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H Lindley Hatcher Comparing Techniques for in vivo Skin Hydration Measurement
Hannah Lindley-Hatcher1, A.I Hernandez-Serrano1 , Emma Pickwell-MacPherson1,2
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1Physics Department, Warwick University, Coventry CV4 7AL, UK 2Department of Electronic Engineering, The Chinese University of Hong Kong, Hong Kong, 999077, China
Abstract—If THz imaging is to be developedas a technique for in vivo skin hydration assessment it is important to compare it to the gold standards presently used in the skin care industry. This study presents a comparison between THz reflected amplitude, transepidermal water loss and capacitance measurements of the volar forearm before and after treatments with common ingredients in commercial moisturizers.
I. INTRODUCTION
THE application of THz imaging to in vivo measurements of human skin has been developing significantly as we grow in understanding of what variables will have an impact on the result. The strength of THz light as an in vivo imaging modality lies in its high sensitivity to water content however this also leads to variables which cause problems if not properly controlled or accounted for.
It has been observed that parameters such as skin occlusion time, pressure and environmental conditions must all be carefully controlled, in doing so a robust protocol for in vivo skin measurements using THz light has been developed [1]. This protocol was demonstrated to be successful at quantifying the changes induced in the skin following the application of a commercial moisturizer product [2].
Now THz imaging can be used to take data which is comparable between subjects the next step is to compare it to other techniques which are used within the moisturizer industry to assess skin hydration. The primary two techniques presently in use are corneometetry which measures the capacitance of the skin surface giving a result representing the hydration and the measurement of the transepidermal water loss (TEWL) which indicates the condition of the skin barrier as it measures the loss of water from the skin surface.
A study was conducted to compare the results of using these three imaging techniques to measure regions on the volar forearms of 20 subjects, these regions were measured before and after treatment with 3 different common components of moisturizers. Additionally, images were taken with a USB optical microscope device to make it possible to observe how well the skin absorbed the products being applied. Fig. 1 shows each imaging technique and also an example of the optical images obtained before and after the treatment. This study allowed the repeatability of the techniques to be compared and also the ability of the techniques to measure the contrast in the responses to different types of products.
II. RESULTS
Through previous investigations using THz devices for in vivo imaging it was found that when the skin is in contact with a quartz imaging window a decrease in the reflected THz amplitude can be associated with an increase in the water content as the contrast in the refractive indices at the skin quartz interface is reduced in better hydrated skin. Based on this same theory by measuring the refractive index of each of the products to be tested we can predict the expected change which will be observed in the THz response of the skin. On average the observed responses were seen to follow these predicted changes, however some variation in the responses was observed which is thought to be associated with the different initial skin states, for example drier skin does not respond in the same way to the treatment as well hydrated skin.
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b) c)
d) e)
Fig. 1. A summary of the techniques used for the study. a) THz imaging with pressure sensor in place, b) corneometer, c) TEWL measurement, d)optical image of skin before treatment and e) optical image of skin after treatment.
Overall there was seen to be correlation between the reflected THz amplitude and the TEWL and these techniques were able to differentiate between products of different compositions. The corneometer measured the most significant change in the properties of the skin following the treatment however these trends could not be systematically linked to the type of product being applied. THz imaging was demonstrated to be capable of giving repeatable results for the untreated region and was successful at giving consistent trends to presently used techniques.
REFERENCES
[1] H. Lindley., A. Hernandez-Serrano, Q. Sun, J. Wang, and E. Pickwell-MacPherson. Pressure Controlled in vivo THz Measurements of Skin: Monitoring the Effects of Moisturi ers, 2019 44th International Conference on Infrared, Millimeter, and Terahertz Waves (IRMMW-THz). (2019). [2]. H. Lindley-Hatcher, A. I. Hernandez-Serrano, Q. Sun, J. Wang, J. Cebrian, L. Blasco and E. Pickwell-MacPherson. A Robust Protocol for In Vivo THz Skin Measurements , Journal of Infrared, Millimeter, and Terahertz Waves, vol. 40, pp. 980-989, (2019) DOI: 10.1007/s10762019-00619-8.
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