contact lens materials
Considering the Facts of Friction
BY JASON J. NICHOLS, OD, MPH, PHD
In my previous column on wettability, we started to discuss concepts that include lubricity and friction. We described wettability as the process of a film spreading over a surface (such as the pre-lens tear film spreading over a contact lens surface).
Factors that influence on-eye wettability include components of the tear film and polymer characteristics. For example, these factors include siloxane content, hydrophilic co-monomers and hydrophobic co-monomers.
Siloxane macromers are critical for increasing oxygen permeability, but a material based purely on silicone is uncomfortable and will bind to the eye.
The hydrophilic co-monomers N-vinyl pyrrolidone (NVP) and dimethylacrylamide (DMA) are included to promote the incorporation of water, which leads to lens movement and comfort.
Lastly, hydrophobic co-monomers methyl methacrylate and trimethylsiloxysilyl alter the mechanical strength of a material and its modulus.
Impact of Lens Surfaces
While the bulk properties of the material are important to consider relative to promoting oxygen permeability, it's critical that we consider the surface of contact lenses as well, particularly as it relates to biocompatibility and comfort. It's the surface of a contact lens that obviously interfaces with the tear film and ocular surface, and thus it's critical that the surface maintains properties that lend to biocompatibility.
Friction is the force opposing the motion of two adjacent surfaces while a lubricant is an agent that separates adjacent surfaces, resulting in a reduction in friction between the surfaces when they move. A lubricant is an agent that can provide some boundary between two surfaces.
There are two types of lubrication. In fluid film lubrication, a layer of fluid prevents the two moving surfaces from touching. In boundary lubrication, the two surfaces rub against each other in certain positions. Fluid film lubrication should eliminate the possibility of wear and damage of the surfaces, but it seems probable that blinks cause some rubbing of the palpebral surface with the underlying cornea or contact lens, and so is a case of boundary lubrication. The force between surfaces in boundary lubrication is friction; thus boundary lubricants should reduce friction between surfaces as much as possible.
One might also consider the properties of a surface itself that may be associated with reducing friction. In this regard, some important general properties of surface-associated friction might include surface smoothness, velocity of the moving surfaces, surface area, pressure between the surfaces and electrostatic attraction or repulsion between the surfaces.
A general, nonophthalmic example of a surface with a low coefficient of friction is Teflon (polytetrafluoroethylene), which has one of the lowest coefficients of friction of any solid surface. This fluoropolymer works by aligning its negatively charged fluorine atoms at the surface to help reduce electrostatic attraction between it and the alternative surface. An ocular system which works in a similar manner is the glycocalyx, with the negatively charged sugars of the ocular surface and palpebral conjunctiva adjacent to one another helping reduce friction during the blink.
Looking to the Future
Silicone hydrogel lenses have created somewhat of a recent awareness of the importance of contact lens surfaces, given their role in reducing expression of siloxane. It is interesting to consider the idea of altering a surface even further so that it more optimally interacts with the tear film and is associated with a reduction of friction, particularly during the blink.
Again, the corneal surface with its glycocalyx seems to be a unique model which has characteristics that might be desirable for a contact lens. This will be something to consider in the years to come as contact lens materials continue to evolve. CLS
Dr. Nichols is an assistant professor of optometry and vision science at The Ohio State University College of Optometry.