The French firm’s Institute for Ethnic Hair and Skin Research has been investigating the structural differences of hair for a number of years, and published its latest study on crack propagation and fracture mechanisms in African-American hair in the British Journal of Dermatology.
Using a combination of scanning electron microscopy and x-ray tomography the research team were able to provide new information about the fracture of hair.
“Mechanical damage from grooming and some environmental factors accumulate in hair creating internal cracks that eventually result in breakage at unpredictable sites and therefore a continuous care regimen for the hair throughout the life cycle of the fibres is recommended,” says L'Oréal Research and Innovation, in the study.
Studying the details
In its investigation the team collected virgin hair of excellent quality, with informed consent, from a female African-American volunteer.
A series of controlled mechanical stresses was then applied to 10-mm hair sections using a high-resolution mechanical stage (20 mN) up to the fracture of the fibre.
The surface was monitored using scanning electron microscopy imaging during the stress application, and x-ray tomographic microscopy images were acquired and quantified to detect changes in energy absorption as a function of applied stress that could be linked to increase in crack density.
After analysing the two imaging techniques, L’Oréal proposes the following mechanism of hair breakage: cuticle sliding; failure of the cuticle–cortex interface; nucleation of intercellular cracks and growth of cracks at the cuticle–cortex junction; and propagation of intercellular cracks towards the surface of the hair and final breakage when these cracks merge at the cuticular junction.
“The use of X-ray photon and electron imaging combined with in situ tensile testing provided a new insight into how human hair breaks, in particular curly hair,” explains the research.
“It has helped reconcile previous observations at the molecular level and at a purely macroscopic statistical level allowing us to propose a fracture mechanism that takes into consideration all constituents of hair at different scales.”
The recommendation based on these results is that hair fibres would benefit from a hair care regimen that reduces the grooming forces and the friction between grooming tools and fibres and in between fibres.
Furthering understanding
In 2005, the L'Oréal Institute for hair and skin research carried out a multi-ethnic study to investigate hair breakage in women residing in the US, finding that a large percentage (96%) of the African-American respondents experience breakage.
A combination of structural differences and grooming-induced stresses seem to contribute to the higher breakage incidence in the African-American group as the chemical composition of African-American hair is not significantly different from other ethnic groups, according to L’Oréal.
It has been suggested that the repeated elongation, torsion and flexion actions may affect the components of the hair fibre. However, considering the different properties of cuticle and cortex, L’Oréal says that one would expect a different wearing mechanism of each, leading to the ultimate failure of hair.
Knowing in detail how each part of the structure fails can potentially lead to better ways to protect the hair from physical insults, and was one of the drivers behind the firm’s latest research investigation.