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Journal of Materials Chemistry BMaterials for biology and medicinersc.li/materials-b

ISSN 2050-750X

REVIEW ARTICLESadra Bakhshandeh and Saber Amin Yavari Electrophoretic deposition: a versatile tool against biomaterial associated infections

Volume 6 Number 8 28 February 2018 Pages 1121–1288

1128 | J. Mater. Chem. B, 2018, 6, 1128--1148 This journal is©The Royal Society of Chemistry 2018

Cite this: J.Mater. Chem. B, 2018,

6, 1128

Electrophoretic deposition: a versatile tool againstbiomaterial associated infections

Sadra Bakhshandeh and Saber Amin Yavari *

Biomaterial-associated infections (BAIs) are today considered as one of the most withering complications of

orthopedic implant surgery. Even though BAIs occur relatively infrequently in primary joint replacement surgeries

(incidence rates around 1–2%), revision arthroplasties carry up to 40% risk of infection recurrence, with

devastating consequences for the patient and significant associated cost. Once the responsible pathogens, mainly

bacteria, attach to the surface of the biomaterial, they start creating layers of extracellular matrix with complex

architectures, called biofilms. These last mentioned, encapsulate and protect bacteria by hindering the immune

response and impeding antibiotics from reaching the pathogens. To prevent such an outcome, the surface of

the biomaterials, in particular implants, can be modified in order to play the role of inherent drug delivery devices

or as substrates for antibacterial/multifunctional coating deposition. This paper presents an overview of novel

electrochemically-triggered deposition strategies, with a focus on electrophoretic deposition (EPD), a versatile

and cost-effective technique for organic and inorganic material deposition. Other than being a simple deposition

tool, EPD has been recently employed to create novel micro/nanostructured surfaces for multi-purpose

antibacterial approaches, presented in detail in this review. In addition, a thorough comparison and assessment

of the latest antibacterial and multifunctional compounds deposited by means of EPD have been reported,

followed by a critical reflection on current and future prospects of the topic. The relative simplicity of EPD’s

application, has, by some means, undermined the fundamental requirement of rationality of multifunctional

coating design. The demanding practical needs for a successful clinical translation in the growing fields of tissue

engineering and antibacterial/multifunctional implant coatings, calls for a more systematic in vitro experimental

design rationale, in order to make amends for the scarcity of significant in vivo and clinical studies.

Department of Orthopedics, University Medical Centre Utrecht, Utrecht, The Netherlands. E-mail: [email protected], [email protected];

Tel: +31-88-7559025

Sadra Bakhshandeh

Sadra Bakhshandeh graduated(cum laude) with a Master’s inBiomedical Engineering at DelftUniversity of Technology,specializing in Biomaterials andTissue Biomechanics. The focus ofhis thesis, in collaboration withthe UMC Utrecht OrthopaedicDepartment, was mainly todevelop an antibacterial coatingby means of electrochemicaldeposition techniques on additivelymanufactured porous implants.Sadra is interested in the designof biomaterials for multiplebiomedical purposes.

Saber Amin Yavari

Dr Saber Amin Yavari receivedhis PhD degree in BiomechanicalEngineering from the DelftUniversity of Technology, theNetherlands, in 2014. His PhDwork mainly focused on thedevelopment of additive manu-facturing technology to fabricateporous titanium implants. From2014 to 2016, he was a post-doctoral fellow at the departmentof orthopedics, University MedicalCenter Utrecht where he couldfocus more on orthopedic implant

challenges such as implant associated-infection and implant loosening.Currently, he is an assistant professor at the same department anddevelops multi-functional coatings for antimicrobial activity, boneregeneration and bone tumours.

Received 11th September 2017,Accepted 25th January 2018

DOI: 10.1039/c7tb02445b

rsc.li/materials-b

Journal ofMaterials Chemistry B

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This journal is©The Royal Society of Chemistry 2018 J. Mater. Chem. B, 2018, 6, 1128--1148 | 1129

Introduction

The significant increase of life quality and expectancy, togetherwith improvements in surgical techniques, have resulted in asteep increase in orthopedic implant usage in the past 20 years.1

Although remarkable improvements have been achieved,surgical site infection is still considered one of the majorchallenges in orthopedics.2

The first 6 hours after surgery are particularly important fornegating early stage infection, as pathogens that eventuallyenter the body during the procedure, have not yet started rapidproliferation.3 Long-term shielding would definitely be preferableas implants are expected to function for up to 20 years.3 In spiteof that, current material and technological limitations havedelayed this ambition for the moment.4

It is generally accepted that the mechanisms for achievingantimicrobial efficiency can be distinguished into three mainmodes of action: anti-adhesion, contact-killing and agent-releasing5,6 (Fig. 1). In this context, two major challenges havebeen delineated by Anthony Gristina in his pioneering notionof ‘‘race for the surface’’.7 He stated that for implants to achieveprolonged use in the body, tissue cells should outcompetebacteria in the race leading to the implant’s surface; whicheverwins, will be able to colonize the substrate in the form of tissueintegration for the former, and biofilm for the latter (Fig. 2).

This is particularly important, as once in the biofilm state,bacteria become 10 to 1000 times more resistant to the hostresponse, antibiotics, or any other antimicrobial agent.8

However, the situation is even more complicated, as bacteriahave been shown to reside in tissues surrounding the implant

Fig. 1 Different modes of action of antimicrobial coatings for application on the surface of implants or devices.149

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and within host cells (such as macrophages) as well.9 Moreover,an inconvenient host response, may induce thrombus generation,activation of complementary systems and inflammatory reactionson the surface of the implant which can negatively affect implantintegration.10 Therefore, an ideal implant should be able tosuccessfully modulate tissue integration, bacterial adhesionand host response all at once.

Deposition strategies

Based on preceding explanations, it is essential to find suitablecarrier structures capable of releasing antimicrobial agents in acontrolled manner, in order to prevent eventual cytotoxicity ordrug resistance implications. Antimicrobial polymers, besideshaving the aforementioned properties, can even have intrinsicallybactericidal and/or anti-adhesive characteristics. Their cost-effectiveness, physico-chemical design versatility and ease ofprocess, have made them one of the main candidates in fieldslike biomolecule delivery, cell culture, wound dressing, implantcoatings and tissue engineering (Fig. 3).11–14

Among the different electrochemically triggered techniquesemployed for depositing polymeric films and coatings on bio-medical implants, some have captured more attention fromresearchers, such as atomic transfer radical polymerization (ATRP),self-assembled monolayers (SAMs), layer-by-layer assemblies(LbLs), and electrophoretic/electrochemical deposition. Althoughwidely used and valuable for certain applications, SAMs and ATRPsuffer from poor stability and robustness both in physiological andambient environments.15–18 Moreover, due to their inherent mono-layer properties, the amount of biological molecules that can beloaded is restricted.18 Last but not least, in order for thesemonolayers to be established, definite compounds are required,like silane (for titanium, silicon and aluminum oxide), thiols(for noble metals) or organic acids (other metal and metaloxides) (Fig. 4).18

LbL assemblies on the other hand, driven by electrostaticinteractions, hydrogen bonding and so on, have been revealedas cost-effective, simple, and versatile in terms of the materialsemployed and loading-capacity, and stable over a wide range ofenvironmental conditions (Fig. 5).18,19 However, like SAMs, theprocedure of making these polyelectrolyte multilayers is stilltime and material consuming.19 A number of comprehensivereviews have been recently published, thoroughly covering thephysics, chemistry and applications of the LbL method.18–22

Compared to the above-mentioned methods, electrophoreticdeposition (EPD) benefits from a short operation time, cost-effectiveness in terms of processing and infrastructure, ease ofscaling up both production and product size (from nanometersto meters), unneeded crosslinking agents, excellent control ofcoating thickness and morphology, and versatility for a varietyof shapes with the ability to coat even the inner surfaces ofporous structures23–25 (Fig. 6).

Fig. 2 Race for the surface illustration: (1) floating bacteria come into contact with the implant surface and start to proliferate. After a few hours, theystart to produce a biofilm which will protect them against drugs; (2) if the host cells/proteins reach the surface of the implant within their window ofopportunity, they can colonize the substrate before bacteria, leaving no place for their adhesion; (3) when fixed, bacteria repelling proteins will preventfloating bacteria from adhering to the substrate.150

Fig. 3 Incorporation of drugs within an interpenetrating network hydrogelmatrix, and successive release under different stimuli.151

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EPD is a colloidal electrochemical process involving themigration of suspended, charged particles and/or moleculestoward an electrode (anode or cathode) by means of an appliedelectric field (Fig. 7).26 The electrical charge of the particles(or ions) involved, together with their mobility within theelectrolytic solvent are considered the main driving forces forthe successful deposition of these colloidal entities.25 Thedriving force and magnitude of the flow during EPD are mostlysuspension and process dependent, as summarized in Table 1.25–27

Among the most important parameters related to the EPDsuspension, particle size is worth mentioning, as it directlyaffects the stability of the electrolyte. No general law has beendefined regarding this criterion, although concerns with respectto the settlement due to gravity of larger particles have beenreported.25 This issue can affect homogeneity, as well as the

probability of crack formation in the resultant final coating.25

Colloidal particles larger than 1 mm in particular, have beendocumented to require uninterrupted hydrodynamic agitationin order not to flocculate.25 The dielectric constant of thesuspension on the other hand, directly affects its conductivityand therefore its deposition capability. More specifically, a highdielectric constant increases the ionic concentration of thesuspension, resulting in a small double layer region, diminishingthe electrophoretic mobility. A low dielectric constant on theother hand, will impede successful deposition due to a lack ofdissociative power.25 An essential factor for the success of theEPD process is the zeta potential of the particles. Too lowparticle charge will result in heavy particle packing densityand agglomeration due to low repulsion.25,28 It is vital tomention that every set of particle-suspensions needs its own

Fig. 4 Illustration of silanization of titanium substrates.152

Fig. 5 Example of a LbL-made pH-responsive composite polymeric coating which releases antimicrobial agents when environment acidification due tobiofilm formation occurs; (A) without coating; (B) with coating.153


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and specific parameter optimization. Usually a narrow bandrange of the above-mentioned parameters successfully worksfor different types of materials. Last but not least, it has to beacknowledged that many of these parameters are closely inter-related.

Another set of parameters worth mentioning, are thoserelated to the EPD process itself, namely the deposition time,voltage, concentration of solid and the conductivity of thesubstrate.25 It is generally accepted that for constant voltages,an initially linear relationship between deposition mass andtime can be established, followed by a plateau for longerperiods. Again, the applied voltage is closely related to the type

of materials and suspensions involved, although it can beordinarily stated that the deposition mass increases withapplied voltage. Nonetheless, if too high a voltage is applied,suspension turbulence may occur, preventing the formation ofa closed-pack deposit, therefore negatively affecting the coatingquality.25 Finally, a low conductivity of the electrode’s substratehas been shown to diminish the deposition rate, leading to anon-uniform final structure.25

Comparison studies have also shown that EPD yields ahigher particle loading efficiency compared to classic drop-castingmethods,29 resulting in a higher antibacterial efficiency to thatof silanization (TESPA, CPTES) and atomic transfer radicalpolymerization (ATRP).30

Among existing electrochemical approaches for coatingdeposition on biomaterials, EPD can be positively consideredas one of the most promising methods.23 A number of goodreviews can be found on the topic,23–25 whereas in this paperthe focus lies on the less-considered antimicrobial applicationof this technique.

Nanostructured composites

One of the major advantages of the electrophoretic depositionmethod, is the allowance to add a variety of agents by simplyincorporating them into the electrolytic solution. To achieveantimicrobial efficiency, researchers have used a variety ofmaterials; metal oxides, nanoparticles, antibiotics, peptidesand other organic and inorganic compounds. Nano (NPs) andmicro particles in particular, due to their wide-ranging anti-microbial efficiency and the low risk of inducing bacteria

Fig. 6 Comparison of coating thickness ranges between different deposition processes.154

Fig. 7 Illustration of the cathodic electrophoretic deposition process.When the voltage is applied, colloidal particles migrate from the positiveto the negative electrode.155

Table 1 Parameters influencing the electrophoretic deposition process

Parameters influencing EPD

Electrolyte Particle size Dielectric constant of liquid Conductivity of suspension Viscosity Zeta potential StabilityProcess Time Voltage Conductivity of substrate Concentration of suspension


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resistance, are currently being extensively studied. The highprocessability control of EPD, minimizes the amount of NPslost during the procedure, reducing common environmentaland toxic concerns about free NPs.31 Some of the most widelyemployed NPs and ions in EPD have been summarized inTable 2.

Antibiotics on the other hand, are easy for loading intocoatings, but their release will only last for a short period.32

According to a recent study of thin coating drug carriers,33 overthe first hour more than 80% (up to 90%) of the total drug isreleased, making it efficient for only two days at most.33–37 Theversatility of electrophoretic deposition, enables the tuning ofthe amount of drug deposited and released by means ofparameters such as electrolyte composition, time and voltageof deposition. Gentamicin, tobramycin and vancomycin areamong the most widely employed antibiotics for local antibioticinfection prophylaxis deposited through EPD (Table 3).

The versatility and flexibility of EPD, have allowed researchersto develop multifunctional, nanostructured composite coatingson biomaterials.23 Besides antimicrobial efficiency, otherfunctionalities including osteogenesis, control of the inflam-matory response and enhanced vascularization are necessaryto achieve considerable increase in the implant’s lifespan.EPD facilitates this implementation in just a one-step proce-dure. Alongside antimicrobial agents, biocompatible materialswith osteoinductive and osteoconductive properties such asbioactive glasses, carbon nanotubes and hydroxyapatite areamongst the most widely adopted within the EPD electrolyte(Table 4).

As stated previously, to effectively control the release ofthe above-mentioned antimicrobial and biological agents,a polymeric matrix is usually employed as a drug reservoir.EPD enables deposition of a wide range of materials encom-passing metals, ceramics, and synthetic (Table 5) and natural

Table 2 Summary of studies implementing antibacterial oxides, micro and nanoparticles

Antimicrobial agent Polymeric matrix Implant substrate Pathogen Ref.

Silver Chitosan/gelatin Titanium S. aureus 50Silver E. coliStainless steelIndium tin oxide

Silver Chitosan/gelatin Porous titanium S. aureus 57Silver PEGDA-AA Titanium S. aureus 156

E. coliP. aeruginosa

Silver Chitosan Stainless steel S. aureus 31Silver/lignin — Titanium S. aureus 95 and 96Silver — Titanium nanotubes P. aeruginosa 157Silver — Super-hydrophobic

titanium nanotubesS. aureus 158

Silver Chitosan Nitinol E. coli 159Silver PEEK Stainless steel E. coli 160Silver/graphene — Titanium S. aureus 133

E. coliSilver/chrome — Ni/AISI1018 steel S. aureus 161

E. coliSelenium Chitosan Titanium nanotubes E. coli 162Strontium Chitosan/PVP Titanium nanotubes S. aureus 163

E. coliSamarium/zinchallosyte nanotubes

— Titanium S. aureus 164E. coli

Gold Chitosan Titanium S. aureus 165Gallium Chitosan/PAA Titanium E. coli 166

P. aeruginosaNickel — Indium tin oxide E. coli 167

B. atrophaeusCopper PEGDA Stainless steel S. aureus 168

E. coliCopper (nanocubes) — Titanium nanotubes S. aureus 169

E. coliCopper — Titanium S. aureus 170

E. coliCopper — Aluminum E. coli 171Cerium Poly(3,4-ethylenedioxypyrrole-co-3,4-

ethylenedioxythiophene) (P(EDOP-co-EDOT))Stainless steel S. aureus 109

E. coliCopper/zinc — Nickel foam S. aureus 172

E. coliZinc Alginate Stainless steel E. coli 173Zinc Chitosan/polypyrrole Indium tin oxide S. aureus 174

E. coliZinc/silver — Stainless steel S. aureus 175

E. coliZirconium/zinc — Titanium alloy — 176


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polymers. The innate complexity and at the same timetechnical equilibrium evolutionarily achieved by naturalmaterials such as polysaccharides though, is hardly found inother compounds.5

As far as concerns the EPD technique, the natural polysac-charide chitosan and its derivatives38 are by far the most widelyemployed polymers especially for antibacterial purposes due totheir intrinsic bactericidal properties.

It’s been near a decade since the feasibility of electrodepositionof chitosan hydrogels has been assessed.39,40 Moreover, differentparametric studies have been performed to evaluate appositecriteria like the film growth rate or thickness as a function ofpolymer concentration, pH, voltage and time-duration amongothers.41,42 The suggested main mechanism behind the electro-deposition of polysaccharides consists of a sol–gel transitioninitiated by an electrical signal or field through a cathodic oranodic neutralization process, depending on the polymer’scharge.43

In the case of chitosan, its primary amines are neutralizedby the high cathodic pH occurring during the process, which inturn instigates the gelation of a pH-responsive film over thecathode’s surface.43 Under basic and neutral conditions, thelayer can be considered stable, however, if subjected to anacidic environment, the film dissolves again reprotonating theamines.43

To mitigate the eventual contact-killing effects of chitosanagainst host cells and at the same time enhance their adhesion,researchers have come up with the idea of combining chitosanwith gelatin (Gel). This protein is one of the main componentsof collagen, the major player constituting the extracellularmatrix.44 The presence of the RGD peptide has made gelatinan inviting factor for cell attraction,45 simultaneously decreasingbacteria anchoring/adhesion.46 Moreover, the higher solubility ofgelatin compared to that of chitosan, helps the combined coatingto better control biodegradability and drug eluting properties.47

In addition, Jiang et al.48 showed that the inclusion of gelatinincreases significantly the shear bonding strength of the hydrogelcoating with the titanium surface. After assessing the cell viabilityand proliferation in their first pilot study,48 they provided resultsachieved by characterizing the osteogenic performance of Chi-Gelin vivo, concluding that their composite coating facilitated osteo-genesis and the early bone healing process.49 In fact, after 8 and

12 weeks since operation, the evident presence of the osteoblastwas observed compared to the almost empty surface of bareimplants.49

Taking advantage of these promising results, Wang et al.50

introduced silver nanoparticles with the Chi-Gel matrix on titanium,stainless steel, silver foil and indium-tin-oxide (ITO), showingantimicrobial efficacy in vitro. Compared to the standard procedure,they added H2O2 and 1-ethyl-3-(3-dimethylaminopropyl)carbo-diimide (EDC) to the solution in order to improve the mechanicalstrength and smoothness of the films by lowering water hydrolysisand consequently bubble formation.

Nonetheless, the fast degradation of the polymeric matrixand its hydrogen bonding with the drug due to polar molecules,have limited the attainment of high-control long-term drugrelease.51

To mitigate this effect, researchers have recently focused onincorporating drugs within micro/nano polymeric spheres, notonly for enhancing their interconnecting binding, but alsoexploiting their high specific surface area for more active sitesand higher electrostatic capture.52

Zhao et al.53 for instance, electrodeposited chitosan withembedded mesoporous silica nanoparticles loaded with amodel drug. They developed a dual stimuli-responsive releasesystem by controlling the pH of the medium and the electricalfield, showing that the higher the negative potential applied,the faster is the release of the drug.

Similarly, Chen et al.52 incorporated levofloxacin into poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) microsphereswithin an alginate polymeric matrix, achieving in vitro anti-bacterial activity against E. coli. The amount of drug loaded canbe flexibly tuned by either modifying the drug to polymer ratioduring microsphere manufacturing, or by changing the con-centration of the EPD solution.52

Within the same line of reasoning, Kong et al.34 recentlyincorporated chitosan nanospheres into a thin mineralizedcollagen coating loaded with vancomycin hydrochloride. Thelarge specific surface and good affinity of chitosan nano-particles with the drug, together with their stable bonding withcollagen, showed successful inhibition against S. aureus withno toxicity on pre-osteoblastic MC3T3-E1 cells.

Following the same trend, Cai et al.54 fabricated tetracyclineloaded chitosan–gelatin nanospheres on titanium (Fig. 8) effectively

Table 3 Summary of studies implementing antibiotics

Antibiotic Polymeric matrix Implant substrate Pathogen Ref.

Vancomycin/moxifloxacin Chitosan/gelatin nanospheres Stainless steel S. aureus 56Gentamicin Chitosan Stainless steel S. aureus 177Vancomycin Chitosan Titanium S. aureus 178–180Vancomycin Chitosan/gelatin Porous titanium S. aureus 57Gentamicin — Titanium S. epidermis 181Vancomycin Chitosan/graphene oxide Titanium S. aureus 127Levofloxacin Alginate/poly(3-hydroxybutyrate-co-3-hydroxyvalerate)

(PHBV) microspheresStainless steel E. coli 52

Vancomycin Chitosan nanospheres/collagen Titanium S. aureus 34Tetracycline Chitosan/gelatin nanospheres Titanium S. aureus 182

E. coliPenicillin PLGA/PPy Titanium S. epidermis 183Streptomycin


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killing S. aureus and E. coli in vitro. The poorly controlled deliverythough, resulted in a significantly brief burst release of only 2 hours,hence not meeting clinical requirements. Interestingly, in theirfollow up in vivo study on rabbits, they successfully preventedinfection up to 7 days after operation. Although the group with a

high concentration of tetracycline showed heavy toxicity to MC3T3cells in vitro, the in vivo results outperformed all the otherexperimental groups (including the systemic delivery of tetra-cycline), both in terms of antibacterial efficiency and whiteblood counting.55 These somehow controversial results, once

Table 4 Summary of studies implementing antibacterial and bioactive agents

Osteogenic agentAntimicrobialagent Polymeric matrix Implant substrate Pathogen Host cell Ref.

Hydroxyapatite Silver — Titanium nanotubes E. coli MC3T3-E1 184P. aeruginosa

Hydroxyapatite Silver — Alkali treatedtitanium

S. epidermis Osteoblasts(SD rats)

185E. coli

Hydroxyapatite Silver — Ti6Al4Zr E. coli MG-63 186Hydroxyapatite Copper/zinc — Titanium E. coli MC3T3-E1 187Hydroxyapatite/strontium Copper — Titanium E. coli MC3T3-E1 188Hydroxyapatite Copper — Ti6Al4V S. aureus MG-63 189

E. coliHydroxyapatite Zinc — Titanium-coated

siliconE. coli NHI3T3

fibroblasts190

Hydroxyapatite/europium Cerium — 316L stainless steel S. aureus MG-63 109E. coli

Hydroxyapatite/strontium Cerium Sulphonated PEEK 316L stainless steel S. aureus MG-63 191E. coli

Hydroxyapatite/strontium/magnesium

Cerium Poly(3,4-ethylenedioxypyrrole-co-3,4-ethylenedioxythiophene)(P(EDOP-co-EDOT))

316L stainless steel S. aureus Mg-63 108E. coli

Hydroxyapatite/calciumsilicate

Silver — Titanium nanotubes S. aureus MC3T3-E1 192

Hydroxyapatite/graphene Silver — Titanium S. aureus Peripheral bloodmononuclear cell(PBMC)

133E. coli

Hydroxyapatite/graphene — — Titanium S. aureus PBMC 193E. coli

Hydroxyapatite — Chitosan/graphene Titanium S. aureus PBMC 140E. coli

Fluorohydroxyapatite Silver/copper/zinc

— Stainless steel S. aureus — 110

Fe3O4 carbonatedhydroxyapatite

Gentamicin — Titanium S. epidermis Human bonemarrow stromalcell (hBMSC)

181

Hydroxyapatite/strontium Samarium/zinc-hallosytenanotubes

— Ti6Al4V S. aureus MG-63 164E. coli

Nanophase hydroxyapatite — — Titanium S. aureus Macrophages 77 and194E. coli

P. aeruginosaHydroxyapatite/calciumphosphate

Silver — Titanium E. coli MG-63 195

Bioactive glass Zinc Alginate/PVA/chitosan Stainless steel S. aureus — 196S. enterica

Bioactive glass Zinc Alginate Stainless steel E. coli — 173Bioactive glass Silver Chitosan Stainless steel S. sureus MG-63 31Bioactive glass Silver PEEK Stainless steel E. coli MG-63 160Bioactive glass Gentamicin Chitosan Stainless steel S aureus MG-63 177Bioactive glass Ampicillin Chitosan Titanium S. mutants MC3T3-E1 197Bioactive glass/strontium Zinc Chitosan Stainless steel — — 198Calcium silicate Strontium Chitosan/PVP Titanium nanotubes S. aureus MG-63 163

E. coliCalcium phosphate — — Anodized titanium S. aureus — 199Calcium phosphate/carbonnanotubes

Gentamicin Chitosan AZ91D magnesiumalloy

— SaOS-2 200

— Carbonnanotubes

Poly(N-vinylcarbazole) (PVK) Stainless steel E. coli — 201

— Tannic acid Poly(isobornyl acrylate-co-dimethylaminoethyl methacrylate)/tannic acid (P(ISA-co-DMA)/TA)

Magnesium S. aureus Mouse fibroblastL929

118E. coli

Dexamethasone Penicillin PLGA/PPy Titanium S. epidermis hFOB 183Streptomycin NIH-3T3

TIB-186


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again expose the well-known inconsistency between in vitro andin vivo studies, stressing the demand for more reliable assays.

To mitigate the shortcomings of the unrestrained delivery ofantibiotics, Song et al.56 altered Cai’s chitosan–gelatin hybridnanosphere approach54,55 by first incorporating vancomycinand moxifloxacin in gelatin nanospheres, and subsequentlydepositing them within a chitosan matrix. Interestingly, thehigh affinity between gelatin and vancomycin resulted in asustained release for nearly 2 weeks, which was not the casewith the previously reported one-step approach.54 Moxifloxacininstead, showed a burst-mode profile, stemming from itsweaker interactions with gelatin compared to vancomycin.

In an even more recent study,57 our group developed acomposite coating based on vancomycin and/or silver withina chitosan/gelatin polymeric matrix on additively manufacturedporous titanium scaffolds (Fig. 9). The huge surface area ofthese last mentioned, together with the migrating natureof EPD, enabled efficient drug loading efficiency, with fulleradication (Z99.9%) of S. aureus for up to 21 days. Interestingly,when vancomycin and silver were employed concurrently at lowerconcentrations, they displayed a combinatorial effect with a

significantly higher sustained delivery and almost no initial burstrelease compared to when applied separately. Besides, the highantibacterial activity was fully preserved. These results show thepromising potential of combining different antibacterial agentsas a feasible approach to achieve synergistic bactericidal effects.57

The versatility of EPD prompted researchers to try differentcompounds and combinations to achieve antibacterial andbiocompatible coatings through the years, resulting in anoverabundance of studies with relatively high success ratesin vitro. However, few, if any of these coatings were furthertested in vivo, let alone reaching clinical trials. A systemicdesign rationale, which takes into account the precise chrono-logical order and dose of delivery of different antibacterialagents, growth factors and host-response modulators, is usuallydisregarded, overshadowed by the notable ‘‘ease-of-use’’ ofEPD. As will be presented in the following sections, combiningclassical EPD with LbL or its recent alterations (alternativecurrent EPD, pulsed EPD) are some feasible approachesto rationally obtain matrix deposition and drug loading withtunable degradation and release kinetics. In addition, with theaid of high-throughput screening, the cost and time consumption

Table 5 Summary of synthetic polymers employed in EPD

Synthetic polymer Implant substrate Pathogen Ref.

Poly(1,3-bis-(p-carboxyphenoxy propane)-co-sebacic anhydride) Stainless steel E. coli 52PLGA/PPy Titanium S. epidermis 183PEDOT-F4 Stainless steel P. aeruginosa 202PEDOTH8 L. monocytogenesPoly(N-vinylcarbazole) (PVK) Stainless steel E. coli 201PEEK Stainless steel E. coli 160poly(3,4-ethylenedioxypyrrole-co-3,4-ethylenedioxythiophene) (P(EDOP-co-EDOT)) Stainless steel S. aureus 109

E. coliSulphonated PEEK Stainless steel S. aureus 191

E. coliChitosan/PVP Titanium nanotubes S. aureus 163

E. coliPoly(ethyleneglycol) (PEG) Titanium S. gordonii 203

S. mutansPoly(ethyleneglycol) (PEG) Titanium S. sanguinis 204

L. salivariusPoly(isobornyl acrylate-co-dimethylaminoethyl methacrylate)/tannic acid (P(ISA-co-DMA)/TA) Magnesium S. aureus 118

E. coliAlginate/PVA/chitosan Stainless steel S. aureus 196

S. entericaAlginate/poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) microspheres Stainless steel E. coli 52

Fig. 8 Schematic diagram of the steps in the electrophoretic deposition of a chitosan/gelatin/tetracycline/nanosphere composite in a titaniumsubstrate.54


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required for parameter optimization and material selection, can besignificantly reduced.58

EPD as a tool to createmicro/nanostructure surfaces

With the advent of tissue engineering and additive manufacturingtechniques, porous structures have been revealed to be beneficialsubstitutes to solid materials, especially when aiming to enhancetissue integration and regeneration.59–64 As a result of the obtaineddesign versatility, a variety of networks can be contrived, attainingenhanced fluid transport, tissue ingrowth and consequentlybetter mechanical fixation.65–67 In addition, their open structurescontribute to an exponentially larger surface area compared tothe solid ones,68 which can be aptly filled by malleable polymericcarriers such as hydrogels. This determining advantage can beadopted for drug/coating incorporation with bio-functionalizingand/or antimicrobial agents, especially in non-union bonedefects,68–74 The electrical field induced by EPD, makes mostof the colloidal particles migrate towards the inner partsof porous/complex materials, which can hardly be achievedwith conventional techniques. Remarkably, EPD itself can beapplied as a tool to create intrinsically antibacterial highlyporous surfaces and controlled micro/nano topographies in atemplate-mediated fashion.

For instance, Braem et al.75 developed micrometer-scale poroustitanium substrates by electrodepositing TiH2 powder suspensionswith subsequent dehydrogenation and sintering (Fig. 10).

Hence, they managed to lessen the sintering temperature ofthe non-treated material without losing its mechanical properties.Moreover, the EPD technique enabled them to achieve a coatingthickness of 80–200 mm, porosity of 37–52%, pore interconnectionthroat size of 1.6–4.9 mm and tensile strength higher than30 MPa.75 They later managed to reach controlled macropore-sized surfaces by means of the same method but with a mixture

of emulsions and suspensions of TiH2. The antimicrobial assess-ment of these coatings was later performed, showing a decrease inthe number of adherent bacteria compared to classical and com-mercial vacuum plasma sprayed coatings. Likewise, when theappropriate particle size was chosen, the adherence of S. aureusand S. epidermis was comparable to those of smooth substrates.76

In a more recent study, Webster et al.77 synthesized a nano-phase layer of hydroxyapatite on titanium implants by means ofEPD without the inclusion of any additional drug. The nano-scale texture (especially in the 110 nm range), exhibitedsignificant antimicrobial efficiency against S. aureus, P. aureginosaand E. coli compared to non-treated and plasma sprayed titanium(Fig. 11). In addition, they investigated the macrophage adhesionand proliferation, showing that their drug-free coating mitigatedthe inflammatory response, resulting in a shorter wound healingperiod and less risk of osteolysis.77 In similar fashion, the samegroup adopted EPD in ethanol medium as a tool to create ananophase TiO2 surface nanotextures with different topographies,achieving 1 log reduction in S. aureus and P. aureginosa, and at thesame time significantly enhancing osteoblast proliferation after3 and 5 days.78

Lately, inspired by the bactericidal effect of the nanopillarstructure of cicada’s wings,79,80 Wu et al.81 fabricated differentnanotopographies on gold substrates taking advantage of

Fig. 9 Chitosan/gelatin/silver/vancomycin composite electrodeposited on additively manufactured porous titanium scaffolds.57

Fig. 10 Porous coatings by means of EPD of powder suspension.75


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electrodeposition on defined templates. They developed nano-pillars, nanonuggets and nanorings and compared their anti-microbial efficiency against MRSA (Fig. 12). Remarkably, theymanaged to attain up to 3 log in bacterial reduction for all threenanotopographies without any additional antibacterial agent.

Another creative study performed by Hoop et al.,82 combinedtemplate-assisted electrodeposition and metal evaporation todevelop silver-coated magnetic nanocoils (Fig. 13). The magneticproperties of the nanocoils provided for the accurate handling andcontrol of the nanostructures through low magnitude magneticfields by means of corkscrew motion,82 with their efficacy assessedalso in vivo.83 Interestingly the antimicrobial efficiency againstS. aureus and silver-resistant E. coli was shown to be up to99.9999%, corresponding to 6 log reduction after 2 hours. Besides,the nanocoils didn’t show any cytotoxic effect on the fibroblastmouse cells even for concentrations as high as 100 ppm ofsilver ions.

The above-mentioned studies, clearly show the potential ofEPD not only as a technique for direct organic/inorganic materialdeposition, but also as a versatile tool for developing template-mediated substrates.

Alternative electrodepositiontechniques

When positive particles migrate toward the negative electrode(cathode), we have a cathodic electrophoretic deposition. Onthe other hand, when negative particles are present in the

Fig. 11 Colony forming unit (CFU) results for S. aureus on plain titanium,plasma sprayed titanium and 110 nm EPD coated HA on titanium after1 hour in 1%, 5% and 10% FBS and 16 hours of bacterial incubation.*P o 0.01 compared with plasma-sprayed-deposited hydroxyapatite onTi, incubated in 1% FBS; **P o 0.01 compared with Ti (control), incubatedin 1% FBS; ***P o 0.01 compared with samples coated with 110 nmhydroxyapatite, incubated in 1% FBS; ****P o 0.01 compared with samplescoated with 110 nm hydroxyapatite, incubated in 5% FBS.77

Fig. 12 Contact killing effects of nanostructured gold substrates. (a) Step-by-step schematic representation of the experiment. (b) Proliferation ofbacteria over 1 � 1 cm2 substrate. (c) Viable cell counting (CFU), *P o 0.001 vs. the reference substrate.81

Fig. 13 Overview of the fabrication of Pd/Ni/Ag nanocoil structures: (a) (i)electrodeposition of Pd (blue)/Cu (orange) nanowires in AAO templates(grey), followed by dissolution of the template (NaOH), and selectivede-alloying of Cu(HNO3). (ii) Physical vapor deposition of 7.5 nm Ni layer(green). (iii) Physical vapor deposition of 10 nm Ag layer (grey). (iv) Directedlocomotion of the nanocoils in low magnetic fields. (v) Bactericidal activity.(b) Elemental composition by means of EDX. (c) SEM image of a Pd/Ni/Agnanocoil (scale bar 200 nm). (d) EDX map of Pd distribution (scale bar200 nm). (e) EDX map of Ag distribution (scale bar 200 nm).82


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electrolyte, they’ll move toward the positive electrode (anode),resulting in an anodic EPD process. Recently, Gray et al.43

compared these two methods (Fig. 14); according to their study,the chitosan film created under the process is chemically andphysically different than the cathodic version, resulting in acovalently cross-linked hydrogel stable over a wider range ofpH.43 Other than the benefits of cathodic electrodeposition,anodic EPD avails of the formation of a stimuli-responsiveswelling/de-swelling film, exploitable for the selective triggerand release of therapeutic agents under specific biological(biofilm, tumor etc.) or non-biological (microfluidic actuators/valves) conditions.43

Capitalizing on a similar mechanism, Wang et al.84 combinedcathodic and anodic electrophoretic deposition with the LbLmethod, alternating chitosan and alginate on titanium. The pHdecrease on the anode resulted in the anodic deposition ofanionic alginate, while the pH increase on the cathode followedcationic chitosan deposition.84 Electrostatic forces enabledthe composite film to maintain its stability; in addition, it hasbeen reported that the anodic electrodeposition of a cationicpolyelectrolyte in the presence of multivalent anions is mosteffective when the polyvalent positive and negative charge are inan optimum range with respect to each other, which is the caseof alginate and chitosan.85

Another novel approach to achieve sustained release hasbeen recently presented by Ordikhani et al.,51 where a multi-layer chitosan/bioactive glass/vancomycin nanocomposite coatinghad been deposited by a multistep EPD procedure. The final eight-layer coating showed a minimal initial burst release (8%), with asubsequent 40 days zero-order release, resulting in successful S.aureus growth inhibition and MG-63 osteoblast-like cell viabilityhad also been achieved.

In a similar fashion, Chen et al.86 electrodeposited daidzen-loaded poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) micro-spheres by being enclosed in an alginate–PVA matrix. Theysubsequently deposited multiple layers of chitosan and alginateby means of LbL deposition to tune the release of the microspheres.

In fact, they showed that 42 wt% of the coating was preservedafter two weeks, with no significant loss in adhesion strengthduring degradation and a more sustained release compared tothose of non-encapsulated microspheres.

In a recent and elegant study, Geng et al.87 developed a novelEPD method, consisting of the coordination of chitosan withmetal ions generated in situ by simultaneous electrochemicaloxidation. More specifically, the amino groups present inchitosan, enables this polymer to coordinate with transitionmetal ions.87–90 As a result, the ensuing coating shows highlyhomogeneous properties (due to the lower bubble generation),and increased strength and stability in acidic environments.

AC and pulsed fields

Although successful for certain experimental conditions, classicaldirect current electrophoretic deposition suffers from the electro-lysis of aqueous suspensions.27 O2 and H2 gases generated at therespective electrodes, lead to bubble formation and pH shiftswhich can result in non-uniform and low-quality coatings.27 Oneapproach to deal with this issue is to employ modulated electricalfields, like alternating current (AC) and pulsed direct current(PDC).27 In the former, the voltage changes between the positiveand negative value, while in the latter, it varies keeping its signconstant27 (Fig. 15).

A more detailed explanation of the chemo-physical mechanismsinvolved with these approaches has been reported else-where.26,27,91,92 In both modulated fields, the lower rate ofelectrolysis generated helps achieve high reproducibility,homogeneous and crack-free coatings, well-aligned micro and

Fig. 14 Partial oxidation of chitosan in anodic electrodeposition comparedto cathodic provided by Schiff test.43

Fig. 15 Electrical signal schematic representation: (A) continuous directcurrent (CDC), (B) pulsed direct current (PDC), (C) symmetrical alternatingcurrent (AC) with no net DC component, (D) asymmetrical AC signal withnet DC component and, (E) asymmetrical AC wave with no net DCcomponent.27


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nano structures and the ability to deposit biochemical andbiological agents for strong bioactive layers.27

Within the antimicrobial context, a recent study reportedthe application of AC-EPD of an antifungal peptide on titaniumimplants against C. albicans. Specifically, they combined ACsignals with a mild DC offset, which resulted in a significantlyhigher deposition efficiency of the antifungal lipopeptide caspo-fungin (CASP). Interestingly, although the DC offset of approxi-mately 2 V cm�1 doesn’t show the relevant material deposition,when superimposed on the AC wave, it results in a synergisticdeposition yield of up to 6 fold thickness increment.93 Last butnot least, this approach showed an intrinsic filtering behavior,separating sugar impurities present in the initial suspension,resulting in a final uncontaminated film.93

Yan et al.,94 recently took advantage of PDC to depositchitosan/hydroxyapatite NPs/silver NPs on titanium. The double-mediation role of chitosan toward HA-NPs and Ag-NPs resulted inanti-wear and antimicrobial effects against E. coli and S. aureus upto 99.9% with good bioactive properties. Specifically, the formationof chitosan–Ag+ complexes during PDC, moderated the depositionof Ag+ to attain a well-dispersed distribution, simultaneouslypreventing aggregation due to electrostatic repulsion.94 Besides,the hybridization of chitosan in HA, resulted in a more sustainedrelease of Ca2+, benefiting bone ingrowth.

Novel antimicrobial compounds

The emergence of bacterial strains resistant to antibiotics, hasled scientists to search for suitable alternatives to replace or atleast supplement classical drugs. Concurrently, these novelmaterials should be available or convertible to their colloidalform to be employed in the EPD electrolyte.

On this basis, Erakovic et al.95–97 recently introduced lignin(Lig) as an emergent biocompatible, antimicrobial and anti-oxidant candidate for incorporation in their hydroxyapatite/silver composite coating. Lignin is a natural polymer composedof associated phenolic moieties.98 Its biocompatibility, thermalstability and resistance against cracks, make it a suitablecomposite component for higher stability and better porousinterconnection of hydroxyapatite-based coatings.95,99

Another novel compound with promising antibacterialeffects is torularhodin. This material has a carboxylic terminalgroup produced by Rhodotorula rubra which possess strongantioxidant and antibacterial100 properties for use even in thefood industry.101 Exploiting these qualities, Ungureanu et al.101

made a new composite coating constituted of Torularhodinincorporated in polypyrrole (PPy) films, achieving bactericidaleffects against both Gram positive (S. aureus, B. subtilis andE. faecalis) and Gram negative strains (E. coli and P. aeruginosa),with no cytotoxicity. PPy itself enhances cell adhesion andproliferation of a large variety of cells in vitro and in vivo101–105

with enhanced antibacterial effects when combined with poly-ethylene glycol (PEG).101

A new emerging polymer used for the biofunctionalizationof orthopedic implants, is silk fibroin. This negatively charged

material, has to be cationized before being employed in theEPD set-up.106 Tetracycline antibiotic moieties can undergoprotonation–deprotonation reactions when crosslinked withsilk fibroin, causing the cationization of the latter.107 The resultingcomposite coating acted as the ‘‘soft component-hard deviceinterconnect’’, effectively inhibiting S. aureus and E. coli, butcompromising osteoblast-cell proliferation at the same time.106

Cerium (Ce3+), a lanthanide ‘‘rare earth’’ element, has beenshown to have effective antimicrobial properties even at lowconcentrations, with low or no cytotoxicity to host cells.108–112

Interestingly, Ce3+ has attracted attention also for its capacity toget substituted within hydroxyapatite structures, resulting inmultifunctional osteogenic–antibacterial composites.108,109

Although antimicrobial peptides (AMPs) are emerging topotentially replace classical drugs in the post-antibiotic era,few studies have employed EPD as a means for deposition. In arecent study, Braem et al.,93 exploited the superior properties ofAC EPD to delicately deposit the antifungal lipopeptide drugcaspofungin (CASP), achieving a crack/bubble-free antibiofilmcoating against C. albicans.

Following this trend, Akbulut et al.113 managed to electro-chemically polymerize and deposit polythiophene (PTs) carryinghomopeptide side chains, ready to be decorated with functionalbiomolecules for biosensing applications. Surprisingly, thisprecursor layer showed unexpected intrinsic antibacterial activityagainst S. aureus, leaving the Gram-negative E. coli unaffectedthough. Other than the above-mentioned studies, the electro-deposition of AMPs still remains an unexplored area with greattherapeutic potential. Recent alternatives to the classical electro-deposition process (AC, PDC), could be further explored topromote AMP application without the risk of damaging thesedelicate biomolecules, and at the same time keeping the competitiveadvantage of EPD of being a one-step procedure.

In a recent study, Yu et al. employed the Chinese herbalmedicine naringin, which has been recently shown to trigger thesecretion of bone morphogenetic proteins (BMPs), for the benefitof osteoblasts and mesenchymal stem cells (MSCs).114–116

Besides, antimicrobial activity against P. gingivalis and A. actino-mycetemcomitans have also been reported for this material.117

In order to tune its release kinetics, Yu et al.114 took advantageof the versatile properties of metal–organic framework (MOFs)nanocrystals in terms of drug carrying abilities, and electro-deposited the composite on titanium. Interestingly, they reportedenhanced proliferation, differentiation and attachment of MSCs,with significant antibacterial activity against S. aureus for up to7 days (Fig. 16).

Another study introduced tannic acid, a natural polyphenoldendroid118,119 with intrinsic antibiofilm properties118,120,121

which has successfully been employed as an agent stabilizerin a number of recent studies.118–122 This tannin-derived molecule,binds firmly to active antibiotics (i.e. gentamicin), forming non-eluting coatings at neutral pH.119 Principal pathogens involved inbiomaterial-associated-infections such as S. aureus, S. epidermisand E. coli, produce lactic acid right after adhering to the substrate.When the local pH drops below neutral conditions, the compositefilm starts to release its antibacterial agents in a responsive


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manner, preventing therefore the excessive and unrestricteddelivery of antibiotics. Inspired by these results, Sun et al.118

electrodeposited a colloidal composite made of the P(ISA-co-DMA) copolymer, self-assembled with tannic acid on magnesium(Mg) surfaces. By chelating metal ions and hindering metallo-enzyme functions, which are essential for bacterial survival, thetannic-containing coated implants revealed significant anti-bacterial activity against S. aureus and E. coli in vitro. Moreover,in vivo Micro CT and histological studies exhibited improved

bone regeneration after 4, 8 and 12 weeks compared to bare Mg(Fig. 17 and 18). This latter is known for its fast degradation inbiological environments.123 Remarkably, the coated implantsnot only mitigated this effect in favor of a sustained tissueingrowth and subsequent bone regeneration over a period of12 weeks, but rationalized the delivery of Mg2+, reported toenhance osteogenic activity.124

Graphene oxide (GO) and its derivatives, has recently beenconsidered as a potential antimicrobial agent with interesting

Fig. 16 Illustration of the burst and sustained release of naringin in the MOF-loaded mineralized collagen coating.114

Fig. 17 Optical density of the adhered E. coli (A) and S. aureus (B) bacteria on different substrates, significant difference (**P o 0.05), no significantdifference (*P 4 0.05). Illustration of the underlying antibacterial mechanism (C).118


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mechanical and biocompatibility properties.125,126 Its hydro-philic groups can form covalent and non-covalent bonding withchitosan’s amine groups,127 resulting in a well-dispersed nano-composite structure with enhanced mechanical, thermal andantibacterial properties.128–130 Graphene oxide-based materials,have also been shown to induce higher toughening effects com-pared to carbon nanotubes.131 Concurrently, a purer synthesisof GO, has lowered the metallic catalyst particles involved in itsproduction, therefore overcoming the common cytotoxicity issuesassociated with carbon nanotubes.132,133 A detailed and well-written review article has been published on the fundamentals ofthe EPD of graphene-related materials134 (Fig. 19).

Nevertheless, as long as antibacterial effects are concerned,controversial results have been reported.135–138 In a recentstudy, Palmieri et al.138 investigated different environmentalconditions and GO concentrations often found in the literature.They deduced that different buffer solutions directly affect GOsurface zeta potentials and stability, resulting in differentcluster sizes which alter GO–bacteria interactions.138 Specifically,at low concentrations (r6 mg ml�1), GO is stable in all mediumsand effectively kills both Gram-positive (S. aureus) and Gram-negative (E. coli) by means of membrane disruption. Except foraqueous solutions, when the concentration of GO increases, clustersare formed which bulwark GO edges, impeding efficient bacteriakilling. Interestingly, at concentrations above 100 mg ml�1 in NaCl,CaCl2 and MgCl2, GO aggregates outsize bacterial diameters,resulting in bacteria in wounds and their consequent growthinhibition. This high versatility can be further exploited for anumber of biomedical and non-biomedical applications.

Shortly after the pioneering work of Akhavan et al.,139 inwhich GO nanowalls were electrodeposited on stainless steel

with significant antibacterial activity against S. aureus andE. coli, several researchers followed their approach. Ordikhaniet al.,127 prepared graphene oxide/chitosan/vancomycin coatingsby means of EPD, reporting the chemical interactions betweenthe chitosan’s amine group with the GO’s carboxylic group, lowcytotoxicity for less than 30 wt% of GO, and antimicrobialefficiency against S. aureus for up to 4 weeks. Similarly, Jankovicet al.,133 synthesized silver/hydroxyapatite/graphene, achieving

Fig. 18 Histological pictures of hematoxylin and eosin stained sections of the tissues around bare Mg (A1, A2 and A3), MgCP30 (B1, B2 and B3), andMgCP60 (C1, C2 and C3) after 4 (A1, B1 and C1), 8 (A2, B2 and C2), and 12 (A3, B3 and C3) weeks post-operation.118

Fig. 19 Inputs, side reactions and side reactions of graphene relatedmaterials deposited by means of EPD.134


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antimicrobial efficiency and apatite formation. Subsequently,the same group developed a chitosan/hydroxyapatite/graphenecomposite, this time without significant antibacterial activity,again stressing on the well-known GO controversial results.140

In a recent study, Qiu et al.141 investigated the effects ofdifferent voltages on GO layer formation and subsequent anti-bacterial and osteogenic behavior. They showed that the higherthe voltage (40 to 120 V), the stronger was the antibacterial andosteogenic activity of the coatings. As the thickness increasedwith increasing voltage, higher wrinkling and surface rough-ness were detected on the titanium substrate. According to theauthors, membrane-cutting or eventual trapping effects againstbacteria could therefore be excluded as the dominant mechanismsbehind the notable antimicrobial activity. That’s why they proposedoxidative pressure as the principal antibacterial mechanism, whichthey validated by measuring ROS production.141 Interestingly, therBMSCs proliferation test showed lower trends for GO-containinggroups, however, ALP activity, ECM mineralization and collagensecretion were significantly higher as the number of GO layersincreased.

Conclusion and future prospects

Although orthopedic surgeries can be considered successfulcompared to other clinical interventions, increasing demanddue to the aging population, has made it urgent to pursue suitableapproaches to increase the implant’s efficiency and lifespan.These concerns are accentuated by the advent of strongerantibiotic-resistant bacteria, resulting in the lower utility ofclassical antibiotic-based therapies.

Along with designing suitable drug carriers, the innate abilityof bacteria to grow resistance against classical antibiotics, hasresulted in the quest to discover new antimicrobial compounds,which alone, or in combination with antibiotics, could instigatesynergistic antibacterial effects. Few studies have been dedicated tothis type of evaluation though. EPD’s practicality offers a versatiletool for facile implementation of combinatorial therapeuticapproaches.

A plethora of antibacterial coatings have been investigatedin the literature, most of them though, have taken into account

too few of the many functionalities and relevant time spansneeded for their potentially successful implementations onimplants. In this regard, nitric oxide (NO) has shown greatclinical potential.142 Efficient antibacterial, antibiofilm andthromboresistant activities have been reported both in vitro143–146

and in vivo,147 with promising results as implantable sensors.Nevertheless, to our knowledge, no study has investigated thefeasibility of depositing NO-based coatings on orthopaedic implantsthrough EPD. Last but not least, co-culture studies of bacteria andcells are still in their infancy and an even higher paucity of clinicaland animal studies is limiting successful bench-to-bed translationin the field.

Likewise, any optimal coating should also fit the industrialand market requirements. Cost-effectiveness, versatility and easeof application are some of these prerequisites. Electrophoreticdeposition (EPD) hereof, benefits from the above-mentionedproperties as well as ease of application and scaling up ofproduction, and the ability for implementing a variety of com-pounds all at once or even in a ‘‘layer-by-layer’’ fashion, essentialto achieve multifunctionality and tunable release. Nonetheless,a limited number of studies have systematically investigatedrelevant mechanical properties such as the bonding betweenthe coating-material interface or endurance when exposed tomechanical strains under physiological conditions. In addition,the trial and error optimization process required for differentmaterials and shapes is time and money consuming, calling forsystemic standardization and integral mathematical modelling.The practicality of EPD has somehow dwarfed the demand for therational design of the sequential and sustainable delivery ofmultifunctional agents. Recent alterations of EPD, together withcombinatorial approaches with other deposition techniques(i.e. LbL), are some practical ways worth following.

The fast-growing field of tissue engineering and the adventof additive manufacturing and bioprinting techniques, haveintroduced novel materials with complex shapes in the form ofscaffolds, or as custom-made patient-specific implants. The lastmentioned are usually associated with high infection rates dueto their substantial superficies, demanding hybrid approachesbalancing tissue regeneration and antimicrobial efficacy (Fig. 20).148

To date, few, if any relevant research has been conducted for theinfection assessment of porous and complex architectures.148

Fig. 20 Critically-sized non-union fractures with antibacterial properties can lead to defect repair without the risk of failure due to osteomyelitis.148


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In this review, we have summarized the latest antibacterialcoatings developed by means of electrophoretic deposition,discussing the different aspects, drawbacks and prospects of theirimplementations for prevention and treatment of peri-prostheticinfections.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The research for this paper was financially supported by theProsperos project, funded by the Interreg VA Flanders – TheNetherlands program, CCI grant no. 2014TC16RFCB046.

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