Non-invasive assessment of skin barrier properties: Investigating emerging tools for in vitro and in vivo applications

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.