Effect of Equilibrium Bulk Water Content on the Humidity-Dependent Surface Mechanical Properties of Hydrophilic Contact Lenses Studied by Atomic Force Microscopy

The surface mechanical properties of neutral poly(HEMA) (HEMA, hydroxyethyl methacrylate) and ionic poly(HEMA+MA) (MA, methacrylic acid) soft contact lenses were compared using the atomic force microscope (AFM). Surface stiffness, adhesion, and viscoelastic effects were extracted from AFM load−displacement plots collected under a variable probing rate in order to gauge interfacial water content as a function of humidity. Below 60% relative humidity, the surfaces of both lenses are water-depleted, relative to the bulk, and behave as glassy (elastic) polymers. As the relative humidity exceeds ∼60% and surface evaporation decreases, the surfaces of both contact lenses soften and the viscoelastic relaxation time decreases. The onset of viscoelastic behavior at the poly(HEMA+MA) surface is delayed, relative to that of poly(HEMA). This suggests that poly(HEMA) has greater interfacial water content than poly(HEMA+MA), although it possesses a lower equilibrium water content in the bulk. The AFM data also show poly(HEMA) possesses greater work of adhesion at a given humidity, relative to that of poly(HEMA+MA); these affects are related to water-mediated surface compliance.