{"id":4865,"date":"2016-01-12T07:58:19","date_gmt":"2016-01-12T07:58:19","guid":{"rendered":"http:\/\/www.kurzweilai.net\/?p=270755"},"modified":"2016-01-13T08:08:51","modified_gmt":"2016-01-13T08:08:51","slug":"self-adaptive-material-heals-itself-stays-tough","status":"publish","type":"post","link":"https:\/\/hoo.central12.com\/fugic\/2016\/01\/12\/self-adaptive-material-heals-itself-stays-tough\/","title":{"rendered":"Self-adaptive material heals itself, stays tough"},"content":{"rendered":"
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Rice University postdoctoral researcher Pei Dong holds a sample of SAC, a new form of self-adapting composite. The material has the ability to heal itself and to regain its original shape after extraordinary compression. (credit: Jeff Fitlow\/Rice University)<\/p><\/div>\n

A flexible adaptive material invented at\u00a0Rice University<\/a> combines self-healing and reversible self-stiffening properties.<\/p>\n

The material, called SAC (for self-adaptive 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. When cracked, the matrix quickly heals, over and over. And like a sponge, it returns to its original form after compression.<\/p>\n

SAC may be a useful biocompatible material for tissue engineering or a lightweight, defect-tolerant structural component.<\/p>\n

The labs of Rice materials scientists Pulickel Ajayan<\/a> and Jun Lou<\/a> led the study that appears in the American Chemical Society journal\u00a0ACS Applied Materials and Interfaces<\/a>. They suggested<\/p>\n

Other \u201cself-healing\u201d materials encapsulate liquid in solid shells that leak their healing contents when cracked.<\/p>\n

In SAC, tiny spheres of\u00a0polyvinylidene fluoride<\/a>\u00a0(PVDF) encapsulate much of the liquid. The viscous\u00a0polydimethylsiloxane<\/a>\u00a0(PDMS) further coats the entire surface. The spheres are extremely resilient, Lou said, as their thin shells deform easily. Their liquid contents enhance their\u00a0viscoelasticity<\/a>, 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.<\/p>\n

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Characterization of the hierarchical solid-liquid composite. (a) Image of solid-liquid SAC. (b) SEM image of the porous SAC. (c) High magnification SEM image of the shell of the bead. (credit: Pei Dong et al.\/ACS Appl. Mater. Interfaces)<\/p><\/div>\n

\u201cThe sample doesn\u2019t give you the impression that it contains any liquid,\u201d he said. \u201cThat\u2019s very different from a gel. This is not really squishy; it\u2019s more like a sugar cube that you can compress quite a lot. The nice thing is that it recovers.\u201d<\/p>\n

Making SAC is simple, and the process can be tuned — a little more liquid or a little more solid — to regulate the product\u2019s mechanical behavior. The polymer components begin as powder and viscous liquid. With the addition of a solvent and controlled heating, the PDMS stabilizes into solid spheres that provide the reconfigurable internal structure.<\/p>\n

In tests, Rice scientists found a maximum of 683 percent increase in the material\u2019s storage modulus — a size-independent parameter used to characterize self-stiffening behavior. This is much larger than that reported for solid composites and other materials, they said.<\/p>\n