With filter modes that can be switched via a magnetic field, the device is very promising for cell sorting

WASHINGTON – Researchers have made a magnetically powered rotating microfilter that can be used to filter particles in a microfluidic device. They made the tiny rotary filter by creating a magnetic material that could be used with a very precise 3D printing technique called two-photon polymerization.

Microfluidic devices, also known as lab-on-a-chip devices, can be used to perform multiple laboratory functions within a chip, which is typically a few square centimeters or less. These devices contain complicated networks of microfluidic channels and are becoming increasingly complex. They can be useful for a variety of uses, such as screening molecules for therapeutic potential or doing blood tests that detect disease.

“By changing the direction of the external magnetic field, the microfilter we make can be remotely manipulated if necessary to either filter certain sized particles or let them all pass,” said Dong Wu, a member of the University of Science and Research Team Technology of China. “This functionality could be used for many types of chemical and biological studies that are performed in lab-on-a-chip devices, and most importantly, enables the chips to be reused.”

In the Journal of the Optical Society (OSA) Optics letters, Wu along with colleagues from Hefei University of Technology and the RIKEN Center for Advanced Photonics in Japan show that their new rotating microfilter filters can sort particles in a microfluidic device with high performance.

“This filter could eventually be used to sort cells of different sizes, for example to isolate circulating tumor cells for analysis or to identify unusually large cells that could indicate disease,” said Chaowei Wang of the University of Science and Technology of China . “With further development, it might even be possible to use it in cancer detection devices that are placed in the body.”

A more versatile filter

Filters with micrometer-sized holes are often used in microfluidic chips as a passive way to sort particles or cells based on the size of the holes. However, because the number and shape of the holes in the filter cannot be dynamically changed, the available equipment lacks flexibility to sort different types of particles or cells as needed. To expand the usefulness of microfluidic devices, the researchers developed a filter that can freely switch between modes such as selective filtering and passing.

They developed the new filter using two-photon polymerization, which uses a focused femtosecond laser beam to solidify or polymerize a liquid photosensitive material known as photoresist. Thanks to the two-photon absorption, the polymerization can be carried out very precisely, which enables the production of complex structures in the micrometer range.

To produce the microfilter, the researchers synthesized magnetic nanoparticles and mixed them with the photoresist. When manufacturing the rotating microfilter, they had to optimize the laser power density, the number of pulses and the scan intervals for the polymerization. After testing its magnetically driven properties on a glass slide, they integrated the microfilter into a microfluidic device.

Multiple filter modes

To filter larger particles, a magnetic field is applied perpendicular to the microchannel. After completion of the filtering process, the large particles can be released by applying a magnetic field running parallel to the microchannel, which rotates the microfilter by 90 °. The filtering process can then be repeated as required.

The researchers checked the filter performance of the filter using polystyrene particles with diameters of 8.0 and 2.5 micrometers that were mixed in an alcohol solution. “It was clear that particles smaller than the pore size could easily pass through microfilters, while larger ones were filtered out,” said Chenchu ​​Zhang of the University of Science and Technology of China. “In straight-through mode, any larger particles caught by the filter were washed away with the liquid, which prevents the filter from clogging and enables the microfilter to be reused.”

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Articles: C. Wang, Z. Hu, L. Yang, C. Zhang, L. Zhang, S. Ji, L. Xu, J. Li, Y. Hu, D. Wu, J. Chu, K. Sugioka, “Magnetically driven rotary microfilter, produced by two-photon polymerization for the multimode filtering of particles”, Opt. Lett., 46, 12, 2968-2971 (2021).

DOI: https: //doi.Organization/10.1364 /OIL.428751.

over Optics letters

Optics letters offers rapid dissemination of new results in all areas of optical science with short, original and peer-reviewed communications. Optics letters accepts papers that are important to a significant part of the optics community. Published by The Optical Society and directed by Editor-in-Chief Miguel Alonso, Institut Fresnel, École Centrale de Marseille and Aix-Marseille Université, France, University of Rochester, USA. Optics letters is available online from OSA Publishing.

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