Diffusion of polyethyleneglycols in calcium alginate hydrogels

The mass transfer of a series of polyethyleneglycols (PEG, average molecular weight=4000, 9000, 12 000, 20 000, 35 000) through calcium alginate hydrogels has been studied at 37 °C. Release kinetics in water (stirred vessel) from uniform diameter alginate beads enabled the effective solute diffusion coefficient to be computed. In a first step, a quasi simultaneous gelation and release situation (gelation time around 1 minute) was experimented. The results were typical of a convection mechanism: high effective PEG diffusion coefficients, which are incompatible with a strict diffusion mechanism were obtained, and no effect of the solute molecular weight on the overall transfer was observed. In a second step, the release kinetics of PEG was monitored after alginate gelation was completed (gelation time >24 h), and three different alginate concentrations of the beads (8.7, 16.5 and 28.5 g l−1), corresponding to different network densities, have been experimented. In that case, polyethyleneglycol diffusion coefficients were shown to decrease with increased molecular weight of the diffusing species and/or increased network density, in qualitative agreement with the predictions of a strict diffusion mechanism. It is shown that a power law expression gives a correct description of the PEG diffusion coefficient versus molecular weight relationship. The power law exponent gradually shifts from −0.6 to −1 with increasing alginate concentration. These results correspond for the solute molecule to an intermediate situation between the negligible solvent drainage coil (known to hold in dilute solution, with a characteristic exponent around −0.5) and the reptation model (observed in dense networks with a −2 exponent value). An hypothetical mechanism based on the conformational possibilities for a highly flexible macromolecule to pass through a gel mesh is proposed in order to account for these results.