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K.A. Copley, Y. Zhang, C.J. Radke; Wettability of SCLs Assessed in a Model Blink–cycle Cell . Invest. Ophthalmol. Vis. Sci. 2006;47(13):2407.
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© ARVO (1962-2015); The Authors (2016-present)
Ideally, soft contact lenses (SCLs) should be highly wettable and immune to oily and proteinaceous tear–film deposits during wear. Therefore, it is important to assess SCL wettability performance when the lens is in contact with human tear film especially the lipid layer. For the first time, we devise and evaluate an in–vitro blink–cycle cell that mimics contact–lens exposure to the human–eye environment during blinking and that permits contact–angle assessment.
An in–vitro blink–cycle cell was designed to measure contact angles of SCLs during model blink cycles. A syringe pump flows an artificial tear solution into and out of a blink–cycle cell, thereby raising and lowering the liquid level above and below the surface of the lens. The artificial tear solution contains phosphate–buffered saline (PBS), mucin, and tear–film proteins. Importantly, a model tear lipid is spread over the air/water interface to reflect exposure to meibomian lipids. One blink cycle corresponds to lowering the lipid–covered aqueous level below the lens anterior surface followed by raising the liquid interface until the lens is re–coated with tear–film all at 36°C. The captive–bubble technique then gauges the advancing contact angles before and after exposure of the SCL to the blink cycles.
The frequency of the liquid–level (i.e., blink) cycles was varied from 0.2 s–1 to 0.2 min–1. Advancing contact angles become independent of frequency for cycle times below about 30 s. Advancing contact angles without exposure to the lipid layer are found equivalent to those with no cycling. However, the presence of a lipid layer during cycling significantly alters the measured contact angles. Contact angles of Acuvue® 2 lenses with lipid–present in the blink–cycle cell (74 ± 6°) are in excellent agreement with corresponding ex–vivo advancing contact angles measured after 1 week of daily wear (8 subjects). No agreement is found between ex–vivo and in–vitro blink–cycle contact angles when the lipid layer is not present.
The new blink–cycle cell provides a useful mimic of wettability performance of SCLs during on–eye blinking. We find that advancing contact angles of SCLs after immersion in the blink–cycle cell with a lipid layer present match those of the ex–vivo measured contact angles obtained after daily wear. Accordingly, tear lipids, along with tear mucin and protein, are essential to in vivo SCL wettabilty.
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