Anisotropic Hydrogels with High Mechanical Strength by Stretching-Induced Oriented Crystallization and Drying

Many natural tissues in living organisms, such as muscle, tendons, ligaments, and articular cartilage, have anisotropic structure features. Because of their water-rich soft structure, hydrogels resemble those living tissues more than any other artificial materials and are ideal alternative materials. However, compared with natural soft tissues, hydrogels synthesized by traditional methods commonly exhibit isotropic structure feature. Therefore, anisotropic hydrogels have been developed to address this issue, although there is still room for improvement in their mechanical properties. Herein, a simple method is employed to prepare polyacrylamide-chitosan (PAM-CS) composite hydrogels with typical anisotropic mechanical properties. The anisotropic PAM-CS hydrogels are engineered through prestretching and fastening followed by drying in air and subsequent soaking into NaOH solution to form microcrystalline CS. The anisotropic network feature is proved by the observation from optical microscope and field-emission scanning electron microscope (FESEM). Meanwhile, the mechanical strength of anisotropic PAM-CS hydrogels along parallel and vertical directions can be well tuned by controlling the prestretching strains, soaking time, and mass concentration of CS. Analysis shows the generation of anisotropy originates from the oriented distribution of the fixed polymer chains within the hydrogels network. Finally, our anisotropic hydrogels exhibit high tensile strength (12.5 MPa), ultrahigh toughness (23 MJ m–3), and high water content (∼75%), implying important applications for designing tissuelike functional soft materials.