Strongly anisotropic thermal conductivity and adequate breathability of bilayered films for heat management of on-skin electronics

Abstract Thin-film wearable electronics are required to be directly laminated on to human skin for reliable, sensitive bio-sensing but with minimal irritation to the user after long-time use. Excellent heat management films with strongly anisotropic thermal conductivity ( K ) and adequate breathability are increasingly desirable for shielding the skin from heating while allowing the skin to breathe properly. Here, interfacial self-assembly of a graphene oxide (GO) film covering an ambient-dried bacterial cellulose aerogel (AD-BCA) film followed by laser reduction was proposed to prepare laser-reduced GO (L-rGO)/AD-BCA bilayered films. The AD-BCA substrate provides low cross-plane K ( K _⊥ ≈ 0.052 W mK ⁻¹ ), high breathability, and high compressive and tensile resistance by ‘partially’ inheriting the pore structure from bacterial cellulose (BC) gel. The introduction of an upper L-rGO film, which is only 0.31 wt% content, dramatically increases the in-plane K ( K _// ) from 0.3 W mK ⁻¹ in AD-BCA to 10.72 W mK ⁻¹ owing to the highly in-plane oriented, continuous, uniform assembling geometry of the GO film; while K _⊥ decreases to a lower value of 0.033 W mK ⁻¹ , mainly owing to the air pockets between L-rGO multilayers caused by the laser reduction. The bilayered films achieve a K _// / K _⊥ of 325, which is substantially larger even than that of graphite and similar polymer composites. They permit high transmission rates for water vapor (416.78 g/m ² /day, >204 g/m ² /day of normal skin) and O ₂ (449.35 cm ³ /m ² /day). The combination of strongly anisotropic thermal conductivity and adequate breathability facilitates applications in heat management in on-skin electronics.