Manufacturing technology
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.
MIT engineers have developed a soft, metal-free hydrogel electrode that can conduct electricity like conventional metals.
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 metal- based 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.
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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 direct- ink-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. ●
Medical Device Developments /
www.nsmedicaldevices.com
© Felice Frankel/MIT
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