Anisotropically Fatigue‐Resistant Hydrogels

Abstract

still susceptible to fatigue fracture during multiple-cycle mechanical loads, exhibiting fatigue threshold (i.e., the minimal fracture energy required for crack propagation under cyclic loads) below 100 J m −2 . [5][6][7] Therefore, the long-term reliability has substantially hampered the in practical utility of hydrogels and hydrogel-based devices, and remains a key challenge in these fields.On the contrary, biological tissues, such as skeletal muscles, tendon and cartilage, are well known for not only their superior strength, modulus, toughness, but also long-term robustness. [8][9][10] For example, skeletal muscles can sustain a high stress (i.e., 1 MPa) over millions cycles per year without fracture, exhibiting fatigue thresholds (i.e., the minimal fracture energy required for crack propagation under cyclic loads) over 1000 J m −2 , despite their high water content (≈80%). [8,11] Such unrivalled fatigue-resistance originates from their hierarchically-arranged collagen fibrillar micro/nanostructures. [10] Despite bioinspired construction of structural materials has been promising for the design of fatigue-resistant hydrogels, [12][13][14][15][16][17] how to produce hydrogel materials with unprecedented fatigue-resistance in a universal and viable manner still remains an open issue. More recently, fatigue-resistant hydrogels have been fabricated by engineering the crystalline domains, [12][13][14] fibril structures, [15,16] or mesoscale phase separation. [17] Ice-templated freeze-casting strategy has been utilized as a powerful technology to impart Nature builds biological materials from limited ingredients, however, with unparalleled mechanical performances compared to artificial materials, by harnessing inherent structures across multi-length-scales. In contrast, synthetic material design overwhelmingly focuses on developing new compounds, and fails to reproduce the mechanical properties of natural counterparts, such as fatigue resistance. Here, a simple yet general strategy to engineer conventional hydrogels with a more than 100-fold increase in fatigue thresholds is reported. This strategy is proven to be universally applicable to various species of hydrogel materials, including polysaccharides (i.e., alginate, cellulose), proteins (i.e., gelatin), synthetic polymers (i.e., poly(vinyl alcohol)s), as well as corresponding polymer composites. These fatigueresistant hydrogels exhibit a record-high fatigue threshold over most synthetic soft materials, making them low-cost, high-performance, and durable alternatives to soft materials used in those circumstances including robotics, artificial muscles, etc.

Anti‐Fatigue and Highly Conductive Thermocells for Continuous Electricity Generation

“…[ 29 ] Furthermore, the amorphous domain and bound water‐related T 2 is slower after mechanical training. Although previous reports show that the overall water content slightly decreases after the mechanical training, [ 28,30 ] our LF NMR results indicate that the mechanical training strengthens hydrogen bonds between PVA chains but weakens those between PVA chains and water molecules. Thus, the mechanical training releases more free water, which could promote ion conduction in polymer networks.…”

“…It is also quantitatively confirmed by differential scanning calorimetry (DSC) measurements (Figure 2d and Note S4, Supporting Information). [30,36] The crystallinity of the swollen thermocell increases from 3.6% to 9.1% when the prestrain enlarges from 0% to 150% (Figure S5, Supporting Information). As the prestrain magnitude increases, it promotes the orientation of the initially disordered nanofibrils and decreases the distance among the parallel polymer chains.…”

A Protein‐Like Nanogel for Spinning Hierarchically Structured Artificial Spider Silk

“…Spider silk has an excellent combination of strength and toughness, which originates from the hierarchical structures including β-sheet crosslinking points, spiral nanoassemblies with highly aligned molecular chains, and rigid sheath-soft core. [1][2][3] Artificial fibers with enhanced mechanical properties were achieved by mimicking some of these characteristics, such as crosslinking, [4][5][6][7] aligned nanoassemblies, [8,9] and sheath-core structures. [10,11] However, simultaneous realization of the above hierarchical structural characteristics nucleation seeds in the nanogel solution, and applying prestretch and spiral structure in the nanogel fiber, further tuned the alignment and self-assembly of the polymer chains of the nanogel fibers.…”

Preliminary Investigation of Linum usitatissimum Mucilage-Based Hydrogel as Possible Substitute to Synthetic Polymer-Based Hydrogels for Sustained Release Oral Drug Delivery

“…Hydrogels stand at the middle of the podium when it comes specifically to sustaining release delivery because of their ability to provide spatial and temporal control over the release of drugs. The development of tuneable hydrogels such as in-situ forming, stimuli (pH, temperature and enzymes) responsive [ 3 , 4 , 5 ], fatigue resistant [ 6 , 7 ], mechanical tune-ability with nano-particulate crosslinkers [ 8 ] and the degradation controlled hydrogel matrices are mainly based upon their physicochemical properties [ 9 ]. Over the past few decades, studies on novel drug delivery systems in general, and hydrogels in particular, have been focused on the so-called biocompatible synthetic polymers, which have dominated the era because of their consistency and considerable purity.…”