Abstract
1. Introduction
Flexible sensors have been widely used in various types of electronic devices. Their advantage lies in their high flexibility, ability to adapt to various complex environments, and broad development prospects. Flexible sensors with nanomaterials have a larger specific surface area to further enhance sensor performance. There are several ways to make nanomaterials. Among them, electrospinning provides a low-cost, large-scale method for preparing flexible nanofibers. The morphology of the fibers can be adjusted during the electrospinning process to obtain different structures[
In this review, we summarize the progress in the preparation of flexible sensors by electrospinning. Photodetectors, stress sensors, gas sensors and nanogenerators were introduced. The prospect of flexible sensors and electrospinning were discussed towards the end.
2. Electrospinning
Electrospinning devices typically consist of a propulsion pump, a high voltage power supply, and a collection plate. When the voltage applied at the end of the needle exceeds a certain critical value, the charge repulsion on the surface of the solution exceeds its surface tension, and the jet formed at the end of the needle is deposited on the collecting plate under the action of an electric field to form a fiber membrane. The parameters used in the electrospinning process will affect the morphology of the fiber to some extent. An increase in the spinning voltage, a decrease in the advancement speed, and an increase in the distance between the electrodes and the collection plate can reduce the diameter of the resulting nanofiber. There are usually two methods for preparing conductive nanofibers by electrospinning, as shown in Fig. 1[
Figure 1.(Color online) (a) Flowchart for preparing conductive nanofibers by surface modification[
3. Strategies for preparing conductive nanofibers
3.1. Surface treatment
An electrospun fiber is used as a template, and a metal material such as gold or copper is deposited thereon to obtain a conductive fiber. The electrospinning film can also be removed by sintering, dissolution, or the like in a subsequent step. Similarly, conductive fibers can also be obtained by chemically depositing or coating a polymer conductive material, carbon nanotubes or the like onto the surface of the fiber membrane. This method of treating only the surface of the fiber causes the conductive material to form a cylindrical structure, increase the specific surface area, and have a certain conductivity. This also increases their sensitivity as a gas sensor.
3.2. Precursor heat treatment
Conductive fibers can also be obtained by heat treating a precursor of a certain material. For example, the carbon fiber is obtained by electrospinning a polyacrylonitrile (PAN) solution of N,N-dimethylformamide (DMF) and heat treatment. Mixing a precursor thereof with a polymer solution and electrospinning, then sintering in an air atmosphere to remove components such as a polymer can obtain an inorganic oxide fiber. This method can completely remove the polymer component and obtain a continuous conductive fiber.
4. Application
4.1. Photodetector
Due to its photoelectric effect, semiconductors can be exposed to light and cause a change in conductivity. The light intensity can be characterized by changes in material current. Zhang et al.[
Figure 2.(Color online) (a) Schematic diagram of field effect test device for zinc oxide fiber. (b) Field effect characteristics of Ce-doped ZnO[
Figure 3.(Color online) (a) Schematic of the printing unit. (b) Time-domain optical response of the ZnO fiber array. (c)
4.2. Stress sensor
The conductive fiber membrane prepared by electrospinning is easily changed under pressure for the variation of shape, and the electrical resistance of the fiber membrane is also changed. Therefore, the change in the current of the conductive fiber membrane can characterize the change in the corresponding stress[
Figure 4.(Color online) (a)
A silver-loaded alginate stress sensor was prepared by Hu et al.[
Figure 5.(Color online) (a) Electrical performance of the sensor at different pressures. (b) Current change of the sensor at different pressures. (c) Response of the sensor to human respiration. (d) Response of the sensor to “Nano” and “Perfect”. (e) Electrode array diagram. (f) Pressure distribution[
4.3. Gas sensor
Due to its structural advantages, the large specific surface area and high porosity allow rapid gas detection, and the nano-conductive fiber membranes are well suited for use as gas sensors. Zinc oxide has a special shape and is widely used in various types of sensors. Zhang et al.[
Figure 6.(Color online) (a) PEDOT:PSS/PVP composite nanofibers response to CO. (b) PEDOT:PSS/PVP composite nanofibers response to different concentrations of CO[
The diameter of nanofibers affects their sensing properties. Zhang et al.[
Figure 7.(Color online) (a) SEM image of ultrafine fiber. (b) Particle size distribution of ultrafine fiber. (c, d) Ammonia response curve of traditional electrospun fiber. (e, f) Ammonia response curve of microfiber[
Zhang et al.[
Figure 8.(Color online) (a) Photo of the mask combined with the sensor. (b) Response in normal and running state. (c) Response in joy state. (d) Response in sad state. (e) Response in sleep state. (f) Response in normal breathing (NB) and hard to breathe (HB) conditions[
4.4. Nanogenerator
Nano-generators can convert small-scale environmental energy into electrical energy, and self-powered systems based on nano-generators will be a good choice for reducing the size of electronic devices. On the other hand, the strength of the output signal of the nanogenerator is related to pressure and temperature, and can be used as a self-powered sensor. Due to its portability and self-powered features, there are many applications in the field of wearable devices. You et al.[
Figure 9.(Color online) (a) Piezoelectric properties of PVDF under pressure. (b) Piezoelectric properties of PVDF during bending. (c) Pyroelectric characteristics of PVDF. (d) Mixed output of piezoelectric signals and pyroelectric signals of PVDF[
Figure 10.(Color online) (a) Electronic skin map and wiring diagram. (b) Piezoelectric properties of electronic skin under pressure. (c) Piezoelectric properties of electronic skin during bending. (d) Pyroelectric characteristics of electronic skin[
Figure 11.(Color online) (a) Pressure profile. (b) Temperature profile. (c) Output of electronic skin after short-circuit of adjacent components. (d) Output signal of direct short-circuit of resistor unit. (e) Output of resistor unit in short-circuit condition of adjacent components[
Guo et al.[
Figure 12.(Color online) (a) Bluetooth device schematic. (b) Software interface diagram. (c) Wireless signal when squatting. (d) Wireless signal while walking. (e) Wireless signal during running. (f) Effect of BaTiO3 content on piezoelectricity. (g) 3% BaTiO3 The piezoelectricity of the PVDF fiber membrane at different frequency impacts[
Figure 13.(Color online) (a) Triboelectric nanogenerator structure diagram. (b) Open circuit voltage output. (c) Breathing output signal. (d) Finger click output signal[
5. Summary
Flexible sensors composed of conductive nanofibers exhibit outstanding performance. The large-scale and low-cost development of high-performance conductive nanomaterials will greatly promote the development of flexible sensors. Of course, electrospinning technology is an alternative solution. Electrospinning technology can control the fiber diameter and even produce fibers with a diameter of less than 1 nm[
Acknowledgements
This work was supported by the National Natural Science Foundation of China (Nos. 51673103 and 51973100).
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