The overview of antimicrobial peptide‐coated implants against oral bacterial infections

Dental implants are the most common therapeutic approach for resolving tooth loss and damage. Despite technical advances in treatment, implant failure rates can be as high as 23% with the major cause of peri‐implantitis: a multi‐species bacterial infection. With an annual growth rate in implant placements of 8.78% per annum, implant failure caused by bacterial infection is a significant oral and general health issue. The rise in antibiotic resistance in oral bacteria further adds pressure to implant failure; thus, there is a need for adjunctive therapy to improve implant outcomes. Due to the broad spectrum of activity and a low risk of inducing bacterial resistance, peptide antibiotics are emerging as a promising implant coating material to reduce/prevent peri‐implantitis and improve dental implant success rates. In this review, we summarised the current strategies of coating antimicrobial peptides (AMPs) onto dental implant material surfaces with multi‐functional properties to enhance osteoblast growth and prevent bacterial infections. This review compared the recent reported literature on dental implant coating with AMPs, which will provide an overview of the current dental implant coating strategies using AMPs and insights for future clinical applications.

F I G U R E 1 Increase in the number of articles on antimicrobial materials to avoid peri-implantitis. This figure was generated using results from the Web of Science using keywords 'peri-implantitis' and 'antibacterial' for the red dot line and 'peri-implantitis' and 'implant' for the blue column, respectively. loss over time. [8] Peri-implantitis, however, is characterised by inflammation, bleeding and/or suppuration with increased probing depth and tissue and bone loss. [8,9] Recent data suggest that peri-implantitis accounts for the major cause of implant failure. [10,11] With the rise in the placement of dental implants, there is an increasing need to prevent and eliminate peri-implantitis. [12] To manage and eliminate the bacterial infection and biofilm formation on/around implants, current therapeutic strategies include surgical and nonsurgical treatments in combination with antibiotics. [13,14] However, the physical removal of the bacterial biofilm by mechanical debridement is inefficient, expensive and timeconsuming. There is an increased risk of the development of bacterial resistance with the repetitive and long-term use of antibiotics. [9,15] Despite advances in treatment and care, peri-implantitis occurs in 3%-47% of dental implants with implant failure rates of 2%-23%. [16][17][18] Thus, given an increase of 8.78% per annum in implant growth globally, [19] it is critical to develop more efficient antimicrobial techniques to reduce and eliminate implant bacterial infections. In recent years, a large amount of effort has been put forth by universities and research institutions to find solutions to these problems. This is reflected in an increasing number of publications that provide new alternatives to bacteria adhering and forming biofilms associated with peri-implantitis ( Figure 1). Although antimicrobial materials have been applied extensively in recent years, [20] there have been limited reports on their use in dental implants. To address these issues, an innovative research momentum is the incorporation of antimicrobial materials onto dental implants, which is the focus of this review. Therefore, we have conducted a comprehensive search of the literature in PubMed, Web of Science, Embase, Cochrane and Google Scholar using the keywords of 'Antimicrobial peptides', 'Dental implants', 'Implantassociated infection' or 'antimicrobial coating' with a focus on publications from 2015 to 2022 ( Table 1). The aim of this review is to highlight the recent progress of using antimicrobial peptides (AMPs) as surface coatings for dental implants, which further provide insight for future implant coating design.
The oral cavity poses a unique and challenging environment in placing implants with constant exposure to chemical and physical surroundings changes through eating and drinking. Consequently, mechanical strength, stability and biocompatibility are major factors in implant materials, and only a few materials, such as certain metals (titanium, Ti), polymers (polyetheretherketone, PEEK) and ceramics (zirconia, hydroxyapatite [HA]), are used for dental implants as they have greater fatigue strength, reduced modulus of elasticity and improved corrosion resistance. [28] There are several advantages and disadvantages for each implant material as listed in Table 2. Due to its high biocompatibility, osseointegration and resistance to corrosion, pure Ti and its alloys are currently considered the gold standard for dental implants. [29] However, the biocompatible property of Ti can enhance bacterial infections after tooth implantation, resulting in serious oral health issues. [10] To improve the biointegration and reduce bacterial adherence and infection of the implant materials, various physical and chemical strategies have been applied to antimicrobial coating onto implants. It TA B L E 2 Advantages and disadvantages of different types of dental implant materials  [33,39,42,44,45] F I G U R E 2 Different implant coating materials and strategies to prevent bacterial colonisation on implants include antibacterial polymers, [46,47] nanoparticles, [48,49] antimicrobial peptides (AMPs), [50][51][52] interconnecting 3D structures, [53] metal ions (Ag + , Cu 2+ , Sr 2+ or Zn 2+ ) [29,54,55] and antibiotics. [56][57][58] includes organic coatings (e.g., polymers, biomimetic films) and inorganic functional coatings (e.g., HA, titanium oxide) for osseointegration acceleration, bacteria adhesion reduction and biocompatibility improvement. [30,31] These functional coatings aim to enhance both antimicrobial and osteointegration properties of dental implants in the short-and long-term life. [29,30,32] Several approaches have been investigated to prevent periimplantitis, such as coating the modified implant surface with antimicrobial agents. [33] Antimicrobial surface coating involves adding a layer on the implant's surface to either prevent bacteria adherence or kill bacteria by releasing antimicrobial chemicals or ions, or both. [33] To date, various coating materials have been applied to prevent bacteria from colonising the dental implant surface, such as antibacterial polymers, [46,47] nanoparticles, [48,49] AMPs, [50][51][52] interconnecting 3D structures, [53] metal ions (Ag + , Cu 2+ , Sr 2+ or Zn 2+ ) [29,54,55] and antibiotics ( Figure 2). [56][57][58] Of these antimicrobial materials, AMPs have garnered considerable research attention for their broad spectrum of antimicrobial activity and their multi-modal mechanisms (advantages and disadvantages listed in Table 3). AMPs are typically short peptides (10-50 amino acids), often rich in cationic and hydrophobic amino acids, are amphiphilic and form αhelical structures in lipid environments such as bacterial cell membranes. [59,60] The widely accepted mode of action is electrostatic interactions with the negatively charged bacterial surface and then transverse the cell envelope to the inner/cytoplasmic membrane via an electrostatic gradient where they insert and disrupt the membrane causing lysis. [61] This mechanism of action has meant that AMPs do not readily induce resistance and are highly effective against antibiotic-susceptible and multi-drug-resistant bacteria [62] in both nosocomial and uropathogenic pathogens. [63] This attractive characteristic has inspired that the incorporation of AMPs can become a major research focus in preventing or reducing the growth of bacteria and biofilms. [64][65][66][67] Thus, AMPs have emerged as alternatives to routinely used antibiotics and are now being considered a new class of antimicrobial surface coatings. [62,[68][69][70][71][72][73][74] Generally, both adsorption and chemical conjugation can be used to coat AMPs onto dental implants. The adsorption approach immobilises the peptides onto the surface of the dental implant via either a simple incubation-drying process or electrostatic/hydrogen bonding adsorption method. [68] Although the adsorption approach is effective, it can lead TA B L E 3 The advantages and disadvantages of antimicrobial peptides (AMPs) as antimicrobial coating  [32,60,62,[75][76][77][78] to a heterogeneous coating and high localised concentrations of an AMP with increased cytotoxic potential and inefficient protection against bacterial colonisation and infection. Using modified adsorption or chemical conjugation approach can avoid the high localised concentrations of AMPs and control the surface coating process. Further, using cross-linking agents to covalently conjugate AMPs onto implants has led to a promising strategy to control the release of active AMPs from the implant and have a long-lasting antimicrobial effect against bacteria colonisation and biofilm formation. [45] In the last 5 years, the development of AMP coating has advanced the antibacterial characteristics of implant material, especially in dental implants. For example, AMP-coated materials were found to have direct antimicrobial properties, inhibit biofilm formation and have a reduced tendency to trigger inflammatory reactions. [79,80] However, by contrast, some coating materials have been found to compromise mechanical properties and biocompatibility on the implant, as well as having a cytotoxic effect. [81,82] To provide a brief guide to further optimise dental implant coating and their therapeutical applications in oral health, this review focuses on the current approaches for coating AMPs onto metal, ceramic and polymer dental implants via advanced adsorption or chemical conjugation approaches and summarises the potential of AMP-based implant therapies to combat oral infections and improve implant success rates.

COATING METAL-BASED IMPLANTS WITH AMPS
Metals, such as Ti, are well known for their good osseointegration, biocompatibility and high strength and stiffness properties and as such remain the most common implant material used in dental clinics. To address bacterial colonisation and biofilm formation, Ti surface modification has been a prominent focus of dental implant research. [83] AMPs have been applied to coat Ti surfaces using a number of strategies, including adsorption, binding, electrospinning and chemical conjugation to prevent bacterial colonisation. [79,84] The recent use of AMPs in metal-based (mainly Ti) dental implants are summarised in Table 4.

Adsorption and binding methods for AMP implant coating
Early studies of AMPs coatings on the Ti surface for dental implants usually use peptide absorption or chemical conjugation with the modified Ti surface. For example, calcium phosphate (CaP), known to enhance bone growth and integration of orthopaedic implants, was electrolytically deposed onto the surface of Ti (CaP-Ti), [85] followed by peptide immobilisation. The CaP-Ti surface was then incubated with the AMP Tet213 (Table 4, a C-terminally modified analogue of the AMP, HHC36) and peptide immobilisation onto the CaP via absorption in an Na 2 HPO 4 buffer (pH 7.4). [85] In a comparative study with non-CaP-coated Ti, the CaP-Tet213-coated Ti prevented bacterial colonisation by killing both Staphylococcus aureus and Pseudomonas aeruginosa within 30 min. Interestingly, CaP-Tet213 surfaces retained their antibacterial activity for four consecutive tests, indicating their longevity ( Figure 3A,B). This study provided a new approach to coating implants with CaP and then adsorption of AMPs as a potential solution for the prevention of peri-implant infection in orthopaedic treatments. However, the strategy of physical absorption of AMPs onto the Ti surface via CaP usually led to inconsistent loading and releasing kinetics, which limited this approach in clinical applications. To retain CaP osteointegration properties, Ti surfaces were coated with calcium phosphate and HA, [98] and then metal bind peptides linked AMPs (Table 4) can easily immobilise the CaP/HA-coated Ti surface. [99] Yazici et al. engineered an AMP, tet127 (Table 4), with an HA binding peptide-1 (HABP1, Table 4) via a flexible tri-glycine linker (GGG) for binding to CaP/HA nanotubular coated Ti. [86] The HABP1-structured tet127 (HBAP1-tet127) enabled active binding onto the CaP/HA nanosurface of Ti and resulted in improved implant osseointegration. The HBAP1-tet127-CaP/HA-coated Ti was also found to provide antibacterial activity against Streptococcus mutans (Gram-positive) and Escherichia coli (Gram-negative) ( Figure 3C,D). [86] This direct/active binding approach using a metal-binding peptide enables a level of control of loading and releasing kinetics at the biomaterial interface.
In a later study, Shi et al. conjugated AMP, Tet213 (Table 4), onto collagen IV functionalised with sulfosuccinimidyl-4-(p-maleimidophenyl)butyrate (sulfo-SMPB) via a thiol-maleimide reaction, termed AMPCol. [87] Using a layer-by-layer (LBL) technique, Ti surfaces were sequentially coated with HA and AMPCol to form a multi-layer Ti coating (AMPCol-HA-coated Ti). The AMPCol-HA-coated Ti was found to inhibit the growth of Porphyromonas gingivalis and S. aureus by 58.5% and 56.4% respectively, compared to non-coated Ti surfaces. Furthermore, using an early biofilm formation assay, it was shown that thicker multi-layer coatings exerted a stronger inhibitory effect on the early biofilm formation of S. aureus than thinner surface coatings ( Figure 3E,F). More importantly, this AMPCol-HA-functionalised Ti caused little cytotoxicity and haemolysis in vivo, suggesting good immunocompatibility.
To avoid the complex chemical reactions and cytotoxic reagents, Liu and co-workers designed an approach by conjugating JH8194 (Table 4), an AMP derived from the saliva antimicrobial protein histatin, with a hexapeptide motif of a Ti-binding peptide (minTBP-1) via either a flexible TA B L E 4 Recent development of antimicrobial peptides (AMPs) in metal-based (mainly Ti) dental implants to combat implant-related infection  amino acid linker (GGGGS) or a rigid proline-rich linker (PAPAP). [88] In an adsorption study, the PAPAP linker significantly increased the adsorption of the AMP onto Ti surfaces and enhanced antimicrobial activity against the oral pathogens Streptococcus gordonii and Streptococcus sangui-nis ( Figure 4A,B). This study indicates that the choice of linker for a chimeric AMP coating on an implant can modulate the peptide binding and bioactivity and is a consideration to take into account in producing biomaterials to lower the risk of implant-related infection.  Table 4) and then directly coated the Ti surface to determine their antimicrobial efficacy against S. mutans, Staphylococcus epidermidis and E. coli. [89] Both chimeric peptides were found to inhibit bacterial growth and biofilm formation, although TiBP1 was found to enhance Ti surface binding of the AMP and consequently increased antimicrobial activity as compared to TiBP2-GGG-AMP ( Figure 4C,D). From these encouraging studies, it appears that TiBP-AMP peptides are promising new antimicrobials with a high potential for treating peri-implant infections. However, the underlying mechanisms are still under investigation, and further strategies must be developed to circumvent AMPs' observed toxicity for future clinical translation.
Zhang et al. coated a Ti surface with a similar chimeric peptide using a TiBP and human β-defensin-3/RGD sequence (Table 4) linked by a GGG motif, the peptide-coated Ti surface was found to prevent the biofilm formation of Streptococcus oralis. [90,91] They also found that the chimeric peptide inhibited the formation of S. oralis biofilms by suppressing the expression of the adhesin protein genes sspA and sspB. [100] Utilising the binding properties of TiBP, [12] Wisdom et al. applied a TiBP to modify AMPs (Table 4) to coat dental implant surfaces and protect against oral bacteria adhesion and biofilm formation. [21] Two different AMPs, AMPA and GL13K (Table 4), were synthesised with TiBP at the N-terminus and separated using a GSGGG linker and applied on separate Ti surfaces to form an AMP-coated Ti surface. Both the TiBP-GSGGG-AMPA and TiBP-GSGGG-GL13K inhibited bacterial colonisation and biofilm formation ( Figure 4E,F) and was further found to reduce the host tissue inflammatory response around the implants. The TiBP approach of binding an AMP to the Ti surface often leads to complete surface coverage, a high and long-term level of antimicrobial activity and retention of bioactivity even after mechanical abrasion (electric toothbrush). The lack of an additional surface coating of the Ti surface to bind an AMP makes the TiBP approach an attractive alternative for the prevention of peri-implant infections. However, further studies are needed to investigate the controlled release of the AMP, the long-term durability and osteointegration of the TiBP-AMP-coated implants, as well as the quantification of adsorbed AMPs on the surface. Although this coating strategy is easy to execute, it may cause side effects in biological environments due to AMPs' low stability. Overall, this coating strategy is a flexible and easy way to investigate their antimicrobial efficiency by coating AMP on Ti surfaces against oral pathogenic bacteria.

2.2
Ti coating using co-electrospinning of AMP with polymers Antifouling materials play an important role in preventing the initial bacterial attachment on implants. [101] Among the different antifouling materials, polyethylene glycol (PEG) has been widely used on dental implant surfaces to inhibit the adhesion of bacteria, proteins, cells and cellular and serum molecules. [102,103] Hoyos-Nogués et al. developed a tri-functional coating approach to coat a Ti implant surface with antifouling (PEG), antimicrobial (AMP (LF1-11)) and cell adhesion (RGD peptide) ( Table 4) properties to enhance osteoblast adhesion and growth. [22] The coating approach first electrodeposited a PEG film onto the Ti surface, the PEG was then functionalised with N-succinimidyl-3-maleimidopropionate and the RGD-Cys-LF1-11 peptide bound to the functionalised PEG using thiol-maleimide conjugation. The RGD-Cys-LF1-11-coated Ti surface demonstrated excellent osteoblast adhesion and prevented the initial attachment of S. sanguinis, a primary early colonising bacterium in oral biofilms ( Figure 5). [22] This strategy addresses key causes of implant failure: bacterial infection and biofilm growth, as well as insufficient osseointegration, which has significant potential in dental implant and implant therapies.  Table 4) onto a Ti dental implant surface in an attempt to prevent two early colonising oral plaque species (S. sanguinis and Lactobacillus salivarius) attaching to the implant surface. [92] By using silanisation of Ti with either (3-chloropropyl)triethoxysilane (CPTES) or (3-aminopropyl)triethoxysilane (APTES), and a C-terminal Cys-modified hLf1-11, the AMP was covalently immobilised via thiol-halogen or thiol-maleimide conjugation ( Figure 6A) onto the Ti surfaces. The hLf1-11-coated Ti implants were found to decrease the adhesion of S. sanguinis and L. salivarius by three-to-sixfold compared to the non-coated Ti surface and reduced the development of early phases of biofilm development of each bacteria by two-to-fourfold ( Figure 6B). In a later study, greater biological efficacy could be achieved when the hLf1-11 peptide was attached to a Ti implant precoated with a silanised brush polymer (prepared by surface-initiated atom transfer radical polymerisation) than an ATPES-prepared surface. [104] In both studies, it was shown that hLf1-11-coated Ti implants were biocompatible with human fibroblasts with no cytotoxicity observed, further indicating that this method is a promising strategy in the prevention of bacterial infection and long-term stability in dental implant applications.

Covalently conjugation approaches for Ti surface coating
The AMP, GL13K (Table 4), derived from parotid secretory protein, is known to have broad-spectrum antimicrobial activity and has been the subject of several dental implant coating studies to develop bioactive surfaces that reduce oral infections and improve implant long-term efficacy. For example, Aparicio et al. utilised a silanised Ti surface to covalently incorporate GL13K to prevent colonisation and kill the peri-implantitis putative pathogen P. gingivalis. [93] Significantly, the homogeneous and highly hydrophobic surface environment of the Ti implant coated with GL13K was found to be resistant to mechanical and thermochemical stress, and enzymatic degradation by gingipain proteases secreted by P. gingivalis. [93] Using a drip-flow biofilm bioreactor to mimic oral cavity conditions, the GL13K (Table 4)-coated Ti was found to kill and prevent the biofilm formation of S. gordonii, a primary coloniser that provides attachment for the biofilm accretion by P. gingivalis. [94] Interestingly, the GL13K peptide coatings could rupture the S. gordonii bacteria cell wall, leaving empty shell-like structures ( Figure 6D). [94] To promote cytocompatibility and retain its antibacterial activity, Chen et al. covalently linked the GL13K onto microgroove Ti surfaces, which was found to display both bactericidal activity and improved adhesion and proliferation of human gingival fibroblasts (HGFs). [94] In another study, Li and co-workers used TiO 2 nanotubes as a drug/peptide carrier to deliver GL13K (Table 4) onto Ti surfaces; this coating strategy resulted in a sustained and slow drug/peptide release profile for the prevention of the growth of Fusobacterium nucleatum and P. gingivalis bacteria associated with implant infections ( Figure 6E). [95] It should be noted given those promising in vitro studies, their application in the animal model should be further investigated for clinical translations.
Aparicio and co-workers also designed a proteinengineered molecular platform for GL13K coating on Ti to prevent implant-associated infections. [51] The proteinengineered polymers based on elastin-like recombinamers (ELRs) were chemically conjugated with both enantiomeric forms (L and D) of GL13K (GL13K-ELR) using 'click chemistry', followed by covalent conjugation of the GL13K-ELRs via organosilanes to a Ti surface. The protein-AMP hybrid (GL13K-ELR) coatings showed strong antibiofilm activity against S. gordonii ( Figure 7A,B).
In a further study, the AMPs, GL13K and LamLG3 (a derivative of the natural product from oral keratinocytes, laminin 332, Table 4), were mono-and co-immobilised onto a Ti surface. [23] The co-peptide Ti surface displayed significantly better antibiofilm potency against S. gordonii (an early colonising bacteria, Figure 7C,D) and enhanced keratinocyte proliferation and hemidesmosome formation than the mono-peptide Ti surfaces. [23] These co-peptide surfaces also afforded mechanical and thermochemical stability and retained bioactivity, which indicates the potential use in dental and other percutaneous implants to reduce infection and failure rates. Mishra and Wang designed a similar approach by silanisation of Ti followed by Cys-maleimide reaction to covalently attach a short AMP, FK-16 (Table 4), [96] a fragment derived from the host defense peptide LL-37 ( Figure 7E). [105] The FK-16-modified Ti surfaces were found not only to have sustained antimicrobial activity against the ESKAPE pathogens, including Enterococcus faecium, S. aureus, Klebsiella pneumoniae, Acinetobacter baumannii, P. aeruginosa and Enterobacter cloacae, but also inhibited the bacterial surface adhesion ( Figure 7E,F). [96] Apart from silanisation of Ti surface, Abbasizadeh et al. co-electrospun nanofibrous biocompatible silk fibroin (SF)/HA onto Ti, followed by a polydopamine (PDA)  (Table 4), on the nanofibrous SF/HA. [97] The coated implants displayed enhanced haematopoiesis and mineralisation, as well as sustained HHC-36 antibacterial activity against E. coli and S. aureus (Figure 8). [97] In comparing with covalent attached AMPs via silane monolayers and LBL assembly coating strategies, the covalently linked nanofibrous coating Ti via PDA [106] demonstrated a burst release profile of HHC-36 in the first 3 h and a steady slow release for subsequent days with retained antibacterial potency. Taken as a whole, these findings demonstrate that AMPs in metal-based (mainly Ti) dental implants are promising formulations to combat implant-related infection.

COATING AMPS ONTO CERAMIC-BASED IMPLANT
Due to the superior surface strength, aesthetic properties, chemical stability and excellent biocompatibility, ceramicbased materials are widely used in dental applications, such as zirconia ceramic restorations. [107] However, ceramics are bioinert and lack anti-infective properties, which limit their use for fast integration with the surrounding tissues, [108] mak-ing them prone to peri-implantitis osteoarthritis and mechanical loosening. [107] Examples of AMP-modified ceramicbased implants are also listed in Table 5. To improve the soft-tissue integration and applicability in dental clinics, Yang et al. immobilised two cell-adhesion peptides (fibronectin sequence: KGGRGDSP and RGD, Table 5) onto zirconia abutment surfaces by a chemical cross-linking strategy using PDA films. [45] The PDA-RGD-functionalised zirconia significantly enhanced the biological activities of HGF and retained the antimicrobial activity of the PDA coating, indicating an alternative approach to promote a peri-implant soft tissue seal. These recent developments in the surface coating of AMPs onto ceramic-based implants ( Table 5) will provide alternative approaches to improve the soft-tissue integration with enhanced antimicrobial properties.
Given the similar composition to human bone, HA has been used for coating nonmetal implants to promote osseointegration. To investigate the AMP coating approaches for enhanced antimicrobial activity of ceramic dental implants, Townsend et al. incorporated the human beta-defensins (Table 5) onto thiol-functionalised HA (tHA) surfaces using three methods: covalent attachment forming 'permanent' coating (cAMP), an electrostatic attachment (eAMP) and a combined covalent/electrostatic attachment as a dual coating  Table 5). [24] In comparison with cAMP and eAMP attachment, the dual technique (dAMP) was found to be more effective in preventing the colonisation of both Gram-positive and Gram-negative bacteria, including clinical isolates of S. aureus, S. epidermidis and P. aeruginosa ( Figure 9A,B) and this effect was found to be long-term lasting for over a year. The development of a dual surface coating method can modulate the short-term release and long-term antimicrobial activity, a vital characteristic of orthopaedic infection in oral health.
Although the current trend in dentistry is to develop allceramic dental systems, ceramic veneering using biocermet materials is still widely used for restoring dental frames with metallic implants. [109] In a recent study by Fernandez-Garcia et al., an oligopeptide (Table 5, KKKGGGGRGDS, an extensively studied cell adhesion motif) was conjugated on two widely used bioceramics 3Y-TZP and 3Y-TZP/Ti biocermets via a silanisation (Table 5, Figure 9C). [25] The functional oligopeptide coatings on zirconia and biocermets facilitated osseointegration, improved permucosal sealant properties

Coating strategy
AMPs Sequence

Dental implant materials Antimicrobial activity Reference
Amidation with NH 2 human beta-defensins RRRRRRGALAGR RRRRRGALAG HA Prevented both Gram-positive and Gram-negative bacteria from colonising the surface [24] Amine alkylation Oligopeptide KKKGGGGRGDS Ti Zirconia Prevented peri-implant infections [25] Abbreviation: HA, hydroxyapatite. and provided a potential antimicrobial property by preventing bacterial early colonisation. Although the oligopeptide does not have antimicrobial properties, this study demonstrated a novel strategy to covalently link potent AMPs onto the ceramic surface and provide a strong and stable bioactive surface. [25]

COATING AMPS ONTO POLYMER-BASED IMPLANT
Other than metal and ceramic materials, synthetic polymers, such as acrylic resins, PEEK, dendrimers and polylactic acids, are a new biomaterial for fabricating restoration materials in dental applications. [110] These new biomaterials, exhibiting biomechanical properties similar to human bone, as well as being chemical-and radiation-resistant and thermally stable, are emerging as a popular and viable substitute for traditional metallic implants. [111] However, the high hydrophobicity of polymer-based implants has been shown to lead to an increase in post-surgical bacterial infections and biofilm formation. [112] The incorporation of cationic peptides into the polymer can modulate the hydrophobicity. Importantly, the excellent mechanical and chemical resistances of polymer-based implants were found to enhance the long-term stability and the controlled and sustained release of AMPs in targeting oral pathogens. [113] In Table 6, we further list the polymer-based implants with an AMP coating that have been used against implant-related infection.
Given the excellent mechanical and physicochemical tolerances of PEEK, Xu et al. covalently decorated carbon fibre-reinforced PEEK/nanohydroxyapatite (CFRPEEK/n-HA) with a bone-forming peptide (Table 6) via a polydopamine tag strategy. [114] The CFRPEEK/n-HA material displayed dual functions of reducing bacterial adhesion and enhancing osteointegration of the PEEK implant. [114] Inspired by the mussel adhesion mechanism, Li et al. used bioorthogonal click chemistry to conjugate differing ratios of an AMP, MP196 (Table 5, a short hexapeptide), and osteogenic growth peptide (OGP, Table 6) to an azido-modified PEEK surface. [26] The molar ratios of the conjugated MP196 and OGP on PEEK surfaces were  comparing the antibacterial and osteogenic properties of the differing PEEK-A n O n materials, a coating ratio of 2:2 of MP196/OGP (PEEK-A 2 O 2 ) displayed the most promising dual-effect of host defence and tissue repair, as well as preventing/inhibiting bacterial growth over a long period ( Figure 10A,B).  Table 6) onto PEEK biomaterial achieving a 30% higher peptide loading over non-EDC activation method. [27] Additionally, the PEEK/GL13K-EDC material had a smoother surface and better hydrophilicity with a significant increase in antibacterial activity and resistance to S. aureus biofilm formation ( Figure 10C,D). [27] In comparison with the standard activation process of PEEK, such as sandpaper roughing, plasma spraying and acid treatments, [115,116] the EDC chemical conjugation strategy facilitates the immobilisation of AMPs onto the polymer implant and so obtains a dual-effect of host defence and antibacterial activity. The current findings using PEEK-based dental implants with AMP modification via either click chemistry [26] or EDC coupling [27] demonstrate a promising way to imbue polymer-based implants with antibacterial activity and biofilm inhibition for orthopaedic/dental tissue engineering applications with osseointegrative properties.

CONCLUDING REMARKS AND FUTURE OUTLOOKS
Dentistry has been recognised as a contributor to the development of antimicrobial resistance with up to 80% of antibiotics being prescribed inappropriately in dentistry, for both prophylaxis and therapeutic reasons. [117] Treatment of bacterial infections caused by biofilm-producing microorganisms is a challenging task and a major concern for health-care organisations. [118,119] Despite advances in dental implant treatment, peri-implantitis is a significant risk to oral health and the survival rate of dental implants. [120] Given the broad spectrum of antimicrobial activity and low induction of resistant mechanisms, AMPs have emerged as a viable solution to antibiotics. Their further incorporation into implant surfaces  [74,[121][122][123][124][125] An additional advantage of peptides is that a variety of chemistries can be applied to regulate their attachment and then release kinetics. These chemical strategies are critical in the development of implant materials by using composite materials, nanotechnology and bioengineering. [126,127] This review demonstrated utilising these chemical approaches to incorporate AMPs and cell adhesion peptide sequences on dental implants to enhance osteoblast growth and prevent oral bacterial infections. Antimicrobial actions are generally achieved by blocking bacterial adhesion, reducing biofilm development, interfering with cellular metabolism and respiration and disrupting the bacterial cell wall or membrane, all of which finally result in cell death. In vitro and in vivo results have been exciting, indicating that they have a lot of promise for clinical use in the future. [123] Despite its potential, it should be noted that dental implant companies have not used antimicrobial coatings, and there is a lack of good clinical studies dealing with such coating materials. [128,129] The coated peptides' dependability, biocompatibility, long-term impact against microbes in vivo, and their toxicity to the human body are factors that require verification for their application in clinical use. Due to safety, burst release, stability and inappropriate mechanical characteristics, there is unmet research and translational demand for both fundamental and clinical researchers. More importantly, it should be noted by readers that the majority of studies in this review utilised a mono-species biofilm model for their coated materials biological application, whereas peri-implant tissues were composed of a polymicrobial community.
Due to the enzymatic instability and cost of production of AMPs, synthetic mimetics or peptoids have been designed to reproduce AMP critical biophysical and bioactive characteristics. [130,131] Further verification of the durability of these AMP mimetics/peptoids adsorbed onto implant surfaces, and their tolerance to pH, buffers and body fluids would enhance their progression to clinical use. [89] Those peptide mimetics with controlled charge density are considered an emerging approach to overcome natural peptide's inherent physical limitations and improve their therapeutic potential. [132,133] Currently, there is no consensus about the best dental implant surface for preventing polymicrobial infections and reducing oral bacterial accumulation. [128] It can be expected that the use of modified peptides as dental implant coatings has the potential to revolutionise the dental implant field and dramatically improve oral health and dental implant patient outcomes, by enabling tissue integration and preventing bacterial contamination. The covalent coupling with various functional groups (such as thiols, carboxylic acids, hydroxyl, guanidines and amines) provided different approaches to enhance the implant coating with bioactive compounds. But the current covalent modification can lead bioactivity loss and/or erroneous bioactivity display to the cellular environment, [134] which do require further optimisation and improvements. By incorporation of the AMPs' or mimetics' immunomodulatory activities, the recent advanced development of composite materials, nanotechnology, bioengineering and interdisciplinary fusion further provided novel approaches for a breakthrough discovery in this field, especially in polymicrobial biofilm environment. We believe our review provides a perspective view on the future studies of the AMP-coated dental implant material for their clinical applications in the presence of a polymicrobial community in vivo or in vitro.

A U T H O R C O N T R I B U T I O N S
All authors have read and agreed to the published version of the manuscript.

C O N F L I C T O F I N T E R E S T
There are no conflicts to declare.