Textile-Based Flexible Capacitive Pressure Sensors: A Review

Abstract

Flexible capacitive pressure sensors have been widely used in electronic skin, human movement and health monitoring, and human-machine interactions. Recently, electronic textiles afford a valuable alternative to traditional capacitive pressure sensors due to their merits of flexibility, light weight, air permeability, low cost, and feasibility to fit various surfaces. The textile-based functional layers can serve as electrodes, dielectrics, and substrates, and various devices with semi-textile or all-textile structures have been well developed. This paper provides a comprehensive review of recent developments in textile-based flexible capacitive pressure sensors. The latest research progresses on textile devices with sandwich structures, yarn structures, and in-plane structures are introduced, and the influences of different device structures on performance are discussed. The applications of textile-based sensors in human wearable devices, robotic sensing, and human-machine interaction are then summarized. Finally, evolutionary trends, future directions, and challenges are highlighted.

Keywords: capacitive pressure sensor; flexibility; micro/nanostructure; textile; wearable electronics.

Figure 1

The framework of textile-based capacitive pressure sensors.

Figure 2

Capacitive pressure sensors based on different microstructures. (Micropyramids: Reprinted with permission from Ref. [81]. Copyright 2021 Elsevier. Micropillars: Reprinted with permission from Ref. [80]. Copyright 2019 American Chemical Society. Microspheres: Reprinted with permission from Ref. [82]. Copyright 2020 Elsevier. Plant templates: Reprinted with permission from Ref. [79]. Copyright 2018 Wiley-VCH. Wrinkle: Reprinted with permission from Ref. [90]. Copyright 2019 American Chemical Society. Inclined structures: Reprinted with permission from Ref. [89]. Copyright 2019 American Chemical Society. Porous foams: Reprinted with permission from Ref. [92]. Copyright 2020 American Chemical Society. Fiber membranes: Reprinted with permission from Ref. [94]. Copyright 2020 American Chemical Society).

Figure 3

Capacitive pressure sensors based on different functional textile layers: (a) Textile-structured electrodes; (b) Textile-structured dielectric layers; (c) All-textile structures; (d) Yarn structures; (e) In-plane structures.

Figure 4

Fabrication of a capacitive pressure sensor by weaving technology. (a) Vertically stacked strip fabric electrodes of conductive polymer-coated fibers. Reprinted with permission from Ref. [118]. Copyright 2015 Springer Nature. (b) Different knitting patterns of multifunctional MXene-coated cellulose yarns. Reprinted with permission from Ref. [99]. Copyright 2019 Wiley-VCH. (c) Seam-line sensor network produced with silver threads. Reproduced with permission Ref. [130]. Copyright 2020 Wiley-VCH.

Figure 5

Fabric substrate is modified to prepare a capacitive pressure sensor. (a) Conductive fabric electrodes electroplated with nickel and copper. Reproduced with permission from Ref. [10]. Copyright 2020 Elsevier. (b) Preparation of cotton fibers with a uniform coating of conductive materials using a topology modification method. Reproduced with permission from Ref. [131]. Copyright 2020 American Chemical Society. (c) Preparation of a fabric capacitance sensor by combining a carbonized cotton fabric electrode with an Ecoflex dielectric layer. Reproduced with permission from Ref. [103]. Copyright 2021 MDPI.

Figure 6

Capacitive pressure sensor prepared by electrospinning technology. (a) Screen printing technology coated CNTs on Electrospun TPU nanofiber membranes as planar electrodes. Reproduced with permission from Ref. [109]. Copyright 2021 MDPI. (b) Electrospinning with Pd²⁺/PAN solution and electroless plating of a mixed nanofiber membrane to prepare a flexible electrode. Reproduced with permission from Ref. [102]. Copyright 2020 Elsevier. (c) Preparation of a dual-structure polyurethane nanofiber membrane by electrospinning. Reproduced with permission from Ref. [134]. Copyright 2022 American Chemical Society. (d) Preparation of an MXene/PVDF-TrFE composite nanofiber membrane and its use as the dielectric layer of a capacitive pressure sensor. Reproduced with permission from Ref. [91]. Copyright 2020 American Chemical Society.

Figure 7

Capacitive pressure sensor based on the sandwich devices. (a) Combining soft conductive fabrics (knitted and woven fabrics) and microporous dielectric layers (sugar particles and salt crystals) to prepare capacitive pressure sensors. Reproduced with permission from Ref. [100]. Copyright 2017 Wiley-VCH. (b) Electrospinning to prepare dielectric membranes composed of insulating microbeads within PVDF nanofibers. Reproduced with permission from Ref. [135]. Copyright 2020 American Chemical Society. (c) All-fabric capacitive pressure sensor based on a micropattern nanofiber dielectric layer. Reproduced with permission from Ref. [110]. Copyright 2021 American Chemical Society.

Figure 8

Capacitive pressure sensor based on the yarn devices. (a) Coating microporous PDMS elastomer dielectric on conductive fibers to prepare a capacitive pressure sensor. Reproduced with permission from Ref. [112]. Copyright 2017 Royal Society of Chemistry. (b) Retractable capacitance sensor array weaved by electrospun nanofiber-coated yarn. Reproduced with permission from Ref. [136]. Copyright 2017 Royal Society of Chemistry.

Figure 9

Capacitive pressure sensor based on the in-plane devices. (a) Interdigital capacitive strain sensor combining conductive braided fabric and a silicone elastomer. Reproduced with permission from Ref. [137]. Copyright 2021 MDPI. (b) Interdigitated capacitor on fabric as a tactile sensor. Reproduced with permission from Ref. [138]. Copyright 2021 Elsevier.

Figure 10

Pressure sensors are used in wearable devices. (a) All-textile-structured skin sensors monitor physiological signals, such as human breathing and heart rate. Reproduced with permission from Ref. [106]. Copyright 2017 Wiley-VCH. (b) All-textile pressure sensor and wireless batteryless monitoring system for real-time human movement detection. Reproduced with permission from Ref. [107]. Copyright 2019 American Chemical Society. (c) Capacitive textile pressure sensor for walking gait analysis. Reproduced with permission from Ref. [108]. Copyright 2020 Elsevier. (d) Capacitive pressure sensing array composed of conductive fabric electrodes used to measure and map the pressure on a player’s head wearing a helmet. Reproduced with permission from Ref. [140]. Copyright 2021 American Chemical Society.

Figure 11

Pressure sensors are used in robotic sensing. (a) Textile gloves for grip detection. Reproduced with permission from Ref. [100]. Copyright 2017 Wiley-VCH. (b) Real-time monitoring of a robotic arm grasping objects. Reproduced with permission from Ref. [96]. Copyright 2021 Elsevier.

Figure 12

Pressure sensors are used for human–machine interaction. (a) The textile capacitive pressure sensor is used for wireless control of a UAV quadrotor and hexapod walking robot. Reproduced with permission from Ref. [111]. Copyright 2015 Wiley-VCH. (b) Capacitive tactile sensors are used to play a piano. Reproduced with permission from Ref. [104]. Copyright 2020 American Chemical Society. (c) Smart gloves for the remote control of drones. Reproduced with permission from Ref. [130]. Copyright 2020 Wiley-VCH.