Electrode SNR has not improved in 20 years despite massive materials engineering. The limit is not the electrode—it is the Johnson-Nyquist noise of the tissue itself. At room temperature, 10 kHz bandwidth, and 500 kΩ electrode impedance, you cannot do better than ~5 µVrms. Most neural signals are 50-200 µV. We are already within an order of magnitude of the physical limit.