Studies show that a significant percentage of contact lens users decide to discontinue the use due to ocular symptoms such as dryness and discomfort 1. There are number of factors affecting the usability of the contact lens. Contact lens should support a continuous tear film, i.e. it should not dehydrate, for optimum visual clarity. It should also resist the sorption of tear components such as lipids, proteins, and mucins as deposition can lead to decreased visual clarity and discomfort. Wettability of the surface affects both the tear film as well as biocompatibility of the lens. In addition to the good wettability, contact lenses should be highly permeable to oxygen to support continuous wear, for example, overnight 2.
The first soft contact lenses were commercialized in 1970 and made of pHEMA. They were easily adopted by the users due to their relative comfort compared to previously used PMMA lenses. The main problem with the pHEMA-based materials is their inadequate oxygen transmissibility which prevents an overnight use of the lenses. This led to the development of silicone hydrogel contact lenses that incorporated silicon as siloxane polymers into the lens material to improve oxygen permeability. However, the use of siloxane also led to a hydrophobic surface which is poorly wetted.
Modifications for improved wettability
Several methods have been used to improve the wettability, including surface treatments such as plasma, coatings, and incorporation of wetting agents. Plasma is a highly reactive gas that interacts with the surface groups on the contact lens. When oxygen is present, it oxidizes the siloxane groups to silicate which makes the surface more hydrophilic. Plasma treatment offers a simple and well-established method, but the effect decreases over time. Coatings offer a more stable solution and can also possess additional benefits such as anti-microbial properties. Incorporation of high molecular weight wetting agents into the polymer matrix of the lens is also one approach to overcome the wettability issues.
To study the effect of different modifications on the wettability of the contact lens, contact angle measurements are commonly used. To read more about wettability and how it can be measured, please download the overview below.
1 Richdale, K., Sinnott, L.T., Skadahl, E., and Nichols, J.J., “Frequency of and factors associated with contact lens dissatisfaction and discontinuation”, Cornea 26 (2007) 168. 2 Menzies, K.L., and Jones, L., “The impact of contact angle on the biocompatibility of biomaterials”, Optometry and vision science 87 (2010) 387.
In industry, wettability is important for example when manufacturing electronics or when planning how to extract oil from the oil reservoir. In all these applications, knowing the contact angle value gives a strong indication on the performance of the product or a successfulness of the process.
Contact angle is easy to understand visually. It is the angle a droplet forms in contact with a solid surface. Thermodynamically, it is a balance between different forces acting on a three-phase contact point where solid, liquid and air meet.