# Electrically Equivalent Head Tissue Materials for Electroencephalogram Study on Head Surrogates
## 3 Experimental Procedure  
### Sample Preparation  
The composites were all mixed in a weight-to-weight ratio. The matrix was PDMS, and the filler was carbon fiber (CF). The PDMS itself is an insulator material. The CF is a conductive filler. The concentration of carbon fiber affects the conductivity of the composite. The length of CF was kept at 1000 \(\upmu\)m and the width was 40 \(\upmu\)m. The CF was chopped using a commercially available stainless steel grinder at 1200 RPM. Each batch of chopped carbon fiber was then measured using a microscope to ensure the length of the fiber. The PDMS and carbon fiber were mixed at room temperature using a commercial resin mixer at 120 RPM for 30 min. Then, the hardener was measured and poured into the mixer, and mixed for another 15 min. The samples were poured into a mold and the degassing chamber was used to clear all air pockets inside the samples. The mold for the sample was printed using a 3D polymer printer. Each mold filled with the mixture was kept for 36 h at room temperature to cure. For each wt%, a minimum of 5 samples were manufactured and tested. The top copper plate of the sample holder in dielectric spectroscopy has a radius of 12.5 mm, and the bottom plate has a radius of around 20 mm. So, the sample was designed accordingly. The dielectric spectroscopy would measure only \(\frac{\pi\text{d}_{\text{i}}\text{t}}{4}\) volumes of the sample. The extended lips of thickness \(\text{t}_{\text{i}}\) and diameter \((\text{d}_{0}-\text{d}_{\text{i}})\) were designed to hold the liquid CSF while manufacturing a 4-layer head tissue model sample, seen in Figure 3a. For the 4-layer sample, the CSF was made by combining NaCl and DI water of desired conductivity [30]. The CSF was then injected into the hollow space between the skull and the brain layer. To ensure water-tight sealing between the layers, a thick layer of sealant was used. The sealant was made with a PDMS and CF (5 wt%) composite. The sealant layer was also dried for a day before injecting the NaCl and DI water mixture. For the 7-layer head tissue model's multilayer samples, seen in Figure 3b, the sample diameter was kept at 40 mm and the thickness was varied for each layer. It is observed that the typical skull thickness varies between men and women across different regions: frontal (men: 7.8 mm; women: 8.6 mm), parietal (men and women: 10.1 mm), occipital (men: 10.1 mm; women: 10 mm), and temporal (men and women: 6 mm) [31]. Additionally, the normal scalp thickness for adult humans is about 5.8 mm [32]. Cerebrospinal fluid (CSF) thickness ranges from 2.1 \(\pm\) 0.9 mm to 6.3 \(\pm\) 1.7 mm [33]. Gray matter (38 mm \(\pm\) 12.7 mm) and white matter thickness (25.1 mm \(\pm\) 11.3 mm) also vary, depending on the head surface location [34]. To maintain similar human head geometry, the minimum total thickness of a 7-layer model would be 77 mm. The maximum thickness for samples in dielectric spectroscopy is 11 mm. Variations in human head tissue thickness have an impact on the conductivity of specific layers. The variations in the thickness of human head tissues affect the conductivity of the specific layer [35; 36]. Given the available resources, greater emphasis has been placed on matching electrical conductivity than on matching geometric thickness, as conductivity is the most important property that affects EEG research the most [7]. The thickness of each layer of the prepared sample is shown in Table 1. For each multilayer sample, the bottom layer has been prepared first. After curing the bottom layer, the next layer according to Figure 1 has been prepared and cured. Each layer has been measured using a digital screw gauge after curing. While pouring the sample solution into the mold, a measuring scale was used to determine the approximate thickness of each sample to ensure a consistent thickness. For multilayer samples, the newly produced layer thickness was obtained by subtracting the preceding layer's thickness from the overall thickness of the measured layers.