Antibacterial Metallic Touch Surfaces

23 Pages·2016·2.8 MB·English

materials
Review
Antibacterial Metallic Touch Surfaces
VictorM.Villapún1,LynnG.Dover2,AndrewCross3andSergioGonzález1,*
1 FacultyofEngineeringandEnvironment,NorthumbriaUniversity,NewcastleuponTyneNE18ST,UK;
[email protected]
2 DepartmentofAppliedSciences,FacultyofHealthandLifeSciences,NorthumbriaUniversity,
NewcastleuponTyneNE18ST,UK;[email protected]
3 ACTSurfacesLtd.,1BearcroftAvenue,WorcesterWR40DR,UK;[email protected]
* Correspondence:[email protected];Tel.:[phone]
AcademicEditor:CarlaRenataArciola
Received:6July2016;Accepted:24August2016;Published:29August2016
Abstract: Ouraimistopresentacomprehensivereviewofthedevelopmentofmodernantibacterial
metallicmaterialsastouchsurfacesinhealthcaresettings. InitiallywecompareJapanese,European
and US standards for the assessment of antimicrobial activity. The variations in methodologies
definedinthesestandardsarehighlighted. Ourreviewwillalsocoverthemostrelevantfactorsthat
definetheantimicrobialperformanceofmetals,namely,theeffectofhumidity,materialgeometry,
chemistry,physicalpropertiesandoxidationofthematerial. Thestateoftheartincontact-killing
materialswillbedescribed. Finally,theeffectofcleaningproducts,includingdisinfectants,onthe
antimicrobialperformance, eitherbydirectcontactorbyalteringthetouchsurfacechemistryon
whichthemicrobesattach,willbediscussed. Weofferouroutlook,identifyingresearchareasthat
requirefurtherdevelopmentandanoverviewofpotentialfuturedirectionsofthisexcitingfield.
Keywords: copper;disinfection;antimicrobial;touchsurfaces
1. Introduction
Despitegreatinvestmentinresourcesdevotedtoenvironmentalandpersonalhygiene,ratesof
nosocomialorhospital-acquiredinfections(HAI)remainsignificant. ArecentEuropeansurvey[1]
estimated that the total annual number of patients with HAI in European acute care hospitals
was 3.2 million in 2011–2012. Another important issue is the continued emergence of antibiotic
resistancesinhealthcare,whichisagrowingconcernforpublichealth. However,greatvariationis
seenintheincidenceofantimicrobialresistanceacrossEurope[2].
Totacklethisproblem,newantimicrobialmaterialshavebeendeveloped. Metallicalloyshave
beenwidelyusedsinceancienttimestolimitmicrobialactivity. Forexample,silverbottlesandvessels
wereusedbyPhoenicians,RomansandGreekstostorewatertopreventitbeingspoiledbybacteria.
Copper was widely used in the pre-antibiotic era to tackle a plethora of infections ranging from
tuberculosistoskinproblems[3]. Toimplementtheiruseinthehealthcaresector,theseantimicrobial
elements could be mixed with other metals to optimise the final properties of the resulting alloy.
Forexample,brassesareamixtureofcopperandzincthatcouldbeusedinhospitaldoorfurniture
suchaspushplatesanddoorknobsduetotheirantimicrobialactivity. Thehardness,antimicrobial
performanceandcolourofthealloycanbetunedbychangingthecopper-to-zincratio. Othermixtures
suchascopper-silverandcopper-nickel,enableimprovementsintheanticorrosionperformanceand
durability of copper while maintaining good antimicrobial activity [4]; however, copper-silver is
primarily used for electrical applications while nickel-containing alloys are not recommended for
touchsurfacessincenickelisametalallergen.
Now, with antibiotic resistance on the rise, including the emergence of multi-drug resistant
superbugsabletotoleratethelast-resortantibiotic(colistin),areneweddrivetodevelopantimicrobial
Materials2016,9,736;doi:10.3390/ma9090736 www.mdpi.com/journal/materials
MMataerteiarlisa l2s021061, 69,, 97,3763 6 2 o2fo 2f22 3
antimicrobial materials is needed urgently. Hospital touch surfaces such as bed rails and handles can
materials is needed urgently. Hospital touch surfaces such as bed rails and handles can become
become a reservoir of pathogens that cause patient infections resulting not only in substantial
a reservoir of pathogens that cause patient infections resulting not only in substantial economic
economic losses, but even in patient death. Traditionally, stainless steel is used and sterilised on a
losses,buteveninpatientdeath. Traditionally,stainlesssteelisusedandsterilisedonaregularbasis.
regular basis. However, researchers have realised that this approach may not effectively prevent the
However,researchershaverealisedthatthisapproachmaynoteffectivelypreventthetransmissionof
transmission of bacteria due to the large number of surfaces in hospitals and the fact that their
bacteriaduetothelargenumberofsurfacesinhospitalsandthefactthattheirsterilisationistransient
sterilisation is transient as the materials themselves do not possess inherent antimicrobial activity.
asthematerialsthemselvesdonotpossessinherentantimicrobialactivity. Nosocomialinfectionsare
Nosocomial infections are therefore a real hazard, and using antimicrobial touch surfaces could be
thereforearealhazard,andusingantimicrobialtouchsurfacescouldbeusefulagainstpathogenic
useful against pathogenic microbes. Among these materials, copper is the most frequently used [5]
microbes. Among these materials, copper is the most frequently used [5] due to its efficiency in
due to its efficiency in “contact killing”; however, silver and zinc oxides also exhibit antimicrobial
“contactkilling”;however,silverandzincoxidesalsoexhibitantimicrobialactivity,whicharegenerally
activity, which are generally used in the form of nanoparticles. The antimicrobial properties have
usedintheformofnanoparticles. Theantimicrobialpropertieshavebeengenerallyattributedtothe
been generally attributed to the release of ions from their surface, the rate of which mainly depends
releaseofionsfromtheirsurface,therateofwhichmainlydependsonthechemicalcompositionof
on the chemical composition of the material [6–8].
thematerial[6–8].
The disruption of microbes due to the presence of Cu ions appears to occur in three parallel
ThedisruptionofmicrobesduetothepresenceofCuionsappearstooccurinthreeparallelways
ways (Figure 1) and as such, prevents the risk of bacterial resistance. Contact with copper surfaces
(Figure 1) and as such, prevents the risk of bacterial resistance. Contact with copper surfaces has
has been shown to damage the integrity of bacterial membranes, Cu ions can directly damage
beenshowntodamagetheintegrityofbacterialmembranes,Cuionscandirectlydamagebacterial
bacterial proteins and they can also induce the formation of highly damaging hydroxyl radicals via a
proteinsandtheycanalsoinducetheformationofhighlydamaginghydroxylradicalsviaaFenton-like
Fenton-like chemistry, which can then damage the cells via their interactions with DNA, enzymes
chemistry, which can then damage the cells via their interactions with DNA, enzymes and other
and other proteins, as well as the peroxidation of lipids and subsequent membrane damage [9–11].
proteins,aswellastheperoxidationoflipidsandsubsequentmembranedamage[9–11].

Figure1. RepresentationofthecontributingantimicrobialmechanismsofCuionsreleasedbyaCu
Figure 1. Representation of the contributing antimicrobial mechanisms of Cu ions released by a Cu
bearingZr-basedbulkmetallicglass[11].
bearing Zr-based bulk metallic glass [11].
Despitethegrowinginterestinthetopic,therearestillmanyquestionsthatremainopensuch
Despite the growing interest in the topic, there are still many questions that remain open such
asthekeyfactorsaffectingantimicrobialperformance. Moreover,reviewsandmanuscriptsdealing
as the key factors affecting antimicrobial performance. Moreover, reviews and manuscripts dealing
withthetopicareveryscarce(e.g.,oneofthereviewswasrecentlypublishedbyVincentetal.[12])
with the topic are very scarce (e.g., one of the reviews was recently published by Vincent et al. [12])
and therefore we aim to fill the gap by providing a critical overview of the topic and to highlight
and therefore we aim to fill the gap by providing a critical overview of the topic and to highlight
futurepotentialdevelopmentsandtrendsinthefield. Thisreviewwillcoverimportantaspectsofthe
future potential developments and trends in the field. This review will cover important aspects of
standardsforantimicrobialassessmentandtheneedformicrobiologicalbenchmarks. Someofthe
the standards for antimicrobial assessment and the need for microbiological benchmarks. Some of
mostimportantfactorsresponsiblefortheantimicrobialbehavioursuchasthehumidity,composition
the most important factors responsible for the antimicrobial behaviour such as the humidity,
and oxidation conditions will be critically discussed. Additionally, we will consider the effects
composition and oxidation conditions will be critically discussed. Additionally, we will consider the
of cleaning products and disinfectants, an issue of the utmost importance that has been mostly
effects of cleaning products and disinfectants, an issue of the utmost importance that has been
overlookedelsewhere.
mostly overlooked elsewhere.
2. StandardsforAntimicrobialAssessment
2. Standards for Antimicrobial Assessment
Therearethreemajorinternationalstandardsusedfortheassessmentofantimicrobialactivity
There are three major international standards used for the assessment of antimicrobial activity
of surfaces; the major features of the Japanese, European and a newly proposed US standard are
of surfaces; the major features of the Japanese, European and a newly proposed US standard are
summarised in Table 1. The proposed US EPA protocol is the most specifically directed toward
summarised in Table 1. The proposed US EPA protocol is the most specifically directed toward
coppersurfaces. ThethreestandardshavebeendevelopedusingthepathogensStaphylococcusaureus
copper surfaces. The three standards have been developed using the pathogens Staphylococcus aureus
and either Escherichia coli or Pseudomonas aeruginosa. S. aureus is a common coloniser of the human
Materials2016,9,736 3of23
andeitherEscherichiacoliorPseudomonasaeruginosa. S.aureusisacommoncoloniserofthehuman
nasopharynx and skin. Often without symptoms, it also causes a broad range of diseases and is
particularlyadeptatgainingentrytopatientsviain-dwellingmedicaldevicesandwounds,including
surgicalsites[13].
Table1.Summaryofantimicrobialtestingstandards.
Countryof Pathogens Antibacterial
Standard Name
Application Analysed Benchmark
Antibacterialproducts—Testfor S.aureus,
JISZ2801:2010 Japan No
antibacterialactivityandefficacy. E.coli
Measurementofantibacterial
S.aureus,
ISO22196:2011 activityonplasticsandother Europe No
E.coli
non-poroussurfaces.
ProtocolfortheEvaluationof
BactericidalActivityofHard, S.aureus,
USEPA1 USA Yes2
Non-PorousCopperContaining P.aeruginosa
SurfaceProducts
1Thisdocumentisreleasedaspartofaconsultationprocessinthedevelopmentofthestandard.2Considered
asasanitizerwhenmorethan99.9%ofthepathogensareeliminated.
Particularlyconcerningismethicillin-resistantS.aureus(MRSA),whichiswidespreadandhas
broad resistance to β-lactam antibiotics [13]. Many strains of E. coli are harmless organisms that
inhabitourintestinesbutthereareseveralpathotypesthatcauseintestinaldisease(bothhaemolytic
and non-haemolytic diarrhoeas via a variety of mechanisms), and infections of the urinary tract,
bloodstreamandcentralnervoussystem[14]. Furthermore,thesebacteriaarewell-studiedexemplars
oftwokeybacterialarchetypes, theGram-positive(S.aureus)andGram-negative(E.coli)bacteria.
InplaceofE.coli, theUSEPAprotocolusesP.aeruginosa, anotherGram-negativeorganismthatis
foundwidelyintheenvironment. P.aeruginosahasintrinsicresistancetoarangeofantibiotics,aswell
ascausesinfectionsofwoundsandburnsandprogressiveinfections. Italsocolonisesthelungsof
patientswithcongestivelungdisorderssuchascysticfibrosis,bronchiecstasisandchronicobstructive
pulmonarydisordercausinginflammationandultimatelyprogressivelossoflungfunction[15].
JISZ2801:2010andISO22196:2011defineamethodologyformeasuringantimicrobialperformance
in a variety of materials and define how this should be calculated and expressed. Essentially,
astandardisedinoculumof6×105cells/mLisappliedtoantimicrobialtestpieces(40mm×40mm)
andotherwiseidenticalsamplesthatlacktheactivecomponent. Thebacteriaareincubatedforafixed
periodof24±1hat35±1◦Cwithhighrelativehumidity(>90%). Therecoveryoftheinoculumand
24hsurvivorsisquantifiedbydilutioninbrothmediumandsubsequentcountingofcoloniesthat
developonagarplates. Therearevalidationcriteriarelatingtoreproducibilityoftechnicalreplicates
andadequaterecoveryofbacteriafromcontrol(non-microbicidal)surfaces.
The United States Environmental Protection Agency (US EPA) [16] have recently developed
a protocol for the evaluation of bactericidal activity of hard and non-porous copper/copper-alloy
surfacesthatiscurrentlybeingconsideredforadoptioninaconsultationprocess. Theantimicrobial
parametersofthisprotocolaresimilartotheotherstandardsbutseveraldifferencesareevident. Firstly,
theEPAprotocolnotonlyprovidestheequationsandconditionstodevelopantimicrobialtestsbutalso
providesnormalisedmethodstoassesstheimpactofbiocidalcleaningliquids(Sodiumhypochlorite,
HydrogenperoxideandPhosphoricacid)onnon-poroussurfaces. Disinfectionbybiocidalliquidsis
adailyprocedureformaterialsmeanttobeusedfortheirbactericidalproperties. TheEPAprotocol
provides a means to assess the changes in mechanical and chemical properties of this process in
copper-basedmaterials[16].
TheUSstandardclearlystatesthatamaterialcanbeconsideredasasanitizeronlywhengreater
than99.9%ofthesepathogensareeliminatedwithin1h. TheJapaneseandEuropeanstandardsdonot
provideanantibacterialactivitythresholdbutprovideaframeworkforstandardisedquantificationof
Materials2016,9,736 4of23
antimicrobialactivity;therangeofproductstheycoveraresomewhatbroaderandthusbenchmarking
ismoredifficulttodevelop.
Thenumberofanti-microbiologicalstudiesthatareperformedfollowingthestandardsarevery
scarce.Forexample,Chuetal.[17]proposedtheuseoftheJapaneseIndustrialStandardJISZ2801:2000
toassesstheantimicrobialpropertiesofZr-Cu-basedthinfilms. Thevalueoftheantimicrobialactivity
proposedbythisstandardiscalculatedthroughtheformulaR=log(B/A)whereRistheantimicrobial
activity,Aistheaveragenumberofviablecellsofbacteriaafter24hinoculationofthetreatedtest
pieceandBistheaverageofthenumberofviablecellsofbacteriaontheuntreatedtestpieceafter24h.
Nevertheless,theauthorsdecidedtocalculatetheantimicrobialrateas No−N ×100%whereN andN
N 0
arethenumberofviablemicrobesonacontrolsampleandonthepreparedthinfilmrespectively[17].
ThesameequationwasusedbySharifahmadianetal. tocomparetheantimicrobialrateofwirearc
spraycoppercoatingswiththatofcommercialcopperandstainlesssteel[18].
Theantimicrobialactivityofcopperischaracterisedbyahighbactericidalrate. Testconditions
based on 24 h of interaction are not able to completely characterise a touching surface. As a
practicalexample, doorknobsordoorplatesinhospitalsarecontinuouslytouched. Inthesecases,
thetransmissionofpathogencausedbytouchingsurfacesmaytakeplaceinlessthan24h. Thismeans
that even if 99.99% of bacteria are eliminated after 24 h, the transmission of pathogen may take
place. Forthisreason,theantibacterialperformanceofmaterialsneedstobeanalysedovershorter
periods. As far as the authors are concerned, this inconsistency may be caused by the lack of an
internationalstandardwhichnormalisesantimicrobialactivitytestsfortouchingsurfaces. Inaddition,
abenchmarktimethatcanbeusedtoevaluatewhennosocomialinfectionispreventedseemstobe
lacking. Someresearchershaverecentlyproposed5minastheoptimumtimethatbacteriashouldbe
eliminatedfromtouchsurfacesinordertolimittransmission[19]. Thiswasconsideredtheaverage
estimatedtimefortwopeopletotouchthesamesurfaceinhospitals. Asuitabletestofantibacterial
activity should be proposed based upon the application of a standardised bacterial inoculum at
materialsurfacesoveranauthentictimecourse.
3. FactorsResponsibleforAntimicrobialBehaviour
3.1. AdhesionofBacteriatoAntimicrobialSurfaces
Theantimicrobialperformanceofamaterialdependsonavarietyofparametersbuttheadhesion
ofbacteriatosurfacesplaysamajorrole. Severalresearchersuseassaysthatessentiallymeasurethe
antimicrobialeffectofsolidmaterialsbytheirabilitytokillorinhibitthegrowthofbacteriaatsome
distancefromthesurface,forexampleintheapplicationofsurfacestoagarplatesortheimmersionof
materialsinbrothcultures. Themajorfactorsmeasuredinthesesystemsaretheabilityofthematerial
toproduceionswhichdiffuseandinfluencethegrowthofbacteriainthesurroundingculturemedium.
Theseareparticularlysuitableforapplicationssuchasemulatingtheflowofwaterthroughpipesbut
moreauthenticmethodsfortouchsurfacesneedtolimitthephysicaldistancebetweenthebacteriaand
thesubstratesurface. Thestandardisedtestsdescribedintheprevioussectionaremoreappropriatein
thisregard,sincebacteriaincloseproximitytotheion-generatingsurfacewillbesubjecttothegreatest
antibacterialactivity.
Theprocessbywhichbacteriaadheretoasurfaceissomewhatcomplexandisdominatedby
theirsurfacechemistry. Itessentiallyoccursintwosteps[20]. Initially,themovementofbacteriafrom
anaqueousphasetoamaterialsurfaceisinfluencedbytheirabilitytomove(motility)orbemoved
byBrownianmotionand/orgravityand,whenincloserproximitytothesurface(<5nm),theyare
attractedtothesurfacebyshort-rangeforcessuchaselectrostaticcharge,vanderWaalsattraction
andhydrophobicinteractions. Onmanysurfaces,especiallyhumantissuesorthesurfacesofmedical
implants,thisinitialadherencefacilitatesmorespecificmolecularinteractionsbetweenbacteriaand
surfaces. In order to more thoroughly understand the factors and forces that determine bacterial
adhesiontosurfaces,someresearchershaveconsideredwhetherbacterialattachmenttosurfacesis
Materials2016,9,736 5of23
governedbythesamephysicochemicalinteractionsthatdeterminedepositionofnon-livingcolloidal
particles[20].
Itisclearthatanumberofdifferentfactorsaffectbacterialadhesion. Environmentalfactorssuch
asthenumberofbacteriapresent,theambienttemperature,thedurationofexposure,pHandthe
presence of disinfectants will all influence adhesion. Likewise, by changing the surface chemical
composition it is possible to tune the charge and hydrophobicity of the material and therefore to
control the adhesion and subsequent proliferation of bacteria. An important factor is that touch
surfacesaresubjecttothebuild-upoforganicdepositsderivedfromtheexudationsofhumanskinand
thereforeresultinverydifferentadhesionpropertiestothemetalsurfaceitselfandwillservetosupport
bacterialadherence. Moreover,theabilityofbacteriatoadheretoeachotherandtodevelopacomplex
communityatasurfaceasabiofilmshouldnotbeunderestimated. Thesebiofilmsareprotectedfrom
environmentalchallengeandphysicalremovalbyapolysaccharidematrix. Dentalplaqueisauseful
exampleofabiofilmwhichcanformonmetalsurfacesusedinorthodonticsanddentalsurgery[21–23].
The topology of the surface itself is another important factor. Rough (unpolished) surfaces
bearsurfaceirregularitiesthatpromotebacterialadhesionandbiofilmdepositionwhileverysmooth
(polished surfaces) do not favour the adhesion and deposition [24]. Beyond these visibly obvious
features, surface configuration is also important: bacteria adhere and colonise porous surfaces
preferentiallyoverdensematerials. Microscopicscratchesorgroovesofasimilarsizetothebacteria
servetoincreasethecontactareaavailableforadhesionandthereforethebindingpotential. However,
whentheseimperfectionsaremuchlargerormuchsmallerthanthebacterialsize,thebindingisnotso
strong[25].
Bacteriaareincrediblydiverseandthecharacteristicsoftheadheringbacteriumitselfaredefined
by its genome. Genetic variation that affects the surface composition of bacteria can occur within
a species (i.e., differences between strains) as well as between species and may affect adhesion.
Bacteriawithhydrophobicsurfacewouldlikelyadherebetteronhydrophobicsurfaceswhilebacteria
withhydrophilicpropertiesadherebetteronsurfaceswithcharge[20].
3.2. EffectofHumidityonAntimicrobialPerformance
TheconditionsimposedintheassayofantimicrobialperformancesetoutintheJISZ2801method
differfromthosereportedunderrealconditionsinhospitals[26]. Althoughsuchinvitrotestsmay
notaccuratelyrecreatethedesiredconditions,theycanbeeasilyimplementedandareusefulininitial
screeningofnewmaterials. Asthestandardsdonotaccuratelyemulatetheconditionsexperiencedby
bacteriaduringhorizontaltransferinhealthcareenvironments,researchershavesoughttodevelop
second-tierteststhatperformantibacterialsurfaceanalysisunderauthenticconditions. Severalstudies
reportthebactericidaleffectofcopperinrealhospitalenvironments[27–29].
The deposition of bacteria on surfaces in hospitals can occur via an aerosol route after, for
instance, coughing, sneezing and splashing. These could be simulated in a laboratory setting by
generatingbacterialaerosols. Despitetheimportanceofthisissue,onlyafewstudiesaddressit[26,30].
Ojeiletal.[26] deposited an S. aureus suspension on copper surfaces and measured antibacterial
activity at three different conditions of temperature and relative humidity (RH): those stated in
ISO22196(37◦C,100%RH)andtwoauthenticconditionsat20◦Cwith40%or50%RHand100%.
Theauthenticconditionsgeneratedanantibacterialactivityaroundhalfofthatmeasuredunderthe
standardconditions. Ojeiletal.[26]reportedthattheaerosolsusedtoinoculatetestmaterialsintheir
studydriedbetween30and60min,andthemanuscriptemphasisestheimportanceofwater(andhence
the relative humidity) in enhancing the antimicrobial activity of copper. The higher antimicrobial
activityreportedforwetsurfacesmayleadtoanoverestimationofthebactericidalpropertiesofthe
surface. For this reason, numerous studies have been focused on developing new tests at similar
conditionsasthosedetectedinrealconditionsinhospitals[26–31].
Robineetal.[30]reportedsimilaroutcomeswhenassessingthebactericidalpropertiesofstainless
steel grades and massive copper. An Enterococcus faecalis suspension was deposited on various
Materials2016,9,736 6of23
Materials 2016, 9, 736 6 of 22
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values achieved at low humidities in these studies may suggest an overestimation of bactericidal
humiditiesinthesestudiesmaysuggestanoverestimationofbactericidalpropertiesoftouchsurfaces
properties of touch surfaces for evaluation in indoor conditions, such as hospitals, when studied
forevaluationinindoorconditions,suchashospitals,whenstudiedusingthestandardmethodologies.
using the standard methodologies.
Additionally,inordertoreplicaterealconditions,notonlythetemperatureandhumidityare
Additionally, in order to replicate real conditions, not only the temperature and humidity are
importantbutalsothedurationofthecopper-microbecontact. Forthisreason,someauthorshave
important but also the duration of the copper-microbe contact. For this reason, some authors have
monitoredexperimentsforlongperiodsoftimesuchas10weeks[27],24weeks[28]ormore[29]. The
monitored experiments for long periods of time such as 10 weeks [27], 24 weeks [28] or more [29].
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Figure 2. Survival of E. faecalis deposited as a bacterial aerosol on stainless steel AISI 304, stainless
Figure2. SurvivalofE.faecalisdepositedasabacterialaerosolonstainlesssteelAISI304,stainless
steel AISI 211 + 3Cu and copper) at 25 °C and (a) relative humidity of 100% and (b) relative humidity
steelAISI211+3Cuandcopper)at25◦Cand(a)relativehumidityof100%and(b)relativehumidity
of 0% [30].
of0%[30].
The method for depositing cells on touch surfaces (i.e., wet plating and dry plating) also has an
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effecmtoetnhothdes a(dnrtyim pilcartionbgi)a, ltahcet aivccituym. Fuolarteioxna mofp iolen,sw inh ethneE c.ecllosl iisc elalrlsgearr ethpalna ttehde coonnccoenptpraetriobny odfr cyomppeetrh ods
(dryiopnlast iinn gth),et cheellsa cwcuhemnu tlhaetyio anreo afpiopnlisedin toth ceopcpelelrs ciosulaprognesr itnh wanett hcoencdointicoennst (ria.et.i,o wneot fpcloatpinpge)r [i1o0n,3s3i]n. the
However, dry plating may not always be feasible as observed by Mathews et al. [34] since the
cellswhentheyareappliedtocoppercouponsinwetconditions(i.e.,wetplating)[10,33]. However,
spreading of cells with a cotton swab may destroy the cell’s structure and therefore a wet-plating
dryplatingmaynotalwaysbefeasibleasobservedbyMathewsetal.[34]sincethespreadingofcells
technique would be preferred [4,35].
withacottonswabmaydestroythecell’sstructureandthereforeawet-platingtechniquewouldbe
Although touch surfaces are of special relevance for the healthcare system due to their direct
preferred[4,35].
contact with human skin, other potential reservoirs of microorganisms are water pipeline systems.
Althoughtouchsurfacesareofspecialrelevanceforthehealthcaresystemduetotheirdirect
Cervantes et al. [31] tried to assess the bactericidal effect of three common plumbing materials (PVC,
contactwithhumanskin,otherpotentialreservoirsofmicroorganismsarewaterpipelinesystems.
copper and Pyrex control) in aqueous solution. Several pathogen suspensions (including S. aureus,
CervPa. nateerusgeitnoasla.,[ 3E1. ]faterciealdis,t oanadss Ees. scotlhi)e wbaercet esrtiucdidieadl eaftf ehcitgohf atnhdre leowco minmocounlupml udmenbsiintigesm ina tperipiaelss o(Pf VC,
coppareoruanndd 3P.5y crmex dcioanmtertoelr), itnhuasq au evoeurys ssmolaullt iporno.pSoertvioenra olfp tahteh boagcetenrisau asrpee inns dioirnesct( icnocnltuadcti nwgitSh. tahuer eus,
P. aepruipgei nsousraf,acEe.s.f aTehcael iesx,paenridmEen.tcaoll ir)eswueltrse sshtouwdeided hiagthh aignhtibaanctderlioalw acitnivoictyu lwumith dae nresdituiectsioinn fproipme s of
arou1n0d5 C3F.5U/cmmL dtoia 0m CeFtUer/,mthLu wsitahivne trhye sfmirsat l2l ph roofp coornttiaocnt foofr tthhee cboapcpteerr isaamarpeleisn wdhirileec ftocro PnVtaCc attw 48it hh the
pipe surfaces. The experimental results showed high antibacterial activity with a reduction from
105CFU/mLto0CFU/mLwithinthefirst2hofcontactforthecoppersampleswhileforPVCat48h
Materials2016,9,736 7of23
Materials 2016, 9, 736 7 of 22
86%ofthebacteriawererecovered. Theseresultsclearlyshowthatmakingdrinking-waterpipesout
ofco8p6p%e orfi sthaen beafcfteecritaiv weewrea ryecoofveenrehda.n Tchinesgew reastuelrtsq culeaalirtlyy sfohorwhu thmaat nmuaksein.g drinking-water pipes out
of copper is an effective way of enhancing water quality for human use.
3.3. EffectofGeometry,ChemistryandPhysicalProperties
3.3. Effect of Geometry, Chemistry and Physical Properties
3.3.1. BulkSamples
3.3.1. Bulk Samples
Thereareveryfewreportsdealingwiththeantimicrobialbehaviourofbulkmetallicsamples
andmosTthoefrteh aerme ,vearsyf aferwa sretphoertasu dtheaolrinsga rweitcho nthcee rannetidm,iacrroebfioacl ubseehdavoionumr oaft ebruialkl smoebttaalliinc esdambpylersa pid
and most of them, as far as the authors are concerned, are focused on materials obtained by rapid
solidification,i.e.,bulkmetallicglasses(BMGs). BMGsaresuitablefortouchsurfacesinhealthcare
solidification, i.e., bulk metallic glasses (BMGs). BMGs are suitable for touch surfaces in healthcare
facilities due to their hardness and wear resistance (i.e., resistance against scratches), corrosion
facilities due to their hardness and wear resistance (i.e., resistance against scratches), corrosion
resistance attributed to the lack of grain boundaries and smooth surface finish that provides an
resistance attributed to the lack of grain boundaries and smooth surface finish that provides an
aestheticappearance. OnlyrecentlyhaveBMGsbeenstudiedaspotentialmaterialsforantimicrobial
aesthetic appearance. Only recently have BMGs been studied as potential materials for antimicrobial
applications.In2012,Linetal.[36]studiedtheantimicrobialeffectofFe-based,Ni-basedandCu-based
applications. In 2012, Lin et al. [36] studied the antimicrobial effect of Fe-based, Ni-based and
BMGsviaamoistcontactassayusingE.colibacteria. Theyfoundthattheantibacterialeffectseen
Cu-based BMGs via a moist contact assay using E. coli bacteria. They found that the antibacterial
afterefefxepcto sseuerne atfotetrh eexpthorseuerem toa ttehrei atlhsrewe amsaptrearicatlisc awllays tphraecstiacmalley, cthoen scalumdei,n cgonthclautdtihnge mthaots tthsei gmnoifistc ant
factosriginniftihceanatn ftaimctoicr rionb tiahle baenhtiamviicoruorbioafl tbheehBavMioGusr wofa tshne oBtMthGesir wcoasm npoots tihtieoinr cboumtpthoesiitrionno nb-uctr ythsetairl line
strucntounr-ec.ryTshtaellainnet ismtriuccrtoubriea. lTehfefe acntstiimniceraocbhiaol fetfhfeecitrs ainss eaaycshw oaf sthtreairn assiesanytsa nwdasw traasnosiveenrtw anhde lmwaesd by
densoevreirnwohceulmlae.dT hbiys dmeanyseirn inpoacrutlbae. Tdhuise mtoatyh iena psasarty bme edauseu troin tghet haesseafyf emcteoasfutrhiengg ltahses eoffnecat couf ltthuer eto
whicghlaistsh oand ap acrutlitaulraen tdo iwnchoicnhs iistt ehnatdc poanrttaiaclt raantdh einrctohnasnisatecnut ltcuornetaacpt pralitehderd tihreacnt lay ctoulittu.re applied
directly to it.
The antimicrobial behaviour of various Cu-containing Zr-based BMGs against S. aureus was
The antimicrobial behaviour of various Cu-containing Zr-based BMGs against S. aureus was
recentlyinvestigatedbyHuangetal.[11]. CuwasalloyedwithZrtoproducematerialswithsufficient
recently investigated by Huang et al. [11]. Cu was alloyed with Zr to produce materials with
goodglass-formingabilitytoproducetheminbulkshape[37]. Theseauthorsstudiedthebacterial
sufficient good glass-forming ability to produce them in bulk shape [37]. These authors studied the
killingofZr Al Ni Cu and(Zr Al Ni Cu ) Y (at. %)BMGsafter4hofmoistcontactand
bacterial 5k5illin10g of5 Zr553A0l10Ni5Cu305 5and1 0(Zr555Al103N0i95C9u310)99Y1 (at. %) BMGs after 4 h of moist contact
comparedthebacterialrecoverywiththatobservedonTi-6Al-4ValloyandonpureCu(Figure3).
and compared the bacterial recovery with that observed on Ti-6Al-4V alloy and on pure Cu (Figure 3).

FiguFriegu3r.e 3G. rGorwowththo off EE.. ccoolil ioon nexepxopsuorseu troe Ttio-6ATil--64VA,l -Z4rV55,AZl10rNiA5Clu30N BiMCGu, (ZrB55MAlG10N,i(5ZCru30 ABMl GN)9i9YC1 u
55 10 5 30 55 10 5 30
and pure copper after 4 h [11].
BMG) Y andpurecopperafter4h[11].
99 1
The highest killing activity was observed for Cu (no viable cells were observed after 4 h contact)
fTolhloewheigdh beys ttkhiel liZnrg-BaMctGiv iatnydw fainsaollbys ethrvee ZdrfYo-rBCMuG(. nTohvesiaeb rleescueltlsls wweerree aotbtrsiberuvteedd taof tethre4 hhigchoenrt act)
follocwonecdenbtyrattihoen oZfr C-BuM ioGns arenldeasfienda flrloymt hpeurZe rCYu-B thMaGn .frTomhe istes arlelosyusl.t sDywnearmeica timtrimbuertseidont toestths ewehrieg her
concaelnsotr paetriofonrmoefdC tuo mioondselr tehlee aaspepdlicfartoiomn epnuvrieroCnmuetnhta [n11f]r foomr 1 idtsaya allnody tsh.eyD oybnsaermveicd itmhem foerrmsiaotinont ests
wereofa als So. apuerrefuos rbmioefidlmto, sumgogdesetlintgh ethaapt tphleic baatciotenriae ncavnir aodnhmereen tto [t1h1e] BfMorG1s. dTahyesae nredsuthltes yaroe bcsoenrtvraerdy the
formtaot itohne oofbaseSrv.aatuiroenu sobf iLoifinl met, aslu. g[g36e]s twinhgot hclaatimtheedb tahcatte rbiaacctearniaal d(hEe. rceoltio inth teheBiMr cGasse.)T ghreoswetrhe swulatss are
prevented on metallic glasses due to their lack of adherence to the substrate.
contrarytotheobservationofLinetal.[36]whoclaimedthatbacterial(E.coliintheircase)growth
waspreventedonmetallicglassesduetotheirlackofadherencetothesubstrate.
3.3.2. Coatings
3.3.2. CoaRteisnegasrch with bulk samples is useful as a screening method to assess the antimicrobial
performance of novel compositions since it is a fast technique that requires limited materials to
Research with bulk samples is useful as a screening method to assess the antimicrobial
produce samples. However, once the most promising compositions are selected based on the
performanceofnovelcompositionssinceitisafasttechniquethatrequireslimitedmaterialstoproduce
samp les. However,oncethemostpromisingcompositionsareselectedbasedontheantimicrobial
Materials2016,9,736 8of23
cost-effectiveness,theyhavetobeproducedascoatingsinordertoimplementtheminthehealthcare
sectorM.aTtehriaulss 2t0o16,d 9a, 7t3e6, m ostoftheresearchhasbeenperformedoncoatings. 8 of 22
Overtheyearsmanyresearchershavereportedthatcopperandcopperalloyscaneffectivelybe
antimicrobial cost-effectiveness, they have to be produced as coatings in order to implement them in
usedasantimicrobialtouchsurfaces[5,38,39]. Purecoppercaneliminate99.9%ofbacteriaafterone
the healthcare sector. Thus to date, most of the research has been performed on coatings.
hour,whiOlevfeorr th6e0 %yeacrosp mpeanr,yt wreoseharocuhresrsw hearvee rreeqpuoirrteedd tthoaat cchopiepveer asinmd icloaprpvearl ualelosyosf caannt iebffaecctteivreialyl abcet ivity.
Theseusreeds ualst sanhtiimghiclirgobhitatl htoeuicnht esruersfatciens d[5e,3v8e,l3o9p]. iPnugrea lcloopypsewr ciathn heliigmhinCatue c9o9.n9t%e notf ibnacotredriear atfotere loinmei nate
bactehrioaurfa, swtheinleo ufogrh 6f0o%r tchoeppaeprp, ltiwcaot ihoonusros uwgehrte. rHeqouwireevde tro, thacehnieuvme sbiemriolafr rveaplouretse dof Caun-tirbicahctetrhiianl film
alloysacitsivsictayr. cTehaensed rmesuolstts ohfigthheligrhets ethaerc ihntiesrebsats iinc adlleyvefolocpuinsegd alolonytsh weiCthu higThi C[u4 0co–n4t2e]nat lilno yo.rder to
90 10
Telhimeiannatteim baicctreorbiai afalspt eernfoourgmha fnorc ethne oatppolniclaytiodne psoeungdhst. oHnowtheevearl,l othyei nngumebleemr oefn rtespoardtedde dCut-oriccho pper
butatlhsoin ofinlmth aellocyosa itsin sgcatrecec hanndiq mueossta onf dthteh reerseefaorrceh oisn bathsiecamllyic froocsutsreudc otunr teh.eD Ciuff90eTrie10n [t40sp–4r2a]y altleocyh. n iques
The antimicrobial performance not only depends on the alloying elements added to copper but
have been reported to be feasible to fabricate coatings: plasma sprays, wire arc sprays and cold
also on the coating techniques and therefore on the microstructure. Different spray techniques have
sprays. Aneconomicconsiderationistheproductionofadurablecoatingwithalongworkinglife
been reported to be feasible to fabricate coatings: plasma sprays, wire arc sprays and cold sprays. An
andusingtheminimumofmaterials,thustheproductionofthinfilmshasreceivedmuchattention.
economic consideration is the production of a durable coating with a long working life and using the
Thepmroinceimssuomf ochf moiacteerfioarlsd, ethpuoss tihtieo nproofdmucetitoanll iocf tthhiinn fciolmatsi nhgass rienceaivweidd meuracnh gaetteonftiinodn.u Tshtrei aplroacpepslsi coaf tions
ismacghnoeictreo fnors dpeuptotseirtiinong .oWf meehtaalvlice trheicne cnotalytincgosn isni da ewreidde trhaengaen otifm inidcurostbriiaall appeprlfiocramtioannsc ies mofapgnuerterocno pper
thin fisplmuttperriondg.u Wceed hbayvem reacgennetltyr ocnonsspiduetrteedri nthge aagnatiimnsictrtohbeiapl patehrfoogrmenanSc.ea oufr epuusre(u cnoppupebrl itshhiend firlmes ults).
Muchprliokdeutcheed mbye tmhaogdnoeltorgony sdpeustctreirbinegd aingaIiSnOst 2th2e1 9p6a:t2h0o1g1ena nSd. aJuISreuZs 2(u80n1p:u2b0l1is0h,eads ruesspueltns)s.i Monuochf Sli.kaeu reus
waspthlaec medetohnodtholeogcoy pdpeesrcrsibuerdfa icne ,ISaOnd 22s1a9m6:p2l0e1s1 oafntdh eJISs uZs p2e8n01s:i2o0n10w, ea rseutsapkeennsiaotn inoft eSr.v aaulsreoufs 5wmasi nfor
placed on the copper surface, and samples of the suspension were taken at intervals of 5 min for an
anhour. Adenserinoculumthantheonedescribedinthestandard(bytwoordersofmagnitude)was
hour. A denser inoculum than the one described in the standard (by two orders of magnitude) was
usedandtheprocedurewascarriedoutunderambientlaboratoryconditions. Bacteriawererecovered
used and the procedure was carried out under ambient laboratory conditions. Bacteria were
fromthesuspensionincontactwiththefilmandwerequantifiedbyanalysingserialdecimaldilutions
recovered from the suspension in contact with the film and were quantified by analysing serial
culturedonagarplates(Figure4). Thenumberofbacteriarecoveredcanbegivenasthenumberof
decimal dilutions cultured on agar plates (Figure 4). The number of bacteria recovered can be given
Colonasy tFhoer nmuimngbeUr noift Cso(CloFnUy )F.oTrhmeinCgF UUnritesc (oCvFeUre).d Tdheec CreFaUs erdeceoxvperoende dneticarellaysefdro emxp4o.n6e×nti1a0ll7y tforo0mw hen
theco4n.6t a×c 1t0t7i mtoe 0 wwahsenin tchree acosendtacfrt otimme0 wtoas5 0inmcriena.seTdh feropmro 0fi tloe 5is0 cmoimn.p Tlihcea tperdofbilye itsh ceofmacptlitchaatetdth bey Sth.ea ureus
colonfyacfto trhmati tnhge uS.n aiutriesurs acroelolynya fsoirnmgilnegc uelnlitb ius trathreelsye ag srionwglea cselal bcluut mthpesien glrioqwu iads cau clltuumrep. iEn alicqhuicdlu mp,
whichcumltuarye.c Eoanctha icnlu1m0po, rwmhiocrhe mcealyl sc,omntiagihn t1f0o romr maocroe lcoenllys, imfoignhlty foonrme oaf ctohloencyl uifm opnelyd ocneell sofs uthrev ives;
clumped cells survives; the kinetics of death therefore appear complex.
thekineticsofdeaththereforeappearcomplex.
An effective technique for producing amorphous alloys is to make them as thin films by
Aneffectivetechniqueforproducingamorphousalloysistomakethemasthinfilmsbymagnetron
magnetron sputtering [43,44]. Amorphous alloys are important since they can be used as precursor
sputtering[43,44]. Amorphousalloysareimportantsincetheycanbeusedasprecursormaterialsfor
materials for controlling the type and volume fraction of crystalline phases, which enables tuning of
controllingthetypeandvolumefractionofcrystallinephases,whichenablestuningofthemechanical
the mechanical and antimicrobial properties. Over the years, the antimicrobial activity of various
and antimicrobial properties. Over the years, the antimicrobial activity of various thin film alloy
thin film alloy compositions has been studied. Most of the papers are focused on Zr-based or
compTois-bitaisoends ahlalosybs eseunchs tausd Zier5d3C. Mu30oNsit9Aofl8t h[1e7]p, aZpre43rCsua40rAelf9oAcgu8 s[4e5d] oanndZ Tr-ib40aCsue3d0Podr14TZir-1b0 a[4s6e]d, faolrl owyhsiscuh chas
Zr53Ccuom30pNlei9teA el8lim[1i7n]a,tZiorn4 3oCf uth40eA clu9lAtivga8te[4d5 b]aacntedriTa i4o0nClyu 3o0cPcudr1s4 Zafrt1e0r [2446 ]h,. fDoriffwerheincchesc oinm apnlteitmeiecrliombiianla tion
of theacctiuvlittiyv awteedre bmacatienrliya aotntrliybuotcedcu tros avfatreirat2io4nhs .inD icfofeprpeenr cecosnitnenatn, tii.me.,i ctrhoeb iaanltiabcatcitveirtiyal waectrievimtya inly
attribiuntcerdeatsoesv wariitaht iionncrseiansicnogp cpoeprpceor nctoenntet,nit. e[4.,7t]h, esiannceti ab ahcitgehreiar lcaocntcievnittyraitniocnr eoafs ceospwpeitrh sihnocurleda sreinlegasceo pper
more Cu ions (i.e., dissolution of Cu).
content[47],sinceahigherconcentrationofcoppershouldreleasemoreCuions(i.e.,dissolutionofCu).

Figure 4. Kinetics of bacterial killing on thin copper films. A suspension of Staphylococcus aureus was
Figure4.Kineticsofbacterialkillingonthincopperfilms.AsuspensionofStaphylococcusaureuswas
deposdietepdosoitneda othni na ctohpinp ecorpfiplmer sfiulmpp sourptepdorbteyda bsyta ain sletasisnsletsese lssteuepl psourptpuonrtd uenrdaemr baimenbtiecnotn cdointidointison(2s2 ◦C).
(22 °C). Samples were taken periodically and survivors were quantified by culturing serial decimal
Samplesweretakenperiodicallyandsurvivorswerequantifiedbyculturingserialdecimaldilutions
dilutions on LB agar plates (unpublished results from the authors).
onLBagarplates(unpublishedresultsfromtheauthors).
Materials2016,9,736 9of23
With regard to evaluation of differences in antimicrobial activity of the materials, various
methodologies have been reported to date. Some authors such as Chu et al. [17] used the plate
countmethodwherethenumberofbacterialcoloniesthatgrewoneachagarplateiscountedand
compared to the control. Other methods are viz. agar diffusion method, solution suspension and
wet interfacial contact method [45,46]. The agar diffusion plate test generates a zone of inhibition
on a confluent bacteria lawn, indicating the diffusion of Cu ions from the sample to a threshold
inhibitory concentration, thus the diameter of the zone indicates efficiency in release of the ions.
Thewetinterfacialmethodinvolvesdirectcontactbetweenthecoatingandtheviablebacteriacanbe
visualisedbyEpi-fluorescencemicroscopy.
Therearealsodifferencesintheconditionsatwhichantimicrobialtestshavebeencarriedout,
which makes it difficult to compare the results of the different authors. While some studies were
carriedupto24h[42,46],otherstudieshavebeendoneforvariouslongerperiods(rangingfrom3
to20days)[17,40,45].
Other antimicrobial elements such as Ag or Zn are able to eliminate pathogens in thin films.
Forexample,Chuetal.[48]observedthattheantimicrobialperformanceofAg-containingZrTiAland
AlAgTithinfilmsissimilartothatofapureAgcoating. InseveralotherstudiesAgwasincorporated
asanelementinthinfilms[49–51]totunetheantimicrobialperformance,butthealterationofanother
parameter(roughness,sizeofcrystalformation,etc.) makesitdifficulttoassesswhetherthekilling
effectiveness can be only attributed to compositional differences. Sometimes it can be useful to
make composites of various materials to enhance the flexibility to tune the behaviour (Ag-TiO
2
composites[52–55]orAgnanoparticles[56],etc.) butadetaileddescriptionisbeyondthescopeof
thismanuscript.
3.3.3. PhysicalPropertiesofMetallicCoatings
The studies previously described have been mostly focused on optimising the chemical
compositionofmaterialstocontroltheirantimicrobialbehaviour. Nevertheless,thephysicalproperties
of thin film surfaces also have an effect on pathogen elimination rate. A useful parameter to
predict the antibacterial activity is the wettability [42], which depends on the surface roughness,
surface-freeenergyorsurfacestructure. Consequently,understandingtheroleoftheseparametersin
theantibacterialpropertiesofthinfilmsisanissueofutmostimportance.
Intermsofwettability,contactanglemeasurementisthepreferredmethodtomeasuresurface-free
energy. High contact angles (i.e., the angle between the surface of a liquid and the outline of
theliquid-solidcontactsurface)areattributedtohydrophobicbehaviour,whilematerialsshowing
lowervaluespresentbetteradhesionpropertiestothematerial[42]. Hydrophobicmaterials(contact
angles>65◦)areusuallyrecommendedforantimicrobialapplicationssincesuchsurfacesinhibitcell
attachment[42].
Surface roughness can be controlled by careful selection of the sputtering conditions in the
magnetron sputterer, which is a useful physical vapour deposition method to produce thin films.
Chen et al. studied the effect of coating surface roughness obtained at different sputtering power
conditionsonantimicrobialactivity[45]andobservedthatsurfacesgeneratedunderlowerpower
conditionsexhibitedahigherantibacterialcapabilitycomparedtothosegeneratedwithhigherpower.
Nevertheless,thisstudydidnotinvestigatetherelationshipbetweensputteringpowerandsurface
morphologyindepth. Forthisreason,acompleteunderstandingofthereasonswhichleadtobetter
antibacterialpropertieswasnotachieved. Ontheotherhand,Stranaketal. investigatedtheroleof
severalsputteringtechniquesonthebactericidalcharacteristicsofCu Ti alloy[41]. Threedifferent
90 10
magnetron sputtering techniques were analysed (direct current magnetron sputtering: DC, dual
magnetron-sputtering: Dual-MSanddualhighpowerimpulsemagnetron-sputtering: Dual-HiPlM)
andtheirantibacterialpropertieswerecompared. Dual-HiPlMwasregardedasthemostpromising
technique for antibacterial applications because the thin film produced thus released the highest
concentrationofcopperions(i.e.,40%morethaninDCandDual-MS)[41].
Materials2016,9,736 10of23
Amongthemetalliccoatingfabricationtechniques,magnetronsputteringisthemostbroadly
used, as indicated by the large number of manuscripts found in the literature. Even for the same
sputteringconditions,asmallchangeincompositioncanresultindifferencesinthenucleationprocess
andsMuartfearicales 2t0o1p6,o 9g, 7r3a6p hy. Forexample,Wojcieszaketal.[51]reportedthatmagnetron-sputte10r eodf 22T i-Ag
andNb-Agcoatingsexhibitedextremelydifferentnucleationofthecrystallinephases,whichresulted
sputtering conditions, a small change in composition can result in differences in the nucleation
in a surface roughness of 2 nm for Ti-Ag and 22 nm for Nb-Ag. Larger surface roughness can be
process and surface topography. For example, Wojcieszak et al. [51] reported that
attainmedagunseitnrognw-spiruettaerrceds pTria-Ayg[ 1a8n].dT Nhbe-rAogu gcohanteinsgsso efxthhiebictoedp peexrtrceomaetliyn gdsifwferaesnst tunudcieledataionnd ocfo rthreel ated
withctrhyestaanllitnibea pchtearsieasl, wprhoicphe rrteiseusl.teIdn inth ai ssuterfcahcne irqouueg,htnheessc oofa 2ti nnmg ifsorf aTbir-Aicga taendd b2y2 npmro pfoerl lNinbg-Amg.o lten
droplLeatrsgaert shuigrfhacev erlooucgithineesssto catnh ebes uabttsatirnaetde ussuinrfga cweirien aarc hsipgrhayo [x1y8g].e Tnhea trmouogshpnheesrse o.f Tthhee croopupgehr ness
achiecvoeadtiningst hwiasst setcuhdnieidq uanedm caoyrrebleatfiedv ewoitrhd tehres aonftimbaacgtenriiatul pdreopheirgthieesr. I(nfr tohmis t1ecnhmniqtuoe1, t0h0e µcoma)tinthga ins that
fabricated by propelling molten droplets at high velocities to the substrate surface in a high oxygen
obtainedusingtheconventionalmagnetronsputteringtechnique. Theantimicrobialactivityofthese
atmosphere. The roughness achieved in this technique may be five orders of magnitude higher (from
coatingswasabout92%,comparedwiththe99.999%obtainedusingmagnetronsputtering[48]. Itis
1 nm to 100 μm) than that obtained using the conventional magnetron sputtering technique. The
importanttonotethatwirearccoatingisdoneinahighoxygenatmosphere[18]andthereforeresultsin
antimicrobial activity of these coatings was about 92%, compared with the 99.999% obtained using
theformationofsurfaceoxides.Whensignificantsurfaceoxidationtakesplace,micronlevelroughness
magnetron sputtering [48]. It is important to note that wire arc coating is done in a high oxygen
mightbecoveredbytheoxidelayerandtheroughnessmaynothaveaninfluenceontheantimicrobial
atmosphere [18] and therefore results in the formation of surface oxides. When significant surface
perfoormxidaantcioen. Tthakeefso rpmlaactei,o nmoicfroonx idleevseli srtohuegrhenfoerses immipghotr tabne tcaonvderwedi llbbye tchoev eorxeiddei nlatyheer naenxdt stehcet ion.
roughness may not have an influence on the antimicrobial performance. The formation of oxides is
3.4. Oxides
therefore important and will be covered in the next section.
Metaloxidesareimportantfortheiruniquephysicochemicalpropertiesandantibacterialactivity.
3.4. Oxides
Among all oxides, those detected on copper, silver and zinc have special relevance since they are
antimicroMbieatlaml oaxteidrieasl sa.rTe hiemrepoisrtaannti nfocrr etahseinirg uinnitqeurees tpihnysthiceosceheemleimcael nptsroapnedrtieessp eancida llayntiinbaccotperpiaelr due
activity. Among all oxides, those detected on copper, silver and zinc have special relevance since
toitsrelativelylowcostandantimicrobialefficiency.
they are antimicrobial materials. There is an increasing interest in these elements and especially in
Theantimicrobialmechanismofcopperoxides[57]isnotunderstoodindetailbutseemstobethe
copper due to its relatively low cost and antimicrobial efficiency.
resultofseveralco-actingmechanismswheretheprimarycauseisdirectcontactbetweenbacteriaand
The antimicrobial mechanism of copper oxides [57] is not understood in detail but seems to be
metalliccoppersincecopperionscaninduceseveredamagetothebacterialenvelope. Accordingto
the result of several co-acting mechanisms where the primary cause is direct contact between
Hansbeatctaelr.ia[5 a8n]dth meeftoarllmic actoipopneor fscinocpep ceorpopexri dioensst hcaant ainrdeuscoel usebvleerien daaqmuaegoeu tso pthhea sbeacftoelrlioawl eendvebloypree. lease
ofcopApcecorridoinnsg ftroo mHatnhse eotx aidl. e[s58a]r ethkee yforfamcattoiorsn tohfa tcompapkere ocxoipdpese rthaantt iamrei csroolbubialel. iInn aoqrudeeorutso pahsassees sthe
contafcotllkoiwlleind gbya cretilveaitsye ooff cCopupoexr iidoness ,frHoman tshee toaxlid.[e5s 7a]reg kreewy faocxtiodrse tlhaayte mrsakine cpohpopsepr hanattiembicurfofbeiraeld. Ins aline
(PBS)oordreTr rtios- HasCselss(p tHhe 7c)ofnotrac3t0 k0ilmlining (aFcitigvuitrye o5fa )Caun doxmideesa,s uHraends teht eanl. u[m57b] egrreowf souxrivdiev alalyoefrsb ainc teria
(Figupreho5sbp)havteer sbuusffeerxepdo ssaulrineet i(mPBeS)i norg lTarsiss,-HCCul, (CpuH O7) afonrd 3C00u Om.inT h(Feigaunrtei b5aac) teanrida lmeeffiascuarceyd ftohre pure
2
Cu anndumCbuer Oof wsuerrveivvael royf bsiamcteilraiar (bFuigtusrlei g5hbt)l yvesrmsuas lelexrpofosurrCe utimOe ;inth gelaascst, iCviut,y Cfuo2rOC aunOd CwuaOs. sTmhea llest.
2 2
antibacterial efficacy for pure Cu and Cu2O were very similar but slightly smaller for Cu2O; the
ThebuffermediumTris-HClwasobservedtoinducefastercopperionreleasethanPBSandtherefore
activity for CuO was smallest. The buffer medium Tris-HCl was observed to induce faster copper
higherkillingefficiency.
ion release than PBS and therefore higher killing efficiency.

(a) (b)
Figure 5. (a) Plot showing oxide growth versus oxidation time in PBS and in Tris; (b) Plot survival of
Figure5.(a)PlotshowingoxidegrowthversusoxidationtimeinPBSandinTris;(b)Plotsurvivalof
E. hirae exposed to glass, Cu, CuO and Cu2O at different times [57].
E.hiraeexposedtoglass,Cu,CuOandCu Oatdifferenttimes[57].
2
The antimicrobial behaviour of other metal oxides is also of interest but studies are very scarce.
TMhoesta cnotnimsidicerro obxiiadlabtieohna ovni opuarrtoicfleost/hneanrompearttailcloexs iadneds tihsearelsfooroe fiti nist edrieffsictublut ttos taussdeisess wahreetvheerr ythsec arce.
Mostacnotnimsiidcreorboiaxl ipdeartfioornmoanncpe aisr taicttlreisb/unteadn otop tahret ioclxeidsea nodr ttoh tehree fpoarretiictleis sdizief.fi Hcuolwtetovears, sReesbsewloh eett ahle. rthe
[50] developed AgxO thin films and compared the antibacterial effect with that of pure Ag thin films