Composite inner fixing device for bionic lantern frame and preparation method

Abstract

The invention discloses a bionic lantern frame composite internal fixing device and a preparation method, and relates to the technical field of orthopedic medical instruments, the bionic lantern frame composite internal fixing device comprises an implant and a multi-rod bridging structure, the multi-rod bridging structure is composed of a plurality of longitudinal connecting rods and transverse locking nails, and the implant is composed of a plurality of multi-level bionic structures; the multi-layer bionic structure comprises two hexagonal upper and lower layer structures which are symmetrically arranged and a middle spiral layer structure arranged between the two hexagonal upper and lower layer structures. According to the invention, inspired from honeycomb and ox horn macro-microstructure, the multi-level bionic structure prepared by adopting a coupling bionic technology has excellent mechanical properties and strong buffering and vibration absorbing capabilities, can be effectively matched with the mechanical properties of natural bones, and can effectively avoid secondary injury and stress shielding effect after implantation of a patient; the composite coating introduced through an electrochemical deposition method solves the problems that a single implant structure is insufficient in biocompatibility, insufficient in antibacterial property and weak in osteogenic differentiation capacity.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of orthopedic medical devices, in particular to a bionic lantern frame composite internal fixation device and a preparation method thereof. BACKGROUND

[0002] With the progress of society and the development of science and technology, life and health are increasingly valued. However, the number of cases of bone trauma and bone defect caused by factors such as traffic accidents and diseases continues to rise, especially the demand for high-performance bone implant materials and mechanical stability for long bone segmental bone defects is increasingly urgent.

[0003] In terms of bone implant materials, currently commonly used hard tissue implants (such as densified coated titanium alloy, porous titanium alloy, etc.) are widely used in the fields of orthopedics, dentistry, etc., but they are difficult to simultaneously have excellent biocompatibility, mechanical properties (especially elastic modulus) matching human bone tissue, and strong bone integration ability, which can easily cause the "stress shielding" effect, leading to inflammation and degradation of bone tissue around the implant. Natural bone tissue is a kind of biological composite material with exquisite multi-level structure, which is a composite structure of cancellous bone (porous) and cortical bone (dense) on a macroscopic scale, and has a periodic arrangement of collagen fibers and deposition of hydroxyapatite (HA) nanocrystals on a microscopic scale. Most of the existing implant structures are relatively simple, and are mostly single-scale surface modification (such as micron-level sandblasting and acid etching or deposition of nanoscale tubular structures), which cannot effectively simulate the multi-level complexity of bone tissue. Macroscopic body structures are mostly solid or simple and uniform pore structures (such as some porous titanium), which are not conducive to cell ingrowth, vascularization and nutrient transport. Therefore, there is a lack of an implant material in the prior art that can balance mechanical properties, biocompatibility, bone integration ability, and adapt to individualized bone defect morphology for precise customization to meet the growing demand for diversified and high-performance bone repair materials in clinical practice.

[0004] In terms of internal fixation devices, the current clinical commonly used internal fixation methods include intramedullary nails, steel plates and external fixation frames, but their application scenarios and biomechanical properties have deficiencies: intramedullary nails need to penetrate bone graft materials, thereby affecting their biological properties; steel plate fixation needs to be attached to the surface of bone graft materials, interfering with their biological properties, and the mechanical support for long segmental defects is insufficient; external fixation frames can avoid direct contact of the implant with the bone graft material, but have problems such as poor stability, low patient comfort, high risk of needle tract infection, etc.

[0005] Therefore, a bionic "lantern" frame composite internal fixation device combining a medical implant multi-level bionic structure with a bridging internal fixation system is proposed to solve the above problems. SUMMARY

[0006] To achieve the above-mentioned objectives, the present invention provides the following technical solutions: a bionic lantern frame composite internal fixation device, comprising an implant and a multi-rod bridging structure, the multi-rod bridging structure consisting of a plurality of longitudinal connecting rods and transverse locking nails, the implant consisting of a plurality of multi-level bionic structures, the multi-level bionic structure comprising two symmetrically arranged hexagonal upper and lower layer structures and an intermediate spiral layer structure arranged between the two hexagonal upper and lower layer structures, wherein the hexagonal upper and lower layer structures comprise an equilateral hexagonal frame and a wavy line structure, a plurality of wavy line structures are symmetrically arranged on the inner side of the equilateral hexagonal frame, the intermediate spiral layer structure consists of a plurality of intermediate spiral layer unit structures arranged in a circular array, a supporting cylinder is arranged between the center positions of the two hexagonal upper and lower layer structures, both ends of the supporting cylinder are respectively connected to the wavy line structures of the two hexagonal upper and lower layer structures through ribs, the two ends of the intermediate spiral layer unit structure are respectively connected to the two ends of the supporting cylinder, and a plurality of intermediate layer concave support structures arranged in a circular array are also arranged between the two equilateral hexagonal frames, and the two ends of the intermediate layer concave support structure are respectively connected to the two equilateral hexagonal frames.

[0007] Preferably, the wavy line structure is designed according to the following equation: ; in, is the horizontal coordinate of the curve, is the ordinate of the curve, is the amplitude, is the angular velocity, For the sake of appearance, For the offset.

[0008] Preferably, the intermediate helical layer unit structure is designed according to the following equation: ; ; in, is the radius, is the coefficient, For the changing angle, For the large diameter, For the trail, is the degree of rotation; the equation converted to the Cartesian coordinate system is: ; ; .

[0009] Preferably, the intermediate helical layer structure consists of six intermediate helical layer unit structures.

[0010] As preferred, the edge length of the equilateral hexagonal frame is 1 cm, and the height is 0.2 cm; the radius of the supporting cylinder is 0.2 cm, and the rib width is 0.1 cm.

[0011] As preferred, the concave support structure in the intermediate layer is designed according to the following equation: ; ; wherein, is a parameter ranging from [0, 1], , is the starting point coordinate of the curve, , is the midpoint coordinate of the curve, , is the terminal point coordinate of the curve.

[0012] The application also provides a preparation method of the bionic lantern frame composite internal fixation device, for preparing the implant as described above, comprising the following steps: Step 1: modeling the multi-level bionic structure based on the principle of sine function and Archimedes spiral line, and obtaining the model of the implant after closely arraying the multi-level bionic structure; Step 2: preparing the implant skeleton based on the laser powder bed fusion metal additive manufacturing technology; Step 3: immersing the implant skeleton obtained in step 2 in a polydopamine solution, magnetically stirring at a temperature of 25-30℃ for 12 h, then washing with deionized water after taking out, and drying for standby; Step 4: immersing the implant treated in step 3 in an electrolyte, using a platinum electrode as a counter electrode, and performing electrochemical deposition under magnetic stirring to obtain a gradient coating.

[0013] As preferred, in step 2, the model obtained in step 1 is saved as stl. format, imported into MaterialiseMagics software for slicing processing, then copied into an L-PBF printing device, and TC4 spherical powder with a particle size of 15-53 μm is used for printing, with the printing parameters being: laser power is 175 W, scanning speed is 1200 mm / s, scanning interval is 0.08 mm, layer thickness is 0.03 mm, scanning strategy is strip scanning, and scanning angle is 67°.

[0014] As preferred, the polydopamine solution in step 3 is prepared by dissolving dopamine hydrochloride powder in a Tris buffer solution which is pre-configured and adjusted to a pH of 8.5, and the concentration of the polydopamine solution is 2 mg / ml.

[0015] As preferred, step 4 is specifically: First, the implant treated in step 3 was immersed in the electrolyte, and a platinum electrode was used as the counter electrode. Electrodeposition was performed for 30 min under magnetic stirring at 10 rpm to obtain the first layer of hydroxyapatite coating. Then, 0.5 wt% chitosan was added to the electrolyte and the electrodeposition was continued for 45 min. The rotation speed was increased to 15 rpm, and 0.03 mol / L SrCl2 was added to the electrolyte. Electrodeposition was continued for 60 min to make Sr 2+ Immerse in the coating to improve the antibacterial properties. Finally, remove the implant and immediately immerse in deionized water for ultrasonic cleaning. After cleaning, place it in a drying oven at 60°C for 24 hours to obtain a gradient coating.

[0016] The present invention has the following beneficial effects: The implant of this invention differs from traditional single, dense or porous titanium alloy implants. Inspired by the macro- and microscopic structures of honeycombs and yak horns, the multi-layered biomimetic structure fabricated using coupled bionic technology exhibits excellent mechanical properties and strong vibration damping and absorption capabilities. Furthermore, bone tissue is a composite structure of cancellous (porous) and cortical (dense) bone. The hexagonal upper and lower layers of the multi-layered biomimetic structure simulate cancellous bone, while the central spiral unit layer simulates cortical bone. This effectively matches the mechanical properties of natural bone, effectively preventing secondary damage and the "stress shielding" effect after implantation.

[0017] The present invention introduces gradient nano-hydroxyapatite (HA), Sr into the multi-level bionic structure by electrochemical deposition. 2+ , chitosan (CTS) composite coating, which makes up for the problems of insufficient biocompatibility, insufficient antibacterial property and weak osteogenic differentiation ability of a single implant structure, making the prepared multi-level bionic structure more suitable for implant applications.

[0018] The present invention combines additive manufacturing technology with electrochemical deposition and coats polydopamine (PDA) material on the surface of the structure, which can effectively enhance the coating interface bonding strength and improve the corrosion resistance of the multi-level bionic structure. At the same time, additive manufacturing technology gives bone implants the ability to quickly form free shapes.

[0019] The multi-rod bridging structure of this invention provides three-dimensional stability. Its core design lies in the "lantern-like" arrangement of fixed rods (i.e., longitudinal connecting rods) across the central bone graft area. Locking screws (i.e., transverse locking screws) penetrate healthy bone tissue, anchoring the mechanical support structure at both ends of the bone defect. This device provides a stable mechanical environment without disturbing the biological microenvironment of the bone graft area. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 This is an electron microscope image of the microstructure of the ox horn in the present invention; Figure 2Flow chart for multi-level biomimetic structure design in the present application; Figure 3 Schematic diagram of multi-level biomimetic structure in the present application; Figure 4 Schematic diagram of electrodeposition coating in the present application; Figure 5 Schematic diagram of implant filling in the present application; Figure 6 Schematic diagram of overall device in the present application.

[0021] In the figure: 3, equilateral hexagonal frame; 31, wavy line structure; 4, intermediate spiral layer unit structure; 5, intermediate spiral layer structure; 6, multi-level biomimetic structure; 61, concave support structure in the intermediate layer; 7, polydopamine solution; 8, platinum electrode; 9, electrolyte; 10, implant; 11, multi-rod bridging structure. DETAILED DESCRIPTION

[0022] The technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, rather than all the embodiments of the present application. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative work fall within the protection scope of the present application.

[0023] The present application will be further described in detail below according to the accompanying drawings and embodiments.

[0024] Embodiments of the present application: As Figure 2 , Figure 3 , Figure 5 and Figure 6As shown, a kind of bionic lantern frame composite internal fixation device, including implant 10 and multiple stick bridging structure 11, multiple stick bridging structure 11 is made of multiple longitudinal connecting rods and transverse locking nails, implant 10 is made of multiple closely arrayed multilevel bionic structure 6, multilevel bionic structure 6 includes two symmetrically arranged hexagonal upper and lower layer structure and the intermediate spiral layer structure 5 between the two hexagonal upper and lower layer structure, wherein the hexagonal upper and lower layer structure includes equilateral hexagonal frame 3 and wave line structure 31, multiple wave line structure 31 is symmetrically arranged inside equilateral hexagonal frame 3, intermediate spiral layer structure 5 is made of six circumferential arrayed intermediate spiral layer unit structure 4, to make intermediate spiral layer structure 5 have good support, the center position between the two hexagonal upper and lower layer structure is also provided with a support cylinder, the both ends of support cylinder are connected with the wave line structure 31 of the two hexagonal upper and lower layer structure respectively through rib, the both ends of intermediate spiral layer unit structure 4 are connected with the both ends of support cylinder respectively, multiple circumferential arrayed intermediate layer inner recess support structure 61 are also provided between the two equilateral hexagonal frames 3, the both ends of intermediate layer inner recess support structure 61 are connected with the two equilateral hexagonal frames 3 respectively.The length of side of equilateral hexagonal frame 3 a=1cm, height is 0.2cm;The radius of support cylinder b=0.2cm, rib width c=0.1cm.

[0025] The specific design method of hexagonal upper and lower layer structure is: first, according to the principle of honeycomb structure, an equilateral hexagonal frame 3 is established, then based on the characteristic of the cross section of the horn of a cow (i.e. Figure 1 The transverse cross section in it), it is simplified as sine curve due to its typical wave line feature, so that wave line structure 31 is obtained, wave line structure 31 is designed according to the following equation: ; Wherein, It is the abscissa of curve, It is the ordinate of curve, It is amplitude, It is angular velocity, It is at phase, It is offset distance.

[0026] Based on the characteristic of the longitudinal cross section of the horn of a cow (i.e. Figure 1 The longitudinal cross section in it), it presents the characteristic of high spiral twist, therefore, the typical Archimedes spiral equation is used to extract this feature, intermediate spiral layer unit structure 4 is designed according to the following equation: ; ; Wherein, It is radius, It is coefficient, for the angle of change, for the large diameter, for the small diameter, for the degree of rotation; in order to facilitate the establishment of a three-dimensional model, the above equation is converted into a Cartesian coordinate equation: ; ; ; By adjusting the values of and , the controllability of the intermediate spiral layer unit structure 4 can be achieved, and the parameterized design of the implant 10 is also achieved.

[0027] In order to make the intermediate layer concave support structure 61 have good support stability, an excessively relatively smooth Bezier curve is used for design, and the intermediate layer concave support structure 61 is designed according to the following equation: ; ; wherein, is a parameter in the range [0, 1], , is the starting point coordinate of the curve, , is the midpoint coordinate of the curve, , is the end point coordinate of the curve.

[0028] Inspired by the macro-micro structure of honeycomb and cow horn, the embodiment also provides a preparation method of a bionic lantern frame composite internal fixation device, for preparing the implant 10 as described above, with reference to Figures 2 to 6 , comprising the following steps: Step 1, based on the principle of sine function and Archimedes spiral, the modeling of multi-level bionic structure 6 is carried out, and the model of implant 10 is obtained after the multi-level bionic structure 6 is closely arrayed; Step 2, based on the laser powder bed fusion (L-PBF) metal additive manufacturing technology, the implant 10 skeleton is prepared by using a suitable printing strategy, the model obtained in step 1 is saved in stl. format, imported into Materialise Magics software for slicing processing, then copied into L-PBF printing equipment, and TC4 spherical powder with particle size of 15-53 μm is used for printing, the printing parameters are: laser power is 175 W, scanning speed is 1200 mm / s, scanning interval is 0.08 mm, layer thickness is 0.03 mm, scanning strategy is strip scanning, and scanning angle is 67°; Step 3, the implant 10 skeleton obtained in step 2 is immersed in a polydopamine solution 7, and is magnetically stirred at a temperature of 25-30 DEG C for 12 hours, and after being taken out, is washed with a large amount of deionized water, and is dried for standby use; the polydopamine solution 7 is prepared by dissolving dopamine hydrochloride powder in a Tris buffer solution which is prepared in advance and adjusted to a pH of 8.5, and the concentration of the polydopamine solution 7 is 2 mg / ml; after being treated by polydopamine, the implant 10 can effectively improve the interfacial bonding force of the coating, enhance the adhesion of the coating, and be beneficial to improving the corrosion resistance; Step 4, first, the implant 10 treated in step 3 is immersed in an electrolyte 9, a platinum electrode 8 is used as a counter electrode, and is subjected to electrodeposition at a magnetic stirring speed of 10 rpm for 30 minutes to obtain a first layer of hydroxyapatite (HA) coating, then 0.5wt% of chitosan is added to the electrolyte 9 to continue electrodeposition for 45 minutes, the stirring speed is increased to 15 rpm, 0.03mol / L of SrCl2 is further added to the electrolyte 9 to continue electrodeposition for 60 minutes, so that Sr 2+ The implant 10 is immersed in the coating to improve the antibacterial performance, and finally the implant 10 is taken out and immediately immersed in deionized water for ultrasonic cleaning, and after being cleaned, is placed in a drying box for heat preservation at a temperature of 60 DEG C for 24 hours to obtain a gradient coating, and the biocompatibility is significantly improved; the electrolyte 9 used in step 4 comprises deionized water, 0.1mol / L of Ca(NO3)2.4H2O, 0.06mol / L of NH4H2PO4 and 0.1mol / L of NaNO3.

[0029] When the implant 10 is used, the designed implant 10 is implanted into a bone defect site to form a "wick" structure of a bionic "lantern" frame composite internal fixation device, and bone cement is used to connect two ends of the bone defect. Then, a multi-rod bridging structure 11 is used to form a three-dimensional support system composed of multiple longitudinal connecting rods and horizontal locking nails, to precisely cross the capsule wall of an infection focus in the body to realize "extra-capsular fixation", to realize stable fixation of both ends of the bone defect and the bone graft area without crossing the bone graft material, to effectively reduce the risk of infection, in addition, by adjusting the number of bridging rods (i.e. longitudinal connecting rods) (three rods / four rods) to optimize the compression stiffness (up to the level of double steel plates) and the torsional stiffness (close to single steel plate), the stability of the implant 10 and the comfort of the patient are improved, and a frame structure of the bionic "lantern" frame composite internal fixation device is formed.

[0030] The implant 10 of the present application is distinguished from conventional single dense or porous titanium alloy implants, and is inspired by the macro-micro structure of honeycomb and cow horn, and has excellent mechanical properties and strong shock absorption capacity by using a multi-level biomimetic structure 6 prepared by coupling biomimetic technology.

[0031] The present application introduces a gradient nano-hydroxyapatite (HA), Sr 2+ , chitosan (CTS) composite coating in the multi-level biomimetic structure 6 by electrochemical deposition method, which makes up for the problems of insufficient biocompatibility, insufficient antibacterial property and weak osteogenic differentiation ability of single implant structure, so that the prepared multi-level biomimetic structure 6 is more suitable for implant applications.

[0032] The present application combines additive manufacturing technology with electrochemical deposition method, and coats polydopamine (PDA) material on the structure surface, which can effectively enhance the interfacial bonding force of the coating and improve the corrosion resistance of the multi-level biomimetic structure 6.

[0033] The multi-bar bridging structure 11 of the present application provides three-dimensional stability, and the core of its design is the “lantern type” multi-bar support distribution of the fixed rod (i.e. the longitudinal connecting rod) in space, which spans the middle bone graft area and is anchored to the bone defect at both ends by the locking screw (i.e. the transverse locking nail) passing through the healthy bone tissue. The device provides a stable mechanical environment without interfering with the biological microenvironment of the bone graft area.

[0034] In summary, the implant 10 and the multi-bar bridging structure 11 designed in the present application together form a biomimetic “lantern” frame composite internal fixation device, which provides a good biological and mechanical microenvironment for the bone defect area after implantation, and can effectively promote bone repair. The present application breaks through the limitations of traditional fixation path and material, and provides a minimally invasive, efficient and safe new internal fixation solution for bone defects.

[0035] Although embodiments of the present application have been shown and described, it will be understood by those of ordinary skill in the art that various changes, modifications, substitutions and alterations can be made without departing from the principles and spirit of the present application, and the scope of the present application is defined by the appended claims and their equivalents.

Claims

1. A bionic lantern frame composite internal fixation device, characterized in that: The invention comprises an implant (10) and a multi-rod bridging structure (11), wherein the multi-rod bridging structure (11) is composed of a plurality of longitudinal connecting rods and transverse locking nails, and the implant (10) is composed of a plurality of multi-level bionic structures (6), wherein the multi-level bionic structure (6) comprises two symmetrically arranged hexagonal upper and lower layer structures and an intermediate spiral layer structure (5) arranged between the two hexagonal upper and lower layer structures, wherein the hexagonal upper and lower layer structures comprise an equilateral hexagonal frame (3) and a wavy line structure (31), wherein the plurality of wavy line structures (31) are symmetrically arranged on the inner side of the equilateral hexagonal frame (3), and the intermediate spiral layer structure (5) It is composed of a plurality of intermediate spiral layer unit structures (4) arranged in a circular array. A supporting cylinder is provided between the center positions of the two hexagonal upper and lower layer structures. Both ends of the supporting cylinder are connected to the wavy line structures (31) of the two hexagonal upper and lower layer structures through ribs. The two ends of the intermediate spiral layer unit structure (4) are connected to the two ends of the supporting cylinder respectively. A plurality of intermediate layer concave support structures (61) arranged in a circular array are also provided between the two equilateral hexagonal frames (3). The two ends of the intermediate layer concave support structure (61) are connected to the two equilateral hexagonal frames (3) respectively.

2. The bionic lantern frame composite internal fixation device according to claim 1, characterized in that: The wavy line structure (31) is designed according to the following equation: ； in, is the horizontal coordinate of the curve, is the ordinate of the curve, is the amplitude, is the angular velocity, For the sake of appearance, For the offset.

3. The bionic lantern frame composite internal fixation device according to claim 2, characterized in that: The intermediate helical layer unit structure (4) is designed according to the following equation: ； ； in, is the radius, is the coefficient, For the changing angle, For the large diameter, For the trail, is the degree of rotation; the equation converted to the Cartesian coordinate system is: ； ； 。 4. The bionic lantern frame composite internal fixation device according to claim 3, characterized in that: The intermediate helical layer structure (5) is composed of six intermediate helical layer unit structures (4).

5. The bionic lantern frame composite internal fixation device according to claim 4, characterized in that: The side length of the equilateral hexagonal frame (3) is 1 cm and the height is 0.2 cm; the radius of the supporting cylinder is 0.2 cm and the rib width is 0.1 cm.

6. The bionic lantern frame composite internal fixation device according to claim 5, characterized in that: The concave support structure (61) in the middle layer is designed according to the following equation: ； ； in, is a parameter in the range [0,1], 、 are the coordinates of the starting point of the curve, 、 are the coordinates of the midpoint of the curve, 、 are the coordinates of the end point of the curve.

7. A method for preparing a bionic lantern frame composite internal fixation device, characterized in that: The method for preparing the implant (10) as claimed in claim 6 comprises the following steps: Step 1, modeling a multi-level bionic structure (6) based on the sine function and the Archimedean spiral principle, and densely arranging the multi-level bionic structure (6) to obtain a model of the implant (10); Step 2, preparing the implant (10) skeleton based on laser powder bed fusion metal additive manufacturing technology; Step 3, immersing the implant (10) skeleton obtained in step 2 in a polydopamine solution (7), stirring it magnetically at a temperature of 25 to 30° C. for 12 hours, taking it out and rinsing it with deionized water, drying it and setting it aside; In step 4, the implant (10) treated in step 3 is immersed in an electrolyte (9), and a platinum electrode (8) is used as a counter electrode to perform electrochemical deposition under magnetic stirring to obtain a gradient coating.

8. The method for preparing a bionic lantern frame composite internal fixation device according to claim 7, characterized in that: In step 2, the model obtained in step 1 was saved in stl format, sliced ​​in Materialise Magics software, and then copied into the L-PBF printer. TC4 spherical powder with a particle size of 15 to 53 μm was used for printing. The printing parameters were: laser power of 175 W, scanning speed of 1200 mm / s, scanning pitch of 0.08 mm, layer thickness of 0.03 mm, strip scanning strategy, and scanning angle of 67°.

9. The method for preparing a bionic lantern frame composite internal fixation device according to claim 8, characterized in that: The polydopamine solution (7) in step 3 is prepared by dissolving dopamine hydrochloride powder in a pre-prepared Tris buffer adjusted to pH 8.

5. The concentration of the polydopamine solution (7) is 2 mg / ml.

10. The method for preparing a bionic lantern frame composite internal fixation device according to claim 9, characterized in that: Step 4 is as follows: First, the implant (10) treated in step 3 was immersed in the electrolyte (9), and a platinum electrode (8) was used as a counter electrode. Electrodeposition was performed for 30 minutes under magnetic stirring at 10 rpm to obtain the first layer of hydroxyapatite coating. Then, 0.5 wt% chitosan was added to the electrolyte (9) and electroplating was continued for 45 minutes. The rotation speed was increased to 15 rpm. Then, 0.03 mol / L SrCl2 was added to the electrolyte (9) and electroplating was continued for 60 minutes to make Sr 2+ Immerse in the coating to improve the antibacterial properties, and finally remove the implant (10) and immediately immerse in deionized water for ultrasonic cleaning. After cleaning, place it in a drying oven at 60°C for 24 hours to obtain a gradient coating.

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