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.

Moderate conformational transition promotes the formation of a self-reinforced highly oriented silk fibroin network structure.

A highly oriented molecular network structure (HOMNS) is a common and favorable design in natural and regenerated silks to achieve self-reinforcement of the material. However, the fundamental issues related to the formation of the HOMNS in silk fibroin materials and its influence on mechanical performance have not yet been addressed. By combining experimental characterization and molecular dynamics simulation, this work revealed that moderate conformational transition of silk fibroin promoted the formation of a low-density crosslinking molecular network among proteins. Such a molecular network is beneficial to further rearrangement of amorphous proteins in subsequent processing to form HOMNS. Here, a structure was confirmed that can strengthen the materials several times compared with the same material without HOMNS. These investigations improved the in-depth understanding of the fundamental questions related to the silk fibroin assembly, revealed their crucial structural remodeling, and paved the way for new fabrication strategies of mechanical-enhanced silk fibroin materials.

Structural Characterization of Silk Fibers by Wide-Angle X-Ray Scattering.

As one of the most advanced techniques to gain insight into the structure of the materials, wide-angle X-ray scattering (WAXS) records the scattering information at wide angles which typically larger than 5° (2θ), where contains abundant and detailed atomic-scale structure information of the matter. To improve the intensity and time-resolution, the WAXS can be further coupled with a synchrotron light source. The resultant technique, that is, Synchrotron WAXS can reach and even surpass the spatial and time resolution of 0.1 nm and microsecond scale, respectively, thus is very suitable for characterization of animal silks both statically and quasi-dynamically. This chapter would show methods to understand the structure-property relationship of animal silks by WAXS.

Structural Characterization of Silk Fibers by Small-Angle X-Ray Scattering.

Nature is rich in all kinds of unbelievably designed microstructures, which endows the natural materials with various fantastic performances. Unveiling the mystery of the sophisticated configurations and the relationship between those microstructures and the corresponding functions is helpful for the manufacture of artificial functional materials. Small-angle X-ray scattering (SAXS) is an advanced tool to gain the microstructural features of materials within a spatial scale much larger than the atomic scale (1000 nm), which can be carried out along with WAXS to conduct more systematic investigation over different kinds of materials. With the help of SAXS/WAXS, one may generate an insightful understanding of the mechanisms of structure-property evolution (which is efficient guidance for the artificial material designs). This chapter will introduce the mathematics and the methodologies used by SAXS when investigating the microstructure of natural materials.

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

Highly hydrated silk materials (HHSMs) have been the focus of extensive research due to their usefulness in tissue engineering, regenerative medicine, and soft devices, among other fields. However, HHSMs have weak mechanical properties that limit their practical applications. Inspired by the mechanical training-driven structural remodeling strategy (MTDSRS) in biological tissues, herein, engineered MTDSRS is developed for self-reinforcement of HHSMs to improve their inherent mechanical properties and broaden potential utility. The MTDSRS consists of repetitive mechanical training and solvent-induced conformation transitions. Solvent-induced conformation transition enables the formation of ß-sheet physical crosslinks among the proteins, while the repetitive mechanical loading allows the rearrangement of physically crosslinked proteins along the loading direction. Such synergistic effects produce strong and stiff mechanically trained-HHSMs (MT-HHSMs). The fracture strength and Young's modulus of the resultant MT-HHSMs (water content of 43 ± 4%) reach 4.7 ± 0.9 and 21.3 ± 2.1 MPa, respectively, which are 8-fold stronger and 13-fold stiffer than those of the as-prepared HHSMs, as well as superior to most previously reported HHSMs with comparable water content. In addition, the animal silk-like highly oriented molecular crosslinking network structure also provides MT-HHSMs with fascinating physical and functional features, such as stress-birefringence responsibility, humidity-induced actuation, and repeatable self-folding deformation.

Mechanism of Mechanical Training-Induced Self-Reinforced Viscoelastic Behavior of Highly Hydrated Silk Materials.

Mechanical training is an operation where a sample is cyclically stretched in a solvent. It is accepted as an effective strategy to strengthen and stiffen the highly hydrated silk materials (HHSMs). However, the detailed reinforcement mechanism of the process still remains to be understood. Herein, this process is studied by the integration of experimental characterization and theoretical analysis. The results from time-resolved Fourier transform infrared spectroscopy and real-time birefringent characterization reveal that the silk proteins rapidly formed a molecular cross-linking network (MCN) during the mechanical training. The cross-links were the ß-sheet nanocrystals generated from the conformation transition of silk proteins. With the progress in mechanical training, these MCNs gradually remodeled to a highly oriented molecular network structure. The final structure of the silk proteins in HHSMs is highly similar to the structural organization of silk proteins in the natural animal silk. The training process significantly improved the mechanical strength and modulus of the material. With regards to the dynamic behavior of conformation transition and MCN orientation, the structural evaluation of silk proteins during mechanical training was divided into three distinct stages, namely, the MCN-forming stage, MCN-orienting stage, and oriented-MCN stage. Such division is in complete agreement with the three-stage viscoelastic behavior observed in the cyclic loading and unloading tests. Hence, a five-parameter viscoelastic model has been established to elucidate the structure-property relationship of these three stages. This work improves in-depth understanding of the fundamental issues related to structure-property relationships of HHSMs and thus provides inspiration and guidance in the design of soft silk functional materials.