Anisotropic hydrogels with enhanced mechanical and tribological performance by magnetically oriented nanohybrids

Most current hydrogels have weak mechanical properties and unordered structures, which limits their applications. However, soft tissues with highly ordered hierarchical nanocomposite structures exhibit anisotropic mechanical performance and function to adapt the complex environments. Therefore, the introduction of an ordered nanocomposite structure into hydrogel is an effective strategy to endow the hydrogel with superb mechanical strength and excellent anisotropic functions. Here, we develop a method to create anisotropic hydrogels with significantly enhanced mechanical and tribological properties. Based on mussel adhesion chemistry, polydopamine (PDA) was used as a mediator to control the formation of Fe3O4 nanoparticles on the surface of montmorillonite (MMT). Subsequently, the obtained PDA-Fe3O4-MMT nanohybrids in the polyvinyl alcohol/polyacrylic acid (PVA/PAA) hydrogel matrix were aligned to form an ordered structure using magnetic-induced technology and then frozen in hydrogel form by freezing-thawing and annealing methods. The resulting hydrogels showed an anisotropic microstructure and exhibited outstanding mechanical properties with a tensile strength of 10.65 MPa, toughness of 52.2 MJ/m3, and compressive strength of 4.86 MPa, which is higher than that of the corresponding isotropic hydrogel. More interestingly, the friction coefficient of hydrogel is significantly reduced to as low as 0.038 due to the presence of ordered structure. This work not only improves magnetic induction technology for the preparation of anisotropic hydrogels but also provides a simple and effective method to obtain hydrogels with anisotropic structure, high mechanical strength, and anisotropic properties for potential application in tissue engineering.