An In Situ Assembled, Sprayable Poly(Lipoic Acid) E-Tattoo for Multimodal Health Monitoring.

Wearable electronics show promise in advancing personalized healthcare, with electronic skins (e-skins) emerging as ideal candidates due to their intimate skin conformability. However, their widespread adoption is constrained by complex fabrication, insufficient conformability, and suboptimal biocompatibility. Herein, we propose a poly(lipoic acid)-based sprayable electronic tattoo (LS e-tattoo) formulated from lipoic acid (LA), tannic acid (TA), and silver flakes. This strategy uses spray-coating for rapid polymerization, followed by a Zn2+ spray for curing. As a result, a conformal, breathable, biocompatible, and robust conductive network forms in situ on the skin within 3 min. The secondary-cured LS e-tattoo exhibits an ultralow contact impedance of 95 kΩ at 50 Hz. The device also shows less than 1% resistance variation after 1,500 bending cycles, confirming its mechanical robustness. An integrated epidermal sensing system was further developed. This platform enables synchronous sweat chloride detection (12.5-70 mM, R2 = 0.992). Concurrent measurements include body surface temperature (error < 0.1°C) and skin hydration (R2 = 0.998), while high-fidelity electromyography signals (SNR: 14.4 dB) are acquired simultaneously. By combining in situ conformal integration with rapid on-demand fabrication, this work offers a versatile platform for customized e-skins.