Substrate-free ultra-thin epidermal bioelectrodes with enhanced conformality and breathability for long-term physiological monitoring

The development of durable and reliable bioelectrodes for high-quality bioelectrical signals acquisition has become a crucial technology in the field of human physiological condition monitoring and human-machine interfaces. However, most existing bioelectrodes are limited to conventional elastomeric substrates that suffer from mechanical mismatch and low permeability, and lack multifaceted attributes and essential synergistic properties akin to those found in biological skins. In this work, we report advanced substrate-free ultra-thin epidermal bioelectrodes (ASU-EBEs) based on free-standing conductive all-polymer (FCAP) films, which integrate the advantages of ultra-conformality, excellent stretchability and breathability into a single device. The resulting ASU-EBEs exhibit excellent conductivity of ∼475 S cm⁻¹, outstanding stretchability of ∼48%, ultra-conformality to the interface of biological tissues and superior breathability. The enhanced electronic and mechanical performance is attributed to the introduction of water-soluble polyethylene oxide into poly(3,4-ethylenedioxythiophene): poly(4-styrenesulfonate) (PEDOT:PSS) to control the molecular π-π stacking distance and promote the formation of nanofiber structures. Hence, ASU-EBEs show much lower skin-contact impedance than the standard gel electrodes, enabling the use for long-term healthcare monitoring in complex daily conditions.