By Sarah A. WebsterEditor in Chief
The marriage between additive manufacturing and the medical industry has been a happy one, leading to new applications all the time. With the design freedom that rapid, 3D printing processes offer, designers and engineers can produce organic, custom shapes in an increasing number of materials that are biocompatible, such as Ti6A14V, or even capable of being sterilized, such as ABS-M30i. Parts made by additive methods are now being used for a wide variety of purposes in the medical field—from surgical aids and medical tools to orthopedic implants and dental prosthetics to be used in humans. The use of additive manufacturing in other medical areas continues to grow rapidly too.It’s little surprise as to why: Additive processes can build complex human shapes, even with particular finishes, that may take more steps and time to produce—or may even be impossible to produce—with traditional subtractive or formative methods. Take, for example, trabecular structures, which are fine, lattice-shaped structures that allow living bone to fuse to an implant, a process called oseointegration. Arcam AB (Mölndal, Sweden), which specializes in electron beam melting (EBM) additive manufacturing, now allows the ability to specify pore geometry, pore size, and density and roughness of structures for trabecular structures and surfaces.What’s more, additive manufacturing can produce parts for relatively low cost, in small, even custom, production runs and make quick, easy adjustments as patient’s needs change, with, say, growth.That was the case for Emma Lavelle, who was two years old when her life-changing adventure with additive manufacturing began.
The story of Emma Lavelle and her “magic arms” captured media attention in 2012.While simple in many respects, the story is also an instructive case study on the benefits of additive manufacturing in the medical industry and the importance of orientation design and material choices.Lavelle was diagnosed with arthrogryposis multiplex congenita (AMC), a nonprogressive condition that causes stiff joints and very underdeveloped muscles. Emma was born with her legs folded up by her ears and her shoulders turned in. After surgery, development was slow, as she was unable to play and interact with her environment in normal ways.When she was about two, Emma was introduced to the Wilmington Robotic Exoskeleton (WREX), an assistive device made of hinged metal bars and resistance bands. Working much like a swing arm on a lamp, it assists kids with underdeveloped arms to play, feed themselves and hug. It was designed and patented by Tariq Rahman, head of pediatric engineering and research, and Whitney Sample, research design engineer, both at Nemours/Alfred I DuPont Hospital for Children (Wilmington, DE).The aluminum and steel device was a lifesaver for Emma, who was able to pick up objects and lift her arms toward her mouth for the first time.But there was a problem. The WREX worked fine for children as young as six who were bound to wheelchairs, which could help support the relatively heavy metal structure. But for little Emma, just 25 pounds and walking, it was too heavy. At first, her medical team at Nemours attached the device to a stationary support, which was too clunky for Emma to wear outside the hospital workshop.As Rahman and Sample began to look for solutions, they quickly turned to a tool they already used at the hospital for protyping, a Dimension 1200 3D printer from Stratasys (Eden Prairie, MN). Stratasys developed fused deposition modeling (FDM), an additive method that lays down, or extrudes, thermoplastics layer by layer through a heated nozzle. Stratasys recently merged with Objet (Rehovot, Israel), which uses ink-jet polyjet printing, a process that uses photopolymers cured by UV light.“We required something light and small that would attach to her body,” Rahman said in a video about Emma’s WREX.So they printed a prototype WREX in ABS, the same material used to make prototypes and build Legos. They also made Emma a plastic vest to help support the arms.
However, it was clear that Emma would need a stronger WREX as she grew and used the frame in the ways typical of a growing child.Rahman and Sample turned to Noah Zehringer, a senior application engineer at Stratasys, for support. “I worked with them to refine the way they were printing the components,” Zehringer said. “The way you print them can make them stronger.”Several materials and printing orientations were tested for strength using load tests. In all, four different sets of components were tested until failure. The first set of parts was built in a flat orientation. The second set was built with default parameters in an alternative orientation to better handle the forces acting on the component. The third set was built using parameters optimized by Stratasys’ Insight software. The fourth set was built in the same manner as the third set but with a new material, ULTEM 9085. “One of the critical factors is the orientation and so just by changing the orientation of the part, you can get different properties,” Zehringer said. “It’s all geometry-dependent.”One way to think of strengthening a part through build orientation is to think of building a pen horizontally (X axis) or vertically (Y axis). An object built with additive methods is weakest where the layers are joined together. So a pen would be easier to, say, snap in half if it is built vertically rather than horizontally or in some other alternative axis.In the component tests, Zehringer said, “Simply changing the orientation yielded a 48% increase in strength.” Using Insight software to optimize the toolpaths yielded a strength increase of another 47%, for a total 117% increase in strength from the original part. Changing ABS to ULTEM 9085 yielded a strength increase of 124%, or a total of 387% over the original part. ULTEM 9085 has a high strength-to-weight ratio and has an FST (flame, smoke, and toxicity) rating. The material’s preexisting certifications make it an excellent choice for the commercial transportation industry—especially aerospace, marine and ground vehicles. The only Stratasys systems that build with ULTEM 9085 are the Fortus 400mc and 900mc.The ULTEM 9085, a material that is manufactured by SABIC Innovative Plastics (Pittsfield, MA), was almost as light as the ABS, too, with a specific gravity of 1.34 compared to 1.04 for the ABS. “It’s very similar in weight,” Zehringer said. “It’s slightly heavier, but much stronger for its weight.”Ultimately, Emma, now about five years old, ended up with a strong, light WREX to help her navigate the world like a normal child.Today, nearly 50 children use custom 3D-printed WREX devices.When Emma grows out of a part or breaks a piece of her WREX, Sample said, “I don’t have to worry about lead time to machine something.”In fact, Nemours just purchased a Stratasys Fortus 400mc. Sample uses the Dimension for his prototypes and the Fortus for actual parts. “I’m going to be using them both out the wazoo,” Sample said.Once only able to do three–four WREX devices a year made out of aluminum and steel, he now does about one a week in ULTEM 9085. The difference in workload and productivity, Sample noted, is “scary.”
With a fast-growing list of additive manufacturing materials that are certified for medical use, additive manufacturing is expected to continue its fast growth in the medical field.While Emma’s WREX is an exoskeleton and has different requirements than a Class III medical device, Zehringer said Stratasys has two biocompatible thermoplastic materials, ABS-M30i and PC-ISO. Other additive manufacturing machine makers are also adding to their material lists all the time.Eventually, Terry Wohlers of Wohlers Associates (Fort Collins, CO), an independent consulting firm specializing in additive manufacturing, said that the 3D printing of living tissues and biodegradable structures will replace defective and damaged body parts.Sample said additive manufacturing has been a godsend in his field. Said Sample: “This is one of these industries that matches perfect with 3D printing—additive manufacturing—because we need custom everything.” ME
This article was first published in the April 2013 edition of Manufacturing Engineering magazine. Click here for PDF.