Self-hardening polycaprolactone/calcium phosphate inks for 3D printing of bone scaffolds: rheology, mechanical properties and shelf-life

The development of 3D-printed calcium phosphate bone grafts represents a breakthrough for personalized bone tissue engineering. However, their high brittleness is a limiting factor for clinical applications. The present work aims to overcome this drawback by developing a composite ink for direct ink writing using a polycaprolactone solution (20 wt% in pyridine) as binder for a self-hardening alpha-tricalcium phosphate powder. The ink consolidation results from a two-step process. First, evaporation of the solvent gives rise to a flexible green body, in which the polymer matrix retains the loose ceramic particles. Subsequently, upon contact with water, the hydrolysis of these particles creates a continuous network of biomimetic hydroxyapatite nanocrystals, with the consequent hardening of the ink. The presence of the polymer improves the mechanical properties, increasing the flexural strength from 2 to 4 MPa and the strain energy density from 0.2 to 1.1 kJ/m3. Unlike the inks based on hydrogel binders, where the hardening reaction starts as soon as the liquid and solid components are mixed, the ink prepared with the non-aqueous binder is stable, with over 5 months shelf life, and allows the hardening reaction to be triggered at will by contact with water.