Method for preparing magnesium alloy stent composite coating

Abstract

The application relates to a preparation method of a magnesium alloy stent composite coating and belongs to the technical field of biomedical materials. In view of the technical problem that an existing polyester coating produces acid by degradation, and accelerates corrosion of a magnesium alloy stent, a composite coating with piezoelectric-pH synergistic protection function is provided. The coating comprises a PVDF-g-APTES primer layer, a pH regulating intermediate layer composed of PVDF, PDLLA and alkaline nanoparticles, and a drug-loaded PDLLA outer coating. The application actively increases the interface pH to the passivation zone by on-demand release of alkaline nanoparticles, and realizes the positive feedback mechanism of "the more stress, the more protection" by using the piezoelectric effect. Compared with the prior art, the dynamic corrosion resistance of the application is increased by more than 40%, the synergistic effect makes the protection effect increased by more than 60%, the piezoelectricity and alkalinity are synergistically protected, a long-acting corrosion-resistant and intelligent response comprehensive solution is provided for the degradable magnesium alloy implant device.

Description

Technical Field

[0001] This invention belongs to the field of biomedical materials technology and relates to a method for preparing a composite coating for a magnesium alloy scaffold. Background Technology

[0002] Magnesium alloys have broad application prospects in the field of medical implant materials due to their good biodegradability and mechanical properties similar to human bone. However, the rapid corrosion rate of magnesium alloys in physiological environments, accompanied by hydrogen evolution reaction leading to local pH elevation, limits their clinical application.

[0003] In the prior art, patent CN201210184704.X discloses a phytic acid micro-arc anodic oxidation film and a polylactic acid coating and process for medical magnesium alloy surfaces. This technology prepares a phytic acid micro-arc oxidation film and a polylactic acid coating in the form of a nanofiber network on the surface of magnesium alloys, aiming to control the corrosion rate. However, the degradation of polylactic acid produces lactic acid, which may lead to local microenvironment acidification at the coating / magnesium alloy interface, thus accelerating the corrosion of the magnesium alloy. Its nanofiber network is a continuous phase structure with high porosity, which cannot effectively block moisture penetration. The fiber network itself constitutes the main body of the coating, making it difficult to achieve precise loading and controllable release of functional units. In addition, the bonding force between the polylactic acid coating and the magnesium alloy substrate is limited, and moisture penetration easily amplifies adhesion defects, leading to coating peeling. Furthermore, the coating is prone to microcracks under mechanical stress such as scaffold expansion, and there is no self-repair mechanism. Crack propagation will lead to the loss of protective function.

[0004] Patent CN201910168650.X proposes a method for preparing a micro-arc oxidation film-LDHs composite film on the surface of magnesium alloys. This method prepares a Mg-Al LDHs protective film through in-situ growth to seal the pores of the micro-arc oxidation film. However, the LDHs in this technology exist as a continuous film layer rather than a dispersed phase, resulting in the one-time exposure of alkaline substances and failing to achieve pH-responsive regulation of "release on demand". The coating system lacks a polymer matrix, and the interfacial bonding between the LDHs film layer and the magnesium alloy substrate mainly relies on physical adsorption and mechanical interlocking, resulting in limited bonding force. This inorganic salt coating is composed of stacked crystals, and its porous nature limits the water barrier performance of the coating. Once moisture reaches the substrate through the porous channels in the coating, pitting corrosion will occur. The coating's main functions are physical barrier and ion exchange, lacking the ability to actively regulate the microenvironment, and thus failing to fundamentally solve the "pH-dependent" problem of magnesium alloy corrosion.

[0005] Patent CN202010404879.1 discloses a method for preparing a medical magnesium alloy modified with a phytic acid / calcium phosphate composite membrane, which modifies the magnesium alloy through an inner phytic acid conversion membrane and an outer calcium phosphate conversion membrane. However, this technology only adjusts the pH during the preparation process; the coating itself lacks dynamic response capability and cannot actively regulate the microenvironment during service. The pH buffering capacity of the calcium phosphate conversion membrane is limited, and its buffering function will gradually be lost as the coating degrades and undergoes ion exchange. Furthermore, the coating's function is relatively simple, and due to the lack of mechanical reinforcement and barrier synergy from the polymer matrix, the coating lacks toughness and is prone to cracking under dynamic loads.

[0006] Furthermore, patent CN202410026507.8 discloses an antibacterial and corrosion-resistant coating for medical magnesium alloy surfaces and its preparation method. This coating is a two-layer composite coating consisting of an inorganic porous inner layer and a flexible piezoelectric polymer outer layer. Although this technology introduces a piezoelectric polymer, the porosity defect of its inorganic inner layer is not overcome, and moisture may still permeate through the pores of the inner layer. At the same time, this technology lacks a targeted design for the key issue of "pH dependence" in the corrosion behavior of magnesium alloys, and fails to effectively solve the problem of interface pH fluctuation during the corrosion process. Summary of the Invention

[0007] In view of this, the purpose of this invention is to provide a magnesium alloy composite coating with piezoelectric-pH synergistic protective function and its preparation method, through the construction of a systematic technical solution of "alkaline nanoparticle dispersion system + pH-responsive release + piezoelectric enhancement mechanism + multilayer gradient structure".

[0008] To achieve the above objectives, the present invention provides the following technical solution:

[0009] This invention provides a method for preparing a composite coating on a magnesium alloy support, the steps of which are as follows:

[0010] S1: Preparation of the base coating:

[0011] 3-aminopropyltriethoxysilane (APTES) modified polyvinylidene fluoride (PVDF) was dissolved in N-methylpyrrolidone to prepare a 5-10% (w / w) solution, which was then sprayed onto the surface of a pretreated magnesium alloy support to form a dense base coating with a thickness of 1-3 μm.

[0012] S2: Preparation of the intermediate coating:

[0013] Preparation of intermediate coating solution:

[0014] Polyvinylidene fluoride (PVDF) and poly-DL-lactic acid (PDLLA) were dissolved in a mixed solvent of N,N-dimethylformamide and acetone in a mass ratio of 3:7 to 7:3 to prepare a solution containing 8-15% PVDF and PDLLA. Then, 5-20% LDH nanosheets, 0.5-2% photoinitiator Irgacure 2959, and 1-5% crosslinking agent trimethylolpropane triacrylate were added. The mixture was ultrasonically dispersed at 200-400 W for 30-60 min to prepare an intermediate coating solution.

[0015] Application of intermediate coating:

[0016] A gradient coating solution of polyvinylidene fluoride and poly-DL-lactic acid in a mass ratio of 3:7 to 7:3 is sequentially sprayed onto the surface of the base coating. Each layer is pre-baked at 60-80℃ for 5-10 min after spraying. The number of coating layers is 5-10, and the coating thickness is 5-15 μm. Then, a high-pressure mercury lamp with a main wavelength of 365 nm and a power of 500-1000 W is used for irradiation at a distance of 10-15 cm for 10-30 min. Within 30-60 min after spraying, a vertical electric field is applied to prepare and form the intermediate coating.

[0017] S3: Preparation of drug-loaded poly-DL-lactic acid surface layer:

[0018] Rapamycin and poly-DL-lactic acid were dissolved in hexafluoroisopropanol to prepare a 3-8% (w / w) solution of rapamycin. The solution was sprayed onto the surface of the intermediate coating and vacuum dried at 40-60°C for 12-24 h to remove residual solvent, thus preparing a drug-loaded PDLLA surface layer with a thickness of 2-5 μm.

[0019] Preferably, the electric field uses a magnesium alloy support substrate as the lower electrode and an aluminum foil or copper foil as the upper electrode; the electric field strength is 10-50 MV / m; the temperature is raised to the target temperature of 80-120℃ at a heating rate of 2-5℃ / min; the time is 30-120min; the electric field is maintained until it drops below 40℃ before being removed.

[0020] Preferably, the spray gun has a nozzle diameter of 0.3-0.5 mm, a spraying distance of 10-15 cm, an air pressure of 0.2-0.4 MPa, and a coating flow rate of 0.5-2 mL / min.

[0021] Preferably, the LDH nanosheets are prepared as follows:

[0022] A mixed aqueous solution of Mg(NO3)2·6H2O and Al(NO3)3·9H2O was prepared and slowly added dropwise to a NaNO3 solution under nitrogen protection at 60-80℃, while simultaneously adding NaOH solution to adjust the pH to 9.5-11.0. The solution was crystallized at 80-100℃ for 12-24 h, centrifuged and washed until the pH reached 7, and then vacuum dried at 60-80℃ for 12-24 h to obtain the LDH nanosheets.

[0023] Preferred, Mg 2+ With Al 3+ The molar ratio of Mg is 2:1 to 4:1. 2+ And Al 3+ The total concentration of metal ions is 0.5-1.5 mol / L;

[0024] Preferably, the pretreatment procedure for the magnesium alloy bracket is as follows: the magnesium alloy bracket is successively ground and polished, ultrasonically cleaned with acetone for 10-15 min, and cleaned with anhydrous ethanol. Then, it is placed in an ethanol solution of 3-aminopropyltriethoxysilane with a volume fraction of 2-5% and immersed at 40-60℃ for 2-6 h. Then, it is rinsed with anhydrous ethanol and vacuum dried at 60-80℃ for 2-4 h.

[0025] Furthermore, the magnesium alloy scaffold composite coating obtained by the preparation method.

[0026] The beneficial effects of this invention are as follows:

[0027] This invention establishes an "active alkalization" intelligent microenvironment at the magnesium alloy / coating interface, enabling the alkaline LDH nanosheets to release OH- only as needed when the PDLLA degradation produces an acidic microenvironment. - This actively raises the interface pH from acidic (≤6.5) to the passivation region (≥10); simultaneously, it utilizes the piezoelectric field generated by the piezoelectric PVDF under mechanical stress to provide cathodic protection for the magnesium alloy matrix and promote OH- - The release rate is increased by 3-5 times, establishing a positive feedback mechanism of "the greater the stress, the stronger the protection," overturning the traditional technical prejudice that coatings "fail under stress," and enhancing dynamic corrosion resistance by more than 40%. Through interfacial chemical bonding, electric field-assisted gradient distribution, and three-layer structure design, it simultaneously solves the interfacial incompatibility between inorganic particles and organic matrix, precise spatial control of functional units, and the multi-dimensional integration challenges of physical barriers, chemical regulation, mechanical response, and biological functions. Ultimately, it achieves a "mechanical-chemical" synergistic enhancement effect of piezoelectric protection and pH regulation, which improves the protection effect by more than 60%. This provides a comprehensive protection solution for biodegradable magnesium alloy implantable devices that combines long-lasting corrosion resistance, intelligent response, mechanical adaptation, and biofriendliness.

[0028] This invention reverses the acidification defects of existing coatings, achieves on-demand release of alkaline substances, establishes a stress-responsive protection enhancement mechanism, integrates multi-dimensional functions, and achieves precise control of microstructure through a system design of "alkaline nanoparticle dispersion system + pH-responsive release + piezoelectric enhancement mechanism + multi-layer gradient structure". It also establishes a new paradigm of piezoelectric-alkaline synergistic protection for the first time, thus providing a long-lasting, intelligent, and multi-functional integrated protection solution for biodegradable magnesium alloy implantable devices.

[0029] This invention, through a quadruple innovation of "active alkalization + intelligent response + micro-control + synergistic coupling," reduces the corrosion current density of the magnesium alloy composite coating to less than 1 / 15 of that of the bare substrate. Under dynamic loads, it achieves the unconventional performance of "the more stress, the more protection." The alkaline nanoparticles actively respond and regulate the acidic environment caused by the degradation of polylactic acid, thus affecting the corrosion resistance of the magnesium alloy scaffold. It also has drug controlled release and antibacterial functions, providing a comprehensive protective solution for biodegradable magnesium alloy implantable devices that is long-lasting, corrosion-resistant, intelligently responsive, mechanically adaptable, and bio-friendly. This solution is significantly superior to the single or dual-function designs of existing technologies.

[0030] Other advantages, objectives, and features of the invention will be set forth in part in the description which follows, and in part will be apparent to those skilled in the art from the following examination, or may be learned from practice of the invention. The objectives and other advantages of the invention can be realized and obtained through the following description. Attached Figure Description

[0031] To make the objectives, technical solutions, and advantages of the present invention clearer, the preferred embodiments of the present invention will be described in detail below with reference to the accompanying drawings, wherein:

[0032] Figure 1 The SEM morphology of the composite coating in Example 1 is shown.

[0033] Figure 2 The SEM cross-sectional morphology of the composite coating in Example 1 is shown.

[0034] Figure 3 This is a comparison chart of pH values ​​during the degradation process of the composite coatings prepared in Examples 1-3 and Comparative Examples 1-5;

[0035] Figure 4 Comparison of weight loss rates during the degradation process of the composite coatings prepared in Examples 1-3 and Comparative Examples 1-5;

[0036] Figure 5 Comparative images of the degradation morphology of the composite coatings prepared in Examples 1-3 and Comparative Examples 1-5;

[0037] Figure 6The image shows a comparison of the potentiodynamic polarization curves of the composite coatings prepared in Examples 1-3 and Comparative Examples 1-5. Detailed Implementation

[0038] The following specific examples illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and various details in this specification can be modified or changed based on different viewpoints and applications without departing from the spirit of the present invention. It should be noted that the illustrations provided in the following embodiments are only schematic representations of the basic concept of the present invention. Unless otherwise specified, the following embodiments and features can be combined with each other.

[0039] The accompanying drawings are for illustrative purposes only and are schematic diagrams, not actual pictures. They should not be construed as limiting the invention. To better illustrate the embodiments of the invention, some parts in the drawings may be omitted, enlarged, or reduced, and do not represent the actual product dimensions. It is understandable to those skilled in the art that some well-known structures and their descriptions may be omitted in the drawings.

[0040] In the accompanying drawings of the embodiments of the present invention, the same or similar reference numerals correspond to the same or similar components. In the description of the present invention, it should be understood that if terms such as "upper," "lower," "left," "right," "front," and "rear" indicate the orientation or positional relationship based on the orientation or positional relationship shown in the drawings, they are only for the convenience of describing the present invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, the terms used to describe positional relationships in the drawings are only for illustrative purposes and should not be construed as limiting the present invention. For those skilled in the art, the specific meaning of the above terms can be understood according to the specific circumstances.

[0041] Example 1: Optimal formulation (PVDF:PDLLA=5:5, LDH content 10%)

[0042] Step 1: Surface pretreatment of magnesium alloy substrate

[0043] A WE43 magnesium alloy cardiovascular stent (3.0 mm in diameter and 18 mm in length) was successively polished with 400#, 800#, and 1200# sandpaper, ultrasonically degreased with acetone for 15 min, and cleaned with anhydrous ethanol for 5 min. It was then immersed in a 3% (v / v) APTES ethanol solution at 50°C for 4 h to graft amino functional groups onto the substrate surface. After removal, it was rinsed three times with anhydrous ethanol and vacuum dried at 70°C for 3 h.

[0044] Step 2: Construction of PVDF-g-APTES Undercoat

[0045] End-functionalized PVDF-g-APTES (prepared in the laboratory, grafting rate 5%) was dissolved in NMP to prepare an 8% (w / w) solution. This solution was applied to the pretreated magnesium alloy surface using a spraying method (spraying distance 12 cm, air pressure 0.3 MPa), with three coats applied at 2-minute intervals. The surface was then heat-treated at 100℃ for 4 hours to form a dense base coat with a thickness of 2 μm.

[0046] Step 3: Preparation of pH-controlled intermediate layer

[0047] (1) Preparation of LDH alkaline nanoparticles

[0048] Co-precipitation method: Prepare a mixed aqueous solution of Mg(NO3)2·6H2O and Al(NO3)3·9H2O (Mg 2+ :Al 3+ A Mg-Al LDH nanosheet (molar ratio 3:1, total metal ion concentration 1.0 mol / L) was slowly added dropwise at a rate of 2 mL / min to a 0.3 mol / L NaNO3 solution at 70 °C under nitrogen protection, while simultaneously adding 1.5 mol / L NaOH solution to maintain the pH at 10.5. After the addition was complete, the nanosheet was crystallized at 90 °C for 18 h, centrifuged and washed until neutral, and then vacuum dried at 70 °C for 18 h to obtain Mg-Al LDH nanosheets (particle size 80-150 nm, Zeta potential +25 mV).

[0049] (2) Preparation of intermediate layer solution

[0050] PVDF and PDLLA were dissolved in a mixed solvent of DMF and acetone (volume ratio 7.5:2.5) at a mass ratio of 5:5 to prepare a solution with a total mass fraction of 12%. The prepared LDH nanoparticles (10% mass fraction relative to the polymer mass), along with the photoinitiator Irgacure 2959 (1% mass fraction) and the multifunctional crosslinking agent TMPTA (3% mass fraction), were added, and the mixture was ultrasonically dispersed for 45 min (300 W power).

[0051] (3) Coating of the gradient intermediate layer

[0052] A gradient spraying process was employed: a spray gun nozzle diameter of 0.4 mm, a spraying distance of 12 cm, an air pressure of 0.3 MPa, and a flow rate of 1.2 mL / min. Under program control, five PVDF:PDLLA layers were sequentially deposited with ratios of 7:3, 6:4, 5:5, 4:6, and 3:7. Each layer was pre-baked at 70°C for 8 min after spraying. A total of five layers were applied, resulting in a final intermediate layer thickness of 10 μm.

[0053] (4) Within 10 minutes after the photo-initiated in-situ crosslinking spraying is completed, ultraviolet light irradiation is immediately carried out: a high-pressure mercury lamp (power 800 W, main wavelength 365 nm) is used, the irradiation distance is 12 cm, and the irradiation time is 20 min to initiate the in-situ crosslinking reaction between PVDF and PDLLA molecular chains.

[0054] Step 4: Electric field-assisted polarization and particle orientation alignment

[0055] 45 minutes after spraying (before the coating is fully cured), an electric field is applied in the vertical direction:

[0056] Polarization electric field strength: 30 MV / m (coating thickness 10 μm, applied voltage 300 V)

[0057] Polarization temperature: 100℃

[0058] Polarization time: 60 min

[0059] Electrode materials: Aluminum foil as the upper electrode, and magnesium alloy substrate itself as the lower electrode.

[0060] Heating rate: 3℃ / min to 100℃

[0061] Cooling method: Maintain the electric field until it drops to 35°C, then remove the electric field.

[0062] Step 5: Preparation of drug-loaded PDLLA surface layer

[0063] Rapamycin (15% by mass relative to PDLLA) and PDLLA were dissolved in hexafluoroisopropanol to prepare a 5% by mass solution. The solution was applied to the surface of the cured gradient intermediate layer using a spraying method (spraying distance 12 cm, air pressure 0.15 MPa) to a thickness of 3 μm. The layer was then vacuum dried at 50 °C for 18 h to remove residual solvent.

[0064] Step 6: Post-processing

[0065] The prepared coating samples were annealed in a vacuum drying oven at 50℃ for 18 h to eliminate internal stress and stabilize the coating structure. They were then sealed and stored in a desiccator for later use. Figure 1 The SEM morphology of the composite coating is shown. Figure 2 The cross-sectional morphology of the composite coating is shown in the SEM image.

[0066] Example 2: High PVDF ratio (PVDF:PDLLA=7:3, LDH content 15%)

[0067] It is basically the same as Example 1, except that:

[0068] Step 3 (2) Preparation of intermediate layer solution: PVDF and PDLLA are dissolved at a mass ratio of 7:3.

[0069] Step 3 (3) Coating of the gradient intermediate layer: deposit five layers of coating with PVDF:PDLLA ratios of 8:2, 7.5:2.5, 7:3, 6.5:3.5 and 6:4 in sequence.

[0070] Step 3 (1) Alkaline nanoparticles: Mg(OH)2 nanosheets are used to replace LDH (prepared by hydrothermal method, particle size 100-200nm), with a mass fraction of 15%.

[0071] Step 4: Electric field polarization: polarization electric field strength 40 MV / m, polarization temperature 110℃, polarization time 90 min.

[0072] Example 3: High PDLLA ratio (PVDF:PDLLA=3:7, LDH content 5%, drug loading 20%)

[0073] It is basically the same as Example 1, except that:

[0074] Step 3 (2) Preparation of intermediate layer solution: PVDF and PDLLA are dissolved in a mass ratio of 3:7.

[0075] Step 3 (3) Coating of the gradient intermediate layer: Sequentially deposit five coating layers with PVDF:PDLLA ratios of 5:5, 4:6, 3:7, 2:8, and 1:9.

[0076] Step 3 (1) Alkaline nanoparticles: CaCO3 nanoparticles (prepared by precipitation method, particle size 50-100 nm) with a mass fraction of 5%.

[0077] Step 5: Drug-loaded surface layer: 20% rapamycin by mass, spray thickness 4 μm.

[0078] Step 4: Electric field polarization: polarization electric field strength 20 MV / m, polarization temperature 90℃, polarization time 45 min.

[0079] Comparative Example 1: No alkaline particles (pure PVDF / PDLLA coating)

[0080] It is basically the same as Example 1, except that:

[0081] Step 3 (2) Preparation of intermediate layer solution: without adding LDH alkaline nanoparticles, only PVDF and PDLLA are dissolved in a mass ratio of 5:5, and photoinitiator and crosslinking agent are added.

[0082] Step 4: Electric field polarization: Same as in Example 1.

[0083] Differences in expected results: No alkaline particles, unable to achieve pH-responsive alkalization, interface pH gradually acidifies as PDLLA degrades (≤6.5), and corrosion current density is high.

[0084] Comparative Example 2: No piezoelectric effect (unpolarized)

[0085] It is basically the same as Example 1, except that:

[0086] Step 4: Electric field-assisted polarization: This step is omitted, that is, no polarization treatment is performed. PVDF is mainly in α crystal form (β crystal form content <30%), and there is no piezoelectric effect.

[0087] Differences in expected results: Without piezoelectric cathodic protection and piezoelectric alkali-promoting function, the corrosion current decreases under dynamic load (due to microcracks), and the "the more stress, the more protection" effect cannot be achieved.

[0088] Comparative Example 3: No in-situ crosslinking (physical blending only)

[0089] It is basically the same as Example 1, except that:

[0090] Step 3 (4) Photo-initiated in-situ crosslinking: This step is omitted. No ultraviolet light irradiation is performed. PVDF and PDLLA are only physically blended, and no chemical crosslinking network is formed.

[0091] Step 4: Electric field polarization: Same as in Example 1.

[0092] Differences in expected results: Without a cross-linked network, PVDF and PDLLA have poor compatibility and are prone to macroscopic phase separation; alkaline particles are prone to migration and aggregation; the coating has poor density and reduced water and oxygen barrier properties; the piezoelectric stability is poor and the β crystal form is easily reversed.

[0093] Comparative Example 4: PVDF as the nanoparticle carrier only

[0094] It is basically the same as Example 1, except that:

[0095] Step 3: (2) Preparation of intermediate layer solution: PVDF only (without PDLLA).

[0096] Comparative Example 5: PDLLA alone as a nanoparticle carrier (without PVDF)

[0097] It is basically the same as Example 1, except that:

[0098] Step 3: (2) Preparation of intermediate layer solution: PDLLA only (without PVDF).

[0099] Example 4: Performance Comparison

[0100] Figure 3The pH values ​​of the degradation process of Examples 1, 2, 3, Comparative Examples 1, 2, 3, 4, and 5 are compared. The pH values ​​after 30 days are as follows: Example 1 (10.2), Example 2 (9.8), Example 3 (9.5), Comparative Example 4 (9.0), Comparative Example 2 (8.3), Comparative Example 3 (7.8), Comparative Example 1 (5.8), and Comparative Example 5 (5.6).

[0101] Figure 4 The weight loss rates during the degradation process of Examples 1, 2, 3, Comparative Examples 1, 2, 3, 4, and 5 were compared. The weight loss rates after 30 days were as follows: Example 1 (15.2%), Example 2 (17.3%), Example 3 (16.95%), Comparative Example 4 (21.38%), Comparative Example 2 (19.2%), Comparative Example 3 (25.25%), Comparative Example 1 (42.7%), and Comparative Example 5 (55.7%).

[0102] Figure 5 The degradation morphology of Examples 1, 2, 3, 1, 2, 3, 4, and 5 was compared. Example 1 had the best ratio, the lowest weight loss rate, and the scaffold was intact without breakage. Comparative Example 5 had the worst ratio, the lowest weight loss rate, and the scaffold was broken.

[0103] Figure 6 The following is a comparison of the potentiodynamic polarization curves of Examples 1, 2, 3, 1, 2, 3, 4, and 5, with the corrosion potentials as follows: Example 1 (-1.35V), Example 2 (-1.38V), Example 3 (-1.42V), 4 (-1.48V), 2 (-1.52V), 3 (-1.54V), 1 (-1.68V), and 5 (-1.72V).

[0104] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A method for preparing a composite coating on a magnesium alloy support, characterized in that: The preparation method comprises the following steps: S1: Preparation of the base coating: 3-aminopropyltriethoxysilane-modified polyvinylidene fluoride was dissolved in N-methylpyrrolidone to prepare a 5-10% (w / w) solution, which was then sprayed onto the surface of a pretreated magnesium alloy support to form a dense base coating with a thickness of 1-3 μm. S2: Preparation of the intermediate coating: Preparation of intermediate coating solution: Polyvinylidene fluoride (PVDF) and poly-DL-lactic acid (DL-LAL) are dissolved in a mixed solvent of N,N-dimethylformamide and acetone in a mass ratio of 3:7 to 7:3 to prepare a solution containing 8-15% PVDF and DL-LAL by mass. Then, 5-20% LDH nanosheets, 0.5-2% photoinitiator Irgacure 2959, and 1-5% crosslinking agent trimethylolpropane triacrylate are added. The mixture is ultrasonically dispersed at 200-400 W for 30-60 min to prepare an intermediate coating solution. Application of intermediate coating: A gradient coating solution of polyvinylidene fluoride and poly-DL-lactic acid in a mass ratio of 3:7 to 7:3 is sequentially sprayed onto the surface of the base coating. Each layer is pre-baked at 60-80℃ for 5-10 min after spraying. The number of coating layers is 5-10, and the coating thickness is 5-15 μm. Then, a high-pressure mercury lamp with a main wavelength of 365 nm and a power of 500-1000 W is used for irradiation at a distance of 10-15 cm for 10-30 min. Within 30-60 min after spraying, a vertical electric field is applied to prepare and form the intermediate coating. The electric field uses a magnesium alloy support substrate as the lower electrode and an aluminum foil or copper foil as the upper electrode; the electric field strength is 10-50 MV / m; the temperature is raised to the target temperature of 80-120℃ at a heating rate of 2-5℃ / min; the time is 30-120 min; the electric field is maintained until it drops below 40℃ before being removed. S3: Preparation of drug-loaded poly-DL-lactic acid surface layer: Rapamycin and poly-DL-lactic acid were dissolved in hexafluoroisopropanol to prepare a 3-8% (w / w) solution of rapamycin. The solution was sprayed onto the surface of the intermediate coating and vacuum dried at 40-60°C for 12-24 h to remove residual solvent, thus preparing a drug-loaded PDLLA surface layer with a thickness of 2-5 μm.

2. The preparation method according to claim 1, characterized in that: The spray gun used for spraying has a nozzle diameter of 0.3-0.5 mm, a spraying distance of 10-15 cm, an air pressure of 0.2-0.4 MPa, and a coating flow rate of 0.5-2 mL / min.

3. The preparation method according to claim 1, characterized in that: The LDH nanosheets are prepared as follows: A mixed aqueous solution of Mg(NO3)2·6H2O and Al(NO3)3·9H2O was prepared and slowly added dropwise to a NaNO3 solution under nitrogen protection at 60-80℃, while simultaneously adding NaOH solution to adjust the pH to 9.5-11.

0. The solution was crystallized at 80-100℃ for 12-24 h, centrifuged and washed until the pH reached 7, and then vacuum dried at 60-80℃ for 12-24 h to obtain the LDH nanosheets.

4. The preparation method according to claim 3, characterized in that: Mg 2+ With Al 3+ The molar ratio of Mg is 2:1 to 4:

1. 2+ And Al 3+ The total concentration of metal ions is 0.5-1.5 mol / L.

5. The preparation method according to claim 1, characterized in that: The pretreatment procedure for the magnesium alloy bracket is as follows: the magnesium alloy bracket is successively ground and polished, ultrasonically cleaned with acetone for 10-15 min, and cleaned with anhydrous ethanol. Then, it is placed in an ethanol solution of 3-aminopropyltriethoxysilane with a volume fraction of 2-5% and immersed at 40-60℃ for 2-6 h. After that, it is rinsed with anhydrous ethanol and vacuum dried at 60-80℃ for 2-4 h.

6. The magnesium alloy support composite coating prepared by the preparation method according to any one of claims 1-5.

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