Carbon Nanotubes Hold Potential For Synthetic Tissue, Muscles

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Tough Tubes: Carbon Nanotubes Hold Potential For Synthetic Tissue,

Muscles

http://www.sciencedaily.com/releases/2007/07/070702145144.htm

The ability of carbon nanotubes to withstand repeated stress yet

retain their structural and mechanical integrity is similar to the

behavior of soft tissue, according to a new study from Rensselaer

Polytechnic Institute.

A block of carbon nanotubes before (left) and after (right) being

compressed more than 500,000 times. (photo at link) There is

virtually no difference in shape, mechanical integrity or electrical

conductivity. This resistance to wear and tear is similar to the

behavior of soft tissues such as a shoulder muscle or stomach wall.

When paired with the strong electrical conductivity of carbon

nanotubes, this ability to endure wear and tear, or fatigue,

suggests the materials could be used to create structures that mimic

artificial muscles or interesting electro-mechanical systems,

researchers said.

The report, " Fatigue resistance of aligned carbon nanotube arrays

under cyclic compression, " appears in the July issue of Nature

Nanotechnology. Despite extensive research over the past decade into

the mechanical properties of carbon nanotube structures, this study

is the first to explore and document their fatigue behavior, said co-

author Victor Pushparaj, a senior research specialist in

Rensselaer's department of materials science and engineering.

" The idea was to show how fatigue affects nanotube structures over

the lifetime of a device that incorporates carbon nanotubes, "

Pushparaj said. " Even when exposed to high levels of stress, the

nanotubes held up extremely well. The behavior is reminiscent of the

mechanics of soft tissues, such as a shoulder muscle or stomach

wall, which expand and contract millions of times over a human

lifetime. "

Pushparaj and his team created a free-standing, macroscopic, two-

millimeter square block of carbon nanotubes, made up of millions of

individual, vertically aligned, multiwalled nanotubes. The

researchers then compressed the block between two steels plates in a

vice-like machine.

The team repeated this process more than 500,000 times, recording

precisely how much force was required to compress the nanotube block

down to about 25 percent of its original height.

Even after 500,000 compressions, the nanotube block retained its

original shape and mechanical properties. Similarly, the nanotube

block also retained its original electrical conductance.

In the initial stages of the experiment, the force needed to

compress the nanotube block decreased slightly, but soon stabilized

to a constant value, said Jonghwan Suhr, an assistant professor of

mechanical engineering at the University of Nevada in Reno, who

received his doctorate from Rensselaer in 2005, and with Pushparaj

contributed equally to this report.

As the researchers continued to compress the block, the individual

nanotube arrays collectively and gradually adjusted to getting

squeezed, showing very little fatigue. This " shape memory, " or

viscoelastic-like behavior (although the individual nanotubes are

not themselves viscoelastic), is often observed in soft-tissue

materials.

While more promising than polymers and other engineered materials

that exhibit shape memory, carbon nanotubes by themselves do not

perform well enough to be used as a synthetic biomaterial. But

Pushparaj and his fellow researchers are combining carbon nanotubes

with different polymers to create a material they anticipate will

perform as well as soft tissue. The team is also using results from

this study to develop mechanically compliant electrical probes and

interconnects.

In addition to Pushparaj and Suhr, other contributing authors of the

paper include Pulickel Ajayan, the Henry Burlage Professor of

Materials Science and Engineering at Rensselaer; Omkaram Nalamasu,

professor of chemistry and materials science and engineering at

Rensselaer; Lijie Ci, Rensselaer research associate; Subbalakshmi

Sreekala, a research associate in the department of mechanical and

aerospace engineering at Princeton University; and X. Zhang,

research associate in the school of materials science and

engineering at Shanghai Jiao Tong University.