It is known that the stark disparities between soft tissues and rigid electronics generally introduce difficulties toward seamless interfaces between the two realms. [4] Ultraflexible (opto)electronics and sensors, defined with a thickness down to 10 µm or below, have arisen as an important device configuration that presents extreme mechanical compliance and foresees remarkable application potentials toward imperceptible and wearable system. [5][6][7][8] Particularly for health monitoring applications, intimate contact between sensing components and the human skin would effectively reduce skin-contact impedance, minimize motion artifact, enhance measurement accuracy, and simplify subsequent data possessing algorithm. Despite the significant progress in reducing the device thickness and maintaining high electrical performance, documented thin-film devices have certain limitations: lack of sufficient gas permeability and relatively high Young's modulus compared to that of human tissue. Besides, commonly used flexible substrates for electronics, e.g., parylene, polyethylene terephthalate (PET), polyethylene naphthalate, polyimide, On-skin electronics that offer revolutionary capabilities in personalized diagnosis, therapeutics, and human-machine interfaces require seamless integration between the skin and electronics. A common question remains whether an ideal interface can be introduced to directly bridge thin-film electronics with the soft skin, allowing the skin to breathe freely and the skin-integrated electronics to function stably. Here, an ever-thinnest hydrogel is reported that is compliant to the glyphic lines and subtle minutiae on the skin without forming air gaps, produced by a facile cold-lamination method. The hydrogels exhibit high water-vapor permeability, allowing nearly unimpeded transepidermal water loss and free breathing of the skin underneath. Hydrogel-interfaced flexible (opto)electronics without causing skin irritation or accelerated device performance deterioration are demonstrated. The long-term applicability is recorded for over one week. With combined features of extreme mechanical compliance, high permeability, and biocompatibility, the ultrathin hydrogel interface promotes the general applicability of skin-integrated electronics.

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