Filtration membranes are sponge-like materials that have micro or nanoscopically small pores at their core. Unwanted chemicals, bacteria, and even viruses are physically blocked by the maze of mesh, but liquids like water can make it through.

The current standards for making these filters are pretty straightforward but they don’t allow for a lot of additional functionality.

A tube-shaped bijel filter. Researchers at the University of Pennsylvania's School of Engineering and Applied Science have a new way of making polymer filters out of bijels, or bicontinuous interfacially jammed emulsion gels, that allow functional nanoparticles to adhere to the surface of the polymer. Source: University of PennsylvaniaThis is a particular need when it comes to “biofouling.” The biological material they were supposed to filter out, like bacteria and viruses, gets stuck on the surface of the mesh, blocking out the pores with a slimy residue.

Beyond reducing the flow, these biofilms can potentially contaminate whatever liquid makes it through to the other side of the filter.

Researchers at the University of Pennsylvania’s School of Engineering and Applied Science have a new way of making membranes that could address this problem. Their method allows them to add in a host of new abilities via functional nanoparticles that adhere to the surface of the mesh.

They have demonstrated the new process with membranes that block bacteria and virus sizes contaminants without letting them stick — a property that would increase the efficiency of the lifespan of the filter.

The antifouling membranes they have tested would be immediately useful for relatively simple applications, like filtering drinking water and could eventually be used on the oily compounds found in fracking wastewater and other heavy-duty pollutants.

The researcher’s method allows for membranes made from a wide range of polymers and nanoparticles. Beyond antifouling abilities, future nanoparticles could catalyze reactions with the contaminants, destroying them or even converting them into something useful.

The researchers’ new membrane making method relies on a specialized type of liquid mixture known as a “biocontinuous interfacially jammed emulsion gel” or “bijel.” Unlike emulsions that consist of isolated droplets, the oil and water phases of bijels consist of densely intertwined but fully connected networks. Nanoparticles introduced the emulsion find their way to the interface between the oil and water networks.

Researchers Daeyeon Lee, a professor in Penn Engineering’s Department of Chemical and Biomolecular Engineering; Kathleen Stebe, Penn Engineering’s Deputy Dean for Research and Richer & Elizabeth Goodwin Professor of Chemical and Biomolecular Engineering; and Martin Haase, an assistant professor at Rowan University, developed this technology together. Lee, Stebe and Haase have previously devised a new way of making bijels that allows for a greater range of component materials. Now they have shown a way to make a solid filter using the same method.

"We knew this technology had promise," Stebe said. "Some of that promise is now being made real."

Like the earlier bijels, this filter begins as an intertwined network of water and oil with a dense layer of nanoparticles that separate the two. By using an oil that can be polymerized with UV light — crosslinking free-floating individual molecules into a solid, 3D mesh — the researchers are now able to solidify the structure of the bijel.

This method leaves a dense layer of nanoparticles in place on the surface of the polymer after the water has flowed away. Conventional ways of making polymer membranes don’t allow for this.

"Polymers typically hate particles and will reject them, but interfaces love particles and will trap them," Stebe said. "The density of nanoparticles on the surface of our polymers is through the roof. They are jammed together like sand in a sandcastle."

The researchers imbued their filters with silica nanoparticles and fashioned them into straw-like tubes. Silica nanoparticles can be modified with a wide range of chemicals with different functionalities including the antifouling property the researchers tested. They demonstrated both the filtering and antifouling capabilities on water containing gold nanoparticles of various sizes.

"In our experiment, we were able to filter out very small gold nanoparticles, in sizes equivalent to viruses," said Lee. "The tube shape also works well in the large-scale implementation of these filter membranes. Because they have large surface-area-to-volume ratios and don't get clogged, we can draw in fluid from the sides and suck it out from the end, allowing for continuous filtration."

"Membranes are typically passive materials that do not adapt their properties when environmental conditions change," said Haase. "An exciting aspect about our membranes is that they can be made to open and close their pores in response to a chemical signal. This unique feature enables the membrane to have controllable permeability, which is useful for the separation of different types of contaminants from water."

Lee is also a co-principle investigator at Penn Engineering’s REACT, or Research and Education in Active Coatings Technologies for human habitat. The multidisciplinary program is aimed at improving shelters used in disaster relief. Lee has interacted with emergency responders and equipment providers such as ShelterBox.

"When we spoke to people at ShelterBox, they said that more than a tent, what people need is clean water," Lee said. "REACT could potentially make these filters part of a system that does both."

With several ongoing refugee crises around the world and millions still without potable water after hurricane Maria hit Puerto Rico, the importance of this development is not lost on the researchers.

"There are real people right now who need this kind of technology so badly," said Stebe.

A paper on this study was published in the journal Nature Communications.