Printed Organic Circuit Systems for Wearable Lactate Sensors
Authors: Shiwaku Rei , Matsui Hiroyuki , Nagamine Kuniaki , Uematsu Mayu , Mano Taisei , Maruyama Yuki , Nomura Ayako , Tsuchiya Kazuhiko , Hayasaka Kazuma , Takeda Yasunori , Fukuda Takashi , Kumaki Daisuke , Tokito Shizuo 
Published: February 2020

Wearable sensor devices that enable continuous real-time monitoring and analysis of biological information from the human body are promising in the fields of sports, healthcare, and medical applications. A variety of biomarkers in body fluids (such as perspiration, saliva) can be continuously detected by a wearable electrochemical sensing system, which enables in situ analysis of physiological signals. An enzyme-based amperometric sensor is one of the most important electrochemical sensors owing to the high selectivity and the connectivity to information technologies. So far, the quantitative measurement of metabolites in human fluids, including lactate, glucose, and uric acid, have been carried out using enzyme-based amperometric sensors and conventional potentiostat systems. A potentiostat is the electronic hardware which controls the three-electrode cell for electrochemical experiments and possesses two functions: (1) maintaining the potential of the working electrode (WE) at a constant level with respect to the reference electrode (RE) by adjusting the current at a counter electrode (CE), and (2) converting the current at the working electrode to the voltage by a transimpedance amplifier with a high gain. Hence, both the enzyme-based amperometric sensors and the potentiostat need to be integrated in the wearable devices.
Organic thin-film transistors (OTFTs) have potential for realizing ultra-thin, lightweight, and flexible circuit components of the potentiostat for wearable sensor devices owing to their advantages such as the small Young’s modulus, biocompatibility, and the processability of direct printing onto plastic films. Printability is an attraction of OTFTs because organic materials can be dissolved in organic solvents, which enables roll-to-roll manufacture of large-area devices on flexible substrates. These features are significant requirements for making the low-cost devices attachable onto human tissues or clothes like a patch.
So far, OTFTs have been utilized as amplifiers for potentiometric electrochemical sensors, which is called extended-gate type OTFTs. Although the potentiometric measurement is applicable to enzymatic sensors, it exhibits an irreversible and slow response in several minutes, which is not suitable to real-time sensing. On the other hand, the amperometric measurement based on enzymatic sensors exhibits a reversible and fast response in several tens of seconds. OTFTs have never been utilized for amperometric sensors since the three-electrode system requires complex integrated circuits rather than the simple extended-gate type OTFTs.
In this work, we demonstrate a novel flexible and printed organic circuit system for wearable amperometric electrochemical sensors, implemented with two OTFT-based negative-feedback inverters. The inverters employed pseudo-CMOS design for obtaining rail-to-rail operation and low output impedance, and consisted of the OTFTs based on a blend of a small molecular p-type semiconductor and polystyrene for the active layer. The first inverter was utilized to maintain the potential at the WE at a constant level with respect to the RE. The second inverter was utilized to convert the current at the WE into voltage with a tunable gain of 106–107 V/A. A lactate sensor with a lactate oxidase membrane was used as the WE. This simple system worked like a potentiostat for electrochemical measurements, and enabled the quantitative and real-time measurement of lactate concentration with high sensitivity of 1 V/mM and a short response time of a hundred seconds. These results will allow organic semiconductor devices to have potential for realizing extremely thin and lightweight wearable devices for in situ monitoring of metabolites in body fluids.