Qualitative and quantitative characterization of the in vitro dehydration process of hydrogel contact lenses.

PURPOSE: To investigate the in vitro dehydration process of conventional hydrogel and silicone-hydrogel contact lens materials. METHODS: Eight conventional hydrogel and five silicone-hydrogel contact lenses were dehydrated under controlled environmental conditions on an analytical balance. Data were taken at 1-min intervals and dehydration curves of cumulative dehydration (CD), valid dehydration (VD), and dehydration rate (DR) were obtained. Several quantitative descriptors of the dehydration process were obtained by further processing of the information. RESULTS: Duration of phase I (r(2) = 0.921), CD at end of phase I (r(2) = 0.971), time to achieve a DR of -1%/min (r(2) = 0.946) were strongly correlated with equilibrium water content (EWC) of the materials. For each individual sample, the VD at different time intervals can be accurately determined using a 2nd order regression equation (r(2) > 0.99 for all samples). The first 5 min of the dehydration process show a relatively uniform average CD of about -1.5%/min. After that, there was a trend towards higher average CD for the following 15 min as the EWC of the material increases (r(2) = 0.701). As a consequence, average VD for the first 5 min displayed a negative correlation with EWC (r(2) = 0.835), and a trend towards uniformization among CL materials for the following periods (r(2) = 0.014). Overall, silicone-hydrogel materials display a lower dehydration, but this seems to be primarily due to their lower EWC. CONCLUSIONS: DR curves under the conditions of the present study can be described as a three-phase process. Phase I consists of a relatively uniform DR with a duration that ranges from 10 to almost 60 min and is strongly correlated with the EWC of the polymer as it is the CD during this phase. Overall, HEMA-based hydrogels dehydrate to a greater extent and faster than silicone-hydrogel materials. There are differences in water retention between lenses of similar water content and thickness that should be further investigated.

Refractive index and equilibrium water content of conventional and silicone hydrogel contact lenses.

PURPOSE: The purpose of the present study was to measure equilibrium water content (EWC) and refractive index of conventional and silicone hydrogel soft contact lenses (SCL) using a hand refractometer and an automated refractometer. METHODS: Sixteen SCL were used in this study including 12 conventional SCL not containing siloxane moieties (equilibrium water content (EWC) range: 38.6-74%) and the four silicone hydrogel based contact lenses currently available (WC range: 24-47%). Two experienced observers performed the measurements in a randomised order being masked by a third party during the three sessions at which the measurements were collected. The Atago N-2E hand refractometer and the CLR 12-70 digital refractometer were used. Data were analysed separately for conventional and silicone hydrogel materials. RESULTS: Measured EWC and refractive index correlate better when measured with the instruments used in this study (r(2) = 0.979, p < 0.001) than the nominal parameters (r(2) = 0.666, p < 0.001). The linear relationship that correlates nominal and measured EWC shows higher spread of data when all lenses are analysed together (r(2) = 0.840) than when conventional hydrogel (r(2) = 0.953) and silicone hydrogel contact lenses (r(2) = 0.967) are analysed separately. Regarding refractive index, the relationship between nominal and measured values when all the lenses are considered together (r(2) = 0.794) becomes weaker when conventional hydrogel are considered separately (r(2) = 0.688), while a stronger relationship is observed for silicone hydrogel lenses (r(2) = 0.939). Hence, hand refractometry overestimates the EWC of silicone hydrogels, while automated refractive index measurements are more accurate in silicone hydrogels than in conventional hydrogels. CONCLUSIONS: New relationships are presented that correlate nominal and measured values of EWC and refractive index for the silicone containing hydrogels. The linear relationships derived fit well to the data. Hand refractometry overestimates the EWC of silicone hydrogel materials and this bias is related to the proportion of siloxane moieties in the material. Conversely, refractive index can be obtained more accurately with automated refractometry for silicone hydrogels than for conventional hydrogels. Present results are of interest in planning future clinical studies involving the measurement of EWC of current hydrogels.

Microscopic observations of superficial ultrastructure of unworn siloxane-hydrogel contact lenses by cryo-scanning electron microscopy.

The purpose of this study was to analyze three commercial siloxane-hydrogel contact lens materials, lotrafilcon A, balafilcon A, and galyfilcon A, by cryogenic scanning electron microscopy (cryoSEM). The fully hydrated lenses were frozen in slush liquid nitrogen and qualitatively observed in a cryogenic scanning electron microscope. The superficial ultrastructure of the siloxane-hydrogels was observed at the areas where the lens fractured during sample cryogenic preparation. There are qualitative differences among the three examined materials in the complex polymer network structure existing between the outer layer and the underlying polymer. CryoSEM, although destructive, is a useful tool to investigate the structure of polymers used in contact lenses. This technique allows the observation of the inner structure of polymers in the hydrated state. The ultrastructure, the polymer network underlying the outer surface of siloxane-hydrogels by cryoSEM microscopy, have never been reported before.

Microscopic observation of unworn siloxane-hydrogel soft contact lenses by atomic force microscopy.

In the present study, samples of lotrafilcon A, balafilcon A, and galyfilcon A contact lenses were observed by atomic force microscopy (AFM) in tapping mode at areas ranging from 0.25 to 400 microm2. Mean roughness (Ra), root-mean-square roughness (Rms) and maximum roughness (Rmax) in nanometers were obtained for the three lens materials at different magnifications. The three contact lenses showed significantly different surface topography. However, roughness values were dependent of the surface area to be analyzed. For a 1 microm2 area, statistics revealed a significantly more irregular surface of balafilcon A (Ra = 6.44 nm; Rms = 8.30 nm; Rmax = 96.82 nm) compared with lotrafilcon A (Ra = 2.40 nm; Rms = 3.19 nm; Rmax = 40.89 nm) and galyfilcon A (Ra = 1.40 nm; Rms = 1.79 nm; Rmax = 15.33 nm). Ra and Rms were the most consistent parameters, with Rmax presenting more variability for larger surface areas. The higher roughness of balafilcon A is attributed to the plasma oxidation treatment used to improve wettability. Conversely, galyfilcon A displays a smoother surface. Present observations could have implications in clinical aspects of siloxane-hydrogel contact lens wear such as lens spoliation, resistance to bacterial adhesion, or mechanical interaction with the ocular surface.

Oxygen transmissibility of piggyback systems with conventional soft and silicone hydrogel contact lenses.

PURPOSE: To investigate the apparent oxygen transmissibility of various piggyback systems using conventional and silicone hydrogel soft contact lenses of different water content and permeability, rigid poly(methyl methacrylate), and rigid gas-permeable lenses of medium, high, and ultrahigh oxygen permeability. The aim of the study was to establish which material (rigid or hydrogel) is more representative of the resulting oxygen performance of piggyback systems. METHODS: The apparent oxygen transmissibility of 66 piggyback systems was measured with an electrochemical method. Eighteen of these combinations involved the use of silicone hydrogel contact lenses currently available. One hyperpermeable rigid gas-permeable contact lens (tisilfocon A) was also included in the study. RESULTS: Measured apparent transmissibility correlates with rigid lens permeability (r = 0.403; SE = +/-3.03 barrer/cm; P < 0.001) and hydrogel lens permeability (r = 0.334; SE = +/-3.2 barrer/cm; P < 0.001). As expected, a linear model comprising permeability values from both rigid and soft materials gave a more precise estimation of the piggyback transmissibility (r = 0.736; SE = +/-2.02 barrer/cm; P < 0.001). The highest values of apparent oxygen transmissibility were found for the combination of tisilfocon A rigid material with any of the 3 silicone hydrogel lenses. Tisilfocon A material significantly improved the transmissibility of all piggyback systems even when conventional hydrogels are involved. CONCLUSION: The combination of hypertransmissible rigid gas permeable lenses with silicone hydrogel soft materials should result in normal corneal function under daily wear conditions. When fitting piggyback systems, clinicians must be aware of material selection to optimize oxygen performance. This is of particular importance in already compromised corneas.

pH stability of ophthalmic solutions.

BACKGROUND: In this study, we evaluated the pH value of 17 ophthalmic solutions, and we investigated whether the pH of these solutions changed over time after the bottle was opened. METHODS: Fifteen bottles of each type of solution were chosen at random from different production lots. A 0.05-ml increment was taken from each bottle and was measured daily using a micropH 2002 Crison pH-meter over a period of 30 days. RESULTS: The results revealed differences between the pH values of the solutions; nine solutions presented pH values within ocular comfort range and eight solutions presented pH values between 3.5 and 6.4. Ten solutions presented nonstatistically significant variations over time (p > 0.01) and seven solutions presented isolated but statistically significant variations. CONCLUSIONS: We may assume that the nine solutions with pH values within the ocular comfort range will not produce initial discomfort. The solutions with acid pH values will produce initial discomfort. The solutions with nonstatistically significant pH variations over 30 days in relation to their initial pH values presented great stability.

Porous structure of Purevision versus Focus Night&Day and conventional hydrogel contact lenses.

The surface and bulk structures of hydrogel contact lenses that contain siloxane moieties, Purevisiontrade mark (balafilcon A) and Focus(R)Night&Daytrade mark (lotrafilcon A), were investigated. Standard hydrogel lenses of low (Seequence(R)), medium (Acuvue(R)), and high water content (Precision UV(R)) were used as controls. All the lenses were dehydrated in a series of ethanol solutions of increased concentration, critical-point dried in CO(2), and sputter coated with gold/palladium before they were examined by scanning electron microscopy. Of all lenses examined, only the balafilcon lenses presented, in addition to the polymer network porosity characteristic of all hydrogels, a macroporous structure that was observed on the front and back surfaces, and in their bulk. The average diameter of the macropores appears to be much larger, from one to several orders of magnitude, than the network porosity of standard hydrogel lenses. The macropores might contribute to the gas and water permeability of these lenses, as well as to their mobility on the cornea.