Freeze-cast hydroxyapatite scaffolds for bone tissue engineering applications

Freeze casting of aqueous suspensions was investigated as a method for preparing porous hydroxyapatite (HA) scaffolds for eventual application to bone tissue engineering. Suspensions of HA particles (10-20 volume percent) were frozen unidirectionally in a cylindrical mold placed on a cold steel substrate (-20 degrees C). After sublimation of the ice, sintering for 3 h at 1350 degrees C produced constructs with dense HA lamellae, with porosity of similar to 50%, and inter-lamellar pore widths of 5-30 mu m. These constructs had compressive strengths of 12 +/- 1 MPa and 5 +/- 1 MPa in the directions parallel and perpendicular to the freezing direction, respectively. Manipulation of the microstructure was achieved by modifying the solvent composition of the suspension used for freeze casting. The use of water-glycerol mixtures (20 wt% glycerol) resulted in the production of constructs with finer pores (1-10 mu m) and a larger number of dendritic growth connecting the HA lamellae, and higher strength. On the other hand, the use of water-dioxane mixtures (60 wt% dioxane) resulted in a cellular-type microstructure with larger pores (90-110 mu m). The mechanical response showed high strain tolerance (5-10% at the maximum stress), high strain for failure (> 20%) and sensitivity to the loading rate. The favorable mechanical behavior of the porous constructs, coupled with the ability to modify their microstructure, indicates the potential of the present freeze casting route for the production of porous scaffolds for bone tissue engineering.