Hydrogel materials have been widely considered as potential soft tissue replacements because of their high permeability, hydrophilicity, and biocompatibility, as well as their low coefficient of friction. Injectable (thermo-responsive) hydrogels can provide support and cushioning at irregularly shaped disease sites, and are thus suitable for use in treating osteoarthritis or degenerative disc disease. However, while some injectable hydrogels have been proven to sustain human body weight during daily activities, their mechanical properties under harsh dynamic conditions have not been well documented. A specified injectable polyacrylic acid (PAA) hydrogel was prepared for this study. To simulate sudden impacts or unexpected shocks to the PAA hydrogel, the split Hopkinson pressure bar technique was utilized. The dynamic responses of various hydrogels at confined high strain rates (100-2590 s(-1)) were presented. Hydrogel specimens with 3.37, 6.75, and 13.5% acrylic acid (AAc) concentrations were tested in the following three different material conditions: raw, phosphate-buffered saline (PBS) swollen, and PBS swollen with elevated temperature (37 °C). The dynamic bulk moduli of the hydrogels varied from 1.55 to 47.8 MPa depending on the given hydrogel's AAc concentration and swollen condition.

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The meniscus is a multifunctional fibrocartilage tissue in the knee joint which stables joint movement, bears load and absorbs impact. Improper collisions will cause damage to meniscus tissue and lose its original functionality. However, it is difficult to fully evaluate the mechanical properties of the meniscus based on static test results alone. In this study, Split Hopkinson Pressure Bar (SHPB) and hydraulic material testing system (MTS) were utilized to examine the quasi-static and dynamic properties of the porcine meniscus along with two different orientations. The results showed that the meniscus is a strain rate sensitive material and its mechanical properties mainly depend on the orientation of collagen fiber bundles in the peripheral direction. The meniscus tissue did not show obvious yield characteristics under quasi-static test conditions. However, the meniscus showed clear yield behavior under dynamic loading. When the strain rate increased, the elastic modulus of the radial meniscus remained around 35 MPa while the elastic modulus of the axial meniscus increased from 30 MPa to 80 MPa. This study demonstrates that the meniscus is sensitive to strain rate at both dynamic and quasi-static conditions, and the meniscus is an anisotropic biological tissue.