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

Abstract

The invention discloses preparation and application of an S-GS / PDMS flexible piezoresistive sensor, S-GS obtained by reducing GO by Na2Sx is compounded with PDMS foam by a sugar template method to obtain S-GS / PDMS composite foam, and the S-GS / PDMS composite foam is used as a functional layer and an interdigital electrode is used as a conductive electrode to assemble the flexible piezoresistive sensor based on the S-GS / PDMS composite foam. As sulfur atoms have a very strong N-type doping effect on graphene, conductivity adjustment is realized along with different doping amounts of the sulfur atoms, so that the highest conductivity of the S-GS / PDMS composite foam can reach 4.44 * 10 <-7 > S / m. The method for modifying and optimizing the common conductive filler provides a new thought for preparing the piezoresistive sensor with excellent performance.

Description

Technical Field

[0001] The present invention belongs to the technical field of sensors, and in particular relates to the preparation and application of an S-GS / PDMS flexible piezoresistive sensor. Background Art

[0002] Piezoresistive sensors, due to their simple structure, high sensitivity, and fast response speed, have broad application prospects in emerging electronic fields such as medical monitoring, human-computer interaction, and smart wearables. To achieve high-performance pressure sensing, the selection of the sensor's core conductive filler is crucial. Graphene, a two-dimensional nanomaterial composed of a single layer of carbon atoms, has a low resistivity close to that of metals (as low as 5.6×10 -8 Ω·m) and extremely high carrier mobility (up to 2×10 5 cm² / (V·S)), is considered an ideal piezoresistive sensor. Its piezoresistive effect stems from the significant changes in electron mobility and resistance caused by deformation of the lattice structure under external force, enabling rapid sensor response. Graphene is also highly processable and can be formed into various forms, including thin films, nanoribbons, and nanosheets, facilitating integration into sensor devices of varying structures and sizes, enhancing design flexibility.

[0003] However, graphene's inherent properties limit its practicality in solution processing. Its extremely high crystallinity and strong hydrophobicity result in poor dispersibility in most common solvents, making it prone to agglomeration and difficult to prepare stable, uniform composite materials. To overcome this drawback, chemical oxidation is often used to treat graphene, introducing oxygen-containing functional groups such as hydroxyl (-OH) and carboxyl (-COOH) to produce graphene oxide (GO). GO has excellent hydrophilicity and solution dispersibility, but severe structural defects cause it to lose the excellent electrical conductivity of pristine graphene. Although conductivity can be partially restored through reduction treatment, the structural integrity and electrical properties of reduced graphene oxide (rGO) are difficult to fully restore to the level of pristine graphene, limiting its potential for sensor sensitivity.

[0004] To improve the conductivity of reduced graphene oxide (RGO), researchers have proposed heteroatom doping as an effective means of regulation. Sulfur-doped graphene (S-GS) introduces sulfur atoms (e.g., in the form of sulfonic acid groups) into the carbon lattice of rGO, causing changes in the local charge density distribution and band structure, resulting in semiconducting properties and significantly improved conductivity compared to rGO. This provides new insights into the development of graphene-based fillers with both good dispersibility and high conductivity.

[0005] Therefore, there is an urgent need to develop an innovative process for preparing sulfur-doped graphene and effectively integrate it into a flexible polymer matrix (such as polydimethylsiloxane, PDMS) to create a piezoresistive sensing layer with a stable structure and excellent conductive network, thereby overcoming the bottleneck of existing technologies in balancing material dispersion and conductivity. Furthermore, optimizing the doping concentration and device structure (such as interdigitated electrode design) to improve sensor sensitivity, response time, cyclic stability, and a wide pressure detection range is of great significance, as is exploring its application value in real-world biosignal monitoring (such as human joint movement). Summary of the Invention

[0006] In view of the problems existing in the above background technology, the present invention discloses a method for preparing a S-GS / PDMS flexible piezoresistive sensor, using Na2S X GO was reduced and doped to prepare S-GS, which was then added to PDMS as a conductive filler to prepare S-GS / PDMS composite foam. On this basis, a piezoresistive sensor was constructed by combining interdigital electrodes.

[0007] The method for preparing the S-GS / PDMS flexible piezoresistive sensor of the present invention comprises the following steps: 1) S-GS x Preparation Take 2.5±0.1 mol / L Na2S solution, measure 25.0 mL, 37.5 mL, 50.0 mL or 62.5 mL respectively and place them in the reaction container, add 1.6 g, 2.4 g, 3.2 g or 4.0 g of sublimed sulfur respectively, stir for 1 hour and filter to obtain Na2S x Solution; Na2S X The solution was filtered and added to a 1-10 mg / mL graphene oxide dispersion, heated in a water bath at 90-100 °C for 4-6 h, filtered, washed, and vacuum dried to obtain sulfur-doped graphene powder, which was designated as S-GS. x ; The specific operation parameter combination is any of the following: (i) 25.0 mL Na2S solution + 1.6 g sublimed sulfur → the product is labeled S-GS1; (ii) 37.5 mL Na2S solution + 2.4 g sublimed sulfur → the product is labeled S-GS2; (iii) 50.0 mL Na2S solution + 3.2 g sublimed sulfur → the product is labeled S-GS3; (iv) 62.5 mL Na2S solution + 4.0 g sublimed sulfur → the product is labeled S-GS4; 2) S-GS x Preparation of / PDMS composite foam Mix white sugar with S-GS x Mix them in a mass ratio of 50:1~60:1, spread them on a glass slide and melt them to form a sugar template, then cast a product mixed with PDMS and octamethylcyclotetrasiloxane in a mass ratio of 8~12:1, vacuum degas, cure at 80±5℃ for 1~2h, and wash with water to remove the sugar template to obtain S-GS x / PDMS composite foam; 3) Assembly of flexible piezoresistive sensor First, the interdigital electrodes were fixed on the surface of the PDMS film, and then the S-GS x / PDMS composite foam was covered on the interdigital electrodes and finally encapsulated as a whole with PET film to prepare a flexible piezoresistive sensor.

[0008] 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: S-GS covering the interdigital electrodes x / PDMS composite foam; Encapsulation layer: A PET film covering the PDMS film, the interdigitated electrodes and the sensitive layer to form an overall sealing structure.

[0009] The present invention uses white sugar as a template to prepare PDMS foam, and Na2S x The S-GS obtained by reducing GO was compounded with PDMS foam to obtain S-GS / PDMS composite foam, which was used as the functional layer and the interdigitated electrodes as the conductive electrodes to assemble a flexible piezoresistive sensor based on S-GS / PDMS composite foam.

[0010] The main innovations are as follows: 1. Using Na2S x S-GS was prepared as a reducing agent. Since sulfur atoms have a strong N-type doping effect on graphene, the conductivity can be adjusted with the different doping amounts of sulfur atoms, making the conductivity of S-GS3 / PDMS composite foam as high as 4.44×10 - 7 S / m; This strategy of modifying common conductive fillers (graphene) based on sulfur doping provides a new idea for the preparation of high-performance piezoresistive sensors; 2. The sensor has a pressure range of 7.02 kPa in the pressure range of 0-8.02 kPa, 8.02-19.09 kPa and 19.09-54.14 kPa respectively. -1 , 0.47 kPa -1 and 0.32 kPa -1The sensor exhibits excellent sensitivity, a response time of 0.76 s and an unloading time of 0.62 s when subjected to a pressure of 40 kPa, and excellent repeatability and stability over 1200 compression-release cycles. These outstanding properties enable the sensor to accurately reflect and monitor a variety of daily human activities. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Figure 1 Flow chart of the preparation and assembly of the S-GS / PDMS sensor prepared in an embodiment of the present invention.

[0012] Figure 2 Actual images of (a) PDMS and (b) S-GS / PDMS prepared in accordance with the present invention.

[0013] Figure 3 Optical microscopy images of (a) PDMS and (b) S-GS / PDMS prepared in accordance with the present invention.

[0014] Figure 4 SEM images and elemental mapping of PDMS and S-GS3 / PDMS prepared in accordance with the present invention: (a)-(c) PDMS; (d)-(f) S-GS3 / PDMS; (g)-(k) elemental mapping of S-GS3 / PDMS.

[0015] Figure 5 FTIR spectra of PDMS and S-GS3 / PDMS prepared in the examples of the present invention.

[0016] Figure 6 Raman curves of (a) S-GS3 and GO; (b) PDMS and S-GS3 / PDMS prepared in accordance with the present invention.

[0017] Figure 7 These are the XRD curves of S-GS3, PDMS, and S-GS3 / PDMS prepared in the examples of the present invention.

[0018] Figure 8 XPS spectra of S-GS3 / PDMS prepared in this embodiment of the present invention: (a) full spectrum; (b) C 1s; (c) S2p; (d) Si 2p.

[0019] Figure 9 Mechanical property curves of different PDMS composite foams prepared in accordance with the present invention: (a) stress-strain curve; (b) tensile strain curve.

[0020] Figure 10 This is a sensitivity test diagram of the S-GS1 / PDMS piezoresistive sensor prepared in an embodiment of the present invention.

[0021] Figure 11 This is a sensitivity test diagram of the S-GS2 / PDMS piezoresistive sensor prepared in an embodiment of the present invention.

[0022] Figure 12 This is a sensitivity test diagram of the S-GS3 / PDMS piezoresistive sensor prepared in an embodiment of the present invention.

[0023] Figure 13 This is a sensitivity test diagram of the S-GS4 / PDMS piezoresistive sensor prepared in an embodiment of the present invention.

[0024] Figure 14 The resistance change of the S-GS3 / PDMS piezoresistive sensor prepared in this embodiment of the present invention under different pressures.

[0025] Figure 15 Test curves of different piezoresistive characteristics of the S-GS3 / PDMS piezoresistive sensor prepared according to an embodiment of the present invention: (a) Stability and repeatability of the resistance change rate of the piezoresistive sensor after 50 cycles at different pressures; (b) Stability of the resistance change rate of the piezoresistive sensor when the pressure is maintained for 10 seconds at different pressures; (c) Response curves of the piezoresistive sensor to different pressure signals; (d) Response curves of the piezoresistive sensor at different compression rates.

[0026] Figure 16 Cyclic stability test curve of the S-GS3 / PDMS piezoresistive sensor prepared in accordance with an embodiment of the present invention: (b) resistance change rate curve of the pressure sensor after 1200 cycles; (a) and (c) are partial enlarged views of Figure b.

[0027] Figure 17 Response time curve of the S-GS3 / PDMS piezoresistive sensor prepared in an embodiment of the present invention at 40 kPa.

[0028] Figure 18 Application demonstration of the S-GS3 / PDMS piezoresistive sensor prepared for an embodiment of the present invention: (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

[0029] The present invention will be further explained below with reference to specific embodiments. Example

[0030] 1. Preparation of S-GS 1.6, 2.4, 3.2 and 4 g of sublimed sulfur were added to 25.0, 37.5, 50.0 and 62.5 mL of 2.5 mol / L Na2S solution respectively, stirred for 1 hour and filtered to obtain uniform reddish brown Na2S. X Solution ((X-1)S8+8Na2S→8Na2S x GO 0.2g was dispersed in 200mL distilled water and ultrasonicated to obtain GO (1mg / mL) solution. X The solution was filtered and added to the GO solution, and heated in a water bath at 95°C for 4 hours. The product was filtered while hot, washed with deionized water several times, and dried in a vacuum drying oven at 80°C for 4 hours to obtain black S-GS. x powders, labeled as S-GS1, S-GS2, S-GS3, and S-GS4 respectively.

[0031] 2. Preparation of S-GS / PDMS composite foam and assembly of flexible piezoresistive sensors Take 10g white sugar and 0.17g S-GS x Mix, spread it flat on a Petri dish with a glass slide, put it in an oven to melt slightly, take it out and let it cool, then pour the mixture of PDMS and curing agent on the surface, cover it with a glass slide, clamp the two glass slides with a clip, and then vacuum for 20 minutes. Then put the sample into an 80℃ oven to cure for 2 hours, and then put it into distilled water to soak out the sugar template, and you can get S-GS1 / PDMS, S-GS2 / PDMS, S-GS3 / PDMS, S-GS4 / PDMS composite foams respectively. x The / PDMS composite foam was cut into 2.0 cm × 2.0 cm pieces, and the interdigital electrodes were selected as the conductive electrodes. The PDMS film, interdigital electrodes and composite foam were sequentially combined and finally encapsulated with PET film to prepare a flexible piezoresistive sensor. x / PDMS piezoresistive sensor preparation and assembly process Figure 1 shown.

[0032] The structural characterization and performance evaluation of the above-mentioned embodiment products are as follows: 1. Structural characterization First, observe the changes in the appearance of PDMS foam before and after loading S-GS. Figure 2 In (a), it can be clearly seen that the overall color of the PDMS foam is white, and there are multiple pore structures inside. After adding S-GS, the front and cross-section colors of the S-GS / PDMS composite foam turn black, and the color distribution is uniform, indicating that S-GS and PDMS foam are successfully composited ( Figure 2 (b)).

[0033] The structures of PDMS foam and S-GS / PDMS composite foam were observed and analyzed under an optical microscope with a magnification of 100 times. Figure 3 As shown. Figure 3 (a) It can be seen that the PDMS foam is smooth and the pores are open-celled. From the optical microscope image of the S-GS / PDMS composite foam, it can be seen that S-GS is dispersed in the PDMS foam in the form of black flakes ( Figure 3 (b)).

[0034] Scanning electron microscopy was used to characterize the micromorphology of PDMS foam and S-GS / PDMS composite foam and analyze their microstructure. Figure 4 As can be seen from (ac), the PDMS foam presents a uniform and smooth surface structure, with pore diameters of approximately 100 μm-500 μm and irregular shapes. Careful observation reveals that the pores on the surface of the PDMS foam are tightly connected by micropores, which is consistent with the results observed under an optical microscope. Figure 4 It can be clearly observed in (df) that compared with PDMS foam, the pore outline of S-GS3 / PDMS composite foam is not clear, and many tiny protrusions and burrs appear on the pore wall. The existence of these structures makes the surface of the composite foam rougher. Figure 4 (gk) are the corresponding element mapping diagrams in S-GS3 / PDMS composite foam, respectively. It can be seen that C, O, S, N and Si elements are uniformly distributed in the composite material, which further indicates the successful loading of S-GS in PDMS foam.

[0035] Figure 5 The infrared spectra of PDMS foam and S-GS3 / PDMS composite foam are shown. For PDMS foam, the infrared spectra at 2968 cm -1 The absorption peak at 1251 cm corresponds to the stretching vibration of CH in methyl group; -1 The absorption peak near 1010 cm represents the CS stretching vibration; -1 The nearby absorption peak comes from the stretching vibration of Si-O-Si; 796 cm -1 The absorption peak near 3323 cm corresponds to the bending vibration of Si-CH3. In the composite foam obtained by S-GS and PDMS, in addition to all the peaks of PDMS mentioned above, there is also a peak at 3323 cm -1 An absorption peak representing OH stretching vibration appears near PDMS, a siloxane polymer that generally does not contain hydroxyl functional groups. The appearance of this absorption peak proves the successful loading of S-GS in PDMS.

[0036] Figure 6Shown are the Raman spectra of GO, S-GS3, PDMS, and S-GS3 / PDMS. Figure 6 As can be seen in (a), both GO and S-GS3 have a peak at 1587 cm -1 and 1354 cm -1 There are two strong peaks nearby, corresponding to the G peak and D peak of carbon atoms. The D peak is the carbon atom lattice defect peak, which represents the degree of defect on the surface and edge of GO. The G peak represents the planar stretching vibration of carbon atoms, reflecting the symmetry and order of the graphene lattice. The ID / IG intensity ratio is usually used to evaluate the number of carbon atom crystal defects. After calculation, the ID / IG of S-GS3 increased significantly compared with GO (ID / IG=0.65), reaching 1.84. This is mainly because the size of the GO sheet becomes smaller after ultrasound, resulting in more edge structural defects. After Na2S x After reduction, most of the oxygen-containing groups are removed, resulting in topological defects and vacancies in the graphene lattice, which enhances the D peak intensity. Figure 6 (b) shows the Raman spectra of PDMS and S-GS3 / PDMS. It can be seen from the figure that PDMS has a Raman spectrum at 2905 and 2960 cm -1 There are two strong peaks at 495 cm, corresponding to the symmetric and antisymmetric stretching vibrations of -CH3; -1 The peak at 150 to 300 cm corresponds to the stretching vibration of Si-O-Si; -1 The band between 1 and 2 corresponds to the bending vibration of the CC backbone. From the Raman spectrum of S-GS3 / PDMS, it can be seen that the material has characteristic peaks of both S-GS3 and PDMS, further proving that S-GS3 has been successfully loaded into the PDMS foam.

[0037] The crystalline structures of S-GS3, PDMS and S-GS3 / PDMS were analyzed by X-ray diffraction. The relevant X-ray diffraction patterns are as follows: Figure 7 As shown. The XRD spectrum of the PDMS foam sample shows a characteristic peak of PDMS at 2θ=11°; the XRD spectrum of S-GS3 shows a sharp peak at 2θ=25°. This diffraction peak corresponds to the (002) crystal plane of graphite, indicating graphite stacking or the distribution of amorphous carbon. Comparing the XRD patterns of the S-GS3 / PDMS composite foam with those of PDMS and S-GS3 reveals that the composite foam exhibits relevant characteristic peaks at both 2θ=11° and 2θ=25°, further demonstrating the successful composite of S-GS3 with PDMS.

[0038] The elemental composition and chemical state of the S-GS3 / PDMS sample were analyzed by XPS characterization. Figure 8 shown. Figure 8(a) shows the full spectrum of the S-GS3 / PDMS sample, which shows the presence of characteristic peaks of S 2p (123.1 eV), C1s (285.1 eV), O 1s (532.1 eV) and Si 2p (102.1 eV). Figure 8 In the C 1s spectrum shown in (b), the characteristic absorption peaks near the binding energies of 283.9 eV, 284.8 eV, 285.9 eV and 288.4 eV are attributed to C-Si, CC / C=C, CO and O=CO groups, respectively. Figure 8 In the S 2p spectrum shown in (c), the absorption characteristic peaks near 162.2, 166.6, and 169.5 eV are attributed to sulfide, thiophene sulfur, and sulfonate, respectively. Figure 8 The Si 2p spectrum in (d) shows two significant peaks at 101.5 and 102.3 eV, which are related to CO-Si and Si-O-Si. These results further indicate that S-GS3 has been successfully loaded into PDMS, which is consistent with the XRD and other characterization tests.

[0039] 2. Performance evaluation 1) Mechanical properties In order to evaluate the effect of different sulfur doping levels of S-GS on the mechanical properties of S-GS / PDMS composite foams, compression and tensile properties of pure PDMS foam and four S-GS / PDMS composite foams were tested. Figure 9 As shown in (a), at 20% strain, the maximum compressive stresses of pure PDMS, S-GS1 / PDMS, S-GS2 / PDMS, S-GS3 / PDMS, and S-GS4 / PDMS composite foams are 11.2 kPa, 12.5 kPa, 14.6 kPa, 15.5 kPa, and 16.9 kPa, respectively. The curves for the five samples show similar trends. Under the same deformation, pure PDMS foam exhibits the lowest compressive stress, indicating its superior elasticity. Under the same reaction conditions, increasing the sulfur doping level requires higher pressure to produce the same strain in the composite foam, indicating a decrease in the composite foam's elasticity. This phenomenon may be due to the formation of C-S bonds when sulfur reacts with graphene, which alters the graphene lattice structure and enhances its mechanical properties. Therefore, increasing the sulfur doping level increases the number of C-S bonds, which in turn increases the compressive strength of the S-GS / PDMS composite foam. Figure 9 (b) shows the tensile stress-strain curves of the five groups of samples. As can be seen from the figure, the tensile strength and elongation at break of PDMS are 37.4 kPa and 80.1% respectively. The higher the values ​​of these two parameters, the better the mechanical properties of the composite material. Compared with PDMS, S-GS xThe tensile strength of S-GS4 / PDMS decreased to 22.7 kPa, 18.3 kPa, 31.1 kPa, and 28.5 kPa, respectively, and the elongation at break decreased to 21.7% (S-GS4 / PDMS). These results indicate that as the degree of sulfur doping increases, the force that the PDMS composite foam can withstand per unit area gradually increases, but the stiffness increases, resulting in a decrease in deformation.

[0040] 2) Sensing performance 2.1) Electrical conductivity The resistance of the S-GS / PDMS composite foam was tested by a digital multimeter, and its conductivity was calculated. The data are shown in Table 1.

[0041] Table 1 Electrical conductivity of S-GS / PDMS composite foam Among the products of the series of experiments with varying sulfur doping levels, the S-GS3 / PDMS composite foam had the highest conductivity, reaching 4.44×10 -7 S·m -1 , indicating that most oxygen-containing functional groups are reduced upon addition of a reducing agent, leading to improved conductivity. However, adding more reducing agent does not necessarily guarantee better results. Sulfur atoms have two more valence electrons than carbon atoms. Therefore, when sulfur atoms replace carbon atoms in the graphene lattice, they donate more electrons to the graphene, increasing the electron concentration and charge density. The additional electrons become charge carriers, enhancing the conductivity of S-GS. However, the C-S bond is approximately 25% longer than the C-C bond. When graphene is doped with sulfur atoms, the S atoms are approximately 1.1 Å above the graphene plane, resulting in lattice distortion and distortion. Excessive sulfur doping can lead to increased local lattice distortion and distortion in the graphene. Furthermore, the doping process can generate lattice defects, such as vacancies and dislocated atoms. These structural defects significantly affect the distribution of the electron cloud and electron transport, leading to electron scattering during transport and thus reducing conductivity. Therefore, the conductivity of the S-GS / PDMS composite foam shows a trend of initial increase followed by a decrease.

[0042] 2.2) Sensitivity Figure 10-13 It is S-GS x Sensitivity curves of the S-GS1 / PDMS composite foam piezoresistive sensor in three different pressure ranges. As can be seen from the figure, the sensitivity of the S-GS1 / PDMS composite foam piezoresistive sensor is: S1=5.60kPa -1 (0-9.02 kPa), S2=0.44 kPa -1 (9.02-25.06 kPa), S3=0.07kPa -1(25.06-54.54 kPa). The sensitivity of the S-GS2 / PDMS composite foam piezoresistive sensor is: S1=6.40kPa -1 (0-8.01 kPa), S2=0.45 kPa -1 (8.01-23.12 kPa), S3=0.09kPa -1 (23.12-52.13 kPa). The sensitivity of the S-GS3 / PDMS composite foam piezoresistive sensor is: S1=7.02kPa -1 (0-8.02 kPa), S2=0.47 kPa -1 (8.02-19.09 kPa), S3=0.32kPa -1 (19.09-54.14 kPa). The sensitivity of the S-GS4 / PDMS composite foam piezoresistive sensor is: S1=6.05kPa -1 (0-9.71 kPa), S2=0.46 kPa -1 (9.71-25.25 kPa), S3=0.29kPa -1(25.25-48.18 kPa). Comparing the values ​​of S1, S2, and S3 reveals that the sensor sensitivity decreases with increasing pressure. This change in sensor sensitivity is due to the fact that when the foam is unpressurized, it has a relatively loose structure, large internal pores, and a high Poisson's ratio. Therefore, even under low pressure, the PDMS composite foam can deform to a certain extent, allowing some S-GS structures within the foam to connect with each other, rapidly increasing the number of conductive pathways and resulting in a correspondingly high sensor sensitivity. However, after the PDMS composite foam deforms to a certain extent, the Poisson's ratio decreases, weakening its deformability. Even with increased pressure, the conductive pathways within the composite foam tend to saturate, resulting in a gradual decrease in sensitivity. The figure also shows that the S-GS / PDMS composite foam piezoresistive sensor has a certain pressure detection range. Beyond this pressure range, the sensor's resistance change rate begins to decrease. This phenomenon is due to the fact that the operating principle of the S-GS / PDMS composite foam piezoresistive sensor relies on changes in resistance caused by changes in its internal pore structure. Within the normal pressure detection range, increasing pressure connects the pores and channels within the foam more tightly, increasing the resistance change rate. However, when the pressure exceeds a certain threshold, the foam material's elastic limit is reached, causing the overall volume of the material to shrink, reducing the space for electron movement. Under these conditions, the foam's microstructure is damaged, and the pores and channels cease to collapse or even close, reducing the conductive paths and, in turn, decreasing the resistance change rate of the composite foam. Furthermore, a comparison of the sensitivity of the four sensors reveals that, across different pressure ranges, the sensitivity initially increases and then decreases with increasing sulfur doping concentration. This is because the S-GS3 conductive filler has the highest conductivity. Under the same pressure, the S-GS3 / PDMS composite foam piezoresistive sensor also generates more electron transfer, resulting in the highest sensitivity. In summary, the S-GS3 / PDMS 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 testing.

[0043] 2.3) Stability In order to test the response performance of the S-GS3 / PDMS composite foam piezoresistive sensor to dynamic pressure, the resistance change rate of the sensor under different pressure cycles was recorded. Each set of cycles was performed 10 times continuously. The results are shown in the figure below. Figure 14As shown in the figure, during the loading and unloading processes, the sensor's resistance changes synchronously with the applied force, and the resistance change rate of the sensor remains consistent under different pressure conditions. When the pressure is 1 kPa, the maximum resistance change rate of the S-GS3 / PDMS composite foam piezoresistive sensor is 14.9%. As the pressure increases from 2 kPa to 30 kPa, the maximum resistance change rates of the sensor are approximately 30.1%, 45.4%, 50.9%, 61.5%, 63.6%, 65.7%, and 76.8%, respectively. As the pressure increases from 40 kPa to 50 kPa, the maximum resistance change rates of the sensor are approximately 78.8%, 81.3%, and 82.2%, respectively, with minimal difference in resistance change within this pressure range. These results demonstrate that the S-GS3 / PDMS composite foam piezoresistive sensor exhibits different electrical signal responses to different external pressures, demonstrating its reliability in pressure detection applications.

[0044] The resistance change of S-GS3 / PDMS composite foam piezoresistive sensor under force loading-unloading cycle and loading-holding-unloading was further evaluated. Figure 15 As can be seen in (a), the electrical response of the sensor maintains good reconstruction after 50 cycles when the applied pressure is 2kPa, 8kPa, 10kPa, 30kPa and 50kPa. Figure 15 (b) shows the electrical characteristic curves obtained when the sensor was subjected to pressures of 2 kPa, 8 kPa, 10 kPa, 30 kPa, and 50 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 continuous application of pressure. Figure 15 (c) shows the data obtained from a staged pressure loading experiment on an S-GS3 / PDMS composite foam piezoresistive sensor. Using a universal testing machine, pressures of 2 kPa, 8 kPa, 30 kPa, 40 kPa, and 50 kPa were applied to the sensor, followed by a period of stagnation at each pressure. The figure shows that the rate of change of the sensor's resistance increases with increasing pressure, and remains essentially unchanged during the stagnation period, indicating that the sensor's stability does not deviate due to pressure changes. Furthermore, glitches can be observed on the curve during pressure loading, likely due to signal instability during the pressure loading process. After the pressure is removed, the sensor's output signal stabilizes, demonstrating the sensor's rapid response to pressure. Figure 15(d) shows the sensor's response at different compression rates under a pressure of 40 kPa. The figure shows that the sensor's response signal is relatively stable at compression rates of 5 mm / min, 10 mm / min, and 50 mm / min. This data provides valuable insights into the sensor's suitability for use in complex stress conditions.

[0045] In order to test the working life of the S-GS3 / PDMS composite foam piezoresistive sensor, a continuous cycle compression test of 1200 times was performed at 40 kPa. The test results are shown in Figure 2. Figure 16 As shown. Figure 16 As shown in (b), the rate of change of the sensor resistance remained essentially stable during the 1200 pressure loading-unloading cycles, without experiencing large fluctuations. This data demonstrates that the prepared S-GS3 / PDMS composite foam piezoresistive sensor has excellent repeatability. Randomly selecting two sections for amplified analysis during the 1200 cycles (16 (a), (c)), it can be seen that the sensor's resistance change rate remained within the 0-80% range, with only slight fluctuations in the resistance change rate during the early stages of the cycle. The remaining time periods exhibited good stability and reproducibility. This demonstrates that the S-GS3 / PDMS composite foam piezoresistive sensor has a long lifespan and cyclic stability.

[0046] 2.4) Response / Recovery Time The response time refers to the time required for the sensor to generate a stable output signal after being stimulated by pressure. The shorter the time required, the faster the response speed. The present invention uses a universal testing machine to apply a 40kPa pressure to the S-GS3 / PDMS composite foam piezoresistive sensor and then release it. This is repeated 5 times. The change in resistance represents the sensor's response to pressure. The results are shown in Figure 2. Figure 17 As shown in the figure, when pressure is applied, the sensor responds quickly within 0.76 s. After the pressure is released, the sensor's resistance returns to its original level within 0.62 s. Furthermore, in five repeated experiments, the sensor's resistance remained constant before and after pressure application, demonstrating good sensor performance and fast response time.

[0047] 3. Application of S-GS / PDMS sensors From the above test results, we can see that the S-GS3 / PDMS composite foam piezoresistive sensor has good flexibility, high sensitivity and fast response time. Therefore, we try to apply it to the field of intelligent detection to monitor and distinguish various daily motion states of the human body. The test process is as follows: Figure 18 As shown. Figure 18As shown in (a) and (b), when the sensor is attached to the surface of the subject's fingers and wrist, the bending of the fingers and wrist will exert a certain amount of pressure on the sensor, causing the bubble structure inside the sensor to be squeezed and causing changes in its internal conductive network. Different bending angles will cause different degrees of sensor deformation, so the bending signals of the fingers and wrists at different angles can be monitored. Similarly, when the sensor is attached to the surface of the subject's elbows and knees, their bending signals can also be effectively monitored ( Figure 18 (c), (d)). It's important to note that the output signal waveform remains essentially stable at the same bending angle, demonstrating the device's robustness in continuously and dynamically monitoring the position of joints like the wrist and knee. Furthermore, these monitoring results demonstrate that the S-GS3 / PDMS composite foam piezoresistive sensor can monitor not only large joint movements but also small movements like finger bending, making it widely applicable for monitoring human activity.

Claims

1. A method for preparing a S-GS / PDMS flexible piezoresistive sensor, characterized in that: The following steps are involved: 1) S-GS x Preparation Take 2.5±0.1 mol / L Na2S solution, measure 25.0 mL, 37.5 mL, 50.0 mL or 62.5 mL respectively and place them in the reaction container, add 1.6 g, 2.4 g, 3.2 g or 4.0 g of sublimed sulfur respectively, stir for 1 hour and filter to obtain Na2S x Solution; Na2S X The solution was added to a 1-10 mg / mL graphene oxide dispersion, heated in a water bath at 90-100 °C for 4-6 h, filtered, washed, and vacuum dried to obtain sulfur-doped graphene powder, which was designated as S-GS. x ; 2) S-GS x Preparation of / PDMS composite foam Mix white sugar with S-GS x Mix them in a mass ratio of 50:1~60:1, spread them on a glass slide and melt them to form a sugar template, then cast the mixture of PDMS and curing agent, degas under vacuum and cure at 80±5℃ for 1~2h, and wash with water to remove the sugar template to obtain S-GS. x / PDMS composite foam; 3) Assembly of flexible piezoresistive sensor First, the interdigital electrodes were fixed on the surface of the PDMS film, and then the S-GS x / PDMS composite foam was covered on the interdigital electrodes and finally encapsulated as a whole with PET film to prepare a flexible piezoresistive sensor.

2. The method for preparing a S-GS / PDMS flexible piezoresistive sensor according to claim 1, wherein: In step 1), the operating parameters are combined into any of the following groups: (i) 25.0 mL Na2S solution + 1.6 g sublimed sulfur → the product is labeled S-GS1; (ii) 37.5 mL Na2S solution + 2.4 g sublimed sulfur → the product is labeled S-GS2; (iii) 50.0 mL Na2S solution + 3.2 g sublimed sulfur → the product is labeled S-GS3; (iv) 62.5 mL Na2S solution + 4.0 g sublimed sulfur → the product is labeled S-GS4.

3. The method for preparing a S-GS / PDMS flexible piezoresistive sensor according to claim 1, wherein: In step 2), the curing agent is octamethylcyclotetrasiloxane, and the mass ratio of PDMS to the curing agent is 8:1 to 12:

1.

4. 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: S-GS covering the interdigital electrodes x / PDMS composite foam; Encapsulation layer: A PET film covering the PDMS film, the interdigitated electrodes and the sensitive layer to form an overall sealing structure.

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

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