Testing Hydrogel Surface Properties of Contact Lenses

Various types of contact lenses are used by many people every day.

Figure 1. Various types of contact lenses are used by many people every day.

Hydrogels are the material of choice for the manufacture of contact lens. They are a cross-linked hydrophilic polymer network filled with liquid that exhibits the behavior of both liquid and solid under applied stress. Hydrogels recover upon release of the mechanical stress as they support diffusion of the liquid through the pores in the solid meshwork [1]. Typically, 30-70% of water is present in the polymeric matrix [1,2].

This article discusses the application of Anton Paar Nano Tribometer to perform a tribology test procedure for measurement of the coefficient of friction (CoF) of contact lenses. It also provides indications on the test conditions and their determination. It also illustrates a typical example with the CoF results and briefly discusses their evolution with the applied load. The test conditions, such as sliding speed and pressure, should be chosen if possible close to the in vivo conditions.

Blinking is typically considered to be the primary force and motion contribution for contact lens tribology research. The contact pressure exerted by the eyelid during the blinking was found to be 3.5 to 4.0 kPa and the estimated blinking average speed was 12 cm/s [3-5].

Earlier studies demonstrated that hydrogels have an elastic modulus ranging between 30 and 120 kPa, based on the water content and composition of the polymer matrices [6]. The contact pressure can be calculated and the frictional contact can be simulated via appropriate experiments when these properties are known.

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This information has been sourced, reviewed and adapted from materials provided by Anton Paar GmbH.

For more information on this source, please visit Anton Paar GmbH.

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