Biophysical aspects of the skin barrier: towards increased non-invasive extraction and optimized biomarker sampling
Authors:   , Morin Maxim 
Published: January 2022

The skin provides a link to the body’s health via its rich variety of high and low molecular weight biomarkers, reflecting both systemic diseases (e.g., cancer, diabetes) and local skin disorders (e.g., atopic dermatitis, psoriasis). Non-invasive monitoring of disease-specific biomarkers on the skin surface provides a highly attractive diagnostic procedure as alternative to current practices that normally are biopsy-based and invasive. In order to succeed with non-invasive topical diagnostics, the sampling of biomarkers should proceed in a highly accurate and reproducible manner. Further, a major challenge to achieve this goal is to overcome the outermost skin layer (the stratum corneum, SC) that acts as a remarkable permeability barrier, restricting molecular diffusion in and out of our body, including diffusion of potential biomarkers. The primary aim of this thesis is to achieve an optimized and reproducible noninvasive sampling of endogenous biomarkers from the skin surface. Here, water plays a crucial role as the hydration degree of the SC has a strong influence on the diffusion of molecules across the skin barrier. In particular, fully hydrated skin is expected to be optimal for increased diffusion of biomarkers in the skin tissue, favoring efficient extraction. Considering this, to develop a suitable sampling matrix for non-invasive extraction, it is very important to optimize the matrix so that it has a good ability to hydrate the skin as well as a high capacity to absorb the biomarker and finally allow for analytic quantification. The main questions in this thesis are as follows. (i) How long time does it take to reach a stable level of skin hydration? (ii) How do the intrinsic properties of sampling matrices influence the extraction of biomarkers? (iii) What are the effects of the sampling matrices on the biophysical properties of the skin barrier?(iv) Are hydrogels and bicontinuous cubic liquid crystals suitable matrices for noninvasive sampling of endogenous biomarkers? (v) Is reverse iontophoresis a suitable technique to further enhance the extraction endogenous biomarkers? The hydration of the skin is investigated in vivo and in vitro in order to estimate the time to reach stable hydration level. We show that skin hydration proceeds in two distinct stages with different rates of change of the electrical impedance response and conclude that stable conditions are obtained approximately after 60 min of hydration. We explore the novel approach of using lipid-based bicontinuous cubic liquid crystalline phases as matrices for non-invasive sampling of biomarkers in vivo and invitro and compare them with hydrogel-based materials. From these investigations, we conclude that both kind of materials show promising capacity of hydrating the skin and collect skin-derived biomarkers. However, the cubic phases are shown to havea bout twice as high extraction capacity, as compared to hydrogels. Further, we show that reverse iontophoresis enhances extraction of a potential cancer biomarker in vitro by at least an order of magnitude, as compared to passive diffusion. Taken together, the results obtained in this thesis can serve as a point-of-departure for future applications based on non-invasive sampling of disease-related biomarkers from skinin clinical diagnostics.