Analysis of Surface Mechanical Properties of Unworn and Worn Silicone Hydrogel Contact Lenses Using Nanoindentation with AFM

8have observed a loss of keratocytes in the corneal stroma with the use of Si-Hi contact lenses. The authors hypothesize that the mechanical stimulation of the corneal surface by the presence of the CL is able to release inflammatory mediators able to induce keratocyte apoptosis. The potential implication of mechanical stimulation of the corneal surface, due to the physical presence of a CL, in the release of inflammatory mediators as the likely cause of reduced keratocyte density or keratocyte redistribution associated with lens wear is being investigated. 8-10 On a similar line of thought but probably with different significance, Ladage et al. 11 have reported the proliferation of keratocytes under the corneal surface affected by the presence of post-lental debris in the form of mucin balls in rabbit corneas. This mechanism could also be the reflection of the mechanical impact of the mucin balls compressed under the contact lens and could be interpreted as a form of corneal response to localized mechanical interaction between the CL and the external epithelium. Different methods have been used to evaluate the mechanical properties of contact lenses as will be later discussed. However, little has been reported on the use of atomic force microscopy (AFM) with this regard, and to the knowledge of the authors no reports on the potential changes in the mechanical properties of contact CL as measured with other techniques are available. We hypothesize that changes in the lens surface due to dehydration phenomena, deposits, and other kinds of spoliation could induce significant changes in the mechanical behavior of the CL surface. The AFM with its high resolution capability could be able to measure such changes at the nanoscale level. The goal of this study was to evaluate the mechanical properties at the surface of several silicone hydrogel contact lenses before and after being worn on a daily wear schedule. These properties had been analyzed by nanoindentation with AFM and should not be interpreted as the classical mechanical properties reported by the industry concerning to the properties of the bulk of the lens usually obtained with macroscopic indenters or with instruments that induce a deformation on the whole sample, measuring the force needed to induce such a deformation, at a macroscopic scale.