Filtering pollution: A microfluidic device for collecting microplastics via acoustic focusing

Microplastics (MPs), which are plastic debris particles smaller than 5 mm, have become a significant environmental issue. These MPs are generated through the degradation of plastic waste due to factors such as wear and tear and exposure to sunlight, as well as from fiber waste in laundry wastewater and microbeads in beauty products. MPs have the ability to adsorb and introduce harmful chemicals into the environment, leading to pollution. In fact, it is projected that by 2050, the quantity of MPs in the oceans could surpass the number of fish. Given this alarming scenario, the collection and removal of MPs from water sources have become critical environmental priorities. Proper management of MPs is crucial in order to mitigate their harmful effects on marine ecosystems and safeguard the health of our oceans.

Traditionally, MPs are obtained by filtering water through mesh screens. However, this method has several drawbacks. Sand and biological debris must be separated from the MPs using density separation and chemical treatment, which is time-consuming and labor-intensive. Additionally, the mesh screens can easily become clogged and are unable to capture particles smaller than their openings. They also require frequent maintenance and are costly. As a result, scientists have turned to microfluidic devices, which use acoustic focusing in micrometer-sized channels, as an alternative approach to collect MPs.

Acoustic technology utilizes ultrasonic waves to transport MPs to the center of the fluid stream, enriching the collected amount of MPs. However, current microfluidic devices require repeated recirculation of fluids to achieve high enrichment of MPs. To address this limitation, a team of researchers, led by Professor Yoshitake Akiyama from the Department of Mechanical Engineering and Robotics at the Faculty of Textile Science and Technology at Shinshu University, has developed a high enrichment device specifically designed for 10-200 μm MPs.

The device, developed by Professor Yoshitake Akiyama’s team, including co-author Professor Hiroshi Moriwaki from the Department of Applied Biology, Faculty of Textile Science and Technology at Shinshu University, has been detailed in a study available online since March 26, 2023. The study is set to be published in Volume 315 of the Separation and Purification Technology journal on June 15, 2023.

“Our microfluidic device, based on a hydraulic-electric analogy, features three microchannels that are 1.5 mm wide, connected via four trifurcated junctions that are 0.7 mm wide each,” explains Professor Akiyama as he describes the device’s design. “Through the use of a bulk acoustic wave with a resonance frequency of 500 kHz, the MPs are aligned at the center of the middle microchannel. This results in a 3.2-fold enrichment of MPs at each junction, leading to an overall enrichment of 105-fold within the device.” The MPs are collected from the middle branch of the trifurcated junctions, while the remaining MP-free fluid is removed from the other branches.

To assess the collection performance of their device, the researchers conducted measurements of total collection rates for microparticles with diameters of 5, 10, 15, 25, 50, and 200 μm. The collection rates were found to exceed 90% for all particle sizes, except for 5 μm microparticles which were too small to be acoustically controlled. Furthermore, the researchers also tested the device using two different sample water mixtures: one with small MPs ranging from 25-200 μm, and another with very small MPs ranging from 10-25 μm. The collection rates for these mixtures ranged from 70-90%, with the actual enrichment of MPs varying from half of the design value of 105 to the design value itself.

Although some MPs were found to slow down and clog the microchannel walls of the device by acoustic radiation force, the researchers believe these minor limitations can be easily addressed through pre-filtration and by improving 2D focusing.

Professor Akiyama concludes with optimism, “Our proposed microfluidic device based on acoustic focusing offers efficient, rapid, and continuous collection of 10-200 μm MPs without the need for recirculation after pre-filtration through a mesh. It can be easily installed in washing machines, factories, and other sources of MPs, allowing for efficient enrichment and removal of MPs from laundry and industrial wastewater. This technology has the potential to prevent the discharge of MPs into the environment, offering a promising solution for mitigating the issue of MP pollution.”

We have our fingers crossed for a microplastic-free cleaner environment, thanks to this useful invention by Shinshu University researchers.

About Shinshu University

Shinshu University is a national university founded in 1949 located nestling under the Japanese Alps in Nagano known for its stunning natural landscapes. Our motto, “Powered by Nature – strengthening our network with society and applying nature to create innovative solutions for a better tomorrow” reflects the mission of fostering promising creative professionals and deepening the collaborative relationship with local communities, which leads up to our contribution to regional development by innovation in various fields. We’re working on providing solutions for building sustainable society through interdisciplinary research fields: material science (carbon, fiber and composites), biomedical science (for intractable diseases and preventive medicine) and mountain science, and aiming to boost research and innovation capability through collaborative projects with distinguished researchers from the world. For more information visit　https://www.shinshu-u.ac.jp/english/ or follow us on Twitter @ShinshuUni for our latest news.