where is the bulk electrical resistivity, t is the thickness of the CNT layer, and m is the CNT ρ mass loading per unit area. This inverse relationship between the sheet resistance of the substrate and the areal mass loading of CNTs is clearly shown in Fig. . With the increased amount of the CNT spray-coated onto the glass substrate, the sheet resistance of the glass slides is significantly reduced to about . However, as the amount of CNT increases, the diffusion length of the 9 Ω/𝑠𝑞 ions also becomes longer or even begins to form voids, indicating that there is an optimal areal mass loading that provides both low electrical resistance and high ionic conductance, which will be discussed later.