# Electrically Equivalent Head Tissue Materials for Electroencephalogram Study on Head Surrogates
## 1 Introduction
Among all the layers, dura mater, arachnoid mater, and pia mater were not considered while making the sample. For the 4-layer head tissue model, the brain, CSF, skull, and scalp layers were considered. In the 7-layer head tissue model, white matter, grey matter, meninges, cortical bone, spongiform bone, cortical bone, and scalp layers were considered. The meninges in our 7-layered head tissue model are only represented by CSF. such single-layer head tissue models may be inadequate for understanding layer-by-layer signal conduction and loss. In [2; 4], a post-mortem human head skull with no scalp was used to study the EEG and MEG signal analysis. As the skull is one of the main barriers to ensure effective signal conduction, the role of the scalp and its conductivity cannot be ignored [14; 15]. In [2; 3], three layers of the human head model (brain-skull-pseudo' scalp) were used for testing EEG equipment and source localization techniques. The CSF layer between the brain and skull has a significant effect [17] on the EEG sensitivity measurement. It can be understood that EEG sensitivity measurement studies may be prone to errors even if three layers are used in the phantom heads. Also, in the EEG source localization calculations, the distribution of spongiform bone thickness can affect the field potential on the scalp [18]. So, any design without detailed skull bone layers might affect EEG source localization calculations. In [19], a detailed human phantom head geometry was built using agar, gelatin, agarose, etc. In the study of [19], the material processing is simpler, but the final product did not provide the skull conductivity as it is harder to reach the skull conductivity using these materials [20]. In other studies [21; 22], detailed phantom head geometry as well as the head tissue layer were built but most of the materials used were not durable and involved multiple components to create targeted geometry. To design different layers of head tissue with matching electrical properties, it is first important to understand the electrical properties of brain layers and possible mechanisms of achieving the properties using phantom materials. Based on the most recent and comprehensive study on head tissue conductivities, it is known that the conductivity of each layer varies greatly [23]. Also, as the signal generated by the human brain is an analog signal, it changes with frequency [21; 24]. Depending on the different mental tasks, the dominant frequencies and amplitudes of the sources vary. Therefore, the layers that have been manufactured for this study were tested in the targeted frequency range for conductivity. The validation of the targeted frequency range is explained in the results section in detail. There is also evidence of geometrical variations in human head sizes and the thickness of each layer of head tissues. Considering all the variations, it is not feasible to test each of them separately. Therefore, the goal within the specified frequency range is to build each layer that closely matches the suggested or median conductivities [23]. Reference [23] provides the range of conductivities of brain tissue layers based on many reported studies. A great focus is put on building a three-layer skull as the skull has the most effect on effective signal conduction [14; 15]. The other head layers considered here are the ones that impose the greatest effect on EEG analysis [15; 25]. Various researchers use different methods to construct different layers. The existing phantom materials serve two distinct purposes--one to match the mechanical properties [26; 27] and the other to match the electrical properties. Oftentimes, many of the electrically equivalent phantom head materials are not durable, which severely limits its applications. Here, we use Composite materials for constructing different phantom layers. As the composite matrix is made using polymer, the durability is high [28]. In this method, conductive short fibers are mixed with nonconductive matrix material.