Streaming potential analysis of hair variation: Ethnicities and environmental exposures

This study examined the ethnic and longitudinal (proximal 'root' vs. distal 'tip') variations in hair properties across Caucasian, Chinese and Indian populations, focusing on electrokinetic behaviour, structural features and particle deposition. The aim was to use electrokinetic analysis to determine how ethnicity and weathering affect hair health and influence hair-product interactions.

Streaming potential measurements were conducted on chemically untreated ponytail samples from Caucasian, Chinese and Indian donors to determine zeta (ζ)-potentials. Particle deposition was studied using amine-grafted polymethyl-methacrylate (PMMA) particles, while cross-sectional properties were quantified using optical microscopy. Root and tip segments were analysed separately to evaluate weathering effects.

Cross-sectional analysis revealed huge strand-to-strand variability within individuals. Chinese hair exhibited larger cross-sectional areas and perimeters compared to Indian and Caucasian hair, with minimal differences between the root and tip regions. For both root and tip, ζ-potential magnitudes ranked Caucasian > Indian > Chinese with no significant root tip variations despite tips enduring over 2 years of additional weathering. Similar root and tip ζ-potentials resulted in similar particle deposition for the same sample. Generally, particle deposition rates correlated with surface coverage, but ζ-potential values did not greatly influence deposition rate, with highly negatively charged Caucasian hair samples having slower deposition rates than the other ethnicities. However, a more negative ζ-potential resulted in higher surface coverage within each ethnic group. Despite Caucasian hair's more negative ζ-potential, it had the lowest average particle surface coverage (θ_Caucasian = 3-10%, θ_Indian = 6-16%, θ_Chinese = 5-20%), suggesting other factors such as hydrophilicity influence deposition. Ethnicity influenced pH-ζ profiles, with Caucasian hair exhibiting the steepest overall gradients and the highest isoelectric point (IEP), suggesting relatively less oxidative damage. A pH-ζ classification framework is proposed to aid in analysing surface functionalities.

Hair properties are shaped by genetic, environmental and cultural factors. While surface charge guides particle deposition, our findings reveal distinct ethnicity-dependent behaviours, necessitating further research into other surface properties. Smaller sample sizes reduced variability, enabling hypotheses on classifications and surface functional groups, which may serve as a foundation for future studies.