## 1 Introduction  
Electrical impedance of biological tissue, named electric bioimpedance, is used for medical diagnosis due to properties such as the possibility of monitoring organs and tissues' state, both in vivo and in vitro modes. Moreover, it can offer the relatively simple possibility of technological achievement for bioimpedance sensors [1]. Bioimpedance presents an advantage because it is a technique that provides health information in a non-invasive way. This medical technique involves an injection of alternating electric current in human tissues at very low intensity. In the technique of bioimpedance measurements, performed under the influence of the injected electric current, the biologic tissues, organs, cells medium behave like conductors, dielectrics, or insulators, each depending on their composition. Because of this phenomenon, bioimpedance dependence on frequency produces much information about the biological medium's psychological state under investigation [2]. This investigation refers to the changes taking place in organs, muscle, nerves, brain, heart, skin, eye retina in establishing a diagnosis, can be made by clinical methods, like ECG(electrocardiogram), EMG (electromyogram), EOG (electrooptigram), ESR (galvanic skin reflex), electrooculography, electroencephalography (EEG) [3, 4, 5, 6, 7, 8, 9]. Bioimpedance sensors must meet several requirements, such as being non-invasive, portable, user-friendly, and low cost [2]. A skin inclusive ECG electrode is a contact sensor that produces an electrical interface between the body and the measurement system, allowing the electrical charges to flow through the tissue and sense endogenous biopotential [10]. The bioimpedance electrodes are classified into polarizable and non-polarizable electrodes, also known as dry or wet electrodes. In the case of a polarizable electrode, no electrode reactions occur when an electrical potential is applied, and no charge flows across the electrode-electrolyte interface. In this case, the electrode behaves as a capacitor. In the case of non-polarizable electrode, the electrode will not change from its equilibrium potential in the presence of a density of electric current, which appeared due to the injection of a constant current. For this type of electrode, the current flows freely across the electrode without producing any electric potential. An ideal non-polarizable electrode exhibits a resistive behaviour, with a nominal resistance value that is ideally zero [10,11]. The polarizable electrodes are more suitable for sensing biopotentials, while non-polarizable electrodes are more suitable for current stimulation by current injection [12]. Ag/AgCl is the most common non-polarizable wet electrode and requires a gel electrolyte to work properly. The use of the gel leads to many negative effects over time, such as: it reduces the resistance between the surface of the metal part of the electrode and the skin below the stratum corneum (SC) layer; it increases the dielectric constant between skin and electrode, and operates for short time because of degradation of conductivity as the electrode interface and the impedance changes over time, from gel evaporation, so, the signal quality of the electrodes decreases. The long-term contact between the gel and the skin can also produce unwanted adverse reactions (e.g., irritation or eczema). For this reason in particular, these electrodes should be replaced with dry electrodes [12,13]. Dry sensors work without the use of conductive gel and other skin preparations. However, these sensors depend on the interfacial potential, the contact surface, the high interfacial impedance, and the noise [13]. For bioimpedance measurements, three methods are used: configuration with two electrodes, with three electrodes, and four electrodes, respectively.