Holistic approach protects and repairs against fatigue

In this paper, a holistic approach that protects the hair shaft from the ‘insideout’ and from the ‘outside-in’ targeting the cortex and cuticle sheath is proposed. For this purpose, two main actives are used, namely, one with a low molecular weight able to penetrate and act inside the cortex, and the other one with high molecular weight able to act outside at the cuticle sheath. The composition of both molecules and their relation to mechanisms involved in the protection and repair process are discussed. Hair damage due to the action of repetitive small stresses with and without actives are analysed by cyclic fatigue testing using a Diastron Tester. Damage to hair at the cuticle sheath and to the cortex in the form of ‘split-ends’ and fracture are analysed by SEM, UV Fluorescence Imaging, and Optical Microscopy. Effects arising from damage and repair of bleached hair are analysed by Differential Scanning Calorimetry (DSC), FTIR Imaging Spectroscopy, and Confocal Raman Spectroscopy. 

Perhaps, one of the biggest realisations in the field of hair science during the past 10 years is the fact that, in real life, hair breakage and damage to the cuticle sheath rarely occurs by the action of unrealistic high value forces, but rather from fatigue failure, i.e. from the application of repetitive small forces.1-3 Therefore, hair breakage studies previously based on the application of single high value forces which extend beyond the hair yield region, i.e., forces > ~ 5 grams per fibre, although academically relevant, are not considered realistic in everyday life. Indeed, fatigue failure is a phenomenon of such a great importance that in the field of Material Science it has been the object of intense studies during the last 20 years.4,5 

Fatigue failure is a direct manifestation of fracture, a phenomenon explained first by Griffith in 1921 who discovered that the mechanical force needed to induce mechanical breakage in any material is always much lower than that anticipated based on molecular bonding calculations.6 He found that at the basis of this discrepancy is the existence of randomly distributed micro defaults and micro cracks which are always present no matter how perfect the material is. When a force is applied to a material, the randomly distributed micro defaults and micro cracks act as stress concentrators causing them to grow further until a large crack develops and breakage occurs. The immediate consequence of randomly growing cracks leading to failure, is that a collection of apparently identical materials will always exhibit different breakage force values when tested each one at a time. At best, therefore, the breakage behaviour of a collection of identical samples can be better described by a statistical distribution.