Polymers that are good conductors of electricity could be useful in biomedical devices to help with sensing or electrostimulation, for example. But there has been a sticking point preventing their widespread use: their inability to adhere to a surface such as a sensor or microchip and stay put despite moisture from the body.
Most electrodes used for biomedical devices are made of platinum or platinum-iridium alloys. These are very good electrical conductors that 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. Because they can’t flex and stretch as the body moves, they can damage delicate tissues.
Conductive polymers such as PEDOT: PSS, by contrast, can very closely match the softness and flexibility of the vulnerable tissues in the body. The tricky part has been getting them to stay attached to the biomedical devices to which they are connected.
Companies making biomedical devices don’t use these materials because they are not 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 sometimes harm the tissues but they work well in terms of reliability and stability over a period of years, which has not been the case with polymer substitutes until now. Most efforts to address this problem have involved making significant modifications to the polymer materials to improve their durability and their ability to adhere, which 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.
Researchers have now developed a way of getting conductive polymer gels to adhere to wet surfaces. The 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. The method involves an extremely thin adhesive layer between the conductive polymer hydrogel and the substrate material. Though only a few nanometers thick, this 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 into 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 used in 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 very strong when they form interpenetrating networks, as they do when coated on the conducting polymer. This 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 to which it is attached.
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 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 long periods of time.
Conductive adhesives that work well in wet conditions are rare and are very much needed for nerve interfaces and recording electrical signals from the heart or brain.