Inorganic polyphosphate, a paradigm changer in 3D printing of β-tricalcium phosphate based materials for bone tissue surgery

Abstract β-Tricalcium phosphate (β-TCP) is widely used as a material for bone implants due to its excellent biocompatibility, biodegradability, and osteoconductivity, as well as its osteoinductive properties. Here, we demonstrate that the regenerative potential of this material can be significantly enhanced when incorporated into a matrix of inorganic polyphosphate (polyP), a physiological, metabolically active polymer composed of phosphate residues linked by high-energy phosphoanhydride bonds. A 3D-printable hydrogel was developed containing suspended β-TCP and amorphous calcium-polyP nanoparticles (Ca-polyP-NP; the water-insoluble depot form of polyP), as well as NaH₂PO₄ as the monomeric precursor of the polymeric, water-soluble Na-polyP. Heating the printed scaffold to 700°C causes condensation of NaH₂PO₄, resulting in the formation of a Na-polyP glass melt that embeds the Ca-polyP-NP and β-TCP particles. The final scaffolds exhibited the necessary porosity, with pore sizes ranging from 10 to 100 µm (average 84 µm), which are suitable for bone ingrowth, along with the required mechanical stability. The morphogenetically active polyP component is released from the 3D-printed porous scaffolds in appropriate amounts, significantly increasing both the proliferation and energy-dependent differentiation of mesenchymal stem cells (MSCs) into mineralizing osteoblasts compared to polyP-free β-TCP scaffolds. Moreover, enhanced formation of collagen fibers and hydroxyapatite deposits on the cell surface, as well as accelerated microvessel tube formation, were observed in MSCs seeded on polyP-containing scaffolds. These results demonstrate that the novel strategy of integrating β-TCP with polyP as an energy-supplying, regeneration-promoting component imparts superior functional properties to β-TCP scaffolds, making them a promising material for future bone implant applications.