Sunglasses that darken automatically in response to bright sunlight have been available for eyeglasses for 40 years. But adapting this flexibility to contact lenses has proven challenging. Now researchers in Singapore have developed UV-responsive, or photochromic, lenses that darken when exposed to ultraviolet light, protecting the eyes against the sun's damaging rays, and return to normal in UV's absence.

The key is a novel polymer laced with an intricate network of nano-sized tunnels that can be filled with dyes. Initial studies have shown that the technology performs faster than the light-sensitive sunglasses on the market today, says Jackie Ying, director of the Institute for Bioengineering and Nanotechnology (IBN) in Singapore, and developer of the lenses. The research is part of a broader effort at IBN to develop new materials for contact lenses that can dispense drugs and diagnose diseases.

Conventional light-responsive sunglasses are coated with millions of molecules of photochromic dyes, which are transparent when out of the sun. These molecules change shape when UV light hits, enabling them to absorb UV light and triggering the darkening of the lens. When UV light disappears, the molecules change back to their original shape and transparent appearance.

Few previous attempts have been made to design light-responsive contact lenses, largely because it's difficult to apply dye coatings uniformly to the delicate, soft surface of a contact lens. Ying and her colleagues got around this by developing a contact lens that embeds dyes uniformly throughout the material. This approach allowed them to pack more dye molecules into the material, Ying says, giving the contact lens greater sensitivity to light and thus a faster response.

Researchers created the spongy nanostructure material by mixing specific combinations of water, an oil solution with monomers commonly used in contact lenses, and a novel surfactant--a compound that encourages mixing between water and oil solutions. The resulting material is studded with tiny pores and tunnels, which can be loaded with agents such as UV-sensitive dyes.

The lens material's porous structure provides a flexible environment for dyes to transform from dark to light and back again, says Edwin Chow, team leader and senior research scientist at IBN. "If the polymer is too rigid, the dye is stuck and can't transform," says Chow. "This pore structure and polymer happens to give the best environment for dyes to react quickly."