A Thermally Powered ISFET Array for On-Body pH Measurement

Abstract

Recent advances in electronics and electrochemical sensors have led to an emerging class of next generation wearables, detecting analytes in biofluids such as perspiration. Most of these devices utilize ion-selective electrodes (ISEs) as a detection method; however, ion-sensitive field-effect transistors (ISFETs) offer a solution with improved integration and a low power consumption. This work presents a wearable, thermoelectrically powered system composed of an application-specific integrated circuit (ASIC), two commercial power management integrated circuits and a network of commercial thermoelectric generators (TEGs). The ASIC is fabricated in 0.35 m CMOS and contains an ISFET array designed to read pH as a current, a processing module which averages the signal to reduce noise and encodes it into a frequency, and a transmitter. The output frequency has a measured sensitivity of 6 to 8 kHz/pH for a pH range of 7-5. It is shown that the sensing array and processing module has a power consumption 6 W and, therefore, can be entirely powered by body heat using a TEG. Array averaging is shown to reduce noise at these low power levels to 104 V (input referred integrated noise), reducing the minimum detectable limit of the ASIC to 0.008 pH units. The work forms the foundation and proves the feasibility of battery-less, on-body electrochemical for perspiration analysis in sports science and healthcare applications.

A Dual-Sensing Thermo-Chemical ISFET Array for DNA-Based Diagnostics

“…Over the last few years, ISFET arrays have been integrated into portable [28] and wearable [5] platforms. These platforms are powered by batteries or power harvesting techniques, requiring architectures that can operate at scenarios where power budget and supply levels are key factors.…”

“…Detail presents CMOS-Based ISFET macromodel connected Dual-Sensing Front-End nature, catalysing the development of a wide range of ISFETbased LoC platforms. These applications range from multi-ion detection [3]- [5], where different electrolytes on a solutions are analysed; to DNA based amplification detection, with implications on rapid diagnosis of infectious diseases [6]- [8]; as well as pH-based DNA methylation detection for epigenetic monitoring [9], [10].…”

“…Finally, the need for portability has catalysed the use of power harvesting techniques and battery solutions to achieve autonomous operation [5]. However, these supply schemes require reduced power consumption as well as robust sensing under various supply conditions.…”

Temperature Compensation Circuit for ISFET Sensor

“…where I ds is from Equations (5) and (11), and the gate voltage of the sensor is deduced as follows:…”

“…Hence, the use of a junction diode Zener [4] for temperature compensation on a single ISFET sensor, the employment of an ISFET with a differential configuration (use of a differential amplifier) [8] that increases the immunity against temperature variation, or the combination of attenuators and a differential amplifier alongside with Caprio's quad [7] were used in order to enhance the temperature stability. The trend of the recent works is set on ISFET sensor arrays [9][10][11][12][13], but most of these researches are focused on the wearable and thermoelectrically powered system [11] and on achieving very high accuracy and a small settling time [10]. They also focused on system-on-chip for real-time ion imaging [13], but most of them did not focus on the temperature effect on the response of the sensor and did not give a robust solution to reduce it.…”