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 degreesC. 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 I 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 1(-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 - I 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. (C) 2001 Elsevier Science B.V. All rights reserved.