# A High Compressibility Pressure--Sensitive Structure Based on CB@PU Yarn Network
## 1 Introduction  
Pressure sensors with wide detection range and high compressibility are highly desirable in future portable devices. Recently, various pressure-sensing mechanisms including transistor sensing [1; 2], capacitive sensing [3; 4], piezoelectric sensing [5; 6], triboelectric sensing [7; 8; 9] and resistive sensing [10] have been used to construct pressure sensors with excellent performance. Resistive-sensors in particular draw extensive attention due to their simple device structures and low energy consumption. Resistive-sensors' functions are based on bulk piezoresistivity and the change in contact resistance (R\({}_{\text{c}}\)). Limited by their unstable mechanical structures and compression volume, current piezoresistive sensors cannot meet the needs of some emerging applications which require significant elastic compression [11]. [PERSON] and co-workers [12] fabricated sensors with microcracks based on graphene-covered human hairs. However, the finite value of electrical conductance suddenly dropped to zero when the microcracks were disconnected. Such sensors based on the pure crack mechanism exhibited limited detection range because of the fragile conductive pathways. [PERSON] and colleagues prepared strain sensors based on the compressible structure of CB@PU sponges. However, when the applied strain to sponge reached a limiting value (60%), the sensors could not be compressed more or detect further [13]. Conventional conductive polymer films are potential materials for resistive-type sensors, which can detect large compressive strains. However, these materials are insensitive, unstable, and incapable of detecting low pressures [14; 15; 16]. In order to improve the performances of resistive-sensors, microstructure designs were used, such as fractured microstructures [13; 17], reversible interlocked microstructures [18], micro-pyramid array structures [19] and carbon nanotube mesh structures [20]. These devices could precisely detect low pressure stress, but most of them could not be applied torelatively high pressure regime (>10 kPa). For example, a previously reported pressure sensor using an Au-coated micropillars array structure showed a high sensitivity of 2.0 kPa\({}^{-1}\) in the pressure range below 0.35 kPa, but it could not detect pressures above 3.5 kPa due to the current saturation [21]. Another work based on a microcracks structure showed a minimum detectable pressure of 9 Pa, but the effective range of the sensor was less than 2 kPa [16]. Therefore, it is very challenging to fabricate a pressure sensor with good compressibility which is applicable to a broad detection range. The network structure presents advantages such as structural stability, excellent integrity and good interaction within the network. When the network is given elasticity, small external pressure variation can cause significant elastic fluctuation, and the elastic network can produce large deformation under strain. Meanwhile, polyurethane (PU) materials are widely utilized for pressure sensors due to their elastic elongation, good mechanical property and chemical stability [15; 16; 22; 23; 24]. In previous studies, polyurethane pressure sensors were mainly manufactured in forms of sponge or foam, but polyurethane yarn have rarely been reported in pressure sensors by far. PU yarn's excellent elasticity makes it a desirable material for pressure sensors. Thus, it is worthy of further exploration and utilization. In this paper, a new pressure sensor structure was proposed. Combining the advantages of PU fibers and net-like structures, a highly elastic and sensitive 3D network sensor was fabricated. Through the electrostatic deposition method, the CB conductive layer was efficiently supported by the PU yarn. The detection range of pressure was significantly extended (from 25 Pa to 20 kPa) and the tolerable compressive strain was as high as 97.4%.