A Flexible and Multipurpose Piezoresistive Strain Sensor Based on Carbonized Phenol Formaldehyde Foam for Human Motion Monitoring

High-performance flexible strain sensors are extensively studied for various applications including healthcare, robots, and human–computer interaction. In most of the reported research, the fabrication of these sensors involves conductive polymer composites containing expensive metallic or carbon nanomaterials. In this study, commercial phenol formaldehyde foam (PFF) is carbonized by a simple high-temperature pyrolysis treatment and encapsulated by polydimethylsiloxane (PDMS) to fabricate a flexible and multipurpose piezoresistive strain sensor. The as-fabricated PDMS-cPFF strain sensor is capable of detecting various strain modes, including tension, compression, and three-point bending. Furthermore, the sensor exhibits a high sensitivity with a gauge factor (GF) of −20.5 under tension and stable signal responses in a frequency range of 0.01–0.5 Hz. The sensor is also capable of accurately monitoring a subtle bending strain of 0.05%. In addition, the sensor shows excellent durability in cyclic loading/unloading tests up to 1000 cycles. The applications of this strain sensor in both large- (finger bending and neck movement) and small-scale human motion monitoring (facial micro-expression and phonation) are demonstrated, showing its potential for applications in wearable electronics. This work also offers an alternative route to reuse waste thermosetting resins which would otherwise be difficult to recycle.