Can the "Mosquito Method" Become a Vector of Knowledge?

Takaaki Ishigure (Associate Professor, Department of Applied Physics and Physico-Informatics)

Mosquitoes have the image of being a nuisance that transmits infectious diseases such as dengue fever. Seeing the title of this article, you might have imagined it was about medical research. However, the "Mosquito method" is the name of a fabrication method for optical communication devices developed in the Ishigure Laboratory. The Ishigure Laboratory aims to realize optical circuits for signal transmission within computers, using this Mosquito method as a core technology.

An optical circuit is formed by creating a "core" circuit, which serves as the optical signal transmission path, within a thin film of transparent organic polymer (the cladding) on a printed circuit board. The process of the Mosquito method is shown in Figure 1. The tip of a syringe needle is inserted into a liquid film of cladding material coated on a substrate. While scanning the needle, a liquid monomer for the core is injected into the cladding to create a waveguide circuit pattern. It was named the "Mosquito method" due to the similarity of the "needle insertion" process.

Until now, photolithography has been the common method for fabricating polymer optical waveguides. This method forms cores with a rectangular cross-section, and these cores are limited to arrangement within a single plane. Since optical waveguides are intended to be connected to optical fibers, which have a circular cross-section core, there are concerns about optical loss due to the difference in core shape. One day, while wondering, "Isn't there a new fabrication method for waveguides that can form circular cores instead of rectangular ones?" I came up with the Mosquito method, inspired by how toothpaste maintains its cylindrical shape on a toothbrush. At that time, the student in charge of this experiment proposed the method of inserting the needle tip. Although this technique might seem unconventional for optical waveguide fabrication, it allowed for the easy creation of the desired circular cores, and thus the Mosquito method was born.

Since then, many possibilities have been discovered for the Mosquito method. Because the needle's scanning direction is not limited to a single plane, vertical core alignment (3D wiring) has become possible. Figure 2 shows a cross-sectional photograph of a multi-layered core waveguide as an example. We were also approached by a company about fabricating microfluidic channels with a circular cross-section. Figure 3 shows a top-view photograph of the fabricated microfluidic channel. This is expected to be applied to artificial blood vessels, and it was also a student's idea that led to the first successful fabrication of the branching structure necessary for blood vessels. This marked the beginning of expansion into new fields beyond communications. Furthermore, we wanted to understand the behavior of the core monomer after injection, which, again at a student's suggestion, led to a joint research project with Professor Fukagata of the Department of Mechanical Engineering, who specializes in fluids. Through fluid analysis, we have been able to quantify factors such as the position where the core is formed, and methods for controlling the core's diameter and shape. Figure 4 shows a comparison between the calculated results of the core monomer flow injected from the needle tip and the actual measured results. A very good agreement is observed. Gradually, the previously unknown aspects of the Mosquito method have begun to be clarified.

In this way, the Mosquito method is expanding beyond optical circuits into new fields. We hope it will become a "mosquito" that is appreciated as a "vector of knowledge" in various fields.