Model of Collagen Nanostructure Explains Its Strength
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10/31/2006
12:47 PM
Page 850
RESEARCH/RESEARCHERS
The hexagon of carbon that makes up a nanotube has a predilection for clinging to other hexagons. One of the many challenges of working with CNTs is that they tend to stick to each other. Attaching a molecule to the sidewall of a nanotube serves a double purpose: It stops nanotubes from sticking, so that they can be processed and manipulated more easily, and it allows researchers to control and change the electronic properties of the nanotubes. Still, most such molecules also destroy the CNTs’ conductance because they make the nanotube structurally more similar to diamond, which is an insulator, rather than to graphite, a semimetal. The researchers indicate that their studies show that carbenes and nitrenes work by breaking a molecular bond on the nanotube’s wall while creating their own new bond to the nanotube. This process—one bond formed, one bond lost—restores the perfect number of bonds each carbon atom had in the original nanotube; thus, “conductance is recovered,” Marzari said. The theory indicates that some molecular handles can even transform between a
“bond-broken” and a “bond-intact” state, allowing the CNTs to act like switches that can be turned on or off in the presence of certain substances or with a laser beam. “This direct control of conductance may lead to novel strategies for the manipulation and assembly of nanotubes in metallic interconnects, or to sensing or imaging devices that respond in real time to optical or chemical stimuli,” Marzari said. The next step is for experiments to confirm that the approach works.
has been an unexplained phenomenon. M.J. Buehler, principal investigator at the Atomistic Mechanics Modeling Laboratory at the Massachusetts Institute of Technology, has used a combination of theoretical and molecular modeling that led to a breakthrough in understanding how molecular and tissue properties are linked. Buehler has reported his findings in the August 15 issue of the Proceedings of the National Academy of Sciences (p. 12285; DOI: 10.1073/pnas.0603216103). Buehler discovered that the characteristic design of collagen displays a clever strategy that enables nature to take advantage of the nanoscale properties of individual molecules at larger scales, leading to a tough material. This is achieved by arranging tropocollagen molecules into a staggered assembly known as collagen fibrils, tiny fibers with diameters of 50–200 nm and lengths of several micrometers. When a tensile force is applied at the end of a collagen fibril, the force is transmitted as shear forces between molecules and a tensile force within molecules. Whereas the elastic tensile strength of the tropocollagen mol-
Model of Collagen Nanostructure Explains Its Strength Collagen’s characteristic nanostructure may be the reason for its high strength and ability to sustain large deformation in its physiological role in tissues such as bone, tendon, and muscle. Previous experimentation has shown that collagen isolated from different tissue sources universall
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