A flexible pressure sensor based on cellulose nanofiber membrane with wide linear detection range and a preparation method thereof

Abstract

The application relates to a flexible pressure sensor based on a cellulose nanofiber film and having a wide linear detection range and a preparation method thereof, and belongs to the technical field of sensing. The cellulose nanofiber film is modified by using a conductive material, multi-walled carbon nanotubes, and a water-soluble polymer, polyvinyl alcohol, as a double-conductive sensitive layer (different resistances) and a substrate of the sensor respectively, and a silver interdigital electrode is prepared on the above-mentioned substrate by adopting a silk screen printing process to serve as a flexible electrode of the sensor. The application changes the current transmission path of the sensor through the design of the double-conductive sensitive layer film, and combines with a microstructure interface to realize the improvement of the linear detection range of the sensor. The application has the characteristics of simple preparation process, easily-obtained raw materials, batch production, lightness, thinness and portability, can realize good linearity in the range of 0.0004-480kPa, the linearity can reach 0.99, and the sensitivity is 0.1kPa ‑1 to solve the problems in the background art.

Description

Technical Field

[0001] This invention belongs to the field of sensing technology, specifically relating to a flexible pressure sensor with a wide linear detection range based on cellulose nanofiber thin films and its preparation method. Background Technology

[0002] Flexible electronic devices have attracted much attention due to their ability to maintain stable performance under bending, stretching, and torsion. Compared to traditional rigid electronic devices, flexible electronic devices have greater deformation capabilities and better conformability to curved surfaces, enabling them to play an important role in our lives and production. Flexible wearable sensors are the core components of flexible electronic devices, characterized by their thinness, softness, and flexibility. By integrating with clothing or everyday accessories, they can adapt to the complex curvature of the human skin surface to the greatest extent, thereby ensuring the accurate collection and acquisition of vital signs. Currently, flexible wearable sensors have achieved the monitoring of various vital signs and motion parameters of the human body, such as body temperature, pulse, sweat, respiration, joint and muscle movement, thereby realizing the functions of human health management and motion monitoring. They have broad application prospects in daily health monitoring and real-time health feedback, as well as motion tracking and posture recognition.

[0003] Flexible pressure sensors are a crucial component in flexible sensor systems, enabling the monitoring of various vital signs such as heart rate, blood pressure, and limb movement. Based on their working mechanisms, flexible pressure sensors are primarily categorized into resistive, capacitive, piezoelectric, and triboelectric types. Resistive pressure sensors typically achieve their sensitivity by altering the volume resistance caused by changes in the conductive network within the sensitive layer under pressure, as well as the contact resistance between the sensitive layer and the electrodes. This causes the device's resistance to change systematically with the magnitude of the pressure load, thus achieving pressure sensitivity. Current research reports have demonstrated the optimization of sensitivity, response speed, and detection limits in resistive pressure sensors through material selection and structural design. However, resistive flexible pressure sensors still face challenges due to their narrow detection range and effective linear interval. This makes it difficult to distinguish small pressure changes when monitoring large pressure signals generated by the human body, such as accurately identifying small center shifts when monitoring plantar pressure signals. Some studies have utilized porous foam elastomers as the sensing layer, improving the sensor's detection range by adjusting the foam pore size. However, this method typically requires a thicker sensing layer, which hinders the device's adhesion to the skin surface. Furthermore, the fabrication and control processes of porous foam structures are complex, limiting large-scale production and the consistency of fabricated devices. Therefore, developing a resistive flexible pressure sensor with a simple fabrication process, low cost, and a wide linear detection range is of great significance. Summary of the Invention

[0004] To address the aforementioned problems, this invention utilizes conductive multi-walled carbon nanotubes (MWCNTs) and water-soluble polymer polyvinyl alcohol (PVA) to modify cellulose nanofiber (CNFs) films, serving as the dual-conductive sensitive layer and substrate of the sensor, respectively. The flexible electrode of the sensor is a silver interdigitated electrode fabricated on the substrate using a screen printing process. The purpose of this invention is to provide a flexible pressure sensor with a wide linear detection range based on cellulose nanofiber films and its fabrication method. This invention achieves a change in the sensor's current transmission path through the design of the dual-conductive sensitive layer film and, combined with the microstructure interface, improves the sensor's linear detection range. This invention features a simple fabrication process, readily available raw materials, mass production capability, and lightweight portability, thus solving the problems existing in the background art.

[0005] The flexible pressure sensor of this invention comprises a CNFs / PVA substrate, interdigitated electrodes with a metal layer structure screen-printed on the CNFs / PVA substrate, and two CNFs / MWCNTs conductive films with different conductivity properties, ultimately encapsulated with thermoplastic polyurethane elastomer (TPU) tape. The substrate layer is prepared by a drop-coating and drying process on a polytetrafluoroethylene (PTFE) mold using a mixed solution of CNFs and PVA. The CNFs / MWCNTs conductive films with different conductivity properties are prepared by a drop-coating and drying process on a PTFE mold using mixed solutions of CNFs and MWCNTs with different composite ratios. When the device is subjected to pressure, the gaps between the electrodes and the CNFs / MWCNTs conductive layers in contact with them, as well as between the two CNFs / MWCNTs conductive layers with different conductivity properties, are compressed. The current through the CNFs / MWCNTs conductive films changes. By measuring the change in the output current of the sensor's interdigitated electrodes under a constant voltage, the sensor's response under different pressures can be calculated, thereby establishing a correlation between the sensor current and the applied pressure. The response of a pressure sensor is defined as: Response(%) = (I-I0) / I0×100%, where I is the current of the interdigital electrode of the sensor under different pressures, and I0 is the initial current of the interdigital electrode of the sensor when no pressure is applied.

[0006] The present invention discloses a method for fabricating a flexible pressure sensor with a wide linear detection range based on cellulose nanofiber thin films, comprising the following steps:

[0007] (1) Add 5g of CNFs aqueous dispersion with a mass fraction of 1-2% and 0.25-0.30g of MWCNTs to 10mL of deionized water to obtain a CNFs / MWCNTs solution;

[0008] (2) Add 0.01-0.05 g sodium dodecylbenzenesulfonate (dispersant) to the solution in step (1) and stir at 20-30°C for 2-5 h; then sonicate the resulting solution for 1-2 h to uniformly disperse CNFs and MWCNTs to obtain a mixed solution.

[0009] (3) Drop the mixed solution obtained in step (2) onto the ultrasonically cleaned and dried polytetrafluoroethylene mold (10×10cm). 2 In the process of making the solution evenly cover the surface of the mold, the mold is placed in a fume hood and left to stand for 8 to 15 hours to allow the water in the solution to evaporate naturally, thus obtaining a CNFs / MWCNTs conductive film (thickness of 0.05 to 0.15 mm and resistance of 45 to 55 Ω). The CNFs / MWCNTs conductive film is then cut and used as the upper sensitive layer of the sensor.

[0010] (4) Add 5g of CNFs aqueous dispersion with a mass fraction of 1-2% and 0.003-0.30g of MWCNTs to 10mL of deionized water to obtain CNFs / MWCNTs solution;

[0011] (5) Add 0.01-0.05g sodium dodecylbenzenesulfonate (dispersant) to the solution in step (4) and stir at 20-30℃ for 2-5h; then sonicate the resulting solution for 1-2h to uniformly disperse CNFs and MWCNTs to obtain a mixed solution.

[0012] (6) Drop the mixed solution obtained in step (5) onto the ultrasonically cleaned and dried polytetrafluoroethylene mold (10×10cm). 2 In the process of making the solution evenly cover the surface of the mold, the mold is placed in a fume hood and left to stand for 8 to 15 hours to allow the water in the solution to evaporate naturally, thus obtaining a CNFs / MWCNTs conductive film (thickness of 0.01 to 0.15 mm and resistance of 45 Ω to 16 kΩ). The CNFs / MWCNTs conductive film is then cut and used as the lower sensitive layer of the sensor.

[0013] (7) Add 0.05g of PVA powder to 10mL of deionized water and stir at 85-95℃ for 0.5-4h to obtain a PVA solution;

[0014] (8) Add 10g of CNFs aqueous dispersion with a mass fraction of 1-2% to the PVA solution obtained in step (7), and stir at 85-95℃ for 2-6h to obtain CNFs / PVA mixed solution;

[0015] (9) Drop the CNFs / PVA mixed solution obtained in step (8) onto a polytetrafluoroethylene mold (10×10cm) that has been ultrasonically cleaned and dried. 2In a fume hood, the solution is uniformly applied to the surface of the mold and placed in a fume hood for 8–15 hours to obtain a CNFs / PVA film (0.02–0.07 mm thick). The CNFs / PVA film is then cut and used as the substrate layer of the sensor.

[0016] (10) Apply conductive silver paste (with a conductivity of approximately 1×10⁻⁶) 5 Silver forked electrodes (with a thickness of 0.001–0.007 mm) are printed on the surface of the CNFs / PVA film obtained in step (9) using a screen printing process, and spring terminals are fixed at both ends of the forked electrodes to connect wires; the number of pairs of forked electrodes is 2–5, the finger width is 0.6–0.8 mm, the finger length is 4.5–5.5 mm, and the finger spacing is 0.55–0.65 mm;

[0017] (11) The films obtained in steps (3), (6) and (10) are encapsulated by bonding two single-sided adhesive TPU tapes together in the following order from top to bottom: upper sensitive layer of CNFs / MWCNTs conductive film (resistance of 45-55Ω), lower sensitive layer of CNFs / MWCNTs conductive film (resistance of 45Ω-16kΩ), and CNFs / PVA film with interdigitated electrodes on the surface. The adhesive side of the upper TPU tape faces down and the adhesive side of the lower TPU tape faces up. The three films are sandwiched in the middle of the TPU tape to obtain a flexible pressure sensor. The structure of the pressure sensor is upper TPU tape, upper CNFs / MWCNTs conductive film, lower CNFs / MWCNTs conductive film, CNFs / PVA film with interdigitated electrodes on the surface, and lower TPU tape. The overall thickness of the sensor is 0.20-0.35mm.

[0018] The advantages of this invention are:

[0019] 1) The flexible pressure sensor in this invention is thin, easy to cut, has a simple manufacturing process, and is inexpensive;

[0020] 2) The flexible pressure sensor in this invention utilizes a dual-conductivity sensitive layer structure, and the initial resistance and sensitive range of the sensor are controlled by changing the current transmission path;

[0021] 3) The flexible pressure sensor in this invention can achieve good linearity within the detection range of 0.0004 to 480 kPa, with a linearity of up to 0.99 and a sensitivity of 0.1 kPa. -1 This improved the linear detection range of the flexible pressure sensor;

[0022] 4) The flexible pressure sensor prepared by this invention has a wide range of applications in wearable gait instability monitoring for elderly people and other people with gait instability. Attached Figure Description

[0023] Figure 1 This is a schematic diagram of the structure of the flexible pressure sensor prepared in Embodiments 1 to 5 of the present invention.

[0024] The components are named as follows: upper CNFs / MWCNTs conductive film 1, lower CNFs / MWCNTs conductive film 2, CNFs / PVA substrate 3, silver forked electrode 4 screen-printed on the surface of substrate 3, terminals 5 and 6 fixed at both ends of silver forked electrode 4, conductive copper wire 7 and conductive copper wire 8.

[0025] Figure 2 This is a schematic diagram showing the dimensions of the interdigitated electrodes prepared in Examples 1 to 10 of the present invention.

[0026] Figure 3 This is a schematic diagram of the sensing mechanism of the flexible pressure sensor prepared in Examples 1 to 5 of the present invention.

[0027] The names of each part are: upper CNFs / MWCNTs conductive film 1, lower CNFs / MWCNTs conductive film 2, and silver cross-finger electrode 3.

[0028] Figure 4 These are the current response curves of the flexible pressure sensors prepared in Examples 1 to 10 of this invention under different pressure loads, as a function of pressure.

[0029] Figure 5 These are the volt-ampere characteristic curves of the flexible pressure sensor prepared in Embodiment 2 of the present invention under different pressure loads.

[0030] Figure 6 These are the dynamic current response recovery curves of the flexible pressure sensor prepared in Embodiment 2 of the present invention under different pressures for six repeated cycles.

[0031] Figure 7 The curves show the response time and recovery time of the flexible pressure sensor prepared in Embodiment 2 of the present invention under a pressure of 320 kPa.

[0032] Figure 8 This is the dynamic current response recovery curve of the flexible pressure sensor prepared in Embodiment 2 of the present invention under pressure load (200kPa) at different frequencies.

[0033] Figure 9 This is the dynamic current response recovery curve of the flexible pressure sensor prepared in Embodiment 2 of the present invention when detecting a small pressure of 0.4 Pa.

[0034] Figure 10 The curves showing the current response of the flexible pressure sensor prepared in Embodiment 2 of the present invention under different pressure loads, along with its linear fitting curves, are shown.

[0035] Figures 4 to 10 The results show that the sensor has good linearity in the detection range of 0.0004 to 480 kPa, and has accurate resolution of small pressures, as well as stability and repeatability. Detailed Implementation

[0036] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0037] Example 1

[0038] (1) Prepare a mixed solution of CNFs and MWCNTs with a mass ratio of 1:5 (5g × 1% : 0.25g). Add 5g of CNFs dispersion (mass fraction 1%) and 0.25g of MWCNTs to 10mL of deionized water to obtain a CNFs / MWCNTs solution;

[0039] (2) Add 0.05g sodium dodecylbenzenesulfonate (dispersant) to the solution in step (1), stir at 22℃ for 5h, and then sonicate the resulting solution for 1h to make CNFs and MWCNTs uniformly dispersed to obtain a mixed solution.

[0040] (3) Drop the mixed solution obtained in step (2) onto the ultrasonically cleaned and dried polytetrafluoroethylene mold (10×10cm). 2 In a process called ), the solution is uniformly applied to the surface of the mold, and the mold is placed in a fume hood and left to stand for 12 hours to allow the water in the solution to evaporate naturally, resulting in a CNFs / MWCNTs conductive film (0.09 mm thick, 45–55 Ω resistivity). The CNFs / MWCNTs conductive film is then cut into several 0.6 × 0.8 cm pieces. 2 A rectangle serves as the upper sensitive layer of the sensor;

[0041] (4) Prepare a mixed solution of CNFs and MWCNTs with a mass ratio of 15:1 (5g × 1% : 0.0033g). Add 5g of CNFs dispersion (1% by mass) and 0.0033g of MWCNTs to 10mL of deionized water to obtain a CNFs / MWCNTs solution;

[0042] (5) Add 0.01g sodium dodecylbenzenesulfonate (dispersant) to the solution in step (4), stir at 22℃ for 5h, and then sonicate the resulting solution for 1h to make CNFs and MWCNTs uniformly dispersed to obtain a mixed solution.

[0043] (6) Drop the mixed solution obtained in step (5) onto the ultrasonically cleaned and dried polytetrafluoroethylene mold (10×10cm). 2 In a process called ), the solution is uniformly applied to the surface of the mold, and the mold is placed in a fume hood and left to stand for 12 hours to allow the water in the solution to evaporate naturally, resulting in a CNFs / MWCNTs conductive film (0.01 mm thick, 15–16 kΩ resistivity). The CNFs / MWCNTs conductive film is then cut into several 0.6 × 0.8 cm pieces. 2 A rectangle, serving as the lower sensitive layer of the sensor;

[0044] (7) Add 0.05g of PVA powder to 10mL of deionized water and stir at 90℃ for 3h to obtain a PVA solution;

[0045] (8) Add 10g of CNFs dispersion (1% by mass) to the PVA solution obtained in step (7) and stir at 90℃ for 4h to obtain CNFs / PVA mixed solution;

[0046] (9) Drop the CNFs / PVA mixed solution obtained in step (8) onto a polytetrafluoroethylene mold (10×10cm) that has been ultrasonically cleaned and dried. 2 In a process called ), the solution is evenly applied to the surface of the mold, and the mold is placed in a fume hood and left to stand for 12 hours to allow the water in the solution to evaporate naturally, resulting in a CNFs / PVA film (0.05 mm thick). The CNFs / PVA film is then cut into several 4×2 cm pieces. 2 A rectangle is used as the substrate layer of the sensor;

[0047] (10) Apply conductive silver paste (with a conductivity of approximately 1×10⁻⁶) 5 Silver interdigitated electrodes (interdigitated electrode thickness is 0.002 mm) are printed on the CNFs / PVA film obtained in step (9) by screen printing process, and spring terminals are fixed at both ends of the interdigitated electrodes to connect the wires.

[0048] (11) The films obtained in steps (3), (6) and (10) are encapsulated by bonding two single-sided adhesive TPU tapes together in the following order from top to bottom: upper sensitive layer of CNFs / MWCNTs conductive film (resistance of 45-55Ω), lower sensitive layer of CNFs / MWCNTs conductive film (resistance of 15-16kΩ), and CNFs / PVA film with interdigitated electrodes on the surface. The adhesive side of the upper TPU tape faces down and the adhesive side of the lower TPU tape faces up. The three films are sandwiched in the middle of the TPU tape to obtain a flexible pressure sensor. The structure of the pressure sensor is upper TPU tape, upper CNFs / MWCNTs conductive film, lower CNFs / MWCNTs conductive film, CNFs / PVA film with interdigitated electrodes on the surface, and lower TPU tape. The thickness of the sensor is 0.29mm.

[0049] The fabricated devices are named according to the rule of "mixing ratio of CNFs to MWCNTs in the upper CNFs / MWCNTs conductive film" / "mixing ratio of CNFs to MWCNTs in the lower CNFs / MWCNTs conductive film". In this embodiment, the sensor is named: Sensor-1:5 / 15:1.

[0050] Example 2

[0051] (1) Prepare a mixed solution of CNFs and MWCNTs with a mass ratio of 1:5 (5g × 1% : 0.25g). Add 5g of CNFs dispersion (mass fraction 1%) and 0.25g of MWCNTs to 10mL of deionized water to obtain a CNFs / MWCNTs solution;

[0052] (2) Add 0.05g sodium dodecylbenzenesulfonate (dispersant) to the solution in step (1), stir at 22℃ for 5h, and then sonicate the solution for 1h to make CNFs and MWCNTs uniformly dispersed to obtain a mixed solution.

[0053] (3) Drop the mixed solution obtained in step (2) onto the ultrasonically cleaned and dried polytetrafluoroethylene mold (10×10cm). 2 In a process called ), the solution is uniformly applied to the surface of the mold, and the mold is placed in a fume hood and left to stand for 12 hours to allow the water in the solution to evaporate naturally, resulting in a CNFs / MWCNTs conductive film (0.08 mm thick, 45–55 Ω resistivity). The CNFs / MWCNTs conductive film is then cut into several 0.6 × 0.8 cm pieces. 2 A rectangle serves as the upper sensitive layer of the sensor;

[0054] (4) Prepare a mixed solution of CNFs and MWCNTs with a mass ratio of 10:1 (5g × 1% : 0.005g). Add 5g of CNFs dispersion (1% by mass) and 0.005g of MWCNTs to 10mL of deionized water to obtain a CNFs / MWCNTs solution;

[0055] (5) Add 0.02g sodium dodecylbenzenesulfonate (dispersant) to the solution in step (4), stir at 22℃ for 5h, and then sonicate the solution for 1h to make CNFs and MWCNTs uniformly dispersed to obtain a mixed solution.

[0056] (6) Drop the mixed solution obtained in step (5) onto the ultrasonically cleaned and dried polytetrafluoroethylene mold (10×10cm). 2 In a process called ), the solution is uniformly applied to the surface of the mold, and the mold is placed in a fume hood and left to stand for 12 hours to allow the water in the solution to evaporate naturally, resulting in a CNFs / MWCNTs conductive film (0.02 mm thick, 8–9 kΩ resistivity). The CNFs / MWCNTs conductive film is then cut into several 0.6 × 0.8 cm pieces. 2 A rectangle, serving as the lower sensitive layer of the sensor;

[0057] (7) Add 0.05g of PVA powder to 10mL of deionized water and stir at 90℃ for 3h to obtain a PVA solution;

[0058] (8) Add 10g of CNFs dispersion (1% by mass) to the PVA solution obtained in step (7) and stir at 90℃ for 4h to obtain CNFs / PVA mixed solution;

[0059] (9) Drop the CNFs / PVA mixed solution obtained in step (8) onto a polytetrafluoroethylene mold (10×10cm) that has been ultrasonically cleaned and dried. 2 In a process called ), the solution is evenly applied to the surface of the mold, and the mold is placed in a fume hood and left to stand for 12 hours to allow the water in the solution to evaporate naturally, resulting in a CNFs / PVA film (0.06 mm thick). The CNFs / PVA film is then cut into several 4×2 cm pieces. 2 A rectangle is used as the substrate layer of the sensor;

[0060] (10) Apply conductive silver paste (with a conductivity of approximately 1×10⁻⁶) 5 Silver interdigitated electrodes (interdigitated electrode thickness is 0.002 mm) are printed on the CNFs / PVA film obtained in step (9) by screen printing process, and spring terminals are fixed at both ends of the interdigitated electrodes to connect the wires.

[0061] (11) The films obtained in steps (3), (6) and (10) are encapsulated by bonding two single-sided adhesive TPU tapes together in the following order from top to bottom: upper sensitive layer of CNFs / MWCNTs conductive film (resistance of 45-55Ω), lower sensitive layer of CNFs / MWCNTs conductive film (resistance of 8-9kΩ), and CNFs / PVA film with interdigitated electrodes on the surface. The adhesive side of the upper TPU tape faces down and the adhesive side of the lower TPU tape faces up. The three films are sandwiched in the middle of the TPU tape to obtain a flexible pressure sensor. The structure of the pressure sensor is upper TPU tape, upper CNFs / MWCNTs conductive film, lower CNFs / MWCNTs conductive film, CNFs / PVA film with interdigitated electrodes on the surface, and lower TPU tape. The thickness of the sensor is 0.31mm.

[0062] The fabricated devices are named according to the rule of "mixing ratio of CNFs to MWCNTs in the upper CNFs / MWCNTs conductive film" / "mixing ratio of CNFs to MWCNTs in the lower CNFs / MWCNTs conductive film". The sensor in this embodiment is named: Sensor-1:5 / 10:1.

[0063] Example 3

[0064] (1) Prepare a mixed solution of CNFs and MWCNTs with a mass ratio of 1:5 (5g × 1% : 0.25g). Add 5g of CNFs dispersion (mass fraction 1%) and 0.25g of MWCNTs to 10mL of deionized water to obtain a CNFs / MWCNTs solution;

[0065] (2) Add 0.05g sodium dodecylbenzenesulfonate (dispersant) to the solution in step (1), stir at 22℃ for 5h, and then sonicate the solution for 1h to make CNFs and MWCNTs uniformly dispersed to obtain a mixed solution.

[0066] (3) Drop the mixed solution obtained in step (2) onto the ultrasonically cleaned and dried polytetrafluoroethylene mold (10×10cm). 2 In a process called ), the solution is uniformly applied to the surface of the mold, and the mold is placed in a fume hood and left to stand for 12 hours to allow the water in the solution to evaporate naturally, resulting in a CNFs / MWCNTs conductive film (0.09 mm thick, 45–55 Ω resistivity). The CNFs / MWCNTs conductive film is then cut into several 0.6 × 0.8 cm pieces. 2 A rectangle serves as the upper sensitive layer of the sensor;

[0067] (4) Prepare a mixed solution of CNFs and MWCNTs with a mass ratio of 5:1 (5g × 1% : 0.01g). Add 5g of CNFs dispersion (1% by mass) and 0.01g of MWCNTs to 10mL of deionized water to obtain a CNFs / MWCNTs solution;

[0068] (5) Add 0.03g sodium dodecylbenzenesulfonate (dispersant) to the solution in step (4), stir at 22℃ for 5h, and then sonicate the solution for 1h to make CNFs and MWCNTs uniformly dispersed to obtain a mixed solution.

[0069] (6) Drop the mixed solution obtained in step (5) onto the ultrasonically cleaned and dried polytetrafluoroethylene mold (10×10cm). 2 In a process called ), the solution is uniformly applied to the surface of the mold, and the mold is placed in a fume hood and left to stand for 12 hours to allow the water in the solution to evaporate naturally, resulting in a CNFs / MWCNTs conductive film (0.01 mm thick, 1–2 kΩ resistivity). The CNFs / MWCNTs conductive film is then cut into several 0.6 × 0.8 cm pieces. 2 A rectangle, serving as the lower sensitive layer of the sensor;

[0070] (7) Add 0.05g of PVA powder to 10mL of deionized water and stir at 90℃ for 3h to obtain a PVA solution;

[0071] (8) Add 10g of CNFs dispersion (1% by mass) to the PVA solution obtained in step (7) and stir at 90℃ for 4h to obtain CNFs / PVA mixed solution;

[0072] (9) Drop the CNFs / PVA mixed solution obtained in step (8) onto a polytetrafluoroethylene mold (10×10cm) that has been ultrasonically cleaned and dried. 2 In a process called ), the solution is evenly applied to the surface of the mold, and the mold is placed in a fume hood and left to stand for 12 hours to allow the water in the solution to evaporate naturally, resulting in a CNFs / PVA film (0.06 mm thick). The CNFs / PVA film is then cut into several 4×2 cm pieces. 2 A rectangle is used as the substrate layer of the sensor;

[0073] (10) Apply conductive silver paste (with a conductivity of approximately 1×10⁻⁶) 5 Silver interdigitated electrodes (interdigitated electrode thickness is 0.003 mm) are printed on the CNFs / PVA film obtained in step (9) by screen printing process, and spring terminals are fixed at both ends of the interdigitated electrodes to connect the wires.

[0074] (11) The films obtained in steps (3), (6) and (10) are encapsulated by bonding two single-sided adhesive TPU tapes together in the following order from top to bottom: upper sensitive layer of CNFs / MWCNTs conductive film (resistance of 45-55Ω), lower sensitive layer of CNFs / MWCNTs conductive film (resistance of 1-2kΩ), and CNFs / PVA film with interdigitated electrodes on the surface. The adhesive side of the upper TPU tape faces down and the adhesive side of the lower TPU tape faces up. The three films are sandwiched in the middle of the TPU tape to obtain a flexible pressure sensor. The structure of the pressure sensor is upper TPU tape, upper CNFs / MWCNTs conductive film, lower CNFs / MWCNTs conductive film, CNFs / PVA film with interdigitated electrodes on the surface, and lower TPU tape. The thickness of the sensor is 0.30 mm.

[0075] The fabricated devices are named according to the rule of "mixing ratio of CNFs to MWCNTs in the upper CNFs / MWCNTs conductive film" / "mixing ratio of CNFs to MWCNTs in the lower CNFs / MWCNTs conductive film". The sensor in this embodiment is named: Sensor-1:5 / 5:1.

[0076] Example 4

[0077] (1) Prepare a mixed solution of CNFs and MWCNTs with a mass ratio of 1:5 (5g × 1% : 0.25g). Add 5g of CNFs dispersion (mass fraction 1%) and 0.25g of MWCNTs to 10mL of deionized water to obtain a CNFs / MWCNTs solution;

[0078] (2) Add 0.05g sodium dodecylbenzenesulfonate (dispersant) to the solution in step (1), stir at 22℃ for 5h, and then sonicate the solution for 1h to make CNFs and MWCNTs uniformly dispersed to obtain a mixed solution.

[0079] (3) Drop the mixed solution obtained in step (2) onto the ultrasonically cleaned and dried polytetrafluoroethylene mold (10×10cm). 2 In a process called ), the solution is uniformly applied to the surface of the mold, and the mold is placed in a fume hood and left to stand for 12 hours to allow the water in the solution to evaporate naturally, resulting in a CNFs / MWCNTs conductive film (0.09 mm thick, 45–55 Ω resistivity). The CNFs / MWCNTs conductive film is then cut into several 0.6 × 0.8 cm pieces. 2 A rectangle serves as the upper sensitive layer of the sensor;

[0080] (4) Prepare a mixed solution of CNFs and MWCNTs with a mass ratio of 1:1 (5g × 1% : 0.05g). Add 5g of CNFs dispersion (mass fraction 1%) and 0.05g of MWCNTs to 10mL of deionized water to obtain a CNFs / MWCNTs solution;

[0081] (5) Add 0.04g sodium dodecylbenzenesulfonate (dispersant) to the solution in step (4), stir at 22℃ for 5h, and then sonicate the solution for 1h to make CNFs and MWCNTs uniformly dispersed to obtain a mixed solution.

[0082] (6) Drop the mixed solution obtained in step (5) onto the ultrasonically cleaned and dried polytetrafluoroethylene mold (10×10cm). 2 In a process called ), the solution is uniformly applied to the surface of the mold, and the mold is placed in a fume hood and left to stand for 12 hours to allow the water in the solution to evaporate naturally, resulting in a CNFs / MWCNTs conductive film (0.02 mm thick, 100–200 Ω resistivity). The CNFs / MWCNTs conductive film is then cut into several 0.6 × 0.8 cm pieces. 2 A rectangle, serving as the lower sensitive layer of the sensor;

[0083] (7) Add 0.05g of PVA powder to 10mL of deionized water and stir at 90℃ for 3h to obtain a PVA solution;

[0084] (8) Add 10g of CNFs dispersion (1% by mass) to the PVA solution obtained in step (7) and stir at 90℃ for 4h to obtain CNFs / PVA mixed solution;

[0085] (9) Drop the CNFs / PVA mixed solution obtained in step (8) onto a polytetrafluoroethylene mold (10×10cm) that has been ultrasonically cleaned and dried. 2 In a process called ), the solution is evenly applied to the surface of the mold, and the mold is placed in a fume hood and left to stand for 12 hours to allow the water in the solution to evaporate naturally, resulting in a CNFs / PVA film (0.05 mm thick). The CNFs / PVA film is then cut into several 4×2 cm pieces. 2 A rectangle is used as the substrate layer of the sensor;

[0086] (10) Apply conductive silver paste (with a conductivity of approximately 1×10⁻⁶) 5 Silver interdigitated electrodes (interdigitated electrode thickness is 0.002 mm) are printed on the CNFs / PVA film obtained in step (9) by screen printing process, and spring terminals are fixed at both ends of the interdigitated electrodes to connect the wires.

[0087] (11) The films obtained in steps (3), (6) and (10) are encapsulated by bonding two single-sided adhesive TPU tapes together in the following order from top to bottom: upper sensitive layer of CNFs / MWCNTs conductive film (resistance of 45-55Ω), lower sensitive layer of CNFs / MWCNTs conductive film (resistance of 100-200Ω), and CNFs / PVA film with interdigitated electrodes on the surface. The adhesive side of the upper TPU tape faces down and the adhesive side of the lower TPU tape faces up. The three films are sandwiched in the middle of the TPU tape to obtain a flexible pressure sensor. The structure of the pressure sensor is upper TPU tape, upper CNFs / MWCNTs conductive film, lower CNFs / MWCNTs conductive film, CNFs / PVA film with interdigitated electrodes on the surface, and lower TPU tape. The thickness of the sensor is 0.31 mm.

[0088] The fabricated devices are named according to the rule of "mixing ratio of CNFs to MWCNTs in the upper CNFs / MWCNTs conductive film" / "mixing ratio of CNFs to MWCNTs in the lower CNFs / MWCNTs conductive film". The sensor in this embodiment is named: Sensor-1:5 / 1:1.

[0089] Example 5

[0090] (1) Prepare a mixed solution of CNFs and MWCNTs with a mass ratio of 1:5 (5g × 1% : 0.25g). Add 5g of CNFs dispersion (mass fraction 1%) and 0.25g of MWCNTs to 10mL of deionized water to obtain a CNFs / MWCNTs solution;

[0091] (2) Add 0.05g sodium dodecylbenzenesulfonate (dispersant) to the solution in step (1), stir at 22℃ for 5h, and then sonicate the solution for 1h to make CNFs and MWCNTs uniformly dispersed to obtain a mixed solution.

[0092] (3) Drop the mixed solution obtained in step (2) onto the ultrasonically cleaned and dried polytetrafluoroethylene mold (10×10cm). 2 In a process called ), the solution is uniformly applied to the surface of the mold, and the mold is placed in a fume hood and left to stand for 12 hours to allow the water in the solution to evaporate naturally, resulting in a CNFs / MWCNTs conductive film (0.07 mm thick, 45–55 Ω resistivity). The CNFs / MWCNTs conductive film is then cut into several 0.6 × 0.8 cm pieces. 2 The rectangles are used as the upper and lower sensitive layers of the sensor, respectively.

[0093] (4) Add 0.05g of PVA powder to 10mL of deionized water and stir at 90℃ for 3h to obtain a PVA solution;

[0094] (5) Add 10g of CNFs dispersion (1% by mass) to the PVA solution obtained in step (4) and stir at 90℃ for 4h to obtain CNFs / PVA mixed solution;

[0095] (6) Drop the CNFs / PVA mixed solution obtained in step (5) onto a polytetrafluoroethylene mold (10×10cm) that has been ultrasonically cleaned and dried. 2 In a process called ), the solution is evenly applied to the surface of the mold, and the mold is placed in a fume hood and left to stand for 12 hours to allow the water in the solution to evaporate naturally, resulting in a CNFs / PVA film (0.04 mm thick). The CNFs / PVA film is then cut into several 4×2 cm pieces. 2 A rectangle is used as the substrate layer of the sensor;

[0096] (7) Apply conductive silver paste (with a conductivity of approximately 1×10⁻⁶) 5 Silver interdigitated electrodes (interdigitated electrode thickness is 0.005 mm) are printed on the CNFs / PVA film obtained in step (6) by screen printing process, and spring terminals are fixed at both ends of the interdigitated electrodes to connect the wires.

[0097] (8) The films obtained in steps (3), (3), and (7) are encapsulated by bonding two single-sided adhesive TPU tapes together in the following order from top to bottom: upper sensitive layer of CNFs / MWCNTs conductive film (resistance of 45-55Ω), lower sensitive layer of CNFs / MWCNTs conductive film (resistance of 45-55Ω), and CNFs / PVA film with interdigitated electrodes on the surface. The adhesive side of the upper TPU tape faces down and the adhesive side of the lower TPU tape faces up. The three film layers are sandwiched in the middle of the TPU tape to obtain a flexible pressure sensor. The structure of the pressure sensor is an upper TPU tape, an upper CNFs / MWCNTs conductive film, a lower CNFs / MWCNTs conductive film, a CNFs / PVA film with interdigitated electrodes on the surface, and a lower TPU tape. The thickness of the sensor is 0.32 mm.

[0098] The fabricated devices are named according to the rule of "mixing ratio of CNFs to MWCNTs in the upper CNFs / MWCNTs conductive film" / "mixing ratio of CNFs to MWCNTs in the lower CNFs / MWCNTs conductive film". The sensor in this embodiment is named: Sensor-1:5 / 1:5.

[0099] Example 6

[0100] (1) Prepare a mixed solution of CNFs and MWCNTs with a mass ratio of 15:1 (5g × 1% : 0.0033g). Add 5g of CNFs dispersion (1% by mass) and 0.0033g of MWCNTs to 10mL of deionized water to obtain a CNFs / MWCNTs solution;

[0101] (2) Add 0.01g sodium dodecylbenzenesulfonate (dispersant) to the solution in step (1), stir at 22℃ for 5h, and then sonicate the solution for 1h to make CNFs and MWCNTs uniformly dispersed to obtain a mixed solution.

[0102] (3) Drop the mixed solution obtained in step (2) onto the ultrasonically cleaned and dried polytetrafluoroethylene mold (10×10cm). 2 In a process called ), the solution is uniformly applied to the surface of the mold, and the mold is placed in a fume hood and left to stand for 12 hours to allow the water in the solution to evaporate naturally, resulting in a CNFs / MWCNTs conductive film (0.01 mm thick, 15–16 kΩ resistivity). The CNFs / MWCNTs conductive film is then cut into several 0.6 × 0.8 cm pieces. 2 A rectangle, serving as the lower sensitive layer of the sensor;

[0103] (4) Add 0.05g of PVA powder to 10mL of deionized water and stir at 90℃ for 3h to obtain a PVA solution;

[0104] (5) Add 10g of CNFs dispersion (1% by mass) to the PVA solution obtained in step (4) and stir at 90℃ for 4h to obtain CNFs / PVA mixed solution;

[0105] (6) Drop the CNFs / PVA mixed solution obtained in step (5) onto a polytetrafluoroethylene mold (10×10cm) that has been ultrasonically cleaned and dried. 2 In a process called ), the solution is evenly applied to the surface of the mold, and the mold is placed in a fume hood and left to stand for 12 hours to allow the water in the solution to evaporate naturally, resulting in a CNFs / PVA film (0.05 mm thick). The CNFs / PVA film is then cut into several 4×2 cm pieces. 2 A rectangle is used as the substrate layer of the sensor;

[0106] (7) Apply conductive silver paste (with a conductivity of approximately 1×10⁻⁶) 5 Silver interdigitated electrodes (interdigitated electrode thickness is 0.003 mm) are printed on the CNFs / PVA film obtained in step (6) by screen printing process, and spring terminals are fixed at both ends of the interdigitated electrodes to connect the wires.

[0107] (8) The films obtained in steps (3) and (7) are encapsulated by bonding two single-sided adhesive TPU sheets together in the following order from top to bottom: CNFs / MWCNTs conductive film sensitive layer (resistance of 15-16kΩ) and CNFs / PVA film with interdigitated electrodes on the surface. The adhesive side of the upper TPU sheet is facing down and the adhesive side of the lower TPU sheet is facing up. The two films are sandwiched in the middle of the TPU sheet to obtain a flexible pressure sensor. The structure of the pressure sensor is an upper TPU sheet, a CNFs / MWCNTs conductive film, a CNFs / PVA film with interdigitated electrodes on the surface, and a lower TPU sheet. The thickness of the sensor is 0.20 mm.

[0108] The fabricated devices are named according to the "mixing ratio of CNFs to MWCNTs in CNFs / MWCNTs conductive films". The sensor in this embodiment is named: Sensor-15:1.

[0109] Example 7

[0110] (1) Prepare a mixed solution of CNFs and MWCNTs with a mass ratio of 10:1 (5g × 1% : 0.005g). Add 5g of CNFs dispersion (1% by mass) and 0.005g of MWCNTs to 10mL of deionized water to obtain a CNFs / MWCNTs solution;

[0111] (2) Add 0.02g sodium dodecylbenzenesulfonate (dispersant) to the solution in step (1), stir at 22℃ for 5h, and then sonicate the solution for 1h to make CNFs and MWCNTs uniformly dispersed to obtain a mixed solution.

[0112] (3) Drop the mixed solution obtained in step (2) onto the ultrasonically cleaned and dried polytetrafluoroethylene mold (10×10cm). 2 In a process called ), the solution is uniformly applied to the surface of the mold, and the mold is placed in a fume hood and left to stand for 12 hours to allow the water in the solution to evaporate naturally, resulting in a CNFs / MWCNTs conductive film (0.01 mm thick, 8–9 kΩ resistivity). The CNFs / MWCNTs conductive film is then cut into several 0.6 × 0.8 cm pieces. 2 A rectangle, serving as the lower sensitive layer of the sensor;

[0113] (4) Add 0.05g of PVA powder to 10mL of deionized water and stir at 90℃ for 3h to obtain a PVA solution;

[0114] (5) Add 10g of CNFs dispersion (1% by mass) to the PVA solution obtained in step (4) and stir at 90℃ for 4h to obtain CNFs / PVA mixed solution;

[0115] (6) Drop the CNFs / PVA mixed solution obtained in step (5) onto a polytetrafluoroethylene mold (10×10cm) that has been ultrasonically cleaned and dried. 2 In a process called ), the solution is evenly applied to the surface of the mold, and the mold is placed in a fume hood and left to stand for 12 hours to allow the water in the solution to evaporate naturally, resulting in a CNFs / PVA film (0.05 mm thick). The CNFs / PVA film is then cut into several 4×2 cm pieces. 2 A rectangle is used as the substrate layer of the sensor;

[0116] (7) Apply conductive silver paste (with a conductivity of approximately 1×10⁻⁶) 5 Silver interdigitated electrodes (interdigitated electrode thickness is 0.003 mm) are printed on the CNFs / PVA film obtained in step (6) by screen printing process, and spring terminals are fixed at both ends of the interdigitated electrodes to connect the wires.

[0117] (8) The films obtained in steps (3) and (7) are encapsulated by bonding two single-sided adhesive TPU sheets together in the following order from top to bottom: CNFs / MWCNTs conductive film sensitive layer (resistance of 8-9kΩ) and CNFs / PVA film with interdigitated electrodes on the surface. The adhesive side of the upper TPU sheet is facing down and the adhesive side of the lower TPU sheet is facing up. The two films are sandwiched in the middle of the TPU sheet to obtain a flexible pressure sensor. The structure of the pressure sensor is an upper TPU sheet, a CNFs / MWCNTs conductive film, a CNFs / PVA film with interdigitated electrodes on the surface, and a lower TPU sheet. The thickness of the sensor is 0.20 mm.

[0118] The fabricated devices are named according to the "mixing ratio of CNFs to MWCNTs in CNFs / MWCNTs conductive films". The sensor in this embodiment is named: Sensor-10:1.

[0119] Example 8

[0120] (1) Prepare a mixed solution of CNFs and MWCNTs with a mass ratio of 5:1 (5g × 1% : 0.01g). Add 5g of CNFs dispersion (1% by mass) and 0.01g of MWCNTs to 10mL of deionized water to obtain a CNFs / MWCNTs solution;

[0121] (2) Add 0.03g sodium dodecylbenzenesulfonate (dispersant) to the solution in step (1), stir at 22℃ for 5h, and then sonicate the solution for 1h to make CNFs and MWCNTs uniformly dispersed to obtain a mixed solution.

[0122] (3) Drop the mixed solution obtained in step (2) onto the ultrasonically cleaned and dried polytetrafluoroethylene mold (10×10cm). 2 In a process called ), the solution is uniformly applied to the surface of the mold, and the mold is placed in a fume hood and left to stand for 12 hours to allow the water in the solution to evaporate naturally, resulting in a CNFs / MWCNTs conductive film (0.02 mm thick, 1–2 kΩ resistivity). The CNFs / MWCNTs conductive film is then cut into several 0.6 × 0.8 cm pieces. 2 A rectangle, serving as the lower sensitive layer of the sensor;

[0123] (4) Add 0.05g of PVA powder to 10mL of deionized water and stir at 90℃ for 3h to obtain a PVA solution;

[0124] (5) Add 10g of CNFs dispersion (1% by mass) to the PVA solution obtained in step (4) and stir at 90℃ for 4h to obtain CNFs / PVA mixed solution;

[0125] (6) Drop the CNFs / PVA mixed solution obtained in step (5) onto a polytetrafluoroethylene mold (10×10cm) that has been ultrasonically cleaned and dried. 2 In a process called ), the solution is evenly applied to the surface of the mold, and the mold is placed in a fume hood and left to stand for 12 hours to allow the water in the solution to evaporate naturally, resulting in a CNFs / PVA film (0.05 mm thick). The CNFs / PVA film is then cut into several 4×2 cm pieces. 2 A rectangle is used as the substrate layer of the sensor;

[0126] (7) Apply conductive silver paste (with a conductivity of approximately 1×10⁻⁶) 5 Silver interdigitated electrodes (interdigitated electrode thickness is 0.004 mm) are printed on the CNFs / PVA film obtained in step (6) by screen printing process, and spring terminals are fixed at both ends of the interdigitated electrodes to connect the wires.

[0127] (8) The films obtained in steps (3) and (7) are encapsulated by bonding two single-sided adhesive TPU sheets together in the following order from top to bottom: CNFs / MWCNTs conductive film sensitive layer (resistance of 1-2kΩ) and CNFs / PVA film with interdigitated electrodes on the surface. The adhesive side of the upper TPU sheet is facing down and the adhesive side of the lower TPU sheet is facing up. The two films are sandwiched in the middle of the TPU sheet to obtain a flexible pressure sensor. The structure of the pressure sensor is an upper TPU sheet, a CNFs / MWCNTs conductive film, a CNFs / PVA film with interdigitated electrodes on the surface, and a lower TPU sheet. The thickness of the sensor is 0.21mm.

[0128] The fabricated devices are named according to the "mixing ratio of CNFs to MWCNTs in CNFs / MWCNTs conductive films". The sensor in this embodiment is named: Sensor-5:1.

[0129] Example 9

[0130] (1) Prepare a mixed solution of CNFs and MWCNTs with a mass ratio of 1:1 (5g × 1% : 0.05g). Add 5g of CNFs dispersion (mass fraction 1%) and 0.05g of MWCNTs to 10mL of deionized water to obtain a CNFs / MWCNTs solution;

[0131] (2) Add 0.04g sodium dodecylbenzenesulfonate (dispersant) to the solution in step (1), stir at 22℃ for 5h, and then sonicate the solution for 1h to make CNFs and MWCNTs uniformly dispersed to obtain a mixed solution.

[0132] (3) Drop the mixed solution obtained in step (2) onto the ultrasonically cleaned and dried polytetrafluoroethylene mold (10×10cm). 2 In a process called ), the solution is uniformly applied to the surface of the mold, and the mold is placed in a fume hood and left to stand for 12 hours to allow the water in the solution to evaporate naturally, resulting in a CNFs / MWCNTs conductive film (0.01 mm thick, 100–200 Ω resistivity). The CNFs / MWCNTs conductive film is then cut into several 0.6 × 0.8 cm pieces. 2 A rectangle, serving as the lower sensitive layer of the sensor;

[0133] (4) Add 0.05g of PVA powder to 10mL of deionized water and stir at 90℃ for 3h to obtain a PVA solution;

[0134] (5) Add 10g of CNFs dispersion (1% by mass) to the PVA solution obtained in step (4) and stir at 90℃ for 4h to obtain CNFs / PVA mixed solution;

[0135] (6) Drop the CNFs / PVA mixed solution obtained in step (5) onto a polytetrafluoroethylene mold (10×10cm) that has been ultrasonically cleaned and dried. 2 In a process called ), the solution is evenly applied to the surface of the mold, and the mold is placed in a fume hood and left to stand for 12 hours to allow the water in the solution to evaporate naturally, resulting in a CNFs / PVA film (0.05 mm thick). The CNFs / PVA film is then cut into several 4×2 cm pieces. 2 A rectangle is used as the substrate layer of the sensor;

[0136] (7) Apply conductive silver paste (with a conductivity of approximately 1×10⁻⁶) 5Silver interdigitated electrodes (interdigitated electrode thickness is 0.005 mm) are printed on the CNFs / PVA film obtained in step (6) by screen printing process, and spring terminals are fixed at both ends of the interdigitated electrodes to connect the wires.

[0137] (8) The films obtained in steps (3) and (7) are encapsulated by bonding two single-sided adhesive TPU sheets together in the following order from top to bottom: CNFs / MWCNTs conductive film sensitive layer (resistance of 100-200Ω) and CNFs / PVA film with interdigitated electrodes on the surface. The adhesive side of the upper TPU sheet is facing down and the adhesive side of the lower TPU sheet is facing up. The two films are sandwiched in the middle of the TPU sheet to obtain a flexible pressure sensor. The structure of the pressure sensor is an upper TPU sheet, a CNFs / MWCNTs conductive film, a CNFs / PVA film with interdigitated electrodes on the surface, and a lower TPU sheet. The thickness of the sensor is 0.20 mm.

[0138] The fabricated devices are named according to the "mixing ratio of CNFs to MWCNTs in CNFs / MWCNTs conductive films". The sensor in this embodiment is named: Sensor-1:1.

[0139] Example 10

[0140] (1) Prepare a mixed solution of CNFs and MWCNTs with a mass ratio of 1:5 (5g × 1% : 0.25g). Add 5g of CNFs dispersion (mass fraction 1%) and 0.25g of MWCNTs to 10mL of deionized water to obtain a CNFs / MWCNTs solution;

[0141] (2) Add 0.05g sodium dodecylbenzenesulfonate (dispersant) to the solution in step (1), stir at 22℃ for 5h, and then sonicate the solution for 1h to make CNFs and MWCNTs uniformly dispersed to obtain a mixed solution.

[0142] (3) Drop the mixed solution obtained in step (2) onto the ultrasonically cleaned and dried polytetrafluoroethylene mold (10×10cm). 2 In a process called ), the solution is uniformly applied to the surface of the mold, and the mold is placed in a fume hood and left to stand for 12 hours to allow the water in the solution to evaporate naturally, resulting in a CNFs / MWCNTs conductive film (0.08 mm thick, 45–55 Ω resistivity). The CNFs / MWCNTs conductive film is then cut into several 0.6 × 0.8 cm pieces. 2 A rectangle, serving as the lower sensitive layer of the sensor;

[0143] (4) Add 0.05g of PVA powder to 10mL of deionized water and stir at 90℃ for 3h to obtain a PVA solution;

[0144] (5) Add 10g of CNFs dispersion (1% by mass) to the PVA solution obtained in step (4) and stir at 90℃ for 4h to obtain CNFs / PVA mixed solution;

[0145] (6) Drop the CNFs / PVA mixed solution obtained in step (5) onto a polytetrafluoroethylene mold (10×10cm) that has been ultrasonically cleaned and dried. 2 In a process called ), the solution is evenly applied to the surface of the mold, and the mold is placed in a fume hood and left to stand for 12 hours to allow the water in the solution to evaporate naturally, resulting in a CNFs / PVA film (0.03 mm thick). The CNFs / PVA film is then cut into several 4×2 cm pieces. 2 A rectangle is used as the substrate layer of the sensor;

[0146] (7) Apply conductive silver paste (with a conductivity of approximately 1×10⁻⁶) 5 Silver interdigitated electrodes (interdigitated electrode thickness is 0.004 mm) are printed on the CNFs / PVA film obtained in step (6) by screen printing process, and spring terminals are fixed at both ends of the interdigitated electrodes to connect the wires.

[0147] (8) The films obtained in steps (3) and (7) are encapsulated by bonding two single-sided adhesive TPU sheets together in the following order from top to bottom: CNFs / MWCNTs conductive film sensitive layer (resistance of 45-55Ω) and CNFs / PVA film with interdigitated electrodes on the surface. The adhesive side of the upper TPU sheet is facing down and the adhesive side of the lower TPU sheet is facing up. The two films are sandwiched in the middle of the TPU sheet to obtain a flexible pressure sensor. The structure of the pressure sensor is an upper TPU sheet, a CNFs / MWCNTs conductive film, a CNFs / PVA film with interdigitated electrodes on the surface, and a lower TPU sheet. The thickness of the sensor is 0.25 mm.

[0148] The fabricated devices are named according to the "mixing ratio of CNFs to MWCNTs in the CNFs / MWCNTs conductive film". The sensor in this embodiment is named: Sensor-1:5.

[0149] Schematic diagrams of the flexible pressure sensors prepared in Examples 1-5 are shown below. Figure 1As shown, CNFs / MWCNTs conductive films 1 and 2 with different conductivity properties are used as the upper and lower sensitive layers, respectively. Conductive film 1 (located in the uppermost layer) is a CNFs / MWCNTs conductive film prepared by mixing CNFs and MWCNTs in a mass ratio of 1:5. Conductive film 2 (located in the middle layer) is a CNFs / MWCNTs conductive film prepared by mixing CNFs and MWCNTs in five mass ratios of 15:1, 10:1, 5:1, 1:1, and 1:5, respectively.

[0150] The dimensions of the interdigitated electrodes of the pressure sensors prepared in Examples 1-10 are as follows: Figure 2 As shown. The interdigitated electrodes have 2 pairs, with a finger width of 0.71 mm, a finger length of 5.08 mm, a finger spacing of 0.60 mm, an electrical connection area of ​​1.67 mm square, and an electrical connection lead of 6.42 mm long and 0.73 mm wide rectangular structure.

[0151] The schematic diagrams of the sensing mechanisms of the flexible pressure sensors prepared in Examples 1-5 are shown below. Figure 3 As shown. Initially, without pressure load, there are large gaps between the lower sensitive layer and the electrode layer, as well as between the two sensitive layers. The contact resistance between the two sensitive layers is extremely high. Therefore, current mainly flows through the lower, less conductive CNFs / MWCNTs conductive film 2. Only a very small amount of current can cross the contact interface between the two conductive layers and flow through the more conductive CNFs / MWCNTs conductive film 1. At this time, the sensor exhibits a high-resistance state. When the sensor is subjected to pressure load, the structural gaps between the sensitive layer and the electrode layer, as well as between the two sensitive layers, are compressed, increasing the contact area and decreasing the contact resistance between the interfaces. Since current tends to flow towards the more conductive material, more current crosses the contact interface between conductive film 1 and conductive film 2 and flows through the more conductive CNFs / MWCNTs conductive film 1. Therefore, the sensor resistance decreases. As the pressure continuously increases, the contact resistance between the sensitive layer and the electrode layer, as well as between the two sensitive layers, continuously decreases. The current flowing across the interface into conductive film 1 continuously increases, and the sensor resistance changes regularly with the pressure, thus achieving pressure sensitivity.

[0152] The current response curves of the flexible pressure sensors prepared in Examples 1-10 as a function of pressure are shown below. Figure 4As shown, compared to a single-layer sensitive layer, the double-layer sensitive layer design effectively improves the linear detection range of the sensor. Meanwhile, as the conductivity of the lower CNFs / MWCNTs conductive film 2 increases, the current response of the double-layer sensor exhibits a trend of first increasing and then decreasing. This is because as the resistance of the lower CNFs / MWCNTs conductive film 2 increases, the initial resistance of the sensor increases. However, under pressure, the interfacial contact resistance between the two conductive sensitive layers decreases, and the current preferentially crosses the interfacial resistance to be transmitted to the more conductive film 1. At this point, the sensor's resistance under pressure mainly depends on the conductivity of the conductive film 1, thus increasing the sensor's current change. However, when the resistance of the lower conductive film 2 continues to increase to a higher resistance state, it acts as an insulating layer, significantly increasing the interfacial contact resistance between the two conductive layers. The current struggles to cross the contact interface to reach the conductive film 1. At this point, the sensor's initial resistance and resistance under pressure mainly depend on the resistance of the conductive film 2, thus limiting the current change of the sensor under pressure. Among them, the flexible pressure sensor prepared in Example 2 has the best linearity and response characteristics in the pressure range of 0.0004 to 480 kPa, and is therefore the best device for subsequent performance testing.

[0153] The volt-ampere (IV) characteristic curves of the flexible pressure sensor prepared in Example 2 under different pressures are shown below. Figure 5 As shown, it can be observed that the sensor current is perfectly symmetrical when positive and negative voltages are applied, indicating that the device exhibits ohmic characteristics. The slope of the corresponding IV curve increases with increasing pressure, proving that the sensor resistance decreases with increasing pressure.

[0154] The dynamic current response and recovery curves of the flexible pressure sensor prepared in Example 2 under six repeated cycles of the same pressure load are shown below. Figure 6 As shown, the current response of the flexible pressure sensor increases with increasing pressure, and its current response remains basically consistent during repeated application of the same pressure, thus proving that the sensor has good repeatability.

[0155] The response time and recovery time curves of the flexible pressure sensor prepared in Example 2 are as follows: Figure 7 As shown, the sampling frequency for the test was 0.01Hz, and the response and recovery times of the sensor were defined as the time required for the sensor's current response to reach 90% of its stable value. It can be seen that the flexible pressure sensor's response time under 320kPa is approximately 90ms, and its recovery time is approximately 60ms, indicating that the sensor exhibits relatively fast response and recovery speeds even under high pressure loads.

[0156] The flexible pressure sensor prepared in Example 2, when subjected to the same pressure at different frequencies, exhibits dynamic current response recovery curves for five repeated cycles as shown below. Figure 8 As shown, when a pressure of 200 kPa is applied to the pressure sensor at five frequencies of 0.16 Hz, 0.39 Hz, 0.83 Hz, 1.67 Hz and 2.50 Hz respectively, the current response value of the sensor remains basically consistent, proving that the sensor can still maintain stable performance under different pressure application frequencies.

[0157] The dynamic current response recovery curve of the flexible pressure sensor prepared in Example 2 to a small pressure is shown in the figure below. Figure 9 As shown in the figure, it can be seen that the sensor can accurately identify and respond stably to minute pressures less than or equal to 0.4 Pa.

[0158] The current response curve and its linear fitting curve of the flexible pressure sensor prepared in Example 2 are shown below. Figure 10 As shown, this indicates that the sensor exhibits good linearity within the detection range of 0.0004–480 kPa, with a linearity of up to 0.99 and a sensitivity of 0.1 kPa. -1 .

Claims

1. A method for fabricating a flexible pressure sensor with a wide linear detection range based on cellulose nanofiber thin films, comprising the following steps: (1) Add 5 g of CNFs aqueous dispersion with a mass fraction of 1~2% and 0.25~0.30 g of MWCNTs to 10 mL of deionized water to obtain CNFs / MWCNTs solution; (2) Add 0.01~0.05 g sodium dodecylbenzenesulfonate to the solution in step (1) and stir at 20~30˚C for 2~5 h; then sonicate the resulting solution for 1~2 h to uniformly disperse CNFs and MWCNTs to obtain a mixed solution; (3) The mixed solution obtained in step (2) is drop-coated into the polytetrafluoroethylene mold after ultrasonic cleaning and drying, so that the solution is evenly covered on the surface of the mold, and placed in a fume hood for 8~15 h to naturally evaporate the water in the solution, so as to obtain a CNFs / MWCNTs conductive film with uniformly distributed conductive material. The resistance of the CNFs / MWCNTs conductive film is 45~55 Ω. Then, the CNFs / MWCNTs conductive film is cut and used as the upper sensitive layer of the sensor. (4) Add 5 g of CNFs aqueous dispersion with a mass fraction of 1~2% and 0.003~0.30 g of MWCNTs to 10 mL of deionized water to obtain CNFs / MWCNTs solution; (5) Add 0.01~0.05 g sodium dodecylbenzenesulfonate to the solution in step (4) and stir at 20~30˚C for 2~5 h; then sonicate the resulting solution for 1~2 h to uniformly disperse CNFs and MWCNTs to obtain a mixed solution; (6) The mixed solution obtained in step (5) is drop-coated into the polytetrafluoroethylene mold after ultrasonic cleaning and drying, so that the solution is evenly covered on the surface of the mold, and placed in a fume hood for 8~15 h to allow the water in the solution to evaporate naturally, so as to obtain a CNFs / MWCNTs conductive film with uniformly distributed conductive material. The resistance of the CNFs / MWCNTs conductive film is 45 Ω~16 kΩ. Then, the CNFs / MWCNTs conductive film is cut and used as the lower sensitive layer of the sensor. (7) Add 0.05 g of PVA powder to 10 mL of deionized water and stir at 85-95˚C for 0.5-4 h to obtain a PVA solution; (8) Add 10 g of CNFs aqueous dispersion with a mass fraction of 1~2% to the PVA solution obtained in step (7), and stir at 85~95˚C for 2~6 h to obtain CNFs / PVA mixed solution; (9) The CNFs / PVA mixed solution obtained in step (8) is drop-coated into the polytetrafluoroethylene mold after ultrasonic cleaning and drying, so that the solution is evenly covered on the surface of the mold, and placed in a fume hood for 8~15 h to obtain a CNFs / PVA film. Then, the CNFs / PVA film is cut and used as the substrate layer of the sensor. (10) Print silver interdigitated electrodes on the surface of the CNFs / PVA film obtained in step (9) using a screen printing process, and fix spring terminals to connect wires at both ends of the interdigitated electrodes. (11) The films obtained in steps (3), (6), and (10) are encapsulated by bonding two single-sided adhesive TPU sheets together in the following order from top to bottom: upper sensitive layer of CNFs / MWCNTs conductive film, lower sensitive layer of CNFs / MWCNTs conductive film, and CNFs / PVA film with interdigitated electrodes on the surface. The adhesive side of the upper TPU sheet faces down, and the adhesive side of the lower TPU sheet faces up. The three film layers are sandwiched in the middle of the TPU sheet to obtain a flexible pressure sensor. The structure of the pressure sensor is an upper TPU sheet, an upper CNFs / MWCNTs conductive film, a lower CNFs / MWCNTs conductive film, a CNFs / PVA film with interdigitated electrodes on the surface, and a lower TPU sheet. The flexible pressure sensor has good linearity in the detection range of 0.0004~480 kPa, with a linearity of 0.99 and a sensitivity of 0.1 kPa. -1 .

2. The method for fabricating a flexible pressure sensor with a wide linear detection range based on a cellulose nanofiber thin film as described in claim 1, characterized in that: The thickness of the CNFs / MWCNTs conductive film obtained in step (3) is 0.05~0.15 mm; the thickness of the CNFs / MWCNTs conductive film obtained in step (6) is 0.01~0.15 mm.

3. The method for fabricating a flexible pressure sensor with a wide linear detection range based on a cellulose nanofiber thin film as described in claim 1, characterized in that: The CNFs / PVA film obtained in step (9) has a thickness of 0.02~0.07 mm.

4. The method for fabricating a flexible pressure sensor with a wide linear detection range based on a cellulose nanofiber thin film as described in claim 1, characterized in that: The thickness of the silver interdigitated electrode obtained in step (10) is 0.001~0.007 mm, the number of interdigitated electrodes is 2~5 pairs, the finger width is 0.6~0.8 mm, the finger length is 4.5~5.5 mm, and the finger spacing is 0.55~0.65 mm.

5. The method for fabricating a flexible pressure sensor with a wide linear detection range based on a cellulose nanofiber thin film as described in claim 1, characterized in that: The overall thickness of the sensor obtained in step (11) is 0.20~0.35 mm.

6. A flexible pressure sensor with a wide linear detection range based on cellulose nanofiber thin films, characterized in that: It is prepared by the preparation method described in any one of claims 1 to 5.

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