Preparation method of optical fiber pressure sensor based on full polylactic acid

Abstract

The invention aims to provide a preparation method of an all-polylactic acid-based optical fiber pressure sensor with high stability, which comprises the following steps: preparing a polylactic acid-based pressure-sensitive diaphragm with a multilayer reflection structure, and drawing a polylactic acid-based optical fiber with a core-cladding structure; processing a polylactic acid-based base and a polylactic acid-based perforated plate with through holes; and low-temperature plasma bonding of the assembly is realized through interface activation and precise alignment. The low-temperature bonding has a remarkable improvement effect on the degradation synchronism and the structural stability of the sensor, so that the degradation rate difference of each part is reduced to 12%, and the overall degradation consistency of the sensor in an object is ensured; the laser micromachining technology improves the optical properties of the sensor, such as light transmittance and interference fringe contrast, and effectively enhances the sensitivity of the sensor.

Description

Technical Field

[0001] The present invention relates to the technical field of sensor manufacturing, and in particular to a method for preparing an optical fiber pressure sensor based on all-polylactic acid. Background Art

[0002] With the rapid development of implantable medical devices, fiber optic pressure sensors, thanks to their tiny size and excellent biocompatibility, can be implanted in the human body to provide real-time, accurate measurement of physiological parameters such as blood pressure, intracranial pressure, and pulmonary pressure, providing important evidence for disease diagnosis and treatment. Traditional fiber optic pressure sensors use quartz or non-degradable plastic optical fibers, requiring secondary surgery for removal after implantation, increasing medical risks. Non-degradable materials can also trigger chronic inflammatory reactions. Quartz optical fibers are also prone to breakage, making it difficult to match the mechanical properties of biological tissue. To address these issues, the applicant submitted an invention patent on the same day, entitled "Fiber Optic Pressure Sensor Based on All-Polylactic Acid." Given the properties of polylactic acid (PLA), traditional sensor preparation methods are not applicable. A method for preparing a pressure sensor compatible with a polylactic acid (PLA) substrate is needed. Summary of the Invention

[0003] The purpose of the present invention is to provide a method for preparing a highly stable optical fiber pressure sensor based on polylactic acid.

[0004] The operating principle of a Fabry-Perot cavity fiber pressure sensor is as follows: a stable light source emits a beam, which is transmitted via optical fiber to the sensor's FP cavity. The beam undergoes multiple reflections within the FP cavity, generating interference. The resulting interference pattern depends on the cavity length. External pressure is applied to the elastic diaphragm, causing it to deform, thereby changing the length of the FP cavity. As the cavity length changes, the characteristics of the interference pattern also change, such as the phase shift of the interference fringes. A spectrum analyzer or other suitable detection device is used to capture and interpret the phase shift of the interference pattern, and after signal processing, the actual pressure value is obtained. Ultimately, the sensor outputs the detected pressure information as a digital or analog signal for user reading or further processing.

[0005] For a fully degradable optical fiber pressure sensor, the degradation synchronization of each component is the key to ensuring the stability and measurement accuracy of the sensor, and its influencing factors are closely related to the manufacturing process.

[0006] In order to achieve the above object, the technical solution adopted by the present invention is: a method for preparing an optical fiber pressure sensor based on polylactic acid, comprising the following steps: S1) preparing a polylactic acid (PLA)-based pressure-sensitive film with a multilayer reflective structure and drawing a polylactic acid (PLA)-based optical fiber with a core-cladding structure; S2) Processing a polylactic acid (PLA)-based base and a polylactic acid (PLA)-based perforated plate with through holes; all components of the sensor are made of polylactic acid (PLA)-based materials. Once implanted in the human body, the sensor will completely degrade into CO2 and H2O within a certain period of time, eliminating the need for secondary surgery to remove it. The degradation products are non-toxic and harmless, reducing medical risks.

[0007] S3) Low-temperature plasma bonding of components is achieved through interfacial activation and precise alignment. Activation improves the surface properties of the polylactic acid (PLA) material, allowing the adhesive to more easily penetrate the surface, forming stronger chemical bonds or physical adsorption, significantly increasing bond strength. Pressurized curing further promotes a tight bond between the adhesive and the PLA material, ensuring stability and durability after bonding. Pressurization eliminates bubbles and voids in the adhesive, forming a more uniform bond layer. Given the characteristics of PLA, low-temperature plasma bonding technology prevents deformation and maintains its structural integrity and mechanical properties. Furthermore, low-temperature plasma introduces acute groups into the PLA surface through electrical forces, excitation, and collisions, increasing surface activation energy and enhancing interfacial bonding and wettability. Furthermore, the degree of surface modification can be precisely controlled by adjusting plasma parameters, providing high controllability.

[0008] Furthermore, the steps of preparing the pressure-sensitive membrane in step S1) include: A1) a composite material composed of polylactic acid (PLA), nanocellulose, and hydrogen-based apatite is extruded and blended into pellets through a screw at a temperature of 150-200°C and a rotation speed of 50-80 rpm; A2) Dissolve the modified particles in chloroform to form a solution, stir magnetically for several hours until completely dissolved, and then cast to form a film; A3) Use ultraviolet laser to cut thin films into small diameter diaphragms. The laser parameters are wavelength 355nm, power 2-3W, and frequency 15-25kHz. A4) A multilayer reflective film was constructed by alternately spin-coating polylactic acid (PLA) layers and polycaprolactone (PCL) reflective films.

[0009] Furthermore, the step of drawing the optical fiber in step S1) includes: B1) melt extruding polylactic acid (PLA) to form a core preform; B2) A core-cladding preform is formed by coextruding a polylactic acid-polyhydroxyalkanoate (PLA-PHA) cladding material through a coaxial die. PLA and PLA-PHA have good compatibility, reducing interfacial defects during melt coextrusion. The addition of PHA improves the cladding's flexibility and impact resistance, adapting to the bending and stretching requirements of the optical fiber.

[0010] B3) Thermal drawing into optical fiber, at a temperature of 160-190°C, ensures synchronous melting and flow of the core and cladding materials, preventing delamination or breakage. Within this temperature range, polylactic acid (PLA) maintains low optical loss and high core refractive index uniformity. The drawing rate is 1-2.5 m / min. A lower drawing rate helps maintain uniform fiber diameter and reduces transmission loss caused by diameter fluctuations.

[0011] B4) Polish the optical fiber end face to a surface roughness of Ra < 15 nm. Alumina polishing liquid can be used as the polishing liquid.

[0012] Furthermore, in step S2), a through-hole is cut in a nanocellulose-containing polylactic acid (PLA) sheet using a CO2 laser, and the surface of the hole is plasma polished at a power of 15-20W for 30-50 seconds. The CO2 laser wavelength matches the absorption peak of the PLA, concentrating the energy and minimizing thermal impact, thus preventing material degradation and performance degradation. A multilayer reflective film is constructed on the substrate surface by alternately spin-coating layers of PLA and polycaprolactone (PCL) reflective film.

[0013] Furthermore, step S3) includes: C1) Low-temperature plasma bonding of the perforated plate and the base in a nitrogen atmosphere achieves a bond strength >6 MPa, meeting long-term stability requirements and ensuring the material does not delaminate or fall off in vivo. A power of 30-40 W balances activation efficiency and material protection. A pressure of 2-5 kPa maintains a stable plasma ionization state, enhancing the surface modification effect. A duration of 30-60 s ensures the formation of a stable chemical functional group layer on the polylactic acid (PLA) surface while avoiding overtreatment.

[0014] C2) Apply adhesive to the surface of the perforated plate, place the pressure-sensitive film over the perforated plate, activate the bonding surface with ethyl acetate vapor, and apply a pressure of 10-25N to cure for 3-8 minutes. Ensure a perfect fit between the base and the pressure-sensitive film, eliminating gaps and avoiding stress concentration. Polylactic acid (PLA) has a low elastic modulus, and excessive pressure may cause excessive deformation or damage. A pressure of 10-25N balances bonding strength and material integrity. After ethyl acetate vapor activation, the adhesive requires time to complete the cross-linking reaction. This 3-8 minute period ensures sufficient curing and gradual release of internal stress, reducing the risk of warping or delamination after bonding. Ethyl acetate vapor activation can also improve the mechanical properties of the PLA material, enhancing the sensor's impact resistance and bending resistance.

[0015] C3) Fill the gap between the optical fiber and the optical fiber sleeve with UV curing adhesive at a curing wavelength of 365 nm, an intensity of 40 to 60 mW / cm², and a curing time of 20 to 30 seconds. The end face of the optical fiber is flush with the end face of the base, and the optical fiber end face directly contacts the end face of the base, resulting in high end face flatness, which reduces the deviation of the optical signal during reception and thus reduces the impact of signal loss.

[0016] C4) A protective layer of PLA is sprayed on the sensor's exterior. Polylactic acid (PLA) is susceptible to water, acidic, and alkaline environmental factors, so a protective layer is applied to the entire sensor's exterior to isolate it from the external environment, maintain overall sensor performance, and slow down the hydrolysis and oxidation of the PLA, extending the sensor's lifespan. The protective layer also improves the sensor's impact and bending resistance, preventing damage when subjected to stress.

[0017] Furthermore, the polylactic acid (PLA) layer in the multilayer reflective film is spin-coated from a chloroform solution at a speed of 3000-4000 rpm; the polycaprolactone (PCL) layer is spin-coated from a tetrahydrofuran solution at a speed of 3000-4000 rpm; and the polylactic acid (PLA) and polycaprolactone (PCL) layers are alternately spin-coated to form multiple layers. The alternating arrangement of the multiple PLA and PCL layers achieves a balance between strength and toughness. The PCL layer acts as a plastic deformation layer, absorbing impact energy and reducing stress concentration in the PLA layer, thereby improving impact resistance. The alternating arrangement also achieves a step-by-step degradation pattern, avoiding structural failure caused by rapid degradation of a single material.

[0018] Furthermore, the reflective film in direct contact with the pressure-sensitive film and the base is a polylactic acid (PLA) layer, which has a highly similar molecular chain structure and can form strong interactions through hydrogen bonds, van der Waals forces and even ester exchange reactions to ensure the reliability of the reflective film.

[0019] Furthermore, the interface treatment described in step S3) involves ultrasonically cleaning the membrane edge with ethanol and then performing argon plasma polishing. The substrate surface is pre-coated with a 5% polycaprolactone (PCL) / acetone solution to form a wetting transition layer. This transition layer reduces interfacial stress between the composite membrane and the PLA substrate, minimizing the risk of membrane cracking or shedding due to differences in thermal expansion coefficients. Plasma activation is also used to enhance bonding strength at the bonding interface.

[0020] Furthermore, after spraying the polylactic acid (PLA) protective layer, it is annealed at 50-70°C for 2-3 hours to promote molecular chain alignment while preventing high-temperature-induced material decomposition or an abnormal increase in crystallinity. This also accelerates the cross-linking and curing reaction of the protective layer, relieves stress generated during the spraying process, and prevents coating shedding. By controlling the time, the coating's hardness is guaranteed while maintaining a flexible match between the coating and the sensor substrate.

[0021] The above scheme has at least the following beneficial effects: Low-temperature bonding significantly improves the sensor's degradation synchronization and structural stability, reducing the difference in degradation rates between components to 12%, ensuring the overall degradation consistency of the sensor within the object. 2. Laser micromachining technology improves the optical performance of the sensor, such as transmittance and interference fringe contrast, effectively enhancing the sensitivity of the sensor.

[0022] 3. The perforated plate is polished by CO2 plasma to eliminate surface defects, reduce local degradation acceleration, ensure uniform degradation starting point, and the laser wavelength matches the PLA absorption peak, with little thermal impact and no material degradation, thus ensuring processing accuracy and material performance.

[0023] 4. Multiple components are dissolved in chloroform to avoid interference from heterogeneous materials and ensure that the bonding points of each component and the main body degrade synchronously; 5. By alternately spin-coating PLA and PCL layers to form a multilayer reflective structure, a balance of strength and toughness is achieved. The PCL layer acts as a plastic deformation layer, absorbing impact energy and reducing stress concentration. It also achieves step-by-step degradation, avoiding structural failure caused by rapid degradation of a single material.

[0024] 6. During the optical fiber drawing process, the drawing temperature and drawing rate are strictly controlled to ensure the synchronous melting and flow of the core layer and the cladding, reduce diameter fluctuation and transmission loss, and ensure high uniformity of the core layer's refractive index; 7. The edge of the diaphragm is polished by argon plasma, and the surface of the base is pre-coated with PCL / acetone solution to form an infiltration transition layer to reduce interfacial stress and reduce the risk of film cracking or falling off due to differences in thermal expansion coefficients; ethyl acetate vapor activates the bonding surface to promote close bonding between the adhesive and the PLA material, while improving the mechanical properties of the material and enhancing the impact resistance and bending resistance of the sensor. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 Schematic diagram of the structure of the optical fiber pressure sensor; Figure 2 The figure is a flow chart for the preparation of optical fiber pressure sensor; Figure 3 is the sensor reflection spectrum; Figure 4 This is the linear relationship diagram between wavelength drift and force change / pressure response curve. DETAILED DESCRIPTION

[0026] In order to make the purpose, technical solution and beneficial effects of the present invention clearer, examples of optical fiber pressure sensors prepared under different process conditions under the same component formulation are given below. Example

[0027] Preparation of pressure-sensitive film: Polylactic acid (PLA), nanocellulose, and hydroxyapatite were blended, granulated, and dried in a twin-screw extruder. The modified particles were dissolved in chloroform and magnetically stirred for 6 hours until completely dissolved. The solution was then cast to form a uniform film. Small-diameter circular films were cut using a UV laser cutter (power 3 W, frequency 25 kHz). The edges of the films were ultrasonically cleaned with ethanol for 5 minutes and then plasma polished in an Ar atmosphere (power 15 W, time time 30 s). Five layers of reflective film were then formed by alternately spin-coating polylactic acid (PLA) layers and polycaprolactone (PCL) layers.

[0028] Fiber drawing: High-purity polylactic acid (PLA) pellets are melt-extruded at 180°C to form a core preform. A polylactic acid-polyhydroxyalkanoate (PLA-PHA) blend is passed through a coaxial die at 180°C to wrap the core layer, forming a core-cladding structure preform. The preform is then hot-stretched to a set diameter at 170°C and 1.2 m / min. The fiber end face is polished with a nano-alumina polishing solution to an Ra of <15 nm.

[0029] Processing of the perforated plate base: CO2 laser cutting of through-hole apertures and surface plasma polishing (power 20 W, time 40 s) were used to form the perforated plate. The base was immersed in a 5% PCL / acetone solution, and polylactic acid (PLA) layers and polycaprolactone (PCL) layers were alternately spin-coated to form a multilayer reflective film.

[0030] Precision assembly of components: Low-temperature plasma bonding (power 35 W, pressure 3 kPa, time 30 s) under nitrogen atmosphere, achieving a bond strength >6 MPa. A pressure-sensitive film was mounted on the upper surface of the perforated plate, adhesive was applied, and the film was precisely positioned to cover the perforation. After activation with ethyl acetate vapor, the film was pressurized and cured (pressure 25 N, time 5 minutes). The optical fiber was inserted into the cannula and cured with UV adhesive (wavelength 365 nm, intensity 60 mW / cm², time 20 s). A protective coating was sprayed on the outer surface, followed by annealing at 50°C for 3 hours. Example

[0031] Pressure-sensitive film preparation: Polylactic acid (PLA), nanocellulose, and hydroxyapatite were blended, pelletized, and dried in a twin-screw extruder. The modified pellets were dissolved in chloroform and magnetically stirred for 6 hours until completely dissolved. The solution was then cast to form a uniform film. Small-diameter circular films were cut using a UV laser cutter (power 2 W, frequency 20 kHz). The edges of the films were ultrasonically cleaned with ethanol for 5 minutes and then plasma polished in an Ar atmosphere (power 15 W, time 30 s). A multilayer reflective film was then formed by alternating spin-coating of polylactic acid (PLA) and polycaprolactone (PCL) layers.

[0032] Fiber drawing: High-purity polylactic acid (PLA) pellets are melt-extruded at 180°C to form a core preform. A polylactic acid-polyhydroxyalkanoate (PLA-PHA) blend is then passed through a coaxial die at 180°C to wrap the core, forming a core-cladding preform. The preform is then hot-drawn at 170°C and 1.2 m / min to the desired diameter. The fiber endfaces are polished to an Ra of <15 nm using a nano-alumina polishing slurry (particle size 0.05 μm).

[0033] Processing of the perforated plate and base: CO2 laser cutting of the through-hole aperture and surface plasma polishing (power 20 W, time 40 s) were used to form the perforated plate. The base was immersed in 5% PCL / acetone solution, and polylactic acid (PLA) layers and polycaprolactone (PCL) layers were alternately spin-coated to form a multilayer reflective film.

[0034] Precision assembly of components: Low-temperature plasma bonding (power 35 W, pressure 3 kPa, time 50 s) in a nitrogen atmosphere, with a bond strength >6 MPa; a pressure-sensitive film is mounted on the upper surface of the perforated plate, and adhesive is applied to precisely cover the through-holes. After activation with ethyl acetate vapor, the film is pressurized and cured (pressure 15 N, time 5 minutes); the optical fiber is inserted into the cannula and cured with UV adhesive (wavelength 365 nm, intensity 60 mW / cm², time 20 s); a protective layer (thickness 5 μm) is sprayed on the outer surface and annealed at 60°C for 2 hours. Example

[0035] Preparation of pressure-sensitive film: Polylactic acid (PLA), nanocellulose, and hydroxyapatite were blended, granulated, and dried in a twin-screw extruder. The modified particles were dissolved in chloroform and magnetically stirred for 6 hours until completely dissolved. The solution was then cast to form a uniform film. Small-diameter circular films were cut using a UV laser cutter (power 2 W, frequency 15 kHz). The edges of the films were ultrasonically cleaned with ethanol for 5 minutes and then plasma polished in an Ar atmosphere (power 15 W, time 30 s). Polylactic acid (PLA) layers and polycaprolactone (PCL) layers were alternately spin-coated to form a multilayer reflective film.

[0036] Fiber drawing: High-purity polylactic acid (PLA) pellets are melt-extruded at 180°C to form a core preform. A polylactic acid-polyhydroxyalkanoate (PLA-PHA) blend is passed through a coaxial die at 180°C to wrap the core layer, forming a core-cladding structure preform. The preform is then hot-stretched at 190°C and 2 m / min to a set diameter. The fiber end face is polished to Ra < 15 nm using a nano-alumina polishing slurry (particle size 0.05 μm).

[0037] Processing of the perforated plate base: CO2 laser cutting of through-hole apertures and surface plasma polishing (power 20 W, time 40 s) were used to form the perforated plate. The base was immersed in a 5% PCL / acetone solution, and polylactic acid (PLA) layers and polycaprolactone (PCL) layers were alternately spin-coated to form a multilayer reflective film.

[0038] Precision assembly of components: Low-temperature plasma bonding (power 35 W, pressure 3 kPa, time 50 s) under nitrogen atmosphere, achieving a bond strength >6 MPa. A pressure-sensitive film was mounted on the upper surface of the perforated plate, adhesive was applied, and the film was precisely positioned to cover the perforation. After activation with ethyl acetate vapor, the film was pressurized and cured (pressure 15 N, time 5 minutes). The optical fiber was inserted into the cannula and cured with UV adhesive (wavelength 365 nm, intensity 40 mW / cm², time 20 s). A protective coating was sprayed on the outer surface, followed by annealing at 70°C for 2 hours.

[0039] To make the purpose, technical solutions and beneficial effects of the present invention clearer, Table 1 shows the performance comparison of sensors with the same structure and size prepared using the preparation methods of Examples 1 to 5 under the same component formulation conditions.

[0040] Table 1 performance Example 1 Example 2 Example 3 Light transmittance 92% 94% 87% Interference fringe contrast 85% 88% 80% Pressure sensitive diaphragm sensitivity 0.15 nm / kPa 0.18 nm / kPa 0.13 nm / kPa Sensor sensitivity 0.1 kPa⁻¹ 0.12 kPa⁻¹ 0.08 kPa⁻¹ Differences in degradation rates among components 15% 12% 20% Full degradation cycle 6 months 10 months 12 months Biocompatibility Better Better Better As can be seen from Table 1, low-temperature bonding significantly improves the degradation synchronization and structural stability of the sensor, reducing the difference in degradation rates of each component to 12%, ensuring the overall degradation consistency of the sensor within the object. Laser micromachining technology improves the optical performance of the sensor, such as transmittance and interference fringe contrast, effectively enhancing the sensitivity of the sensor.

[0041] In summary, through the systematic construction of all-polylactic acid (PLA) materials, combined with low-temperature plasma bonding, laser micromachining and multi-layer reflective film structure design, the prepared optical fiber pressure sensor significantly improves the optical transmittance, interference fringe contrast and sensitivity while maintaining good biocompatibility and degradation performance. It has the advantages of high structural stability, sensitive response, and good degradation synchronization, meeting the dual requirements of performance and safety for implantable applications.

[0042] Finally, the embodiments listed above are only preferred implementation schemes of the method of this application and are not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for preparing an optical fiber pressure sensor based on polylactic acid, characterized in that: The following steps are involved: S1) preparing a polylactic acid (PLA)-based pressure-sensitive film with a multilayer reflective structure and drawing a polylactic acid (PLA)-based optical fiber with a core-cladding structure; S2) processing a polylactic acid (PLA)-based base and a polylactic acid (PLA)-based perforated plate with through holes; S3) Low-temperature plasma bonding of components through interface activation and precise alignment; There is no particular order between step S1) and step S2).

2. The method according to claim 1, characterized in that The steps of preparing the pressure-sensitive membrane in step S1) include: A1) a composite material composed of polylactic acid (PLA), nanocellulose, and hydrogen-based apatite is extruded and blended into pellets through a screw at a temperature of 150-200°C and a rotation speed of 50-80 rpm; A2) dissolving the modified particles in chloroform to form a solution, and casting to form a film; A3) Use ultraviolet laser to cut thin films into small diameter diaphragms. The laser parameters are wavelength 355nm, power 2-3W, and frequency 15-25kHz. A4) A multilayer reflective film was constructed by alternately spin-coating polylactic acid (PLA) layers and polycaprolactone (PCL) reflective films.

3. The method according to claim 1, characterized in that The optical fiber drawing step in step S1) includes: B1) melt extruding polylactic acid (PLA) to form a core preform; B2) co-extruding a polylactic acid-polyhydroxyalkanoate (PLA-PHA) cladding material through a coaxial die to form a core-cladding preform; B3) thermal drawing into optical fiber at a temperature of 160-190°C and a speed of 1-2.5 m / min; B4) The optical fiber end face is polished to a surface roughness of Ra < 15 nm.

4. The method according to claim 1, wherein: In step S2), a through hole is cut on a polylactic acid (PLA) sheet containing nanocellulose using a CO2 laser, and the hole surface is plasma polished at a power of 15 to 20 W for 30 to 50 seconds. A multilayer reflective film is constructed on the surface of the base by alternately spin-coating polylactic acid (PLA) layers and polycaprolactone (PCL) reflective films.

5. The method according to claim 1, wherein Step S3) includes: C1) Low-temperature plasma bonding of the perforated plate and the base in a nitrogen environment, with a bond strength >6 MPa, a power of 30-40 W, a pressure of 2-5 kPa, and a time of 30-60 s; C2) Apply adhesive to the surface of the perforated plate, cover the through-holes of the perforated plate with a pressure-sensitive film, activate the adhesive surface with ethyl acetate vapor, and apply a pressure of 10 to 25 N to cure for 3 to 8 minutes; C3) Fill the gap between the optical fiber and the optical fiber sleeve with UV curing glue, curing wavelength 365 nm, intensity 40-60 mW / cm², time 20-30 s; C4) Spray a PLA protective layer on the outer surface of the sensor.

6. The method according to claims 2 and 4, characterized in that: The polylactic acid (PLA) layer in the multilayer reflective film is spin-coated using a chloroform solution at a rotation speed of 3000 to 4000 rpm; the polycaprolactone (PCL) layer is spin-coated using a tetrahydrofuran solution at a rotation speed of 3000 to 4000 rpm; the polylactic acid (PLA) layer and the polycaprolactone (PCL) layer are alternately spin-coated to form multiple layers.

7. The method according to claim 6, characterized in that: The reflective film in direct contact with the pressure-sensitive film and the base is a polylactic acid (PLA) layer.

8. The method according to claim 1, wherein: The interface treatment described in step S3) includes ultrasonic cleaning of the membrane edge with ethanol and then argon plasma polishing, pre-coating the base surface with 5% polycaprolactone (PCL) / acetone solution to form an infiltration transition layer, and plasma activation of the bonding interface to enhance the bonding strength.

9. The method according to claim 5, characterized in that: After spraying the polylactic acid protective layer, annealing is performed at 50-70°C for 2-3 hours.

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