Soft Hydroxyapatite Composites Based on Triazine–Trione Systems as Potential Biomedical Engineering Frameworks

[Image: see text] Composites of triazine–trione (TATO) thiol–ene networks and hydroxyapatite (HA) have shown great potential as topological fixation materials for complex bone fractures due to their high flexural modulus, biocompatibility, and insusceptibility to forming soft-tissue adhesions. However, the rigid mechanical properties of these composites make them unsuitable for applications requiring softness. The scope of these materials could therefore be widened by the design of new TATO monomers that would lead to composites with a range of mechanical properties. In this work, four novel TATO-based monomers, decorated with either ester or amide linkages as well as alkene or alkyne end groups, have been proposed and synthesized via fluoride-promoted esterification (FPE) chemistry. The ester-modified monomers were then successfully formulated along with the thiol TATO monomer tris [2-(3-mercaptopropionyloxy)ethyl] isocyanurate (TEMPIC) and HA to give soft composites, following the established photo-initiated thiol–ene coupling (TEC) or thiol–yne coupling (TYC) chemistry methodologies. The most promising composite shows excellent softness, with a flexural modulus of 57 (2) MPa and ε(f) at maximum σ(f) of 11.8 (0.3)%, which are 117 and 10 times softer than the previously developed system containing the commercially available tri-allyl TATO monomer (TATATO). Meanwhile, the surgically convenient viscosity of the composite resins and their excellent cytotoxicity profile allow them to be used in the construction of soft objects in a variety of shapes through drop-casting suitable for biomedical applications.