Thanks to their similarity with bone mineral and to their biological properties, calcium phosphate ceramics, and particularly hydroxyapatite, are reference materials for bone grafting procedures. However, there is still room for improvement, especially for the repair of poorly vascularized bone tissue, for which there is no satisfactory clinical solution yet. Composite materials obtained by the combination of hydroxyapatite with chitosan, a polysaccharide known for its angiogenic properties, could be a solution for this major clinical issue. To reach the desired properties, it is essential to control both the chemical composition and the microstructure of the material, with an interconnected porosity at different scales. In this collaborative work, the presence of the organic chitosan phase within the formulation of the composite materials enabled to opt for direct ink-writing technology to 3D-print such scaffolds.
 
This communication will focus on the formulation and optimization of chitosan-calcium phosphate composite inks for 3D printing. Two complementary approaches will be presented: on the one hand, inks in the form of suspensions are prepared to be printed under alkaline conditions. The gelation of chitosan at high pH leads to the consolidation of the extruded filaments as printing proceeds: self-supported structures can be printed layer-by-layer.
The second approach consists of printing cementitious composite pastes, which harden over time due to a setting reaction, similarly to calcium phosphate bone cements. The ink has been optimized so that its evolution from a viscous printable paste to a solid strut occurs in adequate time, compatible with the implementation of the 3D printing process.
In both cases, the composite obtained after consolidation consisted of apatite nanocrystals intertwined with chitosan, and remained very homogeneous at the scale of the struts forming the walls of the scaffold.
The inks thus obtained meet the requirements for direct ink writing, being shear-thinning and exhibiting a good printability without phase separation during extrusion. The complementarity of the two approaches makes it possible to vary the ratio of organic to mineral phases, and thus the biological properties, over a very wide range. Finally, in both cases, no thermal post-treatment is required to consolidate the composite material, opening the way to the addition of molecules of biological interest in formulations for optimized bone substitutes (e.g., angiogenic and/or antibacterial properties).
 
This work is the result of a collaboration between the Carnot institutes Chimie Balard Cirimat and Ingénierie@Lyon in the framework of the ChitCaP project.