a,b, Schematic of measurement-system architecture (a) and photographs of the front and reverse sides of the system along with encapsulation using epoxy and PDMS (b). The printed circuit board includes a programmable load resistor (DCP; part no. ISL23315), crystal (XTAL), microcontroller (μP; part no. PIC12LF1840), radio frequency matching network (MATCH) and antenna (ANT) on the front side, and the battery (BATT) and decoupling capacitor (CAP) on the reverse. The μP receives a supply voltage (VDD) and contains a central processing unit (CPU), an analog-to-digital converter (ADC), a radio frequency transmitter (RF), a serial interface (I2C), and a temperature sensor (Temp. sens.). c, Diagram of the experimental set-up, including a photograph of the encapsulated pill in contact with gastric fluid inside the porcine stomach. d–f, In vivo power characterization for a representative device (C4) including the voltage at the point of peak power extraction during each sweep frame (d), the peak extracted power in each frame (e) and the measured body temperature (f). g, X-rays at two time points showing passage from the stomach to the small intestine corresponding to the observed drop in power in e. h, Statistical summary of the source voltage characterization data for eight deployed devices (window size, 1 h; IQR, interquartile range). i, Corresponding peak power measurements for the eight devices.