A 260 mV start-up charge pump and oscillator applying dynamic bulk-biasing (DBB) for step-up converters in thermal energy harvesting is presented and validated through SPICE simulation. This DBB employs a dynamic substrate control circuit (DSCC) directly controlled by the pump clock. It can dynamicallyreduce sub-threshold leakages during off-state and increase conduction during on-state. This is necessary for thermoelectric harvesters with output voltages of <= 300 mV. Performance of this DBB start-up circuit is compared to those of conventional reverse and forward bulk-biasing charge pumps. According to simulated results using the TSMC180 nm CMOS model, DBB reduces kick-start voltage to 225 mV. This proposed circuit has a 1.44x–52x better overall figure-of-merit (FOM) compared with conventional circuits at a supply voltageof 260 mV. It can generate 50 lW of maximum power with an unregulated output of 2.5 V within 171 ls from cold start-up. This overall improvement in speed, power and pumping efficiency facilitates battery-less solutions in biomedical applications for wearable/implanted micro-devices