Release of Potassium Ion and Calcium Ion from Phosphorylcholine Group Bearing Hydrogels

Hanna Aucoin,Anthony Guiseppi-Elie,Kazuhiko Ishihara,A Wilson,Ann Wilson

doi:10.3390/polym5041241

Abstract

In an attempt to recreate the microenvironment necessary for directed hematopoietic stem cell differentiation, control over the amount of ions available to the cells is necessary. The release of potassium ion and calcium ion via the control of cross-linking density of a poly(2-hydroxyethyl methacrylate) (pHEMA)-based hydrogel containing 1 mol % 2-methacryloyloxyethyl phosphorylcholine (MPC) and 5 mol % oligo(ethylene glycol) (400) monomethacrylate [OEG(400)MA] was investigated. Tetra(ethylene glycol) diacrylate (TEGDA), the cross-linker, was varied over the range of 1–12 mol %. Hydrogel discs (ϕ = 4.5 mm and h = 2.0 mm) were formed by UV polymerization within silicone isolators to contain 1.0 M CaCl2 and 0.1 M KCl, respectively. Isothermal release profiles, were measured at 37 °C in 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid sodium salt (HEPES) buffer using either calcium ion or potassium ion selective electrodes (ISE). The resulting release profiles were found to be independent of cross-linking density. Average (n = 3) release profiles were fit to five different release models with the Korsmeyer-Peppas equation, a porous media transport model, exhibiting the greatest correlation (R2 > 0.95). The diffusion exponent, n was calculated to be 0.24 ± 0.02 and 0.36 ± 0.04 for calcium ion and potassium ion respectively indicating non-Fickian diffusion. The resulting diffusion coefficients were calculated to be 2.6 × 10−6 and 11.2 × 10−6 cm2/s, which compare well to literature values of 2.25 × 10−6 and 19.2 × 10−6 cm2/s for calcium ion and potassium ion, respectively.

Highlights

Bioactive hydrogels have over the past decade gained traction as polymers for the abio-bio interface in indwelling bioelectronics [1], as drug delivery matrices [2,3], and as tissue scaffolds in regenerative medicine [4]
Two phenomenological effects can contribute to the transport of ions out of a hydrogel matrix: diffusion due to concentration gradients and electromigration due to charge density gradients
This is often expressed relative to the reference diffusion in pure solvent, D0, the polymer mesh size, ξ, the hydrodynamic radius of the solute, rs, polymer fraction in the swollen state, ν2,s, polymer-solute interactions, crosslink density and polymer chain mobility

Summary

Introduction

Bioactive hydrogels have over the past decade gained traction as polymers for the abio-bio interface in indwelling bioelectronics [1], as drug delivery matrices [2,3], and as tissue scaffolds in regenerative medicine [4]. Much of this attention is because of their high biocompatibility [5], attributed partially to their ability to imbibe large amounts of water [6], to mitigate denaturing protein adsorption, and because of their stimuli responsive nature [7], attributed partially to swelling-collapsing dynamics that are dependent on environmental conditions [8,9,10]. Apparent diffusivities of ions within hydrogels are expected to be reduced relative to their values in aqueous solution due to interaction with the polymer chains including hydrodynamic drag, electrostatic drag and physical obstruction that are best represented by the Brinkman-Effective-Medium model [16]

Results

Discussion

Conclusion