On the gular sac tissue of the brown pelican: Structural characterization and mechanical properties

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.

The toughness of porcine skin: Quantitative measurements and microstructural characterization

An exceptional tear resistance is required of the skin to protect the body from external attacks, environmental damage, and other forms of aggression. To estimate the toughness of juvenile porcine skin, we conduct two types of experiments on pre-notched specimens, placing the tissue under shear (Mode III) by using the classical trouser test with a 25 mm long pre-notch, and opening (Mode I) with an experimental setup with the same pre-notch length. We obtain two distinct average toughness values of JIIIc≈20.4kJ/m2 and JIc=30.4kJ/m2, as a result of differences between these two modes of crack-tip loading and propagation, and collagen alignment. Digital image correlation coupled with single edge notch tests of 10 mm × 30 mm skin samples enables the mapping of the local strains around the tip of the crack. Effects of sample orientation and initial notch size ratio on the strain profile and on the net-section failure stress are discussed. The evaluation of the structure at the crack tip and regions undergoing more uniform states of deformation is conducted by ex situ transmission electron microscopy and in situ environmental scanning electron microscopy. Prior to crack propagation, the stress concentration is decreased by redistributing loads away from the crack tip, illustrated by gradual recruitment of collagen fibers ahead of the crack tip, thus delaying crack growth. After the crack has propagated, collagen fibers are substantially damaged, marked by delamination and recoil of the collagen fibrils.

Constitutive description of skin dermis: Through analytical continuum and coarse-grained approaches for multi-scale understanding

Although there are many successful descriptions of the mechanical response of dermis at different levels of complexity and incorporating varying degrees of the physical phenomena involved in deformation, observations indicate that the unraveling of fibers involves a complex three-dimensional process in which they interact in ways that resemble a braided pattern. Here we develop two complementary treatments to gain a better understanding of the mechanical response of dermis: a) an analytical treatment incorporating fibril stiffness, interfibrillar frictional sliding, and the effect of lateral fibers on the extension of a primary fiber; b) a coarse-grained molecular dynamics model comprised of an array of parallel curved fibrils simulating a fiber. Interfibrillar frictional sliding and stiffness are also captured. Both analytical and molecular dynamics models operate at a scale compatible with the wavelength of collagen fibers (~10 µm). The constitutive description presented here incorporates important physical processes taking place during deformation of dermis and thus represents an advance in our understanding of these phenomena. STATEMENT OF SIGNIFICANCE: Microstructural observations of the dermis of skin during tensile deformation indicate that the unraveling of fibers involves a complex three-dimensional process which replicates the effects of braiding. Two complementary constitutive modeling treatments were developed to gain a better understanding of the mechanical response of dermis: an analytical treatment incorporating fibril stiffness, interfibrillar sliding, and the effect of transverse fibers; and a coarse-grained molecular dynamics model describing the fibril bundling effect. An important novel aspect of the current contribution is the recognition that tridimensional collagen fiber arrangements play an important role in the mechanical response. The constitutive description presented here incorporates physical processes taking place during deformation of the dermis and thus represents an advance in our understanding of these phenomena.

Tensile behavior and structural characterization of pig dermis

Skin, the outermost layer of the body, fulfills a broad range of functions, protecting internal organs from damage and infection, while regulating the body's temperature and water content via the exchange of heat and fluids. It must be able to withstand and recover from extensive deformation and damage that can occur during growth, movement, and potential injuries. A detailed investigation of the evolution of the collagen architecture of the dermis as a function of deformation is conducted, which reveals new aspects that help us to understand the mechanical response of skin. Juvenile pig is used as a model material because of its similarity to human skin. The dermis is found to have a tridimensional woven structure of collagen fibers, which evolves with deformation. After failure, we observe that the fibers have straightened and aligned in the direction of tension. The effects of strain-rate change, cyclic loading, stress relaxation, and orientation are quantitatively established. Digital image correlation techniques are implemented to quantify skin's anisotropy; measurements of the Poisson ratio are reported. This is coupled with transmission electron microscopy which enables obtaining quantitative strain parameters evaluated through the orientation and curvature of the collagen fibers and their changes, for the first time in all three dimensions of the tissue. A model experiment using braided human hair in tension exhibits a similar J-curve response to skin, and we propose that this fiber configuration is at least partially responsible for the monotonic increase of the tangent modulus of skin with strain. The obtained results are intended to serve as a basis for structurally-based models of skin. STATEMENT OF SIGNIFICANCE: Our study reveals a new aspect of the dermis: it is comprised of a tridimensional woven structure of collagen fibers, which evolves with deformation. This is enabled by primarily two techniques, transmission electron microscopy on three perpendicular planes and confocal images with second harmonic generation fluorescence of collagen, captured at different intervals of depth. After failure, the fibers have straightened and aligned in the direction of tension. Digital image correlation techniques are implemented to quantify skin's anisotropy; measurements of the Poisson ratio are reported. A model experiment using braided human hair in tension exhibits a similar J-curve response to skin, and we propose that this fiber configuration is at least partially responsible for the monotonic increase of the tangent modulus of skin with strain.

Lessons from the Ocean: Whale Baleen Fracture Resistance

Whale baleen is a keratin-based biological material; it provides life-long (40-100 years) filter-feeding for baleen whales in place of teeth. This study reveals new aspects of the contribution of the baleen's hierarchical structure to its fracture toughness and connects it to the unique performance requirements, which require anisotropy of fracture resistance. Baleen plates are subjected to competing external effects of hydration and varying loading rates and demonstrate a high fracture toughness in transverse loading, which is the most important direction in the filtering function; in the longitudinal direction, the toughness is much lower since delamination and controlled flexure are expected and desirable. The compressive strength is also established and results support the fracture toughness measurements: it is also highly anisotropic, and exhibits a ductile-to-brittle transition with increasing strain rate in the dry condition, which is absent in the hydrated condition, conferring impact resistance to the baleen. Using 3D-printing prototypes that replicate the three principal structural features of the baleen plate (hollow medulla, mineralized tubules, and sandwich-tubular structure) are created, and the role of its structure in determining its mechanical behavior is demonstrated. These findings suggest new bioinspired engineering materials.

A comparative analysis of the avian skull: Woodpeckers and chickens

Woodpeckers peck at trees without any reported brain injury despite undergoing high impact loads. Amongst the adaptations allowing this is a highly functionalized impact-absorption system consisting of the head, beak, tongue and hyoid bone. This study aims to examine the anatomical structure, composition, and mechanical properties of the skull to determine its potential role in energy absorption and dissipation. An acorn woodpecker and a domestic chicken are compared through micro-computed tomography to analyze and compare two- and three-dimensional bone morphometry. Optical and scanning electron microscopy with energy dispersive X-ray spectroscopy are used to identify the structural and chemical components. Nanoindentation reveals mechanical properties along the transverse cross-section, normal to the direction of impact. Results show two different strategies: the skull bone of the woodpecker shows a relatively small but uniform level of closed porosity, a higher degree of mineralization, and a higher cortical to skull bone ratio. Conversely, the chicken skull bone shows a wide range of both open and closed porosity (volume fraction), a lower degree of mineralization, and a lower cortical to skull bone ratio. This structural difference affects the mechanical properties: the skull bones of woodpeckers are slightly stiffer than those of chickens. Furthermore, the Young's modulus of the woodpecker frontal bone is significantly higher than that of the parietal bone. These new findings may be useful to potential engineered design applications, as well as future work to understand how woodpeckers avoid brain injury.

Nature's technical ceramic: the avian eggshell

Avian eggshells may break easily when impacted at a localized point; however, they exhibit impressive resistance when subjected to a well-distributed compressive load. For example, a common demonstration of material strength is firmly squeezing a chicken egg along its major axis between one's hands without breaking it. This research provides insight into the underlying mechanics by evaluating both macroscopic and microstructural features. Eggs of different size, varying from quail (30 mm) to ostrich (150 mm), are investigated. Compression experiments were conducted along the major axis of the egg using force-distributing rubber cushions between steel plates and the egg. The force at failure increases with egg size, reaching loads upwards of 5000 N for ostrich eggs. The corresponding strength, however, decreases with increasing shell thickness (intimately related to egg size); this is rationalized by a micro-defects model. Failure occurs by axial splitting parallel to the loading direction-the result of hoop tensile stresses due to the applied compressive load. Finite-element analysis is successfully employed to correlate the applied compressive force to tensile breaking strength for the eggs, and the influence of geometric ratio and microstructural heterogeneities on the shell's strength and fracture toughness is established.