Hydrogels are used as scaffolds for tissue engineering1, vehicles for drug delivery2, actuators for optics and fluidics3, and model extracellular matrices for biological studies4. The scope of hydrogel applications, however, is often severely limited by their mechanical behaviour5. Most hydrogels do not exhibit high stretchability; for example, an alginate hydrogel ruptures when stretched to about 1.2 times its original length. Some synthetic elastic hydrogels6,7 have achieved stretches in the range 10–20, but these values are markedly reduced in samples containing notches. Most hydrogels are brittle, with fracture energies of about 10 J m−2 (ref. 8), as compared with ∼1,000 J m−2 for cartilage9 and ∼10,000 J m−2 for natural rubbers10. Intense efforts are devoted to synthesizing hydrogels with improved mechanical properties11,12,13,14,15,16,17,18; certain synthetic gels have reached fracture energies of 100–1,000 J m−2 (refs 11, 14, 17). Here we report the synthesis of hydrogels from polymers forming ionically and covalently crosslinked networks. Although such gels contain ∼90% water, they can be stretched beyond 20 times their initial length, and have fracture energies of ∼9,000 J m−2. Even for samples containing notches, a stretch of 17 is demonstrated. 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. These gels may serve as model systems to explore mechanisms of deformation and energy dissipation, and expand the scope of hydrogel applications.
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The work at Harvard was supported by ARO (W911NF-09-1-0476), NSF (CMMI-0800161), DARPA (W911NF-10-1-0113), NIH (R37 DE013033) and MRSEC (DMR-0820484). X.Z. acknowledges the support of the NSF Research Triangle MRSEC (DMR-1121107) and Haythornthwaite Research Initiation grants. K.H.O. is supported by the National Research Foundation of Korea (NRF), funded by the Ministry of Education, Science and Technology (R11-2005-065). Z.S. acknowledges a sabbatical leave at the Karlsruhe Institute of Technology funded by the Alexander von Humboldt Award and by Harvard University.
This file contains a movie showing a crack initiating at the front of a blunted notch, and running rapidly across the sample (see Supplementary Information for full legend).
This file contains a movie that demonstrates large, recoverable deformation when a metal ball drops on a membrane of the hybrid gel (see Supplementary Information for full legend).
This file contains a movie that shows large deformation and rupture when a metal ball drops on a membrane of the hybrid gel (see Supplementary Information for full legend).
About this article
Thermodynamically stable whilst kinetically labile coordination bonds lead to strong and tough self-healing polymers
Nature Communications (2019)