Tiny Lantern To Illuminate Blood Vessel Health — From Within

Skoltech researchers have fashioned a microstructure made of modified glass fiber that could in principle function as a tiny lantern for medical probes exploring the interior of blood vessels and other tubular cavities in the body. Described in the journal Annalen der Physik, the microlantern consists of a piece of hollow-core optical fiber — a very small piece of tube made of glass. A polymer layer and nanoparticles are deposited on the inner surface of the tube, and the ends are sealed with polymer membranes. If the membranes are topped with mirrors, the lantern will turn into a laser producing targeted light of a specific color. This can be used in photodynamic therapy to eradicate tumors sensitized with special dyes.

Endoscopic probes made of glass fiber are a promising way to reach hard-to-access regions in the body for medical imaging or therapeutic purposes. Optical fiber is thin enough to fit even within blood vessels. But there’s the question of fitting the end of the probe with the necessary instruments. The part manufactured in this study could serve either as a diffuse light source or — with further modifications and improvements — as a laser. The prospect of having a laser with a tunable wavelength at the end of an endoscopic probe is intriguing, because of the opportunities it would unlock for both diagnostics and treatment.

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The key innovation in the Skoltech study has to do with lowering the losses in light transmission, which plague setups of this kind. The light-emitting microstructure is based on a piece of hollow-core optical fiber several centimeters long, with a 0.25 millimeter inner diameter and a 0.5 millimeter outer diameter. A polymer layer is deposited on the inside of the hollow core, topped by a subsequent layer of so-called quantum dots — nanoparticles provided by Saratov State University. The fibers themselves are made by SPE LLC Nanostructured Glass Technology, also based in Saratov. The more polymer and quantum dot layers are involved, the greater the losses of light transmitted through the fiber.

“What we found is that the nanoparticles in the layered coating can be fused closer together by heat treatment, which dehydrates the polymer layers and reduces the roughness of the nanocomposite coatings and, as a result, leads to decreased light transmission losses. Remarkably, the requisite heating is achieved ‘for free’ in completing the structure for use as an optical resonator, since this entails depositing polymer membranes at both ends, topped by titanium oxide/silica layered mirrors, and that last stage involves sufficient heating,” said the study’s principal investigator Professor Dmitry Gorin of Skoltech Photonics.

The resulting system holds promise for creating optically pumped lasers operating at a fairly broad range of wavelengths from 0.3 to 6 micrometers. The quantum dots serve as the laser’s active medium, and the rest of the structure provides a resonator. The light beam comes out from the end of the cylinder-shaped microstructure, and the “color” of the light is determined by the quantum dot layer characteristics during manufacture.

Alternatively, it is possible to heat the microstructure without depositing the mirrors. While similarly resolving the transmission losses problem, this turns the structure into a diffuse source of light rather than a laser resonator. That means it will illuminate its surroundings in all directions, and the color of the light can similarly be tuned by tweaking the quantum dot layer.

Depending on what the case calls for, a probe equipped with the proposed light-emitting microstructure could be used for surface inspection, deep tissue visualization, removal of pathological tissue via so-called photodynamic therapy, etc.

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