Engineers developed a soft, 3D printable, metal-free electrode
(Nanowerk News) Do an image search for “electronic implants,” and you’ll find a wide variety of devices, from traditional pacemakers and cochlear implants to more futuristic brain and retinal microchips aimed at improving vision, treating depression, and restoring mobility.
Some implants are hard and large, while others are flexible and thin. But regardless of form or function, almost all implants incorporate electrodes – tiny conductive elements that attach directly to target tissue to electrically stimulate muscles and nerves.
Implanted electrodes are mostly made of rigid metal that is electrically conductive. But over time, metals can exacerbate tissue, causing scarring and inflammation which in turn can degrade implant performance.
Now, MIT engineers have developed a metal-free, Jell-O-like material that is as soft and tough as biological tissue and can conduct electricity similar to conventional metals. The material can be made into printable ink, which the researchers patterned into flexible rubber electrodes. The new material, which is a type of high-performance polymeric hydrogel, could one day replace metals as gel-based functional electrodes, with the look and feel of biological tissue.
“This material operates like metal electrodes but is made of a gel similar to our bodies, and with a similar water content,” said Hyunwoo Yuk SM ’16, PhD ’21, co-founder of SanaHeal, a medical device startup. “It’s like an artificial network or nerve.”
“We believe that for the first time, we have a hard, strong, Jell-O-like electrode that can potentially replace metal to stimulate nerves and interact with the heart, brain and other organs in the body,” added Xuanhe Zhao, professor of mechanical engineering and civil and environmental engineering at MIT.
Zhao, Yuk, and others at MIT and elsewhere report their results Natural Ingredients (“3D printable high performance polymer hydrogels for all hydrogel bioelectronic interfaces”). The study’s co-authors include first author and former MIT postdoctoral fellow Tao Zhou, who is now an assistant professor at Penn State University, and colleagues at Jiangxi Science and Technology Normal University and Shanghai Jiao Tong University.
A real challenge
Most polymers are insulating, meaning that electricity cannot pass through them easily. But there is a special, small class of polymers that can actually pass electrons through most of them. Some conductive polymers first demonstrated high electrical conductivity in the 1970s — work that later won the Nobel Prize in Chemistry.
Recently, researchers including those in Zhao’s lab have been trying to use conductive polymers to make metal-free soft electrodes for use in bioelectronic implants and other medical devices. This effort aims to make soft yet tough, electrically conductive films and patches, primarily by mixing conductive polymer particles, with hydrogels – a soft, spongy type of water-rich polymer.
The researchers hope that the combination of the conductive polymer and the hydrogel will result in a gel that is flexible, biocompatible and electrically conductive. But the materials made to date are too weak and brittle, or exhibit poor electrical performance.
“In gel materials, the electrical and mechanical properties always oppose each other,” said Yuk. “If you increase the electrical properties of the gel, you have to sacrifice the mechanical properties, and vice versa. But in reality, we need both: The material has to be conductive, and also be elastic and strong. That’s the real challenge and the reason why people can’t make conductive polymers into reliable devices made entirely of gel.”
Electric spaghetti
In their new study, Yuk and his colleagues found that they needed a new recipe for mixing conductive polymers with hydrogels in ways that enhance the electrical and mechanical properties of each material.
“Previously people relied on mixing two materials that were homogeneous and random,” said Yuk.
The mixture produces a gel made of randomly dispersed polymer particles. The group realized that in order to maintain the electrical and mechanical strength of the conductive polymer and hydrogel respectively, the two materials must be mixed in a slightly resistive manner — a state known as phase separation. In this slightly separated state, each material can then link its own polymers to form long microscopic strands, while also being mixed as a whole.
“Imagine we make spaghetti electrically and mechanically,” Zhao offers. “Electric spaghetti is a conductive polymer, which can now transmit electricity throughout the material because it is continuous. And mechanical spaghetti is a hydrogel, which can transmit mechanical forces and be tough and elastic because it is also continuous.”
The researchers then turned the recipe for cooking spaghetti gel into ink, which they fed through a 3D printer, and printed onto a pure hydrogel film, with a pattern similar to that of conventional metal electrodes.
“Because the gel can be 3D printed, we can adjust the geometry and shape, which makes it easy to create electrical interfaces for all types of organs,” said first author Zhou.
The researchers then implanted electrodes imprinted like Jell-O into the heart, sciatic nerve, and spinal cord of the rats. The team tested the electrical and mechanical performance of the electrodes on animals for up to two months and found the devices remained stable, with little inflammation or scarring of the surrounding tissue. The electrodes are also capable of relaying electrical pulses from the heart to external monitors, as well as sending small pulses to the sciatic nerve and spinal cord, which in turn stimulate motor activity in the associated muscles and limbs.
Going forward, Yuk imagines that direct application of this new material might be for people recovering from heart surgery.
“These patients need electrical support for several weeks to avoid cardiac arrest as a side effect of surgery,” said Yuk. “So doctors stitch metal electrodes on the surface of the heart and stimulate them for weeks. We can replace those metal electrodes with our gels to minimize the complications and side effects that people have recently received.”
The team works to extend material life and performance. Then, the gel can be used as a soft electrical interface between organs and long-term implants, including pacemakers and deep brain stimulators.
“Our group’s goal is to replace the glass, ceramic and metal inside the body, with something like Jell-O so that it’s more docile but performs better, and lasts a long time,” said Zhao. “That is our hope.”
