Highlights:• Bone has a complex multi-level hierarchical organisation. This structural organisation is key for its outstanding properties. • Artificial calcium phosphate ceramics (CPCs) are promising for achieving the set of mechanical and biological requirements for use as bone implants. • At individual lengthscales, methods have been developed to control crystal phases, microscale porosity and macroscopic geometry but the CPCs produced remain intrinsically brittle. • Inspired by the organisation of bones and other biomaterials, CPCs and their composites could have their toughness increased through multi-level hierarchical microstructures. • Challenges remain but recent advances in ceramic processing and toughening may pave the way for a new generation of medical implants.

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Natural materials such as bone and enamel have intricate microstructures with inorganic minerals oriented to perform multiple mechanical and biological functions. Current additive manufacturing methods for biominerals from the calcium phosphate (CaP) family enable fabrication of custom-shaped bioactive scaffolds with controlled pore structures for patient-specific bone repair. Yet, these scaffolds do not feature intricate microstructures similar to those found in natural materials. In this work, we used direct material extrusion to 3D print water-based inks containing CaP microplatelets, and obtained microstructured scaffolds with various designs. To be shear-thinning and printable, the ink incorporated a concentration of 21 – 24 vol% CaP microplatelets of high aspect ratio. Good shape retention, print fidelity and overhanging layers were achieved by simultaneous printing and drying. Combined with the 3D design, versatile CaP microstructured objects can be built, from porous scaffolds to bulk parts. Extruded filaments featured a core-shell microstructure with graded microplatelet orientations, which was not affected by the printing parameters and the print design. A simple model is proposed to predict the core-shell microstructure according to the ink rheology. Given the remaining open porosity after calcination, microstructured scaffolds could be infiltrated with an organic phase in future to yield CaP biocomposites for hard tissue engineering.