Detachable Flexible ISFET‐Based pH Sensor Array with a Flexible Connector

Abstract

systems for IoT, economical fabrication of devices is important. At a minimum, flexible devices currently require signal processing circuits and batteries to operate sensor systems. Integrating these into a sensor while maintaining mechanical flexibility is difficult without increasing the device cost. An approach to overcome this challenge is to separate the sensor system into two film parts: a disposable one for the printable low-cost sensor array and a reusable one for the expensive systems, including circuits, battery, etc. [1-3] Because the connector combining these two components is usually inflexible, mechanical flexibility is often sacrificed. Our group has reported a flexible connector, which electrically connects two flexible sheets (i.e., disposable and reusable sheets) without sacrificing the mechanical flexibility using a liquid metal of eutectic Gallium Indium (EGaIn). [13,14] However, the metal can be readily removed from the connector by scratching it due to the liquid-state of EGaIn at room temperature, which is impractical for IoT applications including human-interactive devices. An interesting approach for sensor applications is to monitor the chemical distribution in a solution for environmental monitoring and even healthcare diagnosis from sweat. Although many studies have monitored the chemical contents in sweat for healthcare and medical applications, [1-9] chemical mapping in a solution using a flexible sensor sheet has yet to be demonstrated. If a flexible chemical sensor array is realized for mapping, applications can be expanded from healthcare/medical to biological and environmental monitoring. Such an array should also be able to map interesting data from an object for the next class of IoT concepts, which has been very difficult to collect using conventional inflexible chip-based sensors. To address the above challenges, this study proposes mechanically flexible and stable macroscale pH distribution and diffusion monitoring in solution with a flexible electrical connector. Figure 1a shows the device concept. First, a flexible connector using a soft 3D Ag electrode is discussed. As a reusable sheet, flexible extended gate ion-sensitive field-effect transistors (ISFETs) are fabricated using thin-film InGaZnO for the n-type transistors. There are two reasons to use ISFETs for pH monitoring in this study. The first one is to show the possibility

Recent advances in ion‐sensitive field‐effect transistors for biosensing applications

“…[46] Oxides play an important role and form the foundation of the ISFET gates. [20,[47][48][49] The basic principle is that the H + released from the dynamic solution will affect the pH in the well. This induces a change in the surface potential of the oxide layer as well as the potential between the gate and base of the underlying field effect.…”

A comprehensive review of FET‐based pH sensors: materials, fabrication technologies, and modeling

“…The achieved sensitivity was 66.5 mV/pH, which has negligible effect of temperature. [512] Chou et al reported a work in which they had used nickel oxide as a sensing film on flexible arrayed pH sensor. They have improved the sensitivity by introducing oxygen gas during the sputtering process and the fabricated device shows the sensitivity of 63 mV/pH for pH range of 1-13.…”

Field‐Effect Transistor‐Based Biosensor for pH Sensing and Mapping

“…[153] Honda et al designed a macroscale detachable pH sensor array that consists of a reusable, flexible ISFET array and a disposable flexible pH-sensitive membrane array through the flexible electrical connector, where ISFET is proposed to be extended gate geometry. [154] Figure 10k illustrates the optical image of a flexible 5 × 5 pH sensor array, in which polyaniline (PANI) is employed for a flexible pH-sensitive membrane because it has the capacity of low environment toxicity and can also detect protons by protonation/deprotonation effect on its film. The reaction between PANI and protons serves as potential pro-duction, leading to a variation of the transistor potential because PANI electrodes are combined with gate electrode through the flexible connectors.…”

“…Copyright 2022, American Association for the Advancement of Science. Reproduced with permission [154]. Copyright 2020, Wiley-VCH.…”