A study on the structure and mechanical behavior of the Terrapene carolina carapace: A pathway to design bio-inspired synthetic composites

Shell biomechanics suggests an aquatic palaeoecology at the dawn of turtle evolution

The turtle shell is a remarkable structure that has intrigued not only evolutionary biologists but also engineering and material scientists because of its multi-scale complexity and various functions. Although protection is its most apparent role, the carapace and plastron are also related to many physiological functions and their shape influences hydrodynamics and self-righting ability. As such, analysing the functional morphology of the shell could help understanding the ecology of Triassic stem-turtles, which will contribute to the century-long debate on the evolutionary origins of turtles. Here, we used 3D imaging techniques to digitize the shells of two of the earliest stem-turtle taxa, Proganochelys and Proterochersis, and submitted their models to biomechanical and shape analyses. We analysed the strength performance under five predation scenarios and tested the function of two morphological traits found in stem-turtles, the epiplastral processes and an attached pelvic girdle. The latter, also present in the crown-lineage of side-necked turtles, has been suggested to increase load-bearing capacity of the shell or to improve swimming in pleurodires. Our results do not confirm the shell-strengthening hypothesis and, together with the results of our shape analyses, suggest that at least one of the first stem-turtles (Proterochersis) was an aquatic animal.

Nature's Load-Bearing Design Principles and Their Application in Engineering: A Review

Biological structures optimized through natural selection provide valuable insights for engineering load-bearing components. This paper reviews six key strategies evolved in nature for efficient mechanical load handling: hierarchically structured composites, cellular structures, functional gradients, hard shell-soft core architectures, form follows function, and robust geometric shapes. The paper also discusses recent research that applies these strategies to engineering design, demonstrating their effectiveness in advancing technical solutions. The challenges of translating nature's designs into engineering applications are addressed, with a focus on how advancements in computational methods, particularly artificial intelligence, are accelerating this process. The need for further development in innovative material characterization techniques, efficient modeling approaches for heterogeneous media, multi-criteria structural optimization methods, and advanced manufacturing techniques capable of achieving enhanced control across multiple scales is underscored. By highlighting nature's holistic approach to designing functional components, this paper advocates for adopting a similarly comprehensive methodology in engineering practices to shape the next generation of load-bearing technical components.

Modulate stress distribution with bio-inspired irregular architected materials towards optimal tissue support

Natural materials typically exhibit irregular and non-periodic architectures, endowing them with compelling functionalities such as body protection, camouflage, and mechanical stress modulation. Among these functionalities, mechanical stress modulation is crucial for homeostasis regulation and tissue remodeling. Here, we uncover the relationship between stress modulation functionality and the irregularity of bio-inspired architected materials by a generative computational framework. This framework optimizes the spatial distribution of a limited set of basic building blocks and uses these blocks to assemble irregular materials with heterogeneous, disordered microstructures. Despite being irregular and non-periodic, the assembled materials display spatially varying properties that precisely modulate stress distribution towards target values in various control regions and load cases, echoing the robust stress modulation capability of natural materials. The performance of the generated irregular architected materials is experimentally validated with 3D printed physical samples - a good agreement with target stress distribution is observed. Owing to its capability to redirect loads while keeping a proper amount of stress to stimulate bone repair, we demonstrate the potential application of the stress-programmable architected materials as support in orthopedic femur restoration.

A Numerical Modelling Framework for Investigating the Ballistic Performance of Bio-Inspired Body Armours

Biological structures possess excellent damage tolerance, which makes them attractive for ballistic protection applications. This paper develops a finite element modelling framework to investigate the performance of several biological structures that are most relevant for ballistic protection, including nacre, conch, fish scales, and crustacean exoskeleton. Finite element simulations were conducted to determine the geometric parameters of the bio-inspired structures that can survive projectile impact. The performances of the bio-inspired panels were benchmarked against a monolithic panel with the same 4.5 mm overall thickness and projectile impact condition. It was found that the biomimetic panels that were considered possessed better multi-hit resistant capabilities compared to the selected monolithic panel. Certain configurations arrested a fragment simulating projectile with an initial impact velocity of 500 m/s, which was similar to the performance of the monolithic panel.

Dietary protein requirement for captive juvenile green turtles (Chelonia mydas)

Head-starting programs are extremely important for restoring the population of sea turtles in wild whereas husbandry conditions and feeding regimens of captive turtles are still limited. In the current study, the optimal dietary protein requirement for green turtle (Chelonia mydas) was investigated to support rearing in head-starting programs. Twenty-five-day-old turtles (44.5-46.2 g body weight, n = 45) were randomly distributed into 15 experimental plastic tanks, comprising three treatment replications of 3 turtles each. They were fed fishmeal-based feeds containing different levels of protein (30%, 35%, 40%, 45%, and 50%) for 8 weeks. At the end of feeding trial, growth performance (specific growth rate = 1.86% body weight/day) and feed utilization (protein efficiency ratio = 3.30 g gain/g protein) were highest in turtles fed with 40% protein in feed (p < .05). These nutritional responses were significantly supported by specific activities of fecal digestive enzymes, especially trypsin, chymotrypsin, amylase, and the amylase/trypsin ratio. Also, this dietary level improved the deposition of calcium and phosphorus in carapace, supporting a hard carapace and strong healthy bones. There were no negative effects in general health status of reared turtles, as indicated by hematological parameters. Based on a broken-line analysis between dietary protein levels and specific growth rate, the optimal protein level for green turtles was estimated as 40.6%. Findings from the current study support the use of artificial diets of specific protein levels to rear captive green turtle before release to natural habitats.

Effect of Needle Type, Number of Layers on FPAFC Composite against Low-Velocity Projectile Impact

Protective structures subjected to intensive loads that may benefit from the use of multilayer composite structures with excellent hardness and impact resistance represent an emerging research field in recent times. In this study, low-velocity projectile impact tests were performed on Functionally-graded Preplaced Aggregate Fibrous Concrete (FPAFC) mixtures to evaluate their performance. The effects of projectile needle type, fibre type and hybridization in addition to the number of layers in the composites on projectile impact were investigated. The bioinspiration of the excellent impact strength of turtle shells was used to design an FPAFC comprising a higher amount of steel and polypropylene fibres at the outer layers. In parallel, one and two-layered concretes were also cast to assess the effectiveness of three-layered FPAFC. The tests were performed on disc specimens using non-deformable compound bevel, convex edge and hollow edge projectiles. The damage severity was quantified by the top damage area, bottom damage area and depth of penetration. In addition, a simple analytical model for predicting the composite mass expulsion was developed and implemented. Findings indicated that regardless of fiber type and distribution, the compound bevel projectile needle produced the lowest impact numbers for all single, double and triple-layer specimens compared to the convex edge and hollow edge projectiles. Repeated projectile impacts increased the penetration depth and damaged area at the top and bottom surfaces of all targets. Targets were more resistant to convex edge and hollow edge projectile penetration than the compound bevel. The experimental and analytical model results for mass expelled from the top surface are reasonably acceptable. This research gives an idea of developing advanced fibrous composite with superior impact resistance for the promising protective structures.

Elytra coupling of the ladybirdCoccinella septempunctatafunctions as an energy absorber in intentional falls

Some insects, such as bees, wasps, and bugs, have specialized coupling structures to synchronize the wing motions in flight. Some others, such as ladybirds, are equipped with coupling structures that work only at rest. By locking elytra into each other, such structures provide hindwings with a protective cover to prevent contamination. Here, we show that the coupling may play another significant role: contributing to energy absorption in falls, thereby protecting the abdomen against mechanical damage. In this combined experimental, numerical and theoretical study, we investigated free falls of ladybirds (Coccinella septempunctata), and discovered that upon collision to the ground, the coupling may fail and the elytra may unlock. This unlocking of the coupling increased the energy absorption by 33%, in comparison to when the elytra remain coupled. Using micro-computed tomography scanning, we developed comparative models that enabled us to simulate impact scenarios numerically. Our results showed that unlocking of the coupling, here called elytra splitting, reduces both the peak impact force and rebound velocity. We fabricated the insect-inspired coupling mechanism using 3D printing and demonstrated its application as a damage preventing on system for quadcopters in accidental collisions.

A review of the osteoderms of lizards (Reptilia: Squamata)

Osteoderms are mineralised structures consisting mainly of calcium phosphate and collagen. They form directly within the skin, with or without physical contact with the skeleton. Osteoderms, in some form, may be primitive for tetrapods as a whole, and are found in representatives of most major living lineages including turtles, crocodilians, lizards, armadillos, and some frogs, as well as extinct taxa ranging from early tetrapods to dinosaurs. However, their distribution in time and space raises questions about their evolution and homology in individual groups. Among lizards and their relatives, osteoderms may be completely absent; present only on the head or dorsum; or present all over the body in one of several arrangements, including non-overlapping mineralised clusters, a continuous covering of overlapping plates, or as spicular mineralisations that thicken with age. This diversity makes lizards an excellent focal group in which to study osteoderm structure, function, development and evolution. In the past, the focus of researchers was primarily on the histological structure and/or the gross anatomy of individual osteoderms in a limited sample of taxa. Those studies demonstrated that lizard osteoderms are sometimes two-layered structures, with a vitreous, avascular layer just below the epidermis and a deeper internal layer with abundant collagen within the deep dermis. However, there is considerable variation on this model, in terms of the arrangement of collagen fibres, presence of extra tissues, and/or a cancellous bone core bordered by cortices. Moreover, there is a lack of consensus on the contribution, if any, of osteoblasts in osteoderm development, despite research describing patterns of resorption and replacement that would suggest both osteoclast and osteoblast involvement. Key to this is information on development, but our understanding of the genetic and skeletogenic processes involved in osteoderm development and patterning remains minimal. The most common proposition for the presence of osteoderms is that they provide a protective armour. However, the large morphological and distributional diversity in lizard osteoderms raises the possibility that they may have other roles such as biomechanical reinforcement in response to ecological or functional constraints. If lizard osteoderms are primarily for defence, whether against predators or conspecifics, then this 'bony armour' might be predicted to have different structural and/or mechanical properties compared to other hard tissues (generally intended for support and locomotion). The cellular and biomineralisation mechanisms by which osteoderms are formed could also be different from those of other hard tissues, as reflected in their material composition and nanostructure. Material properties, especially the combination of malleability and resistance to impact, are of interest to the biomimetics and bioinspired material communities in the development of protective clothing and body armour. Currently, the literature on osteoderms is patchy and is distributed across a wide range of journals. Herein we present a synthesis of current knowledge on lizard osteoderm evolution and distribution, micro- and macrostructure, development, and function, with a view to stimulating further work.

Role of soft bi-layer coating on the protection of turtle carapace

The turtle carapace is a biological armor exhibiting enhanced protection performance. Despite considerable efforts to characterize the structure-property relations of the turtle carapace, how the design of soft keratin-collagen bi-layer coating contributes to the protection of this biological armor remains largely unknown. In this study, calculations are carried out for fracture of the turtle carapace subjected to impact loading. The dynamic fracture of the bone layer, plastic deformation of the keratin-collagen bi-layer, and delamination at the keratin-collagen and collagen-bone interfaces are accounted for in the analyses. We reveal that plastic deformation and interfacial delamination within the soft bi-layer coating are two toughening mechanisms controlling the resistance to dynamic crack growth in the bone layer of the turtle carapace. The architecture of the keratin-collagen bi-layer coating enables large plastic deformation in the collagen layer and multiple delaminations within the bi-layer coating, preventing crack propagation in the bone layer. It is found that the dynamic fracture of bone layer in the turtle carapace depends on the stiffness mismatch and yield stress contrast between the keratin layer and the collagen layer. As the stiffness mismatch increases, small plastic deformation of the bi-layer coating occurs and the plastic deformation of collagen layer tends to emerge in the vicinity of the keratin-collagen interface, suppressing interfacial delamination and leading to weak resistance to fracture of the bone layer. The intermediate level of yield stress contrast can activate large plastic deformation and multiple delaminations within the bi-layer coating, mitigating fracture of the bone layer.

Instrumenting Polyodon spathula (Paddlefish) Rostra in Flowing Water with Strain Gages and Accelerometers

The prominent rostrum of the North American Paddlefish, supported by a lattice-like endoskeleton, is highly durable, making it an important candidate for bio-inspiration studies. Energy dissipation and load-bearing capacity of the structure from extreme physical force has been demonstrated superior to that of man-made systems, but response to continuous hydraulic forces is unknown and requires special instrumentation for in vivo testing on a live fish. A single supply strain gage amplifier circuit has been combined with a digital three-axis accelerometer, implemented in a printed circuit board (PCB), and integrated with the commercial-off-the-shelf Adafruit Feather M0 datalogger with a microSD card. The device is battery powered and enclosed in silicon before attachment around the rostrum with a silicon strap "watch band." As proof-of-concept, we tested the instrumentation on an amputated Paddlefish rostrum in a water-filled swim tunnel and successfully obtained interpretable data. Results indicate that this design could work on live swimming fish in future in vivo experiments.

An inspiration from the microstructure of the cortical bone in goat tibia

When vigorous goats are running and jumping, their leg bones play a crucial role in dynamic loading. The hierarchical structure of diverse tissues at different length scales is commonly used to account for the superior mechanical properties of bone. Yet, how the goat tibia achieves its remarkable function remains mostly unknown. Scanning electron microscopy was employed to image the multi-scale microstructure of the cortical bone in goat tibia. A new processing method was utilized during the preparation of one of the three kinds of samples. The transverse and longitudinal sections of the cortical bone were observed thoroughly, and high-quality images of the internal organizations are acquired. Some interesting findings, including the two-layered character of cortical bone and the microstructure near osteocyte lacunae, are discussed. A three-dimensional three-level hierarchical structure is found accordingly. Then, the cushioning mechanisms are discussed by analyzing the structure–function relationships of the bone tissues. Finally, a structural model of biomimetic composite is proposed based on the authors’ insights into the constitution of cortical bone, which is expected to inspire engineers to design load-bearing structures with excellent mechanical performance.

Optimal Background Color for Head-Starting Northern River Terrapins (Batagur baska Gray, 1831)

Simple Summary

Substrate color is an important physical condition affecting the lifestyle of reared animals. However, no prior data are available regarding northern river terrapins. In this study, the terrapins were reared in five types of colored tanks (transparent, green, red, blue, and black) for twelve weeks. Based on parameters assessing survival, growth, feed utilization, and health, the blue background is more suitable for rearing terrapins relative to other colored tanks. The findings from the current study could be applied to wall, pond, or aquaria decoration to support the head-starting programs of northern river terrapins before release to natural habitat, as well as in public displays, such as aquaria and zoos.

Abstract

Background color has significant effects on the lifestyles of various animal species. In this study, the effects of wall background color on growth, feed utilization, specific activity of gastric and pancreatic enzymes in fecal samples, fecal thermal properties, carapace elemental profile, and hematological parameters were investigated in northern river terrapin (Batagur baska Gray, 1831) in order to optimize the head-starting conditions. The terrapins (70.13 ± 0.04 g initial weight) were completely randomized into five types of colored tanks (transparent, green, red, blue, and black) and reared for twelve weeks. At the end of the experiment, tank color had no effect on survival and growth performance, but had significant effects on feeding rate, feed conversion ratio, and protein efficiency ratio (p < 0.05). Terrapins reared with black, red, or blue backgrounds had the highest feed utilization among the treatments. Among these three groups, analysis of specific activity of gastric and pancreatic enzymes in fecal samples and fecal thermal properties suggested improved digestive functionality in terrapins reared with a blue background relative to the other treatments. Carapace elemental composition and hematological parameters indicated no negative effects on health status of the terrapins reared with this optimal treatment. Findings from the current study support the head-starting program of northern river terrapins before release to natural habitats, and could also be applied in aquaria or zoos for public display.

5334000-53340000Optimal Salinity for Head-Starting Northern River Terrapins (Batagur baska Gray, 1831)

Simple Summary

Northern river terrapins can adapt physiologically or behaviorally across a salinity gradient, so that they can move between freshwater habitats and brackish water or estuaries. However, there is no available information on the optimal salinity of this species for supporting captive husbandry programs before release to natural habitat. In the current study, the optimal salinity for the terrapins was investigated to serve the well-being of turtles. At the end of the rearing period, various observed parameters related to growth, feed utilization, and health indicated that optimal salinity for rearing the terrapins is about 4 ppt, while rearing in freshwater (0 ppt) or other brackish (8 and 12 ppt) water conditions had negative effects on some key parameters that reflect well-being. A polynomial regression fit to terrapin weight gain as function of salinity predicts the optimal salinity as 4.35 ppt. Findings from the current study could be directly used in ex situ conservation programs of northern river terrapins before release to natural habitat. Additionally, concurrent conservation and restoration of the wild habitats with preferred salinity is essential for flourishing terrapin population.

Abstract

Northern river terrapins (Batagur baska Gray, 1831) are Asia’s largest turtles living in both freshwater and brackish water. In the current study, the optimal salinity for head-starting programs of this critically endangered species was investigated in order to serve the well-being of turtles before release to natural habitat. Forty-eight terrapins (54.64 ± 0.18 g initial body weight) were randomly distributed to four salinity levels (0, 4, 8, and 12 ppt) and reared for eight weeks, using three replicates with four terrapins each. At the end of rearing trial, growth performance and feed utilization parameters were superior in terrapins reared at 4 ppt, followed by 8 ppt in the rank order of treatments. Negative stress responses were observed in terrapins reared at 12 ppt, as the fecal activity of amylase-to-trypsin ratio was changed significantly, but not that of proteolytic enzymes. The fecal thermal transition properties indicated an abundance of nutrients in the post-absorptive phase for terrapins reared at 4 ppt, followed by the 8 ppt treatment group. The preferred 4 ppt salinity had no negative effects on the health status of the terrapins in terms of carapace elemental composition or hematological parameters. Second-order polynomial regression suggests 4.35 ppt as the optimal salinity for maximal weight gain. Findings from the current study could be directly used in ex situ conservation programs of northern river terrapins before release to natural habitat.

Compressive deformation and failure of trabecular structures in a turtle shell

Turtle shells comprising of cortical and trabecular bones exhibit intriguing mechanical properties. In this work, compression tests were performed using specimens made from the carapace of Kinixys erosa turtle. A combination of imaging techniques and mechanical testing were employed to examine the responses of hierarchical microstructures of turtle shell under compression. Finite element models produced from microCT-scanned microstructures and analytical foam structure models were then used to elucidate local responses of trabecular bones deformed under compression. The results reveal the contributions from micro-strut bending and stress concentrations to the fractural mechanisms of trabecular bone structures. The porous structures of turtle shells could be an excellent prototype for the bioinspired design of deformation-resistant structures. STATEMENT OF SIGNIFICANCE: In this study, a combination of analytical, computational models and experiments is used to study the underlying mechanisms that contribute to the compressive deformation of a Kinixys erosa turtle shell between the nano-, micro- and macro-scales. The proposed work shows that the turtle shell structures can be analyzed as sandwich structures that have the capacity to concentrate deformation and stresses within the trabecular bones, which enables significant energy absorption during compressive deformation. Then, the trends in the deformation characteristics and the strengths of the trabecular bone segments are well predicted by the four-strut model, which captures the effects of variations in strut length, thickness and orientation that are related to microstructural uncertainties of the turtle shells. The above results also suggest that the model may be used to guide the bioinspired design of sandwich porous structures that mimic the properties of the cortical and trabecular bone segments of turtle shells under a range of loading conditions.

Performance in three shell functions predicts the phenotypic distribution of hard-shelled turtles

Adaptive landscapes have served as fruitful guides to evolutionary research for nearly a century. Current methods guided by landscape frameworks mostly utilize evolutionary modeling (e.g., fitting data to Ornstein-Uhlenbeck models) to make inferences about adaptive peaks. Recent alternative methods utilize known relationships between phenotypes and functional performance to derive information about adaptive landscapes; this information can then help explain the distribution of species in phenotypic space and help infer the relative importance of various functions for guiding diversification. Here, data on performance for three turtle shell functions-strength, hydrodynamic efficiency, and self-righting ability-are used to develop a set of predicted performance optima in shell shape space. The distribution of performance optima shows significant similarity to the distribution of existing turtle species and helps explain the absence of shells in otherwise anomalously empty regions of morphospace. The method outperforms a modeling-based approach in inferring the location of reasonable adaptive peaks and in explaining the shape of the phenotypic distributions of turtle shells. Performance surface-based methods allow researchers to more directly connect functional performance with macroevolutionary diversification, and to explain the distribution of species (including presences and absences) across phenotypic space.

Warped finite element models predict whole shell failure in turtle shells

Finite element (FE) models have become increasingly popular in comparative biomechanical studies, with researchers continually developing methods such as 'warping' preexisting models to facilitate analyses. However, few studies have investigated how well FE models can predict biologically crucial whole-structure performance or whether 'warped' models can provide useful information about the mechanical behavior of actual specimens. This study addresses both of these issues through a validation of warped FE models of turtle shells. FE models for 40 turtle specimens were built using 3D landmark coordinates and thin-plate spline interpolations to warp preexisting turtle shell models. Each actual turtle specimen was loaded to failure, and the load at failure and mode of fracture were then compared with the behavior predicted by the models. Overall, the models performed very well, despite the fact that many simplifying assumptions were made for analysis. Regressions of observed on predicted loads were significant for the dataset as a whole, as well as in separate analyses within two turtle species, and the direction of fracture was generally consistent with the patterns of stresses observed in the models. This was true even when size (an important factor in determining strength) was removed from analyses - the models were also able to predict which shells would be relatively stronger or weaker. Although some residual variation remains unexplained, this study supports the idea that warped FE models run with simplifying assumptions at least can provide useful information for comparative biomechanical studies.

On the Materials Science of Nature's Arms Race

Biological material systems have evolved unique combinations of mechanical properties to fulfill their specific function through a series of ingenious designs. Seeking lessons from Nature by replicating the underlying principles of such biological materials offers new promise for creating unique combinations of properties in man-made systems. One case in point is Nature's means of attack and defense. During the long-term evolutionary "arms race," naturally evolved weapons have achieved exceptional mechanical efficiency with a synergy of effective offense and persistence-two characteristics that often tend to be mutually exclusive in many synthetic systems-which may present a notable source of new materials science knowledge and inspiration. This review categorizes Nature's weapons into ten distinct groups, and discusses the unique structural and mechanical designs of each group by taking representative systems as examples. The approach described is to extract the common principles underlying such designs that could be translated into man-made materials. Further, recent advances in replicating the design principles of natural weapons at differing lengthscales in artificial materials, devices and tools to tackle practical problems are revisited, and the challenges associated with biological and bioinspired materials research in terms of both processing and properties are discussed.

Armours for soft bodies: how far can bioinspiration take us?

The development of armour is as old as the dawn of civilization. Early man looked to natural structures to harvest or replicate for protection, leaning on millennia of evolutionary developments in natural protection. Since the advent of more modern weaponry, Armor development has seemingly been driven more by materials research than bio-inspiration. However, parallels can still be drawn between modern bullet-protective armours and natural defensive structures. Soft armour for handgun and fragmentation threats can be likened to mammalian skin, and similarly, hard armour can be compared with exoskeletons and turtle shells. Via bio-inspiration, it may be possible to develop structures previously un-researched for ballistic protection. This review will cover current modern ballistic protective structures focusing on energy dissipation and absorption methods, and their natural analogues. As all armour is a compromise between weight, flexibility and protection, the imbricated structure of scaled skin will be presented as a better balance between these factors.

Merging cranial histology and 3D-computational biomechanics: a review of the feeding ecology of a Late Triassic temnospondyl amphibian

Finite Element Analysis (FEA) is a useful method for understanding form and function. However, modelling of fossil taxa invariably involves assumptions as a result of preservation-induced loss of information in the fossil record. To test the validity of predictions from FEA, given such assumptions, these results could be compared to independent lines of evidence for cranial mechanics. In the present study a new concept of using bone microstructure to predict stress distribution in the skull during feeding is put forward and a correlation between bone microstructure and results of computational biomechanics (FEA) is carried out. The bony framework is a product of biological optimisation; bone structure is created to meet local mechanical conditions. To test how well results from FEA correlate to cranial mechanics predicted from bone structure, the well-known temnospondyl Metoposaurus krasiejowensis was used as a model. A crucial issue to Temnospondyli is their feeding mode: did they suction feed or employ direct biting, or both? Metoposaurids have previously been characterised either as active hunters or passive bottom dwellers. In order to test the correlation between results from FEA and bone microstructure, two skulls of Metoposaurus were used, one modelled under FE analyses, while for the second one 17 dermal bone microstructure were analysed. Thus, for the first time, results predicting cranial mechanical behaviour using both methods are merged to understand the feeding strategy of Metoposaurus. Metoposaurus appears to have been an aquatic animal that exhibited a generalist feeding behaviour. This taxon may have used two foraging techniques in hunting; mainly bilateral biting and, to a lesser extent, lateral strikes. However, bone microstructure suggests that lateral biting was more frequent than suggested by Finite Element Analysis (FEA). One of the potential factors that determined its mode of life may have been water levels. During optimum water conditions, metoposaurids may have been more active ambush predators that were capable of lateral strikes of the head. The dry season required a less active mode of life when bilateral biting is particularly efficient. This, combined with their characteristically anteriorly positioned orbits, was optimal for ambush strategy. This ability to use alternative modes of food acquisition, independent of environmental conditions, might hold the key in explaining the very common occurrence of metoposaurids during the Late Triassic.

On the relationship between indentation hardness and modulus, and the damage resistance of biological materials

The remarkable mechanical performance of biological materials is based on intricate structure-function relationships. Nanoindentation has become the primary tool for characterising biological materials, as it allows to relate structural changes to variations in mechanical properties on small scales. However, the respective theoretical background and associated interpretation of the parameters measured via indentation derives largely from research on 'traditional' engineering materials such as metals or ceramics. Here, we discuss the functional relevance of indentation hardness in biological materials by presenting a meta-analysis of its relationship with indentation modulus. Across seven orders of magnitude, indentation hardness was directly proportional to indentation modulus. Using a lumped parameter model to deconvolute indentation hardness into components arising from reversible and irreversible deformation, we establish criteria which allow to interpret differences in indentation hardness across or within biological materials. The ratio between hardness and modulus arises as a key parameter, which is related to the ratio between irreversible and reversible deformation during indentation, the material's yield strength, and the resistance to irreversible deformation, a material property which represents the energy required to create a unit volume of purely irreversible deformation. Indentation hardness generally increases upon material dehydration, however to a larger extent than expected from accompanying changes in indentation modulus, indicating that water acts as a 'plasticiser'. A detailed discussion of the role of indentation hardness, modulus and toughness in damage control during sharp or blunt indentation yields comprehensive guidelines for a performance-based ranking of biological materials, and suggests that quasi-plastic deformation is a frequent yet poorly understood damage mode, highlighting an important area of future research.

STATEMENT OF SIGNIFICANCE

Instrumented indentation is a widespread tool for characterising the mechanical properties of biological materials. Here, we show that the ratio between indentation hardness and modulus is approximately constant in biological materials. A simple elastic-plastic series deformation model is employed to rationalise part of this correlation, and criteria for a meaningful comparison of indentation hardness across biological materials are proposed. The ratio between indentation hardness and modulus emerges as the key parameter characterising the relative amount of irreversible deformation during indentation. Despite their comparatively high hardness to modulus ratio, biological materials are susceptible to quasiplastic deformation, due to their high toughness: quasi-plastic deformation is hence hypothesised to be a frequent yet poorly understood phenomenon, highlighting an important area of future research.

Failure retardation in body armor

The protective structures that occur in biological systems are complex composite materials that display impressive mechanical properties, considering the weak properties of the individual constituents from which they are assembled. Body armors are hard materials designed to protect an animal from the fangs and claws of their predator. The usual engineering approach to biological materials has focused on treating them like synthetic composite materials designed to achieve higher strength and stiffness. Here, the authors propose that the basic evolutionary design of body armors and biological materials is related to the retardation of catastrophic failure through a variety of mechanisms, most of which directly relate to the absorption of energy during deformation. The authors subsequently reviewed and classified in a systemic way failure retardation mechanisms related to various types of body armor, including fish scales, fish dermal plates, osteoderms, mollusk shells and porcupine quills. These materials are compared with soft materials such as bacterial cellulose, jumbo squid mantles and actin microtubules that exhibit similar failure retardation characteristics. Through comparison of these failure analysis studies, the authors aim to develop a more nuanced understanding of the evolutionary design of the hierarchical structures observed in a variety of biological systems.

Leatherback sea turtle shell: A tough and flexible biological design

The leatherback sea turtle is unique among chelonians for having a soft skin which covers its osteoderms. The osteoderm is composed of bony plates that are interconnected with collagen fibers in a structure called suture. The soft dermis and suture geometry enable a significant amount of flexing of the junction between adjacent osteoderms. This design allows the body to contract better than a hard-shelled sea turtle as it dives to depths of over 1,000 m. The leatherback turtle has ridges along the carapace to enhance the hydrodynamic flow and provide a tailored stiffness. The osteoderms are of two types: flat and ridged. The structure of the two types of osteoderms is characterized and their mechanical properties are investigated with particular attention to the failure mechanisms. They both are bony structures with a porous core sandwiched between compact layers that form the outside and inside surfaces. The compressive strength is highly anisotropic by virtue of the interaction between loading orientation and arrangement of porous and compact components of osteoderms. The angle of interpenetration at the suture of osteoderms is analyzed and compared with analytical predictions. The sutures have a triangular shape with an angle of ∼30° which provides a balance between the tensile strength of the osteoderms and shear strength of the collagen fiber layer and is verified by Li-Ortiz-Boyce in a previous study. This is confirmed by an FEM analysis. A calculation is developed to quantify the flexibility of the carapace and plastron as a function of the angular displacement at the sutures, predicting the interdependence between geometrical parameters and flexibility.

STATEMENT OF SIGNIFICANCE

The leatherback turtle is a magnificent chelonian whose decreasing numbers have brought it to the brink of extinction in the Pacific Ocean. This first study of the structure of its shell provides important new insights that explain its amazing capacity for diving: depths of over 1,000 m have been recorded. This is enabled by the flexibility between the bony plates comprising its shell, which is covered by a skin and not by hard keratin as all other turtles. We use the arsenal of Materials Science characterization techniques to probe the structure of the shell and explain its amazing structure and capacity for flexing, while retaining its protection capability.

Structural Design Elements in Biological Materials: Application to Bioinspiration

Eight structural elements in biological materials are identified as the most common amongst a variety of animal taxa. These are proposed as a new paradigm in the field of biological materials science as they can serve as a toolbox for rationalizing the complex mechanical behavior of structural biological materials and for systematizing the development of bioinspired designs for structural applications. They are employed to improve the mechanical properties, namely strength, wear resistance, stiffness, flexibility, fracture toughness, and energy absorption of different biological materials for a variety of functions (e.g., body support, joint movement, impact protection, weight reduction). The structural elements identified are: fibrous, helical, gradient, layered, tubular, cellular, suture, and overlapping. For each of the structural design elements, critical design parameters are presented along with constitutive equations with a focus on mechanical properties. Additionally, example organisms from varying biological classes are presented for each case to display the wide variety of environments where each of these elements is present. Examples of current bioinspired materials are also introduced for each element.

Biological armors under impact--effect of keratin coating, and synthetic bio-inspired analogues

A number of biological armors, such as turtle shells, consist of a strong exoskeleton covered with a thin keratin coating. The mechanical role upon impact of this keratin coating has surprisingly not been investigated thus far. Low-velocity impact tests on the turtle shell reveal a unique toughening phenomenon attributed to the thin covering keratin layer, the presence of which noticeably improves the fracture energy and shell integrity. Synthetic substrate/coating analogues were subsequently prepared and exhibit an impact behavior similar to the biological ones. The results of the present study may improve our understanding, and even future designs, of impact-tolerant structures.

25th anniversary article: scalable multiscale patterned structures inspired by nature: the role of hierarchy

Multiscale, hierarchically patterned surfaces, such as lotus leaves, butterfly wings, adhesion pads of gecko lizards are abundantly found in nature, where microstructures are usually used to strengthen the mechanical stability while nanostructures offer the main functionality, i.e., wettability, structural color, or dry adhesion. To emulate such hierarchical structures in nature, multiscale, multilevel patterning has been extensively utilized for the last few decades towards various applications ranging from wetting control, structural colors, to tissue scaffolds. In this review, we highlight recent advances in scalable multiscale patterning to bring about improved functions that can even surpass those found in nature, with particular focus on the analogy between natural and synthetic architectures in terms of the role of different length scales. This review is organized into four sections. First, the role and importance of multiscale, hierarchical structures is described with four representative examples. Second, recent achievements in multiscale patterning are introduced with their strengths and weaknesses. Third, four application areas of wetting control, dry adhesives, selectively filtrating membranes, and multiscale tissue scaffolds are overviewed by stressing out how and why multiscale structures need to be incorporated to carry out their performances. Finally, we present future directions and challenges for scalable, multiscale patterned surfaces.

Structural design and mechanical behavior of alligator (Alligator mississippiensis) osteoderms

Alligator is a well-adapted living fossil covered with dorsal armor. This dermal shield consists of bony plates, called osteoderms, interconnected by sutures and non-mineralized collagen fibers, providing a dual function of protection and flexibility. Osteoderm features a sandwich structure, combining an inner porous core and an outer dense cortex, to offer enhancements for stiffness and energy absorbance. In this study, we investigated the multi-scale structure and mechanical behaviors of the American alligator (Alligator mississippiensis) osteoderm. Microcomputed tomography was applied to reveal the complex neurovascular network. Through the observation under optical and scanning electron microscopes, the osteoderm was found to consist of woven bone in the dorsal region and lamellar-zonal bone in the ventral region. Nanoindentation and compressive tests were performed to evaluate the mechanical properties of osteoderms. The varying mineral contents and porosity result in a graded mechanical property: a hard and stiff dorsal cortex gradually transform to a more compliant ventral base. Three protective mechanisms optimized for alligator osteoderms were proposed and elucidated.

Wear and abrasion resistance selection maps of biological materials

The mechanical design of biological materials has generated widespread interest in recent years, providing many insights into their intriguing structure-property relationships. A critical characteristic of load-bearing materials, which is central to the survival of many species, is their wear and abrasion tolerance. In order to be fully functional, protective armors, dentitious structures and dynamic appendages must be able to tolerate repetitive contact loads without significant loss of materials or internal damage. However, very little is known about this tribological performance. Using a contact mechanics framework, we have constructed materials selection charts that provide general predictions about the wear performance of biological materials as a function of their fundamental mechanical properties. One key assumption in constructing these selection charts is that abrasion tolerance is governed by the first irreversible damage at the contact point. The maps were generated using comprehensive data from the literature and encompass a wide range of materials, from heavily mineralized to fully organic materials. Our analysis shows that the tolerance of biological materials against abrasion depends on contact geometry, which is ultimately correlated to environmental and selective pressures. Comparisons with experimental data from nanoindentation experiments are also drawn in order to verify our predictions. With the increasing amount of data available for biological materials also comes the challenge of selecting relevant model systems for bioinspired materials engineering. We suggest that these maps will be able to guide this selection by providing an overview of biological materials that are predicted to exhibit the best abrasion tolerance, which is of fundamental interest for a wide range of applications, for instance in restorative implants and protective devices.

Micro-structure and mechanical properties of the turtle carapace as a biological composite shield

Turtle shell is a multi-scale bio-composite in which the components are arranged in various spatial patterns, leading to an unusually strong and durable structure. The keratin-coated dorsal shell, termed the carapace, exhibits a flat bone, sandwich-like structure made up of two exterior cortices enclosing a cancellous interior. This unique structure was developed by nature to protect the reptile from predator attacks by sustaining impact loads and dissipating energy. In the present study we attempt to correlate the micro-scale architecture with the mechanical properties of the carapace sub-regions of the red-eared slider turtle. The microscopic structural features were examined by scanning electron microscopy and micro-computed tomography. Nanoindentation tests were performed under dry and wet conditions on orthogonal anatomical planes to evaluate the elastic modulus and hardness of the various carapace sub-regions. The mineral content was also measured in the different regions of the carapace. Consequently, we discuss the influence of hydration on the carapace sub-regions and the contribution of each sub-region to the overall mechanical resistance of the assemblage.

The effects of water and microstructure on the mechanical properties of bighorn sheep (Ovis canadensis) horn keratin

The function of the bighorn sheep horn prompted quantification of the various parametric effects important to the microstructure and mechanical property relationships of this horn. These parameters included analysis of the stress-state dependence with the horn keratin tested under tension and compression, the anisotropy of the material structure and mechanical behavior, the spatial location along the horn, and the wet-dry horn behavior. The mechanical properties of interest were the elastic moduli, yield strength, ultimate strength, failure strain and hardness. The results showed that water has a more significant effect on the mechanical behavior of ram horn more than the anisotropy, location along the horn and the type of loading state. All of these parametric effects showed that the horn microstructure and mechanical properties were similar to those of long-fiber composites. In the ambient dry condition (10 wt.% water), the longitudinal elastic modulus, yield strength and failure strain were measured to be 4.0 G Pa, 62 MPa and 4%, respectively, and the transverse elastic modulus, yield strength and failure strain were 2.9 GPa, 37 MPa and 2%, respectively. In the wet condition (35 wt.% water), horn behaves more like an isotropic material; the elastic modulus, yield strength and failure strain were determined to be 0.6G Pa, 10 MPa and 60%, respectively.