Minimal Heating at the Skin Surface During Transcranial Direct Current Stimulation.

Area of Science:

• Neuroscience
• Biophysics
• Biomedical Engineering

Background:

• Transcranial direct current stimulation (tDCS) is a non-invasive brain stimulation technique.
• Understanding the physiological effects of tDCS, such as thermal changes, is crucial for safety and efficacy.
• The precise mechanisms behind tDCS-induced temperature variations remain under investigation.

Purpose of the Study:

• To determine if tDCS causes skin surface temperature changes.
• To investigate if these temperature changes are specific to stimulation polarity (anode or cathode).
• To differentiate the contributions of passive heating (joule heat) versus active physiological responses (blood flow) to any observed temperature changes.

Main Methods:

• Experiments were conducted using an agar phantom and in vivo forearm stimulation with 20 volunteers.
• Finite element method (FEM) multiphysics models simulated current flow and bioheat transfer in skin and phantom models.
• Temperature was measured using thermocouples during pre-stimulation, stimulation, and post-stimulation phases.

Main Results:

• In phantom studies, tDCS (anode or cathode) showed no significant temperature difference (<0.1°C) compared to control.
• In vivo, tDCS at 2 mA resulted in a gradual skin temperature increase (e.g., 0.9°C anode, 1.1°C cathode at 20 min) compared to control (0.05°C).
• FEM models predicted minimal temperature changes in the phantom but validated the observed in vivo temperature increases in skin models.

Conclusions:

• tDCS at 2 mA induces a moderate, non-hazardous increase in skin surface temperature.
• The observed temperature increase is independent of stimulation polarity.
• The primary driver of this temperature change is stimulation-induced blood flow, not joule heating.