Instability and Breakup of Model Tear Films

PURPOSE. An experimental platform to replicate the human tear film on a contact lens is presented. The influence of interfacial viscoelasticity in stabilizing in vitro model tear films against breakup and dewetting is investigated using this instrument. METHODS. Model tear films consisting of bovine meibomian lipids (meibum) spread on either PBS or artificial tear solution (ATS) are created. The interfacial shear rheology of these films is measured as a function of temperature. The dewetting dynamics of these films is then investigated using the Interfacial Dewetting and Drainage Optical Platform (i-DDrOP) on top of silicone hydrogel (SiHy) contact lenses at 23 and 35 degrees C. The film breakup times are evaluated using two parameters: onset of film breakup, T-onset for thick films (similar to 100 mu m), and tear breakup times, T-BU for thin films (similar to 1 mu m). Thin film thinning rates as a result of evaporation are also calculated. RESULTS. The ATS/meibum films have the largest surface rheology and correspondingly show the largest T-onset times at both 23 and 35 degrees C. The parameter T-BU is also significantly larger for ATS/meibum (T-BU similar to 40 seconds) compared with that of ATS and PBS/meibum films (T-BU similar to 30 seconds) at room temperature. However, at 35 degrees C, all three model tear films exhibit similar T-BU similar to 17 seconds and average rate of thinning of similar to 4 lm/minute. CONCLUSIONS. Tear film stability is influenced by both surface rheology and evaporation. The in vitro tear breakup times and thinning rates of model tear films at 35 degrees C are in good agreement with in vivo measurements previously reported, highlighting the utility of the i-DDrOP for in vitro tear film breakup research.