Depth Sensitivity Of The Dielectric Properties Of Human Skin: Simulations And Measurements
Free (open access)
F. Dewarrat, D. Huber, L. Falco-Jonasson, M. S. Talary & A. Caduff
It has been shown that in vivo glucose level changes affect the dielectric characteristics of the skin and underlying tissue, which can be measured non-invasively by impedance spectroscopy (IS). For a successful implementation of such an impedance sensor it is necessary that the sensor, when placed in contact with human skin, has a sufficient field penetration depth so that it is not only sensitive to changes in the dielectric properties of the upper skin layer, but also has a higher sensitivity to changes in the deeper skin layers. This is due to the fact that the upper layers of the skin are poorly vascularised, therefore their dielectric properties have a low sensitivity to changes associated with blood and intravascular fluids. A simplified model of the human skin and underlying tissue has been developed using electromagnetic field finite element simulations. This model can be used to optimise the electrode design of the impedance sensor. Initially, the applicability of the simulation concepts has been tested by comparing simulations with measurements of reference materials using sensors of various sizes. Subsequently, simulations of the skin model are compared with measurements on human subjects using two experimental procedures. In the first procedure, the blood perfusion of the deeper skin layers is changed by the restriction of the blood outflow (using a pressure cuff). It is shown that the sensitivity to these changes is larger with the sensor with the deeper field penetration, in agreement with simulation results. Similar sensors are then used in a second procedure where the blood glucose concentration is changed. The largest sensitivity to this glucose change is again obtained with the sensor that has the deepest field penetration. Future work will concentrate on quantifying perturbing factors, their effects on the different electrode geometries and the differences between various skin types. Keywords: impedance, skin, non-invasive, glucose, depth profile, simulation.
impedance, skin, non-invasive, glucose, depth profile, simulation.