Ultra-thin chips combine flexibility and mechanical stability of silicon (20 ?m thick) and maturity of CMOS technology. However, circuit design on bendable dies brings up new challenges that are not typically present in rigid electronics. Due to bending, variable stress emerges in silicon, changing the mobility of carriers, which may lead to inaccuracy or failure of the circuit. In order to obtain a robust, precise system, bending must be considered in the design phase prior to fabrication. This work focuses on development of a precision flexible CMOS in-plane stress sensor to measure the complex stress-displacement relationship of a Fin Ray robotic gripper without introducing nonlinearity. Within this framework, a stress-aware mixed-signal design flow is developed for the first time, demonstrating the feasibility of high performance, complex circuitry on ultra-thin chips, in turn flexible foils.