Abstract
This paper presents ISFET array based pH-sensing system-on-ultra-thin-chip (SoUTC) designed and fabricated in 350 nm CMOS technology. The SoUTC with the proposed current-mode active-pixel ISFET circuit array is desined to operate at 2 V and consumes 6.28 μW per-pixel. The presented SoUTC exhibits low sensitivity to process, voltage, temperature and strain-induced (PVTS) variations. The silicon area occupancy of each active-pixel is 44.9 × 33.5 µm2 with an ion-sensing area of 576 µm2. The design of presented ISFET device is analysed with finite element modeling in COMSOL Multiphysics using compact model parameters of MOSFET in 350 nm CMOS technology. Owing to thin (∼30 µm) Si-substrate the presented SoUTC can conform to curvilinear surfaces, allowing intimate contact necessary for reliable data for monitoring of analytes in body fluids such as sweat. Further, it 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. Finally, the circuits have been tested in aqueous Dulbecco's Modified Eagle Medium (DMEM) culture media with 5-9 pH values, which mimics cellular environments, to demonstrate their potential use for continuous monitoring of body-fluids pH.
Highlights
Wearable microsystems with sensors and integrated signal processing and data transmission capability could boost digital health by providing non-invasive tools for continuous and real-time monitoring of health parameters [1, 2]
In terms of the figures of merit (FoM) of the circuit on ultrathin chips (UTCs), the mean signal-to-noise ratio (SNR) dropped by 5.16 dB and the total harmonic distortion (THD) increased by 1.31 dB as compared to bulky or conventional chips
The 8-T pixel architecture ensures proper ion-sensitive field-effect transistors (ISFETs) biasing and high output impedance while the non-ideal effects of drift and strain-induced variations are compensated with a correlated double sampling (CDS) architecture
Summary
Wearable microsystems with sensors and integrated signal processing and data transmission capability could boost digital health by providing non-invasive tools for continuous and real-time monitoring of health parameters [1, 2] This is aided by recent progress in flexible electronics as they allow such sensor systems to conform to curvilinear surfaces, such as human skin, which is necessary for robust physiological measurements [3, 4]. Some of these technological advances have been already exploited to develop devices for real time monitoring of various health parameters such as heart rate [5], blood oxygen and pressure [6, 7], respiration rate [8], body posture [9], skin stretching [10], skin temperature [11, 12] and brain activity [13]. The array programmability can be utilized for recording small variations in pH while accounting for electrochemical drift via a compensation circuit
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