Major depressive disorder (MDD) is a significant public health problem. Existing treatment modalities are not always sufficient to alleviate this disorder. Treatment refractoriness is a common clinical problem. Transcranial direct current stimulation (t-DCS), a non-invasive brain stimulation technique, has been shown to be effective in alleviating depressive symptoms in preliminary studies. There is need to explore the role of t-DCS in Treatment-resistant depression (TRD). Therefore, the investigators aim to undertake this exploratory study.