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.
The self-healing and elasticity of this gel is pretty amazing. I would have liked to have this as a commercialized option for my dog who in the last two years went through two separate surgeries to repair the doggie equivalent of a torn ACL.
What's amazing to me about these hydrogels is their damage tolerance. The ability to stretch a polymeric material many times its original length is not all that noteworthy, but the ability to stretch a polymeric material with a notch in it many times its original length is totally incredible.
Understanding the mechanisms behind the toughness and damage tolerance of these hydrogels could lead to the development of tough polymers for all kinds of applications.
With regard to cartilage replacement, biocompatibility may be a hurdle. One of the biggest difficulties with cartilage replacement therapies to date has been the body rejecting the new cartilage (even when it has been grown in the lab from the patient's own cells).
Dave, I agree. I found the technical discussion a bit dense, but the ability to stretch and recover, notch or no notch, is apparently due to a mix of strong and weak molecular integration and the (resulting?) crosslinked networks.
This is amazing and sorely needed. For some young patients who have had a lot of cartilage removed, the only other alternative to is to use cadaver cartilage or an artificial knee. One of my college-age sons is now in this situation. If there was an artificial alternative that wouldn't be rejected by the body, it would be a godsend.
Ann, that's a new and interesting technology. Most of the old peoples have severe pain in their knees due to the wear and tear in cartilages around and beneath the knee cap. Any idea how we can apply this to the knee.
Charles, now there are some magnetic therapy treatments are available for regeneration of cartilages. I know some of the patient who had undergone the treatment and feels better. But so far it is not proved or accepted by any medical council.
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