Fabrication steps: (1) Graphene, grown on copper foil, is pressed against a supporting sheet of polycarbonate. (2) The polycarbonate acts to peel the graphene from the copper. (3) Using interfacial polymerization, researchers seal large tears and defects in graphene. (4) Next, they use oxygen plasma to etch pores of specific sizes in graphene. Image credit: Massachusetts Institute of Technology
Weekly Briefs - 94
Dialysis is used in medical applications to remove waste from blood. Besides hemodialysis, the process is also used to purify drugs, to clean chemical solutions, to isolate molecules and for other tasks. Nowadays, dialysis is performed using special commercially available membranes. The process is very slow due to the fact that these membranes are thick so the pores that tunnel through do so in winding paths. Also, because of the thickness, the particles move in winding paths making it difficult to target specific points at the end of their paths.
To solve this problem, a team of engineers at the Massachusetts Institute of Technology (MIT) developed a novel dialysis membrane from a sheet of graphene, the single-layer of carbon atom material, which is less than one nanometer thick. The thinnest existing dialysis membranes are around 20 nanometers thick, so from this point of view, the new development can bring faster dialysis.
The team leader, Piran Kidambi, a postdoc student in the Department of Mechanical Engineering at MIT, says that the findings demonstrate that graphene, largely developed for electronics applications, can be used to improve membrane technology.
“Because graphene is so thin, diffusion across it will be extremely fast,” Kidambi says. “A molecule doesn’t have to do this tedious job of going through all these tortuous pores in a thick membrane before exiting the other side. Moving graphene into this regime of biological separation is very exciting.”
The steps used by the researchers to fabricate the graphene membrane are illustrated in the figure. First, they used a known technique called chemical vapor deposition (CVD) to grow graphene on a sacrificial layer of copper foil. Once the graphene formed, they etched the copper foil and transferred the graphene to a sheet of polycarbonate full of pores, so any molecule that has passed through the graphene can be filtered. The polycarbonate plays the role of a support, so the graphene will not bend. To create the pores in the graphene that will become the molecularly selective sieve membrane, they exposed the material to oxygen plasma. This is a process used in some industries to etch materials (to create the pores, in this case) by adding oxygen into a plasma chamber, so the oxygen atoms etch away at materials.
“By tuning the oxygen plasma conditions, we can control the density and size of pores we make, in the areas where the graphene is pristine,” Kidambi says. “What happens is, oxygen radical comes to a carbon atom [in graphene] and rapidly reacts, and they both fly out as carbon dioxide.”
To validate the research, the team tested multiple graphene membranes with pores of various sizes using various mixtures of molecules of different sizes, ranging from potassium chloride (0.66 nanometers wide) to vitamin B12 (1 to 1.5 nanometers) and lysozyme (4 nanometers). They repeated the tests with existing commercial dialysis membranes and found that the graphene membranes filter out molecules 10 times faster. This is hopeful news for people that must undergo hemodialysis treatment. For a normal treatment, the patient has to go through the hemodialysis three times a week and each time the treatment lasts about four hours!
“What’s exciting is, what’s not great for the electronics field is actually perfect in this [membrane dialysis] field,” Kidambi says. “In electronics, you want to minimize defects. Here you want to make defects of the right size. It goes to show the end use of the technology dictates what you want in the technology. That’s the key.”
The results of the research were recently published in Advanced Materials. An abstract can be found here: http://onlinelibrary.wiley.com/doi/10.1002/adma.201700277/full
