Measurement of mechanical properties of soft contact lenses using instrumented indentation

The mechanical properties of contact lenses are important for their proper application and wear comfort. Several attempts were made to characterize the elastic modulus of contact lenses but there is still no widely accepted method for measurement of their mechanical properties. This application report presents the results of an instrumented indentation study using the Anton Paar Bioindenter for characterization of soft contact lenses. The measurement methodology and analysis of the results is demonstrated on two types of contact lenses. In addition, the influence of loading rate on the calculated elastic modulus is shown.

Soft contact lenses are used by many people in everyday life because of their simple application and low cost. They are usually made of hydrogels or a mixture of hydrogel (such as pHEMA Poly(2- hydroxyethyl methacrylate)) and a copolymer. Soft contact lenses contain between 25 % and 75 % of water and their mechanical behavior is from a great part determined by the solid mesh and its capability to conduct fluid. When external pressure is applied, the solid mesh is compressed and the liquid is ‘pushed’ in the direction opposite to pressure gradient, resulting in fluid flow. When the mechanical stress is released, the hydrogel recovers as the solid mesh recovers elastically and the fluid is flowing back through the pores in the mesh. The mechanical behavior of contact lenses can be modeled as that of a hydrogel. The contact lens is subjected to the movement of the eyelid, which is commonly believed to be the primary source of force on the ocular surface. The contact pressure induced by the eyelid during blinking was estimated to be in the range of 1 to 7 kPa.

Several methods can be used to characterize the mechanical properties of contact lenses: compression, dynamical mechanical analysis (DMA), rheology or instrumented indentation testing (IIT). However, only
the IIT method can reveal local properties of contact lenses since the other methods require much larger surface or volume to obtain the mechanical properties. The material of contact lenses is expected to exhibit
time-dependent behavior, either viscoelastic or poroelastic. Analytical solutions for indentation of a viscoelastic half-space have been developed and can be found in current literature, but satisfactory solutions for indentation on a poroelastic half-space are still rare.

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