# Evaluating Commercial Electrical Neuromodulation Devices with Low-Cost Neural Phantoms
## 2 Materials and Methods
### Phantoms  
Three materials were examined for use as brain phantoms: ground beef, agar gel mixed with salt, and PBS [10; 12; 13]. Each material has been used in at least one peer-reviewed study, including those with successful subsequent animal and human trials. Ground beef, purchased directly from a supermarket, has been used as a phantom in several studies. Agar gel, or ballistic gel, was mixed with salt in different ratios: 1%, 5%, and 10%. PBS was mixed with boiling water and used to model a brain. The preparation times and costs (in US dollars) were compared. As shown in Table 1, the materials with the lowest cost and preparation time were used.  
Each material was prepared according to methods reported in prior research. Agar gel was measured with respect to the weight of distilled water in the container, and salt was added in proportions of 1%, 5%, and 10% of the total agar gel mass [12]. In the case of the 10% mixture, the agar gel was prepared by heating 0.95 L of distilled water, used to dissolve with 108 g salt. A total of 59.2 mL of agar gel powder was mixed in. The mixture was stirred and boiled for 5 min on a stovetop at 100 \({}^{\circ}\)C. It was removed from the stove, left to cool for 5 min, and poured into containers. It was refrigerated overnight. It was warmed to room temperature (25 \({}^{\circ}\)C) before experimentation [12]. While ground beef has variable composition, the same brand was purchased from the same supermarket for all tests. Prior studies with ground beef reported that despite its variable composition, it was sufficiently homogenous in its electrical properties [11; 37]. PBS was boiled in distilled water. Agar gel was prepared in batches 24-48 h before the experiment and refrigerated. Beer was purchased the day of the experiment, and emptied into half the container's volume. PBS was prepared shortly before experimental usage. Containers with the same volume (1280 mL) were used in all cases. Each sample in a container was reused until all measurements of its material were completed.  
The conductance and impedance of each material was documented elsewhere. The brain itself has a conductivity of 0.26 S/m for gray matter and 0.17 S/m for white matter, but the skin resistance must be accounted for with non-invasive neuromodulation [38]. PBS mimics the ion concentrations and pH inside the human body, with an impedance of 33 k\(\Omega\) at 0.5 Hz. It has been used repeatedly in stimulation phantom models [10; 39]. Meat products, such as ground beef, have an impedance and conductivity dependent on power dissipated [37]. The reported electrical conductivity of meat ranged from 0.01-10 S/m. At room temperature (25 \({}^{\circ}\)C), the electrical conductivity of ground beef was reported as approximately 1.6 S/m [37]. Others have reported success using ground beef as a brain phantom in regard to both its electric and thermal properties [11]. Agar gel mixed with salt has long been used as an electrical brain phantom, although the electrical properties depend on the salt [40]. The measured electrical resistance was at 200-300 \(\Omega\). The addition of even 10% salt (NaCl) was reported to drop impedance by a factor of 14 [12]. Although skin conductance was not factored into these prior studies, published research reported that all three materials should sufficiently act as brain phantoms for room temperature experiments conducted with low-powered neuromodulation devices. While a 3D measurement technique could detect electric fields at a high resolution across the phantom, different stimulation devices would generate different profiles, even at a low power stimulation.