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Measuring electric potentials using encapsulated flexible probes is advantageous especially when the signal source is comprised of soft and irregular organic tissues, such as the human brain or muscle fibers. However, the capacitive nature of encapsulated probes demands circuits with ultra-high input impedance and low 1/f noise, which are not trivially achieved using flexible transistors. Here, we show an a-IGZO based capacitive bootstrapped cascode buffer where the input impedance is actively controlled to allow capacitive electric potential measurements. By exploring different biasing schemes of the cascode element, we show a relative gain of 55 dB and a bandwidth between 450 Hz and 19 kHz.