The shape of Nature's stingers revealed.

Stinger-like structures in living organisms evolved convergently across taxa for both defensive and offensive purposes, with the main goal being penetration and damage. Our observations over a broad range of taxa and sizes, from microscopic radiolarians to narwhals, reveal a self-similar geometry of the stinger extremity: the diameter (d) increases along the distance from the tip (x) following a power law [Formula: see text] , with the tapering exponent varying universally between 2 and 3. We demonstrate, through analytical and experimental mechanics involving three-dimensional (3D) printing, that this geometry optimizes the stinger's performance; it represents a trade-off between the propensity to buckle, for n smaller than 2, and increased penetration force, for n greater than 3. Moreover, we find that this optimal tapering exponent does not depend on stinger size and aspect ratio (base diameter over length). We conclude that for Nature's stingers, composed of biological materials with moduli ranging from hundreds of megapascals to ten gigapascals, the necessity for a power-law contour increases with sharpness to ensure sufficient stability for penetration of skin-like tissues. Our results offer a solution to the puzzle underlying this universal geometric trait of biological stingers and may provide a new strategy to design needle-like structures for engineering or medical applications.

Hydration-induced reversible deformation of the pine cone.

The scales of pine cones undergo reversible deformation due to hydration changes in order to optimize seed dispersal. This improves the survivability of the pine. The reversible flexing of the scales is caused by two tissue layers arranged in a sandwich configuration: a layer composed of sclereid cells and a sclerenchyma layer. They expand differentially upon hydration (and contract upon dehydration) due to differences in the structure that are analyzed here for Torrey pine (Pinustorreyana) cones. In addition to this well-known mechanism by which the cellulose microfibrils in the scales vary their angle with the wood cell axis, we confirm the presence of a porosity gradient in the sclereid cells and calculate, using a model consisting of three layers, the stresses generated upon dehydration taking into account the effect of hydration on the elastic modulus. Our quantitative analysis reveals that this gradient structure can significantly decrease the stress concentrations due to the mismatch between the two layers, and show that this is an ingenious design to increase the interfacial toughness to improve the robustness of pine cone scales. We also show that each individual layer of sclereid cells and sclerenchyma fibers undergoes bending when hydrated separately, and suggest that the two layers operate synergistically to effect the required deformation for seed release. A synthetic bioinspired analog consisting of hydrogels with different porosities is used to confirm this principal actuation mechanism. These findings may inspire the materials science and mechanical engineering communities to develop more robust, biocompatible and energy-efficient actuation systems. STATEMENT OF SIGNIFICANCE: Some biological structures can exhibit reversible deformation enabled by water inflow and outflow of their structure. We analyse the reversible motion of pine cone scales. The dehydration produces their flexure and opening, resulting in the release of seeds and their dispersal, when conditions are right. This process is reversible, and rehydration of the pine cone recloses the scales. The processes of flexing and straightening are governed by shrinking and swelling which are directed by differences in the arrangement of cellulose microfibrils in a bilayer construct. We demonstrate that the scales are more complex than a simple bilayer structure and that they actually have gradients, which significantly reduce the internal stresses and ensure their integrity. We analyse the process of opening and closing of the scales for a gradient structure in the Torrey pine cone using a simple idealized trilayer model. The results demonstrate a significant decrease in internal stresses produced by the gradient structure. Using the lessons learned from the pine cone, we produce a bilayer junction using hydrogels with different porosities which exhibit the same reversible bending response.

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.

Reversible adhesion to rough surfaces both in and out of water, inspired by the clingfish suction disc.

Adhesion is difficult to achieve on rough surfaces both in air and underwater. In nature, the northern clingfish (Gobiesox maeandricus) has evolved the impressive ability to adhere onto substrates of various shapes and roughnesses, while subject to strong intertidal surges. The suction disc of the clingfish relies on suction and friction to achieve and maintain adhesion. Inspired by this mechanism of attachment, we designed an artificial suction disc and evaluated its adhesive stress on rough surfaces and non-planar geometries. The artificial suction disc achieved adhesion strengths of 10.1 ± 0.3 kPa in air on surfaces of moderate roughness (grain size, 68 µm), and 14.3 ± 1.5 kPa underwater on coarse surfaces (grain size, 269 µm). By comparison, a commercially available suction cup failed to exhibit any significant adhesion in both scenarios. The roughly 2 g heavy clingfish-inspired suction discs gripped concave surfaces with small radii of curvature (12.5 mm) and supported payloads up to 0.7 kg. We correlated the effect of key bioinspired features (i.e. slits, a soft outer layer, and body geometry) to adhesion performance using contact visualization techniques and finite element analysis (FEA). The suction discs were then tested on a remotely operated vehicle (ROV) to demonstrate their utility in the soft manipulation of fragile objects.

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.

A comparative study of piscine defense: The scales of Arapaima gigas, Latimeria chalumnae and Atractosteus spatula.

We compare the characteristics of the armored scales of three large fish, namely the Arapaima gigas (arapaima), Latimeria chalumnae (coelacanth), and Atractosteus spatula (alligator gar), with specific focus on their unique structure-mechanical property relationships and their specialized ability to provide protection from predatory pressures, with the ultimate goal of providing bio-inspiration for manmade materials. The arapaima has flexible and overlapping cycloid scales which consist of a tough Bouligand-type arrangement of collagen layers in the base and a hard external mineralized surface, protecting it from piranha, a predator with extremely sharp teeth. The coelacanth has overlapping elasmoid scales that consist of adjacent Bouligand-type pairs, forming a double-twisted Bouligand-type structure. The collagenous layers are connected by collagen fibril struts which significantly contribute to the energy dissipation, so that the scales have the capability to defend from predators such as sharks. The alligator gar has inflexible articulating ganoid scales made of a hard and highly mineralized enamel-like outer surface and a tough dentine-like bony base, which resist powerful bite forces of self-predation and attack by alligators. The structural differences between the three scales correspond with the attack of their predators, and show refined mechanisms which may be imitated and incorporated into superior bioinspired and biomimetic designs that are specialized to resist specific modes of predation.

Facile synthesis of water-dispersible Cu2O nanocrystal-reduced graphene oxide hybrid as a promising cancer therapeutic agent.

We report a Cu(2)O nanocrystal-reduced graphene oxide hybrid that is dispersible in water and has anticancer activity under both visible and near-infrared light irradiation. In contrast to the highly efficient killing of both normal and cancer cells initiated by the photothermal effect, the photocatalytic effect of this material results in the selective killing of cancer cells under visible light irradiation. These results have implications for safe and widely applicable cancer therapy agents.