The hierarchical structure and mechanical performance of a natural nanocomposite material: The turtle shell

Colloids and Surfaces A: Physicochem. Eng. Aspects 520 (2017) 97–104

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The hierarchical structure and mechanical performance of a natural nanocomposite material: The turtle shell Meiling Chen a , Narisu Hu a,b , Chang Zhou a , Xiankun Lin a , Hui Xie c,∗ , Qiang He a,∗ a State Key Laboratory of Advanced Welding and Joining (HIT), Micro/Nanotechnology Research Centre, Harbin Institute of Technology, 2 Yi Kuang Street, Harbin 150080, PR China b Oral Implant Center, Second Affiliated Hospital of Harbin Medical University, Harbin 150086, PR China c State Key Laboratory of Robotics and Systems (HIT), School of Mechatronics Engineering, Harbin Institute of Technology, 2 Yi Kuang Street, Harbin 150080, PR China

h i g h l i g h t s

g r a p h i c a l

a b s t r a c t

• The design strategies of a natural armor: turtle shells have been investigated. • Microplatelets stack orderly and compactly, constructing a layered structure. • The modulus, stiffness, deformation, and adhesion are studied by AFM.

a r t i c l e

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Article history: Received 1 January 2017 Received in revised form 17 January 2017 Accepted 23 January 2017 Available online 24 January 2017 Keywords: Turtle shell Hierarchical structure Mechanical properties Keratin Biomineralization

a b s t r a c t Turtle shells protect themselves from predatorial attack, which could provide ideas and pathways for bioinspired synthetic materials. Despite with relatively weak constituents and low biomineralization, turtle shells possess unusually robust mechanical properties due to their well organized and layered structures that are accurately designed from nanoscale to macroscale in nature. Minerals (calcium phosphate and calcium sulfate polycrystal mixture) randomly disperse in the keratin, and forming organic-inorganic nanocomposite platelets. Such platelets are basic building blocks that stack orderly and compactly in the radial direction, and constructing individual platelets into a layered micro-configuration. The outstanding tensile mechanical performance of turtle shells has much relationship with rehydration and the growth orientation of the keratin cells. Compressive mechanical properties, growth texture of keratin, topography and mineral components’ distribution of turtle shell are investigated by AFM experiment effectively. Such excellent mechanical properties of turtle shells, which integrated with nanocomposite ingredients and layered structure, may inspire the biomimetic strategies for advanced multi-functional materials, especially for artificial armor. © 2017 Published by Elsevier B.V.

1. Introduction ∗ Corresponding authors. E-mail addresses: [email protected] (H. Xie), [email protected] (Q. He). http://dx.doi.org/10.1016/j.colsurfa.2017.01.063 0927-7757/© 2017 Published by Elsevier B.V.

The protective exterior structures of natural organisms give considerable enlightenment to synthesis materials with robust and

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multifunctional mechanical properties [1–4]. Natural armor, such as abalone shells [5–8], gastropod shells [9], fishes [10–12], bones [13], feathers of birds [14] and other reptile epidermis (pangolin, alligators, crocodiles, lizards, turtles etc.) [15,16] are thought as biological composite shields, owing to their hierarchical structures and super mechanical performance. Hence, they have been a topic of major technological interest in diversified civilian and defense applications, and inspired high-performance materials, objects and processes that function from nanoscale to macroscale [17–27]. Methods to fabricate these materials include large scale model materials, ice templating, freeze casting, layer-by-layer deposition, thin film deposition, bone-inspired metallic nanocomposite, hierarchical alpha-helix based protein filaments and self-assembly [28–32]. Turtle shell possess considerable function of mechanical protection from predators that can induce damage from the environment. Until now, researchers have done much research on the shells from aspect of the biological evolution, but little study on detailed structural and mechanical function [33–38]. Fratzl et al. have presented mechanical performance of the complex 3-D suture joining the bony elements of the turtle shell [33]. However, investigation about the nanoscale feature, the complex interplay between organic protein and inorganic minerals, and mechanical properties for different orientations of turtle shells is still incomplete and unclear. Hence, going deep into understanding of the layered structure, chemical components and mechanical performance is of great important for the development of next-generation materials. Here, we elaborated the elements distribution, chemical components, structure of the turtle shells, as well as the mechanical functions such as strength, flexibility, modulus, energy absorption, stiffness, deformation, and adhesion. The dermal armors of turtle are constructed with much beta-keratin polygonal microplatelets, which well-organized and compactly stacked in the radial direction. In nanoscale, minerals were identified as calcium phosphate and calcium sulfate polycrystal mixture, which disperse in the keratin layers randomly, and abundant betakeratin are connected with bits of alfa-keratin (a kind of soft proteins). Such composite system of turtle epidermis possesses ultra-strong, flexible and anisotropic tensile mechanical properties. Compressive mechanical test was performed to analyze the compressive Young’s modulus, stiffness, deformation and adhesion, and also helped to understand the distribution of organic and mineral components in individual “building block” in turtle epidermis. This analysis focus on the relationship between the composite system, orderly layered structure, and mechanical properties, of turtle shells, may provide new insights and design rules for man-made, bioinspired multiscale nanocomposites.

2. Experimental section 2.1. Preparation of samples of turtle shell The shells were obtained from of a juvenile red-ear turtle with size of ca. 15 cm, was kindly presented as a private gift. The naturally exfoliated turtle epidermis were cut along both the longitudinal and transverse directions to be rectangular samples (15–20 mm × 3–4 mm, thickness is 30–45 ␮m) for the analysis of composition, structural evaluation and mechanical testing. Before further characterizations, these samples were rinsed in deionized water (18 M cm) for three times and dried in circulation oven at 50 ◦ C for 12 h. The ash of turtle shells were obtained by calcination in air at 560 ◦ C for 40 h. The ratio between weight of ash and weight of original dried sample was quantified as mineral content.

2.2. Characterization The morphology, energy dispersive X-ray spectrum (EDX) analysis and line scanning analysis of the outermost turtle shells were characterized by scanning electron microscope (SEM; Helios Nanolab 600i, USA). A HORIBA Raman system (LabRAM) connected to an optical microscope was used to collect Raman spectra of turtle dermis. Thermo-gravimetry (TA Q50, USA) was used to determine the thermal stability and the ratio of mineral/organic of turtle shells. Two samples were heated from 26 ◦ C to 1000 ◦ C at a heating rate of 10 ◦ C min−1 in nitrogen gas and air atmosphere, respectively. The elemental composition of turtle shell and its ash was tested with X-ray fluorescence (XRF) analysis (Axios PW4400). A wide angle X-ray diffractometer (Empyrean, Netherlands) was employed to collect X-ray diffraction patterns of ash, operating in line scan mode, with Cu Ka radiation (1.54060 Å). The mechanical properties of the freestanding shells were measured in the tensile mode with a universal mechanical testing machine (Instron 5969, USA). The distance between the clamps was 10 mm and the load speed was 5 mm min−1 . Morphology and electron diffraction pattern of the mineral platelets in the ash of turtle shell were characterized by transmission electron microscopy (TEM; JEM-1400). The elemental composition of the turtle shell was identified by X-ray photoelectron spectroscopy (XPS; Escalab 250Xi, USA). A commercial AFM (Multimode SPM, Nanoscope V controller and Signal access module, Veeco Instruments) was used for testing surface topography, compressive modulus, stiffness, deformation and adhension of the turtle shell. The images recorded in this work were taken at high resolution (512 × 360 pixels, 0.5 ␮m per pixel) by using a force versus distance mode coupled with phase detection imaging (PDI). The work frequency, the stiffness of probe, the amplitude of cantilever and the scanning rate were 50–130 kHz, 3.2 N/m, 25 nm and 150 Hz, respectively. The Si probe was employed here with a round tip of 19 nm and data was collected in air and at room temperature.

3. Results and discussion The red-ear turtles (chrysemys scripta elegans) are considered to be an invading species in many countries outside the Unites States. As seen the optical photo in Fig. 1a, a juvenile red-ear turtle with a body length of 20 cm and width of 15 cm was investigated here. The polygonous outermost shells (the longitudinal length is ∼3.3 cm) came away from red-ear turtle naturally, which exhibits remarkable strength and flexibility. To study the microstructure of this turtle outermost armor, the sample was broke off transversally at the middle by a liquid nitrogen-cooled technique. Surface SEM image (Fig. 1b) shows that many polygonal tiles with a diameter of ca. 73 ␮m stack up tightly and constitute each individual shell. As seen in SEM and its high resolution images of the fracture surface (Fig. 1c, d), the turtle outermost armor is discerned a typical three-dimensional hierarchical structure with lamellas of 400–700 nm in thickness accumulating compactly. Some particles are clearly observed in each lamella that speculated as mineral here and the verification in our subsequent study (AFM experiment). Turtle shells deploy this uniform multi-layered armor system to protect themselves as the first level of defense from some impact and predators. Besides multiscale structures, studying on chemical components is also important to understand the possible design principles of natural super-strong armor. Elemental compositions were analyzed by line scans on the SEM-EDX system. The location of scanning was selected in the center of the exposed area and line scans along the arrow from left to right (Fig. 2a). Line-scanning measurements of the carbon (C), oxygen (O), nitrogen (N), phosphorus (P), sulphur (S), calcium (Ca) and copper (Cu) contents

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Fig. 1. Macroscopic and microscopic hierarchical structure of the outermost shell of red-ear turtle. (a) Photograph of the entire red-ear turtle and the outermost turtle shell that fell off naturally. (b) Surface SEM image of the turtle shell. Scale bar, 20 ␮m (c) Cross-sectional SEM image of the shell. Scale bar, 10 ␮m. (d) The magnified view of cross-sectional SEM image of the shell. Scale bar, 1 ␮m.

and distributions in the shell were further confirmed (Fig. 2b). XPS was employed to assist with identifying the species of elements in turtle shells, and obtained similar result with EDX analysis (Fig. 2c, d). These results are reasonable since turtle shells are mainly consisting of abundant keratins, few minerals and slight heavy metals [33]. In particular, the sulfur content suggested the presence of cysteine. The data of Ca and N stand for mineral and protein components, respectively. The calcium content throughout depth of shell showed a uniform distribution, testified that every microplatelet and their contact surfaces have the same mineral component. However, the nitrogen content has a more significant reduction in the contact surface than microplatelet, demonstrated that microplatelets have more keratins than their contact surfaces. As seen in Fig. 2e, f, the Raman spectrum further studied the primary chemical structure of the proteins in the turtle shell. The strong peaks at 2938 cm−1 and 1266 cm−1 show the CH3 and C N stretching vibration, respectively. S S stretching vibration (503 cm−1 ), C S stretching vibration (631 cm−1 ) and N H symmetric stretching vibration (3300 cm−1 ) verify that proteins in turtle shell contain two types of bonds: disulphide and hydrogen bonds. This result is accordant with the previous study that the strong S S bond and weak H O bond are mainly assembled keratin macromolecules together and epitheliums of reptile are mainly composed of keratin that rich in sulfur. In addition, peak at 883 cm−1 indicates that the existence of tryptophane. Detail of the turtle shell dialyzed Raman spectrum in 1700–1450 cm−1 region is shown in Fig. 2d. Peaks at 1588 cm−1 and 1623 cm−1 indicate the C C stretching vibration of olefinic. Two amide moieties present in the proteins of turtle shell: amide I (1653 cm−1 ) and amide II (1519 cm−1 , 1544 cm−1 ) [38]. Noteworthy, the strong peak at 1519 cm−1 and shoulder peak at 1544 cm−1 represent beta-like and alpha-like amide II, respectively. Above all, proteins in turtle shell has both alpha-keratin and beta-keratin, and the amount of beta-keratin (␤-keratin) is much greater than alpha-keratin. These

chemical compositions conform to the sheet like structure rather than fibrous structure of turtle shell’s sclero-protein. Additionally, thermogravimetric analysis (TGA) effectively evaluated the ratio of mineral/organic content and the stability of the outermost turtle shell which is displayed in Fig. 3a. In air atmosphere (red curve), there appears to be two main mass loss steps at around 265 ◦ C and 525 ◦ C. The first step is attributed to the decomposition of sclero-protein, and the mass loss is about 55.5%. A 39.25% mass occurs in the next step is proposed the burning of carbon. There remains 1.82% weight of ash after this measurement, indicating the mineral content in the shell is around 1.82%. In a nitrogen atmosphere (black curve), there appears to be two main mass loss steps at around 265 ◦ C and 920 ◦ C. The first step is the decomposition of sclero-protein, which is consistent with sample in air condition, and the mass loss is about 40.4%. At much higher temperatures, loss of 8.05% mass occurs in the last step is attributed to the loss of sulphur and phosphorus from the mineral. X-ray fluorescence (XRF) analysis was used to identify the elemental ingredients in the keratin shell and its ash. The ash wasobtained from burning in the in the air. It can be seen in Fig. 3b that ash is mainly consisting of calcium (30%), sulphur (13.5%), phosphorus (11.5%) and other heavy metal element such as zinc (10.8%). Minerals in turtle shells were identified as hydroxyapatite in the previous report [39]. However, it cannot explain why so much content of sulphur exists in mineral and even beyond that of phosphorus. Furthermore, the theoretical ratio of Ca/P weight in hydroxyapatite is 2.15 and this is not match to our XRF results (ratio of Ca/P weight is 2.61). Obviously, we find new result that different from ever reports. As TEM graphs manifested (Fig. 4a, b), mineral platelets appear in near rectangle shape with length and width of 100 × 60 nm. As shown in Fig. 4c, electron diffraction pattern shows that many polycrystals consist in the ash. Combined with the electron diffraction and X-ray diffraction (XRD) results (Fig. 4d), the mineral can be identified as Ca3 (PO4 )2 and CaSO4 polycrystals mixture.

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Fig. 2. (a,b) Line-scanning analysis of turtle shell’s fracture surface. Scale bar, 10 ␮m. (c,d) X-ray photoelectron spectroscopy (XPS) results of turtle shell. (e) Raman spectrum of turtle shell. (f) Detail of the turtle shell dialyzed Raman spectrum in 1700–1450 cm−1 region.

During mechanical testing, as we respected, this composite composition and well-ordered structure endow keratin shells high strength and modulus. Tensile mechanical property of shell was tested at two different relative humidity conditions. Before testing, dry samples (sample 1, 2) were dried in circulation oven at 50 ◦ C for 12 h and wet samples (sample 3, 4) were soaked in the deionized water for 12 h. Here in, strain energy density of failure is defined as

the amount of energy a material can absorb prior to catastrophic failure, i.e., the area under the stress-strain curve. The fracture strain energy density is a measure of the energy required to induce catastrophic failure. Fig. 5a, c shows a comparison between tensile stress-strain curves for keratin shells at dry and wet conditions. Fig. 5b illustrates a schematic diagram of the three-dimensional hierarchical structure of a turtle keratin shell. We define the direc-

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Fig. 3. (a) Thermogravimetric analysis (TGA) of turtle shell, black curve indicates that turtle shell was heated in N2 atmosphere, and red curve shows that turtle shell was heated in air. (b) X-ray fluorescence (XRF) results of a slice of turtle shell and its ash. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 4. (a) TEM image of the ash. Scale bar, 200 nm. (b) Magnified TEM micrograph of the individual mineral platelet. Scale bar, 20 nm. (c) Electron diffraction pattern of the mineral platelets in ash. Scale bar, 0.2 nm. (d) XRD patterns of the outermost turtle shell and its ash.

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Fig. 5. Mechanical tensile properties of turtle shell were stretched from different directions and under dry or wet conditions. (a) Stress-strain curves of turtle shells. (b) A model of the turtle shell. (c) Young’s modulus, strain energy density, tensile strain and strength of turtle shell. Sample 1 and 2 were tested under dry condition; sample 3 and 4 were tested under wet condition. Sample 1 (red curve) and 4 (blue curve) were stretched from direction x (see b); Sample 2 (black curve) and 3 (green curve) were stretched from direction y (see b). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 6. (a) Schematic of the force-distance curve-based (FD-based) AFM for imaging and mapping multiple properties of turtle shell. (b) Approach (red) and withdrawal (black) FD curves. (c) AFM topography of the microplatelet in the shell. (d) 3D reconstruction of the surface topography of the microplatelet. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

tion x as the growth orientation of sclero-proteins in keratin shell and direction z as cumulate orientation of keratin tiles. In the tensile experiment, sample 1 and 4 were stretched from direction x, and sample 2 and 3 were stretched from direction y. In dry environment, as shown in the tensile stress-strain curves, sample 1 exhibits a higher strength (66.97 MPa) than that of sample 2 (54.15 MPa), yet shows a lower strain (2.06%) than that of sample 2 (2.44%). Young’s modulus of sample 1 and 2 are 3.50 GPa and 2.80 GPa, respectively. These results verify that the turtle shells possesse strong mechanical strengths at dry condition, but shows a higher brittleness and weak flexibility at its growth direction of sclero-proteins. However,

the mechanical response of wet turtle shell is significantly different from that of the dry one. After treated with water, the tensile strength of turtle shells presents a slightly decreasing. Whereas, the strain and fracture strain energy density of sample 3 go up to 9.03% and 345.27 MPa, and that of sample 4 are 10.73% and 369.04 MPa. The flexibility and toughness of turtle shells show a vastly improving at wet condition. These results also demonstrate that water dominates the tensile mechanical performance turtle keratin shell more than anisotropy. Water improves the strain, toughness and flexibility, which is attributed to significant direct rehydration of the keratin shells. This extremely depends on the degree of rehy-

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Fig. 7. The load-displacement impressions of the microplatelet’s surface. (a, b, c, d) The compressive modulus, adhesion, deformation and stiffness were scanned by the AFM system, respectively. Scale bar, 500 nm. Color scales quantify the parameters.

dration that is a characteristic of most biological materials. Since a keratin–water molecule network and a swelling agent may form, partial hydrogen bonding was broken and the interchain space was increased, hence increasing the mobility of protein molecular and reducing the strength [39,40]. Force versus distance curve-based atomic force microscopy (FD-based AFM) can image the architecture of complex biological systems; meanwhile, it can quantify and map their highly heterogeneous physical, chemical and biological properties in subnanometer resolution. Fig. 6a, b demonstrates the principles of the FD-based AFM. An AFM stylus is made to interact with a turtle shell in a pixel-by-pixel manner to record the interaction force (Fi) and FD curves which measured with a position-sensitive photo detector. This interaction can be used to describe the attraction, repulsion or deformation of the shell, as well as can be measured to describe the adhesion, stiffness, Young’s modulus or energy dissipation of the shell based on the withdrawal behavior of the stylus. Surface SEM image (Fig. 1b) show that individual polygonal ␤keratin microplatelets with the diameter of ca. 70 ␮m bulking of macroscopical turtle shell keratin layer. However, the ␤-keratin microplatelets do not arrange in a juxtaposed style, but instead are overlapped. Higher magnification observation (results from the AFM experiment) in the tapping mode revealed the detailed structure of the microplatelet (Fig. 6c, d). The deflection, average distance between the lowest point and the highest point of the microplatelet, is varied from zero to 192 nm. Furthermore, many nanoscale platelets (diameter is 200–300 nm) can be observed on the surface of microplatelet (Fig. 6c), which indicated the growth of the outermost turtle shell consists of two levels of micro/nanoscale hierarchical layered structures. FD-based AFM was employed to determine the compressive Young’s modulus, adhension, deformation and stiffness (Fig. 7). The variation in modulus, adhension, deformation and stiffness is attributed to the interaction of the probe with the miroplatelet of shell. From the nanoscale mechanical profile is displayed in Fig. 7a, compressive Young’s modulus ranges between 1.20 and 2.13 GPa. Nanoindentation test has been effectively used to deter-

mine the Young’s modulus of keratinous materials such as horn, feather, beak, wool, snake epidermis and pangolin scale. Our result of Young’s modulus of turtle shell is consistent with the data that was obtained with a nanoindentation by XP-Nanoindenter (3.6 ± 1.5 GPa) [41]. As shown in Fig. 7d, the highest stiffness value is 14.8 N/m and the average stiffness value is 7.61 N/m. The modulus and stiffness show consistent increasing and decreasing trend in different part of the microplatelet. Reasonably, the deformation values exhibit the opposite tendency with the modulus and stiffness. The region with largest deformation is about 34 nm, while the counterpart with the lowest modulus and stiffness where supposing some soft organic component (acidic alpha-keratin) was connected with ␤-keratin and mineral (Fig. 7c). Furthermore, the adhension of different regions on the microplatelet exhibits quite low value, which the highest adhension is about 25 nN (Fig. 7b). This morphology of adhension has consistency with results of deformation mapping where parts with large deformation also have high adhesion values. This characterization of adhension also indicates the possible nanoscale texture of the protein growth, in which hard proteins (beta-keratins) periodic arrange with soft proteins (alphakeratin) and were mixed with much stiffer mineral. 4. Conclusion The super-strong mechanical performance of turtle amour is attributed to their organic and mineral composite components and orderly hierarchical structure, which was designed accurately at the nano and micro scale. Shells of turtle are orderly and compactly stacked by numerous polygonal keratin micro lamellas with the thickness of 400–700 nm and diameter of ca. 73 ␮m. These keratin lamellas are biomineralized nanocomposites that consist of a high amount of ␤-keratin, a small quantity of ␣-keratin and minerals. Minerals in turtle shell are identified as calcium phosphate and calcium sulfate polycrystal mixture, and were directly observed by TEM for the first time. Furthermore, this natural composite combined the flexibility of ␣-keratin, the toughness of ␤-keratin with the superior strength of inorganic mineral, demonstrates both

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ultra-strong strength (66.97 MPa) and high flexibility (2.44% of strain). Rehydration significantly effect on increasing the flexibility of the turtle shells, which also have a relationship with the growth orientation of the keratin cells. The load-displacement experiment by AFM not only obtained the figure for the maximum compressive Young’s Modulus (2.13 GPa), stiffness (14.8 N/m), deformation (34 nm) and adhension (25 nN), but also aided to understand the protein and mineral components’ distribution and topography of the turtle shell. Such unique multi-level structure, compositional gradation and excellent mechanical properties of turtle shell may offer potential implication and inspiration for optimizing of the synthetic protective nanocomposite materialscc. Acknowledgment This work was supported by the State Key Laboratory of Advanced Welding and Joining in HIT (AWJ-Z15-05). References [1] M.A. Meyers, J. McKittrick, P.Y. Chen, Structural biological materials: critical mechanics-materials connections, Science 339 (2013) 773–779. [2] J.J. Guan, K.L. Fujimoto, M.S. Sacks, W.R. Wagner, Preparation and characterization of highly porous, biodegradable polyurethane scaffolds for soft tissue applications, Biomaterials 26 (2005) 3961–3971. [3] A.R. Studart, Towards high-performance bioinspired composites, Adv. Mater. 24 (2012) 5024–5044. [4] E. Ruiz-Hitzky, M. Darder, P. Aranda, K. Ariga, Advances in biomimetic and nanostructured biohybrid materials, Adv. Mater. 22 (2010) 323–336. [5] R.A. Metzler, J.S. Evans, C.E. Killian, D. Zhou, T.H. Churchill, N.P. Appathurai, S.N. Coppersmith, P.U.P.A. Gilbert, Nacre protein fragment templates lamellar aragonite growth, J. Am. Chem. Soc. 132 (2010) 6329–6334. [6] H.D. Espinosa, A.L. Juster, F.J. Latourte, O.Y. Loh, D. Gregoire, P.D. Zavattieri, Tablet-level origin of toughening in abalone shells and translation to synthetic composite materials, Nat. Commun. 2 (2011) 173. [7] X. Li, W.C. Chang, Y.J. Chao, R.Z. Wang, M. Chang, Nanoscale structural and mechanical characterization of a natural nanocomposite material: the shell of red abalone, Nano Lett. 4 (2004) 613–617. [8] Z. Burghard, L. Zini, V. Srot, P. Bellina, P.A.V. Aken, J. Bill, Toughening through nature-adapted nanoscale design, Nano Lett. 9 (2009) 4103–4108. [9] H. Yao, M. Dao, T. Imholt, J.M. Huang, K. Wheeler, A. Bonilla, S. Suresh, C. Ortiz, Protection mechanisms of the iron-plated armor of a deep-sea hydrothermal vent gastropod, Proc. Natl. Acad. Sci. 107 (2010) 987–992. [10] B.J.F. Bruet, J. Song, M.C. Boyce, C. Ortiz, Materials design principles of ancient fish armour, Nat. Mater. 7 (2008) 748–756. [11] K.D. Jandt, Biological materials: fishing for compliance, Nat. Mater. 7 (2008) 692–693. [12] E.A. Zimmermann, B. Gludovatz, E. Schaible, N.K.N. Dave, W. Yang, M.A. Meyers, R.O. Ritchie, Mechanical adaptability of the bouligand-type structure in natural dermal armour, Nat. Commun. 4 (2013) 2634. [13] H.S. Gupta, W. Wagermaier, G.A. Zickler, D. Raz-Ben Aroush, S.S. Funari, P. Roschger, H.D. Wagner, P. Fratzl, Nanoscale deformation mechanisms in bone, Nano Lett. 5 (2005) 2108–2111. [14] Z.Q. Liu, D. Jiao, M.A. Meyers, Z.F. Zhang, Structure and mechanical properties of naturally occurring lightweight foam-filled cylinder–the peacock’s tail coverts shaft and its components, Acta Biomater. 17 (2015) 137–151. [15] W. Yang, I.H. Chen, B. Gludovatz, E.A. Zimmermann, R.O. Ritchie, M.A. Meyers, Natural flexible dermal armor, Adv. Mater. 25 (2013) 31–48. [16] K.C. Rogers, M. D’Emic, R. Rogers, M. Vickaryous, A. Cagan, Sauropod dinosaur osteoderms from the late cretaceous of madagascar, Nat. Commun. 29 (2011) 564. [17] J.H. Lee, D. Veysset, J.P. Singer, M. Retsch, G. Saini, T. Pezeril, K.A. Nelson, High strain rate deformation of layered nanocomposites, Nat. Commun. 3 (2012) 1164.

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