# Electrically Equivalent Head Tissue Materials for Electroencephalogram Study on Head Surrogates
## 2 Materials and Method  
The electrically equivalent phantom head materials were created using silicon composites. Different filler materials of different concentrations were tested to achieve the targeted conductivity. To produce a multilayer head phantom model, Polydimethylsiloxane(PDMS)-carbon fiber (PDMS-CF) composites are chosen. There are two steps involved in the making of a phantom head material with the desired Electrical conductivity. First, different filler materials of varying concentrations are selected to achieve the targeted conductivity. The test samples are made according to the specifications, as shown in Figure 2. Here, 'Dielectric Spectroscopy' has been used to study the conductivity of the composites at different frequencies. In the next step, the layers with the closest matching conductivity of human head layers are combined to construct the multilayer head phantom materials. In our study, since EEGs work on lower frequencies [3], the electrical permittivity of the material is not considered.  
We argue that the human head tissue can be divided into seven layers as shown in Figure 2 considering the scalp as one layer and ignoring subdural space [29]. In view of this, we have constructed two types of multilayer phantoms, namely the 'Basic four layers' (brain-cerebrospinal fluid (CSF)-skull-scalp) and the 'Detailed seven layers' types, as shown in Figure 1. Each layer has different material properties. The details of the head layers are shown in Table 1. After the confirmation of the conductivity testing, the 7-layered sample was tested for the capability of AC signal conduction using an EEG. As the 7-layered sample has the most possibility of signal loss, this sample has been chosen for EEG testing. The 7-layered sample was tested by wet EEG to confirm the validation of the sample while developing EEG systems.  
\begin{table}
\begin{tabular}{l l l} \hline \hline
**Number of Sample Layers** & **Name of Sample Layers** & **Thickness (\(t_{s}\)) (mm)** \\ \hline \multirow{4}{*}{**Basic 4 Layers**} & Scalp & 4.34 \(\pm\) 0.11 \\  & Skull & 3.53 \(\pm\) 0.50 \\  & CSF & 3 \(\pm\) 0.0 \\  & Brain & 3.93 \(\pm\) 0.42 \\  & Compact cortical bone (top) & 1.09 \(\pm\) 0.15 \\ \multirow{2}{*}{**Skull Layers**} & Spongy or trabecular bone & 2.12 \(\pm\) 0.07 \\  & Compact cortical bone (bottom) & 1.03 \(\pm\) 0.35 \\ \multirow{2}{*}{**Brain Layer**} & Grey Matter & 1.23 \(\pm\) 0.46 \\  & White matter & 3.33 \(\pm\) 0.57 \\  & Scalp & 1.5545 \(\pm\) 0.082 \\  & Compact cortical bone & 0.40475 \(\pm\) 0.092 \\ \multirow{2}{*}{**Detailed 7 Layers**} & Spongy or trabecular bone & 1.41 \(\pm\) 0.2 \\  & Compact cortical bone & 0.3475 \(\pm\) 0.04 \\  & CSF & 1.3925 \(\pm\) 0.204 \\ \multirow{2}{*}{**Brain Layer**} & Grey matter & 0.8665 \(\pm\) 0.15 \\  & White matter & 2.08125 \(\pm\) 0.1 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Details of the as-fabricated head layers.  
Figure 2: The detailed dimensions of the single-layer sample for conductivity testing; **(a)** front view shows the inner (\(\mathrm{d_{i}}\)) and outer (\(\mathrm{d_{0}}\)) diameter differences and thickness differences of extended lips and the main sample. **(b)** The top view provides the overall sample outlook. The samples were made in a circular shape to match the top copper plate shape of the conductivity measuring machine. All the dimensions are also according to the requirements and limitations of the conductivity measuring machine. Only the inner portion (\(\mathrm{d_{i}}\)) was measured while measuring conductivity. The outer diameter (\(\mathrm{d_{0}}\)) and the thickness \(\mathrm{\SIUnitSymbolMicro}\) were built to hold liquid CSF while making a 4-layer sample.