Foundation and Framework: Materials and Processes Evolve for 3D-printed Lower-limb Prostheses

Derick Schmidt, MPO, CPO, Limb and Lattice, Shore P&O, and Presque Isle Medical Technologies, US Virgin Islands, gives a reminder that “while any device can break if not manufactured well, digital design and manufacturing allows for better and more reliable testing than traditional handmade manufacturing does. It’s important to recognize and remember that ‘3D printed’ covers a huge range of processes and materials, which all need to be assessed individually and judged on their own merits.” Joe Johnson, CP, owner of Quorum Prosthetics, Colorado, refers to his first transtibial wooden leg in 1984 as Civil War technology. He’s actively participated in the evolution of additive manufacturing technology ever since, notably as an early adopter who purchased the first 3D printer to use carbon fiber ten years ago. “I thought this was going to revolutionize prosthetics. So I bought their machine. When I started using it, I really couldn’t do anything with it, because it was largely one dimensional.” Johnson continued to develop prosthetic solutions based on his own experience, like the 3D-printed Quatro socket, which was initially created from carbon fiber using the standard wet lamination method. But he found that even buying his own HP Multi Jet Fusion 3D printer plus software, mechanical engineers, and other equipment wasn’t enough without biomechanical engineers who had the technical knowledge to design sockets. Now, in collaboration with HP and their proprietary Multi Jet Fusion 3D printing technology, he continues to pursue fresh 3D printing approaches to lower-limb prosthetic and orthotic solutions. Systems The HP Multi Jet Fusion 3D printing process uses powdered thermoplastics and builds parts layer by layer in a large bed of powdered material, with additional steps that provide seamless postprocessing. The system evolved from similar powder bed fusion systems like selective laser sintering (SLS) that employ single rather than multiple jets in their process. “One of the biggest innovations in the 3D space that has occurred in the last five years is that of the development of area-wide based printing with robust material properties,” says Dustin Kloempken , 3D Multi Jet Fusion printing application engineer, HP. “Many of the 3D technologies in existence today use a point to trace out a part on each layer. SLS technology uses a laser point to trace out each part on a layer. Although SLS is quite fast, the more parts you have in a layer, the longer it takes for the laser to trace out the part. HP’s Multi Jet Fusion technology lays down a layer of powder and fuses an entire layer of parts (regardless of the number of parts) anywhere from eight to 12 seconds per layer. This allows a user to pump out a lot of parts quickly versus what a point-based 3D printing process can do. A similar technology would be SAF ( selective absorption fusion ). “Because you can print more parts faster, the economies of scale can become significant such that part pricing is beginning to bump up close to injection molding, with the added advantage of getting parts in hand faster). As a result of all of these benefits and added ability to produce parts of complex nature (like a latticed flexible part interior), HP is seeing a fast adoption of this technology globally,” Kloempken says. “There are other 3D printing technologies,” says Eythor Bender, cofounder and vice president, UNYQ, “but HP has just emerged as the one we’ve been very happy with. But it’s been a long journey; people talk about the revolution in printing for prosthetics, but it’s not the revolution, it’s more an evolution, I would say. We see printers improving, software improving, scanners are improving steadily, year by year, and getting cheaper and more economical.” He cites other historical and current leaders in the field: EOS, a leading supplier of industrial 3D printing technology since 1989, pioneer in the field of metal 3D printing (direct metal laser sintering, or DMLS), and he also credits other innovators of 3D systems like inventor Chuck Hull, who earned the first patent for a “stereolithography apparatus” in 1983. Hull’s South Carolina-based 3D Systems recently collaborated with Point Designs, an innovator in the field of partial hand prosthetic design, to transition from a 3D-printed steel partial hand prosthesis to one printed with lighter weight titanium. Joe Fairley, CP, board-eligible orthotist, founder, Ascent Fabrication, New York, describes another 3D printing system he employs for clients who send him their digital designs for fabrication. Fused deposition modeling (FDM) builds its final products by creating them layer by layer, with spooled plastic filament. Fairley describes the FDM process: “Typically you’re printing an inner and an outer wall,” he explains, “and you may or may not have an in-fill pattern at a certain percentage. That bond between the inner and outer wall and between layers are going to be your weakest points. So it comes down to a math equation: the thickness of the walls has to be a multiple of your nozzle size, and the layer height needs to be equal to or slightly less than half your nozzle size. “We’re trying to bring the more consumer-level 3D printing form, FDM, to a more definitive nature. The first consideration is materials. Recent developments in material choice, discovered while working with companies in different countries, and certain design considerations, have now made it possible for us to use FDM and polypropylene for definitive lower-extremity prosthetic sockets and ankle-foot orthoses. In some cases, companies are also trying to introduce chopped carbon fiber into a mix of polypropylene or copolymer to produce definitive style devices, which could be printed by FDM machines. “These machines, priced at around $30,000, are a lot cheaper than the Multi Jet Fusion or SLS machines, which sell for close to half a million dollars. There’s a big price discrepancy there, driving what’s slowly happening right now: More of these $30,000 machines are finding a place in US clinics, where they have been primarily used for lower-limb test sockets. They’re able to use the material PET-G, which is an industry-standard polymer, which goes by the trade name of Vivak—and is very similar to material used in our field for lower-extremity prosthetics.” Materials Tony Gutierrez, CP, national clinical specialist, Bionic Prosthetics & Orthotics, Indiana, recalls that Bionic got its first 3D printer five years ago, as part of a partnership with Create 3D Prosthetics. “It took about 48 hours to print a BK check socket and wasn’t very practical yet; the quality was very low, and one of the issues that we discovered was that 3D printers were really good at using a material called PLA, which is something that we do not traditionally use in prosthetics. It’s very brittle, very thin, not very strong, and it’s not translucent, so it’s not a good choice for a diagnostic socket. It is not a material that O&P clinicians are accustomed to working with, and this led to some delays in adapting it into our regular clinical practice. That was our first step in the direction of 3D printing, and we’ve come a long way since then with new materials and methods that have helped us integrate it into our practice.” Bender agrees that materials have come a long way, and prospects are exciting. “I think the evolution of new materials for 3D printing is one of the most positive developments within the last few years, and I see that escalating in the years to come. There has been much more development in plastic and metal materials than in 3D printers; you see companies that used to bring out new material every year—now they’re bringing them out every month. “We see a lot of excitement at international trade shows, and that helps us as we move between materials much faster than before,” he says. “We used to print everything with polymerized plastics, but now we’ve opened it up and are using more flexible materials, which are very useful in prosthetics. The durability has also improved. “Postprocessing is another important part of printing,” he explains. “You can’t just take these materials as they come out of the 3D printer and start using them. You have to clean up the printed item, either paint it, spray it, or finish it in some way, and that has also become easier with the new materials, because the manufacturers of those materials have thought it out more thoroughly from a postprocessing perspective. That makes it easier and faster to finish the product and send it to the end user.” Do these new materials replace or compare with carbon fiber? “We’ll see more in the future,” Bender says, “but at the moment, there’s nothing that can economically replace the carbon materials we use for feet and such. We’re not there yet, but it’s just a matter of time.” Most 3D products, including the UNYQ 3D-printed prosthetic foot he developed in collaboration with the Department of Veterans Affairs, are not completely 3D printed, he points out. “It’s a bit of convergence of technology; our foot is a combination of the 3D-printed shell and a mechanical ankle. So maybe it’s not our goal to 3D print everything but to focus really on parts and pieces where 3D printing enhances the experience.” “When it comes to materials,” says Kloempken, “I think the materials we have right now are fantastic for the problems that we’re trying to solve. Would it be great to have other materials in the future? Absolutely. But Quorum, for example, has printed a flexible inner liner that is breathable, and then a hard-shell exterior for impact, so you’ve got both comfort and strength. I don’t think we need more materials; we just need to execute on all of it now.” Current advances have improved the quality of materials and parts that can now be printed with isotropic properties, i.e. equal strength or elasticity in all directions. “Our Multi Jet Fusion printer in particular is not quite 100 percent isotropic,” Kloempken says, “but it’s within a percent or two. Most people find that’s acceptable for their purposes. It is a big step forward over other printing technologies because we’re literally intermingling and interleaving layers rather than maybe only securing one layer to the previous one; that’s where we can get these amazing strength properties. “Really, the limitation today is in design, and testing those designs in a way that demonstrates that they’re strong enough or appropriate enough for a specific patient.” Fairley addresses concerns regarding the strength of materials used in the additive manufacture of weight bearing lower-limb sockets, pointing out that PETG has been used successfully over the last five years, but its use is limited to test sockets. “Typically, we’re not sending a patient out of the office with a PETG socket. If we do, we’ll fiberglass reinforce that socket like we would a typical test socket, and it’s only going to be used for one to four weeks. And during that time, the socket will break, depending on the activity level of the patient. The longest I’ve had an active patient on one of those sockets before it breaks, with fiberglass reinforcement, is three weeks.” Regular inspections of the PETG socket, however, would reveal some stress marks indicating that it could break. “That’s why we typically don’t want to put a patient out on that particular type of socket for any amount of time. We would rather put them into a preparatory socket, which is typically polypropylene. At Ascent Fab, we’ve been putting patients out on our polypropylene sockets for more than a year—without indications of degradation. Other clinics are also putting their patients out on these sockets for a similar length of time.” At this point, if the patient is still giving good feedback and if the clinician agrees, the patient is allowed to continue using the socket, Fairley notes. He is arranging to begin testing with manufacturers who have cyclic machines and reveals that Ascent Fabrication’s 3D-printed lower-extremity orthotics—solid ankle and articulated AFOs—were previously tested. “We tested both SLS PA11 as well as our in-house FDM polypropylene, and they lasted more than a million cycles with no signs of degradation.” He anticipates similar results for the printed socket designs. As different methods and types of materials evolve overseas, he points out an interesting discrepancy between PA11 and PA12, different strains of nylon thermoplastics. “Both of those materials are being printed by either Multi Jet Fusion or selective laser sintering. PA12 has been more abundant in the US because of its other 3D printing applications. And it is by far one of the cheaper materials that you could print with a Multi Jet Fusion printer. There are more Multi Jet Fusion printers in the US than there are selective laser sintering printers, and for those reasons PA12 Multi Jet Fusion has become the first choice of some people who are printing definitive devices. “However, I don’t prefer PA12 because I’ve seen it completely shatter catastrophically on failure, while PA11 is a bit more of an elastic material and has a higher elongation to break. Because of that, it’s a bit more robust; you’ll see stress marks on the socket or orthosis before anything breaks, and it yields rather than shatters,” Fairley explains. “PA11 is a bit more used through SLS machines and more used in Europe, but that’s largely due to a supply and demand aspect, depending on what printers are where, what companies are placing on those printers, and what those printers and materials are being used for. It’s only by opportunity that HP has brought Multi Jet Fusion to the US. They’re pushing PA12 because it’s cheaper and a little bit easier to print, and since it has a quicker cooling time, objects could be printed a little bit faster. “I’m going to stick with PA11 because of its superior structural capabilities, however.” Sean McClure, BME, ME, R&D engineer and director, Quorum Prosthetics, points out that the industrial strength materials used in printing their Quatro socket are readily available to clinicians or anyone interested in getting started in 3D printing. “What we’re using is a kind of a nylon plastic that’s comparable to some of the thermoplastics that practitioners are used to using in sheet form and thermoforming it over a mold; but there are some differences. We’re using the HP Multi Jet Fusion technology, so only certain materials with that technology are available, even to us. So we’re using that nylon PA12, which is polyamide. I’ve seen other people use PA11, which is a little bit more flexible—just enough so that when it does fail, it’s not catastrophic. “For other elements in the Quatro, we’re using a TPU, which is a thermoplastic urethane, and that material is more of a comfort-aiding material. It’s not used for any structural components. These materials that we use are completely validated. There are new materials that are coming out, and they’re researching them, but it’s not available to the public…until those materials are validated through the manufacturers of that material.” Processes Filament Innovations provides the “extremely robust” FDM printers Ascent Fabrication uses and is the source for the strongest polypropylene FDM devices, Fairley says. “If someone already has one of these printers, we can then go in and show them how to design different devices to print on these printers, and how to use different materials. We also print in some flexible materials that are mainly TPU-based. “Using TPU, PETG, and polypropylene, we can teach them about the different nuances of how we have to design a 3D-printed device, as well as how they end up printing, and we supply them with all those materials and ongoing troubleshooting support.” But the materials alone are only part of a successful print execution, he cautions. “When we look at the print orientation of parts, whether it’s with Multi Jet Fusion or FDM filament printing, the orientation of the part is also crucial to its structural stability. And those print settings, especially with FDM, are crucial to get that recipe just right to get a strong enough part. So in terms of the mechanical characteristics of something being isotropic and the layer lines having good interbonding strength with one another, again, it’s a knowledge in design level of education that is slowly rising as well. As we continue to print, we continue to learn what settings to adapt to make these devices stronger—it could be as simple as having one setting’s check box checked or not, in order to get a good part.” McClure notes that in view of the complexities involved in the process and the constantly evolving body of knowledge, “Clinicians who want to get into 3D printing may choose to work with service bureaus, or they can use us at Quorum to get parts and our TPU material. We’re still in control of manufacturing it, and all of our quality management systems are applied when we produce parts out of these materials.” Shane Grubbs, MPO, CPO/L, ProCare Ottobock.care, Georgia, explains that ProCare has been a part of Ottobock Patient Care since 2021, and has been exploring fitting patients with printed technologies for the past three to four years. Grubbs finds that scanning the limb and modifying through CAD hasn’t yet been able to produce the quality and comfort ProCare aims for, but some of the hybrid systems and printed technologies have been successful. “We have been looking for more dynamic solutions in the printing world,” he explains, “which is what led me eventually to designers like Brent Wright, Advanced 3D. Although we are seeing more and more sockets strong and durable enough to use in clinical practice, our successes with complicated designs and prints have been through collaboration with Brent and insightful support from Derick Schmidt, as well.” “With increased interest in 3D printing, you are seeing more focus on scientific testing for strength and durability. We also employ real-world testing, like the athlete who put over 1,000 miles while running on our socket created with Brent Wright.” That socket combined a flexible component with a printed frame, which Grubbs notes is not uncommon with traditional sockets. “Some manufacturers have been doing printed definitive sockets for quite a few years. I came into the industry when people were using a lot of copoly sockets fabricated in the traditional manner; and the plastics were quite strong. One patient had been wearing the same copoly socket for almost ten years.” Today, Grubbs reflects, similar copolymer materials are used to create the 3D-printed sockets, but they still encounter objections from clinicians who don’t want to provide a “plastic” socket or worry about the finished aesthetics of the printed socket. Grubbs grew interested in the 3D process and possibilities while creating flexible laminations with carbon, and a dynamic inner flexible, making the lamination flexible over the top ½ or ⅓ of the socket, so it was more dynamic with their knee movement, he explains. “Those laminations are fairly thin at the top. You can almost fold them in on themselves; but they can be tricky to fabricate because of the way the resin has to bond to the inner plastic that’s really thin. “In the past some lamination schedules were really thick, even for patients that don’t weigh that much; we’ve probably been overdoing it in an excess of caution,” Grubbs says. “Since there hasn’t been a lot of testing on fabricated sockets, people do what’s necessary to be safe, and as artisans, they marry that with science and try to build a solution specific to their patient. “I think success with exploring that in the past makes fitting the 3D sockets a little easier,” he notes. Gutierrez says that Bionic P&O, which specializes in FDM printing, is also partnering with Filament Innovations to develop a filament that can be used in a 3D-printed definitive lower-limb socket. “We haven’t been able to print anything that we’d be able to call definitive yet. But we’re utilizing a copoly-based filament that was subjected to an ultimate stress test as part of a comparative mechanical testing study of four lower-limb sockets conducted by the Center for Lower Extremity Ambulatory Research at Chicago’s Rosalind Franklin University.” (See A Question of Quality: Measuring Strength and Durability in 3D-printed Lower-limb Prostheses - The O&P EDGE Magazine (opedge.com) ) One of the more promising results Gutierrez reported from that study—aside from the confirmation that when carbon fiber breaks, it’s a catastrophic break—is that carbon fiber will be strong until it breaks. It also confirmed that while the copoly will break eventually, it exhibited a strong bend-before-it-breaks reaction in the study—so the break could be anticipated, predicted, and prevented. He questions why, however, that Vivak or PETG is a standard in regular fabrication and now in 3D printing, as well. “There’s never been any ISO testing on it in standard fabrication. That standard was just proven by the test of time. But there is now some ISO testing data at one point, called the Ultimate Force Test—that shows that 3D-printed sockets can be equivalent to traditionally fabricated sockets. The data in our study showed that PETG and 3D-printed PETG sockets broke in similar conditions, but the traditionally fabricated sockets broke differently, whereas the 3D-printed PETG sockets broke in a consistent manner. This lends some credit to the repeatability in fabrication of a 3D-printed PETG socket. “So it really is a question for everybody: They’re claiming that carbon fiber is the strongest, or claiming that PETG standard fabrication is stronger than 3D printing, but there isn’t any data out there to confirm it.” Judith Philipps Otto is a freelance writer who has assisted with marketing and public relations for various clients in the O&P profession. She has been a newspaper writer and editor and has won national and international awards as a broadcast writer-producer.

Foundation and Framework: Materials and Processes Evolve for 3D-printed Lower-limb Prostheses

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