# Evaluating Commercial Electrical Neuromodulation Devices with Low-Cost Neural Phantoms
## 1 Introduction
### Background  
Non-invasive brain stimulation for therapeutic purposes, or neuromodulation, has been consistently used to manage various neurological and psychiatric disorders [1; 3]. Non-invasive neuromodulation, unlike invasive deep-brain stimulation with implanted devices, does not require complex surgery [6; 17]. Applied electric currents have been used for non-invasive neuromodulation since ancient Egypt [18]. Contemporary techniques employed include transcranial direct current (DC) stimulation (tDCS) and amplitude-modulated transcranial pulsed-current stimulation (am-tPCS) [1; 4; 19]. Together with other electrical stimulation methods, tDCS has been extensively explored in research and medical literature for decades [20]. Conversely, the novelty of am-tPCS has precluded widespread understanding of its possible efficacy and mechanisms of action. Commercial tDCS devices apply a low-amperage direct current between a positive anode and a negative cathode [8; 21; 22]. Larger sponge pads are used to reduce the risk of burns, as well as a lower current density [9]. Discrete patterns of electrode placement, stimulation currents, and stimulation time are referred to as \"montages\". Individual montages were reported to stimulate specific brain states (for example, heightening awareness and response time). Commercial tDCS devices stimulate for approximately 20-30 min to avoid delivering potentially hazardous amounts of electrical power to the brain [6]. The dominant theory for the tDCS mechanism of action concerns lowering the resting potential of a brain region by saturating it with current, thereby facilitating action potential propagation in that region [20; 23]. The opposite case is the potential inhibition of action potential propagation while temporarily raising regional resting potential. This model has been challenged but remains widely discussed [4; 19; 24]. Conversely, am-tPCS combines other montages of electrical stimulation. In addition to tDCS, other types of transcranial electrical stimulation include transcranial alternating current (AC) stimulation (tACS), transcranial pulsed-current stimulation (tPCS), and transcranial random noise stimulation (tRNS) [25; 26]. Whereas tACS uses a varying sinusoidal electrical stimulus pattern [27], tPCS uses binary pulses of direct current, as opposed to the single, constant level used in tDCS [5; 26; 28]. Whereas tRNS uses alternating current sinusoidal stimulations with randomized frequencies and amplitudes, am-tPCS combines tDCS and tRNS, using pulses of DC and amplitude-controlled AC random noise [1]. Every 2.5 s, the polarity of the device changes; the manual claims it can entrain endogenous oscillations [29]. The mechanism of action was hypothesized to combine aspects of tDCS and tRNS, ensuring that neurons acclimate less easily to a constant DC [28]. Live test data on am-tPCS are limited. The stimulation modalities also generate different localized polarizations in the electric field. For example, the sinusoid shape of tACS waveforms may travel at different speeds through different media [27]. Even time-varying measurement at two points may not capture the entire profile of electrical activity. A finite element-based method, even involving relatively direct arithmetic, can offer a potential way to estimate local electric field activity. Prior approaches have involved complex software simulations and multi-electroderecording systems [30; 31; 32]. Simplifying the requirements for electric field measurement and characterization would streamline high-resolution electric field measurements. Animal and clinical human trials can be costly and potentially dangerous [9]. Electrical stimulation, even those that are non-invasive, can endanger a living being [33]. From atrial fibrillation to burns, the possible mechanisms of harm are well documented [6; 9]. Animal and human studies also have a greater cost in time, adding another potential variable to this study. Tissues donated by a living organism have a limited lifespan without specialized care.