This paper addresses the key issues that must be overcome to realize fully printed TFT-based flexible devicesviacommercially viable methods. In particular the threshold voltage ($V_{\rm th} $) variation in printed TFTs is a serious impediment to the successful launch of fully printed TFT-based devices in the market. The underlying causes of the$V_{\rm th} $variation in fully printed TFTs were analyzed by considering the misalignment of printed drain-source to gate electrodes, the rheology of electronic inks and effects from external sources of charge. By alleviating the influences of external sources of charge using a printed passivation layer,$V_{\rm th} $variation is maintained below 30% using a fully printed process. Based on the attainable variation range, the required number of integrated TFTs was estimated to fabricate a fully printed TFT-based radio frequency (RF) sensor device. A practical compromise enables fully printed RF sensors to be realizedviathe scalability of printing processes that mitigate$V_{\rm th} $variation by minimizing the level of TFT integration. Prototypes of fully printed RF sensors with human interactive capability—an RF sensor label, and an RF$e$-sensor (cyclic voltammetry) tag—are enabled with as few as 26 printed TFTs, demonstrating that low-cost and high throughput manufacturing of printed electronics is feasible.