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Hair is more than a collection of fibers on the human head. It is something strongly associated with one’s self-image. To many, it describes who they are and how they want to be perceived by others. This is why hair loss and excessive hair breakage can be so stressful to people. Afro-textured hair, known for its tight curls, is well-known to be prone to breakage; chemically straightened Afro-textured hair even more so. As a result, hair breakage is one of the top concerns of consumers with this hair type, and means to prevent it are needed. 1 The question for the cosmetic scientist is: why is Afro-textured hair prone to breakage? and perhaps more importantly, what can experts do about it? This article digs into the research to answer these questions and suggests routes to mitigate this consumer stressor.
Outer Shape of Afro-textured Hair
Afro-textured hair is usually distinguished by its tightly curled outward appearance. As will be seen in later sections, it is this morphology that has a major effect on tangling and hair breakage.
In the salon, the degree of curl for this hair type is often measured using professional hair stylist grading systems, such as the Andrew Walker scale. These are used to provide advice to consumers about care, styling and treatments. Similarly, cosmetic scientists use a curl classification system based on single fiber analysis to quantify the degree of curl.2 This system measures four parameters related to hair curliness and kinking—i.e., curve diameter, curl index, number of waves and number of twists—to classify hair types. Afro-textured hair is characterized by curliness levels of IV to VIII, where VIII is the curliest. Straighter Caucasian hair, in comparison, is defined as having curliness levels of I to IV. Examining Afro-textured hair more closely under a microscope, however, reveals that the curl is not a simple wave or crimp. Instead, Afro-textured fibers tend to be flatter in their cross-section than straighter hair types, and the wider axis of the fibers tends to twist along the length of the hair, helping to create the tighter curl (see Figure 1). Various studies have measured the cross-sectional dimensions of Afro-textured hair and compared them with other hair types. For example, Robbins showed that the ellipticity, i.e., ratio of maximum to minimum fiber diameter, of Ethiopian hair is 1.75, versus 1.25 for straight Oriental hair.3 Bryant and Porter explained that the ellipticity of African American, Jamaican, Ghanaian, Liberian and Kenyan hair varies between 1.65-1.70, versus approximately 1.45 for Caucasian American hair.1
Studies also showed that Afro-textured hair has a higher cross-sectional area than many other hair types. Bryant and Porter found that the cross-sectional area of Afro-textured hair of subjects from a number of countries was equal to, or higher than, the cross-sectional area of hair from Caucasian American subjects.1 It is not a case, then, of Afro-textured hair being too fine that makes it prone to breakage.
Looking at fibers in even more in detail, Kamath et al. explored ellipticity changes along the length of Afro-textured hair.4 Here, SEM data displayed more fiber flattening in the areas where fibers were twisted, and even twisting in opposite directions along the fiber lengths. This unique morphology of Afro-textured hair is a major factor in its susceptibility to breakage. Tight curls, in combination with the unbending, untwisting and stretching dynamics of brushing and combing, is likely to create internal shear forces that lead to crack formation.
Indeed, SEM studies on broken hairs4 and X-ray tomography of stretched fibers5 suggest that in dry Afro-textured hair, these shear stresses often create cracks in the cell membrane complex between cortical cells, or between the cuticle and the cortex. These cracks then pass through the proteins in the cortical cells and in between the cuticle cells to cause hair breakage.
Internal Structure of Afro-textured Hair
As far as internal fiber structure, it is now widely understood that the curliness of Afro-textured hair is driven by its unique structure, the biology of the hair follicles6 and the bilateral distribution of cells in the cortex. Evidence for this bilateral distribution has been shown in several studies.
For example, a study of Japanese subjects by Bryson et al.7 showed, using TEM, that curlier hair is associated with a bilateral distribution of cortical cell structures. Cells most often found on the inside of the curl had straighter intermediate filaments (IFs), aligned to the central fiber axis, whereas cells on the outside tended to have IFs that formed whorl-like structures around the center of each microfibril. These differences were confirmed by electron tomography, which allowed for the three-dimensional reconstruction of IF arrangements by fluorescent light microscopy. This exemplified a higher bilateral distribution of cell types in curly hair.
Kajiura et al.8 evaluated Afro-textured hair and other straighter hair types using scanning microbeam small angle X-ray diffraction studies. This work found that the degree of hair curl is related to differences in intra-cellular packing arrangements of the cortical cells on the inside and outside of the curl. Afro-textured hair, not surprisingly, had the largest differences in cortical cell packing across the cross-section of each hair.
Interestingly, the bilateral distribution of different cortical cell types in curly human hair is similar to that seen in wool,9 where curl is associated with the presence of paracortical and orthocortical cells on the inside and outside of the curl. As with human hair, the three-dimensional packing of IFs on the inside of the curl is straighter and more parallel, whereas on the outside of the curl, IFs twist in a whorl-like fashion. In wool, paracortical cells have a higher matrix content than orthocortical cells.
It is believed that the differences in IF packing and matrix content in paracortical and orthocortical cells affect how they expand and contract with hydration. Biologists have speculated that the drying of wool fibers during their formation in the follicle causes uneven contractions in the different cell types, which leads to the formation of the curl,9 although recent evidence suggests cortical cell length may also be an important factor driving curl in wool. Notably, cortical cells of the outside of the curl are longer than those on the inside of the curl.10
From the perspective of hair breakage, speculating the differences in tensile properties between different cortical cell types inside Afro-textured hair might also magnify shear forces within the fiber. These could lead to crack formation when the hair is unbent, untwisted and stretched.
Finally, no mention has been made yet of cross-sectional differences in the cuticle layer. Robbins noted the faster abrasion of cuticle cells on fiber high-spots where the scales are most severely bent.3 These are what go on to become weak spots in the hair and increase breakage.
- Bryant, H., Porter, C. and Yang, G. (2012). Curly hair: Measured differences and contributions to breakage. Intl J Dermatol 5 8-11.
- Loussouarn, G., Garcel, A. L., Lozano, I., Collaudin, C., Porter, C., Panhard, S., et al. (2007). Worldwide diversity of hair curliness: A new method of assessment. Intl J Dermatol 46 suppl 1, 2-6
- Robbins, C. R. (2000). Physical and cosmetic behavior of hair. Chemical and Physical Behavior of Human Hairs, Springer, New York 424-433.
- Kamath, Y. K., Hornby, S. B., and Weigmann, H.-D. (1984). Mechanical and fractographic behavior of Negroid hair. J Soc Cosmet Chem 35 21-43.
- Camacho-Bragado, G. A., Balooch, G., Dixon-Parks, F., Porter, C., and Bryant, H. (2015). Understanding breakage in curly hair. Brit J Dermatol 173 suppl 2 10-16.
- Westgate, G. E., Ginger, R. S. and Green, M.R. (2017). The biology and genetics of curly hair. Exper Dermatol 26 483-490.
- Bryson, W. G., Harland, D. P., Caldwell, J. P., Vernon, J. A., Walls, R. J., Woods, J. L., et al. (2009). Cortical cell types and intermediate filament arrangements correlate with fiber curvature in Japanese human hair. J Structural Biol 166 46-58.
- Kajiura, Y., Watanabe, S., Itou, T., Nakamura, K., Iida, A., Inoue, K., et al. (2006). Structural analysis of human hair single fibers by scanning microbeam SAXS. J Structural Biol 155 438-444.
- Plowman, J. E. and Harland, D. P. (2018). Fiber ultrastructure. In: The Hair fiber: Proteins, Structure and Development. Springer, New York 3-13.
- Harland, D. P., Vernon, J. A., Woods, J. L., Nagase, S., Itou, T., Koike, K., et al. (2018) Intrinsic curvature in wool fibers is determined by the relative length of orthocortical and paracortical cells. J Exper Biol 221.