1 Introduction
Due to their low density, high specific strength, low elastic modulus, and good inherent biocompatibility, titanium and titanium alloys are widely used in applications such as orthopedic internal fixation devices, dental implants, and artificial joints. However, titanium alloys are bio-inert materials; their surfaces cannot actively induce bone tissue growth, resulting in prolonged osseointegration periods and insufficient bonding strength post-implantation. Furthermore, the human body environment-characterized by complex chloride-containing electrolytes-can cause localized damage to the surface passivation film of titanium alloys during long-term service, leading to metal ion release and posing potential risks of cytotoxicity and sensitization. Additionally, bacterial adhesion and biofilm formation during implantation surgery can easily lead to postoperative infections, a primary cause of implant failure.
Functionalizing titanium alloy implants through surface modification techniques represents a key research focus in the field of medical titanium alloys. These techniques alter only the morphology, composition, and structure of the material's surface layer without compromising the mechanical properties of the substrate. They enable the simultaneous achievement of multiple objectives-such as enhancing osseointegration, improving corrosion resistance against body fluids, and imparting antibacterial properties-thereby playing a crucial role in extending the service life and ensuring the long-term safety of implants.
2 Typical Surface Modification Techniques and Property Modulation Effects
2.1 Anodic Oxidation
Anodic oxidation is a surface treatment technique that generates a dense TiO₂ oxide film on the surface of titanium alloys via electrochemical oxidation. By regulating voltage, electrolyte composition, and oxidation time, the thickness and microstructure of the oxide film can be precisely controlled to form ordered topological structures such as nanotubes and nanopores.
The resulting oxide film effectively isolates the substrate from the physiological fluid environment, significantly enhancing the alloy's resistance to pitting and uniform corrosion while inhibiting the release of metal ions such as aluminum and vanadium. Simultaneously, the nanoscale porous surface structure provides adhesion sites for osteoblasts, promoting cell proliferation and differentiation and enhancing the material's bioactivity. Furthermore, incorporating antibacterial metal ions (e.g., silver or zinc) into the electrolyte or loading antibiotics into the porous structure imparts sustained-release antibacterial properties to the modified surface, thereby reducing the risk of postoperative infection. Anodic oxidation is a cost-effective and highly controllable process suitable for small implants with complex geometries, making it a common surface treatment technique for dental implants.
2.2 Micro-arc Oxidation
Micro-arc oxidation, also known as plasma electrolytic oxidation, is an advanced technique evolved from anodic oxidation. It utilizes high-voltage discharge to induce localized plasma discharges on the titanium alloy surface, generating an in-situ porous ceramic-like oxide film that is metallurgically bonded to the substrate. The film consists primarily of rutile and anatase TiO₂ and exhibits significantly higher adhesion strength than conventional anodic oxide films.
The micro-arc oxidation coating combines high hardness with excellent wear and corrosion resistance, effectively withstanding corrosion and mechanical wear in physiological environments. Its hierarchical porous surface structure mimics the morphology of the extracellular matrix, significantly promoting osteoblast adhesion, spreading, and mineralization, thereby enhancing osseointegration efficiency. By introducing calcium and phosphorus elements into the electrolyte, bioactive hydroxyapatite components can be doped *in situ* into the oxide film, further enhancing surface bioactivity. Doping with antibacterial elements such as silver or copper enables broad-spectrum antibacterial activity, effectively inhibiting common pathogens like *Staphylococcus aureus* and *Escherichia coli*. Characterized by strong coating adhesion and high functional integration, this technology represents a mainstream approach for the surface modification of load-bearing orthopedic implants.
2.3 Hydroxyapatite Coatings
Hydroxyapatite (HA) closely resembles the inorganic component of human bone and exhibits excellent bioactivity and osteoconductivity, making it the most widely used bioactive coating material. Common preparation methods-such as plasma spraying, sol-gel processing, and electrochemical deposition-allow for the creation of HA bioactive coatings with controllable thickness on titanium alloy surfaces.
HA coatings actively induce bone tissue growth on the implant surface, facilitating direct osseointegration between the implant and the bone; this significantly shortens the osseointegration period and improves both initial stability and long-term bonding strength. Additionally, the ceramic coating acts as a physical barrier, preventing contact between body fluids and the metal substrate, thereby markedly enhancing the material's corrosion resistance. To address clinical infection issues, silver-doped HA or drug-loaded HA composite coatings can be prepared; these impart antibacterial functionality while preserving bioactivity, achieving a synergistic effect of osteogenesis and antibacterial action. Current efforts to optimize HA coatings focus primarily on enhancing the adhesion strength between the coating and the substrate to prevent coating delamination or failure during long-term service.
3 Performance Regulation Mechanisms and Trends in Composite Modification
The three types of modification technologies enhance titanium alloy properties through distinct mechanisms: corrosion resistance is improved via the physical barrier effect of surface oxide films or ceramic coatings, which block contact between corrosive media and the metal substrate; enhanced bioactivity relies on the physical induction provided by surface micro/nano-topography and the chemical induction from calcium-phosphate active components, which synergistically promote osteoblast differentiation and mineralization; and antibacterial properties are achieved through mechanisms such as the sustained release of antibacterial ions and contact-based bactericidal action.
Single modification techniques often struggle to simultaneously meet the multifaceted requirements of high bioactivity, long-lasting antibacterial efficacy, and robust corrosion resistance; consequently, composite modification has emerged as a key development trend. For instance, "micro-arc oxidation (MAO) + hydroxyapatite (HA)" composite coatings combine the high adhesion strength of MAO films with the high bioactivity of HA coatings; similarly, silver-loaded MAO/HA composite systems achieve the triple functionality of corrosion resistance, osteogenesis, and antibacterial activity, making them better suited for the complex environments encountered during in vivo service.
4 Conclusions and Outlook
Anodization, micro-arc oxidation, and hydroxyapatite coating represent three core surface modification technologies for medical titanium alloys. They can specifically enhance resistance to body-fluid corrosion, bioactivity, and antibacterial functionality, effectively addressing three major challenges in the clinical application of titanium implants: insufficient osseointegration, corrosion-induced ion release, and postoperative infection. Among these, anodization is suitable for small, precision implants; micro-arc oxidation is appropriate for load-bearing orthopedic implants; and HA coatings are ideal for applications requiring high bioactivity.
Future surface modification of medical titanium alloys will evolve toward multifunctionality and intelligence. By constructing composite functional coatings with hierarchical structures, it will be possible to achieve a synergy of functions-such as promoted osseointegration, long-lasting antibacterial activity, and self-healing against corrosion. At the same time, further optimization of process stability and long-term in vivo safety is essential to facilitate the standardized clinical translation of these modification technologies, thereby providing technical support for performance upgrades in high-end medical implants.
