Flexible dynamic breathing waterproof packaging structure, wearable sweat sensor and preparation method of wearable sweat sensor

Abstract

The invention relates to the technical field of wearable electronic device packaging, in particular to a flexible dynamic breathing waterproof packaging structure, a wearable sweat sensor and a preparation method thereof.The waterproof packaging structure comprises a breathable nanofiber membrane, a breathable waterproof ePTFE membrane and a microfluidic channel layer; the invention further provides a wearable sweat sensor based on the packaging structure, the wearable sweat sensor further comprises a graphene electrode structure and a PI film on the basis of the packaging structure, an independently researched and developed electrostatic spinning nanofiber membrane is selected as a part of a bottom-layer packaging substrate, a sufficient channel is provided for skin respiration, and the sweat can be absorbed through the graphene electrode structure. A layer of ePTFE film is attached to the surface of the nanofiber film to form a packaging substrate of the sweat sensor, dynamic contradictions are solved through a static structure, the wearing comfort is improved, and meanwhile the signal stability under high sweat secretion is ensured through a passive protection mechanism. According to the wearable sweat sensor, the comfort and reliability of long-term wearing of the wearable sweat sensor are remarkably improved, and the continuous wearing time is greatly prolonged.

Description

Technical Field

[0001] This invention relates to the field of wearable electronic device packaging technology, and in particular to a flexible dynamic breathable-waterproof packaging structure suitable for wearable sweat sensors, a wearable sweat sensor and its preparation method. Background Technology

[0002] Wearable sweat sensors, as a core carrier of non-invasive health monitoring technology, need to be in close contact with the skin for extended periods. The performance of their encapsulation structure determines the device's practicality and user acceptability. The core technical challenge of this encapsulation structure lies in the dynamic balance between breathability and waterproofing: on the one hand, it needs to ensure efficient gas exchange between the skin and the external environment to prevent localized skin temperature increases and humidity buildup due to prolonged wear, which could lead to stuffiness, inflammation, or allergic reactions; on the other hand, it needs to effectively prevent sweat containing electrolytes, proteins, and other detectable components from penetrating into the sensor, preventing short circuits, electrode corrosion, or cross-contamination of biomarkers, thus ensuring detection accuracy and device lifespan.

[0003] Existing packaging technologies have significant limitations, specifically:

[0004] The performance contradictions of traditional sealing materials: While commonly used encapsulation materials such as medical double-sided tape and polydimethylsiloxane (PDMS) can provide a certain degree of waterproofing, their dense molecular chains result in insufficient gas permeability (typically ≤500g / (m²)). 2 Wearing a product for more than 24 hours (24 hours) for an extended period can easily disrupt the skin's microenvironment, such as excessive hydration of the stratum corneum and bacterial imbalance.

[0005] Functional limitations of porous materials: Although porous films, represented by polyethylene (PE) and polyethersulfone (PES), can improve air permeability (gas permeability can reach 800-1000 g / (m³)), they still exhibit certain limitations. 2 However, the pore structure is open, allowing sweat to easily penetrate through capillary action. Furthermore, its pore size and distribution are fixed, making it impossible to dynamically adjust the breathability according to the sweat secretion rate (e.g., <0.5μL / min at rest, >3μL / min during exercise). This results in excessive breathability and severe heat loss when sweat secretion is low, and insufficient protection when sweat secretion is high, leading to sweat penetrating into the sensor.

[0006] Lack of dynamic response mechanism: Existing structures mostly rely on the inherent properties of materials and cannot dynamically adjust to changes in the skin microenvironment (such as humidity and temperature) and respond accordingly. For example, when the human body changes from a static state to strenuous exercise, the amount of sweat secretion surges, and traditional encapsulation cannot enhance waterproofing in a short time; after exercise, it is difficult to quickly restore breathability to expel accumulated moisture, resulting in the sensor-skin contact interface being in a high-humidity state for a long time, affecting detection stability and wearing comfort.

[0007] In addition, biocompatibility is a key factor limiting the practical application of encapsulation structures. While some waterproof coatings (such as fluoropolymers) can improve hydrophobicity, they may cause skin irritation; and residual monomers in adhesives (such as acrylates) may cause allergic reactions, making them particularly unsuitable for people with sensitive skin.

[0008] Therefore, developing a packaging structure that combines dynamic breathability and waterproofing, excellent biocompatibility, and simple manufacturing process is of great scientific significance and commercial value for breaking through the application bottleneck of wearable sweat sensors and promoting their large-scale application in chronic disease management, sports health monitoring, and other fields. Summary of the Invention

[0009] To address the aforementioned technical problems, this invention provides a flexible dynamic breathable waterproof encapsulation structure for wearable sweat sensors, a wearable sweat sensor, and a method for preparing the same.

[0010] This invention provides a flexible, dynamic, breathable, and waterproof encapsulation structure for a wearable sweat sensor, comprising a breathable nanofiber membrane, a breathable and waterproof ePTFE membrane, and a microfluidic channel layer. The breathable and waterproof ePTFE membrane is attached to the upper surface of the breathable nanofiber membrane to form the encapsulation substrate of the sensor. The non-detection area of ​​the breathable and waterproof ePTFE membrane is provided with a laser micropore structure. The microfluidic channel layer is located in the detection area of ​​the breathable and waterproof ePTFE membrane, and the microfluidic channel layer is provided with channels, a liquid reservoir, and several independent inlets. The breathable and waterproof ePTFE membrane is provided with sweat transport channels at the positions corresponding to the independent inlets.

[0011] Furthermore, the thickness of the breathable nanofiber membrane is 80±5μm, the fiber diameter is 100-300nm, and the gas permeability at room temperature is above 1200g / (m²・24h); the thickness of the breathable and waterproof ePTFE membrane is 100±10μm, and the non-detection area of ​​the breathable and waterproof ePTFE membrane is provided with a laser micropore structure, the pore diameter of the laser micropore structure is 30μm, the pore density is 120 pores / cm², and the pore wall is provided with a micro-nano structure with a roughness Ra=0.8-1.0μm; the thickness of the microfluidic channel layer is 140μm±5μm, the radius of the liquid storage pool is 2mm, the depth is the same as the thickness of the microfluidic channel layer, the width of the channel is 180-200μm, and the depth is the same as the thickness of the microfluidic channel layer, and several independent inlets are arranged around the liquid storage pool at equal intervals and angles, the diameter of the independent inlets is 0.5mm, and the depth is the same as the thickness of the microfluidic channel layer.

[0012] Furthermore, the present invention also provides a method for preparing the above-mentioned packaging structure, comprising the following steps:

[0013] Step 1: Preparation of breathable nanofiber membrane:

[0014] 1.1 Preparation of electrospinning solution: A certain amount of polyethersulfone (PES) and poly(N-isopropylacrylamide) (PNIPAM) were dissolved in N-dimethylformamide (DMF) solution at a mass ratio of 6:4. The solution was then placed in an ultrasonic cleaner with the temperature set to 0°C and stirred thoroughly to obtain the electrospinning solution.

[0015] 1.2. The uniformly mixed electrospinning solution is loaded into a syringe with a metal needle. The syringe is then fixed to the injection pump by a clamp and advanced at a rate of 0.5 mL / h. A high-voltage electric field of 15-20 kV is applied to the needle tip to perform electrospinning, thereby generating a continuous jet stream. The resulting nanofibers are deposited on a grounded receiving roller. After spinning for a certain period of time, the prepared nanofiber membrane is removed.

[0016] Preferably, the distance from the needle to the receiving roller is 15-20cm, the receiving roller speed is 300-500r / min, the ambient temperature is 25±2℃, the humidity is 30%-40%, and the receiving roller is covered with a layer of silicone paper.

[0017] 1.3. The nanofiber membrane is dried in a vacuum oven to obtain a hierarchical pore structure nanofiber network with a fiber diameter of 100-300 nm and a gas permeability of more than 1200 g / (m²・24h) at room temperature, which is a gas permeable nanofiber membrane layer.

[0018] Preferably, the drying temperature in the vacuum oven is 60-70℃, and the drying time is 12-16h.

[0019] Step 2: Preparation of breathable and waterproof ePTFE membrane:

[0020] 2.1 Raw material preforming: High-purity PTFE dispersion resin with an average particle size of 450μm is mixed with liquid extrusion aid with a volume ratio of 15-25%, and the mixture is molded under a pressure of 10~15MPa to form a dense preform. The preform is then extruded into rod-shaped or sheet-shaped preforms by a pusher at 30-50℃, and then calendered into a film with a thickness of 0.1-0.5 mm by a two-roll calender.

[0021] Preferably, the roll temperature of the twin-roll calender is controlled at 60-80℃ to ensure that the thickness deviation is less than ±5%.

[0022] 2.2 Biaxial Stretching for Pore Formation: After calendering, the film is dried for a certain period of time to completely volatilize the extrusion aid, forming a uniformly dried PTFE base film. Then, biaxial stretching is performed at 150-200℃. The longitudinal stretching is performed at a stretching speed of 2-5 m / min to form preliminary fiber orientation. Then, the transverse stretching is performed at the same stretching speed, and a stress of 5-20 N is applied transversely to obtain a porous film, which causes the molecular chains to break and form a network structure with microfibers and nodes connected. The pore size is 0.1-10 μm and the porosity is 80%-95%.

[0023] Furthermore, the pore size is controlled by adjusting temperature, speed, and stress. The specific steps are as follows: For a target pore size of 0.1-10μm, set the basic parameter combination of temperature, longitudinal stretching speed, and transverse stretching stress: If a small pore size of 0.1-1μm is required, control the temperature at 150-180℃, the longitudinal stretching speed at 3-5m / min, and apply a low stress of 5-10N in the transverse direction. Utilize low temperature to reduce material ductility, high speed to shorten molecular chain alignment time, and low stress to reduce deformation and limit the pore size; If a medium pore size of 1-10μm is required, use a medium temperature of 180-200℃, a medium speed of 2-3m / min, and a medium stress of 10-20N. A balanced pore size is formed through appropriate ductility and molecular chain breakage.

[0024] 2.3 Shaping and Modification: The stretched porous membrane is sintered at 320-330℃ for 1-10 min, and finally fluorinated on the membrane surface to make the contact angle greater than 150°, thus obtaining expanded polytetrafluoroethylene (ePTFE) membrane.

[0025] Preferably, the fluorination treatment steps are as follows: When fluorinating the ePTFE membrane, first immerse the membrane in an ethanol solution and ultrasonically clean it for 8-12 minutes. Then, irradiate it with ultraviolet light for 1-3 minutes, followed by heating at 70-80℃ for 3-5 hours. After cooling to room temperature, heat it again in an oven at 70-80℃ for 35-40 minutes to complete the surface pretreatment and remove impurities and moisture. Then, use fluorine gas as the fluorinating agent and dilute it with nitrogen gas. Control the reaction temperature at 50-60℃, maintain the pressure at 0.6-0.8 MPa, and the reaction time for 15-20 hours. Under these conditions, the fluorination reaction is carried out to form a fluorinated layer on the membrane surface. After the reaction, use a contact angle meter to detect the contact angle of the membrane surface. If it is greater than 150°, clean the membrane with deionized water or an organic solvent to remove residual reagents and byproducts. If the standard is not met, repeat the above fluorination treatment steps.

[0026] Step 3: Laser micro-hole structure fabrication:

[0027] In the non-testing area of ​​the breathable and waterproof ePTFE membrane, femtosecond lasers were used to process pores with a diameter of 30 μm and a pore density of 100-120 pores / cm². The scanning speed was 150 μm / s, forming a micro-nano structure with a roughness Ra of 0.8-1.0 μm on the pore wall to enhance gas convection.

[0028] Simultaneously, femtosecond lasers are used to process sweat transport channels in the detection area of ​​the breathable and waterproof ePTFE membrane, corresponding one-to-one with the independent entrances of the microfluidic channel layer.

[0029] As a preferred option, the femtosecond laser processing parameters are: a 1030nm femtosecond laser, a pulse width of 80fs, a repetition frequency of 2kHz, and an average power of 70-90mW.

[0030] Step 4: Laser etching to prepare the microfluidic channel layer:

[0031] First, a PET film of a certain size and a thickness of 140μm±5μm is fixed on a substrate. A CO2 laser cutting machine is used to precisely carve the liquid storage pool, channels and independent inlets to obtain a liquid storage pool with a radius of 2mm and a depth equal to the thickness of the microfluidic channel layer, a channel structure with a width of 180-200μm and a depth equal to the thickness of the microfluidic channel layer, and several independent inlets with a diameter of 0.5mm and a depth equal to the thickness of the microfluidic channel layer. The independent inlets are arranged at equal intervals and angles around the circular liquid storage pool, with each independent inlet 3mm away from the center of the liquid storage pool. The channels connect the liquid storage pool and the independent inlets.

[0032] Step 5, Packaging:

[0033] Connect the layers from bottom to top in the following order: breathable nanofiber membrane, breathable and waterproof ePTFE membrane, and microfluidic channel layer.

[0034] First, the surfaces of the breathable nanofiber membrane and the breathable waterproof ePTFE membrane are cleaned. After cleaning, a breathable nanofiber membrane of a certain size and a thickness of 80±5μm is laid flat and fixed on a flat and clean work surface. A breathable waterproof ePTFE membrane of the same size and a thickness of 100±10μm is accurately aligned and covered on the breathable nanofiber membrane. The two membranes are tightly bonded together by heat pressing to form an encapsulation substrate. The encapsulation substrate is bonded to the microfluidic channel layer with medical silicone adhesive. The microfluidic channel layer is attached to the detection area of ​​the breathable waterproof ePTFE membrane. The independent entrances on the microfluidic channel layer are precisely aligned with the sweat transport channels on the breathable waterproof ePTFE membrane to achieve waterproof encapsulation of the wearable sweat sensor.

[0035] The present invention also provides a wearable sweat sensor based on the above-mentioned encapsulation structure. In addition to the above-mentioned encapsulation structure, it further includes a graphene electrode structure and a PI film. The graphene electrode structure is directly fabricated on the surface of the PI film. The graphene electrode structure and the PI film are sequentially disposed above the microfluidic channel layer. The functional area of ​​the graphene electrode structure corresponds to the liquid reservoir on the microfluidic channel layer. The size of the PI film is the same as that of the microfluidic channel layer, and the thickness is 75μm±5μm. The graphene electrode includes three electrodes: a counter electrode, a working electrode, and a reference electrode.

[0036] As a preferred embodiment, the graphene electrode shape is described as follows:

[0037] Working electrode: The functional area is the central circular portion, connected to a long rod; Counter electrode: The functional area is the annular portion surrounding the outer edge of the working electrode, with a diameter smaller than the diameter of the reservoir on the microfluidic channel layer, also connected to a long rod, and is on the same plane as the working electrode functional area, forming a semi-encircling shape; Reference electrode: The functional area is the arc-shaped segment near the working electrode circular portion, connected to a long rod, located next to the gap between the working electrode and counter electrode functional areas. The three functional areas are compactly arranged at the same end, with the rod extending downwards in parallel; The total length of the graphene electrode is less than the length of the microfluidic channel layer, ensuring that the graphene electrode is aligned with the microfluidic channels on the microfluidic channel layer.

[0038] This invention also provides a method for fabricating the above-mentioned wearable sweat sensor, which, based on the above-mentioned method for fabricating the encapsulation structure, further includes step 6: laser printing to fabricate a graphene electrode structure.

[0039] A commercial PI film with the same size and a thickness of 75μm as the microfluidic channel layer was used as the sensor printing substrate. Graphene electrode structures were generated on the PI film in grating mode using a 50W CO2 laser engraving machine.

[0040] It also includes step 7, assembly:

[0041] A PI film with a graphene electrode structure is placed above a microfluidic channel layer. The functional area of ​​the graphene electrode structure corresponds to the liquid reservoir on the microfluidic channel layer. The PI film is aligned with the microfluidic channel layer, and the PI film with the graphene electrode structure is connected to the microfluidic channel layer using pressure-sensitive adhesive.

[0042] Working principle of the invention:

[0043] In this invention, the breathable nanofiber membrane is constructed using a self-developed electrospun nanofiber membrane as part of the underlying encapsulation substrate. This membrane is made by blending polyethersulfone (PES) with thermosensitive poly(N-isopropylacrylamide) (PNIPAM), with a thickness precisely controlled at 80±5 μm. By optimizing the electrospinning parameters and controlling the fiber diameter within the range of 100-300 nm, a nanofiber network with a hierarchical pore structure is formed, ensuring a gas permeability greater than or equal to 1200 g / (m²・24h) at room temperature, providing ample channels for skin respiration.

[0044] This invention utilizes a breathable and waterproof ePTFE (expanded polytetrafluoroethylene) membrane attached to the surface of a nanofiber membrane to form the encapsulation substrate for a sweat sensor. The core principle of the ePTFE membrane lies in its microporous gradient structure with pore sizes of 0.1-10 μm. This, combined with its superhydrophobic properties, results in micropores significantly larger than the diameter of water vapor molecules (0.4-0.5 nm), allowing for efficient water vapor diffusion (breathability up to 8000-10000 g / (m²・24h)), but much smaller than liquid water clusters (100 μm). This, combined with high surface tension, forms a "breathable but waterproof" physical barrier. This design ensures high breathability under low sweat secretion conditions, avoiding heat retention and the risk of skin inflammation; under high sweat secretion conditions, the superhydrophobic surface blocks liquid sweat penetration, preventing interference with electrode signals. The ePTFE membrane is directly attached to the nanofiber membrane surface, eliminating the need for in-situ polymerization or coating, and is compatible with existing encapsulation technologies. This material has a temperature resistance range of -40℃ to 150℃, is resistant to sweat corrosion, and costs less than ¥0.5 / cm². 2 It supports large-scale production. The ePTFE membrane resolves dynamic contradictions with a static structure, improving wearing comfort while ensuring signal stability under high sweat secretion through a passive protection mechanism.

[0045] This invention enhances air permeability through laser microstructuring. It utilizes a femtosecond laser to perform microstructuring on non-detection areas of the encapsulation material, fabricating micron-sized air pores with a diameter of 30 μm and a designed pore density of 120 pores / cm². 2 These micron-sized pores work in conjunction with the hierarchical pore structure of the nanofiber membrane to enhance overall breathability while ensuring waterproof performance. Furthermore, the micro- and nanostructures introduced onto the pore walls during femtosecond laser processing can induce localized air convection, thereby increasing gas exchange efficiency.

[0046] The microfluidic channel of this invention uses PET film as the material and is directly cut into shape in vector mode using CO2 laser engraving technology based on a channel model pre-drawn on computer software. A 75 μm thick commercial polyimide film (PI film) is used as the sensor substrate, and the corresponding three-dimensional graphene electrode structure is directly generated in grating mode using a CO2 laser engraving machine based on the graphic file pre-transmitted to the laser machine.

[0047] The beneficial effects of this invention are:

[0048] Compared to traditional packaging processes, the flexible, dynamic, breathable, and waterproof packaging structure and its fabrication method for wearable sweat sensors provided by this invention significantly improve the comfort and reliability of long-term wear. The continuous wear time is greatly extended from 8 hours using traditional methods to over 48 hours, during which the skin remains dry and comfortable without any stuffiness. In simulated strenuous exercise and sweating scenarios, no sweat penetration was observed inside the sensor, and the detection accuracy remained stable. This effectively ensures continuous and accurate dynamic monitoring of wearable sweat sensors in various complex daily environments, laying a solid foundation for the widespread application of wearable medical devices. Attached Figure Description

[0049] Figure 1 A schematic diagram illustrating the overall manufacturing process of a wearable sweat sensor.

[0050] Figure 2 This is an exploded view of the overall structure of a wearable sweat sensor.

[0051] Figure 3 This is a schematic diagram illustrating the principle of local convection between the sensor substrate and the skin surface.

[0052] Figure 4 The image shows the actual ePTFE membrane and a magnified SEM image of a portion thereof (scale: 100 μm).

[0053] Figure 5 The image shows the actual breathable nanofiber membrane and a magnified SEM image of a portion thereof (10 μm).

[0054] Figure 6 Optical images of the electrochemical sensor and a magnified SEM image (10 μm) showing its porous structure.

[0055] In the figure: 1. Breathable nanofiber membrane; 2. Breathable and waterproof ePTFE membrane; 3. Laser microporous structure; 4. Microfluidic channel layer; 5. Graphene electrode structure; 6. PI film. Detailed Implementation

[0056] The following is combined Figure 1-6As shown, specific embodiments of the present invention are further described. Unless otherwise specified, the experimental methods used in the embodiments are conventional methods, and the materials and reagents used are obtained commercially unless otherwise specified.

[0057] Example 1

[0058] This embodiment provides a flexible dynamic breathable-waterproof encapsulation structure for wearable sweat sensors, including a breathable nanofiber membrane 1, a breathable and waterproof ePTFE membrane 2, and a microfluidic channel layer 4. The breathable and waterproof ePTFE membrane 2 is attached to the upper surface of the breathable nanofiber membrane 1 to form the encapsulation substrate of the sensor. The non-detection area of ​​the breathable and waterproof ePTFE membrane 2 is provided with a laser micropore structure 3. The microfluidic channel layer 4 is located in the detection area of ​​the breathable and waterproof ePTFE membrane 2. The microfluidic channel layer 4 is provided with channels, a liquid reservoir, and several independent inlets. The breathable and waterproof ePTFE membrane 2 is provided with sweat transport channels at the positions corresponding to the independent inlets.

[0059] The breathable nanofiber membrane 1 has a thickness of 80±5μm, a fiber diameter of 100-300nm, and a gas permeability of 1200g / (m²・24h) or higher at room temperature; the breathable and waterproof ePTFE membrane 2 has a thickness of 100±10μm, and a laser micropore structure 3 is provided on the non-detection area of ​​the breathable and waterproof ePTFE membrane 2. The pore diameter of the laser micropore structure 3 is 30μm, the pore density is 120 pores / cm², and the pore wall is provided with a micro-nano structure with a roughness Ra=0.8-1.0μm; the microfluidic channel layer 4 has a thickness of 140μm±5μm, the reservoir has a radius of 2mm, a depth equal to the thickness of the microfluidic channel layer 4, a volume of approximately 2μL, a channel width of 180-200μm, a depth equal to the thickness of the microfluidic channel layer 4, and several independent inlets are arranged around the reservoir at equal intervals and angles. The diameter of each independent inlet is 0.5mm, and the depth is equal to the thickness of the microfluidic channel layer 4.

[0060] This embodiment also provides a method for preparing the above-mentioned packaging structure, including the following steps:

[0061] Step 1: Preparation of breathable nanofiber membrane 1:

[0062] 1.1 Preparation of electrospinning solution: Dissolve a certain amount of polyethersulfone (PES) and poly(N-isopropylacrylamide) (PNIPAM) in N-dimethylformamide (DMF) solution at a mass ratio of 6:4. Place the solution in an ultrasonic cleaner and stir for 15-20 minutes. Set the temperature of the cleaner to 0℃ and stir thoroughly to ensure that the substances in the solution are mixed evenly. If there are no impurities or bubbles in the solution after stirring, it indicates that the solution has been thoroughly stirred and homogeneous, and the electrospinning solution is obtained.

[0063] Electrospinning places extremely high demands on the stability of the solution (viscosity, surface tension, homogeneity, etc.), and the 0°C setting of ultrasonic treatment maintains these parameters within the optimal range required for spinning by counteracting the interference of ultrasonic heat generation. High temperatures can reduce solution viscosity, potentially causing excessive stretching of the jet, leading to excessively fine fibers or fiber breakage; therefore, a cleaning temperature of 0°C is selected.

[0064] 1.2 The uniformly mixed electrospinning solution is loaded into a 20mL syringe equipped with a 21G metal needle (0.5mm inner diameter). The syringe is then fixed to a syringe pump using a clamp, and the syringe is advanced at a rate of 0.5mL / h. A high-voltage electric field of 15-20kV is applied to the needle tip to perform electrospinning, thereby generating a continuous jet stream. The resulting nanofibers are deposited onto a grounded receiving roller, which is covered with a layer of silicone paper to facilitate removal of the nanofiber membrane from the receiver. The distance between the needle and the receiving roller is 15-20cm, the roller speed is 300-500r / min, the ambient temperature is 25±2℃, and the humidity is 30%-40%. After 3 hours of spinning, the silicone paper is removed from the receiving roller, and then the prepared nanofiber membrane is removed.

[0065] 1.3 Place the nanofiber membrane in a vacuum oven and dry it at 60-70℃ for 12-16 hours to obtain a hierarchical pore structure nanofiber network with a fiber diameter of 100-300nm and a gas permeability of more than 1200g / (m²・24h) at room temperature, namely, the air-permeable nanofiber membrane layer 1.

[0066] Step 2, Preparation of breathable and waterproof ePTFE membrane 2:

[0067] 2.1 Raw Material Preforming: High-purity PTFE dispersion resin with an average particle size of approximately 450μm is mixed with a liquid extrusion aid (such as paraffin oil) at a volume ratio of 15-25%. By adjusting the volume content of the liquid extrusion aid, suitable plasticity is ensured in subsequent extrusion and calendering processes, laying the foundation for the microporous structure of the final ePTFE film. A dense preform is formed by compression molding at 11MPa, 12MPa, or 13MPa, effectively expelling air from the preform and preventing structural defects caused by air bubbles in subsequent processing. This also moderately improves the plasticity of the material, making the preform easier to shape during subsequent extrusion and calendering. The preform is extruded into rod-shaped or sheet-shaped preforms at 30-50℃ using an extruder, and then calendered into films with a thickness of 0.3 mm or 0.4 mm using a two-roll calender. The roller temperature of the two-roll calender is controlled at 60-80℃ to ensure a thickness deviation of less than ±5%.

[0068] 2.2 Biaxial Stretching for Pore Formation: The calendered film is dried in a high-temperature oven at 120-200℃ for 10-30 minutes to completely volatilize the extrusion aid and form a uniformly dried PTFE base film, preventing micropore collapse during subsequent stretching; then, biaxial stretching is performed at 150-200℃, within this temperature range of PTFE. Within the material's softening temperature range, sufficient ductility is ensured to facilitate uniform fiber orientation during stretching, while avoiding excessively low temperatures that increase brittleness and cause breakage during stretching, or excessively high temperatures that lead to over-softening and difficulty in controlling fiber orientation stability, thus affecting the uniformity of the subsequent microporous structure. Longitudinal stretching forms the initial fiber orientation; then, transverse stretching is performed at the same stretching speed, causing molecular chains to break and forming a network structure with microfibers and nodes connected, achieving a porosity of 80%-95%. A speed range of 2~5 m / min can achieve ordered fiber orientation. Too slow a speed (e.g., 0.5 m / min) may result in low stretching efficiency, and prolonged residence time at high temperatures may lead to localized over-softening; too fast a speed (e.g., greater than 5 m / min) may cause excessive stretching force, leading to fiber breakage or uneven orientation, affecting the integrity of the network structure.

[0069] The pore size is controlled by adjusting temperature, speed, and stress. The specific steps are as follows: For a target pore size of 0.1-10μm, set the basic parameter combination of temperature, longitudinal stretching speed, and transverse stretching stress: If a small pore size of 0.1-1μm is required, control the temperature at 150-180℃, the longitudinal stretching speed at 3-5m / min, and apply a low stress of 5-10N in the transverse direction. The low temperature reduces the material's ductility, the high speed shortens the molecular chain alignment time, and the low stress reduces deformation to limit the pore size. If a medium pore size of 1-10μm is required, use a medium temperature of 180-200℃, a medium speed of 2-3m / min, and a medium stress of 10-20N. Through appropriate ductility and molecular chain breakage, a balanced pore size is formed, and finally a porous membrane is obtained.

[0070] 2.3 Shaping and Modification: The stretched porous membrane is sintered at 327℃ (PTFE melting point) for 3-5 minutes. The temperature range is close to the PTFE melting point, which ensures that the molecular chains are fully relaxed to eliminate internal stress, and avoids the collapse of the microporous structure due to excessive softening caused by excessive temperature (above 330℃). At the same time, it can stabilize and fix the network pore structure formed by stretching, taking into account both thermal stability and mechanical strength improvement. Finally, the membrane surface is fluorinated to make the contact angle greater than 150°, resulting in expanded polytetrafluoroethylene (ePTFE) membrane, which improves the hydrophobicity of ePTFE membrane and enables the membrane to achieve "air permeable but water impermeable" physical barrier.

[0071] The fluorination process is as follows: First, the ePTFE membrane is immersed in an ethanol solution and ultrasonically cleaned for 8-12 minutes. Then, it is irradiated with ultraviolet light for 1-3 minutes, followed by heating at 70-80°C for 3-5 hours. After cooling to room temperature, it is heated again in an oven at 70-80°C for 35-40 minutes to complete the surface pretreatment and remove impurities and moisture. Then, fluorine gas is used as the fluorinating agent and diluted with nitrogen gas. The reaction temperature is controlled at 50-60°C, the pressure is maintained at 0.6-0.8 MPa, and the reaction time is 15-20 hours. Under these conditions, the fluorination reaction is carried out to form a fluorinated layer on the membrane surface. After the reaction, the contact angle of the membrane surface is measured using a contact angle meter. If it is greater than 150°, the membrane is cleaned with deionized water or a suitable organic solvent to remove residual reagents and byproducts. If the standard is not met, the above fluorination process is repeated.

[0072] Step 3, Laser micro-hole structure fabrication:

[0073] The detection area of ​​the breathable and waterproof ePTFE membrane 2 is the same size (2cm × 0.8cm) as the microfluidic channel layer 4 and corresponds to that position. The remaining part of the breathable and waterproof ePTFE membrane 2 is the non-detection area. The purpose of setting the non-detection area is to increase the contact area with the skin. In the non-detection area of ​​the breathable and waterproof ePTFE membrane 2, a femtosecond laser is used to process a breathable hole with a diameter of 30μm and a pore density of 120 pores / cm². The femtosecond laser processing parameters are: 1030nm femtosecond laser, pulse width 80fs, repetition frequency 2kHz, average power 70mW. By controlling the scanning speed to 150μm / s, a micro-nano structure with a roughness Ra=0.8-1.0μm is formed on the pore wall to enhance gas convection.

[0074] Simultaneously, a femtosecond laser was used to process a sweat transport channel with a diameter of 0.5 mm in the detection area of ​​the breathable and waterproof ePTFE membrane 2, which corresponds to the independent entrance of the microfluidic channel layer 4. The femtosecond laser processing parameters were: a 1030 nm femtosecond laser, a pulse width of 80 fs, a repetition frequency of 2 kHz, and an average power of 90 mW; the scanning speed was controlled at 100 μm / s.

[0075] Step 4: Laser etching to prepare microfluidic channel layer 4:

[0076] The microfluidic channel layer 4 uses PET film as the core material, which is directly cut and shaped in vector mode using CO2 laser engraving technology: First, a 2cm×0.8cm×140μm±5μm PET film is fixed on the substrate, and then a 50W CO2 laser cutter is used to precisely engrave the liquid reservoir, channels, and independent inlets; the contour engraving of the liquid reservoir and channels uses a power of 2.5%, a speed of 5%, and a PPI of 5%. With a setting of 1000, the power for engraving the contours of the independent inlets is increased by 5%, the speed is reduced to 2%, and the PPI remains unchanged. The final result is a reservoir with a radius of 2mm, a depth equal to the thickness of the microfluidic channel layer 4, and a volume of approximately 2μL; a channel structure with a width of 180-200μm and a depth equal to the thickness of the microfluidic channel layer 4; and five independent inlets with a diameter of 0.5mm and a depth equal to the thickness of the microfluidic channel layer 4. These five independent inlets are centered on the circular reservoir, with adjacent inlets spaced at 45° intervals, and each inlet is 3mm from the center of the reservoir. Too few independent inlets (less than five) may limit the ability to operate multiple channels in parallel; too many inlets (more than five) will increase the design complexity and fabrication difficulty of the microfluidic channel layer. The design of five independent inlets strikes a balance between meeting the needs of multi-scenario detection and the complexity of control design, adapting to the functional positioning and process feasibility of this packaging structure.

[0077] Step 5, Packaging:

[0078] The components of the encapsulation structure are connected from bottom to top in the following order: breathable nanofiber membrane 1, breathable and waterproof ePTFE membrane 2, and microfluidic channel layer 4.

[0079] First, the surfaces of the breathable nanofiber membrane 1 and the breathable waterproof ePTFE membrane 2 must be cleaned to remove any dust, impurities, etc., ensuring a good bond. A lint-free cloth dampened with a small amount of anhydrous ethanol or similar cleaning agent can be used for gentle wiping, followed by air drying or a gentle drying method (such as hot air below 50°C) to remove any residual solvent. After cleaning, take a 3cm × 2cm × 80±5μm piece of the breathable nanofiber membrane 1 and lay it flat on a clean, flat work surface. Use clamps to secure it and prevent displacement during the bonding process. Next, accurately align and cover the 3cm × 2cm × 100±10μm breathable waterproof ePTFE membrane 2 (with a microporous structure) onto the breathable nanofiber membrane 1, ensuring precise positioning. Finally, use a hot-press bonding method with rollers or a press machine to apply even pressure from one end of the membrane to the other, ensuring a tight bond between the two layers. During the rolling process, control the pressure and speed carefully: pressure 1.5-2MPa, speed 0.3 km / h. -0.5m / s, temperature controlled at 80-120℃ to avoid generating bubbles or damaging the film, forming an encapsulation substrate.

[0080] The encapsulation substrate and the microfluidic channel layer 4 are bonded together with medical-grade silicone adhesive. The key functional areas of each layer are precisely aligned. The microfluidic channel layer 4 is bonded to the detection area of ​​the breathable and waterproof ePTFE membrane 2. The independent inlets on the microfluidic channel layer 4 are precisely aligned with the sweat transport channels on the breathable and waterproof ePTFE membrane 2 to achieve waterproof encapsulation of the wearable sweat sensor.

[0081] Example 2

[0082] This embodiment provides a wearable sweat sensor based on the above-described encapsulation structure. In addition to the encapsulation structure described in Embodiment 1, it further includes a graphene electrode structure 5 and a PI film 6. The graphene electrode structure 5 is directly fabricated on the surface of the PI film 6. The graphene electrode structure 5 and the PI film 6 are sequentially disposed above the microfluidic channel layer 4. The functional area of ​​the graphene electrode structure 5 corresponds to the liquid reservoir on the microfluidic channel layer 4. The PI film 6 has dimensions of 2cm × 0.8cm and a thickness of 75μm. The graphene electrode includes a counter electrode, a working electrode, and a reference electrode, with the shapes described below:

[0083] Working electrode: The functional area is the central circular part, connected to a long rod, which is used for signal conduction, etc.; Counter electrode: The functional area is the arc-shaped (or ring-like) part surrounding the outer edge of the working electrode's circle, with a diameter of 3.8 mm, smaller than the 4 mm diameter of the reservoir on the microfluidic channel layer. It is also connected to a long rod and is on the same plane as the working electrode's functional area, forming a semi-encircling shape; Reference electrode: The functional area is the arc-shaped (or narrow ring-like segment) near the working electrode's circle, connected to a long rod, located next to the gap between the working electrode and counter electrode's functional areas. The three functional areas are compactly arranged at the same end, with the rod extending downwards in parallel; The total length of the graphene electrode is 1.8 cm, smaller than the 2 cm length of the microfluidic channel layer, ensuring that the graphene electrode is aligned with the microfluidic channels on the microfluidic channel layer.

[0084] This embodiment provides a method for fabricating a wearable sweat sensor, which, based on the method in Embodiment 1, further includes step 6: laser printing to fabricate a graphene electrode structure 5.

[0085] A commercial PI film 6 measuring 2cm × 0.8cm × 75μm was used as the substrate for sensor printing. Graphene electrode structures 5 were generated on the PI film 6 using a 50W CO2 laser engraving machine in grating mode. The power was set to 6-7% to avoid penetrating the substrate, and the scanning speed was controlled at 5-6% to ensure sufficient carbonization of the PI. At the same time, a high-precision pattern was induced on the PI film using a laser engraving machine according to the above graphene electrode pattern, with a PPI resolution of 1000. The laser pyrolysis process breaks the CO / CN bonds in the polyimide molecular chain, triggering carbon atom rearrangement to form a porous graphene lattice, ultimately forming a unique porous graphene electrode structure.

[0086] It also includes step 7, assembly:

[0087] A PI film 6 with graphene electrode structure 5 is placed above the microfluidic channel layer 4. The functional area of ​​the graphene electrode structure 5 corresponds to the liquid reservoir on the microfluidic channel layer 4. The PI film 6 is aligned with the microfluidic channel layer 4, and the PI film 6 with graphene electrode structure 5 is connected to the microfluidic channel layer 4 by pressure-sensitive adhesive.

Claims

1. A flexible, dynamic, breathable, waterproof encapsulation structure, characterized in that: The sensor comprises a breathable nanofiber membrane, a breathable and waterproof ePTFE membrane, and a microfluidic channel layer. The breathable and waterproof ePTFE membrane is attached to the upper surface of the breathable nanofiber membrane to form the encapsulation substrate of the sensor. The non-detection area of ​​the breathable and waterproof ePTFE membrane has a laser microporous structure. The microfluidic channel layer is located in the detection area of ​​the breathable and waterproof ePTFE membrane. The microfluidic channel layer has channels, a liquid reservoir, and several independent inlets. The breathable and waterproof ePTFE membrane has sweat transport channels at the positions corresponding to the independent inlets.

2. The flexible dynamic breathable waterproof encapsulation structure according to claim 1, characterized in that: The breathable nanofiber membrane has a thickness of 80±5μm, a fiber diameter of 100-300nm, and a gas permeability of over 1200g / (m²・24h) at room temperature. The breathable and waterproof ePTFE membrane has a thickness of 100±10μm, and laser micropore structures are provided on the non-detection area of ​​the breathable and waterproof ePTFE membrane. The pore size of the laser micropore structures is 30μm, the pore density is 120 pores / cm², and the pore walls have micro-nano structures with a roughness Ra=0.8-1.0μm. The microfluidic channel layer has a thickness of 140μm±5μm. The reservoir has a radius of 2mm and a depth equal to the thickness of the microfluidic channel layer. The channel width is 180-200μm and the depth is equal to the thickness of the microfluidic channel layer. Several independent inlets are arranged around the reservoir at equal intervals and angles. The diameter of each independent inlet is 0.5mm and the depth is equal to the thickness of the microfluidic channel layer.

3. The method for preparing a flexible dynamic breathable waterproof encapsulation structure according to claim 1 or 2, characterized in that: Includes the following steps: Step 1: Preparation of breathable nanofiber membrane: 1.1 Preparation of electrospinning solution: A certain amount of polyethersulfone and poly(N-isopropylacrylamide) were dissolved in N-dimethylformamide solution at a mass ratio of 6:

4. The solution was then placed in an ultrasonic cleaner and stirred thoroughly at 0°C to obtain the electrospinning solution. 1.

2. The uniformly mixed electrospinning solution is loaded into a syringe with a metal needle. The syringe is then fixed to the injection pump by a clamp and pushed at a rate of 0.5 mL / h. A high voltage electric field of 15-20 kV is applied to the needle tip to perform electrospinning. The resulting nanofibers are deposited on a grounded receiving roller. After spinning for a certain period of time, the prepared nanofiber membrane is removed. 1.

3. The nanofiber membrane is dried in a vacuum oven to obtain a hierarchical pore structure nanofiber network with a fiber diameter of 100-300 nm and a gas permeability of more than 1200 g / (m²・24h) at room temperature, which is a gas permeable nanofiber membrane layer. Step 2: Preparation of breathable and waterproof ePTFE membrane: 2.1 Raw material preforming: High-purity PTFE dispersion resin with an average particle size of 450μm is mixed with liquid extrusion aid with a volume ratio of 15-25%, and the mixture is molded under a pressure of 10~15MPa to form a dense preform. The preform is then extruded into rod-shaped or sheet-shaped preforms by a pusher at 30-50℃, and then calendered into a film with a thickness of 0.1-0.5 mm by a two-roll calender. 2.2 Biaxial Stretching for Pore Formation: After calendering, the film is dried for a certain period of time to completely volatilize the extrusion aid, forming a uniformly dried PTFE base film. Then, biaxial stretching is performed at 150-200℃. The longitudinal stretching speed is 2~5m / min, forming preliminary fiber orientation. Then, transverse stretching is performed at the same stretching speed, with a stress of 5-20N applied transversely to obtain a porous film. This causes the molecular chains to break, forming a network structure of microfibers and nodes, with a pore size of 0.1-10μm and a porosity of 80%-95%. 2.3 Shaping and Modification: The stretched porous membrane is sintered at 320-330℃ for 1-10 min, and finally fluorinated on the membrane surface to make the contact angle greater than 150° to obtain ePTFE membrane; Step 3: Laser micro-hole structure fabrication: In the non-testing area of ​​the breathable and waterproof ePTFE membrane, femtosecond lasers are used to process breathable pores with a pore density of 100-120 pores / cm². Micro-nano structures with a roughness Ra of 0.8-1.0μm are formed on the pore walls to enhance gas convection. At the same time, femtosecond lasers are used to process sweat transport channels in the testing area of ​​the breathable and waterproof ePTFE membrane, which correspond one-to-one with the independent entrances of the microfluidic channel layer. Step 4: Laser etching to prepare the microfluidic channel layer: First, a PET film of a certain size and a thickness of 140μm±5μm is fixed on a substrate. A CO2 laser cutting machine is then used to precisely engrave the reservoir, channels, and independent inlets, resulting in a reservoir with a radius of 2mm and a depth equal to the thickness of the microfluidic channel layer; a channel structure with a width of 180-200μm and a depth equal to the thickness of the microfluidic channel layer; and several independent inlets with a diameter of 0.5mm and a depth equal to the thickness of the microfluidic channel layer. The independent inlets are arranged at equal intervals and angles around the circular reservoir, with each independent inlet 3mm from the center of the reservoir. The channels connect the reservoir and the independent inlets. Step 5, Packaging: Connect the layers from bottom to top in the following order: breathable nanofiber membrane, breathable and waterproof ePTFE membrane, and microfluidic channel layer. The layers are connected from bottom to top in the following order: breathable nanofiber membrane, breathable and waterproof ePTFE membrane, and microfluidic channel layer. First, the surfaces of the breathable nanofiber membrane and the breathable and waterproof ePTFE membrane are cleaned. After cleaning, a breathable nanofiber membrane of a certain size and a thickness of 80±5μm is laid flat and fixed on the operating table. A breathable and waterproof ePTFE membrane of the same size and a thickness of 100±10μm is accurately aligned and covered on the breathable nanofiber membrane. The two membranes are tightly bonded together by hot pressing to form an encapsulation base. The encapsulation base is bonded to the microfluidic channel layer with medical silicone adhesive. The microfluidic channel layer is bonded to the detection area of ​​the breathable and waterproof ePTFE membrane. The independent entrances on the microfluidic channel layer are precisely aligned with the sweat transport channels on the breathable and waterproof ePTFE membrane to achieve waterproof encapsulation of the wearable sweat sensor.

4. The preparation method according to claim 3, characterized in that: In step 1.2, the distance from the needle to the receiving roller is 15-20cm, the rotation speed of the receiving roller is 300-500r / min, the ambient temperature is 25±2℃, the humidity is 30%-40%, and a layer of silicone paper is covered on the receiving roller.

5. The preparation method according to claim 3, characterized in that: In step 1.3, the drying temperature in the vacuum oven is 60-70℃, and the drying time is 12-16 hours.

6. The preparation method according to claim 3, characterized in that: In step 2.1, the roller temperature of the twin-roll calender is controlled at 60-80℃ to ensure that the thickness deviation is less than ±5%.

7. The preparation method according to claim 3, characterized in that: In step 2.3, the fluorination treatment steps are as follows: When fluorinating the ePTFE membrane, first immerse the membrane in an ethanol solution and ultrasonically clean it for 8-12 minutes, then irradiate it with ultraviolet light for 1-3 minutes, followed by heating at 70-80℃ for 3-5 hours. After cooling to room temperature, heat it again in an oven at 70-80℃ for 35-40 minutes to complete the surface pretreatment to remove impurities and moisture. Then, use fluorine gas as the fluorinating agent and dilute it with nitrogen gas. Control the reaction temperature at 50-60℃, maintain the pressure at 0.6-0.8MPa, and the reaction time for 15-20 hours. Under these conditions, the fluorination reaction is carried out to form a fluorinated layer on the membrane surface. After the reaction, use a contact angle meter to detect the contact angle of the membrane surface. If it is greater than 150°, clean the membrane with deionized water or organic solvent to remove residual reagents and byproducts. If it does not meet the standard, repeat the above fluorination treatment steps.

8. The preparation method according to claim 3, characterized in that: In step 3, the femtosecond laser processing parameters are as follows: a 1030nm femtosecond laser is used, with a pulse width of 80fs, a repetition frequency of 2kHz, and an average power of 70-90mW.

9. A wearable sweat sensor, characterized in that: Based on the encapsulation structure described in any one of claims 1-2 or the encapsulation structure prepared by any one of claims 3-8, the encapsulation structure further includes a graphene electrode structure and a PI film. The graphene electrode structure is directly fabricated on the surface of the PI film. The graphene electrode structure and the PI film are sequentially disposed above the microfluidic channel layer. The functional area of ​​the graphene electrode structure corresponds to the liquid reservoir on the microfluidic channel layer. The size of the PI film is the same as that of the microfluidic channel layer, and the thickness is 75 μm ± 5 μm. The graphene electrode includes three electrodes: a counter electrode, a working electrode, and a reference electrode.

10. The method for preparing the wearable sweat sensor according to claim 9, characterized in that: Based on the preparation method described in any one of claims 3-8, the method further includes step 6: laser printing to prepare the graphene electrode structure. A commercial PI film with the same size and a thickness of 75μm as the microfluidic channel layer was used as the sensor printing substrate. Graphene electrode structures were generated on the PI film in grating mode using a 50W CO2 laser engraving machine. Step 7, Assembly: A PI film with a graphene electrode structure is placed above a microfluidic channel layer. The functional area of ​​the graphene electrode structure corresponds to the liquid reservoir on the microfluidic channel layer. The PI film is aligned with the microfluidic channel layer, and the PI film with the graphene electrode structure is connected to the microfluidic channel layer using pressure-sensitive adhesive.

Images