Research ArticleAPPLIED SCIENCES AND ENGINEERING

Conjoined-network rendered stiff and tough hydrogels from biogenic molecules

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Science Advances  01 Feb 2019:
Vol. 5, no. 2, eaau3442
DOI: 10.1126/sciadv.aau3442
  • Fig. 1 Structure features of a conjoined-network hydrogel.

    (A and B) A conjoined-network hydrogel composed of (A) the first network(s), which used to effectively consume energy by bond rupture, and (B) the second network(s) with similar energy dissipation mechanism to the first network(s) but lower cross-linking density, which involved dual functions: partaking energy dissipation with the first network(s) at the initial stage and maintaining the integrity of the hydrogel at the large deformation stage. (C) These networks were further intertwined with each other to form a conjoined-network, which effectively distributed stress in the whole system.

  • Fig. 2 Construction of C-G-P physical conjoined-network hydrogel.

    (A and B) Scheme of (A) chitosan and (B) gelatin solution. (C and D) Optical photos of (C) the dense precipitate formed by chitosan [10 ml, 2 weight % (wt %)] with sodium phytate (1 ml, 20 wt %, pH 7.4) and (D) the loose precipitation formed by gelatin (10 ml, 20 wt %) with sodium phytate (1 ml, 20 wt %, pH 7.4). (E) C-G composite hydrogel. (F) C-G-P conjoined-network hydrogel. Domain (i) illustrates the first network consisting of the positively charged chitosan chains and phytate; domain (ii) reflects the second network built by the gelatin chains and phytate; and domain (iii) illustrates the preformed two networks that were further noncovalently linked with each other by phytate (photo credit: Liju Xu, Institute of Chemistry, Chinese Academy of Sciences).

  • Fig. 3 Mechanical properties of C-G-P conjoined-network hydrogels.

    (A) Optical images of C4-G20-Pz hydrogels after soaking in sodium phytate solution with various concentrations (0, 10, 20, and 40 wt %; pH 7.4). (B) The C4-G20-P20 hydrogel can sustain a high compression strain of 0.8 and recover most of its original shape after relaxing for 1 hour. (C) C4-G20-P20 and C4-G20-P40 hydrogels can load a weight of 2 kg without any noticeable shape deformation. (D) The egg wrapped with the C4-G20-P20 hydrogel was dropped at 45 cm off the ground and kept intact, while the bare egg certainly broke into pieces. (E) Confocal fluorescence microscopy image of MC3T3-E1 cells cultured on the C4-G20-P20 hydrogel for 48 hours. (F) Compressive stress-strain curves (inset shows magnified plot in the low-strain region) and (G) modulus and compressive toughness of C4-G20-Pz hydrogels after soaking in sodium phytate solutions at various concentrations. The error bars represent SD; sample size n = 3. (H) Compressive performance of the C4-G20-P20 conjoined-network hydrogel and the C4-G20-P20 (type B gelatin) DN hydrogel. (I) Tensile property of the conjoined-network hydrogel composed of two and three networks. (J) Scheme of tensile stress-strain curve of the normal connected multiple-network hydrogel [single-network (SN), triple-network (TN), and quadruple-network (QN)] and the conjoined-network hydrogel incorporating three networks (conjoined-network triple network). Arrows show the strain localization in triple-network and quadruple-network hydrogels (photo credit: Liju Xu, Institute of Chemistry, Chinese Academy of Sciences).

  • Fig. 4 Self-recovery and fatigue resistance behavior of the C4-G20-P20 conjoined-network hydrogel.

    (A) Sequential loading-unloading compression tests without interval and (B) the corresponding calculated total and dissipated toughness of the C4-G20-P20 hydrogel under different strains (ɛ = 0.2, 0.4, 0.6, and 0.8). (C) Recovery cyclic compression tests and (D) the corresponding calculated total and dissipated toughness of the C4-G20-P20 hydrogel for different relaxation times under a constant strain (ɛ = 0.6). (E) Fatigue resistance and (F) the corresponding calculated total and dissipated toughness of the C4-G20-P20 hydrogel with five successive loading-unloading cycles without interval under a constant strain (ɛ = 0.6). (G) Proposed mechanism for the self-recovery and fatigue resistance ability: Ionic bonds reformed at original sites or other accessible sites.

  • Fig. 5 Tunable mechanical properties of C-G-P conjoined-network hydrogels via the adjustment of cross-linker functionality and weight ratio of the first network to the second network.

    (A) Chemical structures of phosphates with different phosphate group numbers. (B) Optical photos of C4-G20-MP22, C4-G20-TPP22, and C4-G20-P20 hydrogels. Photo credit: Liju Xu, Institute of Chemistry, Chinese Academy of Sciences. (C) Compressive stress-strain curves (inset shows magnified plot in the low-strain region) and (D) modulus and compressive toughness of C4-G20-MP22, C4-G20-TPP22, and C4-G20-P20 hydrogels. (E) Schematic of Cx-Gy-P20 hydrogels with different weight ratio of chitosan/gelatin after soaking in 20 wt % sodium phytate solution (pH 7.4). (F) Compressive stress-strain curves (inset shows magnified plot in the low-strain region) and (G) modulus and compressive toughness of Cx-Gy-P20 hydrogels with different weight ratio of chitosan/gelatin after soaking in 20 wt % sodium phytate solution (pH 7.4). The error bars represent SD; sample size n = 3.

  • Fig. 6 A universal approach to stiff and tough conjoined-network hydrogels.

    (A) Schematic structures of carboxylates with different numbers of carboxyl group. (B) Photographs of C4-G20-AA22, C4-G20-OA16, and C4-G20-CA23 hydrogels. Photo credit: Liju Xu, Institute of Chemistry, Chinese Academy of Sciences. (C) Compressive stress-strain curves (inset reveals magnified plot in the low-strain region) and (D) modulus and compressive toughness of C4-G20-AA22, C4-G20-OA16, and C4-G20-CA23 hydrogels. The error bars represent SD; sample size n = 3.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/5/2/eaau3442/DC1

    Fig. S1. Microscopic network and tensile mechanical properties of conjoined-network hydrogels.

    Fig. S2. Biocompatibility of C4-G20-P20 conjoined-network hydrogel.

    Fig. S3. Mechanical properties of C4-G20 composite hydrogel.

    Fig. S4. Fracture energy of C-G-P conjoined-network hydrogels.

    Fig. S5. The dissipative capacity and fatigue resistance behavior of C4-G20 composite hydrogel.

    Fig. S6. Fatigue resistance and self-recovery behavior of C4-G20-P20 conjoined-network hydrogel under human body temperature conditions (37°C).

    Fig. S7. Precipitate formation by chitosan with various phosphates and effect of soaking media pH on mechanical behavior of conjoined-network hydrogel.

    Fig. S8. The effect of weight ratio of the first network to the second network on the mechanical properties and the swelling properties of C-G-P conjoined-network hydrogels.

    Fig. S9. Compressive stress-strain curve of the gelatin hydrogel without sodium phytate at a similar solid content to those Cx-Gy-P20 conjoined-network hydrogels and tunable mechanics (compressive modulus and toughness) of C-G-P conjoined-network hydrogels.

    Table S1. Quantitative comparison of the mechanical properties of C-G-P conjoined-network hydrogels with other natural polymer hydrogels, synthetic polymer hydrogels, and articular cartilage.

    Movie S1. This movie showing the stiff and tough C4-G20-P20 conjoined-network hydrogel can be used as a structural material to protect fragile objects (for example, an egg).

    Movie S2. This movie was shot at the same time with movie S1 at a close range.

  • Supplementary Materials

    The PDF file includes:

    • Fig. S1. Microscopic network and tensile mechanical properties of conjoined-network hydrogels.
    • Fig. S2. Biocompatibility of C4-G20-P20 conjoined-network hydrogel.
    • Fig. S3. Mechanical properties of C4-G20 composite hydrogel.
    • Fig. S4. Fracture energy of C-G-P conjoined-network hydrogels.
    • Fig. S5. The dissipative capacity and fatigue resistance behavior of C4-G20 composite hydrogel.
    • Fig. S6. Fatigue resistance and self-recovery behavior of C4-G20-P20 conjoined-network hydrogel under human body temperature conditions (37°C).
    • Fig. S7. Precipitate formation by chitosan with various phosphates and effect of soaking media pH on mechanical behavior of conjoined-network hydrogel.
    • Fig. S8. The effect of weight ratio of the first network to the second network on the mechanical properties and the swelling properties of C-G-P conjoined-network hydrogels.
    • Fig. S9. Compressive stress-strain curve of the gelatin hydrogel without sodium phytate at a similar solid content to those Cx-Gy-P20 conjoined-network hydrogels and tunable mechanics (compressive modulus and toughness) of C-G-P conjoined-network hydrogels.
    • Table S1. Quantitative comparison of the mechanical properties of C-G-P conjoined-network hydrogels with other natural polymer hydrogels, synthetic polymer hydrogels, and articular cartilage.

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    Other Supplementary Material for this manuscript includes the following:

    • Movie S1 (.mp4 format). This movie showing the stiff and tough C4-G20-P20 conjoined-network hydrogel can be used as a structural material to protect fragile objects (for example, an egg).
    • Movie S2 (.mp4 format). This movie was shot at the same time with movie S1 at a close range.

    Files in this Data Supplement:

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