A highly ductile and multifunctional cellulose/CaCl₂ composite film for humidity-responsive wearable sensors

Cellulose films are recognized as environmentally friendly materials due to their high mechanical strength, transparency, and biodegradability. However, their inherent brittleness and low ductility have significantly limited practical applications, for instance in wearable motion sensors. To overcome these limitations, we designed a regenerated carboxymethyl cellulose film incorporating calcium chloride as a plasticizer, resulting in ultrahigh ductility. Using a simple soaking strategy, calcium chloride was introduced and retained within the film throughout the drying process. The hygroscopic nature of calcium chloride attracted a substantial amount of free water molecules, which disrupted the intermolecular hydrogen bonds and thereby significantly enhanced the film's stretchability. Remarkably, the film achieved a greatly enhanced strain of 149.36 ± 48.44%, compared to 21.98 ± 4.47% for the pristine film, yet maintained a tensile strength of 2.79 ± 0.61 MPa, closely matching the mechanical properties of human skin. Furthermore, the embedded calcium chloride endowed the composite film with integrated functionalities of humidity responsiveness, exceptional frost resistance (operational at -20 °C), and superior air and moisture permeability. These combined properties enable reliable monitoring of human activities, making it a robust and flexible material for wearable sensor.