Epidural spinal stimulation has shown effectiveness in recovering the motor function of spinal cord transected rats by modulating neural networks in lumbosacral spinal segments [1, 2]. The state-of-the-art neuromodulation implant  reports a 4-channel stimulator with wireless data and power links for small animal experiments, yet weighs 6g and has a volume of 3cm3. It is preferable that the implant package has a comparable size to its bioelectronics and a high-density stimulator to support stimulation with high spatial resolution. Furthermore, the epidural electrode should be soft and flexible because a mechanical mismatch exists at the tissue-electrode interface . Unlike other implant/SoCs that stimulate with pre-loaded patterns [4-5], the implant for motor function recovery should be capable of adaptively adjusting its stimulation patterns at run time in response to the subject's varying physiological states . Measuring the electrode-tissue impedance is also critical to ensure safe stimulation. Deriving the equivalent circuit model of the electrode-tissue interface determines the safe stimulation boundary (i.e. pulse width and intensity) to ensure the electrode overpotential is within the water window . However, an SoC implementation of this function has not been reported.