Spider silk is a natural fiber with ultimate toughness and excellent environmental tolerance, and biomimicking its relevant structures is helpful to fabricate desirable hydrogel fibers. For the high toughness of spider silk, theoretical and molecular dynamics simulations suggest that \u03b2-nanocrystalline domains with dense hydrogen bonds are the key23<\/a>,24<\/a><\/sup>. Accordingly, Wu et al. prepared hydrogel fibers with high mechanical properties (tensile stress\/strain of 11.76\u2009MPa\/210.20%) and super-shrinkage properties by biomimicking the soft-hard hydrogen bonding structure of spider silk25<\/a><\/sup>. Inspired by \u03b2-nanocrystalline domain crosslinked amorphous peptides, Liu et al. designed high-strength hydrogel fibers by doping zinc ions as additional crosslinking site into polyacrylic acid systems crosslinked with vinyl-functionalized silica nanoparticles26<\/a><\/sup>. Although the high crosslink density provided the hydrogel fiber with high strength (261.00\u2009MPa), the reported strain was undesirable as 49.20%. <\/p>\n\n\n\n The current research on spider-silk-inspired robust hydrogel fibers is mainly surrounding on \u03b2-nanocrystalline domains27<\/a>,28<\/a><\/sup>. In fact, various ions play an important role in the spinning process of spider silk, and they can adjust the mechanical properties of spider silk through ionic coordination and Hofmeister effects29<\/a>,30<\/a>,31<\/a>,32<\/a>,33<\/a><\/sup>. Moreover, the hydration effect of ions and the strong hydrogen bonding effect of proteins with rich polar groups contribute to the tolerance of spider silk to low temperatures and different humidity environments34<\/a>,35<\/a><\/sup>. Apparently, the salt-regulated structural-performance paradigm of spider silk provides a great opportunity to develop strong and tough hydrogel fibers with environmental tolerance.<\/p>\n\n\n\n Inspired by the salt-regulated structural-performance paradigm from spider silks, a bionic structure engineering (BSE) strategy was presented to construct bionic hydrogel fibers with high mechanical properties and environmental tolerance. As a demonstration, anionic and crystalline domain crosslinked hydrogel fiber was constructed by utilizing the coordination of zirconium ions (Zr4+<\/sup>) with polyacrylic acid (PAA) and the Hoffmeister effect-sensitive property of polyvinyl alcohol (PVA). Meanwhile, biomimicking the environmental tolerance mechanism of spider silk, hydroxyl-rich glycerol (Gly) was introduced, the strong hydrogen bonding effect of Gly could synergize with the hydration effect of ions for enhancing the environmental tolerance of the hydrogel fiber. The prepared bionic hydrogel fiber exhibited high toughness (162.25\u2009\u00b1\u200921.99 MJ\/m3<\/sup>) comparable to spider silk, and remained excellent mechanical properties (stress > 50\u2009MPa, strain\u2009>\u2009200%) at \u221240\u2009\u00b0C or 30%RH. <\/p>\n\n\n\n Notably, just as ions were important in spider silk, adjusting the inorganic salt concentration could significantly change the modulus of the hydrogel fibers from gel to plastic levels (3.74\u2009\u00b1\u20090.16\u2009MPa to 118.53\u2009\u00b1\u20095.49\u2009MPa). Moreover, the BSE strategy was universal and directly applicable to different combinations of Hoffmeister effect-sensitive polymers and inorganic salts, offering a solution for fabricating hydrogel fibers with high mechanical properties and environmental tolerance.<\/p>\n\n\n\n Ions can contribute to the fabrication of natural spider silks with high mechanical properties through ionic coordination and Hoffmeister effects (Fig.\u00a01a<\/a>). Therefore, a BSE strategy was proposed to adjust the non-covalent interactions of polymers through inorganic salts. To successfully implement the corresponding bionic design, we employed an improved self-lubricating spinning strategy, which allowed us to freely design the hydrogel fiber network structure for the continuous fabrication of bionic hydrogel fibers36<\/a><\/sup>. For demonstration, the Hoffmeister effect-sensitive PVA and ion-coordinatable PAA were chosen as a model system. The kosmotropic sodium sulfate (Na2<\/sub>SO4<\/sub>) and high coordination number of Zr4+<\/sup>\u00a0were used to construct crystalline domains and ionic crosslinks, respectively. In addition, Gly with strong hydrogen bonding effect was introduced to enhance the environmental tolerance of bionic hydrogel fibers. <\/p>\n\n\n\n The preparation of bionic hydrogel fibers was illustrated in Fig.\u00a01b<\/a>. First, the ionic crosslinked PVA\/PAA\/Zr4+<\/sup>\u00a0(PAZr) hydrogel fiber was spun by an improved self-lubricating spinning strategy. Further, the PAZr hydrogel fiber was washed in a water bath to remove residual monomers followed by soaking in a Na2<\/sub>SO4<\/sub>\/Gly\/H2<\/sub>O ternary solvent for solvent exchange. Solvent exchange in the Na2<\/sub>SO4<\/sub>\/Gly\/H2<\/sub>O ternary solvent promoted the formation of PVA crystalline domains and removed some of the unstable ionic coordination, the PAZr hydrogel fiber was converted into the S-PAZr hydrogel fiber. As a result, the S-PAZr hydrogel fiber prepared by the BSE strategy exhibited excellent mechanical properties. The transparent S-PAZr hydrogel fiber could lift ~50,000 times its own weight (2.5\u2009kg) after folded (Fig.\u00a01c, d<\/a>). Moreover, the hydrogel fiber allowed to be woven into a racket to withstand the impact of a 100\u2009g weight falling freely from 5\u2009cm height (Fig.\u00a01e<\/a>).<\/p>\n\n\nResults<\/h3>\n\n\n\n
Fabrication of hydrogel fibers based on the BSE strategy<\/h3>\n\n\n\n
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