Tissue composition regulates distinct viscoelastic responses in auricular and articular cartilage

It is well-accepted that articular (ART) cartilage composition and tissue architecture are intimately related to mechanical properties. On the other hand, very little information about other cartilage tissues is available, such as elastin-rich auricular (AUR) cartilage. While thorough investigation of ART cartilage has enhanced osteoarthritis research, ear cartilage reconstruction and tissue engineering (TE) could benefit in a similar way from in-depth analysis of AUR cartilage properties. This study aims to explore the constituent-function relationships of AUR cartilage, and how elastin influences mechanical behavior. Stress-relaxation indentation and tensile tests were performed on bovine ART and AUR cartilage. Elastase incubation was performed to simultaneously deplete elastin and sulfated glycosaminoglycans (sGAG), while hyaluronidase incubation was used to deplete sGAG-only, in order to systematically investigate matrix components in material behavior. ART and AUR cartilages showed different viscoelastic behaviors, with AUR cartilage exhibiting a more elastic behavior. Higher equilibrium properties and limited viscous dissipation of strain energy were observed in AUR cartilage, while ART cartilage exhibited a rapid viscous response and high resistance to instantaneous loading. In conclusion, loss of sGAG had no effect on auricular mechanics in contrast to articular cartilage where GAG loss clearly correlated with mechanical properties. Auricular cartilage without elastin lost all compressive mechanical integrity, whereas in articular cartilage this was provided by collagen. This work shows for the first time the involvement of elastin in the mechanical behavior of ear cartilage. In future, this data can be used in AUR cartilage TE efforts to support reproduction of tissue-specific mechanical properties.