*5.2.2 Two-layered phantom*

One of the two-layered artificial skin phantoms using elastomers was developed by Nachman et al. [52] to study the mechanical properties such as friction of dry and moist skin. The model consisted of two different layers, a hydrophilic silicone-based top layer, and a polyurethane-based dermis. But, the materials were selected to mimic the elastic modulus properties [52]. Another two-layered artificial skin phantom demonstrating both electrical and mechanical properties was demonstrated by Liu et al. [53]. The skin phantom consisted of a gelatin membrane mimicking the epidermis, SU-8 photoresist mimicking stratum corneum, and microholes mimicking sweat pores and simulating the electrical properties of skin in the frequency range of (20 Hz–1 MHz). However, this frequency range is still too high of a frequency range for characterizing the biopotential electrodes.

Since biopotential signals lie in the range of 1 Hz–1000 Hz, and hydration plays a significant role in the skin-electrode impedance, none of the phantoms described above are capable of simulating the range crucial for modeling the hydration state. To fill this gap, in one of our previous works [54], a two-layered phantom was simulated, where the two layers represent the deeper tissues and stratum corneum. The lower layer of the phantom mimicking deeper tissues was fabricated using a mixture of polyvinyl alcohol cryogel (PVA-c) prepared with 0.9% W/W saline solution. The upper layer representing the stratum corneum was simulated using a 100 μm-thick layer fabricated by spin-coating a mixture of polydimethylsiloxane (PDMS), 2.5% W/ W carbon black (CB) for conductance, and 40% W/W barium titanate (BaTiO3) as a dielectric. The hydration of the stratum corneum was modeled in a controlled way by varying the porosity of the phantom's upper layer. Steps for fabricating the twolayered phantom are shown in **Figure 6** [54]. The fabricated phantom was found to simulate electrical properties in the range of 1 Hz–1000 Hz, and matched well with the physiological skin impedance of human subjects. Moreover, with the novel

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

#### **Figure 6.**

*Fabrication steps of a two-layered phantom. Clockwise from top-left: A mixture of PDMS, carbon black, barium titanate; followed by removal of air bubbles in the vacuum chamber; spin coat mixture at 1000rpm and 30 s; cure in an oven at 80°C for 2 hours; laser-cut holes; Peel off the layer; cast PVA solution; freeze (12h at 20°C); thaw (24h at RT). (Reproduced from reference [54]).*

approach of change of porosity, it provides a capability to simulate the hydration in a controlled manner and the phantom can be tuned as per skin ranges among different individuals.
