Highly absorbent hydrogels, used in medical and bioengineering applications, tend to be flexible, but also brittle and not very stretchable. A team of engineering and materials science researchers from Harvard University, Seoul National University, and Duke University has invented a set of tough, synthetic hydrogels that can be stretched up to 21 times their length and still recover.
Used to make scaffolds for tissue engineering, as well as drug delivery vehicles, hydrogels absorb as much as 99.9 percent water and are made of natural or synthetic polymers. They are used in biology research and medical applications because their high water content makes them flexible, like living tissue.
A new flexible, self-healing hydrogel that could replace cartilage can be stretched it to 21 times its length before breaking. (Source: Jeong-Yun Sun/Harvard University)
In an article published in Nature (subscription or payment required), the researchers say they synthesized their new materials from polymers that form ionically and covalently crosslinked networks. The new hybrid hydrogels were formed of alginate and polacrylamide, and contain about 90 percent water. Although some elastic hydrogels have stretched to between 10 and 20 times their length, samples that contained notches or other deformations stretched much less. When containing notches, the new materials stretch up to 17 times their length, and notches remain stable. (Watch a video of the notched material stretching here.)
The new hydrogels have a fracture energy of about ~9,000 J m-2, compared to ~10 J m-2 for most hydrogels and ~1,000 J m-2 for cartilage. The researchers say:
We attribute the gels' toughness to the synergy of two mechanisms: crack bridging by the network of covalent crosslinks, and hysteresis by unzipping the network of ionic crosslinks. Furthermore, the network of covalent crosslinks preserves the memory of the initial state, so that much of the large deformation is removed on unloading. The unzipped ionic crosslinks cause internal damage, which heals by re-zipping.
(Watch a video of a large, recoverable deformation formed by a metal ball dropping on a membrane of the gel here.)
Because the new gels are tough, self-healing, biocompatible, and flexible, they could be used as replacements for human cartilage, in soft robotics, as artificial muscles, or as a protective covering for wounds. Different combinations of weak and strong molecular integration could make hybrid hydrogels with different sets of characteristics.
The research team included Jeong-Yun Sun of Harvard University and Seoul National University; Widusha R. K. Illeperuma, Ovijit Chaudhuri, David J. Mooney, Joost J. Vlassak, and Zhigang Suo of Harvard University; Kyu Hwan Oh of Seoul National University; and Xuanhe Zhao of Duke University.
Ann, I hope you do get a better knee joint soon. This polymer has great potential in medical and mechanical engineering areas. Having suffered a back surgery to a blown disc, I too, would welcome a full-sized replacement in my vertebrae. I hope the trials go well with this stuff. The body likes to attack and dissolve foreign objects (mostly for good reason) so I hope this substance proves to be as compatible as it is durable.
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Focus on Fundamentals consists of 45-minute on-line classes that cover a host of technologies. You learn without leaving the comfort of your desk. All classes are taught by subject-matter experts and all are archived. So if you can't attend live, attend at your convenience.