Water-based conductive ink containing graphene nanosheets and ultrafine carbon powder for high-performance flexible wearable heaters

Carbon-based composite conductive material, possessing advantages such as facile processing, cost-effectiveness, and ultralightness, represents a burgeoning electrothermal material. However, developing water-based inks using carbon-based materials that satisfy the requisites of human health safety, low-voltage operability, and durability in the realm of flexible wearable heaters remains an arduous challenge. Here, stable water-based conductive inks, with graphene nanosheets (GNs) and ultrafine carbon powder (UC) as conductive fillers, are prepared by a simple ball milling method. The conductive inks exhibited rheological properties suitable for screen printing, with a print resolution of up to 0.4 mm and an adhesion level of grade 1. When graphene nanosheets accounted for 15% of the total conductive filler content, the printed patterns displayed a "sandwich" type conductive network structure formed by both plane contact and point contact between conductive fillers at the microscale, resulting in a sheet resistance as low as 14.16 Omega sq<SUP>-1</SUP>, which was 54.99% lower than that of pure ultrafine carbon-printed patterns. The electrothermal film prepared from these printed patterns demonstrated rapid response within 50 s under low-voltage drive ranging from 4 to 16 V and achieved an adjustable temperature range of 30-90 degrees C. Also, it maintained stable performance under cyclic heating-cooling and bending conditions for up to 1000 cycles. Wearable heating sleeves with excellent heat uniformity were fabricated to validate their tremendous potential in flexible wearable device applications.