Why settle for good enough, when there can be improvements?
“Conventional hydrogels are very weak and brittle—imagine a spoon breaking through jelly,” says lead author Jeong-Yun Sun, a postdoctoral fellow at the Harvard School of Engineering and Applied Sciences (SEAS) in Harvard’s news release. “But because they are water-based and biocompatible, people would like to use them for some very challenging applications like artificial cartilage or spinal disks. For a gel to work in those settings, it has to be able to stretch and expand under compression and tension without breaking.”
Researchers at Harvard and Duke University have developed a new hydrogel that can be stretched to 21 times its initial length, and their experimental findings are published online in the Sept 5 issue of Nature. The new hydrogel also has fracture energy that is about 9 times higher than cartilage . This combination of stretchability and toughness improves the performance of hydrogels in biological and medical applications.
Hydrogels are a type of crosslinked polymer that can to store large amounts of water without dissolving. Most are biocompatible and designed for used in the human body (think: scaffolds for tissue engineering, contact lens) because of their tunable properties and high water content . But the applications of hydrogels are often limited by their mechanical behaviour; for example, low toughness limits the durability of contact lens.
The team made an extremely stretchable and tough hydrogel from two types of crosslinked polymers. Alginate, an ionically crosslinked polymer is combined with polyacrylamide, a covalently crosslinked polymer.
You can see the team’s hydrogel in action in the video below. It shows a metal ball being dropped onto the surface of the hydrogel. The weight of the ball stretches out the hydrogel, but the material doesn’t rupture. The material recovers to its initial flat form at the end of the video, showing it is extremely stretchable and has little permanent deformation (which is a good thing because you want contact lens to stay their shape).
Interestingly, this hydrogel is tougher (i.e. absorbs more energy) than either of the two polymers it’s made from. The hydrogel is tougher than the constituent materials as a result of how the two polymers behave as the hydrogel is stretched. The hydrogel consist of two different polymers, alginate with weak crossslinks and polyacrylamide with strong crosslinks. As the material is stretched, the weak ionic crosslinks in the alginate network (network refers to the 3D configuration of the crosslinks) break and reduce the stress concentration in the polyacrylamide network . But the stronger covalent crosslinks in the polyacrylamide network remains intact, and bridge the cracks formed from the ionic crosslinks breaking apart .
The stronger covalent crosslinks is also the reason why there is little permanent deformation in the material after it’s stretched . More importantly, only the ionic crosslinks linking different alginate chains together are broken as the material deforms . The aliginate chains themselves remain intact and the crosslinks can reform, meaning the hydrogel can “heal itself” .
The innovation of an elastic, water-containing hydrogel in the 1960s led to the development of soft contact lens, which had an extremely positive impact on the medical field . The continued improvements in the performance of hydrogels will undoubtedly lead to new applications with potentially even larger impacts.
 Jeong-Yun Sun, Xuanhe Zhao, Widusha R. K. Illeperuma Ovijit Chaudhuri, Kyu Hwan Oh, David J. Mooney, Joost J. Vlassak, & Zhigang Suo (2012). Highly stretchable and tough hydrogels Nature DOI: 10.1038/nature11409
 S. Khetan, C. Chung, & J.A. Burdick (2009). Tuning hydrogel properties for applications in tissue engineering. Conference proceedings : … Annual International Conference of the IEEE Engineering in Medicine and Biology Society. IEEE Engineering in Medicine and Biology Society. Conference, 2009, 2094-6 PMID: 19963530
 O. Wichterle, D. Lim (1960). Hydrophilic Gels for Biological Use Nature DOI: 10.1038/185117a0
Feature Image Source: Wikipedia user Nieuw