Flexible temperature sensor based on aerosol jet printing and preparation method and application thereof

Abstract

The invention provides a flexible temperature sensor based on aerosol jet printing and a preparation method and application thereof, and belongs to the technical field of novel materials. The preparation method of the flexible temperature sensor comprises the following steps: (1) mixing polyimide with N, N-dimethylformamide to obtain a PI solution; coating paper with the PI solution, and drying to obtain a PI / paper-based composite material substrate; (2) preparing graphene and deionized water into a solution, and adding a dispersing agent to obtain conductive printing ink; and (3) taking the PI / paper-based composite material as a substrate, printing conductive printing ink on the PI / paper-based composite material substrate through aerosol jet printing, and drying to obtain the flexible temperature sensor. The flexible temperature sensor provided by the invention is mainly used for monitoring the internal temperature of a heat-sensitive food package. Through the improvement and innovation of materials and processes, the overall performance (such as stability, accurate monitoring performance, good bending adaptability, quick response and the like) and environmental protection performance of the sensor are improved, so that the sensor can be used for monitoring the internal temperature of a heat-sensitive food package.

Description

Technical Field

[0001] The present invention belongs to the technical field of new materials, and in particular relates to a flexible temperature sensor based on aerosol jet printing, and a preparation method and application thereof. Background Art

[0002] During the storage and transportation of heat-sensitive foods, temperature is crucial to their quality and safety. For example, chocolate will melt and spoil at excessively high temperatures, yogurt is prone to breeding harmful microorganisms, and fresh meat will spoil faster. Therefore, accurately monitoring the internal temperature of heat-sensitive food packaging is important for ensuring food quality. However, existing temperature sensors have many shortcomings in food packaging applications. Existing temperature sensors mainly include rigid sensors and flexible sensors. Due to their inherent structural characteristics, rigid sensors cannot adapt to the flexibility requirements of food packaging and have difficulty fitting tightly to the packaging surface, making it impossible to accurately monitor internal temperature changes. Traditional sensor preparation processes are complex and involve multiple tedious steps, resulting in long production cycles and high costs. At the same time, their sensitivity and accuracy cannot meet the high temperature monitoring requirements of food packaging, and large temperature measurement deviations may occur.

[0003] While flexible temperature sensors come in a variety of types, including those based on traditional materials (thermal resistors, thermocouples) and novel materials (carbon nanotubes, organic semiconductors), as well as printed sensors using methods like screen printing and inkjet printing, they still present a number of challenges. In terms of accuracy and stability, existing flexible temperature sensors still lag significantly behind traditional rigid sensors in temperature measurement accuracy. Furthermore, they are susceptible to environmental factors (such as humidity, light, and oxygen) over long-term use, resulting in reduced stability and difficulty ensuring the accuracy and reliability of measurement data. Regarding response speed, some flexible temperature sensors have a long response time, making it difficult to accurately and promptly reflect rapid temperature changes within food packaging. This can lead to delayed monitoring of temperature changes and hinder timely assessment of food status. Regarding service life, flexible temperature sensors are susceptible to aging and damage, and their relatively high material cost limits their widespread application in food packaging, increasing both operational costs and the overall cost of the product. Regarding compatibility with food packaging, the materials of some flexible temperature sensors may interact with food, impacting food quality and safety. For example, some sensor materials may release harmful substances that contaminate food or adsorb components from food, affecting sensor performance. At the same time, integrating sensors into food packaging also increases the complexity and difficulty of the production process. It is necessary to achieve effective installation and stable operation of sensors without affecting the integrity of the packaging and food safety.

[0004] Aerosol jet printing is an advanced additive manufacturing technology that enables precise control of functional materials during the printing process. Ink is deposited on the substrate through atomization and inert gas flow, allowing high-precision patterned printing on different flexible substrates. This provides a technical basis for the preparation of high-performance flexible sensors.

[0005] At present, there are no patents or literature on the use of aerosol jet printing technology to prepare flexible sensors for monitoring the internal temperature of heat-sensitive food packaging. However, the development of aerosol jet printing technology and the urgent need for temperature monitoring in food packaging provide possibilities and potential application space for research in this direction. Summary of the Invention

[0006] The present invention proposes a flexible temperature sensor based on aerosol jet printing and its application in monitoring the internal temperature of heat-sensitive food packaging. The aim is to make innovative breakthroughs in materials and processes to meet the strict requirements of heat-sensitive food packaging for temperature monitoring.

[0007] The present invention provides a method for preparing a flexible temperature sensor, comprising the following steps: (1) The molecular weight is 5.0×10 3 - 1.0×10 5 The polyimide is mixed with N, N-dimethylformamide to obtain a PI solution with a concentration of 10-30 wt%; the PI solution is coated on paper and dried to obtain a PI / paper composite substrate; (2) preparing a solution of graphene and deionized water with a concentration of 1-10 wt%, adding a dispersant, and obtaining a conductive printing ink; (3) The PI / paper composite material is used as a substrate, and the conductive printing ink is printed on the PI / paper composite material substrate by aerosol jetting, and dried to obtain a flexible temperature sensor. Furthermore, in step (1), the PI solution is coated at a unit area of ​​0.02-0.24 g / cm 2 (dry weight) applied to paper.

[0008] Furthermore, in step (1), the drying is specifically performed by heating and drying at 80-150°C, and then air-drying at room temperature for 1-6 hours. Preferably, the heating and drying time is 1-5 hours.

[0009] Furthermore, in step (2), the dispersant includes at least one of sodium dodecylbenzenesulfonate, hexadecyltrimethylammonium bromide or polyvinylpyrrolidone; Preferably, in step (2), the amount of the dispersant added is 0.01-0.5 wt%.

[0010] Furthermore, in step (3), during aerosol jet printing, the distance between the nozzle of the aerosol jet printer and the substrate is 4-8 mm; Preferably, in step (3), during aerosol jet printing, the printing speed is 8-20 mm / s; Preferably, in step (3), during aerosol jet printing, the carrier gas parameter is 150-250 psi, and the sheath gas parameter is 120-200 psi; Preferably, in step (3), during aerosol jet printing, the number of printing cycles is 1-5 times. Furthermore, in step (3), the drying is specifically performed at 40-80°C for 0.5-5 h.

[0011] Furthermore, in step (3), the shape of the flexible temperature sensor electrode is designed by CAD; Preferably, the flexible temperature sensor electrode is in a fence shape; More preferably, the line width is 3 μm - 30 μm, and the line spacing is 0.2 μm - 10 μm.

[0012] The present invention also proposes a flexible temperature sensor prepared by any of the above-mentioned preparation methods.

[0013] The present invention also proposes the use of any of the above-mentioned flexible temperature sensors in monitoring the internal temperature of heat-sensitive food packaging.

[0014] Furthermore, the heat-sensitive foods include fat-based fusible foods, live bacteria fermentation products, fresh protein products, plant-based sensitive foods, and special medical purpose formula foods.

[0015] The present invention has the following advantages: The preparation method of the flexible temperature sensor based on aerosol jet printing proposed in the present invention improves the overall performance (such as stability, precise monitoring, good bending adaptability, rapid response, etc.) and environmental protection of the sensor through improvements and innovations in materials and processes, making it suitable for internal temperature monitoring of heat-sensitive food packaging.

[0016] In the described preparation method, when preparing the substrate material, a polyimide of an appropriate molecular weight is selected and mixed with N,N-dimethylformamide to form a PI solution. The PI solution is then evenly coated on paper to produce a PI / paper composite substrate. The resulting substrate combines the thermal stability of PI film with the characteristics of paper fiber and possesses biodegradable properties. When preparing the conductive printing ink, graphene is selected as the conductive material. Its excellent thermal conductivity, low resistivity, and good printability lay the foundation for the sensor's conductive performance. When preparing the flexible sensor by aerosol jet printing, appropriate printing parameters (such as the distance between the nozzle and the substrate, printing speed, carrier gas parameters, sheath gas parameters, and electrode line width and line spacing) are selected to optimize the overall performance of the resulting flexible sensor, making it suitable for internal temperature monitoring of heat-sensitive food packaging. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0018] Figure 1 The cross-sectional and planar SEM images of the PI / paper-based composite material prepared in Example 3 of the present invention are shown; Figure 2 This is an SEM plan view of the graphene ink printed on the PI / paper composite material in Example 3 of the present invention; Figure 3 This is the shape diagram of the flexible sensor electrode prepared in Example 3 of the present invention. DETAILED DESCRIPTION

[0019] The following will be combined with the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the embodiments described are only part of the embodiments of the present invention, not all of the embodiments. The embodiments of the present invention and the features in the embodiments can be combined with each other unless there is a conflict.

[0020] In one aspect, an embodiment of the present invention provides a method for preparing a flexible temperature sensor, comprising the following steps: (1) Preparation of PI / paper composite substrate The molecular weight is 5.0×10 3 - 1.0×10 5 The polyimide is mixed with N, N-dimethylformamide (DMF) to obtain a PI solution with a concentration of 10-30 wt%; the PI solution is coated on paper and dried to obtain a PI / paper composite substrate; (2) Preparation of conductive printing ink Graphene and deionized water are mixed into a solution with a concentration of 1-10 wt%, and a dispersant is added to obtain a conductive printing ink; (3) Preparation of flexible temperature sensors by aerosol jet printing The PI / paper-based composite material is used as a substrate, and the conductive printing ink is printed on the PI / paper-based composite material substrate by aerosol jetting and dried to obtain a flexible temperature sensor. In step (1) of the embodiment of the present invention, a PI / paper composite material substrate is selected. The PI film has excellent thermal stability, which plays a key role in the performance of the sensor under different temperature environments. In a high temperature environment, the chemical bonds in the molecular structure of the PI film can effectively resist the intensified molecular motion caused by thermal excitation and maintain its own structural integrity. This stability prevents the sensor substrate from being significantly deformed, softened or decomposed at high temperatures, thereby ensuring the overall structural stability of the sensor. In a low temperature environment, the PI film can still maintain a certain degree of flexibility and will not break due to brittleness due to low temperature, ensuring that the sensor can also work normally in low temperature scenarios such as cold chain transportation. In addition, the characteristics of paper fiber bring unique advantages to the substrate. On the one hand, paper fiber gives the substrate good flexibility. Paper fiber itself has a certain degree of elasticity and bendability. The structure formed by their interweaving enables the PI / paper composite material substrate to easily adapt to various shape changes of food packaging. Whether it is a flat package or a package with a complex curved surface, it can fit tightly, thereby improving the accuracy of temperature monitoring. On the other hand, paper fiber performs well in terms of compatibility with food packaging. Paper fiber is a natural material with relatively stable chemical properties and is not easy to react chemically with the components in food packaging. At the same time, it also has a certain adsorption capacity, which can absorb a small amount of moisture or volatile substances that may be generated in the packaging, preventing these substances from having a negative impact on the performance of the sensor, and will not release harmful substances to contaminate food, thus ensuring the quality and safety of the food. In one embodiment of the present invention, in step (1), the PI solution is coated at a unit area rate of 0.02 - 0.24 g / cm 2(dry weight) coated on paper. In the embodiment of the present invention, the appropriate PI solution concentration and coating amount per unit area are crucial to forming a uniform, continuous PI coating with appropriate thickness and good coverage. A coating amount per unit area that is too low (such as <0.02 g / cm²) will cause the PI coating to be too thin or discontinuous, unable to fully cover the paper fibers, resulting in high porosity of the substrate, reduced thermal stability and mechanical properties, affecting the quality of subsequent printed electrodes and the overall stability of the sensor; a coating amount per unit area that is too high (such as >0.24 g / cm²) may cause the coating to be too thick, difficult to dry, prone to cracking, reduced flexibility, and increased cost. Controlling the coating amount per unit area within the range of 0.02 - 0.24 g / cm², combined with an appropriate PI solution concentration, can ensure the preparation of a composite substrate with good thermal stability, flexibility, surface smoothness and firm bonding to the paper base.

[0021] In one embodiment of the present invention, in step (1), the obtained solution is mixed by ultrasonic oscillation; the time of ultrasonic oscillation is 20-60 min. In one embodiment of the present invention, in step (1), the drying is specifically performed by heating and drying at 80-150°C, and then air-drying at room temperature for 1-6 hours. Preferably, the heating and drying time is 1-5 hours.

[0022] In one embodiment of the present invention, in step (1), the obtained PI / paper-based composite material is placed in a sealed bag for storage. In the embodiment of the present invention, in step (2), graphene and deionized water are mixed into a solution, and a dispersant is added to successfully prepare a conductive printing ink. Graphene's excellent thermal conductivity, low resistivity, and good printability lay the foundation for the sensor's conductive performance.

[0023] Specifically, graphene's excellent thermal conductivity and low resistivity make it crucial for temperature monitoring sensors, enabling high sensitivity and rapid response. When the ambient temperature changes, graphene's high thermal conductivity allows it to quickly sense the temperature change. Heat conducts rapidly within the graphene, causing changes in its electron cloud distribution. Due to graphene's low resistivity, this change in electron cloud distribution is quickly converted into a change in electrical signal—that is, a change in resistance. When the temperature rises, electrons in the graphene gain more energy, moving faster, and interacting more effectively with the lattice, resulting in a decrease in resistance. Conversely, when the temperature decreases, the resistance increases. This sensitive temperature-dependent resistance change enables sensors to accurately detect even small temperature changes, achieving highly sensitive temperature monitoring. Furthermore, because graphene conducts heat quickly and changes resistance rapidly, sensors can respond quickly to rapid temperature changes, accurately and promptly reflecting temperature fluctuations, meeting the demand for real-time temperature monitoring within food packaging.

[0024] In one embodiment of the present invention, in step (2), the dispersant includes at least one of sodium dodecylbenzenesulfonate, hexadecyltrimethylammonium bromide or polyvinylpyrrolidone.

[0025] In one embodiment of the present invention, in step (2), the amount of the dispersant added is 0.01-0.5 wt%. Specifically, it refers to the mass concentration of the dispersant in the conductive printing ink. In this embodiment of the present invention, the amount of the dispersant added needs to be controlled within the range of 0.01-0.5 wt%. If the amount of dispersant is less than 0.01 wt%, graphene is prone to agglomeration due to insufficient dispersion (refer to the failure caused by the narrow line width in Comparative Example 7); when it is higher than 0.5 wt%, the excess dispersant may act as an insulating residue to hinder electron conduction, significantly reducing the sensitivity of the sensor.

[0026] In one embodiment of the present invention, in step (3), during aerosol jet printing, the distance between the nozzle of the aerosol jet printer and the substrate is 4-8 mm. In the embodiment of the present invention, a suitable nozzle-to-substrate distance is crucial, and the nozzle-to-substrate distance is set to 4-8 mm. When the distance is within this range, effective ink deposition and high pattern accuracy can be achieved. If the nozzle-to-substrate distance is less than 4 mm, the air resistance encountered by the ink during the jetting process is small, which may cause the ink to directly hit the substrate in the form of larger droplets, making the deposited ink dots uneven in size, affecting the accuracy of the pattern, causing deviations in the line width and line spacing of the sensor electrodes, and thus affecting the conductivity and stability of the sensor. In addition, too small a distance may also make the nozzle easily contact the substrate, causing the nozzle to be blocked or damaged. When the nozzle-to-substrate distance is too large, exceeding 8 mm, the ink will be subject to more air resistance and diffusion during transmission, resulting in reduced ink deposition efficiency, and some ink may float into the air before reaching the substrate, resulting in material waste. At the same time, a larger spacing will make the ink deposition position on the substrate less precise, the pattern edges blurred, and affect the shape and dimensional accuracy of the sensor electrodes, ultimately reducing the performance of the sensor and making it impossible to accurately monitor temperature changes. In one embodiment of the present invention, in step (3), during aerosol jet printing, the printing speed is 8-20 mm / s. In this embodiment of the present invention, an appropriate printing speed is crucial, and the printing speed is controlled to be 8-20 mm / s. At a lower printing speed (e.g., close to 8 mm / s), the ink has enough time to be evenly deposited on the substrate, and relatively continuous and uniform electrode lines can be formed, which is beneficial to improving the conductivity and stability of the sensor. However, a printing speed that is too slow will lead to low production efficiency and increase production costs. On the contrary, a higher printing speed (e.g., close to 20 mm / s) can improve production efficiency, but if the speed is too fast, the ink may not be fully deposited on the substrate, resulting in breakpoints or discontinuities in the electrode lines, making the resistance value of the sensor unstable and affecting the accuracy of temperature monitoring. In addition, a printing speed that is too fast may also increase the shear force on the ink during the jetting process, destroying the atomization effect of the ink and further affecting the deposition quality.

[0027] In one embodiment of the present invention, in step (3), during aerosol jet printing, the carrier gas parameters are 150-250 psi and the sheath gas parameters are 120-200 psi. In this embodiment of the present invention, appropriate carrier gas and sheath gas parameters play a key role in the atomization and deposition of the ink, wherein the carrier gas parameters are maintained at 150-250 psi and the sheath gas parameters are set at 120-200 psi. The main function of the carrier gas is to carry the ink out of the nozzle and atomize it into tiny droplets. Appropriate carrier gas pressure can make the ink form uniform and fine droplets, ensuring uniform deposition on the substrate. If the carrier gas pressure is too low, the ink cannot be fully atomized and will be sprayed in the form of larger droplets, resulting in uneven deposition and affecting the performance of the sensor; if the carrier gas pressure is too high, the ink may be over-atomized, the droplets are too small, and are easily dispersed in the air, reducing the deposition efficiency. The sheath gas surrounds the carrier gas and ink, playing a protective and focusing role. Appropriate sheath gas pressure allows the atomized ink droplets to be deposited more concentratedly on the substrate, improving the pattern's accuracy and resolution. When the sheath gas pressure is low, the ink droplets are insufficiently restrained, allowing them to easily diffuse, blurring the pattern's edges and affecting the sensor's electrode shape accuracy. Excessive sheath gas pressure can over-compress the ink droplets, similarly affecting ink deposition and sensor performance. By properly adjusting the carrier and sheath gas parameters, the ink's atomization and deposition uniformity can be optimized, ensuring the sensor's excellent conductivity and stability for accurate temperature monitoring.

[0028] In one embodiment of the present invention, in step (3), during aerosol jet printing, the number of printing cycles is 1-5 times. In one embodiment of the present invention, in step (3), the drying is specifically drying at 40-80°C for 0.5-5 h.

[0029] In one embodiment of the present invention, in step (3), the shape of the flexible temperature sensor electrode is designed using CAD. Preferably, the flexible temperature sensor electrode has a fence-shaped shape. More preferably, the line width is 3 μm to 30 μm, and the line spacing is 0.2 μm to 10 μm.

[0030] On the other hand, an embodiment of the present invention provides a flexible temperature sensor based on aerosol jet printing.

[0031] In yet another aspect, an embodiment of the present invention provides an application of a flexible temperature sensor based on aerosol jet printing in monitoring the internal temperature of heat-sensitive food packaging.

[0032] In one embodiment of the present invention, the heat-sensitive food includes but is not limited to the following categories: fat-based fusible food, live bacteria fermentation products, fresh protein products, plant-based sensitive food, special medical purpose formula food, etc.

[0033] The fat-based meltable food may be chocolate, cocoa products, butter, cheese, ice cream, etc. When the temperature is greater than 30°C, the fat phase changes, resulting in texture collapse, such as chocolate melting and sticking.

[0034] The live bacteria fermented products can be yogurt, probiotic beverages, fermented cheese, etc. Temperature fluctuations accelerate the growth of harmful bacteria and destroy the activity of probiotics. The fresh protein products may be fresh / chilled meat, poultry, aquatic products, pasteurized milk, egg products, etc. The growth rate of spoilage microorganisms exceeds 200% for every 5°C increase in temperature between 4°C and 40°C.

[0035] The plant-based sensitive foods may include pre-cut fruits and vegetables, freshly squeezed juices, salads, soy products, etc. There are dual risks of enzymatic browning and microbial growth.

[0036] The special medical purpose formula food can be amino acid milk powder, etc.

[0037] The aerosol jet-printed flexible temperature sensor proposed in the embodiments of the present invention has the following performance advantages when used to monitor the internal temperature of heat-sensitive food packaging: Groundbreaking application areas: The company pioneered the application of aerosol jet printing technology to internal temperature monitoring of heat-sensitive food packaging, solving the difficulties faced by traditional sensors in fitting and adapting to flexible packaging shapes, and filling a technological gap. For example, in chocolate packaging, the technology can closely fit curved surfaces and accurately monitor temperature.

[0038] Accurate and efficient temperature monitoring: Within the range of 25°C - 40°C, resistance and temperature show a good linear negative correlation, with a correlation coefficient exceeding 0.86. At 35°C, the response time is as short as 5.3 seconds and the resistance change rate is only 0.25%. The accuracy and response speed far exceed those of some traditional flexible temperature sensors, effectively monitoring temperature changes in food packaging.

[0039] Strong stability and anti-interference ability: After special substrate treatment and graphene application, the resistance change rate is within 9.6% under the environment of 25℃-40℃ and 15%-20% humidity. In the test of simulating the complex environment of food packaging (such as high humidity and bending), the performance is stable, which is better than rigid thermocouple sensors and screen-printed sensors. Highly targeted for food packaging: From materials and preparation to testing, the sensor closely follows the needs of food packaging. Chocolate packaging experiments have verified that it can accurately monitor temperature changes of different packaging materials, ensure food quality, and promote the development of smart packaging. Compared with sensors focusing on other fields, it has obvious advantages in application targeting.

[0040] Great potential for application expansion: It provides a new way to monitor the temperature of food packaging. Based on the material and process characteristics, it is expected to be extended to the field of temperature monitoring of similar flexible packaging such as pharmaceuticals and cosmetics, overcoming the limited application scope of traditional sensors and having broad market prospects.

[0041] The present invention will be described in detail below with reference to the accompanying drawings and embodiments.

[0042] Example 1 A method for preparing a flexible temperature sensor comprises the following steps: Preparation of PI / paper composite substrate: 3 The polyimide was dissolved in N, N-dimethylformamide (DMF) to prepare a solution with a concentration of 10 wt%, and ultrasonically vibrated for 20 min to form a uniform mixed solution. The coating amount per unit area was 0.02 g / cm 2 The PI solution (dry weight) was coated on the cut paper to prepare the PI / paper composite substrate, heated and dried at 80 °C for 5 h, then dried at room temperature for 1 h, and stored in a sealed bag.

[0043] Preparation of conductive printing ink: Graphene and deionized water were mixed into a 1 wt% solution (100 mL system), and 0.01 g of sodium dodecylbenzenesulfonate was added as a dispersant (dispersant concentration 0.01 wt%) to prepare a conductive printing ink.

[0044] Aerosol jet printing fabricated the flexible sensor: The distance between the aerosol jet printing nozzle and the substrate was set at 4 mm, the printing speed was set at 8 mm / s, the carrier gas parameters were 150 psi, and the sheath gas parameters were 120 psi. The flexible sensor electrodes were designed using CAD as a fence-shaped pattern with a line width of 3 μm and a line spacing of 0.2 μm. Conductive ink was aerosol jet printed onto a PI / paper composite substrate to fabricate the flexible sensor. The number of printing cycles was one. After printing, the sensor was dried at 40°C for 5 h to remove residual solvent.

[0045] Example 2 A method for preparing a flexible temperature sensor comprises the following steps: Preparation of PI / paper composite substrate: 4 The polyimide was dissolved in DMF to prepare a solution with a concentration of 30 wt%, and ultrasonically vibrated for 60 min to form a uniform mixed solution. The coating amount per unit area was 0.24 g / cm 2 The PI solution (dry weight) was coated on the cut paper to prepare the PI / paper composite substrate, heated and dried at 150 °C for 1 h, then dried at room temperature for 6 h, and stored in a sealed bag.

[0046] Preparation of conductive printing ink: Graphene and deionized water were mixed into a 10 wt% solution (100 mL system), and 0.5 g of hexadecyltrimethylammonium bromide was added as a dispersant (dispersant concentration 0.5 wt%) to prepare a conductive printing ink.

[0047] Aerosol jet printing fabricated the flexible sensor: The distance between the aerosol jet printing nozzle and the substrate was set at 8 mm, the printing speed was set at 20 mm / s, the carrier gas parameters were 250 psi, and the sheath gas parameters were 200 psi. The flexible sensor electrodes were designed using CAD as a fence pattern with a line width of 30 μm and a line spacing of 10 μm. Conductive ink was aerosol jet printed onto a PI / paper composite substrate to fabricate the flexible sensor. Five printing cycles were performed. After printing, the sensor was dried at 80°C for 0.5 h to remove residual solvent.

[0048] Example 3 A method for preparing a flexible temperature sensor comprises the following steps: Preparation of PI / paper composite substrate: 5 The polyimide was dissolved in DMF to prepare a solution with a concentration of 20 wt%, and ultrasonically vibrated for 40 min to form a uniform mixed solution. The coating amount per unit area was 0.12 g / cm 2 The PI solution (dry weight) was coated on the cut paper to prepare the PI / paper composite substrate, heated and dried at 120 °C for 3 h, then dried at room temperature for 3 h, and stored in a sealed bag.

[0049] Preparation of conductive printing ink: Graphene and deionized water were mixed into a 5 wt% solution (100 mL system), and 0.25 g of polyvinyl pyrrolidone was added as a dispersant (dispersant concentration 0.25 wt%) to prepare a conductive printing ink.

[0050] Aerosol jet printing fabricated the flexible sensor: The distance between the aerosol jet printing nozzle and the substrate was set at 6 mm, the printing speed was set at 15 mm / s, the carrier gas parameters were 200 psi, and the sheath gas parameters were 150 psi. The flexible sensor electrodes were designed using CAD as a fence-shaped structure with a line width of 15 μm and a line spacing of 5 μm. Conductive ink was aerosol jet printed onto a PI / paper composite substrate to fabricate the flexible sensor. Four printing cycles were performed. After printing, the sensor was dried at 60°C for 2 h to remove residual solvent.

[0051] Figure 1 The cross-sectional and planar SEM images of the PI / paper-based composite material obtained in Example 3; Figure 2This is an SEM plan view of the graphene ink printed on the PI / paper composite material in Example 3; Figure 3 This is the shape diagram of the flexible sensor electrode obtained in Example 3.

[0052] Test Example 1 Stability test: The sensor prepared in Example 3 was placed in different temperature (25°C - 40°C) and humidity (15% - 20%) environments, and its resistance was measured every 10 minutes for 1 hour. The experimental results showed that the rate of change in the sensor resistance was within 9.6%, indicating that the sensor has good stability under different environmental conditions and can operate stably for a long time.

[0053] Resistance and temperature relationship test: The resistance of the sensor in Example 3 was measured by gradually changing the temperature in 5°C increments over a temperature range of 25°C to 40°C. The experimental data showed that the sensor's resistance decreased with increasing temperature, exhibiting a negative temperature coefficient. Furthermore, the resistance exhibited a good linear relationship with temperature, with a correlation coefficient exceeding 0.86, demonstrating its ability to accurately monitor temperature changes.

[0054] Bending characteristics test: The sensor prepared in Example 3 was subjected to bending experiments, with the bending angle gradually increasing from 0° to 180°, with three bends at each angle. The change in resistance was then measured. The experimental results showed that the sensor's resistance change during bending was minimal, within 10%, demonstrating its excellent bending properties and its ability to adapt to the bending conditions experienced during transportation and storage of food packaging.

[0055] Response time test: The sensor prepared in Example 3 was rapidly placed from a 25°C to a 30°C constant temperature environment. The time required for its resistance change to reach a stable value was measured, and the response time was used as the response time. The experimental results showed that the sensor had a response time of 10.8 s and a resistance change rate of 0.08%. When the sensor was rapidly placed from a 30°C to a 35°C constant temperature environment, the experimental results showed that the sensor had a response time of 5.3 s and a resistance change rate of 0.25%. This sensor can promptly reflect rapid temperature changes and meet the timeliness requirements for temperature monitoring.

[0056] Comparative Example 1 (Polyimide molecular weight is too low) Same as Example 3, except that when preparing the PI / paper composite substrate, the molecular weight of 3.0×10 3A 20 wt% solution of polyimide was dissolved in DMF and ultrasonically vibrated for 40 minutes before being coated on paper. After drying at 120°C for 3 hours, the resulting substrate showed obvious cracks on its surface.

[0057] The testing method was the same as in Test Example 1. Stability testing showed that the resistance change rate reached 18.7% at 40°C, indicating poor thermal stability of the substrate, leading to sensor failure. The resistance change rate exceeded 30% when bent 90°, indicating insufficient mechanical strength of the substrate.

[0058] Comparative Example 2 (Polyimide molecular weight is too high) Same as Example 3, except that when preparing the PI / paper composite substrate, a molecular weight of 2.0×10 5 of polyimide. The test method was the same as in Test Example 1. Due to the high molecular weight of the polyimide, the solution viscosity increased significantly, resulting in uneven substrate thickness after coating. The substrate surface was uneven during electrode printing, and the linear correlation coefficient between resistance and temperature dropped to 0.71. The response time at 35°C was extended to 18.2 s. Comparative Example 3 (PI solution concentration is too low) Same as Example 3, except that a 5 wt% PI solution was prepared when preparing the PI / paper composite substrate.

[0059] The testing method was the same as in Test Example 1. Due to the low concentration of the PI solution, the paper fibers were not fully covered after coating, increasing the substrate porosity. During electrode printing, ink seeped into the pores, causing uncontrolled line spacing. At 90% humidity, moisture adsorption by the substrate led to performance degradation. Comparative Example 4 (The nozzle spacing is too large) Same as Example 3, except that when preparing the conductive printing ink, the nozzle spacing is set to 10 mm.

[0060] The test method was the same as in Test Example 1. Due to the large nozzle spacing, the atomized ink droplets diffused significantly in the air, reducing deposition efficiency, widening the printed electrode lines, and blurring the edges. The resistance values ​​exhibited high dispersion, with a response time of 22.4 seconds at 35°C and a 60% decrease in sensitivity.

[0061] Comparative Example 5 (Printing too fast) The same as Example 3, except that when the flexible sensor is prepared by aerosol jet printing, the printing speed is set to 25 mm / s.

[0062] The testing method was the same as in Test Example 1. Due to excessive printing speed, breakpoints appeared in the electrode lines. During the bending test, the resistance at the breakpoints suddenly changed, with the resistance change exceeding 45% after a 180° bend. The resistance-temperature correlation coefficient dropped to 0.68, significantly degrading linearity.

[0063] Comparative Example 6 (Carrier gas pressure is too low) Same as Example 3, except that when the flexible sensor is prepared by aerosol jet printing, the carrier gas pressure is set to 100 psi.

[0064] The test method was the same as in Test Example 1. Due to low carrier gas pressure, ink atomization was insufficient, resulting in droplet diameters greater than 50 μm. The printed electrode thickness was uneven, resulting in resistance fluctuations of ±18%. During testing at 25°C to 40°C, the temperature measurement deviation reached ±1.8°C, failing to meet monitoring requirements.

[0065] Comparative Example 7 (Line width is too narrow) The same as Example 3, except that when the flexible sensor is prepared by aerosol jet printing, the line width is designed to be 1 μm.

[0066] The testing method was the same as in Test Example 1. Due to the narrow line width, the printed electrode had poor continuity. When bent 30°, the resistance change exceeded 25%, completely failing the sensor. At 35°C, the response time was extended to 28.7 seconds, and the resistance change was only 0.02%, indicating insufficient sensitivity. Comparative Example 8 Conventional rigid thermocouple sensors used for industrial temperature monitoring have an accuracy of up to ±0.5°C.

[0067] Comparative Example 9 A carbon nanotube conductive paste (5 wt% solid content) was prepared, and interdigital electrodes were screen-printed on a PET film as a substrate to prepare a flexible temperature sensor.

[0068] Application Example 1 The flexible sensor prepared in Example 3 was placed in an experimental device that simulated the chocolate packaging environment. The device could adjust the temperature (25°C - 40°C), humidity (10% - 90%), and was set to periodic mechanical bending (simulating the transportation process, bending once per minute, and bending angles of 0° - 90°).

[0069] The temperature was varied in 5°C increments over the 25°C to 40°C range, while humidity levels of 10%, 50%, and 90% were monitored. The deviation between the sensor's temperature measurements and those of a standard thermometer was observed. After 10 cycles of testing, the flexible sensor of Example 3 maintained an average measurement deviation within ±0.4°C. The sensor maintained stable measurement under high humidity and bending conditions, with a deviation fluctuation of no more than ±0.6°C.

[0070] Comparative Application Example 1 The sensor from Comparative Example 1 was placed in an experimental device that simulated the chocolate packaging environment. The device could adjust the temperature (25°C - 40°C), humidity (10% - 90%), and was set to undergo periodic mechanical bending (simulating the transportation process, with one bend per minute and a bending angle of 0°-90°).

[0071] The temperature was varied in 5°C increments over the 25°C to 40°C range, while humidity levels of 10%, 50%, and 90% were monitored. The deviation between the temperature measurements of the sensor in Comparative Example 1 and those of a standard thermometer was observed. After 10 cycles of testing, the rigid thermocouple sensor showed an average measurement deviation of ±1.2°C during temperature changes. Furthermore, at high humidity of 90% and after repeated bending, the sensor exhibited some structural loosening, significantly increasing the fluctuation in the measurement data, with a maximum deviation of ±3°C.

[0072] Comparative Application Example 2 The sensor of Comparative Example 2 was tested in the same experimental device simulating the chocolate packaging environment. The test temperature range was 25°C - 40°C, the humidity range was 10% - 90%, the bending condition was 2 bends per minute, and the bending angle was 0°-120°.

[0073] Experimental results show that within the temperature range of 25°C to 40°C, the linear correlation coefficient between resistance and temperature for this comparison flexible sensor is 0.75, lower than the 0.86 or above for Example 3. At 35°C, the response time is approximately 15 seconds, and the resistance change rate is 0.5%, making both the response speed and stability inferior to those of Example 3. After 24 hours of exposure to high humidity (90%), the sensor's measurement accuracy significantly decreased, with the temperature measurement deviation increasing to over ±1°C. However, under the same conditions, the sensor in Example 3 maintained a high level of accuracy, with a deviation within ±0.5°C.

[0074] The above is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for preparing a flexible temperature sensor, characterized in that: The steps include: (1) The molecular weight is 5.0×10 3 - 1.0×10 5 The polyimide is mixed with N, N-dimethylformamide to obtain a PI solution with a concentration of 10-30 wt%; the PI solution is coated on paper and dried to obtain a PI / paper composite substrate; (2) preparing a solution of graphene and deionized water with a concentration of 1-10 wt%, adding a dispersant, and obtaining a conductive printing ink; (3) The PI / paper composite material is used as a substrate, and the conductive printing ink is printed on the PI / paper composite material substrate by aerosol jetting, and dried to obtain a flexible temperature sensor.

2. The preparation method according to claim 1, characterized in that In step (1), the PI solution is coated at a rate of 0.02 - 0.24 g / cm 2 (dry weight) applied to paper.

3. The preparation method according to claim 1, characterized in that In step (1), the drying is specifically performed by heating and drying at 80-150°C, and then drying at room temperature for 1-6 hours; Preferably, the heating and drying time is 1-5 h.

4. The preparation method according to claim 1, characterized in that In step (2), the dispersant includes at least one of sodium dodecylbenzenesulfonate, hexadecyltrimethylammonium bromide or polyvinylpyrrolidone; Preferably, in step (2), the amount of the dispersant added is 0.01-0.5 wt%.

5. The preparation method according to claim 1, characterized in that In step (3), during aerosol jet printing, the distance between the nozzle of the aerosol jet printer and the substrate is 4-8 mm; Preferably, in step (3), during aerosol jet printing, the printing speed is 8-20 mm / s; Preferably, in step (3), during aerosol jet printing, the carrier gas parameters are 150-250 psi, and the sheath gas parameters are 120-200 psi; Preferably, in step (3), during aerosol jet printing, the number of printing cycles is 1-5 times.

6. The preparation method according to claim 1, characterized in that In step (3), the drying is specifically performed at 40-80°C for 0.5-5 h.

7. The preparation method according to claim 1, characterized in that In step (3), the shape of the flexible temperature sensor electrode is designed by CAD; Preferably, the flexible temperature sensor electrode is in a fence shape; More preferably, the line width is 3 μm - 30 μm, and the line spacing is 0.2 μm - 10 μm.

8. The flexible temperature sensor prepared by the preparation method according to any one of claims 1 to 7.

9. Use of the flexible temperature sensor according to claim 8 in monitoring the internal temperature of heat-sensitive food packaging.

10. The use according to claim 9, characterized in that The heat-sensitive foods include fat-based fusible foods, live bacteria fermented products, fresh protein products, plant-based sensitive foods, and special medical purpose formula foods.

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