In-vitro tests can be performed to assess the parameters listed above for different lens materials, thereby allowing comparison of lenses and enabling in eye performance trends and biocompatibility to be predicted.
Contact lenses are generally prepared from either conventional or silicone hydrogel materials. Contact lens materials are designed for various wear modes, e.g., daily disposable, frequent replacement and extended (overnight) wear. Novel hydrogel materials are being developed to address the following areas; oxygen permeability, enhanced comfort and reduced production costs. For a contact lens to succeed it must have suitable physical properties. The main physical properties and measurement techniques of contact lenses are lens dehydration/rehydration (Dynamic Vapour Sorption), mechanical properties (Static – Tensile and Dynamic - Rheology), oxygen permeability and surface properties (wettability – friction)
The physical tests listed highlight the different lens characteristics; the surface properties and surface interactions of the lens are of great importance. There is growing evidence that the surface properties of lenses affect the subsequent in-eye behaviour, especially wearer comfort. Discomfort still counts for over half of contact lens discontinuation and can be directly linked to the lenses surface properties. Coefficient of friction and contact angle (dynamic and static) are the techniques best suited for understanding the lens surface characteristics.