Water Content of Soft-Contact-Lens-Material Hydrogels

Abstract

 

Purpose

 

SCL wear comfort is dictated, in part, by hydrogel mechanical and transport properties that depend strongly on water uptake (e.g., lubricity, elasticity, and oxygen/ion/water permeability). Accordingly, an essential feature of SCLs is their equilibrium water volume fraction,φ₁. We present a method to predict φ₁ of both HEMA-based and silicone-based SCL hydrogels that compares well with experiment.

 

Methods

 

Water uptake of HEMA/MAA, Si/HEMA, and Si/MAA hydrogels was determined gravimetrically for varying copolymer composition. Prior to measurement, hydrogels were swollen for a minimum of 3 d in pH = 7.4 0.02 M phosphate buffer saline solutions (0.15 M NaCl). Water content was predicted using a multicomponent Flory-Rehner-Donnan theory modified to account for specific complexation of water with charged carboxylic groups of MAA, along with charge-dependent binding of salt counterions.

 

Results

 

Fig. 1 graphs water volume fraction, denoted φ_1,hyphil, against MAA-copolymer volume fraction during synthesis, ν_MAA, for HEMA/MAA hydrogels. The solid line is drawn using theory with no adjustable constants. At pH = 7.4, addition of MAA to the hydrogel increases water content primarily through hydration of charged MAA carboxylic groups. Small MAA amounts increase water uptake dramatically, but less so as MAA copolymer fraction increases. Fig. 2 displays water volume fraction, φ₁, for Si/MAA and Si/HEMA hydrogels with varying copolymer composition (i.e., MAA or HEMA volume fraction, denoted ν_hyphil). φ₁ increases monotonically with increasing ν_hyphil, as expected. Hydrophobic silicone moieties uptake negligible water. Consequently, we propose that φ₁ for silicone-based hydrogels is simply the volume-fraction-weighted water content of the hydrophilic domains, or φ₁= ν_hyphil φ_1,hyphil (solid lines). In both cases, agreement between experiment and theory is excellent.

 

Conclusions

 

We developed a new procedure to quantify equilibrium water uptake of SCL-material hydrogels. Our procedure permits estimation of SCL water content without resort to experiment. To our knowledge, this study is the first attempt to predict water uptake of silicone-based hydrogels.

 

 

Fig. 1. Water volume fraction, φ_1,hyphil, versus MAA-copolymer volume fraction, ν_MAA, for the HEMA/MAA hydrogels.

 

 

Fig. 2. Water volume fraction, φ₁, versus hydrophilic-phase volume fraction (i.e., HEMA or MAA), ν_hyphil, for the silicone-based hydrogels.

 

Keywords: 477 contact lens