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Review
. 2019 Nov;475(2231):20190516.
doi: 10.1098/rspa.2019.0516. Epub 2019 Nov 20.

The what, why and how of curly hair: a review

Elsabe Cloete  1 , Nonhlanhla P Khumalo  1 , Malebogo N Ngoepe  2
Affiliations
Review

The what, why and how of curly hair: a review

Elsabe Cloete et al. Proc Math Phys Eng Sci. 2019 Nov.

Abstract

An attempt to understand and explain a peculiarity that was observed for curly fibres during experimentation revealed disparate literature reporting on several key issues. The phenotypical nature of curly fibres is only accurately understood within the larger scope of hair fibres, which are highly complex biological structures. A brief literature search produced thousands of research items. Besides the large amount of information on the topic, there was also great variability in research focus. From our review, it appeared that the complexity of hair biology, combined with the variety of research subtopics, often results in uncertainty when relating different aspects of investigation. During the literature investigation, we systematically categorized elements of curly hair research into three basic topics: essentially asking why fibres curl, what the curly fibre looks like and how the curly fibre behaves. These categories were subsequently formalized into a curvature fibre model that is composed of successive but distinctive tiers comprising the elements in curly hair research. The purpose of this paper is twofold: namely to present (i) a literature review that explores the different aspects of curly human scalp hair and (ii) the curvature fibre model as a systemized approach to investigating curly hair.

Keywords: curly fibres; curly hair research; curvature fibre model; hair biology; human scalp hair.

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Conflict of interest statement

We declare we have no competing interests.

Figures

Figure 1.
Figure 1.
Structure of a curly hair fibre, showing detail of the growing and keratinized parts. KIF, keratin intermediate filament.
Figure 2.
Figure 2.
Composition of the hair cuticle. Each constituent layer makes a contribution to the physical behaviour of the fibre.
Figure 3.
Figure 3.
Illustration of the different cortical cell types. The packing of the IFs inside the cell contributes to the shape of the cortical cell (adapted from [52,53]).
Figure 4.
Figure 4.
Cross-sectional and longitudinal cortical cell distribution in straight and curly hair fibres (adapted from [52,53]).
Figure 5.
Figure 5.
Stress–strain plot for straight and curly hair fibres. For curly hair fibres, there exists a toe-region prior to the linear elastic region.
Figure 6.
Figure 6.
The curvature model enables easier categorization of research activities relating to curly hair fibres.
Figure 7.
Figure 7.
Application of the curvature fibre model.
See this image and copyright information in PMC

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