A hot pressing method for microfluidic chips

Abstract

This invention relates to the field of microfluidic chip manufacturing technology, and more particularly to a hot-pressing method for microfluidic chips, comprising: directional stretching of a thermoplastic film; determining a three-zone gradient heating temperature based on viscoelastic properties; sequentially applying first, second, and third pressures to the preheated film for roll forming, wherein the switch from the first pressure to the second pressure is triggered by surface temperature, and the switch from the second pressure to the third pressure is triggered by the pressure change rate; real-time detection of the fill rate and graded adjustment of process parameters according to the deviation; two-stage gradient cooling in the stress relief zone, and controlling the third pressure to decrease with decreasing temperature in the second temperature zone, with the decay rate matching the molecular chain orientation retraction effect; finally, forming a cross-linked modified layer on the surface of the microstructure. This invention significantly improves the control precision of chip manufacturing, reduces residual stress, and ensures batch-to-batch consistency.

Description

Technical Field

[0001] This invention relates to the field of microfluidic chip manufacturing technology, and in particular to a hot pressing method for microfluidic chips. Background Technology

[0002] Microfluidic chips are small devices that integrate complex fluid control systems, typically consisting of multiple micrometer- or nanometer-scale channels, pumps, and valves, enabling precise manipulation of liquid flow. By processing liquid samples within these microchannels, analytical efficiency can be improved, sample volume reduced, reagent consumption decreased, and high-resolution results provided. They are widely used in fields such as biomedical diagnostics, drug development, environmental monitoring, and chemical analysis.

[0003] Currently, the main methods for manufacturing microfluidic chips include soft lithography, thermoforming, injection molding, inkjet printing, and laser micromachining. Among these, thermoforming has become one of the mainstream methods for preparing polymer microfluidic chips due to its advantages such as simple process, high replication accuracy, and suitability for mass production.

[0004] Thermoforming typically involves steps such as heating, pressurizing, holding pressure, and cooling / demolding. Its core lies in precisely controlling process parameters such as temperature, pressure, and time to induce viscous flow of polymer material above its glass transition temperature, filling the mold's microstructure and thus replicating high-precision microchannels.

[0005] Chinese Patent Publication No. CN119840140A discloses a hot-pressing method for microfluidic chips and a microfluidic chip. The hot-pressing method for microfluidic chips includes the following steps: mold making, preparing a molding die, the molding die including: preparing a female mold and a male mold, at least two male molds are prepared, and the heights of the at least two male molds are different; pressing the female mold... The die is detachably connected to one of the punches; heating is performed by pressing the punch against the blank and heating the blank, punch, and die; hot pressing is performed by applying thermal pressure between the punch and the blank to cause the punch to squeeze the blank, and the blank replicates the shape of the punch to process the blank into a microfluidic chip; demolding is performed by cooling the microfluidic chip, die, and punch and separating the microfluidic chip and the punch.

[0006] Therefore, the hot-pressing method for microfluidic chips and the microfluidic chips themselves mainly focus on improving the mold structure to achieve the fabrication of various chips. However, the hot-pressing process itself has the following problems: the temperature and pressure control during the hot-pressing process is relatively simple and cannot be dynamically adjusted according to the real-time state of the material, resulting in low microstructure replication accuracy and large residual stress, which affects the dimensional stability and lifespan of the chip. In addition, the existing process lacks a real-time detection and feedback adjustment mechanism for molding quality, making it difficult to ensure batch-to-batch consistency. Summary of the Invention

[0007] To address this, the present invention provides a hot-pressing method for microfluidic chips, which overcomes the problems in the prior art where, in the multi-layer fabrication of microfluidic chips, the microgroove structure formed by the hot-pressing process deforms due to reheating after multi-layer hot-pressing composite, resulting in low precision of the microgroove structure, large residual stress, and lack of real-time feedback adjustment after multi-layer hot-pressing.

[0008] To achieve the above objectives, the present invention provides a hot-pressing method for microfluidic chips, comprising: Step S1: The thermoplastic film is subjected to directional stretching treatment to form molecular chain orientation along the stretching direction inside the film. After drying, a pretreated film is obtained. Step S2: Obtain the glass transition temperature of the pretreated film. Based on the viscoelastic properties of the film, determine the temperatures of the first, second, and third heating zones to perform gradient heating on the pretreated film, obtaining the preheated film. Step S3: Apply a first pressure to the preheated film for roll forming to form a microfluidic groove structure. During the roll forming process, the surface temperature of the preheated film is detected simultaneously. Based on the result that the surface temperature reaches a preset temperature, it is determined to switch to applying a second pressure, wherein the second pressure is greater than the first pressure. Step S4: During the application of the second pressure, the post-pressure rebound rate of the preheated film is monitored in real time to adjust the second pressure. Based on the film pressure change rate continuously monitored after the second pressure adjustment, it is determined whether the microstructure filling is completed. After the filling is completed, the third pressure is used for rolling to obtain the rolled film. Step S5: Detect the microstructure size characteristics of the rolled film to determine the fill rate. If the fill rate is lower than the preset fill threshold, dynamically adjust the film conveying speed during the roll forming process according to the deviation to obtain a roll-formed film with a qualified fill rate. Step S6: The roll-formed film is subjected to gradient cooling in the stress relief zone. The gradient cooling includes a first temperature zone and a second temperature zone. The temperature and cooling rate of the second temperature zone are lower than those of the first temperature zone. At the same time, the third pressure is reduced in the second temperature zone. The reduction rate is determined based on the shrinkage characteristics of the molecular chain orientation formed by directional stretching during the cooling process, and the cooled microfluidic chip is obtained. Step S7: Based on the actual filling rate of the roll-formed film, a cross-linked modified layer is formed on the microstructure surface of the cooled microfluidic chip, wherein the glass transition temperature of the cross-linked modified layer is higher than that of the thermoplastic film.

[0009] Further, in step S2, the temperatures of the first heating zone, the second heating zone, and the third heating zone are determined through a variable-temperature creep experiment, including: Step S21: Conduct a variable temperature creep test on the thermoplastic film by heating it at a constant temperature rise rate and applying constant pressure, collecting temperature and deformation data in real time, and plotting a temperature-deformation curve. Step S22: Based on the temperature-deformation curve, perform viscoelastic property analysis to determine the temperature corresponding to the first inflection point of the curve as the first reference temperature and the temperature corresponding to the second inflection point as the second reference temperature. Step S23: Determine the temperature of the first heating zone based on the first reference temperature; The temperature of the second heating zone is determined based on the second reference temperature; The temperature of the third heating zone is determined based on the glass transition temperature and the second reference temperature.

[0010] Furthermore, the temperature of the third heating zone is determined based on the viscoelastic response characteristics of the thermoplastic film in the glass transition region, so that the film remains in a highly elastic state before entering the imprinting unit.

[0011] Further, step S4 includes: Step S41: During the application of the second pressure, the post-compression rebound rate data of the film is collected in real time, and the second pressure is dynamically adjusted based on the post-compression rebound rate. Step S42: After the second pressure adjustment is completed, the pressure data of the second pressure is collected in real time. Step S43: Calculate the pressure change rate of the second pressure based on the pressure data of the second pressure; Step S44: Compare the absolute value of the pressure change rate with a preset pressure change rate threshold. If the absolute value of the pressure change rate is less than or equal to the preset pressure change rate threshold for a continuous preset time, the microstructure filling is determined to be complete. Step S45: Switch the applied pressure to a third pressure to maintain it until entering the gradient cooling phase.

[0012] Furthermore, the ratio of the third pressure to the second pressure is determined based on the thermal shrinkage characteristics of the thermoplastic film, wherein the thermal shrinkage characteristics include the thermal shrinkage rate, and the ratio is positively correlated with the thermal shrinkage rate of the film.

[0013] Furthermore, the ratio of the third pressure to the second pressure ranges from 0.3 to 0.7.

[0014] Furthermore, step S5 further includes dynamically adjusting the temperature or pressure of the second heating zone based on the deviation amount, including: If the deviation is less than the first threshold, the film conveying speed during the roll forming process is reduced. If the deviation is greater than or equal to the first threshold and less than the second threshold, then the temperature of the second heating zone is increased; If the deviation is greater than or equal to the second threshold, then the second pressure is increased; Wherein, the deviation is the difference between the preset filling threshold and the filling rate / the absolute value of the difference.

[0015] Further, step S6 includes: Step S61: Cool the rolled film in the stress relief zone at a first temperature zone using a first cooling rate. Step S62: Monitor the film temperature in real time. When the film temperature drops to the first preset temperature, enter the second temperature zone for cooling. Step S63: During the cooling process in the second temperature zone, a second cooling rate lower than the first cooling rate is adopted, while the third pressure is reduced. The rate of pressure reduction is determined based on the retraction characteristics of the molecular chain orientation formed by directional stretching during the cooling process. Step S64: When the film temperature drops to the second preset temperature, cooling is complete, and the cooled microfluidic chip is obtained.

[0016] Furthermore, in step S7, the crosslinked modified layer is formed by initiating a crosslinking reaction through ultraviolet light irradiation, electron beam irradiation, or plasma treatment; wherein, the ultraviolet light irradiation parameters / electron beam irradiation energy parameters / plasma treatment parameters are determined based on the actual fill rate of the roll-formed film.

[0017] On the other hand, the present invention also provides a hot pressing system for microfluidic chips, comprising: The pretreatment unit is used to perform directional stretching on the thermoplastic film to form molecular chain orientation along the stretching direction inside the film, and to dry the stretched film. A gradient heating unit, which is connected to the pretreatment unit, is provided with a first heating zone, a second heating zone and a third heating zone in sequence along the film conveying direction, and is used to perform gradient heating on the pretreated film. An imprinting unit, which is connected to the gradient heating unit, is used to roll-form the preheated film. The imprinting unit includes an imprinting roller and a pressure control system. The pressure control system is used to apply a first pressure, a second pressure and a third pressure sequentially during the roll-forming process. The detection feedback unit, which is connected to the imprinting unit, is used to detect the film surface temperature and trigger the switching of the first pressure to the second pressure based on the detection result; to monitor the change rate of the second pressure and trigger the switching of the second pressure to the third pressure based on the detection result; and to detect the microstructure filling rate after roll forming and adjust the film conveying speed, the temperature of the second heating zone, or the second pressure according to the detection result. The stress relief cooling unit, which is connected to the detection feedback unit, is used to perform gradient cooling on the roll-formed film and reduce the third pressure during the cooling process. A surface modification unit, connected to the stress relief cooling unit, is used to form a cross-linked modification layer on the surface of the microstructure of the cooled microfluidic chip.

[0018] Compared with existing technologies, the advantages of this invention lie in its organic integration of techniques such as material viscoelasticity analysis, multi-segment gradient heating, multi-segment pressure control, real-time feedback adjustment, gradient cooling stress release, and surface cross-linking strengthening, forming a complete intelligent hot pressing molding solution. The synergistic effects of these technologies upgrade the hot pressing molding process from traditional "experience-driven" to "material response-driven," solving the problems of low molding accuracy, high residual stress, and lack of real-time feedback adjustment in existing technologies.

[0019] Furthermore, this invention obtains the temperature-deformation curve of the material through variable-temperature creep experiments, scientifically determining the temperatures of the three heating zones: the first heating zone is below the glass transition initiation temperature, ensuring uniform preheating of the film; the second heating zone is equal to the sufficiently elastic state temperature, placing the material in its optimal forming state; and the third heating zone is between the two and above the glass transition temperature, maintaining the film's elastic state. This temperature control method based on material properties enables precise matching of temperature parameters with the material state, significantly improving the accuracy of microstructure replication.

[0020] Furthermore, the present invention employs a three-stage pressure control, and the pressure switching is based on the real-time state of the material: the first pressure switching to the second pressure is triggered by the film surface temperature to ensure that the material is fully softened before applying high pressure; the second pressure switching to the third pressure is triggered by the pressure change rate, and when the pressure change rate is continuously lower than the threshold, the filling is determined to be complete, thus avoiding overfilling or underfilling.

[0021] Furthermore, this invention integrates three-zone gradient heating with three-stage pressure control: after the film achieves a uniform, highly elastic state in the third heating zone, it enters the imprinting unit where a first pressure is applied for initial filling; when the temperature triggers a switch to the second pressure, the material is within the optimal molding temperature range; after filling is complete, the pressure is switched to the third pressure, followed by gradient cooling. This temperature-pressure synergy mechanism ensures the material is always in the optimal temperature-pressure combination state, improving molding efficiency by more than 15%.

[0022] Furthermore, this invention employs a two-stage gradient cooling system. The cooling rate in the second temperature zone is less than 0.5 times that of the first temperature zone. This slow cooling allows the molecular chains to fully relax within the glass transition range. Simultaneously, within the second temperature zone, the third pressure is controlled to decrease with decreasing temperature. The rate of decrease matches the molecular chain orientation retraction effect formed by directional stretching, actively compensating for cooling contraction. This unified mechanism of stress release and contraction compensation reduces chip warpage by more than 70% compared to traditional hot-pressing methods, and improves dimensional accuracy to within ±1.2μm.

[0023] Furthermore, this invention monitors the microstructure filling rate in real time and adjusts process parameters in stages according to the magnitude of the deviation: small deviations reduce the conveying speed, medium deviations increase the temperature of the second heating zone, and large deviations increase the second pressure. This staged adjustment mechanism forms a complete closed-loop control circuit, reducing batch-to-batch dimensional deviations by more than 65%, effectively overcoming process uncertainties caused by material batch differences and environmental fluctuations, and ensuring product consistency.

[0024] Furthermore, the present invention forms a cross-linked modified layer on the surface of the microstructure, the glass transition temperature of which is higher than that of the matrix material, so that the surface of the microchannel remains stable within the operating temperature range. Attached Figure Description

[0025] Figure 1 This is a flowchart of the hot pressing molding method for microfluidic chips according to an embodiment of the present invention; Figure 2 This is a flowchart of step S2 of the hot pressing molding method for microfluidic chips according to an embodiment of the present invention; Figure 3 This is a flowchart of step S4 of the hot pressing molding method for microfluidic chips according to an embodiment of the present invention; Figure 4 This is a flowchart of step S6 of the hot pressing molding method for microfluidic chips according to an embodiment of the present invention. Detailed Implementation

[0026] To make the objectives and advantages of the present invention clearer, the present invention will be further described below with reference to embodiments; it should be understood that the specific embodiments described herein are merely for explaining the present invention and are not intended to limit the present invention.

[0027] Preferred embodiments of the present invention will now be described with reference to the accompanying drawings. Those skilled in the art should understand that these embodiments are merely illustrative of the technical principles of the present invention and are not intended to limit the scope of protection of the present invention.

[0028] It should be noted that in the description of this invention, the terms "upper", "lower", "left", "right", "inner", "outer", etc., which indicate directions or positional relationships, are based on the directions or positional relationships shown in the accompanying drawings. This is only for the convenience of description and is not intended to indicate or imply that the device or element must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, it should not be construed as a limitation of this invention.

[0029] Furthermore, it should be noted that, in the description of this invention, unless otherwise explicitly specified and limited, the terms "installation," "connection," and "linking" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal connection of two components. Those skilled in the art can understand the specific meaning of the above terms in this invention according to the specific circumstances.

[0030] The hot pressing method for microfluidic chips provided in this invention is mainly used to prepare the fluid layer (i.e., the microstructure functional layer) in a multilayer microfluidic chip.

[0031] Multilayer microfluidic chips are typically assembled by sequentially thermoforming a substrate layer, a fluid layer, and a capping layer. Substrate layer: Provides structural support and is typically a flat, thin sheet; Fluid layer: containing microstructures such as microchannels and microcavities, it is the core functional layer for realizing fluid control of the chip, namely the microstructure functional layer prepared in this invention; Cover plate layer: Used to seal the fluid layer, forming a closed flow channel system; In the fabrication process of multilayer chips, the fluid layer needs to undergo two hot-pressing processes, including: In the first hot pressing, the thin film material is hot-pressed and molded using the method of the present invention to prepare a fluid layer with microstructure; The second hot pressing aligns the fluid layer with the substrate layer and cover layer, and then hot-presses them together again to form a complete multilayer chip. In existing technologies, the fluid layer accumulates significant residual stress after the first hot pressing, resulting in insufficient surface thermal stability. During the second hot pressing, the fluid layer is heated again, causing the internal residual stress to release, leading to microstructure deformation, collapse, and surface softening, further exacerbating microstructure deformation.

[0032] This invention aims to solve the above problems by using techniques such as directional stretching, gradient heating, three-stage pressure control, stress release cooling, and surface crosslinking modification to prepare a high-precision, low-stress, and thermally stable fluid layer. This layer can withstand the thermal effects of secondary hot-pressing composite with the substrate layer and the cover layer without significant deformation, thereby greatly improving the fabrication yield and performance of multilayer microfluidic chips.

[0033] Example 1: This embodiment provides a hot pressing method for fabricating microfluidic chips, using PMMA thermoplastic film (glass transition temperature Tg=105℃) as a substrate to prepare microfluidic chips with microchannel structures.

[0034] Please see Figure 1 The flowchart shown is a hot-press forming method for the microfluidic chip of the present invention. The hot-press forming method for the microfluidic chip in this embodiment includes: Step S1: The thermoplastic film is subjected to directional stretching treatment to form molecular chain orientation along the stretching direction inside the film. After drying, a pretreated film is obtained. Specifically, a 0.5 mm thick PMMA film is placed in a stretching apparatus and subjected to unidirectional stretching at 100°C with a stretching ratio of 1.2 times. It is understood that a stretching ratio that is too low (<1.1 times) will not result in effective molecular chain orientation and insufficient shrinkage effect; a stretching ratio that is too high (>1.5 times) may lead to localized excessive thinning or breakage of the film. After stretching, a molecular chain orientation along the stretching direction is formed inside the film, storing reversible elastic strain energy. This strain energy is released during subsequent cooling, producing shrinkage in the same direction as the stretching to compensate for thermal shrinkage. After stretching, the film is placed in a 60°C vacuum drying oven for 2 hours to remove moisture, obtaining a pretreated film.

[0035] Step S2: Obtain the glass transition temperature of the pretreated film. Based on the viscoelastic properties of the film, determine the temperatures of the first, second, and third heating zones to perform gradient heating on the pretreated film, obtaining the preheated film. Wherein, the temperature of the first heating zone is lower than the glass transition temperature to keep the film in a glassy state, the temperature of the second heating zone is higher than the glass transition temperature to make the film reach a highly elastic state, and the temperature of the third heating zone is lower than the second heating zone and higher than the glass transition temperature to keep the film in a highly elastic state. First, differential scanning calorimetry (DSC) was performed on the PMMA thin film: the temperature was increased from room temperature to 200°C at a heating rate of 10°C / min, and the heat flow curve was recorded. The glass transition temperature Tg was determined using the midpoint method: in the region where the heat flow curve showed a step change, two baselines were extended, and the temperature corresponding to the midpoint of the perpendicular distance between the two baselines was taken as Tg. In this embodiment, the glass transition temperature Tg of the PMMA thin film was measured to be 105°C.

[0036] Please see Figure 2 The diagram shown is a flowchart of step S2 of the hot pressing molding method for microfluidic chips according to an embodiment of the present invention. Specifically, determining the temperatures of the first heating zone, the second heating zone, and the third heating zone in step S2 includes: Step S21: Conduct a variable temperature creep test on the thermoplastic film by heating it at a constant temperature rise rate and applying constant pressure, collecting temperature and deformation data in real time, and plotting the temperature-deformation curve. Step S22: Based on the temperature-deformation curve, perform viscoelastic property analysis, and determine the temperature corresponding to the first inflection point of the curve as the first reference temperature and the temperature corresponding to the second inflection point as the second reference temperature. Step S23: Determine the temperature of the first heating zone based on the first reference temperature; The temperature of the second heating zone is determined based on the second reference temperature; The temperature of the third heating zone is determined based on the glass transition temperature and the second reference temperature.

[0037] The difference between the first heating zone and the first reference temperature is determined based on the thermal conductivity of the thermoplastic film, so that the film has a uniform overall temperature after passing through the first heating zone. First, a variable-temperature creep experiment was conducted on the PMMA film: the film was heated at a constant temperature rise rate of 1.5℃ / min, while a constant pressure of 0.5MPa was applied. Temperature and deformation data were collected in real time, and temperature-deformation curves were plotted.

[0038] Understandably, the temperature-deformation curve reflects the change in deformation of a polymer material with temperature when heated under constant pressure. The shape of the curve is closely related to the viscoelastic behavior of the material. The first inflection point, also known as the deformation initiation point, corresponds to the initial temperature at which the material transitions from the glassy state to the elastic state. At this point, molecular chain segments begin to move, and the deformation rate increases significantly from zero. It is identified using the slope threshold method: the slope of the curve is calculated, and the point where the slope first exceeds 50% of the maximum slope is the first inflection point.

[0039] The second inflection point, also known as the deformation stability point, corresponds to the temperature at which the material enters a fully elastic state, at which point the molecular chain segments move sufficiently and the deformation rate tends to stabilize. It can be identified using the slope threshold method: when the slope decreases from its maximum value and first falls below 50% of the maximum slope, this point is the second inflection point. Alternatively, the tangent method can be used: draw tangents to the rising and plateau segments of the curve; the intersection of these two tangents is the second inflection point.

[0040] The accuracy of inflection point identification can be verified through dynamic mechanical analysis (DMA): the first inflection point temperature should be consistent with the temperature at which the energy storage modulus begins to decrease significantly; the second inflection point temperature should be consistent with the temperature at which the energy storage modulus decreases to 50% of its initial value or the peak temperature of the loss factor.

[0041] In this embodiment, the slope threshold method is used to determine the first and second inflection points of the curve: Calculate the slope k at each point on the curve.

[0042] Where ε is the deformation and T is the temperature. The slope ki is calculated using the three-point difference method to reduce the influence of noise, i.e.,

[0043] Where, ε i-1 Let ε be the deformation (mm) of the (i-1)th sampling point. i Let ε be the deformation (mm) of the i-th sampling point. i+1 T represents the deformation (mm) of the (i+1)th sampling point. i-1 T represents the temperature (°C) at the (i-1)th sampling point. i Let T be the temperature (°C) of the i-th sampling point. i+1 Let be the temperature (°C) of the (i+1)th sampling point; Statistically analyze all slope values ​​along the entire curve, and take the maximum value as the maximum slope k. max In this embodiment, k is calculated. max ≈0.15mm / ℃.

[0044] Set slope threshold k th =ξ×k max It should be noted that k th The value of ξ is determined based on material properties and experimental data. The range of ξ is 0.3 to 0.5. A value that is too small (<0.3 times) will misjudge noise as an inflection point; a value that is too large (>0.5 times) will miss the true inflection point. After experimental optimization, this embodiment uses ξ=0.4, then k th =0.4×k max =0.06mm / ℃.

[0045] Starting from the low-temperature end, search for the first condition that satisfies k. i≥k th The point is the reference temperature T1. In this embodiment, T1 = 100℃.

[0046] Continuing the search after the first inflection point, when k... i Decrease from the maximum value and first satisfy k i ≤k th At that point, the temperature is the second reference temperature T2. In this embodiment, T2 = 120℃.

[0047] To verify the accuracy of inflection point identification, this embodiment also employs dynamic mechanical analysis (DMA) for verification: The first reference temperature T1=100℃ is consistent with the temperature at which the energy storage modulus begins to decrease significantly (approximately 98℃~102℃); The second reference temperature T2 = 120℃ is consistent with the temperature at which the energy storage modulus drops to 50% of its initial value (approximately 118℃~122℃); The verification results show that the inflection point identification method in this embodiment is accurate and reliable.

[0048] Based on the above results, the temperatures of the three heating zones are set as follows: Calculation of the temperature T10 in the first heating zone: T10 = T1 - ΔT Wherein, ΔT is the safety margin, and its value ranges from 5 to 15℃; The temperature T10 of the first heating zone must be set to ensure that the overall temperature of the film is uniform after passing through the first heating zone and does not approach the first reference temperature T1 (to avoid premature softening).

[0049] According to the one-dimensional unsteady-state heat conduction equation, the heat penetration depth :

[0050] When δ > h, the overall temperature of the film is uniform.

[0051] In this embodiment, the film thickness h = 0.5 mm, the thermal diffusivity α = 0.1 mm² / s, and the heating time t = 12 s (heating zone length 200 mm, conveying speed 1.0 m / min). Substituting these values ​​into the calculation, we get δ = 1.1 mm, which is greater than the film thickness of 0.5 mm, indicating that the film can be fully heated.

[0052] It is understandable that a value of ΔT that is too small (<5℃) may cause premature softening of the film surface; a value that is too large (>15℃) will result in insufficient preheating, affecting the efficiency of subsequent heating. In this embodiment, ΔT = 10℃ is used, that is, the temperature of the first heating zone T10 = T1 - ΔT = 100℃ - 10℃ = 90℃; The calculation of the second heating zone temperature T20: T20 = T2 = 120℃. This temperature is equal to the second reference temperature. At this temperature, the material is in a fully elastic state, the molecular chain segments move fully, and the fluidity is optimal, which is the ideal temperature for microstructure replication. Calculation of the temperature T30 in the third heating zone:

[0053] Where φ is the proportionality coefficient; Understandably, φ reflects the viscoelastic response characteristics of the material in the glass transition region, determined by the temperature T50 when the storage modulus decreases to 50% of its initial value: φ = (T50 - T1) / (T2 - T1). For most thermoplastic polymers, T50 is typically located between 0.4 and 0.6, between T1 and T2. When φ < 0.4, the temperature of the third heating zone is too low, the material is close to a glassy state, and its fluidity is insufficient; when φ > 0.6, the temperature is too high, the material softens excessively, and creep deformation easily occurs before imprinting.

[0054] In this embodiment, the temperature at which the storage modulus of PMMA drops to 50% of its initial value was determined to be T50 = 112℃ through dynamic mechanical analysis. Therefore, φ = 0.6 and T30 = 112℃ were calculated. This temperature is higher than Tg (105℃) and lower than T2 (120℃), which is in the middle to late stage of the glass transition range. At this time, the material's storage modulus has dropped to about 50% of its initial value, which ensures a sufficiently elastic state and avoids excessive material flow caused by excessive temperature.

[0055] Step S3: Apply a first pressure to the preheated film to roll it to form a microfluidic groove structure. During the rolling process, the surface temperature of the film is detected. When the surface temperature of the film reaches a preset temperature, switch to applying a second pressure, wherein the second pressure is greater than the first pressure. Specifically, the pretreated film is sequentially fed into a first heating zone (90℃), a second heating zone (120℃), and a third heating zone (112℃) for gradient heating to obtain a preheated film. Each of the three heating zones is 200mm long, the film conveying speed is v=1.0m / min, and the residence time in each heating zone is t=12s. The preheated film is then fed into an imprinting unit, where a first pressure P1=0.5MPa is applied for roll forming, while an infrared temperature sensor monitors the film surface temperature in real time.

[0056] Calculation of preset temperature Tp:

[0057] Wherein, η is a proportionality coefficient, with a value ranging from 0.6 to 0.8, which is determined based on the residence time of the thin film in the second heating zone and its thermal conductivity characteristics.

[0058] It is understandable that η reflects the degree of heat penetration of the film at the outlet of the second heating zone. The heat penetration depth δ=√(α·t). When the heat penetration depth is much greater than the film thickness h, the overall temperature of the film is uniform, and η can take a larger value (0.8). When δ is close to h, the surface temperature is lower than the temperature of the second heating zone, and η takes a smaller value (0.6). Furthermore, previous experiments have shown that when η is too small (<0.6), the surface softening is insufficient, and high-pressure filling is difficult; when η is too large (>0.8), the core temperature is too high, and the lack of support makes it prone to collapse.

[0059] In this embodiment, Tg = 105℃, T2 = 120℃, residence time t = 12s, and heat conduction depth δ = 1.1mm, which is much greater than the film thickness of 0.5mm. η is preferably taken as 0.7. Substituting these values, we get Tp = 115.5℃, which is rounded to 115℃.

[0060] When the surface temperature of the film reaches 115℃, the control system automatically switches the pressure to the second pressure P2=2.0MPa. At this time, the surface layer of the material softens sufficiently and can undergo flow deformation under external force, while the core temperature is slightly lower (about 110℃), maintaining a certain supporting strength to prevent overall collapse under high pressure.

[0061] Step S4: During the application of the second pressure, the pressure change rate is monitored in real time to determine whether the microstructure filling is complete, and after the filling is completed, the pressure is switched to a third pressure that is lower than the second pressure. Please see Figure 3 The flowchart shown is a step S4 of the hot pressing molding method for microfluidic chips according to an embodiment of the present invention. Specifically, step S4 includes: Step S41: During the application of the second pressure, the post-compression rebound rate data of the film is collected in real time, and the second pressure is dynamically adjusted based on the post-compression rebound rate. Step S42: After the second pressure adjustment is completed, the pressure data of the second pressure is collected in real time. Step S43: Calculate the pressure change rate of the second pressure based on the pressure data of the second pressure; Step S44: Compare the absolute value of the pressure change rate with a preset pressure change rate threshold. If the absolute value of the pressure change rate is less than or equal to the preset pressure change rate threshold for a continuous preset time, the microstructure filling is determined to be complete. Step S45: Switch the applied pressure to a third pressure that is lower than the second pressure to maintain it until the gradient cooling phase begins.

[0062] In one specific embodiment, during the application of the second pressure P2 = 2.0 MPa, the post-compression rebound rate of the film is monitored in real time. This rebound rate is obtained by real-time acquisition of the recovery amount in the thickness direction of the film using a displacement sensor. Post-compression rebound rate δr:

[0063] Where h b h represents the film thickness under pressure. a This refers to the film thickness after pressure relief. The second pressure is dynamically adjusted based on the real-time monitored post-compression rebound rate. Preset rebound rate control range: upper limit threshold δ up =5%, lower limit threshold δ low =3%.

[0064] When δ r If the rebound rate is greater than 5%, it indicates that the material rebound is too large and the filling is insufficient. Increase the second pressure at a rate of 0.1 MPa / s until the rebound rate drops below 5%. When δ r If the rebound rate is less than 3%, it indicates that the pressure is too high, which may lead to the accumulation of residual stress. The second pressure should be reduced at a rate of 0.05 MPa / second until the rebound rate rises back to above 3%. When 3%≤δ r A rebound rate of ≤5% indicates that the rebound rate is within the ideal range, and the current pressure should be maintained. In this embodiment, the initial rebound rate is detected to be 6%, which is higher than the upper limit threshold of 5%. The system automatically increases the second pressure from 2.0MPa to 2.2MPa, and the rebound rate drops to 4.8%, entering the control range (3% to 5%). The adjustment is then stopped, and the current pressure is maintained.

[0065] After the second pressure stabilized at 2.2 MPa, pressure data was collected in real time at a sampling frequency of 10 Hz, and a pressure-time graph was plotted to observe the pressure change trend over time.

[0066] The pressure change rate dP / dt at each time point was calculated using the three-point difference method:

[0067] Where P i-1 and t i-1 P represents the pressure and corresponding time of the previous sampling point. i+1 and t i+1 This method calculates the slope at the intermediate point using the two points before and after the sampling point, effectively suppressing the impact of sampling noise on the calculation results.

[0068] The absolute value of the pressure change rate is compared with a preset pressure change rate threshold, which is λ = 0.02 MPa / s, approximately 0.9% of the current second pressure of 2.2 MPa, for a continuous preset time t0 = 3 s.

[0069] When the absolute value of dP / dt remains below 0.02 MPa / s for 3 consecutive seconds, the microstructure filling is considered complete, and the pressure is switched to the third pressure P3. P3 can be calculated using the following formula:

[0070] Wherein, γ is the pressure coefficient (%), which ranges from 0.4 to 0.6 and is determined based on the material's thermal shrinkage rate; It is understandable that when the material's thermal shrinkage rate γ is large or its modulus E is high, the required compensation pressure P is large, and γ takes a smaller value; conversely, a larger value is taken. When γ < 0.3, the holding pressure is too low, resulting in insufficient shrinkage compensation and shrinkage marks; when γ > 0.7, the holding pressure is too high, which may cause microstructural deformation or excessive residual stress. In this embodiment, γ = 0.5, P3 = 1.0 MPa, and P3:P2 = 0.45, which is within the range of 0.3 to 0.7.

[0071] Step S5: Detect the microstructure size characteristics of the rolled film to determine the fill rate. If the fill rate is lower than the preset fill threshold, dynamically adjust the film conveying speed during the roll forming process according to the deviation to obtain a roll-formed film with a qualified fill rate. Step S5 further includes dynamically adjusting the temperature or pressure of the second heating zone based on the deviation amount: If the deviation is less than the first threshold, the film conveying speed during the roll forming process is reduced. If the deviation is greater than or equal to the first threshold and less than the second threshold, then the temperature of the second heating zone is increased; If the deviation is greater than or equal to the second threshold, then the second pressure is increased; The deviation is the difference between the preset filling threshold and the filling rate.

[0072] Specifically, after roll forming, the microstructure of the film surface is scanned and detected using a laser confocal microscope. Specifically, multiple characteristic cross-sections of the microchannel are selected, with one cross-section taken every 100 μm along the channel direction, for a total of 10 cross-sections. The actual depth h of the microchannel on each cross-section is measured. a Design depth h corresponding to the microstructure on the mold d Calculate the fill ratio ηi=h for each section. a / h d The fill rate ηf of the film is calculated as the average value of all cross-sections multiplied by 100%. The detected fill rate is then compared with a preset fill threshold η.th Adjust process parameters to account for deviation Δη:

[0073] When the deviation Δη < 5%: Adjust the film conveying speed:

[0074] When the deviation is 5% ≤ Δη < 10%: adjust the temperature of the second heating zone T200 = T20 + ΔTH, where,

[0075] When the deviation Δη ≥ 10%: Adjust the second pressure P20:

[0076] In this embodiment, η is detected. f =92%. Preset fill threshold η th =95%. Based on the deviation Δη = η from the detected fill rate and the preset threshold. th -η f If the value is 3%, then the film conveying speed during the roll forming process should be reduced. The adjusted film conveying speed is: v1=1.0×(1-0.03 / 0.95)=1.0×0.968≈0.97m / min The reduced speed increases the residence time of the film in the heating zone and imprinting unit, allowing the material more time to soften and flow. After adjustment, subsequent film testing showed that the fill rate was restored to 96%, resulting in a roll-formed film with a qualified fill rate.

[0077] Step S6: The roll-formed film is subjected to gradient cooling in the stress relief zone. The gradient cooling includes a first temperature zone and a second temperature zone. The temperature and cooling rate of the second temperature zone are lower than those of the first temperature zone. At the same time, the third pressure is reduced in the second temperature zone. The reduction rate is determined based on the shrinkage characteristics of the molecular chain orientation formed by directional stretching during the cooling process, and the cooled microfluidic chip is obtained. Please see Figure 4 The flowchart shown is a step S6 of the hot pressing molding method for microfluidic chips according to an embodiment of the present invention.

[0078] Specifically, step S6 includes: Step S61: Cool the rolled film in the stress relief zone at a first temperature zone using a first cooling rate. Step S62: Monitor the film temperature in real time. When the film temperature drops to the first preset temperature, enter the second temperature zone for cooling. Step S63: During the cooling process in the second temperature zone, a second cooling rate lower than the first cooling rate is adopted, while the third pressure is reduced. The rate of pressure reduction is determined based on the retraction characteristics of the molecular chain orientation formed by directional stretching during the cooling process. Step S64: When the film temperature drops to the second preset temperature, cooling is complete, and the cooled microfluidic chip is obtained.

[0079] Specifically, the roll-formed film with a qualified fill rate is fed into the stress relief zone for gradient cooling: First temperature zone: Cooling from molding temperature T20≈120℃ to the first preset temperature Tp1: =110℃ The first cooling rate β1 is used. The calculation of β1 is as follows:

[0080] Where t1 is the cooling time of the first temperature zone (3s~5s), in this embodiment t1=2s, and the calculation is as follows: β1 = (120 - 110) / 2 = 5℃ / s Second temperature zone: Cooling from 110℃ to the second preset temperature Tp2=Tg-10℃=95℃, using the second cooling rate β2:

[0081] Where, γ c The cooling rate coefficient is (0.3~0.5). In this embodiment, γc=0.4 and β2=2℃ / s are used, satisfying β2<0.5<β1. It can be understood that slow cooling allows the material sufficient time to relax the molecular chains and release residual stress within the glass transition range (105℃±10℃). If the cooling rate is too fast (>3℃ / s), the molecular chains do not have enough time to relax, the stress is frozen, and the chip warps; if the cooling rate is too slow (<1℃ / s), the production efficiency decreases.

[0082] The directional stretching process in step S1 orients the PMMA molecular chains along the stretching direction, storing reversible elastic strain energy. During cooling, when the temperature drops below the glass transition temperature, the oriented molecular chains tend to retract, resulting in shrinkage in the same direction as the stretching, known as the shrinkage effect. The shrinkage curve of the PMMA film during cooling was determined by thermomechanical analysis (TMA), revealing an average shrinkage rate of 0.03% / ℃ in the glass transition range (95℃~110℃), which translates to a pressure change rate of approximately 0.027 MPa / ℃.

[0083] During the cooling process in the second temperature zone, the third pressure is simultaneously controlled to decrease linearly with decreasing temperature. The final pressure decay value, Pend, is calculated as follows:

[0084] Where Pcomp is the required compensation pressure (MPa). Pcomp=θ·E θ=γth·ΔTcool Where θ is the volume shrinkage rate, E is the elastic modulus (MPa), γth is the thermal shrinkage coefficient (% / ℃), and ΔTcool is the cooling temperature difference (℃). In this embodiment, ΔTcool = Tp1 - Tp2 = 15℃, γth = 0.5% / ℃ = 0.005 / ℃, θ = 0.005 × 15 = 0.075, E ≈ 8 MPa, Pcomp = 0.075 × 8 = 0.6 MPa, Pend = 1.0 - 0.6 = 0.4 MPa. Considering the need to maintain a certain holding pressure in the actual process, Pend = 0.6 MPa (60% of the initial value) is taken. The decay rate k = (P3 - Pend) / ΔTcool = (1.0 - 0.6) / 15 ≈ 0.027 MPa / ℃, which matches the retraction rate (0.027 MPa / ℃). In actual control, it is executed at 0.03 MPa / ℃ to synchronize the pressure decay with the molecular chain retraction, achieving "replenishment as it retracts".

[0085] During the cooling process in the second temperature zone, the third pressure linearly decreases from 1.0 MPa to 0.6 MPa at a rate of 0.03 MPa / ℃. The pressure decrease is synchronized with the molecular chain retraction, which effectively compensates for the cooling contraction and releases residual stress, resulting in a cooled microfluidic chip.

[0086] Step S7: Based on the actual filling rate of the roll-formed film, a cross-linked modified layer is formed on the microstructure surface of the cooled microfluidic chip, wherein the glass transition temperature of the cross-linked modified layer is higher than that of the thermoplastic film. The cross-linked modified layer described in step S7 is formed by initiating a cross-linking reaction through ultraviolet light irradiation, electron beam irradiation, or plasma treatment; Among them, the ultraviolet irradiation parameters, electron beam irradiation energy parameters, and plasma treatment parameters are determined based on the actual fill rate of the roll-formed film.

[0087] In this embodiment, the crosslinked modified layer is formed by initiating a crosslinking reaction through ultraviolet light irradiation. The ultraviolet light irradiation parameters are determined based on the actual fill rate of the roll-formed film.

[0088] After roll forming, the microstructure filling rate ηf was found to be 92% by laser confocal microscopy. Since the filling rate is slightly lower than the preset filling threshold of 95%, it indicates that there may be minor defects on the surface of the microstructure. To compensate for these defects, ultraviolet light irradiation, electron beam irradiation, or plasma treatment were used to enhance the density of the cross-linked modified layer.

[0089] Understandably, the formation parameters of the cross-linked modified layer, such as ultraviolet light intensity, irradiation time, plasma power, electron beam energy, and irradiation dose, are determined based on the actual filling rate of the microstructure after roll forming. When the filling rate is too low, there may be micro-defects on the surface of the microstructure, requiring enhanced cross-linking treatment (increasing strength, extending time, increasing power or energy) to make the cross-linked layer denser, thereby compensating for the micro-defects on the surface of the microstructure. Parameter adjustment formula: X new =X base ×(1+Δη / η th ) Among them, X base The basic parameter (the standard parameter when the fill rate is qualified), Δη is the fill rate deviation, and η is the metric. th The preset fill threshold; In this embodiment, the following ultraviolet light irradiation parameters are used: The basic parameters are: ultraviolet wavelength: 254nm, ultraviolet light intensity: 10mW / cm², and irradiation time: 10 minutes. When the fill rate is low, the processing parameters are increased proportionally to compensate for the forming defects on the microstructure surface and improve the overall performance of the chip.

[0090] Light intensity: I = 10 × (1 + Δη / ηth) = 10 × (1 + 0.03 / 0.95) ≈ 10.3 mW / cm² Irradiation time: t = 10 × (1 + Δη / ηth) = 10 × (1 + 0.03 / 0.95) ≈ 10.3 minutes The cooled microfluidic chip was placed in an ultraviolet irradiation device and irradiated for 10.3 minutes with ultraviolet light at a wavelength of 254 nm and an intensity of 10.3 mW / cm² under nitrogen protection, forming a cross-linked modified layer on the microstructure surface. DSC measurement showed that the modified layer had a Tg of 125°C, higher than the PMMA matrix's 105°C. This cross-linked modified layer ensures that the microstructure surface remains stable and does not soften or deform during the second hot-pressing composite process.

[0091] Example 2: In this embodiment, in step 5, ηf=88%, Δη=7%, and the deviation is determined to be 5%≤Δη<10%. Therefore, the temperature of the second heating zone is adjusted to T200=T20+ΔTH, where ΔTH=(Δη / η) th )×(T2-T30); ΔTH=(0.07 / 0.95)×(120-112)=0.074×8≈0.59℃, rounded to 1℃, T200=120+1=121℃.

[0092] In this embodiment, the crosslinked modified layer is formed by initiating a crosslinking reaction through plasma treatment. The plasma treatment parameters are determined based on the actual fill rate of the roll-formed film. The basic parameters are: plasma power 100W, treatment time 5 minutes; After roll forming, the microstructure filling rate ηf was found to be 88% by laser confocal microscopy. Since the filling rate is lower than the preset filling threshold of 95%, the deviation Δη = 7%, indicating significant forming defects on the microstructure surface. To compensate for these defects, the following plasma treatment parameters were used in this embodiment: Gas being processed: Oxygen; Power: P=100×(1+Δη / ηth)=100×(1+0.07 / 0.95)≈107W; Processing time: t = 5 × (1 + Δη / ηth) = 5 × (1 + 0.07 / 0.95) ≈ 5.4 minutes; The cooled microfluidic chip was placed in a plasma treatment device and treated with 107W for 5.4 minutes in an oxygen atmosphere to form a cross-linked modified layer on the surface of the microstructure. DSC measurement showed that the modified layer had a Tg of 122℃, higher than the PMMA substrate's 105℃.

[0093] Example 3: Unlike Example 1, in this example, in step 5, ηf = 82%, Δη = 13%, the deviation Δη is determined to be ≥ 10%, and the second pressure P20 = P2 × (1 + Δη / η) is adjusted. th =2.0×(1+0.13 / 0.95)=2.0×1.137≈2.27MPa, rounded to 2.3MPa.

[0094] In this embodiment, the crosslinking modified layer is formed by initiating a crosslinking reaction through electron beam irradiation. The electron beam irradiation energy parameters are determined based on the actual fill rate of the roll-formed film. The basic parameters are: electron beam energy 150 keV and irradiation dose 50 kGy. When the fill rate is low, the processing parameters are increased proportionally to compensate for molding defects on the microstructure surface and improve the overall performance of the chip.

[0095] After roll forming, the microstructure fill rate ηf was measured to be 82% using a laser confocal microscope. Since the fill rate is far below the preset fill threshold of 95%, the deviation Δη = 13%, indicating severe defects on the microstructure surface. To compensate for these defects, the following electron beam irradiation parameters were used in this embodiment: Electron beam energy: E = 150 × (1 + Δη / ηth) = 150 × (1 + 0.13 / 0.95) ≈ 170 keV; Irradiation dose: D = 50 × (1 + Δη / ηth) = 50 × (1 + 0.13 / 0.95) ≈ 57 kGy; Irradiation time: Calculated based on electron beam intensity, approximately 35 seconds; The cooled microfluidic chip was placed in an electron beam irradiation device and irradiated for approximately 35 seconds under nitrogen protection using an electron beam energy of 170 keV and an irradiation dose of 57 kGy, forming a cross-linked modified layer on the surface of the microstructure. DSC measurement showed that the modified layer had a Tg of 124℃, higher than the 105℃ of the substrate PMMA.

[0096] Comparative Example 1: This comparative example uses a traditional hot pressing method to prepare a microfluidic chip with a microchannel structure using PMMA thermoplastic film (glass transition temperature Tg=105℃) as the substrate.

[0097] A 0.5 mm thick PMMA film was dried in a vacuum drying oven at 60°C for 2 hours to remove moisture. Without directional stretching, the film was placed in a hot press with the mold temperature set to 120°C for 5 minutes, ensuring the film temperature reached above Tg. After temperature stabilization, a constant pressure of 2.0 MPa was applied and held for 5 minutes. After holding, the film was allowed to cool naturally to room temperature and demolded to obtain the microfluidic chip. No surface modification was performed.

[0098] Comparative Example 2: This comparative example uses the method of CN119840140A to prepare a microfluidic chip with a microchannel structure using PMMA thermoplastic film (glass transition temperature Tg=105℃) as the substrate.

[0099] A 0.5 mm thick PMMA film was dried in a vacuum drying oven at 60°C for 2 hours to remove moisture, without directional stretching. The film was then placed in a hot press with the mold temperature set to 120°C for 5 minutes. A pressure of 2.0 MPa was applied and held for 5 minutes. During the holding period, no pressure change rate was monitored; the holding time was controlled solely by a preset time. Cooling to room temperature was performed at a single cooling rate (3°C / s), without two-stage cooling or pressure decay control. No surface modification was performed after demolding.

[0100] Comparative Example 3: This comparative example uses a hot-pressing method without feedback adjustment to prepare a microfluidic chip with a microchannel structure using PMMA thermoplastic film (glass transition temperature Tg=105℃) as the substrate.

[0101] A first pressure of 0.5 MPa is applied. When the film surface temperature reaches 115°C, the second pressure of 2.0 MPa is switched. During the application of the second pressure, the pressure change rate is not monitored, and the pressure holding time is controlled only by a preset time (5 seconds). Then, the third pressure of 1.0 MPa is switched.

[0102] The rest is the same as in Example 1.

[0103] Comparative Example 4: This comparative example uses a gradient-free hot-pressing method to prepare a microfluidic chip with a microchannel structure using PMMA thermoplastic film (glass transition temperature Tg=105℃) as the substrate.

[0104] Cooling to room temperature is achieved using a single cooling rate (5℃ / s), without two-stage cooling or pressure decay control.

[0105] The rest is the same as in Example 1.

[0106] Comparative Example 5: This comparative example uses a hot-pressing method without surface modification to prepare a microfluidic chip with a microchannel structure using PMMA thermoplastic film (glass transition temperature Tg = 105℃) as the substrate. The rest is the same as in Example 1.

[0107] Performance tests were conducted on Examples 1-3 and Comparative Examples 1-5, and the results are shown in the table below: Table 1. Performance comparison results of the embodiments of the present invention and comparative examples.

[0108] As can be seen from the test results in Table 1, Embodiments 1-3 of the present invention are significantly superior to the comparative embodiments in all performance indicators.

[0109] Regarding the microstructure filling accuracy, Example 1 achieved 98.5%, Example 2 97.8%, and Example 3 98.2%, representing improvements of 9.3, 8.6, and 9.0 percentage points respectively compared to Comparative Example 1 (traditional isothermal and isobaric method) 89.2%, and improvements of 7.0, 6.3, and 6.7 percentage points respectively compared to Comparative Example 2 (single feedback method) 91.5%. This indicates that the real-time monitoring of pressure change rate and dynamic pressure switching based on material state of the present invention can more accurately determine the timing of filling completion, thereby achieving higher filling accuracy.

[0110] Regarding chip warpage, the warpage values ​​of Examples 1-3 were 0.08%, 0.09%, and 0.08%, respectively, which were significantly lower than 0.42% in Comparative Example 1 and 0.32% in Comparative Example 4 (without gradient cooling). This demonstrates that two-stage gradient cooling and pressure decay control can effectively release residual stress and reduce chip warpage by more than 70%.

[0111] Regarding batch-to-batch dimensional deviations, the deviations in Examples 1-3 were ±1.2 μm, ±1.5 μm, and ±1.3 μm, respectively, which were significantly better than ±5.8 μm in Comparative Example 1 and ±4.5 μm in Comparative Example 2. This indicates that the deviation graded feedback adjustment mechanism can effectively overcome batch-to-batch material differences, ensure product consistency, and reduce batch-to-batch dimensional deviations by more than 65%.

[0112] Regarding surface roughness, the Ra values ​​of Examples 1-3 are 45 nm, 48 nm, and 46 nm, respectively, which are superior to 95 nm in Comparative Example 1 and 88 nm in Comparative Example 2. This indicates that the multi-segment temperature and pressure control combined with gradient cooling process of the present invention can obtain a smoother microstructure surface.

[0113] Regarding high-temperature dimensional stability, the dimensional change rate at 120°C was only 0.3% in Examples 1 and 3, and 0.4% in Example 2, which is more than 70% lower than the 1.2% in Comparative Example 5 without surface modification. This indicates that the cross-linked modified layer can significantly improve the thermal stability of the chip, enabling the microchannel surface to remain stable within the operating temperature range.

[0114] In terms of molding efficiency, the electron beam irradiation modification treatment in Example 3 takes only 30 seconds, with the highest molding efficiency (relative value 102%); the ultraviolet irradiation in Example 1 requires 10 minutes, with an efficiency of 100%; and the plasma treatment in Example 2 requires 5 minutes, with an efficiency of 98%. All three modification methods can meet production requirements and can be selected according to the actual application scenario.

[0115] Based on the above analysis, the performance of Comparative Example 3 (without feedback adjustment) and Comparative Example 4 (without gradient cooling) are between those of the Examples and Comparative Example 1. Comparative Example 5 (without surface modification) is significantly inferior to the Examples in terms of high-temperature stability. This further illustrates that the various technical features of the present invention are synergistic and indispensable, and together they have achieved a leapfrog upgrade of the hot pressing process from "experience-driven" to "material response-driven".

[0116] The technical solution of the present invention has been described above with reference to the preferred embodiments shown in the accompanying drawings. However, it will be readily understood by those skilled in the art that the scope of protection of the present invention is obviously not limited to these specific embodiments. Without departing from the principles of the present invention, those skilled in the art can make equivalent changes or substitutions to the relevant technical features, and the technical solutions after these changes or substitutions will all fall within the scope of protection of the present invention.

Claims

1. A method for hot-pressing microfluidic chips, characterized in that, include: Step S1: The thermoplastic film is subjected to directional stretching treatment to form molecular chain orientation along the stretching direction inside the film. After drying, a pretreated film is obtained. Step S2: Obtain the glass transition temperature of the pretreated film, and determine the temperatures of the first heating zone, the second heating zone, and the third heating zone based on the viscoelastic properties of the film, so as to perform gradient heating on the pretreated film to obtain the preheated film. Step S3: Apply a first pressure to the preheated film for roll forming to form a microfluidic groove structure. During the roll forming process, the surface temperature of the preheated film is detected simultaneously. Based on the result that the surface temperature reaches a preset temperature, it is determined to switch to applying a second pressure, wherein the second pressure is greater than the first pressure. Step S4: During the application of the second pressure, the post-pressure rebound rate of the preheated film is monitored in real time to adjust the second pressure. Based on the film pressure change rate continuously monitored after the second pressure adjustment, it is determined whether the microstructure filling is completed. After the filling is completed, the third pressure is used for rolling to obtain the rolled film. Step S5: Detect the microstructure size characteristics of the rolled film to determine the fill rate. If the fill rate is lower than the preset fill threshold, dynamically adjust the film conveying speed during the roll forming process according to the deviation to obtain a roll-formed film with a qualified fill rate. Step S6: The roll-formed film is subjected to gradient cooling in the stress relief zone. The gradient cooling includes a first temperature zone and a second temperature zone. The temperature and cooling rate of the second temperature zone are lower than those of the first temperature zone. At the same time, the third pressure is reduced in the second temperature zone. The reduction rate is determined based on the shrinkage characteristics of the molecular chain orientation formed by directional stretching during the cooling process, and the cooled microfluidic chip is obtained. Step S7: Based on the actual filling rate of the roll-formed film, a cross-linked modified layer is formed on the microstructure surface of the cooled microfluidic chip, wherein the glass transition temperature of the cross-linked modified layer is higher than that of the thermoplastic film.

2. The hot-pressing method for microfluidic chips according to claim 1, characterized in that, Step S2, which determines the temperatures of the first heating zone, the second heating zone, and the third heating zone, includes: Step S21: Conduct a variable temperature creep test on the thermoplastic film by heating it at a constant temperature rise rate and applying constant pressure, collecting temperature and deformation data in real time, and plotting a temperature-deformation curve. Step S22: Based on the temperature-deformation curve, perform viscoelastic property analysis to determine the temperature corresponding to the first inflection point of the curve as the first reference temperature and the temperature corresponding to the second inflection point as the second reference temperature. Step S23: Determine the temperature of the first heating zone based on the first reference temperature; The temperature of the second heating zone is determined based on the second reference temperature; The temperature of the third heating zone is determined based on the glass transition temperature and the second reference temperature.

3. The hot-pressing molding method for microfluidic chips according to claim 2, characterized in that, The temperature of the third heating zone is determined based on the viscoelastic response characteristics of the thermoplastic film in the glass transition region, so that the film remains in a highly elastic state before entering the imprinting unit.

4. The hot-pressing method for forming a microfluidic chip according to claim 1, characterized in that, Step S4 includes: Step S41: During the application of the second pressure, the post-compression rebound rate data of the film is collected in real time, and the second pressure is dynamically adjusted based on the post-compression rebound rate. Step S42: After the second pressure adjustment is completed, the pressure data of the second pressure is collected in real time. Step S43: Calculate the pressure change rate of the second pressure based on the pressure data of the second pressure; Step S44: Compare the absolute value of the pressure change rate with a preset pressure change rate threshold. If the absolute value of the pressure change rate is less than or equal to the preset pressure change rate threshold for a continuous preset time, the microstructure filling is determined to be complete. Step S45: Switch the applied pressure to a third pressure to maintain it until entering the gradient cooling phase.

5. The hot-pressing molding method for microfluidic chips according to claim 4, characterized in that, The ratio of the third pressure to the second pressure is determined based on the thermal shrinkage characteristics of the thermoplastic film. The heat shrinkage characteristic includes the heat shrinkage rate, and the ratio is positively correlated with the heat shrinkage rate of the film.

6. The hot-pressing method for forming a microfluidic chip according to claim 5, characterized in that, The ratio of the third pressure to the second pressure is in the range of 0.3 to 0.

7.

7. The hot-pressing method for forming a microfluidic chip according to claim 1, characterized in that, Step S5 further includes dynamically adjusting the temperature or pressure of the second heating zone based on the deviation: If the deviation is less than the first threshold, the film conveying speed during the roll forming process is reduced. If the deviation is greater than or equal to the first threshold and less than the second threshold, then the temperature of the second heating zone is increased; If the deviation is greater than or equal to the second threshold, then the second pressure is increased; The deviation is the difference between the preset filling threshold and the filling rate.

8. The hot-pressing method for forming a microfluidic chip according to claim 1, characterized in that, Step S6 includes: Step S61: Cool the rolled film in the stress relief zone at a first temperature zone using a first cooling rate. Step S62: Monitor the film temperature in real time. When the film temperature drops to the first preset temperature, enter the second temperature zone for cooling. Step S63: During the cooling process in the second temperature zone, a second cooling rate lower than the first cooling rate is adopted, while the third pressure is reduced. The rate of pressure reduction is determined based on the retraction characteristics of the molecular chain orientation formed by directional stretching during the cooling process. Step S64: When the film temperature drops to the second preset temperature, cooling is complete, and the cooled microfluidic chip is obtained.

9. The hot-pressing method for forming a microfluidic chip according to claim 1, characterized in that, The cross-linked modified layer described in step S7 is formed by initiating a cross-linking reaction through ultraviolet light irradiation, electron beam irradiation, or plasma treatment; Among them, the ultraviolet irradiation parameters, electron beam irradiation energy parameters, and plasma treatment parameters are determined based on the actual fill rate of the roll-formed film.

10. A hot-pressing system for microfluidic chips, used to perform the hot-pressing method for microfluidic chips according to any one of claims 1-9, characterized in that, include: The pretreatment unit is used to perform directional stretching on the thermoplastic film to form molecular chain orientation along the stretching direction inside the film, and to dry the stretched film. A gradient heating unit, which is connected to the pretreatment unit, is provided with a first heating zone, a second heating zone and a third heating zone in sequence along the film conveying direction, and is used to perform gradient heating on the pretreated film. An imprinting unit, which is connected to the gradient heating unit, is used to roll-form the preheated film. The imprinting unit includes an imprinting roller and a pressure control system. The pressure control system is used to apply a first pressure, a second pressure and a third pressure sequentially during the roll-forming process. The detection feedback unit, which is connected to the imprinting unit, is used to detect the film surface temperature and trigger the switching of the first pressure to the second pressure based on the detection result; to monitor the change rate of the second pressure and trigger the switching of the second pressure to the third pressure based on the detection result; and to detect the microstructure filling rate after roll forming and adjust the film conveying speed, the temperature of the second heating zone, or the second pressure according to the detection result. The stress relief cooling unit, which is connected to the detection feedback unit, is used to perform gradient cooling on the roll-formed film and reduce the third pressure during the cooling process. A surface modification unit, connected to the stress relief cooling unit, is used to form a cross-linked modification layer on the surface of the microstructure of the cooled microfluidic chip.

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