Protect and repair hair from radiation and heat damage

The outermost cuticle layer is composed of differentiated keratinocytes filled with a type of cross-linked keratin. Cystine, a dimer of the amino acid cysteine, is the main building block of the cuticle keratin that provides a first line of protection against external stresses.  Five to ten layers of cuticle cells are overlapped and held together by a lipid matrix known as the Cell Membrane Complex (CMC). The CMC is composed of fatty acids, cholesterol and ceramides.1 This lipid content is analogous to that of the stratum corneum barrier.  The CMC composition however differentiates from that of the skin by the presence of 18-methyleicosanoic acid (18-MEA).  18-MEA, a C21 branched fatty acid, may represent up to 40% of the fatty acid mixture found in the CMC.1 Moreover, 18-MEA also forms the F-layer that covers the outermost layer of the hair shaft also referred to as the epicuticle.2 The F-layer and its unique fatty acid composition imparts a hydrophobic property to the hair and helps protecting from external damage.3 It also contributes to hair shine.4 Cuticle multilayers surround the cortex that encloses melanin pigments giving the hair its colour.  The cortex is made of elongated cortical cells aligned along the longitudinal axis of the hair shaft. The cortex also supports the mechanical properties and the tensile strength of the hair (Fig 1).  

Hair weathering and external damage

Various hair treatments, environmental factors and especially UV will damage the hair fibre directly, or through the formation of free radicals.5,6 18-MEA fatty acids of the outermost F-layer are depleted reducing hair hydrophobicity, manageability and shine.4,7 The detrimental action of UV reaches the inside of the cuticle mantle causing photodegradation of the cystine groups and peroxidation of the CMC lipids.8 The ensuing cuticle lifting and partial detachment will affect the global hair ultrastructure.  In the inner cortex, the melanin chromophore pigments absorb UV dissipating the energy as heat.  During that UV protection process, melanin becomes degraded, or bleached, eventually causing hair colour fading.9 Directly exposing hair to heat is also part of many daily routines. Air dryers and straightening irons may act in causing a flash water evaporation eventually leading to cuticle destabilisation, reduction of tensile strength and loss of shine.10,11 Exposure to environmental stresses and treatments part of the daily hair care routine act as a multifactorial burden for hair health and physico-chemical integrity (Fig 2).  Protecting hair from such harms may require an adequate combination of bio-active components.