Role of Lipids in Water Permeation of Different Curl Pattern Hair Types

Background: Dark-base hair fibers with Curl Pattern (CP) types 2 and 3 from Asian and European populations, respectively, are very similar, although each presents different behaviors regarding water diffusion and cosmetic treatments, including in relation to dyeing. This study aims to identify the key drivers of water diffusion in hair, particularly the role of lipids in the diffusion processes. Methods: Virgin, externally delipidized, and internally delipidized CP2 and CP3 hair strands were subjected to Dynamic Vapor Sorption (DVS) and ATR-FTIR investigations. In addition, external and internal lipid extracts were quantified and analyzed via thin-layer chromatography–flame ionization detection (TLC/FID). Results: The results obtained indicate that CP2 hairs present lower water regain at all humidity steps and a different diffusion behavior depending on the humidity. Lower diffusion was obtained at low humidity and higher diffusion at high humidity. TLC/FID analyses indicate that CP2 fibers present a significantly higher amount of external lipids (1.4% vs. 0.4%) and internal lipids (3.2% vs. 2.6%) as compared with the CP3 fibers. Conclusions: The higher amount of internal lipids is mainly due to the greater amount of polar lipids (ceramides). Lipid extraction tends to modify the water content, leading to a more hydrated and less permeable lipid-depleted fiber. The similar water properties of the two types of lipid fiber support the fundamental role of lipids, even when present in small quantities, in the differentiation of hair types. This study highlights a potential link between the lipid composition of CP3 and CP2 hair fibers and their differences in behaviors regarding water diffusion, which could also explain varying responses to cosmetic treatments.