Electrical impedance related to experimental induced changes of human skin and oral mucosa

There is a need for an objective and non-invasive method for assessing reactions in the human skin and oral mucosa, particularly for weak reactions. During the past few decades, several methods of measuring irritant responses have indeed been developed for the skin and to some extent also for the oral mucosa. The focus of this thesis is upon a new measurement system based on electrical impedance, which reflects structural and chemical changes in living tissues. The system is an improvement over earlier impedance techniques, and measures magnitude and phase of impedance in the frequency range 1 kHz to 1 MHz at 5 depth settings. From the raw data, four indices are calculated, representing most of the information in compressed form, and also reducing inter-individual and inter-site variation by normalisation.

The method has been used to study skin irritation induced by various concentration of sodium lauryl sulphate (SLS). Results have been compared with visual scoring as well as with those of other bioengineering techniques - transepidermal water loss (TEWL), electrical capacitance moist measurements (ECM), and laser Doppler flowmetry (LDF).The results show that electrical impedance is at least as sensitive as other bioengineering techniques, and more sensitive than visual scoring. The later was confirmed in a study using low concentration of SLS, which demonstrated that the electrical impedance method is a delicate technique for detecting responses induced by SLS, when these are weak enough to be macroscopically negative. Furthermore, a good correlation was found between the severity of the changes observed visually, the histopathological alterations, and changes in the four electrical impedance indices. We have also found the method to be highly sensitive for detection of changes in the lipid content in the stratum comeum.

Other irritants producing different types of histological skin responses have been shown to create their own distinctive impedance index patterns. Furthermore, the reduced data sets in the form of the four impedance indices contain sufficient information for classifying different responses of the skin, although additional information may be extracted from the complete spectrum. The impedance technique, can also be used in the oral mucosa, and has been shown capable of detecting mucosal responses induced by SLS in the invisible or barely visible range.

Since different anatomical regions display typical baseline values for the skin as well as the oral mucosa it is important to use contra lateral reference sites from the same anatomical location or even better to follow a site over a period of time before and after exposure to a test agent. If references are not available, e.g. in situations where the entire skin or mocosa is affected, established baseline values for different sites in the skin and the oral cavity may be substituted. Such values have now been provided for both the skin and the oral mucosa. The electrical impedance technique seems to be a powerful tool for quantification and classification of reactions in both the skin and the oral mucosa.