As 3D printing expands in medical, new materials are raising the bar
Materials engineered for use with specific printers and qualified for verifiable repeatability and accuracy help ensure long-term mechanical properties ranging from heat resistance to biocompatibility.
This compatibility of material and machine is especially critical as increased serial production with additive spurs innovation of higher-volume printing platforms. For instance, the Figure 4 platform by 3D Systems—available in standalone, modular and production configurations—has a growing array of liquid polymers designed exclusively for its non-contact membrane technology. And at IMTS 2018, HP unveiled its Metal Jet platform to complement its successful Multi Jet Fusion for plastics.
Metals and plastics for customized hip and knee implants, dental devices, surgical guides and pre-surgical medical models are well established. More than 10 years after Italian surgeon Dr. Guido Grappiolo implanted the world’s first 3D-printed hip cup, the Delta-TT Cup (TT stands for Trabecular Titanium), GE surmised in a March 5, 2018, blog post that more than 100,000 hip implants have been made on its recently acquired Arcam electron beam technology. That doesn’t include hundreds of thousands of implants printed on other companies’ equipment.
New materials are being engineered for an entire new health-care paradigm—one in which implantable and wearable devices, remote diagnosis and care and robotic surgeries feature prominently. Applications range from grips or guides to orthoses and prostheses, protective equipment and more complex products and parts.
“Materials for 3D printing have been limited due to technology and resources of 3D printing OEMs,” said Thomas Murphy, senior product manager of physical products for 3D service provider Shapeways. “We have seen a growth of material companies partnering with OEMs to provide a wider range of materials. As many forms of traditional manufacturing, such as injection molding or machining, have been around for decades, there has been much more time to develop the processes to use a broader range of base materials.”
Ultimately, “advancements in additive manufacturing and material science have made it possible to build stronger parts with greater complexity and this will continue to push the industry toward digital manufacturing,” said Lee Dockstader, director of market development for HP 3D printing and digital manufacturing.
World of Choices
With each passing year, 3D printing suppliers’ portfolios of materials grow. The choices are many: variants of PA 11 and PA 12 nylons or polyurethanes; titaniums, stainless steels, and nickel and aluminum alloys; or more exotic creations featuring copper and tungsten. Design engineers can solve for various property requirements: rigidity or flexibility, resistance to heat or impact, clarity and biocompatibility.
Whether developed in-house or in partnership with brand owners, materials are generally tuned specifically for suppliers’ own machines to ensure parts are repeated accurately within the same printer or from printer to printer.
A case in point is the Figure 4 platform, which is quickly finding favor in a range of volume production applications thanks to an expanding portfolio of materials.
In 2019, 3D Systems unveiled several new liquid resins for the Figure 4, in which an oxygen-permeable membrane increases polymerization of photopolymers. Figure 4 MED-AMB 10 (amber colored) and Figure 4 MED-WHT 10 are engineered for autoclave sterilization and biocompatibility, meeting ISO 10993-5 and -10 standards for cytotoxicity and skin sensitivity, respectively. Figure 4 PRO-BLK 10 and Figure 4 RUBBER-BLK 10 also meet those standards but are not limited to medical applications, said Marty Johnson, technical fellow for print process.
The concept of production application materials “is thrown around a lot,” Johnson said. “However, materials need to have long-term environmental stability. If I create a medical device with a two-year shelf life, I need to know that my mechanical properties are going to remain after those two years. Or, if the customer opens the package, it will have the same properties as it does two weeks after I print it. These materials are engineered to have that long-term capability,” including UV and humidity stability.
On the Figure 4, he continued, all mechanical properties are printed in the Z axis, which lets customers set parts up for quality, speed and efficiency and obtain the baseline properties based on material data sheets. With these materials, 50 μm layers are standard for the best balance of print quality and speed, but layers as thin as 10 μm are possible.
Figure 4 materials generally print at up to 40 mm an hour; Figure 4 PRO-BLK 10 allows printing up to 60 mm an hour. “When we started projector-based printing in 2007, we were going for half an inch (12.5 mm) per hour,” Johnson said. And Figure 4 forgoes thermal post-curing, requiring just a UV post cure of 60 to 90 minutes, depending on material, and improves accuracy.
HP’s Multi Jet Fusion (MJF) printers “are changing the orthotics industry,” Dockstader said. One company builds its braces by adding layers of material as thin as 80 μm each—the thickness of a sheet of paper. “This creates a fit matching the unique bone, tendon and muscle contours of a patient’s foot. The result is a lighter, more durable device.” ActivArmor uses MJF technology to create external limb and spinal support devices from high-temperature thermosetting plastics “similar to LEGO, which are waterproof and customized to suit the patient.” Meanwhile, Glaze Prosthetics produces customized devices with MJF “that are lighter, less expensive and more comfortable.”
With metals, HP has partnered with Parmatech, which produces low-cost, high-volume metal parts for customers including OKAY Industries and Primo Medical Group. “It will be using HP Metal Jet technology to produce parts such as surgical scissors, endoscopic surgical jaws, and new applications and geometries not possible with conventional metal fabrication technologies.”
Formlabs, which entered the dental market in 2016 and the health-care market in 2017, creates materials for its Form 2, Form 3 and Form 3B machines for applications in surgery, radiology, veterinary medicine, medical education and medical devices.
“Most models of bony structures are printed in white or rigid resin; organ, vascular, and cardiac models benefit from the pliability of elastic resin; and medical research applications often use biocompatible surgical guide resin,” said Director of Healthcare Gaurav Manchanda. “Medical device engineers often use our engineering resins (durable, tough, gray pro, rigid) for device R&D and prototyping, with more production cases expected this year thanks to the completely new Formlabs Light Processing Unit.”
Formlabs also developed draft resin optimized for fast printing at up to 300 μm layers. “Draft parts can print three to four times faster than our typical materials, which allows multiple iterations of larger parts in the same day.”
GE Healthcare, which uses a range of 3D printing methods and materials, increasingly finds itself looking to tailor unique material solutions to emerging applications, said Jimmie Beacham, executive chief engineer for advanced manufacturing who runs a special GE lab in Waukesha, Wis.
“With subtractive manufacturing, there are nearly limitless materials you can choose; compare that with what’s available to work with printers today, and that’s still a small fraction,” Beacham explained. “Progressive companies are going to have to figure out whether they wait for the material to show up or go after it themselves.”
By the end of 2019, GE Healthcare put more than a dozen 3D printed parts into production from a target list of about 100 parts. Beacham expects up to 20 more print parts to come online this year. Achieving GE goals requires a database of “dozens” of unique material recipes.
“Half of what we’ve launched are biopharma components made on our SLS (selective laser sintering) platform with nylon,” he said, and a lot of work centers on high-efficiency aluminum heat exchangers featuring complex manifolding for cooling passages. “A lot of our projects are tied to radiation shielding and collimation for imaging. Many parts in image chains are being redesigned using additive principles for controlling and shielding the X-ray path.”
Besides employing aluminum, stainless steel and tungsten for direct metal laser melting, “we’re looking at copper, but breakthroughs have to happen at the machine level. We have some one-off situations where we make special alloys. There are niche applications where we mix trace elements with stainless steel or tungsten to get different properties.”
With SLS, GE uses polypropylene for a tiny fraction of products. “If we break the code on using polypropylene, that could be big. The challenge is that each OEM that makes additive machines has tuned their machines with a particular powder or set of powders. Polypropylene doesn’t have the same flow or melt characteristics as other powders, and there isn’t a big base of suppliers making polypropylene powders that will work with existing equipment.”
For stereolithography, GE uses UV-curable binder from Somos. GE worked with them to create the liquid, suspended with ceramic particles, to print injection molding tools; ceramic improves thermal conductivity and tool life.
And with binder jetting, GE employs stainless steel and tungsten. “With most of our projects, we’re not sintering to one solid piece—we’re infiltrating it with another metal. Often we’re using copper as the secondary metal; we add it in post-processing. Copper works great for heat transfer, and tungsten is a great radiation shield, so the combination works well. It’s got really good machinability characteristics for post-processing.”
To usher in new biocompatible resins, Formlabs acquired Spectra Group Photopolymers in November. Spectra has been Formlabs’ primary material supplier of proprietary resin materials since 2012. The union has already generated surgical guide resin, while other resins in development will enable a range of dental and medical applications, Manchanda said.
Materials engineered for Carbon’s Digital Light Synthesis (DLS) technology process are optimized for the company’s layerless technology. Carbon’s proprietary Continuous Liquid Interface (CLIP) keeps material flowing between a curing part during printing. DLS and CLIP allow printing from 25 to 100 times faster than other methods depending on geometry, said Jason Rolland, Carbon’s senior vice president of materials.
Among the projects Carbon has on the horizon is a collaboration with Johnson & Johnson to explore applications for “a stretchy, bioabsordable elastomeric material” useful as a wound-healing device that can be implantable and dissolve in the body over time like a suture would.
“Our approach to materials is to have one big dual-cure platform, where we can use a blend of UV curable and thermally curable chemistries,” Rolland said. “That lets users greatly expand the scope of materials and formulations that they can work with and has allowed us to do some really incredible things with polyurethane chemistry, epoxy chemistry and high-temperature thermosetting chemistry like cyanate esters. That gives us the best of both worlds: Very fine resolution but also the mechanical properties and material properties needed for end-use parts.”
This is in contrast to powder systems, which “make the FDA very nervous because you have a lot of stray powder that isn’t sintered into the part,” said Rolland.
Given Carbon’s expertise in elastomers, “we’re looking at a lot of wearable applications where you need the softness and durability of an elastomer—whether that’s interfacing with your ear or with your mouth or other parts where you need that customization. Our lattice designs are incredibly exciting in this area, where you can think about something like an elastomeric foam that has uniform properties across the entire part that are unchanged vs. a digital lattice structure where you can have areas of lower or higher density thicknesses of the struts in the lattice that let you specifically control the mechanical properties within discrete areas of the part.”
That flexibility came into play with the football helmets Carbon collaborated on with Riddell. “You can have certain pads within the helmet that are stiffer than others, and that’s all designed around data gathered from what the player needs. There are a lot of exciting areas developing in the area of protection, and many of those applications are medically relevant. Durability and skin compatibility is key.”
For health-care facilities, Carbon can employ its medical-grade polyurethane for surgical tools and grips, guides, clips and handles. “We have some customers in the robotic surgery area who really love our medical grade thermoplastic and are using it to form all kinds of things.”
Foam printing took a step forward late last year with the new Digital Foam program announced by EOS. Using highly flexible polymer materials like thermoplastic polyurethane (TPU) or polyether-amide block copolymers (PEBA), 3D printing foam allows fine-tuning of each voxel, or volume pixel. The resulting safety, performance and comfort characteristics are ideal for helmets or orthotic and prosthetic devices.
“The interesting thing about the medical space is that it’s often difficult to get folks to really look into newer materials because they have to have some sort of clinical history in order for them to proceed through regulatory channels in a timely fashion,” said Laura Gilmour, EOS global medical business development manager. “A lot of customers are still using materials that have clinical history to take advantage of the 510(k) predicate-based regulatory pathway.”
HP’s Dockstader noted that the ability to print robust, accurate medical devices in full color is an emerging need that HP answered with its Jet Fusion 300/500 printers. “You can print in full color with microscopic voxel-level control and produce everything from readable patient IDs on parts that can be autoclaved to realistic model hearts with true-color vascular structure that allow doctors to better educate and communicate with their patients.”
Already this year, HP introduced a new certified TPU, ULTRASINT, developed by BASF to expand final-parts applications for customers on the new Jet Fusion 5200 Series systems. Kupol, a design and development company is using ULTRASINT to develop protective padding for an innovative motorcycle helmet concept.
In addition to enduring repeated sterilizations or not releasing toxins into a patient, longevity is vital for medical additive materials, said 3D Systems’ Johnson; another is aesthetics.
“One of the big myths of additive manufacturing is that parts only last for a really short period of time,” Johnson said. “Our long-term stability lets you measure stability in years rather than months — in fact, five to seven years.”
And appearance is critical, he added, noting Figure 4’s superior sidewall quality. “If I’m in a doctor’s office and see something modeled off me, I don’t want to see a chopped up, layered, digitized part. There is comfort in seeing something that looks real. MED-WHT 10 has a really smooth surface finish, and you can get that quality without a lot of post-processing.”
Making materials perform as required might take a combination of printing and coding, different post-processing or even design at the system level, noted GE’s Beacham. A combination of supplier and user testing is also ideal, as “it is impossible for an OEM to know every specification for how people will use their material.”
Machine variations also come in to play, he added. “No two machines from different OEMs act the same way,” whether exhibiting different laser energy levels or how material is handled internally. “Often you have to find a unique recipe for that material and that machine to make your part work. It’s rarely plug and play yet.”
With plastics powders, a mix of fresh and reused powder is often specified.
“Reused powder is mixed with the correct ratio of new powder to produce the optimal powder flowability for the printer,” said Shapeways’ Murphy.
Explained GE’s Beacham: “Most machines heat powder to a few degrees below melting point, then a laser adds enough energy to send it over its melting point. When you heat the material that high, it doesn’t have the same properties as fresh powder.” OEMs often specify an optimal ratio of used vs. fresh powder. “Some recommend you have 50 percent reused. If you manage it well, you typically shouldn’t have a lot of scrap. You’ve got to put thought into your powder-handing strategy.”
Reuse of metal powder is another matter. “During the printing process, especially with aluminum or titanium, you can change the chemistry of the powder over time. Powder that doesn’t get welded into the structure can drift. The art is figuring out how many cycles you can recycle the powder before it loses the chemical composition that makes a good weld.”
Creating a sound powder management strategy is key to reducing scrap. “The art is figuring out how you blend powder between runs so statistically the powder never hits end of life. Some industries will be conservative and say never reuse powder more than ten times, then scrap it. We have powder that has been used up to 30 cycles. Every time we print we run a test coupon and perform chemical analysis and material property studies” to dial in a proper reblending strategy for each material.
Powder management includes correctly storing and tracking raw materials from delivery to loading into the printer, Murphy said. “Room climate is monitored to ensure temperature and humidity stay consistent and within the guidelines of the OEMs.”