Degradable and stretchable bio-based strain sensor for human motion detection.

In recent years, flexible strain sensors have attracted considerable attention for the great application potential in the emerging fields of wearable devices, electronic skin and health monitoring. However, most of flexible strain sensors are nondegradable, and the produced numerous electronic wastes after uselessness will seriously threaten environment and ecology. Herein, we propose a new strategy to fabricate degradable and stretchable bio-based strain sensor using candle soot (CS) particles to construct conductive pathways and chitosan, potato starch (PS), and polyvinyl alcohol (PVA) to form stretchable matrix in the presence of Fe ions. Owing to the formation of multiple hydrogen bonding constructed by chitosan, PS and PVA as well as coordination bonding by Fe ions, the obtained strain sensor showed high elongation at break up to 200% and good fatigue resistance. Furthermore, the firm embedding of the CS particles into the surface of the stretchable matrix endowed the strain sensor with steady sensitivity (gauge factors of 1.49 at 0-60% strain and 2.71 at 60-100% strain), fast response (0.22 s) and good repeatability even after 1000 stretching-releasing cycles. In addition, the strain sensor was successfully applied to detect various human motions including finger and wrist bending, swallowing and pronunciation. Interestingly, after connecting to an Arduino microcontroller circuit with a Bluetooth module, the strain sensor was able to wirelessly detect real-time movements of index finger joints. Different from most previously reported sensors, the prepared strain sensor in this work was completely degraded in 2 wt% CHCOOH solution at 90 °C only within 10 min, thus effectively avoiding the production of electrical waste after the updating and upgrading of the sensors. The findings conceivably stand out as a new methodology to prepare environmental-friendly sensors in the field of flexible electronics, which is very beneficial for the sustainable development of environment and society.