Among the metal oxide semiconductors, ZnO has been widely investigated as a channel in thin film transistors (TFTs) due to its excellent electrical properties, optical transparency and simple fabrication via solution processed techniques. Herein, we are reporting a solution processable ZnO based thin-film transistor, gated through a liquid electrolyte having an ionic strength comparable to that of a physiological fluid. The surface morphology and chemical composition of the ZnO films upon exposure to water and phosphate buffer solution (PBS), are discussed in terms of operation stability and electrical performance of the ZnO TFT devices. Improved device characteristics upon exposure to PBS are associated with the enhancement of the oxygen vacancies in ZnO lattice, possibly due to Na+ doping. Moreover, dissolution kinetics of ZnO thin film in liquid electrolyte opens to possible applicability of these devices as active element in “transient” implantable systems.