In vivo bone response to 3D periodic hydroxyapatite scaffolds assembled by direct ink writing

The in vivo bone response of 3D periodic hydroxyapatite (HA) scaffolds is investigated. Two groups of HA scaffolds (11 mm diameter x 3.5 mm thick) are fabricated by direct-write assembly of a concentrated HA ink. The scaffolds consist of cylindrical rods periodically arranged into four quadrants with varying separation distances between rods. In the first group, HA rods (250 μm in diameter) are patterned to create pore channels, whose areal dimensions are 250 x 250 μm² in quadrant 1, 250 x 500 μm² in quadrants 2 and 4, and 500 x 500 μm² in quadrant 3. In the second group, HA rods (400 μm in diameter) are patterned to create pore channels, whose areal dimensions of 500 x 500 μm² in quadrant 1, 500 x 750 μm² in quadrants 2 and 4, and 750 x 750 μm² in quadrant 3. Each group of scaffolds is partially densified by sintering at 1200°C prior to being implanted bilaterally in trephine defects of skeletally mature New Zealand White rabbits. Their tissue response is evaluated at 8 and 16 weeks using micro-computed tomography, histology, and scanning electron microscopy. New trabecular bone is conducted rapidly and efficiently across substantial distances within these patterned 3D HA scaffolds. Our observations suggest that HA rods are first coated with a layer of new bone followed by subsequent scaffold infilling via outward and inward radial growth of the coated regions. Direct-write assembly of 3D periodic scaffolds composed of micro-porous HA rods arrayed to produce macro-pores that are size-matched to trabecular bone may represent an optimal strategy for bone repair and replacement structures.