A Self-Orienting Single-Chip Ingestible Pill for Electrochemical Sensing in the GI Tract

This paper presents a self-orienting ingestible pill that integrates a custom chip and an electrochemical $\mathbf{p H}$ sensor. The chip, fabricated using the TSMC 180nm CMOS process, integrates an analog front end (AFE) for 3-electrode electrochemical sensors, a closed-loop voltage-controlled oscillator (VCO)based analog-to-digital converter (ADC), a radio frequency (RF) transmitter (TX) with an off-chip loop antenna, clock and control logic, and a power management unit (PMU). The $1 \mu \mathrm{~W}$, chopper-stabilized AFE can measure sensor currents in the $\pm 29 \mu \mathrm{~A}$ range and achieves a minimum detectable current of 150 pA. The $1.2 \mu \mathrm{~W}$, time-based ADC achieves a 9.6-bit ENOB with a 100ms conversion time. The TX operates in the $402-405 \mathrm{MHz}$ Medical Implant Communication System (MICS) band, utilizing either On-Off Keying (OOK) or Frequency Shift Keying (FSK) modulation, with an average power of as little as $32 \mu \mathrm{~W}$. The chip and fabricated pH sensor are integrated onto a flexible polyimide printed circuit board (PCB) and encapsulated in a custom 3Dprinted self-orienting shell ($\varnothing 6.2 \mathrm{~mm} \times 17 \mathrm{~mm}$) that ensures the sensor consistently faces the gastrointestinal (GI) wall for more reliable measurements.