Carey compared the new material to the bones in the racket arm of a tennis player, which are denser from stress  (Source:
Could eventually be used to develop artificial cartilage

Rice University researchers have developed a self-strengthening nanocomposite that works much like muscles and bones that strengthen after working out.

Pulickel Ajayan, study leader and professor of mechanical engineering, materials science and chemistry at Rice University, and Brent Carey, a graduate student in Ajayan's lab, have found that a certain synthetic material is capable of strengthening itself through repeated stress

The material consists of polymer-based nanocomposites with carbon tube filters. Carey discovered the strengthening capability when testing the fatigue properties of a composite he created by allowing the access of polydimethylsiloxane (PDMS), a rubbery polymer, into several multi-walled and vertical nanotubes.  

When loading the material repeatedly, it didn't wreck it like Carey thought it would. Instead, the stress made the material stronger. Carey tested this out using dynamic mechanical analysis (DMA) and found that 3.5 million compressions over one week's time increased the composite's strength, or stiffness, by 12 percent.

"It took a bit of tweaking to get the instrument to do this," said Carey. "DMA generally assumes that your material isn't changing in any permanent way. In the early tests, the software kept telling me, 'I've damaged the sample!' as the stiffness increased. I also had to trick it with an unsolvable program loop to achieve the high number of cycles." 

Carey and Ajayan also discovered that compressing the material wasn't enough to change its properties. Deforming it repeatedly, known as dynamic stress, was the only way to make the material stiffen. 

"As long as you're regularly stressing a bone in the body, it will remain strong," said Carey. "For example, the bones in the racket arm of a tennis player are denser. Essentially, this is an adaptive effect our body uses to withstand the loads applied to it. Our material is similar in the sense that a static load on our composite doesn't cause a change. You have to dynamically stress it in order to improve it." 

While both Carey and Ajayan have noted that they're not sure as to why their material acts the way it does, and that basic research at this level tends to "ask more questions than it answers," they do know that the dynamic fluid interface between nanostructures and polymers in the engineered nanocomposites evolves while the material is stressed, leading them to believe that this interface is key to what makes it become stiff. 

"The data shows that there's very little chemical interaction, if any, between the polymer and the nanotubes, and it seems that this fluid interface is evolving during stressing," said Carey. 

Ajayan also mentioned that nanomaterials used as a filler "increases this interfacial area tremendously for the same amount of filler material added," meaning that the interfacial effects become amplified. 

Researchers hope this invention can eventually be used to develop artificial cartilage that can strengthen itself through stress.  

This study was published in ACS Nano.

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