In vitro analysis of wettability and physical properties of blister pack solutions of hydrogel contact lenses
Menzies, Kara Laura
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Contact lens success is primarily driven by comfort of the lens in eye. Over the years, many modifications have been made to the lens surface and bulk material to improve comfort of the lens, however 50% of contact lens wearers still report dry eye symptoms while wearing their lenses. Wettability of the lens material plays a large role in lens comfort, primarily due to its influence in tear film stability. In vitro wettability of contact lenses has typically been assessed by measuring the water contact angle on the lens surface. Currently there are three techniques to measure the in vitro wettability of contact lenses, the sessile drop technique, captive bubble technique, and the Wilhelmy balance method. To date, there is much published on assessing wettability using the sessile drop and captive bubble technique, however there is no data published looking at the in vitro wettability of hydrogel contact lenses measured by the Wilhelmy balance method. Accumulation and deposition of tear components on the lens surface can also affect lens performance, by altering the wettability of the lens surface and causing lens spoilage. The majority of in vitro studies looking at deposition of tear components on the lens surface dope the lenses in tear solutions for a set period of time. None of these studies have investigated the impact of exposing the lenses to tear solutions, then exposing them to the air and then back into the tear solution, which mimics the process during blinking. In Chapter 2, an evaluation of the influence of lens preparation on the wettability of contact lenses measured by the sessile drop technique was conducted. The wettability of 6 silicone hydrogel and one conventional lens material was assessed. Lenses were blot dried on either a microfiber cloth or lens paper for different drying periods and contact angles were measured using the sessile drop technique. There were large variations in results using the microfiber cloth after all drying periods, but there was little variation in results after lenses were blot dried on lens paper for approximately 20 seconds. Thus, it was determined that for future contact angle analysis using the sessile drop technique that lenses should be blot dried for roughly 20 seconds on lens paper. This method was used consistently for the rest of the experiments in which the sessile drop technique was used to measure contact angles. The remainder of Chapter 2 compared the contact angles of different lens materials measured by the sessile drop technique and Wilhelmy balance method. The wettability of five different silicone hydrogel lens materials was assessed directly out-of-blister and after a 48 hour soak in saline. There were significant differences in contact angles for the lens materials between the two techniques. There were also significant differences in contact angles directly out-of-blister and after the 48 hour soak. Results from this study suggested that different methods of measuring wettability can produce different results and that blister pack solutions can alter the wettability of lens materials. Chapter 3 measured the physical properties of blister pack solutions of silicone hydrogel lenses. The pH, osmolality, surface tension, and viscosity of the blister solutions for 9 silicone hydrogel lenses, 2 conventional lenses, and 2 saline solutions were measured. The osmolality of the blister solutions followed a trend, in that blister solutions manufactured by the same company had the same osmolality. Products produced by Johnson & Johnson had the highest osmolality. Blister solutions that contained additional wetting agents had higher viscosities compared to blister solutions without added wetting agents. The main conclusion from this study was that adding wetting agents to blister solutions could alter the physical properties of the blister solutions. The purpose of Chapter 4 was to measure the physical properties of the blister pack solutions of daily disposable lenses and to evaluate the wettability of the lens materials and substantivity of the blister solutions, using a method in which lenses were cycled through 5 minute soaks in saline to mimic blinking. Five daily disposable lens materials were evaluated, one of which was shipped in a blister solution with added surfactants and wetting agents. The wettability of the lenses was assessed using the sessile drop technique and Wilhelmy balance method. The lens with the modified blister solution had a lower surface tension and higher viscosity compared to all the other blister solutions. The same trend in osmolalties as those reported in Chapter 3, were found with blister solutions made by the same manufacturer having the same osmolality. The wettability varied across lens materials. Overall, the lens material with the added components to the blister solution had the lowest contact angle. Chapter 5 investigated the deposition of tear components onto the surface of conventional and silicone hydrogel lens materials and looked at the impact of this on changes in wettability. Three lens materials used in Chapter 4 were exposed to a saline solution, lysozyme solution, and a complex tear solution for 5 minutes, 1 hour, 4 hours, and 8 hours. The wettability was assessed after each time point using the sessile drop and Wilhelmy balance methods. There was little to no deposition on the lens materials that had the highest in vitro CAs in Chapter 4, exemplified by no change in wettability after being soaked in the lysozyme and complex tear solutions. There was deposition on the lens materials with the lowest CAs in Chapter 4, exemplified by a significant increase in wettability after being soaked in the lysozyme and complex tear solutions. Results indicate that there is some deposition onto one lens material, as shown by the change in wettability of the lens surface. These results were further used to validate a method used in Chapter 6. The experiment conducted in Chapter 6 was similar to the experiment in Chapter 5, except that the lenses were not soaked in the three solutions but rather exposed to the solutions in a “model blink cell”. The model blink cell moves lenses in and out of solution at a set time interval, in an attempt to mimic blinking. The interval was set so the lenses would be placed for 1 second in solution and 5 seconds exposed to the air. The same lens materials used in Chapter 5 were used in for this experiment. The lenses were exposed to a saline solution, lysozyme solution and complex tear solution for 5 minutes, 1 hour, 4 hours, and 8 hours. Much like in Chapter 5, deposition on the lens materials was determined by a change in the lens wettability. There were differences in the results of this chapter and that of Chapter 5, with deposition occurring on two of the lens materials rather than just one. This result indicates that the drying of the lens surface for 5 seconds out of solution has an effect on the deposition of tear components on certain lens materials. Thus, the model blink cell may be a useful tool for future deposition studies. Overall this thesis demonstrated that preparation of the lens material can cause variation in contact angles. Different methods of measuring in vitro wettability of contact lenses can produce different results and thus the method used to assess wettability should always be stated. The physical properties of blister pack solutions can change with added wetting agents and surfactants, and components from blister solutions can alter the initial wettability of contact lenses. In vitro deposition of proteins onto the lens surface can vary with techniques, and finally, deposition of tear components onto the surface of contact lenses can alter the lens wettability.