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Strangely Behaving Filter Promises New Uses

reverse filter
A self-healing filter developed by researchers at Pennsylvania State University lets large objects pass through while small ones remain, paving the way for novel applications.

Researchers have developed a new type of filter that acts in the complete opposite way compared to how we typically use filters: It lets large objects pass through while keeping smaller ones contained. The invention, which hails from mechanical engineers from Pennsylvania State University, could be used for a number of novel applications in which a large surface area must be protected from small particles or objects, researchers said.

Large particles pass through this reverse liquid filter while smaller particles remain behind. This is completely opposite to how typical filters work, paving the way for myriad new applications. (Image source: Tak-Sing Wong Lab, Pennsylvania State University)

Large Vs. Small

“Conventional filters, such as sieves or coffee filters, work in a way that large particles will be retained and small particles will pass through the filters,” Tak-Sing Wong, an assistant professor of mechanical engineering and biomedical engineering at the university, told Design News. “Our self-healing liquid filters work exactly the opposite way, where small particles are retained and large particles pass through.”

While this type of filtration may seem counter-intuitive, it’s actually common in microscopic cellular organisms, which “use a similar strategy to ingest external objects without leaking their cellular content to the outside environment,” explained Birgitt Boschitsch, a graduate student in mechanical engineering who worked on the research. “This is made possible by their liquid-like, self-healing cell membranes,” she said.

Motion Not Size

Researchers applied the same principle to their work, using a stabilized liquid material that screens out smaller objects while allowing larger ones to pass through. It does this not by separating objects by size, but by responding to an object’s kinetic movement or energy.

"Typically, a smaller object is associated with lower kinetic energy due to its smaller mass," Wong said. "So the larger object with a higher kinetic energy will pass through the membrane, while the smaller object with lower kinetic energy will be retained." In addition, the membrane wraps around the object as it passes through, allowing the membrane to completely self-heal over the top of the object passing through it, he said.

In its simplest form, the membrane can be created with water and a substance that stabilizes the interface between liquid and air, giving it a structure similar to that of a biological cell membrane. Researchers used a simple soap film as the liquid in their prototype, they said.

Once the membrane is established, the materials can be modified and optimized so the filter can serve unique purposes, such as enhanced mechanical robustness, antibacterial properties, or odor-neutralization. "You could add components that make the membrane last longer or components that allow it to block certain gases," Boschitsch explained. "There are endless potential additives to choose from to tailor a membrane to the application of interest."


One real-world use for the invention is as a surgical film that allows surgeons to operate on an open wound without other contaminants getting into the wound, Wong said. It also could be used as an odor barrier for waterless toilets that allow solid waste to pass while blocking the odor-causing gases. Wong added that the filter could be used to protect livestock from disease-carrying insects, keeping them away from the animals.

Researchers published a paper on their work in the journal Science Advances. The researchers have filed an international patent application for their work and plan to continue to develop the filters further to make them as intelligent as possible, “similar to those of biological cell membranes which can sense, and selectively filter, different types of particles,” Boschitsch stated. “We are also interested to tailor the compositions of the reverse filters for specific applications,” she said.

Elizabeth Montalbano is a freelance writer who has written about technology and culture for 20 years. She has lived and worked as a professional journalist in Phoenix, San Francisco, and New York City. In her free time, she enjoys surfing, traveling, music, yoga, and cooking. She currently resides in a village on the southwest coast of Portugal.

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