Flexible bifunctional strain-humidity sensor based on PDMS/CS/AgNW composite film for wearable monitoring

With the growing demand for multifunctional wearable electronics, there is increasing research focus on flexible sensors that integrate multiple sensing capabilities. This study developed a novel flexible conductive film with a PDMS/CS/AgNW structure for bifunctional strain-humidity sensing. The composite film employs a polydimethylsiloxane (PDMS)/chitosan (CS) bilayer substrate that offers excellent reversible stretchability, while a silver nanowire (AgNW) network coated on the CS surface provides electrical conductivity and sensing functionality. Benefiting from the moisture absorption and swelling properties of CS, the film exhibits pronounced humidity responsiveness in both contact and non-contact modes, manifested as reversible resistance changes. With an optimal AgNW loading, it delivers a precise and measurable response to trace amounts of water droplets. The sensor performs effectively across a broad relative humidity (RH) range of 10%-93% and can accurately capture breathing patterns under different motion states. In addition to humidity sensing, the film demonstrates reliable strain sensing capabilities. Its flexibility and stable conductivity enable accurate detection of bending angles and strain, even after over 1000 fatigue cycles. The sensor also shows a fast response time and high reversibility when tested on various human body parts. Under tensile strain, it maintains a stable linear response up to 60% strain, along with excellent repeatability. Furthermore, an integrated symmetric structure is designed to enable simultaneous and discriminative detection of humidity and bending stimuli. This work provides a simple and effective strategy for fabricating multifunctional flexible sensors, demonstrating great potential for use in wearable electronics and real-time health monitoring systems.