Visualization of chemical phenomena in the microscopic world using semiconductor image sensor

<Overview>

A research team led by Professor Kazuaki Sawada and Project Assistant Professor Hideo Doi of the Department of Electrical and Electronic Information Engineering, Toyohashi University of Technology has developed a semiconductor sensor enabling the real-time observation of two types of biomolecule dynamics in solutions. By using semiconductor technology to pattern a thin metal film functioning as a neurotransmitter-sensitive membrane on sensor pixels arranged two-dimensionally in a 2 µm pitch, the sensor captures the movement of hydrogen ions and lactate (neurotransmitters) in a solution as image data. A time resolution of milliseconds and a spatial resolution of several microns (approximately 1/17 the size of a strand of hair) were achieved, and it is expected that the measurement of relation for neurotransmitters and ions distribution which changes temporally and spatially between cells with high spatiotemporal resolution.

<Details>

The brain mainly consists approximately 100 billion neurons and ten times as many non-neuronal cells (Glial cells). Action potentials generated by neurons are converted into electrical signals, transmitting information between cells. There are tiny gaps in the cell to cell junctions, and chemical substances called neurotransmitters or ions migrate between them, playing an important role in the control of brain function as chemical signals and being deeply involved in the pathophysiology of the brain. To investigate the detailed functions and pathology of the brain based on the elucidation of the transmission mechanism of chemical signals, the development of a bioimaging technology that visualizes and measures the spatiotemporal dynamics of chemical substances distributed in the extracellular space from a few microns (cellular level) to a few 100 µm (cellular population level). However, most conventional devices for chemical signal detection are of a type that has a single tiny electrode of diameter several tens of microns, or electrodes arranged at intervals of several 100 microns, and pass electric current, rendering them difficult to miniaturize and downsize. Consequently, sensors that measure the distribution of chemical information at the micron level have not been realized.

Professor Sawada's laboratory has previously developed a semiconductor image sensor that captures ion movement like a camera. This was achieved by miniaturizing and integrating potentiometric sensor pixel elements for hydrogen ion detection using semiconductor technology, with ongoing efforts to enhance the sensor's functionality. The researchers fabricated an imaging device for multisensing in which metal electrodes are formed in a lattice shape at intervals of 6 µm on the developed sensor array, and realized the real-time simultaneous measurement of lactate and hydrogen ions involved in memory formation by applying a recognition element (enzyme) that selectively detects biomolecules. In addition, recognition elements corresponding to biomolecules can be also patterned on the electrode array, enabling the simultaneous multichemical measurement of more than two types of neurotransmitters.

In the past, one type of biomolecule was measured with a single-point electrode, or one to three types of biomolecules were measured with an electrode array sensor with low spatial resolution. The newly developed feature of “capturing, visualizing, and measuring” two types of chemicals on a surface with high spatial resolution enables the detection of chemical signals in microscopic regions, such as between cells, along with spatiotemporal analysis. This advancement offers insights into chemical phenomena that were previously inaccessible using point-based sensors. In a collaborative experiment with researchers from the Faculty of Medicine, University of Yamanashi, experts in brain science and pharmacology, the response of hippocampal cells (responsible for memory) to drug stimulation was observed. The researchers simultaneously measured lactate release and extracellular pH. This breakthrough enables direct placement and measurement of cells and tissues without any labeling, allowing observation of spatiotemporal changes in ions and neurotransmitters, in biological samples. Building on this world-first ion image sensor technology, the team aims to advance its development for social implementation and medical applications.

<Future prospects>

We plan to proceed with the further enhancement of the functionality and performance of sensors to expand the range of substances to be measured. Using  brain slice and solid tumor-like diseased tissue, we will conduct applied measurements to elucidate the physiological significance of temporal and spatial changes in molecular dynamics within the extracellular environment. Furthermore, this technology holds potential for analyzing interactions among chemical substances in the complex extracellular microenvironment and verifying pharmacological effects.

<Paper information>

Hideo Doi, Hayato Muraguchi, Tomoko Horio, Young-Joon Choi, Kazuhiro Takahashi, Toshihiko Noda, Kazuaki Sawada, “Real-time simultaneous visualization of lactate and proton dynamics using a 6-µm-pitch CMOS multichemical image sensor”, Biosensors and Bioelectronics, Vol.268, 116898. DOI:10.1016/j.bios.2024.116898