**3. Types of dry electrodes**

Dry electrodes can be broadly classified into two major categories: contact and non-contact electrodes. As the name suggests, contact electrodes are in direct contact with the skin and can be made from a range of materials, including metal, textile, and polymer electrodes. Metal-based electrodes use metal plates such as stainless steel, silver, titanium, and gold [16, 20–22]. These provide ease of manufacturing, integration, and use, but due to the lack of an electrolyte and flat hard surfaces, it is difficult to achieve good contact with dry and hairy skin. This results in an increase in resistance of the stratum corneum (Rsc) and a decrease in stratum corneum capacitance (Csc) and contact capacitance (Cc). It leads to high skin-electrode contact impedance, which leads to poor signal quality. Efforts have been made to embed the metal-based electrodes on a foam structure for ease of wearing [20, 23].

Several investigators have worked on fabricating dry electrodes based on the use of textiles in place of stiff metals [24, 25]. Textiles-based electrodes are made using conductive thread, steel, yarn, or silver coated on nylon fabric. These electrodes can be embedded within clothing and used as smart garments to sense biopotential signals [26, 27]. Due to flexibility, there is an increase in the effective skin contact area compared to the metal-based electrode, which decreases the Rsc and increases the Csc

and Cc, thus lowering the overall impedance. But still, the challenges of high skinelectrode contact impedance remain the same due to the absence of an electrolyte.

To enhance elasticity, electrodes have been fabricated on polymeric substrates like polyethylene terephthalate (PET) and coated with conductive polymers such as MWCNT/PDMS [28]. Another polymer material, such as PEDOT, poly (3,4 ethylenedioxythiophene), is coated on metal electrodes to enhance the electrochemical performance, as it would lead to an increase in Cc [29]. But often, fabrication is complex, and in the long run, polymers usually flake off. Another type of dry contact electrodes includes microtips/pin based. The surface of the electrodes coming into contact with the skin consists of pins/spikes. The pin's height is of the order of 150 μm, which is sufficient to penetrate the 20 μm thick stratum corneum. Thus this leads to a lower value of Rsc and a higher value of Csc. Hence, the pins directly come into contact with the fluids underneath the stratum corneum, which acts as an electrolyte, and eventually, the spike electrodes behave like wet electrodes. Though the application of pin electrodes can overcome the high impedance of the stratum corneum [30], their use can lead to skin irritation and infections as they come into contact with fluids underneath the skin. There are other types of electrodes, which do not make direct contact with the body, which thus can be categorized as non-contact electrodes [31]. These electrodes are worn on top of clothing, and can acquire biopotential signals through clothing [32–35]. The addition of clothing further increases the contact impedance, thus increasing the Rsc and decreasing the Cc and Csc.
