**4. Significant parameters for dry electrodes**

In this section, several factors affecting dry electrode performance such as the electrode area, electrode material, skin hydration, and pressure are discussed. In addition to the high skin-electrode impedance of the dry electrodes, the skin-electrode impedance is variable with the applied pressure. Major factors that should be considered in the development of new electrode designs and their contribution to the skinelectrode interface have been summarized in **Table 1**.

#### **4.1 Electrode area**

Researchers have tried to overcome the challenges of dry electrodes by increasing the area of the electrode [28, 36]. Electrode-skin impedance plays a major role in the biopotential signal quality. With an increase in the electrode area, the Rsc decreases


#### **Table 1.**

*Different factors that affect the electrode performance and the equivalent impedance fitting parameters that will be affected corresponding to these factors are shown. Rsc represents the resistance of the stratum corneum, respectively; Csc and Cc represent the capacitances of the stratum corneum and contact.*

*Factors Affecting Wearable Electrode Performance and Development of Biomimetic Skin… DOI: http://dx.doi.org/10.5772/intechopen.111429*

(Resistance is inversely proportional to the area), and both Cc and Csc increase, which leads to a decrease in Zc. Thus, it leads to an overall decrease in skin-electrode impedance with an increase in the area of the electrode. This approach of increasing the area to achieve lower skin-electrode contact impedance is useful and effective, however, it has a limited application in devices such as wearables where small electrode size is necessary for devices to be practical as wearables. This is particularly true for watch type devices, which require very small (<1cm<sup>2</sup> ) electrodes.

#### **4.2 Electrode material**

Another aspect to improve the quality of biopotential signal acquisition is using different electrode materials [20, 37, 38]. The classical approach followed by researchers for characterizing different electrode materials is entirely empirical, and consists of carrying out experiments with different chosen materials followed by qualitative or quantitative comparison with wet electrodes (gold standard) [20, 37– 39]. After that, the most satisfactory material based on the performance is selected. Some researchers have used the equivalent circuit electrode-skin interface impedance models and fitted them in the electrochemical impedance spectroscopy experimental data to characterize the wet and dry electrodes [40]. Typically, equivalent-skin interface models have been used to demonstrate the role of the size of electrodes on skinelectrode contact impedance, which clearly depicts the decrease in skin-electrode impedance with an increase in the electrode area. However, the role of electrode material (typically metal-based) in the skin-electrode interface is not reported. In one of the recent studies [18], efforts were made to understand the rationale behind a "metal-based electrodes'" material performance. The electrode-skin impedance dependence on the electrode material was investigated by developing a skin-electrode interface model that includes aspects of prior models and incorporates a model of the electrode material. The findings of this work suggested that the relative permittivity of the electrode material native oxide plays a significant role and a higher value leads to an enhanced capacitive coupling and thus a lower skin-electrode impedance. As per Eq. (2), a higher relative permittivity *εr*leads to increase in Cc which further leads to lower Zc, thus lower skin-electrode impedance. However, the skin properties Rsc ad Csc are not affected by the change in the electrode material, but Cc can be improved significantly. Thus, investigating dielectric properties, such as thickness, and relative permittivity of the native oxide, can be one of the approaches when selecting material for biopotential electrodes.

#### **4.3 Skin hydration**

The impedance of the stratum corneum is greatly affected by changes in skin hydration. According to one study, the stratum corneum resistance (Rsc) decreases by 14 times, and capacitance (Csc) increases by 1.5 times under hydrated conditions as compared to dry skin [41]. The increased capacitance (Csc) is attributed to the fact that hydration significantly affects the dielectric properties of the stratum corneum. As per previously reported findings, the relative permittivity of dry stratum corneum is 10 [41] and that of hydrated SC is 49 [42], which suggests an increase in the Csc. Moreover, hydrated skin leads to higher Cc, thus enabling enhanced capacitive coupling. The increased contact capacitance can be attributed to the improved contact between the electrode and stratum corneum [18]. Thus hydration plays an important

role in lowering the stratum corneum impedance and is the most significant factor in achieving a low skin-electrode impedance.

#### **4.4 Pressure**

For dry electrodes, researchers have made efforts to understand the role of applied pressure on electrode performance [25, 36, 43]. An increase in applied pressure results in a lower skin-electrode impedance and this can be attributed to the increase in the effective contact area due to applied pressure. As per Eq. (2), the increase in the effective electrode contact area leads to an increase in both Cc and Csc. In addition to this, there is a decrease in Rsc as resistance is inversely proportional to the area. Changes in the applied pressure affect the skin-electrode impedance significantly, which further impacts the signal quality. Therefore, applied pressure should be accounted for during the testing of wearable devices.
