This randomized, double-blind, sham-controlled clinical trial aims to evaluate the efficacy and underlying biological mechanisms of HD-tDCS targeting the primary somatosensory cortex in adolescents with bipolar depression. Participants will be randomly assigned to receive either active HD-tDCS or sham stimulation, in addition to routine clinical care. Biological data, including neuroimaging, blood biomarkers, voice and facial features, Photoplethysmography (PPG), Electroencephalography (EEG), and behavioral data, will be collected to explore potential predictors of treatment response.