Gelling concept combining chitosan and alginate-proof of principle.

Biocompatible hydrogels are very interesting for applications in, e.g., tissue engineering and for immobilization of cells, such as calcium-alginate gels where the calcium ions form specific interactions with the guluronic acid units. We here report on a new gelling system of chitosan and alginate containing only mannuronic acid (poly-M), which are prepared using the following steps: (i) mixing at a pH well above 7 where the chitosan is mainly uncharged; (ii) controlled lowering of the pH by adding the slowly hydrolyzing d-glucono-δ-lactone (GDL); (iii) formation of a homogeneous chitosan-alginate gel upon leaving the mixture at room temperature. Some properties of the new gelling system are demonstrated herein by adding controlled amounts of GDL to (i) a mixture of a polymeric and neutral-soluble chitosan with poly-M oligomers (MO) and (ii) a mixture of poly-M and neutral-soluble chitosan oligomers. The neutral-solubility of the polymeric chitosan is achieved by selecting a polymeric chitosan with an intermediate degree of acetylation of 40%, while the neutral-solubility of the fully de-N-acetylated chitosan oligomers (CO) is obtained by selecting oligomers with a chain length below 10. A proof of concept of the new gelling system is demonstrated by measuring the gel strengths of the polymeric chitosan-MO, and a poly-M-CO. The results show that the gel strength increases with decreasing the pH from neutral to 5, and that the gel strength decreases with increasing ionic strength, indicative of an ionic gel formation. Poly-M formed relatively strong gels with CO while an alginate highly enriched in Guluronic acid formed gels of very limited mechanical strength, suggesting the importance of the match in charge distances in the poly-M and chitosan, both with diequatorially linked sugar units in the (4)C1 conformation.