Mechanical properties and chemical stability of alginate-based anisotropic capillary hydrogels

Anisotropic capillary hydrogels are formed by ionotropic gel formation of an aqueous sodium alginate solution during unidirectional diffusion and complexation with divalent cations. The type of cation used for gel formation dictates the size of the resulting capillary structure which might facilitate applications as biomaterial scaffolds in tissue engineering and regenerative medicine. Until now, such capillary hydrogel structures have not been characterized regarding their mechanical properties, and we hypothesise that both, the macroscopic capillary structure and the concentration of a chemical crosslinking agent, influence the mechanical properties and the stability of the hydrogels against degradation and dissolution in physiological environment. We prepared anisotropic gels with capillary sizes ranging between 12 and 100 μm using electrolyte solutions containing Ba²⁺, Sr²⁺, Zn²⁺ or Ni²⁺ cations. They were chemically crosslinked with hexamethylene diisocyanate in concentrations varying between 1 and 100 mmol L^-1. Hydrogel properties were determined by swelling experiments, oscillatory rheometry and microindentation analysis and monitored during incubation in phosphate-buffered solution at 37 °C for up to three months. We found, that the mechanical strength generally decreases with increasing capillary diameter. The higher the concentration of the crosslinking agent, the higher is the mechanical strength. The resistance of the alginate hydrogels against degradation is positively correlated with the concentration of the crosslinking agent. Furthermore, microindentation experiments revealed a microscopic anisotropy of the mechanical properties resulting from a perpendicular orientation of the polymer fibres relative to the capillary axis.