Highly stretchable and tough hydrogels

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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 10Jm−2 (ref. 8), as compared with ~1,000Jm−2 for cartilage9 and ~10,000Jm−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 100–1,000Jm−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 ~9,000Jm−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.

At a glance


  1. Schematics of three types of hydrogel.
    Figure 1: Schematics of three types of hydrogel.

    a, In an alginate gel, the G blocks on different polymer chains form ionic crosslinks through Ca2+ (red circles). b, In a polyacrylamide gel, the polymer chains form covalent crosslinks through N,N-methylenebisacrylamide (MBAA; green squares). c, In an alginate–polyacrylamide hybrid gel, the two types of polymer network are intertwined, and joined by covalent crosslinks (blue triangles) between amine groups on polyacrylamide chains and carboxyl groups on alginate chains. Materials used were as follows: alginate (FMC Biopolymer, LF 20/40); acrylamide (Sigma, A8887); ammonium persulphate (Sigma, A9164); MBAA (Sigma, M7279); N,N,N′,N′-tetramethylethylenediamine (Sigma, T7024); CaSO4·2H2O (Sigma, 31221); ultraviolet lamp (Hoefer, UVC 500).

  2. The hybrid gel is highly stretchable and notch-insensitive.
    Figure 2: The hybrid gel is highly stretchable and notch-insensitive.

    a, A strip of the undeformed gel was glued to two rigid clamps. b, The gel was stretched to 21 times its initial length in a tensile machine (Instron model 3342). The stretch, λ, is defined by the distance between the two clamps when the gel is deformed, divided by the distance when the gel is undeformed. c, A notch was cut into the gel, using a razor blade; a small stretch of 1.15 was used to make the notch clearly visible. d, The gel containing the notch was stretched to 17 times its initial length. The alginate/acrylamide ratio was 1:8. The weight of the covalent crosslinker, MBAA, was fixed at 0.0006 that of acrylamide; the weight of the ionic crosslinker, CaSO4, was fixed at 0.1328 that of alginate.

  3. Mechanical tests under various conditions.
    Figure 3: Mechanical tests under various conditions.

    a, Stress–stretch curves of the three types of gel, each stretched to rupture. The nominal stress, s, is defined as the force applied on the deformed gel, divided by the cross-sectional area of the undeformed gel. b, The gels were each loaded to a stretch of 1.2, just below the value that would rupture the alginate gel, and were then unloaded. c, Samples of the hybrid gel were subjected to a cycle of loading and unloading of varying maximum stretch. d, After the first cycle of loading and unloading (red curve), one sample was reloaded immediately, and the other sample was reloaded after 1 day (black curves, as labelled). e, Recovery of samples stored at 80°C for different durations, as labelled. f, The work of the second loading, W2nd, normalized by that of the first loading, W1st, measured for samples stored for different durations at different temperatures. The alginate/acrylamide ratio was 1:8 for a and b, and 1:6 for cf. Weights of crosslinkers were fixed as described in Fig. 2 legend.

  4. Composition greatly affects behaviour of the hybrid gel.
    Figure 4: Composition greatly affects behaviour of the hybrid gel.

    a, Stress–strain curves of gels of various weight ratios of acrylamide to (acrylamide plus alginate), as labelled. Each test was conducted by pulling an unnotched sample to rupture. b, Elastic moduli calculated from stress–strain curves, plotted against weight ratio. c, Critical stretch, λc, for notched gels of various weight ratios, measured by pulling the gels to rupture. d, Fracture energy, Γ, as a function of weight ratio. Weights of crosslinkers were fixed as described in Fig. 2 legend. Error bars show standard deviation; sample size n = 4.


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Author information


  1. School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts 02138, USA

    • Jeong-Yun Sun,
    • Widusha R. K. Illeperuma,
    • Ovijit Chaudhuri,
    • David J. Mooney,
    • Joost J. Vlassak &
    • Zhigang Suo
  2. Department of Material Science and Engineering, Seoul National University, Seoul 151-742, South Korea

    • Jeong-Yun Sun &
    • Kyu Hwan Oh
  3. Department of Mechanical Engineering and Materials Science, Duke University, Durham, North Carolina 27708, USA

    • Xuanhe Zhao
  4. Wyss Institute for Biologically Inspired Engineering, Harvard University, Cambridge, Massachusetts 02138, USA

    • David J. Mooney
  5. Kavli Institute for Bionano Science and Technology, Harvard University, Cambridge, Massachusetts 02138, USA

    • Zhigang Suo


J.-Y.S., X.Z., W.R.K.I., D.J.M., J.J.V. and Z.S. designed the study and interpreted the results. X.Z. developed the protocol for fabrication of the gels and prepared initial samples. J.-Y.S. and W.R.K.I. improved the protocol, and performed mechanical tests and recovery tests. J.-Y.S. obtained Fourier transform infrared spectra and performed thermogravimetric analysis. O.C. and J.-Y.S. conducted the experiment with fluorescent alginate and that using the atomic force microscope. K.H.O. contributed to the discussion of results. J.-Y.S., W.R.K.I. and Z.S. wrote the manuscript. All authors commented on the manuscript.

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Supplementary information

PDF files

  1. Supplementary Information (1.5M)

    This file contains Supplementary Text, Supplementary Figures 1-16, Supplementary References and Full Legends for Supplementary Movies 1-3.


  1. Supplementary Movie 1 (27.9M)

    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).

  2. Supplementary Movie 2 (16.4M)

    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).

  3. Supplementary Movie 3 (21.8M)

    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).

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