A consumer cross-over trial suggests that there are significant seasonal changes of the tensile properties (wet) of human hair.

OBJECTIVES: Through the cooperation with an industrial partner, we gained a set of data for the tensile properties (wet) of human hair. The hair samples originated from a cross-over study with two groups of individuals, using for a topical application sequentially two products (A and B). Each phase of the study lasted 6 months. The phases of the study by chance covered first largely the winter and then the summer period. Initially, tensile variables were chosen, which not only reflect the mechanical properties of hair (modulus, break strain, and break stress) but which are also considered to have a good connection to practice-relevant hair properties. The initial analysis of the data showed that changes were observed for the variables due to the treatment phases. However, these were either small or difficult to interpret. METHODS: Against this background and using two-factor analysis of variance, we investigated the hypothesis that the tensile properties of hair (wet) may show significant seasonal changes. For this, we chose those two independent variables, which reflect the properties of the intermediate filaments (modulus) and the matrix (break strain) in the composite structure of the hair cortex. RESULTS: The results support the 'seasonal' hypothesis and clearly show that the variables show significant changes from Winter to Summer (modulus: 10% increase; break strain: 3% decrease). The seasonal effect was thus a major reason, why the first stage of the analysis of the data was inconclusive. CONCLUSIONS: The tensile properties of the main morphological components of the cortex show distinct seasonal changes. Towards the summer, the hair becomes stiffer and more brittle. Furthermore, the results suggest that seasonal effects may need to be taken into account when conducting studies on lengths of hair grown during different seasons.

OBJECTIFS: Grâce à la coopération avec un partenaire industriel, nous avons obtenu un ensemble de données sur les propriétés de traction des cheveux (humides) humains. Les échantillons de cheveux provenaient d'une étude croisée avec deux groupes de personnes, utilisant pour une application topique deux produits (A et B) de manière séquentielle. Chaque phase de l'étude a duré 6 mois. Les phases de l'étude ont principalement couvert l'hiver, puis l'été. Initialement les variables de traction ont été choisies reflètent non seulement les propriétés mécaniques des cheveux (modulus, extension de rupture et stress de rupture), mais sont également considérées comme ayant un lien étroit avec les propriétés pertinentes des cheveux. L'analyse initiale des données a montré que des changements ont été observés pour les variables en raison des phases de traitement. Cependant, ils étaient faibles ou difficiles à interpréter. MÉTHODES: Dans ce contexte et à l'aide d'une analyse de variance à deux facteurs, nous avons étudié l'hypothèse selon laquelle les propriétés de traction des cheveux (humides) pourraient montrer des changements saisonniers significatifs. Pour cela, nous avons choisi ces deux variables indépendantes, qui reflètent les propriétés des filaments intermédiaires (modulus) et de la matrice (extension de rupture) dans la structure composite du cortex capillaire. RÉSULTATS: Les résultats appuient l'hypothèse « saisonnière ¼ et indiquent clairement que les variables montrent des évolutions significatives de l'hiver à l'été (modulus : augmentation de 10 % ; extension de rupture : diminution de 3 %). L'effet saisonnier était donc une raison majeure pour laquelle la première étape de l'analyse des données n'a pas été concluante. CONCLUSIONS: Les propriétés de traction des principaux composants morphologiques du cortex montrent des changements saisonniers distincts. Vers l'été, les cheveux deviennent plus raides et plus cassants. En outre, les résultats indiquent que les effets saisonniers devrait être pris en compte lors de la réalisation d'études sur les longueurs des cheveux ayant poussé à différentes saisons.

Comparing hair tensile testing in the wet and the dry state: Possibilities and limitations for detecting changes of hair properties due to chemical and physical treatments.

OBJECTIVES: This investigation focuses, first, on the question to which extent wet and dry tensile tests on human hair may be considered as leading to independent results. Second, we try to assess the sensitivities of wet and dry-testing to detect changes of mechanical properties. Specifically, we were interested in separating changes, which were induced by a combination of a chemical (oxidation/bleach) and a physical treatment (heat). METHODS: The basis for our study are data for the tensile properties (wet and dry) of a set of untreated and bleached hair tresses, which were submitted to the same schedule of thermal treatments. As characteristic tensile parameters, we chose modulus (E), break extension (BE), and break stress (BS). First, parameters were analysed across treatments for the correlations between wet and dry data. Second, we applied two-factor analysis of variance to assess the effects of the factors and their potential interaction. RESULTS: Correlations for the dry versus wet data show only a weak relationship for E, while coefficients of determination (R2 ) are quite high for BE and BS. Two-factor ANOVA enables to quantify the various contributions to the Total Sum-of-Squares for all three parameters. We show that the parameters respond quite differently to the chemical and the thermal treatments as well as to testing conditions (wet or dry). It is of interest to note that the interaction between the chemical and the physical treatment is generally quite weak. For the interpretation of the results, we use the concept of the humidity-dependent as well as strain-induced glass transition of the amorphous matrix. CONCLUSIONS: The independence hypothesis for dry and wet tensile measurements only applies for modulus. Overall, we consider modulus (wet) as the best tensile measure of fibre damage when assessing chemical and/or physical treatments. Under ambient conditions (dry), break stress is shown to be a feasible alternative measure.

OBJECTIFS: Cette expérience porte d'abord sur la question de savoir dans quelle mesure les tests de traction humide et sec sur cheveux humains peuvent être considérés comme conduisant à des résultats indépendants. Deuxièmement, nous essayons d'évaluer les sensibilités des tests humides et secs pour détecter les modifications des propriétés mécaniques. Plus précisément, nous nous sommes intéressés à séparer les changements, qui ont été induits par une combinaison d'un produit chimique (oxydation/eau de javel) et d'un traitement physique (chaleur). MÉTHODES: Notre étude repose sur les données relatives aux propriétés de traction (humides et secs) d'un ensemble de tresses capillaires non traitées et décolorées, qui ont été soumises au même programme de traitements thermiques. En tant que paramètres de traction caractéristiques, nous avons choisi le modulus (E), l'extension de rupture (BE) et le stress de rupture (BS). Tout d'abord, les paramètres ont été analysés entre les traitements, pour observer les corrélations entre les données humides et secs. Deuxièmement, nous avons appliqué une analyse de variance à deux facteurs, pour évaluer les effets des facteurs et leur interaction potentielle. RÉSULTATS: Les corrélations entre les données sèches et humides montrent uniquement une relation faible pour E, tandis que les coefficients de détermination (R2) sont assez élevés pour BE et BS. L'analyse ANOVA à deux facteurs permet de quantifier les différentes contributions à la somme totale des carrés pour les trois paramètres. Nous montrons que les paramètres répondent de façon assez différente aux traitements chimiques et thermiques ainsi qu'aux conditions de test (humide ou sec). Il est intéressant de noter que l'interaction entre la substance chimique et le traitement physique est généralement assez faible. Pour l'interprétation des résultats, nous utilisons la notion de transition vitreuse hygro-dépendante ainsi que de la matrice amorphe induite par une contrainte. CONCLUSIONS: L'hypothèse d'indépendance pour les mesures de traction sec et humide ne s'applique qu'au module. Dans l'ensemble, nous considérons le module (humide) comme la meilleure mesure de traction des dommages des fibres, lors de l'évaluation des traitements chimiques et/ou physiques. Dans des conditions ambiantes (sèches), le stress de rupture est une mesure alternative réalisable.

Perm-waved human hair: a thermorheologically complex shape memory composite.

A "permanent" bent shape can be imposed on a straight human hair by a two-stage reduction/oxidation (perm-waving) process. The process relies on the molecular level on sulfhydryl/disulfide interchange as bond exchange reaction (BER). We expected a well-documented transition temperature around 60°C to be the trigger for the shape memory (SM) process of perm-waved hair. We confirm the existence of the SM process as such and investigate its time and temperature dependence. The results show a two-stage SM behavior, implying two distinct variations of the BER. The model to fit the data contains two fractional, normalized, elastic bending rigidities, which are strictly compensatory. They show Arrhenius-type temperature dependence and a common activation energy (EA) of ∼-12 kJ/mol. The characteristic relaxation time for the first SM process shows little, if any, temperature dependence (EA = -4 ± 2.7 kJ/mol). This is in contrast to the second process (EA = -58 ± 5.5 kJ/mol) but in line with the expected properties of the suggested BERs. None of the parameters shows any sign of the expected trigger transition (∼60°C). We hypothesize that this specific transition occurs only for large tensile deformations, when specific SS bonds in the intermediate filaments of hair are activated. There is thus no specific "trigger" transition for the SM behavior of bent, perm-waved hair.

Why is hair curly?-Deductions from the structure and the biomechanics of the mature hair shaft.

The final shape of a head hair is predetermined through a variety of factors during its formation in the follicle. These are genetic pathways, specific growth factors, cell differentiation and segregation, etc, with spatial as well as chronological dynamics. The cortex of hair consists of two major cell groups. These are characterized by parallel (para-type) or roughly helical arrangements (ortho-type) of the intermediate filaments (IF). There are also cell-specific differences in the disulphide content, that is, of the cross-link density of the IF-associated matrix proteins. Given the current state of the academic discussion, we consider it as timely to support and broaden the view that, the structural differences of the cell types together with their lateral segregation are the main driving factor of curl formation. The mechanical effects, which derive thereof, are triggered in the transition zone of the follicle, that is, upon formation of the mature hair shaft. Furthermore, an irregular, "flat" cross section of the hair shaft is shown to be a synergistic but not determining factor of curl formation. The degree of cell type segregation along the mature hair shaft together with dynamic changes of the location of the plane of segregation, namely in a non-circular cross section can account for very complex curl patterns. Against the background of these views, we argue that contributions to hair curl are implausible, if they relate to physical mechanisms which are active below the transition zone from the living to the mature (dead) hair.

A somewhat unexpected result from the deconvolution of DSC curves for human hair: There is no apparent relation between cortical cell fractions and hair curliness.

A deconvolution process has been developed for curves obtained by differential scanning calorimetry in water for Merino wool and the main ethnic hair types. This enables estimation of the fractions of ortho- and para-type cell groups. The results also indicate that hair may contain a further, low-sulphur subgroup of ortho-type cells. The sizes of the major cell fractions are in line with expectations from microscopical investigations. The fractions are comparable for hair types, and no consistent association between cell-type fractions and hair curvature is observed.

Debunking the Myth of Wool Allergy: Reviewing the Evidence for Immune and Non-immune Cutaneous Reactions.

Although wool is commonly believed to cause irritant (non-immune) and hypersensitivity (immune) cutaneous reactions, the evidence basis for this belief and its validity for modern garments have not been critically examined. Publications from the last 100 years, using MEDLINE and Google Scholar, were analysed for evidence that wool causes cutaneous reactions, both immune-mediated (atopic dermatitis exacerbation, contact urticaria, allergic contact dermatitis) and non-immune-mediated (irritant contact dermatitis, itch). Secondary aims of this paper were to examine evidence that lanolin and textile-processing additives (formaldehyde, chromium) cause cutaneous reactions in the context of modern wool-processing techniques. Current evidence does not suggest that wool-fibre is a cutaneous allergen. Furthermore, contact allergy from lanolin, chromium and formaldehyde is highly unlikely with modern wool garments. Cutaneous irritation from wool relates to high fibre diameters (≥ 30-32 µm). Superfine and ultrafine Merino wool do not activate sufficient c-fibres to cause itch, are well tolerated and may benefit eczema management.

Model-based analysis of the torsional loss modulus in human hair and of the effects of cosmetic processing.

Torsional analysis of single human hairs is especially suited to determine the properties of the cuticle and its changes through cosmetic processing. The two primary parameters, which are obtained by free torsional oscillation using the torsional pendulum method, are storage (G') and loss modulus (G″). Based on previous work on G', the current investigation focuses on G″. The results show an increase of G″ with a drop of G' and vice versa, as is expected for a viscoelastic material well below its glass transition. The overall power of G″ to discriminate between samples is quite low. This is attributed to the systematic decrease of the parameter values with increasing fiber diameter, with a pronounced correlation between G″ and G'. Analyzing this effect on the basis of a core/shell model for the cortex/cuticle structure of hair by nonlinear regression leads to estimates for the loss moduli of cortex (G″co) and cuticle (G″cu). Although the values for G″co turn out to be physically not plausible, due to limitations of the applied model, those for G″cu are considered as generally realistic against relevant literature values. Significant differences between the loss moduli of the cuticle for the different samples provide insight into changes of the torsional energy loss due to the cosmetic processes and products, contributing toward a consistent view of torsional energy storage and loss, namely, in the cuticle of hair.

Analysis of the torsional storage modulus of human hair and its relation to hair morphology and cosmetic processing.

Through measurements of three different hair samples (virgin and treated) by the torsional pendulum method (22°C, 22% RH) a systematic decrease of the torsional storage modulus G' with increasing fiber diameter, i.e., polar moment of inertia, is observed. G' is therefore not a material constant for hair. This change of G' implies a systematic component of data variance, which significantly contributes to the limitations of the torsional method for cosmetic claim support. Fitting the data on the basis of a core/shell model for cortex and cuticle enables to separate this systematic component of variance and to greatly enhance the discriminative power of the test. The fitting procedure also provides values for the torsional storage moduli of the morphological components, confirming that the cuticle modulus is substantially higher than that of the cortex. The results give consistent insight into the changes imparted to the morphological components by the cosmetic treatments.

pH-equilibration of human hair: Kinetics and pH-dependence of the partition ratios for H+ - and OH- -ions based on a Freundlich isotherm.

Hair is an insoluble, fibrous, α-keratinous, protein composite material, providing outer coverage, e.g., for mammals. In the context of a wider study on the effects of pH on human hair properties, we investigated the time-dependence of pH-equilibration study across the acid and the basic pH-range, using appropriate pure solutions of hydrochloric acid and sodium hydroxide. The results show that pH-equilibration follows essentially equal 1st-order kinetics across the pH-range. The characteristic process time does not change significantly and is in the range of 2.5-5 h. The analysis enables to determine the equilibrium uptakes of H+- and OH- -ions. These follow the expected U-shaped path across the pH-range. For both acidic and alkaline conditions, data are well described by two very similar sorption isotherms of the Freundlich-type. In consequence, partition ratios for both ions are highest near neutrality (pH 7: >6000) and drop off strongly towards low and high pHs (<50). Hair is thus a very strong 'sink' for H+ and OH-. This observation fundamentally challenges traditional views of limited ion uptake, namely, in the mid-pH-range due to hindered diffusion. It also does not support considerations on special roles of certain pHs, specific groups of amino acids, or morphological components. Our analysis thus suggests that established views of the interaction of hair and pH need to be reconsidered, The Freundlich isotherm approach appears to provide a versatile tool to refine our understanding of the interactions of hair and possibly other keratinous materials (horn, nail, feathers) with acids and bases.

Thermal denaturation and structural changes of α-helical proteins in keratins.

To gain insight into the thermal stability of intermediate filaments and matrix in the biological composite structure of α-keratins, the thermal denaturation performance of human hair fibers was investigated by Modulated Differential Scanning Calorimetry (MDSC) in the dry and the wet state. Denaturation enthalpy ΔH(D) in water was found to be independent of heating rate (11.5J/g) and to be approximately double as high as in the dry state (5.2J/g). The lower enthalpy (dry) and its dependency on heating rate are attributed to effects of pyrolysis. The stepwise change of reversing heat capacity ΔC(p) marks the denaturation process as a classic two-stage transition. The increase of ΔC(p) with heating rate reflects a continuous shift of the nature of the denaturation of the α-helical material, first, into random coil and then towards random ß-materials for lower heating rates. Denaturation temperatures follow Arrhenius relationships with heating rate, yielding activation energies of 416kJ/mol (dry) and 263kJ/mol (wet), respectively. A decrease of activation energy (wet) for high heating rates supports the hypothesis of systematic changes of the pathway of denaturation.

The information content of tensile tests of human hair (wet) is limited: Variables mainly cluster in just two principal components.

Tensile testing of keratin fibres, such as wool and hair, in the wet state is a well established tool in the academic as well as applied portfolio of mechanical analyses. For the tensile curve of a hair fibre, fourteen material-specific stress-, strain, and work-related variables were identified. Analysing twelve samples of cosmetically untreated hair, we show that the variables show good precision and a satisfactory degree of homogeneity across samples. Correlation analysis shows strong relationships between essentially all variables. Some strong correlations were in fact expected. However, their actual extent is very surprising, including even rather dissimilar variables (e.g., elastic-vs break-modulus). This observation led us to investigate whether variables may be clustered into a number of groups in order to define a set of underlying orthogonal factors, using Factor Analysis. The results show that 87% of data variance can be accounted for by just two factors, which we refer to as 'stress-' and 'strain'- factor, respectively. The tensile properties of untreated hair (wet) are thus well described by just any pair of strong variables from each factor, such as elastic modulus and break strain. We hypothesize that similar principles may also be applicable for treated hair. This type of approach and its implications may also have relevance for other epidermal appendages (nails, claws, horn, etc) or even more generally for other biological composite materials, such as skin, tendon and bone.

Spatial probing of the properties of the human hair surface using Wilhelmy force profiles.

The natural surface of human hair (epicuticle) consists of a bilayer of heavily cross-linked proteins toward the individual cuticle cell inside combined with a monomolecular, hydrophobic layer of mixed fatty acids to the outside (F-layer), which is generally assumed to be homogeneous. Wetting force profiles along segments of hair from female test persons with lengths equivalent to about 1 month of growth (approximately 10 mm) are presented. In a multistep analysis, applying curve smoothing as well as Fourier and principal components analysis, for hair lengths comprising daily and weekly growth (2 mm) pronounced systematic changes are observed in the profiles, which show that the wettability curves are nonstochastic in nature and that hair exhibits a strongly nonhomogeneous surface. Specifically, a compound daily rhythm is observed for wettability, which through its typical bimodality can be linked to continuous changes of the hair surface during wake and sleep phases. The data set furthermore suggests systematic monthly changes, which may be related to the menstrual cycle. In consequence, the results not only provide proof for the inhomogeneity of the immediate hair surface but also lead to the hypothesis that it preserves a rather detailed and long-term, individual chronobiological record, through a specific, spatially modulated distribution of hydrophobic (lipids) and hydrophilic (proteins) regions, "written" by the composition of the cell membrane of the cuticle cell prior to apoptosis.