Flexible sensors (FSs) are the key components of intelligent equipment and wearable devices, thus attracting increasing research interests in recent years. However, the preparation of multifunctional FS with good degradability in a natural environment is still challenging. In this work, we fabricated a flexible multimodal sensor that can detect multiple stimuli with only one device by spraying the mixture of carbon black (CB) and reduced graphene oxide (rGO) on a paper substrate. Scanning electron microscopy visualization indicated the CB particles absorbed on the surface of rGO, which then overlayered together, constructing a hierarchical structure. Benefiting from this unique structure, the obtained FS was demonstrated to have good sensitivity for strain, humidity, temperature, and pressure as well as multiple stimuli and was used to monitor human respirations as well as body motions, such as finger and elbow bending and head nodding. Besides, the sensor can be easily degraded in water being free of electronic pollution, but it also can be reused after the soaking–drying process, implying its reliability. This degradable and multimodal FS may find great potential in flexible electronics.

Flexible strain sensors (FSSs) are essential components in intelligent systems, especially in soft robots, human sport monitoring, ect., but their scalable preparation remains a challenge. In this work, we first proposed and demonstrated a strategy to prepare FSS with a scalable and cost-effective papermaking procedure. Cellulose fibers from waste papers and conductive graphite were mixed and subject to a paper former (papermaking machine in laboratory), producing a strain sensitive paper with diameter of 20 cm in 10 min. With the scrips from the strain sensitive paper, the strain sensor was assembled showing good sensing performance for both bending (gauge factor (GF) = 27, response time of 360 ms) and twisting (GF = 26.5, response time of 440 ms) strains. It can be used in movement detections of soft matters (such as a plastic ruler), elbow joints of a puppet, and human fingers. The cost of the sensor was calculated as low as $0.00013, and the strain sensitive paper can be degraded in around 1 min in water under stirring. Furthermore, the strategy can be expanded to the sensor based on carbon black (CB), indicating a universality, which may pave a way for developing more intelligent materials and devices.

Flexible sensors (FSs) hold great potential in wearable and intelligent equipment, but the preparation of degradable and multimodal FSs is urgently demanded, which may simplify the sensor matrix and reduce electronic waste. Here in this work, a multimodal and degradable FS was fabricated by transferring laser-induced porous carbon onto starch film. The obtained sensor showed three modes that can detect strain (with gauge factor (GF) = 134.2, response time of 130 ms, and stability >1000 times), temperature (25–90 °C), and pressure (0–250 kPa). The sensor can be used for monitoring human motions, detection of spatial pressure, and multiple stimuli. More importantly, the sensor was demonstrated to be degradable in water, suggesting it is a “green” electronic device being free of electronic pollution. This kind of multimodal and degradable FS may find wide applications in health and sports monitoring, flexible electronics, artificial intelligence, and so on.

Flexible electronics (FEs) with excellent flexibility or foldability may find widespread applications in the wearable devices, artificial intelligence (AI), Internet of Things (IoT), and other areas. However, the widely utilization may also bring the concerning for the fast accumulation of electronic waste. Green FEs with good degradability might supply a way to overcome this problem. Starch, as one of the most abundant natural polymers, has been exhibiting great potentials in the development of environmental-friendly FEs due to its inexpensiveness, good processability, and biodegradability. Lots of remarks were made this field but no summary was found. In this review, we discussed the preparation and applications of starch-based FEs, highlighting the role played by the starch in such FEs and the impacts on the properties. Finally, the challenge was discussed and the outlook for the further development was also presented.

Paper-based flexible sensors have attracted much attentions due to their abundant resource, low cost and good degradability in natural environment that are free of electronic waste (e-waste). However, the mechanical...