Preparation method of 3D printing hip stress relief heat treatment double gradient bone support

Abstract

The application discloses a preparation method of a 3D printing hip joint stress relief heat treatment double-gradient bone support, and the method comprises the following steps: acquiring image data by performing three-dimensional scanning on a bone defect by means of a CT scanning device, designing the size and shape of a transplantable bone, designing a concave hexagonal support structure with gradient porosity by means of UG three-dimensional drawing software, and gradually transitioning from a Ti-35Nb-5Ta-7Zr alloy with good biocompatibility and a low elastic modulus to a Co-Cr-Mo alloy with good wear resistance and corrosion resistance, improving metallurgical incompatibility, obtaining better interface combination, minimizing bone resorption and aseptic loosening caused by wear, prolonging the service life of an implant, and performing annealing heat treatment on the functional support after printing to eliminate residual stress and depositing a HA coating to enhance cell activity.

Description

Technical Field

[0001] This invention relates to a dual-gradient functional skeletal scaffold in terms of hip joint composition and structure, specifically to a method for fabricating a 3D-printed dual-gradient skeletal scaffold and a heat treatment process for eliminating residual stress in the scaffold. Background Technology

[0002] Total hip replacement (THR) is one of the most common orthopedic surgeries to date. While the average lifespan of the implant is 7–12 years, approximately 10% of implants require revision within 10 years. The lifespan of THR implants is limited, with the most common failure modes being osteolysis and aseptic loosening. This is particularly relevant for the increasing number of younger patients with active lifestyles. This may create a demand for implants with longer lifespans.

[0003] Traditional processing methods struggle to fabricate complex porous metal scaffolds, while additive manufacturing (3D printing) technology can produce scaffolds with controllable structure, shape, and properties. Therefore, 3D printing is one of the most effective methods for fabricating porous metal scaffolds. The design of porous metal scaffolds is a critical issue because scaffold characteristics such as unit cell type, pore size, porosity, and distribution significantly influence their mechanical properties and biocompatibility. Human bone possesses a gradient microstructure; therefore, developing porous scaffolds with gradient structures is a future trend. Experiments have demonstrated that gradient structures exhibit better mechanical properties than single-structure scaffolds, meeting both material transport and mechanical performance requirements.

[0004] Porous metal implants should be non-toxic, non-rejectable, and non-allergenic, requiring the selection of suitable metals as raw materials. Good biocompatibility is also reflected in the appropriate porous shape and distribution, which can promote the adhesion and growth of bone tissue cells. Furthermore, porous metal scaffolds should possess good wear resistance and corrosion resistance; therefore, the selected materials must have both good biocompatibility and excellent wear and corrosion resistance. However, combining materials with different compositions can lead to incompatibility. The SLM process is characterized by a high temperature gradient, which leads to the accumulation of thermal stress, and rapid solidification promotes segregation and the presence of non-equilibrium phases. This necessitates post-processing to eliminate residual stress and improve the mechanical properties of the bone scaffold. Summary of the Invention

[0005] This invention provides a method for preparing a concave hexagonal hip implant with gradient composition and porosity, exhibiting good biocompatibility, corrosion resistance, and wear resistance using 3D printing SLM, as well as a heat treatment process to eliminate residual stress. Through this method, a concave hexagonal structure with gradient porosity is designed by changing the scaffold diameter based on traditional bone implants. In terms of composition, Ti-35Nb-5Ta-7Zr, which is bone-friendly and has a low elastic modulus, is selected for the stem and shell, while the femoral head and acetabular lining use a Co-Cr-Mo alloy with excellent wear resistance.

[0006] The gradient structure is conducive to cell and blood vessel growth and bone ingrowth. The gradient composition improves metallurgical incompatibility, reduces failures, and achieves better interfacial bonding to minimize osteolysis and aseptic loosening caused by wear. The printed functional bone scaffold is heat-treated to eliminate residual stress, eliminate the harm caused by deformation, improve the life of the implanted scaffold, and deposit an HA coating on the scaffold surface to further enhance cell activity.

[0007] 1. Skeletal scaffold modeling

[0008] 3D scanning of the bone defect using CT scanners was employed to acquire image data. Based on the damaged bone, 3D software was used to redesign the size and shape of the bone suitable for transplantation. A porous composite model was designed using UG 3D modeling software. The model was compared with healthy bone to verify the suitability of the model's dimensions and to check for any deviations. The scaffold adopted a gradient porosity concave hexagonal structure design, with concave hexagonal angles of 60° and 120° respectively. The gradient porosity was achieved by controlling the scaffold diameter, resulting in a decrease in porosity followed by an increase. The scaffold dimensions were 7cm × 7cm × 7cm. From top to bottom, the first layer of the scaffold had a diameter of 3mm, a pore size of 450μm, a porosity of 60%, and a volume of 26.438cm³. 3 The second-layer support has a diameter of 2mm, a pore size of 650μm, a porosity of 70%, and a volume of 12.596cm³. 3 The third-layer support has a diameter of 1 mm, a pore size of 900 μm, a porosity of 85%, and a volume of 2.79 cm³. 3 The fourth layer is the same as the second layer, and the fifth layer is the same as the first layer, achieving a symmetrical gradient porosity structure with porosities of 60%, 70%, 85%, 70%, and 60%, which has better mechanical properties, is conducive to cell and blood vessel growth, and facilitates bone ingrowth.

[0009] 2. Powder Preparation

[0010] By using a plasma rotating electrode process, alloy powder samples with the composition of Ti-35Nb-5Ta-7Zr (wt.%) were prepared by solidification in an inert gas environment by centrifugal force and spheroidization under the action of surface tension. The powder diameter was screened to be between 30-50 μm.

[0011] Co-Cr-Mo alloy spherical powder was prepared by gas atomization. The nominal composition (wt.%) was 27Cr, 5.2Mo, 0.3C, 1.2Fe, 0.7Mn, 2.8Ni, and 1.6Si. The powder diameter was between 30 and 50 μm.

[0012] 3. Preparation of mixed powders

[0013] Ti-35Nb-5Ta-7Zr alloy powder is named powder 1; Ti-35Nb-5Ta-7Zr alloy powder and Co-Cr-Mo alloy powder are mixed evenly according to the following mass percentages: 75% Ti-35Nb-5Ta-7Zr and 25% Co-Cr-Mo to obtain metal mixed powder 2; Ti-35Nb-5Ta-7Zr and Co-Cr-Mo are mixed evenly according to the following mass percentages: 50% Ti-35Nb-5Ta-7Zr and 50% Co-Cr-Mo to obtain metal mixed powder 3; Ti-35Nb-5Ta-7Zr and Co-Cr-Mo are mixed evenly according to the following mass percentages: 25% Ti-35Nb-5Ta-7Zr and 75% Co-Cr-Mo to obtain metal mixed powder 4; Co-Cr-Mo alloy powder is named powder 5.

[0014] 4. Printing of a dual-gradient skeletal scaffold

[0015] The pre-designed gradient skeletal scaffold model was imported into an SLM printer and printed layer by layer. During the printing process, the laser power was 180W, the scanning speed was 1200mm / s, the laser diameter was 50μm, and the scanning interval was 0.03mm. For the first layer, powder 1 was loaded into the hopper, forming a skeletal scaffold composed of 100% Ti-35Nb-5Ta-7Zr alloy with a diameter of 0.6mm and a porosity of 60%. For the second layer, powder 2 was loaded, forming a skeletal scaffold composed of 25% Co-Cr-Mo alloy with a diameter of 0.4mm and a porosity of 70%. For the third layer, powder 3 was loaded. The first layer consists of a skeletal scaffold composed of 50% Co-Cr-Mo alloy with a diameter of 0.2 mm and a porosity of 80%. The fourth layer, filled with 40% powder, forms a skeletal scaffold composed of 75% Co-Cr-Mo alloy with a diameter of 0.4 mm and a porosity of 70%. The fifth layer, filled with 50% powder, forms a skeletal scaffold composed of 100% Co-Cr-Mo alloy with a diameter of 0.6 mm and a porosity of 60%. Each time the powder is changed, the powder must be completely removed, and the forming chamber must be wiped with alcohol. A gradient structure containing 25%, 50%, 75%, and 100% Co-Cr-Mo alloys was formed on the first Ti-35Nb-5Ta-7Zr alloy layer. This achieved a gradual transition from an alloy with good biocompatibility and low elastic modulus to an alloy with good wear resistance and corrosion resistance, improving metallurgical incompatibility, reducing failures, and achieving better interfacial bonding to minimize osteolysis and aseptic loosening caused by wear.

[0016] After removal, the powder is immersed in 96% ethanol to clean and remove some of the easily removable unmelted powder. Then, ultrasonic cleaning is used to remove the remaining powder in the pores through ultrasonic vibration.

[0017] 5. Heat treatment

[0018] The printed dual-gradient bone scaffold was subjected to annealing heat treatment: held at 650℃ for 2 hours and then furnace cooled to eliminate residual stress generated during the forming process, eliminate anisotropy, homogenize the tissue, and improve mechanical properties.

[0019] 6. Electrochemical deposition of HA coating

[0020] The heat-treated porous scaffold and an electrolyte solution containing 2.5 mM CaCl2·6H2O, 1.5 mM NH4H2PO4, and 0.15 M NaCl were placed in a container. A vacuum pump was used to evacuate the container to a negative pressure state, allowing the electrolyte to completely permeate into the pores of the sample. The deposition process was carried out in an electrochemical workstation using a pulsed current method and a three-electrode system. The porous scaffold, platinum plate, and saturated calomel electrode were used as the cathode, anode, and reference electrode, respectively. The electrolyte temperature was controlled at approximately 85 °C. Attached Figure Description

[0021] Figure 1 A frontal view of a dual-gradient scaffold implanted at the hip joint, consisting of five concave hexagonal structures that achieve a gradient transition from 100% Ti-35Nb-5Ta-7Zr alloy to 25%, 50%, 75%, and 100% Co-Cr-Mo alloys from bottom to top.

[0022] Figure 2 The diagram shows an isotropic image of a dual-gradient stent implanted at the hip joint. In the image, 1 represents the first layer with a porosity of 60% and a composition of 100% Ti-35Nb-5Ta-7Zr alloy; 2 represents the second layer with a porosity of 25% Co-Cr-Mo alloy; 3 represents the third layer with a porosity of 85% and a composition of 50% Co-Cr-Mo alloy; 4 represents the fourth layer with a porosity of 70% and a composition of 75% Co-Cr-Mo alloy; and 5 represents the fifth layer with a porosity of 60% and a composition of 100% Co-Cr-Mo alloy.

[0023] Figure 3 This is a schematic diagram of a unit cell with a concave hexagonal structure supported by a dual gradient. In the diagram, unit a has a diameter of 3mm. Figure 2 The components of units 1 and 5 in the diagram; unit b in the diagram has a diameter of 2mm. Figure 2 The components of 2 and 4; the diameter of bracket c in the figure is 1mm. Figure 2 The constituent units of 3. Detailed Implementation

[0024] Example: Total hip replacement surgery

[0025] Step 1: A 3D scan of the bone defect area is performed using CT scanning equipment to acquire image data. Based on the damaged bone, the size and shape of the bone suitable for transplantation are redesigned using 3D software. A porous composite model is designed using UG 3D modeling software, and the model is compared with healthy bone to verify whether the model size is appropriate and whether there are any deviations.

[0026] The scaffold adopts a gradient porosity concave hexagonal structure design, with concave hexagonal angles of 60° and 120° respectively. From top to bottom, the first layer of the scaffold has a diameter of 3mm, a pore size of 450μm, a porosity of 60%, and a volume of 26.438cm³. 3 The second-layer support has a diameter of 2mm, a pore size of 650μm, a porosity of 70%, and a volume of 12.596cm³. 3 The third-layer support has a diameter of 1 mm, a pore size of 900 μm, a porosity of 85%, and a volume of 2.79 cm³. 3The fourth layer is the same as the second layer, and the fifth layer is the same as the first layer, achieving a symmetrical gradient porosity structure with porosities of 60%, 70%, 85%, 70%, and 60%.

[0027] Step 2: Using a plasma rotating electrode process, alloy powder samples with the composition Ti-35Nb-5Ta-7Zr (wt.%) were prepared by solidification in an inert gas environment under centrifugal force and spheroidization under surface tension. The powder diameter was screened to be between 30-50 μm. Co-Cr-Mo alloy spherical powder was then prepared by gas atomization, with a nominal composition (wt.%) of 27Cr, 5.2Mo, 0.3C, 1.2Fe, 0.7Mn, 2.8Ni, and 1.6Si, and a powder diameter between 30-50 μm. Ti-35Nb-5Ta-7Zr alloy powder is named powder 1; Ti-35Nb-5Ta-7Zr alloy powder and Co-Cr-Mo alloy powder are mixed evenly according to the following mass percentages: 75% Ti-35Nb-5Ta-7Zr and 25% Co-Cr-Mo to obtain metal mixed powder 2; Ti-35Nb-5Ta-7Zr and Co-Cr-Mo are mixed evenly according to the following mass percentages: 50% Ti-35Nb-5Ta-7Zr and 50% Co-Cr-Mo to obtain metal mixed powder 3; Ti-35Nb-5Ta-7Zr and Co-Cr-Mo are mixed evenly according to the following mass percentages: 25% Ti-35Nb-5Ta-7Zr and 75% Co-Cr-Mo to obtain metal mixed powder 4; Co-Cr-Mo alloy powder is named powder 5.

[0028] Step 3: Import the pre-designed gradient skeletal scaffold model into the SLM printer and print layer by layer. During printing, the laser power is 180W, the scanning speed is 1200mm / s, the laser diameter is 50μm, and the scanning interval is 0.03mm. For the first layer, powder 1 (100% Ti-35Nb-5Ta-7Zr alloy) is loaded into the hopper, with a scaffold diameter of 3mm and a porosity of 60%. The second layer is printed using powder 2 (25% Co-Cr-Mo alloy). The first layer is 2mm thick with a porosity of 70%; the second layer is printed with powder 3, which is composed of 50% Co-Cr-Mo alloy, with a support diameter of 1mm and a porosity of 85%; the third layer is printed with powder 4, which is composed of 75% Co-Cr-Mo alloy, with a support diameter of 2mm and a porosity of 70%; the fifth layer is printed with powder 5, which is composed of 100% Co-Cr-Mo alloy, with a support diameter of 3mm and a porosity of 60%; each time the powder is changed, the powder must be completely removed and the forming chamber must be wiped with alcohol.

[0029] After removal, the powder is immersed in 96% ethanol to clean and remove some of the easily removable unmelted powder. Then, ultrasonic cleaning is used to remove the remaining powder in the pores through ultrasonic vibration.

[0030] Step 4: Annealing heat treatment of the printed dual-gradient skeletal scaffold: Hold at 650℃ for 2 hours, then furnace cool. After cooling to room temperature, place the scaffold in a beaker containing industrial alcohol, place the beaker in an ultrasonic cleaner, add water to cover the sample height and clean for 5 minutes, then remove it. Replace with a new alcohol solution and repeat the ultrasonic cleaning steps twice until no oil stains are visible on the sample, then place it in a ventilated place to dry.

[0031] Step 5: The heat-treated porous scaffold and an electrolyte solution containing 2.5 mM CaCl2·6H2O, 1.5 mM NH4H2PO4, and 0.15 M NaCl were placed in a container. A vacuum pump was used to evacuate the container to a negative pressure state, allowing the electrolyte to completely permeate into the pores of the sample. The deposition process was carried out in an electrochemical workstation using a pulsed current method and a three-electrode system. The porous scaffold, platinum plate, and saturated calomel electrode were used as the cathode, anode, and reference electrode, respectively. The electrolyte temperature was controlled at approximately 85°C.

Claims

1. A method for fabricating a 3D-printed hip joint stress-relief heat-treated dual-gradient bone scaffold, characterized in that, The steps include the following: (1) The bone defect was scanned in three dimensions by CT scanning equipment to obtain image data. A bone scaffold model with continuous symmetry gradient porosity was designed using UG three-dimensional modeling software. Based on the concave hexagonal structure, there are five layers from top to bottom. The porosity of the first layer scaffold is 60%, the porosity of the second layer scaffold is 70%, the porosity of the third layer scaffold is 85%, the porosity of the fourth layer scaffold is 70%, and the porosity of the fifth layer scaffold is 60%. (2) Ti-35Nb-5Ta-7Zr alloy powder was prepared by plasma rotating electrode process, and Co-Cr-Mo alloy spherical powder was prepared by gas atomization method, with the powder diameter between 30-50μm; (3) The designed skeletal scaffold model was imported into the SLM printer and printed layer by layer. The first layer was printed with Ti-35Nb-5Ta-7Zr alloy powder; the second layer was printed with a metal mixed powder obtained by uniformly mixing Ti-35Nb-5Ta-7Zr and Co-Cr-Mo at a mass percentage of 75%; the third layer was printed with a metal mixed powder obtained by uniformly mixing Ti-35Nb-5Ta-7Zr and Co-Cr-Mo at a mass percentage of 50%; the fourth layer was printed with a metal mixed powder obtained by uniformly mixing Ti-35Nb-5Ta-7Zr and Co-Cr-Mo at a mass percentage of 25%; and the fifth layer was printed with Co-Cr-Mo alloy powder. Thus, a dual-gradient skeletal scaffold with a structure and composition was formed. (4) The printed dual-gradient skeletal scaffold is subjected to annealing heat treatment to eliminate residual stress; (5) Electrochemically depositing an HA coating on the heat-treated scaffold enhances cell activity.

2. The method for preparing a 3D-printed hip joint stress-relief heat-treated dual-gradient bone scaffold according to claim 1, in step (1), the first layer of the scaffold has a diameter of 3 mm, a pore size of 450 μm, and a volume of 26.438 cm³. 3 The second-layer support has a diameter of 2mm, a pore size of 650μm, and a volume of 12.596cm³. 3 The third-layer support has a diameter of 1 mm, a pore size of 900 μm, and a volume of 2.79 cm³. 3 The fourth layer is the same as the second layer, and the fifth layer is the same as the first layer.

3. According to the method for preparing a 3D printed hip joint stress-relief heat-treated dual-gradient bone scaffold as described in claim 1, in step (3), the laser power is 180W, the scanning speed is 1200mm / s, the laser diameter is 50μm, and the scanning interval is 0.03mm.

4. According to the method for preparing a 3D printed hip joint stress-relief heat-treated dual-gradient bone scaffold as described in claim 1, in step (4), the printed dual-gradient bone scaffold is subjected to annealing heat treatment: held at 650℃ for 2 hours, then furnace cooled to room temperature.

5. The method for preparing a 3D printed hip joint stress-relief heat-treated dual-gradient bone scaffold according to claim 1, in step (5), an HA coating is electrochemically deposited. The heat-treated porous scaffold and an electrolyte solution containing 2.5 mM CaCl2·6H2O, 1.5 mM NH4H2PO4 and 0.15 M NaCl are placed in a container. The deposition process is carried out in an electrochemical workstation using a pulsed current method and a three-electrode system. The porous scaffold, platinum plate and saturated calomel electrode are used as the cathode, anode and reference electrode, respectively. The electrolyte temperature is controlled at about 85 °C.

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