Vat photopolymerization of polylactic acid/hydroxyapatite scaffolds with a unique combination of structural and compositional gradient for multiple-tissue regeneration

Digital Light Processing (DLP) is a promising approach to fabricate scaffolds with structural and compositional gradients tailored to site-specific tissue properties. In this study, both uniformly dense and gyroid samples as well as structurally graded gyroid scaffolds were designed based on triply periodic minimal surfaces (TPMS) and fabricated by DLP using polylactic acid/hydroxyapatite (PLA/HA) composite slurries. Ultrafine HA powders were added to a commercial PLA resin at 35 wt% and 55 wt%. To produce graded samples, these two slurries were blended in specific ratios (42 and 48 wt%) using a multi-head peristaltic pump, feeding the printer with progressively higher HA contents. Rheological and curing depth tests confirmed good printability across all slurry compositions, including the highest HA loading. The feasibility of the approach was first demonstrated using dense cylindrical specimens. Microstructural and elemental analyses confirmed the desired compositional gradient, while physical and mechanical tests showed that graded samples had intermediate density, compressive strength, and modulus compared to uniform reference samples. This approach was further applied to gyroid-type scaffolds, which also demonstrated smooth compositional transitions and similar intermediate mechanical behavior. Preliminary biological assessments confirmed no cytotoxic effects for both neat PLA and PLA/HA composites. Finally, leveraging the variable HA content slurry, a compositionally and structurally graded PLA/HA gyroid scaffold was printed for the first time, demonstrating the feasibility of the strategy to obtain scaffolds potentially customizable for any type of complex and multilayered anatomical defects.