Enhancing DLP Ink Performance for Bone Tissue Engineering via Calcium Phosphate Oligomers and Concurrent Crosslinking...

Digital light processing (DLP) 3D printing of calcium phosphate (CaP) scaffolds tailored to fit personalized bone defects represents a promising strategy for bone tissue engineering. However, conventional CaP-GelMA inks suffer from particle-induced ink heterogeneity and reduced photopolymerization efficiency, resulting in poor printability. Additionally, since CaP crystallizes before polymer crosslinking, it only mixes with the GelMA matrix, leading to weak interfacial interactions between them and poor mechanical properties of the scaffold. To overcome these challenges, traditional calcium phosphate (CaP) particles were replaced with highly water-soluble calcium phosphate oligomers (CPO), which provide superior dispersion and minimal light scattering leading to high printability. More importantly, the organic-inorganic hybrid structure within the scaffold was achieved through concurrent GelMA crosslinking and CPO crystallization. The optimal CPO, with a Ca/P ratio of 1.67, demonstrated favorable DLP printability owing to its low and narrowly distributed particle size, high solid content, and low absorbance of light. Among various formulations, the ink containing 20% CPO (GC-20) exhibited superior printability and structural integrity. Characterization confirmed that CPO converts to hydroxyapatite (HAp) and forms hybrid HAp-GelMA structures during DLP printing, resulting in significantly improved compressive modulus and toughness. In vitro assays indicated that the scaffolds are noncytotoxic, promote cell adhesion and proliferation, and substantially enhance osteogenic differentiation and mineralization of mesenchymal stem cells. These findings highlight the potential of CPO-based DLP inks for applications in personalized bone and cartilage regeneration.