Solid Material Can Be Cracked, Then Heal Itself

An adaptive material invented at Rice University in Texas combines self-healinThe material is like a sugar cube that can be compressed and then regain its original shape. Image credit: Jeff Fitlow. The material is like a sugar cube that can be compressed and then regain its original shape. Image credit: Jeff Fitlow. g and reversible self-stiffening properties that could make it useful as a biocompatible material for tissue engineering or a lightweight, defect-tolerant structural component.

Materials scientists led by Pei Dong, a postdoctoral researcher, and graduate student Alin Cristian Chipara developed the self-adaptive composite (SAC), which consists of sticky, micron-scale rubber balls that form a solid matrix. The SAC was created by mixing two polymers and a solvent that evaporates when heated, leaving a porous mass of gooey spheres.

When cracked, the matrix quickly heals, over and over. And like a sponge, it returns to its original form after compression.

Other “self-healing” materials encapsulate liquid in solid shells that leak their healing contents when cracked. “Those are very cool, but we wanted to introduce more flexibility,” says 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 encapsulate much of the liquid. The viscous polydimethylsiloxane (PDMS) further coats the entire surface. The spheres are resilient, as their 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 polymer components begin as powder and viscous liquid, says Dong. With the addition of a solvent and controlled heating, the PDMS stabilizes into solid spheres that provide the reconfigurable internal structure. In tests, Rice scientists found a maximum 683% increase in the material’s storage modulus—a size-independent parameter used to characterize self-stiffening behavior. This is much larger than those reported for solid composites and other materials, they say.

According to Pulickel Ajayan, professor of engineering and chemistry, in whose lab the discovery was made, making SAC is simple, and the process can be fine-tuned—using a little more liquid or a little more solid—to regulate the product’s mechanical behavior.

“Gels have lots of liquid encapsulated in solids, but they’re too much on the very soft side,” Ajayan says. “We wanted something that was mechanically robust as well. What we ended up with is probably an extreme gel in which the liquid phase is only 50% or so.”

Dong says sample sizes of the putty-like material are limited only by the container in which they are made. “Right now, we’re making it in a 150-milliliter beaker, but it can be scaled up. We have a design for that.”