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Web Date: September 6, 2012

A Hydrogel That’s Soft, Stretchy, And Tough

Materials Science: Soft material has unusual properties, thanks to a blend of ionically and covalently cross-linked polymers
Department: Science & Technology
News Channels: Materials SCENE
Keywords: hydrogel, alginate, polyacrylamide, double-network
20120906lnp2-alginate

 

A hydrogel with record-busting toughness that’s superstretchy to boot has been created by a team of engineers (Nature, DOI: 10.1038/nature11409). The new material could improve the performance of products made from hydrogels, such as flexible contact lenses and tissue engineering scaffolds. It could also open up applications for which hydrogels were previously considered too prone to breakage to be useful, such as in materials for cartilage replacement.

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Credit: Jeong-Yun Sun
Hydrogel in its unstretched state.
 
Credit: Jeong-Yun Sun
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Credit: Jeong-Yun Sun
Hydrogel stretched to 20x its original thickness.
 
Credit: Jeong-Yun Sun

Alginate-polyacrylamide hydrogel stretches 20 times its original length without breaking.

Hydrogels are made mostly of water—roughly 90%—held into a solid form by water-insoluble polymers. They don’t typically stretch very far, and they are usually quite brittle, with a fracture energy of only about 10 J/m2. By contrast, the new hydrogel has a fracture energy of about 9,000 J/m2 and can stretch to 20 times its initial length without breaking.

The real breakthrough in creating tougher, stretchier hydrogels came in 2003, says Zhigang Suo, the Harvard University mechanical engineering professor who led the development of the new hydrogel. That’s when a team led by Jian Ping Gong, a professor at Hokkaido University, in Japan, came up with the idea of covalently linking two types of polymers in a hydrogel (Adv. Mater., DOI: 10.1002/adma.200304907).

These so-called double-network hydrogels feature a primary polymer network that’s dense in covalent cross-links and a secondary polymer network with fewer cross-links. The primary network is quite brittle and will fracture under significant strain, breaking its covalent cross-links. When that happens, the secondary network disperses the energy through the rest of the hydrogel, lending the material toughness.

The problem with Gong’s hydrogels was that all the polymers were covalently crosslinked, so they could take stress after, but once those bonds were broken they didn’t re-form and the material failed under repeated stress.

Suo’s team came up with the idea of using alginate, a polymer that forms ionic cross-links in the presence of calcium ions, as the primary network. The alginate will break at these ionic cross-links under stress, but the cross-links will re-form when the stress is relieved.

Combining alginate with covalently cross-linked polyacrylamide in a 1:8 ratio creates a material that’s far tougher than either material on its own, Suo notes. Furthermore, if the hydrogel is pierced, that hole won’t propagate through the material, even when it is stretched to 17 times its original length. So, Suo says, one could imagine stitching through the hydrogel without making it prone to breakage when it’s stretched.

The synthesis of the hydrogel is simple, Suo says. “We’re mechanical engineers. We don’t know much chemistry,” he confesses. The researchers essentially just mix two powders in water and shine ultraviolet light on the solution. “There are no fancy materials,” Suo says. “Labs around the world can try it in a matter of weeks.”

“It is amazing to see that the double-network structure can work out so excellently,” comments Hokkaido University’s Gong. “This work will encourage polymer scientists to pursue more possibilities for hydrogels that still have few practical applications at the moment,” she says.

“The depth of the mechanics analysis is what makes this research particularly impressive,” adds Kenneth R. Shull, a materials science professor at Northwestern University. The researchers not only provide detailed answers about the properties soft materials can possess, he says, but they also leave soft-matter scientists with some interesting questions too, such as how these materials behave on a molecular level.

 
Chemical & Engineering News
ISSN 0009-2347
Copyright © American Chemical Society
Comments
Sam Anson (Fri Sep 07 04:46:33 EDT 2012)
This is impressive and has massive potential in the medical polymers industry for urinary stents. I resported on a new hydrogel urinary stent and catheter manufacturing process in the last issue of Medical Plastics News http://bit.ly/U0180H
scott epstein (Thu Jun 27 16:37:19 EDT 2013)
Q Urological pAguamedicina stents and catheters (US Patent for process and utility applications) issued 2012)were discussed in Sam Ansons article focusing on 100% all hydrogel devices that exhibit signiifcant elongation equivalent to any latex or silicone. More so these devices are cleared by the FDA for use as ueteral stents, and pending urethral stent and catheter use. There is nothing like it, to our knowledge.

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