The development and characterisation of a screen‐printed tattoo sensor for the non‐invasive assessment of the skin barrier is presented. A screen‐printed silver tattoo sensor comprising two concentric circle electrodes was fabricated and applied and characterised initially on a soft tissue mimic. It was shown that the tattoo was capable of tracking changes in water content in the soft tissue mimic using impedance spectroscopy. The tattoo sensors were then applied to porcine and human skins and impedance spectroscopy was used to interrogate the skin at the outer stratum corneum (SC). The SC is a layer of great interest from a dermatological point of view since it plays a critical role in the barrier function of the skin by protecting underlying tissue from infection, dehydration, chemical irritants and mechanical stress. Hydration changes were tracked in the skin using the impedance approach and validated against a tissue dielectric constant (TDC) measurement taken with the MoistureMeterD (MMD, Delfin Technologies). The impedimetric results obtained using the tattoo sensor were modelled into proposed circuit models representative of the systems measured. From this study, the potential of using wearable tattoo electrodes coupled with impedance spectroscopy as a transduction technique, to investigate skin barrier status offers considerable promise towards the monitoring and self‐management of skin health.

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.

Cellulose-based filter papers were used as base materials to construct microfluidic paper-based analytical devices (μPADs) coupling a separation channel with electrochemical detection.

Robustness in an impedance-based tattoo electrode platform is achieved through the integration of novel material approaches for skin physiology monitoring.