What Do We Know So Far on Hair Straightening?

What Do We Know So Far on Hair Straightening?
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Hair represents a valued aspect of human individuality. The possibility of having an
easy to handle hairstyle and changing it from time to time promoted an increasing search
for chemical hair transformations, including hair straightening. Hair straightening is the
process used to convert curly into straight hair. The desire for straight hair used to be
associated with the standards of the “universal beauty.” Currently, the preference for
this style is more for the ease of handling and the simpler daily care routine [1].
Straightening may be physical or chemical processes and temporary or permanent, regarding
its duration.
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We performed a literature search in the scientific database MEDLINE through PubMed
until July 15, 2020, using the terms “straightening” AND “hair” (125 results),
“straightening” AND “alopecia” (22 results), and “straightening” AND “human hair”
(103 results). We limited the search to articles available in English and considered those
mentioning alternatives to straighten the hair. After excluding duplicate titles, we had a
total of 33 relevant articles.
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Anatomically, the hair shaft has 3 layers: cuticle, cortex, and medulla [2, 3]. The
outermost part is the cuticle, which is composed of keratin and consists of layers of
scales overlapping 1 and other, just like tiles on a roof. The cuticle protects the
underlying cortex and acts as a barrier [3-5]. The normal, undamaged cuticle has 6–8
layers according to the ethnicity, a smooth surface, allowing reflection of light and
limiting friction between shafts [5]. The outer surface of the cuticle’s scale cells is
coated by a thin membrane, the epicuticle, and each cuticle cell consists of 3 layers of
protein: the A-layer, a resistant layer with high cystine content; the exocuticle, also
rich in cystine; and the endocuticle, low in cystine content. The cortex accounts for most
of the hair shaft and is responsible for the hair. The cortex is comprised of
microfibrils, long filaments oriented parallel to the axis of the fiber. Each microfibril
consists of keratin intermediate filaments, also known as microfibrils, and the matrix,
constituted by keratin-associated proteins [4]. It is the thickest layer located around
the medulla, which is the innermost part of the hair, has melanin granules which
composition is related to the shades of hair color. It is also responsible for hair
volume, the great tensile strength, and mechanical resistance of the shaft, as it contains
the most part of keratin [3-5].
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The primary component of the hair fiber is keratin. The remaining constituents are
represented by other proteins, water, lipids, pigments, and trace elements. Because of its
specific conformation and chemical bonds, keratin is responsible for hair stiffness,
strength, and insolubility. Among the amino acids that make up keratin, cystine is one of
the most important. Each cystine unit contains 2 cysteine amino acids from different
portions of the peptide chains that are connected by 2 sulfur atoms, forming a strong bond
named disulfide bridge [3-5]. Another important structural component of the hair shaft is
the 18-methyl eicosanoic (18-MEA) acid. It forms a hydrophobic layer that retards water
from wetting and penetrating and changing the hair shaft physical’s properties. Removal
of the fatty acid layer decreases the brightness of the hair, making it more susceptible
to static electricity and frizzing induced by humidity [4].
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The spiral shape of the hair is determined by the asymmetric protein expression in the
hair follicles [2]. As it is not possible yet to modify the shape of the follicle, the
only way to change hair appearance is by modifying its physicochemical properties [6].
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This method was developed in the late 19th century and became popular in the early
20th century by Madame C.J. Walker, who combined hot comb with pressing oil. It is a
temporary straightening since it changes only weak hydrogen bonds, in a process named
keratin hydrolysis. The initial technique was the application of a petrolatum ointment
base in the hair, followed by straightening it using a heated metal combing device. Over
time, the technique was improved. However, with the introduction of new methods, the hot
comb went out of use [1, 5, 7, 8].
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Physicochemical techniques combining mechanical and thermal straightening, as
hairdryer and flat iron, are temporary solutions that last until the next washing. The
hair needs to be wet, so hydrogen bridges break and there is the transitional opening of
the helical structure of the shaft, relaxing it. The combined use of the dryer and the
flat iron dehydrates the hair, keeping it straight [1].
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High temperatures, between 235 and 250°C in the dry hair and 155–160°C in the wet
hair, may denature hair shaft proteins [1, 9]. Usually, hairdryers are more harmful to the
hair shaft than naturally drying it [9]. However, a study showed that the use of the dryer
with continuous movement, at a minimum distance of 15 cm from the hair, could be less
damaging than natural drying [10].