Discovery
Molecule By Molecule
BREAKTHROUGH: “We have a material as thin as some of the molecules it’s sorting. . .but can withstand enough pressure to make real-world nanofiltering a practical reality,” says research associate Christopher Striemer.
New super-thin filter—50 atoms thick—opens possibilities
for better dialysis, fuel cells, and neuro-stem cell cultivation. By Jonathan
Sherwood ’04 (MA)
A newly designed porous membrane, so thin it’s invisible edge-on, may
revolutionize the way doctors and scientists manipulate objects as small as
a molecule.
The 50-atom thick filter, developed at Rochester, can withstand surprisingly
high pressures and may be a key to better separation of blood proteins for
dialysis patients, speeding ion exchange in fuel cells, creating a new environment
for growing neurological stem cells, and purifying air and water in hospitals
and clean-rooms at the nanoscopic level.
At more than 4,000 times thinner than a human hair, the new barely-there membrane
is thousands of times thinner than similar filters in use today.
“It’s amazing, we have a material as thin as some of the molecules
it’s sorting, and even riddled with holes, but can withstand enough pressure
to make real-world nanofiltering a practical reality,” says research associate
Christopher Striemer, co-creator of the membrane. “That 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 membrane, which was featured in February’s issue of the journal Nature, is
a 15-nanometer-thick slice of the same silicon that’s used every day
in computer-chip manufacturing. In the lab of Philippe Fauchet, professor of
electrical and computer engineering at the University, Striemer discovered
the membrane as he was looking for a way to better understand how silicon crystallizes
when heated.
He used such a thin piece of silicon—only about 50 atoms thick—because
it would allow him to use an electron microscope to see the crystal structure
in his samples, formed with different heat treatments.
Striemer found that as parts of the silicon contracted into crystals, holes
opened up in their wakes. Imagine a party of people spread out evenly throughout
a room, but as the evening progresses and people huddle into cliques, scattered
areas of empty floor open up.
In talks with Striemer and Fauchet, James L. McGrath, assistant professor
of biomedical engineering, and Tom Gaborski, a graduate student in McGrath’s
group, realized that since the membrane’s holes were only nanometers
in size, it might be possible to separate objects as small as proteins much
more effectively than is being done now.
Current molecular-level filters use a polymer-based design that is a jumble
of varying holes and tunnels. The sizes of holes in the polymer model vary
greatly, and since its “holes” are really convoluted tunnels through
the material, they require much more time for proteins to pass through, and
they are prone to clogging.
While McGrath knew he might have the exact filter researchers have been searching
for, he needed to test if the predictions held up.
“When you build something at this scale, you’re closing in on
the quantum world and you never know what the properties are going to be,”
he says.
When Striemer tested his design, he found that the same 50-atom thickness
could hold back an astonishing 15 pounds per square inch of pressure.
To test the membrane, Gaborski placed a solution of two blood proteins, albumin
and IgG, behind the membrane and forced it gently through the nanoscopic holes.
In just over six minutes, the albumin had passed through, but the larger IgG
protein was stopped.
And as if filtering by nanoscale size weren’t enough, the Rochester
team has found a way for the nanofilter to carry a fixed charge, effectively
making the hole “smaller” for molecules of a certain charge than for
others. In a single filter it’s now possible to quickly and easily separate
molecules by their size and their charge—a serious boon for fuel cell
researchers, who wish to move only certain ions from one part of a fuel cell
to another.
Separating molecules by size and charge efficiently is also the goal of kidney
dialysis researchers. Johnson & Johnson recently gave the Rochester team
a $100,000 grant to pursue developing the membrane’s use in separating
blood proteins with the hope of creating a more efficient method of dialysis.
“Kidneys do a much better job than dialysis machines of filtering blood
proteins and keeping the ones you need, like albumin, and getting rid of toxins,
which in some cases are smaller proteins,” says McGrath. “They use a
type of cellulose or plastic membrane with relatively poor discrimination.
“We think we can engineer these membranes to provide superior discrimination
of proteins, which may make the process of dialysis faster and more effective
than it is today.”
One of the most intriguing ideas for the Rochester work is that it may play
a role in growing neurons from stem cells.
“Its potential applications to neuroscience, cell biology, and medical
research may be profound.” says Steve Goldman, professor of neurology and chief
of the Division of Cell and Gene Therapy at Rochester.
Recent evidence suggests that neurological stem cells may grow better when
in the immediate vicinity of certain “helper” cells. A problem arises
after the new neurons are grown, when scientists need to separate the neurons
from these helper cells. McGrath suggests that the neurological stem cells
can be adhered to one side of the membrane, and the helper cells on the other.
The silicon membrane is about the thickness of the cell’s own membranes,
meaning the two groups of cells can actually touch each other through the membrane’s
pores without passing through themselves.
The chemical communication between the helper and stem cells can continue
as if the two sets of cells were in direct contact, but after the neurons are
fully formed, they can easily be separated from the helper cells.
The Rochester team is working to realize the potential of the membrane by
refining its fabrication. Striemer found he could “tune” the size of
the filter holes depending on the temperature to which the silicon is heated,
but the process is not yet accurate enough for engineers to simply select any
pore size and fabricate it.
The researchers have just founded a company, SiMPore, to help investigate
the filter’s applications and to license and commercialize it.
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