Distribution of water states within Poly(HEMA-co-HPMA)-based hydrogels

Abstract

The distribution of water states within hydrogels is implicated in their biotechnical performance. Formulated synthetic hydrogels based on poly(2-hydroxyethylmethacrylate-co-N-(2-hydroxypropyl) methacrylamide) [poly(HEMA-co-HPMA)] of varying cross-linker concentration and monomer mole ratio were studied for their degree of hydration and water distribution. Cross-linker concentrations of 0.1, 0.5, 1.0, and 3.0 mol% tetraethylene glycol diacrylate (TEGDA) were studied when the HEMA:HPMA mole ratios were selected to be 1:0 and 4:1. Additionally, the cross-linker concentration of 1.0 mol% TEGDA was studied in detail when the HEMA:HPMA ratio was selected to be 1:1. The degree of hydration was determined by gravimetry. The distribution among water states was determined from Gaussian deconvolution of the DSC thermograms and use of ΔHf(T) for T < 0 °C. Both the cross-linker concentration and the HEMA:HPMA mole ratio served to alter degree of hydration and the distribution among freezable free, freezable bound and non-freezable bound water states. The presence of 20% and 50% HPMA served to increase total hydration relative to poly(HEMA) (0% HPMA) by 25% and 46%, respectively at 1 mol% TEGDA. The freezable free and freezable bound water content increased 21% and 47%, respectively, and non-freezable bound water content decreased 37% when 20% HPMA was added to poly(HEMA). Correlation maps between water content and distribution with previously published properties of elastic modulus (kPa), glass transition temperature (Tg (°C)), void fraction (ϵ), freeze fracture surface roughness (Rq (μm)), the time constant of swelling (τ (min)) and time constant of release of FITC-dextran (40 kDa) (τ (h)) showed that within the freezable water domain, the freezable free water had the larger influence. However, all biotechnical properties of hydrogels were most strongly influenced by the non-freezable bound water content.