Preparation and application of CuNPs / S-GS / PDMS flexible piezoresistive sensor

Abstract

The invention discloses a preparation method of a CuNPs / S-GS / PDMS flexible piezoresistive sensor and application of the CuNPs / S-GS / PDMS flexible piezoresistive sensor. The sensor is composed of a PDMS flexible substrate layer, a surface interdigital electrode, a CuNPs / S-GS / PDMS composite foam sensitive layer covering the electrode, and a PET packaging layer. The preparation method comprises the following steps: (1) mixing a graphene oxide dispersion liquid with a gradient concentration copper nitrate solution, adding a sodium polysulfide solution under the protection of nitrogen, and reacting at 90-100 DEG C to synthesize CuNPs / S-GS; (2) loading the CuNPs / S-GS on the PDMS by adopting a template method; and (3) fixing an interdigital electrode on the surface of the PDMS thin film, covering the composite foam body, and then packaging and molding by PET (Polyethylene Terephthalate). According to the sensor, CuNPs are connected with a sulfur-doped graphene network, so that the conductivity of the composite foam is improved by two orders of magnitude, three-stage high sensitivity is achieved when the CuNPs loading capacity is 30 wt%, 0.6 s quick response and 2000 times of cycle stability are achieved, human joint movement (bending of fingers, elbows and knees) can be accurately monitored in real time, and the sensor has wide application prospects in the field of wearable health monitoring.

Description

Technical Field

[0001] The present invention relates to the field of sensor technology, and in particular to a method for preparing a CuNPs / S-GS / PDMS flexible piezoresistive sensor. Background Art

[0002] Flexible piezoresistive sensors have important application value in the fields of human physiological monitoring (such as pulse detection, motion sensing) and engineering safety monitoring (such as pipeline stress detection) due to their excellent deformation adaptability, fast response characteristics and durability. In the existing technology, flexible piezoresistive sensors based on polydimethylsiloxane (PDMS) and conductive materials have become a research hotspot. For example, the invention patent with authorization announcement number CN112086553B proposes a three-dimensional interconnected graphene microchannel embedded PDMS sensor. By depositing precious metal nanoparticles (gold, silver or platinum) on the inner wall of the graphene microchannel, the sensitivity of the sensor is significantly improved (up to 53.7kPa). -1 ) and dual-mode detection capability (simultaneous monitoring of pressure and strain). However, this technology has the following limitations: High material costs: The use of precious metal nanoparticles (gold, silver, platinum) significantly increases manufacturing costs, making it unfavorable for large-scale commercial applications; The process is complex and environmentally unfriendly: Graphene growth relies on chemical vapor deposition (CVD) and requires the use of a highly corrosive FeCl3 solution to remove the nickel foam template. This process is cumbersome and produces polluting wastewater. Single function: The sensor structure is a single-point unit and lacks array design, making it difficult to achieve real-time monitoring of spatial pressure distribution.

[0003] To overcome these challenges, recent research has attempted to optimize the process using low-cost materials (such as copper nanoparticles) and simple templates (such as sugar templates). However, existing alternatives still face challenges such as insufficient conductive network stability and difficulty balancing sensitivity and detection range. In particular, an efficient system solution for arrayed sensor integration has yet to be developed.

[0004] Therefore, there is an urgent need to develop a low-cost, green-process, flexible piezoresistive sensor with both high sensitivity and array detection capabilities to meet the needs of real-time monitoring of pressure distribution in scenarios such as wearable devices and smart correction devices. Summary of the Invention

[0005] In view of this, the object of the present invention is to provide a method for preparing a CuNPs / S-GS / PDMS flexible piezoresistive sensor to solve the problems existing in the above-mentioned background technology.

[0006] The method for preparing the CuNPs / S-GS / PDMS flexible piezoresistive sensor of the present invention comprises the following steps: 1) Preparation of CuNPs / S-GS A 1 mg / mL graphene dispersion was mixed with a 0.15-0.75 wt / v% copper nitrate solution. A sodium polysulfide solution was added under nitrogen protection and reacted at 90-100°C for 3-8 hours to form a CuNPs / S-GS precipitate. After washing and drying, a black powder of CuNPs / S-GS was obtained. The sodium polysulfide solution was prepared by adding 3.2 g of sublimed sulfur to 50 mL of 2.5 mol / L Na2S solution, stirring for 1 to 2 hours, and filtering to obtain a uniform sodium polysulfide solution. The amount of sodium polysulfide solution added is 100~200mL; 2) Preparation of CuNPs / S-GS / PDMS composite foam White sugar and CuNPs / S-GS were mixed in a mass ratio of 30:1-50:1, spread on a glass slide and micro-melted to form a sugar template. PDMS was then poured, and after vacuum degassing, it was cured at 80±5℃ for 1-2h. The sugar template was removed by washing with water to obtain CuNPs / S-GS / PDMS composite foam. 3) Device assembly First, the interdigital electrodes were fixed on the surface of the PDMS film, and then the CuNPs / S-GS / PDMS composite foam was covered on the interdigital electrodes. Finally, the whole was encapsulated with a PET film to prepare a flexible piezoresistive sensor.

[0007] The flexible piezoresistive sensor of the present invention comprises: Flexible substrate: PDMS film; Electrode layer: interdigitated electrodes arranged on the surface of the PDMS film; Sensitive layer: CuNPs / S-GS / PDMS composite foam covering the interdigitated electrodes; Encapsulation layer: A PET film covering the PDMS film, the interdigitated electrodes and the sensitive layer to form an overall sealed structure.

[0008] This paper mainly prepares and studies the CuNPs / S-GS / PDMS piezoresistive sensor, explores the effect of the loading amount of CuNPs in PDMS foam on the sensitivity of the sensor, and also explores the sensing performance, stability and practical application of the sensor in detecting the movement state of human joints.

[0009] The main innovations are as follows: 1. Scanning electron microscopy (SEM) characterization of the composite foam revealed that S-GS and CuNPs were uniformly attached to the interior of the PDMS foam. As a two-dimensional, planar nanomaterial with numerous wrinkles, S-GS is well-suited as a carrier for CuNPs, achieving high-density, uniform loading of CuNPs on its surface. Furthermore, CuNPs act as bridges connecting adjacent graphene layers, providing a large number of free electrons to S-GS through the CuNP-graphene interface, increasing the electrical conductivity of the PDMS composite foam by two orders of magnitude. 2. By testing the sensitivity of the sensor, it was found that with the increase of CuNPs loading, the sensitivity of the CuNPs / S-GS / PDMS composite foam sensor first increased and then decreased. When the CuNPs loading rate was 30wt%, the sensitivity of the CuNPs / S-GS / PDMS composite foam flexible piezoresistive sensor reached its maximum value, with a sensitivity of 17.78kPa in the pressure ranges of 0-5.06kPa, 5.06-39.51kPa and 39.51-92.25kPa, respectively. -1 , 0.85kPa -1 and 0.09kPa -1 sensitivity; 3. When subjected to an applied pressure of 80 kPa, the sensor exhibited a response time of 0.6 s and an unloading time of 0.81 s. Furthermore, the sensor demonstrated excellent repeatability and stability over 2000 compression-release cycles, significantly improving upon the S-GS / PDMS composite foam piezoresistive sensor. These exceptional properties enable the sensor to monitor human joint movement in real time, demonstrating its broad potential for application. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] Figure 1 This is an optical microscope image of CuNPs / S-GS / PDMS (30 wt %) prepared in an embodiment of the present invention.

[0011] Figure 2 SEM images and elemental mapping of CuNPs / S-GS / PDMS (30 wt%) prepared in this embodiment of the present invention: (a)-(d) SEM images; (e)-(i) elemental mapping images.

[0012] Figure 3 FTIR spectra of PDMS, S-GS / PDMS and CuNPs / S-GS / PDMS prepared in the examples of the present invention.

[0013] Figure 4 XRD curves of PDMS, S-GS / PDMS and CuNPs / S-GS / PDMS prepared in the examples of the present invention.

[0014] Figure 5XPS spectra of CuNPs / S-GS / PDMS prepared in this embodiment of the present invention: (a) full spectrum; (b) C 1s; (c) Si 2p; (d) S 2p and (e) Cu 2p.

[0015] Figure 6 Mechanical property curves of the CuNPs / S-GS / PDMS composite foam prepared in this embodiment of the present invention: (a) stress-strain curve; (b) tensile strain curve.

[0016] Figure 7 Diagram of the pressure-sensitive mechanism of the CuNPs / S-GS / PDMS composite foam flexible piezoresistive sensor prepared in an embodiment of the present invention.

[0017] Figure 8 This is the sensitivity curve of the CuNPs / S-GS / PDMS composite foam flexible piezoresistive sensor prepared in an embodiment of the present invention.

[0018] Figure 9 The resistance change of CuNPs / S-GS / PDMS piezoresistive sensor under different pressures.

[0019] Figure 10 Test curves of different piezoresistive characteristics of the CuNPs / S-GS / PDMS (30wt%) piezoresistive sensor: (a) Stability and repeatability of the resistance change rate of the piezoresistive sensor after 50 cycles under different pressures; (b) Stability of the resistance change rate of the piezoresistive sensor when maintained at different pressures for 10 seconds; (c) Response curves of the piezoresistive sensor to different pressure signals; (d) Response curves of the piezoresistive sensor under different compression rates.

[0020] Figure 11 The cyclic stability test curve of the CuNPs / S-GS / PDMS (30wt%) piezoresistive sensor: (b) the resistance change rate curve of the pressure sensor after 2000 cycles; (a) and (c) are partial enlarged views of Figure b.

[0021] Figure 12 is the response time of CuNPs / S-GS / PDMS (30wt%) piezoresistive sensor at 80 kPa.

[0022] Figure 13 Application demonstration of CuNPs / S-GS-PDMS (30wt%) piezoresistive sensor: (a) Finger bending test photo and resistance change curve; (b) Finger bending test photo and resistance change curve; (c) Elbow bending test photo and resistance change curve; (d) Knee bending test photo and resistance change curve. DETAILED DESCRIPTION

[0023] The present invention will be further explained below with reference to specific embodiments. Example

[0024] 1. Preparation of CuNPs / S-GS: Disperse 0.20g of GO in 200mL of distilled water and sonicate to obtain a GO solution (200mL of 1mg / mL). Weigh 0.15g, 0.20g, 0.30g, 0.35g, 0.45g, and 0.75g of Cu(NO3)2 and dissolve them in 100mL of deionized water to prepare a Cu(NO3)2 solution. Mix the GO solution and the Cu(NO3)2 solution, flow nitrogen for 5min, and then slowly add Na2S X The solution, in 100ml, 115ml, 130ml, 150ml, 180ml, and 200ml volumes, was heated in a water bath at 95°C for 4 hours with magnetic stirring to obtain a reddish-brown solution containing a black precipitate. The solution was then filtered while hot, washed multiple times with deionized water, and dried in an oven at 80°C for 2 hours to obtain a black powder, which was labeled CuNPs / S-GS (10wt%), CuNPs / S-GS (15wt%), CuNPs / S-GS (20wt%), CuNPs / S-GS (25wt%), CuNPs / S-GS (30wt%), and CuNPs / S-GS (40wt%).

[0025] Among them, Na2S X The solution was prepared by adding 3.2 g of sublimed sulfur to 50.0 mL of 2.5 mol / L Na2S solution, stirring for 1 h, and filtering to obtain a uniform reddish-brown Na2S solution. X Solution ((X-1)S8+8Na2S→8Na2S x The preparation of S-GS only requires the removal of Cu(NO3)2 in the preparation of CuNPs / S-GS.

[0026] 2. Preparation of CuNPs / S-GS / PDMS composite foam Take 10 g of white sugar and mix it with 0.33 g of CuNPs / S-GS3, spread it flat on a Petri dish with a glass slide, and then put it into an oven to melt it slightly. After taking it out and letting it cool, pour the mixture of PDMS (Sylgard184, A:B = 10:1) and curing agent on the surface, cover it with a glass slide, clamp the two glass slides with a clamp, and then vacuum it for 20 minutes. Then put the sample into 80 oAfter curing in a C oven for 2 h, the sugar template was immersed in distilled water to obtain CuNPs / S-GS / PDMS (10 wt%), CuNPs / S-GS / PDMS (15 wt%), CuNPs / S-GS / PDMS (20 wt%), CuNPs / S-GS / PDMS (25 wt%), CuNPs / S-GS / PDMS (30 wt%), and CuNPs / S-GS / PDMS (40 wt%) composite foams. The experimental formulas are listed in Table 1: Table 1 PDMS foam formulations with different copper nanoparticle doping amounts 3. Assembly of CuNPs / S-GS / PDMS sensor The CuNPs / S-GS3 / PDMS composite foam was cut into a size of 2.0 cm×2.0 cm, and the interdigital electrodes were placed on the PDMS film as conductive electrodes. The cut composite foam was then transferred to the interdigital electrodes and finally encapsulated with PET film to prepare a flexible piezoresistive sensor.

[0027] The structural characterization and performance evaluation of the products prepared in the above examples are as follows: 1. Structural characterization First, the microstructure of CuNPs / S-GS / PDMS (30 wt%) composite foam was observed under an optical microscope at 100 times magnification. Figure 1 As shown in the figure, the pores of PDMS foam are open and CuNPs / S-GS are dispersed in the PDMS foam.

[0028] Secondly, the SEM image is collected. Figure 2 As shown in (ab), the composite foam has an obvious open-cell structure, the inner wall of the cell is rough and without cracks, the outer shell is thick, and the pore size is about 100 μm-500 μm. Figure 2 In (c), CuNPs are uniformly distributed on the surface of S-GS and PDMS or embedded within PDMS. At higher magnification, the S-GS sheets are thin, translucent, and wrinkled; the CuNPs are hexagonal flakes of uniform size, with opposite sides of approximately 1 μm. Furthermore, no significant gaps are observed at the interface between the CuNPs and S-GS, indicating excellent interfacial interaction ( Figure 2 (d)). Figure 2 (ei) are the corresponding element mapping diagrams of CuNPs / S-GS / PDMS (30wt%) composite foam. It can be seen that C, O, S, N and Cu elements are uniformly distributed in the composite material. This result further indicates the successful loading of CuNPs / S-GS in PDMS foam.

[0029] Figure 3 The infrared spectra of PDMS foam, S-GS / PDMS and CuNPs / S-GS / PDMS composite foam are shown in Figure 2. For CuNPs / S-GS / PDMS composite foam, except for the peak at 2968 cm -1 and 796 cm -1 The absorption peaks corresponding to the stretching vibration of CH and the bending vibration of Si-CH3 of methyl group; at 1010 cm -1 The absorption peak at 1251cm corresponds to the stretching vibration of Si-O-Si; -1 Aside from the absorption peak representing CS stretching vibration, which is identical to the absorption peaks seen in S-GS / PDMS composite foam, no other characteristic peaks are observed. This is because absorption peaks in infrared spectroscopy are typically caused by molecular vibration and rotation. For metallic copper, its infrared spectrum typically exhibits a flat baseline without distinct characteristic peaks.

[0030] The crystalline structures of S-GS, PDMS and CuNPs / S-GS / PDMS were analyzed by XRD. Figure 4 As shown. Compared with S-GS and PDMS, in the XRD pattern of CuNPs / S-GS / PDMS, in addition to the characteristic peak belonging to PDMS at 2θ=11° and the weak (002) crystal plane characteristic peak belonging to S-GS3 at 2θ=24°, the diffraction peaks at 2θ=43.3°, 50.4° and 74.1° are consistent with those of face-centered cubic copper (Cu-PDF#04-0836), corresponding to the (111), (200) and (220) crystal planes respectively; the diffraction peaks at 47.6° and 58.9° correspond to the diffraction of the (20-2) and (202) crystal planes of CuO (JCPDS 51-1731), respectively; the characteristic diffraction peak at 2θ=36.7° is consistent with that of Cu2O (JCPDS The diffraction peak (2θ=36.418°) of CuNPs (05-0667) is consistent with that of CuO, which corresponds to the diffraction of the crystal plane at (111). This proves that CuNPs and S-GS are successfully added to the PDMS composite foam, and the composite material contains a certain amount of CuO and Cu2O.

[0031] The elemental composition and chemical state of the CuNPs / S-GS / PDMS sample were analyzed by XPS characterization. Figure 5 shown. Figure 5(a) shows the full spectrum of the CuNPs / S-GS / PDMS sample, which shows the presence of S 2p (119.1 eV), C 1s (284.1 eV), O 1s (531.1 eV), Si 2p (101.1 eV) and Cu 2p (978.1 eV) peaks. Figure 5 In the C 1s spectrum shown in (b), the characteristic peaks near binding energies of 283.7 eV, 285.2 eV, 286.2 eV, and 287.8 eV are attributed to C-Si, CC / C=C, CO, and C=O groups, respectively. Figure 5 In the Si 2p spectrum in (c), two significant peaks related to CO-Si and Si-O-Si were observed at 101.2 and 102.6 eV. Figure 5 In (d), the S 2p spectra near 164.7, 166.9 and 169.3 eV are attributed to elemental sulfur, thiophene sulfur and sulfonate, respectively. Figure 5 The Cu 2p spectrum shown in (e) shows two characteristic peaks at 932.0 eV and 951.5 eV, attributed to the binding energies of Cu 2P3 / 2 and Cu 2P1 / 2 in Cu and Cu2O, respectively. Due to the close proximity of the Cu and Cu2O signal peaks, XPS cannot fully distinguish between Cu and Cu2O. The characteristic peaks at 933.8 eV and 954.5 eV correspond to the binding energies of Cu2P3 / 2 and Cu2P1 / 2 in CuO, respectively. The remaining peaks are likely satellites or companion peaks of these peaks. This XPS spectrum indicates that the composite material contains a certain amount of CuO and Cu2O, which is consistent with the XRD characterization results.

[0032] 2. Performance evaluation 2.1 Mechanical properties In order to evaluate the effect of the mass percentage of CuNPs on the mechanical properties of CuNPs / S-GS / PDMS composite foams, the compression and tensile properties of six groups of composite foams with different CuNPs mass percentages were tested and compared with pure PDMS and S-GS / PDMS composite foams. Figure 6As can be seen in (a), under 20% strain conditions, pure PDMS, S-GS / PDMS composite foam, and samples with different CuNPs percentages all exhibit similar stress-strain curves. Under the same deformation, PDMS foam still has the lowest compressive stress value, indicating its excellent elasticity. Under the same reaction conditions, as the mass percentage of CuNPs increases from 10wt% to 40wt%, the maximum compressive stress gradually increases, reaching 25.7 kPa, 32.4 kPa, 40.6 kPa, 44.4 kPa, 46.2 kPa, and 57.2 kPa, respectively. This is because CuNPs themselves have a high elastic modulus and hardness, and their addition may increase the elastic modulus and hardness of the entire composite material. Figure 6 (b) Tensile stress-strain curves for the eight samples show that the elongation at break of the CuNPs / S-GS / PDMS increases from 37% (S-GS / PDMS) to 77% (CuNPs / S-GS / PDMS (40 wt%)). These results demonstrate that the addition of CuNPs improves the mechanical properties of PDMS composite foams.

[0033] 2.2 Sensing performance 1) Electrical conductivity The resistance of the CuNPs / S-GS / PDMS composite foam was tested by a digital multimeter, and its conductivity was calculated. The data are shown in Table 2: Table 2 Electrical conductivity of S-GS / PDMS composite foam In the series of experiments with varying CuNPs mass percentage, the conductivity of CuNPs / S-GS / PDMS (30wt%) was the highest, reaching 4.23×10 -5 S·m -1. In the S-GS / PDMS composite foam piezoresistive sensor, S-GS provides a conductive pathway, and charges are transported by jumping between S-GS sheets. After the additional addition of CuNPs, since CuNPs have better conductivity than S-GS, they can provide additional conductive channels for the composite foam, thereby reducing the overall resistance of the composite foam. In addition, there is a certain synergistic effect between S-GS and CuNPs, and mutual contact may form a mechanism for synergistically enhancing conductivity, so that when subjected to external pressure, the conductive pathways of S-GS and CuNPs can change together to form a more complete conductive network. In summary, the higher the mass percentage of CuNPs, the better the conductivity of the composite foam. However, when the loading amount of CuNPs in PDMS foam is too much, they will stack with each other, and too many CuNPs may form agglomerations or clusters in PDMS. This agglomeration may break the complete conductive network that has been formed before, resulting in a decrease in the number of effective conductive paths in the CuNPs / S-GS / PDMS composite foam and a decrease in conductivity.

[0034] The working mechanism of the CuNPs / S-GS / PDMS composite foam flexible piezoresistive sensor is mainly based on the material properties and piezoresistive effect of CuNPs, S-GS and PDMS in the functional layer. During the sensing performance test, when the sensor is not under pressure, the conductive fillers are attached to the PDMS foam and are loosely connected to each other. The conductive paths are as follows: Figure 7 As shown by the blue line in the middle, it is composed of three types: CuNPs and CuNPs, CuNPs and S-GS, and S-GS and S-GS. The number is small and the distance between them is large. When the sensor is subjected to external pressure, the deformation of the PDMS substrate causes the arrangement and shape of the internal S-GS and CuNPs to change, generating more conductive paths, such as Figure 7 As shown by the red line in the middle, the number is larger and denser than that of the blue line, resulting in a decrease in the resistance of the composite foam and an increase in the resistance change rate; once the pressure is eliminated, the PDMS substrate will return to its original state, the conductive channel will also return to its original state, and the resistance value will return to its original state, thereby realizing the perception of pressure.

[0035] 2) Sensitivity The sensitivity test of CuNPs / S-GS / PDMS composite foam piezoresistive sensor was carried out, and the results were as follows: Figure 8As shown. By linear fitting method, the sensitivity curve of CuNPs / S-GS / PDMS composite foam piezoresistive sensor can be roughly divided into three linear regions, corresponding to three different sensitivity values. By comparing the sensitivity of six sensors, it can be seen that in different pressure ranges, the sensitivity of the sensor increases first and then decreases with the increase of the mass percentage of CuNPs. The sensor prepared by CuNPs / S-GS / PDMS (30wt%) composite foam with the highest conductivity has the highest sensitivity: the sensitivity in the range of 0-5.06 kPa is 17.78 kPa. -1 , in the range of 5.06-39.51 kPa, the sensitivity of the sensor is 0.85 kPa -1 ; In the range of 39.51-92.25 kPa, the sensitivity of the sensor is 0.09 kPa -1 In summary, the CuNPs / S-GS3 / PDMS (30wt%) composite foam piezoresistive sensor combines high conductivity and sensitivity, demonstrating excellent sensing potential. Therefore, this sensor was used in subsequent stability, response / recovery time, and application tests.

[0036] 3) Stability In order to explore whether the sensor can work normally under dynamic pressure, the CuNPs / S-GS / PDMS (30wt%) composite foam flexible piezoresistive sensor was tested under load conditions of 1-90 kPa. The test results are shown in the figure. Figure 9 In the 11 experiments, when the pressure was 1 kPa, 2 kPa and 4 kPa, the resistance change rate was 12.6%, 20.4% and 58.1% respectively ( Figure 9 (a)); When the pressure is 8 kPa, 10 kPa, 20 kPa, 30 kPa and 40 kPa, the resistance change rate is 77.4%, 83.4%, 90.6%, 94.7% and 96.6% ( Figure 9 (b)); When the pressure is 60 kPa, 80 kPa and 90 kPa, the change rate of resistance is 97.6%, 98.1% and 98.5% respectively ( Figure 9 (c)). It can be concluded from the figure that the CuNPs / S-GS / PDMS (30wt%) piezoresistive sensor can adapt to working environments with different pressures and has a smooth curve and good stability, which proves its reliability in pressure detection applications.

[0037] In order to further prove the working stability of CuNPs / S-GS / PDMS (30wt%) composite foam flexible piezoresistive sensor, Figure 10(a) shows the resistance change curve after 50 cycles at pressures of 2 kPa, 8 kPa, 10 kPa, 20 kPa, 40 kPa, and 80 kPa. The figure shows that the resistance change rate of the sensor after every 10 cycles at different pressures does not fluctuate much. Figure 10 (b) shows the electrical characteristic curves obtained when the sensor was subjected to pressures of 2 kPa, 8 kPa, 10 kPa, 20 kPa, 40 kPa, and 80 kPa for 10 seconds. As can be seen from the figure, the rate of change of the sensor's resistance remains essentially unchanged during the pressure hold. Figure 10 (c) shows the data obtained from a staged pressure loading experiment on the sensor. Using a universal testing machine, the sensor was subjected to pressures of 2 kPa, 8 kPa, 10 kPa, 20 kPa, 40 kPa, and 80 kPa, respectively, while stationary. The graph shows that the rate of change of the sensor's resistance increases with increasing pressure, remaining essentially unchanged during the stagnant period. Figure 10 (d) shows the sensor's response to different compression rates while maintaining a pressure of 10 kPa. The response signal is relatively stable at compression rates of 5 mm / min, 10 mm / min, and 50 mm / min, and the rate of resistance change is essentially the same. These data demonstrate that the sensor has superior stability to the S-GS / PDMS sensor and also demonstrates its potential for application in complex stress conditions.

[0038] In order to test the working life of CuNPs / S-GS / PDMS (30wt%) composite foam piezoresistive sensor, Figure 11 The resistance change rate curve is obtained by performing a continuous cyclic compression test on the sensor at 80kPa for 2000 times. It can be seen that the resistance change rate of the sensor does not change significantly during multiple loads of the same pressure, indicating that the sensor can withstand fatigue testing. Figure 11 (a), (c)), it can be seen that the resistance change rate of the sensor always remains in the range of 0-98%, and the resistance change rate does not fluctuate greatly during the cycle process, showing a more stable characteristic, indicating the feasibility of adding CuNPs material.

[0039] 4) Response / Recovery Time In order to verify whether the sensor has a fast and immediate response to pressure, a pressure of 80 kPa was applied to the CuNPs / S-GS / PDMS (30wt%) composite foam piezoresistive sensor and then released, and repeated 5 times. The resistance change of the sensor and the time it takes for the sensor to respond to the signal were recorded using a digital multimeter. The results are shown in Figure 2. Figure 12 As shown in the figure, when there is no pressure load, the sensor resistance change rate is zero. When pressure is applied, the sensor responds quickly, with a response time as low as 0.6 s. After the pressure is removed, the sensor resistance recovers in 0.81 s. Furthermore, in five repeated experiments, the sensor resistance value remained unchanged before and after the pressure was applied, demonstrating good stability.

[0040] 3. Application research Based on the CuNPs / S-GS / PDMS (30wt%) piezoresistive sensor, this paper also conducted a series of practical application tests. The sensor was placed on the finger surface of a 26-year-old female, such as Figure 13 As shown in (a), when the finger is bent and released, the resistance change rate curve can clearly reflect the degree of bending and the time interval. In addition, the sensor can also detect the wrist ( Figure 13 (b)), elbow ( Figure 13 (c)), knee ( Figure 13 (d) ). Specifically, when the joint bends, the sensor's resistance decreases, and when the joint relaxes, the sensor's resistance increases. Therefore, this sensor has great potential for real-time monitoring of human activity and correcting incorrect posture.

Claims

1. A method for preparing a CuNPs / S-GS / PDMS flexible piezoresistive sensor, characterized in that: The following steps are involved: 1) Preparation of CuNPs / S-GS A 1 mg / mL graphene dispersion was mixed with a 0.15-0.75 wt / v% copper nitrate solution. A sodium polysulfide solution was added under nitrogen protection and reacted at 90-100°C for 3-8 hours to form a CuNPs / S-GS precipitate. After washing and drying, a black powder of CuNPs / S-GS was obtained. 2) Preparation of CuNPs / S-GS / PDMS composite foam White sugar and CuNPs / S-GS were mixed in a mass ratio of 30:1-50:1, spread on a glass slide and micro-melted to form a sugar template. PDMS was then poured, and after vacuum degassing, it was cured at 80±5℃ for 1-2h. The sugar template was removed by washing with water to obtain CuNPs / S-GS / PDMS composite foam. 3) Device assembly First, the interdigital electrodes were fixed on the surface of the PDMS film, and then the CuNPs / S-GS / PDMS composite foam was covered on the interdigital electrodes. Finally, the whole was encapsulated with a PET film to prepare a flexible piezoresistive sensor.

2. The method for preparing a CuNPs / S-GS / PDMS flexible piezoresistive sensor according to claim 1, wherein: In step 1), the sodium polysulfide solution is prepared by the following method: 3.2 g of sublimed sulfur is added to 50 mL of 2.5 mol / L Na2S solution, stirred for 1-2 hours, and filtered to obtain a uniform sodium polysulfide solution; The amount of sodium polysulfide solution added is 100~200mL.

3. A flexible piezoresistive sensor prepared by the method according to claim 1, characterized in that: include: Flexible substrate: PDMS film; Electrode layer: interdigitated electrodes arranged on the surface of the PDMS film; Sensitive layer: CuNPs / S-GS / PDMS composite foam covering the interdigitated electrodes; Encapsulation layer: A PET film covering the PDMS film, the interdigitated electrodes and the sensitive layer to form an overall sealed structure.

4. An application of the flexible piezoresistive sensor according to claim 3 in human motion monitoring.

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