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This study investigated the effects of the three-dimensional (3-D) pore structure of a porous hydroxyapatite/collagen (HAp/Col) composite on their mechanical properties and in vivo tissue ingrowth. The unique 3-D pore structure, comprising unidirectionally interconnected pores, was fabricated by the unidirectional growth of ice crystals by using a cooling stage and a subsequent freeze-drying process. The unidirectional pores had a spindle-shaped cross section, and their size gradually increased from the bottom to the upper face. The porous composite showed an elastic property and anisotropic compressive strength for the pore directions. While the strength and modulus parallel to the pore axis were 1.3-and twofold higher than those of the porous composite with spherical pores formed randomly, the strength and modulus perpendicular to the pore axis showed the lowest values. The subcutaneous implantations revealed that when compared with the random pores, the unidirectional pores promote the ingrowth of the surrounding tissues into the pores.

Transparent hydroxyapatite (HAp) sintered body with a high crystal orientation and a relative density of 99.7% was fabricated by the pulse electric current sintering method; the c-axis of HAp crystals with a hexagonal column morphology and ca. 200 lm  25 lm in size was oriented perpendicular to the pressure direction. The sintered temperature increased to 12001C at a heating rate of 501C/min for 10 min and a pressure of 50 MPa was applied. The orientation indexes were calculated from X-ray diffraction patterns of the planes perpendicular and parallel to the pressure direction. The optical transmittance was greater than 70% over the wavelength of 700 nm.

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