Skin-mimetic adaptive gradient electrode for ambulatory noninvasive brain-computer interfaces

Noninvasive brain–computer interfaces (BCIs) need stable, comfortable electrodes for long-term wear. Current rigid or uniformly soft electrodes suffer from scalp mismatch, causing motion artifacts and short operation times. Inspired by human skin’s gradient structure, we created a skin-mimetic adaptive gradient electrode (AGE). Based on the Marangoni effect, we induced heterogeneous crosslinking in the hydrogel within a gradient temperature field. This yields an electrode with a gradual stiffness change—soft at the scalp interface and firm at the circuit connection—along with a gradient pore structure for sustained electrolyte release. This design enabled stable, high-quality EEG recording for over 8 h during free movement and over 14 h at rest, maintaining >94% typing accuracy. This work demonstrated the potential of gradient-designed hydrogels toward noninvasive BCIs in freely moving practicality.