This energy harvesting solution permits the autonomous supply of low power sensor nodes, without a chemical battery, allowing their extensive use in various environments. The electrical interface between the harvester and the sensor is crucial in order to maximize the harvested energy and boost the voltage to a minimum value required by the sensor. To achieve input impedance and voltage gain independently, this paper presents a flyback converter in discontinuous mode. Using the proposed flyback model validated experimentally, we have studied the impact of each loss source in order to give some trade-off for designing an efficient sub-mW harvesting interface. We underline the effect of the transformer losses due to the magnetic hysteresis as well as the driving loss impact. Following this method, the flyback prototype achieves 71% power efficiency when harvesting from a microbial fuel cell delivering 90µW. Keywords— flyback design method; energy harvesting; DC-DC converter; maximum power point tracking; microbial fuel cell