Physical hydrogels composed of polyampholytes demonstrate high toughness and viscoelasticity

Journal name:
Nature Materials
Year published:
Published online


Hydrogels attract great attention as biomaterials as a result of their soft and wet nature, similar to that of biological tissues. Recent inventions of several tough hydrogels show their potential as structural biomaterials, such as cartilage. Any given application, however, requires a combination of mechanical properties including stiffness, strength, toughness, damping, fatigue resistance and self-healing, along with biocompatibility. This combination is rarely realized. Here, we report that polyampholytes, polymers bearing randomly dispersed cationic and anionic repeat groups, form tough and viscoelastic hydrogels with multiple mechanical properties. The randomness makes ionic bonds of a wide distribution of strength. The strong bonds serve as permanent crosslinks, imparting elasticity, whereas the weak bonds reversibly break and re-form, dissipating energy. These physical hydrogels of supramolecular structure can be tuned to change multiple mechanical properties over wide ranges by using diverse ionic combinations. This polyampholyte approach is synthetically simple and dramatically increases the choice of tough hydrogels for applications.

At a glance


  1. Schematics of physical hydrogels composed of polyampholytes.
    Figure 1: Schematics of physical hydrogels composed of polyampholytes.

    a, An illustration of polyampholyte networks with ionic bonds of different strength. The strong bonds serve as permanent crosslinking points, and the weak bonds act as reversible sacrifical bonds that rupture under deformation. b, The chemical structures of monomers used in this work. Cationic monomers: MPTC, DMAEA-Q; anionic monomers: NaSS, AMPS.

  2. The effect of monomer concentration on the physical properties of polyampholytes.
    Figure 2: The effect of monomer concentration on the physical properties of polyampholytes.

    a, Photographs of the polyampholytes P(NaSS-co-MPTC) Cm-0.52 polymerized with different total monomer concentrations Cm. Numbers in the images are the values of Cm (M). A single hydrogel phase is formed at Cm > 0.7 M. b, Cm dependence of the swelling volume ratio Qv and Young’s modulus E of the polyampholyte hydrogels. c, Tensile behaviours of the polyampholyte hydrogels with different Cm. d, Dependence of the fracture stress σb and tearing energy Ton the weight fraction of polymers Cpoly of the polyampholyte hydrogels at the equilibrium swelling. Inset: a polarized microscope image of sample P(NaSS-co-MPTC) 2.1–0.52 being torn (white arrow indicates the crack tip front). All of the error bars in this work represent standard deviations.

  3. Self-recovery, fatigue resistance, adhesion and self-healing behaviours of polyampholyte hydrogels.
    Figure 3: Self-recovery, fatigue resistance, adhesion and self-healing behaviours of polyampholyte hydrogels.

    ae, The data shown in ae are for sample P(NaSS-co-MPTC) 2.1–0.52. a, Recovery of the sample for different waiting times performed by cyclic tensile tests. b, Waiting time dependence of the residual strain and hysteresis ratio (area ratio of the second hysteresis loop to the first). c, Dependence of the area of the hysteresis loop on the number of repeated cyclic tests for different loading strains. Recovery times are required between successive cyclic tests. d, Self-healing and adhesion between either two freshly cut surfaces (red and blue), or a fresh and an aged surface (white) of samples. e, Images demonstrating partial self-healing of the sample. f, Stress–strain curves of the virgin and self-healed sample P(NaSS-co-DMAEA-Q) 2.0–0.52. The self-healing in f is performed at 25 °C for 24 h in water, and others are indicated in the text.

  4. The effect of saline solution and deformation rate on properties of polyampholyte hydrogels P(NaSS-co-MPTC) 2.1–0.525.
    Figure 4: The effect of saline solution and deformation rate on properties of polyampholyte hydrogels P(NaSS-co-MPTC) 2.1–0.525.

    a, Dependence of the swelling volume ratio Qsalt,water( = V salt/V water) and Young’s modulus Eon the concentration of saline solution CNaCl (M). The blue arrow indicates the physiological solution (0.15 M) condition. b, Tensile behaviours of the hydrogels after swelling in different concentrations of saline solution CNaCl (M). c, Tensile behaviours of the hydrogels under different deformation rates ranging from 5 to 900 mm min−1. d, Deformation rate dependence of fracture stress σb and fracture strain ɛb of the hydrogels.


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Author information

  1. These authors contributed equally to this work

    • Tao Lin Sun &
    • Takayuki Kurokawa


  1. Graduate School of Science, Hokkaido University, Sapporo 060-0810, Japan

    • Tao Lin Sun &
    • Shinya Kuroda
  2. Faculty of Advanced Life Science, Hokkaido University, Sapporo 060-0810, Japan

    • Takayuki Kurokawa,
    • Md. Anamul Haque,
    • Tasuku Nakajima &
    • Jian Ping Gong
  3. Graduate School of Life Science, Hokkaido University, Sapporo 060-0810, Japan

    • Abu Bin Ihsan,
    • Taigo Akasaki &
    • Koshiro Sato


T.L.S., T.K. and J.P.G. designed the experiments. T.L.S., S.K., A.B.I., M.A.H., K.S. and T.A. performed the experiments. T.L.S., T.K., T.N. and J.P.G. analysed the data. T.L.S. and J.P.G. wrote the paper.

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The authors declare no competing financial interests.

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