https://phys.org/news/2018-06-sprinkle-platinum-nanoparticles-graphene-brain.html

Over the past five years, researchers have been exploring graphene electrodes for use in neural implants that can be placed directly on the surface of the brain to record neuronal activity. They have several advantages over the traditional metal electrodes used in today's neural implants. They are thinner and flexible, so they can conform better to brain tissue. They are also transparent, which makes it possible to both record and see the activity of neurons directly beneath the electrodes that would otherwise be blocked by opaque metal materials.

However, graphene electrodes suffer from high impedance, which means electrical current has difficulty flowing through the material. This hinders communication between the brain and recording devices. Readings are noisy as a result. And while there are various techniques to reduce the impedance of graphene, they ruin the material's transparency.