Contact Lens Materials
What would we ask of a material to make a contact lens? The material would fit a variety of corneal shapes, be durable, easy to handle, comfortable, and transparent. Its surface would be “wettable” and stay wet between blinks, but we do not want deposits to stick to it, or for pseudomonas to find it a congenial home. We would want the lens to transmit oxygen and carbon dioxide and flush new tears under it with each blink, to maintain corneal metabolism. We would not want the lens to contribute to dry eye problems by soaking up water from the cornea.
Dk, Dk/t, and wetting angle are some terms used to describe oxygen permeability and wettability: Dk refers to the oxygen permeability of a lens material, where D is the diffusion coefficient for oxygen movement in the material, and k is the solubility constant of oxygen in the material. Dk/t refers to the oxygen transmissibility of a lens, depending on its material and central thickness (t). Wetting angle refers to the wettability of a lens surface (Fig 5-3). A low wetting angle means water will spread over the surface, increasing surface wettability. Both a high Dk/t and a low wetting angle are desirable.
“Hard” contact lenses were introduced in the 1940s and were made of polymethylmethacrylate (PMMA), the same plastic used subsequently for the first intraocular lenses. These lenses were very durable, but oxygen passed only around them, not through them.
Soft contact lenses are made of a hydrogel polymer, hydroxyethylmethacrylate or, more often now, a silicone hydrogel. Hydrogels have more oxygen permeability when they have higher water content, but the higher-water-content lenses tend to cause dryness of the cornea if they are made thin, and they may form deposits and require frequent replacement. Modifying the hydrogels yielded “silicone hydrogels,” used for daily and extended wear. These lenses achieve their oxygen permeability with less water content, using pores induced by the presence of silicon atoms, rather than high water content. Some patients are more comfortable with the hydrogels than the silicone hydrogels. Available lenses differ in chemistry throughout the lens or at its surfaces with differing wettability, flexibility, clarity, resistance to deposit retention, and suitability for those with dry eye. They also differ in their longevity and should be replaced at recommended intervals.
RGP lenses were developed in the latter 20th century, using the architecture of large-polymer plastics to allow passage of oxygen. RGP lenses allow oxygen to pass through and allow more tear exchange around the lens with each blink than soft lenses allow, so that oxygen-depleted tears are not trapped between the lens and the cornea. The addition of fluorine to the silicone/acrylate material increases oxygen permeability and encourages the coating of the lens with mucin, which improves wettability. There are various chemistries, with differing advantages.
Figure 5-3 The wettability of a lens surface determines whether a wetting angle will be low (greater wettability, greater comfort) or high (less wettability, less comfort).
(Modified with permission from Stein HA, Freeman MI, Stein RM. CLAO Residents Contact Lens Curriculum Manual. New Orleans: Contact Lens Association of Ophthalmologists; 1996. Redrawn by Christine Gralapp.)
Table 5-3 Comparative Advantages of Soft and Rigid Gas-Permeable Contact Lenses
Table 5-3 summarizes the comparative advantages of soft and RGP contact lenses.
Excerpted from BCSC 2020-2021 series : Section 3 - Clinical Optics. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.