Fatigue Strength of Panelist Hair and Correlation With Hair Damage Measures

The objective of this work was to investigate fatigue strength differences between root and tip hair for 41 Caucasian and 12 Asian panelists and correlate fatigue strength data to other measures of hair damage. In most cases both fatigue alpha- and beta-values significantly decreased at tips, indicating a higher likelihood of fiber breakage and premature failure as hair became more damaged. This decrease in tip fatigue strength correlated positively with panelist coloring frequency for Caucasian panelists. In addition, a correlation was found between fatigue cycles to break (alpha-value) and oxidative damage (R-2 = 0.73), as measured by Fourier Transform Infrared Spectroscopy, and with cuticle integrity as measured by a scanning electron microscopy damage score (R-2 = 0.77). An increase in oxidative and cuticle damage correlated with a lower alpha-value (i.e., lower cycles to break). The fatigue alpha-values were also compared with internal lipid levels including fatty acids, cholesterols, and wax esters as measured by gas chromatography-mass spectrometry. For root hair, there was a reasonable correlation (R-2 = 0.36) between internal lipid levels and fatigue strength, but no correlation was seen for tip hair. For Asian panelists, a decrease in fatigue strength was also found with increasing oxidative damage, especially for those using powder bleaches. Both panelist groups had high person-to-person variability and no clear difference between Caucasian and Asian panelists.