Electronic hair: Transforming hair into a scalable bioelectronic interface

In electroencephalography, hair remains a major barrier to electrode-skin contact, limiting signal quality. Here, we introduce electronic hair (eHAIR), a strategy that converts hair into a bioelectronic interface using mixed ionic-electronic conductive polymer dyes. Hair, a keratinized filament anchored within the skin, provides a stable and naturally distributed platform for interfacing. Two complementary approaches based on polythiophene derivatives enable rapid, washable functionalization of hair bundles or semipermanent coating of individual strands. The resulting eHAIR forms conformal, low-impedance interfaces that support electrophysiological recordings with increased signal amplitude and spectral power while preserving the underlying neural dynamics without gels or rigid hardware. Beyond passive sensing, eHAIR-based organic electrochemical transistors enable local signal amplification, with gain defined by hair geometry. By repurposing hair as an active bioelectronic interface, eHAIR establishes a wearable and biointegrated platform that can be implemented from individual hairs to larger hair-covered regions.