Materials Science and Engineering Program, University of California, San Diego, CA 92093, United States.
Department of NanoEngineering, University of California San Diego, CA, 92093, United States; Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, United States.
Acta Biomater. 2020 Dec;118:161-181. doi: 10.1016/j.actbio.2020.10.008. Epub 2020 Oct 10.
The brown pelican (Pelecanus occidentalis) wields one of the largest bills of any bird and is distinguished by the deployable throat pouch of extensible tissue used to capture prey. Here we report on mechanical properties and microstructure of the pouch skin. It exhibits significant anisotropy, with the transverse direction having maximum nominal tensile strains of 200% to 300%, triple the value in the longitudinal direction. This is a higher extensibility than most conventional skin and is the result of the requirement of the sac to net fish; it should expand laterally, with controlled longitudinal stretch. Transmission electron microscopy provides microstructural evidence of the directionality of the collagen fibers and reveals the individual collagen fibrils with a bimodal diameter distribution having peaks at 100 and 170 nm. These dimensions are similar to collagen in mammal skin. In the lateral direction, the fibers form a curvy pattern with a radius of approximately 2 µm wherein the fibrils reorient, straighten, slide, and stretch elastically under tensile load. A second mechanism operates in the transverse direction; the membrane forms a corrugated pattern that, upon straightening of collagen fibrils, confers additional extensibility. This elicits the anisotropic response observed in tensile testing. This work focuses on the mechanical characterization based on the effect of relative bird age, sample location on the pouch, and strain rate. Anterior-posterior location and strain rate are not major influencers on exhibited strengths and extensibilities. However, bird age and dorsal-ventral location are found to affect the mechanical response of the pouch significantly. A physically-based constitutive model is developed for the middle layer of the gular sac, based on observations, which predicts maximum stresses, strains, and the shape of the stress-strain curve consistent with the experimental results.
褐鹈鹕（Pelecanus occidentalis）拥有鸟类中最大的喙之一，其可伸展的喉囊组织用于捕捉猎物。本文报告了喉囊皮肤的力学性能和微观结构。它表现出明显的各向异性，横向的名义拉伸应变高达 200%至 300%，是纵向的三倍。这比大多数常规皮肤具有更高的延展性，是因为该囊需要捕捉鱼类；它应该横向扩张，同时控制纵向拉伸。透射电子显微镜提供了胶原纤维方向性的微观结构证据，并揭示了单个胶原原纤维具有双峰直径分布，峰值分别为 100nm 和 170nm。这些尺寸与哺乳动物皮肤中的胶原相似。在横向，纤维形成弯曲图案，半径约为 2µm，其中原纤维重新定向、变直、滑动并在拉伸载荷下弹性拉伸。第二个机制在横向起作用；膜形成波纹图案，在胶原原纤维变直时赋予额外的延展性。这就产生了拉伸测试中观察到的各向异性响应。这项工作主要集中在基于相对鸟类年龄、囊袋样本位置和应变速率的力学特性上。前后位置和应变速率对表现出的强度和延展性没有主要影响。然而，鸟类年龄和背腹位置被发现显著影响囊袋的力学响应。基于观察结果，为喉囊的中层开发了一种基于物理的本构模型，该模型预测的最大应力、应变和应力-应变曲线的形状与实验结果一致。
