![Schematic representation of (a) structure of hair cuticle cell and (b) lipid layer of 18-methyleicosanoic acid (18-MEA) covalently linked to the epicuticle outer surface via thioester linkages. Adapted from reference [13].](https://www.researchgate.net/publication/367396585/figure/fig3/AS:11431281114747120@1674652984757/Schematic-representation-of-a-structure-of-hair-cuticle-cell-and-b-lipid-layer-of_Q320.jpg)
![Comparison of hair fibers surface and cross-section from Caucasian, Asian and African people by SEM. Adapted from reference [23] with permission from Elsevier.](https://www.researchgate.net/publication/367396585/figure/fig4/AS:11431281114765842@1674652985257/Comparison-of-hair-fibers-surface-and-cross-section-from-Caucasian-Asian-and-African_Q320.jpg)
![SEM images of a virgin hair fiber at three locations: near the scalp, in the middle of the fiber and near the tip. Adapted from reference [23] with permission from Elsevier.](https://www.researchgate.net/publication/367396585/figure/fig5/AS:11431281114773722@1674652985390/SEM-images-of-a-virgin-hair-fiber-at-three-locations-near-the-scalp-in-the-middle-of_Q320.jpg)
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Polymers2023,15,608.https://doi.org/10.3390/polym15030608www.mdpi.com/journal/polymers
Review
OnHairCarePhysicochemistry:FromStructureand
DegradationtoNovelBiobasedConditioningAgents
CatarinaFernandes
1,2,
*,BrunoMedronho
2,3
,LuísAlves
1
andMariaGraçaRasteiro
1,
*
1
CIEPQPF,DepartmentofChemicalEngineering,UniversityofCoimbra,R.SílvioLima,
3030‐790Coimbra,Portugal
2
MED–MediterraneanInstituteforAgriculture,EnvironmentandDevelopment,
FaculdadedeCiênciaseTecnologia,UniversidadedoAlgarve,CampusdeGambelas,Ed.8,
8005‐139Faro,Portugal
3
FSCN,SurfaceandColloidEngineering,MidSwedenUniversity,SE‐85170Sundsvall,Sweden
*Correspondence:csfernandes@uc.pt(C.F.);mgr@eq.uc.pt(M.G.R.)
Abstract:Hairisconstantlyexposedtovariousadverseexternalstimuli,suchasmechanicalorther‐
malfactors,thatmaycausedamageorcauseittoloseitsshineandsmoothappearance.Theseun‐
desirableeffectscanbeminimizedbyusinghairconditioners,whichrepairthehairandrestorethe
smootheffectdesiredbytheconsumer.Someofthecurrentlyusedconditioningagentspresentlow
biodegradabilityandhightoxicitytoaquaticorganisms.Consumersarealsobecomingmoreaware
ofenvironmentalissuesandshiftingtheirpreferencestowardnatural‐basedproducts.Therefore,
developingnovel,sustainable,natural‐basedderivativesthatcanactasconditioningagentsinhair
careproductsandthuscompetewiththetraditionalsystemsobtainedfromnon‐renewablesources
ishighlyappealing.Thispaperpresentsthekeyphysicochemicalaspectsofthehairconditioning
process,includinghairstructureanddegradation,andreviewssomeofthenewalternativecondi‐
tioningagentsobtainedfromnaturalresources.
Keywords:biobasedconditioningagents;conditioners;haircare;hairstructure
1.Introduction
Hairischaracteristicofmammals.Ithasaprotectivefunctionasitactsasabarrier
fortheexternalelementsandplaysaroleinthermoregulation.Inhumans,hairalsohasa
socialsignificanceandcancauseagreatimpactonpersonalbodyimageandconfidence.
Healthyhairoffersemotionalandpsychologicalhealthandcontributestoimprovedself‐
esteemandattractiveness[1,2].Theappearanceofshiny,healthy,andsmoothhairisde‐
pendentonenvironmentalexposure,appliedhairtreatments,andsimpledailyhaircare
habits.Agreatvarietyofhaircareproductsareavailableonthemarkettoallowthecon‐
sumertoachievethedesiredpropertiesandaestheticresultsfortheirhair,suchassham‐
poos,conditioners,dyes,mousses,lotions,etc.Otherchemicalandthermaltreatmentsare
alsoappliedtochangethepropertiesofthehair,forexample,straightening,curling,and
bleachingprocesses.Thesetreatmentsarepreferentiallyperformedtoimprovehair’sap‐
pearanceand,consequently,self‐image.
Acrucialstepinhaircareisthecleaningprocess.Haircleaningisprovidedbyshampoos.
Theseproductsareresponsiblefortheremovalofdust,
residuesfromstylingcosmetics,and
excessivesebumaccumulatedinthescalpandhairfibersbysolubilizingthemthroughthe
surfactantspresentintheformulation.Thesolubleparticlescanthenbeeasilyremovedby
rinsingwithwater.Althoughshampooshavethepurposeofcontributingtoandpromoting
well‐cared‐forhair,theycanalsoleavethehairwithadryandroughappearance.Thehigh
cleaningabilityofanionicsurfactantspresentintheshampooscombinedwiththeabrasion
promotedbyscrubbingtheheadandhair,oftenresultsintheremovaland/ordegradationof
Citation:Fernandes,C.;
Medronho,B.;Alves,L.;
Rasteiro,M.G.OnHairCare
Physicochemistry:From
StructureandDegradationto
NovelBiobasedConditioning
Agents.Polymers2023,15,608.
https://doi.org/10.3390/
polym15030608
AcademicEditor:
GeorgeZ.Papageorgiou
Received:18November2022
Revised:17January2023
Accepted:20January2023
Published:24January2023
Copyright:©2023bytheauthors.Li‐
censeeMDPI,Basel,Switzerland.
Thisarticleisanopenaccessarticle
distributedunderthetermsandcon‐
ditionsoftheCreativeCommonsAt‐
tribution(CCBY)license(https://cre‐
ativecommons.org/licenses/by/4.0/).
Polymers2023,15,6082of23
lipidandkeratin[3].Duetothis,somepeoplefeeltheneedtoapplyhairconditionerafter
shampooingtoimprovethehair’ssoftnessandglossandreducefrizz.
Tobetterunderstandtheconditioningprocess,itisnecessarytobefamiliarwiththe
structureandcompositionofhair.Thisisespeciallyrelevantforthedevelopmentofcon‐
ditioningformulationsbecausehairfiberswillactasthesubstrateforthedepositionof
conditioningagents.
2.HairStructureandChemicalComposition
Hairisafibrouscompositebiomaterialgeneratedbythehairfollicleandcomposed
ofproteins,lipids,water,andotherminorcompounds,suchaspigments.Proteinsarethe
maincomponentofhairandcanaccountforca.65to95%ofthetotalhairweight[4].The
mainconstituentofhairiskeratin,aresistantfibrousproteinmainlycomposedoftyro‐
sine,glycine,andcysteineaminoacids.Thehighcontentofsulfur‐containingcysteinere‐
sultsintheestablishmentofdisulfidebondsbetweenneighboringkeratinchains,forming
cystineunits(Figure1)[5].Alongwithdisulfidebonds,peptidebondsarealsoabundant
inthestructureofthehair.Consequently,numerous−COand−NHgroupsareaccessible
toformhydrogenbondsbetweenadjacentproteinaceousfilaments[6].Theresultingcross‐
linkedstructureofkeratinisresponsiblefortheshape,structure,andstrengthofthehairfibers
[7,8].
Figure1.Chemicalstructuresof(a)cysteineaminoacidand(b)cystinedimerformedbydisulfide
bondsbetweentwocysteinemolecules.
Inacross‐sectionview,thehairshaftfiberspresentthreemainconcentriclayers:the
medulla,cortex,andcuticle,fromtheinnermosttotheoutermostlayer,respectively.A
genericrepresentationofthehairfiberstructureispresentedinFigure2.
Figure2.Schematicrepresentationofthehairshaftstructure.
Themedullaisalooselypackedanddisorderedregionlocatedatthecenterofthe
fiberandiscomposedofanucleusofcentralcellsandairspaces[2,5].Itvariesamong
differenthairtypes,beingmorenoticeableinthickhair,as,forexample,inaman’sbeard
[1,2].Medullarepresentsasmallpartofthehair’stotalweight,anditscontributiontothe
mechanicalpropertiesofhairfibersisnegligible[9].Surroundingthemedullaisthehair
Polymers2023,15,6083of23
majorcomponent,thecortex.Thecortexiscomposedofelongatedcorticalcellsthatcon‐
tainalpha‐helicalkeratinfilamentsalignedwiththelongitudinalaxisofthehairshaftand
anamorphousmatrixcomposedofsulfur‐richproteins,mainlykeratin[5,10].Incontrast
tothemedulla,thecortexrepresentsca.90%ofthetotalhairweightandisprimarilyre‐
sponsibleforthemechanicalpropertiesofthehair[5,11].Thefilamentousstructureofthecor‐
tex,withadjacentkeratinfilamentslinkedbycovalentdisulfidebonds,impartsstrengthtothe
hair,whereasitselasticityisprovidedbythehelicalshapeofthekeratinchains[2,10].This
layeralsohasaroleinstoringmoistureandholdingthepigmentsthatconfercoloronhair
[2,6].Theouterpartofthehairshaftisthecuticle.Thislayeriscomposedofca.6–10flat
overlappingcellspointingtowardsthetipendofthehairfiber,resemblingscalesorroof
shingles[12].Eachcuticleiscomposedofseveralsublayers,i.e.,theepicuticle,A‐layer,
exocuticle,endocuticle,andthecellmembranecomplex[6].Aschematicillustrationofthe
complexstructureofthecuticlecellispresentedinFigure3a.
Figure3.Schematicrepresentationof(a)structureofhaircuticlecelland(b)lipidlayerof18‐meth‐
yleicosanoicacid(18‐MEA)covalentlylinkedtotheepicuticleoutersurfaceviathioesterlinkages.
Adaptedfromreference[13].
Theoutercomponentofthecuticle,theepicuticle,iscoveredbyalipidiclayerthatgrants
ahydrophobiccharactertothesurfaceofthehair.Thislayer,knownastheF‐layer,iscom‐
posedof18‐methyleicosanoicacid(18‐MEA),whichiscovalentlybondedtothecysteine
groupsofkeratinproteinsbythioesterlinkages(Figure3b)[14].TheF‐layeristhefirsthydro‐
phobic“defensivebarrier”ofthehair,anditdecreasesthefrictionbetweenhairfibers[10,15].
Thecuticleactsasabarrierthatprotectsthecortexofthefiberandisresponsiblefor
thetextureandsmoothappearanceofhair[10].Inhealthyhair,thecuticlecanlastupto
6years[16].Whenthehairisexposedtoharshtreatments,suchaschemical,physical,or
thermaltreatments,thehairweatheringisaccelerated,andthehairshaftwillbedamaged.
ThedeteriorationofthehairshaftstartswiththeremovaloftheF‐layer,butfurtherdam‐
agecanresultinthedestructionoftheentirecuticleand,ultimately,thedegradationof
theproteinsinthecortex[16].
Ethnicityisarelevantfactortoconsiderwhenstudyinghairanddevelopingnewhair
careproductsasitsstructureandpropertiesvarywithinindividualsofdifferentethnici‐
ties.Inthisrespect,haircanbecategorizedintothreemaingroups:Caucasian,African,
andAsian.Differencesinchemicalcomposition,structure,andpropertieshavebeenstud‐
iedforalongtime[17–22].Thechemicalcompositionofhairisreportedtobeindependent
Polymers2023,15,6084of23
ofethnicorigin;allhairhasthesamechemicalproperties,i.e.,proteincomposition[17,18].
Regardingtheamountoffibrousproteins(FPs)andmatrixsubstances(MSs),thesewere
foundtonotvaryamongsubjectswithinthesameethnicgroup,butdifferenceswereob‐
servedbetweengroups[19].TheyieldsofFPsfromAsianhairweresignificantlyhigher
thanthoseofAfricanandCaucasianhairs.RegardingMSs,higheryieldswereobserved
forAfricanhairthanfortheothertwogroups.Nevertheless,theseparametersdonot
showsignificantdifferenceswithinallthreegroups.Moreinterestingisthecomparison
oftheratiosofFPsandMSsbetweenethnicgroups.Whenaccessingthisnewparameter,
itwasfoundthatallthreegroupshavesignificantlydifferentratios.Asianhairhasthe
highestratioofFPstoMSsyields,whileAfricanhairisontheothersideofthespectrum,
withthelowestvalue.Thevariationofthecontentoffibrousproteinsofdifferenthair
typeswassuggestedtoberelatedtoitsmacroscopicproperties[19].Themaindistinguish‐
ingfeaturesamonghairtypesaretheshapeanddiameterofthehairshaft.Asianhairis
generallythicker,withadiameterofca.100μmandaroundedcross‐sectionshape,while
Caucasianhairisusuallythinner,withadiameterofca.50μmandaslightlyelliptical
shape[22,23].TheAfricanhairhasanintermediatethicknessofca.80μm,althoughwith
ahigherheterogeneityindiameterthanthepreviousonesandanellipticalcross‐section
[20,23].Thevisiblecuticlecellslengthalsovariesfrom5to10μmaccordingtothehair
type[23].ThesefeaturescanbeobservedintheSEMimagesofhairsamplesfromthethree
ethnicgroupspresentedinFigure4.Anotherdistinctivefeatureisthehair’sshape:Afri‐
canhairpresentsacharacteristicallyhighlycurledstructure,whileAsianhairisusually
verystraight.Ontheotherhand,Caucasianhaircanbestraight,wavy,orslightlycurly
[21].Thecross‐sectionalshapeandmechanicalpropertiesofthehairshaftcanbeat‐
tributedtothepreviouslymentionedratiobetweenfibrousproteinandmatrixcontents.
ThehigherdegreeoffibrousproteinsrelativelytomatrixobservedinAsianhairmaybe
responsibleforthecircularcross‐sectionandhigherstrengthobservedinthosefibers,
whiletheflattenedandmoresusceptibletobreakageAfricanhairmayresultfromthe
lowestratiooffibrousproteins[19,21].
Figure4.Comparisonofhairfiberssurfaceandcross‐sectionfromCaucasian,AsianandAfrican
peoplebySEM.Adaptedfromreference[23]withpermissionfromElsevier.
Understandingthedifferencesinstructureandpropertiesinthehairofsubjectsof
differentethnicities,suchasthemechanicalbehavior,shape,anddamagemechanism,is
ofutmostimportanceforthedevelopmentofefficienthaircareformulationstailoredto
Polymers2023,15,6085of23
thedesiredapplicationsandfulfillingtheconsumers’expectations.Inaddition,therepro‐
ducibilityoftheresultsdependsonthesimilarityofthepropertiesbetweensamples.
Therefore,itisimportanttotakeintoconsiderationthepossiblevariationsinhair’smor‐
phologywhenconductingdifferentexperiments[8].
2.1.HairWeathering
Hairweatheringisthechemicalorphysicaldeteriorationofthehairshaftthatresults
intheremovalofsomehaircomponentsandmakesitmoresusceptibletobreakage[3].The
damageiscaused,forexample,bycosmeticprocedures,badhaircarehabits,orenviron‐
mentalfactorssuchasprolongedexposuretoUVradiation[24].Somecosmetictreatments
areextremelyharshandresultinincreasedstructuralandchemicaldamagetothehair
fibers.Chemicalprocedures,suchasbleaching,perming,relaxing,andstraightening,are
knowntocausesignificantdamagetohairfibers[16].Nonetheless,inadequatedailycare
routinesandpracticesarealsoacauseofhairdeterioration.Physicaldamagecanresult
frompoorbrushingorcombingorfromblowdryingatextremelyhightemperatures[16].
Hairweatheringisaprogressiveprocessofdeteriorationwherethehairshaftlosesthe
externalprotectionprovidedbythecuticlesandaffectstheabilityofthecortextoretain
moisture[24].Inafirststage,thisresultsinlossofglossandsoftness,butwithfurther
damage,hairmayloseitsstrengthandelasticity,andeventuallyitscompletestructural
integrity,resultinginhairbreakage[25].Hairisconstantlyexposedtovarioussourcesof
weatheringduringitsexistence.Thismeansthatolderhairwillpresentahigherdegree
ofdamagethantherecentlyformedhairshaft.Consequently,thehairnearthescalppre‐
sentsamorepreservedstructurethanthetipsoflonghair.InFigure5,examplesofhair
samplesshowingremarkableinjuriesonthehairshaftalongthefiber’slengtharepre‐
sented.Nearthescalp,thehairhascompletecuticles,andthehairshaftisfullycovered
bytheoverlappingscales.Asthedistancefromthescalpincreases,thehairbecomesmore
damaged.Somecuticleswerepartiallyremoved,exposingthecortexofthefiber,andthe
surfaceshowsaroughappearance.Nearthetip(Figure5,right),nocuticlesarevisible
duetothelongerexposuretoexternalinsultsduringitsexistence,whichresultedinthe
completeremovalofthecuticlelayers.Thedegreeofdamagewilldifferfrompersonto
person,anditisstronglydependentonthedailycarehabitsofeachpersonandthedegree
ofexposuretoweatheringevents.
Figure5.SEMimagesofavirginhairfiberatthreelocations:nearthescalp,inthemiddleofthe
fiberandnearthetip.Adaptedfromreference[23]withpermissionfromElsevier.
Deteriorationofthehairfibersoccursthroughfourcategoriesofdamagecauses:me‐
chanical,chemical,thermal,orenvironmentalprocesses,oracombinationofthose.The
degreeofdamagecausedbyeachtypeofexternalinsultwilldependonthehabitsofeach
individual,but,ingeneral,chemicalprocessesaretheonespronetoinflictmoredeterio‐
ration,whereasexposuretoenvironmentalfactorshasalowerimpact[16].
Polymers2023,15,6086of23
2.1.1.MechanicalInsults
Mechanicaldamageisthemostfrequentdamagesourcebecauseitoccurseveryday
when,forexample,combingthehairwithplasticorwoodcombsorbrushes.Therepeat‐
edlybrushedmovementsinthesameareacausethecuticleedgestopeeloffand,ulti‐
mately,tobepartiallyortotallyremoved,especiallyontheoutsideedgesofthefiber.
Cuttingthehairisanotherformofmechanicalinsult,especiallywhennotperformed
withappropriatetools.Theuseofrazorsinsteadofscissorsorevenabluntscissorwillresult
inacutthatisnotclean,andthecuticlewillbemorepronetopeelingoff[16].
Althoughshampooshavethepurposeofpromotingcleaningandmaintainingthe
hairingoodcondition,theshampooingactionitselfcanbeasourceofdamagetothehair
causingdeformationandabrasion/erosionwhenbendingthefibersandscrubbingthe
hairsagainsteachother.Moreover,thehighcleaningabilityoftheanionicsurfactantsin
shampooscanalsopromotethedissolutionofstructurallipidsandproteins[3].
2.1.2.EnvironmentalInsults
Thepigmentmolecules,i.e.,melanin,presentinthehairshaftsgrantthemphotopro‐
tectionagainstUVradiationbyabsorbingandfilteringit[26].Thisfilteringpartially
avoidsthedegradationoftheproteinsbyradiation,butthepigmentsaredegradedor
bleachedduringtheprocess,changingtheappearanceofthehair.Forthereasonthatthe
cuticlesareexposedtohigherintensitiesofradiationandmelaninisonlylocatedinthe
cortex,theyaremorevulnerabletophotodamage[1].Sunexposureinducesmodifications
inaminoacids,especiallyinthecuticles,resultinginruptureanddetachmentoftheex‐
ternallayers,causingthesplittingoftheends.Italsocausesthehairtobecomedryand
brittle,loseitslusterandcolor,andhavearoughsurface[26].Thephotochemicaldegra‐
dationofproteinsresultsinadecreaseintheirstructuralintegrity.Thisoccursbytherup‐
tureofdisulfidebondsbetweenstructuralunitsandtheestablishmentofnewbondsby
thereactionofcarbonylgroupswithaminogroupswithinandbetweenstructuralunits
[27].Independentofthehairtype,theUVBradiationisthemajorcauseofhairprotein
loss,whereastheUVAradiationisresponsibleforcolorchanges[26].Eventhoughthe
typeofdamageinflictedonthehairisonlydependentontheradiationwavelength,the
degreeofdamagevariesaccordingtothehairtype.Differenttypesofhairhavedifferent
photostabilityduetotheirdifferencesinmelanintypeandconcentration.Blackhairseems
tobemorephotostablethanblondhairandpresentshigherprotectionagainstUVlightin
themelanin‐richcortex,showingonlyaslightmodificationoffiberproteinsunderirradi‐
ation[27].Whencomparingthephotodamageinthecuticles,theproteinlossissimilar
betweenblackandlight‐brownhairs,whichisexplainedbytheabsenceofmelaninatthe
surfaceofthefibers[28].Changesinhaircoloralsodependonthetypeofhair,beingmore
pronouncedinlight‐coloredhairthaninblackhair[26].
2.1.3.ChemicalInsults
Chemicalproceduresforhairstyling,suchaschemicalstraightening,bleaching,or
dyeing,areresponsibleforsevereinjuriestothehair,especiallywhennotperformedby
well‐trainedprofessionals.Forexample,chemicalstraighteningisachemicalprocedure
adoptedasalong‐lastingalternativetothermalstraightening.Itisperformedwithchem‐
icalrelaxersthatproduceastraightappearancebyaffectingthecysteinedisulfidebonds
ofthehair[29].Alongwiththerearrangementofdisulfidebonds,structuraldamageto
thehairshaftalsooccursduetomodificationsinkeratin’slinkages,resultinginweaker
hairwithreducedtensilestrength[11,29].
Bleachingistheprocessthatallowsforthelighteningofthehairbyoxidizingmelanin
pigmentspresentinthecortex.Thisprocessdisruptsdisulfidebondsandmakesthecuti‐
clesmoreporous,leadingtopoorertensilestrengthandabrittleappearance[1].Thepho‐
toprotectivefunctionofmelaninislostwhenitisoxidized,sobleachedhairismoresus‐
ceptibletobeingfurtherdamagedbyUVexposure[1].
Polymers2023,15,6087of23
Permanentdyesareusedtochangethecolorofthehairfibers.Similarlytobleaching,
theireffectisalsobasedonoxidationreactionsthatoccurwithinthehairshaft.Toenable
theirpenetrationintothehairshaft,alkalinesolutions(e.g.,ammonia)areappliedtodis‐
rupttheprotectivebarrierofthecuticle,makinghairespeciallysusceptibletofuturedam‐
age[1].Applicationofalkalinetreatmentsresultsintheremovalofthe18‐MEAlayer,
increasingthehair’shydrophilicityandfriction[15].Thelossofthishydrophobiclayer
reducesthesmoothappearanceofhairandmakesitmorefragile.
2.1.4.ThermalInsults
Theuseofblow‐dryerstodrythehairisacommonpractice.However,itsregularuse
causesdamagetothehairshaft.Whileair‐driedhairshowsawell‐protectedsurface,blow‐
driedhairshowsahigherdegreeofdamage[30].Thehairsurfacebecomesmoredamagedas
thetemperatureincreases[30].Blow‐dryingthehaircausesthescaleedgestobecomeconcave‐
shapedduetocuticlelayersplitting[31].Structuralchangescausedbyblow‐dryingarenot
exclusivelylimitedtothesurfaceofthefibersbutalsooccurbetweeninnercuticlelayers[31].
Haircolorisalsoaffectedbyheat,becominglighterafterrepeatedshampooinganddrying.
Thesechangesincolorarevisibleafterjust10blow‐dryingprocessesat95°C[30].
Stylingthehairwithhotstraighteningorcurlingironsalsoinflictsdamagetothe
hair.Theseprocessesoccuratveryhightemperatures,andthestylingeffectsarecausedbya
temporaryrearrangementofthehydrogenanddisulfidebondswithinthehairshaft[11,29].
Thermalstraighteningpromotesoverheatingofthehairshaft,causingweakeningand
rupture[29].
3.HairCare
Peoplecareabouttheirhealthandappearance.Abeautiful,healthyandwell‐treatedhair
andskincausesimpactontheothersandself‐impressionofone’simage.Sincewayback,dif‐
ferentproductshavebeenusedtochangehair,skin,andphysicalfeatures.Thesegoodsare
oftengroupedascosmeticproducts,and,accordingtotheproperEUlegislation(Regulation
No.1223/2009),thesearedescribedas“anysubstanceormixtureintendedtobeplacedincon‐
tactwiththeexternalpartsofthehumanbody(epidermis,hairsystem,nails,lips,andexternal
genitalorgans)orwiththeteethandthemucousmembranesoftheoralcavitywithaview
exclusivelyormainlytocleaningthem,perfumingthem,changingtheirappearance,protect‐
ingthem,keepingthemingoodconditionorcorrectingbodyodors”[32].
Hairhasagreatimpactonaperson’simageandexpressionofpersonality.Some
practices,namelythoseproceduresperformedinhairsalons,suchascutting,dying,or
stylingthehair,areimplementedwiththefundamentalpurposeofembellishing.Ona
dailybasis,somecarepracticesalsohelptoimprovehair’sfeelandlook,suchasgrooming
habits.Brushing,cleaning,andtheapplicationofcosmeticproductsplayamajorrolein
accomplishingtheseresults.Thesepracticesareofextremeimportancetokeepingthehair
andscalpclean,maintainingthehairingoodcondition,andrepairingor,attheleast,
mitigatingthedamagecausedbyexternalaggressions.Forthereasonthathairiscom‐
posedofdeadcells,itcannotbebiologicallyrenovated;theonlywaytorestoreitsprop‐
ertiesistophysicallyrepairitbyusingahairconditionerand/orotherhaircareproducts
thatcancoverorfillthedamagedsurfaceofthehairshaft[23,33].
4.HairConditioners
Environmental,thermal,andchemicalweatheringcausedamagetothehairsurface,
resultinginanunpleasantfeelingandappearance.Theprimaryreasonforlusterandtex‐
turelossisthepartialortotalremovalofthe18‐MEAlayerfromthecuticlesurface.The
lackofthishydrophobicprotectionmakesthehairmorehydrophilicandsusceptibleto
swelling,thusincreasingitsfragilityandthefrictionbetweenthefibers[10,15].Asmen‐
tioned,hairfibersarecomposedofdeadcellsthatcannotreplicate,andthushaircannot
bebiologicallyrepaired[5].Toavoidtheexposureanddamageofthehair’scortex,the
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surfaceofthefibercanbecoveredandpartiallyrepairedbyapplyinghairconditionerthat
fillsthedamagedareaofthecuticle[5].InFigure6,SEMimagesofvirginhairandhair
treatedwithconditionerarepresented,showingthedepositionofhairconditionerthat
fillstheedgesofthecuticles.Whenusingintensiveconditioners,theF‐layercanbetemporar‐
ilyreplaced,anditispossibletorestoresomeofthelostphysicalpropertiesofhair[34].
Figure6.SEMimagesofvirginandtreatedhair.Adaptedfromreference[23]withpermissionfrom
Elsevier.
Hairconditionerisoneofthenumeroushaircareproductsavailable.Themainfunc‐
tionofconditionersistoimprovethephysicalandaestheticpropertiesofthehair.They
areprimarilydesignedfordryanddamagedhairandcontainmoleculesthatbuildupon
thedamagedscalesofthecuticle,fillingthefracturesandfissures[35].Thisresultsina
smoothsurfacewithreducedfrictionbetweenhairfilaments.Briefly,hairconditioners
impartsoftness,gloss,andlustertothehair,reducefrizz,andimproveitsmanageability
andeaseofcombing[5,34,35].
4.1.ClassificationofHairConditioners
Therearevariouscosmeticapproachestoimpartinghairconditioning.Haircondi‐
tionerscanbecategorizedindifferentways,accordingtothetypeofproduct(i.e.,theway
itisappliedtothehair),theactionmode,oritscomposition.Themostsuitableconditioner
formulationisdependentonthedesiredeffectanddamagedegree.Thedifferencesbe‐
tweenthevariousproductswillbepresentedinmoredetailinthissection.
Regardingthetypeofformulation,hair‐conditioningagentscanbeincorporatedin
shampoosorusedintheformofpost‐shampooinglotions,mousses,orgels.Inshampoo
formulations,conditioningagentsareincludedasadditivestopreventorreducetheneg‐
ativeeffectspromotedbyanionicsurfactants,whichincreasetheelectricalnegative
chargesonthehairsurfaceandcreatefrizzandfriction.Theyaredesignedfordryand
damagedhairtoimpartsoftnessandglossanddiminishhairentanglement[36].Although
somedegreeofconditioningcanbeachievedwiththeincorporationofconditioningad‐
ditivesinshampooformulations,theireffectisnotaspronouncedasinahairconditioner
formulation,possiblyduetoincompatibilitiesbetweenthecomponents,inparticular,an‐
ionicsurfactantcleanersandcationicconditioners,whichmayreactwitheachotherand,
asaresult,createcomplexesthatareinsolubleinwater[36].In“2in1”shampoos,the
formulationisintendedtohaveadualfunction:cleaningandconditioning.Thus,itis
expectedthattheseproductsarenotaspowerfulastheonesdesignedspecificallyforthat
purpose.Ontheotherhand,post‐shampooconditionersareformulatedwiththespecific
purposeofimpartingconditioningtothehairand,therefore,aremuchmoreeffective.
Someofthemaresupposedtobeappliedforacertainperiodandthenrinsedoff(i.e.,
instantanddeepconditioners),whileothersaredesignedtobeleftinthehair(i.e.,leave‐
inandblow‐dryingconditioners).Instantconditionersarethemostcommonandaregen‐
erallyappliedaftereveryhairwash.Theyareappliedtowethair,leftonforabout5min,
andthenrinsedoffwithwater.Duetotheshortcontacttime,instantconditionersarenot
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theonesthatimpartthebestconditioning,buttheyareeffectiveatreducingthedryness
causedbyshampooingandotherdailydamages,thushelpingtoimprovehairmanagea‐
bility.Thedetanglingofthehairoccursbysmoothingthecuticles,causingareductionin
frictionwhenappliedtoslightlydamagedhair[37].Ifthehairisseverelyinjuredanddry,
itisrecommendedtheuseofdeepconditionersbecausetheyaremoreconcentratedthan
theinstantconditionersandtheyaremeanttobeleftinthehairforlongerperiods,usually
20to30min[38].Additionalapplicationofheatresultsinthecuticles’liftingandincreases
thepenetrationoftheconditioningagents[33,39].Foralongerconditioningeffect,con‐
sumersoftenuseleave‐informulationsinadditiontoorinsteadofrinse‐offproducts.
Theseproductsareformulatedtobeappliedtowetordryhairbeforeblow‐dryingor
styling.Theyusuallycontainsiliconesorotherlightweightpolymers,suchaspolyvi‐
nylpyrrolidone,thatformathinfilmthatcoatsthehairsurfaceandreducesstaticelectric‐
ity[33].Leave‐inandblow‐dryingconditionersareusefultopreventthedamagecaused
bydailycareroutinesandtograntthermo‐andphotoprotection[40].
InTable1,themaincharacteristicsandtheactiveingredientscommonlyusedinthe
formulationofthevarioustypesofconditionersaresummarized.
Table1.Typesofhairconditionerproductsandtheirintendedapplication.
ClassificationApplicationConditioningAgents
Used
Shampoos“2
in1”
Shampoowithdualfunction—cleaningand
conditioning.Conditioningadditivesareincor‐
poratedtoimpartmanageability,gloss,andan‐
tistaticpropertiestothehair.Recommendedfor
dry,damaged,orchemicallytreatedhair.
Hydrolyzedprotein,sili‐
cones,glycerin,polyvi‐
nylpyrrolidoneorqua‐
ternaryconditioners.
Instantcondi‐
tioners
Intendedtobeappliedaftershampooingand
rinsedafterashortperiodoftime(around5
min).Idealfordailyuseinslightlydamaged
hairtoreducetheeffectofshampooandim‐
provedailymanageability.
Quaternarycondition‐
ers,suchasbehentrimo‐
niumchlorideandstea‐
ralkoniumchloride.
Deep/inten‐
sivecondition‐
ers
Moreconcentratedthaninstantconditioners
andshouldbeleftonthehairfor20to30min.
Recommendedforextremelydryhairorbefore
chemicaltreatment,suchascoloringandwav‐
ing.
Higheramountsofqua‐
ternaryconditionersin
additiontoproteins.
Leave‐incon‐
ditioners
Designedtobeappliedaftershampooingand
conditioning,andnotrinsedout.Theseprod‐
uctscanbeapplieddailyinwetordryhairand
areidealforpreventingdamagefromroutine
grooming.
Silicones,oils,polyvi‐
nylpyrrolidoneorother
film‐formingagents.
Blowdrying
conditioners
Theyareappliedtotowel‐driedhairbefore
blow‐dryingandstylingandmayprovidepho‐
toprotectionandpreventheatdamage.Useful
forpeoplewithfinehairandexcessivescalpse‐
bum.
Sameagentsasinstant
conditionersbutdonot
containoil.
Hairthicken‐
ers
Theycoatthehairshaft,increasingtheirdiame‐
terandgivingtheillusionofthickhair.They
usuallycontainproteinsasconditionersandare
alsoappliedtotowel‐driedhairbeforestyling.
Film‐formingagents,
suchassiliconesorhy‐
drolyzedproteins,such
askeratinK31.
AsshowninTable1,thereisawidevarietyofproductswithconditioningproperties,
and,evenforeachtypeofproduct,awidevarietyofcompoundscanbeintroducedto
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impartconditiontothehair.Dependingontheseverityofthedamageandtheaesthetical
andphysicalpropertiesrequiredbytheconsumer,differentclassesofcompoundswith
diversebehaviorsandmodesofactionareusedtodesigntailoredhairconditionersfor
eachtypeofhairanddamagedegree.Thereisaplentifullistofcompoundswithcondi‐
tioningproperties.Someofthemincludenaturalpolymers(e.g.,cellulose,starch,and
theirderivatives),syntheticpolymers(e.g.,polyvinylpyrrolidone,polyacrylates,silicones,
etc.),gums,andhydrolyzedproteins[41].Theinteractionsoftheconditioningcompounds
withhairareaffectedbythepolymer’sproperties,forexample,itsmolecularweight,hy‐
drophobicity,andchargedensity.So,itisexpectedthatthedifferentclassesofcondition‐
ingagentswillactdifferentlyandgrantdifferentpropertiestothehair.Hairconditioning
isachievedbyatleastoneofthethreekeyprocessespresentedbelow:
1. Neutralizationofthenegativechargesofthehairfibersthroughtheadsorptionof
cationiccompoundsontothesurface.
2. Lubricationofthecuticlesbyrestoringthehydrophobiccharacterofthehairshaft.
3. Restoringthelostproteinsandenablingmoistureretentionthroughtreatmentwith
smallproteinsthatpenetratethehairshaft.
Eachoneofthesepathwaystoachievingsmooth,shiny,andmanageablehairisac‐
complishedbyusingcompoundswithdifferentchemicalnatures.Hairconditionerscan
bedividedintothreeclassesaccordingtotheircompositionandmechanismofaction:
cationic,film‐forming,andprotein‐basedconditioners.Anoverviewofthemaincharac‐
teristicsofeachtypeofconditioningagentandexamplesofcompoundsusedinthefor‐
mulationsaregiveninTable2.
Table2.Classificationofhairconditionersbasedontheircomposition.
ClassificationModeofActionIngredients
Cationiccondi‐
tioners
Actbyneutralizationofthenegativecharges
ofdamagedhairbydepositionofpositively
chargedmoleculesonthehairsurface.The
softnessandsmoothappearanceofthehair
areachievedbythereductionofstaticelec‐
tricityofthecuticles.Excellentforchemically
processedhair.
Quaternaryammonium
compounds:cetrimo‐
niumchloride,stea‐
ralkoniumchloride,etc.
Film‐forming
conditioners
Actbydepositionofpolymersthatforma
filmthatfillsthedefectsincuticlessurface
andcoatthehairshaft,restoringitssoftness
andshine.Theycanalsobepositively
chargedandreducethestaticelectricityof
anionicdamagedhairshaft.
Film‐formingagents,
suchaspolyvinylpyrrol‐
idone(PVP),silicones
andoils.
Protein‐based
conditioners
Containaminoacidsandsmallpolypeptide
fragmentsofhydrolyzedproteinsthatcan
penetratethehairshaftandrepairthedam‐
agedhairbyrestoringthelostproteinsand
improvethehair’sstrength.Theexcesspro‐
teinsarerinsedoutwhenwashingthehair,
sotheireffectisonlytemporary.
Manydifferentprotein
sources:animalprotein,
eggs,placenta,collagen,
keratin,beer,among
others.
Cationicconditionersarecharacterizedbythepresenceofcationicsurfactants,also
knownasquaternaryammoniumcompounds.Duetotheirpositivecharges,thistypeof
conditioningagentneutralizesthenegativechargesofthehair,promotingadecreasein
staticelectricity[42].Thethinfilmofconditionerthatadsorbsatthehairsurfaceflattens
thecuticles,makingthemsmoothandimprovingthesoftnessandshineofthehair.Cati‐
onicsurfactantsareveryeffectivefortheconditioningofchemicallytreatedhairbecause
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oftheirhighdensityofnegativecharges.Severalquaternaryammoniumsaltsareusedas
conditioningagentsinhaircareformulations,suchascetrimoniumchloride,behentrimo‐
niumchloride,stearalkoniumchloride,etc.Thesecompoundshaveahydrophilic,posi‐
tivelychargedgroupthatinteractswiththehairshaftthroughelectrostaticinteractions
andoneormorehydrophobicalkylchainsthatpointtowardstheoutersurface,whichena‐
blesthemtorestoresomeofthelosthydrophobicity(Figure7).Italsohelpstoimprovethe
compatibilitybetweenhairandothercompoundsintheformulationthatsynergisticallycon‐
tributetotheconditioningeffects(suchasoils)bychangingthehydrophiliccharacterofthe
hair’ssurface[43].
Figure7.Repairingactionofcationicconditioners:(a)undamagedhaircoveredwithitsnatural
hydrophobicprotectiveF‐layer;(b)negativelychargedamagedhairwithouttheF‐layer;(c)hair
repairedwithconditioner,thecationicsurfactantadsorbstothesurface.Adaptedfromreference
[44]withpermissionfromElsevier.
Film‐formingconditionersarepreparedfromhigh‐molecular‐weightpolymersthat
formathinfilm,coatingthesurfaceofthehairfiberwhilefillingthecuticledefects.This
coatingsmoothsthecuticle’ssurfaceandreducesthefrictionbetweenhairshafts.Conse‐
quently,thecoatedhairwillpresentlowerstaticelectricityandreducedfrizz.Silicones
areoneofthemostcommonfilm‐formingagentsusedinconditioners.Therearedifferent
typesofsiliconesthatshowdifferentdeposition,adherence,andrinse‐outcapacities,re‐
sultinginconditionerswithdistinctperformances[45].Dimethiconeisthemostcom‐
monlyusedwithinthesiliconesfamily[10].Itisahydrophobicpolymer,soitwillcon‐
tributetorestoringthehydrophobicityofthehairandactasalubricant.However,the
adsorptionofhydrophobicpolymerswillbegreaterinvirginhairwithlimiteddamage.
Topromotethedepositionofdimethiconeonanionicfibers,additionalcationicbridging
agentsareaddedtotheformulationtoincreasetheaffinitybetweenchargedhairandsilicone
[45].Analternativeapproachistoaddcationically‐modifiedpolymersthatcombinethe
film‐formingactionofhighmolecularweightpolymerswiththeenhancedinteractionsof
cationicsurfactantsandconsequentneutralizationofhair’ssurfacecharge.Cationicpoly‐
mers,suchasPolyquaternium‐10(quaternizedcellulose)andPolyquaternium‐7(co‐pol‐
ymerofdiallyldimethylammoniumchlorideandacrylamide),arecommonlyused,but
moreinshampooformulationsthaninconditioners[3].
Conditionerscontainingproteinsintheircompositionareefficientatimprovingthe
healthofthehairbytemporarilyrepairingitsdamage,particularlyatthesplitends.Hair
weatheringinducestheremovalofexternalcuticlescalesandleavesthehairfragileand
susceptibletobreakingorsplitting.Protein‐basedconditionersarecomposedoflowmo‐
lecularweight(i.e.,1to10kDa)hydrolyzedproteinfragments,suchasaminoacidsor
smallpeptides,thatcanpenetratethehairshaft,bindtothekeratin,andrestorethelost
proteins[10].Thisgreatlyimproveshairstrengthandhelpstoavoidfurtherdamage.Hydro‐
lyzedproteinsareobtainedfromvarioussources,suchasanimalcollagen,keratin,placenta,
etc.[46].Theefficiencyoftheconditionerisnotasdependentontheproteinsourceasitis
onitsparticlesizeanditsconsequentabilitytopenetrateandremaininsidethehairshaft
[46].Thecontacttimeisalsoanimportantfactor:toachieveagreatereffect,thecondi‐
tionershouldbeleftincontactwiththehairforalongerperiod,thusallowingtheproteins
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todiffuseintothefibers.Theeffectofproteinconditioningisonlytemporary:theexcess
proteinsattachedtothehairareremovedwhenshampooing,andthusitisnecessaryto
reapplytheconditionertomaintaintheconditioningeffect[33].
Mosthairconditionerproductsareformulatedwithamixtureofdifferentcondition‐
ingagentstoenhancetheirperformance.Highconcentrationsoffattyalcohols,suchas
cetyland/orstearylalcohols,orotherlipidcomponentsarefoundinhairconditionersin
additiontocationicsurfactants[3].Thisisbeneficialbecausehydrophobiclubricants,such
asoilsandsilicones,arenotascompatiblewithdamagedhairascationiccompounds.
Whenincorporatingcationicsurfactantsinadditiontothesefilm‐formingagents,they
willactasbindingbridgesbetweenhighlypolar,negativelychargedhairandnon‐polar
conditioners.AspreviouslyshowninFigure7,cationicsurfactantsinteractwiththehairbyits
cationicpolargroup,leavingathinfilmofhydrophobicalkylchainspointingtotheouter
surfacethatisacompatiblesubstrateforfurtherdepositionofhydrophobiccompounds.
4.2.PhysicochemicalPrinciplesoftheHair‐ConditioningProcess
Asdiscussedabove,variousfactorscaninfluencethestructureandaestheticproper‐
tiesofhair.Someofthemcauseseveredamagetothehairfibers,suchaschemicaltreat‐
ments,butevenenvironmentalweatheringaltersthehair’ssurfaceandintegrity,leaving
thehairwithanunhealthyfeelingandappearance.Thesealterationsmaybecaused,for
example,bytheremovaloftheoutermostlipidic18‐MEAlayerthatiscovalentlylinked
tothecuticleorbytheoxidationofthedisulfidebondsfromthecystineresiduestocysteic
acidcausedbyprolongedsunlightexposure[5].Theexcessofacidicgroupsundergodissoci‐
ationintheaqueousmediumandincreasethechargedensityinthehairshaft[5].Thesemod‐
ificationschangethecharacterofthehairsurfacefromhighlyhydrophobicandpoorly
chargedtoahydrophilic,polar,andnegativelychargedsurface[3].Itwasstatedthatthe
useofconditionersiseffectivetopreventortreattheseundesiredmodifications.Theper‐
formanceofthehairconditionersisdependentontheircompositionbutalsoontheirabil‐
itytodepositontothehairsurfaceandremainattachedtoit.Therefore,understanding
theinteractionsbetweenconditioneringredientsandhairisofextremeimportance,since
theywillaffectthedistribution,thickness,and,consequently,theproperfunctionofcon‐
ditioners[47].
Aspreviouslydiscussed,thevarietyofhairconditionersislarge,andtheirdifferent
compoundsactinadistinctmannerduetothedifferentinteractionsthatgoverntheir
depositionorabsorptionintothehair.Forexample,hydrolyzedproteinsofsmallsizecan
diffuseintothehairshaft,whereaspolymersofhighmolecularweightonlyactatthe
cuticlesurfacebyadsorbingviavanderWaalsinteractionsorbindingviachemicalor
electrostaticinteractions[47].Forthereasonhairconditionersareaqueoussolutionsor
suspensions,theamountofproductthatdepositsontothefiberdependsonthebalance
betweenitsinteractionswiththekeratin,itshydrophilicityorbindingforcestotheaque‐
ousphase,anditsdiffusionrate[3].Adsorptionofdifferentcompoundstothehair
keratinsisconsideredacontinuumspectrumofmechanismsbetweencharge‐drivenand
hydrophobically‐drivenprocesses[48].Somecompounds,suchaswater‐solublecationic
surfactants,atapHabovetheisoelectricpointofthehairproteins,adsorbontotheirsur‐
faceessentiallybyacharge‐drivenmechanismduetotheattractionbetweenpositively
chargedquaternaryammoniumionsandnegativelychargedhair[3].Incontrast,thead‐
sorptionofwater‐insolubledimethiconefromananionicshampoomediumontothehair
surfaceisessentiallydrivenbyhydrophobicinteractions[3].Theseexamplesarerepre‐
sentativeoftheextremeendsofthespectrum.However,subtlestructuralchangesinthe
adsorbingcompoundsorinthemediumconditions,suchaspHalteration,mayinduce
changesintheadsorptionmechanismtowardsanintermediatemixedprocesswithcon‐
tributionsofbothchargeandhydrophobicinteractions.Theinteractionswithkeratinare
affectedbyvariousfactors,suchasthemolecularsizeandchargeoftheconditioning
agents,thepHofthemedium,andthehair’sproperties,suchasdamagedegreeandiso‐
electricpoint[3].TheisoelectricpointandpHofthemediumaffectthemechanismof
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depositionofthedifferentcompoundsbecausethesefactorsaredirectlyrelatedtothe
hair’snetcharge.Theisoelectricpointofhairisacidic,around3.67[49].Thismeansthat
abovethispH,thehairwillpresentanetnegativechargedensity,andcationiccondition‐
ersarepronetoelectrostaticallyinteractwithit.Inthecaseofdamagedhair,higher
amountsofconditionersareadsorbedduetotheirlowerisoelectricpoint,whichincreases
theconcentrationofnegativechargesatthesurface.Thedepositionofcationicconditioner
ontoanionichaircuticlesisillustratedinFigure8.Inbrief,theconditionerdepositsflatten
thecuticlescalesagainsteachother,reducingtheflyawaybehaviorandimprovingthe
hair’sshineandcolor[50].
Figure8.Negativelychargedhairandthedepositionofpositivelychargedconditioneronthecuticle
surface.Adaptedfromreference[50].
4.3.AlternativeConditioningAgents
Someoftheconventionalhairconditioningagentsarereportedtocauseskinandeye
irritationandadverseeffectsonthenaturalenvironment,astheyaretoxictoaquaticor‐
ganismswithlong‐termeffects[51].Duetothis,thedevelopmentofhighlyefficientcon‐
ditionerswithlowimpactonhumanhealthandtheenvironmentishighlydesirable.Con‐
sumers’awarenessregardingenvironmentalissuesisgrowing,thusdrivingtheirprefer‐
encestowardssustainableandnatural‐basedformulations.Researchontheassessmentof
thepublicperceptionofbio‐basedproductsandthebioeconomyhasbeenconductedinthe
pastfewyearsinseveralprojects,suchasBIOWAYS[52]andRoadToBio[53].Resultsofsur‐
veysclearlyshowthatthevastmajorityoftherespondents(80−82%)expressapositive
overallperceptionofbio‐basedproducts[54,55].Thepublicperceivestheadvantagesof
usingbio‐basedproducts,especiallyregardingenvironmentalbenefitsintermsofland,
water,andbiodiversity(83.6%)andlessuseoffossilfuels(74.5%)[56].Inaddition,asig‐
nificantnumberofrespondents(64.2%)areconfidentthattheuseofbio‐basedproducts
contributestothecreationofsustainableeconomicgrowthandnewjobs[54].Regarding
theirpreferenceforbiobasedorconventionalproducts,66.6%ofparticipantswouldprefer
biobasedproductsovernon‐biobasedproducts,ifavailable[56].Themajorityofrespond‐
ents(50.2%)believethatbiobasedproductsarejustasgoodasconventionalones,and
53.1%answeredthattheyarewillingtopaymoreforabiobasedproductwiththesame
functionalityandpropertiesasafossil‐fuel‐derivedone[54].Despitetheirpositiveper‐
ceptionandpreference,only12%ofconsumershaveeverconsciouslychosenbiobased
productsoverconventionalones[55],and71%oftherespondentsstatedthattheynever
usebiobasedproducts[54].Thiscouldberelatedtotheirlackofawarenessordifficulty
findingbiobasedproductssinceonly25.6%agreethattheycaneasilyfindbiobasedprod‐
uctsonthemarket[54].Fromtheseresults,itisclearthatpeopleareinterestedinbiobased
products,butmorebiobasedsolutionsmustbeavailableonthemarketor,attheleast,
well‐identifiedsothatconsumerscaneasilyfindthesealternativesandbecomeawareoftheir
benefitsregardingtheimpactsonhealthandtheenvironment.Itismainlythelimitedavaila‐
bilityandthelackoflabellingofbio‐basedproductsthatdiscouragepeoplefromusingthem
[56].
Acommonpracticethathasbeenadoptedbythecosmeticindustrytoimprovethe
sustainabilityofcosmeticproductsistoproduceformulationscontainingnatural
Polymers2023,15,60814of23
ingredients.Thisapproachallowsthecosmeticindustrytomeetconsumerexpectations
regardingtheproductionofmoreeco‐sustainableformulationsaswellascorporatesocial
responsibility(CSR).TheEuropeanCommissiondefinesCSRas“theresponsibilityofen‐
terprisesfortheirimpactsonsociety”andsaysitshouldintegratesocial,environmental,
ethical,humanrights,andconsumerconcerns[57].Somecosmeticindustriesarecommit‐
tedtosustainabledevelopmentandfindingrawmaterialsofnaturalorigin[58,59].The
sustainabledevelopmentprocessshouldconsiderallstagesofthedevelopmentprocess,
includingthelifecycleoftheingredients,andshouldfollowthegreenchemistryprinci‐
ples[59,60].Althoughthereisavastvarietyofnaturalingredients,somerecommenda‐
tionsaredescribedforthechoiceofsustainableingredients:(1)userenewableresources;
(2)usegreenfabricationprocesses;and(3)havealowenvironmentalimpact[59,60].
Despitetheirnaturalorigin,itiscrucialtoassesstheinformationonthecompounds’
properties.Regardingcosmeticproducts,itisparticularlyimportanttoconsidertheirbi‐
odegradation,toxicity,anddermatologicalfeatures[61].Nonetheless,theproductprop‐
ertiesandoverallefficiencyshouldnotbeneglectedwhendevelopingnewhaircareprod‐
ucts.Thegoalistocreatemoresustainableformulationswithoutcompromisingthecon‐
ditioningeffect,i.e.,wetanddrycombingforces,rheologicalproperties,andproductsta‐
bility.Protein‐basedconditioners,forexample,canbeeasilypreparedfromnaturalingre‐
dients.Hydrolyzedkeratinproteinsfromhumanoriginorsheepwoolarecommonlyused
duetotheirgreatabilitytorepairandrestorehumanhair.However,humanandanimal‐based
productsfaceethicalandhealthconcerns,andtheiruseislimitedbyregulations[62].Dueto
this,proteinsobtainedfromvegetablesourcesarehighlyappealingrawmaterialsforthede‐
velopmentofcost‐effectiveandsustainablehaircareformulations[63].Plant‐derivedpro‐
teins,suchasthosederivedfromquinoa,jojoba,baobab,soy,orrice,canbeusedascon‐
ditioningagentsincosmeticformulations[5].Peptidessynthesizedfromnaturallyde‐
rivedaminoacidsarealsoproposedasefficient,sustainableconditioningagentswithtun‐
ablestructureandproperties[62].Thisrouteallowsforfinetuningthemolecularweight
ofthesekeratinstomakethemsuitableforpenetrationintothehairshaftandtoselecting
theterminalpeptidestomakethemmoresubstantivetothehair[62].Ithasbeenshown
thatahighlevelofpeptidedepositioncanbeeasilyachievedandmaintainedonhairafter
dailyuse.
Vegetable‐basedlipidscanalsoactasconditioningagentsandrestorethehydropho‐
biccharacterofthehairshaftbyreplacingthehair’snaturallipidiclayer.Recently,
Rincón‐Fontán,etal.[64]studiedtheadsorptionofalipopeptidebiosurfactantextracted
fromcornsteepliquor(CSL)onhumanhair.Thisbiosurfactantiscomposedof64.2%fatty
acids(mainlyC18–C16fattyacids)and21.9%ofproteins,andbasedonitsbiocompatibil‐