66 HAIR CARE
keratinocytes with production of radical oxygen species and release of pro- inflammatory cytokines resulting in injury of the putative site of follicular stem cells in the superficial portion of the hair follicle.6 Sunlight damage reduces hair tensile
strength and makes it more vulnerable towards further environmental damage. Once the hair is exposed to sunlight, the light penetrates through several layers of the hair. UV-B can penetrate approximately 5 μm of thickness, mainly affecting the cuticle layer, whereas UV-A and visible light (VIS), which have longer wavelengths, penetrate the entire cuticle and influence the cortex as well. The cuticle layer surrounding the cortex,
which protects the hair from environmental and chemical damage, is the most vulnerable to UVR-induced damage due to its external location. Among the various amino acids that constitute keratin, tryptophan, cysteine, tyrosine and histidine are particularly sensitive. Cysteine shows the most conspicuous
alteration. It is mainly damaged through breakdown of C-S fission. Oxygen radicals, created by UVR, break down the disulphide bonds of structural units, forming carbonyl groups and new intra- and intermolecular cross-links that reduce structural integrity. The result is increased cuticle fragility, fibre porosity and surface roughness of the hair, as well as decreased mechanical strength, leading to further vulnerability of the hair to additional mechanical or chemical treatment. Ultimately, oxidative reactions caused by singlet oxygen occur more frequently in photo-damaged hair. In 2014, Marsh et al. investigated the
role of low levels of redox metals such as copper in accelerating damage to hair on exposure to UV irradiation. They identified for the first time in hair the role of exogenous copper in increasing UVR- induced damage in terms of protein degradation.7
Besides, UV radiation make
the PM phototoxic (from 25 mg/ml) and the UV-A seemed to mostly contribute to this phototoxic process.2
Effect of heat treatment on hair Human hair is frequently subjected to thermal treatment when it is dried after washing, or when it goes through a process of hair styling, as part of daily hair care routine for both women and men. The use of straightening/curling irons at high temperatures (over 200°C) in addition to hair dryers causes hair impairment. Damage to the hair cuticle, which make
up the surfaces of hair, caused by the heat treatment, includes micropore formation and cuticle cell disintegration.8,9
that mechanical strength of hair mainly depends on the cortex, and when hair is treated by heat under wet conditions, its
PERSONAL CARE EUROPE It is known
800 700 600 500 400 300 200 100 0
Cleansed Control *** ***
Pre-UV treatment
Post-UV treatment nOLEA-HT 10 n Placebo Figure 2: Results of FRAP assay.
nOLEA-HT 10 n Placebo 600
500 400 300 200 100 0 T0
T0 is the protein carbonyl content before product application and heat exposure
TIS
TIS is the protein carbonyl content after product application and heat exposure (IS= iron straightener).
Figure 3: Results of protein carbonyl assay before and after thermal treatment.
mechanical strength decreases. 10 The changes within hair have been
reported as follows: change in enthalpy and dynamic of water, decomposition of tryptophan in the composing proteins and cross-linkage among proteins.11,12
it is considered that thermal changes in hair happen not only at the cuticle but also at the cortex consisting of keratin fibres. The internal and surface damage
resulting from thermal treatment increases hair breakage especially with the additional stress of hair combing. The induction of oxidised protein formation by thermal treatment was investigated by Fuji and colleagues, which observed a significant formation of carbonyl groups at the heat treatment of 140°C or more in a temperature-dependent manner. 9
Therefore, All these data agree that oxidation is
one of the main mechanisms of hair damage due to pollutants, UV exposure and thermal stress. Oxidative stress influences, at a molecular level, mainly on the proteins, the principal components of hair shaft. Damaged proteins mean at a macroscopic level, duller, brittle, unmanageable hair, prone to split and break. To deal with these issues, cosmetic
ingredient suppliers mainly come up with two types of solutions: l Film-forming ingredients that physically protect the hair, avoiding/reducing the contact with the surrounding pollution/straightening iron;
l Fight against the effects of pollution/thermal treatments with
September 2020 ***p< 0.001 intragroup statistical analysis **p< 0.01 intergroup statistical analysis
** *** ***
nmol/mg hair
[Fe2+
] (µM)
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