Solvent-free fabrication of a biodegradable all-carbon paper based field effect transistor for human motion detection through strain sensing

There has been a huge demand for low-cost, eco-friendly, flexible and wearable electronics which find applications in personal health monitoring. Flexible electronics based on plastic substrates have been extensively studied in this regard because of their versatility. However, their fabrication involves energy consuming complex procedures and processing of eco-unfriendly materials which limit their use to certain specific applications. Here we report the fabrication of a flexible all-carbon field effect transistor (FET) using a low-cost, recyclable and biodegradable cellulose paper as both the substrate and the dielectric and pencil graphite as the source, drain, channel and gate without using any expensive, toxic or non-biodegradable materials. The FET transfer characteristics show ambipolar behavior which can be utilized in analog electronics applications like rectifiers, mixers and frequency multipliers and its mobility was found to be very high compared to reduced graphene oxide based FETs. The FET was utilized as a strain sensor which shows excellent sensitivity for very low strains (of both tensile and compressive type) which is comparable to and even better than recently reported carbon nanotube and graphene based strain sensors. The sensitivity of the FET based strain sensor can be modulated by varying the gate voltage under strain. Furthermore, we investigated the performance of the sensor by integrating it with hand gloves to detect human motion. The results indicate that the sensor can be utilized in patient surveillance in healthcare and human–machine interface (HMI) applications. The successful fabrication of this paper based all-carbon transistor using only paper and pencil graphite and its application in human motion detection using strain sensing indicate that this approach can be used for developing highly scalable, low cost, low energy, flexible electronics for healthcare without using any sophisticated fabrication methods or toxic chemicals.