Abstract
There is increasing interest in the development of non-invasive tools for studying the properties of skin, due to the potential for non-destructive sampling, reduced ethical concerns and the potential comparability of results in vivo and in vitro. The present research focuses on the use of a range of non-invasive approaches for studying skin and skin barrier properties in human skin and human skin equivalents (HSE). Analytical methods used include pH measurements, electrical sensing of the epidermis and detection of volatile metabolic skin products. Standard probe based measurements of pH and the tissue dielectric constant (TDC) are used. Two other more novel approaches that utilise wearable platforms are also demonstrated here that can assess the electrical properties of skin and to profile skin volatile species. The potential utility of these wearable tools that permit repeatability of testing and comparability of results is considered through application of our recently reported impedance-based tattoo sensors and volatile samplers on both human participants and HSEs. The HSE exhibited a higher pH (6.5) and TDC (56) than human skin (pH 4.9–5.6, TDC 29–36), and the tattoo sensor revealed a lower impedance signal for HSEs, suggesting the model could maintain homeostasis, but in a different manner to human skin, which demonstrated a more highly resistive barrier. Characterisation of volatiles showed a variety of compound classes emanating from skin, with 16 and 27 compounds identified in HSEs and participants respectively. The continuing development of these tools offers potential for improved quality and relevance of data, and potential for detection of changes that are undetectable in traditional palpable and visual assessments, permitting early detection of irritant reactions.
Highlights
The development of new methods and instrumental techniques is crucial for the growing cosmetics sector
human skin equivalents (HSE) mimic key elements of human skin biology and overcome the limitations associated with monolayer tissue culture, such as a failure to capture the complexity of the in vivo microenvironment and cell-cell interactions [27]
Labskin is a HSE that consists of a fully differentiated epidermis on a dermal compartment composed of polymerised fibrin containing primary human keratinocytes (Figure 1a)
Summary
The development of new methods and instrumental techniques is crucial for the growing cosmetics sector. Cosmetic product testing is most informative when carried out in human participants, supporting the advancement of non-invasive analytical tools. There is a growing interest in the development of such technologies for interrogating the properties of human skin for a variety of Cosmetics 2017, 4, 44; doi:10.3390/cosmetics4040044 www.mdpi.com/journal/cosmetics. Cosmetics 2017, 4, 44 applications, including skin physiology research [1,2], diagnostics and therapy [3], personal wearable devices [4], and cosmetic testing [5,6,7]. Used methods for assessment of skin barrier function (SBF) include determination of stratum corneum (SC) water content, trans-epidermal water loss (TEWL) and skin pH. Optical coherence tomography and Raman spectroscopy have emerged in recent years as complementary methods to assess
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