On-demand formation of ultrathin liquid metal hydrogel tattoos for conformal and low-impedance bioelectronics.

Ultrathin conductive hydrogels provide conformal and stable contact for flexible bioelectronics, which is critical for achieving the low impedance necessary for high-quality electrophysiological signal acquisition. However, their low modulus has made these hydrogel devices difficult to fabricate and handle. To overcome this, we present an on-demand formation strategy for creating ready-to-use, ultrathin (down to 1 micrometer) hydrogel-based bioelectronics that combine tissue-like softness with ready-to-use functionality. By using cross-linked electrospun polyvinyl alcohol (PVA) fiber mats, we produce liquid metal-patterned hydrogel tattoos (LMHTs). These devices transform from easy-to-handle dry films into adhesive, conformal hydrogel interfaces upon hydration. The resulting LMHTs are ultralight (0.9 milligrams per square centimeter), exhibit ultralow skin contact impedance (1.2 kilohms at 1 kilohertz), and show outstanding biocompatibility. We demonstrate their utility in diverse on-skin and implantable applications, including electrocardiogram recording, multichannel electromyogram mapping, neuromuscular electrical stimulation, and in vivo cardiac monitoring on a beating rat heart. This work offers a scalable and practical approach for generating high-performance, ultrathin hydrogel bioelectronics.