Surface modification of biomedical titanium alloy for hard tissue repair and reconstruction

Mechanical strengthening technology

Mechanical strengthening technology is indeed a direct method to modify the surface structure of titanium alloy. This technique focuses on changing the surface morphology rather than altering the chemical composition of the titanium alloy matrix. It is a relatively simple and effective surface modification method. There are several methods commonly used for mechanical strengthening of titanium alloy surfaces. These methods include grinding, machining, polishing sandblasting, etc. ³⁷

Grinding involves the use of abrasive particles to remove material from the surface of the titanium alloy. This process helps to smooth the surface and improve its flatness and uniformity. Machining refers to the process of shaping or cutting the titanium alloy surface using various tools and techniques. This method can be used to create specific surface features or structures, such as grooves or threads, to enhance the implantation effect. Polishing is a process that uses abrasive materials to smoothen the surface of the titanium alloy. It helps to remove any surface imperfections or roughness, resulting in a smoother and more aesthetically pleasing surface.

Sandblasting involves the use of high-pressure air or other media to propel abrasive particles onto the titanium alloy surface.^38–40 This process creates a roughened surface by removing material and generating microscale irregularities. The rough surface promotes better adhesion and integration with surrounding tissues. Sandblasting not only improves the surface roughness but also enhances the surface hardness and induces residual compressive stress of titanium alloy. This surface modification technique has been found to have positive effects on bone regeneration. Some studies have reported that the combination of grit-blasting and acid-etching techniques can accelerate bone regeneration on titanium alloy surfaces.^41,42 By adjusting the parameters of the grit blasting process, such as the grit size and blasting pressure, the three-dimensional surface morphology and the surface roughness of Ti6Al4 V alloy, the surface can be further modified. These modifications have been shown to significantly impact cell adhesion and proliferation for better suitable requirements of specific biomedical applications. Additionally, the induced residual compressive stress can improve the fatigue resistance mechanical stability and osseointegration of the implant of Ti alloy (Figure 2).

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Figure 2.

Upper SEM images of garnet particles25, ⁴² lower (a) TEM micrographs of untreated and (b) 2-min USP-treated samples and corresponding SAED patterns. ⁴⁶

Ultrasonic shot peening (USP) technology is a process that utilises high-energy density ultrasonic waves to impact the surface of the metal using a medium such as a steel firing pin or projectile. This method has shown the ability to generate ultra-fine particles on the metal surface without altering its internal chemical composition. The generation of microstructural defects and the refinement of crystal microstructure contribute to the improvement of the metal's performance and durability.

Through the repetitive striking of the metal surface with a hard steel ball, numerous microstructural defects are induced on the near surface. Additionally, the coarse crystal microstructure is refined to a micro-nano scale. This refinement process enhances the mechanical properties of the metal, such as increasing its hardness, fatigue resistance and strength. ⁴³

Some scholars have discovered a new titanium alloy called Ti-13Nb-13Zr, which shows great potential for implant applications, this alloy exhibits not only excellent corrosion resistance but also remarkable mechanical properties by USP treatment for different time.^44,45 One notable outcome of the USP treatment is the formation of a nanocrystalline layer on the surface of the alloy, with a thickness of 100um, this nanocrystalline layer contributes to the improvement of the material's properties. Additionally, both the surface hardness and roughness of the alloy are enhanced through USP treatment. Compared to untreated samples, the micro-hardness of the Ti-13Nb-13Zr alloy increased by 20% after USP treatment, which was particularly beneficial for orthopaedic and dental implants application, as it enhanced the durability and longevity of the implants.

In general, surface finishing methods such as grinding, polishing and sandblasting use external forces to change or modify the morphology of the material surface as a way of altering the mechanical properties of the material. At the same time, a better bond between the implant and the tissue is achieved by changing the coefficient of friction of the material surface to facilitate its use in different implant environments. ³⁸ These are also the most basic and simplest experimental methods of all modification methods. Therefore, in practice, mechanical modification is usually combined with other modification methods.

Three-beam modification

Three-beam modification technology does mainly include three types: ion beam, laser beam and electron beam (EB) modification technologies. These techniques are used to modify the properties of materials, such as changing their surface structure or creating microstructure, further improving biocompatibility for biomaterials.

Ion beam polishing (IBP) is a well-known physical sputtering technology used for surface modification and polishing. In IBP, a beam of high-energy ions is directed towards the surface of a solid material, these ions bombard the surface, causing the ejection of atoms from the material called sputtering. The high-energy ions in the beam transfer their kinetic energy to the atoms on the surface, causing them to be dislodged. As a result, the surface undergoes erosion and the topography is modified. The sputtered atoms are typically neutralised and removed from the system to prevent re-deposition on the surface. ⁴⁷ Some scholars have studied on the effect of IBP on the surface roughness of titanium alloy. By adjusting parameters such as the energy and current density of the ion beam, as well as the incident angle, it is possible to effectively reduce the roughness of the titanium alloy surface. ⁴⁸ In these studies, Ti6Al4 V samples were subjected to IBP treatment, resulting in a significant reduction from an initial value of 0.2μm to 53 nm, but the phase structure of titanium alloy remained stable. ⁴⁹ However, it is worth noting that the oxygen content on the surface increased after IBP treatment, this could be attributed to the interaction between the high-energy ions and the surface atoms, leading to the incorporation of oxygen into the surface layer. The reduction in surface roughness greatly improved the integration of titanium alloy with hard tissues, making it more suitable for applications in biomedical fields such as hard tissue repair. Additionally, the enhanced biocompatibility resulting from reduced roughness can promote better tissue adhesion and reduce the risk of complications. (Figure 3a, b, c).

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Figure 3.

3(a, b, c) laser scanning confocal microscopy (LSCM) images of samples polished under different ions currents ⁴⁹ (d, e, f) SEM images of mg-63 osteoblast like cell morphology after 48 h culture on Ti alloy. ⁵¹

The development and application of laser beam technology have brought about significant advancements in various fields. This technology offers several advantages, including higher processing speed and accuracy, compacted structure, universality and environmental friendliness. ⁵⁰ Scholars have found that laser beam modification of titanium alloy surface can affect the wettability and response of osteoblast cells. This modification led to an increase in surface roughness and surface oxygen content, which played a crucial role in enhancing the bioactivity of osteoblast cells. Rafiee et al. ⁵¹ also confirmed that frequency values ranging from 1 to 20 Hz resulted in surface roughness variation between 452 nm and 337um. Additionally, a higher melt rate and solidification led to a 12–25% increase in hardness. So, by optimising the laser beam parameters, it is possible to improve the biocompatibility and cell viability of Ti alloy (Figure 3d, e, f). For example, Cunha et al. ⁵² prepared nanopillars and periodic surface structures on Ti surface by femtosecond laser surface texturing, where the nanopillars enhanced the hydrophilicity of the Ti surface and the periodic surface structures reduced the adherence of Staphylococcus aureus on the Ti surface.

Highly energetic EB is a promising technique for controlled surface modification, its principle of beam heat generation is similar to a conventional scanning electron microscope. When the EB focused on the materials, it caused heating, melting and local evaporation, leading to the formation of ‘keyholes’ due to high vapour pressure. ⁵³ By varying EB defection using computer programming, it is possible to create surface topography on Ti6Al4 V alloy.^54,55 As shown in an in vitro study of multiple cultures of preosteogenic MC3T3-E1 cells, these cells have a polygonal shape and establish connections through elongated filopodia. After 48 h of culture, the cells have a significantly increased spreading area on the surface structure with finer tubular shape. This process further improves surface roughness and hardness, which has shown favourable effects on osteoblast spreading and viability, indicating good biocompatibility.

Samples of EB surface structures have also been characterised using bacterial and cell adhesion tests. The results showed that 10 μm wide grooves were able to support fibroblast alignment onto the metal substrate. Surprisingly, all EB-treated surfaces showed reduced bacterial adhesion after up to 48 h of incubation. ⁵⁶

Thermal diffusion technology

Thermal diffusion technology is a process that involves heating and diffusing metallic of non-metallic elements into the surface of metallic materials or workpieces to form a surface alloy layer. The diffusion layer is bonded to the matrix material forming an alloy layer, showed a high bonding force. Li et al. ⁵⁷ found that a ceramic coating formed on the surface of Ti6Al4 V alloy by solid carburising and improved the cavitation resistance of the alloy. The coating structure contains TiC and a small amount of oxide. The presence of TiC thin films also improves the abrasion resistance of Ti and its alloys, especially on the surfaces of dental implants and in joints of the human body. This technology has been studied to improve the tribological properties of titanium alloys. Researchers have shown^58,59 that thermal expansion infiltration can increase the hardness of titanium alloys by creating a continuous gradient of composition from the outer surface to the core. However, this method may also affect the surface morphology, coefficient of friction and surface microstructure due to the relatively high temperatures involved. Compared with other high- performance alloys, Titanium alloys have limited tribological properties and tend to exhibit higher wear rates and degradation.^60–63 To address this issue, researchers have explored various approaches, for example, Marin et al. ⁶⁴ combined nitriding and carburising treatment with subsequent deposition of PVD coatings to improve the wear resistance of titanium alloys. Both nitriding and carburising treatments showed increased wear resistance, making titanium alloy components more suitable for biological implantation.

High temperature heat treatment can also improve the properties of alloys. For example, the surface treatment with niobium (Nb) or molybdenum (Mo) has also been showing to reduce the elastic modulus of titanium and form a stable oxide layer in air or tissue fluids.^65,66 Additionally, Nb exhibited a faster passivation rate, further improving the corrosion resistance and biocompatibility of Ti alloy. Ureña et al. ⁶⁷ also demonstrated that Nb–Ti and Mo–Ti exhibited better corrosion resistance than commercially available pure titanium (CP-Ti) in NaCl solution when treated with thermal diffusion.

Compared with high-temperature heat treatment, high-temperature gas heat treatment only modifies the surface of the titanium alloy, changing the surface properties while maintaining the original properties of the titanium alloy as a whole and realising the combination of various properties of the titanium alloy.

Anodic oxidation

Anodic oxidation is an electrochemical deposition technique that can effectively modify the surface characteristics of titanium and its alloys. ⁶⁸ The main principle is to convert electrical energy into chemical energy, so that the anode and cathode chemical reaction, especially for the anode, by reasonably regulating the applied potential, current intensity, oxidation time and other conditions, can change the micro-morphology of the anode.^69,70 Titanium alloy, acting as the anode, is an extremely inert material, similar to graphite or platinum (Pt), resulting in a high bond strength between the oxide layer and the substrate. ⁷¹ One of the main advantages of this technique is the ability to adjust the composition of coating by using different electroplating solution. ⁷² Doping the coating with specific components can further enhance the properties of the oxide layer. ⁷³ Using electrochemical anodisation, micron-scale and nanoscale surface morphology as well as microstructures such as nanotubes and nanopores can be prepared on the implant surface. Micron-scale surface morphology can promote osteoblast differentiation,^74,75 while nanoscale surface morphology can promote cell proliferation ⁷⁶ and the combination of the two can promote the proliferation and differentiation of osteoblasts. ⁷⁷ Microstructures are conducive to the adherence and proliferation of normal cells, structural inhibition of bacteria and acting as a carrier for drugs after implantation of Ti and titanium alloys. For example, TiO₂ nanotube arrays prepared through two-step anodic oxidation can be used as carriers for recombinant human bone morphogenetic proteins while promoting osseointegration. ⁷⁸ Kim et al. ⁷⁹ performed alkali treatment and heat treatment on TiO₂ nanotubes formed after Ti anodic oxidation. The alkali treatment transformed the nanotubes into nanofibrous structure, which increased surface roughness as well as hydrophilicity and the heat treatment promotes the exchange of ions in the body fluids with the Na + on the surface of TiO₂ nanotubes and accelerates the formation of bone-like apatite. These two treatments also promoted the diffusion of osteoblasts on the implant surface.

In orthopaedics and bone repair, the surface colour of the implanted titanium alloys is not typically considered. However, for teeth application, the colour becomes important and the anodising process can be adjusted to obtain different colour coatings. Wu et al. ⁸⁰ successfully prepared a composite coating of rutile and sodium titanate, which closely resembled natural human teeth by anodising TC4 titanium alloy substrate in an alkaline solution (NaOH). The bonding force between the coating and the substrate reached 30 MPa. The coating surface exhibited bioactivity, forming a bone-like apatite layer and showed bone induction properties for suitable dental implant. (Figure 4a).

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Figure 4.

4(a) Colour control of teeth after anodic oxidation in alkaline solution. ⁸⁰ (b, c) Surface morphology (FESEM) and cross-sectional view (TEM) of the plated layers: a HA-Ag and b HA-Ag/SiO₂/TiN/Ti. ⁹² (d, e, f) Cell proliferation of titanium scaffolds after anodic oxidation. ⁸²

The combination of anodic oxidation and 3D printing technologies can improve the biocompatibility of titanium alloy surfaces. 3D printing allows for fabrication of titanium scaffolds with ideal pore size, inter-pore connectivity, porosity and permeability, which are crucial for enhancing osseointegration, surface biofunctionalisation. ⁸¹ Zhao et al. ⁸² prepared uniform and ordered arrays of titanium dioxide nanotubes by a two-step anodisation method, mesoporous bioactive glass was then loaded into the nanotubes. This surface-modified scaffolds exhibited good surface cytocompatibility, supporting cell adhesion and proliferation. (Figure 4d–f).

Micro-arc oxidation (plasma electrolytic oxidation/micro-arc oxidation) is derived from anodic oxidation technology, the difference lies in whether or not high voltage is applied to the electrodes, the target alloy is placed in a specific electrolyte as the electrode and a counter electrode is placed and by adjusting the electrical parameters, an arc discharge is generated on the surface of the target alloy in the electrolyte, reaching a high temperature and high-pressure state in a very short time. The target alloy is placed in a specific electrolyte as an electrode and a counter electrode is placed. By adjusting the electrical parameters, the surface of the target alloy in the electrolyte generates an arc discharge which reaches a high temperature and high-pressure state in a very short time, resulting in the growth of a ceramic film layer dominated by the oxides of the base alloy. The resulting ceramic film layer is often very hard, corrosion and wear-resistant. For example, Fan et al. ⁸³ prepared TiO layer inducing apatite formation on the porous titanium scaffold using the micro-arc oxidation technique and the results showed that the osteoinductive effect of the titanium alloy was greatly enhanced. Xie et al. ⁸⁴ by treating titanium alloys at different voltages showed that the roughness and clear water properties of the alloys were greatly improved with increasing voltage and showed better corrosion resistance and excellent antimicrobial properties and better cytocompatibility at 250 V and 300 V.

In summary, anodising is a technology that can modify the surface properties of titanium and its alloys, with the advantages of high controllability, diverse morphology, fast reaction rate and low cost in changing the surface structure. Compositions and structures can be designed according to specific uses (e.g. to improve surface roughness, to act as a drug carrier, to change the colour of the surface layer) and are therefore suitable for a variety of applications in orthopaedics, dental implants and tissue engineering.

PVD/CVD technology

Magnetron sputtering (MS) is a methods of physical vapour deposition (PVD).^85,86 That is widely used to film deposition in the biomedical field. It offers advantages such as lower temperature, good bonding and the ability to produce uniform and dense film structure.^87–89 Studies have shown^90,91 that MS can be used to prepare multilayer coatings with desirable surface properties and multifunctionality. For example, Lenis et al. ⁹² successfully obtained HA-Ag/SiO₂/TiN/Ti multilayer gradient coatings on Ti-6Al-4 V matrix using the MS technique, which coating exhibited a long-term antimicrobial effect due to the presence of HA-Ag. The triple layers of SiO₂/TiN/Ti improved the crystallinity of the HA-Ag coatings, further enhanced their biocompatibity (Figure 4b, c). Additionally, Pana et al. ⁹³ also demonstrated that MS-deposited SiC- and Ag-SiC-doped HA coatings on Ti alloy substrate exhibited superior mechanical properties, as well as excellent corrosion resistance and tribological parameters. While Premphet et al. ⁹⁴ successfully prepared good hardness HA-TiO₂ films using pulsed DC MS technique, which are biocompatible, non-toxic to cells and can be used for tissue implants. SiHA thin films with different Si compositions were deposited on titanium substrates by magnetron co-sputtering technique. The wettability of the modified material surfaces was greatly improved, with water contact angles between 30 and 40° and there were no significant differences in the roughness values between the samples, which were mainly concentrated between 240 and 290 nm. The results showed that the adhesion of HOB cells gradually increased with the increase of Si content, which promoted cell proliferation and calcification. ⁹⁵ Besides good homogeneity of the films by MS, many scholars have also utilised sputtered coatings to improve the antimicrobial properties of titanium alloys. For example, Huang et al. ⁹⁶ prepared ZRO₂-Ag and ZRO₂-Cu coatings on pure titanium and the experimental results showed that both coatings reduced the survival rate of Staphylococcus aureus and Bacillus radiobacteriae compared to pure titanium, which significantly improved the antimicrobial properties of the implants. These results suggest that this modification method strikes a balance between biocompatibility and antimicrobial properties and shows a highly desirable combination for applications in prosthetic implants such as microporous plates and fixation screws. PVD technology has become one of the main surface treatment technologies for material modification with its advantages of simple process, less consumables, uniform film formation and strong bonding with the substrate and has been further developed in the direction of environmental protection and cleanliness.

Chemical vapour deposition (CVD) is a thermochemical process and ions in a vapour state decompose and react on heated surfaces, resulting in the formation of the films on a substrate. This technique has been extensively studied for its ability to enhance the adhesion of osteoblasts on Ti alloy surfaces. CVD coatings tend to have a stronger bond with the substrate and the tight bonding between the coating and the substrate avoids the potential separation of the coating from the substrate that may result from the prolonged presence of the implant in the human body. This method involves the preparation of materials on the substrate that can effectively enhance the bioactivity of the implant surface, such as Ca₃(PO₄)₂ and apatite, which are most typically similar to the composition of natural bone.^97–99 Liu et al. ¹⁰⁰ prepared phosphorochalcogenide coatings by in situ transformation of pure Ti and Ti-6Al-4 V alloys by a chemical transformation technique to improve the biocompatibility of the materials. Meanwhile, researchers have shown^101–105 that the introduction of gases such as oxygen (O₂) and ammonia (NH₃) during the CVD process has a positive impact on the adhesion of osteoblasts. These gases can modify the surface chemistry of the deposited films, improving their biocompatibility and cell adhesion. In recent years, carbon films, such as microcrystalline diamond (MCD), nanocrystalline diamond, or diamond-like (DLC) have been particularly investigated for their excellent corrosion resistance and high biocompatibility,^106–108 researchers ¹⁰⁹ have explored the used of microwave plasma-assisted chemical vapour deposition (MW-PACVD) to deposit films on the surface of titanium alloys. They utilised a precursor mixture of methane, hydrogen and diborane (B₂H₆) to generate films. By varying the CVD parameters and MW-PACVD process conditions, it is possible to prepare diamond-containing MCD film on the titanium alloy surface which exhibit good bioactivity. By plasma enhanced CVD technology, surface modification of titanium and its alloys with positively charged amino-groups can improve the performance of implants on initial osteoblast function, which achieved ideal osseointegration on titanium surfaces. ¹¹⁰

In summary, PVD/CVD is a versatile technique that can be used to deposit films on substrates, including titanium alloys. By carefully selecting the deposition parameters and precursor gases, it is possible to enhance the adhesion of osteoblasts and improve the biocompatibility of the coated surfaces.

Friction stir processing

Friction stir processing (FSP) is derived from the proposed friction stir welding.^111,112 FSP allows for the tuning and control of the microstructure of a material without changing the shape and size of the overall part, including the elimination of defects, breaking of dendrites and the introduction of second phases. ¹¹³ In this way, the microhardness, strength, fatigue properties and corrosion resistance of the material are tuned and ultimately its surface properties are improved ¹¹⁴ During the FSP process (Figure 5), the friction stirring tool moves forward along a designed path, during which the parts in contact with the surface of the material generate a large amount of heat due to friction with each other, leading to plastic deformation of the material,^115,116 which results in finer particles of the processed material. Materials with ultrafine grains typically exhibit strong mechanical properties, high wear resistance, high corrosion resistance and good biocompatibility compared to coarse-grained materials. Ultrafine-grained materials, especially micron- and nanograined materials, have fewer atoms per grain and therefore more surface atoms, leading to higher surface energy. ¹¹⁷ As a result, osteoblasts are more likely to adhere to the surfaces of these materials, leading to increased osseointegration. In addition, these surfaces have been shown to hinder bacterial adhesion and reduce biofilm formation. During processing, protective gases such as nitrogen or argon are usually provided to prevent oxidation at high temperatures. For example, Rezaei et al. ¹¹⁸ used FSP to fill magnesium hydride powder into the grooves on the substrate to integrate nanoscale magnesium into the surface layer of CP-Ti, making the magnesium degradable rapidly in vivo by a means of creating a large number of voids on the implant surface, which could further promote osseointegration.

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Figure 5.

Schematic diagram of the friction stir processing. ¹²⁴

It has been found that FSP-treated NiTi shape memory alloys have high mechanical properties and maintain a critical shape memory effect with small changes in phase transition temperature.^119,120 Due to dynamic recrystallisation in multiple FSPs, the grain size was refined to the submicron level, leading to the formation of thicker and more stable protective oxide coatings on the pure titanium surface and improved corrosion behaviour.^121–123 In order to improve the biocompatibility and the mechanical properties of samples, TIC powder was incorporated as a reinforcing material onto the surface of Ti6Al4 V by FSP technology (Figure 5). ¹²⁴

Since the advent of the FSP technique, a number of studies have been approved that Ti or Ti alloy have significantly improved the properties of the surface layers of materials, making them better suited to the requirements of specific operating environments including cell proliferation, differentiation and antibacterial properties.^125–127 However, there are still some problems with this technology, such as the grain refinement that occurs during FSP due to the large heat input and intense plastic deformation during processing. ¹²⁸ So, FSP is limited to the shape and size of the sample during processing and in can only process plates and sheets and not yet complex shapes.

Spraying technology (coating technology)

Plasma spraying is a well-established deposition technique, commonly it is used for depositing HA on titanium-based implants,^129,130 this process involves injecting a suitable precursor powder into a DC plasma jet to generate in an inert or low-pressure atmosphere, typically argon, within the plasma jet, the powder is heated and accelerated simultaneously. Eventually, the molten powder is sprayed onto the substrate in front of the plasma torch. ¹³¹ Plasma spraying can create smaller nanoscale surface roughness structures, which greatly increases the specific surface area and accelerates cell adhesion and osseointegration.^132,133 Plasma spraying HA on Ti and titanium alloy surfaces improves the surface roughness while significantly increasing the implant's bioactivity, which promotes the implant's rapid bonding with the tissues of the body and induces osteogenic ions recovery of damaged tissues after surgery. So, HA-coated/Ti-6Al-4 V alloy composites have not only biocompatibility but also good mechanical properties similar to those of natural bone, especially the tensile strength and toughness.^134,135 In recent studies, researchers have explored the incorporation of multiple ions into HA coatings to enhance their biological functions. Cao et al. ¹³⁶ prepared (Mg, Sr)-HA coating on Ti6Al4 V alloy by plasma spraying with (Mg, Sr)-HA powder, these (Mg, Sr)-HA coating exhibited high bond strength and favourable biological functions, which hold great potential for application prospect in orthopaedic and dental fields (Figure 6c). Hence, treated bioactive coatings on surface Ti alloy by plasma spraying technology can significantly improve the biocompatibility, density, high bond strength, excellent oxidative stability.

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Figure 6.

6(a) Schematic diagram of the evolution of ultrasound-assisted electrophoretic deposition of HA- GO and exposure of the synthesised titanium to SBF solution (growth of new HA grains) ¹⁶⁹ (b) schematic diagram of the preparation of EDC coatings ¹⁷³ (c) Plasma spraying experimental set-up ¹³⁶ (d, e, f) CS NC-HA coated cells in culture on days 1, 7 and 14. ¹⁵⁷

Cold spray coating technology (CS) is an innovative spray technology that operated at low temperatures and utilises the kinetic energy to deposit coatings, unlike conventional spray technology, CS achieves particle acceleration below the melting point,^137–139 small particles (5–50μm) are propelled onto the substrate at supersonic speeds (300–1200 m/s) using preheated gas (25–1100°C). This technique has shown a better promise for spraying materials sensitive to oxygen and temperature.^140,141 In recent years, cold spraying has a rapid development for easily deformable materials such as Cu,^142,143 Al, ¹⁴⁴ Zn, ¹⁴⁵ etc. and some refractory metals, such as Ti, ¹⁴⁶ Ti alloys, ¹⁴⁷ Ta, ¹⁴⁸ high temperature alloys ¹⁴⁹ and even some ceramic materials. Villareal et al. ¹⁵⁰ conducted a study by cold spray technique to deposit two HA coatings with different grain sizes on Ti6Al4 V alloy, this nanocrystalline HA coating demonstrated enhanced the biocompatibility and faster cell proliferation after surface attachment.

Electrostatic spray deposition (ESD) is another promising technique for low-temperature deposition,^151–153 it is a cost-effective chemical method that allows the deposition of a wide range of oxide materials, while ensuring strong adhesion to the substrate. Moreover, ESD also offers control over stoichiometry and film thickness. ¹⁵⁴ Compared to plasma spraying techniques, which involves high processing temperature and rapid rates. ESD can overcome some drawbacks for unpredictable phase changes.^155,156 Müller et al. ¹⁵⁷ utilised ESD to deposit coatings with good crystalline structure, nanostructure and single-phase HA on titanium alloys, improving biocompatibility. (Figure 6d, e, f).

Electroplating technology (electrodeposition)

Electroplating technology is a process that involves the deposition of a metal coating onto a substrate, such as a titanium alloy, through an electric current. This process allows for the movement of cations in the plating solution towards the cathode, resulting in the accumulation of the desired coating on the titanium alloy surface. ¹⁵⁸ The main purpose of electroplating technology in the context of titanium alloys is to enhance the binding force between different layers and improve the surface properties. Titanium and its alloys tend to form oxide layers on their surfaces, which can hinder the adhesion of other metal layers. Electroplating helps overcome this challenge by facilitating the formation of a strong bond between the layers.

Nickel based alloy as a pre-coating on the surface of titanium alloy is commonly used due to the thermodynamic stability. Ti-Ni intermetallic compound formed by the reaction of Ti and Ni. Wu et al. ¹⁵⁹ prepared TiNi-Ti₂Ni modified coating on Ti6Al4 V surface using a combination of electroplating and EB re-melting, which greatly improved the surface hardness and better wear resistance. Researcher have explored various techniques to further enhance the performance of electroplated coatings on titanium alloys. The addition of ultrasonic oscillation during the plating process has been found to improve uniform distribution and dispersion of nanoparticles in the plating solution, as well as the cleanliness and the quality of the electrode surface, which can result in denser and more structurally refined coatings, such as nanocrystalline nickel coating.^160,161 Moreover, studies have shown that electroplated composite coatings containing micro- and nano-inorganic inert particles, such as nickel-based coatings with added carbon nanotubes, exhibited improved mechanical, catalytic and biological properties.^162,163 These composite coatings can significantly enhance the hardness, wear resistance, corrosion resistance and biocompatibility of the titanium alloy surface.^164,165

Electrophoretic deposition (EPD) is a unique method used colloid processing technique. It involves the transfer of charged particles, which are suspended in a liquid medium to oppositely charged electrodes under the influence of an electric field. This process leads to accumulation of the particles, forming a coherent film known as a cast film. The EPD offers several advantages in coating production. It enables the creation of coatings with varying thicknesses, ranging from less than 1 mm to over 1000 mm in a relatively short period of time. This rapid deposition rate makes EPD an efficient and time-saving technique. The versatility of EPD allows for the deposition of a wide range of materials, including ceramics, metals, polymers and composites. It is widely used in various industries, such as automotive, electronics, energy and biomedical, for applications such as corrosion protection, surface modification and functional coatings. ¹⁶⁶

However, in practical application, improved adhesion for cell is often desired. To address this issue, ultrasound-assisted EPD has been explored.^167,168 researchers have reported enhanced adhesion and corrosion resistance by incorporating ultrasound during the deposition process. Fardi et al. ¹⁶⁹ deposited HA-graphene oxide (GO) nanocomposites using ultrasound-assisted EPD technique, the addition of GO significantly improved the mechanical properties, corrosion resistance and biomedical functionality of the coating on titanium alloy (Figure 6a).

Furthermore, the EPD technique has shown promise in producing composite coatings of bioactive glass and biopolymers. Chitosan-bioactive glass composite coatings, in particular, have attracted attention due to their potential applications.^170,171 Mahlooji et al. ¹⁷² prepared chitosan bioactive glass (CS-BG) nanocomposite coatings with different concentrations on the Ti6Al4 V alloy using cathodic EPD, the results demonstrated that the coatings of 1.5 g/L CS-BG successfully improved the adhesion strength, bioactivity, corrosion resistance and cellular properties of the substrates.

In recent years, researcher have made numerous efforts to enhance the corrosion resistance of titanium alloys. EPD has emerged as an easy method to control the structure and composition of the coating, thereby improving cell adhesion. Peng et al. ¹⁷³ prepared EDC composite coatings on Ti6Al4 V alloy by cathodic EPD, the EDC coatings exhibited smooth surfaces and a dense distribution of nanoparticles. These coating demonstrated stability in the corrosive environments and inhibited the electron transfer further improved the corrosion resistance of titanium alloys (Figure 6b).

Electrochemical deposition techniques allow the synthesis of different materials on irregularly shaped metals using different electrolytes. The principle is similar to chemical surface modification, by adjusting the process parameters such as potential, current and temperature to determine the final result of the modification. Due to its ease of use and versatility, electrochemical deposition techniques have been widely used in recent years.

Other methods

In addition to the modification methods mentioned above, here are some other methods, such as using acid etching to change the surface roughness of the implant.^174,175 By 3D printing stacked additives on the implant surface, where the shape, roughness, pore size, porosity and other parameters can be well controlled, especially for special implant structures. ¹⁷⁶ Li et al. ¹⁷⁷ prepared porous Ti6Al4 V scaffolds with 5 µm roughness and 58% porosity by 3D printing. In addition, a single surface technique may have limitations or drawbacks. Therefore, to achieve the desired surface characteristics of titanium and its alloys, it is often necessary to integrate multiple techniques to optimise the performance of the implant (Table 2).

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Table 2.

Examples of surface modification of titanium and titanium alloys.

Alloys	Method	Aims	Refs
CP Ti	Atmospheric thermal oxidation + micro-arc oxidation (MAO)	Improved the tribocorrosion resistance, biocompatibility and bonding between cells.	178
Ti64	Plasma spray coating	Improved corrosion resistance and microhardness values.	179
Ti-4Al-7Nb	High-speed milling	Improved the hardness, corrosion resistance and biocompatibility.	180
Ti-15Mo	MAO (in baths containing zinc)	Increased surface bioactivity and cytocompatibility and slightly improved the bacteriostatic effect.	181
Ti–24Nb–4Zr–8Sn	Sandblasting, dual acid-etching and alkali thermal treatment	Enhances the surface roughness and improves wetting	182
Ti–29Nb–13Ta-4.6Zr (TNTZ)	Ultrasonic nanocrystal surface modification	Increased the wear resistance and cell adhesion area.	183
Ti	Selective laser sintering Co-sputtering	Enhanced antimicrobial properties	184
Ti15Mo	Plasma electrolytic oxidation + dip coating	Improved corrosion resistance and enhanced biocompatibility and antimicrobial properties	185
Ti	Plasma spray + Dip coating + thermal treatment	Enhanced antimicrobial properties	186
Ti	Surface grafting by silanisation or absorption	Improves biocompatibility and inhibits bacterial growth	187
Ti	NaOH treatment + polydopamine coating + layer-by-layer deposition	Improved biocompatibility and antimicrobial properties	188
Ti	Electrochemical-driven rapid layer by layer	Improved biocompatibility	189
Ti64	Hydrogel 3D bioprinting: hydrogels containing clindamycin were extruded on acid-etched	Improves the surface roughness and enhances antimicrobial properties	190
Ti	Electrospinning process: the poly-caprolactone nanofibres loaded with HA nanoparticles and rifampicin were coated	Improved antimicrobial properties and enhanced mechanical properties	191
Ti	Electrodeposition to coat HA/collagen/vancomycin	Enhanced biocompatibility improves antimicrobial properties	192