This paper presents a closed-loop remote powering link for wireless cortical implants. The link operates from a single power supply at the external reader and delivers power to the implant adaptively under changing load conditions. A feedback information is sent from the implant to the external reader about the power consumption in the implant and the external reader adapts the amount of transmitted power depending on this feedback. In addition, an in vitro measurement setup is fabricated in order to characterize the performance of the wireless energy transfer when the implant is immersed into saline solution. The implant is packaged by using biocompatible materials and the operation of the remote powering link is demonstrated in air and in vitro for a wide range of load power delivered from the voltage regulator. The power transfer efficiency of the overall closed-loop remote powering link is measured to be 10.6% in vitro at nominal load power of 10 mW. Finally, the operation of the implant in vitro is demonstrated over a five-week period.