Fabrication and Characterization of LaF₃-Reinforced Porous HA/Ti Scaffolds

Abstract

To improve the performance of porous hydroxyapatite/titanium (HA/Ti) composites, LaF₃ reinforced porous HA/Ti scaffolds with a porosity of approximately 60% were prepared via a powder metallurgical method, using NH₄HCO₃ as the pore-forming agent. The scaffolds induced HA formation and showed high bioactivity, and the compressive strength could be regulated by changing the LaF₃ dosage. When the LaF₃ dosage was 0.3%, the compressive strength of the porous scaffold was 65 MPa. Moreover, LaF₃ reinforced porous HA/Ti scaffolds can further induce the deposition of calcium phosphate after immersion in simulated body fluid (SBF) for 7 days, indicating that the corresponding scaffold is an ideal choice for spongy bone repair.

1. Introduction

Titanium is used as a biomaterial due to its excellent corrosion resistance and significant strength [1]. However, the Young’s modulus is much higher than that of natural bone. The mismatch of the elastic modulus between titanium and the natural bone would lead to bone resorption and finally, to implant loosening [2]. In addition, due to the fact that the surface of titanium is usually covered with a dense oxide film, titanium exhibits biological inertness. When used as a bone repair material, the combination of this material with bone shows mechanical chimerism. The material is only mechanically integrated, rather than chemically osseointegrated, with the bone, resulting in poor adhesion between the implant material and bone tissue, often leading to implant failure [3]. There are various methods for modifying titanium to endow with biological activity [4,5,6,7]. These methods are all aimed at the surface of the material. This also inevitably leads to issues regarding the bonding strength between the modified layer and the matrix. Hydroxyapatite has been widely considered as an implant material because its mineral composition is very close to that of human bone. Hydroxyapatite offers not only good biocompatibility; it also exhibits osteoinductive properties. However, its mechanical properties are poor. Therefore, it can only be used for bone repair in nonbearing locations [8,9], which greatly limits the application of hydroxyapatite. In order to improve its mechanical properties, using the titanium and hydroxyapatite composite method has been considered for prepare new composite materials. However, most of the current reports on titanium/hydroxyapatite composites are based on hydroxyapatite, and titanium is only sintered as a reinforcing phase [10,11]. Its use has been limited to the improvement of the mechanical properties of the composite strength of the materials. In the studies regarding HA/Ti composites, researchers have extensively explored the preparation methods for hydroxyapatite and titanium. For example, Thian, ES et al. used Ti-6Al-4V and HA as raw materials and PVA as a binder to prepare green specimens with a certain shape using injection molding and then obtained composite materials with high mechanical properties through high-temperature sintering [12]. The biggest advantage of this method is that it can prepare products with complex shapes through the use of injection molding, which has great application prospects. Although thermal spraying and injection molding can be used to prepare HA/Ti composites with mechanical properties close to those of human bone and an elastic modulus of 10–30 GPa [13], its high processing costs, as well as its current product sizes and shapes, are still worthy of in-depth study. However, the above research mainly involves dense block-like composite materials, which do not have pore structures that facilitate the growth of bone tissue for “biological fixation”. The pore structure plays an important role in the growth of bone tissue for the following reasons. (1) The volume density, strength, and elastic modulus of porous titanium can be adjusted by changing the porosity to achieve mechanical properties that match those of the replacement tissue. (2) The porous structure is conducive to the adhesion, differentiation, and growth of osteoblasts, promoting the growth of bone tissue into the pores, strengthening the connection between the implant and bone, achieving biological fixation [14]. Our previous research has shown that through the introduction of a porous structure, treated porous titanium can be implanted into the femurs of dogs. After 6 months of implantation, new bone tissue can be found in the pore, and the bonding strength between the implant and bone tissue is satisfactory [15]. Three-dimensional printing has emerged as a promising technique for fabricating porous hydroxyapatite scaffolds with controlled architecture and porosity. However, the development of biocompatible and bioactive 3D-printed hydroxyapatite scaffolds remains a challenge. In a recent study, Andrej Thurzo fabricated 3D-printed hydroxyapatite scaffolds using polyvinyl alcohol (PVA) as a thermoplastic binder. The scaffolds were found to be biocompatible and supported the adhesion and proliferation of mesenchymal stem cells (MSCs) [16]. However, the introduction of pores will reduce the mechanical strength of the composites. Research has shown that rare-earth elements play an important role in improving material properties. There are two main reasons for this phenomenon. Firstly, rare-earth elements can act as effective deoxidizers to extract oxygen from alloys; secondly, rare-earth elements can alleviate the instability of the alloy structure and surface, and improve the thermal stability of the alloy. Research has shown that adding rare-earth elements to titanium alloy composites can effectively refine the structure, as well as improve the room temperature performance, oxidation resistance, and thermal stability of the alloy [17]. For hydroxyapatite, the addition of rare-earth elements can alter the crystal structure of hydroxyapatite and generate new biological effects [18]. Tang et al. investigated the effect of the lanthanum content in hydroxyapatite coatings on the biological properties of cells attached on titanium surfaces. They found that hydroxyapatite coatings, with an appropriate amount of lanthanum added, exhibit good biocompatibility and can promote the early proliferation and differentiation of bone cells on the material surface. However, when the content of lanthanum is excessive, it can affect the proliferation of osteoblasts because the biological effects of rare-earth element ions are very similar to those of calcium ions, therefore exhibiting antagonistic effects affecting the calcium phosphorus ratio of hydroxyapatite. Another study has shown that strontium offers good compatibility at a concentration of 10% [19,20]. Therefore, adding an appropriate amount of rare-earth elements to Ti/HA composite materials can effectively improve the strength of the matrix and improve its biological properties. In order to further improve the mechanical properties of the strength of materials, this study intends to add appropriate rare-earth elements in Ti/HA composites to effectively improve the strength of the matrix and improve its biological properties.

2. Materials and Methods

2.1. Fabrication of LaF₃ Reinforced Porous HA/Ti Scaffolds

LaF₃ reinforced porous HA/Ti scaffolds were prepared using powder metallurgy through the process shown in Figure 1. Commercial pure titanium powders (purity: ≥99.9%, powder size ≤ 30.0 µm) and hydroxyapatite with a particle size of 1.0–3.0 µm (provided by Sichuan University) were mixed as raw materials in a mass ratio of 1:20. Ammonium bicarbonate with a particle size range of 100.0–300.0 µm was chosen as the spacer material, and the mass fractions of NH₄HCO₃ were 30.0%. LaF₃, with mass fractions of 0.1%, 0.3%, and 0.5%, respectively, were added. After the ingredients were homogeneously blended, powders obtained from the mixture were uniaxially pressed at a pressure of 100 MPa into cylindrical green pellets with the size of Φ10 mm × H15 mm. The specimens were then sintered in the vacuum sintering furnace (model: ZT-40-20Y, Suzhou Huolanxin Vacuum Technology Co., Ltd., Suzhou, China) in a vacuum. The heat-treatment process consisted of two steps, i.e., 150 °C for 1 h and 1200 °C for 3 h. The main purpose of holding the specimen at 150 °C for 1 h is to completely remove the spacer. According to the different amounts of LaF₃ added, the sintered samples are named HA/Ti, HA/Ti + 0.1% LaF₃, HA/Ti + 0.3% LaF₃, and HA/Ti + 0.5% LaF₃, respectively. The specific process is shown in Figure 1.

2.2. Immersion of Samples in the SBF

The bioactivity of the various samples was evaluated in terms of their apatite-forming abilities by immersing them in the SBF solution. The scaffold, with a diameter of 10.0 mm and a thickness of 2.0 mm, was soaked in a bionic mineralization SBF at 37 °C for 7 days. Afterwards, the bioactivity was tested. The SBF formulation was the same as that reported by Kokubo et al. [21]. Briefly, 8.035 g NaCl, 0.355 g NaHCO₃, 0.225 g KCl, 0.231 g K₂HPO₄·3H₂O, 0.5 M-HCl, 0.292 g CaCl₂, 0.072 g Na₂SO₄, 6.118 g CNH₂(CH₂OH)₃, and 1M-HCl were added into 1 L deionized water to reach a pH of 7.4 at 36.5 °C.

2.3. Characterization

All the samples were analyzed using a scanning electron microscope (SEM). X-ray diffraction (XRD) was used to characterize the crystalline phase and chemical compositions, with the measurement scanning speed of 5°/min 2θ (10°–90°). The phase analysis was carried out with the Jade 6.5 software package, using the powder diffraction file (PDF) of the International Center for Diffraction Data. The compressive strength tests were carried out on an Instron mechanical testing machine (Instron 5567, Norwood, MA, USA) with a crosshead speed of 0.5 mm/min, employing three groups of sintered samples of each type, namely HA/Ti, HA/Ti + 0.1%LaF₃, HA/Ti + 0.3%LaF₃, and HA/Ti + 0.5%LaF₃, with three samples in each group for compressive testing. The porosity of the specimens was determined by measuring their surface areas and weights according to the following Equation (1):

P = 1 − ρ_scaffold/ρ_material

(1)

where P is total porosity, ρ_scaffold is the apparent density of porous titanium measured by dividing the weight by the volume of the samples, and ρ_material is the density of the material of which the scaffold is fabricated [22].

3. Results and Discussion

Figure 2 shows the morphology of the sintered samples of the porous composite materials before and after the addition of rare-earth LaF₃. Figure 2a,c shows the samples without the addition of the rare-earth element, and Figure 2b,d shows the samples after the addition of 0.3% LaF₃; the porosity of the porous scaffold is about 60%. From the figure, it can be seen that the shape of the pores somewhat circular, and the pore size is basically distributed between 100–300 µm. The shape and size of the pores are comparable to those of the added pore forming agent. The method of adding pore forming agents is used to prepare porous materials, and the formation of pores is significantly related to the shape of the pore forming agent. It is generally believed that adding pore forming agents to form pores results in a space left by the decomposition of the pore forming agent when heated. Most holes are interconnected with each other. In addition, the morphology of porous materials at high magnification shows the presence of micropores ranging in size from a few micrometers to several tens of micrometers on their macroscopic pore walls, as shown in Figure 2c,d. These pores are significantly smaller than those of the pore forming agent, so their formation mechanism is different from that of large pores. This is because during the pressing of green specimens, space is formed by the accumulation of raw material particles, such as titanium powder. After sintering, approximately circular micropores were formed due to shrinkage. These complex pore structures, interconnected by large and small pores, are conducive to the transport of nutrient solution and the growth of bone tissue.

Figure 3 shows the surface scan image of the porous material after adding 0.3% LaF₃. From this image, it can be seen that elements such as calcium and phosphorus are uniformly distributed on the titanium matrix, and the lanthanum element after adding rare-earth LaF₃ is also uniformly distributed on the matrix, which further improves the mechanical properties of the porous composite material. On the surface, this method can successfully prepare composite materials with uniform composition.

Figure 4 shows the XRD patterns of different porous HA/Ti scaffolds. The main phases of the composite are Ti, Ti₂O, Ti₃P, CaO, and CaTiO₃. During sintering, the formation of the Ti₂O phase is mainly due to the sintering process. Oxygen elements in the surrounding HA phase and the glass phase diffuse into Ti. When the concentration of the oxygen atoms in the interstitial gap of the Ti lattice reaches a certain level, titanium oxides will form. The type of oxide formed is closely related to the concentration of the oxygen atoms in the sintering atmosphere. This study used a vacuum sintering furnace for the sintering process. The oxygen concentration is very low; therefore, only Ti₂O can be generated, rather than oxides with a high oxygen content, such as TiO₂. The volume fraction of titanium in the composite materials is relatively high The main crystal phase is Ti. However, due to the diffusion of elements with smaller atomic radii, such as oxygen and phosphorus, into the Ti lattice, the formation of compounds such as Ti₃P, CaO, and CaTiO₃ occurs. The results are similar to those of Ning CQ et al. [23]. Research has shown that Ti₃P, CaO, and CaTiO₃ contributes to the formation of HA and improves the biological activity of materials. The increase in the concentration of Ca²⁺ in SBF leads to the supersaturation of Ca²⁺ on the surface of the composite, which reduces the minimum free energy required to cause apatite nucleation and growth [24]. There are two stable phases of calcium phosphate in an aqueous solution at room temperature. When the pH was lower than 4.2, the stable phase was CaHPO₄·2H₂O, and when the pH was higher than 4.2, the stable phase was Ca₁₀(PO₄)₆(OH)₂(HA) [25]. In this experiment, the pH of SBF is 7.4, so HA is easily formed on the surface. However the dissolution of the CaO phase forms micropores on the surface, which also contributes to the nucleation of HA. Ti₂O-containing compounds can be prepared by sintering, and the results show that the presence of titanium oxides contributed to the formation of HA [10]. However, in this study, the content of LaF₃ is relatively small, so the presence of LaF₃ or La₂O₃ is not detected by XRD. From Figure 4, it can be seen that there is no diffraction peak of NH₄HNO₃. This indicates that the added pore forming agent (ammonium bicarbonate) has been completely removed after low temperature heating and high temperature sintering, and the sintered sample is not polluted, which ensures the biological safety of porous titanium implanted in the body.

Figure 5 shows the effect of LaF₃ addition on the compressive strength of the HA/Ti porous scaffolds. With the increase in rare-earth content, the compressive strength of the porous scaffolds increases first and then decreases. When LaF₃ is 0.3%wt, the compressive strength of the porous scaffolds is the largest, reaching 65 MPa. The addition of rare-earth elements can significantly improve the compressive strength of the HA/Ti composites, mainly because the rare-earth element as an effective deoxidizer can absorb the oxygen in the composite, purify the interface of the original particles, reduce the oxygen content of the matrix, and improve the sintering density of the sintered body [19]; the refinement of the structure by LaF₃ can also improve the compressive strength of the porous scaffolds. Figure 4 shows that when adding rare-earth LaF₃ to 0.3%, the diffraction peak corresponding to CaTiO₃ widens significantly, and the diffraction intensity decreases. This indicates that the entropy value of the system is large; that is, the lattice distortion of CaTiO₃ is severe, resulting in an increase in the compressive strength of the composite material, which is consistent with the XRD results shown in Figure 4. When adding rare-earth LaF₃ to 0.3%, the compressive strength is the highest. On the other hand, research has shown that one of the biggest obstacles in regards to the sintering density of titanium powder is the presence of an oxide film on the surface of the powder particles [26]. The affinity between rare-earth element La and oxygen is much greater than that between Ti and oxygen. Therefore, adding LaF₃ to the alloy can effectively activate the titanium powder. Adding a small amount of LaF₃ has a significant promoting effect on the sintering densification of the HA/Ti powder metallurgy. The La element carries oxygen on the surface of the Ti powder and diffuses into the matrix to form a Ti (La, O) solid solution, promoting the diffusion of elements between the titanium particles. As the amount of LaF₃ added increases, the surface of the activated Ti particles increases, and the densification effect is improved. However, due to the fact that the sintering process mainly occurs in the solid phase, excessive secondary particles will hinder the plastic and viscous flow of the sintering process, thereby reducing the sintering density. Therefore, when the amount of LaF₃ added is 0.5%, the relative density of the material decreases, and its compressive strength also decreases [18].

Figure 6 shows the surface morphology of the porous scaffolds after soaking in SBF for 7 days. It can be seen from the figure that the materials with 0.3% LaF₃ added exhibit more spherical coverings over the surface of the porous materials after soaking in SBF solution. The energy spectrum analysis results show that the coverage is mainly composed of calcium, phosphorus, and oxygen, and its composition ratio is similar to that of HA, which indicates that the addition of LaF₃ can further improve the biological activity of the composite.

Due to limited experimental conditions, this study demonstrates certain limitations. For example, if the sample size is small and only in vitro testing methods are used to evaluate biological activity, subsequent studies will be required to conduct cell culture experiments and related animal experiments using the material in order to exhaustively evaluate its comprehensive performance.

4. Conclusions

A porous HA/Ti scaffold with high porosity (60%) was prepared by adding pore forming agents. The pore structure of the scaffold is three-dimensional, with pore sizes mainly distributed between 100–300 μm, and there are micrometer sized micropores distributed on the pore walls, which are conducive to the growth of bone tissue and the achievement of biological fixation. The addition of LaF₃ can improve the biomechanical properties of porous HA/Ti scaffolds, and the HA/Ti scaffold exhibits the optimal mechanical performance when the amount of the addition is 0.3%. The compressive strength is 65 MPa, exhibiting mechanical properties similar to those of human bone, which is beneficial for reducing the stress shielding problem caused by the mismatch between the biomechanical properties of the implant and human bone. The rare-earth LaF₃ enhanced porous HA/Ti scaffold offers good pore structure and biomechanical compatibility, and it is expected to be employed as an ideal material for bone replacement and repair.