Date of Award
Neuromodulation, Neural Engineering, Transcranial direct current stimulation, Spinal cord stimulation, Deep brain stimulation
Significant contributors to the broad application of transcranial direct current stimulation (tDCS) are portability, ease-of-use, and tolerability; with adverse events limited to transient and mild cutaneous sensations (e.g. perception of burning, itching, and tingling) and erythema. However, the fundamental questions remain about the mechanism of transdermal current flow during transcranial electrical stimulation, including tDCS. Example of previously unexplained questions in tDCS include: 1) the relationship between tDCS-induced skin reddening (erythema) profile and local current density profile predicted by the model; 2) the source of burning sensation during tDCS and whether it is related to an actual skin heating; 3) the role of skin multi-layers and ultrastructures (blood vessels, sweat glands, and hair follicles) in current flow. The finite element modeling (FEM) of current flow using simplified tissue geometries predict higher current density at the electrode edge, but the experimental evidences for the cutaneous effects of tDCS (skin heating or skin reddening) are unclear. Prior skin models of cutaneous current flow lacked anatomical details that will a priori be expected to govern current flow patterns.
In this dissertation we address the aforementioned questions by: first quantifying tDCS-induced skin erythema profile alongside FEM predicting local current density profile; then assess the extent of skin heating during tDCS, including the role of joule heating, and relate temperature increase (if any) to burning sensation; and finally develop a realistic skin model to address the role of complex skin tissue layers and ultrastructures in current flow. In the first study, we conclude that the tDCS-induced skin reddening profile is diffuse, higher in active stimulation than sham stimulation, and does not occur at the electrode edges suggesting two alternate hypothesis: 1) skin reddening profile is not related to local current density; and 2) skin current density is relatively uniform, so prior FEM models are incorrect. Next, we conduct phantom measurement suggesting no significant temperature increase due to joule heat as expected at the skin during tDCS. The in vitro human skin temperature measurement suggests that independent of tDCS polarity, temperature increases by about 1oC; an increase during tDCS that is less than the cooling produced following a room-temperature sponge application during the set-up. We conclude that any incremental temperature increase by tDCS may reflect vascular flare response due to current flow, cannot exceed the core body temperature, and is more than the offset by sponge-material coolness, thus, the sensation of skin “burning” during tDCS is not related to an actual increase in temperature. In the final study, we develop a detailed multi-layer skin model including sweat glands, hair follicles, and vasculature, and assess the role of multi-layers and ultrastructures in current flow. The FEM analysis predict that sweat glands eliminates localized current density around the electrode edges, and blood vessels uniformly distribution current across the modeled vasculature under the electrode. We expect that a current flow and bioheat model of such a detailed skin would increase the uniformity of current density and temperature predicted at the skin - consistent with the experimental measurement of skin reddening and skin heating.
Khadka, Niranjan, "Influence of transdermal current flow in tDCS-induced cutaneous adverse events" (2020). CUNY Academic Works.