*Review* **Recent Advancements in Materials and Coatings for Biomedical Implants**

**Kamalan Kirubaharan Amirtharaj Mosas 1,\* , Ashok Raja Chandrasekar 1,\* , Arish Dasan <sup>1</sup> , Amirhossein Pakseresht <sup>1</sup> and Dušan Galusek 1,2,\***


**Abstract:** Metallic materials such as stainless steel (SS), titanium (Ti), magnesium (Mg) alloys, and cobalt-chromium (Co-Cr) alloys are widely used as biomaterials for implant applications. Metallic implants sometimes fail in surgeries due to inadequate biocompatibility, faster degradation rate (Mg-based alloys), inflammatory response, infections, inertness (SS, Ti, and Co-Cr alloys), lower corrosion resistance, elastic modulus mismatch, excessive wear, and shielding stress. Therefore, to address this problem, it is necessary to develop a method to improve the biofunctionalization of metallic implant surfaces by changing the materials' surface and morphology without altering the mechanical properties of metallic implants. Among various methods, surface modification on metallic surfaces by applying coatings is an effective way to improve implant material performance. In this review, we discuss the recent developments in ceramics, polymers, and metallic materials used for implant applications. Their biocompatibility is also discussed. The recent trends in coatings for biomedical implants, applications, and their future directions were also discussed in detail.

**Keywords:** bioceramics; metallic implants; biomedical applications; coatings; surface modification; biocompatibility; Mg-alloys; Ti-alloys

## **1. Introduction**

Bioimplants are defined as engineered medical devices that are developed to replace the non-functional or broken biological structural parts of the human body, providing support to the given host. Biomaterial surface modification plays a key role in determining the outcome of the interaction between human biology and materials. Substantial development in research in the field of biomaterials has increased the scope of use for a wide range of orthopedic and dental implants that include total bone replacement, fracture fixation, dental screws, joint arthrodesis, and so on [1]. Essentially, the success of bioimplants depends not only on their bulk properties but also on the properties of their surfaces, which interact with human body tissues. As a result, the evolution of bioimplants has reached a level of choice of materials based on specific properties on the basis of selected specific materials [2]. Though alloys and metallic substances meet many of the biomedical requirements, their interfacial bonding between the surrounding tissue or bone and the metallic surface ranges from poor to virtually absent. The failure of the metallic implant originates at the implant-tissue interface due to poor bonding at the interface, which leads to the formation of a nonadherent layer and movement at the tissue-implant interface [3].

Corrosion in biometallic implants can affect the surface and biocompatible behavior that induce tissue reactions, which lead to the release of corrosion byproducts from the implant surface and result in premature failure. A minimum durability of 15 to 20 years for older patients and more than 20 years for younger patients is expected from a bioimplant [4].

**Citation:** Amirtharaj Mosas, K.K.; Chandrasekar, A.R.; Dasan, A.; Pakseresht, A.; Galusek, D. Recent Advancements in Materials and Coatings for Biomedical Implants. *Gels* **2022**, *8*, 323. https://doi.org/ 10.3390/gels8050323

Received: 2 May 2022 Accepted: 18 May 2022 Published: 21 May 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

However, there are problems associated with the use of metallic implants due to the lack of poor implant fixation, lack of osteoconductivity, corrosion, and wear resistance leading to the formation of wear debris and release of corrosive ions [5–7]. These problems are mostly associated with the surface of the metallic implants. In view of this, the surface of the bioimplant plays a major role in the biological environment because the reactions occur directly on the surface of the implant after implant fixation. Hence, it is necessary to modify the surface of the metallic substrate with specific properties that are different from those in bulk [8,9]. This modification is required to accomplish good bone formability and desired biological interactions. In some applications, biocompatibility, wear, and corrosion resistance are also required.

Surface modifications of bioimplants are explored intensively with many bioactive materials to avoid adverse effects such as lack of biocompatibility, post-surgery infections, long-term survivability, and risks related to implant surface corrosion [10,11]. At first, the research in this field was focused on the improvement in biomechanical properties of metallic implants, but in recent days, it has turned towards improvement in the biological properties of these biomedical devices [12,13]. By applying the appropriate modification on the surface of the material, one can tailor and improve the biocompatibility, cell interactions, and adhesion [14]. Thus, the development and design of biomaterials rely on surface modification. For that, it is necessary to develop techniques for functionalization of the surface of metallic implants through changing the materials' surface composition, morphology, and structure without losing their mechanical properties. By adopting this, the service life and performance of orthopedic and dental implants can be significantly increased. This can be achieved by applying suitable biocompatible coatings with a unique combination of properties.

In view of reliability and performance, the best way to functionalize the implants in direct contact with bones and tissues is ceramic coatings owing to their excellent osteoconductive properties and high stability [15,16]. Surface modification by coating can enhance the antibacterial activity of a bioimplant. The coated surfaces facilitate grafting of cell-binding peptides, directed mutations of the cellular host, protein of extracellular matrix (ECM), and growth of tissues to improve the acceptance of a bioimplant further. Ceramic coatings on bioimplants show promising results in orthopedics with improved bone regeneration and repair [17]. The overview of applications of ceramic coatings used for metallic implants is listed in Table 1.


**Table 1.** Ceramic coatings used for biomedical applications [18].

The major requirements for the selection of coating materials are (a) biocompatibility and nondetrimental effects such as allergy, inflammation, and toxicity, (b) adequate fracture toughness, fatigue, and mechanical strength to withstand the forces, and (c) resistance to corrosion in the human body fluid atmosphere, which contains many constituents such as amino acids, chlorine, water, proteins, sodium, and plasma acids. The choice of coatings, by considering their degeneration and surface properties, plays a major role in terms of reliability and performance of bioimplants. The coatings for biomedical applications can be subdivided into three groups: (a) bioinert, (b) bioactive, and (c) bioresorbable coatings [19]. The coatings having a minimum interaction with the surrounding tissues after implantation in the human body are considered as bioinert coatings. The typical examples of bioinert coatings are metal oxides, nitrides, carbides, carbonitrides, and oxynitrides. Transition metal nitrides (TiN, ZrN, TiAlN, NbN), carbides (TiC), oxides (ZrO2, Al2O3, TiO2), or oxynitride (TiON) coatings find a wide range of applications in bioceramic coatings due to their remarkable properties such as wear, tear, hardness, biocompatibility, and corrosion resistance [20,21].

The current review incorporates a description of the biomaterials and coatings that are commonly used in the manufacturing of different orthopedic and dental implants.

#### **2. Biomaterials for Biomedical Applications**

Biomaterials are used to make devices that interact with the biological systems in the human body and coexist for a long time with minimal failure. The type of material used in implant applications shows specific properties that make them primary candidates for specific applications. The key requirements for the selection of biometallic materials consist of (a) cost effectiveness, (b) mechanical behavior equal to that of the human skull and bones, and (c) their biocompatibility [22,23]. In addition, the major requirement for the bioimplant materials is that it should be compatible with the human body, i.e., it should integrate with the human body without negative impacts. Moreover, it must possess corrosion and wear resistance in the human body environment. These properties will determine the effectiveness of the implant materials.

If a metallic material experiences wear and corrosion, the surrounding tissues present at the implant area can become inflamed, causing unfavorable biological reactions within the human body [24]. The ions and toxins released from the metallic substrates as a byproduct may be potentially harmful and can cause life threatening diseases and increase the risk of using metallic implants. Therefore, it is important to choose correct material for correct applications while performing bioimplants. In addition to that, the mechanical performance of the biomaterial should be close to that of the replacing material where it must sustain complicated and varying mechanical loading cycles [25]. Typical examples for implanting areas are teeth, knee joints, and hips. The selection of biomaterial based on mechanical properties is important to ensure no implant failures within the body when subjected to numerous loading cycles during service life. Moreover, the material should be biocompatible with the surrounding tissues and economically viable. Finally, it is essential that the choice of material should be cost effective, efficient, and able to integrate with the human body. Based on the requirements defined above, several materials were developed in recent years to be used as biomaterials for implant applications. Still, it is hard for a single metallic material to fulfill the desired properties. Biomaterials used for biomedical applications are broadly classified into ceramics, polymers, and metallic systems.

#### *2.1. Ceramics*

Ceramics are inorganic compounds formed at high temperatures. Typical examples are bioactive glass (BG), zirconium oxide (ZrO2), aluminum oxide (Al2O3), hydroxyapatite (HAp), and other calcium and silica-based ceramics. These ceramics are noted for their great biocompatibility, which makes them an excellent candidate for biomedical implant applications. Depending on the reactivity with the human body, ceramic implants are classified into three categories: (a) bioactive, (b) bioinert, and (c) bioresorbable ceramics [26]. Bioactive ceramics are used to interact with the surrounding cells and exhibit a higher level of reactivity within the implant sites. Typical examples for bioactive ceramics are HA and fluorapatites [27]. In an opposite trend, bioinert ceramics do not show any reactivity with the host tissues at the implant sites but form a physical bonding when implanted [28]. Bioresorbable ceramics exhibit a low level of reactivity with the host body tissues [29]. After implantation, these ceramics are gradually resorbed and finally replaced with the bone tissue. These bioresorbable ceramics are widely used in orthopedics and dentistry due to their better biocompatibility and chemical interactions [30].

No risk of transmitting disease plus immunogenicity after implantation are the major advantages of the ceramics [31]. Other remarkable advantages are higher resistance to compressive force, low toxicity, good corrosion resistance, and promotion of the formation of new hard tissues. For example, hydroxyapatite-based ceramics exhibit higher Ca/P ratios, which are desirable due to similar chemical properties of bone and teeth hard tissues [32–35]. Due to these attractive properties, ceramics are increasingly utilized for bioimplant applications.

Ceramics are known for their high hardness and stress-shielding effects due to their high elastic moduli, and slow initiation of crack growth over time, which significantly decrease the reliability of the implants [36]. In addition, brittleness, fracture toughness, and fabrication issues limit their use as bioimplants. The ceramics share the brittleness factor, which limits the performance in terms of load-bearing applications (hip implants). If the difference in mechanical properties of ceramic and bone is large, the load will not be transmitted through the bone, thus leading to failure of the bone [37].

Ceramic composite materials provide superior properties compared to single materials. The inferior mechanical properties of monolithic ceramics can be overcome by composite ceramics while diminishing the limitations of each component. The remarkable properties of composites such as the weight to strength ratio enable them to be used extensively for the restoration of bones, ligaments, and dental fillings [38]. Moreover, the composites prepared through the combination of bioactive and bioinert ceramics show better bioactivity and mechanical strength [39]. Typical examples are HA and Al2O<sup>3</sup> composites which show better osteointegration with bone, good bioactivity, and high yield strength [40,41].

#### *2.2. Polymers*

The most widely used materials in biomedical applications are polymers. Polymers are the building blocks of small repeating units' monomers and are classified into two categories called biodegradable and non-biodegradable. Typical examples for biodegradable polymers are polyacetal, chitosan (CS), alginate, polylactide, and polycaprolactone, whereas nonbiodegradable polymers include polypropylene, polytetrafluoroethylene, polyethylene terephthalate, polymethylmethacrylate, etc. Polymer implants are mostly used in replacing heart valves, kidneys, bone, skin, contact lens, and artificial blood vessels, in addition as pacemakers [42]. Among biodegradable polymers, CS shows remarkable properties such as biocompatibility, biodegradability, wound healing, and antibacterial activity [43]. It is also environmentally friendly and hence acts as a capping agent [44,45]. Polymers show lower strength and elastic moduli as compared to metals and ceramics. Therefore, they are not generally used for load-bearing applications such as joint and knee prostheses. The polymers are also degraded in the body environment due to biochemical factors.

Polymer implants are quite interesting as bioimplants due to their low cost while offering sufficient mechanical properties. For example, Polyether ether ketone (PEEK), composed of 20% TiO<sup>2</sup> particles and an additional ketone group results in 80% higher compressive strength and better fatigue properties than pure PEEK [46]. Depending upon the replacement anatomy to which the polymer is being applied, a wide variety of polymers can be applied. Polymers have the advantage of complete degradation over time, leaving no signs of their presence at the implant locations in a body. This was possible with the subsequent research and development in biodegradable polymer materials, where the proteins and extracellular matrix mimic the cell signaling functions of the surrounding tissue, permitting better bio-integration [47].

Though polymers show exceptional properties and are cost-efficient and easy to manufacture, they show different forms of cytotoxicity: depending on the host body conditions, inflammatory reactions can occur within the implant region. This will induce bone degeneration, abnormalities, rapid rate of corrosion, and decreases in mechanical properties over time. Moreover, the elastic modulus of polymers is extremely low compared to human bone (between 10 and 30 GPa) [48]. This will create an impact while applying load. Another major issue that is being faced is that the polymer implant degrades as the bone heals. If the process is too fast, the neighboring tissues feel more stress, which causes potential discomfort. These limitations prevent them from being widely used as bioimplants.

#### Polymeric Gels

Natural polymers such as collagen are the main components of natural bone due to their hydrophilic nature, enabling the formation of hydrogels with aqueous solutions that exhibit several desirable characteristics for bone-tissue engineering [49]. Polymeric gels are often referred to as hydrogels owing to their ability to hold water inside their networks [50]. These hydrogels swell upon water intake and shrink upon drying [51]. Taking advantage of this property, water soluble drugs, growth factors, and other biological entities such as proteins and even live cells can be incorporated into these hydrogels [52]. These gels can be designed for delivery systems based on certain external stimuli such as pH [53,54], temperature, or the presence of specific chemicals or target molecules [55]. Many researchers choose collagen because it is the most important organic component of human bone [56–58].

Hydrogels are attractive soft biomaterials because of their soft consistency (stiffness and viscoelasticity are essential in directing the immune response), high water content, porosity, and biocompatibility [59]. They are widely used in 3D cell cultures for modeling the biological extracellular matrix or as coatings for promoting cell attachment. Other natural polymer-based hydrogels used as bone tissue engineering (BTE) materials include polysaccharides (e.g., cellulose) and polypeptides (e.g., alginate). Compared with natural polymeric gels, synthetic polymeric gels offer more possibilities for molecular alterations that facilitate tailoring the candidate properties to specific requirements, i.e., tuning mechanical properties and biophysical and biochemical cues. For instance, Poly(ethylene glycol) (PEG) hydrogels, modified with adhesion ligand arginine–glycine–aspartic acid (RGD), offer tunable mechanical properties as well as improved cell attachment and cell differentiation [60]. However, generally, the poor mechanical strength of hydrogels limits their usage and needs further improvement for bone regeneration. Recent emerging technologies such as 3D printing in the manufacturing of hydrogel-based components may offer entirely new possibilities for addressing the challenges [61].

#### *2.3. Metals and Alloys*

Even though ceramics show excellent biocompatible performance, they have poor fracture toughness and exhibit brittle behavior, and their use in load-bearing applications is limited. Thus, metals and alloys are generally used for implants where high strength and load-bearing capacity are required. Most medical industrial segments rely on metallic implants. They are generally used to replace some load-bearing applications such as the hip, plates, knee prostheses, pins, dental materials, screws, and cardiovascular applications [62]. Though metals show high strength and durability, they can lose their properties under physiological conditions with a potential release of various ions and debris which may trigger a biological response. Most of the alloys release metal ions to the plasma in the blood [63]. The excessive release of ions in the blood has a high risk of accumulation in organs such as the spleen and liver that later form particulates, affecting the normal functioning of these organs. This phenomenon leads to cytotoxicity followed by organ failure upon prolonged accumulation.

Metallic materials are not fully accepted by the human body, and the tissue growth is impaired because of inadequate attachment of the implant, leading to discomfort or pain in the implant region [64]. As compared to ceramic materials, the risk of infection is higher, and the healing time is slower in the case of metallic implants. Although metallic implants

have some limitations, preference should be given based on their corrosion resistance, cost effectiveness, and mechanical strength. The chemically inert platinum and gold do not show any corrosion in situ, and these materials can be used as bioimplants, but they are expensive. Hence, recent biomedical industries use Ti-based alloys and Mg-based alloys due to their better biocompatibility and good mechanical strength under human body conditions [65]. The widely used metallic materials used as biomedical devices are stainless steel and Ti- and Co-based alloys [66,67].

#### 2.3.1. Stainless Steels (SS)

In India, SS 304 and 316L are the most used implant materials for biomedical applications due to their cost effectiveness, wide resource availability, reliability, and ease of fabrication as compared to Ti- and Co-based alloys. Among various grades of SS, the primary recommended grade for implant applications is AISI type 316L SS. The presence of chromium (minimum content of 10.5 wt. %) yields a thin and passive oxide layer and protects the implant surface against corrosion [68]. The presence of carbon (min. 0.03 wt. %) in SS increases its mechanical properties, especially fracture toughness, corrosion resistance, and tribological performance of the implants. Their load-bearing capability makes them a suitable orthopedic implant material [69]. However, almost 90% of 316L grade SS implants lose their properties due to a pitting corrosion attack and the release of nickel and chromium ions, which cause allergic reactions in the implant region. Hence, a small addition of molybdenum (2 to 4 wt. %) improves the corrosion resistance and strengthens the 316L SS grade.

The 316L SS used in biomedical devices is classified into two categories: (a) conventional SS and (b) Ni-free stainless steels [70]. The primary use of conventional stainless steels is to provide a load-bearing property to the implanted surfaces: they are often used as fracture plates, nails, screws, and stents in the implant process. In addition, the Ni-free SS provides higher corrosion resistance and biocompatibility [71]. When compared to other bioimplants, the chemical composition of SS alloys offers an advantage when good mechanical properties are desired. Moreover, they have a high cost-to-benefit ratio and exhibit a linear relationship with the manufacturing processes and final structure/properties.

Its elastic modulus (200 GPa), which is higher than that of the human bone (10–30 GPa), results in high stress-shielding effect at the tissue/implant interface leading to the failure of the implanted SS [72–74]. In recent days, SS was modified with hydroxyapatite (HAp) which improves its bio-integration and osteointegration properties. Typical implanted materials are screws, pins, sutures, bone plates, steel threads, and medullary nails, which are used in fracture fixation. However, the corrosion resistance, biocompatibility, and osseointegration of SS are lower compared to Ti-based alloys, where implant success rates are much higher [75].

#### 2.3.2. Co-Cr Alloys

Co-based alloys are considered as one of the most successful materials used for implant applications. This alloy was first used in the early 1900s, where it was used as an implant material for hip replacement. Co-based alloys show better corrosion, wear, and mechanical properties and are used in bioimplant applications. The in vivo and in vitro studies confirmed that Co-based alloys show better biocompatibility and can be used for the manufacturing of surgical implants such as in the hip, knee, shoulder, and fractured bone surfaces [76,77]. The most widely used combination of Co alloys are Co-Cr-Mo owing to their unique combination of strength and ductility. By comparing with other metallic implants, this alloy shows a better elastic modulus, density as well as stiffness, becoming an ideal material for the implant process [78]. This alloy is primarily focused on permanent implant fixation procedures because these alloys maintain their initial properties for a long time after implantation. The cumulative likelihood of endurance reached 96% at 12 years for patients aged above 60 years [79]. A Co-Cr-Mo alloy combined with ultra-high molecular weight polyethylene (UHMWPE) is used in artificial ankles and knees [80,81].

Other major alloying elements of Co-based alloys include Ni, Mo, and Cr. These elements were proven to be toxic to the human body when leached out from the metal surface to the body fluid during corrosion of Co alloys and can lead to skin-related diseases. An excessive leaching of these trace elements leads to damage to organs such as the liver, kidney, blood cells, and lungs [82,83]. The addition of nickel into Co-Cr-Mo improves corrosion resistance and mechanical properties, but due to the cytotoxicity of Ni, the use of this alloy in bioimplants is limited [84]. The elastic modulus (200–250 GPa) and ultimate tensile strength (400–1000 GPa) of Co-based alloys are 10 times higher than those of the human bone. The use of these implants manufactured from Co-based alloys thus results in a stress-shielding effect at the tissue/implant interface. The surface modification of Co-Cr-Mo alloys under plasma treatment improves hardness, wear, and corrosion resistance [85–87]. However, they are still not recommended for joint fixtures due to their inferior frictional and tensile properties. Apart from their biocompatibility and corrosion behavior, Co-based alloys are not ideal materials for bearing and joint surfaces due to their sub-par frictional properties [88].

## 2.3.3. Ti Alloys

Commercially pure titanium (Ti) and its alloys (Ti-6Al-4V, Ti-6Al-7Nb, Ti-5Al-6Nb, and Ti-13Nb-13Zr) have become major assets in the biomedical field owing to their superior biocompatibility, low density, and suitable mechanical properties. At first, it was intended to be used for aerospace applications, but later in the 1970s, the discovery of its biocompatibility led to a demand for Ti and Ti alloys in biomedical applications. If commercial pure titanium (Cp Ti) is used to replace its alloys, the mechanical properties lost due to alloying elements must be compensated for [89,90]. The alloys of Ti show enhanced mechanical and biocompatibility properties in comparison to pure titanium. Depending on the presence of the iron and oxygen content in the Ti alloy, four different grades of alloys are used. The most widely used Ti alloy is Ti-6Al-4V, comprising an estimated 50% of total titanium alloys' usage for bioimplants of this grade [91,92]. By comparing with other grades of Ti alloys, it offers excellent corrosion resistance, biocompatibility, formability, structural stability, and a better weight to strength ratio. The applications of Ti alloys as bioimplants include heart valves, dental prostheses, osteosynthesis, artificial joints, and bone replacements [93].

Biomedical grade titanium alloys are generally categorized as alpha (α, Ti-6Al-4V), near-α, α-β, and metastable β (Ti-6Al-7Nb) [94,95]. These alloys are widely used as biometallic implants, but they cause stress shielding issues at the implant-tissue interface due to their high elastic modulus values. The elastic modulus of Ti and α-β Ti-alloys (100–110 GPa) is higher than that of human bone which limits its usage in joints. The presence of vanadium and aluminum compounds results in the release of toxic ions of vanadium (oxidovanadium (IV) and vanadate (V)) and aluminum (Al3+) under the physiological environment, leading to adverse health issues [96–98]. Therefore, much interest has been paid to β alloys in combination with Zr, Nb, Ta, or Mo to replace V and Al in the alloy. Such alloys possess better mechanical properties, ductility, good structural stability, higher wear resistance, a lower elastic modulus, and improved corrosion resistance [99–101].

One of the disadvantages of using Ti alloys is their below par tribological properties, due to their high friction and abrasive wear nature [102,103]. Moreover, the formation of TiO<sup>2</sup> during exposure protects the surface of the Ti alloy, which hinders the bioimplanttissue relationship. The formation of titanium compounds around the surrounding tissues of the implant causes failure of the implant [104].

#### 2.3.4. Mg Alloys

Metal-based biodegradable orthopedic implants nullify the complications associated with the long-term existence of implants inside the human body. In recent days, biodegradable metallic implants were investigated as biomedical implants [105]. Magnesium (Mg) is present in the human body as the fourth most abundant cation and is essential to the human metabolism. Mg corrodes faster in the chloride containing physiological environment; thus, it has emerged as biocompatible and biodegradable material for use as implants [106]. Moreover, Mg and its alloys have received much attention in the category of biodegradable alloys due to their leading properties such as low density, an elastic modulus close to that of bones, light weight, biocompatibility, and excellent mechanical properties [107,108]. The revision surgeries performed to remove hardware components in implants such as screws and plates from the implanted site after healing are often discomforting and expensive for the patients. The revision surgery can also lead to complications such as nosocomial infection and delay the patient's recovery to a normal lifestyle. Mg-based biodegradable metallic implant components can overcome the revision surgery by degrading in situ, thus also eliminating the need for the procedure to remove the implant components after healing [109].

The high mechanical strength of metallic materials limits the use as bioimplants, whereas the Mg implant shows a reduced elastic modulus and prevents the mismatch between a bone and the Mg-based implant. This leads to the reduction in stress shielding at the bone/implant interface. Their mechanical and corrosion properties can be enhanced by alloying with Al, Zn, and other elements [110,111]. Current research is focused on the development of Mg-based alloys with zero or low cytotoxicity. Alloying Mg with other metals must be selected carefully to avoid metal-related toxic issues and corrosion. Different type grades of Mg alloys such as Mg-Ca and Mg-Y-Nd were studied as biodegradable bioimplants for orthopedic applications [112].

The major limitation associated with Mg and Mg-based alloys is their rapid corrosion in physiological conditions. Rapid corrosion results in quick release of byproducts such as hydrogen gases due to fast in vivo degradation. This indicates the necessity for surface modification. To overcome the rapid corrosion, alloying with various elements has been explored. For example, elements such as calcium (Ca), zinc (Zn), silver (Ag), aluminum (Al), zirconium (Zr), yttrium (Y), and Neodymium (Nd) were added to Mg to enhance the corrosion and mechanical properties [113–117]. Typical examples are Mg-Ca, Mg-Zn, and Mg-Zn-Ca. By carefully selecting a suitable element and its composition, the microstructure can be tailored to meet mechanical properties such as bone. This makes them ideal for bone replacement. Table 2 shows the overall comparison of materials used for biomedical applications and their applications.


**Table 2.** The pros and cons of various biomaterials used in the biomedical industry [118].

#### **3. Need for Surface Modification of Bioimplants**

In an implant operation, any material inserted into the human body is treated as a foreign substance. If the foreign substances are not biocompatible, layers of fibrous tissues, also known as scar tissues, begin to develop between the tissue and implant. Eventually, due to scar tissue development, the implant fails to osteointegrate with the host bone, leading to implant failure. Therefore, the primary requirement for the successful implant process is to have a complete integration between bioimplants and human body tissues [120]. The biological responses of biomedical devices to the lifespan and performance are better controlled by their surface morphology and chemistry. To achieve better biocompatibility and osteoconductivity, surface modification on biometallic materials has been recommended to achieve the desired properties (Figure 1) to increase the success rate of implants. When the surface is effectively modified, the bulk functionality and properties of the biomedical implant device will remain unaffected for a long time [121,122]. With the advantage of bio-integration and the load-bearing capability of biomaterials, the success rate for bioimplants can be greatly increased.

In recent years, researchers tried to enhance the bio-integration of implants by modifying the implant surface that is in contact with the body environment. Two approaches are considered for modifying the surface of the implants. The first approach is to deposit organic/inorganic-based coatings on the metallic surface without modifying the implant substrate [123]. The second approach is to use conversion coatings or surface modified layers, where the chemical surface modification of a substrate results in a slight increase in thickness [124]. In this case, the substrate elements are involved in developing conversion coatings. For conversion coating, surface preparation by grinding and polishing is required to improve the surface roughness for better mechanical interlocking of coatings. This process is critical, and surface modification by depositing an overlay coating is recommended [125]. Recently, a combination of both surface modification and deposition of thin films was performed to achieve the synergy of both properties.

In a modern biomedical implant industry, surface modification of metallic implants with an appropriate coating material is used to enhance biocompatibility, corrosion resistance, antimicrobial behavior, and mechanical properties. Although there are many methods for the deposition of bioactive surface coatings, an optimal coating technique for biomedical applications has not been developed yet. Currently, the coatings on implant materials are deposited by one of the deposition techniques such as physical vapor deposition (PVD), chemical vapor deposition (CVD), electrophoretic deposition (EPD), electrodeposition (ED), or sol-gel methods [2]. Among these, PVD is recommended to deposit metal/ceramic materials over the implant surface and provide exact stoichiometry, excellent adhesion, high density, and good uniformity. Another method for surface modification other than coating methods is chemical etching to prevent bacterial adhesion and improve osseointegration [126].

The success of an implant is dependent on the stability of the coating, which provides better biocompatibility. This section is focused on the recent advancements in various types of ceramic and polymer coatings to improve bioimplant performance and reliability.

Figure 1. The role of bioactive coated metallic implants as a potential implant material [127]. The qualities of coated implants are superior to those of uncoated metallic implants. **Figure 1.** The role of bioactive coated metallic implants as a potential implant material [127]. The qualities of coated implants are superior to those of uncoated metallic implants.

sition (PVD), chemical vapor deposition (CVD), electrophoretic deposition (EPD), electrodeposition (ED), or sol-gel methods [2]. Among these, PVD is recommended to deposit metal/ceramic materials over the implant surface and provide exact stoichiometry, excellent adhesion, high density, and good uniformity. Another method for surface modification other than coating methods is chemical etching to prevent bacterial adhesion and

The success of an implant is dependent on the stability of the coating, which provides better biocompatibility. This section is focused on the recent advancements in various types of ceramic and polymer coatings to improve bioimplant performance and reliability.

#### 3.1. Polyether Ether Ketone (PEEK) *3.1. Polyether Ether Ketone (PEEK)*

improve osseointegration [126].

PEEK is a thermoplastic material that shows a combination of excellent stiffness, chemical and physical properties, and toughness and offers a wide range of applications [128]. Therefore, it is widely used as a bone substitute in orthopedic and dental implants, and in clamps for removable dental prostheses [129]. The PEEK coated substrates show better tribological properties, which are useful for the development of coatings on light weight alloys which lack tribological performance. Most of the sliding and bearing implant materials are coated with PEEK due to its better wear resistance and thermal stability [130,131]. Generally, PEEK coating and its composites are prepared using thermal spraying or electrophoretic processes [132–135]. PEEK coating (70–90 µm thick) deposited through electrophoretic deposition on the Ti-13Nb-13Zr titanium alloy showed excellent wear resistance, 200 times higher than the uncoated alloy [136]. PEEK is a thermoplastic material that shows a combination of excellent stiffness, chemical and physical properties, and toughness and offers a wide range of applications [128]. Therefore, it is widely used as a bone substitute in orthopedic and dental implants, and in clamps for removable dental prostheses [129]. The PEEK coated substrates show better tribological properties, which are useful for the development of coatings on light weight alloys which lack tribological performance. Most of the sliding and bearing implant materials are coated with PEEK due to its better wear resistance and thermal stability [130,131]. Generally, PEEK coating and its composites are prepared using thermal spraying or electrophoretic processes [132–135]. PEEK coating (70–90 µm thick) deposited through electrophoretic deposition on the Ti-13Nb-13Zr titanium alloy showed excellent wear resistance, 200 times higher than the uncoated alloy [136].

PEEK in combination with other bioactive materials shows better antibacterial activity than PEEK alone [137]. Many authors reported on PEEK-based composite coatings on metallic substrates. These coatings enhance bioactivity and electrochemical corrosion resistance, especially for implant structural components. Typical examples for the composite coatings are TiO2/PEEK [138], sol-gel glass/PEEK [139], bioactive glass/PEEK [140], h-BN/PEEK [141], Ag/bioactive glass/PEEK [142], and h-BN/bioactive glass/PEEK coatings PEEK in combination with other bioactive materials shows better antibacterial activity than PEEK alone [137]. Many authors reported on PEEK-based composite coatings on metallic substrates. These coatings enhance bioactivity and electrochemical corrosion resistance, especially for implant structural components. Typical examples for the composite coatings are TiO2/PEEK [138], sol-gel glass/PEEK [139], bioactive glass/PEEK [140], h-BN/PEEK [141], Ag/bioactive glass/PEEK [142], and h-BN/bioactive glass/PEEK coatings [137]. A combination of bioactive glass embedded in a polymeric matrix of PEEK makes it an interesting material for orthopedic applications as it meets biological and biomechanical requirements for the application. A cold sprayed Bioglass/PEEK composite prepared by Garrido et al. [143] showed an increase in wear resistance by more than 70%, higher hardness, and a lower coefficient of friction compared to pure PEEK. Coatings based on Bioglass/PEEK on porous Ti substrates resulted in higher adhesion between Bioglass/PEEK coating and Ti substrates [144].

Flame sprayed hexagonal boron nitride (h-BN) incorporated PEEK coating on lowcarbon steel substrate increased the hardness and decreased wear and frictional coefficient values for the composite coating containing 8 wt. % h-BN due to its self-lubrication prop-

erties [145]. The coefficient of the friction value can also be reduced by the addition of alumina. The Al2O3/PEEK composite coating deposited on a Ti alloy using electrophoretic deposition showed increased corrosion resistance and significantly improved wear resistance under dry sliding conditions. The viability test revealed that the Al2O3/PEEK coating was found to be cytocompatible with MG-63 osteoblast cells [146]. The scratch resistance of PEEK coatings can be increased with the addition of amorphous Si3N<sup>4</sup> nanoparticles. Tomasz et al. [147] performed the electrophoretic deposition of the PEEK/Si3N<sup>4</sup> nanocomposite using a chitosan stabilizer: the coating showed higher scratch resistance than PEEK coating alone. This suggests that PEEK-based nanocomposite coatings potentially improve the bioactive as well as bio-tribological performance of Ti-based alloys used in biomedical applications. The use of PEEK with HAp as a coating can reduce the stress shielding effect. The combination of PEEK/HAp offers similar stiffness to that of the bone tissue. Recent studies suggest that the incorporation of HAp into PEEK coating improves bioactivity and mechanical properties [148]. PEEK coating prepared by different methods and their properties are summarized in Table 3.

**Table 3.** Methods and properties of PEEK-based composite coatings.

