(Invited) Wet Interfacing Technique for Physiological Chemical Sensing
Authors: Nagamine Kuniaki 
Published: December 2024

Since the COVID-19 pandemic, the importance of simple and miniaturized chemical sensors, immunochromatographic kit that use body fluids collected with little physical pain has been increasingly recognized in daily healthcare. Users can operate the kit by themselves and understand their own physiological condition from the easily recognizable colorimetric signal. On the other hand, as there is a shortage of workers in the agricultural field, the sensing technology for digital management of the crop growth and health is also becoming increasingly important. Recently, our research has been focused on a wet-interfacing technique [1]. This technique had been proposed in the research field of analytical chemistry to enable the extraction of chemical compounds from the human skin by simply wetting the human skin surface with water. We integrated this wet interfacing technique with the conventional miniaturized chemical sensors to establish the new non-invasive or non-destructive sensors for continuous transepidermal extraction and detection of the chemical components in the human body or plant leaf. The wet interface is composed of a hydrogel or paper impregnated with water to be handled as solid materials.
For the human healthcare, the sweat components derived from blood have been expected as biomarkers for indirectly understanding the physiological conditions as an alternative to the blood test. The hydrogel touchpad-based sensors for the sweat components are fabricated by embedding various miniaturized chemical sensors into a hydrogel. The users allow the chemical sensing of their sweat components only by touching the hydrogel. The sweat compounds extracted into the hydrogel was detected by the embedded sensors in situ. Since our first report on at-rest sweat lactate sensor in 2019 [2], various types of hydrogel touchpad-based sweat sensors have been proposed for many target compounds using different types of sensing materials and gels [3,4].
The non-destructive plant healthcare devices have also been proposed mainly by the optical sensors based on fluorescence, Raman, Near-infrared, and X-ray spectroscopy. On the other hand, there are few reports on the chemical sensors for non-destructive in vivo sensing of the chemical components in the plant body. Now, we are applying the wet interfacing technique overcome this challenging. A thin film type of potassium sensor was contacted with the surface of plant leaf via a filter paper in phosphate buffer saline solution. As a result, potassium ions in the leaf were non-destructively and continuously extracted, followed by detecting in real time [5]. Another application was for early detection of pathogenic disease of plant. We succeeded in extracting and detecting a blue fluorescent substance, antibacterial chlorogenic acid, as an indicator of early symptom of bacterial wilt in tomatoes from infected leaves using a hydrogel touchpad [6].
We think that these wet-interfacing techniques are quite important for the future advanced healthcare not only for human but also crops. In this presentation, we will introduce the recent progress of wet-interfacing technique-based chemical sensors applied for human healthcare and smart agriculture.

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