Breathable Nanowood Biofilms as Guiding Layer for Green On‐Skin Electronics

: Thin-film electronics are urged to be directly laminated onto human skin for reliable, sensitive biosensing together with feedback transdermal therapy, their self-power supply using the thermoelectric and moisture-induced-electric effects also has gained great attention (skin and on-skin electronics (On-skinE) themselves are energy storehouses). However, "thin-film" On-skinE 1) cannot install "bulky" heatsinks or sweat transport channels, but the output power of thermoelectric generator and moisture-induced-electric generator relies on the temperature difference (∆T ) across generator and the ambient humidity (AH), respectively; 2) lack a routing and accumulation of sweat for biosensing, lack targeted delivery of drugs for precise transdermal therapy; and 3) need insulation between the heat-generating unit and heat-sensitive unit. Here, two breathable nanowood biofilms are demonstrated, which can help insulate between units and guide the heat and sweat to another in-plane direction. The transparent biofilms achieve record-high transport// /transport⊥ (//: along cellulose nanofiber alignment direction, ⊥: perpendicular direction) of heat (925%) and sweat (338%), winning applications emphasizing on ∆T/AH-dependent output power and "reliable" biosensing. The porous biofilms are competent in applications where "sensitive" biosensing (transporting// sweat up to 11.25 mm s-1 at the 1st second), "insulating" between units, and "targeted" delivery of saline-soluble drugs are of uppermost priority.