Preparation method of variable diameter PTFE / PI composite catheter

Abstract

The application is suitable for the technical field of precise forming of polymer composite materials, and provides a preparation method of a variable-diameter PTFE / PI composite catheter, which comprises the following steps: preparing a variable-diameter metal mandrel with a silver-plated layer on the surface; coating and solidifying PTFE inner lining and PI outer layer on the mandrel in sequence to obtain a preform; applying an axial temperature gradient to the mandrel, so that the temperature (T1) of the large-diameter end is higher than the temperature (T2) of the small-diameter end, and a demolding initial gap is generated between the mandrel and the pipe wall by differential thermal expansion; finally, the mandrel is pulled out from the small-diameter end to the large-diameter end to obtain an independent composite pipe; by means of the gradient heating demolding strategy, the technical problem that the inner lining is easily damaged during demolding of the variable-diameter structure is fundamentally solved, and the lossless demolding of the precise PTFE / PI composite structure is realized; the prepared catheter has the characteristics of super-smooth inner wall, high dimensional precision and excellent mechanical properties, and is particularly suitable for high-performance medical interventional catheters.

Description

Technical Field

[0001] This invention belongs to the field of precision molding technology of polymer composite materials, specifically relating to a variable diameter composite catheter with a polytetrafluoroethylene (PTFE) liner and a polyimide (PI) outer layer and its manufacturing method, which is particularly suitable for preparing medical interventional catheters with precise geometric parameters and an ultra-smooth inner surface. Background Technology

[0002] In the fields of high-end medical devices (such as neurointerventional catheters and cardiovascular catheters) and precision industry, there is an urgent need for micro polymer catheters with variable diameter structures. An ideal catheter requires an extremely smooth inner wall to reduce delivery resistance, while the outer wall must have excellent mechanical strength, dimensional stability and fatigue resistance. Composite material with low-friction coefficient PTFE as the inner lining and high-modulus, high-temperature resistant PI as the outer layer is an ideal solution to meet the above requirements.

[0003] Polytetrafluoroethylene (PTFE) has an extremely low coefficient of friction (0.04-0.06) and excellent biocompatibility, making it an ideal lining material; polyimide (PI) has high strength (tensile strength > 300 MPa), high modulus (3-4 GPa) and good dimensional stability, making it an ideal support layer material; combining these two materials can produce an ideal catheter that combines low friction and high strength.

[0004] Currently, there are two main technical approaches to manufacturing this type of composite pipe: One approach is to coat the inner wall of the pre-formed hollow PI tube with PTFE. However, this method suffers from uneven coating, poor adhesion, and difficulty penetrating small-diameter areas with large length-to-diameter ratios for micro and complex variable-diameter tubes. Another approach is direct molding on a mandrel. While this method achieves excellent inner surface quality, demolding becomes the biggest bottleneck for variable-diameter structures, especially those with large aspect ratios and short transition zones. Traditional demolding methods (such as rigid extraction after uniform heating) generate significant frictional resistance in the variable-diameter section, easily leading to scratches and tears in the delicate PTFE inner liner or damage to the PI outer layer structure. Furthermore, significant stress concentration occurs in the variable-diameter section during demolding, resulting in extremely low product yield.

[0005] In addition, while using soluble or fusible cores can avoid mechanical demolding damage, it also presents problems such as high cost, complex process, potential chemical residues, and difficulty in withstanding the high-temperature PI curing process. Summary of the Invention

[0006] The purpose of this invention is to overcome the shortcomings of existing technologies and provide a method for producing high-quality, high-efficiency variable-diameter PTFE / PI composite pipes, as well as high-performance composite pipe products obtained by this method. The core of this method lies in the innovative "gradient heating demolding" strategy, which fundamentally reduces the demolding resistance of the variable-diameter mandrel from a physical perspective.

[0007] This invention is achieved through a method for preparing a variable-diameter PTFE / PI composite catheter, comprising the following steps: S1: Provides a variable diameter metal mandrel with a silver-plated layer on its surface; the silver plating provides extremely low surface energy, which is beneficial for the later demolding of PTFE, while the excellent thermal conductivity of silver is the basis for achieving efficient and controllable gradient heating. S2: Coating the surface of the mandrel with a polytetrafluoroethylene (PTFE) coating and performing a first heat treatment to form a PTFE inner liner; S3: Coating the PI precursor coating onto the surface of the PTFE inner liner and performing a second heat treatment to imidize it, forming a PI outer layer, and obtaining a composite pipe preform; S4: An axial temperature gradient is applied to the mandrel (high temperature T1 at the larger diameter end, low temperature T2 at the smaller diameter end, T1 > T2). Due to the thermal expansion and contraction characteristics of metallic materials, the radial expansion (ΔD1) generated by the larger diameter end after heating will be much greater than the radial expansion (ΔD2) of the smaller diameter end, or even contraction. This differential thermal expansion creates a "wedge-shaped" gap starting point between the mandrel and the inner wall of the composite tube, starting from the larger diameter end. When the mandrel is pulled out in the direction from the smaller diameter end to the larger diameter end, the friction in the area with the greatest resistance at the larger diameter end is reduced sharply due to the pre-existing large gap, thus making the entire core pulling process smooth and stable, achieving non-destructive demolding of the fragile PTFE inner liner and PI outer layer.

[0008] Preferably, the ratio of the larger diameter end to the smaller diameter end of the variable diameter metal mandrel is less than 1.4:1, and the axial length of the variable diameter section is ≥3mm. If the ratio of the two diameters is too large, the smaller diameter end will reach the fracture stage in the subsequent demolding and core-pulling process, while the larger diameter end will not reach the elastic stage, thus demolding cannot be completed. If the variable diameter section is too short, stress concentration is easily caused in the variable diameter section, resulting in failure in the core-pulling and demolding process due to the fracture of the variable diameter section.

[0009] Preferably, the silver plating layer thickness is 3-10 μm, and the surface roughness Ra≤0.05 μm. It is preferable to first mirror polish the variable diameter metal substrate, and then obtain the silver plating layer on the surface by electroplating. When the silver plating layer is too thin, under high temperature imidization (380℃) for a long time, copper atoms (Cu) and silver atoms (Ag) will undergo severe solid-phase diffusion. Copper atoms diffuse through the silver layer to the surface and form cuprous oxide, which leads to a sharp increase in surface roughness and even the appearance of "copper spots", causing local adhesion of the PTFE liner and tearing during demolding. When the silver plating layer is too thick, due to the "edge effect" of the electroplating process, when the plating layer is too thick, plating layer accumulation will occur at the corner of the variable diameter transition zone, causing the actual profile of the mandrel to deviate from the design value, and ultimately making the inner diameter of the guide tube unqualified.

[0010] Preferably, in step S2, a polytetrafluoroethylene (PTFE) aqueous dispersion is coated onto the surface of the mandrel using a dip-coating method. The dispersion contains PTFE resin particles with a solid content of 50-60%, a pH value of 9-10, a particle size of 0.15-0.35 μm, and a density range of 1.51-1.54 g / cm³. 2 The solvent is water (36%), and the surfactant is 4%, specifically anionic surfactant with a viscosity range of 20 mPa·s to 600 mPa·s. The immersion speed is 5-10 mm / s, the leaching speed is 10-12 mm / s, and the immersion time is 8 s. A single immersion coating can form a 5-10 μm thick PTFE layer. If a 10-20 μm thick layer is required, 2-3 repeated immersion coatings can be used. After each immersion coating, the layers are dried at 100℃ and preheated at 250℃, and finally sintered at 385℃ to ensure a dense interlayer bond.

[0011] Preferably, in step S2, the first heat treatment is carried out in three temperature stages, with programmed curing in an oven: drying at 90-100℃ for 5-15 minutes, then preheating to 200-250℃ for 5-10 minutes, and finally sintering at 380-400℃ for 10-20 minutes; furthermore, the heating rate when heating from room temperature to 90-100℃ should not exceed 2℃ / min; the heating rate when heating from 90-100℃ to 200-250℃ should not exceed 5℃ / min; and the heating rate when heating from 200-250℃ to 380-400℃ should not exceed 8℃ / min; ultimately forming a dense PTFE film with a thickness of approximately 5-20μm.

[0012] Preferably, in step S3, a polyamic acid (PAA) solution is coated onto the surface of the PTFE liner by dip coating; further, the polyimide solution has a solid content of ≥15%, the solvent is N,N-dimethylacetamide with a content of ≤85%, and the viscosity range is 250-1500 mPa·s.

[0013] Preferably, in step S3, imidization is performed using a gradient heating method: the temperature is increased from room temperature to 150-200°C at a rate of 2-5°C / min and held for 10-30 minutes; then increased to 200-250°C at a rate of 2-5°C / min and held for 10-30 minutes; finally, the temperature is increased to 250-300°C at a rate of 1-3°C / min and held for 30-60 minutes; after natural cooling, a fully imidized PI outer layer with a thickness of 20-50 μm is formed.

[0014] Preferably, in step S4, a 1500-2000W hot air gun is used to heat the large-diameter end of the mandrel, with the air outlet 15-25mm away from the mandrel, an air velocity of 1.2-1.5m / s, and a heating angle of 20-50°. This ensures that the temperature T1 at the large-diameter end is stable at 130-150℃, while the temperature T2 at the small-diameter end remains at room temperature (20-30℃), creating an axial temperature difference of ≥110℃ (i.e., T1-T2≥110℃). A K-type thermocouple is attached to the surface of the mandrel to monitor the temperature in real time. The special fixture consists of a metal mold core and a silicone liner. The silicone layer has an annular groove that matches the outer diameter of the composite tube, ensuring accurate product positioning and preventing damage during heating and extraction. After maintaining the temperature difference for 3 minutes, the mandrel is extracted from the small-diameter end to the large-diameter end at a speed of 0.5-5mm / s, achieving non-destructive demolding.

[0015] In step S4, the temperature difference ΔT, preheating time, and core-pulling speed need to be optimized based on the results of orthogonal experiments. Experiments show that a temperature difference ΔT ≥ 110℃ is the critical condition for generating an effective demolding gap, at which point the expansion difference reaches more than 0.30μm; a preheating time of 3 minutes allows the core temperature to reach the set value and the gradient to be most stable; the core-pulling speed should be controlled within the range of 0.5-5mm / s, preferably 1-3mm / s, to balance efficiency and quality; for different diameter specifications, the parameters need to be adjusted according to the diameter ratio and the length of the diameter-changing section: when the diameter ratio is large or the diameter-changing section is short, the temperature difference should be increased (115-140℃), the preheating time extended (3.5-4.0min), and the core-pulling speed reduced (0.5-2.0mm / s).

[0016] The core mechanism of the gradient heating demolding step lies in utilizing the thermal expansion characteristics of metallic materials. According to the theory of thermal expansion, the formula for calculating the diameter change ΔD is: ΔD = α·D·ΔT, where α is the coefficient of thermal expansion of the material, D is the original diameter, and ΔT is the temperature change.

[0017] When an axial temperature gradient (T1>T2) is applied to the variable-diameter mandrel, the radial expansion ΔD1 caused by heating at the larger diameter end will be significantly greater than the expansion ΔD2 at the smaller diameter end; using the preferred oxygen-free copper mandrel of this invention (α≈1.7×10⁻⁶), -5 Taking / ℃ as an example, for a large diameter section of 0.9mm and a small diameter section of 0.74mm, under a temperature difference (ΔT) of approximately 125℃: Expansion of the coarse diameter section (0.9mm): ΔD_coarse = 0.00191mm Expansion of the narrower section (0.74mm): ΔD_fine = 0.00157mm The expansion difference ΔD_diff between the two can reach approximately 0.00034 mm.

[0018] This micron-level differential expansion actively creates an initial demolding gap in the region with the greatest resistance at the large-diameter end, thereby significantly reducing friction from the large-diameter end during subsequent extraction, achieving smooth and damage-free demolding.

[0019] The present invention also provides a variable diameter PTFE / PI composite catheter, which is prepared by any of the above methods; The composite pipe has a PTFE inner lining and a PI outer layer, with an inner surface roughness Ra≤0.04μm and a dimensional accuracy of ±0.003mm; The composite tube is suitable for medical interventional devices. The inner diameter of the larger diameter end of the composite tube ranges from 0.5 to 1.8 mm, and the inner diameter of the smaller diameter end ranges from 0.3 to 0.9 mm. The ratio of the inner diameter of the larger diameter end to the inner diameter of the smaller diameter end is less than or equal to 1.4:1.

[0020] Composite tubing is suitable for medical interventional devices. The inner diameter of the variable-diameter structure of the composite tubing varies from 0.9mm to 0.74mm, 1.2mm to 0.9mm, 0.7mm to 0.5mm, or 0.5mm to 0.3mm. That is, the inner diameter of the composite tubing can be selected from 0.9mm to 0.74mm, from 1.2mm to 0.9mm, from 0.7mm to 0.5mm, or from 0.5mm to 0.3mm.

[0021] Compared with the prior art, the embodiments of this application have the following main advantages: The method for preparing variable-diameter PTFE / PI composite conduits provided by this invention uses a variable-diameter metal mandrel with a silver-plated surface. A PTFE inner liner and a PI outer layer are formed by precision coating. Then, an axial temperature gradient (high temperature at the coarse end and low temperature at the fine end) is applied to the mandrel. The differential thermal expansion is used to actively create a demolding gap, which fundamentally avoids the scratching and tearing of the PTFE / PI composite layer by mechanical extraction, achieves non-destructive demolding, and significantly improves the product yield.

[0022] Through precision coating and segmented curing processes, as well as non-destructive demolding processes, the PTFE inner liner has an ultra-smooth surface (Ra≤0.04μm), the PI outer layer has excellent mechanical properties, and the composite catheter has a dimensional accuracy of ±0.003mm, meeting the requirements of medical interventional catheters.

[0023] The production cost is lower than that of soluble / fusible core technology, and the process is efficient, stable, and environmentally friendly, with no risk of chemical residue. Attached Figure Description

[0024] Figure 1 This is a flowchart of the preparation method of the variable diameter PTFE / PI composite catheter provided by the present invention.

[0025] Figure 2 This is a schematic diagram of the variable diameter mandrel structure provided by the present invention (0.9mm→0.74mm).

[0026] Figure 3 This is a schematic diagram of the variable diameter PTFE / PI composite catheter product provided by the present invention. Detailed Implementation

[0027] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs; the terminology used herein in the specification of the application is for the purpose of describing particular embodiments only and is not intended to be limiting of the application; the terms "comprising" and "having," and any variations thereof, in the specification, claims, and foregoing drawings of this application are intended to cover non-exclusive inclusion. The terms "first," "second," etc., in the specification, claims, or foregoing drawings of this application are used to distinguish different objects, not to describe a particular order.

[0028] In this document, the term "embodiment" means that a particular feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of this application. The appearance of this phrase in various places throughout the specification does not necessarily refer to the same embodiment, nor is it a separate or alternative embodiment mutually exclusive with other embodiments. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments.

[0029] Example 1: Preparation of a 0.9mm to 0.74mm diameter reducing catheter 1. Mandrel Preparation Material: Oxygen-free copper rod (purity > 99.95%); Dimensions: Total length 500±20mm, of which the coarse diameter section is Φ0.900mm and 80mm long, the transition section is 3.00mm long and has a taper angle of 1.53°, and the fine diameter section is Φ0.740mm and 420mm long; Machining process: Precision CNC grinding (dimensional accuracy ±0.005mm) → Chemical mechanical polishing (Ra optimized from 0.02μm to 0.008μm) → Silver electroplating (thickness 5.0±0.2μm, Ra=0.012μm).

[0030] 2. Preparation of PTFE inner liner Coating: PTFE dispersion (60% solids content, 25 mPa·s viscosity); Coating parameters: ambient temperature 23±0.5℃, RH 45±5%, immersion speed 8mm / s, immersion speed 10mm / s, immersion time 8s; Curing procedure: Dry at 90℃ for 10 minutes, preheat at 200℃ for 10 minutes, then sinter at 385℃ for 10 minutes; Results: The PTFE layer thickness was 7.3±0.4μm, and it was dense and defect-free.

[0031] 3. Preparation of PI outer layer Solution: Polyamic acid (solid content 18%, viscosity 1500 mPa·s); Coating: Dip coating, lifting speed 1.2mm / s, 3 coats; Imidization process: room temperature → 150℃ (2℃ / min) → hold for 30 min → 150℃ → 250℃ (2℃ / min) → hold for 30 min → 150℃ → 300℃ (1℃ / min) → hold for 60 min, then cool with the furnace; Results: The PI layer thickness was 20.2±0.8μm, and the degree of imidization was >98%.

[0032] 4. Gradient heating demolding Temperature control: Temperature of the large diameter section T1 = 150 ± 5℃, temperature of the small diameter section T2 = 25 ± 2℃, temperature difference ΔT = 125 ± 5℃; Preheating time: 3 minutes; Core pulling parameters: speed 0.6mm / s, direction from the narrow diameter section to the wide diameter section.

[0033] 5. Product performance testing Dimensional measurements: Inner diameter of the rough section: 0.897 mm; wall thickness of the rough section: 0.065 mm; Inner diameter of the narrow section: 0.743 mm; wall thickness of the narrow section: 0.058 mm; length of the transition section: 3.02 mm. Mechanical properties: tensile strength 145±15MPa, coefficient of friction 0.029±0.003.

[0034] Example 2: Verification of structures with different diameters Specifications: 1.2mm → 0.9mm, L = 4mm; 1. Mandrel Preparation Substrate: Oxygen-free copper (purity > 99.95%); Dimensions: Total length 600±20mm, large diameter end Φ1.200±0.003mm, small diameter end Φ0.900±0.003mm, variable diameter section length L=4.00±0.05mm, cone angle 2.15°; Processing technology: Precision CNC grinding (dimensional accuracy ±0.005mm) → chemical mechanical polishing (Ra 0.015μm) → silver electroplating (thickness 5.0±0.3μm, Ra≤0.015μm after plating).

[0035] 2. Preparation of PTFE inner liner Dispersion formulation: PTFE solid content 60%, particle size 0.15-0.30μm, anionic surfactant 4%, deionized water 36%, pH value 9.5, viscosity 550 mPa·s; Dip-coating parameters: immersion speed 8mm / s, immersion speed 12mm / s, immersion time 8s, immersion number of times; Sintering process: 400℃ × 10 minutes; PTFE layer parameters: thickness 20±5μm.

[0036] 3. Preparation of PI outer layer PAA solution: solid content 20%, viscosity 1800 mPa·s, solvent DMAc; Dip coating parameters: lifting speed 1.5mm / s, coating times 6; Interlayer treatment: Pre-dry at 80°C for 20 minutes after each coating; Imidization process: Room temperature → 150℃ (2℃ / min) → Hold for 30 min → 150℃ → 250℃ (2℃ / min) → Hold for 30 min → 150℃ → 300℃ (1℃ / min) → Hold for 60 min, then cool with the furnace. PI layer parameters: thickness 40±3μm, imidization degree >98%, surface Ra=0.10μm.

[0037] 4. Gradient heating demolding Temperature control: T1 = 135 ± 2℃ at the large diameter end, T2 = 25 ± 2℃ at the small diameter end, temperature difference ΔT = 110℃; Preheating time: 3.0 minutes; Core pulling parameters: speed 2.5mm / s, direction from small diameter end to large diameter end.

[0038] 5. Product performance testing Dimensional accuracy: Inner diameter of large diameter end 1.198±0.003mm, inner diameter of small diameter end 0.901±0.003mm, length of variable diameter section 4.02±0.05mm, wall thickness uniformity ±2.5μm; Mechanical properties: tensile strength 185±12MPa, elongation at break 16.5%; Demolding results: With a sample size of 25 pieces, the demolding success rate was 97%, and the yield rate was 90.6%.

[0039] Test case I. Test on the effect of diameter ratio (D / d) Test conditions: Variable diameter section length L = 3 mm, drawing speed 1 mm / s, temperature gradient T1 = 150℃, T2 = 25℃. Results are shown in Table 1 below: As shown in the table above, the larger the diameter ratio, the greater the radial interference (or the amount of mandrel compression deformation required) under the same axial displacement. When the gap created by gradient heating is limited, the "wedge effect" of the variable diameter section becomes too severe after exceeding 1.4:1, exceeding the elastic limit of PTFE material.

[0040] II. Multi-factor orthogonal experimental design 1. To comprehensively evaluate the impact of various process parameters on the demolding effect, a three-factor, three-level orthogonal experiment was conducted, as shown in Table 2 below: Test specimens: 0.9mm→0.74mm reducing composite pipe (L=3mm), n=10 per test group, and the product damage rate is statistically analyzed.

[0041] 2. The results and analysis of the orthogonal experiment are shown in Table 3 below: The orthogonal test results in Table 3 show that the order of influence of each factor on the product damage rate is: temperature difference ΔT > preheating time > core pulling speed. Among them, when the temperature difference ΔT is 110℃ or above (tests 2, 3, 5, and 9), the product damage rate is controlled below 8%. When the temperature difference drops to 100℃ (tests 1, 6, and 8), the damage rate increases significantly to 18%-25%. A preheating time of more than 3 minutes combined with an appropriate temperature difference can achieve better results.

[0042] It should be noted that, for the sake of simplicity, the foregoing embodiments are all described as a series of actions. However, those skilled in the art should understand that the present invention is not limited to the described order of actions, as some steps may be performed in other orders or simultaneously according to the present invention. Furthermore, those skilled in the art should also understand that the embodiments described in the specification are preferred embodiments, and the actions and modules involved are not necessarily essential to the present invention.

[0043] It should be understood that the disclosed apparatus can be implemented in other ways, as illustrated in the embodiments provided in this application. For example, the apparatus embodiments described above are merely illustrative; the division of units described above is only a logical functional division, and in actual implementation, there may be other division methods. For example, multiple units or components may be combined or integrated into another system, or some features may be ignored or not executed. Furthermore, the coupling or communication connections shown or discussed may be through some interfaces; the indirect coupling or communication connections between devices or units may be telecommunications or other forms.

[0044] The units described above as separate components may or may not be physically separate. The components shown as units may or may not be physical units; that is, they may be located in one place or distributed across multiple network units. Some or all of the units can be selected to achieve the purpose of this embodiment according to actual needs.

[0045] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit the scope of protection of the invention. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on these embodiments, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art can still combine, add, delete, or otherwise adjust the features of the various embodiments of the present invention according to the circumstances without conflict or creative effort, thereby obtaining different technical solutions that do not fundamentally depart from the concept of the present invention. These technical solutions also fall within the scope of protection of the present invention.

Claims

1. A method for preparing a variable diameter PTFE / PI composite catheter, characterized in that, include: S1: Provide a variable diameter metal mandrel with a silver-plated surface; S2: Coating the surface of the mandrel with a polytetrafluoroethylene (PTFE) coating and performing a first heat treatment to form a PTFE inner liner; S3: Coating the surface of the PTFE inner liner with a polyimide (PI) precursor coating and performing a second heat treatment to imidize it, forming a PI outer layer, and obtaining a composite tube preform fixed on the mandrel; S4: Apply an axial temperature gradient to the mandrel in the preform, so that the temperature T1 at the larger diameter end of the mandrel's variable diameter section is higher than the temperature T2 at the smaller diameter end; while maintaining this temperature gradient, extract the mandrel from the preform to obtain an independent variable diameter PTFE / PI composite pipe.

2. The method for preparing the variable diameter PTFE / PI composite catheter as described in claim 1, characterized in that, The variable diameter metal mandrel is made of copper; The thickness of the silver plating layer is 3-10 μm, and the surface roughness Ra≤0.05 μm.

3. The method for preparing the variable diameter PTFE / PI composite catheter as described in claim 1, characterized in that, The ratio of the diameter of the larger diameter end of the variable-diameter metal mandrel to the diameter of the smaller diameter end is less than or equal to 1.4:1; The axial length of the variable diameter section is greater than or equal to 3 mm.

4. The method for preparing the variable diameter PTFE / PI composite catheter as described in claim 1, characterized in that, In step S2, a PTFE aqueous dispersion with a solid content of 50-60% is coated by dip coating. The viscosity of the dispersion is 20-600 mPa·s, and the dip coating speed is 5-10 mm / s.

5. The method for preparing the variable diameter PTFE / PI composite catheter as described in claim 1, characterized in that, In step S2, the first heat treatment includes: drying at 90-100°C for 10-15 minutes, then preheating to 200-250°C for 5-10 minutes, and finally sintering at 380-400°C for 10-20 minutes.

6. The method for preparing the variable diameter PTFE / PI composite catheter as described in claim 1, characterized in that, In step S3, a polyamic acid (PAA) solution is coated onto the surface of the PTFE liner using a dip-coating method.

7. The method for preparing the variable diameter PTFE / PI composite catheter as described in claim 1, characterized in that, In step S3, the second heat treatment is gradient heating imidization, which includes: heating from room temperature to 150-200℃ at 2-5℃ / min and holding for 10-30 minutes, then heating to 200-250℃ at 2-5℃ / min and holding for 10-30 minutes, and finally heating to 250-300℃ at 1-3℃ / min and holding for 30-60 minutes.

8. The method for preparing the variable diameter PTFE / PI composite catheter as described in claim 1, characterized in that, In step S4, the large-diameter end of the mandrel is heated to maintain its temperature T1 at 130℃-150℃, while the small-diameter end temperature T2 is maintained at 20℃-30℃, forming a temperature difference of 110℃ or higher. After maintaining this temperature difference for 3 minutes, the mandrel is pulled out from the small-diameter end to the large-diameter end at a speed of 0.5-5mm / s.

9. A variable diameter PTFE / PI composite catheter, characterized in that, Prepared by the method of any one of claims 1-8; The composite pipe has a PTFE inner lining and a PI outer layer, with an inner surface roughness Ra≤0.04μm and a dimensional accuracy of ±0.003mm; The composite tube is suitable for medical interventional devices. The inner diameter of the larger diameter end of the composite tube ranges from 0.5 to 1.8 mm, and the inner diameter of the smaller diameter end ranges from 0.3 to 0.9 mm. The ratio of the inner diameter of the larger diameter end to the inner diameter of the smaller diameter end is less than or equal to 1.4:

1.

10. The variable diameter PTFE / PI composite catheter as described in claim 9, characterized in that, The composite tube is suitable for medical interventional devices. The inner diameter of the variable diameter structure of the composite tube varies from 0.9mm to 0.74mm, 1.2mm to 0.9mm, 0.7mm to 0.5mm, or 0.5mm to 0.3mm.

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