TESTING
A guide to hair product testing techniques
Adriana Tomás, Ana Carvalheira, Sofia Lopes, Ana Luísa Fonseca, Marta Ferreira - Inovapotek
The health of human hair plays a crucial role in both male and female appearance, and has a strong psychological and social importance. Beautiful, strong and healthy hair is broadly desired. However, hair is highly exposed to harsh factors such as heat, chemical treatments, pollution and watering that can cause damage on the hair physicochemical properties, mainly in the hair cuticle. There is great interest not only in developing
products for the protection and recovery of damaged hair, but also in developing effective protocols to investigate the efficacy of innovative hair care products. Claims for cosmetic products, as well as hair care products, are a strategy to obtain successful marketing and promotion. These claims need to be substantiated by efficacy tests, whether they be in vivo, ex vivo or in vitro studies. For high reliability, clinical studies should be
designed, conducted, recorded and reported following good clinical practices principles to provide a unified standard for ICH (International Conference on Harmonisation) regions to facilitate the mutual acceptance of clinical data by regulatory authorities. In this article, we present a range of hair testing
techniques that can show damage sustained, from the cortex to the hair surface, for both quantitative and/or qualitative techniques. In the interest of better understanding the
complexity behind these techniques, it is important to make reference to the hair structure, as well as hair care products and how they act on the hair. This is because the knowledge of these matters will greatly facilitate the understanding of the techniques.
Hair structure The hair structure can be divided into two components: the root that is inside the follicle, and the fibre, the part which protrudes from the skin surface. The hair fibre structure (Figure 1)1
is
divided into three main structures: cuticle (external layer), cortex (intermediate layer) and medulla (axial central component).2 One of the major components of the
outer surface of the cuticle is the lipid 18-methyleicosanoic acid (18-MEA),3
which is
responsible for the hair hydrophobicity. This lipid layer acts as a lubricant to reduce friction resistance between hair fibres and induces the feel of smooth hair to the touch. 18-MEA can be easily removed by chemical treatments as permanent hair colouring and waves. The removal of the 18- MEA layer leads the hair surface to become more hydrophilic and to an increase in friction.
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The hair is damaged easily by a variety of
agents, such as exposure to chemicals, colouring and environment or daily hair drying that weaken the hair by increasing inter-fibre friction, leading to breakage and to unhealthy and bad-looking hair in a macroscopic level detectable by the consumers. At a microscopic level, the use of hair dyes can lead to cuticular swelling with focal degeneration and exposure of the hair cortex, due to extensive cuticular detachment.4 Photodegradation of hair due to exposure
Keratin Tetramer Matrix Macrofibril Medulla Epicuticle
Exocuticle Endocuticle
Cuticle
Cortical cell Cortex
Microfibril
to UV radiation can lead to dryness, texture alteration, colour degradation and lustre and to stiffness and brittleness. These changes mainly come from damage to hair proteins, causing a gradual loss of structural differentiation; to integral lipids, increasing the fibre porosity, and to hair pigments due to oxidative attacks on the eumelanin or the pheomelanin melanosomes.5 In addition, the use of hair dryers and
straightening plates are damaging agents to the hair, by inducing cracks holes, and hazy borders on the cuticle.6
Hair products Hair products such as shampoos, conditioners, styling products and temporary colours can have a temporary effect on the hair. However - due to their permanent effect on the hair shaft - bleaches, permanent hair colours, relaxers and permanent waves improve the appearance of hair but can cause damage. Moreover, consumers aim for healthy
Figure 1: The structure of hair
and strong hair that appears full, shiny and manageable. Manageable hair is easy-to-comb hair, perceived as soft and easy to finger-comb. On the contrary, weak hair appears dull, with irregular length (due to split ends), frizzed, rough, sometimes with low density on the scalp, tangled and difficult to finger-comb. Thus, hair products submitted to tests can be classified into two categories: hair care, which
October 2022 PERSONAL CARE
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