In Vivo Human Hair Hydration Measurements by Using Opto-Thermal Radiometry
The water content in human hair is very important for its cosmetic properties and general health. However, to measure water content in hair, especially in vivo hair, is very difficult. Opto-thermal transient emission radiometry (OTTER) is a promising, infrared remote sensing technology that can be used for this kind of measurements. It can not only measure the water content but also the water depth profile in hair. By measuring the water content and its depth profile in hair, we can understand the status of hair, i.e., healthy or damaged, etc. In this paper, we will present our latest study on in vivo human hair hydration measurements by using OTTER. We will first present the theoretical background and then show the experimental results. We will also compare the OTTER hair results with other established measurement technique results, such as condenser trans-epidermal water loss method.
KeywordsAquaFlux Desorption Hair hydration Opto-thermal radiometry Shampooing
The hair contains three main layers: medulla, cortex and cuticle. The medulla is the innermost layer of the hair. It is made of soft keratin and is unstructured. The medulla only exists in large thick hairs, like the hairs on scalp rather than from the abdomen or upper arms. If exists, the medulla exists all the way along the hair. The cortex is the middle layer of hair and is made of hard keratin which gives the hair its strength. The cortex contains pigments that give the hair colors; it is also the main source for water absorption. The cortex is the thickest hair layer. Therefore, the cortex determines the hair’s color, curl pattern, thickness and texture. The cuticle is the outmost layer of hair. It looks very similar to the layout of tiles on a house roof. When the hair swells, the cuticle’s complex structure slides. It is covered by a layer of lipids that makes the hair repels water. There are sweat glands that can produces sweats to cool down the body, and sebaceous gland that can produce oil to lubricate the hair.
The hair on the top of the head, i.e., scalps, presents the most cosmetic value and therefore is mostly researched on. Kristensen et al.  have studied the effect of hair dyeing and hair washing on hair cortisol concentrations. Hamel et al.  have studied the effect of hair shampooing and hair washing on hair cortisol concentrations. Boll et al.  have studied of dyed and non-dyed hairs using ATR FT-IR spectroscopy. Barba et al. [6, 7] have also studied the water content, water absorption/desorption of hair. Water content in the hair is very important for the hair’s health and cosmetic properties; however, to measure water content in the hair, especially in vivo hair, is very difficult. There are several studies using near infrared technologies to study the water content in hair, including in vivo hair [8, 9, 10]. But to date, there is no technologies can resolve water depth distribution in hair. Our previous studies show that it is also possible to measure the water content as well as the water depth profiles in hair by using Opto-thermal transient emission radiometry (OTTER) [11, 12, 13, 14], and to measure desorption of hair by using condenser trans-epidermal water loss (TEWL) method [15, 16, 17]. In this paper, we will present in vivo human hair hydration measurements by using OTTER. We will first present the theoretical background and then show the experimental results. We will also compare the OTTER hair results with other established measurement technique results, such as condenser-TEWL method and DVS (dynamic vapor sorption) method.
2 Methodology and Materials
All the hair samples used in this study are freshly cut from healthy volunteers, prior to the measurements. The each hair samples is made of a bundle of hairs, about 2 cm long, about 100 mg in weight, and wrapped at one end with aluminum foil. The hair sample was placed in a sealed cap, which is then coupled on the probe. The flux density values were then recorded for a period of 30 min, the normalized flux density curves are then compared between different hair samples. The details of the hair desorption measurement using Condenser-TEWL method can be found in . The laboratory ambient condition is 21 °C ± 2 °C and 40 % ± 5 % relative humidity (RH).
3 Results and Discussions
3.1 Hair, Nail and Skin Sites
3.2 Hair Hydration
The results show that opto-thermal transient emission radiometry (OTTER) is a promising technology that can be used for in vivo hair hydration measurements. OTTER can measure not only the water content in hair but also its distributions at different depth, i.e., depth profiles. The OTTER depth profiling results show that the water distribution within hair is not uniform. The curvature of the water distribution in hair is likely due to the layered structure of hair, as OTTER can measure both cuticle layer and cortex layer of the hair. The results also show that OTTER can effectively measure the hair hydration changes due to soaking, shampooing and straightening. OTTER can also measure the differences of different hair samples. In the cases of hair soaking and hair straightening, the water depth profiles in the hair are also changed. Finally, the OTTER results generally agree well with other measurement technique, such as hair desorption measurements using condenser-TEWL method and the standard DVS (dynamic vapor sorption) method.
We thank London South Bank University for the financial support. We also thank Biox Systems Ltd for the studentship support for C. Bontozoglou.
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