contact lens design and materials
How Nature is Influencing New Lens Materials and Designs
BY RONALD K. WATANABE, OD, FAAO
In many ways, contact lenses have been bio-inspired from the beginning. They were designed to closely match the natural contour of the cornea or to simulate the normal eye in people who have corneal irregularities. Soft contact lenses were designed to act as “membranes” that drape over the corneal surface, in effect acting as the ocular surface. Most efforts in contact lens design and material technology have aimed at making lenses more compatible with the ocular surface and tear film and providing good vision and comfort. Research and development has led to our current array of highly oxygen permeable, wettable and optically clear lenses that are successful for most of our patients.
Yet truly bio-inspired contact lens designs and materials have been few and far between. One example is the incorporation of phosphorylcholine (PC) into hydrogel materials. PC is a zwitterionic phospholipid found on the outer surface of red blood cell membranes. Its unique surface properties led to its use in stents for coronary artery disease. In this application, it improves surface biocompatibility and lowers the risk of inflammation and thrombosis. In contact lenses, the PC molecule attracts and surrounds itself with water, which helps the lens maintain hydration and resist surface deposition.
An example of bio-inspired contact lenses on the horizon is a hydrogel lens containing hyaluronic acid (HA). Though wetting agents such as polyvinyl alcohol and polyvinylpyrrolidone are incorporated into hydrogel matrices, they are synthetic polymers not readily found in nature. HA is an anionic, non-sulfated glycosaminoglycan distributed widely throughout the extracellular matrices of connective, epithelial and neural tissues. It's found in skin, cartilage and synovial fluid. It helps retain water, increase viscosity and provide lubrication. In contact lenses, it can improve surface wettability, increase lens hydration and improve resistance to surface deposits (vanBeek M et al, 2008; vanBeek, Jones, Shear-down, 2008).
In Australia, antimicrobial contact lenses are being developed. Furanones, compounds released by a certain red sea kelp, interfere with bacteria's communication signaling mechanisms. This disrupts the bacteria's ability to form colonies and enables the kelp to stay free of Pseudomonas aeruginosa biofilms.
This mechanism is being investigated as a novel method to eliminate microbial colonization of contact lens surfaces (Zhu et al, 2008). By incorporating furanones in the lens surface, the researchers hope to reduce the risk of microbial keratitis and other complications of contact lens wear.
Swedish researchers are examining animal eyes for inspiration to improve optical designs in cameras and possibly multifocal contact lenses. They investigated the multifocal eyes of nocturnal helmet geckos and found them to have a series of concentric zones of varying focal lengths that allow them to focus light of different wavelengths simultaneously on the retina. This gives them good color vision at night and allows them to focus on objects at different distances simultaneously (Roth et al, 2009), which may provide clues to help develop better multifocal contact lenses.
These are interesting and exciting examples of bio-inspiration in contact lens surface technology, material composition and optical design. Perhaps we'll also see new types of lens packaging and lens cases inspired by nature's wonders. CLS
For references, please visit www.clspectrum.com/references.asp and click on document SE2010.
Dr. Watanabe is an associate professor of optometry at the New England College of Optometry. He's a Diplomate in the American Academy of Optometry's Section on Cornea and Contact Lenses and Refractive Technologies and is in private practice in Andover, Mass.