An Apparent pH-lnduced Effect on Extended Wear Hydrogel Lens Water Content
Numerous studies have examined the separate issues of precorneal tearfilm pH and hydrogel lens water content. None have examined the two issues in an associated, clinical fashion. Earlier research by this author has shown that stabilized hydrogel lens anterior surface pH correlates with established values for the precorneal tearfilm pH. This technique was therefore paired with two different methods of hydrogel lens water content assessment to determine if there was a predictable tear pH-related effect on soft lens hydration. Sixty-five volunteer subjects were fitted with either 38% (polymacon) or 58% (etafilcon A) water content hydrogel lenses on an extended-wear basis. The in situ hydrogel lens anterior surface pH was measured with a flat-surfaced, self-referenced pH electrode 5 min after initial fitting, and on subsequent extended wear follow-up examinations. On each follow- up, associated lens water content was determined by gravimetric and/or refractive analysis, yielding a total of 517 paired data points over a 33 month period. While the anterior lens surface pH increased in a nonlinear fashion with immediate duration of extended lens wear [p < 0.001 analysis of variance (ANOVA) by number of days extended lens wear], the water content decreased in a nonlinear fashion under the same conditions (p < 0.001 by ANOVA). Correlation of these two nonlinear functions yielded a negatively sloped linear regression (r=0.99 and 0.97) for each material. The slopes of the resultant linear functions were significantly different (p < 0.01) by t-test; the higher water content material exhibited a greater sensitivity to pH. The routine labeling of pH-induced water content changes by hydrogel lens manufacturers would provide an objective model for clinical lens behavior, and assist in lens type and parameter selection when fitting extended-wear soft lenses. Additionally, as other lens materials are characterized, it may be possible to model the “ideal” hydrogel material in terms of apparent pH-dependent or -independent behavior.
Dehydration of hydrogel contact lenses in vitro and in vivo
An attempt was made to derive a model for predicting the extent of hydrogel lens dehydration in vivo (ΔWv) by using a contact lens refractometer to (1) measure the water content (W) of eight lensus ranging from 36.5 to 85.8% water before and after 90 min lens wear by six subjects; and (2) measure the rate of dehydration of the same lenses in vitro (ΔW1), in a controlled, randomized, double‐masked experiment. A significant correlation was demonstrated between W and the rate of change of ΔW1 (the dehydration rate, D), whereby lenses of higher water content dehydrate at a slower rate. No correlation was found between W and ΔWv or between D and ΔWv, thus precluding the development of a predictive model. Possible reasons for this failure, and directions for further research, are discussed
Synthetic hydrogels: 3. Hydroxyalkyl acrylate and methacrylate copolymers: surface and mechanical properties
Abstract The surface and mechanical properties of copolymers of hydroxyalkyl acrylates and methacrylates have been examined by a variety of techniques. This work is complementary to earlier parts of this series which describe the effect of copolymer structure on water binding properties. Water structure has been demonstrated to exert a profound effect upon mechanical properties whether measured in compression or in tension. In particular, water that is characterized by differential scanning calorimetry as ‘freezing’ water is observed to have a marked plasticizing effect upon the gel, whereas ‘non-freezing’ water has little such effect. Similarly, the ‘freezing’ water produces a more marked effect on thermally induced transitions. Two distinct transition points are observed as a result of its presence. One corresponds to the freezing point of water and the other to a glass transition temperature, whose value depends upon the proportion of ‘freezing’ or ‘plasticizing’ water in the gel. Several predictive and direct measurement techniques have been used to study the surface properties of the copolymers in both hydrated and dehydrated states. Taken together they have established a sound understanding of the way in which polar and dispersive components of surface free energy vary as a function of copolymer composition and water content. Use of protein adsorption and fibroblast cell interaction techniques demonstrate that biological phenomena respond to changes at a molecular level which current macroscopic surface energy techniques are unable to discern.