Research paperNon-invasive in vivo methods for investigation of the skin barrier physical properties
Introduction
Skin, being the largest and the outermost organ of the human body, accomplishes multiple defensive and regulatory functions [1]. The skin barrier function resides almost entirely in the epidermis and, in particular, in its superficial layer – stratum corneum (SC) [2]. According to Oxford advanced learner’s dictionary, “barrier” is a term referred to as an object that separates two compartments and/or prevents or hinders the movement from one place to another [3]. Skin is not inertly covering the organism, but rather performs a number of important functions such as protection, excretion, absorption, thermoregulation, and hormone synthesis. In the ontogenesis, the epidermal barrier develops relatively late during embryogenesis (at approx. gestational age of 34 weeks in humans) [4], [5]. However, skin barrier function is not completely developed in the early stages of postnatal life [6], [7]. The immature epidermal barrier in infants is responsible not only for the disease susceptibility in this lifespan period, but also for the increased permeability of the barrier for exogenous substances [8].
The epidermal barrier protects the human body against many external stressors, namely, physical stress (e.g., mechanical, thermal, radiation), chemical stress (e.g., tensides, solvents, topical xenobiotics), and environmental conditions [9]. Skin as a barrier prevents the organism from loss of essential components such as ions, water and serum proteins. However, the epidermis is not completely impermeable for chemical substances directly applied on the skin surface. This phenomenon is used in topical dermatological therapy as well as in the transdermal drug delivery for the systemic drugs (e.g., hormones). Hence, the investigation of skin barrier functions is important not only for the clinical specialists, but also for the researchers working on (trans)cutaneous drug delivery.
In the past decades, a number of in vitro, ex vivo, in silico and mathematical models have been developed for studying and predicting skin barrier permeation and the penetration of exogenous agents [10], [11], [12], [13]. However, none of these methodologies can simulate thoroughly the real life conditions in humans [14]. The in vivo assessment of skin bioavailability of xenobiotics is interesting for obtaining toxicology and transdermal drug delivery data. Current methods applied in vivo human studies, e.g., suction blister fluid, microdialysis, skin biopsy, and tape stripping exhibit certain disadvantages such as the need for standardization and/or the invasiveness of the test procedures [14]. Thus, the constant development of novel, non-invasive, in vivo methods for skin barrier research and transcutaneous penetrations is justified.
Section snippets
The epidermal barrier – morphological basis
The mammalian skin has layered and complex organization including the SC (the most superficial part of the epidermis), the viable epidermis, and the vascularized dermis. The vast majority of skin barrier functions are attributed to the SC. Over the last decades the simple two-compartment model (“brick-and-mortar”) of the SC structure evolved to a concept presenting SC as a system with a regulated metabolic activity and as a biosensor for external factors (e.g., regulating proteolytic activity,
Non-invasive in vivo methods for assessment of the skin barrier physical properties
In the era of evidence-based medicine, a precise quantification and standardization is requested in the scope of biomedical research. The scientific community is witnessing the development of novel techniques with greater descriptive and accuracy properties [35]. Different non-invasive methods for monitoring skin functions have been introduced, offering the advantages of precise and non-invasive methods – thus harmless investigation of the epidermal barrier properties in vivo (Table 1). Due to
Conclusion
The availability of various non-invasive techniques for the in vivo investigation of skin barrier poses the question on which is the most appropriate method for the characterization of the barrier physical properties. A variety and not one single of the reviewed methods should be used to embrace the full range of biophysical functions of the epidermal barrier. Thus, an integrated and a multi-parametric approach in the evaluation of skin barrier properties should be implemented.
Acknowledgement
Part of this manuscript has been presented at the Seventh Conference and Workshop on Biological Barriers and Nanomedicine – Advanced Drug Delivery and Predictive non vivo Testing Technologies 20–29th February, 2008, Saarbrücken, Germany.
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