Simultaneous morphological analysis of large numbers of hair cross sections as a tool for investigation of population-level trends.

Abstract

Objective: Characterizing the fibre properties of individuals with different hair textures across several ethnicities is important for understanding how hair shape varies within and between groups, and how these may influence consumer needs. Here, we present a high-throughput scanning electron microscope (SEM) method for simultaneous measurement of cross-sectional single hair shape parameters from hundreds of hairs per sample, which has not been feasible previously. We demonstrate the power of the method through application on a population with diverse hair types.

Methods: Scalp hairs were collected from individuals located in the United States of America. Each hair sample (consisting of up to several hundred fibres) was classified using two different methods, one during clinical collection [hair texture Types 1-4] and later another blind standard laboratory method [hair curliness classification Types I-VIII]. Additional clinical data were collected on age and self-identified ethnicity. Hair shape parameters (cross-sectional area, ellipticity, shape factors) were measured using a SEM sample preparation, imaging and image analysis method. SEM data were analysed with respect to clinical texture, age and self-identified ethnicity and subsequent hair curliness classifications.

Results: The SEM method generated sufficient data from each sample to identify trends, and we found some statistically significant differences between SEM hair shape parameters and clinical sample types, as well as with laboratory curliness classifications. In the curliness classification, there was an expected tendency between hair curliness and aspect ratio: curlier hairs were more elliptical than straight hairs. In terms of the hair grouping types, in the age group, older individuals had thinner hairs than young ones. In the texture group, individuals in Texture Type 1 had thinner hairs than Texture Types 2, 3 and 4. Texture Types 3 and 4 had hairs with a more elliptical profile than individuals in Texture Types 1 and 2.

Conclusion: The SEM method was reliable to quantify cross-sectional hair parameters within populations of donors with different types of hair. This approach corroborates clinically assessed hair type and curliness classification systems and provides a more thorough characterization of hair shape variation between and within individuals and ethnicities.