Fundamentals of Skin Bioimpedances.

Abstract

The bioimpedances of tissues beyond the stratum corneum, which is the outermost layer of skin, contain crucial clinical information. Nevertheless, bioimpedance measurements of both the viable skin and the adipose tissue, though easy and innocuous, are not widely used, mainly because of the complex multi-layered skin structure and of the electrically insulating nature of the stratum corneum. Here, first, we establish a theoretical framework for analyzing the impedances of multi-layered tissues and apply it to skin. Afterwards, we determine strategies for the system-level design of electrodes and electronics which minimize 4-wire (or tetrapolar) measurement errors even in presence of a top insulating tissue, thus enabling non-invasive characterizations of tissues beyond the stratum corneum. As an example, we demonstrate non-invasive measurements of bioimpedances of living tissues which are beyond orders-of-magnitude (e.g., up to 350 times) higher stratum corneum impedances independently on extreme variations of the barrier (tape stripping) or of the skin-electrodes contact impedances (sweat). Our results can advance the development of bioimpedance systems enabling the characterization of viable skin and adipose tissues for several applications, including transdermal drug delivery and the assessment of skin cancer, obesity, dehydration, type 2 diabetes mellitus, cardiovascular risk and multipotent adult stem cells. This article is protected by copyright. All rights reserved.