High-performance structural protein fibers have attracted substantial attention owing to their good mechanical properties and biocompatibility. The manipulation of internal interactions at the molecular level within protein fibers is thus of particular importance but challenging, severely limiting their tunability in macroscopic performances and applications, e.g., environmental resistance. Herein, we introduce a random coiled elastin-like protein and for the first time we propose the strategy regarding dynamic imine fiber chemistry (DIFC) for mechanical recoverability and stimulus-triggered responses. In particular, the DIF fibers exhibited exceptional long-term mechanical stability and recoverability against extreme environments (Fig. 1).

Fig. 1 Design and preparation of high-performance DIF fibers by developing dynamic imine fiber chemistry. The resulting DIF fibers showed a typical humidity-regulated actuation behavior.

Mechanical Performance of the DIF fibers under different conditions

In this work, the DIF-36, DIF-72, DIF-144, and DIF-144cys fibers were employed to systematically investigate the effect of as-spinning and post-stretching treatment on the mechanical performance of DIF fibers (testing 30 times for each group). All results demonstrated an increase in ultimate tensile strength as the protein molecular weight increased from 19 kDa (K-36) to 72 kDa (K-144cys). Besides, the post-stretched fibers exhibited distinct mechanical behaviors after post-stretching treatment, yielding much higher ultimate tensile strength in the range of 200~420 MPa and stiffness in the range of 2~ 5.5 GPa, outperforming many artificial synthetic protein and polymer fibers. These findings suggest that high molecular weight and well-aligned molecular arrangement induced by post-stretching treatment are essential to improve the mechanical performance of the DIF fibers.

As measured, the DIF fibers exhibited recoverable mechanical properties under different conditions and long-term mechanical stabilities under nonsterile ambient conditions (Fig. 2). All these fibers could be recovered or restored to their original values after immersion at pH=8 and re-dried in the air due to the broken and reformed dynamic imine bonds. Interestingly, the DIF-72 fibers demonstrated exceptional reliability even under extremely cold conditions. After being treated in liquid nitrogen for 12 hours, the ultimate tensile strength and toughness of as-spun and post-stretched DIF-72 fibers showed no obvious decrease, suggesting their excellent stability at -196°C. We hypothesized that the introduction of imine-based cross-linking networks might prevent the approach or formation of ice clusters within the fibers at low temperatures, avoiding stress concentrations and cracking, thus maintaining the stability of the DIF-72 fibers. More importantly, no distinguishable difference was observed in the mechanical performance of DIF-72 fibers after 3-, and 8-months’ storage, suggesting the good stability of DIF-72 fibers. Furthermore, our DIF fibers showed high-temperature resistance properties. Such phenomenon might be attributed to imine-based cross-linking networks within DIF fibers. These cross-linking networks might maintain the stability of the protein system to some extent in the range of 100°C to 150°C.

Fig. 2 Recoverable mechanical properties and stabilities of the DIF fibers regulated by dynamic imine network under different conditions. a, Schematic illustrates the underlying mechanism of the anti-fatigued behavior for the DIF fibers. b-e, Ultimate tensile strength of the post-stretched DIF-72 fibers under different pH treatment (b), at -196°C for 12 hours (c), at non-sterile ambient conditions for 3-month and 8-month (d), and at ex-situ high temperatures (100, 150, and 200°C) for 12 hours (e).

Humidity-triggered actuations of the DIF fibers.

We further observed different water-regulated actuation behaviors of the as-spun DIF-72 and post-stretched DIF-72 fibers (Fig. 3). When in contact with an aqueous solution, self-folding and extending of as-spun DIF-72 fibers were observed. For a typical as-spun DIF-72 fiber of length=50 mm, the fiber extended to 63 mm in water and recovered to its original length upon dehydration. On this basis, a humidity-responsive actuator was designed and this actuator exhibited extraordinary reciprocating behaviors. It can reversibly lift and release the clip by 20% of its original length in many cycles when the fiber is dehydrated at 30% humidity and hydrated at 100% humidity, respectively (Fig. 4). Unlike the self-folding and extending of as-spun DIF-72 fibers in water over time, the post-stretched DIF-72 fibers shrank when they were contacted with water due to the increased entropy. As observed, the twisted fiber bundles of 20 post-stretched fibers rapidly contracted when triggered by water and the contraction forces are strong enough to seal a 3 mm notch in agar gel or porcine skin.

Fig. 3 Water-regulated actuation behaviors of the as-spun DIF-72 fibers and the post-stretched DIF-72 fibers. a, Schematics illustrating the underlying mechanism for the self-folding and extending of the as-spun DIF-72 fiber in water over time. b, Length of a typical as-spun DIF-72 fiber before hydration, after hydration, and after dehydration. c, Water-induced contraction of the post-stretched fibers. d, Contraction of the post-stretched DIF-72 fiber bundles upon hydration sealed a 3 mm notch in agar gel (top) and porcine skin (bottom).

Fig. 4 The reversible extension and contraction of the as-spun DIF fibers upon hydration and dehydration.

Conclusion and Perspective

By combing protein engineering and dynamic imine bonds, we demonstrate the use of dynamic imine chemistry for engineering molecular interactions and forming strong and tough protein fibers. In contrast to conventional protein fibers with inferior environmental adaptability, the reversible disassembly and reassembly properties of imine bonds within the protein fibers allowed DIF fibers with tunable mechanical performances under different conditions. In particular, owing to the hydration inhomogeneity and entropy increase within the DIFs, their versatile self-motivated actuation modes were achieved. Overall, our strategy enables the precise engineering of intra- and inter-molecular interactions via the manipulation of amino acid sequences and the introduction of dynamic amine bonds between protein chains, thus achieving high mechanical performances and diverse functionalities and intriguing new inspirations for the design of protein fibers for high-tech applications.

For more details of this work, please see our recent publication in Nature Communications. Nat Commun 14, 5348 (2023). https://doi.org/10.1038/s41467-023-41084-1.