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When considered at the molecular level, most solid
materials can be described as either crystals or glasses, explains
lead author Mark Ediger, a UW-Madison chemistry professor. The
difference lies in the degree of internal organization of their
constituent molecules.
"A crystal is like toy soldiers all lined up
marching together," Ediger says. "A glass is a teenager's room, with
stuff packed in everywhere."
Just as levels of messiness can range from
cluttered to chaotic, levels of molecular disorder can vary between
different types of glass. Glasses composed of more organized molecules
are more stable and durable, while glasses with haphazard molecular
assemblies are less stable and may degrade over time.
Conventional glasses are relatively disordered and
molecularly unstable because of how they are made. Glass ingredients
are melted, then cooled and allowed to harden. As the molten glass
cools, Ediger says, "The molecules slow down, then get stuck. The
question is, did they get stuck in an organized state or in an
unorganized state""
Normally, a piece of glass is allowed to cool all
at once and the inner molecules, unable to move freely, tend to be
trapped in disarray. Ediger and his team, in collaboration with
researchers in the UW-Madison School of Pharmacy and the National
Institute of Standards and Technology, designed a new technique that
gives all the molecules a chance to arrange themselves a little more
neatly.
In their new work, funded by grants from the
National Science Foundation and the U.S. Department of Agriculture,
Ediger and his team build glass layer by layer using a method called "vapor
deposition." Glass is heated to the point of evaporation and allowed
to condense on a cold surface, where the vapor forms an ultrathin
liquid film. By adding layers to the surface one at a time, each sheet
of particles can move into a more organized arrangement before
solidifying.
Though the new glasses do not reach the precision
of crystals, they are denser and far stronger than traditional glass.
"We were just astonished," says Ediger. "These materials were so
unusual, it took a whole year to understand what was going on."
Ediger estimates that the more stable glass would
take at least 10,000 years to make using conventional technology,
because the liquid glass would have be cooled extremely slowly. With
the new vapor deposition method, it takes about an hour.
For now, the Wisconsin researcher has no
expectations of using the method for large items like window glass.
The microscopic scale of the layering technique makes it best suited
for thin films and small products. With UW-Madison pharmacy professor
Lian Yu, Ediger is working toward possible medical applications for
the new stable glass.
Like other solids, pharmaceutical compounds can
form crystals or glasses when melted and cooled. For potential drugs,
molecular stability is critical, Ediger says. Forms that are too
unstable can degrade and change their effects over time, while those
that are too stable may be insoluble in the human body.
By using their method as a general technique to
control stability and solubility of molecular glass, Ediger says, it
may be possible to develop drug compounds that were previously
unusable. They may also be able to use stable glass films to extend
the shelf life of existing medical tools like off-the-shelf blood and
pregnancy testing kits.
So far, Ediger's team has successfully made stable
glass with an anti-inflammatory called indomethacin, a common test
drug Ediger refers to as "the fruit fly of the drug industry."
Encouraged by their results, the group plans to test more materials in
the search for additional applications. |
