A flexible thin film sensor based on lotus root carbon, a preparation method and application thereof

Abstract

The application provides a flexible film sensor based on lotus root carbon, which comprises a first electrode layer, a lotus root carbon / thermoplastic polyurethane composite electrostatic spinning sensitive layer and a second electrode layer arranged in sequence from bottom to top, and has a sandwich structure; wherein the first electrode layer and the second electrode layer are both metal foil electrode layers subjected to surface flattening treatment; and the sensitive layer is a nanofiber porous film prepared by electrostatic spinning process with lotus root carbon powder and thermoplastic polyurethane as raw materials. The application uses natural lotus root powder to prepare biochar conductive filler through high-temperature carbonization, replaces traditional artificial synthetic carbon materials, and adopts a sandwich structure prepared by assembling the electrode and the sensitive layer in a split mode, so that the damage of the electrode preparation process to the sensitive layer is effectively avoided, and the sensitivity, signal stability and finished product yield of the device are improved. In addition, the application also provides a preparation method and application of the flexible film sensor based on lotus root carbon.

Description

Technical Field

[0001] This invention relates to the field of flexible sensor technology, and in particular to a flexible thin-film sensor based on lotus root carbon, its preparation method, and its application. Background Technology

[0002] Flexible pressure film sensors, due to their advantages such as light weight, good flexibility, and ability to adhere to irregular surfaces, show broad application prospects in wearable health monitoring, robot tactile sensing, and human-computer interaction. Current fabrication of flexible pressure film sensors mainly relies on synthetic carbon materials such as carbon nanotubes, graphene, and acetylene black as conductive functional fillers, which are dispersed in flexible polymer substrates such as polydimethylsiloxane (PDMS) and thermoplastic polyurethane (TPU) to form a composite sensitive film with piezoresistive effect.

[0003] However, the aforementioned existing technical solutions have the following significant drawbacks: (1) High raw material cost and heavy environmental burden: The preparation process of artificial carbon materials such as carbon nanotubes and graphene is complicated and expensive, which is not conducive to large-scale promotion and application; at the same time, the synthesis process involves high energy consumption and high pollution processes such as strong acid oxidation and high-temperature chemical vapor deposition.

[0004] (2) Poor biocompatibility: The surface of artificially synthesized carbon materials has high chemical activity and many residual catalyst impurities, posing potential biosafety risks when in long-term contact with human skin.

[0005] (3) Poor dispersion and microstructure uniformity of conductive fillers: When the filler is unevenly dispersed or agglomerated in the polymer matrix, it will lead to significant differences in the local resistance distribution of the thin film, resulting in decreased sensor sensitivity, nonlinear response curve, and poor repeatability.

[0006] (4) Adverse effects of the preparation process on the performance of the substrate and the sensitive layer: The strong polar organic solvents or complex high-temperature processing methods commonly used in the existing preparation process may have a negative impact on the mechanical properties of the flexible substrate, resulting in signal drift and reduced stability of the thin film sensor during long-term cyclic testing.

[0007] (5) Direct film formation on rough electrode substrate leads to many film defects: In existing sandwich structure sensors, the sensitive layer is often deposited or grown directly on the electrode substrate that has not been leveled. The unevenness of the electrode surface will lead to uneven fiber spreading, large differences in film thickness, and many film defects.

[0008] In summary, existing thin-film sensors based on synthetic carbon materials and their preparation methods suffer from drawbacks such as high raw material costs, poor biocompatibility, complex preparation processes that can adversely affect the substrate and sensitive layer, and insufficient device stability. Summary of the Invention

[0009] Therefore, the purpose of this invention is to overcome the shortcomings of the prior art and provide a flexible thin-film sensor based on lotus root carbon, its preparation method and application, so as to solve the technical problems of high raw material cost, poor biocompatibility, complex preparation process, and the substrate being easily affected by the process and insufficient device stability of existing carbon-based thin-film sensors.

[0010] To achieve the above objectives, the present invention adopts the following technical solution: In a first aspect, the present invention provides a method for preparing a flexible thin-film sensor based on lotus root carbon, comprising a first electrode layer, a lotus root carbon / thermoplastic polyurethane composite electrospun sensitive layer and a second electrode layer arranged sequentially from bottom to top, forming a sandwich structure. The first electrode layer and the second electrode layer are both metal foil electrode layers that have undergone surface leveling treatment; The sensitive layer is a nanofiber porous film prepared by electrospinning using lotus root carbon powder and thermoplastic polyurethane as raw materials.

[0011] Furthermore, the lotus root carbon powder is a biochar micro powder obtained by washing, drying, and grinding natural lotus root, carbonizing it at a high temperature of 800℃~1000℃ under an inert atmosphere, and then grinding and sieving it. The particle size of the lotus root carbon powder is no greater than 100μm.

[0012] Furthermore, the materials of the first electrode layer and the second electrode layer are each independently selected from at least one of nickel sheet, copper foil or aluminum foil, and the thickness of each is independently 10μm to 50μm.

[0013] Furthermore, the mass ratio of lotus root carbon powder to thermoplastic polyurethane in the sensitive layer is (0.1~0.5):1, the thickness of the sensitive layer is 50μm~500μm, and the fiber diameter is 100nm~2000nm.

[0014] Furthermore, the first electrode layer and the sensitive layer, as well as the second electrode layer and the sensitive layer, are fixed and electrically connected by at least one of conductive adhesive bonding, thermoforming, or physical pressing.

[0015] Secondly, the present invention provides a method for preparing a flexible thin-film sensor based on lotus root carbon, comprising the following steps: (1) Fresh lotus root is washed, dried and ground, and then heated to 800℃~1000℃ under an inert protective atmosphere at a rate of 1℃ / min~5℃ / min for 1h~3h. After grinding and sieving, lotus root carbon biochar conductive filler is obtained. (2) The lotus root carbon powder and thermoplastic polyurethane are dissolved in an organic solvent at a mass ratio of (0.1 to 0.5):1 to prepare a spinning solution. Electrospinning is carried out under the conditions of applying a voltage of 10kV to 18kV and receiving distance of 10cm to 20cm. The nanofiber composite film is collected and dried to obtain a lotus root carbon / thermoplastic polyurethane composite electrospinned sensitive film. (3) The surface of the metal foil electrode layer is leveled so that the surface roughness Ra is not greater than 1.0 μm; (4) The sensitive film is placed between the first electrode layer and the second electrode layer after the leveling treatment, and fixed into a sandwich stacked configuration to obtain the lotus root carbon flexible film sensor.

[0016] Furthermore, in step (2), the mass concentration of thermoplastic polyurethane in the spinning solution is 5% to 20%, the electrospinning propulsion rate is 0.5 mL / h to 3 mL / h, the roller speed is 50 rpm to 150 rpm, and the spinning time is 1 h to 4 h.

[0017] Furthermore, in step (1), the inert protective atmosphere is nitrogen, argon, or a mixture of both; in step (2), the organic solvent is selected from at least one of N,N-dimethylformamide, N,N-dimethylacetamide, tetrahydrofuran, or a mixture thereof.

[0018] Thirdly, the present invention provides a wearable electronic device, including the lotus root carbon flexible thin film sensor as described above, wherein the wearable electronic device is used to detect human motion signals, and the human motion signals include at least one of finger bending and straightening movements, changes in human standing and bending postures, and swallowing behavior.

[0019] Fourthly, the present invention provides an application of the lotus root carbon flexible thin film sensor as described above in human motion detection, wherein the sensor is attached to a human joint, torso, or throat, and human motion signals are identified by monitoring changes in sensor resistance. The human motion signals include at least one of finger bending and straightening movements, changes in human standing and bending postures, and swallowing behavior.

[0020] The advantages and positive effects of this invention are as follows: (1) The raw materials are widely available, low cost, and environmentally friendly: This invention uses natural lotus root as raw material to prepare biochar conductive filler. Lotus root is widely available and inexpensive, and lotus root powder is an edible natural biomass. The carbonization products have excellent biocompatibility and can effectively replace high-cost artificially synthesized carbon materials such as carbon nanotubes and graphene.

[0021] (2) Good biocompatibility and suitable for skin-friendly scenarios: Compared with artificially synthesized carbon materials, the carbonized products of natural lotus root powder have milder surface chemical properties, no heavy metal catalyst residues, and are highly safe for contact with human skin.

[0022] (3) The microstructure of the sensitive film is uniform and the film quality is high: The present invention uses electrospinning process to prepare nanofiber composite film. The fibers are randomly stacked to form a three-dimensional porous network structure. The lotus root carbon filler is effectively coated and dispersed in the interior of the single fiber and in the fiber gaps, avoiding the filler agglomeration and sedimentation problems common in traditional solution casting or scraping processes.

[0023] (4) Sandwich-style assembly structure protects the performance of the sensitive layer: The sandwich configuration, in which the electrode layer and the sensitive film layer are processed independently and then bonded together, fundamentally avoids damage to the sensitive layer caused by high temperature and chemical corrosion during the electrode preparation process, ensuring the integrity of the fiber pore structure and filler dispersion state inside the spun film; at the same time, the pre-leveling treatment of the electrode surface eliminates problems such as uneven fiber spreading, large film thickness difference, and many film defects caused by directly forming films on irregular substrates, significantly improving the device sensitivity, signal stability and product yield.

[0024] (5) The preparation process is simple, the parameters are controllable, and it is easy to scale up: The carbonization process involved in this invention does not require complex multi-stage chemical pretreatment, but only conventional processes such as cleaning, drying, grinding, carbonization, and sieving; the electrospinning process parameters are clear and controllable, and it is suitable for continuous production.

[0025] (6) Excellent overall device performance: The lotus root carbon flexible thin film sensor prepared by this invention has a sensitivity of 0.0582 kPa in the low pressure range of 0-15 kPa. -1 (R) 2 =0.9919), with a sensitivity of 0.0059 kPa in the 15–30 kPa high-pressure region. -1 The response time is approximately 2.3ms and the recovery time is approximately 0.6ms, combining good sensitivity, wide response range, fast response / recovery characteristics, and excellent cycling stability and repeatability.

[0026] (7) Excellent human motion signal detection capability: The sensor prepared by the present invention can sensitively identify motion signals of different amplitudes, such as finger bending and straightening, changes in human posture and swallowing behavior. The response is clear and distinguishable and has good repeatability. It has broad application prospects in wearable health monitoring, intelligent human-computer interaction and other fields. Attached Figure Description

[0027] Figure 1 This is a schematic diagram of the structure of the lotus root carbon flexible thin film sensor described in this invention (sandwich stacked configuration). Figure 2This is a process flow diagram for preparing the lotus root carbon biochar conductive filler described in this invention; Figure 3 This is a process flow diagram for preparing the electrospun composite sensitive film according to the present invention; Figure 4 The image shows the morphology of the lotus root biochar filler prepared in Example 1 using a scanning electron microscope (SEM). Figure 5 The image shows the surface SEM image of the lotus root carbon / TPU composite electrospun sensitive film prepared in Example 1. Figure 6 The curves showing the relationship between the relative resistance change rate and pressure of the lotus root carbon flexible thin film sensor prepared in Example 1 under different pressures, and the piecewise sensitivity fitting diagram. Figure 7 The response time and recovery time curves of the lotus root carbon flexible thin film sensor prepared in Example 1; Figure 8 The resistance response stability curve of the lotus root carbon flexible thin film sensor prepared in Example 1 during cyclic loading-unloading test; Figure 9 The lotus root carbon flexible thin film sensor described in this invention is attached to the joint of a human finger to detect the relative resistance change response curve when the finger bends and straightens. Figure 10 The lotus root carbon flexible thin film sensor of this invention is attached to the waist of a human body to detect the relative resistance change response curve when the human body changes posture between standing and bending over; and Figure 11 The image shows the response curve of the relative resistance change when the lotus root carbon flexible film sensor described in this invention is attached to the human throat to detect swallowing behavior. Detailed Implementation

[0028] In a first aspect, the present invention provides a method for preparing a flexible thin-film sensor based on lotus root carbon, comprising a first electrode layer, a lotus root carbon / thermoplastic polyurethane composite electrospun sensitive layer and a second electrode layer arranged sequentially from bottom to top, forming a sandwich structure. Both the first electrode layer and the second electrode layer are metal foil electrode layers that have undergone surface leveling treatment. The sensitive layer is a nanofiber porous film prepared by electrospinning using lotus root carbon powder and thermoplastic polyurethane as raw materials. The lotus root carbon powder is obtained by washing, drying, and grinding natural lotus roots, then carbonizing them at 800℃~1000℃ under an inert atmosphere, followed by grinding and sieving to obtain biochar micropowder. The particle size of the lotus root carbon powder is no greater than 100μm. The first and second electrode layers are each independently selected from at least one of nickel sheet, copper foil, or aluminum foil, and their thicknesses are each independently 10μm~50μm. The mass ratio of lotus root carbon powder to thermoplastic polyurethane in the sensitive layer is (0.1~0.5):1, the thickness of the sensitive layer is 50μm~500μm, and the fiber diameter is 100nm~2000nm. Interlayer fixation and electrical contact are achieved between the first electrode layer and the sensitive layer, and between the second electrode layer and the sensitive layer, through at least one of conductive adhesive bonding, thermoforming, or physical pressing.

[0029] Specifically, this invention uses natural lotus root as a carbon source to prepare biochar conductive filler, replacing traditional artificially synthesized carbon materials, and constructs a sandwich structure sensor through a technical route of "electrode leveling treatment + independent film formation + separate bonding assembly". The sensitive layer and electrode layer are prepared independently and then bonded together, fundamentally avoiding thermal and chemical damage to the fiber structure of the sensitive layer caused by the electrode preparation process; at the same time, after the electrode surface is leveled, the problem of uneven fiber spreading and film defects caused by directly forming a film on a rough electrode surface is avoided, ensuring the integrity of the microstructure of the sensitive layer.

[0030] In this invention, the materials of the first electrode layer and the second electrode layer are each independently selected from at least one of nickel foil, copper foil, or aluminum foil, and their thicknesses are each independently 10 μm to 50 μm. Preferably, the thicknesses of the first electrode layer and the second electrode layer are each independently 15 μm to 25 μm. The metal foil electrode has good conductivity and flexibility, and its surface flatness is significantly improved after leveling treatment, allowing it to form good interfacial electrical contact with the sensitive layer.

[0031] In this invention, the lotus root carbon powder is obtained by washing, drying, and grinding natural lotus root, followed by high-temperature carbonization under an inert atmosphere, and then grinding and sieving to obtain biochar micropowder; the particle size of the lotus root carbon powder is no greater than 100 μm. Preferably, the lotus root carbon powder is obtained by passing it through a 100-400 mesh sieve. Lotus root, as a natural biomass raw material, is widely available and inexpensive. Its carbonization products have graphite-like electrical conductivity, no heavy metal catalyst residues, and excellent biocompatibility.

[0032] In this invention, the mass ratio of lotus root carbon powder to thermoplastic polyurethane in the sensitive layer is (0.1-0.5):1. Preferably, the mass ratio is (0.2-0.35):1. When the lotus root carbon content is too low, the conductive network is incomplete, resulting in insufficient sensitivity; when the content is too high, the filler agglomeration tendency increases, the spinnability of the spinning solution decreases, and the mechanical properties of the film deteriorate. The above preferred range can maintain excellent spinning processing performance while ensuring a good conductive network.

[0033] In this invention, the thickness of the sensitive layer is 50 μm to 500 μm, and the fiber diameter is 100 nm to 2000 nm. Preferably, the thickness of the sensitive layer is 100 μm to 300 μm, and the fiber diameter is 300 nm to 1000 nm. The three-dimensional porous network structure of the electrospun fibers not only provides abundant conductive pathways but also endows the sensitive layer with excellent compression recovery properties and air permeability.

[0034] In this invention, the first electrode layer and the sensitive layer, as well as the second electrode layer and the sensitive layer, are fixed and electrically connected by at least one of conductive adhesive bonding, hot pressing, or physical pressing.

[0035] Secondly, the present invention provides a method for preparing a flexible thin-film sensor based on lotus root carbon, comprising the following steps: (1) Fresh lotus root is washed, dried and ground, and then heated to 800℃~1000℃ under an inert protective atmosphere at a rate of 1℃ / min~5℃ / min for 1h~3h. After grinding and sieving, lotus root carbon biochar conductive filler is obtained. (2) The lotus root carbon powder and thermoplastic polyurethane are dissolved in an organic solvent at a mass ratio of (0.1 to 0.5):1 to prepare a spinning solution. Electrospinning is carried out under the conditions of applying a voltage of 10kV to 18kV and receiving distance of 10cm to 20cm. The nanofiber composite film is collected and dried to obtain a lotus root carbon / thermoplastic polyurethane composite electrospinned sensitive film. (3) The surface of the metal foil electrode layer is leveled so that the surface roughness Ra is not greater than 1.0 μm; (4) The sensitive film is placed between the first electrode layer and the second electrode layer after the leveling treatment, and fixed into a sandwich stacked configuration to obtain the lotus root carbon flexible film sensor.

[0036] Furthermore, in step (2), the mass concentration of thermoplastic polyurethane in the spinning solution is 5% to 20%, the electrospinning propulsion rate is 0.5 mL / h to 3 mL / h, the roller speed is 50 rpm to 150 rpm, and the spinning time is 1 h to 4 h.

[0037] Furthermore, in step (1), the inert protective atmosphere is nitrogen, argon, or a mixture of both; in step (2), the organic solvent is selected from at least one of N,N-dimethylformamide, N,N-dimethylacetamide, tetrahydrofuran, or a mixture thereof.

[0038] Specifically, the method for preparing the lotus root carbon flexible thin film sensor of the present invention includes: Phase 1: Preparation of Lotus Root Carbon Biochar Conductive Filler (1) Raw material pretreatment: Fresh lotus root is washed with deionized water to remove surface mud and impurities, chopped into small pieces, and placed in a drying oven to dry at 60℃~80℃ for 10h~15h to remove free moisture; the dried lotus root pieces are coarsely ground to obtain lotus root powder.

[0039] (2) High-temperature carbonization: The coarse lotus root powder obtained in step (1) is placed in a tube furnace and carbonized under an inert protective atmosphere; the inert protective atmosphere is nitrogen, argon or a mixture of both; the heating regime is: heating from room temperature to 800℃ to 1000℃ at a heating rate of 1℃ / min to 5℃ / min, maintaining the temperature at the target temperature for 1h to 3h, and then naturally cooling to room temperature with the furnace.

[0040] (3) Grinding and sieving: After grinding the carbonized product in step (2) by ball mill or mortar, pass it through a 100-400 mesh sieve and collect the powder that passes through the sieve to obtain lotus root carbon biochar conductive filler.

[0041] Phase Two: Preparation of Electrospinning Composite Sensitive Films (4) Preparation of spinning solution: Weigh lotus root carbon powder and thermoplastic polyurethane particles according to a mass ratio of (0.1 to 0.5): 1; first, add lotus root carbon powder to an organic solvent and stir magnetically at room temperature for 2 to 4 hours until fully dispersed to obtain a dispersion; then add thermoplastic polyurethane particles to the dispersion and continue stirring at 50°C to 70°C for 8 to 16 hours to obtain a uniform and stable spinning solution; wherein, the organic solvent is selected from at least one of N,N-dimethylformamide, N,N-dimethylacetamide, tetrahydrofuran or a mixture thereof, and the mass concentration of thermoplastic polyurethane in the spinning solution is 5% to 20%.

[0042] (5) Electrospinning: The spinning solution obtained in step (4) is injected into the syringe of the electrospinning device. A 15G to 22G needle is selected, and electrospinning is carried out under the following process conditions: the applied voltage is 10kV to 18kV, the distance from the needle to the receiving roller is 10cm to 20cm, the roller speed is 50rpm to 150rpm, the spinning solution propulsion rate is 0.5mL / h to 3mL / h, the ambient temperature is 20℃ to 35℃, the ambient relative humidity is 30% to 60%, and the spinning time is 1h to 4h. The nanofiber composite film is collected on the receiving roller.

[0043] (6) Post-processing: After spinning is completed, the nanofiber composite film collected in step (5) is removed from the receiving roller and placed in a vacuum drying oven. It is dried at 50℃~70℃ for 1h~4h to remove residual solvent and obtain lotus root carbon / thermoplastic polyurethane composite electrospun sensitive film.

[0044] Phase 3: Sensor Device Assembly (7) Electrode pretreatment: Select metal foil as the electrode layer substrate, and perform at least one of the following leveling treatments on its surface: grinding, polishing or chemical cleaning, to remove the surface oxide layer and uneven defects, and obtain a flat electrode layer with a surface roughness Ra of no more than 1.0 μm.

[0045] (8) Device bonding: The lotus root carbon / thermoplastic polyurethane composite electrospun sensitive film obtained in step (6) is cut into the required size and placed between the first electrode layer and the second electrode layer. The three-layer structure is fixed into a sandwich stacked configuration by at least one of conductive adhesive bonding, hot pressing bonding or physical pressing to obtain the lotus root carbon flexible film sensor.

[0046] Thirdly, the present invention provides a wearable electronic device, including the lotus root carbon flexible thin film sensor as described above, wherein the wearable electronic device is used to detect human motion signals, and the human motion signals include at least one of finger bending and straightening movements, changes in human standing and bending postures, and swallowing behavior.

[0047] Fourthly, the present invention provides an application of the lotus root carbon flexible thin film sensor as described above in human motion detection, wherein the sensor is attached to a human joint, torso, or throat, and human motion signals are identified by monitoring changes in sensor resistance. The human motion signals include at least one of finger bending and straightening movements, changes in human standing and bending postures, and swallowing behavior.

[0048] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0049] Example 1 This embodiment provides a lotus root carbon flexible thin film sensor and its preparation method, the specific steps of which are as follows: Phase 1: Preparation of Lotus Root Carbon Biochar Conductive Filler (1) Raw material pretreatment: Fresh lotus root is washed with deionized water to remove surface mud and impurities, chopped into small pieces of about 1cm×1cm, and placed in a forced-air drying oven to dry at 70℃ for 12h. The dried lotus root pieces are coarsely ground in a mortar and pestle to obtain coarse lotus root powder.

[0050] (2) High-temperature carbonization: The coarse lotus root powder was placed in an alumina crucible and then placed in a tube furnace for carbonization under a protective atmosphere of high-purity nitrogen (N2, purity ≥99.99%). The heating regime was as follows: the temperature was uniformly increased from room temperature (about 25°C) to 900°C over 300 minutes at an average rate of about 2.93°C / min, and then held at 900°C for 2 hours. After that, the heating power was turned off and the furnace was allowed to cool naturally to room temperature.

[0051] (3) Grinding and sieving: The carbonized product is ground in a mortar for 30 minutes and passed through a 200-mesh (approximately 74 μm) standard sieve. The powder passing through the sieve is collected to obtain lotus root carbon biochar conductive filler, which is then sealed and stored for later use.

[0052] Phase Two: Preparation of Electrospinning Composite Sensitive Films (4) Preparation of spinning solution: Weigh 0.3g of lotus root carbon powder obtained in step (3) and add it to 10mL of N,N-dimethylformamide (DMF). Stir magnetically at 600rpm for 3h at room temperature (25℃) to fully wet and pre-disperse the lotus root carbon in DMF. Then add 1g of thermoplastic polyurethane (TPU) particles to the above dispersion, heat to 60℃, and continue to stir magnetically at 400rpm for 12h to obtain a black, uniform and stable spinning solution. The mass ratio of lotus root carbon to TPU in the spinning solution is 0.3:1, and the mass concentration of TPU is approximately 9.1wt%.

[0053] (5) Electrospinning: The above spinning solution was drawn into a 10mL plastic syringe, and a No. 21 stainless steel needle (with an inner diameter of about 0.52mm) was installed. The syringe was fixed to the feed pump of the electrospinning machine, and the process parameters were adjusted as follows: a DC high voltage of 12kV was applied, the vertical distance from the needle tip to the grounded receiving roller (covered with aluminum foil) was 15cm, the roller speed was set to 80rpm, the spinning solution feed rate was 1.0mL / h, the ambient temperature was controlled at 25℃±2℃, and the ambient relative humidity was controlled at 45%±5%. Under the above conditions, continuous spinning was carried out for 2h, and a layer of grayish-white nanofiber composite film was collected on the surface of the receiving roller.

[0054] (6) Post-processing: After spinning, the film is gently peeled off from the receiving roller and laid flat in a clean glass petri dish. It is then placed in a vacuum drying oven and vacuum dried at 60°C for 2 hours to remove residual DMF solvent, thus obtaining a lotus root carbon / TPU composite electrospun sensitive film. The film has a thickness of approximately 150 μm and an average fiber diameter of approximately 600 nm, exhibiting a uniform three-dimensional porous network structure.

[0055] Phase 3: Sensor Device Assembly (7) Electrode pretreatment: Select a pure nickel sheet (Ni≥99.5%) with a thickness of 20μm as the electrode layer. Use 800-2000 grit sandpaper to polish the surface of the nickel sheet in sequence to remove the surface oxide layer and mechanical scratches. Then, use anhydrous ethanol for ultrasonic cleaning for 10 min and blow dry with nitrogen to obtain a bright and smooth nickel sheet electrode layer.

[0056] (8) Device bonding: The lotus root carbon / TPU composite electrospun sensitive film obtained in step (6) is cut into a circular material with a diameter of 1 cm and placed between two nickel sheet electrodes pretreated in step (7) to align and bond the film with the electrode layer. Conductive silver paste is applied to the contact edge between the film and the electrode for fixation, and a uniform pressure of about 0.1 MPa is applied and maintained for 30 min to firmly bond the three-layer structure, thus obtaining the lotus root carbon flexible film sensor (sandwich stacked configuration) described in this embodiment.

[0057] Performance testing: The sensor prepared in this embodiment was connected to a digital source meter for piezoresistive sensing performance testing.

[0058] Figure 4 The image shows the SEM morphology of lotus root carbon biochar filler. It can be seen that the carbonization product is in the form of irregular particles with a rich microporous structure on the surface. The particle size is distributed in the range of submicron to several micron, which is conducive to forming a uniform dispersion in the spinning solution.

[0059] Figure 5The image shows a surface SEM image of a lotus root carbon / TPU composite electrospun sensitive film. It can be seen that the fibers exhibit a typical random orientation and stacking morphology, with a relatively uniform fiber diameter distribution. The lotus root carbon particles are effectively coated inside the TPU fibers and embedded on the fiber surface, forming a continuous conductive network.

[0060] Figure 6 The curves showing the relationship between the relative resistance change rate of the sensor and pressure under different pressures are presented. The results indicate that the sensor has a sensitivity of 0.0582 kPa in the low-pressure range of 0–15 kPa. -1 (R) 2 =0.9919), with a sensitivity of 0.0059 kPa in the 15–30 kPa high-pressure region. -1 (R) 2 =0.7425), exhibiting good piecewise linear response characteristics.

[0061] Figure 7 The figures show the response and recovery time curves of the sensor. Under step pressure, the sensor's response time is approximately 2.3 ms and its recovery time is approximately 0.6 ms, demonstrating rapid signal response and recovery capabilities.

[0062] Figure 8 The resistance response curve of the sensor after 200 cyclic loading-unloading tests at 10 kPa pressure is shown. Throughout the cyclic test, the peak resistance change rate of the sensor remained stable with no significant signal drift, indicating that the sensor has excellent cyclic stability and repeatability.

[0063] Example 2 This embodiment provides a lotus root carbon flexible thin film sensor and its preparation method. The difference from Embodiment 1 lies in the different mass ratio of lotus root carbon to TPU, as detailed below: In step (4) of stage two, 0.15g of lotus root carbon powder was weighed and dispersed in 10mL of DMF, followed by the addition of 1g of TPU granules to prepare a spinning solution with a lotus root carbon to TPU mass ratio of 0.15:1. The remaining steps and parameters were the same as in Example 1.

[0064] The obtained sensor has a uniform fiber diameter in the sensitive layer, and the lotus root carbon filler is well dispersed within the fibers. Piezoresistive performance tests show that the sensor has a sensitivity of approximately 0.035 kPa in the low-pressure range of 0–15 kPa. -1 Compared to Example 1, the sensitivity is reduced, but it still has good linear response characteristics.

[0065] Example 3 This embodiment provides a lotus root carbon flexible thin film sensor and its preparation method. The difference between this embodiment and Embodiment 1 lies in the electrode layer material and thickness, as detailed below: In step (7) of stage three, a copper foil with a thickness of 25 μm is selected as the electrode layer. After being polished with sandpaper, it is chemically cleaned (in sequence with dilute hydrochloric acid, deionized water, and anhydrous ethanol), and dried with nitrogen gas to obtain a copper foil electrode layer with a smooth surface. The remaining steps and parameters are the same as in Example 1.

[0066] The obtained sensor was tested for piezoresistive performance, and its sensitivity, response time and cycle stability were basically the same as those of Example 1, indicating that copper foil as an electrode layer can also achieve good device performance.

[0067] Example 4 This embodiment provides a lotus root carbon flexible thin film sensor and its preparation method. The difference between this embodiment and Embodiment 1 lies in the electrospinning parameters, as detailed below: In step (5) of stage two, a DC high voltage of 15kV is applied, the receiving distance is 12cm, the drum speed is 100rpm, the spinning solution propulsion rate is 1.5mL / h, and the spinning time is 1.5h. The remaining steps and parameters are the same as in Example 1.

[0068] The resulting sensitive film has a thickness of approximately 120 μm and an average fiber diameter of approximately 400 nm, with a more dense fiber arrangement. The piezoresistive performance test results are similar to those of Example 1, further verifying the repeatability and process adaptability of the technical solution of this invention.

[0069] Application Example 1: Sensors are used to detect finger bending and straightening movements. The lotus root carbon flexible film sensor prepared in Example 1 was tightly attached to the outer surface of the volunteer's index finger joint using medical breathable tape to ensure good contact between the sensor and the skin. The two electrode leads of the sensor were connected to a digital source meter through thin wires. The resistance signal was acquired using a constant current mode with a sampling interval of 100ms. The real-time resistance data was recorded and converted into a relative resistance change rate ΔR / R0 (where R0 is the initial reference resistance value and R is the real-time measured resistance value).

[0070] Test Procedure: Volunteers held their fingers straight for about 3 seconds in a relaxed state, then slowly bent their finger joints to the maximum bending angle and held for about 2 seconds, then returned to the straight position and held for about 5 seconds. This "straighten → bend → straighten" action was repeated for a total of 3 cycles. Throughout the test, a digital source meter continuously recorded the resistance change signal.

[0071] Figure 9The sensor, attached to the finger joint, detects the relative resistance change during finger flexion and extension movements. When the finger is extended, the sensor's relative resistance change rate ΔR / R0 remains at a baseline level of approximately 0.05, with minimal baseline fluctuation (approximately ±0.02), indicating good signal stability in a static state. When the finger flexes, the sensor experiences tensile deformation due to joint bending, causing deformation of the conductive path and a rapid increase in the relative resistance change rate to approximately 1.0 (i.e., an increase in resistance of approximately 100% from the initial value). Upon finger extension, ΔR / R0 quickly returns to near the baseline. Throughout the test, three distinct flexion-extension cycles were observed, with peak amplitudes in different cycles highly consistent with the baseline level, demonstrating the sensor's sensitive, stable, and repeatable response to finger flexion and extension movements.

[0072] The results indicate that the lotus root carbon flexible thin film sensor prepared in this invention can be used for finger joint motion monitoring and has good application potential in scenarios such as sign language recognition, rehabilitation training assessment, and human-computer gesture interaction.

[0073] Application Example 2: Sensors are used to detect changes in human posture when standing up and bending over. The lotus root carbon flexible film sensor prepared in Example 1 was tightly attached to the midline of the volunteer's lumbar spine (approximately L3-L4 lumbar vertebrae) using medical breathable tape to ensure that the sensor could fully detect the deformation of the skin surface during bending and straightening. The sensor lead was connected to a digital source meter via a thin wire, and the resistance signal was acquired using a constant current mode with a sampling interval of 100 ms.

[0074] Test procedure: Volunteers first maintained a natural standing posture for about 15 seconds, then bent forward to the maximum comfortable bending angle of their waist and held for about 8 seconds, then returned to an upright standing posture and held for about 12 seconds, completing one posture cycle of "standing → bending → standing". This posture cycle was repeated twice, and the sensor resistance response was recorded throughout.

[0075] Figure 10 The sensor's response curves, when attached to the human waist, detect relative resistance changes during standing and bending postures. When the volunteer is standing upright, the sensor's relative resistance change rate ΔR / R0 remains at a low baseline level (approximately 0.02–0.03). When the volunteer bends forward, the sensor experiences compressive strain due to the stretching of the waist skin, causing ΔR / R0 to rise rapidly to approximately 0.63–0.65 (i.e., a resistance increase of approximately 63%–65%), and maintains a relatively stable high output during the bending period. When the volunteer returns to an upright position, ΔR / R0 falls back to the baseline level. The response waveforms of the two "standing-bending-standing" cycles are highly consistent, with good peak reproducibility.

[0076] The results show that the lotus root carbon flexible thin film sensor prepared in this invention has a sensitive and reliable response capability to large-scale changes in human torso posture, and can be used in wearable health monitoring scenarios such as sitting posture monitoring, lumbar mobility assessment, posture correction training, and fall warning for the elderly.

[0077] Application Example 3: Sensors used to detect swallowing behavior The lotus root carbon flexible film sensor prepared in Example 1 was tightly attached to the area below the thyroid cartilage (Adam's apple) at the front of the volunteer's larynx using medical breathable tape, ensuring that the sensor covered the main deformation area of ​​the laryngeal muscles. The sensor lead was connected to a digital source meter via a thin wire, and the resistance signal was acquired using a constant current mode with a sampling interval of 50 ms (to increase the sampling frequency to capture rapid dynamic changes during swallowing).

[0078] Test procedure: Volunteers maintained resting respiration for approximately 1 second in a quiet state, followed by a complete swallowing action (swallowing approximately 5 mL of room temperature water), and were then observed while remaining at rest for approximately 4 seconds. The swallowing action was repeated a total of 3 times, and the sensor resistance response was recorded throughout.

[0079] Figure 11 The sensor is attached to the human larynx to detect the relative resistance change response during swallowing. The resistance response of swallowing exhibits typical multi-peak waveform characteristics. During each swallowing action, the sensor's relative resistance change rate ΔR / R0 shows a series of sharp negative pulse peaks, with peak values ​​reaching approximately -0.08 to -0.35 (relative to the resting baseline). A single swallowing process contains 2-3 distinct sub-peaks, corresponding to the phased contraction-relaxation movements of the laryngeal muscles during swallowing. After swallowing, ΔR / R0 gradually returns to the resting baseline level. The waveform patterns of the three swallowing events are highly consistent, and the peak characteristics of each swallow show good repeatability.

[0080] The results show that the lotus root carbon flexible thin film sensor prepared in this invention can capture the subtle and rapid physiological activity signal of swallowing, and has important potential application value in medical and health fields such as swallowing disorder screening, food safety monitoring, and speech production research.

[0081] In summary, the three application examples above demonstrate that the lotus root carbon flexible thin film sensor prepared in this invention exhibits excellent detection capabilities for different types and amplitude ranges of motion signals from the human body: for large-amplitude reciprocating bending movements of finger joints, the sensor can output a large-amplitude step-like resistance change signal; for posture changes of the human torso, the sensor can generate a significant resistance change response with stable and reliable performance, suitable for long-term posture monitoring; for weak and rapid physiological activity signals such as swallowing, the sensor can capture fine waveforms with typical multi-peak characteristics, exhibiting excellent micro-strain sensing capabilities. The advantages and positive effects of this invention are as follows: (1) The raw materials are widely available, low cost, and environmentally friendly: This invention uses natural lotus root as raw material to prepare biochar conductive filler. Lotus root is widely available and inexpensive, and lotus root powder is an edible natural biomass. The carbonization products have excellent biocompatibility and can effectively replace high-cost artificially synthesized carbon materials such as carbon nanotubes and graphene.

[0082] (2) Good biocompatibility and suitable for skin-friendly scenarios: Compared with artificially synthesized carbon materials, the carbonized products of natural lotus root powder have milder surface chemical properties, no heavy metal catalyst residues, and are highly safe for contact with human skin.

[0083] (3) The microstructure of the sensitive film is uniform and the film quality is high: The present invention uses electrospinning process to prepare nanofiber composite film. The fibers are randomly stacked to form a three-dimensional porous network structure. The lotus root carbon filler is effectively coated and dispersed in the interior of the single fiber and in the fiber gaps, avoiding the filler agglomeration and sedimentation problems common in traditional solution casting or scraping processes.

[0084] (4) Sandwich-style assembly structure protects the performance of the sensitive layer: The sandwich configuration, in which the electrode layer and the sensitive film layer are processed independently and then bonded together, fundamentally avoids damage to the sensitive layer caused by high temperature and chemical corrosion during the electrode preparation process, ensuring the integrity of the fiber pore structure and filler dispersion state inside the spun film; at the same time, the pre-leveling treatment of the electrode surface eliminates problems such as uneven fiber spreading, large film thickness difference, and many film defects caused by directly forming films on irregular substrates, significantly improving the device sensitivity, signal stability and product yield.

[0085] (5) The preparation process is simple, the parameters are controllable, and it is easy to scale up: The carbonization process involved in this invention does not require complex multi-stage chemical pretreatment, but only conventional processes such as cleaning, drying, grinding, carbonization, and sieving; the electrospinning process parameters are clear and controllable, and it is suitable for continuous production.

[0086] (6) Excellent overall device performance: The lotus root carbon flexible thin film sensor prepared by this invention has a sensitivity of 0.0582 kPa in the low pressure range of 0-15 kPa. -1 (R) 2 =0.9919), with a sensitivity of 0.0059 kPa in the 15–30 kPa high-pressure region. -1 The response time is approximately 2.3ms and the recovery time is approximately 0.6ms, combining good sensitivity, wide response range, fast response / recovery characteristics, and excellent cycling stability and repeatability.

[0087] (7) Excellent human motion signal detection capability: The sensor prepared by the present invention can sensitively identify motion signals of different amplitudes, such as finger bending and straightening, changes in human posture and swallowing behavior. The response is clear and distinguishable and has good repeatability. It has broad application prospects in wearable health monitoring, intelligent human-computer interaction and other fields.

[0088] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention in any other way. Any person skilled in the art may make changes or equivalent variations to the disclosed technical content and apply them to other fields. However, any simple modifications, equivalent variations, and alterations made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the protection scope of the present invention. For those skilled in the art, the specific meaning of the above terms in the present invention can be understood through specific circumstances.

Claims

1. A flexible thin-film sensor based on lotus root carbon, characterized in that: It includes a first electrode layer, a lotus root carbon / thermoplastic polyurethane composite electrospun sensitive layer and a second electrode layer arranged sequentially from bottom to top, forming a sandwich structure; The first electrode layer and the second electrode layer are both metal foil electrode layers that have undergone surface leveling treatment; The sensitive layer is a nanofiber porous film prepared by electrospinning using lotus root carbon powder and thermoplastic polyurethane as raw materials.

2. The flexible thin-film sensor based on lotus root carbon according to claim 1, characterized in that: The lotus root carbon powder is a biochar micro powder obtained by washing, drying, and grinding natural lotus roots, carbonizing them at a high temperature of 800℃~1000℃ under an inert atmosphere, and then grinding and sieving them. The particle size of the lotus root carbon powder is no greater than 100μm.

3. The flexible thin-film sensor based on lotus root carbon according to claim 1, characterized in that: The materials of the first electrode layer and the second electrode layer are each independently selected from at least one of nickel sheet, copper foil or aluminum foil, and the thickness of each is independently 10μm to 50μm.

4. A flexible thin-film sensor based on lotus root carbon according to claim 1, characterized in that: The mass ratio of lotus root carbon powder to thermoplastic polyurethane in the sensitive layer is (0.1-0.5):1, the thickness of the sensitive layer is 50μm-500μm, and the fiber diameter is 100nm-2000nm.

5. A flexible thin-film sensor based on lotus root carbon according to claim 1, characterized in that: The first electrode layer and the sensitive layer, as well as the second electrode layer and the sensitive layer, are fixed and electrically connected by at least one of conductive adhesive bonding, hot pressing, or physical pressing.

6. A method for fabricating a flexible thin-film sensor based on lotus root carbon, characterized in that, Includes the following steps: (1) Fresh lotus root is washed, dried and ground, and then heated to 800℃~1000℃ under an inert protective atmosphere at a rate of 1℃ / min~5℃ / min for 1h~3h. After grinding and sieving, lotus root carbon biochar conductive filler is obtained. (2) The lotus root carbon powder and thermoplastic polyurethane are dissolved in an organic solvent at a mass ratio of (0.1 to 0.5):1 to prepare a spinning solution. Electrospinning is carried out under the conditions of applying a voltage of 10kV to 18kV and receiving distance of 10cm to 20cm. The nanofiber composite film is collected and dried to obtain a lotus root carbon / thermoplastic polyurethane composite electrospinned sensitive film. (3) The surface of the metal foil electrode layer is leveled so that the surface roughness Ra is not greater than 1.0 μm; (4) The sensitive film is placed between the first electrode layer and the second electrode layer after the leveling treatment, and fixed into a sandwich stacked configuration to obtain the lotus root carbon flexible film sensor.

7. The method for fabricating a flexible thin-film sensor based on lotus root carbon according to claim 6, characterized in that: In step (2), the mass concentration of thermoplastic polyurethane in the spinning solution is 5% to 20%, the electrospinning propulsion rate is 0.5 mL / h to 3 mL / h, the roller speed is 50 rpm to 150 rpm, and the spinning time is 1 h to 4 h.

8. The method for fabricating a flexible thin-film sensor based on lotus root carbon according to claim 6, characterized in that: In step (1), the inert protective atmosphere is nitrogen, argon, or a mixture of both; in step (2), the organic solvent is selected from at least one of N,N-dimethylformamide, N,N-dimethylacetamide, tetrahydrofuran, or a mixture thereof.

9. A wearable electronic device, characterized in that, The wearable electronic device includes the lotus root carbon flexible film sensor according to any one of claims 1 to 5, wherein the wearable electronic device is used to detect human motion signals, and the human motion signals include at least one of finger bending and straightening movements, changes in human standing and bending postures, and swallowing behavior.

10. The application of a lotus root carbon flexible thin film sensor as described in any one of claims 1 to 5 in human motion detection, characterized in that, The sensor is attached to a human joint, torso, or throat, and human motion signals are identified by monitoring changes in sensor resistance. The human motion signals include at least one of the following: finger bending and straightening movements, changes in human posture such as standing up and bending over, and swallowing behavior.

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