Formation of dextran hydrogels by crystallization.

In this paper, a novel method is presented for the preparation of dextran hydrogels and microspheres, based on crystallization. Although dextrans are known to be well soluble in water, precipitation was observed in concentrated aqueous solutions of low molecular weight dextran (dextran 6000), whereas for solutions of dextran with higher molecular weights (dextran 40,000 and 220,000) no precipitation was observed in the time-frame studied. The kinetics of the precipitation process were studied and showed that precipitation was faster when more concentrated dextran solutions were used. Furthermore, the precipitation process was accelerated by stirring and by the presence of salts. Depending on the precipitation time, microspheres or gels were obtained. The precipitates were insoluble in water at room temperature, but readily dissolved in boiling water or DMSO. IR spectroscopy and (modulated) differential scanning calorimetry ((M)DSC) demonstrated that the precipitates were crystalline. We hypothesize that crystallization is due to association of the chains through hydrogen bonding, induced by the large polymer/water ratio in concentrated dextran 6000 solutions.