Digital Light Processing of Reactive Ceramic-Polymer Bone Scaffolds Enables In situ Hydroxyapatite Formation

The incorporation of calcium phosphates to additive manufacturing, particularly digital light processing (DLP), has gained increasing attention for the fabrication of bone tissue engineering scaffolds. DLP enables the production of patient-specific constructs with complex architectures and high spatial resolution. In this study, we develop photo-crosslinkable resin formulations based on poly(ethylene glycol) diacrylate (PEGDA) loaded with reactive α-tricalcium phosphate (α-TCP) particles for DLP printing. Using a liquid crystal display-based DLP (LCD-DLP) system, composite scaffolds were directly printed. High-resolution gyroid architectures containing 50 wt% α-TCP and a designed open porosity of 50% were successfully fabricated. The printed scaffolds exhibited controllable multiscale porosity and underwent an in situ hydrolysis reaction after printing, reaching more than 92% conversion of α-TCP to biomimetic calcium deficient hydroxyapatite (CDHA) composed of high-aspect-ratio nanocrystals. This phase transformation generated an entangled nanocrystalline network that increased the specific surface area and induced structural hardening, resulting in an interpenetrating polymer-ceramic composite architecture. As a consequence, the compressive strength of the scaffolds nearly doubled relative to the as-printed state while maintaining appreciable flexibility. Compared to conventional sintered calcium phosphate scaffolds, the reactive composite scaffolds exhibited markedly improved toughness and flexibility, with almost two-fold increase in strain energy density. Furthermore, the reactive scaffolds showed good cytocompatibility in vitro. These results demonstrate that combining DLP with reactive calcium phosphate-based resins enables the fabrication of mechanically resilient and biologically relevant bone scaffolds through a single-step printing process followed by low temperature in situ hardening. STATEMENT OF SIGNIFICANCE: Traditional 3D-printed bone scaffolds often rely on high-temperature sintering, producing brittle structures prone to failure. We introduce a reactive resin, combining poly(ethylene glycol) diacrylate and α-tricalcium phosphate, designed for Digital Light Processing (DLP). DLP offers significantly higher spatial resolution and architectural complexity than conventional micro-extrusion methods, allowing for more precise patient-specific geometries. Post-printing, an in situ reaction transforms the material into a toughened, interpenetrating polymer-ceramic network. This unique architecture doubles compressive strength and significantly enhances flexibility compared to traditional ceramics. By replacing fragile, sintered components with this resilient, nanocrystalline structure, our approach provides a robust, single-step pathway for creating mechanically durable, cytocompatible scaffolds essential for effective bone tissue engineering.