In the recently published, ‘Hydroxyapatite Structures Created by Additive Manufacturing with Extruded Polymer,’ Katherine Vanesa López Ambrosio (School of Advanced Materials Discovery / Colorado State University at Fort Collins) tackles 3D printed implants for bone regeneration. While surgeons have been using conventional implants with some success, there is always a risk of infection and the potential for lack of compatibility. Costs are high too with traditional techniques as broken bones require implants to guide new growth.

With the use of hydroxyapatite (HAp), researchers see the potential for success but a need to produce synthetic HAp implants. For this study, the team created a hydroxyapatite photo-polymeric resin suitable for 3D printing, and able to produce complex shapes without supports. Ambrosio and the researchers developed a HAp-based photopolymer slurry for 3D printing Hap green bodies:

“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,” stated Ambrosio. “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.”

Healing process of bone after fracture trauma. Day 0 to 5 represent the inflammatory stage, day 5 to 16 represent repair stage and day 16 to 35 represent remodeling stage

Bones are not always able to tolerate the stress exerted upon them causing weakening or fatigue failure. After a break, ‘an auto-activated healing restorative process’ begins to rebuild the tissue and consequently the bone. This is not always a perfect process though, and defects may emerge if the patient has compromised health or is living in a challenging environment. Overall, bone regeneration requires:

• Osteogenic cells
• Osteoconductive scaffolds
• Mechanical environment
• Growth factors

Bone healing occurs through the inflammatory stage (four days), repairing (four to six weeks), and remodeling (up to several months).

Mechanical properties of human bone.

Ideally, bone implants should be:

• Osteoconductive
• Osteoinductive
• Osteogenic
• Biocompatible
• Bioresorbable
• Not prone to infection
• Accessible
• Compatible with mechanical properties
• Cost-effective

Previous research has produced a variety of different methods to produce scaffolds, whether biocompatible, osteogenic, or resorbable. Here, the HAp slurries showed positive behavior in storage (not to exceed 20 days) and then flow deposition, involving both viscous extrusion and photopolymerization.

“It was possible to build complex structures that had complete layer cohesion with no support material. Scaffolds with different pores sizes from ~130 µm were 3D printed using 41 vol% HAp slurries. Visual inspection, SEM, and fluid flow verified that the pores were interconnected through the structures, leading to the belief that these scaffolds could be used for cell growth in orthopedic applications, decreasing the risk posed by poor perfusion during fracture healing,” concluded the research team.

“Non-uniform distribution of densification produced localize stresses that caused cracks in the parts. Future work determining a relationship between part size and sintering holding time (T1) should be addressed to obtain uniform densification and minimize cracking. The issue above, in addition to the brittle nature of HAp, showed less flexural strength than their green bodies. Thus, mechanisms for avoiding thermal gradients in the sintering process and strengthening mechanisms of HAp should be investigated.”

Bone regeneration is a source of constant challenge in the medical realm, and researchers continue to create new methods for better success, whether they are studying the potential of titanium in AM processes, the effects of annealing in bioprinting, and the uses of innovative scaffolding.