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ORIGINAL ARTICLE BJD
British Journal of Dermatology
Understanding breakage in curly hair
G.A. Camacho-Bragado,
1
G. Balooch,
2
F. Dixon-Parks,
1
C. Porter
1
and H. Bryant
2
1
The L’Oreal Institute for Ethnic Hair and Skin Research, Chicago, IL, U.S.A.
2
L’Oreal Research and Innovation, Clark, NJ, U.S.A
Correspondence
G. Alejandra Camacho-Bragado.
E-mail: acamacho@rd.us.lorea.com
Accepted for publication
11 June 2014
Funding sources
This study was funded by L’Oreal.
Conflicts of interest
All the authors are employees of L’Oreal.
DOI 10.1111/bjd.13241
Summary
Background In 2005, the L’Oreal Institute for hair and skin research carried out a
multiethnic study to investigate hair breakage in women residing in the U.S.A. In
this study it was reported that a large percentage (96%) of the African-American
respondents experience breakage. A combination of structural differences and
grooming-induced stresses seem to contribute to the higher breakage incidence in
the African-American group as the chemical composition of African-American hair
is not significantly different from other ethnic groups. Some authors have
proposed that the repeated elongation, torsion and flexion actions may affect the
components of the hair fibre. However, considering the different properties of
cuticle and cortex, one would expect a different wearing mechanism of each, lead-
ing to the ultimate failure of hair. Knowing in detail how each part of the structure
fails can potentially lead to better ways to protect the hair from physical insults.
Objective To investigate crack propagation and fracture mechanisms in African-
American hair.
Methods Virgin hair of excellent quality was collected, with informed consent,
from a female African-American volunteer. A series of controlled mechanical
stresses was applied to 10-mm hair sections using a high-resolution mechanical
stage (20 mN) up to the fracture of the fibre. The surface was monitored using
scanning electron microscopy imaging during the stress application. X-ray tomo-
graphic microscopy images were acquired and quantified to detect changes in
energy absorption as a function of applied stress that could be linked to increase
in crack density.
Results Analysis of the mechanical response of hair combined with the two imag-
ing techniques led us to propose the following mechanism of hair breakage: cuti-
cle sliding; failure of the cuticle–cortex interface; nucleation of intercellular
cracks and growth of cracks at the cuticle–cortex junction; and propagation of
intercellular cracks towards the surface of the hair and final breakage when these
cracks merge at the cuticular junction.
Conclusions The combination of scanning electron microscopy and X-ray tomo-
graphy provided new information about the fracture of hair. Mechanical damage
from grooming and some environmental factors accumulate in hair creating
internal cracks that eventually result in breakage at unpredictable sites and there-
fore a continuous care regimen for the hair throughout the life cycle of the fibres
is recommended.
©2015 The Authors
BJD ©2015 British Association of Dermatologists
10 British Journal of Dermatology (2015) 173 (Suppl. 2), pp10–16
Hair breakage has been identified as one of the main hair
problems in women of African descent in the U.S.A. as
indicated by an internet survey performed in 2005.
1
In this
survey, it was found that 96% of the surveyed participants
experience breakage and 23% indicated this condition as their
main concern.
Multiple authors have reported on the potential causes of
increased fragility of curly hair vs. other ethnic types. In some
cases, genetic hair shaft abnormalities can be the origin; how-
ever, this is not necessarily a cause exclusive to curly hair.
2
In
most cases, an acceleration of hair degradation is linked to a
combination of structural characteristics of curly hair
3
and
grooming practices,
4–7
therefore highlighting the importance
of selecting an appropriate routine that can help extend the
life of the fibres keeping the structural integrity of the hair.
In spite of all this information, there has not been a report
on the structural mechanism and process of hair breakage
from a materials science approach; this article discusses a
combination of techniques that were applied to study the
failure mechanism of virgin (nonrelaxed) hair. The hair was
considered to be a reinforced, highly hierarchical natural fibre
with a remarkable tensile toughness (130–180 MJ m
3
).
8
The
method is a combination of in situ tensile testing, scanning
electron microscopy (SEM) and X-ray tomography (XTM).
The scale and resolution of these techniques are appropriate
for studying the interaction of the different mesoscopic
components of hair and help describe their individual fracture
steps leading to the catastrophic failure of the structure.
Human hair is a biological material formed, at the nano-
scale, by keratin a-helices coiled into rope-like structures,
which are bound by keratin-associated proteins (KAPs) to
form microfibrils. Microfibril bundles, held together by a
proteinaceous matrix, constitute the macrofibrils. On the
microscopic scale, the macrofibrils aggregate into cortical cells
kept together by the cellular membrane complex (CMC) to
form the cortex. The cortex is protected from direct inter-
action with the environment by the cuticle, which consists of
5–10 layers of keratinized cells in a roof-tile-like arrange-
ment.
9
Several groups have attempted to elucidate the
mechanical behaviour of hair based exclusively on its molecu-
lar or nanoscopic components (keratin coils, microfibrils and
amorphous protein matrix).
10
Other authors
3,11
use the sur-
face quality and macroscopic geometry (twists, kinks, etc.) of
hair fibres as key parameters in describing the failure of hair
under different stress conditions (uniaxial tensile testing, cyclic
fatigue, flexabrasion, etc.).
3,10–13
However, the series of
phenomena leading to failure cannot be fully established from
the properties of the molecular elements or the presence of
surface defects alone. In the present study it was observed, for
example, that fibres tested in tension (Fig. 1a, b) showed a
step fracture similar to that previously reported.
3
However,
fatigued fibres resulted in bevelled fracture surfaces (Fig. 1c).
(a)
(b)
(c)
(d)
Fig 1. Postmortem (after fracture) scanning electron microscopy images of hair subjected to tensile stress showing stepwise fractures at a twist (a)
and within a homogeneous region (b). (c) Postmortem micrograph of a fatigued fibre revealing a bevelled surface fracture, a different fracture
mode compared with tensile stress, in a region away from a twist (homogeneous region). (d) Fractured fibres collected from panellists that
self-identified as having breakage issues.
©2015 The Authors
BJD ©2015 British Association of Dermatologists
British Journal of Dermatology (2015) 173 (Suppl. 2), pp10–16
Breakage in curly hair, G.A. Camacho-Bragado et al. 11
Moreover, the presence of twists and kinks did not seem
necessarily to increase the frequency of fracture occurrence at
these locations, as the observation of 20 postmortem (after
breakage) specimens revealed only four fractures associated
with a twist or constriction such as the one in Figure 1a. A
similar distribution of ‘at-the-twist’ and ‘away-from-the-twist’
fracture surfaces was observed in hairs collected from panel-
lists who complained of hair breakage (Fig. 1d). These
findings led us to believe that some internal crack propagation
must occur and that stress may be concentrated internally and
not only at the obvious macroscopic constraints and heteroge-
neities.
Scanning electron microscopy and XTM (Fig. 2a) were
used for multimodal imaging of the surface and the interior
of the fibres. These techniques combined with in situ mechan-
ical testing provided a better understanding of the evolution
of internal stress distribution upon application of an external
tensile force and the ultimate hair fracture. The combination
of SEM and XTM provides a unique way to analyse the
response of hair to stress and the changes induced by
the strain applied. SEM offered a high-resolution view of the
outer surface of the fibre, while synchrotron XTM allowed
the visualization of the interior of the hair, particularly the
cuticle–cortex interface both in a three-dimensional (3D)
view and in virtual cross-sections (Fig. 2d). It must be men-
tioned, that resolving the cuticle–cortex interface by XTM
requires special conditions not achievable with bench-top
systems. Synchrotron light is the most viable solution for
performing hard XTM of materials such as hair. In addition,
the monochromaticity of synchrotron light enables accurate
quantification of X-ray absorption coefficients, which can be
linked to localized protein and lipid content.
Methods
Virgin hair of excellent quality as indicated by amino acid
analysis (cysteic acid =02, lanthionine =01 and tyro-
sine =20 mg amino acid per 100 g total amino acids) was
collected from a female African-American donor who had
given written consent. The hair was washed with a 10%
ammonium lauryl sulfate solution and the fibres attached to
brass ferrules (n=9).
Tensile test
A Deben microtensile tester, customized by Gatan Inc. (War-
rendale, PA, U.S.A.), was used to strain 10-mm sections of
hair at 1 mm min
1
with a 2 N load cell (20 mN accuracy).
The strain was applied to each fibre in four stages. Firstly, the
fibres were strained up to the onset of plastic deformation;
the second strain stage was performed to a point within the
plastic region approximately before post-yielding; the third
stage was stopped within the post-yielding region (Fig. 2b);
the fibres were then strained a final time up to fracture. The
hair fibres were imaged during (SEM) and after (SEM and
XTM) each stage of the stepwise tensile test using an FEI
QuantaTM 400 FEG Environmental scanning electron micro-
scope (FEI Company, Hillsboro, OR, U.S.A.). X-ray tomo-
grams of the entire fibre after each strain stage were also
recorded. The final curve for the stepwise test was constructed
(a) (b)
(c) (d)
Fig 2. (a) Schematic showing the combination of scanning electron microscopy (SEM) and X-ray tomography (XTM) to capture surface and
internal information of the specimen under tensile stress. (b) Stress–strain curve indicating the stages chosen for the stepwise tensile test. Stage 1,
at the onset of plastic deformation; stage 2, within the plastic zone; stage 3, within the post-yielding region; stage 4, fracture. (c) Stress–strain
graph showing the elastic (Ee) and plastic (Ep) strain portions of the curve. (d) Schematic showing typical XTM virtual cross-sections of hair and
the corresponding location within the length of the hair fibre.
©2015 The Authors
BJD ©2015 British Association of Dermatologists
British Journal of Dermatology (2015) 173 (Suppl. 2), pp10–16
12 Breakage in curly hair, G.A. Camacho-Bragado et al.
by shifting the partial curves an amount equivalent to the
plastic deformation, so the elastic strain was accounted for
one time only. Figure 2c shows how the elastic and plastic
portions of a partial curve were defined.
Synchrotron X-ray tomography
X-ray tomography imaging was performed at the Advanced
Light Source on Beamline (8-3-2) at the Lawrence Berkeley
National Laboratory (Berkeley, CA, U.S.A.). Monochromatic
radiation was used to obtain 2D projections that represent
X-ray attenuation maps. These maps were used to reconstruct
the 3D data volume at a resolution of about 2 lm; the inten-
sity of the signal can be correlated with the local density in
the specimen. Enough slices were captured to reconstruct the
entire hair length. A detailed description of synchrotron XTM
can be found in the literature.
14
Results
The SEM images taken during the different stages of the
stepwise tensile test indicate that the applied stress led to
increasingly higher cuticle lifting (endocuticle failure). How-
ever, upon stress release, the cuticle closely returned to its
initial configuration. Thus, the surface of the fibre revealed
little evidence of the overall mechanical history of the hair.
Figure 3 shows sequences of SEM images taken at two loca-
tions (close to the root and close to the tip) after different
amounts of applied stress. The proximal end (close to root) is
expected to have about 1 month less of grooming history than
the distal portion. The cuticle lifting became permanent after
the final stress application; the effect is gradually more promi-
nent towards the position of the final fracture as shown in the
postmortem (after fracture) image (Fig. 3b).
On the other hand, XTM 3D reconstructions showed voids
developing along the cuticle–cortex junction after the second
application of stress (Fig. 4). Interestingly, the voids were
more noticeable after stage 2 than at the unstrained stage but
did not show significant change after a third stress cycle. This
contrasts with the observed decrease in attenuation coefficient
between the same three stages. The attenuation coefficient is
related to the density of the material under analysis as it is a
measure of how much of the incident beam is scattered or
absorbed by the specimen. In this particular case, cracks and
voids would decrease the attenuation coefficient as the empty
spaces absorb less radiation. Quantitative analysis of the stress
attenuation coefficient (Fig. 5a) showed a steady reduction in
relative energy absorption with increased stress, especially
after stages 2 and 3, where the decrease is statistically signifi-
cant (P<005).
(a)
(b)
Fig 3. Sequences of scanning electron microscopy images taken after stages 2 and 3 of the stress–strain test compared with postmortem (after
fracture). (a) Proximal section. (b) Distal section and location of failure; the defect enclosed in the square was used as a fiducial to backtrack the
area of failure, the exact location of failure is marked by black arrows.
©2015 The Authors
BJD ©2015 British Association of Dermatologists
British Journal of Dermatology (2015) 173 (Suppl. 2), pp10–16
Breakage in curly hair, G.A. Camacho-Bragado et al. 13
Changes in cross-sectional area and Young’s modulus as a
function of sequential stress applications were also studied.
The cross-sectional area was measured from averaging XTM
data at five different locations along the fibre (Fig. 5b). This
parameter remained relatively constant between baseline and
stage 2. However, after the fibre exceeded 30% strain
(Fig. 5b, stage 3), the cross-sectional area decreased more
noticeably, particularly in the section where the final fracture
occurred. Figure 5c and d shows the stress–strain curves per
stage and the Young’s moduli calculated from these curves;
we observed that the first stress application caused an 18%
decrease in Young’s modulus, while this parameter increased
11% after the second cycle. An additional cycle caused only a
slight decrease of the elastic modulus. The impact of the
changes in this parameter in the overall fracture mechanism
will be discussed later.
Discussion
In spite of the cuticle splaying and the changes in cross-
sectional area during the stress application, the status of the
fibre surface did not allow the prediction of the location of
failure before it occurred. This could indicate that, under pure
tensile forces, minor surface defects including missing or
chipped cuticle layers and macroscopic heterogeneities
contribute but do not necessarily play a determining role in
Fig 4. Tomographic reconstructions of a hair fibre after different stress application stages. X-ray tomography (XTM) three-dimensional
reconstructions showing multiple internal cracks and voids (white arrows) formed following stage 2 as a result of applied stresses.
(a) (b)
(c) (d)
Fig 5. X-ray tomography quantitative analysis and stress–strain curves. (a) Relative energy absorption as a function of stress stages (statistically
significant differences are marked with stars, error bars correspond to standard deviation). Increase in low-density voids causes a decrease in the
energy absorbed by the specimen. (b) Cross-sectional area measurements at five regions along the length of the fibre after each of three stress
cycles (the area values for the location of failure are marked by the arrow); these show a formation of a ‘neck’ at the site of final fracture.
(c) Stress vs. strain curves, the contrast between the maximum value reached in the green vs. the other curves indicates the material has lost
mechanical integrity (weakening). (d) Young’s modulus values showing the changes in elasticity of the hair as stress is accumulated.
©2015 The Authors
BJD ©2015 British Association of Dermatologists
British Journal of Dermatology (2015) 173 (Suppl. 2), pp10–16
14 Breakage in curly hair, G.A. Camacho-Bragado et al.
ultimate hair breakage. One must remember than in real life,
hair is rarely exposed to pure tensile stress and that pulling is
usually accompanied by some sort of surface friction or
abrasion from grooming tools. Based on the lack of surface
breakage indicators, we propose that the critical cracks form
and migrate from the inside out and that catastrophic failure
occurs after the fibre has undergone a relatively large strain.
The behaviour of the cuticle is consistent with adhesion
failure at the cuticle–cuticle interface as a first stage leading to
fracture. As described by Robbins et al.,
15
straining hair at
humidity <65%, corresponding to the humidity regime
inside the microscope chamber, causes fracture at the CMC
between cuticle layers due to weak bonds between hydropho-
bic components, particularly between the side chains of the
fatty acid 18-methyleicosanic acid and the contiguous fibrous
protein layer. This initial partial fracture of the cuticle is
manifested as the initial decrease in elastic modulus. The gen-
eralized internal void formation eventually leads to detachment
of the cuticle from the cortex; failure of the cuticle–cortex
interface results in the load being transferred and carried
mainly by the cortex. At these later stages, the Young’s modu-
lus is dictated by the relatively stiff cortex, which is consistent
with the observation of an increase in elastic modulus at stage
3 (Fig. 5c, d). The last small change in elastic modulus could
be linked to propagation of cracks along the cortical CMC. As
the contribution of the CMC to the elastic modulus is rather
small, the corresponding change is expected to be small, as
observed (Fig. 5c, d). The increase in crack density within the
cortex accounts for the decrease in energy absorption at large
strains as the air-filled cracks have a lower density than the
crack-free areas of the fibre.
Crack propagation along the intercellular space would cause
the decrease in cross-sectional area by allowing the cells to
slide and the hair to contract radially; thus one can define a
mesoscopic-level Poisson effect in hair linked to the degrada-
tion of its hierarchical structure. It is not until the fibre has
undergone large strains (>30%) that the load is directly
transferred to the cortical cells and eventually to individual
microfibrils. At these stages, one would expect that fibres with
different cortical cell distribution such as straight vs. curly
hairs,
16
would have a different mechanical response as the dif-
ferent cell geometry (ortho- vs. meso- vs. paracortical cells)
and packing could lead to different distribution of areas of
internal stress concentration. As the CMC continues to fracture
and the load is transferred to individual cells, the load-bearing
macrofibrils and microfibrils within the cell start failing. Two
competing processes have been reported to occur during
straining of a-keratin intermediate filaments:
17
molecular
stretching and molecular sliding. Time-resolved small-angle
X-ray scattering data
17
showed that at low humidity the
sliding process is more likely to occur. Thus, the last stage
leading to failure of hair would be the fracture of intracellular
CMC, which allows the fibrils to slide and eventually break.
From the aforementioned observations, the mechanism of
hair breakage can be summarized as taking place in the
following four steps (Fig. 6): cuticle sliding; failure of the
cuticle–cortex interface (Fig. 6a); nucleation of intercellular
cracks and growth of cracks at the cuticle–cortex junction
(Fig. 6b); and propagation of intercellular cracks towards the
surface of the hair and final breakage when these cracks merge
at the cuticular junction (Fig. 6c).
In summary, the use of X-ray photon and electron imag-
ing combined with in situ tensile testing provided a new
insight into how human hair breaks, in particular curly hair.
It has helped reconcile previous observations at the molecular
level and at a purely macroscopic statistical level allowing us
to propose a fracture mechanism that takes into consideration
all constituents of hair at different scales. In the particular
case of curly hair, fibres may break in two distinct ways: (i)
at macroscopic constrictions, as they act as points of stress
concentration and are more susceptible to surface-initiated
cracks due to their inhomogeneous macrostructure, or (ii) at
sites of accumulated internal stress; as demonstrated by this
study, internal cracks may accumulate in the hair due to
excessive grooming force without showing surface evidence
of weakened spots; the hierarchical structure of hair is able
to deflect the cracks to extend the life of the fibre up to a
critical crack density and size. In both cases, hair fibres
would benefit from a hair care regimen that reduces the
grooming forces and the friction between grooming tools
and fibres and in between fibres.
Acknowledgments
The studies were supported by L’Oreal Research and Innova-
tion. XTM was performed at the Advanced Light Source at
Lawrence Berkeley National Laboratory, supported by the
Office of Science, U.S. Department of Energy (DE-AC02-
(a) (b) (c)
Fig 6. Schematic representation of the hair-breakage mechanism. (a) Failure of the cuticle–cortex interface; observe the crack separating cuticle
from cortex. (b) Formation of intercellular cracks. (c) Propagation of cracks further into the cortex and towards the surface.
©2015 The Authors
BJD ©2015 British Association of Dermatologists
British Journal of Dermatology (2015) 173 (Suppl. 2), pp10–16
Breakage in curly hair, G.A. Camacho-Bragado et al. 15
05CH11231). The authors appreciate the assistance of Candace
Woodson in the preparation of the illustrations.
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16 Breakage in curly hair, G.A. Camacho-Bragado et al.
