Medical Parts Go Additive
Additive manufacturing is charting new ground in the medical field, where it offers design freedom, relatively low costs and short, even custom production runs.
By Bruce Morey
To a mechanical engineer, the human body is filled with perplexing shapes. Replacing its parts, or designing tools to operate on it, is a challenge. The body’s uneven, organic shapes are difficult to replicate with standard machine tools, which are more accustomed to cutting straight lines or drilling round holes. But additive manufacturing, which gives designers the freedom to create complex, organic shapes, seems a natural fit. As additive manufacturers develop improved processes with a wider choice of materials, applications in medical manufacturing are growing.
Surgery Leads the Way
Producing exact 3D models of a patient’s body, for surgeons to use in planning, are just one of the applications where additive is making headway, according to Terry Wohlers of Wohlers Associates (Fort Collins, CO), an independent consulting firm specializing in additive manufacturing.
Medical professionals convert CT or MRI scan data from the patient into physical models of the patient on whom they are going to operate. "They actually sterilize those parts and take them into the operating room," he said. Among the applications: craniofacial or maxillofacial reconstruction, which repairs birth defects or the consequences of cancer surgery, car accidents, or gunshot wounds, among other traumas. "These 3D models have aided teams of surgeons in dramatically shortening the actual surgical time. It both reduces cost and trauma to the patient," Wohlers said.
A leading company in this endeavor is Medical Modeling (Golden, CO), which has "done over 40,000 cases representing hundreds of thousands of models," Wohlers said.
While surgical aids and models use plastics common in the field of additive manufacturing, the development of metal-based additive manufacturing has opened up the field of metal implants to additive manufacturers, too.
Wohlers points, in particular, to metal acetabular hip cup implants. "Magnus René, CEO of Arcam [Mölndal, Sweden], estimates that about 80,000 of these have been created using the company’s electron beam melting process," Wohlers reported. The parts are made primarily from Ti6Al4V, a favorite titanium alloy used for medical implants because of biocompatibility. These hip cups, fixed into the pelvis of the patient, are the part to which the bone stem mates and rotates around. "About 30,000 have been implanted into patients; the remainder is inventory," said Wohlers, noting that doctors want them on the shelves for rapid selection when needed.
It is interesting that these are standard items. Unlike the surgical guides, they are not customized to a patient’s unique geometry. So what is the advantage in using additive manufacturing? "Speed, on-demand manufacturing, ease of producing complex shapes and features, and cost," answered Wohlers. What’s more, additive manufacturing can eliminate some finishing work required by conventional methods. "These cups need a rough, porous surface for bone in-growth," Wohlers said. Conventional methods use an expensive and time-consuming sintering process on the outer surface of a cast or forged part. Besides the cost of this secondary process, manufacturers usually outsource the work. Lead-time can be significant while increasing inventory costs. "Using additive manufacturing means you can build that surface feature into the data," Wohlers said, noting that solid and porous sections of the implant can be built in the same step.
Another area of medical production where additive manufacturing is making inroads is dental copings made from cobalt chrome, used for crowns and bridges. Wohlers said that "at least 15,000 of these are made every day," with machinery provided by EOS (Krailling, Germany). Other dental items being produced by additive manufacturing include: drill guides for placing implants, braces, partials, and dental stones.
Tech Transfer from Industrial Applications
Wohlers notes that most of the 80,000 hip cups manufactured to date use electronic beam melting (EBM), with some others made through laser sintering. These are processes that melt the powder in a bed that the machine lays down layer-by-layer. Both require elaborate support structures while the part is ‘grown.’ Arcam (Mölndal, Sweden) is a primary supplier of EBM machines. According to René, the CEO of Arcam, medical was not the company’s first thought: "We were founded in 1997 and initially we worked with the tooling industry," he said. "However, in 2003 we were contacted by the University of North Carolina, in Raleigh. They kind of pushed us into implants made with our additive manufacturing technology." After establishing the basis for the technology, they developed a partnership with the Lima Corp. (Udine, Italy). "They were the first to really get started with making implants in volumes using additive manufacturing, around 2006," René said, noting that Lima owns eight of Arcam’s EBM machines: "We believe that 2% of all acetabular hip cups worldwide are made with our machines, and most of those are made by Lima." For press-fit implants specifically, Arcam says, the EBM process lends itself to high-volume production.
René also stressed that Arcam makes many other medical components with EBM technology as well—hip cups are just the most successful at present.
With a number of customers making such implants he notes that about one-third of Arcam’s business is supplying EBM machines for medical applications. He also pointed out that the advantage these manufacturers were looking for is in making trabecular structures. These are fine, interlaced, sometimes-spongy looking bone or artificial lattices. They give structures lightness combined with strength. They also provide an ability to fuse living bone into the implant, called osseointegration. Arcam provides the ability to specify in the design of the implant pore geometry, pore size, relative density, and roughness for these structures or surfaces.
Manufacturers like to have machines tuned for their industry. This is no different for medical. René pointed out this means easy FDA or CE Mark approval as one factor. The Arcam A1 EBM system was specifically designed for the orthopedic implant industry. It boasts high productivity to create either custom or standard implants. Materials include cobalt chrome and titanium alloys. It has a beam power of up to 3000 watts in its vacuum build chamber, with active cooling for shorter build cycles. Its MultiBeam technology allows for faster builds and model-to-part accuracy of ± 0.13 mm, providing a surface finish specification of 25 Ra in the vertical direction and 35 Ra in horizontal. Its build rate is as high as 80 cm3/hr using Ti6Al4V titanium alloy.
"The real winner is design freedom when using additive manufacturing," said René of Arcam. "The value of AM is not in converting an existing process, but developing a design tuned for the process."
ExOne (North Huntingdon, PA) is another additive manufacturing company with a beachhead in the medical field. Unlike laser sintering or EBM that melt powders of metal or plastic, ExOne uses binder-jetting technology. "We selectively print a binder into a powder bed, essentially gluing the particles together," explained Robert Wood, regional manager for ExOne. The advantage of this process is the wider range of materials possible. "If you can get the material into a powder form suitable for our process, we can print it," he said. One advantage of this process over selective melting processes is that the loose powder is not disturbed, eliminating the need to design elaborate supporting structures, according to Wood. The downside is that there is a sintering post-process required—up to 1100°C—before the part is ready for use. "Still, our process can be significantly less expensive than other methods, such as laser," he said. "That is due to the speed and output of our machine, our layer speeds are much faster. Our build envelopes are much bigger so we are able to produce a lot more parts in one print."
Wood reports that his company also is in medical production, delivering, for example, finger structures for a prosthetic hand. Each finger structure is composed of 60% 420 stainless steel infiltrated with 40% bronze. "In our process, the green finger structure before sintering is about 60% density," explained Wood. "When we sinter it, we get the density up to about 90% and to prevent warpage and cracking, we infiltrate it with bronze." The company reports that their process replaced an existing investment casting procedure and is about ten times less expensive. Wood also reported that they also use the process to generate tooling for medical parts, such as molds for casting or injection molding.
The Future—Education, Materials, Data Integration
The future of these additive processes in medical manufacturing hinges on two factors, according to Wood. One is educating the industry on the power and limitations of additive technologies. The other is materials. "For direct production, binder jet technology opens up the possibility of using new materials, such as ceramics," he predicted, even bioabsorbables. Wohlers, the consultant, also sees a future in 3D printing for living tissues using resorbable or biodegradable scaffolding structures. "They could use, for example, hydroxylapatite that your body accepts, and can break down," said Wohlers. "There has been a lot of work in this area over the last 10 years."
Renishaw (Hoffman Estates, IL), long known as a provider of metrology technology, is another provider of additive manufacturing systems. Robin Weston, global product manager for Renishaw in this area, sees a future in information integration. Renishaw has a number of health technologies, including surgical planning software, implantable therapeutic delivery devices, and stereotactic surgical robots. "Our disciplines are founded on our metrology capability and our ability to control processes," explained Weston. "What you will see is us integrating the expertise in those areas and marry that up to the additive manufacturing technology." He believes providing a more complete solution, rather than selling machines with which organizations can experiment, will be the future as the industry matures. ME
This article was first published in the 2013 edition of the Medical Manufacturing Yearbook. Click here for PDF.