Fig. 1. Flexible pressure sensors and their applications in electronic skin and flexible wearable devices. “Wearable touch keyboard”, Reproduced with permission [11]. Copyright 2018, Springer Nature. “Health monitoring”, Reproduced with permission [12]. Copyright 2014, Macmillan. “Wearable device”, Reproduced with permission [13]. Copyright 2019, American Chemical Society. “Flexible devices”, Reproduced with permission [14]. Copyright 2010, Macmillan. “Stretchable transistor”, Reproduced with permission [15]. Copyright 2018, Macmillan. “Textile pressure sensors”, Reproduced with permission [16]. Copyright 2017, Wiley-VCH.

Fig. 2. Schematic illustrations of three common transduction methods: (a) piezoresistivity, (b) capacitance, and (c) piezoelectricity.

Fig. 3. (a) Procedure for fabricating microdome arrays; (b, c) SEM images of microdome arrays; (d) Pressure-response curve for an interlocked microdome array (8 wt % CNTs) with a minimum detection limit of ～0.2 Pa; (e) Response/relaxation curve. Reproduced with permission [27]. Copyright 2014, American Chemical Society.

Fig. 4. (a) Schematic illustration of fabrication process for flexible patterned PDMS films. Reproduced with permission [35]. Copyright 2014, Wiley-VCH; (b) Schematic drawing of ACNT/G pressure sensor. (c) Response to water droplets (≈0.6 Pa). Reproduced with permission [38]. Copyright 2017, Wiley-VCH.

Fig. 5. Schematic illustration of fabrication process of textile pressure sensors. Reproduced with permission [16]. Copyright 2017, Wiley-VCH.

Fig. 6. Fabrication of graphene-wrapped polyurethane (PU) sponge with fractured microstructure: (a-e) Schematic of fabrication process. Reproduced with permission [53]. Copyright 2013, Wiley-VCH.

Fig. 7. (a) Schematic illustration of microstructured PDMS film fabrication process; (b) Pressure-response curves for different types of microstructured PDMS films. Reproduced with permission [69]. Copyright 2010, Macmillan; (c-d) Structure of sensor and fabrication process reproduced with permission [71]. Copyright 2019, American Chemical Society.

Fig. 8. (a) Structure of pressure sensor device; (b) Pressure response of the source-drain current with a pressure sensitivity of up to 192 kPa-1. Reproduced with permission [81]. Copyright 2015, Macmillan; (c-d) Schematic illustration of pressure-sensitive graphene field-effect transistor (FET) with air-dielectric, opening, and folding; (e) Schematic drawing of FET. Reproduced with permission [82]. Copyright 2017, Springer Nature.

Fig. 9. (a) Pressure response of a pure PTNW-based pressure sensor under a pressure pulse; (b) Pressure response of a PTNWs/G transistor under a pressure pulse. Reproduced with permission [100]. Copyright 2017, American Chemical Society; (c) Schematic configuration of PSM devices; (d) Demonstrations of recording the signing habits of signees by PSM devices. Reproduced with permission [106]. Copyright 2015, Wiley-VCH.

Fig. 10. (a) Schematic illustration of skin-inspired matrix network (SCMN) conforming to the surface of a human arm; (b) An integrated sensor array with eight functions; (c) Temperature sensing, (d) strain sensing, (e) humidity sensing, (f) light intensity detection. Reproduced with permission [111]. Copyright 2018, Springer Nature.

Fig. 11. (a) Intrinsically stretchable transistor array; (b) An array of 108 stretchable transistors on a fingertip; (c) 6300 transistors in an area of around 4.4 × 4.4 cm2; (d) Electronic skin conforms perfectly to skin. Reproduced with permission [15]. Copyright 2018, Macmillan.