Effect of Directional Stretching on Properties of PVA-HA-PAA Composite Hydrogel

Polyvinyl alcohol (PVA) hydrogels with excellent characteristics are considered as promising cartilage replacement materials. However, there are still some main issues to be solved for PVA hydrogel, such as poor mechanical strength and disordered structure. Inspired by the highly ordered structure of biological soft tissues such as articular cartilage, here, we prepared a high-strength but anisotropic polyvinyl alcohol-nanohydroxyapatite-polyacrylic acid (PVA-HA-PAA) composite hydrogel by directional stretching, freezing–thawing, and annealing method. Stretching of an as-prepared isotropic PVA-HA-PAA composite hydrogel leads to the orientation of PVA crystallites and PVA chains, which enables the formation of ordered structure and more hydrogen bonds via freezing under stretching. The microstructure, water content, swelling and creep performance, tensile and bio-tribology properties of the composite hydrogel are studied, the results indicated that the properties of the hydrogel are affected by stretching due to the formation of ordered structure in the anisotropic hydrogel. For instance, the elastic modulus and tensile strength of the anisotropic hydrogel reach 5.703 MPa and 18.958 MPa, respectively, which is significantly enhanced by comparing with isotropic hydrogel. Moreover, the friction property is anisotropic, and the Coefficient Of Friction (COF) reduced in the parallel direction. Thus, this work provides a simple and practicable strategy to design strong and anisotropic hydrogels for potential applications in biomedical materials such as cartilage substitute.