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This work is dedicated to characterizing unexplored structural biological materials in both mineralized and non-mineralized systems including the transparent teeth of the deep-sea dragonfish, a comparative study on the piranha and pacu teeth, and the feather rachis of primary flight feathers from birds with various flight styles. The insights gained are used to better understand how the structure across multiple length scales (micro, meso, and macro) relates to the optimization of tough, stiff, lightweight, energy absorbent, and impact resistant materials. We propose that these findings can be used to develop bioinspired structural designs for tailorable mechanical performance. The transparent dragonfish teeth are found to be composed of nanostructured hydroxyapatite (~20 nm grain size) and collagen. They are devoid of microscale features such as microtubules which are typically found in the dentin layer. The nanoscale structure is responsible for the reduced Rayleigh light scattering and the increase in hardness and stiffness in the dentin layer. The optical transmittance of the teeth is found to be within the range of ~38% to ~78% and is found to depend on tooth location (tip, middle, base) and wavelength. The tip of the tooth is the most transparent to red light (700 nm) and least transparent to blue light (450 nm). We suggest that the nanostructured design of the transparent teeth enables predatory success as it makes its wide-open mouth armed with teeth effectively disappear against the darkness of the deep-sea. The relationship between feeding mechanics and structure-property relationships are investigated with the comparison of two closely related species: the piranha (carnivorous) and the pacu (herbivorous). Typically, the morphology and form of the tooth are only used to indicate feeding performance, ignoring the hierarchical nature and relative tooth position within the jaw. Herein we investigate both types of teeth across multiple length scales and relate the findings to how the tooth performs during feeding. We suggest that tooth form, enamel thickness, structuring of the dentin-enamel junction, and microstructural characteristics of enamel influences the biomechanics required for each style of feeding. Tubules are not only found in the dentin layer of teeth but also in horns, hooves, and many other natural energy absorbent materials. However, the effects of porosity, radius, or degree of reinforcing mineralization and the synergy with other design motifs (suture and gradient structures) on the mechanical properties have not been fully explored. We propose to 3D print tubule architectures to parametrically explore the above-mentioned effects in quasi-static compression, fracture toughness, drop tower testing, Hopkinson bar, and dynamic mechanical analysis and compare them with numerical models. The mechanics of the feather shaft are explored through 3D printed bioinspired designs to investigate structure-property relationships with respect to flight. The feather shaft is the predominantly load bearing component of the feather and is the focus of this work. Sandwich structures inspired by the feather shaft are printed with a foam core and without a foam core to investigate the flexural properties. The flexural strength of the sandwich structure increases with the addition of a foam core, due to the increase in the moment of inertia. However, the stiffness remains the same despite having or not having a foam core. Stiffness is only determined by the cortex since the foam provides negligible stiffness .

Summary Keratin is a highly multifunctional biopolymer serving various roles in nature due to its diverse material properties, wide spectrum of structural designs, and impressive performance. Keratin-based materials are mechanically robust, thermally insulating, lightweight, capable of undergoing reversible adhesion through van der Waals forces, and exhibit structural coloration and hydrophobic surfaces. Thus, they have become templates for bioinspired designs and have even been applied as a functional material for biomedical applications and environmentally sustainable fiber-reinforced composites. This review aims to highlight keratin's remarkable capabilities as a biological component, a source of design inspiration, and an engineering material. We conclude with future directions for the exploration of keratinous materials., Graphical abstract, Biomaterials; Engineering materials; Materials design

Avian flight feathers have developed, through evolution, an intricate architecture with multi-functional structures that are essential for flight. These lightweight and resilient appendages motivate the invention of bioinspired designs. Here we fabricate various structures inspired by significant concepts identified in the feather vane and shaft. Bioinspired prototypes based on the feather vane's unique adhesive mechanism and directional permeability are explored, and feather-shaft inspired designs motivated by the highly ordered hierarchical fiber-matrix structure in the feather are fabricated. The exquisite architecture of the rachis, consisting of a hollow tube filled with foam, is simulated in a bioinspired design that demonstrates the synergy of the two components in enhancing the flexural strength. These structures provide an enhanced understanding of the mechanisms operating in feathers and suggest highly efficient solutions which can contribute to creating innovative materials inspired by the feather.

Additive manufacturing (AM) is a current technology undergoing rapid development that is utilized in a wide variety of applications. In the field of biological and bioinspired materials, additive manufacturing is being used to generate intricate prototypes to expand our understanding of the fundamental structure-property relationships that govern nature's spectacular mechanical performance. Herein, recent advances in the use of AM for improving the understanding of the structure-property relationship in biological materials and for the production of bioinspired materials are reviewed. There are four essential components to this work: a) extracting defining characteristics of biological designs, b) designing 3D-printed prototypes, c) performing mechanical testing on 3D-printed prototypes to understand fundamental mechanisms at hand, and d) optimizing design for tailorable performance. It is intended to highlight how the various types of additive manufacturing methods are utilized, to unravel novel discoveries in the field of biological materials. Since AM processing techniques have surpassed antiquated limitations, especially with respect to spatial scales, there has been a surge in their demand as an integral tool for research. In conclusion, current challenges and the technical perspective for further development of bioinspired materials using AM are discussed.