Role of skin tissue layers and ultra-structure in transcutaneous electrical stimulation including tDCS

Abstract

Background. During transcranial electrical stimulation (tES), including transcranial direct current stimulation (tDCS) and transcranial alternating current stimulation (tACS), current density concentration around the electrode edges that is predicted by simplistic skin models does not match experimental observations of erythema, heating, or other adverse events. We hypothesized that enhancing models to include skin anatomical details, would alter predicted current patterns to align with experimental observations. Method. We develop a high-resolution multi-layer skin model (epidermis, dermis, and fat), with or without additional ultra-structures (hair follicles, sweat glands, and blood vessels). Current flow patterns across each layer and within ultra-structures were predicted using finite element methods considering a broad range of modeled tissue parameters including 78 combinations of skin layer conductivities (S m–1): epidermis (standard: 1.05 × 10−5; range: 1.05 × 10−6 to 0.465); dermis (standard: 0.23; range: 0.0023 to 23), fat (standard: 2 × 10−4; range: 0.02 to 2 × 10−5). The impact of each ultra-structures in isolation and combination was evaluated with varied basic geometries. An integrated final model is then developed. Results. Consistent with prior models, current flow through homogenous skin was annular (concentrated at the electrode edges). In multi-layer skin, reducing epidermis conductivity and/or increasing dermis conductivity decreased current near electrode edges, however no realistic tissue layer parameters produced non-annular current flow at both epidermis and dermis. Addition of just hair follicles, sweat glands, or blood vessels resulted in current peaks around each ultrastructure, irrespective of proximity to electrode edges. Addition of only sweat glands was the most effective approach in reducing overall current concentration near electrode edges. Representation of blood vessels resulted in a uniform current flow across the vascular network. Finally, we ran the first realistic model of current flow across the skin. Conclusion. We confirm prior models exhibiting current concentration near hair follicles or sweat glands, but also exhibit that an overall annular pattern of current flow remains for realistic tissue parameters. We model skin blood vessels for the first time and show that this robustly distributes current across the vascular network, consistent with experimental erythema patterns. Only a state-of-the-art precise model of skin current flow predicts lack of current concentration near electrode edges across all skin layers.

The impact of individual electrical fields and anatomical factors on the neurophysiological outcomes of tDCS: A TMS-MEP and MRI study

“…Furthermore, recent studies have suggested that several other individual anatomical factors (e.g. absolute head volume and relative volume of skin, skull and CSF [56], as well as anatomical details of the skin [57]) are relevant for the tDCS-induced EF. Conceptually, EF, and especially the normal component of EF, which takes into account directional differences of EF due to stimulation polarity, should be better suited to predict physiological tDCS effects than single anatomical factors, because it is based on the contribution of all potentially relevant anatomical factors.…”

Adaptive current-flow models of ECT: Explaining individual static impedance, dynamic impedance, and brain current density

“…Current passage through the scalp (skin) depends on numerous layers and ultra-structures, each with a complex (non-linear, time-dependent) impedance to current flow [16,[38][39][40][41][42] which is computationally intractable for tES head models [21]. The pipeline developed here to simulate adaptive-scalp conductivity during tES represent two scalp layers, with just two respective associated subject-specific parameters: a deep-scalp layer with fixed conductivity (σ #! )…”

“…In this same vein, additional complexity (e.g. adding microscopic sweat ducts [19,38]) may not change the relevant clinical parameters predicted by these models.…”

Mimicking and mitigating the cutaneous response to transcranial electrical stimulation using interferential and combinatorial techniques

“…Insufficient dosing is another major limiting factor of tES due largely to anatomical and physiological differences across individuals 34,35 . Thicker layers of CSF and skull 36,37 , and a more resistive dermis 38,39 shunt more current away from the brain, causing field reductions of 100% or more across individuals at fixed doses 40 . These field fluctuations are further exacerbated by movement or misplacement of electrodes 41,42 .…”