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For fully autonomous implantable or body-worn devices running on harvested energy, the peak and average power dissipation of the radio transmitter must be minimized. Additionally, link symmetry must be maintained for peer-to-peer network applications. We propose a highly integrated 90 μW 400 MHz MICS band transmitter with an output power of 20 μW, leading to a 22% global efficiency - the highest reported to date for low-power MICS band systems. We introduce a new transmitter architecture based on cascaded multi-phase injection locking and frequency multiplication to enable low power operation and high global efficiency. Our architecture eliminates slow phase/delay-locked loops for frequency synthesis and uses injection locking to achieve a settling time <;250 ns permitting very aggressive duty cycling of the transmitter to conserve energy. At a data-rate of 200 kbps, the transmitter achieves an energy efficiency of 450 pJ/bit. Our 400 MHz local oscillator topology demonstrates a figure-of-merit of 204 dB while locked to a stable crystal reference. The transmitter occupies 0.04 mm2 of active die area in 130 nm CMOS and is fully integrated except for the crystal and the matching network.