Formation and Bioactivity of Composite Structure with Sr-HA Phase and H₂Ti₅O₁₁·H₂O Nanorods on Ti Surface via Ultrasonic-Assisted Micro-Arc Oxidation and Heat Treatment

Abstract

To address the biological inertness of pure titanium implants, a composite coating with a strontium-doped hydroxyapatite (Sr-HA) phase and H₂Ti₅O₁₁·H₂O nanorods was engineered via ultrasonic-assisted micro-arc oxidation (UMAO) with hydrothermal treatment (HT). The ultrasonic field was applied to modulate the MAO discharge behavior, enhancing ion transport and coating formation. Structural characterization revealed that UMAO-HT coatings exhibited a lower anatase/rutile ratio and higher Sr-HA crystallinity, as compared to MAO-HT. In vitro simulated body immersion studies showed that UMAO-HT induced rapid apatite formation within 24 h, with a better apatite-inducing ability than the conventional MAO-HT. Density functional theory (DFT) simulations demonstrated that Sr substitution in HA lowered the (001) surface work function, enhancing Ca²⁺ adsorption energy and promoting apatite phase nucleation. This work reported the synergistic effects of ultrasonic-induced microstructure optimization and Sr-HA higher bioactivity, providing a mechanistic framework for designing next-generation bioactive coatings with enhanced osseointegration potential.

1. Introduction

Titanium is often used as a metallic biomaterial due to its excellent biocompatibility and mechanical properties [1,2,3]. However, the high modulus of elasticity of pure titanium relative to bone tissue can easily lead to the stress shielding phenomenon, and pure titanium is biologically inert and usually does not bond well with bone tissue [4,5,6]. As highlighted in recent advances in ceramic coating research, these inherent limitations necessitate advanced surface modifications to enhance osseointegration and implant longevity [7].

Due to the problems associated with the use of pure titanium as an implant, a number of processes have been developed for the preparation of bioactive coatings on pure titanium surfaces, including ion spraying, electrochemical deposition, and micro-arc oxidization [8,9,10]. Among these, micro-arc oxidation (MAO) has emerged as a dominant technique for preparing Sr-doped hydroxyapatite (Sr-HA) coatings. For instance, Zhang et al. demonstrated that adjusting Sr(CH₃COO)₂ concentration in MAO electrolytes could tune Sr-HA content, yet their coatings exhibited limited phase complexity and required 48–72 h for apatite nucleation. Similarly, Wang et al. fabricated porous Sr-HA films via MAO, but the absence of post-treatment resulted in low crystallinity and inhomogeneous Ca/Sr distributions. Micro-arc oxidization has the advantages of low cost, simplicity, and high bonding strength of the coating compared to the other processes, and it is often used for the preparation of bioactive coatings [11,12].

Coatings containing calcium and phosphorus elements can be prepared on the surface of pure titanium by MAO and subsequent microwave hydrothermal treatment, which allows the bioactivity of the implant to be further enhanced [13,14,15]. And among these, the microscopic morphology and elemental composition of the coating surface are important factors affecting the bioactivity of the implant and determining the extent to which the implant is well integrated with the host cells [16,17].

However, existing Sr-HA coatings primarily rely on electrolyte composition modulation (e.g., Sr²⁺ doping), neglecting the synergistic effects of process innovation and microstructure design. Calcium–phosphorus bioceramic coatings generated on titanium surfaces by the MAO process alone are still unsatisfactory in terms of performance, with problems such as insufficient bonding strength between the coating and the substrate, insufficient corrosion resistance, and the non-uniform and non-dense distribution of the coatings [18]. In order to further explore the factors and mechanisms affecting the formation of the coatings, which can lead to increased bioactivity on the surface of the coatings or enhance other related properties, many researchers have introduced different processes in the preparation of coatings [19,20].

Li et al. [21] proposed ultrasonic micro-arc oxidation (UMAO) for the fabrication of ceramic coatings on magnesium alloys, demonstrating that this method not only enhanced corrosion resistance but also achieved a more homogeneous distribution of surface micropores compared to conventional approaches. Building on this foundation, Li et al. [22] systematically investigated the role of ultrasonic power modulation during UMAO, revealing that precise control of this parameter effectively increased the Ca/P ratio within both the coatings and the corrosion layer, thereby significantly enhancing the bioactivity of the resulting surfaces. Komarova et al. [23] expanded the application scope by demonstrating that ultrasonic field modulation during MAO enabled precise regulation of critical coating parameters, including thickness, surface roughness, and porosity, which collectively governed the structural integrity and functional properties of calcium phosphate coatings in biomedical contexts. Furthermore, Kasprolewicz et al. provided mechanistic insights by confirming that ultrasonic MAO synergistically enhanced bioactivity, while also elucidating the underlying physicochemical mechanisms responsible for these performance improvements.

Currently, studies have shown that introducing ultrasonic assistance into the MAO process can make the coating more dense and uniform, leading to better bonding with the substrate and improved corrosion resistance [24,25,26]. Additionally, parameters such as the electrolyte composition can be adjusted to enhance the biological activity and improve the bioactivity of medical titanium alloys [27,28]. However, most of the research has concentrated on the MAO process itself, with relatively fewer studies examining the enhancement of biological properties through post-treatment methods like microwave hydrothermal treatment.

In this study, by introducing ultrasound-assisted technology into the MAO process, the concentration of calcium and phosphorus and their distribution uniformity in the coating were improved, which in turn promoted the generation of the strontium-doped hydroxyapatite (Sr-HA) phase during the microwave hydrothermal process. The adsorption behavior of the calcium and phosphorus ions of coatings prepared by ultrasound-assisted MAO during the microwave hydrothermal process has been investigated through structural characterization and software simulation to elucidate the mechanism of ultrasound-assisted enhancement of the biological properties of the coatings, which provides a theoretical basis for optimizing the bioactivity of biomedical titanium alloys [29,30,31].

2. Materials and Methods

2.1. Material Preparation and Pretreatment

Pure titanium TA2 (Baoji Yingnaite Nonferrous Metals Co., Ltd., Baoji, China; meeting Chinese National Standard GB/T13810-97) was cut into 10 mm × 10 mm × 1 mm using a wire cutting machine and polished using SiC sandpaper with 800# and 1000#, respectively. Then, it was washed with acetone and deionized water separately for 10 min.

2.2. UMAO

After pretreatment, the sample underwent micro-arc oxidation to form an MAO coating. The electrolyte consisted of EDTA-2Na (15 g), Ca(CH₃COO)₂·H₂O (8.8 g), Ca(H₂PO₄)₂·H₂O (6.3 g), Na₂SiO₃·9H₂O (7.1 g), and NaOH (5 g). The applied voltage, processing time, frequency, and duty cycle of the MAO treatment were 400 V, 5 min, 600 Hz, and 8%, respectively, which was denoted MAO-450. For UMAO, a 40 kHz ultrasonic transducer (power: 150 W) was integrated into the MAO system, synchronously with the applied voltage (400 V, 600 Hz, 8% duty cycle, 5 min), which was denoted UMAO-450. The composition of the micro-arc oxidation electrolyte is shown in

Table 1. Composition of micro-arc oxidation electrolytes.

Reagent Name	Content (g/L or mL/L)	Purity
EDTA-2Na	15	>98%
Ca(CH3COO)2·H2O	8.8	>98%
Ca(H2PO4)2·H2O	6.3	>98%
Na2SiO3·9H2O	7.1	>98%
NaOH	5	>98%

.

2.3. Hydrothermal Treatment (HT)

In order to obtain MAO-450-HT and UMAO-450-HT samples, post-MAO samples were immersed in a 0.5 mol/L NaOH solution with (C₂H₃O₂)₂Sr (Na:Sr = 4:1) in a Teflon-lined autoclave (40 mL volume). HT was performed at 180 °C for 12 h, followed by cooling to room temperature, ultrasonic washing (deionized water, 10 min), and air drying.

2.4. Structural Characterization

The surface microstructure and phase compositions of samples before and after MAO and MHT treatment were examined using electron/ion double beam microscopy (SEM) and X-ray diffraction (XRD, Empyrean, The Netherlands).

The microstructure of the UMAO-450-HT sample was further investigated using transmission electron microscopy (TEM). X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi, Thermofisher Scientific Co, USA) was used to analyze the chemical composition of the coating. The binding energy of C1s (hydrocarbonC-C, C-H) 258 eV was used as a standard for the analysis of the Ti, O, Ca, P, and Sr elements on the coating surface.

2.5. In Vitro Bioactivity Evaluation

The simulated body fluid (SBF) immersion test was applied to evaluate the in vitro bioactivity. The SBF solution consists of NaCl (8.036 g), NaHCO₃ (0.352 g), KCl (0.225 g), K₂HPO₄∙3H₂O (0.23 g), MgCl₂∙6H₂O (0.311 g), CaCl₂ (0.293 g), Na₂SO₄ (0.072 g), (CH₂OH)₃CNH₂ (6.063 g/L), and 1 mol/L HCl (40 mL). After soaking in SBF for 1, 3, and 5 days, the microstructure and chemical composition of MAO-450-HT and UMAO-450-HT were studied using SEM, EDS, and XRD.

2.6. Materials Studio Simulation

DFT calculations were performed using the CASTEP module to model Ca²⁺ and PO₄³⁻ adsorption on HA and Sr-HA (001), (010), and (100) surfaces. The generalized gradient approximation (GGA-PBE) was used with a 5 × 5 × 1 k-point mesh and 500 eV cutoff energy. Work function (Φ) and adsorption energy (E_ads) were calculated to evaluate surface reactivity. Adsorption energy (E_ads) was calculated as E_ads = E_{surface+adsorbate} − E_surface − E_adsorbate, where E_{surface+adsorbate}, E_surface, and E_adsorbate are the total energies of the composite system, clean surface, and isolated adsorbate, respectively.

3. Results

3.1. Microstructure of MAO, UMAO, MAO-450-HT, and UMAO-450-HT

Figure 1 shows the SEM images of MAO and UMAO coatings before and after hydrothermal treatment with a 0.5 mol/L NaOH solution for 12 h. In Figure 1a, the MAO coating showed a rough and porous surface, and the micropore size on the MAO coating was relatively inhomogeneous, which ranged from 1 to 3 µm. In Figure 1b, after the ultrasonic wave was incorporated into the micro-arc oxidation process, the micro-scale porous structure was formed on the UMAO coating, and the edge of the micro-arc discharge channel was relatively smooth. Moreover, the amount of micro-scale pores was significantly decreased compared to that of micro-scale pores on the MAO coating, and the micro-scale pore size was about 1~3 µm. After hydrothermal treatment, as shown in Figure 1c,d, both MAO and UMAO coatings develop a significant number of H₂Ti₅O₁₁·H₂O nanowires, along with small amounts of nanorods. The original porous structures of both coatings are no longer visible. Notably, some nanoflakes are observed on the MAO-450-HT coating surface, but these nanoflakes are absent on the UMAO-450-HT coating surface, suggesting that the ultrasonic-assisted treatment might influence the nanostructure development during hydrothermal treatment.

Figure 2 gives the XRD patterns of MAO and UMAO coatings before and after hydrothermal treatment. It was observed that the characteristic diffraction peaks of the Ti phase at 35.09°, 38.42°, and 40.17° were found on the MAO and UMAO coatings before and after hydrothermal treatment as shown in Figure 2a–d. Moreover, in Figure 2a–d, the diffraction peaks of the anatase phase at 25.28, 34.95, and 48.05° and rutile phase at 27.45, 36.09, and 41.23° were observed on the MAO and UMAO coatings before and after hydrothermal treatment. However, for the MAO-450-HT and UMAO-450-HT coatings, new characteristic diffraction peaks corresponding to the H₂Ti₅O₁₁·H₂O phase at 10.66°, 24.23°, and 28.67° appear on the coating surfaces, suggesting the formation of this phase during hydrothermal treatment.

The relative strengths of the sample phases are shown in

Table 2. Relative strength ratio of sample phases.

Specimen	Anatase (101)/ Ti (101)	Rutile (110)/ Ti (101)	Anatase (110)/ Ti (101)	H2Ti5O11·H2O(110)/ Anatase (101)
MAO	3.10	0.53	-	-
UMAO	1.27	0.22	-	-
MAO-450-HT	-	0.07	0.72	0.60
UMAO-450-HT	-	0.13	1.31	0.41

. The relative intensity ratio of the anatase (101) to Ti (101) crystal planes on the MAO coating is 3.10, which is higher than the ratio of 1.27 observed on the UMAO coating. Similarly, the relative intensity ratio of the rutile (110) to Ti (101) crystal planes on the MAO coating is 0.53, higher than the 0.22 observed on the UMAO coating. After hydrothermal treatment, the intensity ratios of anatase (110)/Ti (101) and rutile (110)/Ti (101) are 0.72 and 0.07 on the MAO-450-HT coating, which are lower than the ratios of 1.31 and 0.13 on the UMAO-450-HT coating. Furthermore, the relative intensity ratio of H₂Ti₅O₁₁·H₂O (110) to anatase (101) on the UMAO-450-HT coating significantly decreases to 0.41 compared to 0.60 on the MAO-450-HT coating. New diffraction peaks corresponding to the Sr-HA phase at 31.72° and 49.32° are detected on both the MAO-450-HT and UMAO-450-HT coatings. The intensity of the Sr-HA peaks is higher on the UMAO-450-HT coating than on the MAO-450-HT coating, indicating that the ultrasonic-assisted micro-arc oxidation process leads to a greater amount of Sr-HA formation during hydrothermal treatment.

Figure 3 shows the atomic concentrations of the calcium (Ca), phosphorus (P), strontium (Sr), sodium (Na), and silicon (Si) elements on the MAO and UMAO coatings before and after hydrothermal treatment (MAO-450-HT and UMAO-450-HT) as measured by energy-dispersive X-ray spectroscopy (EDS, EDAX, USA). In Figure 3a, it was observed that the concentrations of the Ca, P, Sr, Na, and Si elements on the UMAO coating significantly increased compared to the MAO coating. This suggests that the UMAO process effectively adjusts the elemental concentrations in the coating. In Figure 3b, after hydrothermal treatment, the concentrations of the Ca and Si elements on the UMAO-450-HT coating slightly decreased compared to the MAO-450-HT coating. However, the concentrations of the P, Sr, and Na elements on the UMAO-450-HT coating increased relative to the MAO-450-HT coating. These changes in elemental concentrations are consistent with the XRD results, further confirming the influence of the UMAO process on the elemental composition of the coatings and the subsequent hydrothermal treatment.

3.2. TEM Analysis of UMAO-450-HT

Figure 4 shows the TEM image, SAED pattern, and element mapping distribution of nanorods formed on the UMAO-450-HT coating. In Figure 4a, the length and diameter of the nanorod on the UMAO-450-HT coating are about 2.5 µm and 45 nm, and the nanorod was in situ formed on the UMAO coating during the hydrothermal treatment. In order to confirm the phase and compositions of the as-formed nanorod, the SAED patterns of the red region in Figure 4a confirmed that the nanorod was a Sr-HA phase as shown in Figure 4b. And the element mapping distribution of elements was relatively homogenous and revealed that the chemical compositions of the as-formed nanorod were the O, Ca, P, and Sr elements.

Figure 5 shows the TEM image, SAED pattern, and element mapping distribution of nanowires formed on the UMAO-450-HT coating. In Figure 5a, the diameter of the nanowires was clearly observed to be about 10 nm. The HRTEM image and SAED patterns of the red region are shown in Figure 5b,c. The plane spacing of 0.829 nm corresponds to the (200) plane of the H₂Ti₅O11·H₂O phase, and the SAED pattern of the red region further confirms that the nanowires are composed of the H₂Ti₅O₁₁·H₂O phase. Additionally, the nanowires were identified to grow along the (001) crystal direction of the H₂Ti₅O₁₁·H₂O phase based on the results from the HRTEM image and SAED pattern. The HAADF image and element mapping distribution are shown in Figure 5d–f, where it was confirmed that the chemical composition of the nanowires consists of the titanium (Ti) and oxygen (O) elements.

3.3. XPS Analysis of UMAO-450-HT

Figure 6 shows the XPS survey and high-resolution spectrum of UMAO-450-HT coating. It was confirmed that the chemical compositions of the UMAO-450-HT coating included the Ti2p, O1s, Ca2p, P2p, and Sr3d elements. In order to confirm the chemical state of the incorporated elements, the high-resolution spectrum of Ti2p, O1s, Ca2p, P2p, and Sr3d are shown in Figure 6b–f. In Figure 6b, the Ti2p XPS spectra of the UMAO-450-HT coating surface are divided into two double peaks of Ti2p_3/2 at 458.10eV and Ti2p_1/2 at 464.8 eV, which correspond to the chemical state of Ti⁴⁺ in TiO² or H₂Ti₅O_11·H₂O. In Figure 6c, the O1s spectra of the UMAO-450-HT coating are deconvoluted into three Gaussian component peaks at 529.60, 531.2, and 532.0eV, which are assigned to Ti-O, OH, and H₂O, respectively. In Figure 6d, the Ca2p spectra of the UMAO-450-HT coating reveal the double peaks of Ca2p_3/2 at a binding energy of 346.33 and Ca2p_1/3 at a binding energy of 349.90 eV, corresponding to Ca²⁺ in Sr-HA. In Figure 6e, the P2p XPS spectra of the UMAO-450-HT coating are fit into double peaks of P2p at 133.2 and 134.9 eV, which are assigned to PO₄^3- and HPO₄^2-. In Figure 6f, the Sr3d spectra of the UMAO-450-HT coating are fit into double peaks of Sr3d_5/2 at a binding energy of 133.21 and Sr3d_3/2 at a binding energy of 134.74 eV, assigned to the chemical state of Sr2+ in Sr-HA.

3.4. Bioactivity of MAO-450, UMAO-450, MAO-450-HT, and UMAO-450-HT

Figure 7 shows the SEM images of the MAO-450, UMAO-450, MAO-450-HT, and UMAO-450-HT coatings after immersion in simulated body fluid (SBF) for 1, 3, and 5 days. In Figure 7a–c, it is clear that no new depositions were observed on the MAO-450 coating after immersion in SBF for 1, 3, and 5 days. The MAO-450 coating maintained its rough and porous surface throughout the immersion period. Similarly, no changes were observed on the UMAO-450 coating, which retained its porous surface after SBF immersion for 1, 3, and 5 days. In Figure 7g–i, the MAO-450-HT coating showed significant changes after immersion in SBF. After 1 day of immersion, a large number of spherical-like depositions appeared on the surface of the MAO-450-HT coating, with some nanowires still observed. As the immersion time increased to 3 and 5 days, the surface became fully covered with depositions, and the quantity of spherical-like depositions increased. These depositions exhibited characteristics typical of apatite, suggesting that they may be apatite deposits. In Figure 7j–l, spherical-like apatite deposits were found at the base of the nanowires on the UMAO-450-HT coating, as seen in the magnified image of Figure 7j. A large number of nanowires remained on the UMAO-450-HT coating after 1 day of SBF immersion. With the increase in immersion time to 3 days, the quantity of spherical-like apatite deposits significantly increased, while the nanowires remained visible on the UMAO-450-HT coating. After 5 days of SBF immersion, the UMAO-450-HT coating was completely covered with apatite, indicating that the hydrothermal treatment of the MAO and UMAO coatings enhanced their apatite-inducing ability. This suggests that the hydrothermal treatment significantly improves the bioactivity of the coatings.

Figure 8 presents the XRD patterns of the MAO-450, UMAO-450, MAO-450-HT, and UMAO-450-HT coatings after immersion in simulated body fluid (SBF) for 5 days. In Figure 8a,b, the diffraction peaks corresponding to the Ti (101), anatase (101), and rutile (110) crystal planes at 40.17°, 25.28°, and 27.45° were still present in the XRD patterns of the MAO and UMAO coatings. No new diffraction peaks were observed after SBF immersion for 5 days, indicating that the MAO and UMAO coatings did not undergo significant phase changes or new phase formation during the immersion period. In Figure 8c,d, for the MAO-450-HT and UMAO-450-HT coatings, the diffraction peaks of the Sr-HA phase disappeared after SBF immersion for 5 days. New diffraction peaks characteristic of apatite were detected at 26.31°, 31.93°, and 49.70° on both the MAO-450-HT and UMAO-450-HT coatings. Additionally, the intensity of the apatite peaks was significantly higher on the UMAO-450-HT coating compared to the MAO-450-HT coating, suggesting that the UMAO process, followed by hydrothermal treatment, improved the apatite-inducing ability of the coatings. This indicates that the UMAO-450-HT coating exhibited enhanced bioactivity and a stronger capacity to form apatite compared to the MAO-450-HT coating.

Figure 9 shows the atomic content of surface elements (EDS analysis) of ultrasound-assisted micro-arc oxidized hydrothermal treated specimens after immersion in SBF for a different number of days. It can be found from the data that PO₄³⁻ and HPO₄²⁻ ions in the solution gradually form an apatite layer on the coating surface as the number of days of immersion increases. As shown in the figure, the atomic percentages (at.%) of Ca and P increased significantly with the soaking time: from 11.71% Ca and 8.03% P at 1 day (Ca/P = 1.45) to 18.28% Ca and 11.28% P at 5 days (Ca/P = 1.62), which gradually converged to the theoretical Ca/P value of hydroxyapatite, which is 1.74. This is further confirmation of the apatite dynamic deposition process. These quantitative results indicate that the biomineralization capacity of the coated surface increased with time, validating its excellent biological activity.

4. Discussions

The XRD results for MAO and UMAO coatings reveal that the relative intensity ratios of the anatase, rutile, and Ti(101) crystalline facets in the UMAO coatings are lower than those in the MAO coatings after micro-arc oxidation, yet higher following hydrothermal treatment. This difference can be attributed to the ultrasound-assisted micro-arc oxidation process, which accelerates the deposition of calcium and phosphorus ions, resulting in a denser coating. The atomic concentrations of the Ca, P, Sr, Na, and Si elements on the MAO and UMAO coatings further validate this ultrasound-assisted acceleration of calcium and phosphorus deposition.

Additionally, the effects of high-frequency cavitation, thermal effects, and plasma chemical reactions contribute to a more uniform distribution of ions and micro-arc discharges within the solution. This results in a smoother coating surface with more evenly sized micropores. As shown in Figure 10 during the UMAO process, an increased deposition of calcium and phosphorus elements occurs within the coating. In the subsequent hydrothermal treatment under high temperature and pressure, these elements diffuse to the coating’s surface, where calcium and phosphorus are converted to calcium and phosphate ions. The hydroxide ions in the solution then react with these ions on the coating surface to form strontium-substituted hydroxyapatite nanocrystalline nuclei [31]. As treatment time progresses, these crystal nuclei grow, leading to a gradual depletion of calcium and phosphorus within the coating as they are incorporated into the expanding hydroxyapatite layer. Consequently, the overall calcium and phosphorus content within the coating decreases as the reaction continues. Atomic concentration measurements of the Ca, P, Sr, Na, and Si elements on the MAO-450-HT and UMAO-450-HT coatings further support these findings.

SEM images and XRD results of the MAO-450, UMAO-450, MAO-450-HT, and UMAO-450-HT coatings after SBF immersion indicate that the ultrasonically assisted micro-arc oxidized coatings following hydrothermal treatment exhibit superior apatite-inducing capability compared to the MAO-450-HT coatings. Notably, after 5 days of SBF immersion, the diffraction peaks corresponding to the Sr-HA phase on the coating disappeared, while new apatite diffraction peaks emerged at 26.31º, 31.93º, and 49.70º. This suggests that the dissolution of the Sr-HA phase during SBF immersion enhances apatite formation, thereby improving the bioactivity of the coating. To further investigate how the Sr-HA phase promotes apatite formation, Materials Studio 2024B software was used to simulate the adsorption behavior of calcium and phosphorus ions on both the HA and Sr-HA phases.

Figure 11 shows the results of the adsorption of calcium and phosphorus ions by the HA phase and Sr-HA phase simulated by Materials Studio. The DFT calculations were performed with the following parameters: a plane-wave cutoff energy of 500 eV, force convergence criteria of 0.02 eV/Å, and energy convergence tolerance of 1.0 × 10⁻⁵ eV. A vacuum layer of 20 Å was applied along the z-direction to minimize periodic interactions. The self-consistent field (SCF) convergence threshold was set to 1.0 × 10⁻⁶ eV. Spin polarization was not considered in this study due to the non-magnetic nature of the HA and Sr-HA systems. The comparison shows that compared with the HA phase, the work functions of the three types of crystal surfaces (001), (010), and (100) of the Sr-HA phase become smaller, which makes it easier for the electrons of the Sr-HA phase to migrate from the interior to the surface, and so the enhancement of the crystals of the Sr-HA phase further increases the adsorption capacity of calcium and phosphorus ions on the surface of the coating. This increased adsorption of calcium and phosphorus ions promotes apatite formation, thereby improving the bioactivity of the bioceramic coatings. The findings suggest that the enhanced Sr-HA phase plays a crucial role in promoting apatite formation, leading to better bioactivity of the coatings. Additionally, the Sr-HA peak intensity on the ultrasonically assisted micro-arc oxidized coating was higher than that on the MAO-450-HT coating, indicating that the ultrasound-assisted micro-arc oxidized coating has superior bioactivity after hydrothermal treatment. While the in vitro SBF immersion tests demonstrated rapid apatite nucleation and enhanced bioactivity of the UMAO-HT coatings, future in vivo studies are essential to validate their osseointegration performance in physiological environments. To address this need, we plan to evaluate the coatings in rabbit femoral defect models to assess both biosafety and structural stability.

5. Conclusions

In this study, we conducted a comparative investigation into the microstructures and surface properties of MAO and UMAO specimens, along with a simulation and analysis of calcium and phosphorus ion adsorption on the ultrasound-enhanced Sr-HA phase using Materials Studio. Compared to MAO specimens, the UMAO specimens displayed a reduced number of micropores, with more uniform sizes and smoother edges, and no nanosheet features were observed after hydrothermal treatment. Additionally, UMAO specimens exhibited lower relative intensities of anatase and rutile phase crystals due to the denser coating structure, while showing increased relative crystal strengths following hydrothermal treatment.

Furthermore, the UMAO process regulates elemental concentration within the coating, which supports the formation of the Sr-HA phase during the subsequent hydrothermal process. Simulation results indicate that the enhanced Sr-HA phase significantly improves calcium and phosphorus ion adsorption, accelerating apatite formation and thereby boosting the coating’s apatite-inducing ability.

In summary, this study demonstrates that the UMAO can effectively enhance the performance of bioceramic coatings. By elucidating the mechanism through which the Sr-HA phase on the coating adsorbs phosphorus and calcium ions, this research offers valuable insights for optimizing the bioactive properties of biomedical titanium and preparing high-performance bioactive coatings.