Hydrogel Contact Lenses

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.

Materials Studies

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)

Modified nano-tribometer
Schematic of the modified nano-tribometer
Schematic of Wilhelmy balance
Schematic of Wilhelmy balance

Surface properties

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.

• Coefficient of Friction – Measured using a modified nano-tribometer 

• Contact Angle (Static) –The sessile drop technique is used to measure the contact lens wettability. 

• Contact Angle (Dynamic) – Measured using a modified Wilhelmy plate technique. This simulates the blinking cycle and measures the advancing and receding angles of the lens material.