**2. Electrodes for biopotential acquisition**

A lot of attention has been paid to ECG because of the clinical significance of heart disease, the ability to diagnose arrhythmias from biopotential, and the utility of longterm monitoring of ECG for some conditions that only occur intermittently and may therefore be missed in the clinic. Clinically, ECG is captured using wet electrodes. Some wearable devices, such as the Holter Monitor, capture ECG using wet electrodes on the skin and provides an ambulatory electrocardiogram [2]. Such a wearable device's benefits include a portable device that is worn for a short time (24–72 hours) to determine if the subject has any occasional cardiac arrhythmias, but they do face the challenges associated with wet electrodes mentioned earlier, such as being uncomfortable and leading to skin irritation. Because of this, there is a lot of research and development activity to implement an ECG system using dry electrodes. Although not an exhaustive list, two of these are described here. One such device is KardiaMobile by AliveCor, a US-based company [3]. It is a hand-held device and can detect atrial fibrillation (an irregular and rapid heart rate). It uses two 3 cm 3 cm stainless steel electrodes and provides single-lead ECG. Dry electrodes are also being integrated into wearable devices such as watches. One such device is an Apple watch series 4. It also measures a single-lead ECG and consists of two electrodes. The button on the side of the watch (digital crown) made up of titanium serves as one of the electrodes, and the back crystal electrode on the back of the apple watch serves as another electrode. It consists of an ultrathin chromium silicon carbide nitride layer that is applied to the sapphire crystal. The wrist of the user is always in contact with the watch and the user needs to apply his fingertip across the crown, which creates a closed circuit, and this ECG can be recorded using the ECG app. Although dry electrodes are more user-friendly, they generally lead to poorer quality signals, due to the different mechanistic principles of the wet and dry electrodes, which are discussed in the following section.

### **2.1 Mechanistic principle of wet electrodes**

#### *2.1.1 Electrochemical reactions of electrode/electrolyte interface*

The mechanistic principle of a wet electrode is based on an electrode-electrolyte interface's electrochemical reactions. When a metal electrode comes in contact with an electrolyte solution, redox reactions may occur at the interface. The redox reactions occurring at the electrode/electrolyte interface are complex, but mechanistic models have been proposed that describe this interface as the *double layer* structure. The very first model was proposed by Helmholtz [4] and then modified by Gouy [5], Chapman [6], Stern [7], and Grahame [8], and followed by Bockris, Devanathan, and Muller with what is known as the BDM model [9]. Details for the double layer structure can be found elsewhere [10, 11] but are described here in brief in the context of the workings of a wet electrode by means of the double-layer structure at the interface of an Ag/AgCl electrode immersed in an electrolyte containing NaCl (**Figure 2**) [12].

The electrochemical reactions occur at the liquid electrode interface when the Ag/ AgCl electrode is immersed in an electrolyte containing Na + and Cl- ions. First, the oxidation reaction occurs at the Ag electrode interface with the AgCl coating, which results in an excess of Ag + ions. The Ag + ions move into the AgCl and fill up the vacancies or move the adjacent Ag + ions to an interstitial site. Further, the Ag + ion interacts with the Cl- ions in the electrolyte. Hence it leads to the precipitation of AgCl

#### **Figure 2.**

*An example image of a typical wet electrode is shown. The zoomed-in view represents the electrode-electrolyte interface for a wet electrode depicting the double layer structure and an electric field of the interface, where IHP is the inner Helmholtz plane and OHP is the outer Helmholtz plane (Image modified from reference [12]).*

at the interface of AgCl and the electrolyte. Also, there is a dissolution of AgCl, as the Ag + ions in AgCl prefer to be reduced. The Cl- ions get dissolved in the electrolyte, which leads to a higher concentration of Cl- ions at the interface than at the bulk of the electrolyte. Thus, to neutralize the negatively charged plane, the Ag electrode becomes positively charged, and these two planes form a double layer structure, and there appears a potential difference across the electrode/electrolyte interface. The electrochemical reactions can be described by the following eqs. [12]:

$$\text{Ag} \rightarrow \text{Ag}^+ + e^- \text{ (Oxidation)}$$

$$\text{Ag}^+ + \text{Cl}^- \rightarrow \text{AgCl (Precipitation)}$$

$$\text{AgCl} \rightarrow \text{Ag}^+ + \text{Cl}^- \text{ (Dissolution)}$$

$$\text{Ag}^+ + e^- \rightarrow \text{Ag}(\text{Reduction})$$

The potential difference across the electrode/electrolyte interface causes the rearrangement of Cl– ions and leads to the orientation of water molecules. The water molecules align due to their dipole nature. Some Cl– ions are specifically adsorbed on the AgCl surface. There also happens to be the electrostatic attraction of Cl– ions by the positive charges of the Ag/AgCl electrode. This process can be described as the formation of planes, the inner Helmholtz plane IHP and the outer Helmholtz plane (OHP). The plane crossing the center of the water molecules is called IHP, and the plane crossing through the center of the aligned Cl- ions is known as OHP. The two positively (Ag/AgCl) and negatively charged planes (OHP) act as two plates of a parallel plate capacitor, and thus can be described as a capacitor. It is also known as the Stern layer. Due to the thermal driving forces and electrostatic attraction, there is a diffusion of the other Cl- ions in the electrolyte. The concentration of Cl- ions decreases exponentially; it is known as the diffusion layer. The diffusion layer ends

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

when there is no potential gradient, which is represented by the bulk electrolyte. The double-layer structure is formed by the combination of the stern layer and diffusion layer. It can be considered the two layers connected in the series and can be described by the following expression, where CH is the Helmholtz layer capacitance, and CGC is diffusion layer capacitance (Gouy- Chapman):

$$\frac{1}{C\_{dl}} = \frac{1}{C\_H} + \frac{1}{C\_{GC}}\tag{1}$$

#### *2.1.2 Equivalent electrical circuit for electrode/electrolyte interface*

Electrically, the electrode/electrolyte interface and the skin can be represented by a combination of resistor and capacitor networks. The double layer is formed at the interface of two different phases. If there are any electrochemical reactions at the interface, they take place in the double-layer structure, leading to the faradaic current. Faradaic current is the current generated by the redox reactions at the electrode. The double-layer structure is represented by a capacitor (Cd), while the faradaic current flowing through it due to electrochemical reactions can be expressed as leakage current of the capacitor and is represented by a resistor in parallel (Rd) [13]. It is also known as charge transfer resistance and can be represented as Rct. Thus, in wet electrodes, due to this charge transfer phenomenon, resistive coupling is dominant. In this work, as shown in **Figure 3**, Rct is used to represent the charge transfer resistance, Cdl as the double-layer capacitance, and the resistance offered by the electrolyte is represented by Rg.

#### *2.1.3 Equivalent electrical circuit for skin*

The electrode-skin interface plays an important role in measuring biopotential signals. The anatomical structure of skin comprises different layers. It can be broadly divided into the epidermis, dermis, and subcutaneous tissues. The topmost layer of the epidermis is the stratum corneum (SC), which consists of dead cells and is also referred to as the horny layer [14]. The outermost layer has the highest electrical

#### **Figure 3.**

*An analogous electrical equivalent circuit of the wet electrode. Electrode-electrolyte interface is shown, where Rct and Cdc represent the charge transfer resistance and double-layer capacitance respectively. Rg represents the resistance in the gel/electrolyte. The stratum corneum is shown as a parallel combination of resistor (Rsc) and capacitor (Csc), and deeper tissue layers as a resistor (Rd).*

resistance as it consists of dead cells. The layer beneath the epidermis is known as the dermis and mainly consists of blood vessels and sweat glands. Sweat ducts penetrate the stratum corneum, and as the sweat emerges, it results in a low-resistance parallel pathway. Sweat is considered a weak electrolyte and thus the flow of sweat across the duct walls leads to an increase in the stratum corneum's hydration. Hence, this causes variation in the conductance of the skin. The skin can be represented electrically by equivalent capacitor and resistor configurations.

Electrical conduction inside the body is ionic. The stratum corneum contains sweat ducts and hair follicles that contain an ionic liquid and traverse the SC, therefore allowing electrical conduction across the SC. Hence it can be represented as a resistor. Stratum corneum consists of dead cells, and from an electric standpoint, it can be considered an insulator between the electrode (one plate of the capacitor) and the other living conductive tissues underneath it (another plate of the capacitor). Hence, it can be represented by a capacitor. The skin can be modeled as a capacitor and resistor in parallel. Several investigators have studied human skin's electrical properties in response to AC signals and found skin impedance is of the order of 100 Ω at high frequencies and 10 kΩ–1MΩ at low frequencies (below 1 kHz) [15]. **Figure 4** shows an equivalent circuit model where Rsc and Csc represent the resistance of the stratum corneum, and Rd represents the resistance of the deeper tissue layers.

$$C = \frac{\varepsilon\_0 \varepsilon\_r A}{d} \tag{2}$$

$$Z\_{\varepsilon} = \frac{1}{o\mathcal{C}}\tag{3}$$

In wet electrodes, the electrolyte/gel helps in facilitating the electrochemical reactions and hydrates the stratum corneum, thus providing conductive ions that create an easier ionic path between the electrode and the skin below the stratum corneum [16]. The gel further helps in lowering the skin-electrode impedance by reducing Rsc and

#### **Figure 4.**

*An analogous electrical equivalent circuit for the metal-based dry electrode. Contact is represented by a capacitor (Cc), along with the stratum corneum as a parallel combination of resistor (Rsc) and capacitor (Csc), and deeper tissue layers as a resistor (Rd). The air gaps are due to the surface roughness of the stratum corneum and stiff metal-based dry electrodes. Cc comprises both the air gaps and the thin native oxide that comes in direct contact with the skin.*

increasing Csc. However, the gel dries over time, which leads to an increase in Rg, Rsc, and a decrease in Cd, and Csc. In addition to this, Huigen et al. revealed that the main origin of the noise in the surface electrodes is due to the electrolyte-skin interface and is highly dependent on the electrode gel [17]. Thus, to overcome the limitations and challenges of wet electrodes, efforts have been made in the field of dry electrodes.

#### **2.2 Mechanistic principle of dry electrodes**

In contrast to *resistive coupling* in wet electrodes, dry electrodes are free of gel, and thus in the absence of any sweat has no ionic fluid coupling from the skin to the electrode, and the mechanistic principle is fundamentally different [18] and referred to as *capacitive coupling*.

In the capacitive coupling associated with dry electrodes no actual charge crosses the skin/electrode interface, as the metal electrode being an inert metal is difficult to oxidize or dissolve [14]. A displacement current exists as a result of capacitance at the interface and the signal gets capacitively coupled from the body to the sensing electrodes [19]. The heart causes immediate changes in the electric potential within the tissue, which is sensed by the metal electrodes. The metal electrode acts as one plate of a capacitor, with the deeper tissue layers as the other plate of the capacitor. The thin metal native oxide, air gaps, and the dry outer layer of the skin (stratum corneum) together act as a *dielectric* (the space between plates of a capacitor) [18]. Thus Cc varies as per Eq. (2), where the capacitance is directly proportional to the relative permittivity and area, and inversely proportional to the distance between two plates. Further, the capacitive impedance holds an inverse relation to the capacitance given by Eq. (3). Based upon this mechanistic principle of dry electrodes, different types of electrodes are reviewed in Section 3.

Where ε<sup>0</sup> is the relative permittivity of free space, ε<sup>r</sup> is the relative permittivity of a material, A is the area of the plate, and d is the distance between the two plates, *ω* = 2*π*f, *ω* is in rad/s, and f is the frequency (Hz).
