Friction Measurements on Contact Lenses in a Physiologically Relevant Environment: Effect of Testing Conditions on Friction

PURPOSE. To characterize the effect of lubricant composition and in vitro ageing on the coefficient of friction (CoF) of a wide range of commercially available soft contact lenses (SCLs). METHODS. The CoF of SCLs was characterized by means of microtribometry against a mucin-coated glass disk. One reusable (RU) silicone-hydrogel (SiHy) lens, senofilcon A, and two daily disposable (DD) lenses, etafilcon A (hydrogel) and nelfilcon A (hydrogel), were tested under different lubricant solutions, including a tear-like fluid (TLF) containing proteins and lipids. Five RU (balafilcon A [SiHy], comfilcon A [SiHy], etafilcon A [hydrogel], lotrafilcon B [SiHy], senofilcon A [SiHy]) and five DD (delefilcon A [SiHy], etafilcon A [hydrogel; two lens types], narafilcon A [SiHy], nelfilcon A [hydrogel]) lenses were tested before and after exposure to an in vitro ageing process, consisting of continuous immersion and withdrawal from TLF for 18 hours. The CoF in TLF was further compared to previously published data collected in a different lubricant. RESULTS. After in vitro ageing, three RU (balafilcon A, etafilcon A, comfilcon A) and three DD (delefilcon A, etafilcon A, nelfilcon A) lenses displayed a significant increase in CoF (P < 0.05). Lenses that contained poly (vinyl pyrrolidone; PVP) showed unaltered CoF after ageing. CONCLUSIONS. An in vitro methodology to simulate in vivo wearing of contact lenses has been proposed. The results suggest that certain lens materials show increased CoF after ageing, with potential clinical implications. The results indicate that the presence of a persistent wetting agent is of advantage to maintain a low CoF after prolonged wearing.