Microcapsules of alginate-chitosan. II. A study of capsule stability and permeability

The stability of alginate-chitosan capsules was shown to depend strongly on the amount of chitosan bound to the capsules. When the capsules were made by dropping a solution of sodium alginate into a chitosan solution (one-sta ge procedure), all the chitosan was located in a thin alginate-chitosan mem brane on the surface. These capsules were much weaker than the capsules mad e by reacting calcium alginate beads in an aqueous solution of chitosan and calcium chloride (two-stage procedure). Capsules with high mechanical stre ngth were obtained after shorter reaction times when the number-average mol ecular weight of the chitosan was reduced to around 15 000, when the capsul es were made more homogeneous and when the capsule diameter was reduced to around 300 mu m. When these capsules were treated with calcium sequestrant such as citrate under conditions when calcium alginate gels normally dissol ve, they still had a gel core indicating the presence of chitosan throughou t the capsule matrix. The permeability of the two-stage capsules was reduce d when the chitosan molecular weight was increased and the degree of acetyl ation was increased, and when the capsules were made more inhomogeneous. Th e addition of another several layers of alginate and chitosan resulted in c apsules virtually impermeable to IgG, suggesting an average capsule pore di ameter less than 90 Angstrom. (C) 1999 Elsevier Science Ltd. All rights res erved.