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

Bioinspired and Multifunctional Tribological Materials for Sliding, Erosive, Machining, and Energy-Absorbing Conditions: A Review

Friction, wear, and the consequent energy dissipation pose significant challenges in systems with moving components, spanning various domains, including nanoelectromechanical systems (NEMS/MEMS) and bio-MEMS (microrobots), hip prostheses (biomaterials), offshore wind and hydro turbines, space vehicles, solar mirrors for photovoltaics, triboelectric generators, etc. Nature-inspired bionic surfaces offer valuable examples of effective texturing strategies, encompassing various geometric and topological approaches tailored to mitigate frictional effects and related functionalities in various scenarios. By employing biomimetic surface modifications, for example, roughness tailoring, multifunctionality of the system can be generated to efficiently reduce friction and wear, enhance load-bearing capacity, improve self-adaptiveness in different environments, improve chemical interactions, facilitate biological interactions, etc. However, the full potential of bioinspired texturing remains untapped due to the limited mechanistic understanding of functional aspects in tribological/biotribological settings. The current review extends to surface engineering and provides a comprehensive and critical assessment of bioinspired texturing that exhibits sustainable synergy between tribology and biology. The successful evolving examples from nature for surface/tribological solutions that can efficiently solve complex tribological problems in both dry and lubricated contact situations are comprehensively discussed. The review encompasses four major wear conditions: sliding, solid-particle erosion, machining or cutting, and impact (energy absorbing). Furthermore, it explores how topographies and their design parameters can provide tailored responses (multifunctionality) under specified tribological conditions. Additionally, an interdisciplinary perspective on the future potential of bioinspired materials and structures with enhanced wear resistance is presented.

Nanoscale dynamic mechanical analysis on interfaces of biological composites

Biological composites incorporate structural arrays of rigid-elastic reinforcements made of minerals or crystalline biopolymers, which are connected by thin, compliant, and viscoelastic macromolecular matrix material. The near-interface regions of these biological composites grant them energy dissipation capabilities against dynamic mechanical loadings, which promote various biomechanical functions such as impact adsorption, fracture toughness, and mechanical signal filtering. Here, we employ theoretical modeling and finite-element simulations to analyze the mechanical response of the near-interface in biological composites to nanoscale dynamic mechanical analysis (DMA). We identified the dominating load-bearing mechanisms of the near-interface region and employed these insights to introduce simple semi-empirical formulations for approaching the mechanical properties (storage and loss moduli) of the biological composite from the nanoscale DMA results. Our analysis paves the way for the nanomechanical characterization of biological composites in diverse natural materials systems, which can also be employed for bioinspired and biomedical configurations.

Finite element analysis of energy absorption mechanism of durian peel and durian thorns under compressive load

This study aims to investigate the energy absorption mechanism of durian peel under compression using finite element analysis. The durian peel comprises four substructures: thorn skin, thorn core, locule, and membrane. Their mechanical properties were measured experimentally and used as inputs for finite element models. Three-dimensional solid models of the durian peel were created and compared to the corresponding macroscopic responses. Good agreements between the experimental results and model predictions validate the measured properties. Energy analysis of the substructures shows that thorn skins and cores are the dominant components of energy absorption in durian peel under large deformation. Single thorn models with various aspect ratios were created to analyze their influence on energy absorption. The single thorn models were compressed at four angles of attack to capture the effect of peel curvature. The analysis shows that the aspect ratio of a single thorn that balances energy absorption at all angles of attack falls between 1 and 1.25, which is consistent with the experimentally measured values.

In Situ Synthesis of Keratin and Melanin Chromophoric Submicron Particles

In humans, melanin plays an esthetic role, dictating hair and skin color and traits, while keratin is the protein that comprises most of the epidermis layer. Eumelanin and pheomelanin are types of melanin synthesized from the same building blocks via enzymatic oxidation. Pheomelanin has an additional building block, cysteine amino acid, which affects its final structure. Keratin contains high cysteine content, and by exploiting free thiols in hydrolyzed keratin, we demonstrate the formation of keratin-melanin (KerMel) chromophoric submicron particles. Cryo-TEM analyses found KerMel particle sizes to be 100-300 nm and arranged in the form of a central keratin particle with polymerized l-dopa chains. Attenuated total reflection (ATR)-FTIR, UV-vis, and fluorescence measurements identified new chemical bonds, indicating the formation of KerMel particles. Finally, KerMel replicated natural skin tones and proved cytocompatibility for human epidermal keratinocytes at concentrations below 0.1 mg/mL. Taken together, KerMel is a novel, tunable material that has the potential to integrate into the cosmetic industry.

Allelochemical run-off from the invasive terrestrial plant Impatiens glandulifera decreases defensibility in Daphnia

Invasive species are a major threat for native ecosystems and organisms living within. They are reducing the biodiversity in invaded ecosystems, by outcompeting native species with e. g. novel substances. Invasive terrestrial plants can release allelochemicals, thereby reducing biodiversity due to the suppression of growth of native plants in invaded habitats. Aside from negative effects on plants, allelochemicals can affect other organisms such as mycorrhiza fungi and invertebrates in terrestrial ecosystems. When invasive plants grow in riparian zones, it is very likely that terrestrial borne allelochemicals can leach into the aquatic ecosystem. There, the often highly reactive compounds may not only elicit toxic effects to aquatic organisms, but they may also interfere with biotic interactions. Here we show that the allelochemical 2-methoxy-1,4-naphthoquinone (2-MNQ), produced by the ubiquitously occurring invasive terrestrial plant Impatiens glandulifera, interferes with the ability of Daphnia to defend itself against predators with morphological defences. Daphnia magna and Daphnia longicephala responded with morphological defences induced by chemical cues released by their corresponding predators, Triops cancriformis or Notonecta sp. However, predator cues in combination with 2-MNQ led to a reduction in the morphological defensive traits, body- and tail-spine length, in D. magna. In D. longicephala all tested inducible defensive traits were not significantly affected by 2-MNQ but indicate similar patterns, highlighting the importance to study different species to assess the risks for aquatic ecosystems. Since it is essential for Daphnia to adapt defences to the current predation risk, a maladaptation in defensive traits when simultaneously exposed to allelochemicals released by I. glandulifera, may therefore have knock-on effects on population dynamics across multiple trophic levels, as Daphnia is a key species in lentic ecosystems.

Impact Resistant Structure Design and Optimization Inspired by Turtle Carapace

The turtle carapace has a high level of protection, due to its unique biological structure, and there is great potential to use the turtle carapace structure to improve the impact resistance of composite materials using bionic theory. In this paper, the chemical elements of the turtle carapace structure, as well as its mechanical properties, were investigated by studying the composition of the compounds in each part. In addition, the bionic sandwich structure, composed of the plate, core, and backplate, was designed using modeling software based on the microstructure of the keratin scutes, spongy bone, and the spine of the turtle carapace. Additionally, finite element analysis and drop-weight experiments were utilized to validate the impact-resistant performance of the bionic structures. The numerical results show that all of the bionic structures had improved impact resistance to varying degrees when compared with the control group. The experimental results show that the split plate, the core with changing pore gradients, and the backplate with stiffener all have a considerable effect on the impact-resistance performance of overall composite structures. This preliminary study provides theoretical support for composite material optimization.

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.

Assessing the Interfacial Dynamic Modulus of Biological Composites

Biological composites (biocomposites) possess ultra-thin, irregular-shaped, energy dissipating interfacial regions that grant them crucial mechanical capabilities. Identifying the dynamic (viscoelastic) modulus of these interfacial regions is considered to be the key toward understanding the underlying structure-function relationships in various load-bearing biological materials including mollusk shells, arthropod cuticles, and plant parts. However, due to the submicron dimensions and the confined locations of these interfacial regions within the biocomposite, assessing their mechanical characteristics directly with experiments is nearly impossible. Here, we employ composite-mechanics modeling, analytical formulations, and numerical simulations to establish a theoretical framework that links the interfacial dynamic modulus of a biocomposite to the extrinsic characteristics of a larger-scale biocomposite segment. Accordingly, we introduce a methodology that enables back-calculating (via simple linear scaling) of the interfacial dynamic modulus of biocomposites from their far-field dynamic mechanical analysis. We demonstrate its usage on zigzag-shaped interfaces that are abundant in biocomposites. Our theoretical framework and methodological approach are applicable to the vast range of biocomposites in natural materials; its essence can be directly employed or generally adapted into analogous composite systems, such as architected nanocomposites, biomedical composites, and bioinspired materials.

Understanding hydration effects on mechanical and impacting properties of turtle shell

Study of the properties of natural biomaterials provides a reliable experimental basis for the design of biomimetic materials. The mechanical properties and impact wear behaviors of turtle shell with different soaking time were investigated on a micro-amplitude impact wear tester. The damage behavior of turtle shells with different soaking time and impact cycles were systematically analyzed, also the impact dynamics behavior was inspected during the impact wear progress. The results showed that the energy absorption and impact contact force were significantly different with varied soaking time. Under different impact cycles, the peak contact force of shell samples with same soaking time were approximate to each other in value and the values of impact contact time change in a small range. However, the damage extent of shells were distinct with varied impact cycles. It was found that impact worn scars of shells increase with impact cycles increasing. However, under the same impact cycles, energy absorption and contact time increased with the extending of soaking time, but the peak contact force decrease. Especially shell without soaking, the absorption rate is the lowest.

Biochemistry and adaptive colouration of an exceptionally preserved juvenile fossil sea turtle

The holotype (MHM-K2) of the Eocene cheloniine Tasbacka danica is arguably one of the best preserved juvenile fossil sea turtles on record. Notwithstanding compactional flattening, the specimen is virtually intact, comprising a fully articulated skeleton exposed in dorsal view. MHM-K2 also preserves, with great fidelity, soft tissue traces visible as a sharply delineated carbon film around the bones and marginal scutes along the edge of the carapace. Here we show that the extraordinary preservation of the type of T. danica goes beyond gross morphology to include ultrastructural details and labile molecular components of the once-living animal. Haemoglobin-derived compounds, eumelanic pigments and proteinaceous materials retaining the immunological characteristics of sauropsid-specific β-keratin and tropomyosin were detected in tissues containing remnant melanosomes and decayed keratin plates. The preserved organics represent condensed remains of the cornified epidermis and, likely also, deeper anatomical features, and provide direct chemical evidence that adaptive melanism – a biological means used by extant sea turtle hatchlings to elevate metabolic and growth rates – had evolved 54 million years ago.

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.

Recent progress of abrasion-resistant materials: learning from nature

Recent investigations into natural abrasion-resistant materials to explore their general design principles, and the fabrication of bio-inspired abrasion-resistant materials are reviewed.

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.