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Material self-heals and returns to its original form

12 January 2016

An adaptive material developed by researchers at Rice University in the US combines self-healing and reversible self-stiffening properties.

Rice University postdoctoral researcher Pei Dong holds a sample of SAC, a new form of self-adapting composite (photo: Jeff Fitlow/Rice University)

The material, called SAC (self-adaptive composite), comprises sticky, micron-scale rubber balls that form a solid matrix. The researchers made SAC by mixing two polymers and a solvent that evaporates when heated, leaving a porous mass of gooey spheres. When cracked, the matrix quickly heals and, like a sponge, it returns to its original form after compression.

Rice materials scientists, Pulickel Ajayan and Jun Lou suggest SAC may be a useful biocompatible material for tissue engineering or a lightweight, defect-tolerant structural component.

Other 'self-healing' materials encapsulate liquid in solid shells that leak their healing contents when cracked. "We wanted to introduce more flexibility," says co-researcher, Pei Dong. "We wanted a biomimetic material that could change itself, or its inner structure, to adapt to external stimulation and thought introducing more liquid would be a way. But we wanted the liquid to be stable instead of flowing everywhere."

In SAC, tiny spheres of polyvinylidene fluoride (PVDF) encapsulate much of the liquid. The viscous polydimethylsiloxane (PDMS) further coats the entire surface. The spheres are extremely resilient, as their thin shells deform easily. Their liquid contents enhance their viscoelasticity, a measure of their ability to absorb the strain and return to their original state, while the coatings keep the spheres together. The spheres also have the freedom to slide past each other when compressed, but remain attached.

"The sample doesn't give you the impression that it contains any liquid," Lou says. "That's very different from a gel. This is not really squishy; it's more like a sugar cube that you can compress quite a lot. The nice thing is that it recovers."

Moreover, the SAC process can be tuned - adding a little more liquid or a little more solid - to regulate the product's mechanical behaviour.

An article describing the work appears in the American Chemical Society journal ACS Applied Materials and Interfaces.

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