High-stability hydrophobic breathable full-paper-based flexible pressure sensor and preparation method thereof

Abstract

The invention relates to the technical field of flexible electronic devices, and particularly discloses a high-stability hydrophobic breathable full-paper-based flexible pressure sensor and a preparation method thereof. According to the sensor, cellulose paper serves as a substrate and is soaked in MXene dispersion liquid prepared in advance to form a conductive layer, an electrode structure is constructed through a silver paste interdigital electrode with the printing size being 15 mm * 30 mm, then a hydrophobic modified packaging layer is used for packaging, and the sensor has good conductivity, flexibility, hydrophobicity and air permeability. The prepared sensor shows excellent stability and response characteristics in a high-humidity and underwater environment, and can be widely applied to wearable equipment, human health monitoring, intelligent packaging, signal detection in a high-humidity environment and other scenes. The method is simple in process and environmentally friendly, and the sensor has good biodegradability and good application prospects.

Description

Technical Field

[0001] This invention relates to the field of flexible sensor technology, specifically to a highly stable hydrophobic and breathable all-paper-based flexible pressure sensor and its fabrication method. Background Technology

[0002] With the rapid development of sensor technology, biosignal sensors have been widely used in medical health monitoring, motion tracking, smart wearable devices, and emergency rescue communications. Flexible pressure sensors, with their ability to adapt to human body surface morphology, high sensitivity, and real-time response, have become a research hotspot. These sensors can accurately monitor biosignals such as finger pressure, bending, jumping, and pulse, and are widely used in health monitoring, rehabilitation therapy, and human posture recognition, providing strong technical support to users.

[0003] However, despite the excellent performance of existing flexible pressure sensors in various applications, their performance in extreme environments remains significantly insufficient. Particularly in high humidity, rainy, and underwater environments, the poor stability of existing sensors leads to a substantial decrease in signal accuracy and reliability. For example, traditional flexible sensors may experience changes in conductivity or even performance degradation in high humidity environments due to moisture penetration; in underwater environments, many sensors cannot effectively isolate water, resulting in signal loss or reduced accuracy, thus failing to meet the requirements for underwater biological signal monitoring, swimming status tracking, and emergency rescue.

[0004] Furthermore, existing flexible sensors mostly use materials such as polydimethylsiloxane (PDMS), which, while possessing a certain degree of flexibility and durability, have a high water vapor permeability in humid environments, leading to damage to the internal circuitry and further affecting their lifespan and reliability. Although some research has attempted to improve humidity adaptability by refining packaging methods or using special materials, existing technologies still face the following challenges:

[0005] Poor humidity sensitivity: Most existing sensors cannot work stably in high humidity environments, resulting in reduced signal accuracy and stability.

[0006] Unstable underwater performance: The performance degradation problem of existing sensors during long-term underwater operation has not been effectively solved, making it difficult to meet the needs of accurate biological signal monitoring.

[0007] Insufficient durability: Existing sensors exposed to humid or underwater environments for extended periods are prone to electrical failure or material degradation, severely impacting their lifespan.

[0008] Therefore, designing a biosignal sensor that combines high sensitivity, flexibility, and stability, and can operate stably for a long time in high humidity and underwater environments, has become a major challenge in the current technological field. Summary of the Invention

[0009] The primary objective of this invention is to provide a flexible biosignal sensor capable of stable operation in extreme environments (such as high humidity and underwater environments), aiming to solve the problems of signal instability, performance degradation, and insufficient durability of existing flexible sensors in high humidity, rainy, and underwater environments. Most existing flexible pressure sensors suffer from poor humidity sensitivity, unstable performance in underwater environments, and easy material degradation after long-term use, limiting their widespread application in health monitoring, emergency rescue, motion analysis, and other fields.

[0010] This invention proposes a flexible biosignal sensor with excellent sensing performance through innovative material design and structural optimization. Compared with existing technologies, the sensor of this invention can operate stably for a long time underwater and in high humidity environments, and has excellent performance, making it widely applicable in various fields such as health monitoring, emergency rescue, and motion analysis.

[0011] This invention proposes a hydrophobic, breathable, and biodegradable all-cellulose-based flexible pressure sensor with the following advantages: high sensitivity: capable of accurately monitoring biological signals such as finger pressure, bending, jumping, and pulse; good adaptability to underwater and high-humidity environments: able to work stably in extreme environments; long-term durability: maintaining stable performance even after prolonged exposure to humid or underwater environments; environmental friendliness: using all-cellulose-based materials, possessing good biodegradability, reducing electronic waste.

[0012] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0013] A method for fabricating a highly stable, hydrophobic, and breathable all-paper-based flexible pressure sensor, characterized by comprising the following steps:

[0014] (1) Immerse cellulose paper in MXene colloidal solution and dry to obtain an intermediate layer;

[0015] (2) Methyltrichlorosilane chemical vapor deposition was performed on cellulose paper and cellulose paper after printing interdigitated electrodes to obtain the top layer and bottom layer of the encapsulation, respectively.

[0016] (3) Stack the top layer, middle layer and bottom layer of the package in sequence, and seal the edges with waterproof tape to obtain the pressure sensor.

[0017] Preferably, in step (1), the preparation of the MXene colloidal solution is as follows: lithium fluoride and concentrated hydrochloric acid are mixed to obtain an activated etching solution, Ti3AlC2 powder is added to react, the product is centrifuged, the upper layer solution is replaced until the pH value is close to neutral, the precipitate obtained is MXene, and then MXene is redispersed in water to form an MXene colloidal solution for later use.

[0018] Preferably, the ratio of lithium fluoride to concentrated hydrochloric acid is 1g:(8-12)mL, and the mass ratio of Ti3AlC2 to lithium fluoride is 1:2; the reaction temperature after adding Ti3AlC2 is 30-50℃, and the reaction time is 40-50h.

[0019] Preferably, the concentration of the MXene colloidal solution is (4-10) mg / mL.

[0020] Preferably, the concentration of the MXene colloidal solution is (8±1) mg / mL.

[0021] Preferably, the cellulose paper described in step (1) is processed by a hot pressing mold and has a micro-convex dot matrix structure.

[0022] Preferably, in step (2), the method for performing methyltrichlorosilane chemical vapor deposition is as follows: cellulose paper or cellulose paper after printing interdigitated electrodes is laid flat in a petri dish, 0.5-2g of methyltrichlorosilane is added, and the mixture is dried at 40-60°C for 3-10 minutes.

[0023] Preferably, the amount of methyltrichlorosilane added is (1±0.2)g.

[0024] Preferably, the size of the interdigital electrode is 15mm × 30mm.

[0025] The method described in this invention produces a highly stable, hydrophobic, and breathable all-paper-based flexible pressure sensor.

[0026] This invention provides a flexible biosignal sensor with high stability, capable of operating stably in high humidity and underwater environments. Through innovative material design and structural optimization, this sensor achieves superior performance under extreme environmental conditions. Specifically, it employs an all-cellulose-based material, combined with the high conductivity and unique microstructure of MXene, enabling the sensor to not only possess excellent flexibility and sensitivity but also maintain stable performance in high humidity and underwater environments. Furthermore, the sensor's moisture-isolation layer and special encapsulation method give it significant water resistance and moisture resistance, greatly improving the sensor's stability and reliability in complex environments.

[0027] Compared with the prior art, the sensor of the present invention has the following significant advantages:

[0028] (1) High humidity and underwater stability: The sensor can work stably in the underwater environment for a long time without being affected by water. It is suitable for monitoring biological signals in high humidity, rainy days and underwater environments, and performs particularly well in applications such as human motion monitoring, swimming status tracking and emergency rescue.

[0029] (2) Environmental friendliness and biodegradability: The sensor is made of all-cellulose-based material, which not only has strong mechanical properties, but can also be naturally degraded to avoid pollution of the environment by electronic waste, which meets the requirements of green environmental protection.

[0030] (3) High sensitivity and response speed: The sensitivity and response speed of this sensor are significantly improved compared with traditional sensors, enabling it to accurately capture subtle changes in human body movements. Its sensing performance, such as pressure range, response time, and recovery time, has reached the international leading level.

[0031] (4) Excellent durability: The sensor is more durable than traditional sensors, can withstand long-term wear and bending, and will not experience performance degradation when exposed to humid or underwater environments for a long time.

[0032] The flexible biosignal sensor of this invention exhibits stability, environmental friendliness, and high performance in high humidity and underwater environments, making it promising for applications in various fields such as medical health monitoring, motion analysis, emergency rescue, and smart wearable devices. It solves the problem of insufficient performance of existing sensors in extreme environments. Attached Figure Description

[0033] Figure 1 This is a schematic diagram of the fabrication process of the flexible pressure sensor of the present invention.

[0034] Figure 2 The image shows the water contact angle measurement results of the encapsulation layer with an MTS addition amount of 1g in Example 3 of the present invention, which indicates that the surface has excellent hydrophobic properties.

[0035] Figure 3 The graph shows the sensitivity curve of the sensor in Embodiment 1 of the present invention, which verifies its response capability to external force stimuli.

[0036] Figure 4 This is a sensor response curve of the sensor in Embodiment 2 of the present invention under different applied pressures.

[0037] Figure 5 This is a comparative response curve of the unmodified sensor and the hydrophobic modified sensor in the underwater elbow bending application of Embodiment 5 of the present invention.

[0038] Figure 6 The permeability evaluation charts for different packaging materials verify the superior permeability performance of the material of this invention.

[0039] Figure 7 This is a diagram of the soil degradation process of the flexible pressure sensor of the present invention, and its performance is compared with that of the PDMS sensor. Detailed Implementation

[0040] The technical solution of the present invention will be further described below with reference to specific embodiments of the present invention, but this does not limit the scope of protection of the present invention.

[0041] The method for fabricating the all-paper-based flexible pressure sensor of the present invention, as follows: Figure 1 As shown, it includes the following steps:

[0042] A method for preparing a hydrophobic, breathable, biodegradable, all-cellulose-based flexible pressure sensor includes the following steps:

[0043] Step 1: Hydrogen chloride (HCl) and lithium fluoride (LiF) were added to the reaction vessel and stirred until completely dissolved. Then, titanium aluminum carbide (Ti3AlC2) powder was slowly added, and the reaction was carried out at 40°C for 48 hours. After the reaction was completed, the solution was poured into a centrifuge tube, deionized water was added, and the mixture was centrifuged. The supernatant was removed, and this step was repeated until the pH of the water was close to 6. Next, the MXene colloidal solution was concentrated to 8 mg / ml and sonicated to ensure uniform dispersion of the MXene nanosheets.

[0044] Step 2: Immerse the cut 15mm × 15mm rectangular cellulose paper in an 8mg / mL MXene colloidal solution, sonicate for 5 minutes to ensure full saturation, and then vacuum dry at 60°C for 30 minutes. This paper-based sensing core layer can be optimized based on structural changes, such as using a paper base with dotted micro-convex structures, and evaluate the impact of the structure on sensing performance.

[0045] Step 3: Lay a 200mm × 200mm cellulose paper flat in a petri dish, add methyltrichlorosilane (MTS), and vacuum dry at 50°C for 5 minutes. Deposit the MTS onto the paper surface using chemical vapor deposition (CVD) to form a hydrophobic and breathable top layer for the paper-based encapsulation. The hydrophobicity of the encapsulation layer is controlled by adjusting the MTS content, while ensuring good breathability. Cut the top layer into 25mm × 30mm pieces.

[0046] Step 4: Screen printing interdigitated electrodes on a 25mm*35mm cellulose paper substrate. The size of the interdigitated electrodes is 15mm×30mm. The electrodes are fixed to the copper wires by welding to ensure reliable connection between the electrodes and the sensor.

[0047] Step 5: After printing the interdigitated electrodes, spread them evenly in a petri dish, add methyltrichlorosilane (MTS), vacuum dry at 50°C for 5 minutes, and deposit the MTS onto the paper substrate surface by chemical vapor deposition (CVD) to form a paper substrate encapsulation bottom layer with hydrophobic and breathable properties.

[0048] Step 6: Stack the materials from Steps 2, 3, and 5 sequentially according to the "upper layer - sensor core layer - lower layer" structure, and seal the edges of the sensor with 3M waterproof tape. At this point, the sensor exhibits high sensitivity, excellent humidity stability, and biodegradability, making it suitable for long-term wear and biosignal monitoring in harsh environments.

[0049] Step 7: Test the response characteristics, resistance changes, response time, and degradation performance using equipment such as a universal mechanical analyzer and an electrochemical workstation.

[0050] Example 1

[0051] This embodiment explores the influence of paper-based materials with different surface structures on the performance of a flexible pressure sensor, aiming to improve sensing sensitivity and signal stability through microstructure design. This embodiment fabricates a composite sensor and tests its performance through multiple steps, as detailed below:

[0052] Step 1: Add 2g of lithium fluoride (LiF) and 20mL of 9M concentrated hydrochloric acid (HCl) to a polytetrafluoroethylene reaction vessel and stir thoroughly with a magnetic stirrer until completely dissolved to form an activated etching solution. Then, slowly add 1g of Ti3AlC2 precursor powder, controlling the dropping rate to avoid local aggregation, and maintain the reaction temperature at 40℃ for 48 hours to achieve selective etching of the MAX phase. After the reaction is complete, the product is centrifuged repeatedly with deionized water (3500rpm, 5min) and the supernatant is replaced until the pH value is close to neutral (approximately 6). The resulting precipitate is MXene. Subsequently, MXene is redispersed in deionized water using ultrasonic treatment and concentrated to 8mg / mL to form a homogeneous and stable colloidal solution for later use.

[0053] Step 2: Prepare two types of paper-based sheets, each 15mm × 15mm in size: one is ordinary cellulose paper, and the other is a paper base with a 2mm diameter circular micro-convex lattice structure after being processed by a hot-pressing mold. Immerse both paper bases completely in an 8mg / mL MXene colloidal solution and treat them in an ultrasonic cleaner for 5 minutes to enhance the penetration and adhesion of the nanosheets. Then, place them in a 60℃ vacuum oven to dry for 30 minutes to remove excess solution and fix the conductive coating, obtaining two sensor core layers with different structures.

[0054] Step 3: Select 200mm × 200mm cellulose paper and lay it flat in a glass petri dish. Add approximately 1g of methyltrichlorosilane (MTS) reagent and perform vapor deposition at a constant temperature of 50°C for 5 minutes using a closed CVD apparatus. After deposition, remove the paper, desorb the residue under vacuum, and cut it into 25mm × 30mm pieces for use as the top layer for encapsulation.

[0055] Step 4: Apply silver paste to a 25mm × 35mm cellulose paper substrate and screen print interdigitated electrodes with a size of 15mm × 30mm. After drying, solder the electrodes with copper wire to secure the electrical interface and ensure stable and reliable signal output.

[0056] Step 5: Place the paper substrate with the completed electrode printing in a petri dish for MTS vapor deposition treatment (50℃, 5min) to give it hydrophobic and breathable properties, and cut it into an encapsulation base layer that matches the electrode size.

[0057] Step 6: Stack the materials in the order of "top sealing layer - sensor core layer - bottom sealing layer", seal the edges with 3M waterproof medical tape, and complete the overall sensor assembly.

[0058] Step 7: Using a precision pressure loading device, perform multiple sets of repeated tests on the two types of paper-based sensors within the range of 0-60 kPa, record the current changes, and calculate the sensitivity. For example... Figure 3 As shown in the figure, the test results indicate that the paper-based sensor with a micro-convex structure has a sensitivity of 39.58 kPa in the range of 0-1.01 kPa. -1 The value is 11.95 kPa in the range of 1.01-60 kPa. -1 The Pa level was significantly better than that of the control group at 4.16 kPa. -1 (0-1.01kPa) and 0.35kPa -1 (1.01-60 kPa). In addition, the signal response curve of the micro-convex structure sensor is smoother, and 11.95 indicates that the surface microstructure design helps to concentrate stress and enhance electrical signal induction, thereby improving the overall performance.

[0059] Example 2

[0060] This embodiment investigates the effect of different concentrations of MXene colloidal solution on the performance of a flexible pressure sensor. Six concentrations of 1–14 mg / mL were prepared to explore their effects on current response and sensitivity.

[0061] This embodiment involves fabricating the composite sensor and testing its performance through multiple steps, as detailed below:

[0062] Step 1: Three different concentrations of MXene colloidal solutions were prepared. Ti3AlC2 powder was etched using a LiF / HCl mixture to obtain MXene material, which was then centrifuged and washed until the pH value was close to 6. The resulting MXene precipitate was ultrasonically exfoliated and diluted with deionized water to prepare colloidal solutions with concentrations of 1 mg / mL, 2 mg / mL, 4 mg / mL, 8 mg / mL, 10 mg / mL, and 14 mg / mL, respectively, ensuring accurate and comparable concentrations.

[0063] Step 2: Six groups of 15mm×15mm cellulose paper were immersed in MXene colloids of three different concentrations and ultrasonically treated for 5 minutes to promote uniform penetration. They were then dried in a vacuum drying oven at 60℃ for 30 minutes to obtain conductive core layers with different MXene loadings.

[0064] Step 3: The top layer of the encapsulation is uniformly made of 25mm×30mm paper base and treated with MTS hydrophobicity by CVD method: the filter paper is placed in a petri dish containing 1g MTS and treated in a vacuum drying oven at 50℃ for 5 minutes to obtain an encapsulation layer with good hydrophobicity and air permeability.

[0065] Step 4: The bottom paper base has a size of 25mm×35mm, and the printed interdigitated silver paste electrodes have a size of 15mm×30mm. They are connected by copper wire welding, and after drying and curing, they are subjected to MTS hydrophobic treatment.

[0066] Step 5: Assemble the three core layers with the unified top and bottom layers of the package, and bond and seal them according to the "top-middle-bottom" structure to ensure package consistency.

[0067] Step 6: Use an electrochemical workstation to test the IV response, dynamic response time and sensitivity of the sensor at three concentrations, and evaluate the effect of MXene concentration on conductivity and performance output.

[0068] like Figure 4 As shown, the fabricated sensor exhibits a clear and stable current response under different applied pressures. The output signal increases progressively with increasing pressure, and the response curve shows no significant hysteresis, demonstrating good repeatability. This further verifies the device's sensitive response to external mechanical stress. In particular, the 8 mg / mL MXene concentration sample exhibits significant current changes in the low-pressure region (0–1.01 kPa), demonstrating excellent sensitivity, consistent with the aforementioned test results. This indicates that the fabrication method of this invention can effectively realize the construction of high-performance flexible sensors.

[0069] Experimental results show that the performance of the fabricated sensor is significantly affected by the concentration of MXene, exhibiting a clear optimal concentration window. Among the sensors prepared under different concentration conditions, the sample with a concentration of 8 mg / mL showed the best performance, achieving a sensitivity of 39.58 kPa in the pressure range of 0-1.01 kPa. -1 The value is 11.95 kPa in the range of 1.01-60 kPa. -1 The response times are 93ms and 69ms, respectively, combining high sensitivity with fast response characteristics. In contrast, at a concentration of 4 mg / mL, due to insufficient conductive material loading, the sensor output signal is weaker, and the sensitivity is generally lower than 35 kPa. -1However, the 14 mg / mL sample exhibited excessive MXene layer stacking, leading to delayed stress transmission and a decreased response speed, with response times generally exceeding 100 ms. These results validate that 8 mg / mL is the optimal concentration, achieving synergistic optimization of sensitivity and response speed.

[0070] Example 3

[0071] This embodiment studies the effect of the amount of MTS added to the encapsulation layer on the hydrophobic properties of the sensor surface, and then analyzes its impact on gas exchange and long-term performance.

[0072] This embodiment completes the adjustment and testing of the packaging processing time parameters through multiple steps, as follows:

[0073] Step 1: Hydrogen chloride (HCl) and lithium fluoride (LiF) were added to the reaction vessel and stirred until completely dissolved. Then, titanium aluminum carbide (Ti3AlC2) powder was slowly added, and the reaction was maintained at 40°C for 48 hours. After the reaction was completed, the solution was poured into a centrifuge tube, deionized water was added, and the mixture was centrifuged to remove the supernatant. This process was repeated until the pH of the water was close to 6. Next, the MXene colloidal solution was concentrated to 8 mg / mL and sonicated to ensure uniform dispersion of the MXene nanosheets.

[0074] Step 2: Immerse the cut 15mm×15mm rectangular cellulose paper in an 8mg / mL MXene colloidal solution and sonicate for 5 minutes to ensure full saturation. Then, vacuum dry the soaked cellulose paper at 60℃ for 30 minutes to remove excess solution and moisture.

[0075] Step 3: Prepare 200mm × 200mm cellulose paper and lay it flat in a petri dish. Add 0.5g, 1.0g, 1.5g, and 2.0g of methyltrichlorosilane (MTS) respectively, and place each petri dish in a vacuum dryer at 50°C for 5 minutes. MTS is uniformly deposited on the paper surface via chemical vapor deposition (CVD) to form a hydrophobic encapsulation layer. Adjust the hydrophobicity and permeability of the encapsulation layer according to the different amounts of MTS used. After drying, cut the paper base to a size of 25mm × 30mm to serve as the top encapsulation layer.

[0076] Step 4: Screen printing of interdigitated electrodes within a 25mm × 35mm area on the cellulose paper substrate. The interdigitated electrodes are 15mm × 30mm in size, and their design ensures a stable and reliable connection with the sensor. The electrodes are connected to copper wires by soldering to ensure reliable transmission of sensor signals.

[0077] Step 5: Based on the printed interdigitated electrodes, the paper substrate is laid flat again in a petri dish, and methyltrichlorosilane (MTS) is added for treatment. Under the same treatment conditions, vacuum drying is performed at 50°C for 5 minutes, and MTS is deposited on the surface of the paper substrate by chemical vapor deposition (CVD) to form a hydrophobic and breathable encapsulation underlayer.

[0078] Step 6: Stack the materials from Steps 2, 3, and 5 sequentially in the order of "upper layer - sensor core layer - lower layer," and seal the edges of the sensor with 3M waterproof tape to ensure the overall sealing and stability of the sensor. At this point, the sensor exhibits excellent sensitivity, humidity stability, and biodegradability, making it suitable for long-term wear and biosignal monitoring in harsh environments.

[0079] Step 7: Using a universal mechanical analyzer and an electrochemical workstation, the prepared sensor was tested for response characteristics, resistance changes, response time, and degradation performance. A systematic evaluation of sensors prepared under different MTS addition levels was conducted to analyze their impact on overall performance, particularly the balance between hydrophobicity and permeability. Test results show that, as... Figure 2 As shown, when the amount of MTS added is 1g, the surface water contact angle can reach 130°, which significantly improves the stability of the sensor in high humidity or water environment, while still maintaining good air permeability and response performance, verifying that the structure regulation effect is the best at this amount of addition.

[0080] Example 4

[0081] This embodiment verifies the adaptability of the sensor encapsulation layer in high humidity environments by simulating the use of the sensor under different humidity conditions.

[0082] This embodiment completes the performance verification and testing of the sensor under different humidity environments through multiple steps, as detailed below:

[0083] Step 1: Hydrogen chloride (HCl) and lithium fluoride (LiF) were added to the reaction vessel and stirred until completely dissolved. Then, titanium aluminum carbide (Ti3AlC2) powder was slowly added, and the reaction was carried out at 40°C for 48 hours. After the reaction was completed, the solution was poured into a centrifuge tube, deionized water was added, and the mixture was centrifuged. The supernatant was removed, and this step was repeated until the pH of the water was close to 6. Next, the MXene colloidal solution was concentrated to 8 mg / ml and sonicated to ensure uniform dispersion of the MXene nanosheets.

[0084] Step 2: Immerse the cut 15mm×15mm micro-convex fiber paper in an 8mg / mL MXene colloidal solution, sonicate for 5 minutes to ensure full saturation, and then vacuum dry at 60℃ for 30 minutes.

[0085] Step 3: Lay a 200mm × 200mm cellulose paper flat in a petri dish, add 1g of methyltrichlorosilane (MTS), and vacuum dry at 50℃ for 5 minutes. Deposit the MTS onto the paper surface using chemical vapor deposition (CVD) to form a paper-based encapsulation top layer with hydrophobic and breathable properties. The hydrophobicity of the encapsulation layer is controlled by adjusting the MTS content, while ensuring good breathability. Cut the top layer into 25mm × 30mm pieces.

[0086] Step 4: Screen printing interdigitated electrodes on a 25mm*35mm cellulose paper substrate. The size of the interdigitated electrodes is 15mm×30mm. The electrodes are fixed to the copper wires by welding to ensure reliable connection between the electrodes and the sensor.

[0087] Step 5: After printing the interdigitated electrodes, spread them evenly in a petri dish, add methyltrichlorosilane (MTS), vacuum dry at 50°C for 5 minutes, and deposit the MTS onto the paper substrate surface by chemical vapor deposition (CVD) to form a paper substrate encapsulation bottom layer with hydrophobic and breathable properties.

[0088] Step 6: Stack the materials from Steps 2, 3, and 5 in sequence according to the structure of "upper layer - sensor core layer - lower layer", and seal the edges of the sensor with 3M waterproof tape.

[0089] Step 7: Place the fabricated flexible sensor in environments with relative humidity of 20%, 50%, and 88%, respectively. Use a humidity control chamber to precisely adjust the ambient humidity and ensure that the sensor is equilibrated in each humidity environment for 30 minutes to ensure the stability and repeatability of the test environment.

[0090] Step 8: Under equilibrated humidity conditions, apply the same finger pressure to the sensor to perform a current response test. Record the current response curves of the sensor under different humidity conditions using an electrochemical workstation, and observe the stability of the current as humidity changes, especially the current decay.

[0091] Step 9: Analyze the test data, focusing on the stability and attenuation of the current. The results show that the sensor can still output a stable current response curve at 88% relative humidity, with almost no obvious attenuation, proving that its packaging structure has good moisture-proof and breathable properties, making it suitable for long-term wear in high-humidity environments, especially on moist skin surfaces.

[0092] Example 5

[0093] This embodiment aims to verify the dynamic response performance of the fabricated sensor in an underwater environment and the waterproof reliability of the encapsulation structure, and sets up a control group to evaluate the hydrophobic modification effect of the encapsulation layer.

[0094] This embodiment completes the hydrophobic modification of the encapsulation structure and underwater dynamic response performance testing through multiple steps, as detailed below:

[0095] Step 1: Hydrogen chloride (HCl) and lithium fluoride (LiF) were added to the reaction vessel and stirred until completely dissolved. Then, titanium aluminum carbide (Ti3AlC2) powder was slowly added, and the reaction was carried out at 40°C for 48 hours. After the reaction was completed, the solution was poured into a centrifuge tube, deionized water was added, and centrifugation was repeated until the solution pH was close to 6. The resulting MXene colloidal solution was concentrated to 8 mg / mL and sonicated to uniformly disperse the nanosheets.

[0096] Step 2: Immerse the cut 15mm×15mm rectangular cellulose paper in MXene colloidal solution, sonicate for 5 minutes to fully impregnate it, and then vacuum dry at 60℃ for 30 minutes to obtain the MXene-loaded paper-based sensing core layer.

[0097] Step 3: Prepare two types of encapsulation layers from 200mm × 200mm cellulose paper:

[0098] Experimental group: The paper base was modified with methyltrichlorosilane (MTS) for hydrophobicity and then vacuum dried at 50°C for 5 minutes to form a hydrophobic and breathable encapsulation layer, which was cut into 25mm×30mm pieces as the top layer.

[0099] Control group: The original paper base without MTS treatment was retained, with consistent dimensions, and used for comparison of unmodified packaging structures.

[0100] Step 4: Print interdigitated electrodes of size 15mm×30mm on 25mm×35mm cellulose paper, and connect copper wires to the electrodes by soldering to form a conductive path.

[0101] Step 5, process the electrode paper base from Step 4 as follows:

[0102] The bottom layer of the experimental group was modified by MTS vapor deposition to form a hydrophobic and breathable bottom layer;

[0103] Control group bottom layer: Unmodified and directly used as the encapsulation bottom layer.

[0104] Step 6: Assemble the components in the following order: top layer (hydrophobic or unmodified) - sensor core layer - bottom layer (hydrophobic or unmodified). Seal the edges with 3M waterproof tape to form two sensor structures: experimental group and control group.

[0105] Step 7: Completely immerse both types of sensors in water, ensuring a tight seal. Then, wear them on the test subject's elbow to simulate actual wear.

[0106] Step 8: Repeatedly bend the elbow (30°) in an underwater environment and record the current change curve in real time using an electrochemical workstation.

[0107] Step 9: Compare the current response results of the experimental group and the control group in the underwater dynamic strain test. For example... Figure 5 As shown in the figure, the experimental results show that the sensor in the experimental group with hydrophobic encapsulation can stably output clear electrical signals underwater, with high response rate and good repeatability; while the control group suffers signal attenuation or even loss due to water absorption by the encapsulation, indicating that hydrophobic encapsulation has significant advantages in underwater applications.

[0108] Example 6

[0109] This embodiment aims to evaluate the environmental degradability of the prepared sensor, examine its structural and performance changes under typical degradable conditions, and especially compare the effects of different encapsulation layer materials on the overall degradation behavior.

[0110] This embodiment completes the degradation performance testing of the sensor and the comparison of the encapsulation layer material through multiple steps, as detailed below:

[0111] Step 1: Hydrogen chloride (HCl) and lithium fluoride (LiF) were added to the reaction vessel and stirred until completely dissolved. Then, titanium aluminum carbide (Ti3AlC2) powder was slowly added, and the reaction was carried out at 40°C for 48 hours. After the reaction was complete, the reaction mixture was poured into a centrifuge tube, deionized water was added, and the mixture was centrifuged to remove the supernatant. Centrifugation and washing were repeated until the pH of the supernatant was close to 6. Subsequently, the MXene colloidal solution was concentrated to 8 mg / mL and sonicated to ensure uniform dispersion of the nanosheets.

[0112] Step 2: Immerse the cut 15mm × 15mm rectangular cellulose paper into the prepared 8mg / mL MXene colloidal solution and sonicate for 5 minutes to promote full adsorption. Then, vacuum dry at 60℃ for 30 minutes to obtain the MXene-loaded paper-based sensing core layer. The structural performance of this core layer can also be optimized by using a dotted micro-convex structure paper base.

[0113] Step 3: Lay a 200mm×200mm paper base material flat in a petri dish, add 1g of methyltrichlorosilane (MTS), vacuum dry at 50℃ for 5 minutes, and deposit MTS on the paper surface by chemical vapor deposition (CVD) to form a paper base encapsulation top layer with hydrophobic and breathable properties. Cut it into 25mm×30mm pieces for later use.

[0114] Step 4: Print 15mm×30mm interdigitated electrode patterns on a 25mm×35mm cellulose paper substrate and connect them by copper wire welding to ensure stable conduction between the sensor and the external circuit.

[0115] Step 5: The electrode paper layer obtained in Step 4 is laid flat in a culture dish, and the MTS vapor deposition process is repeated to obtain the hydrophobic modified paper-based encapsulation bottom layer.

[0116] Step 6: Inject PDMS prepolymer (10:1 ratio with curing agent) between the core layer and the interdigitated electrode layer. After degassing, let it stand at room temperature for 4 hours, and then cure it at 80°C for 1 hour to obtain the top and bottom layers of PDMS encapsulation.

[0117] Experimental group: The encapsulation layer is a hydrophobic and breathable layer formed by modifying paper base with methyltrichlorosilane (MTS);

[0118] Comparison group: The encapsulation layer uses polydimethylsiloxane (PDMS) for encapsulation, and other structures are the same.

[0119] Step 7: Stack the materials in the "upper layer - core layer - lower layer" structure. The experimental group is sealed with 3M waterproof tape; the control group is completely wrapped and cured with PDMS.

[0120] Step 8: To evaluate the biodegradability of the fabricated flexible pressure sensor in the environment, the HBF sensor (experimental group) with the three-layer MTS encapsulation structure used in this invention and the commercially available PDMS encapsulated sensor (control group) were buried in horticultural soil at a depth of 10 cm and placed outdoors in a natural environment for long-term degradation testing. The soil used was purchased from Peilei Horticulture Co., Ltd., and the soil composition was uniform to maintain consistency in the test. The entire test period was 70 days, and the appearance changes of the samples were recorded regularly every week to compare the degradation behavior of the two types of sensors.

[0121] The results are as follows Figure 7As shown, the HBF sensor maintained its structural integrity for the first 7 days. From day 28, signs of degradation appeared, including edge lifting, color fading, and material loosening. By day 49, the sensor surface structure was further damaged, transparency decreased, and it approached a state of natural decomposition. By day 70, only fragments remained, indicating near-complete degradation. In contrast, the PDMS sensor remained intact throughout the entire cycle, showing no significant structural or color changes and no degradation characteristics were observed. This comparison demonstrates that the MTS-modified paper-based structure of this invention exhibits excellent biodegradability, gradually decomposing in natural soil, significantly outperforming traditional non-degradable polymer encapsulation materials, demonstrating good environmental friendliness and sustainable application potential. The results show that the proposed MTS-modified paper-based three-layer structure not only significantly outperforms traditional PDMS films and unmodified cellulose paper structures in terms of air and water vapor permeability, but also provides better skin comfort and environmental adaptability during prolonged wear, making it suitable for complex high-humidity applications such as frequent sweating and real-time physiological signal acquisition. This encapsulation strategy possesses good biocompatibility, breathability, and biodegradability, making it an ideal encapsulation solution for flexible wearable electronic devices.

[0122] Example 7

[0123] This embodiment aims to verify the comprehensive advantages of the three-layer encapsulation structure based on MTS modified paper used in this invention in flexible wearable sensor applications by comparing the breathability and skin compatibility of different encapsulation materials and structures.

[0124] This embodiment evaluates the performance of different packaging structures through multiple steps, as follows:

[0125] Step 1: Hydrochloric acid (HCl) and lithium fluoride (LiF) were added to a reaction vessel and stirred until completely dissolved. Then, Ti3AlC2 precursor powder was slowly added. After reacting at 40°C for 48 hours, the resulting mixture was centrifuged and washed, repeatedly adding deionized water and centrifuging until the pH of the supernatant was close to 6. Subsequently, the obtained MXene colloidal solution was concentrated to a mass concentration of 8 mg / mL and dispersed using ultrasound to obtain a stable and homogeneous MXene dispersion.

[0126] Step 2: Cut 15mm × 15mm cellulose paper with dotted microstructures and immerse it in the above MXene dispersion. Perform ultrasonic treatment for 5 minutes to ensure full penetration. Then, vacuum dry at 60°C for 30 minutes to obtain the MXene / paper-based sensing core layer with dotted microstructures.

[0127] Step 3: Place a 200mm × 200mm cellulose paper in a petri dish, add 1g of methyltrichlorosilane (MTS), and vacuum dry at 50°C for 5 minutes. Then, use chemical vapor deposition (CVD) to form a hydrophobic coating of MTS on the paper surface. After modification, cut the paper to 25mm × 30mm size for use as the top layer of the sensor encapsulation.

[0128] Step 4: On a 25mm × 35mm paper substrate, screen print interdigitated electrode patterns measuring 15mm × 30mm. After the electrodes dry, connect them to the external circuit via copper wire soldering to ensure reliable electrode conductivity.

[0129] Step 5: Using the same method as in Step 3, perform MTS treatment on a piece of paper substrate material again to obtain a modified encapsulation bottom layer with the same encapsulation performance as the top layer.

[0130] Step 6: Assemble the three-layer encapsulated sensor structure of the present invention by sequentially stacking the top layer of the MTS encapsulation obtained in Step 3, the MXene sensing core layer obtained in Step 2, and the bottom layer of the MTS encapsulation obtained in Step 5, and sealing the edges with 3M waterproof tape to construct a complete three-layer flexible sensor.

[0131] Step 7: To verify the advantages of the encapsulation structure of the present invention in terms of breathability and skin compatibility, a uniform MXene / dot-structured paper-based material was prepared as the sensing core layer using the method described in Step 2, and then the following comparative samples with different encapsulation structures were constructed:

[0132] Comparative Sample 1: MXene-loaded dot-shaped microstructure paper-based sensing core layer prepared according to the method described in step 2, without encapsulation;

[0133] Comparison Sample 2: Single-layer unmodified cellulose paper;

[0134] Comparative sample 3: Single-layer MTS modified paper layer;

[0135] Comparison Sample 4: Single-layer PDMS film (commercially available standard thickness);

[0136] Comparative Sample 5: Three-layer structure: cellulose paper – cellulose paper – cellulose paper;

[0137] Comparison Sample 6: Three-layer structure: cellulose paper – core layer – cellulose paper;

[0138] Comparison Sample 7: Three-layer structure: MTS encapsulation layer – core layer – MTS encapsulation layer (structure of this invention);

[0139] Comparison Sample 8: Three-layer structure: PDMS – core layer – PDMS.

[0140] Comparison Sample 9: Three-layer structure: PET – core layer – PET.

[0141] Comparison Sample 10: Three-layer structure: PI – core layer – PI.

[0142] Step 8: Air permeability test. Under constant temperature and humidity conditions (37℃, 90% relative humidity), the air permeability of the above samples was tested using a paper air permeability tester. Figure 6 As shown, the air permeability of the three-layer MTS packaging structure used in this invention (comparative sample 7) is 10.7 μm / (Pa·s), compared to 0.0310.7 μm / (Pa·s) for comparative sample 8, 0.01 μm / (Pa·s) for comparative sample 9, and 0.008 μm / (Pa·s) for comparative sample 10. It can be seen that the three-layer MTS packaging structure used in this invention is significantly superior to other packaging structures.

[0143] Step 9: Skin compatibility test. Sensors made from the present invention and comparative materials (PDMS encapsulation, PET encapsulation, PI encapsulation) were respectively attached to the surface of a human-like skin model. The model was kept in a constant temperature and humidity chamber at 36°C and 85% relative humidity. Skin surface response was tested after 1 hour and 12 hours. Test results showed that the structure of the present invention showed no obvious redness, dampness, or irritation after 12 hours; while the PDMS, PET, and PI encapsulated structures showed slight redness and swelling after 1 hour, and significant heat, dampness, and local discomfort after 12 hours.

Claims

1. A method for fabricating a highly stable, hydrophobic, and breathable all-paper-based flexible pressure sensor, characterized in that, Includes the following steps: (1) Immerse cellulose paper in MXene colloidal solution and dry to obtain an intermediate layer; (2) Methyltrichlorosilane chemical vapor deposition was performed on cellulose paper and cellulose paper after printing interdigitated electrodes to obtain the top layer and bottom layer of the encapsulation, respectively. (3) Stack the top layer, middle layer and bottom layer of the package in sequence, and seal the edges with waterproof tape to obtain the pressure sensor.

2. The method for fabricating a highly stable hydrophobic and breathable all-paper-based flexible pressure sensor according to claim 1, characterized in that, In step (1), the preparation of the MXene colloidal solution is as follows: lithium fluoride and concentrated hydrochloric acid are mixed to obtain an activated etching solution, Ti3AlC2 powder is added to react, the product is centrifuged, the upper layer solution is replaced until the pH value is close to neutral, the precipitate obtained is MXene, and then MXene is redispersed in water to form an MXene colloidal solution for later use.

3. The method for fabricating a highly stable hydrophobic and breathable all-paper-based flexible pressure sensor according to claim 2, characterized in that, The ratio of lithium fluoride to concentrated hydrochloric acid is 1g:(8-12)mL, and the mass ratio of Ti3AlC2 to lithium fluoride is 1:2; the reaction temperature after adding Ti3AlC2 is 30-50℃, and the reaction time is 40-50h.

4. The method for fabricating a highly stable hydrophobic and breathable all-paper-based flexible pressure sensor according to claim 3, characterized in that, The concentration of the MXene colloidal solution is (4-10) mg / mL.

5. The method for fabricating a highly stable hydrophobic and breathable all-paper-based flexible pressure sensor according to claim 4, characterized in that, The concentration of the MXene colloidal solution was (8±1) mg / mL.

6. The method for fabricating a highly stable hydrophobic and breathable all-paper-based flexible pressure sensor according to claim 5, characterized in that, The cellulose paper described in step (1) is processed by a hot pressing mold and has a micro-convex dot matrix structure.

7. The method for fabricating a highly stable hydrophobic and breathable all-paper-based flexible pressure sensor according to claim 1, characterized in that, In step (2), the method for performing methyltrichlorosilane chemical vapor deposition is as follows: cellulose paper or cellulose paper after printing interdigitated electrodes is laid flat in a petri dish, 0.5-2g of methyltrichlorosilane is added, and the mixture is dried at 40-60℃ for 3-10 minutes.

8. The method for fabricating a highly stable hydrophobic and breathable all-paper-based flexible pressure sensor according to claim 7, characterized in that, The amount of methyltrichlorosilane added is (1±0.2)g.

9. The method for fabricating a highly stable hydrophobic and breathable all-paper-based flexible pressure sensor according to claim 8, characterized in that, The dimensions of the interdigitated electrode are 15mm × 30mm.

10. A highly stable, hydrophobic, and breathable all-paper-based flexible pressure sensor prepared by the method described in any one of claims 1 to 9.

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