Transcranial direct current stimulation: A review of electrode characteristics and materials

•Electrode size, shape and montage determine the location and size of the stimulus target region.•HD-tDCS optimizes current delivery by using algorithms to determine the amount of current though selected number of electrodes.•Pruritic sensation can be controlled by the use of appropriate electrode material and correct amount of electrolyte at the required concentration. Electrode characteristics are crucial in transcranial direct current stimulation (tDCS) since electrode design and placement determine the cortical area being modulated, current density and spatial resolution of stimulation. Early research on tDCS sought to determine optimal parameters for stimulation by specifying maximum current, duration and sizes of electrodes. Further research focused on determining efficient ways to deliver stimulation to targeted regions on the cortex with minimal discomfort to the user by altering electrode size, placement, shape and material. This review aims to give an insight on the main characteristics of electrodes used in tDCS and on the variability found in electrode parameters and placements from tDCS to high definition tDCS (HD-tDCS) applications and beyond.