Don't you just love serendipity? Christopher Striemer does. As a research associate at the University of Rochester in the US, Striemer recently discovered a new ultrathin silicon membrane that could revolutionise the way that doctors or scientists manipulate molecules. Only 50 atoms thick, it might even improve treatment regimes for haemodialysis patients with kidney failure.

A self-driven and inquisitive scientist who likes doing things well, Striemer was trying to understand how silicon - as used in computer chips - crystallises. As part of his research into semiconductors under Philippe Fauchet, professor of electrical and computer engineering, he made a thin silicon membrane and placed it in an electron microscope. At first glance, it appeared full of holes. But instead of trying again, he stopped in amazement.

Something special

"Holes are seldom a good thing when you are trying to make electrical or optical devices. However, I immediately recognised that this was something special and unique," says Striemer.

Size really does matter here. Striemer's membrane was less than one millimetre square and, at 50 atoms (15 nanometres) thick, was 4,000 times thinner than a human hair. Laid flat, it appeared blue, but edge-on was invisible to the naked eye. And with almost no mass, it might have floated away unnoticed.

Another experiment confirmed his findings: "When I saw small gold nanoparticles pass through, it was obvious it had holes. The fact that it was incredibly thin with holes in it struck me - this could be a perfect screen."

Just how perfect was soon apparent when Striemer confirmed his new membrane was thousands of times thinner than existing filters. The pores were usefully molecule-sized, with later experiments producing a library of membranes with pore sizes ranging from nine to 35 nanometres. Even more startling, these characteristics were easily controlled by temperature during manufacture.

A meeting with James McGrath, assistant professor of biomedical engineering, led to a series of experiments with McGrath's graduate student, Tom Gaborski. McGrath likes solving biomedical problems and this seemed an interesting challenge.

"Chris [Striemer] came to us asking what in the world of biology this might separate and our best guess was proteins. It was a lucky guess because it works brilliantly, but I recall being sceptical at first," says McGrath.

Conventional polymer molecular filters are a jumble of varying holes and tunnels prone to clogging. Striemer's silicon membrane was about as thick as a protein molecule, yet could resist 15 pounds per square inch of pressure - roughly the same pressure that the human body faces in the atmosphere. This promises fast, clog-free filtering. To test this, two blood proteins, albumin and immunoglobulin - both small enough to pass through the tiny holes in the membrane - were tried.

"Jim [McGrath] and Tom [Gaborski] were amazed at how quickly the smaller species [albumin] passed through the membrane. I thought the six minutes that we spent watching this was a long time, until they told me dialysis processes usually take hours," says Striemer.

Haemodialysis is the treatment used for patients with kidney failure where toxic molecules are removed by dialysis machines. If the new membrane lives up to expectations, such machines might be made smaller or the same volume of blood might be dialysed quicker.

"Right now we are focusing on the question of whether these membranes can filter out small molecular weight toxins while retaining larger essential blood proteins," says McGrath. The scientists have founded a company, SiMPore Inc, to commercialise their work.

Dr John Firth is the director of renal services at Addenbrooke's hospital in Cambridge. He says that a typical haemodialysis session for a patient on a kidney machine lasts four hours, three times a week. This is hard on patients, say the American scientists. "It is true that many patients are tired after dialysis, and that dialysis membranes do not clear everything from the blood that would be removed by normally functioning kidneys - but it is not clear that the two are connected," says Firth.

Improving haemodialysis is an attractive goal for the new membrane. Consultant nephrologist Dr Richard Fluck at Derby City General Hospital is a medical advisor to Kidney Research UK (kidneyresearchuk.org). What most grabs Fluck's attention is that Striemer's discovery also supports "charge". By applying a fixed electrical charge, the pores can in effect be made smaller for some molecules - enabling their separation by size and charge.

"It's not just pore size, it's charge as well and that starts to mimic more closely what our own kidneys do. People think of them as a physical sieve but it's more sophisticated than that," says Fluck. "Kidney medicine is a very technological, very scientific area in which the adoption of new technology is driven by clinical need. This is the sort of thing that nephrologists would go out and look for."

Working in a completely different field, Andrew de Mello of Imperial College has other ideas. As professor of chemical nanosciences, he's also involved with the London Centre for Nanotechnology which brings together engineers, physical scientists and life scientists. His research interest is microfluidics for the emerging "lab-on-a-chip" technology - an ultrathin silicon filter is useful here too.

Unique properties

"There's no real reason that we as chemists traditionally use test tubes apart from the fact they fit in our hand. If you can control and manipulate very small numbers of molecules, you can actually do some really interesting and novel things," says de Mello.

While new haemodialysis systems might be years away from hospitals, filtering applications for lab-on-a-chip technology seem closer. The membrane's unique properties are unlike any molecular filter that's gone before - helping to attract considerable scientific interest. Thanks to serendipity in the hands of a smart individual, silicon has offered an astonishing new breakthrough.

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