Eco-friendly cellulose wearable heaters using Korean traditional Han paper coated with graphene nanosheets via binder-free supersonic spraying

Transparent and nontransparent exoskeleton wearable heaters are used in automobile and aircraft deicing, oil-pipeline defrosting, smart windows, underwater protection clothing, winter sportswear, bioelectronics, personal healthcare, athletic rehabilitation, and thermotherapy applications. In this study, we introduce Korean traditional Han paper (or Hanji), which comprises eco-friendly cellulose fibers from the mulberry bark, as a substrate for nontransparent wearable heaters. Hanji consists of cellulose fibers and is highly flexible, making it suitable for low-cost roll-to-roll scalable production. Graphene flakes are readily and firmly embedded inside the fibrous structure of Hanji by the catastrophic impact inflicted by supersonic spraying. Additionally, Hanji can withstand temperatures of up to 100 °C before suffering thermal damage. The heater with the thicker graphene layer had the lowest electrical resistance and exhibited the highest heating temperature at a constant voltage level. Graphene flakes are firmly held inside the fibrous structure of Hanji, securing the material's mechanical durability. Additionally, tests at various bending radii were conducted to demonstrate the mechanical strength of the Hanji graphene heater. Graphene flakes deposited on an unyielding alumina substrate withstood temperatures of up to 280 °C, thereby facilitating the use of the substrate in high-temperature applications. Graphene-coated textiles were heated and stretched up to 50%, thus demonstrating the application of such materials in human-body thermotherapy.