The filter could be used to recycle sewage on a space station, but the main focus is the use of ultra pure water in the fabrication of semiconductors.
The technology is based on a discovery that was celebrated with the Noble Prize in chemistry in 2003. Prof Peter Agre is a molecular biologist from the Johns Hopkins University in Baltimore, USA, and during his studies he discovered that there is a special protein that is responsible for rapid permeation of water in cells. For him, this discovery was not the achievement of an individual, but a team effort: “I do not see myself as Einstein-reincarnation”, he says with a little smirk in his face. “I see myself more as Huckleberry Finn, a happy-go-lucky individual who is looking for adventure, and boy did we find one.”
Peter Agre and his team named these proteins ‘aquaporins’, as they function as water pores on the nano scale. In plants they work like the plumbing system of cells and ensure a highly efficient but selective transport of water: only H2O molecules can pass through these channels. For plants this filter is a life-saver, as they make sure that the cell will not lose any minerals.
Turning the discovery of aquaporins into an industrial filter has been the challenge of the European research project MEMBAQ. Prof Claus Helix Nielsen and his team of researchers at Technical University of Denmark felt it was possible to make this transitions despite the technological challenges. “The engineering of this will have some technical difficulties because the native membranes are very tiny and so only nano-water purification can occur", says Peter Agre – and has in mind the large amounts of ultra pure water that is needed every day for industrial applications.
For example, ultra pure water is used in the semiconductor fabrication to clean silicon wafers residue-free. Even the smallest particles could affect the conductivity of the computer chips that will be fabricated from these silicon wafers. Big manufacturers therefore use millions of litres per day.
The IMS Fraunhofer Institute in Duisburg has built its own production line of silicon wafers and needs up to 2 million litres of ultra pure water every day. Although this is only a relatively small production unit, it takes a whole room full of machines to produce the ultra pure water. “It is complex, it uses a lot of energy, and is therefore also quite expensive”, admits Klaus Debusmann who is responsible for wafer production at the IMS Fraunhofer Institute. The production of the daily requirement of two million litres costs 30.000 Euros.
The researchers in Denmark want to make the process cheaper and less energy consuming with the help of the highly specialized proteins. “Our job here in this project is to take these proteins out of their natural environment and put them into an artificially membrane mimicking the membrane of cells. Thereby we create a matrix in which we have these proteins sitting in large quantities so that they can collectively work as a water filter”, says Prof Nielsen.
While scientists have unravelled the principle of cleaning water with aquaporins it is still a difficult task to construct a durable filter membrane. The fragile proteins in the membrane need to be supported. The scientists have therefore constructed a net made from a perforated Teflon film that holds them together. The net holes are only 300 microns in diameter and are literally burned into the Teflon with a special kind of laser using CO2.
In order to further stabilize the filter Prof Nielsen and his team had to come up with a second ingenious idea: They supported the construction with a second layer underneath the net made from porous hydrogel. It is flexible and resembles tissue, similar to soft contact lenses.
So far the researchers are testing their filters in lab experiments only. But Prof Agre is already impressed by the development of new technologies based on his discovery: “The power of these techniques is very impressive. This is clearly contemporary engineering. These technologies are no longer science fiction. It’s fact.”
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