Bio-impedance is expected to provide a wide range of useful information related to heterogeneous tissue structures, and physiological states and functions [1]. Several authors have used impedance spectroscopy in their study of the electrical properties of skin [2-4]. Skin impedance can be investigated using different conditions and methods. It is inexpensive, not harmful to patients, and convenient for image monitoring. This is determined in subjects by the ratio of the voltage to the current.

Electrical impedance tomography has been used for imaging of both geometry and skin impedance (conductivity) distribution in internal tissues through non-invasive measurement of skin impedance on the body surface [5-7].

This research employs an electrical impedance model, which includes constant phase element (CPE). In general, this model is better suited for tissue and cell suspensions, and, its use has often been implicit. The impedance model was developed gradually in the 1930s until complete with the mathematical equation. In 1941, Cole introduced a similar model for dielectric permittivity, the Cole-Cole model [8].

Using this two-point method for non-invasive measurement of skin measurement; measurement can be made between two electrodes by application of a small alternating current, and comparison of the measured voltage with the current according to Ohm's law. Depth penetration of the currents in skin and other biological materials is correlated with frequency, distance between, and size of electrodes, as well as physical properties of the tissue under study, in particular multi layered structure with different electrical properties.

Measurement electrode systems include two-electrode, three-electrode, and four-electrode methods. Three-electrode methods [9,10] and a four-electrode method [11] have been developed for measurement of impedance. The three-electrode method can measure the impedance value of particular positions. In this study, experiments using three-electrode methods can be described by computer simulation based on a simple model.

The purpose of the present study was to evaluate the efficiency of skin depth information through computer simulations, using a typical multilayer tissue model consisting of epidermis, dermis tissues, and subcutaneous. The proposed skin electrical model is discussed with regard to different values of skin impedance and frequency using Cole-Cole.

Figure 3A shows the results of finite-element analysis (FEM) analysis based on the three-electrode method using finite element simulation for the epidermis, dermis and subcutaneous In Figure 3B, which shows the results of the computer simulation, current density as the edge of the electrode is higher than that under the electrode. Sections immediately below and at the edge of the measurement electrode were measured at six different depths, revealing that impedance was lower at the edge of the electrode than immediately below it. This is because the current density was higher at the edge of the electrode.

Figure 3C shows how impedance changed with measurement distance at different depths, revealing that impedance increased as the measurement distance increased for all depths. Figure 3D shows the results obtained when impedance was measured at different depths; here impedance decreased as measurement depth increased. This is because deeper tissues are characterized by lower resistance. In deep layers, the measured impedance was small; in shallow layers, the measured impedance was large. The increment of measuring depth was 2 mm, and the maximum measuring depth was 12 mm. Figure 4 shows results from comparison of skin impedance according to the measuring depth (four electrodes). When the measuring depth is deep, impedance measurement is small and when the measuring depth is shallow, the value for impedance measurement is large.

In this paper, the electrical skin model used (Figure 5) was the one proposed by Grimnes and Martinsen [14]. In this electric model, R₁ represents the resistance of internal tissue, the R₂ model the resistance of external tissue, and C represents the electrode/tissue interface and constant phase element model. The equivalent circuit that describes the impedance of skin consists of a CPE with Z_CPE =Y^-1(jω)^-α, where α is related to the fractal dimension of the skin's surface and can be regarded as a measure of the roughness of the skin's surface, ω is the angular frequency, and Y is the admittance of the CPE. Values of the chosen components were determined via a non-linear least-squares fit to the equivalent circuit using the Z-view program. Results of this fit are shown in Figure 5 and Table 1. This electrical impedance model was fitted and developed in comparison with our model for measurement of skin impedance. We obtained accurate results from our developed electric model.

In conclusion of simulation, an increase or decrease in impedance in the region between the voltage electrodes produced a corresponding increase or decrease in voltage, which is a measurement of the change in impedance within this region. When the conductivities of the different tissue layers are estimated, the tissue layer depth can also be estimated simultaneously by increasing the distance between the current electrodes. For simultaneous estimation of the tissue structure and conductivities of different tissue layers, the impedance should be sensitive to changes in the conductivity of each tissue layer and changes in each layer depth. Determination of the sensitivity of the change in the conductivity of the tissue to the measured impedance may be possible at some special locations by use of acupuncture points, which have low resistance [15]. Voltage can also be increased in order to increase the measured impedance. However, the distance between the current electrodes is more important since the current can flow in the deep layers, which provides information on these layers.

In the present study, the skin impedance measurement method is easily applied and skin impedance depth information is based on simulations by FEM computer calculation. The effect of the new method is assessed by computer simulations using a typical two-dimensional (2-D) model. Electrical equivalent circuit is defined as the skin model most similar to that of our proposed Cole mode. From the results obtained, we propose that the report observed in the literature on large dispersion of non-invasiveness impedance characteristics of the human body. Another conclusion that can be deduced from these measurements is that, to better locate the exact position of an area of skin with low-frequency, measurements in the range of about 200 Hz offer a better opportunity. In the future more in depth research as well as additional studies using other methods of skin impedance should be conducted.