Saliva Stress-Marker Monitor Using Aptamer-Immobilized FET Biosensor
Authors: Ohashi Keishi , Kuroiwa Shigeki , Toyama Ryo , Hayashi Hiroki , Momma Toshiyuki , Osaka Tetsuya 
Published: May 2020

Introduction

People have felt that their mind controls the body. Advances in immunology and neuroscience are scientifically elucidating this experience. For example, it has been clarified that the mechanism by which changes in the activity of the central nervous system due to stress regulate the immune response through sympathetic nerves. If simple monitoring of stress-related substances is realized by utilizing such medical advances and advances in electronic devices, we can expect to help prevent many diseases. Nevertheless, the medically established technique of collecting blood for the detection of stress hormones, cortisol and catecholamine has the disadvantage of causing stress by the collecting action itself. Instead, cortisol concentration measurement from saliva has been developed as a non-invasive detection, but catecholamines cannot be detected from saliva. Salivary α-amylase and chromogranin A (CgA) have also been studied and used in part as alternatives to catecholamines. Secretary immunoglobulin A (s-IgA) in saliva is also a stress marker that reflects suppression of the immune system by stress. Simultaneous monitoring of time-dependent changes in these stress markers of different origins is expected to be an important technique for mental and physical health [1].

Biosensor module platform

Precise but inexpensive biological material detectors are required for IoT biosensing system, and it has not been possible to monitor them over time. A field effect transistor (FET) biosensor is a compact device that detects many types of biomarkers with low power consumption without disturbing the system under test [2]. The biggest challenge of FET biosensors when used in electronics systems is instability due to current drift. We have succeeded in developing a method that minimizes drift using only the normal silicon fab process. This manufacturing process does not use tantalum pentoxide or other special materials.
Figure 1 shows a photo of a newly developed four-element FET sensor chip with very low instability. The electronics part of the biosensor module consists of this chip and a Bluetooth Low Energy (BLE) type communication function. We selected aptamers as sensor receptors on the gate insulator film on the chip [3]. Aptamers can be stored and used at room temperature for a long period of time. They also have the advantages of being reversible to thermal denaturation, and can be produced inexpensively and industrially. Finally, as a technology for manufacturing biosensors with as little variation as semiconductor devices, we have developed a tool that uniformly immobilizes receptor monolayers in a narrow place at a fixed position on a chip.

Simultaneous detection of stress markers in saliva

When n types of receptors are immobilized on n FET elements, n − 1 types of independent signals can be extracted. The effects of non-specifically adsorbed substances and pH in saliva, temperature fluctuation, optical noise, and crosstalk between elements can be eliminated from the original signals of multiple FET elements. Using this technique, we succeeded in obtaining multiple stress marker concentrations such as cortisol, α-amylase, s-IgA, and CgA [4], at the same time just by dropping saliva on the sensor. The signal time constant is less than 1 minute, which indicates that a continuous monitor is realized substantially.

Operability equivalent to physical sensor

Internet of Things (IoT) systems require a large number of low-cost sensors. An FET sensor chip manufactured using only the normal silicon fab process achieves a low cost of about $ 1 per chip. However, if a biosensor is disposable, the cost burden on the user will increase significantly over the long term. As a result, it becomes difficult to secure good customers as fixed users. In addition, disposable chips are not suitable for continuous monitoring required for medically important data. We are developing an easy-to-reuse cleaning method and a recycling precision cleaning method with the aim of securing customers by realizing biosensor technology with the same operability as conventional physical sensors.

References

[1] L. Steinman, Annu. Rev. Immunol., 32, 257-281, (2014).
[2] K. Ohashi, T. Osaka, ECS Transactions, 75, 39, 1-9, (2017).
[3] N. Kaneko, H. Minagawa, J. Akitomi, K. Ohashi, S. Kuroiwa, S. Wustoni, S. Hideshima, T. Osaka, K. Horii, I. Waga, The 43rd International Symposium on Nucleic Acids Chemistry, 2P-55, (2016).
[4] S. Kuroiwa, R. Takibuchi, A. Matsuzaka, S. Hideshima, N. Kaneko, H. Minagawa, K. Horii, I. Waga, T. Nakanishi, K. Ohashi, T. Momma, T. Osaka, 232nd ECS Meeting, 2115, (2017).

Figure 1