A novel strategy for preparing mechanically robust ionically cross-linked alginate hydrogels.

The properties of alginate films modified using two cross-linker ions (Ca(2+) and Ba(2+)), comparing two separate cross-linking techniques (the traditional immersion (IM) method and a new strategy in a pressure-assisted diffusion (PD) method), are evaluated. This was achieved through measuring metal ion content, water uptake and film stability in an ionic solution ([Ca(2+)] = 2 mM). Characterization of the internal structure and mechanical properties of hydrated films were established by cryogenic scanning electron microscopy and tensile testing, respectively. It was found that gels formed by the PD technique possessed greater stability and did not exhibit any delamination after 21 day immersion as compared to gels formed by the IM technique. The Ba(2+) cross-linked gels possessed significantly higher cross-linking density as reflected in lower water content, a more dense internal structure and higher Young's modulus compared to Ca(2+) cross-linked gels. For the Ca(2+) cross-linked gels, a large improvement in the mechanical properties was observed in gels produced by the PD technique and this was attributed to thicker pore walls observed within the hydrogel structure. In contrast, for the Ba(2+) cross-linked gels, the PD technique resulted in gels that had lower tensile strength and strain energy density and this was attributed to phase separation and larger macropores in this gel.

Encapsulation of a glycosaminoglycan in hydroxyapatite/alginate capsules.

The development of suitable vehicles for the delivery of growth-inducing factors to fracture sites is a challenging area of bone repair. Bone-specific glycosaminoglycan molecules are of particular interest because of their high stability and proven effect on bone growth. Calcium alginate capsules are popular as delivery vehicles because of their low immunogenic response; they offer a versatile route that enables the controlled release of heparin (a member of the glycosaminoglycan family). In this study, hydroxyapatite (HA)/alginate composite capsules are explored as novel drug delivery vehicles for heparin, using both medium- and low-viscosity alginates. The composition, structure, and stability of the capsules are fully characterized and correlated to the release of heparin in vitro. Heparin is found to associate both with the alginate matrix through polymeric flocculation and also with the HA crystals in the composite beads. The mechanism by which heparin is released is dictated by the stability of the capsule in a particular release media and by the composition of the capsule. The use of medium-viscosity alginate is advantageous with respect to both drug loading and prolonging the release. The inclusion of HA increases the encapsulation efficiency, but because of its destabilizing effect to the alginate hydrogel matrix, it also increases the rate of heparin release. The bioactivity of heparin is fully retained throughout the assembly and release processes.