Anisotropic Double-Network Hydrogels via Controlled Orientation of a Physical Sacrificial Network

Abstract

We report a method to create anisotropic double-network (DN) hydrogels, through the controlled orientation of a physical sacrificial network. A cross-linked polyacrylamide hydrogel is synthesized from a solution containing a semirigid anionic polyelectrolyte. Subsequently, the gel is stretched to orient the semirigid polyelectrolyte, which does not relax in the stretched state because of the high contour length in comparison to the mesh size of the polyacrylamide network. The polyelectrolyte is then physically cross-linked with a multivalent cation, ZrCl2O, to fix the anisotropy. Anisotropy was visualized by observing birefringence and quantified by small-angle X-ray scattering. By comparing the scattering in the oriented direction versus perpendicular to the oriented direction, a structural anisotropy factor was calculated. Uniaxial tensile testing was performed on samples of varying prestretch, both parallel and perpendicular to the stretching direction. Young’s modulus, fracture stress, fracture strain, and work of extension were characterized, and the resulting mechanical anisotropy was compared to the structural anisotropy factor. We find that the anisotropy of Young’s modulus and fracture stress is directly controlled by the anisotropy of the sacrificial network, while fracture strain and work of extension show little influence from structural anisotropy. The results of this work demonstrate that prestretching of a physical sacrificial network is a controllable and simple method to create anisotropic DN hydrogels.

Recent Strategies for Strengthening and Stiffening Tough Hydrogels

“…From this perspective, several strategies have been suggested to improve the hydrogel mechanical properties, such as increasing the polymer crystallinity, [ 111–113 ] synthesizing unique microstructures containing lamellar or fibril structures, [ 114–116 ] and aligning polymer chains into an anisotropic densified structure. [ 117–122 ] However, the previous two strategies have limitedly applicable to specific polymers, including those with controllable crystallinity and microstructure, whereas aligning polymer chains could have been easily used to DN and PA gels. Therefore, in this section, we describe a universal and effective drawing method for aligning polymer chains into the anisotropic densified structure, which were successfully applied to enhance the mechanical properties of DN and PA gels on a macroscopic scale.…”

“…In the typical DN and PA gels, the unidirectional stretching of the polymer network induces its anisotropic (linearly oriented) deformation, [ 123,124 ] and the subsequent fixation step such as an additional crosslinking for interchain bonds fastens the stretched state. [ 117–122 ] Because hydrogels have high water content and are easily swollen, the fixation step to hold the aligned chains is usually needed. It is noteworthy that such anisotropy is observed for natural load‐bearing tissues and strongly contributes to their superior mechanical properties.…”

“…Reproduced with permission. [ 120 ] Copyright 2020, American Chemical Society. g) Preparation of a PA gel containing repetitive cationic and anionic groups.…”

Bioinspired Structural Composite Hydrogels with a Combination of High Strength, Stiffness, and Toughness

“…Several methods, such as impregnating fibers/fabrics and aligning polymer networks through a drawing process, have been conventionally used to improve the hydrogel mechanical properties. [ 11–15 ] However, the resultant hydrogels could be vulnerable to buckling or wear, [ 16 ] and these methods are relatively inconvenient and require extra processes to produce large‐scale materials. For that reason, several previous studies on mechanically reinforced hydrogels by the polymer chain alignment have apparently focused only on the enhanced elastic modulus (stiffness) except for the fracture energy (toughness).…”

“…For that reason, several previous studies on mechanically reinforced hydrogels by the polymer chain alignment have apparently focused only on the enhanced elastic modulus (stiffness) except for the fracture energy (toughness). [ 11–13 ] Moreover, embedding nanosized reinforcing fillers often induces a compromise between stiffness and toughness. [ 17 ]…”