What makes curly hair curly?
This is something that has been extensively studied, resulting in several theories how our hair actually curls. This includes things outside the scope of our control, such as genetics, type of proliferating hair cells, the asymmetrical keratin contents of the cortex, and the shape of the follicle the hair originates from. First, we will review what causes the hair to actually curl.
Shape of Follicle
The shape of the follicle is a big contributor to the curl pattern. It is theorized that the angle of the follicle bend will determine the level of curl the hair will have. The follicle is not always placed perpendicular to the scalp, which is how most diagrams are illustrated. They can bend at different angles, putting a variable amount of force on the developing hair. The following figure illustrates examples of the three types of bends in three dimensions.
A(i) will create straight hair, A(ii) will create wavy hair, and A(iii) will create curly hair.
Westgate, Gillian E., et al. “The Biology and Genetics of Curly Hair.”
Shape of the Hair Shaft
The shape of the hair shaft will also dictate the level of curl. As the cells proliferate and build the developing hair shaft, it will be soft and similar in shape to the follicle. Follicle tunnel can vary in shape from being a round cylinder to an oval. Straight hair will have an almost perfectly round hair, while curly hair can seem almost flattened or ‘squished’.
B2 cross section of curly hair, B3 cross section of straight hair
Wortmann, Franz J., et al. “Why Is Hair Curly?-Deductions from the Structure and the Biomechanics of the Mature Hair Shaft.”
Amino Acid Bonds
An important amino acid bond that contributes to curls is Cysteine. Not only does it affect the follicle shape, but can contribute to the tightness of curls and coils. Cysteine’s chemical structure contains sulfide, a sulfur group that loves to bind to itself in disulfide bonds. This type of bond can create ‘hairpin turns’ on the molecular level, which will make the section more densely packed. The more densely packed the area, the flatter the hair shape will appear, causing more of a curl! This is the principle that is used in perm treatments. The chemicals separate the bond between sulfur groups before the stylist places the hair into the desired curl pattern. Then new disulfide bonds will be formed, locking the curls into place.
Wortmann, Franz J., et al. “Why Is Hair Curly?-Deductions from the Structure and the Biomechanics of the Mature Hair Shaft.” Experimental Dermatology, vol. 29, no. 3, Mar. 2020, pp. 366–372. EBSCOhost, doi:10.1111/exd.14048.
De La Mettrie, Roland, et al. “Shape Variability and Classification of Human Hair: A Worldwide Approach.” Human Biology, vol. 79, no. 3, June 2007, p. 265. EBSCOhost, search.ebscohost.com.ezproxy.uvu.edu/login.aspx?direct=true&db=edsgac&AN=edsgac.A172095080&site=eds-live.
Leite, M. G. A., and P. M. B. G. Maia Campos. “Mechanical Characterization of Curly Hair: Influence of the Use of Nonconventional Hair Straightening Treatments.” Skin Research & Technology, vol. 23, no. 4, Nov. 2017, pp. 539–544. EBSCOhost, doi:10.1111/srt.12368.
Westgate, Gillian E., et al. “The Biology and Genetics of Curly Hair.” Experimental Dermatology, vol. 26, no. 6, June 2017, pp. 483–490. EBSCOhost, doi:10.1111/exd.13347.