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

Abstract 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.