Macromolecule Sorption and Diffusion in HEMA/MAA Hydrogels

Abstract

Transient solute absorption and desorption concentration profiles were measured in a 70 wt % hydroxyethyl methacrylate (HEMA)/30 wt % methacrylic acid (MAA) anionic hydrogel using two-photon confocal microscopy. Dilute aqueous solutes included fluorescently labeled dextrans with molecular masses of 4, 10, and 20 kDa, and fluorescently labeled cationic avidin protein. Cross-linking densities with ethylene glycol dimethacrylate (EGDMA) varied from 0 to 1 wt % with polymer volume fractions increasing from 0.15 to 0.25. Average gel mesh sizes, determined from zero-frequency oscillatory shear storage moduli, ranged from about 3.6 to 8.4 nm over the cross-link ratios studied. All solutes exhibit Stokes−Einstein hydrodynamic radii obtained from measured free diffusion coefficients, D o , comparable to or larger than the average gel mesh size. In spite of considerable size exclusion, the studied solutes penetrate the gels indicating a range of mesh sizes available for transport. Transient uptake and release concentration profiles for FITC-dextrans follow simple diffusion theory with diffusion coefficients, D, essentially independent of loading or release characteristic of reversible absorption. Although strongly sizeexcluded, these solutes do not interact specifically with the polymer network. Diffusivities are accordingly predicted from a largepore effective-medium (LPEM) model developed to account for solute size, hydrodynamic drag, and distribution of mesh sizes available for transport in the polymer network. For this class of solute, and using no adjustable parameters, diffusivities predicted from the new effective-medium model demonstrate good agreement with experiment. For the specific-interacting cationic protein, avidin, gel loading is 3 orders of magnitude slower than that of dextran of similar hydrodynamic radius. Desorption of avidin is not complete even after 2 weeks of extraction. On the basis of size alone, avidin is strongly size-excluded, yet it exhibits a partition coefficient of over 20. For the positively charged protein, we observed specific ion binding on the negatively charged carboxylate groups of MAA-decorated polymer strands in the larger mesh spaces. Simple linear sorption kinetics gives an adsorption time constant of 5 min and a desorption time constant of about 20 days, suggesting nearly irreversible uptake of cationic avidin on the anionic gel matrix.

Controlled drug release from hydrogels for contact lenses: Drug partitioning and diffusion

“…Besides, the effective-medium theory underestimates the dynamic drag and obstruction of small solutes, (Liu et al, 2013) i.e. solutes smaller than the average mesh size, which is the case of the studied drugs.…”

“…Liu et al (Liu et al, 2013) studied the diffusion of dextrans with molecular masses of 4, 10, and 20 kDa and the cationic avidin protein in a HEMA/MAA (70/30,w/w) anionic hydrogel. They found that for dextrans, although being size excluded, the measured diffusivities were in good agreement with those predicted from a new effective-medium model which considered solute transport only in the accessible liquid-filled voids.…”

“…Equilibrium partition coefficients were determined at different ionic strength and pH, using water (pH 5.6) and PBS (pH 7.4). The measured partition coefficients were related with the polymer volume fraction in the hydrogel, through the introduction of an enhancement factor following the approach developed by the group of C. J. Radke (Kotsmar et al, 2012;Liu et al, 2013). This factor may be decomposed in the product of three other factors , and which account for, respectively, hard-sphere size exclusion, electrostatic interactions, and specific solute adsorption.…”

“…This law may be extended to account for the solute specifically adsorbed to the polymer which is different from that diffusing in the liquid-filled spaces (Liu et al, 2013 …”

Preparation and characterization of dual-responsive spiropyran-based random copolymer brushes via surface-initiated atom transfer radical polymerization

“…PMAA is one of the most typical pH-responsive polymers used as the carriers for drug delivery system which can achieve the precise targeted drug release. It has been widely applied in biomedical applications, [40] coatings, [41] and sensors. [42] Inspired by the previous work, we designed and synthesized novel hybrid silica nanoparticles grafted with photoand pH-sensitive random copolymer P(SPMA-co-MAA) by SI-ATRP.…”