Fundamental neuroscience research and high-performance neuro-prostheses require large-scale brain interfaces with ever-greater spatial resolution across a large cortex coverage, which cannot be achieved with current passive (micro) electrocorticography (ECoG) technologies. In this article, we present an active micro-electrocorticography (μECoG) implant system that circumvents these challenges while achieving significantly lower noise compared to other existing active μECoG arrays. The proposed brain implant system is composed of a flexible, actively multiplexed 256-electrode μECoG array and an incremental-ΔΣ readout integrated circuit (ROIC). The 1 cm x 1 cm μECoG array was fabricated in a 3-μm IGZO thin-film transistor (TFT) technology on a 15-μm flexible foil and coupled to a 1.25 mm x 1.25 mm CMOS ROIC fabricated in a 22-nm fully depleted silicon on insulator (FDSOI) process. Due to the 256:16 time-division multiplexing achieved in the electrode array, only 16 multiplexed channels are required in the ROIC to acquire signals from the 256 electrodes simultaneously. By combining TFT multiplexing with newly proposed bulk-DAC (BDAC) feedback in the readout channel, we can integrate and address 4x more electrodes than other passive arrays, achieve >10x less noise than existing active arrays, and obtain >2x effective channel area reduction in the ROIC while maintaining comparable electrical performance over current state-of-the-art (μ)ECoG readouts.