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Graduate student Catherine Tweedie, left, and
materials science and engineering Assistant Professor Krystyn Van
Vliet, right, use a nanoindenter to measure the mechanical
properties of biomaterials. Photo/Donna Coveney
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The trick? The team, led by Assistant
Professor Krystyn J. Van Vliet of the Department of Materials Science
and Engineering, miniaturized the process.
Van Vliet, MSE graduate student Catherine A.
Tweedie, research associate Daniel G. Anderson of the Department of
Chemical Engineering and Institute Professor Robert Langer describe
the work in the cover story of the November issue of Advanced
Materials.
In 2004 Anderson, Langer and a colleague reported
using robotic technology to deposit more than 1,700 spots of
biomaterial (roughly 500 different materials in triplicate) on a glass
slide measuring only 25 millimeters wide by 75 millimeters long.
Twenty such slides, or microarrays, could be made in a single day.
The arrays were then used to determine which
materials were most conducive to the growth and differentiation of
human embryonic stem cells. (See web.mit.edu/newsoffice/2004/celltest.html.)
Enter Van Vliet, whose lab studies how the
mechanical properties of a surface affect cells growing on that
surface. Curious as to whether the Langer team had probed the
mechanical properties of the biomaterials, she contacted Langer, who
introduced her to Anderson.
And what began as an isolated question turned into
a collaboration with wider implications.
Together the researchers showed that the mechanical
properties of each biomaterial could indeed be determined - and
quickly - by combining the arrays with nanoindentation, a technique
key to Van Vliet's work.
In nanoindentation a hard, small probe is pressed
into a more compliant material, to depths many times smaller than the
diameter of a human hair. By measuring the force applied and how
deeply the probe penetrates the material, scientists can learn a great
deal about the material's mechanical properties.
"The spots of material Dan was making had diameters
about three times that of a human hair, a scale perfect for
nanoindenation," Van Vliet said. So the team created new arrays of
roughly 600 unique polymers. "Each dot was a combination of two
different monomers, or building blocks, so we could map out the
effects of the percentage of each monomer on the properties of the
material," Van Vliet said. And in 24 hours Tweedie, using the
nanoindenter, had that data in hand.
It would have taken many weeks to analyze that many
materials using traditional techniques, which involve "the serial
process of bulk-material synthesis, batch-sample preparation, and
individual-sample testing," the team writes in Advanced Materials.
Further, Anderson explained, many materials have been discovered when
a scientist thinks about what the perfect properties of a material
should be, and then invents it. "But that can take lots of time," he
said.
Enter combinatorial libraries. "Instead of trying
to engineer perfect materials, let's make thousands at the smallest
scale we can, and see if we can find some materials with unexpected or
interesting properties," Anderson said.
Tweedie notes that even in this first "proof of
principle" experiment there were some surprises. For example, she said,
"the stiffness of certain polymers depended more on the combination of
monomers used (how much of A and B) rather than the structure of each
monomer, with certain combinations resulting in very compliant
polymers. These were very large, unanticipated changes in mechanical
properties that could then be optimized further in a subset of
combinations."
Describing the collaboration that brought about
these results, Van Vliet concluded: "It's really made both [of our
groups] think in different ways about what we're doing." |
