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Harvard physicists have shown that specially treated
diamond coatings can keep water frozen at body temperature, a finding
that may have applications in future medical implants. Doctoral student
Alexander Wissner-Gross and Efthimios Kaxiras, physics professor and Gordon
McKay Professor of Applied Physics, spent a year building and examining
computer models that showed that a layer of diamond coated with sodium
atoms will keep water frozen up to 108 degrees Fahrenheit.
In ice, water molecules are arranged in a rigid framework
that gives the substance its hardness. The process of melting is somewhat
like a building falling down: pieces that had been arranged into a rigid
structure move and flow against one another, becoming liquid water. The
computer model shows that whenever a water molecule near the diamond-sodium
surface starts to fall out of place, the surface stabilizes it and reassembles
the crystalline ice structure.
Simulations show that the process works only for layers
of ice so thin they're just a few molecules wide - three nanometers at
room temperature and two nanometers at body temperature. A nanometer is
a billionth of a meter. The layer should be thick enough to form a biologically
compatible shield over the diamond surface and to make diamond coatings
more useful in medical devices, Wissner-Gross said.
The work is not the first showing that water can freeze at high temperatures.
Dutch scientists had shown previously that ice can form at room temperature
if placed between a tiny tungsten tip and a graphite surface. Kaxiras
and Wissner-Gross's work shows that ice can be maintained over a
large area at body temperature and pressure.
Device manufacturers have been considering using diamond coatings in
medical implants because of their hardness. Concerns have been raised,
however, because the coatings are difficult to get absolutely smooth,
abrasion of the tissue surrounding the implant could result, and that
diamond might have a higher chance of causing blood clots than other materials.
Wissner-Gross said a two-nanometer layer of ice would
just fill the pits in the diamond surface, smoothing it out and discouraging
clotting proteins from attaching to the surface. "It should be just soft
enough and water-friendly enough to smooth out diamond's disadvantages,"
Wissner-Gross said.
Wissner-Gross and Kaxiras are planning experiments to confirm the computerized
findings in the real world. Wissner-Gross said they expect results within
a year.
"We're reasonably confident we'll be able to realize
the effect experimentally," Wissner-Gross said. Wissner-Gross, who has
been a doctoral student at Harvard since 2003, said the research grew
out of an interest in the physical interaction of nanostructured surfaces
with molecules that are biologically relevant, such as water. Diamond
films are growing cheaper, Wissner-Gross said, and as their cost declines
the array of possible uses of the material grows wider.
"We both had this notion that it would be very interesting
to combine theory with respect to diamond surfaces with what's going on
in cryobiology," Wissner-Gross said. "We were thinking about how we could
leverage this long-term trend [of declining prices] to do something interesting
in the medical field." The work has won Wissner-Gross the 2007 Dan David
Prize Scholarship from Tel Aviv University and the 2007 Graduate Student
Silver Award from the Materials Research Society.
Wissner-Gross, who expects to graduate in June 2008, said he plans to
continue work not only on this project, but on other efforts concerning
the physics of surfaces that have novel properties.
Source: Harvard News Office
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