Massachusetts Institute of Technology scientists have created fabricated a functional dialysis membrane from a sheet of graphene. The new membrane works some 10 times faster than current membranes.
Dialysis refers to the mechanism by which molecules filter out of one solution into a more dilute solution (by the chemical process of membrane diffusion). Most well-known is hemodialysis, for removing waste from blood. However, there are many other applications, such as with the purification of drugs; the removal of residues from chemical solutions; and experiments to isolate molecules as required for medical diagnosis. Many of these studies of dialysis use materials that pass through a porous membrane. A common application of dialysis in industry is for the removal of unwanted small molecules such as salts, reducing agents, or dyes from larger macromolecules such as proteins, DNA, or polysaccharides.
Targeting molecules
Many dialysis membranes used today work quite slowly when separating molecules. This is intrinsic to their design. Membranes tend to be thick with pores arranged in winding paths. These physical features ensure the target molecules move relatively slowly.
With the new membrane, graphene is key. Graphene is a single layer of carbon atoms, where the atoms are linked in a hexagonal configuration. The ultrathin material is exceptionally sturdy, remaining intact under applied pressures of at least 100 bars (equivalent to about 20 times the pressure produced by a typical kitchen faucet). Graphene has a big association with electronics – the material can be used to enhance the strength and speed of computer display screens, photonics circuits, solar cells and various medical, chemical and industrial processes, among other things. However, many other applications are now emerging, such as dialysis membranes.
Ultrathin graphene membrane
The new graphene membrane is no large than a fingernail and it is, remarkably, less than 1 nanometer thick. The application is with filtering out nanometer-sized molecules from aqueous solutions. It is hoped that, on scale-up, the membrane can be used for industrial separation processes and, perhaps, for hemodialysis.
The research was led by Dr. Piran Kidambi. The researcher told Controlled Environments magazine: “Because graphene is so thin, diffusion across it will be extremely fast, 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 most complex part of the development was with manufacturing the graphene membrane. To this the scientists used a common technique termed chemical vapor deposition. This was deployed to grow graphene on copper foil. Following this, they proceeded to etch away the copper and then transferred the graphene to a sheet of polycarbonate. This was to act as support, with the sheet studded throughout with pores. The pores were of a calculated size designed to allow through any molecules that passed through the graphene.
Super-efficient molecular sieve
The graphene was then transformed into a molecularly selective sieve (one designed to let through only molecules of a certain size). For this the scientists created tiny pores within the material by exposing the structure to oxygen plasma (as an etching method). This left very tiny holes in the graphene. With this novel process, the longer graphene was exposed to oxygen plasma for then the larger and denser the pores became. Exposure times of between 45 to 60 seconds produced the smallest pores.
To test out the membrane the scientists made a solution made up of potassium, vitamin B12 (and lysozyme (the protein found in egg white). Each of these constituents was made up of molecules of a different size. The researchers next assessed the flow of molecules as they diffused through the membrane. This process showed how graphene with different sized pores could filter differently sized molecules. The study showed how graphene was ten-times faster than conventional membranes.
The research has been published in the journal Advanced Materials. The research paper is headed: “Nanoporous Atomically Thin Graphene Membranes for Desalting and Dialysis Applications.”
Essential Science
This article is part of Digital Journal’s regular Essential Science columns. Each week Tim Sandle explores a topical and important scientific issue. Last week we looked at a new tattoo made from bioink that signals health changes relating to the wearer via color changes.
The week before we asked what can the structure of the World Wide Web tell scientists about protein structures? The answer was ‘quite a bit’, according to Ohio State University researchers. The scientists discovered molecular ‘add-ons’ that customize key protein interfaces.
