Soft, Printable, Metal-Free Electrode Could Pave Way for Longer-Term Medical Implants

While these implants vary in shape and functionality, most of them incorporate electrodes, small conductive elements that directly connect to target tissues to provide electrical stimulation to muscles and nerves. Typically, implantable electrodes are made of rigid metals, which are naturally conductive. However, over time, these metals can cause tissue irritation, leading to scarring and inflammation that can degrade the performance of the implant.

Now, researchers have created a metal-free material with a gel-like consistency, resembling Jell-O, that possesses the softness and toughness of biological tissue while conducting electricity like conventional metals. This innovative material can be transformed into printable ink, allowing researchers to create flexible, rubbery electrodes. The material, classified as a high-performance conducting polymer hydrogel, holds the potential to replace metal-based electrodes in the future, offering functional, gel-based alternatives that mimic the appearance and texture of biological tissue.

The majority of polymers are naturally insulating, meaning they do not conduct electricity easily. However, there exists a unique class of polymers known as conductive polymers, which can facilitate the flow of electrons. The discovery of highly conductive polymers dates back to the 1970s and was subsequently recognized with a Nobel Prize in Chemistry. Recently, researchers at the Massachusetts Institute of Technology (MIT, Cambridge, MA, USA) have been investigating the use of conductive polymers to fabricate soft, metal-free electrodes for bioelectronic implants and other medical devices. Their goal has been to create soft, durable, electrically conductive films and patches by combining conductive polymer particles with hydrogel, a soft and water-rich polymer. Previous attempts to combine the two materials resulted in weak and brittle gels or exhibited subpar electrical performance.

In their latest study, the MIT researchers determined that a new approach was necessary to enhance the electrical and mechanical properties of both the conductive polymers and hydrogels. Instead of the conventional homogeneous mixing method, they found that a slight repulsion between the ingredients, known as phase separation, was crucial. This allowed each component to form long, microscopic strands of their respective polymers while still mixing as a whole, similar to making electrical and mechanical spaghetti. The researchers transformed this gel into ink, which they fed into a 3D printer and used to print patterns onto pure hydrogel films, similar to traditional metal electrodes.

To evaluate the performance of these printed gel-like electrodes, the researchers implanted them into the hearts, sciatic nerves, and spinal cords of rats. Over a two-month period, they monitored the electrical and mechanical behavior of the electrodes and observed that they remained stable with minimal inflammation or scarring in the surrounding tissues. The electrodes successfully transmitted electrical pulses from the heart to an external monitor and delivered small pulses to the sciatic nerve and spinal cord, stimulating motor activity in the associated muscles and limbs. The researchers anticipate that this new material could find immediate application in the recovery of individuals undergoing heart surgery. They are continuing their work to improve the material's longevity and performance, envisioning its use as a soft electrical interface between organs and long-term implants such as pacemakers and deep-brain stimulators.

“We believe that for the first time, we have a tough, robust, Jell-O-like electrode that can potentially replace metal to stimulate nerves and interface with the heart, brain, and other organs in the body,” said Xuanhe Zhao, professor of mechanical engineering and of civil and environmental engineering at MIT. “The goal of our group is to replace glass, ceramic, and metal inside the body, with something like Jell-O so it’s more benign but better performance, and can last a long time.”

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