Wearable medical sensors gain sensitivity

In the proposed biosensor, the waveguide is inside the dielectric substrate. The resonator is positioned at the interface between the dielectric material and the biological fluid that is analyzed.
In the proposed biosensor, the waveguide is inside the dielectric substrate. The resonator is positioned at the interface between the dielectric material and the biological fluid that is analyzed. A change in the fluid's refractive index shifts the resonant curve. (Kirill Voronin, Sensors)

Biosensors could be integrated into smartphones, smart watches, and other gadgets in the not-too-distant future. Researchers from the Moscow Institute of Physics and Technology, in a paper featured in the January issue of Sensors, describe a way to increase the sensitivity of biological detectors to the point where they can be used in mobile and wearable devices.

Up to now, biosensors have not been considered for consumer products because the available devices were not sensitive enough and were prohibitively expensive for the consumer market. But researchers from the MIPT Center for Photonics and 2D Materials have proposed a new biosensor design, which could increase detector sensitivity many times over and offer a similarly impressive reduction in price.

"A conventional biosensor incorporates a ring resonator and a waveguide positioned in the same plane," explained MIPT graduate student Kirill Voronin from the Laboratory of Nanooptics and Plasmonics, who came up with the idea used in the study. "We decided to separate the two elements and put them in two different planes, with the ring above the waveguide."

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The reason researchers did not test two-level sensor layout before is that manufacturing a flat, single-level device is easier in a laboratory setting. By depositing a thin film and etching it, both a ring resonator and a waveguide are produced at the same time. The alternative two-level design is less convenient for prototype devices, but it turned out to be cheaper for mass-producing sensors. The reason for this is that the technological processes at an electronics plant are geared toward layer-by-layer active component placement.

The two-tier biosensor design resulted in higher sensitivity. A biosensor operates by registering the slight changes in the refractive index at its surface, which are caused by organic molecule adsorption. These variations are detected via a resonator whose resonance conditions depend on the refractive index of the external medium. Since even the slightest fluctuations in the refractive index cause a significant resonant peak shift, a biosensor responds to nearly every molecule that lands on its surface.

"We have positioned the strip waveguide under the resonator, in the bulk dielectric," said paper co-author Aleksey Arsenin, a leading researcher at the MIPT Laboratory of Nanooptics and Plasmonics. "The resonator, in turn, is at the interface between the dielectric substrate and the external environment. By optimizing the refractive indices of the two surrounding media, we achieve a significantly higher sensitivity."

The proposed biosensor layout has both the source and the detector of light within the dielectric. The only part that remains on the outside is the sensitive element.

Valentyn Volkov, who heads the MIPT Center for Photonics and 2D Materials, estimates that it will take about three years to develop an industrial design based on the proposed technology.

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