An adaptive material invented at Rice University combines self-healing and reversible self-stiffening properties. When cracked, the matrix of the self-adaptive composite material (SAC) quickly heals, over and over; like a sponge, the material returns to its original form after compression.

The composite consists of 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.

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, according to the Rice engineers, as their thin shells deform easily.

Liquid contents enhance the material's viscoelasticity, a measure of SAC's ability to absorb the strain and return to its original state. The spheres also have the freedom to slide past each other when compressed, but remain attached.

In tests, Rice scientists found a maximum of 683 percent increase in the material’s storage modulus – a size-independent parameter used to characterize self-stiffening behavior.

The researchers suggested that SAC may be a useful biocompatible material for tissue engineering or a lightweight, defect-tolerant structural component.