Continuous accurate pH measurements of human GI tract using a digital pH-ISFET sensor inside a wireless capsule

Abstract

This paper describes the design and implementation of a reliable, low-power pH-ISFET sensor inside a wireless diagnostic capsule for monitoring the pH of the human gastrointestinal (GI) tract. The sensitive element of the pH sensing device was an ion-sensitive FET (ISFET) with a threshold voltage that varies by concentration of hydrogen ions. An Ag/AgCl reference electrode that was surrounded by gel (mixed KCl) was applied in this pH-sensing device. An application-specific integrated circuit (ASIC) that is designed by a very large scale integration (VLSI) architecture was used as the interface between the pH sensing device and a wireless transceiver. Experimental results demonstrated that the pH sensitivity and operating range of the pH sensing device was 44.94mV/pH and 1–11 pH; the device is linear. With the 18-bit Analog/Digital converter (ADC) module in the ASIC, the resolution could achieve 11,780bits/pH. Moreover, the power dissipation of the pH-sensing device was only 0.048mW while working in intermittent mode (duty cycle=20%). Results from human experiments showed that this pH sensing device can work reliably for 136h, and the pH data of sampled from the human GI tract can be received by a portable data recorder outside the human body in real time.