A male Eleutherodactylus Coqui (EC, a frog) expands and contracts its gular skin to a great extent during mating calls, displaying its extraordinarily compliant organ. There are striking similarities between frog gular skin and the human bladder as both organs expand and contract significantly. While the high extensibility of the urinary bladder is attributed to the unique helical ultrastructure of collagen type III, the mechanism behind the gular skin of EC is unknown. We therefore aim to understand the structure–property relationship of gular skin tissues of EC. Our findings demonstrate that the male EC gular tissue can elongate up to 400%, with an ultimate tensile strength (UTS) of 1.7 MPa. Species without vocal sacs, Xenopus Laevis (XL) and Xenopus Muelleri (XM), elongate only up to 80% and 350% with UTS~6.3 MPa and ~4.5 MPa, respectively. Transmission electron microscopy (TEM) and histological staining further show that EC tissues’ collagen fibers exhibit a layer-by-layer arrangement with an uninterrupted, knot-free, and continuous structure. The collagen bundles alternate between a circular and longitudinal shape, suggesting an out-of-plane zig-zag structure, which likely provides the tissue with greater extensibility. In contrast, control species contain a nearly linear collagen structure interrupted by thicker muscle bundles and mucous glands. Meanwhile, in the rat bladder, the collagen is arranged in a helical structure. The bladder-like high extensibility of EC gular skin tissue arises despite it having eight-fold lesser elastin and five times more collagen than the rat bladder. To our knowledge, this is the first study to report the structural and molecular mechanisms behind the high compliance of EC gular skin. We believe that these findings can lead us to develop more compliant biomaterials for applications in regenerative medicine.

Here, we report the design and development of highly stretchable, compliant, and enzymatic-resistant transiently cross-linked decellularized extracellular matrixes (dECMs) (e.g., porcine small intestine submucosa/dSIS, urinary bladder matrix/dUBM, bovine pericardium/dBP, bovine dermis/dBD, and human dermis/dHD). Specifically, these dECMs were modified with long aliphatic chains (C9, C14, and C18). Upon modification, dECMs became significantly resistant to enzymatic degradation for extended periods, showed increased water contact angle (>20%–90%), and stretched >200% than their control counterparts. Modified dECMs are compliant, undergoing 100% elongation at only 0.3–0.5 MPa of applied tensile stress (∼10%–25% of their control counterparts), similar to the control bladder tissue. Furthermore, modified dECMs remain structurally stable at the physiological temperature with increased storage and loss modulus values but decreased tan δ values compared to their control counterparts. Although modification reduces cell adhesion, the gene expressions in polarized macrophages remain unchanged (e.g., TGFβ, CD163, and CD86), except for the modified bovine pericardium (dBP) where a significant decrease in TNFα gene expression is observed. When implanted in the rat subcutaneous model, modified dECMs degraded relatively slowly and did not cause significant fibrotic tissue formation. The numbers of pro-regenerative macrophages increased to several folds in a later time point of evaluation. Modified dECM also supported the bladder wall regeneration with formations of the urothelium, lamina propria, blood vessels, and muscle bundles and reduced the occurrence of calculi formation by 50% in a rat bladder augmentation model. We anticipate that the enhanced stretchability, compliance, and physiological stability of dECMs indicate their suitability for urologic tissue regeneration.