Berkeley Lab Highlights
Health: For a Soft and Lingering Touch

Biology has inspired Bertozzi to develop two new materials for soft contact lenses that mimic the strategies living cells use to communicate with their environment. Both start with a material known as HEMA (2-hydroxyethyl methacrylate), used in many ordinary contact lenses.

HEMA monomers link to form a tough, flexible, transparent polymer; with numerous hydroxyl groups (consisting of one hydrogen and one oxygen atom), polyHEMA is water-like to begin with, and it can absorb a remarkable amount of water to form what�s called a hydrogel.

Most standard soft contacts are more than half water. A high proportion of water facilitates oxygen exchange with the eye; lack of oxygen is a principal cause of corneal damage.

"PolyHEMA is a pretty good contact-lens material by itself, but we thought we could improve it by incorporating the kinds of sugars found on the surfaces of living cells," says Bertozzi.

In one approach, she and her colleagues set out to create a hydrogel with bulk properties similar to natural mucin. This would not only lubricate contact with the cornea but repel proteins as well, preventing the "biofouling" that can cause allergic reactions and bacterial infections in contact-lens wearers.

"We made bulk hydrogels of polyHEMA and carbohydrate acrylamides, synthetic sugar analogues that are made from natural sugars themselves. We hoped these sugars would entangle themselves the way the sugars in mucin do," Bertozzi says, "but in order to work, they had to be displayed on the surface when the material was in the hydrated state"—that is, when it was wet.

The surface properties of most polymers change according to the medium in which they are immersed, as the molecules at the interface rearrange themselves to reduce free energy. Bertozzi and her colleagues used a special spectroscopic technique to look at the surfaces of the new hydrogel in air and under water.

Spectroscopy showed that when the gel was dry, the surface was dominated by water-repelling methyl groups (consisting of a carbon and three hydrogen atoms), not water-loving sugars. But when the gel was hydrated, many of the carbohydrates, arranged as side groups on the polymer chains, rotated and emerged on the surface.

Indeed, lenses with only 20 percent carbohydrate can absorb so much water that the resulting hydrogel is 70 percent water by weight. And the water is bound tightly, requiring temperatures of over 100 degrees Fahrenheit to drive it out. Says Bertozzi, "This will help with the dry-eye problem."

For a different approach to creating a lens surface that would resist binding to proteins while absorbing water, Bertozzi and her colleagues made use of published reports that a monolayer of sulfoxide on a gold substrate repels proteins.

"We took the logical next step and incorporated the sulfoxides into hydrogel polymers," Bertozzi says. Instead of using a carbohydrate, her team synthesized a copolymer of HEMA and sulfoxide acrylate.

The sulfoxides were displayed at the surface of the gel, and the new material took up water dramatically. The optimum water content for low protein binding is between 40 and 70 percent by weight; the new copolymer could easily achieve 90 percent, depending on the proportion of sulfoxide.

Besides high water content and very low protein binding, the sulfoxide copolymer has another important advantage, Bertozzi says: "It�s cheap."

High water content, good oxygen transfer, low protein binding, and low cost add up to attractive commercial potential. Under a cooperative research and development agreement (CRADA) between Berkeley Lab and a manufacturer of specialty soft contact lenses in Albuquerque, New Mexico, the Sunsoft Corporation, the new sulfoxide copolymer lenses will soon be tested.

Bertozzi sees lots of other uses for artificial materials that mimic the properties of biological materials, "ranging from something as simple as epidermal patches for drug delivery, which could be worn without irritating the skin, to entire artificial organs. There are environmental uses too, such as biosensors for detecting dangerous chemical and biological agents in air or water, and industrial uses such as bioreactors to produce tailored chemical products in bulk."

Safe, inexpensive materials for comfortable, continuous-wear contact lenses are a small step toward a healthier, safer world made possible by biology-inspired synthetic organic chemistry, a field in which Carolyn Bertozzi excels.�— Paul Preuss

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