Glass-transition temperatures for soft-contact-lens materials. Dependence on water content

Glass-transition temperatures for three soft-contact-lens (SCL) materials are measured by modulated differential scanning calorimetry as a function of SCL hydration, as determined by thermogravimetric analysis. The SCL materials are: a conventional hydrogel (SofLens® 38, polymacon) with a low water content at saturation; a conventional hydrogel (SofLens™ One Day, hilafilcon A) with a high water content at saturation; and a siloxane–hydrogel (PureVision™, balafilcon A). con, hilafilcon A, and balafilcon A turn glassy at 35 °C when their water contents drop below 10.4, 13.5, and 6 wt%, respectively. These water contents correspond to the equilibrium water uptake at 35 °C for polymacon, hilafilcon A, and balafilcon A at relative humidities, RH, of 74, 64, and 57%, respectively. Our results suggest that the outer surface of a soft contact lens worn on the eye may develop a glassy skin when exposed to air at low relative humidity. This glassy skin may alter fluid transport through the soft contact lens, and influence SCL-wear comfort.