Electron micrograph of device

Researchers used two-photon polymerization to fabricate tiny, magnetically driven rotating filters for use in microfluidic channels. The filters (one of them shown here in a scanning electron micrograph) are just 70 microns wide and 60 microns tall, with square openings that measure 6.5 microns on each side. [Scale bar: 10 microns] [Image: Dong Wu, University of Science and Technology of China]

In recent years, lab-on-chip technology has emerged as a portable, cost-effective way to perform diagnostic tests at sites with little to no access to laboratory facilities. These miniaturized devices integrate multiple laboratory processes onto a single chip and require only tiny volumes of samples and reagents.

Using two-photon polymerization, a team of researchers from China and Japan has fabricated a new type of microfilter that adds a new function to lab-on-a-chip systems (Opt. Lett., doi: 10.1364/OL.428751). While previous filters could only sort particles of a specific size, this magnetically driven rotary microfilter allows for flexible sorting of different-sized particles.

The need for flexible sorting

Traditionally, microfilters in lab-on-chip systems have micropores of a set size for designated tasks, such as separating plasma from red and white blood cells, sorting out a particular cell type for analysis, or filtering out impurities from the rest of the sample.

“Since the number and shape of micropores in the filter cannot be dynamically changed during sorting, flexible sorting of different particles or cells on demand cannot be performed, which restricts the usage of the microchip,” said Dong Wu, a member of the research team from the University of Science and Technology of China. “Therefore, developing a versatile filter that can freely switch between the functional modes of filtering, passing, and selective filtering can diversify the applications.”

Wu and his colleagues decided to design a magnetically driven rotary microfilter with switchable modes. Depending on the orientation of an external magnetic field, it has the ability to either remain fixed and filter the microchannel or rotate parallel to the microchannel to let fluid to freely pass through.

Control by magnetic field

First, the researchers synthesized magnetic iron oxide nanoparticles and mixed them into a photoresist. Then, the team employed femtosecond laser direct writing to fabricate the microfilter using two-photon polymerization. The resulting, rectangle-shaped device is 70 microns wide and 60 microns tall, with two grids of 6.5-micron square micropores on either side of a central axis.

“To filter the larger particles, the magnetic field is applied perpendicular to the microchannel so as to cover the cross-section of microchannel with micropores in filtering mode,” said Wu. “In this way, particles which are larger than the pore size of the microfilter can be intercepted, while the smaller particles can pass freely.”

After letting the smaller particles go through, the rotary microfilter can be rotated by 90° by changing the direction of applied magnetic field in passing mode. When the magnetic field is set parallel to the microchannel, the captured larger particles can be released and analyzed. In addition, this mode prevents the filter from getting clogged with particles, which facilitates its reuse.

Next steps

When integrated into a microfluidic chip, the microfilter could successfully capture and release 8-micron spheres while allowing 2.5-micron spheres to pass in an alcohol solution.

Wu and his colleagues foresee many biological applications for a microfilter that can sort cells of different sizes, such as isolating circulating tumor cells for analysis or detecting abnormally large cells that may indicate disease. In the future, they plan to refine their design while maintaining a focus on improving the device’s performance with real-world samples and cells.

“As the viscosity of body fluids is larger than the currently used ethanol solution, how to design the structure and treat the surface of the microfluidic channel to maintain a high flow rate in the chip is important for further research,” he said.