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

· The hair follicle (described earlier) is a dynamic structure, developing from the epidermis and dermis downwards, that is subjected to different 
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Video The what, why and how of curly hair: a review

1. Introduction

A quick review of current and most recurrent topics addressed in hair research includes hair growth, loss, corrective practices/surgery, mechanical, physical and chemical properties, follicle morphology and activities, genetic controls, forensic investigations and clinical pathology [1-8]. The phenotypical nature of the curly fibre is explored by some, but, compared with the bulk of the investigations, the curly fibre is mostly an addendum in the field of hair research. The work presented in this paper arose from an interesting phenomenon observed during tensile studies of hair fibres of different degrees of curliness, in which curly fibres lost a significant degree of curl during the preparation process. Exploring the observed phenomenon in light of available theories and experimental findings on curly hair, a literature review revealed a diversity of intentions and investigation perspectives when studying hair in general (all scalp hair forms). It also became evident that, in many of the reports, there was a gap in the comprehensive understanding of how certain aspects under investigation relate to one another or to other important aspects relating to the hair fibre. This lack of interconnectedness is not unusual for an interdisciplinary field, but nonetheless produces inappropriate associations or combinations of non-parallel concepts. This is especially true for curly hair fibres, where phenotype is usually described in race-dependent hair subjective terms.

The main purpose of this paper is to present a literature review that explores the different aspects of curly human scalp hair. This review aims to address three main questions which have shaped the field: what does curly hair look like (structurally), why does some hair curl and how does curly hair behave? To answer any of these questions, it is necessary to distinguish between the different macroscopic parts of a fibre, irrespective of its shape. Each hair fibre comprises two distinct portions: namely, the growing and the keratinized portions. Below the scalp, the growing portion is generated in the follicle; above the scalp, the keratinized portion exists as a dead biological structure. The terms more commonly used for the keratinized part are ‘hair shaft’, ‘hair fibre’ or simply ‘fibre’. Since hair curliness is generated in the growing portion, the ‘why’ question of the literature review is addressed at the follicular level. Curly fibre behaviour (the ‘how’ question) pertains mostly to the keratinized portion. The ‘what’ question requires an answer for both growing and keratinized portions.

Answering these questions through the literature review has led to the development of a curvature fibre model that formalizes successive but distinctive tiers of elements in curly hair research. This model is presented in the final part of this paper. The model offers researchers a tool to obtain an overview of the available relevant research focus areas; it also assists with comparative studies on curly hair and highlights areas in which more research is required. It helped us to identify where a theory about the observed phenomenon would belong within the myriad of curvature aspects, and, from that point, enabled the development of new theoretical constructs to explain the observed phenomenon. While the explanation for the observed phenomenon is important, it is not the focus of this paper, but is described elsewhere as part of a larger research project described in [9].

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