Hydrogels: A Facile Method to Fabricate Anisotropic Hydrogels with Perfectly Aligned Hierarchical Fibrous Structures (Adv. Mater. 9/2018)

Abstract

In article number 1704937, Jian Ping Gong and co-workers report a bioinspired facile method to fabricate anisotropic hydrogels with perfectly aligned multiscale (from nano-to sub-micro-to microscale) hierarchical fibrous structures, similar to those of the collagen fibril structure in tendons and ligaments. The method is fairly general, and includes drying a diluted physical hydrogel consisting of rigid/semi-rigid polymers in air by confining its length direction. HYDROGELS

Coordinatively Stiffen and Toughen Hydrogels with Adaptable Crystal‐Domain Cross‐Linking

“…b) Tensile stress–strain curves and c) the modulus and fracture energy of PVA–CA sal‐exogels after 48 h of salting out in sodium citrate solutions with various concentrations from 0 m to saturated (≈2 m at room temperature). d) The mechanical performances of PVA sal‐exogels in the state‐of‐the‐art toughness–modulus diagram together with results from previously reported high‐performance hydrogels and elastomers (mineralized PAAm‐ l ‐MBAm, [ 2 ] PVA–ANF, [ 8 ] C–G–P CN, [ 10 ] PCL–PNAGA, [ 11 ] P(DMAA‐ co ‐MAAc), [ 13 ] DDC cellulose, [ 14 ] P(MAAm‐ co ‐MAAc), [ 15 ] HA–PVA, [ 16 ] P(NaSS‐ co ‐MPTC), [ 19 ] dry‐annealed PVA hydrogel, [ 34 ] COR, TOR elastomer [ 32 ] ). The yellow dashed line indicated the trade‐off between toughness and stiffness for the current state‐of‐the‐art hydrogels ( Γ = 240 416 kN 3/2 m −2 × E −1/2 ).…”

“…DOI: 10.1002/adma.202209913 sipate energy, whereas the loose one(s) survive to withstand large strain and maintain the hydrogel integrity. [7][8][9][10][11] In addition, the incorporation of a high density of dynamic bonds, such as hydrogen bonds, [12][13][14][15][16][17][18] electrostatic attractions, [19,20] and host-guest interactions, [21,22] into single networks is an alternative pathway to achieve the stiffness-toughness combination. By particular optimization of the distribution of those dynamic bonds, the discrete dynamic bonds act as sacrificial cross-links, dissipating energy upon dissociation, while the clustered ones serve as permanent crosslinks, surviving large deformation.…”

Aggregated structures and their functionalities in hydrogels

“…For example, Mredha et al reported a type of anisotropic hydrogels with wellaligned multiscale fibrous structure from nanoscale to microscale. [89] These hydrogels were obtained by confined drying of a dilute physical hydrogel with relatively rigid polymers, such as cellulose and alginate, in air and then reswell the gel in water. During drying process, the polymer chain orientates along the confined direction and the polymers form nanofibrils through supramolecular interaction.…”

“…(C) High fatigue resistance of a hydrogel with different alignments of nanofibrils: (i) the longitudinal direction of nanofibrils is perpendicular to the notch direction; (ii) the longitudinal direction of nanofibrils is parallel to the notch direction; (iii) the nanofibrils have no preferred orientation. Reproduced with permission from the literatures [23,28,89] structure. [109] In cyclic loading, the bicontinuous phase network orients along the stretching direction, resulting in a pronounced crack blunting and crack deceleration effect, which gives an extremely slow crack growth even above the fatigue threshold of PA gel.…”

Mechanical Training Enabled Reinforcement of Polyrotaxane‐Containing Hydrogel

“…In natural systems, such as muscle, skin, articular, and wood adopt hierarchically anisotropic structures from micro‐ to macroscale. Inspiration from biological systems, various structural engineering techniques, [12] such as electrical/magnetic field alignment, [13] ion diffusion, [14] template induced arrangement, [15] stress induced arrangement, [16] freeze‐casting/draw, [17] and drying in confined conditions [18] have been employed to obtain a long‐range integrated structure in hydrogels. In fact, the region of long‐range highly ordered structure was induced by anisotropic integrated noncovalent interactions between polymer chains.…”