Sensing materials with fiber structures are excellent candidates for the fabrication of flexible pressure sensors due to their large specific surface area and abundant contact points. Here, an ultrathin, flexible piezoresistive pressure sensor that consists of a multilayer nanofiber network structure prepared via a simple electrospinning technique is reported. The ultrathin sensitive layer is composite nanofiber films composed of poly (3,4-ethylenedioxythiophene):poly (styrenesulfonate) and polyamide 6 (PEDOT:PSS/PA6) prepared by simultaneous electrospinning. PEDOT:PSS conductive fibers and PA6 elastic fibers are interwoven to form a multilayer network structure that can achieve ultrahigh sensitivity by forming a wealth of contact points during loading. In particular, gold-deposited PA6 fibers as upper and lower flexible electrodes can effectively increase the initial resistance. Due to this special fiber electrode structure, the sensor is able to generate a large electrical signal variability when subjected to a weak external force. The devices with different sensing properties can be obtained by controlling the electrospinning time. The sensor based on the PEDOT:PSS/PA6 nanofiber network has high sensitivity (6554.6 kPa −1 at 0−1.4 kPa), fast response time (53 ms), and wide detection range (0−60 kPa). Significantly, the device maintains ultrahigh sensitivity when cyclically loaded over 10,000 cycles at 5 kPa, which makes it have great prospects for applications in human health monitoring and motion monitoring.

Although the increasing trend of sensor signal with decreasing oxygen partial pressure was observed quite early, the underlying mechanism is still elusive, which is a hindrance to accurate gas detection under varying oxygen partial pressure. In this work, a sensing model based on previous experimental and theoretical results is proposed, in which the O2– species is determined to be the main oxygen species because O– species has not been observed by direct spectroscopic studies. On this basis, combined with the band bending of SnO2 at different oxygen partial pressures, the functional relationship between the surface electron concentration, oxygen partial pressure, and reducing gas concentration is established, which includes three forms corresponding to the depletion layer, accumulation layer, and flat band. In the depletion layer case, the variation of the sensor resistance to different concentrations of CO and oxygen can be well fitted with our function model. Besides, this model predicts that the response of sensors will no longer maintain the increasing trend in an extremely hypoxic atmosphere but will decrease and approach 1 with the background oxygen content further going down to 0.

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