Resistive flexible strain sensor based on AgNWs-Gr-PDMS for wearable electronics

With the advancement of wearable devices,^1,2 intelligent robots,^3,4 medical monitoring devices,^5,6 and electronic skins,^7,8 the core technology—flexible pressure sensors—has attracted a lot of attention.^9–11 Among multiple flexible sensors, flexible strain sensors have been widely applied in wearable devices, smart textiles, and human health monitoring due to its good flexibility and stretchability and its ability to adapt to objects of different shapes and sizes.^12–15 According to the sensor mechanism, flexible strain sensors can be categorized into capacitive,¹⁶ resistive,¹⁷ and piezoelectric ones,¹⁸ and resistive flexible strain sensors have become a research hotspot for scholars and researchers due to their simple structure, high sensitivity, low cost, and rapid response.^19–22 

Resistive flexible strain sensors are based on the principle of resistance change, and the basic mechanism is to detect variations in strain by changing the resistance value of the resistive material.^23,24 Resistive flexible strain sensors generally consist of a layer of flexible resistive material and two electrodes. When the sensor is subjected to strain, the length and cross-sectional area of the resistive material change, resulting in a change in its resistance value that can be detected by measuring the voltage or current between the electrodes.^25–28 In general, such sensors contain materials such as metal nanoparticles, metal nanowires, carbon nanotubes, and graphene as resistive elements. In particular, graphene and silver nanowires are widely used in flexible strain sensors due to their good electrical conductivity, mechanical properties, optical properties, chemical stability, and biocompatibility.^29–33 

In this paper, a resistive flexible strain sensor based on the AgNWs-graphene-PDMS (AGP) sensitive layer is prepared with polydimethylsiloxane (PDMS)³⁴ as the substrate material and graphene (Gr) and silver nanowires (AgNWs) as the conductive material. The strain-sensitive layer is prepared by the spraying process, and the encapsulation is carried out by the spin-coating process. At the same time, the performance of the sensor prepared by the AGP sensitive layer is also tested. The results show that the sensor features simple preparation, high sensitivity, good repeatability, and stability.

The preparation process of the strain sensitive layer is shown in Fig. 1. Take PDMS (20 ml) and hardener (2 ml) for proportioning, and stir the mixed liquid for 2 h with a magnetic stirrer such that the air inside the liquid is discharged; pour the stirred mixture onto a glass sheet, and place it on a homogenizer to rotate at 300 rpm for 30 s to obtain a uniformly distributed PDMS pre-film. Then place it on a heating table and heat for 2 h at 80 °C to solidify the PDMS, and the PDMS film is obtained for use.

Add Gr (2 g) and AgNWs (2 g) to a beaker with 100 ml of ethanol to obtain AgNWs-graphene (AgNWs-Gr) mixture at a concentration of 2%; in the same way, AgNWs-Gr mixtures at concentrations of 3%, 4%, 5%, and 6% are obtained. The beaker containing the AgNWs-Gr mixture is placed on a magnetic stirrer for 2 h to fully mix the silver nanowires, graphene, and ethanol, and for 2 h for ultrasonic processing to have homogeneously dispersed AgNWs and Gr. The processed AgNWs-Gr mixture is sprayed on the prepared PDMS film with a spray gun and placed on a heating table and heated at 120 °C for 2 h so that the ethanol is fully volatilized. The AgNWs-Gr mixture is fully dried and adhered to the PDMS film, and the strain-sensitive layer required herein is obtained. The strain-sensitive layer prepared as mentioned above is placed on the operation platform. Two sections of copper foil tape are used as the electrodes of this sensor, and two wires are connected for subsequent measurements. A layer of PDMS is spin-coated again on the sprayed AgNWs-Gr-PDMS and heated at 80 °C for 2 h to solidify the PDMS. In this way, a resistive flexible strain sensor based on the AGP sensitive layer is fabricated by peeling it off the substrate.

The microstructure characterization of the prepared AgNWs-Gr sensitive layer is shown in Fig. 2. It can be seen from Fig. 2 that AgNWs and graphene are uniformly and densely dispersed on the PDMS film, and these two materials cross and overlap with each other to achieve good conductivity. It can be observed that AgNWs and graphene are interwoven and embedded to form a complete conductive network.

The testing platform for the flexible strain sensor consists of a digital tensile machine, a digital multimeter, a caliper, and an upper computer, as shown in Fig. 4. The fixture is used to clamp both ends of the prepared flexible strain sensor, the upper computer and the digital multimeter are connected to the flexible strain sensor, the digital tensile machine is used to provide the flexible strain sensor with different tensions, and the caliper is used to measure the tensile length of the flexible strain sensor. The tension displayed by the digital tensile machine, the resistance value at the corresponding moment displayed by the upper computer, and the corresponding measurement value of the caliper are recorded in real time and calculated to obtain the sensitivity, response time, test range, and other data of the flexible strain sensor.

To test the influence of different AgNWs-Gr concentrations on the sensitivity of the flexible strain sensor, the flexible strain sensors prepared with five different AgNWs-Gr concentrations (2%, 3%, 4%, 5%, and 6%) are subjected to strain tests in this paper, and the curves obtained from the tests are shown in Fig. 5(a). As can be seen from Fig. 5(a), the flexible strain sensor prepared with a concentration of 6% AgNWs-Gr mixture has the highest sensitivity since as the concentration of the AgNWs-Gr mixture increases, the number of AgNWs-Gr on the strain-sensitive layer increases, and the better the electrical conductivity is, the higher the rate of change in the resistance of the resistive flexible strain sensor is under the application of the same tensile force, and thus the higher the sensitivity is.

Figure 5(b) presents the test results of the relative resistance rate of change of the flexible strain sensor. It can be seen from Fig. 5(b) that the relative resistance rate of change is low at a stretch degree of 0%–50%, and its sensitivity is 5; at a stretch degree of 50%–100%, the relative resistance rate of change is slightly increased, and its sensitivity is 41; the relative resistance rate of change is the highest at a stretch degree of 100%–120%, and its sensitivity is 236. This is because under small tension, the structure of the flexible strain sensor undergoes small changes and the strain of the strain-sensitive layer is small, resulting in a low relative resistance rate of change and low sensitivity. Similarly, under large tension, the structure of the flexible strain sensor undergoes large deformation and the strain of the strain-sensitive layer is large, resulting in a large relative resistance rate of change and high sensitivity.

To test the repeatability of the sensor under different stretching conditions, the sensor is stretched by 30%–110%; the test is repeated five times, and the test results are shown in Fig. 5(c). It can be seen that the small deviations of waveform and the peak indicate a small error in the resistance value of the flexible strain sensor, which shows that the sensor has good repeatability.

To test the stability of the sensor in different stretching situations, the flexible strain sensor is stretched by 80%; the test is repeated more than 500 times, and the test results are obtained as shown in Fig. 5(d). It can be seen from Fig. 5(c) that the sensor has similar waveforms and similar peaks under the repeated application of the same pressure, indicating that the sensor has good stability. Through testing, the flexible strain sensor described in this paper breaks at a tensile degree of 118%, and the resistive strain sensor has a larger strain range (0%–118%) than conventional flexible strain sensors.

In addition to parameters such as sensitivity, repeatability, and stability, to measure the excellence of a flexible strain sensor, response time is also an important element. The flexible strain sensor is rapidly stretched to 80% and rapidly released for recovery when the digital multimeter shows a smooth value. By virtue of testing and analysis, the response time of the sensor is 83 ms, and the recovery time is 116 ms, as shown in Fig. 5(e). The sensor exhibits excellent stability and recoverability when applying strain and releasing strain, as is shown in Fig. 5(f).

It can be seen from the above-mentioned tests that the resistive flexible strain sensor has characteristics of high sensitivity, rapid response time, simple preparation, good repeatability, stability, etc. It can be applied in practical applications in view of its excellent performance, with the application directions to be determined by the testing data of the sensor in practical applications. Although the resistive flexible strain sensor has high sensitivity under large tensile forces, the range, linearity, and stability of the sensor require to be considered in practical applications. At the same time, large tensile forces may cause structural damage or fatigue failure of the flexible strain sensor.

The resistive flexible strain sensor described in this paper is tightly attached to the joint of the index finger and connected to a digital multimeter. The finger is bent to different angles (30°, 45°, 60°, and 90°), and the resistance value is recorded and tested using the digital multimeter and the upper computer in real time. The curves are shown in Fig. 6. It can be seen that the resistive flexible strain sensor can accurately monitor the finger bending changes in real time and have a broad application prospect in the fields of intelligent robots and wearable devices. To further investigate the performance of this sensor, we attached the sensor to the skin of the face. The resistance value of the sensor changes with the smile, as shown in Fig. 6(b).

By attaching the strain sensor to the throat, speech recognition can be achieved through the vibration of the vocal cords. The sensor can distinguish different English words, such as A and interesting, and the results are shown in Fig. 7.

In this paper, a resistive flexible strain sensor based on the AGP sensitive layer is prepared with PDMS as the substrate material, AgNWs and Gr as the conductive materials. The strain-sensitive layer is prepared by the spraying process, and encapsulation is carried out by the spin-coating process. The performance of the sensor prepared with different concentrations of the AgNWs-Gr mixtures is also tested. The results show that the prepared resistive flexible strain sensor has the highest sensitivity of up to 236 with the AgNWs-Gr mixture at a concentration of 6%, the flexible strain sensor has rapid response time (83 ms) and recovery time (116 ms), and the flexible strain sensor also has good repeatability and stability. Given the excellent performance of the flexible strain sensor, it is placed at the finger joints for practical testing. According to the analysis on the detected data, the flexible strain sensor can accurately monitor the finger bending changes in real time and have a broad application prospect in intelligent robots, wearable devices, electronic skin, and so on.

This work was supported by the Tai Yuan Institute of Technology Scientific Research Initial Funding, Grant No. 2023KJ016.

The authors have no conflicts to disclose.

All works with relation to this paper have been accomplished by all authors’ efforts. R.W. performed the measurements, analyzed the data, and wrote the main manuscript text. The experiments of the sensor were completed with the help from X.L. T.C. analyzed the data, and R.W. contributed to the manuscript writing. All authors reviewed the manuscript.

Ruirong Wang: Writing – original draft (equal); Writing – review & editing (equal). Xiaohong Li: Software (equal). Tong Chen: Supervision (equal).

The data that support the findings of this study are available from the corresponding author upon reasonable request.