While heat acts as an essential component of various hair care processes, the thermal properties of hair are little studied and these properties dictate how heat is spread and stored in hair. Here, the infrared microscopy enhanced Ångström's method is introduced for accurate measurement of thermal diffusivity of hair. Three factors that could influence the thermal diffusivity were statistically tested in vacuum: (1) hair type, (2) the specific fiber from the same hair type, and (3) the specific locations along the same hair fiber (i.e., near the root vs. near the tip). The average thermal diffusivity of hair across the types measured in vacuum is 0.15 mm2/s, which is in good agreement with the published data. Hair type, fiber, and location had no statistically significant impact on the thermal diffusivity. Hair is particularly sensitive to moisture content and while absolute measurements of thermal diffusivity in air are not reported here, we show that increasing humidity level reduced the apparent thermal diffusivity of the samples. Understanding the variations in thermal properties with humidity is crucial to understanding how treatment processes impact hair health.

Saturated and unsaturated fatty acids make up 85% of the total hair lipid content and are found in the cuticle and cortical cell membrane complex. Although these lipids only make up 2-6% of the hair's overall weight, they play a crucial role in keeping hair healthy, influencing shine, feel, manageability, and strength. The objective of this work was to understand the mechanisms of how these lipids are lost on exposure to external stressors, such as chemical treatments, washing, and UV exposure and to understand how their loss impacts hair strength. The experimental approach was to measure these lipids and oxidation products, lipid peroxides (LPOs) and correlate their loss with fatigue strength measurements. The results show lipids are lost over time by washing, exposure to chemical treatments, such as coloring, and environmental insults, such as UV, and it was confirmed that a mechanism of degradation is via oxidation of unsaturated lipids to form LPOs. In addition, it was shown that replenishment of these lipids is possible by incorporating lipids, such as fatty alcohols (FaOHs), into a gel network with anionic surfactants to create a delivery system that can efficiently penetrate FaOHs into hair and increase internal strength as measured by fatigue.