Once little more than a novelty, 3D printing is now well-established as a manufacturing technique for medical devices. The technology, also known as additive manufacturing, involves building up objects layer by layer, based upon digital models. 3D printing is still not dominant within the manufacturing industry – even today, it represents less than 1% of total output. However, as the costs continue to fall, ever more applications are emerging.

Because it enables manufacturers to create products on demand, it has been billed as especially suitable for an era of personalised medicine. Rather than producing a large run of, say, dental implants, you can create a bespoke implant at the point of care. Some of these applications are now mainstream – 99% of the world’s hearing aids are made using 3D printing.

You can also 3D-print surgical instruments, allowing them to be optimised for a particular operative technique. Then there’s the prospect of printing components for machines in need of repair. This strategy proved particularly useful during the Covid-19 pandemic, when hospitals were able to print parts for their ventilators and respirators on site.

A: Schematic illustrating the as-deposited radial bimetallic structure arrangement, material removal boundaries for subsequent milling procedures, and the cut plane location used to provide microstructural analysis.
B: Polished crosssectional image showing build direction and the interlocking zigzag pattern of wedgeshaped protrusions of stainless material in the core into the mild steel casing, and vice versa. © Nature Communications/ Lile Squires, Ethan Roberts & Amit Bandyopadhyay – Washington State University

According to GlobalData, the medical 3D printing market stood at $2bn globally in 2022 and is growing at more than 21% a year. This rise is being fuelled not only by more ‘traditional’ applications – orthopaedics, dental devices, and personalised devices – but also by its use within diagnosis and medical training. For instance, surgeons might hone their skills on a 3D-printed anatomical model before they operate on an actual patient.

Many of the most exciting potential use cases, however, remain some way from the clinic. Research teams worldwide are working on everything from a programmable 3D-printed wound dressing to a 3D-printed heart replica that pumps just like the real thing. There is even a group working on 3D-printed hair follicles, set within labcultured human skin.

It would be remiss, too, not to mention the fastemerging field of bioprinting – bodily tissue printed using the patient’s own cells. 3D-printed miniorgans have already been tested successfully in animals, and the hope is that 3D-printed organs could one day eliminate the need for organ donation. (Exciting though it is, that scenario is probably 20 years away or more.)

Stronger joint replacements

At the less splashy – yet no less innovative – end of the spectrum, 3D printing is enabling big steps forward in material science. Amit Bandyopadhyay, Boeing Distinguished Professor of Mechanical and Materials Engineering at Washington State University, is developing a technique that could one day have profound implications for joint replacements. His team have found a way to 3D-print two types of steel in the same circular layer, creating a material that’s stronger than either metal alone. Their findings were published in the journal Nature Communications in June 2023.

The team began by thinking about trees and bones – natural materials that derive their strength from layered rings of different materials. If you look at cross sections of cut wood, you will notice radial rings, formed by seasonal variations in wood growth. It’s a similar story with bone. “We hypothesise that such radial layering improves the mechanical properties of the overall structure,” says Bandyopadhyay.

Figuring that it might be possible to do the same thing with metals, the researchers used two welding machines of the type generally found in automotive shops. These machines were used to 3D-print two different metals in a circular layer, without any need to stop or change metal wires.

Although the metals were deposited at the same time, they cooled at different rates, and the resulting pressure fused the two together. The researchers think this pressure accounts for the increase in strength – in tests, the resulting composite material was 33–42% stronger than either metal taken alone.

“This process is called wire arc-directed energy deposition,” says Bandyopadhyay. “We have also tried a powder-based directed energy deposition process and found that the concept works. So it is not process-dependent but design-dependent.”

As he explains, the technique could be used with different materials depending on what you’re hoping to achieve. For instance, you could opt for a harder outer layer and a more ductile inner layer, providing high wear-resistance on the surface but good toughness overall. You could even start combining three or more materials, all 3D-printed within the same circular layer.

Within the medical devices field, you might be able to create joint replacements that have titanium on the outside and magnetic steel on the inside. As Bandyopadhyay explains, magnetic fields have been shown to help in bone healing.

“It’s a simple idea – titanium alloys offer excellent corrosion and fatigue resistance, but not such good biocompatibility,” he says. “Medical devices could be printed using the radial bimetallic structure concept, where the outside is titanium alloy and the inside is a magnetic material. So the overall device would offer better biocompatibility than a regular titanium implant, but similar corrosion and fatigue resistance.”

MIT engineers have developed a soft, metal-free hydrogel electrode that can conduct electricity like conventional metals. © Felice Frankel/MIT

Jelly-like electrodes

Another new development comes from a team at Massachusetts Institute of Technology (MIT). They have designed a soft, printable metal-free electrode that could one day be used within pacemakers and other electronic implants. Their findings were also published in the journal Nature Materials last June.

“Conventional electrodes based on metals have been widely used for their high electronic conductivity, ease of manufacturing, complex geometry, and mechanical stability,” explains Hyunwoo Yuk, study author and co-founder of the medical device startup SanaHeal. “While metalbased electrodes are the bread and butter of most electronics, they face several fundamental challenges when it comes to the interface with biological tissues.”

“We use a 3D printing technique called direct-ink-writing where ink is being printed out of nozzles like toothpaste. The printed ink requires limited post-processing.”
Hyunwoo Yuk

In his view, there is a clear mismatch between metals and the human body. Metals are stiff, electron-based, and dry, whereas biological tissue is soft, ion-based and moist. Over time, the implant can aggravate the tissue and its performance can start to degrade.

“This was a motivation for our team to develop high-performance metal-free materials for bioelectronic interfacing while providing all the advantages of metal electrodes – electrical property, mechanical stability, and manufacturability,” says Yuk.

While polymer-based electrodes might sound like the obvious solution – and indeed, many conductive polymers have been developed since the 1970s – these materials tend not to perform very well in practice. There can be problems around versatility and biocompatibility, while there is often a trade-off between their mechanical and electrical properties. Simply put, highly robust polymers are less conductive, and highly conductive polymers less robust.

The MIT team wanted to develop a polymer that ticked all the boxes: one that was highly conductive, soft and tough at the same time. Building on their previous research, in which particles of conductive polymer were mixed with spongy hydrogel, they tweaked the formula so that the particles didn’t just randomly combine. Rather, each ingredient formed into long, thin strands, which served to improve the connectivity of the electrical particles and the toughness of the mechanical ones. The resulting gel was 3D printable.

“We use a 3D printing technique called directink- writing (DIW) where ink is being printed out of nozzles like toothpaste,” says Yuk. “The printed ink requires limited post-processing and avoids the use of toxic reagents or chemical reactions. This allowed us to simplify the manufacturing process while keeping the whole device biocompatible.”

The researchers used the gel to print electrodes, which they implanted on the heart, spinal cord, and sciatic nerve of rats. The device worked well over the three-month trial period with minimal side effects – a resounding success when you consider that many rigid metal or plastic-based devices often fail within a month.

Although the material is at an early stage of development, and will require further animal studies before it can be tested in humans, Yuk is hopeful about the prospects. One day, it might be used as a jelly-like interface between organs and medical implants, or as a replacement for the metal electrodes currently implanted after heart surgery.

“We hope that this work can help the development of implantable bioelectronic devices that are functional and biocompatible enough to be used over the lifetime without causing adverse response to patients,” he says.

As these two very different projects demonstrate, 3D printing is far more than a single technology. Rather, it encompasses a diverse array of techniques that can be used for an almost limitless set of purposes. Assuming costs keep falling and regulation keeps up with the pace of development, many of the more speculative applications could soon become a concrete reality. That’s good news for patients and the medical devices industry alike.