Jackson MP, Truong D, Brownlow ML, Wagner JA, McKinley RA, Bikson M, Jankord R. Safety parameter considerations of anodal transcranial Direct Current Stimulation in rats.  Brain, Behavior, and Immunity 2017 pii: S0889-1591(17)30110-1. doi: 10.1016/j.bbi.2017.04.008 Jankord_Safety_tDCS_2017

Nitsche M. Bikson M. Extending the parameter range for tDCS: Safety and tolerability of 4 mA stimulation. Brain Stimulation. Editorial, Volume 10, Issue 3, Pages 541–542, 2017 Nitche_Bikson_BrainStim_2017

And don’t forget our seminal 2016 safety review here