Polyplex formation influences release mechanism of mono- and di-valent ions from phosphorylcholine group bearing hydrogels

The release of monovalent potassium and divalent calcium ions from zwitterionic phosphorylcholine containing poly(2-hydroxyethyl methacrylate) (pHEMA)-based hydrogels was studied and the effects of polymer swelling, ion valence and temperature were investigated. For comparison, ions were loaded during hydrogel formulation or loaded by partitioning following construct synthesis. Using the Koshmeyer-Peppas release model, the apparent diffusion coefficient, D_app, and diffusional exponents, n, were D_app (pre-K⁺) = 2.03 × 10^-5, n = 0.4 and D_app (post-K⁺) = 1.86 × 10^-5, n = 0.33 respectively, indicative of Fickian transport. The D_app (pre-Ca²⁺) = 3.90 × 10^-6, n = 0.60 and D_app (post-Ca²⁺) = 2.85 × 10^-6, n = 0.85, respectively, indicative of case II and anomalous transport. Results indicate that divalent cations form cation-polyelectrolyte anion polymer complexes while monovalent ions do not. Temperature dependence of potassium ion release was shown to follow an Arrhenius-type relation with negative apparent activation energy of -19 ± 15 while calcium ion release was temperature independent over the physiologically relevant range (25-45 °C) studied. The negative apparent activation energy may be due to temperature dependent polymer swelling. No effect of polymer swelling on the diffusional exponent or rate constant was found suggesting polymer relaxation occurs independent of polymer swelling.