Commercial dialysis machines use thick membranes to filter out molecules, but the process is slow; however, scientists have recently developed a graphene dialysis membrane that speeds up the process considerably.
Standard dialysis membranes contain pores that wind through membranes, slowing the rate that molecules can pass through. Typically, commercial dialysis membranes are around 20 nanometers thick.
The graphene dialysis membrane is much thinner – less than 1 nanometer. Consequently, target molecules can pass through the membrane much more quickly. Depending on the type of molecule, diffusion speed can be increased by between 10 and 100 times.
Dialysis membranes have many uses, including hemodialysis to filter blood, drug purification and the isolation of molecules for medical tests. The graphene dialysis membranes therefore have multiple applications, in particular, hemodialysis and laboratory medical testing.
The new membrane was developed by scientists at MIT’s Department of Engineering. The project was led by Piran Kidambi who said the new graphene dialysis membrane allows extremely fast diffusion. “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,” he said.
The graphene membrane was grown on copper foil using a process called chemical vapor deposition. After the membrane was formed, the copper was etched away leaving a 1 nanometer thick graphene membrane about the size of a fingernail. The membrane was transferred to a polycarbonate scaffold which has pores to allow molecules to pass through.
Pores are created in the graphene by exposing the sheet to oxygen plasma. When the plasma comes into contact with a carbon atom, carbon dioxide is formed and a pore is created in the membrane. The researchers were able to create pores of varying diameters by changing the length of time that the graphene membrane was exposed to oxygen plasma. The longer the exposure, the bigger the pore diameter.
The researchers created membranes with different pore sizes and tested the membranes using molecules of various sizes including potassium chloride, Vitamin B12, and lysozyme, which ranged from 0.66 nanometers to 4 nanometers wide.
The membrane is only 1 square centimeter, with the polycarbonate scaffold only having pores in 10% of its surface. Even with such a tiny area the diffusion speed was 10 times faster than standard dialysis membranes. The graphene itself is 100 times faster than standard membranes, although the scaffold is needed to prevent the membrane from curling.
The researchers are now making improvements to the oxygen plasma process and are now scaling up the dimensions of the graphene dialysis membrane.