This study is the first report demonstrating proof-of-concept for a hydrogel-based touch sensor pad used for the non-invasive extraction and detection of sweat components. The sensor device was composed of an electrochemical L-lactate biosensor covered with an agarose gel in a phosphate buffer saline. When human skin contacts the agarose gel, L-lactate in sweat was continuously extracted into the gel, followed by in-situ potentiometric detection without controlled conditions. This novel type of sweat sensor is expected to enable the simple, non-invasive daily periodic monitoring of sweat biomarkers for advanced personal healthcare methods in the future.

Wearable sensor device technologies, which enable continuous monitoring of biological information from the human body, are promising in the fields of sports, healthcare, and medical applications. Further thinness, light weight, flexibility and low-cost are significant requirements for making the devices attachable onto human tissues or clothes like a patch. Here we demonstrate a flexible and printed circuit system consisting of an enzyme-based amperometric sensor, feedback control and amplification circuits based on organic thin-film transistors. The feedback control and amplification circuits based on pseudo-CMOS inverters were successfuly integrated by printing methods on a plastic film. This simple system worked very well 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.

Flexible and printed biosensor devices can be used in wearable and disposable sensing systems for the daily management of health conditions. Organic thin-film transistors (OTFTs) are promising candidates for constructing such systems. Moreover, the integration of organic electronic materials and biosensors is of extreme interest owing to their mechanical and chemical features. To this end, the molecular recognition chemistry-based design for the interface between sensor devices and analyte solution is crucial to obtain accurate and reproducible sensing signals of targets, though little consideration has been given to this standpoint in the field of device engineering. Here, we report a printed OTFT on a 1 μm-thick film functionalized with a sensing material. Importantly, the fabricated device quantitatively responds to the addition of a protein immunological marker. These results provide guidelines for the development of effective healthcare tools.

A printed extended‐gate‐type organic transistor‐based enzyme sensor for the continuous monitoring of D‐glucose has been developed through a simple modification of the electrical circuit. The sensor is composed of a glucose oxidase (GOx)/Prussian blue (PB)‐modified extended‐gate electrode and an Ag/AgCl reference electrode, which are connected to the gate and source electrodes of an organic transistor, respectively. The GOx/PB‐modified extended‐gate electrode and Ag/AgCl reference electrode were short‐circuited by an external resistor to spontaneously induce redox cycling of PB. Continuous monitoring of the source‐drain current of the organic transistor for various concentrations of D‐glucose was successfully demonstrated in this system. This is the first report of the transistor‐based enzyme sensor exhibiting reversible response to the analytes in physiologically buffered solution without the addition of any chemical reagents. This new type of printed organic transistor‐based enzyme sensor will greatly contribute to the development of wearable biosensors for non‐invasive monitoring of biomarkers in externally secreted bodily fluids such as tears, saliva, and sweat.

Electrochemical sensor systems with integrated amplifier circuits play an important role in measuring physiological signals via in situ human perspiration analysis. Signal processing circuitry based on organic thin-film transistors (OTFTs) have significant potential in realizing wearable sensor devices due to their superior mechanical flexibility and biocompatibility. Here, we demonstrate a novel potentiometric electrochemical sensing system comprised of a potassium ion (K+) sensor and amplifier circuits employing OTFT-based pseudo-CMOS inverters, which have a highly controllable switching voltage and closed-loop gain. The ion concentration sensitivity of the fabricated K+ sensor was 34 mV/dec, which was amplified to 160 mV/dec (by a factor of 4.6) with high linearity. The developed system is expected to help further the realization of ultra-thin and flexible wearable sensor devices for healthcare applications.