Tunable mechanical properties of polymer-based surrogate materials for articular cartilage and vocal fold repair

The macroscopic biomechanical characteristics of soft and ultrasoft tissues, such as articular cartilage and vocal folds, significantly determine their physiological function. Treatments of widespread tissue degradations due to osteoarthritis in the knee or vocal fold impairment remain an unresolved challenge. For the design of implants for tissue repair after injury or disease, the thorough understanding of the unique biomechanical properties of native tissues and potential substitute materials is essential. Based on large-strain mechanical testing methods combined with hyperelastic nonlinear continuum mechanics modeling and finite element simulations, we identify qualitative similarities between articular knee cartilage and vocal folds. Under compression, both tissue types show a strain-induced stiffening alongside hysteresis and conditioning. Based on synthetic metamaterials, we demonstrate that the replication of the complex mechanical properties, and thus the macroscopic mechanical functionality of the corresponding soft tissues, becomes possible. In accordance with native tissues, a pronounced nonlinear stress-strain behavior and stiffening is obtained, while hysteresis can be predominantly demonstrated for hybrid, polyolefin-hydrogel-based articular cartilage surrogates. We furthermore demonstrate the tunability of biomimetic stiffnesses of synthetic articular cartilage and vocal fold surrogates through tissue-specific process-material combinations. These involve complementary approaches based on adapting the density of load-bearing polyolefin networks in laser-sintered polypropylene-polyvinylpolypyrrolidone composites for cartilaginous properties, and the variation of the cross-linking density of synthetic silicone-based epithelium surrogates in ultrasoft vocal folds. These observations furthermore indicate the significance of viscous, hydrogel-based fractions for the biomimetic replication of hysteresis and relaxation. Our results therefore demonstrate the biomimetic similarity of soft tissues across a broad mechanical range alongside the feasibility of synthetic metamaterials in replicating essential passive biomechanical functions with great potential for future treatment options.