## 2 Materials and Methods  
### Material Preparation and Properties  
Frequency-dependent contrast enhancement in EIT involves complex interactions between electromagnetic fields and materials, mainly guided by the frequency-dependent electrical properties of the substances under examination [16; 17]. The crucial electrical parameters in this context are conductivity and permittivity, which exhibit varying behaviors in response to the frequency of the applied electrical signal. Within specific frequency ranges, materials may display distinct responses to the electric field, leading to increased contrast in EIT images. This phenomenon is elucidated through materials' polarization and relaxation processes [18; 19]. At lower frequencies, the electric field induces polarization, aligning charged particles within the material. This alignment contributes to alterations in the material's electrical properties, consequently influencing impedance measurements. By contrast, at higher frequencies, relaxation processes occur as the material's charges respond to the rapidly changing electric field. The intricate interplay between these mechanisms, along with the inherent electrical characteristics of the materials, collectively shapes the observed contrast in EIT images. Furthermore, the influence of frequency on the distribution of charges within materials can induce variations in impedance, which the EIT system can delicately detect. Materials with distinct electrical properties, such as conductive and non-conductive substances, may exhibit different responses to different frequencies, highlighting their contrasts [20]. Understanding this frequency-dependent behavior is essential for fine-tuning imaging parameters, as specific frequency ranges can accentuate impedance differences between materials, resulting in clearer and more pronounced contrasts in EIT images. Essentially, the enhancement of contrast in EIT, dependent on frequency, reflects the intricate relationship between the frequency-dependent electrical properties of materials and the sensitivity of the EIT system to these variations. Delving into the specifics of this mechanism entails considering the molecular and atomic responses of materials to the applied electric field across a spectrum of frequencies, laying the groundwork for interpreting and optimizing contrast in EIT imaging. Copper, renowned for its electrical conductivity, possesses a lattice crystalline structure rich in free electrons. Its low resistivity, following standards IEC 60228 and IEC 60287, approximately \(1.7241\times 10^{-8}\)\(\Omega\cdot\) m at 20 \({}^{\circ}\)C, facilitates an efficient electron flow with minimal electrical resistance[21]. Similarly, aluminum, with a resistivity of about \(2.8264\times 10^{-8}\)\(\Omega\cdot\)m at the same temperature, is also acknowledged for its electrical conductance [22]. By contrast, acrylic resin exhibits significantly higher electrical resistivity, often exceeding \(10^{13}\)\(\Omega\cdot\)m, which is attributable to its scarcity of free electrons, which classifies it as a non-conductive material [23]. The impedance disparity between acrylic resin and conductive materials such as copper underscores a substantial contrast in their electrical properties. To ensure precise impedance measurements and the consistent reconstruction of the EIT image, cylindrical samples of copper and aluminum, each with a 20 mm diameter, were manufactured. Maintaining the quality of the surface finish and the purity of the material was imperative to minimizing the variations in resistivity caused by surface corrosion or impurities. Similarly, acrylic resin samples, identical in size to their metal counterparts, were prepared with a uniform composition to maintain consistent resistivity, averting localized impedance inconsistencies in EIT reconstructions. This study not only validates these considerations but extends their applicability to a broader spectrum of materials.