are crucial to the creation of the artificial compound eye, said Lee, because these processes will also align the microlenses and waveguides with the pixels of CCDs and spectroscopes. "Who knows? Maybe this is how insect eyes are created, too," said Lee. "First, there are the lenses, and then as light keeps coming in, they make their own optical paths and connect with the visual system."
Source: University of California - Berkeley
Pic courtesy Science/Luke Lee