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Researchers at Federal University of Uberlândia in Brazil first came across the use of salt remelting on CNC Kitchen’s YouTube channel, a popular spot to view videos about 3D printing and other manufacturing technology. The process was used to improve the strength of 3D-printed polylactic acid (PLA) parts but left behind the adherence of some salt particles in the surface.
At the time, the researchers were working with 3D printing and other technologies to deposit hydroxyapatite (HA) and titanium dioxide (TiO2) in polymers.
“Upon seeing the video, we thought about using the salt remelting process with finer powders to not only improve the strength of 3D-printed PLA but also to create some type of coating, as long as the powder could adhere uniformly in the PLA parts,” said Felipe dos Anjos Rodrigues Campos, a doctoral student in mechanical engineering at Federal University of Uberlândia in Brazil.
Exploring that possibility was the first step in annealing 3D-printed PLA parts that culminated in the powder bed hardening into a coating.
The Heat Is Key
PLA is the polymer most often used for 3D printing by the Fused Filament Fabrication (FFF) process, noted Campos, who added that the material is extensively researched as a biocompatible and resorbable polymer for implants. It has a melting point of about 170°C (338°F) and is usually printed with a nozzle at 200°C (392°F).
“If you heat the polymer at this temperature interval inside some type of mold, it will retain its geometry despite becoming viscous,” said Campos, who is also studying medicine to pursue a career as an orthopedic surgeon.
His research team covered the bottom of a small metallic tray with HA and/or TiO2 powder, placed the PLA parts on the powder and covered the parts with more powder. They then placed a metallic lid in the tray above the powder and applied pressure to the lid, which caused the powder to mold around the PLA parts.
When the tray was heated to between 170°C and 200°C (338°F to 392°F), the PLA became viscous, enabling a small amount of the adjacent powder from the mold to diffuse into the outer layers of the PLA parts. The mold maintained its geometry, keeping the original geometry of the PLA parts.
When the tray was removed from the oven and allowed to cool, the PLA became solid again but presented as powder particles diffused into the outer surface like a crust. Because the process is diffusion dependent, the thickness of the coating layer depends on the time and temperature to which the PLA parts are subjected.
“Our powder bed annealing method is much more simple than other traditional coating methods that deposit ceramic powder onto polymeric substrate,” Campos said. “It can create a uniform coating around an entire part, even on complex geometries. Other techniques such as plasma spray, flame spray and cold gas shooting are line-of-sight techniques that require complex robotic arms and optimization of many process parameters such as nozzle distance, speed and path.”
Campos said the sputtering process typically does not create coatings of more than 3 μm in thickness, and sol-gel is limited to 10 μm in thickness and may also require deleterious heating of the part to solidify the coating.
“Powder bed annealing, besides producing coatings of up to 50 μm in thickness, applies heat with the part immersed in powder, which avoids heat deflection and maintains the geometry,” he said. “Electrodeposition methods, on the other hand, cannot be applied to polymers, which are natural electrical insulators.”
Enhancing Bone Growth
Powder bed annealing is a much less expensive and scalable method than other techniques because it only requires an electrical oven and a metallic tray, 100 g of HA powder and 16 g of PLA to produce four PLA parts during each annealing process.
“The average roughness parameter for samples coated with pure HA was mostly between 3 μm to 7 μm, which is close to those observed for the femoral component of the same commercial hip implant, which ranges from 4.7 μm to 5.8 μm,” Campos said. “This indicates that the surface of the annealed 3D-printed PLA parts bears similarity with those of biomaterials that are already validated for commercial use.”
The researchers conducted the powder bed annealing process under various time and temperature parameters to determine the optimal annealing condition. Some conditions achieved a robust coating free of defects and with improved Maximum Flexural Strength and higher Impact Energy than that of pure PLA.
There were no significant changes to the hardness of the subsurface and substrate, with some small reduction at the coating that can be attributed to border effects, but that pose no important harm to the coating properties.
“We are currently investigating the adhesion of the coating and have found good results, which we will be submitting for journal publication in the coming weeks,” Campos said. “However, it is important to note that there was no coating delamination of any kind, not even in the impact tests, which indicates another advantage of this coating technology.”
Campos said in vitro tests have shown evidence of improved cell differentiation thanks to the coating, which signifies that a PLA implant coated with HA and/or HA+TiO2 by the powder bed annealing process will lead to earlier osteocyte deposition of bone matrix around the surface of the implant.
This technology presents an extremely low-cost solution that’s easy to use and provides high scalability. It could be applied to well-established pure PLA implants such as interference screws used in knee surgery and anchors used in bone, tendon and ligament repairs.
It could also be applied to future 3D-printed PLA implants for patient-specific purposes, such as pediatric and oncologic surgeries, for which standard implants might not be the most effective option.
The technology is patent pending, and the research team is looking forward to working with the orthopedic industry to develop new products.
