Abstract:

Objective Silk fabric is an ideal substrate for fabricating flexible and wearable pressure sensors because of its excellent comfort and skin-friendly properties. However, it requires complex and time-consuming process to prepare the silk fabrics from the silkworm cocoon. Flat silk cocoon (FSC) formed by the free movement of silkworm is a natural non-woven structure with high porosity and favorable flexibility. In this paper, FSC without post-treatment was used as a flexible substrate to develop flexible pressure sensor together with conductive materials MXene and AgNWs.

Method During silkworm spinning, MXene and AgNWs dispersions were repeatedly sprayed on the surface of silk fibers at a fixed time interval. MXene and AgNWs were coated on the surface of silk fibers by natural adhesion of sericin during this biospinning process, and then MA-FSC with multil-layered structure were prepared. The MA-FSC pressure sensor was assembled face-to-face with the interdigitated electrode, and its sensing performance and real-time monitoring application were studied.

Results The sprayed MXene and AgNWs alternatively distributed on the surface of flat silk cocoon, and the prepared MA-FSC exhibited a hierarchical structure with five layers. The optimizing experiment showed that the pressure detection range of the sensor gradually was increased with the increase in the number of sensing layers. However, too many or too few sensing layers led to the low sensitivity of sensor. When the number of sensing layers was too small (such as 1 layer), the conductive layer active material content inside the sensor was insufficient, resulting in a small number of conductive paths formed during the compression process, and the sensor showed low sensitivity. When too many sensing layers (such as 3 layers) was fabricated, more conductive paths existed inside the sensor even when no pressure, and the signal change of the sensor was not obvious when pressure was applied. Therefore, the 3-layer MA-FSC pressure sensor demonstrated little difference between the no-pressure state and the saturated pressure state. The 2-layer MA-FSC pressure sensor was proved to have the best sensitivity (0.20 kPa^-1). The response/recovery time was shorter than 1 s, and it could react quickly to changes in external pressure. In addition, the 2-layer MA-FSC pressure sensor maintained a stable signal output under the gradient increasing compression rate and gradient increasing pressure, with the cycle durability over 1 500 times. Owing to the excellent sensing performance, MA-FSC pressure sensor would be expected to to monitor human body movement status (such as finger pressing, finger bending, elbow bending and knee bending). The 2-layer MA-FSC pressure sensor could also be combined with Morse code to output some signals similar to ″SOS″, ″SWU″ and ″WYJ″, which were used in the field of encrypted transmission of information. 16 pressure sensor units were fitted into a 4×4 sensing array for pressure distribution visualization.

Conclusion Using natural flat silk cocoon as a flexible substrate, the MA-FSC pressure sensor was prepared. The optimization indicates that the prepared sensor with two sensing layers has the best sensing performance, which can be applied in the fields of human body movement status monitoring, information encrypted transmission, and real-time pressure trajectory tracking. This research broadens the application field of silk and provides new ideas and inspiration for the development of sensors based on natural material substrates.

Key words: silkworm, flat silk cocoon, pressure sensor, smart wearable, transition metal carbide/nitride, silver nanowires