Mechanical Training-Driven Structural Remodeling: A Rational Route for Outstanding Highly Hydrated Silk Materials

Aesthetic Cellulose Filaments with Water-Triggered Switchable Internal Stress and Customizable Polarized Iridescence Toward Green Fashion Innovation

Healthy, convenient, and aesthetic hair dyeing and styling are essential to fashion trends and personal-social interactions. Herein, we fabricate green, scalable, and aesthetic regenerated cellulose filaments (ACFs) with customizable iridescent colors, outstanding mechanical properties, and water-triggered moldability for convenient and fashionable artificial hairdressing. The fabrication of ACFs involves cellulose dissolution, cross-linking, wet-spinning, and nanostructured orientation. Notably, the cross-linking strategy endows the ACFs with significantly weakened internal stress, confirmed by monitoring the offset of the C-O-C group in the cellulose molecular chain with Raman imaging, which ensures a tailorable orientation of the nanostructure during wet stretching and tunable iridescent polarization colors. Interestingly, ACFs can be tailored for three-dimensional shaping through a facile water-triggered adjustable internal stress: temporary shaping with low-level internal stress in the wet state and permanent shaping with high-level internal stress in the dry state. The health, convenience, and green aesthetic filaments show great potential in personal wearables.

Biomimetic Silk Architectures Outperform Animal Horns in Strength and Toughness

Structural biomimicry is an intelligent approach for developing lightweight, strong, and tough materials (LSTMs). Current fabrication technologies, such as 3D printing and two-photon lithography often face challenges in constructing complex interlaced structures, such as the sinusoidal crossed herringbone structure that contributes to the ultrahigh strength and fracture toughness of the dactyl club of peacock mantis shrimps. Herein, bioinspired LSTMs with laminated or herringbone structures is reported, by combining textile processing and silk fiber "welding" techniques. The resulting biomimetic silk LSTMs (BS-LSTMs) exhibit a remarkable combination of lightweight with a density of 0.6-0.9 g cm-3 , while also being 1.5 times stronger and 16 times more durable than animal horns. These findings demonstrate that BS-LSTMs are among the toughest natural materials made from silk proteins. Finite element simulations further reveal that the fortification and hardening of BS-LSTMs arise primarily from the hierarchical organization of silk fibers and mechanically transferable meso-interfaces. This study highlights the rational, cost-effective, controllable mesostructure, and transferable strategy of integrating textile processing and fiber "welding" techniques for the fabrication of BS-LSTMs with advantageous structural and mechanical properties. These findings have significant implications for a wide range of applications in biomedicine, mechanical engineering, intelligent textiles, aerospace industries, and beyond.

Tendon Repair and Regeneration Using Bioinspired Fibrillation Engineering That Mimicked the Structure and Mechanics of Natural Tissue

Replicating the controlled nanofibrillar architecture of collagenous tissue represents a promising approach in the design of tendon replacements that have tissue-mimicking biomechanics─outstanding mechanical strength and toughness, defect tolerance, and fatigue and fracture resistance. Guided by this principle, a fibrous artificial tendon (FAT) was constructed in the present study using an engineering strategy inspired by the fibrillation of a naturally spun silk protein. This bioinspired FAT featured a highly ordered molecular and nanofibrillar architecture similar to that of soft collagenous tissue, which exhibited the mechanical and fracture characteristics of tendons. Such similarities provided the motivation to investigate FAT for applications in Achilles tendon defect repair. In vitro cellular morphology and expression of tendon-related genes in cell culture and in vivo modeling of tendon injury clearly revealed that the highly oriented nanofibrils in the FAT substantially promoted the expression of tendon-related genes combined with the Achilles tendon structure and function. These results provide confidence about the potential clinical applications of the FAT.

Biomimetic Spun Silk Ionotronic Fibers for Intelligent Discrimination of Motions and Tactile Stimuli

Innovation in the ionotronics field has significantly accelerated the development of ultraflexible devices and machines. However, it is still challenging to develop efficient ionotronic-based fibers with necessary stretchability, resilience, and conductivity due to inherent conflict in producing spinning dopes with both high polymer and ion concentrations and low viscosities. Inspired by the liquid crystalline spinning of animal silk, this study circumvents the inherent tradeoff in other spinning methods by dry spinning a nematic silk microfibril dope solution. The liquid crystalline texture allows the spinning dope to flow through the spinneret and form free-standing fibers under minimal external forces. The resultant silk-sourced ionotronic fibers (SSIFs) are highly stretchable, tough, resilient, and fatigue-resistant. These mechanical advantages ensure a rapid and recoverable electromechanical response of SSIFs to kinematic deformations. Further, the incorporation of SSIFs into core-shell triboelectric nanogenerator fibers provides outstanding stable and sensitive triboelectric response to precisely and sensitively perceive small pressures. Moreover, by implementing a combination of machine learning and Internet of Things techniques, the SSIFs can sort objects made of different materials. With these structural, processing, performance, and functional merits, the SSIFs prepared herein are expected to be applied in human-machine interfaces.

Tough Gelatin Hydrogel for Tissue Engineering

Tough hydrogel has attracted considerable interest in various fields, however, due to poor biocompatibility, nondegradation, and pronounced compositional differences from natural tissues, it is difficult to be used for tissue regeneration. Here, a gelatin-based tough hydrogel (GBTH) is proposed to fill this gap. Inspired by human exercise to improve muscle strength, the synergistic effect is utilized to generate highly functional crystalline domains for resisting crack propagation. The GBTH exhibits excellent tensile strength of 6.67 MPa (145-fold that after untreated gelation). Furthermore, it is directly sutured to a ruptured tendon of adult rabbits due to its pronounced toughness and biocompatibility, self-degradability in vivo, and similarity to natural tissue components. Ruptured tendons can compensate for mechanotransduction by GBTH and stimulate tendon differentiation to quickly return to the initial state, that is, within eight weeks. This strategy provides a new avenue for preparation of highly biocompatible tough hydrogel for tissue regeneration.

Bio-Inspired Multiscale Design for Strong and Tough Biological Ionogels

Structure design provides an effective solution to develop advanced soft materials with desirable mechanical properties. However, creating multiscale structures in ionogels to obtain strong mechanical properties is challenging. Here, an in situ integration strategy for producing a multiscale-structured ionogel (M-gel) via ionothermal-stimulated silk fiber splitting and moderate molecularization in the cellulose-ions matrix is reported. The produced M-gel shows a multiscale structural superiority comprised of microfibers, nanofibrils, and supramolecular networks. When this strategy is used to construct a hexactinellid inspired M-gel, the resultant biomimetic M-gel shows excellent mechanical properties including elastic modulus of 31.5 MPa, fracture strength of 6.52 MPa, toughness reaching 1540 kJ m^-3 , and instantaneous impact resistance of 3.07 kJ m^-1 , which are comparable to those of most previously reported polymeric gels and even hardwood. This strategy is generalizable to other biopolymers, offering a promising in situ design method for biological ionogels that can be expanded to more demanding load-bearing materials requiring greater impact resistance.

Strong and Tough Cellulose Hydrogels via Solution Annealing and Dual Cross-Linking

Strong and tough hydrogels are promising candidates for flexible electronics, biomedical devices, and so on. However, the conflict between improving the mechanical strength and toughness properties of polysaccharide-based hydrogels remains unsolved. Herein, a strategy is proposed to produce a hierarchically structured cellulose hydrogel that combines solution annealing and dual cross-linking treatment approaches. The solution annealing considerably increases the hydrophobic stacking and chemical cross-linking of the cellulose chains, thereby facilitating their subsequent self-assembly and recrystallization during the chemical and physical cross-linking processes. The cellulose hydrogels exhibit superposed chemically and physically cross-linked domains comprising homogeneous nanoporous network structures, which in turn are composed of interconnected cellulose nanofibers and cellulose II crystallite hydrates. These cellulose hydrogels exhibit a high water content of 76-84% and excellent mechanical properties that compare favorably to those of biomacromolecule-based hydrogels. The prepared hydrogels exhibit a mechanical strength and work of fracture of 21 ± 3 MPa and 2.6 ± 0.4 MJ m^-3 under compression, and 7.2 ± 0.7 MPa and 5.9 ± 0.6 MJ m^-3 under tension, respectively. It is anticipated that this strategy will be applicable to other biomacromolecules and crystalline polymers, and that it will enable the construction of other hydrogels exhibiting high mechanical performances.

Cytoskeleton-inspired hydrogel ionotronics for tactile perception and electroluminescent display in complex mechanical environments

The emerging applications of hydrogel ionotronics (HIs) in devices and machines require them to maintain their robustness under complex mechanical environments. Nevertheless, existing HIs still suffer from various mechanical limitations, such as the lack of balance between softness, strength, toughness, and fatigue fracture under cyclic loads. Inspired by the structure of the cytoskeleton, this study develops a sustainable HI supported by a double filamentous network. This cytoskeleton-like structure can enhance the strength of the HI by 26 times and its toughness by 3 times. It also enables HI to tolerate extreme mechanical stimuli, such as severe deformation, long-term cyclic loading, and high-frequency shearing and shocking. The advantages of these structurally- and mechanically-optimized HI devices in tactile perception and electroluminescent display, i.e., two practical applications where complex mechanical stimuli need to be sustained, are demonstrated. The findings reported in this study can inspire the design of human skin-like robust and anti-fatigue-fracture HI devices for long-term stable use.

Mechanically and electrically biocompatible hydrogel ionotronic fibers for fabricating structurally stable implants and enabling noncontact physioelectrical modulation

Narrowing the mechanical and electrical mismatch between tissue and implantable microelectronics is essential for reducing immune responses and modulating physioelectrical signals. Nevertheless, the design of such implantable microelectronics remains a challenge due to the limited availability of suitable materials. Here, the fabrication of an electrically and mechanically biocompatible alginate hydrogel ionotronic fiber (AHIF) is reported, which is constructed by combing ionic chelation-assisted wet-spinning and mechanical training. The synergistic effects of these two processes allow the alginate to form a highly-oriented nanofibril and molecular network, with a hierarchical structure highly similar to that of natural fibers. These favourable structural features endow AHIF with tissue-mimicking mechanical characteristics, such as self-stiffening and soft tissue-like mechanical properties. In addition, tissue-like chemical components, i.e., biomacromolecules, Ca²⁺ ions, and water, endow AHIF with properties including biocompatibility and tissue-matching conductivity. These advantages bring light to the application of AHIFs in electrically-conductive implantable devices. As a prototype, an AHIF is designed to perform physioelectrical modulation through noncontact electromagnetic induction. Through experimental and machine learning optimizations, physioelectrical-like signals generated by the AHIF are used to identify the geometry and tension state of the implanted device in the body. Such an intelligent AHIF system has promising application prospects in bioelectronics, IntelliSense, and human-machine interactions.