# Electrically Equivalent Head Tissue Materials for Electroencephalogram Study on Head Surrogates
## 4 Results
### Conductivity Test
\begin{table}
\begin{tabular}{l l l} \hline \hline
**Name of the Layer** & **Recommended (S/cm)** & **Achieved (S/cm)** \\ \hline Scalp & \(4.1\times 10^{-3}\) & \(4.5\times 10^{-3}\) \\ Outer Compact Bone & \(5\times 10^{-5}\) & \(5\times 10^{-5}\) \\ Spongiform Bone & \(4.8\times 10^{-4}\) & \(7.5\times 10^{-4}\) \\ Inner Compact Bone & \(7\times 10^{-5}\) & \(5\times 10^{-5}\) \\ CSF & \(1.71\times 10^{-2}\) & \(2\times 10^{-2}\) \\ Grey matter & \(4.70\times 10^{-3}\) & \(4.50\times 10^{-3}\) \\ White matter & \(2.2\times 10^{-3}\) & \(2.3\times 10^{-3}\) \\ \hline
**Name of the Layer** & **Median Value (S/cm)** & **Achieved (S/cm)** \\ \hline Brain & \(3.5\times 10^{-4}\) & \(0.5\times 10^{-4}\) \\ Dura & \(6\times 10^{-4}\) & \(8\times 10^{-4}\) \\ Spinal Cord and cerebellum & \(1.60\times 10^{-3}\) & \(2.3\times 10^{-3}\) \\ Eye & \(5.05\times 10^{-4}\) & \(8\times 10^{-4}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 2: The conductivity lists of recommended, median values, and achieved conductivities of the head layers manufactured. \begin{table}
\begin{tabular}{l l l} \hline \hline
**Number of Layers** & **Name of Layers** & **CF (wt\%)** \\ \hline \hline \multirow{4}{*}{**Basic 4 Layers**} & Scalp & 5 \\  & Skull & 2 \\  & CSF & Saline water \\  & Brain & 5 \\  & Compact cortical bone & 1 \\
**Skull Layers** & Spongy or trabecular bone & 2 \\  & Compact cortical bone & 1 \\
**Brain Layer** & Grey Matter & 5 \\  & White matter & 4 \\  & Scalp & 5 \\  & Compact cortical bone & 1 \\  & Spongy or trabecular bone & 2 \\
**Detailed 7 Layers** & Compact cortical bone & 1 \\  & CSF & 6 \\  & Grey matter & 5 \\  & White matter & 4 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Carbon fiber concentration for each of the head layers manufactured. tissue, white matter, gray matter, and scalp. In most cases, the recommended conductivity value of each layer or region is the median value obtained from the statistical analysis [23]. For further details on how the recommended values are obtained, the readers are referred to Ref. [23]. We set the composite composition for each layer to match the recommended values. From Figure 6 and Table 2, it can be observed that most of the achieved values are close to the recommended values. Although the spongiform's targeted conductivity is much higher than recommended, the three-layer skull bone (outer compact-spongiform-inner compact) showed a similar conductivity compared to the recommended one. The brain layers (white matter and grey matter combined) do not have a recommendation in [23], but our combined GM-WM layer exhibits similar conductivity compared to the median value, as shown in Table 2. Table 2 also shows the specific values of the recommended and achieved conductivities of each layer. While not included in our current study, we also acquired the carbon fiber concentrations to align with the conductivities of the eye, cerebellum, spinal cord, and dura. It should also be noted that there are no data available on the combined conductivity of the whole head tissue. As our separate layers and sublayers match very well with the recommended values of the corresponding layers, we argue that the overall conductivity of the head tissue we report here reflects the overall conductivity of human head tissue, and the value can be used for models requiring conductivity of the whole head tissue.