Multi-functional bioactive silver-and copper-doped diamond-like carbon coatings for medical implants

: Diamond-like carbon (DLC) coatings doped with bioactive elements of silver (Ag) and copper (Cu) have been receiving increasing attention in the last decade, particularly in the last 5 years, due to their potential to offer a combination of enhanced antimicrobial and mechanical performance. These multi-functional bioactive DLC coatings offer great potential to impart the next generation of load-bearing medical implants with improved wear resistance and strong potency against microbial infections. This review begins with an overview of the status and issues with current total joint implant materials and the state-of-the art in DLC coatings and their application to medical implants. A detailed discussion of recent advances in wear resistant bioactive DLC coatings is then presented with a focus on doping the DLC matrix with controlled quantities of Ag and Cu elements. It is shown that both Ag and Cu doping can impart strong antimicrobial potency against a range of Gram-positive and Gram-negative bacteria, but this is always accompanied so far by a reduction in mechanical performance of the DLC coating matrix. The article concludes with discussion of potential synthesis methods to accurately control bioactive element doping without jeopardising mechanical properties and gives an outlook to the potential long-term impact of developing a superior multifunctional bioactive DLC coating on implant device performance and patient health and wellbeing.

*Correspondence: martin.birkett@northumbria.ac.uk Abstract: Diamond-like carbon (DLC) coatings doped with bioactive elements of silver (Ag) and copper (Cu) have been receiving increasing attention in the last decade, particularly in the last 5 years, due to their potential to offer a combination of enhanced antimicrobial and mechanical performance. These multi-functional bioactive DLC coatings offer great potential to impart the next generation of load-bearing medical implants with improved wear resistance and strong potency against microbial infections. This review begins with an overview of the status and issues with current total joint implant materials and the state-of-the art in DLC coatings and their application to medical implants. A detailed discussion of recent advances in wear resistant bioactive DLC coatings is then presented with a focus on doping the DLC matrix with controlled quantities of Ag and Cu elements. It is shown that both Ag and Cu doping can impart strong antimicrobial potency against a range of Gram-positive and Gram-negative bacteria, but this is always accompanied so far by a reduction in mechanical performance of the DLC coating matrix. The article concludes with discussion of potential synthesis methods to accurately control bioactive element doping without jeopardising mechanical properties and gives an outlook to the potential long-term impact of developing a superior multifunctional bioactive DLC coating on implant device performance and patient health and wellbeing.

Introduction
Global interest in multifunctional bioactive coatings to improve the performance and extend the lifetime of total joint implants is growing at an exponential rate. There were over 5.6 million total joint arthroplasty (TJA) surgeries registered in Europe by 2017 [1] and the annual rate of TJA surgery has been predicted to increase by as much as 100% over the next 30 years [2]. Globally, over 40% of people undergoing TJA surgery are under 65 years of age due to low bone density and osteoporosis, with the incidence among this age group expected to further increase [3].
Titanium (Ti) implants are widely used for load-bearing TJA applications because of their superior properties such as high strength to weight ratio, low modulus of elasticity, corrosion resistance, and biocompatibility. In general, metallic heads articulating against ultra-high molecular weight polyethylene (UHMWPE) or cross-linked polyethylene (XLPE), as well as metal/metal combinations, generate tribocorrosive particles that cause aggressive inflammation and lead to periprosthetic osteolysis [1]. These problems associated with high component wear rates and subsequent metal poisoning (metallosis) of the patient were clearly highlighted by the recent catastrophic failure of the Johnson & Johnson, DePuy metal-on-metal hip implants and subsequent lawsuits (>€3.5 billion to date) [4]. The other critical disadvantage of these implant designs is decreasing lifetime due to implant associated infections (IAIs), which form biofilms followed by severe inflammation [5] and in most of these conditions, the patients will require revision surgery. One of the main IAIs is osteomyelitis, which develops primarily by nosocomial pathogen invasion of the implant area during orthopaedic surgery and has a mortality risk that scales enormously with age [5]. Research has shown that the death rate of patients between the ages of 65 and 74 with chronic osteomyelitis is ~24% and this figure nearly doubled for those aged over 85 [6]. Traditionally, IAIs can be fought using antibiotics but the increase in the emergence of antibiotic resistant microbial strains render them ineffective and limit their use [7][8][9]. Figure 1 illustrates the post-surgery development of wear/corrosion and biofilm formation in a total knee replacement (TKR) with a metallic femoral component articulating against a UHMWPE tibial plate. Both conditions can ultimately result in failure of the implant and require costly revision surgery for the patient. Globally, the estimated failure rate of total joint implants ten years post-operatively is approximately 5% with as high as 30-50% of these failures reportedly caused by periprosthetic osteolysis induced by wear debris [10][11][12][13].
One potential solution against IAIs which has been receiving increasing attention, is the use of antimicrobial coatings. The global market for antimicrobial coatings for medical devices is expected to grow at a Compound Annual Growth Rate (CAGR) of 15.7 % and reach €2.45 billion by 2026 [14]. Such coatings may also have the potential to significantly reduce IAIs if the surfaces of Ti metal hip, knee and shoulder implants etc. could be protected through their application. However, in addition to antimicrobial properties, these multifunctional coatings also need to be biocompatible and exhibit high mechanical wear-resistance. Diamond-like carbon (DLC) coatings are renowned for their high mechanical hardness (>20 GPa) and their revenue is projected to reach ~ £2 billion by 2025 [16]. These coatings are hard, wear resistant, chemically inert and biocompatible at the same time. Such carbon-based materials are also well-known for their potent antimicrobial action and are characterized by a low risk of antimicrobial resistance [17]. Their antimicrobial action is often attributed to a combination of several physical and chemical mechanisms: directly on the microbial particle such as (peptidoglycan membrane structure disruption, entrapment of microorganisms, transfer of electrons) and/or indirectly through induction of oxidative stress by reactive oxygen species (ROS) [18,19]. Therefore, DLC coatings possess broad-spectrum antimicrobial and antibiofilm properties that are very promising in providing a long lasting solution in the field of advanced medical implants [20]. However, although DLC coatings are intrinsically biocompatible, they are not bioactive to respond to inflammation [21,22]. Hence, the antimicrobial action of DLC can be further enriched with the incorporation of bactericidal ions such as to gain triple antimicrobial potency in terms of antibacterial and anti-inflammatory activities as well as antibiofilm formation. Previous research has investigated doping DLC coatings with small quantities of fluorine [23], germanium [24], silicon [25], titanium dioxide [26] and zinc oxide [27] but the most studied active bactericidal elements are silver (Ag) [28], copper (Cu) [29] and their combination (Ag/Cu) [30]. Moreover, mechanically hard DLC coatings are known to have poor toughness which reduces their useable life and durability and DLC coating failures have been observed on retrieved hip implant femoral heads [31]. Hence, addition of softer Ag/Cu metallic particles can reduce residual stress and improve toughness to prolong the durability and lifetime of DLC coated implant surfaces [32]. Thus, DLC coatings doped with bioactive elements of Ag and Cu have been receiving attention for the last decade, particularly in the last 5 years due to their potential to offer superior antimicrobial and mechanical performance.
There are several very recent reviews on advancements in surface modifications and coatings for medical implants [33][34][35][36] which give comprehensive overviews on surface texturing, porous surfaces, ion implantation and various commercially available and currently researched nitride (CrN, SiN, TaN, TiN, Zr), carbide (TiC, ZrC), carbonitride (TiCN, ZrCN), oxide (ZrO 2 ) and DLC coatings. However, these reviews only briefly discuss the potential of DLC coatings for medical implants and do not consider the effect of doping them with bioactive metal elements on their antimicrobial and mechanical performance. Therefore, this article provides a critical review of the state-of-the-art in Ag and Cu doped DLC coatings for application to medical implant devices. It begins with an overview of the current applications of DLC coatings in implant technology and is then followed by a detailed discussion of Ag/Cu doped DLC coatings with particular focus on the relationship between their mechanical and antimicrobial performance. Finally, it ends with a discussion on the potential long-term impact of developing a truly multifunctional wear resistant bioactive DLC coating to extend the lifetime of total joint implants.

Application of DLC coatings in total joint implant technology
Load bearing surfaces of total hip and knee joint implants work under complex mechanical, tribological, and biological environments and require a combination of high hardness, high Young's modulus; low friction coefficient, high wear resistance, and durability against high contact loads; high albumin/fibrinogen ratios and non-cytotoxic profiles. In this context, numerous implant materials are currently in use such as metals (stainless steel, Ti6Al4V, CoCr), ceramics (alumina, zirconia) and polymers (UHMWPE, XLPE, PEEK). These materials are used in combination in implant bearing pairs like metal-on-polymer (MoP), metal-on-metal (MoM), ceramic-on-ceramic (CoC), ceramic-on-polymer (CoP) and more recently hybrid combinations such as ceramic-on-metal (CoM) have been investigated and commercially developed [37], all with their own specific advantages and limitations. Generally, metallic implants have higher weight, poor tribo-corrosion performance, and can release excessive cytotoxic metal ions under in-vivo environments. Ceramics are hard, wear-resistant and can bear high contact stresses but they can suffer from rim chipping, squeaking and lack functionality for load-carrying implants due to higher brittleness and change in material phases under bodily fluid that increases the risk of fracture on impact and under torsional loads [38]. CoP and MoP bearings have received increasing attention in recent years due to the advantage of subject-specific customization of polymeric materials. They are used in the current state-of-the-art hip implant products such as the Stryker LFIT™ (CoCr head on PE liner), the DePuy Synthes CORAIL® Pinnacle (CoCr/ceramic head on XLPE liner) and the Smith & Nephew VERILAST ◊ (Oxidised Zirconium (OXINIUM) on XLPE liner). However, long term durability against high contact stresses, excessive release of material debris, oxidation stability, and lack of antimicrobial properties are current limitations of CoP and MoP technologies. Hence, typical metal, alloy, ceramic, and polymer materials have certain limitations for implant applications [39]. Therefore, these materials are incrementally coated with hard, bioactive materials [40], not only to reduce their intrinsic limitations (e.g., tribo-corrosion, metallization, release of debris, etc.,) but also to promote mechanical compliance with the host patient and bio-activation for antimicrobial features [41].
DLC coatings have received increasing attention in the biomechanical sector in recent years [42,43] due to their simultaneous bio-mechanical and bio-tribological performance. Diamond, graphite, graphene, and nanotubes are common crystalline allotropes of carbon. However, DLC is an amorphous variant of carbon materials which is a mixture of sp 1 , sp 2 , and sp 3 phases [44]. The carbon sp 3 networks bridge the sp 2 graphitic nanoclusters in the coating. Usually, sp 2 and sp 3 phase proportions determine their diamond-like (sp 3 enriched) and graphite-like or polymeric (sp 2 enriched) behaviours. DLC films are made with numerous types of deposition methods as described in section 4. The DLC coating growth and proportion of sp 2 and sp 3 phases highly depend on the deposition method. The higher sp 3 amount is supposed to give higher hardness, Young's modulus and density of DLC coatings [45,46]. Carbon species i.e., atoms and ions having optimum energy of 100 eV [47,48] are demonstrated to grow the highest amount of sp 3 networks. This optimum energy varies for deposition methods and depends on types of electrification to electrodes, precursors, electrostatic, magnetic, and kinetic potentials, and level of vacuum pressure, temperature, humidity etc. which regulate plasma dynamics such as ionization energies, mean free path, velocities and momentum of carbon atoms and ions.
DLC coatings have received their name of 'diamond-like' on the basis of their superior mechanical performance rather than their atomic structure. DLC coatings have been demonstrated with high sp 3 amounts beyond 80%, having hardness as high as 70 GPa and Young's modulus of 630 GPa, respectively [49]. Similarly, these coatings not only have lower friction coefficients ~ 0.1 [50] but their friction coefficient remains consistent over a prolonged aging time [51]. DLC coatings have shown only about a 30 % friction coefficient when compared to steel, TiCN, TiAl, and TiN materials [52]. In the same way, DLC coatings offer incredibly low wear rates [53], good impact resistance [54], and ability to withstand high contact pressures [55]. DLC is doped with foreign elements like silicon and nitrogen to further elevate their bio-tribological and corrosion performance [56]. DLC atomic structure is composed of disordered carbon atomic rings [57] and chain structures which completely differ from the well-ordered carbon atomic structure of diamond, which is also presented in the literature with atomistic studies [58]. The amorphous structure of DLC demonstrates lower carbon ion release rates [59] than other carbon variants or nanocomposites which make it suitable for biomedical applications. Further, DLC coatings have shown superior biological performance over other types of materials. A significantly higher albumin/fibrinogen adsorption of DLC material when compared to Ti, TiN, TiC, silicon elastomer, poly(methyl methacrylate), carbon nitride [60] and polycarbonate [61] ensures higher resistance to thrombus formation. DLC coatings have significantly lower amounts of platelets when compared to polycarbonate [61] and polytetrafluoroethylene [62]. Therefore, the combination of superior biomedical properties and mechanical performance make them an attractive choice for load-carrying [63] and non-load carrying [64] implants and biochemical sensors [65]. Even though DLC coatings exhibit superior mechanical and biomedical performance, they also evidence higher compressive residual stress which stimulates coating failure and delamination from the interface [31,66,67]. Therefore, developing durable DLC coatings for mechanical and biomedical applications is an active research area.
While most research to date has focussed on coating metallic implant components, recent studies by Rothammer et al. have shown that applying DLC coatings to the polymer counter-body surface of MoP joints could also be of decisive importance for enhancing the wear performance and increasing the service life of load-bearing implants [42,43,73,74]. Their initial research showed that, although coating the UHMWPE surface leads to an increase in friction due to considerably higher surface roughness, there is a significant reduction in wear of up to 49% and 77% when articulating against CoCr and Ti counter-bodies, respectively [43]. Their very recent work suggests that DLC coating of only one side of the MoP joint can result in higher wear of the uncoated counter-body and substantial wear reduction can be achieved when both polymer and metallic components of the joint are coated [73,74].
Another important consideration in understanding the wear mechanisms occurring in artificial implants is the interaction of the joint surfaces with the surrounding periprosthetic fluid (pseudo-synovial fluid), lubricant [75,76]. The main organic components in periprosthetic fluid, bovine serum albumin (BSA), human γ-globulin (HGG), and hyaluronic acid (HA), are adsorbed onto the joint surface, and they influence the friction and wear behavior of the bearing surfaces [77]. There have been several recent studies on the interaction between DLC coatings with periprosthetic fluid and formation of lubricating surface films [77][78][79]. Wu et al. [77] characterized the adsorption of BSA, HGG and HA molecules on a DLC film surface using a quartz crystal microbalance with dissipation and antigen-antibody reactions. They found that BSA and HGG monolayers could be formed on the DLC film surface with side-on and end-on orientations, respectively, whereas HA molecules were hardly adsorbed. They also assessed the frictional behavior of the DLC film sliding against an Al 2 O 3 ball in a mixture of BSA, HGG and HA and found that the HGG molecule was the main component that formed the lubricating surface film. More recently, Jing et al [78] studied the mechanism of protein biofilm formation on 10 at.%. Ag-DLC films prepared using a hybrid deposition technique for hardwearing joint implant applications. Their results showed that when the Ag-DLC films are exposed to physiological solutions they release Ag + ions which promote the formation of a biofilm of denatured proteins at the friction interface of the film leading to improved wear resistance.
Although there has been significant research on DLC coatings in recent years, and several commercial products have been developed to exploit its superior mechanical and biocompatibility features, there is still a need to systematically explore and develop a multifunctional wear resistant bioactive DLC coatings for application on load-bearing orthopaedic implants.

Multifunctional wear resistant bioactive DLC coatings
Nowadays, the deposition of coatings on Ti implants is considered a prerequisite to improving their surface properties, including wear resistance, accelerating osseointegration and reducing bacterial infections. Transition metal nitride-based ceramic coatings have been widely used for medical devices due to their high hardness and wear-resistance and Ag and Cu can be added to them to achieve antimicrobial and self-lubrication functionalities [80]. Enhanced antimicrobial activity with increase in the content of Ag and Cu nano-particles was observed for TiN/Ag, ZrN/Ag, CrN/Ag, TiN/Cu, [81] and TiAlN(Ag-Cu) [82] coatings, but was accompanied with a reduction in mechanical hardness (Fig 3), which can lead to premature coating failure. High Ag and Cu content is also known to be toxic to humans and the environment [83]. Thus, regulating the amount of Ag and Cu to achieve antimicrobial efficacy and mechanical strength simultaneously is a major limitation in the present state-of-the-art.
Another issue is the emergence of multidrug resistant microbial strains at an alarming rate. The synergistic effect of Ag and Cu nanoparticles has shown enhanced microbicidal efficacy against a wide range of antibiotic resistant strains compared to individual Ag or Cu [84,85]. Even when the concentration of Ag is reduced by 10-fold, a high level of bactericidal effect is still maintained via the Ag and Cu combination [86]. Hence, the key advantage of this unique combination of Ag and Cu is that it enables tailoring of the coating structure at a lower composition of these soft metals to achieve antimicrobial efficacy and mechanical strength simultaneously. In addition to enhanced antimicrobial activity, the co-existence of Ag and Cu could also provide a sustained supply of these elements on the coating surface to enhance self-lubrication of medical devices [87]. Despite the promising response of the Ag-Cu system, its potential has not been properly explored in the development of antimicrobial coatings for invasive medical devices and consequently there are very few published reports on their combined effect [82,88] and none addressing the effect of their chemical ratio on thin film properties. It is therefore essential to uncover the structure dependent mechanism behind the synergy of the Ag-Cu system. A unique coating architecture with enhanced tribological and antimicrobial properties could be achieved by combining Ag-Cu into a DLC coating matrix, if the deposition conditions and Ag:Cu ratio are properly coordinated [87,88]. Thus, by developing an optimized deposition technique, a nanocomposite structure of Cu-Ag-DLC coating could be formed, consisting of Cu and Ag nanoparticles, dispersed in the DLC matrix. Hence, this nanocomposite system could effectively utilize the overall mechanical strength provided by Cu-Ag-DLC as well as the antimicrobial functionalities of Ag and Cu. The function of the DLC matrix is to provide improved hardness, wear and corrosion resistance behavior to the surface of the implant in addition to its inherent antimicrobial and biocompatibility properties, while controlled release of biocidal Ag/Cu ions around the implant surface would effectively inhibit the bacterial biofilm growth during the initial implant surgery stage without causing cytotoxicity. The synergistic effect of the Ag and Cu combination provides the ability to fight against a wide strain of bacteria; even at a very low content, which is an added advantage over Ag-DLC and Cu-DLC coatings. Moreover, addition of softer Ag/Cu metallic particles can reduce residual stress and improve toughness while providing self-lubrication to prolong the durability and lifetime of coated implant surfaces.
DLC coatings are carbon-based amorphous materials which display similar properties to diamond, like low friction coefficient and high hardness due to their sp3 carbon bonds. DLC coatings have received much attention for TJA applications due to their remarkable wear resistance, antimicrobial activity and biocompatible properties. Despite their advantages, they have poor adhesion properties due to high internal stress (~10 GPa), resulting in coating failure in the physiological environment under dynamic loading conditions [20]. Recently, nanocomposite DLC coatings with suitable crystalline metallic/ceramic materials embedded in an amorphous DLC matrix, are gaining significant interest to reduce the internal stress of conventional DLC coatings. Incorporation of transition metals such as Ag and Cu to create DLC nanocomposite coatings has been shown to reduce the internal stress [89]. For example, Wu et al. observed a decrease in residual stress from ~1.0 to 0.3 GPa with a Ag concentration of 11 at.% in DLC films [90]. Interestingly, Ag-DLC coatings effectively provide an enhanced synergistic antimicrobial platform capable of inhibiting the formation of bacterial colonies with the release of Ag+ metal ions. However, higher Ag content (>10 at.%) is shown to drastically decrease the hardness of these coatings from 17 to 6 GPa [91] (Fig 4b) and 19.6 to 9.2 GPa [92], and higher doses of Ag+ ions are also potentially toxic to the hosting tissues. For example, Ag-DLC coatings have been shown to resist the formation of bacterial colonies with rapid Ag+ ion release, reducing the growth of surface-bound and planktonic Staphylococcus aureus and Staphylococcus epidermidis [93].
Combining the unique mechanical strength, non-stick features, and antimicrobial activity of DLC coatings with the enhanced antimicrobial properties of Ag and Cu has the potential to significantly reduce the spread of IAIs and provide a long-lasting solution in the implant industry. However, existing studies on these coatings have all required relatively large quantities of Ag/Cu doping to achieve the required antimicrobial performance due to the tendency of their particles to agglomerate during the manufacturing process, which reduces their relative available surface area for ion release. These clusters disrupt the uniformity of the coating structure, significantly reducing their mechanical performance and durability. Reducing ion release rates and mechanical performance are attributed to the size and morphology of the doped elements. For example, Ag-DLC nanocomposite coatings have shown higher Ag + release rates than pure Ag coatings and the difference could reach as high as 100 % after 24 hours of leaching time [59] (Fig 4f). Similarly, doping of 15 % Ag into DLC without control over size, shape, and morphology i.e., using a conventional co-sputtering method, significantly reduces overall Ag-DLC coating hardness from 17 to 6 GPa and Young's modulus from 180 to 90 GPa [91] (Fig 4e). The concept of developing multifunctional nanocomposite coatings composed of these suitable elements (DLC with Ag and Cu) with optimal concentration is of great technological interest to improve the wear resistance of the articulating surface and the antibacterial efficiency and biocompatibility of the implants.

Multifunctional bioactive DLC coating synthesis
Generally, DLC coatings are made with physical vapor deposition (PVD) and plasma-enhanced physical vapor deposition (PECVD) methods. Table 1 presents the common PVD and PECVD methods to deposit DLC coatings, such as sputtering and its various types; arc discharge and filtered (e)

(f) (g)
cathodic vacuum arc systems which use C and S bends to filter microparticles and impurities; ion and laser beam based methods, and hybrids of the aforementioned techniques. There are other methods to fabricate DLC coatings such as a wide range of atmospheric plasma systems including dielectric barrier discharge [94], electrolysis [95], microwave resonator [96], and electrodeposition [97] etc. However, these methods are not as commercially developed as PVD/PECVD for biomedical coatings due to certain limitations in handling large surface area, complex geometries, dimensional control, flexibility for a range of multilayers or doped coating designs, large volume and batch productions, deposition cost per unit product, and quality of the DLC film. The atmospheric plasma systems have potential to make DLC coatings for non-load-carrying applications such as grafts and cotton dressings [98] etc. However, lower sp 3 phases are usually produced in DLC coatings made with atmospheric plasma systems which compromise mechanical and tribological properties, particularly for load-carrying biomedical applications. Table 1 also presents the common deposition parameters which influence the DLC growth and determine the coating properties and performance.  Figure 5 presents the typical growth models for thin film DLC coatings prepared by PVD and PECVD methods. The coating growth pattern highly depends on the plasma kinetics, deposition conditions, and physio-chemical characteristics of elements as an electron affinity. The Volmer-Weber model is also called island or three-dimensional growth and the coating grows in this pattern when the atomic bonding between adjoining atoms is stronger than the new bonding formed between incoming atoms and the substrate. The coatings grow layer-by-layer or in epitaxy [136] in a Frank-van der Merwe model which is attributed to the non-linear kinetics. The Frank-van der Merwe model is also called two-dimensional growth model. The Stranski-Krastanov growth model is a hybrid of both the Volmer-weber and Frank-van der Merwe models. The coating first grows layer-by-layer for a certain thickness following the Frank-van der Merwe model and then grows as a three-dimensional island according to the Volmer-weber growth pattern. Stranski-Krastanov (c). Adopted from: [137].
Most Ag and Cu doped bioactive DLC coatings are made with sputtering, vacuum arc, pulsed laser deposition, ion beam implantation and their hybrid methods using solid target materials. Basically, the materials to be deposited are transported from the source surface to the substrate surface under a high vacuum and the transport mechanisms are characteristic of each deposition method. Figure 6 illustrates the process schematics of sputtering, pulsed laser deposition, filtered cathodic vacuum arc, and hybrid sputtering/ arc methods. Referring to Figure 6A, a direct current or radio frequency power is supplied to the magnetron. A target made of the material to be deposited is mounted on the magnetron and works as a cathode. The substrate holder is the anode and is often supplied with a bias voltage to boost plasma kinetics. A process gas, normally argon (Ar), is used to generate a plasma. The Ar atoms and ions are bombarded on the target surface which sputters the target material. The sputtered atoms are attracted by the anode due to the application of electric potential. The incoming atoms grow atomic layers on the substrate surface according to the thin film growth models described in Figure 5.  [138], (B) [139], (C) [140], (D) [141]. Figure 7 illustrates a typical PECVD process which differs from PVD due to the use of gaseous precursors instead of a solid target material. The reactive gases are introduced between the electrodes in addition to the process gas. The radio frequency electric potential ionizes the reactive gases and the chemical reactions form the atomic vapors of materials. The ionizing energy may vary with the type of precursor. Hence, a matching box is used to regulate the capacitive load. The atomic vapor condenses on the substrate surface to form a coating layer. A substrate heater is used to improve the coating quality which is attributed to the thermal migration of atoms. More details on the deposition of antimicrobial DLC coatings made with PECVD methods can be found in the literature studies [142,143].  [144,145] are other types of metal and metal oxide bioactive agents used to fabricate antimicrobial coatings. Similarly, the antimicrobial coatings are also made with 2D materials like MoS2, nano clay, graphene, graphene oxide, boron nitride and phosphorus-based coatings. Another category is composite coatings such as Ag-doped hydroxyapatites/carbon nanotubes, gold nanocomposites with amino acids etc [145]. These materials also perform reasonable antimicrobial actions but may have limitations for in vivo biocompatibility. The excessive ion release rates from 2D nanomaterials may produce local toxicity; often these materials may need photo, thermal, and magnetic activations to perform biocidal actions which limits their application on invasive biomedical implants; the availability of structure-property data for coating hardness, adhesion, and tribology over a prolonged in vivo life is limited. Therefore, DLC coatings as a matrix incorporating bioactive agents have the potential for a wide range of biomedical applications such as artificial implants [146], medical patches [147], textiles [148], space and other industrial sectors due to their proven unique combination of mechanical, tribological and biomedical properties. Besides sputtering, pulsed laser deposition, and hybrid plasma, the bioactive DLC coatings are also made with ion irradiation [149] and Plasma immersion ion implantation [150,151] for antimicrobial activity.  E. coli Space applications [30] Although Ag/Cu-DLC coatings can be produced with different materials deposition techniques, the simultaneous or co-sputtering of carbon and Ag or Cu is perhaps the most commonly used method.
When depositing carbon at a higher deposition rate than Ag or Cu, the Ag/Cu is continuously doped inside the carbon matrix during the sputtering process. Figure 8 shows tunnelling electron and scanning electron micrographs of Ag/Cu-DLC coatings, illustrating the current state-of-the-art in manufacture of these materials. It can be observed that the Ag or Cu received in the DLC matrix may have different physical forms, like particles, clusters, flakes and agglomerations of random sizes and distributions. Thus, the current manufacturing methods used to produce Ag/Cu-DLC coatings have limited control over Ag/Cu distribution and specified physical forms. Scanning electron microscope image of Cu-DLC coating. Reproduced with permission from: (A) [93], (B) [59], (C) [160].
A large amount of Ag or Cu is known to have good antibacterial performance. For example, it has been reported that the bacteria level reduces from 8.6×10 3 to 2.4×10 3 to colony forming units per mL (cfu/mL) by doping up to 23% Ag in a carbon coating [60]. However, such large amounts of Ag or Cu in the DLC matrix, delivered due to the lack of control in current deposition processes, greatly reduces the mechanical properties, durability, and service life of the resulting Ag/Cu-DLC coating.
Solid mechanics also plays an important role in the mechanical design of Ag/Cu-DLC coatings. A large amount of Ag or Cu brings ductility in the DLC matrix. The coatings become soft and lose Young's modulus, bringing larger elastic strains. Ultimately, the coating loses its unique combination of strength to ductility and thus toughness declines as well, leading to failure at lower cracking thresholds. Besides the amount, the physical forms of Ag/Cu doped in the DLC matrix are equally responsible for controlling mechanical strength. In co-sputtering, the Ag or Cu is received in the matrix in random forms like particles of different sizes, clusters and flakes etc. with random distributions. This heterogeneous mixture produces weak Ag/Cu matrix interfaces, defects and porosities, which act as fracture initiation sites, eventually leading to failure of the coating. The cracking failure is known to damage the coating and is a result of either the Ag/Cu particles being too small to withstand the fracture energy of the propagating crack or the Ag/Cu clusters being large enough to produce localized ductility and cracking initiation from these soft spots [161]. Thus, large amounts and irregular forms of Ag or Cu reduce the hardness, toughness, and wear resistance of Ag/Cu-DLC coatings.
One potential route to overcome this problem is if the bioactive Ag/Cu element could maintain its antimicrobial behaviour at smaller quantities in the DLC matrix, thus preserving the material's mechanical performance. Recently, Merker et al. reported on a new fabrication technique to embed Ag nanodroplets in the surface of ultra-nanocrystalline diamond (UNCD) coatings [162]. They used a five-step process to fabricate the coatings, which includes the deposition of a continuous UNCD film followed by a continuous Ag film. The Ag layer is then etched to produce Ag nanodroplets, which are again covered with a UNCD capping layer (Fig 9a). Although this work did not consider the mechanical properties of these coatings, the results highlighted that restricting Ag-droplets to the surface of the film may provide sufficient Ag ion release and antimicrobial performance (Figs 9b and 9c) without jeopardizing the mechanical properties of the coatings. Even though there are associated concerns with this technique, such as the extensive five-stage fabrication process and the costly slurry required for seeding, especially with respect to scale-up and commercialization of this technology, the work highlighted the possibility of doping an optimized amount of Ag or Cu at a specified location in the coating matrix to fabricate wear resistant bioactive DLC coatings containing low nontoxic levels of precisely distributed and isolated antimicrobial Ag and Cu nanoparticles.
DLC is a suitable choice of coating matrix material due to its proven mechanical properties and biocompatibility [163][164][165][166]. A unique coating architecture could be achieved with lower amounts of Ag/Cu and control over shape, size, morphology, distributions, and optimal ratio of Ag:Cu. It was previously reported that a Ag content of 4 at.% has shown high antimicrobial activity while maintaining the hardness value of 15 GPa for a Ag-TiN coating which could reduce the toxicity and is suitable for medical devices [167]. The ratio of 50:50 Ag:Cu has also been shown to give the best results in terms of antimicrobial activity against a wide range of microbial strains for TiO 2 /Ag/Cu coatings [168]. As the higher detachment of Cu leaves a transportation path for Ag, the coordination between their respective content or Ag:Cu ratio along with substrate temperature, governs their transportation mechanism [169] but the effect of its variation (Ag:Cu) was not previously investigated.
If the Ag-Cu content could be controlled in the range 2-5 at. % and the Ag:Cu ratio varied it would allow exploration of the synergistic combination of Ag and Cu to enhance antimicrobial activity against a wide range of multidrug resistant microbial strains and enable a reduction in Ag and Cu content (currently one of the biggest challenges) to the lowest possible extent (<5 at.%) to achieve an optimized coating structure combining enhanced mechanical hardness, wear-resistance, antimicrobial efficacy and biocompatibility. Reproduced with permission from [162].

Outlook and long-term impact potential of a multifunctional bioactive DLC coating
The development of a hardwearing and bioactive DLC coating material is vital for extending the lifetime of medical implants and reducing the number of implant associated infections (IAI). The potential impact of such a technology is far reaching. It will be of significant interest to both academic research groups and biomedical companies alike and could have a profound societal impact by reducing the risk of infection and improving the lifestyle and extending the lifetime of patients requiring medical implants.
Such a multifunctional bioactive DLC coating would be sustainable-by-design as its key features are its high wear resistant DLC matrix to extend the lifetime of medical implants and its bioactive Ag/Cu elements to reduce the risk of post implant surgery infections. Both of these features would allow the DLC coated implant to remain inside the patient for longer than conventional implants, thus reducing the number of costly and traumatic revision surgeries. This will in turn reduce the load on the implant device supply chain by only producing the DLC coated implants in the required volumes without depleting non-renewable raw material resources. Furthermore, the bioactive DLC coating will also provide a non-toxic/pollutant solution, which is a prerequisite for any in-vivo material. The main DLC coating element, graphite, is known to be non-toxic to humans and highly biocompatible and the underlying titanium implant is considered as the most biocompatible, non-toxic metal. Silver and especially copper are known to be toxic in higher doses, so their concentrations would need to be carefully controlled in the bioactive DLC coating to provide sufficient antimicrobial efficacy without jeopardizing the overall biocompatibility of the material. A new bioactive DLC coating could also contribute to a circular economy by providing a technology that is restorative and regenerative by design, which will present an extended lifetime by including a whole life cycle perspective and sustainability indicators from the early stages of product and production process development. A unique hard wearing, bioactive DLC coating could design out waste and pollution, keep materials and medical products in use for longer and minimise disruption to the established steady-state equilibrium of the environment and key natural resource systems. Moreover, although current legislation stipulates single use of artificial joints, in the future it is anticipated that if a coated implant eventually wears out then the device could be recovered from the patient and recycled for future use by re-coating it with a new bioactive DLC layer.
A multifunctional bioactive DLC coating has the potential to provide several long-term wider economic/technological and societal impacts if the technology could be successfully developed and used to coat the next generation of load bearing orthopaedic implants. There were 1.8 million total hip replacement (THR) surgeries registered in OECD countries in 2015 and this figure is projected to increase to 2.8 million per year by 2050 [2], with the cost of surgery ranging from €5,00 to €15,000 (average €10,000) [170,171]. The estimated failure rate of THR implants is approximately 5% after ten years and 15% after 20 years post-surgery, with as high as 50% of these failures reportedly caused by aseptic loosening due to periprosthetic osteolysis induced by wear debris [10][11][12][13]172]. Hence, by 2050 there could be 2.8 million THR surgeries being carried out in OECD countries with up to ~70,000 (2.5%) implants failing after 10 years and ~210,000 (7.5%) after 20 years post-surgery due to periprosthetic osteolysis. Even assuming there is no increase in the average cost of surgery of €10,000, the total annual cost of these 2.8 million THR surgeries would be €28 Billion and the annual cost of the required ~210,000 revision surgeries would be €2.1 Billion. Furthermore, although the incidence of post-surgery IAI's has become quite low (<5%) for total joint replacements it can be up to 15% in high-risk patients undergoing primary or revision joint arthroplasty. Therefore, IAI's still cause a huge impact in terms of morbidity, mortality, and medical costs. For example, direct hospital costs, related to the management of total joint infections has been reported to range from €20,000 to €60,000, while the long-term economic effect of a post-surgical infection after joint arthroplasty has been calculated to exceed €350,000 per case [167]. The use of bioactive Ag/Cu elements in a DLC coating technology has the potential to reduce biofilm formation, minimising the risk of post-surgery IAIs leading to inflammation. This, in turn would result in lower primary infection rates, or where an early infection occurs, it could aid in the eradication of infection with early surgical intervention prior to biofilm establishment, thus significantly reducing the requirement for costly revision surgery. For example if a new hard, wear-resistant and bioactive DLC coating could reduce the number of IAI's by 80%, extend the lifetime of THR implants by 100% and in turn, reduce the number of 20-year post-surgery failures due to periprosthetic osteolysis by approximately 50%, this will lead to a substantial reduction in the requirement for costly revision surgery, resulting in significant economic benefits to the patient and healthcare system of approximately €1 Billion per year by 2050 in OECD countries alone.
In addition to the economic benefits to the patient and healthcare system, there are also key economic and technological benefits to medical device companies that will qualify and bring a new bioactive DLC technology to market. The projected global market value for joint replacement devices is ~€21 Billion by 2025, growing at a compound annual growth rate (CAGR) of 5.2% [173]. If a new bioactive DLC coating technology could be developed and qualified and its superior performance is proven and accepted, it could rapidly secure a significant share of the total joint implant market. For example, securing a 25% share of the OECD total joint implant market, would be worth around €1.4 Billion per year by 2050 (assuming average implant device cost of €2,000 and market size of 2.8 million devices). Other benefits of a new bioactive DLC coating technology could include providing security in the medical device and equipment supply chain and creating new jobs with the widening of manufacture of the new material for other orthopaedic devices such as total knee replacements (TKR), percutaneous pins for fractures, plates and screws for ENT surgery, maxillofacial surgery and in other applications like dentistry and hard wearing antimicrobial touch surfaces in high traffic areas such as hospital and public transport.
Although there are some very significant economic and technological benefits of a new bioactive DLC coating technology, the most important and far-reaching impact will most certainly be on society itself. Total joint replacement surgery is a common procedure, most often recommended for people with hip or knee pain that has not responded to conservative measures and is affecting their quality of life. However, post-surgery IAIs can lead to severe inflammation and serious medical complications resulting in the requirement for revision surgery, typically involving thorough debridement, removal of dead and extraneous tissue and replacement of the orthopaedic implant. The patient may be in significant pain until the infected implant is removed and there is a serious risk of complication and potential mortality during revision surgery, especially for physically weak and elderly patients. These total joint revision procedures caused by IAIs and mechanical wear of the articulating surfaces of the implant have a major negative effect on the quality of patient life along with significant ramifications for healthcare providers. A new bioactive DLC coating that is designed to release controlled amounts of antimicrobial Ag and Cu ions in the early days post-surgery to actively fight IAIs, while providing a high wear resistant DLC surface to significantly extend the lifetime of the articulating implant joint, will lead to a substantial reduction in the requirement for risky revision surgery, resulting in a significant positive impact on patient quality of life and a reduced burden on health care systems.

Conclusions
DLC coatings are receiving increasing attention in medical implant applications due to their synergistic bio-mechanical and bio-tribological performance. Several DLC coating technologies and DLC coated medical devices are already commercially available, including coatings for orthopaedic and spine implants, surgical tools and dental implants and trauma plates, screws and tapers.
This article has provided a critical review of the state-of-the-art in Ag and Cu doped DLC coatings for application to medical implants. Combining the unique mechanical strength, non-stick features, and biocompatibility of a DLC matrix with the enhanced antimicrobial properties of bioactive Ag and Cu elements has the potential to significantly reduce the spread of IAIs and provide a long-lasting coating solution for medical implant devices. However, the current manufacturing methods used to produce Ag/Cu-DLC coatings have limited control over Ag/Cu particle distribution and existing studies on these coatings have all required relatively large quantities of Ag/Cu doping to achieve the required antimicrobial performance due to the tendency of their particles to agglomerate during the manufacturing process, which reduces their relative available surface area for ion release. These clusters disrupt the uniformity of the DLC coating structure, significantly reducing its mechanical performance and durability.
One potential route to overcome this problem is if the soft bioactive Ag/Cu elements could maintain their antimicrobial behaviour at smaller quantities in the DLC matrix, thus preserving the material's mechanical performance. A multifunctional biomaterial that has a unique combination of high mechanical hardness and wear resistance with biocompatibility and enhanced antimicrobial performance could be achieved by developing a fabrication process that will enable soft bioactive Ag/Cu elements to be introduced into a hard DLC matrix with minimal reduction in its mechanical performance. The doping of these bioactive elements in the coating could be fine-tuned to maximise bioactivity whilst maintaining biocompatibility and biomechanical properties, leading to a unique multifunctional bioactive DLC coating that can be further developed by biomedical companies to coat load bearing medical devices to respond to unmet clinical needs such as post-surgery development of wear/corrosion and biofilm formation leading to IAIs and inflammation and ultimately early failure of implants.
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Declaration of Competing Interest:
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.