Feb 16, 2007
Silicon filter breaks new record
Researchers in the US have designed a new porous membrane made from silicon that is just 50 atoms thick and can sort individual molecules. The filter, which overcomes many of the problems associated with conventional polymer-based membrane filters, could find use in blood dialysis, fuel cells and purification systems in hospitals and clean rooms.
Current polymer-based membrane filters contain holes that vary greatly in size. These holes, which are actually convoluted tunnels that run through the material, can clog up because the solution being filtered takes a relatively long time to pass through the structure.
The new membrane is made of silicon, is just 15 nm thick and contains nanometre-sized holes. It is made using the same tools used to create integrated circuit chips and consists of three thin layers – an amorphous silicon layer sandwiched between two silica layers – on a silicon wafer. Exposing the wafer to temperatures higher than 700 °C crystallizes the amorphous silicon so that it forms pores about 10 nanometres wide.
The Rochester team tested its membrane by placing a solution of two blood proteins, albumin and IgG behind the membrane and forcing it gently through the nanoscopic holes. In just over six minutes, the albumin had passed through but the larger IgG protein was stopped.
"It's amazing, we have a material as thin as some of the molecules it's sorting, and even riddled with holes, but [it] can withstand enough pressures to make real-world nano-filtering a practical reality," says team member Christopher Striemer of the University of Rochester. "The ultra-thinness means much higher efficiency and lower sample loss, so we can do things that can't normally be done with current materials."
The researchers also discovered that they could make the nano-filter carry a fixed charge, therefore making the hole "smaller" for molecules of a certain charge than for others. This means it is possible to easily separate molecules by their size and charge, which could be of interest for fuel cell researchers who only want to move certain ions from one part of a fuel cell to another. It could be useful for kidney dialysis too, where molecules also need to be separated according their size and charge. Indeed, in laboratory tests, dye molecules 1 nm across passed through the membrane 10 times faster than through a commercial blood dialysis membrane.
The team now plans to make its membrane stronger – although it can already sustain pressures of 15 psi – so it can push more molecules through. This could potentially improve dialysis speeds by a factor of 100 over commercial membranes. The researchers have just founded a company called SiMPore to commercialize the numerous applications of their nanomembranes. They have already been approached by semiconductor giants such as Intel to see if the filter could remove nanoparticles from solutions in microchip manufacturing.
The team reported its results in Nature.