by Kip Hanson
Additive manufacturing promises to make metal implants obsolete
Physicians have used metal to repair the human body for over a century, beginning with vanadium steel in the early 1900s.
That corrosion-prone material has since been replaced by a number of high strength, low corrosion alloys – stainless steel and cobalt-chrome are favourites for orthopaedic implants, and titanium screws and bone plates are common. Despite their life-saving attributes, however, metal implants sometimes don’t sit well in humans: allergic reactions, heavy metal poisoning, discomfort and temperature sensitivity are some of the complaints raised against these reconstructive medical products. “Metallic-type implants can be problematic,” says Dr. Ehsan Toyserkani, associate professor of the Multi-Scale Additive Manufacturing Lab at the University of Waterloo. “After ten years or so, they may have to be replaced due to inflammation, calcification, and so forth. Also, machined metal surfaces do not provide an ideal surface to which bone can adhere.”
If Toyserkani and his collaborators have their way, the decades-old use of titanium in the human body will one day go the way of mercury treatments and bloodletting. His lab’s Additive Biofabrication project is designed to use 3D printing in a novel approach to implant building. Known ast (SFF), Toyserkani has shown that it is now possible to print complex, anatomically shaped “scaffolds” from calcium polyphosphate. “This is a biodegradable material with very good mechanical strength. Using a two-step process including our 3D printing technology and a sintering protocol that is a proprietary of our collaborator at the University of Toronto, we are able to fabricate complex-shaped implants with internal channels and porosity similar to bone,” Toyserkani says. In collaboration with Mount Sinai Hospital of the University of Toronto, some types of these implants were then placed in a culture of cartilage cells prior to “in vivo” testing in rabbits and sheep. The results were impressive. “The original calcium polyphosphate scaffold was replaced by natural bone and cartilage after a while.”
This doesn’t mean you can buy a rapid prototype machine and print up the next Frankenstein’s monster. Toyserkani has developed a special hybrid technology that combines two different types of additive manufacturing, Z-Corp printing and fused deposition modeling (FDM). “We spread powder across the larger cross sections of the slice as you would with a conventional Z-Corp machine, but in the same process we inject sacrificial photopolymers inside the narrow areas followed by UV light curing,” Toyserkani explains.
In a process similar to lost wax casting, the printed part is placed into a furnace, which burns away the photopolymers and leaves behind small gaps or channels. This is what makes the process unique. “Other types of 3D printing cannot develop the very fine detail that we can. This is applicable not only for biodegradable components but also generic ceramic and metallic parts. There are a number of materials that would benefit from this type of porous structure – batteries and fuel cells, for instance, or even piezoelectric sound sources, where you can actually enhance the sound quality of the systems by controlling the density and internal structure of the material. I think the technology is unlimited in terms of market application. Our 3D printing system can also be used for printing of a variety of hard and soft tissues with heterogeneous internal structures. This means scientists may one day be printing implants using live cells and biological polymers for use in the human body.”
Dr. Toyserkani is a busy man. Aside from his work on additive implant manufacturing, he’s also doing research on smart cutting tools, laser-based micro-texturing, and maskless microdeposition for use in ultra-small sensors. He is also developing a laser-based solid freeform fabrication process, or LSFF, which deposits metallic particles onto a substrate and then melts them. “We have been working on a variety of alloys, including titanium and tungsten carbide-reinforced in cobalt, and can build parts with finer detail and less thermal stress than what is currently commercially available.”
Through it all, Toyserkani remains a teacher at heart. “These things take longer in a university because you are working with students, not engineers, where the resources are very limited. That’s just the nature of academia.” Still, he sees a day when additive biofabrication will be commercially viable. “We are working hard to take it out of the university. Mount Sinai Hospital is extremely happy with the performance of our implants. We filed a patent for it about three months ago, and are now approaching companies to see if we can actually take it to market. So far, it looks very good.” SMT
Kip Hanson is a contributing editor. [email protected]