Recent Development of Self-Powered Tactile Sensors Based on Ionic Hydrogels

Abstract

Hydrogels are three-dimensional polymer networks with excellent flexibility. In recent years, ionic hydrogels have attracted extensive attention in the development of tactile sensors owing to their unique properties, such as ionic conductivity and mechanical properties. These features enable ionic hydrogel-based tactile sensors with exceptional performance in detecting human body movement and identifying external stimuli. Currently, there is a pressing demand for the development of self-powered tactile sensors that integrate ionic conductors and portable power sources into a single device for practical applications. In this paper, we introduce the basic properties of ionic hydrogels and highlight their application in self-powered sensors working in triboelectric, piezoionic, ionic diode, battery, and thermoelectric modes. We also summarize the current difficulty and prospect the future development of ionic hydrogel self-powered sensors.

Keywords: ionic hydrogels; piezoelectric; self-powered sensors; tactile sensors.

Figure 13

(a) I. Self-powered strain-sensing mechanism of the self-powered ionic hydrogel-based sensor (SPI). II. Charge–discharge curves of SPI sensors. III. Relative resistance changes of SPI sensor in response to elbow-joint motions in no-load state and in the state of powering a watch. (Reprinted with permission from [85].) (b) I. Schematic illustration of pressure-sensing mechanism. II. The voltage changes when pressure is applied and released. III. Voltage variation of the sensor in response to finger bending. IV. Cyclic voltage variations with finger bending angles ranging from 0° to 90°. (Reprinted with permission from [86].) (c) I. Mechanism of the potentiometric mechanotransduction. II. Responsive characteristics of mechanotransducers with varied Gly content. III. Photograph of the single-electrode-mode e-skin with 6 × 6 sensing pixels. IV. Response behaviors of the e-skin. (Reprinted with permission from [88].) (d) I. Schematic illustration of the self-powered pressure and strain sensor. II. Relative current variations of the sensor in response to compression. III. Relative current variations of the sensor in response to strain. (Reprinted with permission from [89].)

Figure 1

Number of published papers on the https://app.dimensions.ai/ database with the search of “hydrogel for self-powered tactile sensor” (the data were accessed on 1 February 2023).

Figure 2

Schematic of the unique properties and modes of the ionic hydrogel self-powered tactile sensors.

Figure 3

Working modes of TENGs.

Figure 4

(a) Schematic of the hydrogel-based triboelectric generator. (b) Output voltage and current versus the resistance of the external loads. (c) Schematic diagram of the Hydrogel-TENG in a tube shape, along with the open-circuit voltage generated by (d) bending, (e) twisting, and (f) tensile strains. (Reprinted with permission from [61].)

Figure 5

(a) Cellulose hydrogel for flexible sensor. I. Schematic working mechanism of CNH TENG. II. A transparent hydrogel TENG tapped by fingers. III. Various peak amplitudes of voltage across the resistor (220 MΩ) with different pressures applied. (Reprinted with permission from [63].) (b) Use of a hydrogel-based TENG for driving fatigue monitoring. I. Eye-closure movement detection. II. Yawning detection. III. Head-turning movement detection. IV. Vertical bending angle detection. (Reprinted with permission from [65]). (c) Self-Powered Smart Arm Training Band Sensor. I. Schematic diagram of the SA−Zn hydrogel TENG and the photographs of the SH-TENG in its original and foldable state. II. Output Performance of the SH-TENG. III. The voltage outputs of the SH-TENG under bending of the finger. (Reprinted with permission from [66].)

Figure 6

(a) Schematic diagram of the DE-THS and SE-THS. (b) Open-circuit voltage of both the plain SE-THS and the micro-pyramid-patterned SE-THS varied with different pressure. (c) Output signals of DE-THS under touching, pressing, tapping, and bending (reprinted with permission from [68]).

Figure 7

(a) Schematic diagram of PAA hydrogel-based sensor. (b) The working principle of PAA hydrogel converting mechanical energy into electrical energy. (c) A schematic diagram of the self-powered position recognizer and the corresponding variations in the output voltage of the detector, in response to pressing at different locations on the sensor. (d) Application of the PAA hydrogel-based sensor as a sound detector. (Reprinted with permission from [78].)

Figure 8

(a) Schematic illustration of the mechanism of the voltage generated by dynamic structure-nonuniform-induced ion squeezing. (b) Output voltage resulting from the two-end-symmetry-stretch tests conducted on the coiled CNT@PVA/H₂SO₄ yarn, with the middle portion secured while both ends are simultaneously stretched. (c) The output voltage signal of CNT@PVA/H₂SO₄ and CNT@PVA/KOH yarns with reversed phase. (d) The output voltage signal of the sensors attached on each finger of a hand with different finger motions. (e) Signal generated by the bending of an index finger at varying degrees. (Reprinted with permission from [79].)

Figure 9

(a) The schematic illustrates a hydrogel undergoing indentation, with differential ionic displacement and field observed. The smaller red cations are transported through the green polymer chain network more rapidly than the blue anions, creating a charge imbalance and generating an electric field. (b) Voltage response of PAAm hydrogel upon step compression at 20 kPa. (c) Peak voltage produced in relation to the pressure being applied. (d) A wrist-mounted 16-element piezoionic mechanoreceptor array. (e) Photograph and related normalized voltage bar plot of the piezoionic mechanoreceptor array that detects single and multiple touches. (Reprinted with permission from [75].)

Figure 10

(a) The electrical behavior and response of the ionic diode including potential diagram. (b) The electrical behavior and response of the ionic diode under external mechanical stress. (c) Voltage and current output generated at various external pressures. (d) Picture of an arm-wrapped self-powered hydrogel tactile sensor array. Insets: (top panel) voltage signal mapping obtained by pushing the center pixel. (Reprinted with permission from [82].)

Figure 11

(a) Thickness-dependent self-induced potential of ionic diode. (b) Decreased voltage loss at the diode/electrode interfaces in response to pressure. (c) Output voltage and sensitivity of self-powered pressure sensors as a function of pressure. (d) Output voltage and sensitivity of self-powered sensors as a function of strain. (Reprinted with permission from [83].)

Figure 12

(a) Synthesis mechanism of hydrogel-based GPMs. (b) Schematic illustration of pressure-sensing mechanism. (c) The voltage produced by self-powered i-skins while experiencing gradually increasing pressure, as well as in response to walking and running motions. (Reprinted with permission from [84].)

Figure 14

(a) I. Thermal voltage generation mechanism of thermodiffusion of ions. II. Relative resistance change during stretching and releasing. III. Schematic illustration of the thermo-powered ionic hydrogel strain sensor. IV. Thermal charge and voltage changes during compression and relaxation. (Reprinted with permission from [98].) (b) I. Voltage changes of the hydrogel at various compressive strains with a ΔT of 7.5 K. II. The voltage changes of the hydrogel sensor in parallel with thermoelectric hydrogels at various tensile strains. III. Corresponding equivalent circuit diagram. (Reprinted with permission from [99].)

Figure 15

(a) I. Hydrogel-based TEC utilizing Sn⁴⁺/Sn²⁺ temperature-dependent redox reactions. II. Experimental set-up and equivalent circuit for the measurement of the self-powered strain sensor. III. Variation in current and voltage for the self-powered TEC sensor. IV. Finger movement monitoring using a self-powered strain sensor. (Reprinted with permission from [106]). (b) I. Schematic of temperature−pressure-sensing mechanism. II. Seebeck coefficient fitting relative current change versus pressure of the TGH sensor. III. Responses of a self-powered TGH sensor to finger touch in terms of relative current change and output thermal voltage. IV. TGH sensor array output voltage and relative current change response curves with relaxed and bending states on the human wrist. (Reprinted with permission from [100]).