Ultra-Thin Chips with Current-Mode ISFET Array for Continuous Monitoring of Body Fluids pH

This paper presents a fully integrated pH-sensing system-on-ultra-thin-chip (SoUTC) designed in a 0.35-µm CMOS technology. Owing to lower Si-substrate thickness (~30μm) the presented chip can conform to curvilinear surfaces, such as human skin, allowing intimate contact necessary for robust data for continuous monitoring of analytes in body fluids. A current-mode active-pixel ISFET array architecture is presented, with a power consumption of 6.28μW per-pixel from a 2V power supply and low sensitivity to process, voltage, temperature as well as strain-induced (PVTS) variations. The silicon area occupancy of each active-pixel is 44.9 × 33.5 μm2 with an ion-sensing area of similar size. The pH sensing SoUTC can operate either in a rolling shutter fashion or in a pseudo-random pixel selection mode allowing the simultaneous detection of pH from different skin regions. From the electrical and electrochemical characterization of the proposed readout circuit on both bulky and ultra-thin version of chips, it was observed degradation in the signal-to-noise ratio (SNR) and total harmonic distortion (THD), in the latter version of the chip. The circuits have been tested in aqueous Dulbecco's Modified Eagle Medium (DMEM) solutions with 59 pH values to demonstrate their potential use for continuous monitoring of body-fluids pH.