Mechanical anisotropy of hair affected by genetic diseases highlights structural information related to differential crosslinking in keratins.

Previous work with Atomic Force Microscope (AFM) nanoindentation, on longitudinal and cross-sections of the human hair fibre, allowed for the derivation of a model for the mechanical behaviour of human hair, called the Anisotropic Index. Expanding that research further, and by applying this model, the nanomechanical behaviour of hairs from patients with the disease Trichothiodystrophy (TTD) has been examined and structural insights, gained from combining the AFM results with Differential Scanning Calorimetry (DSC) experiments and tensile measurements, suggests that TTD-affected hairs have a relatively increased amount of Keratin Intermediate Filaments, contained in compartments of differing crosslinking extent. The associated calculations of axial and transverse Young's Moduli deliver values in good agreement with the measured fibre mechanics. Furthermore, comparing these findings with the results previously obtained from the study of hairs from patients with the disease Monilethrix, it is shown that the Anisotropic Index correlates well with the known deficiencies in both hair types obtained from such patients and allows for discerning between the Control hair and from those affected by the two diseases. AFM nanoindentation along and across the fibre axis and the Anisotropic Index thus appear to reveal structural details of hair not otherwise acquirable, whilst DSC may offer a quick and simple method for distinguishing between different severities of TTD.

Learning from hair moisture sorption and hysteresis.

OBJECTIVE: The process of moisture sorption and desorption by human hair was analysed for extracting hints on the hair structure. METHODS: The isotherms of moisture sorption and desorption by hair were recorded for untreated and chemically treated (permed and bleached) hair. Data of swelling were also considered. RESULTS: By examining the swelling and moisture sorption of keratin fibres, it is possible to conclude that hysteresis is quite improbably caused by capillary condensation. The mobility of the protein chains and the strength of the bonds binding water molecules to the active sites inside the matrix are proposed as causes instead. The concept of "breaking symmetry", derived from moisture sorption- desorption data, and the method of evaluating this parameter, is proposed as a way of characterizing the chemical treatment of hair. The results show that bleaching produces a larger breaking of symmetry than perming, and this is suggested to be due to new hydrogen bonds, created as a result of the chemical treatment, replacing the original disulphide bonds, which are of different strength compared to the bonds of untreated hair. The quantitative sorption data matched well to the model of grains of matrix enveloped in layers of water molecules at increasing relative humidity, up to 100%. The analysis suggested that, aside from the glass transition event occurring at around 60%-70% relative humidity, there is another, less examined, transition occurring at around 30% relative humidity, assigned to the opening of the hair inner structure, and accommodation of more water molecules. Both transitions are reflected by corresponding changes in the fibre mechanical behaviour. CONCLUSION: The moisture sorption-desorption by hair was shown to not only allow a quantitative differentiation among various cosmetic treatments of the hair but to also provide valuable information on the structure of the fibre.

OBJECTIF: Le processus de sorption et de désorption de l'humidité par le cheveu humain a été analysé pour en extraire des informations sur la structure du cheveu. MÉTHODES: Les isothermes de sorption et de désorption de l'humidité par les cheveux ont été enregistrées pour des cheveux non traités et traités chimiquement (permanente et blanchie). Les données de gonflement ont également été prises en compte. RÉSULTATS: En examinant le gonflement et la sorption de l'humidité des fibres de kératine, il est possible de conclure que l'hystérésis est très improbablement causée par la condensation capillaire. La mobilité des chaînes de protéines et la force des liaisons liant les molécules d'eau aux sites actifs à l'intérieur de la matrice sont plutôt proposées comme causes. Le concept de "rupture de symétrie", dérivé des données de sorption-désorption de l'humidité, et la méthode d'évaluation de ce paramètre, sont proposés comme moyen de caractériser le traitement chimique des cheveux. Les résultats montrent que la blanchiment produit une rupture de symétrie plus importante que la permanente, ce qui serait dû à de nouvelles liaisons hydrogène, créées à la suite du traitement chimique, remplaçant les liaisons disulfure d'origine, qui sont de force différente par rapport aux liaisons des cheveux non traités. Les données quantitatives de sorption correspondent bien au modèle des grains de la matrice enveloppés dans des couches de molécules d'eau à une humidité relative croissante, jusqu'à 100 %. L'analyse a suggéré qu'à part la transition vitreuse qui se produit à environ 60-70 % d'humidité relative, il y a une autre transition, moins examinée, qui se produit à environ 30 % d'humidité relative, attribuée à l'ouverture de la structure interne du cheveu et à l'accommodation de plus de molécules d'eau. Les deux transitions sont reflétées par des changements correspondants dans le comportement mécanique de la fibre. CONCLUSION: La sorption-désorption de l'humidité par le cheveu permet non seulement de différencier quantitativement les différents traitements cosmétiques du cheveu, mais aussi de fournir des informations précieuses sur la structure de la fibre.

Formation of free radicals in human hair under strain: Combined electron paramagnetic resonance (EPR) - Strain technique.

In this work, the formation of free radicals in human hair and the evolution of the radical concentration under strain, using a combined electron paramagnetic resonance (EPR) - strain technique, has been investigated. The radicals formed in the hair as a result of homolytic bond cleavage in cystine residues of polypeptide chains were identified. Stability of the radicals formed in dry hair and in the presence of water were studied. The spin-strain curves for the grey human hair in dry state and in water are presented and compared with conventional hair stress vs. strain curves. The evolution of sulfur radical species has been found to only occur at strains of above ca. 25%, corresponding with the Post-Yield region of the stress vs. strain behaviour for hair; this indicates that the matrix of the hair in this region behaves like a highly crosslinked gel and helps to explain the reversibility in hair mechanical behaviour below the Post-Yield region.

Cuticle - Designed by nature for the sake of the hair.

OBJECTIVE: The cuticle of human hair has been examined, via a range of analytical methods, in order to reveal previously unknown information about its structure and to deepen understanding of its contribution to fibre properties. METHODS: Cross-sections of hair fibre have been examined with X-ray microdiffraction oriented perpendicular to the surface of the cross-sections. AFM investigations were carried out for further investigating and deciphering the structure of the cuticle. Moisture sorption analytics of cuticle separated from fibre and mechanical tests of decuticled fibres against virgin fibres were used for understanding the role of the cuticle in the economy of hair fibre. RESULTS: Previously unknown swelling behaviour of the hair cuticle during moisture sorption has been revealed, as has an increased significance of the cuticle's role in moisture management at higher values of relative humidity. Through AFM investigation, the reaction of hair cuticles with chlorine water has further strengthened the idea that the Allwörden membrane does not exist, and is actually an artefact of the delamination of the A-layer and exocuticle from the underlying endocuticle. Using decuticled fibres for stress-strain tests, and by comparing the results with those of virgin fibres, the effect of the cuticle on the post-yield area of the hair fibre stress-strain diagram has also been demonstrated. Finally, X-ray microdiffraction and AFM investigations suggest that the cuticle possesses a small-scale ordered structure, based on possibly not fully crystalline and irregularly arranged α-helices oriented almost perpendicular to the growth axis of the fibre and enhancing the general description of cuticle as the protective layer of the fibre. CONCLUSION: The role of the cuticle for the hair fibre is more complex than previously thought. The cuticle is demonstrated not only to possess a hidden rod-matrix structure, that supports its protective nature, but also to play specific roles in the fibre's response to moisture, and in fibre mechanical behaviour.

OBJECTIF : la cuticule des cheveux humains a été examinée à l'aide d'un ensemble de méthodes analytiques, afin de révéler des informations jusqu'alors inconnues sur sa structure et d'approfondir la compréhension de son rôle dans les propriétés de la fibre. MÉTHODES : des coupes transversales de fibres capillaires ont été examinées par microdiffraction radiographique orientée perpendiculairement à la surface des coupes. Des expérimentations par AFM ont été réalisées pour approfondir les recherches et découvrir la structure de la cuticule. Des analyses d'absorption de l'humidité de la cuticule séparée de la fibre et des tests mécaniques des fibres décuticulées par rapport aux fibres vierges ont été utilisés pour comprendre le rôle de la cuticule dans la préservation de la fibre capillaire. RÉSULTATS : un comportement de gonflement jusqu'alors inconnu de la cuticule des cheveux durant l'absorption de l'humidité a été révélé, de même qu'une importance accrue du rôle de la cuticule dans la gestion de l'humidité à des valeurs plus élevées d'humidité relative. À l'aide des expérimentations par AFM, la réaction de la cuticule des cheveux avec de l'eau chlorée a à nouveau renforcé l'idée selon laquelle la membrane d'Allwörden n'existe pas et est en réalité un artéfact de délaminage de la couche A et de l'exocuticule provenant de l'endocuticule sous-jacente. L'utilisation de fibres décuticulées pour des tests de contrainte et la comparaison des résultats avec ceux de fibres vierges ont également démontré l'effet de la cuticule sur la zone post-rendement du diagramme de contrainte de la fibre capillaire. Enfin, les expérimentations par microdiffraction radiographique et AFM suggèrent que la cuticule possède une structure ordonnée à petite échelle, basée sur des hélices alpha potentiellement non entièrement cristallines et disposées de manière irrégulière, orientées presque perpendiculairement à l'axe de croissance de la fibre, améliorant la description générale de la cuticule comme couche protectrice de la fibre. CONCLUSION : le rôle de la cuticule pour la fibre capillaire est plus complexe qu'on ne le pensait. Il a été démontré que la cuticule possède non seulement une structure rod-matrix cachée, qui maintient sa nature protectrice, mais joue également des rôles spécifiques dans la réponse de la fibre à l'humidité et dans son comportement mécanique.

Nanomechanical properties of Monilethrix affected hair are independent of phenotype.

Utilising the AFM nanoindentation technique for the study of hair cross- and longitudinal sections, the mechanical anisotropy of human hair fibres affected by a rare congenital condition, Monilethrix, has been investigated for the first time. Supported by X-ray microdiffraction data, and applying a model based on an ideal composite material consisting of rods (KIFs) and matrix (KAPs) to Monilethrix affected fibres, it has been shown that the results could be grouped into clearly different classes, namely: almost isotropic behaviour for Monilethrix affected hairs and anisotropic behaviour for Control hair. Moreover, AFM nanoindentation of hair cross sections has demonstrated, also for the first time that hairs affected by Monilethrix have a continuous, and not periodic, weakness within the cortex. This has been attributed to disruptions in the KIF-KIF, KIF-intermacrofibrillar matrix or KIF-desmosome complexes within the hair shaft, as suggested by X-ray microdiffraction examination. Hairs from a patient exhibiting no obvious phenotype exhibited similar mechanical weakness despite the otherwise normal visual appearance of the fibre. This further supports a hypothesis that the beaded appearance of Monilethrix hair is a secondary factor, unrelated to the inherent structural weakness.

Relevance and Evaluation of Hydrogen and Disulfide Bond Contribution to the Mechanics of Hard α-Keratin Fibers.

We propose a simple mechano-chemical model for the dependence of Young's modulus of α-keratin fibers on the hydrogen and disulfide bonds existing in the matrix and evaluate the relative change in bonding following an oxidative chemical treatment. Atomic force microscopy nanoindentation of longitudinal and cross sections of the fiber showed that, although the oxidative treatment breaks a significant amount of disulfide bonds, it introduces compensatory hydrogen bonds that maintain fiber elasticity at values comparable with those of the untreated fiber under dry conditions. The striking influence of humidity on the hydrogen bonding in keratin fibers is also evaluated. The hydrogen bonds are labeled as "type 1" and "type 2" hydrogen bonds; newly formed hydrogen bonds, type 2, are more labile than those native (type 1) to the matrix. Examining their contribution to Young's modulus of the matrix allowed for quantifying the loss of disulfide bonds, and the result matched the decrease in cystine, measured by amino-acid analysis, caused by the oxidative treatment.

Hair Mechanical Anisotropy-What Does It Tell Us?

Hair fibers were examined by atomic force microscopy, nanoindentation. By indenting along (longitudinal) and across (transversal) the fiber, we evaluated the Young's modulus and its dependence on the moisture content (relative humidity) of the environment. The ratio of the two values collected for Young's modulus, at a given relative humidity, is defined as the anisotropy index (IA) of the fiber and the acquired results give the evolution of the index of anisotropy with the relative humidity. The use of the model of composite materials allowed us to relate the anisotropy index to the fiber internal architecture. The evaluation of the results acquired on the components of the fiber, within the frame of this model, ultimately points to a possible micro-structure of exocuticle, hindered under usual circumstances by its heavy cross-linking and only noticeable when the absorbed moisture swells the surrounding network and annuls, in this way, its effect.

Deletion of the Sox21 gene drastically affects hair lipids.

The effects of Sox21 gene deletion on hair lipids have been studied. For the cuticle-specific bound lipid 18-methyl eicosanoic acid (18-MEA), which was found to predominantly exist as the free form in Sox21(-/-) hair, total levels and distribution were unexpectedly unchanged. This indicates that while the biosynthesis of 18-MEA is unaffected, its covalent attachment to the cuticle surface is disrupted by loss of keratin-associated protein binding partners. Although the class compositions differed, the total ceramide (CER) levels were found to be comparable between Sox21(+/+) and Sox21(-/-) hairs. Deletion of the gene was also found to increase cholesterol sulphate (CS) levels. The biosynthesis process might be associated with cuticle keratinocyte maturation, because both CS and CERs are known bioactives in keratinocyte differentiation.