j. Cosmet.
Sci.,49, 141-153 (May/June1998)
The crackingof humanhair cuticlesby cyclical
thermal stresses
MANUEL GAMEZ-GARCIA,
CrodaNorth AmericanTechnical
Center,180 NorthfieldAvenue,Edison,NJ 08832.
Accepted
for publication
May 15, 1998.
Synopsis
Cyclesof wetting and blow-dryingwere appliedto hair fibersand resultedin the formationof multiple
crackson the hair cuticles.The peculiarityof thesecrackswasthat they alwaysappearedalignedparallelto
the longitudinalaxisof the hair fiber. The cracksappearedto be initiated at the end of the cuticlesclose
to the cortexand propagated
invariablytowardsthe outercuticleedges.The maximumgrowthlengthof
eachcrackwasseento be limited to the sizeof onecuticle.Crackformationdid not only occurat the outer
edgesof the cuticlesbut alsotookplacein the secondand third overlaidhiddencuticlesections.
The results
showthat thesecracksform when the externalportionsof the cuticlesundergodrasticreductionin their
hydrationwater.Under theseconditionsthe outercuticleportionsbecomerigid andbrittle andcrackby the
action of circumferentialtensionstresses
arising from the swelling pressureof both the cuticle layers
underneathand the cortexitself. Hair cuticleanalysisfrom a panelof 100 individualsshowedthat these
cracksarepresentin the hair of peoplewho commonlyblow-drytheir hair and appearto a muchlesserextent
in the hair of subjectswho do not practicethis type of groomingprocess.The combingof hair fibers
presentingthis type of crackingwasseento resultin the breakageof largeportionsof cuticle.The effectof
some cosmetic actives on the formation
of these cracks is also discussed.
INTRODUCTION
Hair cuticlesrepresentthe hair structurecomponentsmost exposedto variousgrooming
and environmentalstresses
during the life of a hair fiber. The outer cuticlesat the hair
surfacearefrequentlysubjectedto harshstresses
suchasabrasion,UV light, bleaching,
andblow-drying(1-4). It is well knownthat oncethecuticlesareoneor two centimeters
awayfrom the hair root,patternsof breakageand abrasionstart to appearat the cuticle
edges(5). By the time the hair is about 14 or 20 cm long, if not enoughcareis taken,
the cuticlesmight be totally absentat the tips, giving rise to the earlierformationof
split ends(6). It is mostlybecause
of thesereasons
that the effectsof combingabrasion
on hair cuticleshave been thoroughlystudied in the past. There are, however,other
groomingpractices,suchas hair blow-drying,whoseeffectson the cuticledegradation
processare still poorly understood.
As it is well known,hair blow-dryingis nowadays
a commonpracticeby manypeople.
When keratinfibersareheated,not only wateradsorbed
to hair by capilarityis rapidly
evaporated
but thereis alsoa rapid lossin the freeandboundwater of the hair (7,8). The
rapid lossof hydrationwatermay causemorphologicalchangeswithin the cortexdue to
141
142
JOURNAL OF COSMETIC SCIENCE
destabilization
of the keratin crystallineregions(9). The consequences
of this type of
processin hair cuticlesafter cyclicalthermalstresses
havenot yet beenanalyzed.This
paperrepresents
part of a studywhoseaim is to reproducepatternsof cuticledamage
found in a panelof 100 individuals(10). This articlereportsthe productionof small
longitudinalcracksby cyclicalthermalstresses.
The cracksweremainlyfoundin the hair
cuticlesof subjectswho frequentlyblow-dry their hair.
EXPERIMENTAL
METHODOLOGY
The panelof 100 individualsparticipatingin this studywasmainlycomposed
of women
with Caucasianbrown hair never treated chemically.A total of ten fibers per each
individualwasanalyzed.The differentpatternsof cuticledamagefoundin theseindividuals such as cuticle decementation, abrasion, craters, and crackswere classifiedand
quantified. Cyclesof mechanicaltension,torsion,bending, and thermal stresses
were
then appliedto singlehair fibersin orderto reproducesuchpatterns(10). Only the
cyclicalthermalstresses
wereseento reproduce
the typesof cracksmentionedabove,and
therefore,onlythe experimental
conditions
relatedto thistypeof stress
will bedescribed
here. The other typesof damageand experimentalconditionsare describedelsewhere
(10-12). Eachthermalcycleconsisted
of immersingsinglehair fibersfor a periodof ten
seconds
into de-ionizedwaterfollowedby ten seconds
of blow-drying.The temperatures
used during blow-drying and measuredat the level of the wet hair surfacevaried
between30ø and 120øC. The number of applied wetting/blow-dryingcyclesvaried
betweenfive and 100 asrequired.A total of ten fibersper eachsetof thermalcycleswas
analyzedfor shortlongitudinalcuticlecracks.The numberof thesecrackspermillimeter
of hair was then counted. From this data, meansand standarddeviationswere calculated.
Prior to the thermalcyclesthe fiberswerethoroughlywashedwith SLSandrinsedwith
tap water.
The hair usedin the single-fiberexperiments
wasfrom a subjectwhosehair waswashed
only with a 10% SLSaqueoussolutionfor a period of oneyear. Sectionsof hair fibers
threeincheslong andcut closeto the rootwereusedin the experiments.The hair fibers
were subsequently
cut into two snippets1.5 incheslong each;onesnippetwasusedas
a control while the other one was subjectedto thermal cycling. All selectedhair fibers
presentedan averagediameter of 82 + ll l•m. Caucasianvirgin brown hair from
InternationalHair Importersin the form of tresseswas alsousedto studyeffectsof
combingabrasionon wet/blow-drycycledhair. Aqueoussolutionsof glycerin,propylene
glycol, polyquaterniumll, cetrimoniumchloride,steralkoniumchloride,and hydrolyzedwheatproteinpolysiloxanecopolymer(13) at a 2% w/w were alsousedaswetting
solutionsduring thermal cycling. These solutionswere preparedin order to test the
effectof somecosmeticactiveson cuticlecracking.After thermalcycling,the fiberswere
preparedfor SEM analysis.
RESULTS
SHORT
AND
DISCUSSION
LONGITUDINAL
CUTICLE
CRACKS
FOUND
IN A HAIR
ANALYSIS
FROM
A PANEL
OF
100 INDIVIDUALS
SEM analysisof hair fiberspertainingto subjects
in the panelshowedthat mostof them
presenteddifferentnumbersof shortlongitudinalcracksin their cuticles.The popula-
CRACKING
OF HUMAN
HAIR
CUTICLES
143
tion of theseindividualscould, however,be divided into two main groups,namelyone
with a high numberof cuticlecracksand the other with a very low number.The first
group,representing
about40% of the panel,showedan averageof 184 + 15.5 cracksper
mm of hair (cpmh), while the other 60% showedan averageof about 7 + 1.3 cpmh.
Figures la and lb showtypical imagesof thesecracksas found in the panel analysis.
Incidentally,the highernumberof cuticlecrackscorresponds
to the hair of peoplewho
manifestedto blow-dry their hair on a daily basis.It waspreciselythis observationthat
suggested
the possibilityof reproducingthe cracksin the laboratoryby cyclesof wetting
and blow-drying.In Figure l it canbe seenthat the particularityof thesecracksis their
positionon the cuticles,i.e., they alwaysappearalignedparallel to the longitudinal axis
of the hair fiber, and their length is no longerthan a singlecuticlesize.The crackswere
foundto occurmorefrequentlyat the discontinuities
formedby the cuticleedges.They
seemed to be initiated
near the middle
section of the cuticle
at that end close to the
cortex.Their directionof propagationalsoseemedto be towardsthe outer edgesof the
cuticles.Figure lb shows,for instance,a long crackwhosewidth is largerat the cuticle
endcloseto the cortexandverysharpat its tip towardsthe outeredgeof the cuticle;this
crackhasnot yet beenable to propagateall the way throughoutthe outercuticleedge.
CRACK
REPRODUCTION
IN
THE
LABORATORY
After observingthat the short longitudinalcrackswere consistentlyfound in a large
portion of people from the panel, a way to reproducethem in the laboratorywas
researched.
First, single hair fibers were subjectedsystematicallyto cyclicaltension,
torsion, and bending stresses,and it was found that none of theseconditionscould
reproducethe cracks.The cuticlepatternsof damageresultingfrom thesestresses
have
beenreportedelsewhere(10-12). The next stepwasto take fibersthree incheslong and
'
la
='"
x2.4k
x4.0k
lb
Figure 1. Typical cuticlecracksfound in hair from individualswho blow-dry their hair. la, x2.4k; lb,
x4.0k.
144
JOURNAL OF COSMETIC SCIENCE
I
. ! litk x
--
13k v
2?3
--
7.
2b
I .3tlkx
8kv
3•'9
Figure 2. Cuticlecracksreproduced
in the laboratoryasfollows:2a and2b after20 thermalcycles;2c after
60 thermalcycles.
CRACKING
OF HUMAN
HAIR
CUTICLES
145
2o
Figure 2. Continued.
cut them in half. One half was usedas a control,and the other half was subjectedto
cyclesof blow-dryingandwetting at differenttemperatures
andfor differentintervalsof
time. The numberof cracksalreadypresentin the controlwere countedand compared
with thoseappearingin the thermallycycledfibers.A total of ten fibersper eachtrial
wereanalyzed.The averagenumberof cracksper millimeterfoundin the controlsamples
was 6 + 2.1.
After severaltrials it wasfoundthat shortthermalcyclesof ten seconds
of blow-drying
at 75øC combinedwith ten secondsof wetting at 25øC were able to reproduceand
increasedramaticallythe numberof shortlongitudinalcuticlecracks.In Figures2a, 2b,
and 2c areshowntypicalimagesof hair fiberssubjectedto 20 and 60 of thesecycles.A
total of 21 crackscanbe countedin the 76 x 52 micrometersectionof hair represented
in Figure 2b. In Figure 2c it canbe seenthat the total numberof visible cracksreaches
a valueof 26; the surfacehair dimensions
capturedby thismicrographareapproximately
67 x 42 microns.The numberof cracksshownin Figures2b and 2c correspond
to an
averageof 472 and 562 cracksper millimeter of hair, respectively.
Suchhigh concentrationsof cuticle cracksare rarelyfound in people'shair and can only be producedin
the laboratory.Figures3a, 3b, and 3c showhighermagnifications
of thesecracks,while
Figure4 showsthe averagenumberof cuticlecracksfoundin hair samplesasa function
of thermal cyclesin relationto the control.In this last figure it can be seenthat the
numberof cracksper unit areaon the hair surfaceincreases
up to a limit and then levels
off with the numberof thermal cycles.
A closeexaminationof Figures1, 2, and 3 revealsthat, in all cases,the appliedthermal
cyclesresultedin the productionof crackssimilarto thosealreadyobservedin the panel
146
JOURNAL OF COSMETIC SCIENCE
x2 4k
"
3a
x2.4k
3b
3c.
Figure 3. Magnifiedviewsof cracksshownin Figure2 asfollows:3a after20 thermalcycles;3b and 3c
after60 thermalcycles.
Eachcycleconsisted
of tenseconds
of blow-drying
at 75øCfollowedby tenseconds
of wetting at 25øC.
700
Number of shortlongitudinalcuticlecracksper mm of hair
(a) Average number of •
.
--
,
600
- cracksinha,
500
I I L '
-
4OO
3O0
200
-
/
'
(b) Averagenumber
/•'
'100
-f
0
5
.
ofcra•sincontrols •
(SD=2.1)'•
10
15
20
25
30
35
40
45
50
55
60
Numberof appliedthermalcycles
Figure 4. Variationsin the averagenumberof cuticlecracksfound in the laboratoryas follows:(a) in
thermallycycledfibersasthe numberof thermalcyclesincreases,
and(b) in theircorresponding
halfsnippet
usedasa control(non-exposedto thermal cycles).Eachpoint representsaverageof cracksfound in ten hair
samples.Each cycleconsistedof ten secondsof blow-dryingat 75øC followedby ten secondsof water
immersionat 25øC. Error barsrepresentone standarddeviationabout the mean.
analysis.This observation
clearlyshowsthat the cuticlecracksfound in hair subjects
fromthepanelarisemainlyasa consequence
of subjecting
hairto thermalstresses
during
blow-drying.An analysisof the cracksshownin Figures1, 2, and 3 indicatesthat their
CRACKING
OF HUMAN
HAIR
CUTICLES
147
formationis mainly limited to the outerpart of the cuticles.This observationsuggests
that the stresses
involvedin crackformationare more intenseat thosecuticleportions
near the outer hair surface.Since blow-drying and wetting involves swelling and
deswellingof the hair fiber, the followingexperimentwascarriedout in orderto test
whetherthe phenomenonof swellingper seplays a role in crackformation.Several
solventswith limited swellingcapacitywere usedin the thermal cyclingexperiments
insteadof water. The solventswere ethanol,iso-propanol,and methanol;thesesolvents
havealreadybeenreportedin the literatureaspoorkeratin swellingsolvents(14-16).
The resultsshowedthat thermalcyclingexperimentswith thesesolventsdo not lead to
crackproductionat all, indicatingthat cuticleswellingis a necessary
phenomenon
for
cracks to occur.
Increasingboth the water-swellingand blow-dryingtimes to periodslongerthan ten
seconds
did not haveanyeffecton the numberof producedcracks.Also,it wasobserved
that non-swollenhair fibers, which were thermally cycledwith water immersiontime
periodsasshortasfive seconds,
underwentcuticlecracking.In suchshorttime periods
of water immersion,only the cuticularsystemand a smallportionof the cortexcan be
expectedto swell. Theseobservations
indicatethat cuticlecrackingis not due to a
thermal shockarisingfrom rapid changesin cuticle temperature.It seemsrather that
cuticle crackingoccursbecausethoseouter cuticlessectionslack elasticityto comply
with the dimensionalchangeseitherof the swellingcuticlelayersunderneathor of the
swellingcortex.
In the caseof blow-drying, the lack of elasticityin the outer cuticle sectionswill
originatefrom rapid cuticledehydrationat high temperatures.Thus, it would appear
that when hair is wet or dried at room temperature,all cuticleportionsand the cortex
contractin a synchronous
manner.However,if during a water evaporationprocessonly
the outer cuticle sectionscontractmore rapidly than thosecuticle layersunderneathor
than the cortexitself, crackingwill occur.Crackingduring blow-dryingtakesplace,
thus, as a consequence
of circumferentialextensionstresses
set up on dry portionsof
cuticleby the swollenpressure
of boththe cuticlelayersunderneath
andthe cortexitself.
It shouldbe mentionedherethat circumferentialor "hoop"stresses
are knownto occur
in cyclindrical
pipessubjected
to internalpositivehigh pressures
(17). Cracksat lower
hair-swellingpressures
may alsooccurif the cuticleslosetheir naturalelasticitydue to
weathering.This might explainwhy cuticlevertical cracksare found at lower hairsurfaceconcentrations
in peoplewho do not blow-dry their hair.
The repetitiveactionof cuticle"rigidization"and water swellingset up on the outer
cuticlesectionsby the absorptionand desorptionof water during thermal cyclingdid
not lead immediately to crack formation. For instance,it was found that beforethe
cracksbecamefully developed,they appearedfirst as sharpwhite lines on the cuticle
surfaces
(seeFigure 5a);then, uponfurtherthermalcycling,the white linesturned into
full cracks.The appearance
of thesewhite linesindicatesthat beforethe cuticlescrack,
the mechanicalenergyaccumulatedby the circumferentialtensionstresses
or "hoop
stresses,"
is first dissipatedby the formationof localizedshearyield regions.This form
of mechanicalenergy dissipationis a very commonphenomenonthat takes place in
polymericmaterialsbeforethey fracture(18).
Increasingthe watertemperatureduringthermalcyclingto about50øCresultedin more
diffuse and wider shearyield regionsthat did not turn into crackseven after the
148
JOURNAL OF COSMETIC SCIENCE
.... x2.4k
5a
x2.4k
5b
Figure 5. Cuticleswith shearyield regionsformedbeforecrackingtakesplace(5a) and with shearyield
regionsproducedduring thermal cyclingwith water at 50øC (5b).
applicationof a high numberof thermalcycles(seeFigure 5b). Belowthis temperature,
shearyield regionsandcrackswerealwaysproduced.Thus, increasingthe water-swelling
temperatureduring thermal cycling softensthe cuticle proteins,preventingthe shear
yield regionsfrom becomingfull cracks.It shouldbe mentionedhere that thosecuticles
crackedthermally were seento be easilybrokenduring hair combing. For instance,
Figure 6 showsa hair fiber from a tressthat has been subjectedto thermal cycles
followedby combing.This micrographshowsthat the removalof cuticlesby abrasion
occursmainly at the crackedsites.
EFFECTS
OF BLOW-DRYING
AND
WATER
TEMPERATURE
The temperatureat which air from the blow-dryerreachesthe hair surfaceseemedto be
crucialin the incubationand propagationof thermal cracks.In the trial experimentsit
was observedthat the averagenumberof cracksproducedfor a particularnumber of
cycleswas maximum when the hair surfacetemperaturewas maintainedfor about ten
seconds
between75ø and 95øC. In Figure7 is shownthe averagenumberof cracksasa
function of air temperatureat the wet hair surface.In this figure it can be seenthat
temperatureslower than 50øC do not increasethe averagenumber of cracksalready
present in unexposedhair, while temperatureshigher than 95øC lead rather to hair
surfaceand bulk distortion.It is quite plausible,thus, that temperatureslower than
65øCdo not producethe criticalrateof waterevaporation
neededfor the top part of the
cuticlesto contractand becomerigid, while temperatureshigher than 85øC might
softenthe cuticleproteins,releasing,thereby,the mechanicalstresses
by viscousflow.
The temperaturerate usedduring thermal cyclingwas found also to be an important
parameter.For instance,if the hair surfacetemperaturewasincreasedat a very slowrate,
CRACKING
2.kx
OF HUMAN
HAIR
CUTICLES
149
{}81
Figure 6. Typical combingabrasionpatternof hair with crackedcuticles.
i.e., 10øCper minute up to 75øC, crackformationwasalmostnil. In contrast,if hair
samplesat 25øC were suddenlyexposedto a temperatureof 75øC, they immediately
showedan increasein the numberof new thermal cracks.Thus, high temperaturerates
causecracksbecause
theyproducea rapidcontractionof the top part of the cuticleswhile
maintainingin a swollenstateboth the cuticlelayersunderneathand the cortexitself.
Conversely,low temperatureratesdo not causecrackingbecausecuticlesand cortex
contractin a more synchronous
manner.
It is worth mentioning here that crackssimilar to thoseproducedthermally were also
formedin fibers swollenwith mixturesof strongswelling solventsand strongdehydrating solutions.As is well known, formic acid is a strongswellingsolventbecauseit
is able to break hydrogenbondsand salt bridgesnot accessible
to water and to cause
higher levelsof swelling in keratin fibers(19,20). In contrast,solutionsof saturated
NaCI areknownto dehydratethe fibers(21). Experimentswith mixturesof formicacid
and saturatedsolutionsof NaCI (50/50%), or mixtures of formic acid and glycerin
(20/80%), were seento producesimilar crackson the cuticles(seeFigure 8). The cuticle
crackswere observedto appearafter ten hoursof fiber immersionin thesesolutions.
Also, it wasobservedthat if a thin film of gold is depositedonto a cleanhair fiber and
then allowed to swell in water, that part of the cuticle coveredwith gold showsalso
vertical crackssomewhatsimilar to thoseproducedthermally (seeFigure 9). The explanationto thesephenomenais straightforward,i.e., in both cases,with the swelling
experimentand with the gold film, only a portionof the cuticleis restrainedto expand
during the swellingprocessof both the cuticlelayersunderneathand the cortex,and
crackingoccursby the alreadyadvancedmechanism.
150
600
JOURNAL OF COSMETIC SCIENCE
Averagenumberof shortlongitudinal
cuticlecracksper mm of hair
5OO
4OO
300
2OO
lOO
20
30
40
50
60
70
80
90
lOO
11o
12o
Air temperature (C)
Figure 7. Plot of averagenumberof cuticlecracksreproduced
in the laboratory
vs blow-dryingtemperature. The numberof applied thermalcycleswas kept constantat 20 cycles,and the air temperaturewas
measuredat the level of the hair surface.Error barsrepresentone standarddeviationabout the mean.
Figure 8. Cuticlecracksproducedafterimmersinga hair fiberin a 50/50% solutionof saturated
NaCI and
formic acid for ten hours.
CRACKING
OF HUMAN
HAIR
CUTICLES
! 5!
•
x2.4k
9b
Figure 9. Fiber with a thin gold film depositedon its surfacebeforewaterimmersion(9a) and after water
immersion (9b).
EFFECT
OF SOME
COSMETIC
ACTIVES
When 2% w/w aqueoussolutionsof glycerinand propyleneglycol were usedduring
thermal cycling insteadof water, the hair cuticlesdid not show any increasein the
averagenumberof crackscharacteristic
of virgin hair fibers.This observationsuggests
that during thermal cyclingtheseactivesretard water evaporationand alsoare able to
plasticizethe cuticles, thereby preventing crack formation. Glycerin and propylene
glycolcould,however,beeasilyremovedfrom the hair fiberby simplewaterrinsing,and
under theseconditionsthe cuticlescrackedagain. The useof 2% aqueoussolutionsof
variousquarternaries
insteadof waterduring thermalcyclingdid not indicateanycuticle
crackpreventionat all. The quaternariesanalyzedwere as follows:polyquaternium11,
cetrimonium chloride, and steralkonium chloride.
The depositionof four alternatinglayersof a positivepolymer(polyethylenimine)
and a
negativepolymer(polyacrylate)on the hair surfacewasnot capableof preventingcrack
formation.For instance,in Figure 10 it canbe seenthat cracksstill formed,both on the
hair cuticlesand alsoon the depositedpolymerlayers.Other substances
that did not
preventcrackformationwhen depositedonto the hair surfacewere oils suchas triglycerides,silicon oils, mineral oil, and petrolatum.In contrast,an aqueoussolutionof
hydrolyzedwheat proteinpolysiloxane
copolymerat 2%, usedinsteadof water during
thermalcycling,preventedcuticlecracking.The crackpreventioneffectwasseento take
placeevenafter the hair waswater rinsed.This proteincopolymer,which crosslinksupon
heat application, is believed to retard water evaporationand also to give a strong
cohesiveness
to the cuticles,therebypreventingthermalcrackformation.
CONCLUSIONS
Hair blow-dryingproducescuticlecracksthat canbe reproducedin the laboratoryby the
applicationof alternatingcyclesof hair wetting and blow-drying.The crackswereseen
to result as a consequence
of circumferentialtension stressesimposedon the dried
portionof the cuticlesat the top by the swollencortex.The temperaturerangeat which
152
JOURNAL OF COSMETIC SCIENCE
I
. ::-:kx
;kv
Figure 10. Cracksformedon cuticlesof hair treatedwith four alternatinglayersof polyethylenimineand
polyacrylate.
thesecracksseemto take place is between75ø and 95øC. It was alsoshownthat the
combingof hair with crackedcuticlesresultsin the removalof big portionsof cuticle.
The preventionof crackformationby the useof somecosmeticactiveswasshownto be
possible.
ACKNOWLEDGMENTS
The author wishes to thank Ann Harder for her valuable technical assistance and Herb
Eldesteinfor his helpful discussions.
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OF HUMAN
HAIR
CUTICLES
153
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