Introduction

The integration of high strength, toughness and rapid stimuli-responsiveness is highly sought after for hydrogel fibers as the load-bearing and intelligent responsive materials in the fields of soft robotics1,2, artificial muscles3,4 and smart clothing5,6. Unfortunately, it remains challenging to combine these properties into a single hydrogel fiber owing to the inherent property trade-offs7,8 (e.g., strength and toughness) and the necessity for intricate network configuration that simultaneously enables structural robustness, dynamic responsiveness and network integrity under stimulus. In contrast, natural materials with incorporating the rigid phase-soft matrix network into delicate hierarchically anisotropic structure spanning from molecular to macroscopic scales, normally reconcile the competing demands in mechanical properties and dynamic responsiveness, offering a blueprint for engineering artificial smart fibers with combination of mechanical properties and actuation performance.

Spider silk, a typical natural biological fiber, achieves a tensile stress up to 1.6 GPa, elongation of 30% ~ 50% and high toughness reaching 180 MJ·m−3 resulting from the delicate hierarchically anisotropic architecture, comprising of anisotropic nanofibrils, β-crystallites, and amorphous chains9,10,11. Fascinated by their high strength-toughness synergy, bioinspired assembly strategies including mechanical stretching12,13, twisting14,15 and microfluidic spinning16,17 have been developed to engineer oriented polymer configurations in synthetic fibers mimicking aligned organization of spider silk, achieving high strength and toughness rivalling spider silk. However, the highly aligned and rigid polymer networks inherently impede the stretchability and further improvement in toughness of synthetic fiber owing to the limited energy dissipation mechanism at the molecule and nanometer scales12,18. Therefore, it is highly desired to develop delicate hierarchical structural assembly ranging from the molecular to macroscopic scales to improve the stretchability and toughness of fibers.

Furthermore, the hydration-responsive supercontraction of spider silk is attractive for synthetic smart fibers as the environmental interactive intelligent materials, requiring the adaptive all-in-one network configuration including stable crosslinks for structural integrity, hydration-responsive motifs and high elasticity in the hydration conditions for energy-efficient actuation19,20. However, tough synthetic fibers with considerable dynamic crosslinks for energy dissipation present the limited stimuli-responsiveness owing to lack of elastic systems and stable crosslinks for ensuring network integrity under hydration. Recently, topological entanglement strategy has demonstrated a success in reconciling the inherent toughness-hysteresis paradox in swollen hydrogels via reversible chain slippage as energy dissipation21,22, providing a promising route to enhance contraction behaviors in hydration-responsive systems. Unfortunately, construction of delicate hierarchical structure incorporating stimuli-responsive motifs based on the topological entanglement remains challenging owing to poor structural programmability, resulting in the relatively inferior mechanical properties and environmental responsiveness compared to structured hydrogels23,24. To bridge these gaps, a synergistic design principle is desired to achieve the programmable entanglement crosslinking system in the precursor hydrogels which permits integrating hierarchical structure with stimuli-response motifs in the post-processing crafts, realizing high toughness and supercontraction in a single gel fiber.

Herein, we engineered a kind of ionogel fibers based on high-aspect-ratio bottlebrush-shaped nanocomposite hydrogel slurry with tunable entanglement crosslinks and hydrogen bonds, yielding combinational high mechanical properties and strong hydration-triggered supercontraction via a nanoconfinement entanglement-enhanced phase separation strategy. The hierarchical structure was constructed including macroscopic helical configuration, microscale separated phase, highly entangled nanoscale skeletons and strong hydrogen bonded nanoconfined polymer networks by the dry-spinning of hydrogel slurry combined programmed assembly-assisted solvent-exchange method, facilitating multiscale energy dissipation mechanism. Consequently, the ionogel fibers provided high stiffness of 363.8 ~ 2111.7 MPa, strength of 278.5 ~ 434.1 MPa and toughness of 866.3 ~ 1652.2 MJ·m−3 coupled with a damping capacity of 93% ~ 96%, exceeding the smart-of-the-art synthetic fibers and natural spider silk. Benefiting from high network entropy elasticity ascribed to highly ordered hierarchical structure with hydration-responsive phase separation regions and efficient stress transfer between topological entanglement skeleton and network which maintained the integrity of network under hydration, the ionogel fibers offered a linear contraction actuation stress of 16.1 MPa and a large stroke of 76% upon the stimuli of hydration. This work paves a way for engineering hierarchically anisotropic structured ionogel fibers with combination of high strength, toughness and robust hydration-triggered contraction property, demonstrating the synergy of nanoconfinement entanglement and enhanced phase separation in energy dissipation and high elasticity under hydration.

Results

Fabrication of confined-entanglement aligned ionogel fibers

A nanoconfinement entanglement-enhanced phase separation strategy was developed to fabricate the hierarchically anisotropic ionogel fibers with high toughness and hydration-triggered supercontraction by the dry-spinning of the bottlebrush-shaped nanocomposite hydrogel (BSNH) combined the programmed assembly-assisted solvent-exchange method. Typically, the cellulose nanofibers (CNFs) with a high aspect-ratio (diameter: ~25 nm, length: > 3 μm) were selected to serve as the crosslinking skeletons in favor of producing considerable entanglements (Supplementary Fig. 1a). Through a facile hydrothermal reduction process, the CNF@AgNPs nanocomposites were prepared by uniformly anchoring the silver nanoparticles (AgNPs, 0.09 mM) onto 0.5 wt% of CNFs (Supplementary Fig. 1b–e), evidenced by a color transition from opaque white to light yellow with a presence of characteristic plasmonic absorption band of AgNPs at 400 nm in UV-Vis absorption spectrum25,26 (Supplementary Fig. 1f). Through RS-Ag coordination interaction, photoinitiator 2-methyl-4′-(methylthio)−2-morpholinopropiophenone (MMMP) was coated on the AgNPs, generating one-dimensional CNF@AgNPs@MMMP nanocomposite initiators. Under the ultraviolet (UV) light irradiation, the BSNH was synthesized using a “grafting from” approach via free radical copolymerization of acrylamide (AAm) and sodium acrylate (iAA) monomers (a total content: 17 wt%) on the modified nanocomposite initiator27,28 (Fig. 1a). The HRTEM images and elemental mapping demonstrated the pronounced polymer sheath coating on the CNF@AgNPs skeletons, while the Ag, N and S elements were aligned along the CNF skeletons, indicating the formation of BSNH network (Supplementary Fig. 2). Here, the length of poly(AAm-co-iAA) brushes was dependent on the concentration of monomers supported by good shape retention in inverted-bottle tests (Supplementary Fig. 3a).

Fig. 1: Design and fabrication of STIFs.
Fig. 1: Design and fabrication of STIFs.The alternative text for this image may have been generated using AI.
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a Schematic diagram of the preparation of BSNH slurry using a “grafted from” method by copolymerization of AAm and iAA on the modified nanocomposite initiator CNF@AgNPs@MMMP. b Schematic diagrams of the fabrication of STIFs by dry-spinning combined assembly-assisted solvent-exchange method. Firstly, the BSNH fibers with entangled CNF@AgNP skeletons were obtained by a dry-spinning process. By stretching the hydrogel fibers along the long axis and twisting, a temporarily elongated architecture was constructed with a helical configuration. After immersing in ionic liquid EMIES, the STIFs were finally fabricated. The trajectories of water molecules illustrated the dehydration process. c Evolution of hierarchically anisotropic structure with seamless interfaces. The CNF@AgNPs skeletons with high aspect-ratio were aligned and packed by dry-spinning, forming a nanoconfined polymer network which was further strengthened in the subsequent prestretch and twisting process. Upon solvent exchange with EMIES, the nanoconfined PAAm chains were separated from matrix to generate hard phase owing to the selectively strengthened hydrogen bonds between PAAm chains, enabling the tight interfacial connection between filaments.

Rheological tests showed that the viscosity of BSNH increased with improving the content of CNFs or monomer under static conditions (at low shear rates), while simultaneously displaying typical shear-thinning behavior with viscosity decreasing gradually upon applied shear (Supplementary Fig. 3b, c), highlighting its high entanglement and dynamic crosslinks that were essential for the subsequent spinning29,30. Furthermore, the higher storage modulus of BSNH was recorded at the lower crossover frequency for the hydrogel network with a higher CNF content, strongly indicating its more stable entanglement crosslinks, as evidenced by the slight swelling instead of dissolution of BSNH even after 1-day swelling23 (Supplementary Fig. 4). Once removing the shear force, an immediate transition from liquid (loss modulus G” > storage modulus G’) to solid state (G’ > G”) was observed (Supplementary Fig. 5a), underscoring the rapidly recovered entanglement crosslinks after shearing31.

Based on the shear-thinning behavior of BSNH, the self-supporting BSNH fibers were massively produced by a dry-spinning technique (Fig. 1b, and Supplementary Fig. 5b), after which the microstructure was optimized with the fiber diameter decreased from 673 μm–520 μm and pore size uniformly declined from 7.5 μm–1.5 μm (Supplementary Fig. 6a–d). Subsequently, a programmed assembly method was carried out for the fabrication of the hierarchically anisotropic ionogel fibers by the prestretch and twisting-assisted solvent-exchanging of the obtained BSNH fibers. Typically, when subjected to 100% axis strain, the prestretched hydrogel fiber, named as S1F, where the number 1 was the prestretch, exhibited distinct structural densification with the diameter reducing to 200 μm (Supplementary Fig. 7). With the prestretch increasing from 1 to 4, the diameter of SFs decreased from 200 μm to 80 μm and highly aligned texture was observed, as confirmed by the brighter interference color along the fiber axis in POM images for SFs at the higher stretch (Supplementary Fig. 8a). Upon twisting three prestretched S1Fs with a density of 20 turns/cm, the anisotropic hydrogel fiber with a macroscopic helical configuration was produced (denoted as S1T20F, where the number 20 was the twist density), which displayed the decreased diameter of ~120 μm for each filament (Supplementary Fig. 6e), suggesting that the incorporation of twisting further aggregated polymer networks. Notably, HRTEM image demonstrated the mean distance between adjacent CNF@AgNPs skeletons of ~30 nm after the programmed assembly, suggesting the formation of 1D nanoconfinement space among the highly aligned and entangled skeletons induced by prestretch and twisting32 (Supplementary Fig. 8b).

After immersing into ionic liquid 1-ethyl-3-methylimidazolium ethyl sulfate (EMIES) for solvent exchange, a tough ionogel fiber (denoted as S1T20IF) was obtained which delivered a macroscopic helical architecture with a bias angle of 70o and seamlessly welded interfaces between adjacent filaments (Fig. 2a). However, the control S1T20F without solvent-exchange showed visible interfacial gaps (Supplementary Fig. 6f), demonstrating the critical role of solvent exchange in strengthening interfacial binding. With the increase of twist density y, the S1TyIF showed the larger bias angle with the vague interfaces, illustrating that the higher twist density promoted the greater polymer aggregation which facilitated the interfacial fusion by solvent exchange. Crucially, no cracks between filaments of the S1T20IF were observed even subjected to a large deformation of 400% and additional twisting (Fig. 2b). Besides, considerable microparticles with the diameter of 1 ~ 2 μm were scanned on the scanning electron microscope (SEM) and atomic force microscope (AFM) height images of cross-section of S1T20IF, as indicative of the generation of phase separation regions33,34 (Fig. 2c, Supplementary Fig. 9a). Such a heterogeneous phase separation structure of S1T20IF was also recorded on the AFM phase-contrast image, which displayed dark regions with lower phase angle and light regions with higher phase angle representing the hard and soft phases, respectively35,36 (Supplementary Fig. 9b). In comparison, the non-exchanged S1T20F showed the smooth cross-section and minimal phase contrast in AFM phase angle image (Supplementary Fig 9c, d). According to the Raman imaging spectroscopy based on the NH2 deformation vibration at 1580 cm−1, the distinct intensity contrast at the cross-section of S1T20IF demonstrated the strong aggregation of PAAm chains37,38 (Fig. 2d). Specifically, the highest peak intensity at red region was 6 times that at blue region (Fig. 2e). As for the control S1T20F, more uniform distribution of NH2 bands was detected with low contrasts in peak intensity at different regions (Supplementary Fig 10). Therefore, the enhanced microphase separation was produced in the S1T20IF arising from the selectively enhanced hydrogen bonds among nanoconfined PAAm chains by the solvent-exchange of the ionic liquid EMIES that was a good solvent for PiAA and a poor solvent for PAAm33.

Fig. 2: Characterization of STIFs.
Fig. 2: Characterization of STIFs.The alternative text for this image may have been generated using AI.
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a SEM images of S1TyIFs with different twist densities. b SEM image of S1T20IF at an elongation of 400%. The inset showing the magnified interface between adjacent filaments. c SEM image of cross section of S1T20IF. d Raman mapping image of S1T20IF, based on the intensity of the NH₂ deformation vibration mode at ~1580 cm⁻¹. The color scale represents the relative intensity of the ν(NH₂) band. e Raman spectra of two regions squared in (d). The dotted arrow highlights the contrast in peak intensity between the two regions. f Temperature dependence of loss tangent (tan δ) of S1T20IF and control samples S1T0IF, S0T20IF and S1T20F. g, POM images of the S0T0IF, S1T20IF and S4T20IF viewed between crossed polarizers. A, analyzer. P, polarizer. h Variation of light transmitted through S0T0IF, S1T20IF and S4T20IF with different polarizer angles. i, Scattering intensity versus azimuthal angle plots of S0T0IF, S1T20IF and S4T20IF.

In dynamic mechanical analysis (DMA) plots, the S1T20IF presented a glass transition temperature (Tg) of 45.1 oC, greatly exceeding 20.5 oC for the S1T20F, confirming the highly confined networks by the solvent exchange which restrained the thermal motion of polymer chains39,40 (Fig. 2f). Furthermore, two control samples without the prestretch (S0T20IF) or twisting (S1T0IF) exhibited low Tg of 21.6 °C and 38.2 °C, respectively, while it dramatically increased to 62.7 °C for S4T20IF with a high prestretch and 49.7 °C for S1T40IF with a high twisting (Supplementary Fig. 11), indicating the importance of mechanical alignment in promoting the solvent-exchange induced phase aggregation by constructing aligned 1D nanoconfinement space among skeletons. In the Fourier-transform infrared spectroscopy (FT-IR) spectra, the absorption peaks of N-H at 3342 and 3196 cm−1, C-O stretching vibration at 1656 cm−1, and N-H deformation vibration at 1601 cm−1 of non-stretched S0T20IF were red-shifted to 3312, 3161, 1639 and 1589 cm−1 for the highly stretched S4T20IF, suggesting the stronger hydrogen-bonded PAAm chains which decreased the frequency of NH2 and CO bands41,42 (Supplementary Fig. 12a, b). The red-shifted characteristic peaks were also recorded for the S1TyIF fibers with the increase of twist density (Supplementary Fig. 12c, d). As such, the programmed assembly contributed to the formation of 1D nanoconfinement space among CNF@AgNPs skeletons which, assisted by solvent exchange, facilitated the phase separation with the stronger hydrogen-bonded polymer chains.

As shown from POM images, the S1T20IF displayed the brightest birefringence when rotated 45° with respect to the polarizer while retaining a visible birefringence even the long axis paralleled to the polarizer (Fig. 2g), indicating its highly aligned structure with torsional orientation configuration43. With increasing the prestretch to 4, the S4T20IF showed the brighter birefringence at each direction. In contrast, a typical birefringence of uniaxial alignment was captured for the control S0T0IF, that was, a faint birefringence at 45o to the polarizer while completely dark image paralleled to the polarizer, revealing poor alignment and the absence of torsional organization without mechanical assembly44. Quantitatively, the S1T20IF exhibited the asymmetric transmission intensity with the higher value at 135o compared to 45o, along with residual intensity when aligned parallel to the polarizer, demonstrating the higher anisotropy both along the prestretch and twisting directions (Fig. 2h). A similar asymmetric orientation structure was also delivered through the S4T20IF with the higher transmission intensity across all angles. However, the S0T0IF exhibited a four-leaf clover-shaped transmission intensity plot with the minimal peak transmission intensity at 45o and complete absence of transmission when parallel to the polarizer, as indicative of a weak uniaxial orientation configuration45. The anisotropic aligned structure of the S1T20IF was further confirmed by an oval pattern along the long axis in the 2D small-angle X-ray scattering (SAXS) image, which was further elongated along the equatorial direction for the S4T20IF13,46 (Supplementary Fig. 13a). In comparison, the S0T0IF showed the inconspicuous orientation. Correspondingly, the S4T20IF presented the highest scattering intensity in the azimuth angle curves (Fig. 2i), which gave rise to the Herman’s factor of 0.596, higher than 0.512 of the S0T0IF (Supplementary Fig. 13b).

Based on above analysis, benefiting from the tunable entanglement and hydrogen bonds among BSNH slurry, the dry-spinning combined programmed assembly-assisted solvent exchange strategy contributed to integrating hierarchically anisotropic structure at multiple scales into entanglement-crosslinked rigid-soft molecular networks including macroscopic helical configuration, enhanced microphase separation, highly anisotropic and entangled CNF@AgNPs skeletons and robust hydrogen bonds among nanoconfined networks (Fig. 1c). Firstly, the entanglement of high-aspect-ratio CNF@AgNPs skeletons combined with tunable hydrogen-bonded polymer arms provided BSNH with pronounced shear-thinning behavior, enabling the fabrication of self-supporting fibers with the pre-aligned CNF@AgNPs skeletons by dry-spinning. Then, attributed to the efficient stress transfer between CNF skeletons and polymer arms, the prestretch and twisting assembly facilitated further prealignment and entanglement of crosslinking skeletons with aggregated polymer networks and macroscopic torsional configuration. Crucially, the programmed assembly induced the generation of 1D nanoconfinement space within the oriented and entangled CNF@AgNPs skeletons which enabled the nanoconfined polymer networks. With solvent exchange, the confined PAAm chains were finally coalesced from matrix to generate enhanced microphase separation regions based on strong hydrogen bonds among nanoconfined PAAm chains, achieving seamless interfaces with delicate and compact hierarchical structure.

Mechanical properties

Attributable to the hierarchically anisotropic structure with entangled crosslinking and enhanced phase separation, the S1T20IF exhibited a high strength of 278.5 MPa and a high modulus of 703 MPa at an ultimate strain of 800%, yielding a high toughness of 1652.2 MJ·m−3 (Fig. 3a). Without the prestretch, a low strength of 120.4 MPa and modulus of 154.7 MPa were recorded for S0T20IF (Fig. 3b). Once increasing the prestretch, both the strength and modulus were significantly enhanced. As for the S4T20IF, 434.1 and 2111.7 MPa were measured, that was 3.6- and 13.6-fold increase compared to S0T20IF, respectively. Load-bearing experiments showed that the SxT20IF with a high prestretch lifting a 1 kg weight displayed a low elongation (Fig. 3c), confirming the significance of prestretch in mechanical enhancement.

Fig. 3: Mechanical properties.
Fig. 3: Mechanical properties.The alternative text for this image may have been generated using AI.
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a, b Tensile stress-strain curves (a) and variations in strength and toughness (b) of SxT20IFs at the prestretch from 0 to 4. Data in (b) are presented as mean values ± SD (n = 3). c Photographs showing that SxT20IFs at the prestretch of 0, 1 and 4 lifted a 1 kg weight. d, e Tensile stress-strain curves (d) and variations in strength and toughness (e) of S1TyIFs with the twist density from 0 to 40 turns/cm. Data in (e) are presented as mean values ± SD (n = 3). f Ashby diagrams of toughness versus strength of S1TyIFs, synthetic polymer fibers, bio-based synthetic fibers and biological fibers. g, h Cyclic loading-unloading curves (g) and calculated dissipated energy and dissipated ratio (h) of S1T20IFs under different strains. Data in (h) are presented as mean values ± SD (n = 3). i Comparison of toughness and damping capacity of the S1T20IFs with synthetic polymer fibers, tough hydrogels and biological fibers. j Schematic diagrams of measurement for impact force oscillations of a free-falling weight of 10 g tied with fibers. k Time-resolved impact force oscillations of a free-falling weight of 10 g tied with Nylon and S1T20IF. The inserted photographs showing the states of weights tied with fibers including free-falling, oscillating and achieving stability.

Different from prestretch primarily enhancing the strength, the S1TyIF fibers exhibited the counterintuitively synchronized enhancements in strength and stretchability upon twisting (Fig. 3d). Specifically, both ultimate strength and elongation increased from 226.0 MPa and 700% for S1T0IF to the maximal 278.5 MPa and 800% for S1T20IF. Further increasing twist density resulted in the declined strength and strain at break to 192.4 MPa and 500% for S1T40IF (Fig. 3e). It was found that increasing the filament number to 3 toughened the fiber with the maximal strength (Supplementary Fig. 14a). Based on the highly entangled and oriented networks, stiff phase separation regions with nanoconfined polymer chains were triggered at 20 min of solvent exchange, achieving the highest toughness (Supplementary Fig. 14b). Therefore, attributed to hierarchically anisotropic structure with enhanced phase separation, the S1T20IF achieved the reconciled strength and toughness which was even one order magnitude higher than biological fibers and reported synthetic fibers (Fig. 3f), highlighting the effectiveness of programmed mechanical assembly-assisted solvent exchange in inducing structural densification, orderization and hierarchy, and ultimately enhancing mechanical strength and toughness.

Notably, the bottlebrush-shaped nanocomposite configuration greatly influenced mechanical performance through interconnected network regulation. Once the CNF@AgNPs were substituted with a physical blending of CNFs and AgNPs to prepare the hydrogel slurry for constructing the ionogel fibers (denoted as PB-S1T20IF), the greatly decreased strength of 54.4 MPa and toughness of 403 MJ·m−3 were recorded, revealing the essence of strong interfacial interactions between CNFs and polymer networks in stress transfer and entanglement-mediated energy dissipation (Supplementary Fig. 15a). Moreover, the increasing of the CNF content would produce a more entangled network achieving the optimal combination of strength and toughness (Supplementary Fig. 15b). A dramatic increase of the modulus from 30.6 MPa to 393.8 MPa was recorded upon increasing the content from 0.3 wt% to 0.5 wt% owing to the formation of nanoconfinement networks at the high CNF content32. With the monomer content increasing from 10 to 23 wt%, the fracture strength increased from 97.1 to 309.7 MPa arising from the formation of the longer polymer arms (Supplementary Fig. 15c). Furthermore, when improving the ratio of monomer AAm to iAA, the S1T20IF was toughed owing to the stiffer phase separation regions based on the hydrogen bonded PAAm chains by solvent exchange (Supplementary Fig. 15d). As the adsorption time of MMMP onto CNF@AgNPs increased from 2 to 10 min, the strength increased markedly from 108 MPa to 278.5 MPa coupled with a rise in toughness from 532.7 MJ·m−3 to 1652.2 MJ·m−3 (Supplementary Fig. 16a). The further prolonging adsorption caused the excessive crosslinking, resulting in the enhanced modulus from 393.8 MPa to 947.1 MPa, but the decreased fracture strength to 248.6 MPa and toughness to 1409 MJ·m−3. Meanwhile, the modulus increased from 57.4 MPa to 1073.5 MPa with the MMMP content increasing from 0.5 to 2 mg (Supplementary Fig. 16b). Both the strength and toughness improved from 128.0 MPa and 747 MJ·m−3 to the maximal 278.5 MPa and 1652.2 MJ·m−3 when the MMMP content increased from 0.5 to 1 mg, and sharply dropped to 148.1 MPa and 695.4 MJ·m−3 at a high MMMP content of 2 mg, further validating the formation of excessive crosslinking and brittle networks.

As indicated by apparent hysteresis loops in the tensile loading-unloading tests at the strains from 0 to 500%, the S1T20IF exhibited the higher damping capacity at the large strain (Fig. 3g). Even at a low strain of 100%, the S1T20IF providing a high stretching work of 119.3 MJ·m−3 featured a large dissipated energy of 112.0 MJ·m−3 with a dissipated ratio of 93.9% (Fig. 3h). When the strain was set as 500%, the dissipated ratio increased to 95.4% with the dissipated energy increasing to 822.7 MJ·m−3, which was 7.3 times that at 100% strain. Such an excellent damping capacity far exceeded spider silk featuring a high damping capacity of 50% ~ 70% as well as most natural and synthetic fibers (Fig. 3i). To evaluate damping capacity of S1T20IF in potential shock-absorbing applications, the time-resolved force oscillation measurements were performed by free-fall dropping a 10 g weight tied to a 4 cm-long S1T20IF from a height of 6 cm (Fig. 3j). It was found that the S1T20IF generated a peak force of 0.63 N following by rapid stabilization at 0.49 N, which was evidenced by the weight rapidly calming down in 1 s (Fig. 3k). In comparison, a Nylon fiber with a damping capacity of 40% could not effectively slow down the force produced by falling weight, suffering a 5.7-fold higher maximum impact force (3.59 N) than S1T20IF and requiring twice the stabilization time.

Furthermore, the S1T20IF possessed good recyclability by dissolving the wasted fibers in water under a high-speed mechanical stirring at 60 oC which facilitated the disentanglement of CNF@AgNPs skeletons and dissociation of hydrogen-bonded PAAm chains. The dissolved solution retained high processability, allowing reformation of ionogel fibers via the original fabrication protocol (Supplementary Fig. 17a). Crucially, the recycled S1T20IF maintained high strength of 234.8 MPa and toughness of 1048 MJ·m−3 even after 5 cycles (Supplementary Fig. 17b, c). The recycling stability established the SxTyIF as a sustainable material platform for applications requiring both high mechanical properties and high durability.

Toughening mechanism

The multiscale structural evolution of the S1T20IF under progressive strains was characterized to illuminate the strengthening and toughening mechanism. At the small elongation (0 ~ 50%), the stretching vibrations of N-H at 3336 cm−1 and 3184 cm−1, and deformation vibrations of C-O at 1647 cm−1 and NH2 at 1598 cm−1 in FT-IR spectra were blue-shifted to 3342, 3198, 1652 and 1600 cm−1, respectively, revealing the dissociated hydrogen bonds among nanoconfined polymer chains (Fig. 4a, b). Meanwhile, DMA plots demonstrated the Tg greatly decreasing from 45.4 oC to 34.8 oC, indicating the increasing chain mobility owing to the broken phase separation regions coupled with the disentangled CNF@AgNPs skeleton and polymer network (Fig. 4c). The rupture of rigid phase-aggregation regions originating from the enhanced hydrogen bonding among the nanoconfined networks required the higher force compared to the deformation of amorphous regions, resulting in a steep increase of stress and distinct yield point at the low strain. In the subsequent stretching (50% ~ 600%), the continuously blue-shifted absorption peaks in FT-IR spectra and decreased Tg suggested that the hydrogen bonds among disentangled polymer networks and CNF@AgNPs skeletons were gradually dissociated for energy dissipation with the deformation of ionogel fibers. Meanwhile, as the elongation increased from 0 to 600%, the 2D SAXS images exhibited more pronounced streak along the stretching direction with the estimated Herman’s factor increasing from 0.6 to 0.71, illustrating the formation of higher orientation degree at the higher stretch (Fig. 4d, Supplementary Fig. 18). The Ag-S coordination crosslinks between highly entangled CNF@AgNPs skeletons and nanoconfined polymer networks were beneficial to the stress transfer and high alignment during stretch, stiffening and strengthening the ionogel fiber after the yielding point47. In SEM images, the microcracks were observed at the interfaces between filaments which were obvious at the higher elongation, suggesting the slight slippage of filaments dissipating energy and delaying fracture (Supplementary Fig. 19). At a large elongation ratio of 800%, the pull-out of CNF@AgNPs provided the further energy dissipation and concurrently, the microfibers bridged the microcracks that formed under high load, jointly contributing to the high stretchability and toughness (Fig. 4e). As the strain further increased to the rupture point (810%), considerable microfibers were visible at the blunted tip of the macroscopic crack on the ruptured filaments, indicating that the aligned nanofibers effectively inhibited the macroscopic rupture through a crack-bridging mechanism (Fig. 4f).

Fig. 4: Toughening mechanism.
Fig. 4: Toughening mechanism.The alternative text for this image may have been generated using AI.
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a–d FT-IR spectra (a, b), temperature dependence of loss tangent (tan δ) (c), and scattering intensity versus azimuthal angle plots (d) of S1T20IF at different elongation strains. The dotted lines in (a, b) show the displacement of the peaks. e, f Side-view SEM images of S1T20IF suffering a micro-crack and macroscopic fracture at the strain of (e) 800% and (f) 810%. The notch boundaries are marked by white dotted lines. g Schematic illustration of toughening mechanism, including progressive break of phase separation regions, dissociation of hydrogen bonds, disentanglement of CNF@AgNPs skeletons and polymer chains, and fibrillary bridging for dissipating energy.

The importance of macroscopic helical geometry in toughening the fibers was also highlighted48,49 (Supplementary Fig. 20a). The variations in elongation ratio L′/L0′ of the individual filament was calculated as \({{{{\rm{L}}}}}^{{\prime} }/{{{{\rm{L}}_{0}}}}^{\prime}=\frac{{L}_{1}}{{COS}{{{{\rm{\theta }}}}}_{1}}/\frac{{L}_{0}}{{COS}{{{{\rm{\theta }}}}}_{0}}\) and the bias angle θ of filaments could be calculated as \(\theta=\arctan \frac{R}{L}\), where the L0 and L1, θ0 and θ1 were the length of ionogel fiber and bias angle of filaments before and after deformation, respectively. Both the theoretical and experimental L′/L0′ were significantly lower than the L/L0 of the entire fibers (Supplementary Fig. 20b). For example, when the fiber was stretched to L/L0 = 7, the theoretical L′/L0′ was 3.05, while the measured value was 2.72, which were 0.44 and 0.39 times the L/L0 of entire fiber, demonstrating that macroscopic helical structure improved the stretchability of the fibers by reducing the stroke of single filament. Strikingly, the experimental L′/L₀′ was consistently lower than theoretical predictions and this discrepancy was broadened at the larger elongation, which was ascribed to the slight slippage of filaments for dissipating energy at the large strain.

Based on these analyses, the nanoconfinement entanglement-enhanced phase separation strategy achieved the integration of high strength, stiffness and toughness within hierarchically anisotropic ionogel fiber under the coordinative strengthening and toughening mechanism (Fig. 4g). The dissociation of hydrogen bonds, disentanglement of CNF@AgNPs, pull-out of skeletons, crack bridging of microfibers, as well as slippage of filaments provided effective energy dissipation, while macroscopic helical architecture accommodated the large deformation, synergistically leading to the high toughness. The stiff phase separation regions arising from robust hydrogen bonding among nanoconfined polymer networks and effective stress transfer between highly aligned CNF@AgNPs skeletons and networks jointly gave rise to high modulus and strength. Therefore, integrating the entanglement-crosslinked rigid phase-soft matrix networks into such structural hierarchy spanning from molecular interactions to macroscopic organization achieved property combinations that exceeded conventional synthetic fibers through coordinated energy dissipation and strengthening across multiple length scales.

Supercontraction of ionogel fibers

Considering the advantage of highly anisotropic structure for contractile actuation, the S4T20IF integrating water-sensitive phase separation and elastic entanglement crosslink was studied to disclose the supercontraction behavior. Typically, the actuation strokes of fibers were collected by horizontally dragging a 2 g weight with one end fixed, in which the torsion behavior was suppressed (Supplementary Fig. 21). The S4T20IF offered the highest actuation stroke of 76% with a maximum contraction velocity of 6.2 %/s under the water stimulus (Fig. 5a, Supplementary Movie 1), outperforming the control samples S4T0IF, S0T20IF and S4T20F featuring actuation strokes of 65%, 1.6%, and 53%, which underscored the advantage of hierarchically anisotropic structure with robust phase separation in promoting supercontraction capacity. Notably, the control PB-S4T20IF exhibited an inferior actuation stroke of 46%, highlighting the roles of efficient stress transfer between the entangled CNF@AgNPs skeletons and polymer networks in contraction behavior. Furthermore, both ends of ionogel fibers were fixed at the force sensors for monitoring the hydration-triggered actuation stress. The actuation stress of S4T20IF was linearly enhanced after hydration, reaching the plateau with the highest actuation stress of 16.1 MPa in 11 s (Fig. 5b). Notably, the contractile stress was hardly collected for S0T20IF, demonstrating the necessity of anisotropic structure for stimuli-response contractile behavior. In contrast, without twisting or solvent-exchange, low actuation stresses of 11 MPa for S4T0IF and 8.3 MPa for S4T20F were recorded. Importantly, once replacing the CNF@AgNPs skeletons, a significantly dropped actuation stress of 3.7 MPa was measured for PB-S4T20IF, as indicative of the importance of effective stress transfer between disentangled CNFs and polymer networks in enhancing hydration-triggered contractile stress.

Fig. 5: Hydration-triggered supercontraction behavior.
Fig. 5: Hydration-triggered supercontraction behavior.The alternative text for this image may have been generated using AI.
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a, b Time-dependent actuation stroke (a) and actuation stress (b) of S4T20IF, S4T0IF, S0T20IF, S4T20F and PB-S4T20IF by dragging a weight with one end fixed upon the hydration stimulus. c–f Weight-lifting tests upon the hydration stimulus. c Photographs of contraction of a S4T20IF lifting a weight of 2 g which is 400 times its own weight. White lines show the lengths of fibers and orange arrows represent the torsional actuation of fibers. d Time-dependent actuation stroke and rotation angle of S4T20IF. e Actuation stroke and energy density as a function of loaded stress of S4T20IF. Data are presented as mean values ± SD (n = 3). f Comparison in actuation stroke and energy density of S4T20IF, S4T0IF, S0T20IF, S4T20F and PB-S4T20IF. g, Cyclic contractile actuation stress and stroke generated from S4T20IF and PB-S4T20IF over 50 actuation cycles. h Schematic illustrations of actuation mechanism of S4T20IF, involving hydration-triggered dissociation of hydrogen bonded nanoconfined PAAm chains and melting of phase separation regions followed by disentanglement of CNF@AgNPs skeletons with the structural order-disorder transformation.

The weight-lifting tests were further performed to quantify the work capacity of ionogel fibers. The S4T20IF exhibited a contraction from 100 to 40 mm with a high actuation stroke of 60% accompanied by a large rotation angle of 324 o/mm within 60 s when lifting a 2 g weight that was 400 times that of the fiber (Fig. 5c, d, Supplementary Movie 2), which surpassed the contraction behavior of the spider silk featuring the rotation angle of ~300 o/mm and actuation stroke of ~50%50. With the load scaling from 0 to 1.47 MPa, a slight decrease in actuation stroke from 68% to 60% was recorded for S4T20IF with the energy density greatly increasing from 0 to 236.4 J/kg. When the load further increased to 2.2 MPa, the highest energy density was recorded as 248.1 J/kg representing 6-fold enhancement over that of human skeleton muscles (39 J/kg)13, whereas the excessive load caused both decreased actuation stroke and energy density (Fig. 5e). However, bearing the same load of 2 g equivalent to actuation stress of 1.47 MPa, the control samples exhibited limited actuation stroke after hydration, for example, 45% for S4T0IF, 2.3% for S0T20IF, 40% for S4T20F and 22% for PB-S4T20IF, corresponding to the energy density of 176.4, 9.01, 156.8 and 86.8 J/kg (Fig. 5f, and Supplementary Fig. 22). The strong actuation property of S4T20IF further demonstrated the significance of the engineered anisotropic structure and stress transfer between highly entangled CNF@AgNPs and networks for enhancing the work capacity of ionogel fibers.

After hydration, the contracted fibers could be stretched to the initial length and dried for the cyclic actuation. The maximum actuation stress and actuation stroke of S4T20IF with the suppressed torsional relaxation were monitored to evaluate the reliability of actuation performance during cyclic actuation process (Fig. 5g). Over 50 actuation cycles, the S4T20IF maintained a consistent 74% actuation stroke with the actuation stress decreasing from 16.1 MPa to 12 MPa with the first 3 cycles, stabilizing at ~10 MPa for the subsequent 47 cycles. However, the control PB-S4T20IF exhibited rapidly deteriorated contractile behavior during cyclic actuation, for example, actuation stress dropping from 3.7 to 1.5 MPa after the first cycle with the actuation stroke progressively decreasing from 48% to 41% within 50 cycles. This stark contrast illustrated that high entanglement of CNFs and effective stress transfer between CNFs and polymer network enabled strong and stable actuation behavior during cyclic uses.

The structural variation after actuation during the hydration-responsive contraction was further investigated to elucidate the underlying mechanism. A curled configuration was observed for S4T20IF after hydration when the torsion behavior was suppressed, suggesting that the energy for untwisting provided the axial contraction force (Supplementary Fig. 23a, b). Whereas S4T20IF after lifting weight test showed minimal twist retention, demonstrating that the energy for untwisting drove the torsion behavior (Supplementary Fig. 23c, d). Notably, these samples both maintained the structural integrity after hydration, confirming the stable entanglement of CNFs. Meanwhile, though the G’ and G” presented great reduction after hydration, the S4T20IF maintained stable solid state (G’ > G”) even after actuation cycles, further highlighting the advantage of entangled CNF@AgNPs in structural stability after hydration (Supplementary Fig. 24). The streak scattering disappeared in the 2D SAXS image of S4T20IF after hydration, indicating massive energy release during the transition from highly ordered to disordered states driving strong contraction behavior (Supplementary Fig. 25). The differential scanning calorimetry (DSC) curves recorded the Tg of S4T20IF decreasing from 45.1 to 21.6 oC after hydration (Supplementary Fig. 26a, b), which could be attributed to the melting of separated phase44. The change in the intensity of specific Raman peaks (denoted as Iₓ, where x was the Raman shift in cm⁻¹) demonstrated the hydrogen bonding evolution upon hydration. The relative intensity of NH2 bending at 1580 cm−1 and C-O stretching vibrations at 1665 cm−1 (I1580/I1665) in Raman spectra exhibited distinct decrease after hydration (Supplementary Fig. 26c), implying the dissociation of hydrogen bonds among nanoconfined PAAm chains corresponding to the melting of phase separation regions51. Meanwhile, the decreased relative intensity of two bands (I3219/I3431) demonstrated the weakened hydrogen bonds among -OH groups after hydration, confirming the disentanglement of CNFs (Supplementary Fig. 26d). To sum up, the supercontraction behavior of S4T20IF was attributable to the synergy of hierarchically anisotropic structure and all-in-one network configuration (Fig. 5h). Hydration drove the melting of phase separation regions with the dissociated hydrogen bonds among the nanoconfined PAAm chains, followed by rapid disentanglement of CNF@AgNPs skeletons which transmitted the contraction force to networks, ultimately producing macroscopic contraction and untwisting through releasing considerable energy stored in the highly ordered anisotropic and helical structure.

Discussion

In summary, we developed a nanoconfinement entanglement-enhanced phase separation strategy to integrate high mechanical properties and strong hydration-triggered supercontraction capacity via a dry-spinning combined programmed assembly-assisted solvent exchange approach. The obtained ionogel fibers integrated the entanglement-crosslinked rigid phase-soft matrix networks into hierarchically anisotropic structure including macroscopic helical configuration, separated microphase, highly anisotropic and entangled skeletons, and hydrogen-bonded PAAm chains in 1D nanoconfinement space. Based on the multiscale energy strengthening and toughening mechanism originating from the hierarchical structure and efficient stress transfer between highly entangled skeletons and networks, the fibers exhibited a strength of 278.5 MPa and a high toughness of 1652.2 MJ·m−3, an order magnitude higher than reported tough fibers and spider silk, as well as a high damping capacity of 96%. Meanwhile, the phase separation regions with hydrogen bonds among nanoconfined PAAm chains served as the responsive motifs while the highly entangled CNF@AgNPs skeletons ensured the network integrity and collaborated with the highly anisotropic structure with helical configuration to achieve rapid and robust elasticity under hydration. Consequently, despite the high toughness arising from considerable dynamic bonds which typically compromised the network integrity under stimulus, the ionogel fibers demonstrated a rapid contraction with an actuation stroke of 76% and stress of 16.1 MPa upon hydration, yielding a high energy density of 236.4 J/kg. This work paved a way to engineer reversible entanglement crosslinks and rigid hydration-sensitive motifs for the fabrication of hierarchically anisotropic ionogel fibers with combinational mechanical properties and smart responsiveness, exhibiting significant potentials for sustainable uses in soft robotics and artificial muscles.

Given the critical role of the nanoconfined entanglement strategy in toughening gel fibers, the future research can focus on incorporating the stronger nanoconfinement structures into the 2D nanosheet skeletons with inherent stiffness to enhance hydrogen bonding and entanglement among polymer networks, thereby achieving the improved strength and supercontraction while retaining the high toughness. Moreover, in future work targeting applications in biosafety-critical fields like tissue scaffolds and implantable electronics, we can transition to the greener and biocompatible ionic liquids alternatives, such as choline-based cations, which exhibit significantly improved biodegradability and non-toxic profiles.

Methods

Preparation of CNFs

The aerosol-assisted biosynthesis process of CNFs started with inoculation of Gluconacetobacter xylinus 1.1812 (China General Microbiological Culture Collection Center, CGMCC) onto a solid substrate in a bioreactor at an ambient temperature of 28 °C52. The solid substrate consisting of glucose (50 g L−1), yeast extract (5 g L−1), CaCO3 (10 g L−1), and agar (20 g L−1) in deionized water (DIW) was dissolved thoroughly under heating and stirring and then sterilized in an autoclave at 121 °C for 30 min. In the subsequent continuous fermentation process, liquid nutrient was introduced into the aerosol and gradually settled to the surface of solid medium. After a period of continuous fermentation, the bacterial cellulose was obtained. The bacterial cellulose was mechanically treated using the blender and fully dispersed in DIW. This dispersion was further processed in a high-pressure homogenizer at 1000 bar at least 3 times to generate the final CNF suspension.

Preparation of CNF@AgNPs

CNF@AgNPs nanocomposites were prepared by a one-pot hydrothermal method. Specifically, different volumes of 0.3 wt% of CNF suspension, 1 wt% of AgNO3 aqueous solution (0.5 mL) and DIW were mixed which were homogenized by 10-min ultrasonication (40 kHz, 300 W) and then transferred into a 50 mL Teflon-lined autoclave. Hydrothermal reduction was conducted at 110 °C for 12 h to facilitate Ag⁺ reduction and nanoparticle anchoring on CNF surfaces. The resulting light-yellow suspension was centrifuged at a speed of 12,000 × g for 20 min, followed by three washing cycles with DIW to remove unreacted precursors, yielding purified CNF@AgNPs with different concentrations.

Fabrication of BSNH

Typically, 1 mg MMMP was added to 2.5 mL of 0.5 wt% CNF@AgNPs colloidal suspension, followed with 10-min ultrasonication to ensure the adsorption of MMMP onto the AgNPs surface for CNF@AgNPs@MMMP nanocomposite initiators. Then, 0.125 g iAA and 0.375 g AAm as the monomers were dissolved into the above solution under ultrasonication for 10 min, followed by bubbling nitrogen and vacuum processing to eliminate oxygen. Under the irradiation of UV light for 30 min, the solution was gelated and the BSNH was prepared.

Preparation of SxTyIFs

Typically, the BSNH slurry was injected from a 22 G needle to generate self-supporting BSNH fibers with 2-h room-temperature drying. The obtained fibers were arranged side by side and clamped at the both ends. Through controlling the distance between two clamps and rotation angle, the stretch and twisting procedures were carried out on the fibers. After immerging the prestretched and twisted fibers into ionogel liquid EMIES for 20 min, the temporary orientation was permanently fixed and the SxTyIFs were finally fabricated.

Preparation of the control PB-SxTyIFs

Firstly, the AgNPs were prepared. Briefly, 80 μL of 0.1 mol L−1 ascorbic acid was added into 47.5 mL of DIW preheated to 100 oC. After stirring for 1 min, a mixture containing 1.05 mL DIW, 1 mL of 55.3 mM sodium citrate solution, 0.25 mL of 5.89 mM AgNO3 solution and 0.2 mL of 20 mM NaCl solution was added to the ascorbic acid solution. After stirring at 100 °C for 1 h and rapid cooling, 5 × 10−5 g mL−1 of AgNPs suspension was obtained, and concentrated to 1.1 × 10−3 g mL−1 by centrifugation. Then, 1 mg of MMMP was added to the mixture containing 1.5 mL of 0.83 g mL−1 CNF suspension and 1 mL of 1.1 × 10−3 g mL−1 AgNPs suspension, followed with 10-min ultrasonication to ensure the adsorption of MMMP onto the AgNPs surface for AgNPs@MMMP nanocomposite initiators. By dissolving 0.125 g iAA and 0.375 g AAm into above solution under ultrasonication for 10 min, the bubbling nitrogen and vacuum process were carried out to eliminate oxygen. Under the irradiation of UV light for 30 min, the solution was gelated and the slurry was prepared. By the subsequent dry-spinning, programmed assembly assisted solvent-exchange, the PB-SxTyIFs were finally fabricated.

Mechanical tests

The tensile tests were carried out on an Instron 5965 A universal testing system equipped with a 500 N load cell, in which gel fibers with a length of 2 mm was stretched at a speed of 10 mm min−1 along the direction parallel to the fibrils at room temperature. The tensile stress was calculated from the applied force divided by the cross-sectional area. The irregular cross-sectional areas of twisted samples were measured by ImageJ software which transformed the pixel-based area into real area based on scale bar. Ten independent samples were measured to obtain the mean cross-sectional area. The toughness was calculated as the area covered by the stress-strain curve at the fracture point.

Recycling procedure

The cut S1T20IFs were dispersed in DIW and subjected to vigorous mechanical agitation at 60 oC for 1 h, generating the homogeneous gel slurry. Through dry-spinning, programmed assembly assisted solvent-exchange, the recycled S1T20IFs were fabricated.

Actuation tests

For measurement of actuation stroke at the suppressed torsion, one end of fibers was fixed while the other was attached to a 2 g weight placed on the horizontal desk. After spraying water, the contraction process was recorded using a camera. The actuation stroke was collected frame by frame according to the video. The fibers were secured between the two clamps of a universal testing machine equipped with a 10 N load cell. Through spraying water, the contraction stress-time curves were recorded. Weight-lifting tests were conducted by attaching a weight on one end of vertically suspended fibers.

Materials characterization

SEM images were obtained on a Zeiss Merlin Compact field-emission scanning electron microscope. The hydrogel fibers were freeze-dried using a Labconco FreeZone freeze-dryer beforehand while ionogel fibers have not undergo any treatment. TEM images were performed on a JEM-2100F transmission electron microscope. AFM images were taken with a Bruker Dimension FastScan in tapping mode. FT-IR spectra were collected in wavenumber range of 4000–500 cm−1 on a Thermo Nicolet instrument assisted by ATR attachment. DMA measurement was conducted on a TA HR10 rheometer. Raman mapping images and Raman spectra were collected using a LabRAM HR Evolution system across the Raman shift ranges of 1700–1500 cm⁻¹ and 4000-500 cm⁻¹. The SAXS tests were conducted at SAXSpoint 2.0 system (Anton Paar, Austria) with an X ray of λ = 1.5406 Å. The sample-to-detector distances were set to be 576 mm and exposure time was set at 5 min for SAXS measurements. DSC measurements were performed using a TA Q2000 instrument. Samples were heated from −40 to 80 °C at a heating rate of 20 °C min−1 under a nitrogen atmosphere with a flow rate of 30 mL min−1.