J. Cosmet. Sci., 69, 305–313 (September/October 2018) 305 Hair Mechanical Anisotropy—What Does It Tell Us? STEVEN BREAKSPEAR, BERND NOECKER, and CRISAN POPESCU, Kao European Research Laboratories, KAO Germany GmbH, Darmstadt D-64297, Germany (S.B., B.N., C.P.) Synopsis Hair fi bers were examined by atomic force microscopy, nanoindentation. By indenting along (longitudinal) and across (transversal) the fi ber, 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 defi ned as the anisotropy index (IA) of the fi ber 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 fi ber internal architecture. The evaluation of the results acquired on the components of the fi ber, 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. INTRODUCTION Hair fi bers’ structural analysis reveals an intricate assembly of protein materials. Moving from the cortex (inside) of the fi ber toward the periphery there are some highly ordered rod-like regions, keratin intermediate fi laments (KIF), surrounded by keratin-associated proteins, cellular membrane complex, and other cellular remnants, all wrapped by fl at cuticle cells (1). The cuticles themselves consist of subcomponents, namely the endo- and exocuticles. Commonly, hair mechanics are evaluated by a tensile test, performed along the growth axis, and produce the well-known sigmoidal stress-strain curve (2). An impressive amount of work has been dedicated to analyzing this curve to extract as much information as pos- sible about the hair subcomponents and their role in the overall architecture and behavior of hair (3,4). This method, however, provides information only along one direction and, in addition, lacks the required resolution for discriminating among the contributions of the fi ber subcomponents. Although the measurement of fi ber torsion appears to provide a complementary picture of fi ber mechanics, with the mechanical action taking place in a plane perpendicularly to Address all correspondence to Crisan Popescu Crisan.Popescu@kao.com.
JOURNAL OF COSMETIC SCIENCE 306 the long axis (5), this method also lacks the ability to distinguish among the fi ne subcom- ponents but may offer data on cuticle contribution (6). Atomic force microscopy (AFM), since its emergence (7), has enabled higher spatial reso- lution. This has made AFM the ideal tool for investigating the mechanics of the hair subcomponents (8,9). Moreover, AFM nanoindentation is able to measure the mechanics of the hair subcomponents in any direction. Acquiring the mechanical response of the same subcomponent along different axes has, so far, not been performed, however, by other investigators. This study aims to explore this aspect, expecting that such data pro- vide new hints about hair architecture. MATERIALS AND METHODS HAIR SAMPLES Virgin hairs were collected from a European female, close to the root, and washed. The washing process consisted of sonication of the hairs in 1% aqueous sodium dodecylsul- phate (50 cm3) for 1 min, sonication in deionized and UV irradiated water (50 cm3) for 1 min, followed by rinsing with copious amounts of the same and, fi nally, gentle drying under a stream of nitrogen. AFM All AFM measurements were performed in a closed cell under controlled relative humid- ity, RH, which can be adjusted to the intended value ±1%, and a temperature of 25 ± 1°C, using a MFP-3D Scanning Probe Microscope (Asylum Research, Santa Barbara, CA). Samples were equilibrated for 1 h before each measurement. Fibers were embedded in Epon 812 resin (TAAB Laboratories Equipment Ltd., Aldermaston, Berkshire, UK) and, after curing, excess epoxy material was removed from around the embedded hair with a razor blade, and the top 2 mm of the resin block, containing the hair sample, was severed and mounted on a steel AFM sample stub with silver paint, ensuring that the hair samples were perpendicular (for cross sections) or parallel (for longitudinal sections) to the sample stub. Smooth hair sections were then exposed by ultramicrotome (Reichert Ultracut N Ultramicrotome, Wien, Austria), using a diamond knife (Ultra 45, Diatome Ltd., Biel, Switzerland). Force curves and corresponding images on hair sections were acquired using a rectangular silicon nitride cantilever of length and nominal spring constant, k, 160 μm and 42 N m-1, respectively (Olympus Corp., Tokyo, Japan). The anatomy of force curves and the extrac- tion of nanoindentation data have been described elsewhere (10). To overcome the uncertainty in knowing the tip radius, necessary for evaluating the samples elastic moduli, five polymer samples, listed in Table I, with known Young’s moduli, E, and Poisson’s ratios, υ, were measured, as a means of indirectly calibrating the AFM system (11). Approximately, 2 × 2 mm pieces of each polymer were embedded in Epon 812 resin, mounted, and cut by ultramicrotome, in the same manner as for the hair samples.
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