MIT researchers, a doctoral student Hyunwoo Yuk, a former visiting scholar Akihisa Inoue, a postdoc Baoyang Lu, and a professor of mechanical engineering Xuanhe Zhao, have developed a new method of making conductive polymer gels that can adhere to wet surfaces to enable better biomedical sensors and implants.
Polymers being good conductors of electricity are useful in biomedical devices and help with sensing or electrostimulation. The widespread use of conductive polymers is prevented by their inability to adhere to a surface such as a sensor or microchip and stay put despite moisture from the body.
New Adhesive Method for Sticking Conducting Polymers to Wet Surfaces
Metal Biomedical Devices Damage Delicate Tissues
Most electrodes used for biomedical devices are made of platinum or platinum-iridium alloys. Being good electrical conductors, these electrodes are durable inside the moist environment of the body, and chemically stable so they do not interact with the surrounding tissues. But their stiffness is a major drawback, as they cannot flex and stretch as the body moves, and can damage delicate tissues.
Conductive polymers, such as PEDOT:PSS, closely match the softness and flexibility of the vulnerable tissues in the body. The tricky part is getting them to stay attached to the biomedical devices they are connected to. Researchers have been struggling for years to make these polymers durable in the moist and always-moving environments of the body.
“There have been thousands of papers talking about the advantages of these materials,” said Yuk, but the companies that make biomedical devices do not use conductive polymers because they need materials that are exceedingly reliable and stable. A failure of the material could require an invasive surgical procedure to replace it, which carries additional risk for the patient.
Stiff metal electrodes harm the tissues, but they work well in terms of reliability and stability over years which has not been the case with polymer substitutes until now, Yuk added.
Modifying Polymers to Improve Durability and Adherence Capacity
Most efforts to address the problem have involved making significant modifications to the polymer materials to improve their durability and their ability to adhere, but Yuk said this creates problems of its own. Companies have already invested heavily in equipment to manufacture these polymers, and major changes to the formulation would require significant investment in new production equipment.
The changes would be for a market that is relatively small in economic terms, though large in potential impact. Other approaches that have been tried are limited to specific materials. Instead, the MIT team focused on making the fewest changes possible, to ensure compatibility with existing production methods, and making the method applicable to a wide variety of materials.
New Adhesive Gel Adheres to a Variety of Surfaces
The new method involves an extremely thin adhesive layer between the conductive polymer hydrogel and the substrate material. Though only a few nanometres thick, the layer turns out to be effective at making the gels adhere to any of a wide variety of commonly used substrate materials, including glass, polyimide, indium tin oxide, and gold. The adhesive layer penetrates the polymer itself, producing a tough, durable protective structure that keeps the material in place even when exposed for long periods to a wet environment.
The adhesive layer can be applied to the devices by a variety of standard manufacturing processes, including spin coating, spray coating, and dip coating, making it easy to integrate with existing fabrication platforms. The coating the researchers used in their tests is made of polyurethane, a hydrophilic (water-attracting) material that is readily available and inexpensive, though other similar polymers could also be used. Such materials become strong when they form interpenetrating networks, as they do when coated on the conducting polymer, Yuk explained. The enhanced strength should address the durability problems associated with the uncoated polymer.
The result is a mechanically strong and conductive gel that bonds tightly with the surface it has attached to.
Bend and Twist Resistant Adhesive Polymer
The bonding proves to be highly resistant to bending, twisting, and even folding of the substrate material. The adhesive polymer has been tested in the lab under accelerated aging conditions using ultrasound, but Yuk says that for the biomedical device industry to accept such a new material will require longer, more rigorous testing to confirm the stability of these coated fibers under realistic conditions over periods.
“We’d be very happy to license and put this technology out there to test it further in realistic situations,” Yuk says. The team has begun talking to manufacturers to see how they can best help them to test this knowledge.
“Wet adhesives are already a big challenge. Conductive adhesives that work well in wet conditions are even rarer. They are very much needed for nerve interfaces and recording electrical signals from the heart or brain,” said Zhenan Bao, a professor of chemical engineering at Stanford University, who was not associated with this research.