16.8 nW Ultra-Low-Power Energy Harvester IC for Tiny Ingestible Sensors Sustained by Bio-Galvanic Energy Source.Herein, we present a 16.8 nW ultra-low-power (ULP) energy harvester integrated circuit (IC) for ingestible biomedical sensors.  The energy harvester can be powered from the electro-galvanic operation inside a human body, which provides a sustainable and long-term energy source.  The challenge of dealing with relatively high input impedance (∼kΩ) of the bio-galvanic energy source is addressed by introducing two design techniques.  The first technique is an adaptive VMPP-controlled algorithm (AVCA) for a maximum power point tracking (MPPT) controller, and the second technique is a ULP delay-line-based zero current switching (ZCS) controller.  Different from the conventional fractional open-circuit voltage (FOCV) method for MPPT, the proposed AVCA allows continuous source tracking without detachment of the harvester from the source.  The ZCS operation is achieved using a delay-line controller without using either a comparator or an opamp.  The proposed AVCA is realized using a 12.1 nW MPPT controller.  Successful ZCS operation is achieved using a 2.1 nW delay controller.  Overall power consumption of the IC is 16.8 nW.  The converter has been fabricated in a 0.18 μm CMOS process with 2 μm thick top-metal option.  The measured result shows that the converter achieves a peak efficiency of 72.1% to generate 507 nW output power.  The ULP operation allows a significant reduction in electrode size down to the submillimeter scale (∼0.4 mm2), demonstrating the good potential of the proposed energy harvester IC.
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