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.

Yes / To determine if a gradual adaptation period is necessary for neophytes when fitted with modern hydrogel or silicone hydrogel reusable disposable contact lenses. Across four sites, 74 neophytes (18-28 years) were randomly assigned to a reusable lens cleaned nightly with Opti-Free® Puremoist® multi-purpose contact lens solution: Proclear® (hydrogel) or Biofinity® (silicone hydrogel) and an adaptation schedule: fast (10 h wear from the first day) or gradual (4 h on the first day, increasing their wear time by 2 h on each subsequent day until they had reached 10 h). Masked investigators graded ocular surface physiology and non-invasive tear breakup time (NIBUT) and a range of comfort, vision and lens handling subjective ratings (0-100 visual analogue scales) were recorded at the baseline visit and after 10 h of lens wear, 4-6 days and 12-14 days after lens fitting. Subjective scores were also repeated after 7 days. There was no difference (p > 0.05) in ocular surface physiology or NIBUT between fast and gradual adaptation groups at any time point in either lens type with the exception of increased corneal staining (p = 0.019) in the silicone hydrogel fast adaptation group after 4-6 days, but was similar by 12-14 days. Subjective scores were also similar across the visits and lens types with the exception of 'lens awareness' (p = 0.019) which was less in the gradual versus the fast adaptation silicone hydrogel lens group at 12-14 days. There seems to be no clinical benefit for recommending a gradual adaptation period in new wearers fitted with modern soft reusable disposable contact lenses. The findings of this work add to a growing body of evidence suggesting that such advice is unnecessary in regular soft contact lens wear, which has important ramifications for the initial clinical management of these patients. / BUCCLE

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