Nov 2 2017
Fundamentally, filtration membranes are sponge-like materials including pores that are micro- or nanoscopically small. Bacteria, undesirable chemicals and viruses are physically obstructed by the mesh maze. However, liquids such as water can pass through the maze.
Micrographs show how densely packed the functional nanoparticles are on the surface of the filter’s polymer fibers. Credit: University of Pennsylvania
Although the standard for producing such filters is comparatively uncomplicated, there is no possibility for giving additional functionality to the filters. This is a need when it comes to “biofouling.” The biological materials to be filtered (e.g. viruses and bacteria) get attached to the mesh surface, thereby obstructing the pores with a slimy residue.
Apart from minimizing the flow, these biofilms could pollute any liquid that passes through them to the other side of the filter.
At present, scientists from the University of Pennsylvania’s School of Engineering and Applied Science have devised an innovative technique for producing membranes that can overcome this challenge. This technique enables them to add a wide range of innovative abilities through functional nanoparticles attached to the mesh surface.
The team demonstrated the innovative technique by using membranes with the potential obstruct virus- and bacteria-sized contaminants without allowing them to stick—a characteristic that will largely enhance the filter’s service life and efficiency.
The “antifouling” membranes investigated by the team can be at once used in comparatively uncomplicated applications (e.g. filtering drinking water), and can ultimately be used for the oily compounds produced during fracking of wastewater and also other heavy-duty contaminants.
The new technique has been reported in a paper published recently in the Nature Communications journal, and enables production of the membranes from a broad array of nanoparticles and polymers. Apart from antifouling potential, future nanoparticles can also catalyze reactions with the pollutants, thereby either destroying them or transforming them into usable compounds.
Daeyeon Lee, a professor in Penn Engineering’s Department of Chemical and Biomolecular Engineering, and Kathleen Stebe, Penn Engineering’s Deputy Dean for Research and Richer & Elizabeth Goodwin Professor of Chemical and Biomolecular Engineering, led study in collaboration with Martin F. Haase, an assistant professor at Rowan University who, as a postdoctoral researcher, devised the technique in the labs of Stebe and Lee. Other contributors to the study were Harim Jeon, Noah Hough, and Jong Hak Kim.
The membrane-making technique adopted by the research team is dependent on an exclusive kind of liquid mixture called a “bicontinuous interfacially jammed emulsion gel,” or “bijel.” In contrast to emulsions that comprise of isolated droplets, the oil as well as water phases of bijels comprise of densely interweaved yet completely interlinked networks. Nanoparticles injected into the emulsion reach the interface through the water and oil networks.
In the recent past, Lee, Stebe, and Haase conceived an innovative method for synthesizing bijels that favors a broader array of component materials, reported in a paper published in 2015 in the Advanced Materials journal. At present, they have demonstrated a technique for producing a solid filter by adopting the same technology.
We knew this technology had promise, some of that promise is now being made real.
Kathleen Stebe, Penn Engineering’s Deputy Dean for Research and Richer & Elizabeth Goodwin Professor of Chemical and Biomolecular Engineering
Similar to their prior bijels, this filter initially starts as an interweaved network of oil and water, where a dense nanoparticle layer separates the two. However, by using a type of oil that could be polymerized using UV light— crosslinking individual, free-floating molecules in a three-dimensional, solid mesh— they were able to solidify the bijel’s structure.
Critically, the technique does not displace the dense nanoparticle layer from the polymer surface once the entire water flows away. Traditional methods of producing polymer membranes do not enable this feature.
“Polymers typically hate particles and will eject them, but interfaces love particles and will trap them,” stated Stebe. “The density of nanoparticles on the surface of our polymers is through the roof. They are jammed together like sand in a sandcastle.”
The team infused silica nanoparticles into their filters and designed them as straw-like tubes. Silica nanoparticles can be altered by using different types of chemicals that have unique properties, which also include the antifouling characteristic investigated by the team. They exhibited their filtering as well as antifouling potentials on water comprising gold nanoparticles of differing sizes.
In our experiment, we were able to filter out very small gold nanoparticles, in sizes equivalent to viruses, the tube shape also works well in 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.
Daeyeon Lee, a professor in Penn Engineering’s Department of Chemical and Biomolecular Engineering
“Membranes are typically passive materials that do not adapt their properties when environmental conditions change,” stated 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-principal investigator at Penn Engineering’s Research and Education in Active Coatings Technologies, or REACT, for human habitat. The goal of this multidisciplinary project is to enhance shelters used during disaster relief, and in essence, Lee has contacted equipment providers (e.g. ShelterBox) and emergency responders.
“When we spoke to people at ShelterBox, they said that more than a tent, what people need is clean water,” stated Lee. “REACT could potentially make these filters part of a system that does both.”
As there are many refugee crises happening globally and millions of people are left without potable water following hurricane Maria that struck Puerto Rico, the significance of this advancement is not lost on the team.
There are really people right now who need this kind of technology so badly.
Kathleen Stebe, Penn Engineering’s Deputy Dean for Research and Richer & Elizabeth Goodwin Professor of Chemical and Biomolecular Engineering
The National Science Foundation grant CBET-1449337 primarily supported the study. The German Research Foundation partially supported the study under the project number HA 7488/1-1.