Tissue engineers hail dawn of making multi-material structures
When it comes to using new materials, medical and dental device makers are ultra-conservative—because they need to clear devices through a thicket of federal regulators.
“Most medical manufacturers are conservative people and are used to a small palette of materials that the FDA reviewers are comfortable with and have seen before,” said Steve Pollack, who about two years ago left the Food & Drug Administration to become technical lead in life sciences at Redwood City, CA-based Carbon Inc. “And it’s easier to get along and go along and not challenge FDA’s comfort level with new materials.”
Nevertheless, companies like Carbon and research labs like one at Northwestern University continue to make new materials for medical manufacturers to use, primarily 3D-printable polymers.
Those polymers are part of a growing market.
While polymers represent a $100 million global market for medical additive manufacturing in 2017, that market is forecast to grow to $1.5 billion by 2027, said Scott Dunham, VP of research for SmarTech Publishing, which focuses solely on additive manufacturing.
“From a pure dollars and cents viewpoint, I think it ultimately will be more about materials,” he said. “Long-term, we won’t need tons of machines to produce dental models.”
Another believer in a materials-dependent future is Katie Weimer, vice president of medical devices for Denver-based 3D Systems Healthcare.
“As you guys are seeing and feeling, it’s not about the printers anymore,” she told the audience at the 3DHeals 2017 Global Conference. “It’s about the materials, and that’s what’s driving applications, I think, in health care.”
New hybrid polymers are tougher, more durable, more biocompatible and more resistant to temperature and deflection, Weimer said.
Freed from small box of materials
The new polymers are made by companies like Carbon, whose innovative 3D printers use a two-step process to convert a liquid to a solid.
The first step uses ultraviolet light to set the shape of an object.
“Then, for our engineering resins, we have the second step of applying heat to turn on the second chemistry that’s a different kind of polymerization than UV polymerization,” Pollack said. “It stitches together the latent chemical functions of the polymer to now create biocompatible high-temperature resins, polyurethanes that range in properties from nylons to thermoplastic urethanes, and elastomers based on silicones.
“These are actually a new set of polymers.”
Most of Carbon’s resins have been tested for biocompatibility (for up to 24-hour human contact; longer contact would require additional testing). The new set of polymers has a viable engineering window for steam autoclave, ethylene oxide, e-beam and gamma sterilization.
“We think that’s terribly exciting because we’re not trapped now in a very small little box of what materials we can use,” Pollack said. “We really have a very broad palette either of the materials that we’re commercially releasing or when there’s a specific vertical need from a customer who says, ‘Well, I really need your polymer but I’d like it to be a little stiffer.’ ‘We really need your polymer but we need it to be in a translucent blue.’ We now have the opportunity to make that real.”
Carbon is proceeding cautiously into the medical manufacturing realm. It started with dental models used for planning procedures; those models don’t need FDA clearance. The company then partnered with medical giant Johnson & Johnson for surgical tools.
Pollack said Carbon’s next step will likely be to make surgical guides to help with dental implants. The company is interested in clinical diagnostics and resorbable materials, too.
“Our approach is to crawl, walk and then run,” Pollack said. “Our strategy is to introduce these materials into the medical device ecosystem for devices with a low bar of concern, in the low-risk device category, where there’s not the long-term concern of implant challenge. Get manufacturers comfortable in that space, and get the FDA comfortable in that space.
“Once everybody’s had experience with the materials, their performance and their applications, we would then want to start exploring that next level of risk, in medical device terminology a class II, 501(k) device, where in point of fact you’re talking about parts that are introduced into the body.”
Knowing that 3D printing accommodates almost any geometry, one of the questions the people at Carbon ask themselves relates to cost-effectiveness: They can do it, but should they?
“We really look at the economics of additive manufacturing vs. traditional injection molding and subtractive manufacturing, and when it’s a complex geometry, where you’re going to make 100,000 of a device, then generally additive manufacturing makes it smarter to 3D print the parts,” Pollack said. “When it’s a million parts, then it’s not as obvious there’s an economic benefit to do it. That is unless the complexity of the part means additive is the only approach. Those are the most exciting opportunities.”
Near-, long-term applications envisioned
While industry leaders like 3D Systems and Carbon work on polymers to use with their machines, university researchers in the Midwest are more focused on polymeric materials science alone.
“What we really do altogether is develop new 3D printable materials, primarily for medicine but also for a lot of nonmedical applications,” said Adam Jakus, a postdoctoral fellow in the Shah Tissue Engineering and Additive Manufacturing Lab at Northwestern University. “Within that material development, we’ve developed two distinct platform technologies. One of those we call 3D painting and the other is our hydrogel PEGX platform.”
Hydrogel PEGX was used to print a bioprosthetic ovary from a hydrogel material that made an infertile mouse fertile, an achievement that received publicity earlier this year when the rodent gave birth to a healthy litter of pups.
In addition to the ovary, the lab is making progress on printing kidney and liver tissue, Jakus said.
Among 3D paints, which will soon be distributed through the Shah Lab’s spinoff company Dimension Inx, the most useful for medical applications are hyperelastic bone and highly conductive 3D graphene. Both can be seeded with or without stem cells to grow bone-like cells, in the case of hyperelastic bone, and nerve and muscle-like cells, with 3D graphene (available from Millipore-Sigma).
Jakus sees many fundamental and translational applications for 3D graphene, including:
- restoring movement to peripheral muscles and nerves;
- conducting signals across dead heart-muscle tissue via a regenerative patch applied following a heart attack, and
- manufacturing electrodes for deep brain stimulation. Unlike existing electrodes for the procedure that’s used to treat Parkinson’s and other diseases, ones made of 3D graphene may not be subject to failure from scar tissue formation.
The fourth application, which Jakus describes as more sci-fi, is for machine-human interfaces in prosthetics.
“One of the problems with robotic limbs is how do you integrate a machine into the rest of your arm?” Jakus said. “A big question here, but 3D graphene seems to be a prominent choice because it’s electrically conductive, it’s biocompatible, it’s flexible, it’s as at-home in machines as it is in people. So, it might be a good human-machine interface material.”
There’s an apt reason why some of the 3D-printable materials originating in the Shah lab are called paints. Just like the smears of pigments on an artist’s palette, the materials from Northwestern University can be blended or used consecutively to create a structure from two or more materials.
“In much the same way if a painter wanted to make a beautiful work of art, they would want to pick different colors and put them exactly where they want to put them to make the art,” Jakus said. “In our case, since the 3D paints all print the same way, and they’re all 3D-printing-compatible with each other, we can start making multi-material structures. Using these 3D painting and hydrogel printing systems, we can print all of these things; from the ovary to the hyperelastic bone and ceramics. We only have one 3D printer. So, we’re able to 3D print them using just one machine.”
Making multi-material structures for tissue engineering and other uses in medicine is important because the body’s natural tissues and organs are polylithic. Jakus used the example of the knee, where bone gradually changes to cartilage or vice versa.
“So being able to 3D print different materials together you can start creating complex tissues and organs,” he said.
Jakus also sees potential use for mixing paints, like 3D graphene and hyperelastic bone, to take advantage of their different material properties.
“It might be a good interface material in the face where you need to replace bone, muscle and nerve,” he said. “If we can 3D print them together and create those gradients between the tissues, the bone and muscle or the muscle and nerve, it’s almost absolutely necessary.”
Lack of standards cited as reason for limited use of metals in additive
While even the newest polymers appear to be biocompatible, metals are a different story. The body-friendly metals—titanium and its alloys, and the alloy cobalt-chrome—are already known and widely used to make implants for humans.
“In the orthopedic medical device area, the metals used are all pretty well-known and established, with newcomers happening only sporadically,” said Andy Christensen, a 3D printing expert. “That said, there is a lot of interest in 3D printing metals for orthopedic implant manufacturing.”
That’s because manufacturers using 3D printing can make devices with complex geometries and porosity that can’t be produced with any other existing manufacturing process. They are also interested in the mass customization of patient-specific devices that 3D printing allows. And they can exploit any benefit it may offer in the economy of production.
“Economy of production really talks about the fact that certain parts are way cheaper to make with 3D printing than they are with traditional methods,” Christensen said.
That’s due to batch processing vs. one-part-at-a-time machining in subtractive manufacturing.
“So, if you have small parts sometimes you can make 50, or 100 or 500 of them in one single batch within one 3D printing session,” he said. “And sometimes that equals being able to produce parts in a dramatically reduced price than with traditional techniques.”
However, Katie Weimer, VP of medical devices for Denver-based 3D Systems Healthcare, sees one thing stopping more widespread use of metals in 3D printers.
“On metals, you have to understand it’s a very regulated environment, and the thing I think is holding back 3D printing of metals is having standards that we certify to, like ASTM standards that exist for milled and other stock metals. They don’t exist for 3D printing,” Weimer told the audience at the 3DHeals 2017 Global Conference in April. “So, we have to continue to develop these ASTM standards that in health care we can certify to that guarantee a safe and effective product in the operating room.”