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.
See related sidebar: A Prescriptive Solution to Strengthen African Hair [sponsored]
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.
Chemical Composition of Afro-textured Hair
Another hypothesis for the relative fragility of Afro-textured hair is that it arises from differences in the protein or lipid composition of the hair shaft; e.g., that in relation to straighter hair types, there is supposedly a deficiency or abundance of certain important components that make the hair weaker. Evidence for this theory, based on amino acid and protein analyses of hair samples, is inconclusive.11-16 Subtle differences in protein types are emerging as techniques improve but it is not clear yet whether any of the differences has a direct effect on curl or breakage.
Recently, mass spectrometry analysis of hair shaft proteins has allowed researchers to identify and quantify proteins present in the hair shaft. This has opened up the whole area of proteomics for hair fiber research.
In relation, Laatsch et al.17 published a study in 2012 that compared the protein compositions of scalp hair samples from six Caucasian, five African-American, five Kenyan and five Korean subjects. All had non-chemically treated hair. The data presented considerable variations in protein composition between subjects from the same geographic regions, although differences between ethnic hair types were less marked. Keratin associated proteins (KAPs) accounted for most (66%) of the differences between ethnic groups. Clearly, more work is required on larger sets of samples to distinguish differences between ethnic groups since variations in this type of data are so large.
Recent genomic studies have begun to uncover genes that might control hair shape.18 Analysis of the genetic profiles across 28,964 subjects from around the world have identified several gene loci associated with hair shape. Unfortunately, the causal links to hair shaft morphology and protein composition are unproven.
Synchrotron IR microspectrometry studies of hair samples obtained from Caucasian and Afro-American subjects, i.e., girls and boys aged 8-10, showed lower levels of lipids inside the hair taken from Afro-American panelists.19 In contrast, two separate chromatographic studies of extracted hair lipids, taken from mixed source hair tresses, have revealed that Afro-textured hair has a higher level of internal lipids than Asian or Caucasian hair types.20, 21 It is speculated this may be related to higher levels of absorbed sebum lipids; and given the well-known lower wash frequency associated with Afro-textured hair, this seems possible. Studies also have identified that sebum lipids are absorbed by the hair and are able to mix with internal lipids.22
Overall, the balance of the current evidence suggests that Afro-textured hair in adults has higher levels of lipids. Ideally, however, such studies would be performed in healthy human volunteer studies rather than on tresses containing hair of unknown origin.
Taken together, the effects on breakage of raised lipid levels in Afro-textured hair are not clear. Cruz et al. argue that hair lipids can interact with IF proteins,21 so it is possible that they may affect the mechanical properties in this way. Another theory is the absorption of sebum lipids may strengthen or weaken the cell membrane complex holding the hair cells together.
Evidence for Increased Breakage
Consumers also associate Afro-textured hair with higher levels of hair breakage, and this perception of brittleness has been validated by testing fiber mechanical properties.
Starting with single fiber tensile studies, a large-population study showed that hair’s break stress decreases with an increase in curliness, as defined by the eight-point Loussouarn scale.1 This data suggested the curls and twists in Afro-textured hair may create concentrations of stress and local points of weakness when the hair is stretched, leading to fracture formation.
Also in single fiber tensile experiments, Kamath et al. showed that Afro-textured hair is more prone to premature fracturing (see Figure 2).4 Their data indicated that, at ambient humidity, Afro-textured hair frequently breaks under low levels of extension; i.e., less than 20%. These authors suggest points of weakness exist in this hair type. Interestingly, however, they showed that premature fracturing is reduced when the hair is stretched when wet.
The authors therefore suggest that plasticization of the hair structure might distribute loads more evenly and protect local points of weakness from fracturing. Analysis of the fracture patterns from these experiments with SEM showed, when dry, Afro-textured hair tended to fracture in a stepwise pattern. When wet, hair tended to break with much longer axial splits, suggesting that water affects cortical cell adhesion. Interestingly, the fracture patterns were different on fibers that broke prematurely. Furthermore, premature fractures in dry hair tended to have more fibrillated or split-end fractures, suggesting poor cohesion between cortical cells at local points of weakness.
Hair fibers are not often stretched until they break as they do in single fiber tensile experiments. Instead, they are stretched just a little, many times over, until they break. This insight has led to the development of fatigue testing methods for hair.23 These measurements show that Afro-textured hair breaks in a different way than straighter hair types.
Over the whole range of shear stresses experienced when combing or brushing, Afro-textured hair breaks roughly ten times faster than straighter Caucasian hair. Data also shows Afro-textured hair fibers tend to break much earlier in the experiment. And, as with the single fiber tensile data, fatigue data revealed that failure rates increase with moisture content, meaning Afro-textured hair becomes even more fragile when wet (unpublished data).
The reality is, however, that breakage does not happen in isolation. In reality, it often happens while the hair is being brushed or combed. The unique morphology of Afro-textured hair, described previously, makes it difficult to untangle and comb when dry. However, when wetted, it becomes straighter and much easier to comb.24
This is the opposite in straighter hair types, which are usually easier to comb when dry. Why? It is speculated that surface friction forces are always higher on wet, swollen hair fibers25, 26 but in Afro-textured hair, the benefits of water straightening the hair outweigh the problems created by increased friction.
It is important to note this article has focused on the innate properties of Afro-textured hair that make it more prone to breakage. However, consumers with this hair type also use a wide range of chemical and heat-styling treatments to straighten hair—and these are all damaging to the structure of hair.27-29 In addition, textured-hair consumers also use various leave-in treatments and oils, which may affect fiber properties.
So, why is Afro-textured hair so fragile? With its tight curls and twists, is more prone to breakage for several reasons. Its shape increases tangling and makes combing harder. The shape also creates internal stresses when the hair is unbent, untwisted or stretched that lead to fracturing. The uneven internal morphology of this hair type, with bilateral distributions of different cuticle structures, also magnifies these stresses.
Regarding the chemical makeup of hair, this review examined evidence in the literature for differences in protein or lipid composition and their possible relation to increased breakage. So far, there is no clear evidence that this is the case. Further work is needed.
From the consumer’s perspective, one big question remains: What can the beauty industry do to help reduce hair breakage? Some answers lie in the combing and fatigue data referenced above. For example, the combing data reveals straightening hair by wetting reduces combing forces. The fatigue data also finds that reducing the fatigue stresses imparted to hair markedly reduced the rate of breakage.
The simplest solution for consumers is to comb or brush Afro-textured hair when it is wet and straightened to reduce breakage. Consumers should also use a good conditioning product to reduce fatigue stresses even further. Beyond these solutions, it is disappointing to say there are few treatments proven to reduce the breakage of Afro-textured hair; especially hair that has been chemically treated. This is still a challenge for cosmetic scientists going forward.
- 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.
- Menkart, J., Wolfram, L. J., and Mao, I. (1966). Caucasian hair, Negro hair and wool: Similarities and differences. J Soc Cos Chem 17 769-787.
- Hardy, D. and Baden, H. P. (1973). Biochemical variation of hair keratins in man and non-human primates. Amer J Phys Anthropol 39 19-24.
- Dekio, S. and Jidoi, J. (1988). Hair Low-sulfur Protein Composition does not Differ Electrophoretically among Different Races. The Journal of Dermatology 15, 393-396.
- Dekio, S. and Jidoi, J. (1990). Amounts of fibrous proteins and matrix substances in hairs of different races. J Dermatol 17, 62-64.
- Nappe, C. and Kermici, M. (1989). Electrophoretic analysis of alkylated proteins of human hair from various ethnic groups. J Soc Cosm Chem 40 91-99.
- Porter, C. E., Dixon, F., Khine, C. C., Pistorio, B., Bryant, H., and de la Mettrie, R. (2009). The behavior of hair from different countries. J Cosm Sci 60, 97-109.
- Laatsch, C. N., Durbin-Johnson, B. P., Rocke, D. M., Mukwana, S., Newland, A. B., Flagler, M. J., et al. (2014). Human hair shaft proteomic profiling: Individual differences, site specificity and cuticle analysis. PeerJ 2 e506.
- Liu, F., Chen, Y., Zhu, G., Hysi, P. G., Wu, S., Adhikari, K., et al. (2018). Meta-analysis of genome-wide association studies identifies 8 novel loci involved in shape variation of human head hair. Human Molecular Genetics 27 559-575.
- Kreplak, L., Briki, F., Duvault, Y., Doucet, J., Merigoux, C., Leroy, F., et al. (2001). Profiling lipids across Caucasian and Afro-American hair transverse cuts, using synchrotron infrared microspectrometry. Intl J Cos Sci 23 369-374.
- Marti, M., Barba, C., Manich, A. M., Rubio, L., Alonso, C., and Coderch, L. T (2016). The influence of hair lipids in ethnic hair properties. Intl J Cos Sci 38, 77-84.
- Cruz, C. F., Fernandes, M. M., Gomes, A. C., Coderch, L., Marti, M., Mendez, S., et al. (2013). Keratins and lipids in ethnic hair. Intl J Cos Sci 35 244-249.
- Masukawa, Y., Narita, H. and Imokawa, G. (2005). Characterization of the lipid composition at the proximal root regions of human hair. J Cos Sci 56, 1-16.
- Evans, T. A. (2009). Fatigue testing of hair—A statistical approach to hair breakage. J Cos Sci 60 599-616.
- Epps, J. and Wolfram, L. J. (1983). Letter to the editor. J Soc Cos Chem 34 213-214.
- Newman, W., Cohen, G. L., and Hayes, C. (1973). A quantitative characterization of combing force. J Soc Cosm Chem 24.
- Bhushan, B., Wei, G. and Haddad, P. (2005). Friction and wear studies of human hair and skin. Wear 259 1012-1021.
- Kamath, Y. K., Hornby, S. B. and Weigmann, H. D. (1985) Effect of chemical and humectant treatments on the mechanical and fractographic behavior of Negroid hair. J Soc Cos Chem 36 39-52.
- Ruetsch, S. B., Yang, B. and Kamath, Y. K. (2008). Cuticular damage to African-American hair during relaxer treatments—A microfluorometric and SEM study. IFSCC Magazine 11 131-137.
- Khumalo, N. P., Stone, J., Gumedze, F., McGrath, E., Ngwanya, M. R. and de Berker, D. (2010). 'Relaxers' damage hair: Evidence from amino acid analysis. J Amer Acad Dermatol 62 402-408.