Mechanical characterization of contact lenses by microindentation: Constant velocity and relaxation testing

Non-destructive methods for testing material properties allow for multiple tests to be performed on the same sample, which will speed up the design and testing process for hydrogel contact lenses. The mechanical properties of contact lenses were investigated by microindentation testing. Indenter force responses were recorded for two modes of testing: constant velocity and relaxation indentation. From these tests, we characterized the biphasic properties of a hydrogel contact lens: Young’s modulus of the solid matrix and hydraulic permeability. Measured indenter force response was fit to finite element (FE) simulation results over a range of Young’s modulus (E) and hydraulic permeability (k) over a short testing time scale (2 s). Estimated hydraulic permeability, 1–5 × 10−15 m4 (N s)−1, was similar to previously measured values for Etafilcon A. However, values determined for Young’s modulus, 50–60 kPa, were lower than previously measured.