Bone tissue has the ability to regenerate and heal itself after fracture trauma. However, this ability can be affected by different risk factors that are related to the patient and the nature of the fracture. Some of the factors are age, gender, diet, health, and habits. Critical-sized defects are particularly difficult, if not impossible, to heal correctly. Particularly in large defects, bone regeneration ability is impeded, disrupting normal healing processes, resulting in defective healing, integration, and non-union. To prevent and treat defective healing or non-union, surgical intervention is needed. Surgeons implant various forms of devices between the ends of the broken bone, usually with external fixation. Implants function by guiding and enabling new bone ingrowth while giving support to the healing tissue. Some of the most common implants are autografts, allografts, and metallic endoprostheses. Unfortunately, these common techniques have drawbacks such as the risk of infection and relatively poor biological or mechanical compatibility with host tissue, in addition to the limited source of donor tissue and high cost, often resulting from secondary surgical interventions. Critical defects are particularly problematic. Hence, there is a necessity for bone implant substitutes that diminish the risk of infection and incompatibility while also providing similar mechanical properties to real bone tissue. Hydroxyapatite (HAp) is a ceramic with a chemical composition similar to bone tissue that has shown biocompatibility and osteoconductive properties with host bone tissue, but it is difficult to manufacture into complex structures with mechanical properties comparable to bone tissue. Therefore, significant efforts are directed to produce materials and methods that could produce HAp synthetic implants to treat bone defects.

This research aimed to create and characterize a hydroxyapatite photo-polymeric resin suitable for 3D printing, which could produce dense HAp ceramic parts in complex shapes without requiring support material. We created a HAp-based photopolymer slurry that achieved 41 vol% HAp loading in homogenous slurries. The HAp slurries presented a strong shear thinning behavior and dispersion stability over 20 days under dark storage conditions. The resultant rheological behavior of HAp slurries enabled 3D printing of HAp green bodies in complex shapes using a combination of viscous extrusion and layer-wise photo-curing processes. Complex structures with concave and convex forms and scaffolds with interconnected pores ranging from 130 µm to 600 µm pore sizes and 10% to 40% porosity were successfully built with high resolution and no support material. Moreover, HAp/PEGDMA green bodies presented complete layer cohesion. After 3D printing, sintering was used to densify HAp structures and eliminate the polymer matrix. The resultant HAp structures maintained their complex details, had a relative density of ~78% compared to fully dense HAp and a dimensional shrinkage of ~15% compared to its green body. Sintered HAp structures were found to be non-cytotoxic for ADSCs cells. Flexural properties of HAp green and sintered structures were also determined. It was found that green bodies had a flexural strength of ~30.42MPa comparable to trabecular bone.

To summarize, a photopolymerizable resin with 41 vol% of HAp was created to produce ~78% dense HAp complex structures. This was achieved by using additive manufacturing that combined viscous extrusion and layer-wise photo-curing and a sintering process. HAp/PEGDMA showed flexural strength comparable to the trabecular bone, and HAp sintered structures demonstrated non-cytotoxic behavior.