Strontium and gallium doping enhances in vivo bone regeneration in biomimetic hydroxyapatite 3D-printed scaffolds

Doping of calcium phosphates (CaPs) with bioinorganic ions is a widely used strategy to enhance their biological performance in bone regeneration. However, conventional methods for ionic incorporation in CaP scaffolds often require high-temperature treatments or involve multiple complex steps. Here, we present two simple strategies to dope 3D-printed CaP scaffolds via incorporation of ions into the apatitic phase during the hydrolysis of α-tricalcium phosphate (α-TCP) to calcium deficient hydroxyapatite (CDHA). In the first strategy, ions were incorporated directly into the printing ink, whereas in the second, undoped robocasted scaffolds were immersed in ionic solutions, allowing ion incorporation into precipitated CDHA during phase transformation. We investigated several ions, including strontium (Sr²⁺), magnesium (Mg²⁺), silicon (SiO₄ ^4-) and gallium (Ga³⁺). Sr²⁺ and Ga³⁺ were successfully incorporated into the scaffolds, either by direct ink doping (Sr²⁺) or by soaking in ionic solutions (Sr²⁺ and Ga³⁺). Direct incorporation of Sr²⁺ in the ink resulted in a higher ion loading and release, enhancing bone formation and bone quality, as evidenced by increased mineral-to-matrix ratio and Young's modulus, as well as osteoinductive properties relative to non-doped scaffolds. Furthermore, we demonstrated for the first time the osteoinductive capacity of Ga³⁺ in an ectopic in vivo model.