Capacitive flexible temperature sensor for body temperature monitoring

Abstract

A capacitive flexible temperature sensor for body temperature monitoring. The sensor is of a sandwich structure, and comprises a functional layer and two flexible encapsulation layers arranged on two sides of the functional layer, wherein the functional layer is composed of five composite material layers, which sequentially comprise: a flexible electrode layer, a flexible buffer layer, a flexible temperature-sensitive layer, a flexible buffer layer and a flexible electrode layer; and the flexible encapsulation layers are flexible high-thermal-conductivity waterproof insulating layers. The flexible temperature-sensitive layer is prepared by coating a flexible temperature-sensitive material on both sides of gauze and thermocuring same, and the flexible temperature-sensitive material comprises PVDF-HFP, low-molecular-weight PEO, high-molecular-weight PEO, and H3PO4; and the flexible buffer layer is a composite material thin film prepared by PVDF-HFP, TPU, ultra-high-molecular-weight PEO, and H3PO4. The flexible temperature sensor has characteristics of ultra-sensitivity, high response rate and high stability within a common body temperature range of a human body, and has good practicability.

Description

Capacitive flexible temperature sensor for body temperature monitoring Technical Field

[0001] This invention belongs to the field of body temperature monitoring technology, and specifically relates to a capacitive flexible temperature sensor for body temperature monitoring. Background Technology

[0002] Body temperature is one of the most important physiological parameters of the human body, directly affecting various physical and chemical reactions in life activities. It plays a crucial role in controlling homeostasis and ensuring the normal functioning of metabolism. Traditional body temperature measurement mainly uses rigid thermometers and infrared thermal imagers. Rigid thermometers, such as mercury thermometers and thermocouple thermometers, are easily damaged, uncomfortable, and difficult to use for long-term continuous body temperature monitoring. Infrared thermal imagers are expensive, bulky, and greatly affected by the environment. In recent years, flexible temperature sensors for human body temperature monitoring have attracted widespread attention from researchers due to their advantages of high accuracy, comfort, and the ability to perform long-term continuous measurements.

[0003] Patent application CN116164856A proposes a flexible temperature sensor with a simple fabrication process. Both the positive and negative thermocouple films are set on a PET or PI flexible insulating substrate. Indium tin oxide is used as the positive thermocouple film and indium oxide is used as the negative thermocouple film, which enables the sensor to have good linearity and low power consumption.

[0004] Patent CN111537098B proposes a flexible capacitive temperature sensor. The sensor comprises three layers: an in-line hexagonal boron nitride / PVDF thin film layer, an MXene / PVA thin film layer, and another hexagonal boron nitride / PVDF thin film layer. The highly conductive MXene layer in the middle layer has interfacial polarization with the highly insulating polymer matrix, which can provide a high dielectric constant and improve the sensitivity of the sensor. The hexagonal boron nitride in the upper and lower layers can suppress the charge input from the electrodes into the medium, giving the sensor a high breakdown strength and protecting the device in a strong electric field environment.

[0005] The flexible temperature sensor proposed in patent application CN116295893A has a planar layered structure, in which the central double-layer hydrogel temperature-sensitive film is composed of a polyvinyl alcohol hydrogel film and a sodium polystyrene sulfonate / polyvinyl alcohol hydrogel film. High-precision temperature sensing is achieved by controlling the thickness of the hydrogel film and the concentration of doped ions.

[0006] Patent CN111623899B designs a capacitive thin-film temperature sensor that uses a polyacrylamide / carrageenan dual-network hydrogel film as the temperature-sensitive material layer and is treated with lithium bromide salt solution to give the sensor good sensitivity, response speed and transparency.

[0007] Patent CN113108935B proposes a flexible temperature sensor based on PEDOT:PSS / rGO composite material. PEDOT:PSS can form a strong π-π interaction with rGO, promoting charge transport and potential accumulation, increasing the change in material resistance, and enabling the sensor to have high sensitivity and response speed.

[0008] Patent application CN116399476A discloses a flexible temperature sensor based on multi-component vanadium dioxide, comprising a PMMA substrate, metal electrodes, a multi-component VO2 thin film, and a mica substrate. By doping the VO2 thin film with W and Ti in a specific ratio, the phase transition temperature of VO2 is tuned to near human body temperature, reducing the thermal hysteresis of the film and improving the sensitivity and response speed of the temperature sensor when monitoring human body temperature. The PMMA substrate and mica substrate give the sensor advantages of being lightweight and having good bending temperature characteristics, allowing it to adhere closely to the skin for comfortable wear without affecting human activity.

[0009] Patent CN109764971B designs a flexible temperature sensor comprising a flexible substrate with electrodes, a mixed liquid made of ionic liquid and porous conductive black particulate material, and a protective plate. The porous conductive black particulate material has a significant multilayer hollow structure, which not only allows for full contact with the ionic liquid but also forms a blackbody cavity effect, efficiently absorbing heat and transferring it to the ionic liquid, thereby improving the sensor's detection accuracy and response speed.

[0010] Despite significant progress in temperature sensors used for body temperature monitoring, they still have the following shortcomings:

[0011] Existing temperature sensors are mainly based on thermoelectric sensing mechanisms, which have low sensitivity, poor flexibility, and rigid encapsulation shells, making it difficult to achieve good conformal effects on the body surface in the wearable field.

[0012] Existing flexible temperature sensors are mainly based on resistive sensing mechanisms, and their sensing performance, such as sensitivity, is limited by the inherent thermal resistance of the temperature-sensitive material. Although some studies have used phase change principles to improve the sensor's body temperature monitoring sensitivity, these phase change resistive temperature sensors suffer from poor stability, repeatability, and consistency, and are also subject to strong stress effects during use.

[0013] While existing flexible temperature sensors based on capacitive sensing mechanisms have advantages such as low power consumption and good stability, most capacitive temperature sensors have low sensitivity, especially in the field of body temperature monitoring, where it is difficult to achieve high-resolution human body temperature monitoring within a narrow range of human body temperature.

[0014] Furthermore, existing temperature sensor materials, including thermosensitive and electrode materials, contain metallic components. These materials exhibit poor adhesion to flexible polymer substrates and strong stress crosstalk, causing drift in the sensor's static performance indicators. Moreover, temperature sensors containing metallic materials cannot be used in conjunction with medical equipment such as X-ray machines, making it difficult to achieve continuous real-time monitoring of patient body temperature in such medical environments.

[0015] Summary of the Invention

[0016] To address the shortcomings of existing technologies, this invention proposes a capacitive flexible temperature sensor for body temperature monitoring. Combining the PEO phase transition process and the principle of double-layer capacitance, the sensor's temperature-sensitive range is controlled by the molecular weight of PEO, enabling it to achieve extremely high sensitivity and resolution within the human body temperature range. A mesh structure is introduced into the temperature-sensitive material to restrict the PEO crystallization region, stabilizing the PEO ion conductive network and improving the sensor's repeatability. A buffer layer is placed between the temperature-sensitive layer and the electrode layer, and the electrode layer is fabricated using an adhesive material, enhancing the sensor's resistance to environmental interference. This capacitive flexible temperature sensor for body temperature monitoring can be widely applied in the field of wearable medical electronics.

[0017] To achieve its objectives, the present invention employs the following technical solution:

[0018] This invention first discloses a capacitive flexible temperature sensor for body temperature monitoring, which has a sandwich structure, including a functional layer and two flexible encapsulation layers disposed on both sides of the functional layer. The functional layer is composed of five composite material layers, namely, a flexible electrode layer, a flexible buffer layer, a flexible temperature-sensitive layer, a flexible buffer layer, and a flexible electrode layer; the flexible encapsulation layer is a flexible, highly thermally conductive, waterproof, and insulating layer.

[0019] Furthermore, the flexible temperature-sensitive layer is obtained by coating flexible temperature-sensitive material onto both sides of gauze and then heat-curing it. After heat curing, the thickness of the flexible temperature-sensitive material on one side of the gauze is 10-30 μm. The tensile breaking rate of the flexible temperature-sensitive layer is not less than 60%, and the resistivity is 5-20 MΩ / m.

[0020] Furthermore, the flexible temperature-sensitive material comprises PVDF-HFP, low molecular weight PEO (molecular weight 1200-2000), high molecular weight PEO (molecular weight 5000-20000), and H3PO4. In the flexible temperature-sensitive material, the mass fraction of low molecular weight PEO is 25%-30%, the mass fraction of high molecular weight PEO is 20%-25%, the mass fraction of H3PO4 is 10%-15%, and the balance is PVDF-HFP. The crystallization / decrystallization phase transition temperature range of PEO with a molecular weight of 1200-2000 is approximately 30-55℃. The change in crystallinity during its phase transition alters the dissociation rate of the H3PO4 dispersed within it, and also affects the H3PO4 content.+ PO4 3- HPO4 2- Plasma mobility affects the electric double-layer capacitance. Blending low-molecular-weight PEO with high-molecular-weight PEO forms a eutectic, increasing the maximum crystallinity and phase transition rate of the PEO blend, thereby improving the sensor's sensitivity and response rate.

[0021] Furthermore, the gauze is made of polyester or polyamide fiber, with a tightly and regularly arranged mesh of 100-500 mesh size. Under the constraint of this regular mesh structure, the phase transition process of the thermosensitive material is confined to the mesh, ensuring the stability of the ion-conducting network of the thermosensitive material in its crystalline state, thereby improving the repeatability of the sensor. The mesh size has a certain impact on the sensor characteristics; a smaller mesh size will prevent the thermosensitive material from completely filling the pores, while a larger mesh size will cause the thermosensitive material to continue depositing on the gauze after filling the pores, both of which will reduce the repeatability of the sensor.

[0022] Furthermore, the flexible buffer layer is a composite film made of PVDF-HFP, TPU, ultra-high molecular weight PEO (molecular weight of 200,000-400,000), and H3PO4. In the composite film, the mass fraction of TPU is 25-35%, the mass fraction of ultra-high molecular weight PEO is 20%-30%, the mass fraction of H3PO4 is 10%-18%, and the balance is PVDF-HFP. The thickness of the flexible buffer layer is 10-30 μm, the tensile strength is 12-25 MPa, the tensile breaking rate is not less than 60%, and the ionic conductivity is 8 × 10⁻⁶ within the temperature range of 30-50℃. -3 S / cm~20×10 -3 S / cm. The extremely high molecular weight PEO within the flexible buffer layer does not undergo a phase transition below 80℃, maintaining extremely high ion mobility. Ions transported from the flexible temperature-sensitive layer can quickly reach the buffer layer / electrode layer contact interface to form an electric double-layer capacitance. The buffer layer prevents the PEO phase transition process at the electrode contact surface, maintaining tight contact and improving sensor stability. Adding TPU to the buffer layer enhances its mechanical properties, absorbing most of the stress under external forces, preventing direct application of external forces to the temperature-sensitive layer, and improving the sensor's resistance to external interference.

[0023] Furthermore, the flexible electrode layer is made of a stretchable, highly conductive composite material. This stretchable, highly conductive composite material comprises three parts: a conductive polymer material, an elastic polymer network material, and a supramolecular crosslinking material. The elastic polymer network material comprises 50%-60% by mass, the supramolecular crosslinking material comprises 25%-35% by mass, and the remainder is the conductive polymer material. The conductive polymer material is EG-modified PEDOT:PSS, with a mass ratio of EG to PEDOT:PSS of 1:4-8. The elastic polymer network material comprises PVA and GA, with a mass ratio of PVA to GA of 5:1-3. The supramolecular crosslinking material comprises β-cyclodextrin and citric acid, with a molar ratio of β-cyclodextrin to citric acid of 10:1. The flexible electrode layer has a thickness of 10-30 μm, a tensile breaking rate of not less than 60%, a tensile shear strength of 1.2-1.8 MPa, and an electrical conductivity of 3000-20000 S / cm. EG can reduce the electrostatic interaction between PSS and PEDOT, causing phase separation between PSS and PEDOT and improving the electrical conductivity of PEDOT:PSS. The hydroxyl groups on PVA and the aldehyde groups on GA can undergo polymerization to form a double crosslinked network, improving the mechanical properties of the composite material. β-cyclodextrin and citric acid provide abundant hydroxyl functional groups and charged groups within the composite material, enhancing its adhesion.

[0024] Furthermore, the flexible high thermal conductivity waterproof insulation layer is made of a stretchable high thermal conductivity composite material. This stretchable high thermal conductivity composite material is prepared by impregnation and casting processes, where PVDF-HFP is cast into a BN / TPU sponge, with a BN to TPU mass ratio of 1:5 to 7. The tensile breaking rate of the flexible high thermal conductivity waterproof insulation layer is not less than 60%, and its electrical conductivity is 1×10⁻⁶. -5 ~1×10 -4 The thermal conductivity is 2–5 W / (m·K). After PVDF-HFP is cast into the pores of TPU sponge, hydrogen bonds can form between TPU and PVDF-HFP, improving the mechanical properties and waterproofness of the TPU / PVDF-HFP sponge. BN has a long-range ordered lattice structure, allowing phonons to move orderly along the BN lattice during heat transfer. Its large mean free path improves the thermal conductivity of the composite material while ensuring its electrical insulation.

[0025] The method for preparing the capacitive flexible temperature sensor for body temperature monitoring includes the following steps:

[0026] Step 1: Preparation of flexible temperature-sensitive layer

[0027] PVDF-HFP, low molecular weight PEO, and high molecular weight PEO were added to a beaker in sequence, followed by DMF solution. The mixture was stirred at 60°C for 3 hours. H3PO4 was then added to the resulting mixed solution, and the mixture was stirred at 40°C for 1 hour to obtain a flexible thermosensitive material.

[0028] The flexible temperature-sensitive material was coated onto the surface of the gauze using a coating rod and dried at 75°C for 3 hours. The above process was repeated 3 times on both sides of the gauze to obtain a flexible temperature-sensitive layer with a gauze structure.

[0029] Step 2: Preparation of flexible buffer layer

[0030] PVDF-HFP, TPU, and ultra-high molecular weight PEO were added to a beaker in sequence, DMF solution was added, and the mixture was stirred at 60°C for 3 hours. Then H3PO4 was added to the resulting mixed solution, and the mixture was stirred at 40°C for 1 hour to obtain a flexible buffer layer composite material.

[0031] The flexible buffer layer composite material was heated at 75°C for 1 hour to make the composite material viscous. Then, it was coated onto the surface of the flexible temperature-sensitive layer with a coating rod. The material was then heated at 60°C for 2 hours to obtain a flexible buffer layer on the surface of the flexible temperature-sensitive layer.

[0032] Step 3: Fabrication of flexible electrode layer

[0033] Take a 3% solid content PEDOT:PSS aqueous solution, add EG and stir at 80°C for 2 hours; then add PVA, GA, β-cyclodextrin and citric acid, and continue stirring at 80°C for 1 hour to obtain a stretchable high conductivity composite material; apply the stretchable high conductivity composite material to the surface of a flexible buffer layer with a coating rod, and then dry at 70°C for half an hour to obtain a flexible electrode layer on the surface of the flexible buffer layer.

[0034] Step 4: Preparation of flexible, high thermal conductivity, waterproof insulation layer

[0035] BN powder was added to a mixed solution of isopropanol and deionized water in a volume ratio of 1:1. After ultrasonic treatment for 6 hours, the keratinized BN sheet was removed by filtration to obtain a BN solution.

[0036] Using a water-based dip coating process, TPU sponge is completely immersed in BN solution and squeezed. After the sponge no longer absorbs the solution, it is dried at 60°C for 1 hour to obtain BN / TPU sponge.

[0037] Take PVDF-HFP, heat it to 200℃ in a vacuum oven to melt it, then immerse BN / TPU sponge in the molten PVDF-HFP for 6 hours, and then dry it at room temperature for 2 hours to obtain a flexible, highly thermally conductive, and waterproof insulating layer; attach the flexible, highly thermally conductive, and waterproof insulating layer to the surface of the flexible electrode layer to complete the fabrication of the capacitive flexible temperature sensor.

[0038] Compared with existing technologies, the beneficial effects of this invention are reflected in:

[0039] 1. This invention is based on the PEO phase transition process and the principle of double-layer capacitance. It utilizes the changes in ion excitation rate and mobility during the PEO crystallization / decrystallization process to enable the flexible temperature sensor to have extremely high sensitivity and resolution.

[0040] 2. The present invention adjusts the phase transition temperature range of PEO in the flexible temperature-sensitive layer to the range of human body temperature (35-44℃) by controlling the molecular weight of PEO, so that the sensor has extremely high sensitivity in human body temperature monitoring.

[0041] 3. Based on the crystallization mechanism of high and low molecular weight blended polymers in the flexible temperature-sensitive layer, this invention blends high molecular weight PEO and low molecular weight PEO as a phase change temperature-sensitive material, thereby improving the maximum crystallinity and phase change rate of the temperature-sensitive material and enhancing the sensitivity and response speed of the sensor.

[0042] 4. The present invention sets a gauze structure with regular mesh in the flexible temperature-sensitive layer, so that the crystallization process of PEO is confined within the gauze mesh, so that the PEO ion conductive network remains stable during multiple phase transitions, thereby improving the repeatability of the sensor.

[0043] 5. The present invention designs a flexible buffer layer between the flexible temperature-sensitive layer and the flexible electrode layer. The extremely high molecular weight PEO in the buffer layer will not undergo a phase transition within the normal operating temperature range of the sensor, and will transport ions from the flexible temperature-sensitive layer to the electrode layer / buffer layer interface, ensuring a tight bond between the electrode layer and the buffer layer interface and improving the repeatability of the sensor.

[0044] 6. The present invention incorporates TPU in the flexible buffer layer to enhance the tensile strength and elongation of the flexible buffer layer. When the sensor is subjected to external force, it can absorb most of the stress, avoiding a large amount of stress directly acting on the flexible temperature-sensitive layer and affecting the phase change process and ion transport of the phase change temperature-sensitive material, thereby improving the sensor's resistance to external interference.

[0045] 7. In this invention, EG is used to improve the conductivity of PEDOT:PSS in the flexible electrode layer, and PVA and GA are used to form a double cross-linked network to improve the tensile strength and tensile shear modulus of PEDOT:PSS. This makes the flexible electrode layer have both good conductivity and mechanical properties, so that the sensor can be bent and attached to the human body surface to directly measure temperature.

[0046] 8. This invention incorporates supramolecular crosslinking materials into the flexible electrode layer, giving it extremely high viscosity. This ensures a tight bond between the electrode layer and the buffer layer, preventing changes in contact area from affecting the sensor signal under external force and enhancing the sensor's resistance to external interference. The high viscosity of the electrode layer also simplifies the preparation process of the encapsulation layer, allowing the encapsulation material to be directly applied to the electrode layer surface.

[0047] 9. This invention introduces BN as a thermally conductive material into the encapsulation layer, giving the encapsulation layer both good thermal conductivity and insulation properties, allowing external heat to quickly enter the sensor through the encapsulation layer and improve the sensor's response rate.

[0048] 10. In this invention, TPU is used as the matrix material in the encapsulation layer, and PVDF-HFP is introduced into the TPU sponge, so that the encapsulation layer has good tensile properties, as well as good waterproof performance and anti-aging ability.

[0049] 11. The materials used in the sensor designed in this invention are all safe and harmless to the human body and have good biocompatibility. Attached Figure Description

[0050] Figure 1 shows the overall structure of a capacitive flexible temperature sensor used for body temperature monitoring.

[0051] Figure 2 is a schematic diagram of the sensing mechanism of a capacitive flexible temperature sensor used for body temperature monitoring.

[0052] Figure 3 shows the DSC crystallization curves of PEO with different molecular weights.

[0053] Figure 4 shows a polarized light micrograph of the blended crystals of high and low molecular weight PEO.

[0054] Figure 5 shows the cyclic voltammetry of the flexible temperature-sensitive layer at different temperatures with a scan rate of 0.01 mV / s.

[0055] Figure 6 shows the cyclic voltammetry of the flexible thermosensitive layer at different scan rates at 30℃.

[0056] Figure 7 shows the cyclic voltammetry of the flexible thermosensitive layer at different scan rates at 50°C.

[0057] Figure 8 shows a micrograph of the confined crystallization of PEO in a mesh structure.

[0058] Figure 9 is a schematic diagram showing the effect of mesh size on the crystalline morphology of PEO.

[0059] Figure 10 is a schematic diagram of the conductivity principle of EG-modified PEDOT:PSS.

[0060] Figure 11 is a schematic diagram of the PVA and GA double cross-linking network principle.

[0061] Figure 12 is a schematic diagram of the BN lattice structure and heat conduction path.

[0062] Figure 13 shows the temperature-capacitance response curve of a capacitive flexible temperature sensor used for body temperature monitoring.

[0063] Figure 14 shows the capacitance response curve of a capacitive flexible temperature sensor used for body temperature monitoring under external force.

[0064] Figure 15 shows the response time curve of a capacitive flexible temperature sensor used for body temperature monitoring.

[0065] Figure 16 shows the repeatability curve of a capacitive flexible temperature sensor used for body temperature monitoring. Detailed Implementation

[0066] The embodiments of the present invention will be described in detail below with reference to the accompanying drawings. The following embodiments are implemented based on the technical solution of the present invention, and detailed implementation methods and specific operation processes are given. However, the protection scope of the present invention is not limited to the following embodiments.

[0067] The PVDF-HFP used in the following examples was purchased from Dongguan Zhanyang Polymer Materials Co., Ltd. (Type 21510).

[0068] The low molecular weight PEO, high molecular weight PEO, and very high molecular weight PEO used in the following examples were purchased from China National Pharmaceutical Group Chemical Reagent Co., Ltd. (AR, 98%).

[0069] The H3PO4 used in the following examples was purchased from Jiangbiao Testing Technology Co., Ltd. (AR, concentration >95%).

[0070] The TPU used in the following examples was purchased from Suzhou Yuanqiao Engineering Plastics Co., Ltd. (Type 685A).

[0071] The PEDOT:PSS aqueous solution used in the following examples was purchased from Shanghai Ouyi Organic Optoelectronic Materials Co., Ltd. (3% solid content).

[0072] The EG used in the following examples was purchased from China National Pharmaceutical Group Chemical Reagent Co., Ltd. (AR, 99%).

[0073] The PVA used in the following examples was purchased from Shanghai Maclean Biochemical Technology Co., Ltd. (Type 1797, degree of alcoholysis: 96.0-98.0% (mol / mol)).

[0074] The GA used in the following examples was purchased from China National Pharmaceutical Group Chemical Reagent Co., Ltd. (AR, 98%).

[0075] The β-cyclodextrin used in the following examples was purchased from Tianjin Huasheng Chemical Reagent Co., Ltd. (AR, 98%).

[0076] The citric acid used in the following examples was purchased from Tianjin Zhiyuan Chemical Reagent Co., Ltd. (AR, 99%).

[0077] The BN used in the following examples was purchased from Guangdong Baisi Chemical Reagent Co., Ltd. (AR, 99%).

[0078] The isopropanol used in the following examples was purchased from Wuxi Jinko Chemical Co., Ltd. (AR, 98%).

[0079] Example 1

[0080] As shown in Figure 1, the capacitive flexible temperature sensor for body temperature monitoring provided in this embodiment has a sandwich structure, including a functional layer and two flexible encapsulation layers disposed on both sides of the functional layer. The functional layer is composed of five composite materials, namely, a flexible electrode layer, a flexible buffer layer, a flexible temperature-sensitive layer, a flexible buffer layer, and a flexible electrode layer; the flexible encapsulation layer is a flexible high thermal conductivity waterproof insulation layer.

[0081] Figure 2 shows a schematic diagram of the sensing mechanism of a capacitive flexible temperature sensor used for body temperature monitoring. Based on the phase transition principle and the double-layer capacitance principle, when the temperature is low, the PEO blend in the flexible temperature-sensitive layer is in a crystalline state, resulting in extremely low ion dissociation rate and ion mobility within the temperature-sensitive layer. Only a small number of ions accumulate at the electrode layer / buffer layer interface through the buffer layer, resulting in extremely low sensor capacitance. When the temperature rises, the PEO blend undergoes decrystallization, leading to a decrease in crystallinity and an increase in the blend volume. This increases the ion dissociation rate and mobility within the temperature-sensitive layer, causing a large number of ions to accumulate at the electrode layer / buffer layer interface through the buffer layer, resulting in increased sensor capacitance.

[0082] The flexible temperature sensor in this embodiment is manufactured according to the following steps:

[0083] Step 1: Preparation of flexible temperature-sensitive layer

[0084] 0.4g of PVDF-HFP, 0.3g of low molecular weight PEO with a molecular weight of 2K (hereinafter referred to as PEO2K), and 0.2g of high molecular weight PEO with a molecular weight of 6K (hereinafter referred to as PEO6K) were added to a beaker in sequence, followed by 15ml of LDM solution. The mixture was stirred at 60°C for 3 hours. Then, 0.1g of H3PO4 was added to the mixed solution, and the mixture was stirred at 40°C for 1 hour to obtain a flexible temperature-sensitive material.

[0085] A flexible temperature-sensitive material is coated onto the surface of gauze using a coating rod and dried at 75°C for 3 hours. The above process is repeated 3 times on both sides of the gauze to obtain a flexible temperature-sensitive layer with a gauze structure.

[0086] Step 2: Preparation of flexible buffer layer

[0087] 0.25g of PVDF-HFP, 0.3g of TPU, and 0.3g of ultra-high molecular weight PEO (hereinafter referred to as PEO30W) with a molecular weight of 300,000 were added to a beaker in sequence, followed by 10ml of LDM solution. The mixture was stirred at 60°C for 3 hours. Then, 0.15g of H3PO4 was added to the mixed solution, and the mixture was stirred at 40°C for 1 hour to obtain a flexible buffer layer composite material.

[0088] The flexible buffer layer composite material was poured into a glass petri dish and heated at 75°C for 1 hour. When the composite material was in a relatively viscous state, it was coated onto the surface of the flexible temperature-sensitive layer with a coating rod. Then, it was heated at 60°C for another 2 hours to obtain a flexible buffer layer on the surface of the flexible temperature-sensitive layer.

[0089] Step 3: Fabrication of flexible electrode layer

[0090] Take 10g of a 3% PEDOT:PSS aqueous solution, add 0.05g of EG and stir at 80℃ for 2 hours; then add 2g of PVA, 0.8g of GA, 0.59g of β-cyclodextrin and 1g of citric acid to the mixed solution in sequence, and continue stirring at 80℃ for 1 hour to obtain a stretchable high-conductivity composite material. Apply the stretchable high-conductivity composite material to the surface of a flexible buffer layer using a coating rod, and then dry at 70℃ for half an hour to obtain a flexible electrode layer on the surface of the flexible buffer layer.

[0091] Step 4: Preparation of flexible, high thermal conductivity, waterproof insulation layer

[0092] Take 2g of BN powder and add it to a 1:1 mixture of isopropanol and deionized water. After sonicating for 6 hours, filter to remove the keratinous BN sheet to obtain the BN solution.

[0093] Using a water-based dip-coating process, TPU sponge is completely immersed in BN solution and squeezed several times to allow the solution to better penetrate the TPU sponge. After the sponge no longer absorbs the solution, it is dried at 60°C for 1 hour to obtain BN / TPU sponge.

[0094] 20g of PVDF-HFP was melted by heating it at 200℃ in a vacuum oven. BN / TPU sponge was then immersed in the molten PVDF-HFP for 6 hours, followed by drying at room temperature for 2 hours to obtain a flexible, highly thermally conductive, and waterproof insulating layer. This layer was then attached to the surface of a flexible electrode layer to complete the fabrication of a capacitive flexible temperature sensor for body temperature monitoring.

[0095] The flexible temperature-sensitive layer provided in this embodiment is prepared by coating a flexible temperature-sensitive material onto gauze and then heat-curing it. After heat curing, the thickness of the flexible temperature-sensitive material on one side of the gauze is 15 μm. The tensile breaking rate of the flexible temperature-sensitive layer is 80%, and the resistivity is 8 MΩ / m. The flexible temperature-sensitive material includes PVDF-HFP, PEO2K, PEO6K, and H3PO4.

[0096] Figure 3 shows the DSC crystallization curves of PEO with different molecular weights. As can be seen from the DSC crystallization curves, the molecular weight of PEO affects its phase transition range. Lower molecular weight PEO has a lower phase transition temperature, while higher molecular weight PEO has a higher phase transition temperature. The temperature-sensitive range of PEO2K includes the common human body temperature range (35-44℃), and the crystallinity changes significantly, which can improve the sensitivity of the flexible temperature sensor.

[0097] Figure 4 shows a polarized light micrograph of the high- and low-molecular-weight PEO blend crystallization. The micrograph reveals that PEO2K and PEO6K in the PEO blend form a eutectic, with the number of nucleation sites and the size of the spherulite radius falling between those of PEO2K and PEO6K. The PEO2K / 6K blend exhibits a higher initial nucleation number and a smaller growth radius, resulting in a faster crystallization rate and increased maximum crystallinity, thereby improving the response rate and sensitivity of the flexible temperature sensor.

[0098] Figure 5 shows the cyclic voltammetry of the flexible temperature-sensitive layer provided in this embodiment at different temperatures with a scan rate of 0.01 mV / s. As can be seen from the curves, the specific capacitance of the flexible temperature-sensitive layer increases continuously with increasing temperature. Due to the very low scan rate, ions have sufficient time to migrate to the electrode surface. The increase in specific capacitance depends only on the number of free ions in the flexible temperature-sensitive layer. The increase in specific capacitance is less in the two temperature ranges of 30-35℃ and 45-50℃, while the increase is more in the range of 35-45℃. This is because the PEO blend undergoes a decrystallization process in this temperature range, which greatly increases the ion dissociation rate and improves the number of free ions and the specific capacitance value in the flexible temperature-sensitive layer.

[0099] Figures 6 and 7 show the cyclic voltammetry of the flexible temperature-sensitive layer provided in this embodiment at different scan rates at 30°C and 50°C, respectively. As can be seen from the curves, at 30°C, the cyclic curve gradually becomes spindle-shaped as the scan rate increases. The saturation circuit is the same at different scan rates, indicating that the number of free ions reaching the electrode is the same at all three scan rates. The spindle-shaped curve indicates that more scan time is needed to reach the saturation current, suggesting a lower ion mobility, which cannot quickly move to the electrode surface at higher scan rates. At 50°C, increasing the scan rate has virtually no effect on the cyclic curve, indicating a higher ion mobility at this temperature, allowing for rapid accumulation at the electrode surface to form a saturation current even at higher scan rates.

[0100] The gauze material provided in this embodiment is polyamide fiber, which has a tightly and regularly arranged mesh with a square mesh shape and a mesh count of 300.

[0101] Figure 8 shows a micrograph of PEO confined crystallization in a mesh structure. In the unpolarized micrograph, regular square meshes composed of yarns can be seen. In the polarized micrograph, the crystallization region of PEO is confined within the mesh, which makes the PEO ion-conducting network stable during multiple phase transitions and improves the repeatability of the sensor.

[0102] Figure 9 illustrates the effect of mesh number on the crystalline morphology of PEO: When the mesh number is appropriate, crystalline PEO can just fill the mesh, forming effective crystalline domain constraint; when the mesh number is too small, the mesh area is too large, and PEO cannot fill the entire mesh; when the mesh number is too large, the mesh area is too small, and PEO will overflow the mesh and crystallize on the yarn surface. Therefore, both too small and too large a mesh number will reduce the effect of crystalline domain constraint.

[0103] The flexible buffer layer provided in this embodiment is a composite material film. The composite material film is prepared from PVDF-HFP, TPU, PEO30W, and H3PO4. The flexible buffer layer has a thickness of 20 μm, a tensile strength of 15 MPa, a tensile breaking rate of 80%, and an ionic conductivity higher than 10 × 10⁻⁶ at 30-50°C. -3 S / cm.

[0104] The flexible electrode layer provided in this embodiment is made of a stretchable, highly conductive composite material. The stretchable, highly conductive composite material comprises three parts: a conductive polymer material, an elastic polymer network material, and a supramolecular crosslinking material. The conductive polymer material is EG-modified PEDOT:PSS. The elastic polymer network material comprises PVA and GA. The supramolecular crosslinking material comprises β-cyclodextrin and citric acid, with a molar ratio of β-cyclodextrin to citric acid of 10:1. The flexible electrode layer has a thickness of 25 μm, a tensile breaking rate of 80%, a tensile shear strength of 1.6 MPa, and an electrical conductivity of 5000 S / cm.

[0105] Figure 10 is a schematic diagram of the conductivity principle of EG-modified PEDOT:PSS. Pure PEDOT:PSS has a core-shell structure, in which conductive PEDOT is wrapped by non-conductive PSS, resulting in low overall polymer conductivity. After EG modification, PEDOT and PSS are separated, and the exposed PEDOT forms a complete conductive pathway, significantly improving the polymer conductivity.

[0106] Figure 11 is a schematic diagram of the PVA and GA double crosslinking network principle. PVA has abundant hydroxyl groups on its long chain, while GA has aldehyde groups at both ends. The two can form a double crosslinking network through condensation reaction, which enhances the mechanical properties of the electrode layer.

[0107] The flexible, high thermal conductivity, and waterproof insulation layer provided in this embodiment is made of a stretchable, high thermal conductivity composite material. This stretchable, high thermal conductivity composite material is prepared by impregnation and casting processes, where PVDF-HFP is cast into BN / TPU sponge. Its tensile breaking rate is 80%, and its electrical conductivity is 2×10⁻⁶. -5 The thermal conductivity is 2.6 W / (m·K), with a density of S / cm.

[0108] Figure 12 shows a schematic diagram of the BN lattice structure and thermal conductivity path. The lattice structure of BN is similar to that of graphene, which is a multi-layered hexagonal crystal structure. The different layers of BN are linked together by van der Waals forces. Due to its long-range ordered lattice structure through strong interactions, it can construct phonon thermal conductivity pathways in the polymer matrix through face-to-face connections, thereby improving the thermal conductivity of the encapsulation layer.

[0109] Figure 13 shows the temperature-capacitance response curve of the capacitive flexible temperature sensor for body temperature monitoring provided in this embodiment. As can be seen from the curve, the sensor's temperature-capacitance response can be divided into three parts: the first part is 30-35℃, the second part is 35-45℃, and the third part is 45-50℃. The sensor sensitivity is lower in the first and third parts because PEO is only in a decrystalline or crystalline state within these temperature ranges, without a phase transition. The ion dissociation rate and ion mobility within the flexible temperature-sensitive layer change very little, resulting in a small change in the sensor capacitance. The sensor sensitivity is higher in the second part because PEO undergoes a phase transition within this temperature range, leading to a very high change in the ion dissociation rate and ion mobility within the flexible temperature-sensitive layer, resulting in a large change in the sensor capacitance. The flexible temperature sensor provided in this embodiment has a capacitance of 3.73K within the 35-45℃ range. -1 Sensitivity.

[0110] Figure 14 shows the capacitance response curve of the capacitive flexible temperature sensor for body temperature monitoring provided in this embodiment under external force. The curve shows that the sensor signal is minimally affected by external force at 30℃ and 50℃, indicating that the sensor has good resistance to stress interference. When the sensor is subjected to external force, its buffer layer absorbs most of the stress, preventing the flexible temperature-sensitive layer from being affected by external force in terms of temperature sensitivity. The supramolecular cross-linked material in the electrode layer gives it good adhesion, ensuring a tight bond with the flexible buffer layer and maintaining the consistency of the electrode layer / buffer layer interface under stress loading.

[0111] Figure 15 shows the response time curve of the capacitive flexible temperature sensor for body temperature monitoring provided in this embodiment. The sensor was placed in a 30°C semiconductor cooling pad until the signal stabilized, then rapidly heated to 50°C, and the capacitance signal was recorded as a function of time. After the capacitance signal remained stable at 50°C for a period of time, it was rapidly cooled to 30°C, and the capacitance signal was recorded again as a function of time, resulting in the response time curve of the flexible temperature sensor. The curve shows that both the rise time and fall time of the sensor are within 13 seconds, indicating that the sensor has an extremely high response rate. The phase transition rate of PEO after blending high and low molecular weight molecules is significantly increased, resulting in a good response rate for the sensor.

[0112] Figure 16 shows the repeatability curve of the capacitive flexible temperature sensor for body temperature monitoring provided in this embodiment. The sensor was repeatedly heated from 30°C to 50°C and then cooled back to 30°C at a rate of 1°C / s, repeated 2000 times, to obtain the repeatability curve of the flexible temperature sensor. The insets in the figure show the repeatability curves of the sensor for the 500-506th and 1500-1506th cycles. As can be seen from the curves, the sensor has good repeatability within 2000 cycles. The mesh structure in the flexible temperature-sensitive layer confines the crystallization process of PEO within the mesh pores; the flexible buffer layer prevents phase transition at the electrode layer / buffer layer interface and reduces stress interference; the adhesiveness of the electrode layer ensures a tight bond with the buffer layer; these factors greatly improve the repeatability of the sensor.

[0113] The above description is merely an exemplary embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A capacitive flexible temperature sensor for body temperature monitoring, characterized in that: The flexible temperature sensor has a sandwich structure, including a functional layer and two flexible encapsulation layers disposed on both sides of the functional layer. The functional layer is composed of five composite material layers, namely, a flexible electrode layer, a flexible buffer layer, a flexible temperature-sensitive layer, a flexible buffer layer, and a flexible electrode layer. The flexible encapsulation layer is a flexible, highly thermally conductive, waterproof, and insulating layer.

2. The capacitive flexible temperature sensor for body temperature monitoring according to claim 1, characterized in that: The flexible temperature-sensitive layer is made by coating flexible temperature-sensitive material onto both sides of gauze and then heat-curing it. After heat curing, the thickness of the flexible temperature-sensitive material on one side of the gauze is 10-30 μm. The tensile breaking rate of the flexible temperature-sensitive layer is not less than 60%, and the resistivity is 5-20 MΩ / m.

3. The capacitive flexible temperature sensor for body temperature monitoring according to claim 2, characterized in that: The flexible temperature-sensitive material includes PVDF-HFP, low molecular weight PEO with a molecular weight of 1200-2000, high molecular weight PEO with a molecular weight of 5000-20000, and H3PO4; in the flexible temperature-sensitive material, the mass fraction of low molecular weight PEO is 25%-30%, the mass fraction of high molecular weight PEO is 20%-25%, the mass fraction of H3PO4 is 10%-15%, and the balance is PVDF-HFP.

4. The capacitive flexible temperature sensor for body temperature monitoring according to claim 2, characterized in that: The gauze is made of polyester fiber or polyamide fiber and has a mesh size of 100-500 mesh.

5. The capacitive flexible temperature sensor for body temperature monitoring according to claim 1, characterized in that, The flexible buffer layer is a composite film made of PVDF-HFP, TPU, ultra-high molecular weight PEO with a molecular weight of 200,000-400,000 and H3PO4; in the composite film, the mass fraction of TPU is 25-35%, the mass fraction of ultra-high molecular weight PEO is 20%-30%, the mass fraction of H3PO4 is 10%-18%, and the balance is PVDF-HFP.

6. The capacitive flexible temperature sensor for body temperature monitoring according to claim 1 or 5, characterized in that, The flexible buffer layer has a thickness of 10–30 μm, a tensile strength of 12–25 MPa, a tensile breaking rate of not less than 60%, and an ionic conductivity of 8 × 10⁻⁶ at 30–50 °C. -3 S / cm~20×10 -3 S / cm.

7. The capacitive flexible temperature sensor for body temperature monitoring according to claim 1, characterized in that, The flexible electrode layer is made of a stretchable high-conductivity composite material. The stretchable high-conductivity composite material comprises three parts: a conductive polymer material, an elastic polymer network material, and a supramolecular crosslinking material. The elastic polymer network material accounts for 50%-60% of the mass, the supramolecular crosslinking material accounts for 25%-35% of the mass, and the remainder is the conductive polymer material. The conductive polymer material is EG-modified PEDOT:PSS, with a mass ratio of EG to PEDOT:PSS of 1:4 to 8. The elastic polymer network material comprises PVA and GA, with a mass ratio of PVA to GA of 5:1 to 3. The supramolecular crosslinking material comprises β-cyclodextrin and citric acid, with a molar ratio of β-cyclodextrin to citric acid of 10:

1.

8. The capacitive flexible temperature sensor for body temperature monitoring according to claim 1 or 7, characterized in that, The thickness of the flexible electrode layer is 10-30 μm, the tensile fracture rate is not less than 60%, the tensile shear strength is 1.2-1.8 MPa, and the electrical conductivity is 3000-20000 S / cm.

9. The capacitive flexible temperature sensor for body temperature monitoring according to claim 1, characterized in that, The flexible, high thermal conductivity, and waterproof insulation layer is made of a stretchable, high thermal conductivity composite material. This stretchable, high thermal conductivity composite material is prepared by impregnation and casting processes, where PVDF-HFP is poured into a BN / TPU sponge, with a BN to TPU mass ratio of 1:5–7. The tensile breaking rate of the flexible, high thermal conductivity, and waterproof insulation layer is not less than 60%, and its electrical conductivity is 1×10⁻⁶. -5 ~1×10 -4 The thermal conductivity is 2–5 W / (m·K), with a density of S / cm.

10. A method for preparing a capacitive flexible temperature sensor for body temperature monitoring as described in any one of claims 1 to 9, characterized in that, Includes the following steps: Step 1: Preparation of flexible temperature-sensitive layer PVDF-HFP, low molecular weight PEO, and high molecular weight PEO were added to a beaker in sequence, followed by DMF solution. The mixture was stirred at 60°C for 3 hours. H3PO4 was then added to the resulting mixed solution, and the mixture was stirred at 40°C for 1 hour to obtain a flexible thermosensitive material. The flexible temperature-sensitive material was coated onto the surface of the gauze using a coating rod and dried at 75°C for 3 hours. The above process was repeated 3 times on both sides of the gauze to obtain a flexible temperature-sensitive layer with a gauze structure. Step 2: Preparation of flexible buffer layer PVDF-HFP, TPU, and ultra-high molecular weight PEO were added to a beaker in sequence, DMF solution was added, and the mixture was stirred at 60°C for 3 hours. Then H3PO4 was added to the resulting mixed solution, and the mixture was stirred at 40°C for 1 hour to obtain a flexible buffer layer composite material. The flexible buffer layer composite material was heated at 75°C for 1 hour to make the composite material viscous. Then, it was coated onto the surface of the flexible temperature-sensitive layer with a coating rod. The material was then heated at 60°C for 2 hours to obtain a flexible buffer layer on the surface of the flexible temperature-sensitive layer. Step 3: Fabrication of flexible electrode layer Take a 3% solid content PEDOT:PSS aqueous solution, add EG and stir at 80°C for 2 hours; then add PVA, GA, β-cyclodextrin and citric acid, and continue stirring at 80°C for 1 hour to obtain a stretchable high conductivity composite material; apply the stretchable high conductivity composite material to the surface of a flexible buffer layer with a coating rod, and then dry at 70°C for half an hour to obtain a flexible electrode layer on the surface of the flexible buffer layer. Step 4: Preparation of flexible, high thermal conductivity, waterproof insulation layer BN powder was added to a mixed solution of isopropanol and deionized water in a volume ratio of 1:

1. After ultrasonic treatment for 6 hours, the keratinized BN sheet was removed by filtration to obtain a BN solution. Using a water-based dip coating process, TPU sponge is completely immersed in BN solution and squeezed. After the sponge no longer absorbs the solution, it is dried at 60°C for 1 hour to obtain BN / TPU sponge. Take PVDF-HFP, heat it to 200℃ in a vacuum oven until it melts, then soak BN / TPU sponge in the melted water. A flexible, highly thermally conductive, and waterproof insulating layer is prepared by immersing the material in PVDF-HFP for 6 hours and then drying it at room temperature for 2 hours. The flexible, highly thermally conductive, and waterproof insulating layer is then attached to the surface of the flexible electrode layer to complete the fabrication of the capacitive flexible temperature sensor.

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