In Vitro Bulk Dehydration Rates of Hydrogel and Silicone Hydrogel Daily Disposable and Frequent Replacement Contact Lens Materials
Rebecca Jones, Lyndon Jones
Discomfort and dryness remain major reasons for contact lens wearers to cease lens wear.
One factor that may be associated with this relates to the dehydration of the lens materials. The purpose of this study was to use an in vitro model to measure the rate of dehydration of 6 silicone hydrogel (SH) and 3 conventional polyHEMA-based daily disposable (DD) and frequent replacement (FR) hydrogel materials.
The rate of water loss was assessed gravimetrically (Sartorius MA 100H) in a humidity controlled environment for a variety of materials of varying water content (WC). Three DD (etafilcon A; narafilcon A; nelfilcon A) and 6 FR (lotrafilcon A; lotrafilcon B; balafilcon A; galyfilcon A; senofilcon A; omafilcon A) hydrogel materials were continuously assessed for water loss over a 20 minute period of time.
For both DD and FR groups, the rate of water loss/minute was strongly correlated with initial WC (r>0.8). Within the DD group, the low WC SH material (narafilcon A) exhibited the slowest rate of dehydration (p<0.01). Within the FR group, the material with the lowest WC (the SH material lotrafilcon A) exhibited the least amount and slowest rate of dehydration (p<0.01) and the material with the highest water content (omafilcon A) exhibited the fastest rate of dehydration (p<0.05). There was no significant difference (p>0.05) in rate of water loss between the conventional material with the highest water content (omafilcon A; 62% WC) and the SH material with the highest water content (galyfilcon A; 47% WC).
Bulk dehydration rates for hydrogel materials are closely related to the initial water content of the material.
Given the wide variety of comfort responses reported by subjects between materials of similar water content, it is unlikely that bulk dehydration is directly related to comfort, but that other factors such as surface hydration, surface wettability, lens design and modulus are more important in the comfort of hydrogels.