In vitro Dehydration of Conventional and Silicone Hydrogel Contact Lenses

Abstract

Purpose:: To investigate the dehydration process of hydrogel contact lenses under environmental conditions of relative humidity (RH) and air temperature (AT)and to derive objective parameters to characterize different polymeric materials as a function of their polymeric composition and equilibrium water content (EWC).

Methods:: Thirteen hydrogel contact lenses (EWC 24 to 74%) were dehydrated under environmental conditions (RH=50±5%, AT=22±2ºC) on a microgravimetric comparator. Data were taken at 1-minute intervals untill complete dehydration for the given conditions. Dehydration curves represented cumulative dehydration (CD), valid dehydration (VD) and dehydration rate (DR). Objective parameters of the dehydration process were obtained for further statistical comparison.

Results:: Among those parameters present stronger correlations with the EWC of materials we highlight duration of phase I (r²=0.921), CD at end of phase I (r²=0.971) and time to achieve a dehydration rate of -1%/min (r²=0.946). For each individual sample, the VD at different time intervals can be accurately determined using a 2^nd order regression equation (r²≥0.99 for all samples). The first 5 minutes of the dehydration process show a relatively uniform average CD of about -1.5%/min. Thereafter there was a trend towards higher average CD during the following 15 minutes as the EWC of the material increases (r²=0.701). Thus, average VD for the first 5 minutes displayed a negative correlation with EWC (r²=0.835), and a trend towards uniformization among CL materials for the following periods (r²=0.014).

Conclusions:: DR curves of CL polymers 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 minutes and is strongly correlated with the EWC of the polymer as it is the CD during this phase. Several parameters can be used to characterize the in vitro dehydration of hydrogel contact lens materials. Overall, HEMA-based hydrogels dehydrate more and faster than silicone-hydrogel materials. No significant differences did exist between conventional hydrogels and those that are claimed to reduce on-eye dehydration. However, there are differences in water retention between lenses of similar EWC and thickness that should be further investigated.

Keywords: contact lens • cornea: tears/tear film/dry eye