No description available.

No description available.

Purpose: To examine the variables that influence lipid deposition on conventional and silicone hydrogel contact lens materials and to build a physiologically relevant in vitro model of lipid deposition on contact lenses. Methods: Lipid deposition on contact lens materials can lead to discomfort and vision difficulty for lens wearers. Using a variety of radiochemical experiments and two model lipids (cholesterol and phosphatidylcholine), a number of clinically significant parameters that may influence lipid deposition were examined. • The optimization and characterization of a novel artificial tear solution (ATS) was examined (Chapter 3) • Optimization of an extraction system to remove deposited cholesterol and phosphatidylcholine from various contact lens materials (Chapter 4) • The influence of different tear film components on lipid deposition was researched (Chapter 5) • The efficiency of hydrogen peroxide disinfecting solutions to remove deposited lipid from contact lenses was investigated (Chapter 6) • The effect of intermittent air exposure on lipid deposition was examined through the use of a custom built “model blink cell” (Chapter 7) Results: A novel complex ATS designed for in-vial incubations of contact lens materials was developed. This solution was stable and did not adversely affect the physical parameters of the contact lenses incubated within it. An efficient extraction protocol for deposited cholesterol and phosphatidylcholine was optimized based on chloroform and methanol with the addition of water and acetic acid for phosphatidylcholine extraction. Overall, cholesterol and phosphatidylcholine deposition is cumulative over time and found to deposit in greater masses on silicone-containing hydrogels. Cholesterol and phosphatidylcholine deposition is influenced by the composition of the incubation medium and air exposure which occurs during the inter-blink period. Hydrogen peroxide disinfecting solutions were able to remove only marginal amounts of lipid from the contact lenses, with the surfactant containing solution removing more. Conclusion: This thesis has provided hitherto unavailable information on the way in which lipid interacts with conventional and silicone hydrogel contact lens materials and the in vitro model built here can be utilized in various ways in the future to assess other aspects and variables of lipid and protein deposition on a variety of biomaterials.

The aim of this thesis was to design, develop and evaluate a Non-Contact Corneal Aesthesiometer (NCCA), using a controlled pulse of air, of a pre-determined pressure. First, the system design and alterations are described, then in a series of model experiments, the standardisation and characteristics of the air-pulse were examined. These studies revealed that the NCCA could produce a repeatable stimulus of known volume and rate of air-flow. Furthermore, the air-flow exiting the stimulus air jet was of a laminar shape, with minimal dispersion. The control of the air-flow dispersion was further enhanced by using a 0.5mm diameter air jet and a working distance of lcm. Varying the stimulus duration was not found to influence these qualities of the air-pulse. The air-pulse stimulus was shown to possess the ability to produce corneal nerve stimulation either by surface deformation, temperature change, or both. Using thermal imaging equipment, a temperature drop in the ocular tear film was demonstrated that was localised, and limited to the cornea. A second series of experiments investigated the ability of the NCCA to measure a corneal sensitivity threshold. These studies indicated that a forced-choice, doublestaircase, Method of Limits experimental technique produced an accurate threshold, with low variability in the results, over a minimum time period. Further studies showed that this threshold measurement was repeatable to within 0.1 mbars. A database of typical normal sensitivity thresholds, under a number of physiological variables - corneal location, gender, age and iris colour, was developed. The results from these studies compared well qualitatively with those using invasive stimuli. A third series of experiments examined the ability of the NCCA to assess corneal nerve function when it was under a number of external influences. The first situation was that of anaesthesia,p roduced by 0.4% benoxinate hydrochloride. Non-contact corneal sensitivity loss and recovery were shown to return to normal levels 60mins after instillation of the anaesthetic. The second situation was that of long-term contact lens wear (i.e. longer than three years). Non-contact sensitivity was shown to be reduced with both soft and gas-permeable lens wear, although the extent of loss did not differ between them, nor was it influenced by the length of wear. Thirdly,corneal sensitivity loss and recovery was assessed in subjects following excimer laser photorefractive keratectomy (PRK). Three laser trials were completed: a longitudinal myopic study, a transverse myopic study, and a longitudinal hyperopic study. The results indicated that non-contact corneal sensitivity loss/recovery was not related to the attempted depth of ablation, and that sensitivity had still not returned to normal levels one year post-op.

Purpose: To examine the variables that influence lipid deposition on conventional and silicone hydrogel contact lens materials and to build a physiologically relevant in vitro model of lipid deposition on contact lenses. Methods: Lipid deposition on contact lens materials can lead to discomfort and vision difficulty for lens wearers. Using a variety of radiochemical experiments and two model lipids (cholesterol and phosphatidylcholine), a number of clinically significant parameters that may influence lipid deposition were examined. • The optimization and characterization of a novel artificial tear solution (ATS) was examined (Chapter 3) • Optimization of an extraction system to remove deposited cholesterol and phosphatidylcholine from various contact lens materials (Chapter 4) • The influence of different tear film components on lipid deposition was researched (Chapter 5) • The efficiency of hydrogen peroxide disinfecting solutions to remove deposited lipid from contact lenses was investigated (Chapter 6) • The effect of intermittent air exposure on lipid deposition was examined through the use of a custom built “model blink cell” (Chapter 7) Results: A novel complex ATS designed for in-vial incubations of contact lens materials was developed. This solution was stable and did not adversely affect the physical parameters of the contact lenses incubated within it. An efficient extraction protocol for deposited cholesterol and phosphatidylcholine was optimized based on chloroform and methanol with the addition of water and acetic acid for phosphatidylcholine extraction. Overall, cholesterol and phosphatidylcholine deposition is cumulative over time and found to deposit in greater masses on silicone-containing hydrogels. Cholesterol and phosphatidylcholine deposition is influenced by the composition of the incubation medium and air exposure which occurs during the inter-blink period. Hydrogen peroxide disinfecting solutions were able to remove only marginal amounts of lipid from the contact lenses, with the surfactant containing solution removing more. Conclusion: This thesis has provided hitherto unavailable information on the way in which lipid interacts with conventional and silicone hydrogel contact lens materials and the in vitro model built here can be utilized in various ways in the future to assess other aspects and variables of lipid and protein deposition on a variety of biomaterials.

No description available.

No description available.

The launch of silicone hydrogel contact lenses has led to a rise in the incidence of mechanically-related clinical complications, which is thought to be due to the increased stiffness of these materials compared to conventional hydrogel lens materials. The mechanical characteristics of hydrogel contact lenses have traditionally been investigated using tensile testing which investigated the bulk material characteristics. This thesis presents a study intended to establish a repeatable method for local mechanical measurement of hydrogel contact lenses using nanoindentation. Hydrogel materials in phosphate buffered saline were indented using a Hysitron Triboindenter mounted on a Veeco Explorer AFM using Triboscope software (version 3.5a) with a specially constructed wet cell. A model hydrogel (poly(HEMA-MMA)) was used to validate the methodology and investigate a the effect of controlled change in specimen thickness. A range of commercially available hydrogel contact lenses were then characterised (including conventional and silicone hydrogel lenses) using the same method. Two different analytical techniques were employed to determine the mechanical properties data; elastic analysis and a time-dependent viscoelastic analytical technique.A strong influence of specimen thickness on apparent mechanical properties was seen with the elastic analysis and an empirical relationship was derived to correct for this which was found to be appropriate for all contact lens specimens studied and reported in the thesis. The viscoelastic analysis results were more complex and exhibited a less clear influence of specimen thickness. However, as this is a very simple approximation as contact lenses are suspected to be poroelastic rather than viscoelastic this work could not be fully resolved in the scope of this thesis. For all contact lenses analysed, nanoindentation produced data similar to that found with conventional tensile testing, however, there was evidence for a slight dependence of elastic properties across the lens that does not correlate with sample thickness. This thesis shows the development of a way of accounting for the variation of thickness of a range of contact lenses, and demonstrated that traditional analysis is accurate enough to determine local differences in modulus across contact lenses. The viscoelastic analysis may be more appropriate for hydrogels, however, it produced irregularities that will require further work to fully resolve.

No description available.

No description available.