Ultra-Thin Conformal CNT Dry Electrodes for Surface Electrophysiology Monitoring

Epidermal electronics is a new class of non-invasive wearables with mechanical properties that match human skin or tissues, such as Young's modulus, bending stiffness, and elasticity. These characteristics enhance the coupling between the electronics and the biological tissues and allow the sensors to acquire richer information. This work uses a unique CNT synthesis process, blown aerosol chemical vapor deposition, to produce free-standing dry films roll-to-roll with high yield and controllable thickness, sheet resistance, and transmittance. The CNT films are patterned and transferred to other substrates through a laser-cut mask. The resulting ultra-thin conformal carbon nanotube (CNT) dry electrodes can be used for surface electrophysiology monitoring. We demonstrate electrocardiogram (ECG), electromyography (EMG), and forehead electroencephalogram (EEG) monitoring on the skin in a tattoo-like form, as well as multifocal electroretinography (ERG) recording on the eyes via a smart contact lens. We report the tradeoffs between CNT film thickness, conformality, and sheet resistance in reducing skin contact impedance. The CNT electrode design achieves a signal-to-noise ratio (SNR) of 67dB for ECG monitoring and a contact impedance of 100 kohm cm^2 at 10Hz which are on par with, if not better than, commercial Ag/AgCl wet electrodes, which has an SNR of 60dB. We show that the ultra-thin form factor and skin attachment reduce susceptibility to motion noises, a common downside of dry electrodes. Highly transparent (>90%) and conductive (60 ohm/square) CNT film can be readily patterned with a 200 μm resolution and transferred to the wet and curved surfaces of contact lenses. The smart contact lens can be used for multifocal ERG diagnostics of ocular diseases. We recorded simulated ERG signals on an eye mimicry phantom and showed a clear mapping between the recording from each pixel and the programmed electromagnetic field in the phantom.