Three-dimensional plotted hydroxyapatite scaffolds with predefined architecture: comparison of stabilization by alginate cross-linking versus sintering

Scaffolds for bone tissue engineering are essentially characterized by porous three-dimensional structures with interconnected pores to facilitate the exchange of nutrients and removal of waste products from cells, thereby promoting cell proliferation in such engineered scaffolds. Although hydroxyapatite is widely being considered for bone tissue engineering applications due to its occurrence in the natural extracellular matrix of this tissue, limited reports are available on additive manufacturing of hydroxyapatite-based materials. In this perspective, hydroxyapatite-based three-dimensional porous scaffolds with two different binders (maltodextrin and sodium alginate) were fabricated using the extrusion method of three-dimensional plotting and the results were compared in reference to the structural properties of scaffolds processed via chemical stabilization and sintering routes, respectively. With the optimal processing conditions regarding to pH and viscosity of binder-loaded hydroxyapatite pastes, scaffolds with parallelepiped porous architecture having up to 74% porosity were fabricated. Interestingly, sintering of the as-plotted hydroxyapatite–sodium alginate (cross-linked with CaCl₂ solution) scaffolds led to the formation of chlorapatite (Ca₉.₅₄P₅.₉₈O₂₃.₈Cl₁.₆₀(OH)₂.₇₄). Both the sintered scaffolds displayed progressive deformation and delayed fracture under compressive loading, with hydroxyapatite–alginate scaffolds exhibiting a higher compressive strength (9.5±0.5 MPa) than hydroxyapatite–maltodextrin scaffolds (7.0±0.6 MPa). The difference in properties is explained in terms of the phase assemblage and microstructur