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Biomaterials


colleagues on solid-state additive manufacturing of porous Ti-6Al-4V by supersonic impact could yield huge advantages for the fabrication of functional metallic parts. Normally, fabrication involves layer- by-layer melting and solidification, but the high temperatures used can have numerous detrimental effects. They often lead to large residual stresses, poor mechanical properties, unwanted phase transformations and part distortion. “Using kinetic energy to consolidate powders is an established process,” Moridi observes. “However, this process – called cold spray – is almost always used to make fully dense structures. The novelty of our work is that we modified the process to be able to print porous structures. By optimising the pore size and distribution, we were able to not only provide space for the surrounding bone to grow inside these pores, but also to tune the mechanical properties of the implant.”


Powder particles are loaded into a supersonic nozzle and a pressurised carrier gas accelerates them at high speed. Above a certain velocity, the particles adhere to each other as they hit the substrate. By adjusting the geometry of the nozzle, which can both converge and diverge the particles, it is possible to accelerate the carrier gas and the particles, and achieve the large plastic deformation that is required for successful adhesion. Controlling powder impact velocity can produce porosity of up to 30%. “I think the most unique aspect of cold spray is that it does not melt powders to deposit them,” Moridi notes. “Therefore, the properties of powders are retained after deposition. We normally spend so much time and effort designing powders, and if those properties remain intact during processing, we know what properties we should expect in our parts.” The high plastic deformation achieved by solid- state bonding allows the grain structure of the powder particles to be refined to create printed structures with higher strength.


“Processing materials at high temperatures results in the development of tensile residual stresses that are detrimental for fatigue performance,” says Moridi. “In addition, unwanted reactions typically take place at high temperatures. These can alter the microstructure of the materials and, therefore, their properties.” The team’s work has shown that cold-sprayed porous titanium alloy can achieve higher strength than a laser-printed alloy, with post-printing heat treatment able to further improve mechanical properties, and that the resulting material is biocompatible with preosteoblast cells. “Controlling pore size and pore distribution can impact the ability of cells to migrate into the implant and make a biological fixation,” Moridi explains. “It becomes an art to control the cold-


Medical Device Developments / www.nsmedicaldevices.com


spray process to achieve a porous structure that provides a good scaffold for cells.”


A future with promise


There are trade-offs to be made with any 3D-printing technique. The solid-state bonding process on which Moridi has been working produces implants with higher strength and better biocompatibility, but does not match the build rate of traditional techniques. “There are technologies that have high build rates but can’t achieve high resolution,” she remarks. “So, it comes down to the size of the implant and the resolution of the internal features you want to have in the design.”


Stainless steel, titanium, chromium and cobalt are currently the most commonly used metals in


orthopaedic implants.


“I think the most unique aspect of cold spray is that it does not melt powders to deposit them. Therefore, the properties of powders are retained after deposition.”


Atieh Moridi


What is clear, however, is that the range of 3D- printing techniques is broadening and the implications for the improvement of implants is enormous. “We are still at the early stages of printing biomedical implants, but the benefits that this new technology brings are enormous,” Moridi remarks. “We can print patient-specific implants that ensure perfect anatomical fit or we can design the architecture of implants to match local mechanical properties of bone. More than biocompatibility, however, the design freedom in 3D printing has enabled developers to tailor properties of implants by engineering their structures as opposed to only their composition.” Many may have doubted the power of 3D printing for implanted devices in the early days, but there is little doubt now that it is evolving into an ever more powerful tool in the search for biocompatibility. ●


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Denis Simonov/Shutterstock.com


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