International Journal of Cosmetic Science, 2016, 38, 77–84

doi: 10.1111/ics.12261

The influence of hair lipids in ethnic hair properties M. Martı, C. Barba, A. M. Manich, L. Rubio, C. Alonso and L. Coderch Advanced Chemical Institute of Catalonia (IQAC-CSIC), Jordi Girona 18-26, 08034 Barcelona, Spain

Received 31 March 2015, Accepted 30 June 2015

Keywords: chemical analysis, ethnic hairs, hair growth and hair treatment, hair lipids, mechanical properties, water absorption–desorption

Abstract OBJECTIVE: Biochemical studies have mainly focused on the composition of hair. African hair exhibited lower moisturization and less radial swelling when flushing with water compared with Asian or Caucasian hair, and they assumed a possible lipid differentiation among human populations. This study consists in the lipid characterization of different ethnic hairs (Caucasian, Asian and African hairs) and the influence of these lipids in different hair properties such as humidity and mechanical properties. Evaluation of water sorption and desorption of the different ethnic hairs and with and without lipids is also studied mainly to determine permeation changes of the keratin fibres. METHODS: Extractions of exogenous and endogenous lipids with different organic solvents were performed; lipid analysis and its quantification using thin-layer chromatography coupled to an automated flame ionization detector (TLC/FID) were performed. Absorption and desorption curves were obtained in a thermogravimetric balance equipped with a controlled humidity chamber, the Q5000SA Sorption Analyzer (TA Instruments, New Castle, IL, U.S.A.). Also, mechanical properties (breaking stress and breaking elongation) were analysed using a computer programmable dynamometer (Instron 5500R). RESULTS: Lipid extraction showed the highest amount of total lipids for the African hair which may come from external sebaceous lipids compared with Asian or Caucasian hair. Caucasian fibres were found to be the most hydrated fibre, and a decrease in moisture was found in the extracted fibres, again, which is more important for the Caucasian hair. A superior lineal mass was found for the Asian fibres which supported their higher strength. The results obtained from the analysis of the mechanical properties of delipidized fibres indicate a surprising increase in the strength of African and Caucasian fibres. Perhaps this increase in strength could be related to the humidity decrease in lipid-extracted hair fibres. Results of water uptake and desorption indicate that Asian and Caucasian hairs present the lower diffusion coefficients compared with the African ones. At least for the African fibre, an extraction of its lipids that mainly account for apolar lipids ameliorates the fibre structure, decreasing its permeability to water and increasing its tensile strength. CONCLUSION: The ethnic hairs were assessed related to their lipid composition, and some differences between them were found in terms of water uptake and mechanical properties.

 sume  Re OBJECTIF: Les etudes biochimiques ont principalement porte sur la composition des cheveux. Les cheveux afro montrent un degree d’hydratation inferieure et un gonflement moins radiale lors du  cheveux asiatiques ou caurincßage avec de l’eau par rapport a casiens, laissant supposer une differenciation lipidique possible parmi les populations humaines. Cette etude consiste en la caracterisation des lipides de differents cheveux ethniques (caucasiens, cheveux asiatiques et africains) et l’influence de ces lipides dans differentes proprietes de cheveux telles que l’humidite et les proprietes mecaniques. L’evaluation de la sorption et la desorption de l’eau des differents cheveux ethniques, avec ou sans lipides est egalement etudiee principalement pour determiner les changements de permeabilite des fibres keratiniques.  METHODES: les extractions de lipides exogenes et endogenes ont ete effectuees avec differents solvants organiques; l’analyse des lipides et leur quantification par chromatographie sur couche mince (TLC-FID) ont ete effectuees. Les courbes d’absorption et de desorption ont ete obtenus dans une balance thermogravimetrique equipe  humidite contr^ d’une chambre a olee, la sorption Q5000SA Analyzer (TA Instruments, New Castle, IL, U.S.A.). Les proprietes  la rupture et allongement a  la rupture) mecaniques (contrainte a  l’aide d’un dynamometre programmable par ont ete mesurees a ordinateur (Instron 5500R).  RESULTATS: L’extraction des lipides a montre la plus grande quantite de lipides totaux pour les cheveux africains qui peut venir des lipides sebacees externes. Les fibres des Caucasiens se sont reveles ^etre les plus hydratees. Une masse lineaire superieure trouvee pour les fibres asiatiques peut expliquer leur force superieure. Les resultats des proprietes mecaniques des fibres delipides indiquent une augmentation surprenante de la resistance des fibres caucasiennes  la et africaines. Peut-^etre cette augmentation de la force est liee a diminution de l’humidite des fibres capillaires delipidees. Les resultats de l’absorption et de desorption d’eau indiquent que les cheveux asiatiques et caucasiens presentent des coefficients de  ceux d’Afrique. diffusion inferieurs par rapport a Au moins pour la fibre africaine, une extraction des lipides principalement apolaires ameliore la structure de la fibre en diminuant  l’eau et en augmentant la resistance a  la traction. sa permeabilite a  leur CONCLUSION: Les cheveux ethniques ont ete evalues quant a composition en lipides, et certaines differences entre eux ont ete trouves en termes d’absorption de l’eau et des proprietes mecaniques.

Correspondence: Meritxell Martı, Advanced Chemical Institute of Catalonia (IQAC-CSIC), Jordi Girona 18-26, 08034 Barcelona, Spain. Tel.: +34 934006100; fax: +34 932045904; e-mail: meritxell.marti@iqac. csic.es

Introduction Human hair accounts to a large part of the phenotypic variation between different human races. Therefore, it is usually categorized

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into three major distinct ethnic groups: Asian, Caucasian and African. Although many studies have been conducted to examine the differences in skin colour across populations, few studies have examined the differences in hair morphology. Previous studies confirmed both morphological and biochemical differences across ethnic groups. Morphological studies have focused mainly on hair colour, diameter and section, shape of the fibre and responses to external stimuli determining mechanical properties [1–3]. Besides, biochemical studies have mainly focused on the composition of hair proteins; and some proteins, keratins and molecular structures, including amino acids, have common features across groups [4–6]. Franbourg et al. [4] reported that African hair exhibited less radial swelling when flushing with water compared with Asian or Caucasian hair and they assumed a possible lipid differentiation among human populations. Syed et al. [3] reported differences in moisturizing rates possibly associated with differences in lipid content. Only a few studies have assessed the differences in hair lipids across groups [7, 8]. This is due to the low amount of lipids in the hair fibre and the few established methods are available for the determination. Ji et al. [7] reported a similar lipid composition across human populations, but more integral hair lipids especially free fatty acids on Asian hair. This was associated with less damage of Asian samples in front UV irradiation. Cruz et al. [8] reported higher amount of hair internal lipids in African hair. The lack of axial diffraction in this type of hair, which appears after lipid removal, suggested the influence of the lipids on keratin structure. It is generally considered that lipids are part of the intercellular complex, and they are particularly present in the cuticle. Fatty acids are thought to be linked to the proteins of the outermost part of the cuticle by thio-ester linkages [9]. Hair lipids mainly consist of cholesterol esters, free fatty acids, cholesterol, ceramides and cholesterol sulphate, which can be separated in exogenous and endogenous lipids according to their origin, sebaceous glands or hair matrix cells, respectively [10]. In a recent work, differences were found in the chemical composition and physical behaviour of virgin and delipidized hair [11]. Evidence supports an indispensable role of non-covalent-bound lipids in the surface properties of human hair. Their presence is fundamental on maintaining the internal water content inside the fibre. Thus, hair internal lipids could enhance barrier function, preventing external materials from penetrating the keratin fibres [12,13]. The aim of this study consists in the lipid characterization of different ethnic hairs (Caucasian, Asian and African hairs) and the study of the influence of these lipids on different hair characteristics such as mechanical properties, contact angle and water sorption and desorption mainly to determine permeation changes of the keratin fibres. Materials and methods Materials Solvents used are diethyl ether, formic acid, acetone, benzene, methanol, n-hexane (Merk, Darmstadt, Germany) and chloroform (Carlo Erba, Italy). Hairs used are natural virgin brown Caucasian hair, natural dark brown Asiatic hair (Chinese origin), both without any pre-treatment and natural kinked Afro hair, with only a thermic pre-treatment (without chemicals) supplied by De Meo Brothers (Passaic, NJ, U.S.A.).

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Experimental Hair samples were washed at hair/surfactant solution (ratio 1 : 30) with 3% diluted commercial shampoo Pantene Pro-V (Procter & Gamble, Cincinnati, U.S.A.) and then rinsed three times thoroughly with water and dried in ambient conditions. Lipids were removed by extraction with different solvent mixtures of chloroform/methanol (2 : 1 v/v, 1 : 1 v/v and 1 : 2 v/v) for 2 h every mixture and 100% methanol overnight at room temperature in a stirring system, using the same hair samples. The different extracts were then combined, concentrated and dissolved in chloroform/methanol (2 : 1) before analysis. To evaluate the total amount of lipids extracted, 1 mL of each of the extracts was evaporated to dryness in a desiccator under a P2O5 atmosphere and weighed to a constant weight. Moisture content was evaluated from non-extracted and lipid-extracted hair from different ethnicities. About 0.5 g of hair maintained in a conditioned room (23°C, 50 RH) for at least 24 h was weighted and submitted to dryness in an oven at 105°C during 12 h. After cooling in a desiccator under a P2O5 atmosphere, hair was weighted again and the moisture content was calculated in percentage. Lipid analyses of the different extracts were performed with a thin-layer chromatography coupled to an automated flame ionization detector (TLC/FID) and a Iatroscan MK-5 analyzer (Iatron, Tokyo, Japan). The lipid extracts were directly spotted on silica gelcoated Chromarods (type S-III) by means of a precision Hamilton 2-lL syringe coupled to an SES 3202/IS-02 sample spotter (NiederOlm, Germany). The determination of lipid content was performed using an optimized TLC/FID protocol [14] using a methodology where rods (in set of 10) were developed using the following mobile phases: (i) chloroform/methanol/water (57 : 12 : 0.6, v/v/v) for a distance of 2,5 cm twice, (ii) n-hexane/ethyl ether/formic acid (50 : 20 : 0.3, v/v/v) to 8 cm and (iii) n-hexane/benzene (35 : 35, v/v) to 10 cm, and finally a total scan (100%) to quantify the most polar lipids was performed. Thirty bundles of five parallel hair fibres previously conditioned for 48 h in a standard atmosphere (20°C, 65% RH) were subjected to tensile testing with an Instron 5500R dynamometer. The distance between clamps was 100 mm. The bundle was strained at 60 mm min1 (60% min1) and, after breakage, hairs were cut and weighted to calculate the mean hair linear density of the five fibres. The following parameters involving the five hairs are determined: tex (mean hair linear density in mg m1), initial modulus (IM) and initial increase in tenacity per unit of strain (mm 100 mm1) in cN/tex. The following parameters related to the third (median) breakage are determined: breaking tenacity (T), load at which the third breakage occurs related to the linear density of the three hairs in cN/tex, and breaking deformation (D), extension at which the third breakage occurs related to sample initial length in per cent. Absorption and desorption curves were obtained using a thermogravimetric balance equipped with a controlled humidity chamber, the Q5000SA Sorption Analyzer (TA Instruments, New Castle, IL, U.S.A.). The weight of the hair samples analysed ranged between 17 and 20 mg. All experiments were conducted at 25°C with a total gas flow of 200 mL min1 and followed the same measuring procedure [13]: 1 Initial Drying: temperature 60°C and 0% relative humidity (RH) overnight. 2 Pre-stabilization: temperature 25°C, 0% RH and then initial absorption kinetics at 5% RH.

© 2015 Society of Cosmetic Scientists and the Societe Francßaise de Cosmetologie International Journal of Cosmetic Science, 38, 77–84

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3 Absorption curve: The sample that was previously stabilized at 5% RH is subjected to absorption tests that increase in steps of 10–95%, and the sample is stabilized at 95% RH after the last step. The sample remains in each step until its mass reaches equilibrium (arbitrarily defined as a change in mass of < 0.02% per minute for 10 min). 4 Desorption curve: The sample stabilized at 95% RH after the absorption process is subjected to desorption tests decreasing in steps of 10% to 5%, and the sample is stabilized at 5% RH after the last step. The sample remains in each stage until its mass reaches equilibrium (arbitrarily defined as a change in mass of < 0.02% per minute for 10 min). The high reproducibility of these measurements was established in the validation study of this instrument, in which three replicates of a single sample gave essentially identical sorption isotherms. There takes a long time to measure a complete isotherm (3 days), and because of the reproducibility of the data generated, only one measurement was taken on each sample. Sorption isotherms are generally described by mathematical models based on the empirical and/or theoretical criteria that can be found in the literature. One of the most commonly used equations is that of the GAB model. It has a theoretical basis, and its parameters provide a physical meaning to the sorption process when compared with empirical models. The GAB model is based on the monolayer moisture concept and gives the value of the monolayer moisture content of the material [15]. The GAB model has been proved to be applicable in hydrophilic polymers [16, 17] and food [18] systems and has considerable theoretical justification [19]. Thus, in this work, sorption isotherm data were modelled according to the GAB model, in line with other authors [20, 21]. Table I shows the sorption isotherm and the parameters used to fit the experimental sorption/desorption data. The goodness of fit was evaluated using the determination coefficient (R2).

Table I GAB model and parameters used to fit the experimental sorption data

Model

Mathematical equation

GAB [13]

W = Wm Cg K aw/[(1Kaw + Cg K aw)]

Parameter

Definition

aw

Water activity expressed as vapour relative pressure p/p0, where p0 is the saturated vapour. Equilibrium moisture content at aw in g sorbed/100 g of sorbent on dry basis Monolayer moisture content in g sorbed/100 g of sorbent on dry basis Energy constant related to the difference between the free enthalpy of the water molecules in the pure liquid state and in the monolayer. This is proportional to the rate between both the attachment and the escape rate constants of the primary sites. Ratio between the standard vapour pressure of the liquid and the vapour pressure of the sorbate in the secondary (upper) layers. Proportional to the rate between the attachment rate constant and the escape for all higher layers.

W Wm Cg

K

The GAB model [15] was fitted to the absorption isotherm (Fig. 2). It fits the amount of water absorbed at the equilibrium X at a relative humidity Rh in function of the monolayer capacity Xm, the energy constant of the monolayer C and that of the multilayer K. Fitting procedures using nonlinear regression procedures are described elsewhere [22]. X ¼ Rh Xm C K/[(1  K Rh) (1  K Rh + C K Rh)]: Standard deviations were calculated for all mean values. Analyses of variance with a one-way layout or the Kruskal–Wallis test were applied for group comparisons. The software used was the STATGRAPHICS plus 5 (STatpoint Technologies Inc. Warrenton, VA, U.S.A.). A P-value below 0.05 was considered significant. Results Extraction of lipids from African, Asian and Caucasian hair was performed in duplicate following the methodology described in the experimental part. Hair samples were washed with shampoo, then rinsed thoroughly with water and dried in ambient conditions. Then, total lipids were removed by extraction with different solvent mixtures of chloroform/methanol and 100% methanol overnight at room temperature. The different total lipid extracts of each kind of hair were concentrated and dissolved in chloroform/methanol (2 : 1) before analysis. It is important to remark the highest amount of total lipids for the African hair compared with Asian and Caucasian total lipids (Table II). Although the extracted lipids account for more than 6% in African hair, these account only for about 2% for Asian and 3% for Caucasian hair. This high amount of lipids may suggest that like in other works [23, 24], shampooing could not remove some free lipid from the surface of hair, because even after shampooing an appreciable amount of free lipid remains in the hair fibre. To determine the differences on lipid composition of total lipids between ethnicities, the lipid extracts were analysed in triplicate using TLC/FID (Fig. 1). This technique has been recently used to analyse lipids from hair [8,25]. The external lipids are known to be mainly accounted for the sebaceous surface lipids such as squalene and sterol esters, and the internal lipids are mainly constitutive lipids biosynthesized in the hair matrix cells, which include free fatty acids, cholesterol and more polar-like components such as ceramides, glycosylceramides and cholesterol sulphate [7]. It is important to remark the great differences on the lipid analyses between the three kinds of ethnic hair fibres. Related to the whole fibre, the African hair has the greatest significant proportion of apolar lipids (sterol esters and squalene), and even the free fatty acids, being lower the amount of polar lipids and moreover the sterols. The Caucasian and Asian hair fibres have similar amounts

Table II Total amount of lipids obtained by extraction with chloroform/ methanol mixtures of African Asian and Caucasian hairs

Kind of hair

African

Asian

Caucasian

Lipid extracted (% o.w.f)

6.75  0.29

2.09  0.72

2.73  0.11

o.w.f.: over weight of fibre.

© 2015 Society of Cosmetic Scientists and the Societe Francßaise de Cosmetologie International Journal of Cosmetic Science, 38, 77–84

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of apolar lipids and sterols, the Caucasian a significant higher amount of free fatty acids, and the Asian higher amount of polar lipids. This analysis is performed with the extract obtained from fibres that have been only washed with shampoo and then extensively extracted with chloroform and methanol. This means that external and internal lipids are present in those extracts. It is well known that the higher amount of sebaceous lipids in the African hair [7] accounts for the greatest amount of apolar lipids, cholesterol esters, squalene, etc. It can be concluded that the greatest extraction of apolar lipids and even the free fatty acids mainly come from the external lipid fraction and the higher amount of sterols and polar lipids in Caucasian and Asian hairs comes mainly from the internal lipids. Moisture content for non-extracted and extracted hair fibres was evaluated (Table III). It is important to remark the moisture content in the hair fibres between 10 and 11% when measured at 22°C and 50% RH. Caucasian fibre is the most hydrated one, around 11.2%, different to the Asian hair (significance > 95%) and the African hair with a similar hydration around 10.7%. Differences in hydration properties of ethnic hair have been previously reported [3]. African hair was demonstrated to have less moisture than Caucasian hair [3]. When the fibres are extracted, there is always a diminution on humidity retention which accounts for about 3% of the total humidity for Asian and African and for more than 10% for Caucasian, being this difference significant. It can be pointed out that the slight moisture content diminution is difficult to be related to the different amount of lipids extracted. The African hair with Ethnic Hair: Lipids

4.00 % owf (over weight of fibre)

3.50

* * AFRICAN ASIAN CAUCASIAN

3.00 2.50

*

2.00

*

1.50 1.00 0.50 0.00 Ap. Lip –0.50

FFA

St

P. lip

* Significance 5% between ethnicities in the same lipid family

Figure 1 Quantification of hair lipids in ethnic hair (percentage over the total weight of dry hair fibre (o.w.f.) (Apolar lipids (Ap. lip), free fatty acids (FFA), sterol (St) and polar lipids (P. lip) using TLC-FID).

much lipid extraction is the one with lesser moisture content decrease. However, the Caucasian hair with lower amount of lipid extracted shows higher decrease in moisture content. Perhaps a diminution on water content could be much related to the lipid composition than to the whole amount of lipids. The higher extraction of the internal lipids mainly from the hair matrix cells for the Caucasian and Asiatic hair could be the reason for the more important moisture content diminution. Mechanical properties were determined on 30 bundles of five single parallel hairs using a dynamometer. The mass of hair in 500 mm was evaluated by obtaining the lineal mass in tex (mg m1). The traction assay was performed at 60% strain per minute (60 mm min1). Initial modulus (cN/tex) was measured on the five hairs at the first stages of the test. Break strain (%) was measured at the third breakage of the five fibres (median), and breaking tenacity (cN/tex) was determined when related to hair linear density. Hairs may be regarded as a composite of microfibrils parallel to the fibre axis and embedded in a matrix that includes lipids. When hairs are mechanically deformed, distortions at the macroscopic level are transferred to the matrix and the microfibrils stressing bonds and interactions between components. Depending on the strain, the hair behaves successively like a crystallized solid in the initial region when especially the matrix is stressed, like an amorphous solid or a liquid in the yield region when fibrils are unfolded, where it is said to have a plastic-like response, and again like a solid in the post-yield region. The main parameters that enable to characterize the initial region, the yield region and the post-yield (breaking) region are the following: 1 Hair fineness characterizes the variation in thickness of hair after lipid extraction. 2 The initial modulus measures the efficiency of secondary bonds to support the distortions occurring at the initial region of deformation, when all the strain is taken up in distorting the bonding network of the matrix. 3 Deformation (at break) mainly accounts for the size of the yield region when higher strains causes chain unfolding of fibrils and hair behaves like a plastic material, an amorphous solid or a liquid being affected by the plasticiser effect of the matrix. 4 Breaking tenacity reflects the new configuration attained by hair in the post-yield region when fibrils are further stretched reaching the breaking region. To evaluate the effect of the lipid extraction and ethnicity on the mechanical behaviour of hairs, the analysis of variance has been applied to the above-described parameters relating the variations to

Table III Humidity quantification of African, Asian and Caucasian fibres before and after lipid extraction

African

Asian

Caucasian

Fibres evaluation

NT

Lip. Extr.

NT

Lip. Extr.

NT

Lip. Extr.

Humidity (%)

10.68  0.44

10.51‡  0.21

10.65†  0.26

10.23  0.25

11.24*†  0.03

9.94*‡  0.13

Student’s t-test P < 0.5. *Evaluated between extracted and non-extracted fibres. Evaluated between non-extracted fibres. ‡ Evaluated between extracted fibres. †

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Table IV Fineness, initial modulus, deformation (at break) and breaking tenacity of the different non-treated hairs and after lipid extraction according to their ethnicity (mean  95% C.I.)

African

Hair evaluation

NT

Fineness tex (mg m1) Initial Modulus (cN/tex) Deformation (%) Break. Tenacity (cN/tex)

5.52 4.174 35.24 17.77

Asian

Lip.Extr.

   

0.52•• 0.399•• 0.97• 2.04••

6.83 3.685 35.10 15.25

   

0.48↑ 0.370↓ 0.87 1.91↓

Caucasian

NT

6.39 4.547 38.58 17.01

Lip.Extr.

   

0.48••• 0.393•• 0.88•• 1.91••

6.18 4.295 36.37 15.70

   

0.46 0.355 0.82↓ 1.81↓

NT

4.67 4.629 40.47 17.6

Lip.Extr.

   

0.48• 0.387•• 0.87••• 1.97••

4.25 5.415 40.77 22.53

   

0.48↓ 0.370↑ 0.87 1.88↑

Remarks: Different number of dots in NT hairs means significant differences at 1% level between groups.  No significant differences after lipid extraction. ↓ No significant differences after lipid extraction although a small non-significant decrease is observed. ↑ Significant increase or ↓ significant decrease at 1% level after lipid extraction.

the type of hair and treatment. Mean values and confidence intervals at 95% are included in Table IV. It should be pointed out that the significantly higher lineal mass in tex for the Asian fibres related to the others may induce a higher strength for the Asian hair. The small diminution of the tex for the Asian and Caucasian extracted hair was foreseeable due to the small weight loss of the lipid matter. However, the significant increase in the tex for the African extracted hair are not easily explained. The small diminution of moisture content of all extracted ethnic fibres does not explain the increase in tex for the African fibres. Besides, the African fibre is the one that contained more lipids which were extracted in higher extent. A possible explanation could be a shrinkage of the African extracted hair fibres, a possible shape uniformation with an increase in diameter. African fibres have a physical shape resembling a twisted oval road, whereas Caucasian and Asian hairs are more cylindric [2]. African hair shows frequent twists, with random reversals in direction and pronounced flattening [1]. Perhaps lipid extraction diminishes twists with less flattening zones. This will be studied in further works. As regards the initial modulus that reflects the bonding level of the matrix, results between non-treated hairs are similar between them. After lipid extraction, no significant differences are observed in the thickest (Asian) hair and in the African hair, although the great increase in thickness can explain the slight (but non-significant) decrease in the initial modulus. The Caucasian finest hair shows a very significant increase in the initial modulus, which can be attributed to an increase in the bonding level of the matrix caused by lipid extraction. Deformation (at break) can be mainly related to the yield behaviour caused by slippages between microfibrils and unfolding of chains. The Caucasian thinnest hair shows the best yield behaviour, which remains the same after lipid extraction. African hair shows the shorter yield behaviour, which remains unchanged after lipid extraction but the Asian thickest hair although appear to be an intermediate yield behaviour, after lipid extraction deformation significantly decreases, which can be attributed to a loss of plasticity of the matrix. When breaking tenacity is considered, which is related to the bonding level of the microfibrils and within macromolecular chains, all hairs show the same level although after lipid extraction the Caucasian thinnest hair undergoes a significant increase in breaking tenacity, whereas the thicker hairs (African and Asian)

does not show a significant variation although a slight tendency to decrease can be observed. The increase in strength (initial modulus and break tenacity) for the extracted Caucasian fibres could be related to the increase in abrasion resistance found for another keratin fibre such as wool when it is also lipid depleted with methanol [26]. Even though the mechanism by which this improvement on abrasion resistance was obtained has not yet been established [27], the increased intercellular adhesion following this lipid removal may be involved [28]. Perhaps it could be related to the humidity decrease in lipid-extracted hair fibres related to the native ones. Water produces changes in the properties of human keratinized fibres such as hair. Reactive cosmetic treatments often impair the fibre structure. The resulting damage has usually an adverse effect on hair water absorption at ambient humidities and leads to an increase in swelling or to liquid retention on wetting [29]. Therefore, differences in water absorption and desorption behaviour of ethnic hairs with and without lipids were determined. Water uptake and desorption isotherm for the native and lipid-extracted three ethnic hairs were evaluated by the sorption analyser. Moisture sorption hysteresis results in two paths between sorption and desorption. The shape of the water equilibrium and water sorption isotherms obtained for the three kinds of hair is described by a type II isotherm, which shows a small amount of water at a very low relative humidity and a large increase at high relative humidity. For native hairs, the maximum water regain indicates some differences of Caucasian fibres with higher amount than the other two fibres. This is in accordance with the higher humidity content found for Caucasian hairs related to the other two kinds of hair. The extracted hairs presented a small maximum water regain diminution for Asian and Caucasian hairs as expected, but in the case of African this was increased (Table V). What is common for all fibres is that the hysteresis is increased with lipid extraction (Fig. 2). The extent of hysteresis is associated with nature and state of the components of the sample which may alter the accessibility of water to the energetically favourable polar sites [30]. The regression of the experimental sorption data by the GAB model yields values of Wm, the monolayer capacity, and Cg and K, the energy constants [31] (Table V). A good fit of the GAB model to the uptake and desorption data was achieved (R2 > 0.999). The water content in the monolayer barely decreases with lipid extraction, and the energy constants Cg and K of the primary and

© 2015 Society of Cosmetic Scientists and the Societe Francßaise de Cosmetologie International Journal of Cosmetic Science, 38, 77–84

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Table V Maximum moisture regain, GAB monolayer capacity (Wm), GAB energy constant (Cg), constant (K), GAB determination coefficient (R2), total time to reach equilibrium (tT) and apparent diffusion coefficient (DA) for native (NT) and extracted (Lip. Extr.) ethnic hair fibres

African

Asian

Caucasian

Fibres evaluation

NT

Lip. Extr.

NT

Lip. Extr.

NT

Lip. Extr.

Regain at 95% HR (%) Wm (%) Cg K R2 tT (min) DA (min1 9 103)

24.56 0.0753 5.268 0.730 0.9992 3195 20.7

27.09 0.0734 5.676 0.750 0.9994 3106 18.6

24.52 0.0795 5.457 0.712 0.9993 3524 15.9

24.45 0.0726 5.277 0.751 0.9997 3515 15.5

26.65 0.0820 5.868 0.725 0.9991 3385 17.3

26.14 0.0746 5.553 0.759 0.9997 3486 16.6

Ethnic hair: non treated

A

30

25

AFRICAN ASIAN CAUCASIAN

25

AFRICAN ASIAN CAUCASIAN

g water / g sample

g water / g sample

Ethnic hair: lipid extracted

B

30

20 15 10 5

20 15 10 5

0 0

10

20

30

40

50

60

70

80

90

100

0 0

10

20

% HR

30

40

50

60

70

80

90

100

% HR

Figure 2 Water sorption isotherms of native ethnic hairs (A) and lipid extracted ethnic hairs (B).

secondary sorbed monolayer slightly increases after lipid extraction. This means that there is a rise in the binding energy of water to the active groups of the fibres. Evaluation of kinetics of the moisture uptake and loss is a good strategy for obtaining more detailed information about the structural integrity of a given sample [32]. There are important differences in the rate at which the African hair reaches equilibrium vs. the other two fibres. Caucasian and moreover Asian fibres reached equilibrium more slowly than the African ones (Table V). Moreover, differences in the keratinized structure of these three ethnic kinds of hairs can be also confirmed by the evaluation of diffusion coefficients (Table V). Usually there is an inverse relationship between the time parameter and the diffusion coefficient, that is more time is needed to reach equilibrium for tissues with low water permeability with the result that the diffusion coefficient is smaller. This is the case in the ethnic hair fibres where Asian and Caucasian hairs present the lower diffusion coefficients compared with the African ones. Therefore, it can be concluded the higher permeability of the African fibres when compared with the other fibres. While comparing the diffusion coefficients of each fibre with the lipid-extracted one, it is important to remark its diminution in all cases, although for the Asian and Caucasian fibres the differences are almost null. This indicates that the lipid extraction of the fibres does not have much influence on the permeability of the Asian and Caucasian fibres, but this is clearly diminished for the African

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ones. Diffusion of other keratinized tissues such as nails [32], wool [27] and stratum corneum [33] has been evaluated before and after being lipid depleted. In a different extent but in all cases, the permeability tends to increase after lipid extraction, this indicated a partial loss in their integrity. In the present work, it seems that at least for the African fibre, an extraction of its lipids which mainly accounts for apolar lipids (sterol esters, squalene and free fatty acids) ameliorates the fibre structure decreasing its permeability to water. This is also confirmed by the results obtained in tensile strength which were explained by the increase in thickness and lineal mass. The mechanism to explain these facts is not straightforward; perhaps an increase of intercellular adhesion following this lipid removal may be involved. Further work is thought to be based on the study of the role of exogenous and endogenous lipids from hair in the different ethnic hairs to determine their influence in the physicochemical properties. Under the extraction procedures, sebaceous lipids and free structural lipids from the exterior and the interior of the fibre will be removed. Therefore, it will be possible to discern differences in the structural/functional contributions of measured fibre properties by either sebum or free structural lipids of different ethnic hairs. Conclusions Biochemical studies have mainly focused on the composition of hair proteins; some proteins, keratins and molecular structures,

© 2015 Society of Cosmetic Scientists and the Societe Francßaise de Cosmetologie International Journal of Cosmetic Science, 38, 77–84

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Hair lipids from different ethnicities

including amino acids, have common features across groups [4–6]. African hair exhibited lower moisturization [3] and less radial swelling when flushing with water [4] compared with Asian or Caucasian hair and they assumed a possible lipid differentiation among human populations. Lipid extraction showed the highest amount of total lipids for the African hair compared with Asian and Caucasian total lipids. These lipids may come from external sebaceous lipids which are known to be present in a much extent in the African fibres. The Caucasian and Asian hair fibres presented a similar external lipid fraction which comes mainly from the internal lipids. According to the literature, Caucasian fibre is the most hydrated fibre. When the fibres are extracted, there is always a diminution on humidity retention more important for the Caucasian hair. The higher extraction of the internal lipids mainly from the hair matrix cells for the Caucasian and Asiatic hair could be the reason for the more important moisture content diminution. The superior lineal mass for the Asian fibres related to the others supported their higher strength. Lipid extraction induced a small diminution of the tex for the Asian and Caucasian hairs but a significant increase for the African one. Mechanical properties evaluation indicates that Asian is the strongest fibre. But, although there is a general diminution of mechanical properties of the Asian and Caucasian hair after lipid extraction, the opposite was found for the African hair. Moreover, when the lineal mass is taken into account, mainly the Caucasian hair becomes stronger after lipid extraction. Perhaps this increase in strength could be related to the humidity decrease in lipid-extracted hair fibres. Results of water uptake and desorption isotherm for the native and lipid-extracted ethnic hairs indicated the maximum water regain for Caucasian fibres. The extracted hairs presented a small maximum water regain diminution for Asian and Caucasian hairs as expected but an increase for the African ones. Kinetic evaluation indicates that Asian and Caucasian hairs present the lower

diffusion coefficients compared with the African ones. Therefore, it can be concluded the higher permeability of the African fibres when compared with the other fibres. It seems that at least for the African fibre, an extraction of its lipids which mainly account for apolar lipids (sterol esters, squalene and free fatty acids) ameliorates the fibre structure decreasing its permeability to water and increasing in tensile strength. This could be partially explained by the increase in thickness and lineal mass. The mechanism to explain these facts is not envisaged; perhaps an increase of intercellular adhesion following this lipid removal may be involved. Hair delipidization is not only interesting for structural hair studies, but also represents a situation many consumers find themselves after undergoing a harsh procedure or due to natural or environmental aging of mature hair shaft. Besides, there is a wide gap in our understanding of the structure of ethnic and particular African hair. Differences in properties of ethnic hair have been attributed to the differences found in regard to lipids. The results of this research have particular relevance to chemists and new-product development specialists who work in the ethnic hair-care market. It is clear that African hair differs from Caucasian in a number of important ways, suggesting that the products formulated for Caucasian hair and sold in the general market may not adequately address the special hair-care needs of the African American consumer. The present study opens new prospects for the development of lipid-related products able to increase protection of ethnic hair. Acknowledgements The authors wish to thank the 2009 SGR 2012 (AGAUR) for providing financial support. Additionally, the authors acknowledge I. Yuste and J. Carilla for expert assistance with the techniques used in this work.

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© 2015 Society of Cosmetic Scientists and the Societe Francßaise de Cosmetologie International Journal of Cosmetic Science, 38, 77–84

The influence of hair lipids in ethnic hair properties.

Biochemical studies have mainly focused on the composition of hair. African hair exhibited lower moisturization and less radial swelling when flushing...
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