22.2 A 176-channel 0.5cm3 0.7g wireless implant for motor function recovery after spinal cord injury

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 [3] reports a 4-channel stimulator with wireless data and power links for small animal experiments, yet weighs 6g and has a volume of 3cm
³
. 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 [1]. 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 [2]. 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 [6]. However, an SoC implementation of this function has not been reported.