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Biomimetic preparation of a ceramic combined with sea urchin stereom structure and nacre mineral bridge structure
Materials and Design 178 (2019) 107844
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Biomimetic preparation of a ceramic combined with sea urchin stereom structure and nacre mineral bridge structure Hui Yu a,b, Jianlin Li b, Jianbao Li b,⁎, Yongjun Chen b, Xue Hou b, Shuaifeng Chen b, Haotian Yang b a b
School of Materials Science and Engineering, Jingdezhen Ceramic Institute, Jingdezhen 333403, China State Key Laboratory of Marine Resource Utilization in South China Sea, School of Materials Science and Engineering, Hainan University, Haikou 570228, 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
• A new ceramic combined with stereom structure of sea urchin and mineral bridges of nacre is successfully fabricated. • The double-biologically inspired ceramics have a high porosity of 88.16% but excellent Weibull modulus of 7.85. • The Al2TiO5 mineral bridge forest first reported on the boundary/surface of Al2O3 grains causes the crack deflection. • The mineral bridges can reduce the maximum grain boundary stress by 15.85% and homogenize the stress. • The flexural strength and Weibull modulus of ceramic are increased by about 2.7 times with the mineral bridge introduction.
a r t i c l e
i n f o
Article history: Received 20 January 2019 Received in revised form 8 May 2019 Accepted 9 May 2019 Available online 11 May 2019 Keywords: Biomimetic ceramic Finite element method Stereom Mineral bridge Weibull modulus Compressive property
a b s t r a c t Multibiological multiscale biomimetic design is a novel bionic idea that involves multi-mechanisms. Porous ceramics with a sea urchin stereom structure on a micrometre scale and a nacre mineral bridge structure on a submicron scale have been fabricated via organic foam impregnation and controlled crystallisation. The microstructure and mechanical properties of biomimetic ceramics are, respectively, characterised using scanning electron microscopy and universal testing machine. Moreover, the porosity, reliability and grain boundary stress of biomimetic ceramics are calculated via Archimedes method, Weibull theory and finite element method, respectively. Results show that the ceramic produced in this work has a porosity of 88.16% and an average pore size of 284.65 μm. Mineral bridges of Al2TiO5 with a thickness of 30–230 nm are widely and randomly distributed in the grain boundary glass phase of Al2O3 and improved the compressive strength (2.32 MPa) and Weibull modulus (7.85) of ceramics by the multi-mechanisms of crack deflection, reducing maximum stress and homogenising stress on the grain boundary. These investigations would be of great value to the design and synthesis of novel biomimetic materials. © 2019 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
1. Introduction
⁎ Corresponding author. E-mail address: [email protected] (J. Li).
Throughout billions of year's evolution, biomineralised materials in organisms have evolved elaborate structures and excellent mechanical properties, providing an infinite source of inspiration for the structural bionics of ceramic and organic–inorganic composite materials [1].
https://doi.org/10.1016/j.matdes.2019.107844 0264-1275/© 2019 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
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Table 1 The formulas of MBC and NMBC ceramics. Materials
α-Al2O3 SiO2 MgO TiO2
Formulas MBC (wt%)
NMBC (wt%)
81 10 5 4
84.37 10.42 5.21 0
Sea urchin has light weight skeletons due to its elaborate, porous micro-architecture of calcite stereom [2] frequently used as biotemplates or bionic inspiration sources. Porous gold prepared using the stereom as a template is particularly valuable in terms of photonic band gap behaviour and in biomolecules and chemical separations [3]. Low absorption, high dielectric contrast photonic crystals with a stop band in the mid-infrared regime were fabricated through cyclic reduction/infiltration of the stereom [4]. Inspired by the stereom of the sea urchin spines, a permeable, light-weight Al2O3 ceramic with excellent failure behaviour was fabricated using starch as a pore former [5]. Preparations of biomimetic materials inspired by the stereom are a new process for preparing porous ceramic, metal and polymer prosthetic materials [6]. Mineral bridges in nacre consist of aragonite whiskers present in the organic layer perpendicular to the two layers of plates. During the fracture of nacre layers, deflection of the crack propagation increases due to the presence of a mineral bridge and its randomness [7]. Aragonite plates maintain a tight connection after the crack passes through the organic matrix because of the mechanical chelation of organic matrix and mineral bridges [8]. Pull-out of the plates requires not only overcoming the binding force and friction between the organic phase and aragonite but also cutting off all the mineral bridges on the plates, enhancing the material toughness [9]. Mineral bridges have been biomimetic prepared into hydroxyapatite [10], Al2O3 [11] and plastic materials [12], which are, however, generally considered a secondary toughening structure in brick-and-mortar structural materials. Traditional structural bionics is to fabricate a biological structure on one scale of the material [13] even to apply the multilevel structure of a biological structure to the multiscale structure design of the material [14]. For the first time, this study develops a multibiological and multiscale biomimetic idea. On the millimetre to micrometre scale, the organic foam is used as a template [15] to prepare the stereom structure. On the submicron scale, the crystallisation at the alumina grain boundary is controlled by titanium oxide to form mineral bridges [16]. To highlight the superiority of multibiological multiscale structure bionics, the microstructure, compressive strength, porosity and Weibull modulus of double biostructure biomimetic ceramics are compared with the biomimetic ceramics with single stereom structure. Moreover, finite element method [17] is applied to analyse the effect of mineral bridges on the normal stress (Y-axis) distribution and concentrated tensile stress (CTS) of grain boundary glass. The effect of CTS value on the mechanical properties of ceramics is discussed.
2. Experimental details 2.1. Raw materials and sample preparation 2.1.1. Natural biological materials Anthocidaris crassispina and pearl shell were collected as the main structural biomimetic objects. Sea urchin tests were extensively bleached with sodium hypochlorite (0.26% active chlorine) for 72 h [18] after removing the epidermal cells, spines and internal organs. Subsequently, the bleached samples were placed in a blast oven, air dried at 25 °C for 48 h, then stored in a glass desiccator with desiccant and set aside for future use.
Grain size
Purity
Manufacturer
2–5 μm 500 nm – 60 nm
AR 99% 98% 99.8%
Xilong Chemical Co. Shanghai Xiangtian nano materials Co. Xilong Chemical Co. Aladdin reagent Co.
2.1.2. Biomimetic ceramics The formulas of mineral bridge ceramics (MBC) and no mineral bridge ceramics (NMBC) were weighed according to Table 1. The mixed powders were first ball-milled for 8 h at a milling speed of 300 r/min at a mass ratio of ball: powder: ethanol = 3:1:0.8. Then, the milled mixtures were dried in a vacuum dry evaporator at 60 °C, followed by grinding in an agate mortar with addition of 42.8 wt% polyvinyl alcohol (PVA) solution (concentration = 2.44 wt%). After a few hours of grinding, the impregnated ceramic slurry with 30 wt% moisture was obtained. The 50 ppi polyurethane foam was cut into cubes (side length = 10 mm) and immersed in a sodium hydroxide solution (concentration = 15%) at a temperature of 55 °C for 4 h, subsequently washed, and then immersed in a 0.4% carboxymethyl cellulose (CMC) solution for 24 h. After the polyurethane foam was washed and dried at 50 °C, the ceramic slurry was impregnated into the foam. To ensure adequate slurry penetration, the foams were repeatedly knead. Then, the excess slurry was extruded through the surface-polished wood chips. After a certain number of extrusions, the slurry mass was 15.32 ± 0.35 times higher than that of the foam. After natural air-drying, the sample was pressureless-sintered in air atmosphere in a muffle furnace (GWL-1700B, Juxing, China) at a heating rate of 2 °C/min to 600 °C and a holding time of 0.5 h, then up to 1450 °C at a rate of 5 °C/min and held for 3 h at this temperature.
2.2. Characterisations The morphology of sea urchin skeleton, nacre and biomimetic ceramics were observed using field emission scanning electron microscopy (FESEM, S-4800). The pearl shell and sea urchin tests were cut by a diamond wire cutting machine (STX-202A). The samples were ultrasonically cleaned and then air dried at 25 °C. Moreover, to investigate the crack propagation and mineral bridges in biomimetic ceramics, two ceramics were dipped in 1% hydrofluoric acid at 10 s for slight etching and 60 s for severe etching, correspondingly. All samples were placed in a gold sprayer (ETD-900) thrice for 20 s each time, and the spraying angle of 120° was rotated each time. Elemental analysis was performed using an energy-dispersive spectroscopy (EDS) assembled on scanning electron microscopy (SEM). The sizes of pore and mineral bridge were measured using image analysis software that came with the SEM. A universal testing machine (AGS-10KNG) was used to characterise the compressive properties of the samples at a cross-head speed of 0.5 mm/min. To prevent stress concentration on the surface of the samples, the top and bottom of the samples were padded with a graphite paper. The bulk density and porosity of ceramics was calculated according to Archimedes' principle. The Weibull theory was introduced to describe the statistical distribution of observed strength values of biomimetic ceramics in a given population. In the plot of failure strength ln(σc) against a logarithmic expression of the failure probability Fi(ln(ln(1/(1 − Fi)))), the socalled Weibull modulus was obtained as the slope of the linear regression function and was a measure of the material reliability [19]. Finite element software of ANSYS was used for simulative calculation of the normal stress (Y-axis) distribution and CTS of grain boundary of pure glass phase and glass phase with mineral bridge. In these finite
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element models, the glass elastic modulus and Poisson's ratios were set as 72 GPa and 0.2 [20], and the elastic modulus and Poisson's ratios of Al2O3 and mineral bridges were set as 404 ± 28 GPa [21] and 0.26 ± 0.02 [22], respectively. During the progress of the static analysis, the average element quality of mesh was 0.84367. The applied load of 100 N acted on the grain was in a direction parallel to the long axis of the mineral bridge. 3. Results and discussion As shown in Figs. 1a and b, the stereom of the sea urchin exhibits an intercommunicating porous structure with a pore size of 19.37 ± 5.48 μm. The stereom consists of a large number of neck-shaped trabeculae, and the diameter and length-diameter ratio of trabecula are 13.69 ± 3.07 μm and 1.86 ± 0.78, respectively. Figs. 1c and d show that the shell nacre possesses a brick and mortar structure (the brick thickness is 322.31 ± 29.76 nm). A randomly distributed mineral bridge can be seen in interlamination. The diameter of mineral bridges is 36–49 nm. The SEM images of double-scale, double-structure biomimetic ceramics are shown in Figs. 1e–h. The biomimetic ceramic also has the
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intercommunicating porous structure, but the diameter of the pores is 284.65 ± 101.66 μm. The diameter of the trabecula that makes up the ceramic is 30 μm. The average size of the alumina grains constituting the trabecula is 5.93 ± 2.56 μm. After the slight etching, mineral bridges with a diameter of 30–100 nm randomly distributing in the glass phase at the alumina grain boundary are observed. The prepared ceramic with sea urchin stereom structure in micron scale and nacre mineral bridge structure in submicron scale vividly mimics the biomimetic structure. As shown in Fig. 2a, the mineral bridge is widely and randomly distributed in the grain boundary glass phase. The well-developed mineral bridge has a quadrilateral face and a rectangular side, whereas the newborn mineral bridge buds have a long cylinder. The diameter (or width) of mineral bridges ranges from 30 nm to 230 nm, and the length ranges from 40 nm of mineral bridge buds to 1320 nm of mineral bridges connecting the two grains. The randomly distributed mineral bridges lead to a complex stress distribution of the grain boundary glass phase, beneficial to make the crack propagation path tortuous [9]. A large number of nanocrystalline particles with a typical size of approximately 20 nm are embedded in the glass phase by magnifying the
Fig. 1. Porous ceramics derived from the combination of the stereom and bridge structures. (a) and (b) are SEM images of stereom and trabecula of sea urchin, respectively. (c) and (d) are SEM images of nacre and mineral bridge, respectively. (e–g) are gradually enlarged SEM images of biomimetic ceramics. (h) SEM image of slightly etched grain boundaries. The red arrow points to the mineral bridge.
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Fig. 2. (a) and (b) are the FESEM images of slightly etched MBC. (c) and (d) are the FESEM images of severely etched MBC. The red arrows in (a) point to the mineral bridge. The diagram in (b) shows the growth mechanism of the mineral bridge. The illustration in (c) is the EDS for the yellow cross.
mineral bridge in a growing stage (Fig. 2b). The diagram inset of Fig. 2b visually depicts the growth mechanism of the mineral bridge. The mineral bridge nucleates at the interface between the crystal and the glass phase and then grows toward the glass phase direction. The growth of the mineral bridge is a nano self-assembly. This mechanism is common in nanomaterials [23–26] because of the small nanoparticles and the easy adjustment of the configuration and orientation. Meanwhile, the nanocrystals have several dangling bonds, and the dangling bonds decrease dramatically by fusing the particles close together [27].
The EDS point scanning results (Fig. 2c) show that the mineral bridge is composed of Ti, Al and O (Pt is introduced in the goldspraying), consistent with the reported X-ray diffraction results of this system that the phase composition of mineral bridge is aluminium titanate (Al2TiO5, PDF#41–0256) [16]. Al2TiO5 belongs to the orthorhombic system with lattice parameters of a = 9.439 Å, b = 9.647 Å, c = 3.5929 Å, and α = β = γ = 90.0°. Therefore, the well-developed Al2TiO5 tends to grow into a tetragonal body, substantially identical to the morphology of the mineral bridge observed in Fig. 2a.
Fig. 3. (a) Compressive strength, (b) bulk density and porosity of MBC and NMBC. The inset in (a) is a SEM image of severely etched NMBC.
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Fig. 4. Curves of (a) load–displacement and (b) absorbed energy displacement of MBC and NMBC. The inset in (a) is a SEM image of crack propagation in MBC. The red arrows point to the mineral bridges, and the yellow arrow indicates the direction of crack deflection.
Fig. 2d shows the forest of the mineral bridge buds on the grain surface. The mineral bridge buds are oriented and randomly distributed with an average length of 70 nm and mean diameter of 40 nm. This condition increases the roughness and specific surface area of the grain surface. The compressive properties of the double-structured biomimetic ceramics were analysed using a ceramic (NMBC) with the only stereom structure as comparison. The inset in Fig. 3a shows that only the pure glass phase is present on the alumina grain boundary of the slightly etched NMBC sample, and no mineral bridge is found. Figs. 3a and b show that although the value difference of porosity (88.16 ± 0.44%, 88.13 ± 0.47%) and the bulk density (0.406 ± 0.014 and 0.404 ± 0.014 g/cm3) between the MBC and NMBC samples are small, the compressive strength of MBC (2.32 ± 0.34 MPa) is 2.73 times higher than that of NMBC (0.85 ± 0.35 MPa). The biomimetic introduction of mineral bridges in the submicron scale effectively enhances the compressive properties of stereom biomimetic ceramics. As seen from the load–displacement curves in Fig. 4a, the maximum load on the MBC (206.69 N) is much greater than that of NMBC (79.13 N), indicating the strengthening effect of mineral bridges. The average load of the MBC sample during the densification strain phase is greater than that of the NMBC sample. Moreover, several convex peaks of load appear on the curve of MBC in the blue dotted circle, indicating the toughening effect of mineral bridges. As shown in the inset of Fig. 4a, the crack deflects when encountering the mineral bridges. This phenomenon is attributed to the fact that the mineral bridge can reduce the stress at the crack tip and causes the crack to deflect to the glass phase when it encounters the mineral bridge [16] or reaches the area rich in mineral bridges [9]. In addition, given that the mineral bridge is a one-dimensional nanomaterial, its elastic modulus may approach its theoretical value [28]. Therefore, the energy required for a crack to break through a forest of mineral bridges is higher than through a glass. The existence of the random distribution of mineral bridges, which strengthen and toughen the microregions of ceramics, fluctuates the loads. As shown in Fig. 4b, the MBC and NMBC samples reach their maximum loads at displacements of 0.62 and 0.87 mm, respectively. In addition, the corresponding accumulated energy consumed by the MBC (22.92 × 10−3 J) is slightly higher than that of NMBC (20.22 × 10−3 J). Before displacement of 0.63 mm, the cumulative energy absorbed by the NMBC is higher than that of MBC. However, as the displacement increases, the energy absorbed by the MBC is gradually higher than that of NMBC, and the energy consumed by the MBC sample at 2 mm (126.14 × 10−3 J) is 2.54 times higher than that of NMBC (49.60 × 10−3 J). The results show that the NMBC sample is accompanied by trabecula
rupture and absorbs a large amount of energy in the elastic deformation stage, whereas the MBC sample has good stiffness in the elastic deformation stage, and the trabecula is mainly fractured in the dense strain stage. The energy absorption of MBC at this stage is much higher than that of NMBC. The above results indicate the toughening and strengthening effects of mineral bridges on MBC. According to the inset in Fig. 4a, the crack propagation is carried out in the grain boundary glass phase, indicating that the intergranular fracture is the main type of crack propagation. Therefore, the influence of the presence or absence of the mineral bridge on the stress distribution and maximum stress value of the grain boundary glass is worthy of discussion. The model of grain boundary unit with the mineral bridge is shown in Fig. 5a. One Al2O3 grain is fixed, and the other is subjected to a tensile force of 100 N. The maximum tensile stress produced during loading is used to evaluate the mechanical properties of brittle materials of the glass because they exhibit tensile capacity lower than their compression capacity [21,29]. Figs. 6a and b show the normal stress (Y-axis) distributions of grain boundary of pure glass phase and glass phase with mineral bridges, respectively. In the glass phase without mineral bridge, the high stress areas shown in red are distributed on the glass/Al2O3 interface. Obvious blue low stress areas are observed at both ends of the glass. The difference between the maximum stress and the minimum stress is 20.95 MPa. After the introduction of the mineral bridge, the high stress areas shown in red are not only distributed on the glass/Al2O3 interface but also on the glass/mineral bridge interface. In addition, the large blue
Fig. 5. (a) Three-dimensional model of grain boundary with glass and mineral bridges.
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Fig. 6. (a) and (b) are normal stress (Y-axis) distributions of grain boundary glass with and without of mineral bridges, respectively. The red triangle points to the high stress areas and the blue triangle to the low stress areas.
area becomes sharply small, and the difference between the maximum stress and the minimum stress is up to 21.13 MPa. This result indicates the uniform stress distribution in MBC. The CTS in the pure glass phase is 54.78 MPa, whereas in the glass phase containing the mineral bridges, the CTS is reduced, and the stress is 46.10 MPa. This value is 15.85% lower than that of the pure glass phase. If inherent cracks are not considered, the strengths of glass and glass/Al2O3 interface are set as constants. Once the CTS is greater than the strengths of glass or glass/Al2O3 interface, large CTS means easy generation of cracks. When inherent cracks are considered, the strength of grain boundary, the size, number and shape of crack are fixed. The following Orowan's formulas are applicable [30]: pffiffiffiffiffiffiffiffi σ A ¼ 2σ c=a; (1)where σA is the stress on the crack tip; σ is the external stress; c is the major axis length of elliptical crack, and a is the atomic pitch. Notably, σA increases with the rise of σ, providing easy propagation of the crack. In general, the decreased stress will be transferred from Al 2O 3 grain to glass and glass/Al 2O 3 interface as the mineral bridges grow, and small CTS on the grain boundary signifies difficulty in generating and propagating crack. This condition causes the external load for the MBC fracture to be greater than that of NMBC. Thus, MBC with mineral bridges have more excellent compressive properties than NMBC.
Weibull plot curves are shown in Fig. 7a. The Weibull modulus of the MBC (7.85) is 2.70 times higher than that of the NMBC (2.91), indicating that MBC with mineral bridges has high reliability. This result is attributed to the strengthening of the grain boundary by the mineral bridge, increasing the threshold of the force causing crack expansion. Comparing with the reported porosity and compressive Weibull modulus (CWM) of man-made porous materials (Fig. 7b), although NMBC has only stereom bionics, its CWM (2.91) is still higher than that of SC (2.0) and calcium phosphate cements (1.6 and 2.7) with low porosity. Notably, except for the D16.2 sample with a low porosity of 40.8% and an excellent CWM of 8.67, the CWM of other reported materials is lower than 6, and the porosity is less than 60%. Meanwhile, MBC has the largest porosity of 88.16% but with a good Weibull modulus of 7.85. This finding indicates that MBC is a high porosity material but with significant reliability. In materials with porosity of up to 88%, the effect of mineral bridge on strengthening and improving reliability of ceramics are still evident. Therefore, to further improve the strength and reliability of the ceramic is very feasible to bionic the mineral bridge in a low-porosity ceramic material. Moreover, the structure of mineral bridge forest (MBF) on the grain surface may be important in many fields. In the composite, the MBF on composite substrate can increase the contact area and form mechanical chelating, beneficial to strengthen the composite. If used for the catalytic carrier, MBF can provide a large surface area for adsorption of catalytic
Fig. 7. (a) Weibull plot of MBC and NMBC. (a) Comparison of porosity and compressive Weibull modulus of several man-made porous materials. ■-Hydroxyapatite scaffolds fabricated with 16.2 or 5.96 μm PMMA as a pore former [31]; ●-Porous ceramic Raschig rings fabricated by three different green processes of uniaxial pressing (UP), extrusion (E) and slip casting (SC), correspondingly [32]; ▼-Glass-ceramic foam scaffolds [33]; ►-Calcium phosphate cement with different porosities [34]. The red arrow points to the mineral bridge, and yellow arrow denotes the crack deflection.
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substances, improving the catalytic efficiency. For the sound absorbing material, MBF tends to improve the friction and the viscous effects of sound waves, thereby improving sound absorption. These outlooks shows a good application prospect for the ceramic combined with sea urchin stereom structure and nacre mineral bridge structure. 4. Conclusions A new ceramic (MBC) simultaneously possessing the sea urchin stereom structure and nacre mineral bridge structure has been successfully biomimetic-fabricated with porosity, pore size and mineral bridge diameter of 88.16%, 284.65 μm and 30–230 nm, respectively. The microstructure, compressive strength, porosity and Weibull modulus of MBC are compared with the biomimetic ceramics with single stereom structure (NMBC). The compressive strength (2.32 MPa) and Weibull modulus (7.85) of MBC with many mineral bridges on the grain boundary are 2.73 and 2.70 times higher than those of the NMBC, respectively, although their porosity and bulk density are almost the same. The Al2TiO5 mineral bridge has the crack deflection mechanism characterised by SEM, and has the mechanisms of homogenising stress and reducing the maximum stress on the grain boundary calculated by the finite element method, thereby improving the mechanical properties and reliability of the porous ceramic. These biomimetic ceramics may have a good application prospect in composite ceramic substrate, catalyst carrier and sound absorbing material. Moreover, multibiological-inspired multiscale bionics provides novel ideas for the design and synthesis of structural biomimetic materials. CRediT authorship contribution statement Hui Yu: Formal analysis, Investigation, Writing - original draft, Software, Methodology. Jianlin Li: Methodology, Conceptualization, Writing - review & editing. Jianbao Li: Conceptualization, Resources, Supervision, Funding acquisition. Xue Hou: Validation, Investigation. Shuaifeng Chen: Validation, Data curation. Haotian Yang: Data curation, Formal analysis. CRediT authorship contribution statement Hui Yu: Formal analysis, Investigation, Writing - original draft, Software, Methodology. Jianlin Li: Methodology, Conceptualization, Writing - review & editing. Jianbao Li: Conceptualization, Resources, Supervision, Funding acquisition. Yongjun Chen: Funding acquisition, Resources. Xue Hou: Validation, Investigation. Shuaifeng Chen: Validation, Data curation. Haotian Yang: Data curation, Formal analysis. Declaration of Competing Interest The authors declare that they have no conflict of interest. Acknowledgements The work is supported by the National Natural Science Foundation of China (NO. 51662006). Ethical approval All procedures performed in studies involving animals were in accordance with the ethical standards of the institution or practice at which the studies were conducted. References [1] P.Y. Chen, Y. Po, J. Mckittrick, M.A. Meyers, Biological materials: functional adaptations and bioinspired designs, Prog. Mater. Sci. 57 (2012) 1492–1704, https://doi. org/10.1016/j.pmatsci.2012.03.001.
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Biomimetic preparation of a ceramic combined with sea urchin stereom structure and nacre mineral bridge structure Hui Yu a,b, Jianlin Li b, Jianbao Li b,⁎, Yongjun Chen b, Xue Hou b, Shuaifeng Chen b, Haotian Yang b a b
School of Materials Science and Engineering, Jingdezhen Ceramic Institute, Jingdezhen 333403, China State Key Laboratory of Marine Resource Utilization in South China Sea, School of Materials Science and Engineering, Hainan University, Haikou 570228, 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
• A new ceramic combined with stereom structure of sea urchin and mineral bridges of nacre is successfully fabricated. • The double-biologically inspired ceramics have a high porosity of 88.16% but excellent Weibull modulus of 7.85. • The Al2TiO5 mineral bridge forest first reported on the boundary/surface of Al2O3 grains causes the crack deflection. • The mineral bridges can reduce the maximum grain boundary stress by 15.85% and homogenize the stress. • The flexural strength and Weibull modulus of ceramic are increased by about 2.7 times with the mineral bridge introduction.
a r t i c l e
i n f o
Article history: Received 20 January 2019 Received in revised form 8 May 2019 Accepted 9 May 2019 Available online 11 May 2019 Keywords: Biomimetic ceramic Finite element method Stereom Mineral bridge Weibull modulus Compressive property
a b s t r a c t Multibiological multiscale biomimetic design is a novel bionic idea that involves multi-mechanisms. Porous ceramics with a sea urchin stereom structure on a micrometre scale and a nacre mineral bridge structure on a submicron scale have been fabricated via organic foam impregnation and controlled crystallisation. The microstructure and mechanical properties of biomimetic ceramics are, respectively, characterised using scanning electron microscopy and universal testing machine. Moreover, the porosity, reliability and grain boundary stress of biomimetic ceramics are calculated via Archimedes method, Weibull theory and finite element method, respectively. Results show that the ceramic produced in this work has a porosity of 88.16% and an average pore size of 284.65 μm. Mineral bridges of Al2TiO5 with a thickness of 30–230 nm are widely and randomly distributed in the grain boundary glass phase of Al2O3 and improved the compressive strength (2.32 MPa) and Weibull modulus (7.85) of ceramics by the multi-mechanisms of crack deflection, reducing maximum stress and homogenising stress on the grain boundary. These investigations would be of great value to the design and synthesis of novel biomimetic materials. © 2019 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
1. Introduction
⁎ Corresponding author. E-mail address: [email protected] (J. Li).
Throughout billions of year's evolution, biomineralised materials in organisms have evolved elaborate structures and excellent mechanical properties, providing an infinite source of inspiration for the structural bionics of ceramic and organic–inorganic composite materials [1].
https://doi.org/10.1016/j.matdes.2019.107844 0264-1275/© 2019 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
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Table 1 The formulas of MBC and NMBC ceramics. Materials
α-Al2O3 SiO2 MgO TiO2
Formulas MBC (wt%)
NMBC (wt%)
81 10 5 4
84.37 10.42 5.21 0
Sea urchin has light weight skeletons due to its elaborate, porous micro-architecture of calcite stereom [2] frequently used as biotemplates or bionic inspiration sources. Porous gold prepared using the stereom as a template is particularly valuable in terms of photonic band gap behaviour and in biomolecules and chemical separations [3]. Low absorption, high dielectric contrast photonic crystals with a stop band in the mid-infrared regime were fabricated through cyclic reduction/infiltration of the stereom [4]. Inspired by the stereom of the sea urchin spines, a permeable, light-weight Al2O3 ceramic with excellent failure behaviour was fabricated using starch as a pore former [5]. Preparations of biomimetic materials inspired by the stereom are a new process for preparing porous ceramic, metal and polymer prosthetic materials [6]. Mineral bridges in nacre consist of aragonite whiskers present in the organic layer perpendicular to the two layers of plates. During the fracture of nacre layers, deflection of the crack propagation increases due to the presence of a mineral bridge and its randomness [7]. Aragonite plates maintain a tight connection after the crack passes through the organic matrix because of the mechanical chelation of organic matrix and mineral bridges [8]. Pull-out of the plates requires not only overcoming the binding force and friction between the organic phase and aragonite but also cutting off all the mineral bridges on the plates, enhancing the material toughness [9]. Mineral bridges have been biomimetic prepared into hydroxyapatite [10], Al2O3 [11] and plastic materials [12], which are, however, generally considered a secondary toughening structure in brick-and-mortar structural materials. Traditional structural bionics is to fabricate a biological structure on one scale of the material [13] even to apply the multilevel structure of a biological structure to the multiscale structure design of the material [14]. For the first time, this study develops a multibiological and multiscale biomimetic idea. On the millimetre to micrometre scale, the organic foam is used as a template [15] to prepare the stereom structure. On the submicron scale, the crystallisation at the alumina grain boundary is controlled by titanium oxide to form mineral bridges [16]. To highlight the superiority of multibiological multiscale structure bionics, the microstructure, compressive strength, porosity and Weibull modulus of double biostructure biomimetic ceramics are compared with the biomimetic ceramics with single stereom structure. Moreover, finite element method [17] is applied to analyse the effect of mineral bridges on the normal stress (Y-axis) distribution and concentrated tensile stress (CTS) of grain boundary glass. The effect of CTS value on the mechanical properties of ceramics is discussed.
2. Experimental details 2.1. Raw materials and sample preparation 2.1.1. Natural biological materials Anthocidaris crassispina and pearl shell were collected as the main structural biomimetic objects. Sea urchin tests were extensively bleached with sodium hypochlorite (0.26% active chlorine) for 72 h [18] after removing the epidermal cells, spines and internal organs. Subsequently, the bleached samples were placed in a blast oven, air dried at 25 °C for 48 h, then stored in a glass desiccator with desiccant and set aside for future use.
Grain size
Purity
Manufacturer
2–5 μm 500 nm – 60 nm
AR 99% 98% 99.8%
Xilong Chemical Co. Shanghai Xiangtian nano materials Co. Xilong Chemical Co. Aladdin reagent Co.
2.1.2. Biomimetic ceramics The formulas of mineral bridge ceramics (MBC) and no mineral bridge ceramics (NMBC) were weighed according to Table 1. The mixed powders were first ball-milled for 8 h at a milling speed of 300 r/min at a mass ratio of ball: powder: ethanol = 3:1:0.8. Then, the milled mixtures were dried in a vacuum dry evaporator at 60 °C, followed by grinding in an agate mortar with addition of 42.8 wt% polyvinyl alcohol (PVA) solution (concentration = 2.44 wt%). After a few hours of grinding, the impregnated ceramic slurry with 30 wt% moisture was obtained. The 50 ppi polyurethane foam was cut into cubes (side length = 10 mm) and immersed in a sodium hydroxide solution (concentration = 15%) at a temperature of 55 °C for 4 h, subsequently washed, and then immersed in a 0.4% carboxymethyl cellulose (CMC) solution for 24 h. After the polyurethane foam was washed and dried at 50 °C, the ceramic slurry was impregnated into the foam. To ensure adequate slurry penetration, the foams were repeatedly knead. Then, the excess slurry was extruded through the surface-polished wood chips. After a certain number of extrusions, the slurry mass was 15.32 ± 0.35 times higher than that of the foam. After natural air-drying, the sample was pressureless-sintered in air atmosphere in a muffle furnace (GWL-1700B, Juxing, China) at a heating rate of 2 °C/min to 600 °C and a holding time of 0.5 h, then up to 1450 °C at a rate of 5 °C/min and held for 3 h at this temperature.
2.2. Characterisations The morphology of sea urchin skeleton, nacre and biomimetic ceramics were observed using field emission scanning electron microscopy (FESEM, S-4800). The pearl shell and sea urchin tests were cut by a diamond wire cutting machine (STX-202A). The samples were ultrasonically cleaned and then air dried at 25 °C. Moreover, to investigate the crack propagation and mineral bridges in biomimetic ceramics, two ceramics were dipped in 1% hydrofluoric acid at 10 s for slight etching and 60 s for severe etching, correspondingly. All samples were placed in a gold sprayer (ETD-900) thrice for 20 s each time, and the spraying angle of 120° was rotated each time. Elemental analysis was performed using an energy-dispersive spectroscopy (EDS) assembled on scanning electron microscopy (SEM). The sizes of pore and mineral bridge were measured using image analysis software that came with the SEM. A universal testing machine (AGS-10KNG) was used to characterise the compressive properties of the samples at a cross-head speed of 0.5 mm/min. To prevent stress concentration on the surface of the samples, the top and bottom of the samples were padded with a graphite paper. The bulk density and porosity of ceramics was calculated according to Archimedes' principle. The Weibull theory was introduced to describe the statistical distribution of observed strength values of biomimetic ceramics in a given population. In the plot of failure strength ln(σc) against a logarithmic expression of the failure probability Fi(ln(ln(1/(1 − Fi)))), the socalled Weibull modulus was obtained as the slope of the linear regression function and was a measure of the material reliability [19]. Finite element software of ANSYS was used for simulative calculation of the normal stress (Y-axis) distribution and CTS of grain boundary of pure glass phase and glass phase with mineral bridge. In these finite
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element models, the glass elastic modulus and Poisson's ratios were set as 72 GPa and 0.2 [20], and the elastic modulus and Poisson's ratios of Al2O3 and mineral bridges were set as 404 ± 28 GPa [21] and 0.26 ± 0.02 [22], respectively. During the progress of the static analysis, the average element quality of mesh was 0.84367. The applied load of 100 N acted on the grain was in a direction parallel to the long axis of the mineral bridge. 3. Results and discussion As shown in Figs. 1a and b, the stereom of the sea urchin exhibits an intercommunicating porous structure with a pore size of 19.37 ± 5.48 μm. The stereom consists of a large number of neck-shaped trabeculae, and the diameter and length-diameter ratio of trabecula are 13.69 ± 3.07 μm and 1.86 ± 0.78, respectively. Figs. 1c and d show that the shell nacre possesses a brick and mortar structure (the brick thickness is 322.31 ± 29.76 nm). A randomly distributed mineral bridge can be seen in interlamination. The diameter of mineral bridges is 36–49 nm. The SEM images of double-scale, double-structure biomimetic ceramics are shown in Figs. 1e–h. The biomimetic ceramic also has the
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intercommunicating porous structure, but the diameter of the pores is 284.65 ± 101.66 μm. The diameter of the trabecula that makes up the ceramic is 30 μm. The average size of the alumina grains constituting the trabecula is 5.93 ± 2.56 μm. After the slight etching, mineral bridges with a diameter of 30–100 nm randomly distributing in the glass phase at the alumina grain boundary are observed. The prepared ceramic with sea urchin stereom structure in micron scale and nacre mineral bridge structure in submicron scale vividly mimics the biomimetic structure. As shown in Fig. 2a, the mineral bridge is widely and randomly distributed in the grain boundary glass phase. The well-developed mineral bridge has a quadrilateral face and a rectangular side, whereas the newborn mineral bridge buds have a long cylinder. The diameter (or width) of mineral bridges ranges from 30 nm to 230 nm, and the length ranges from 40 nm of mineral bridge buds to 1320 nm of mineral bridges connecting the two grains. The randomly distributed mineral bridges lead to a complex stress distribution of the grain boundary glass phase, beneficial to make the crack propagation path tortuous [9]. A large number of nanocrystalline particles with a typical size of approximately 20 nm are embedded in the glass phase by magnifying the
Fig. 1. Porous ceramics derived from the combination of the stereom and bridge structures. (a) and (b) are SEM images of stereom and trabecula of sea urchin, respectively. (c) and (d) are SEM images of nacre and mineral bridge, respectively. (e–g) are gradually enlarged SEM images of biomimetic ceramics. (h) SEM image of slightly etched grain boundaries. The red arrow points to the mineral bridge.
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Fig. 2. (a) and (b) are the FESEM images of slightly etched MBC. (c) and (d) are the FESEM images of severely etched MBC. The red arrows in (a) point to the mineral bridge. The diagram in (b) shows the growth mechanism of the mineral bridge. The illustration in (c) is the EDS for the yellow cross.
mineral bridge in a growing stage (Fig. 2b). The diagram inset of Fig. 2b visually depicts the growth mechanism of the mineral bridge. The mineral bridge nucleates at the interface between the crystal and the glass phase and then grows toward the glass phase direction. The growth of the mineral bridge is a nano self-assembly. This mechanism is common in nanomaterials [23–26] because of the small nanoparticles and the easy adjustment of the configuration and orientation. Meanwhile, the nanocrystals have several dangling bonds, and the dangling bonds decrease dramatically by fusing the particles close together [27].
The EDS point scanning results (Fig. 2c) show that the mineral bridge is composed of Ti, Al and O (Pt is introduced in the goldspraying), consistent with the reported X-ray diffraction results of this system that the phase composition of mineral bridge is aluminium titanate (Al2TiO5, PDF#41–0256) [16]. Al2TiO5 belongs to the orthorhombic system with lattice parameters of a = 9.439 Å, b = 9.647 Å, c = 3.5929 Å, and α = β = γ = 90.0°. Therefore, the well-developed Al2TiO5 tends to grow into a tetragonal body, substantially identical to the morphology of the mineral bridge observed in Fig. 2a.
Fig. 3. (a) Compressive strength, (b) bulk density and porosity of MBC and NMBC. The inset in (a) is a SEM image of severely etched NMBC.
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Fig. 4. Curves of (a) load–displacement and (b) absorbed energy displacement of MBC and NMBC. The inset in (a) is a SEM image of crack propagation in MBC. The red arrows point to the mineral bridges, and the yellow arrow indicates the direction of crack deflection.
Fig. 2d shows the forest of the mineral bridge buds on the grain surface. The mineral bridge buds are oriented and randomly distributed with an average length of 70 nm and mean diameter of 40 nm. This condition increases the roughness and specific surface area of the grain surface. The compressive properties of the double-structured biomimetic ceramics were analysed using a ceramic (NMBC) with the only stereom structure as comparison. The inset in Fig. 3a shows that only the pure glass phase is present on the alumina grain boundary of the slightly etched NMBC sample, and no mineral bridge is found. Figs. 3a and b show that although the value difference of porosity (88.16 ± 0.44%, 88.13 ± 0.47%) and the bulk density (0.406 ± 0.014 and 0.404 ± 0.014 g/cm3) between the MBC and NMBC samples are small, the compressive strength of MBC (2.32 ± 0.34 MPa) is 2.73 times higher than that of NMBC (0.85 ± 0.35 MPa). The biomimetic introduction of mineral bridges in the submicron scale effectively enhances the compressive properties of stereom biomimetic ceramics. As seen from the load–displacement curves in Fig. 4a, the maximum load on the MBC (206.69 N) is much greater than that of NMBC (79.13 N), indicating the strengthening effect of mineral bridges. The average load of the MBC sample during the densification strain phase is greater than that of the NMBC sample. Moreover, several convex peaks of load appear on the curve of MBC in the blue dotted circle, indicating the toughening effect of mineral bridges. As shown in the inset of Fig. 4a, the crack deflects when encountering the mineral bridges. This phenomenon is attributed to the fact that the mineral bridge can reduce the stress at the crack tip and causes the crack to deflect to the glass phase when it encounters the mineral bridge [16] or reaches the area rich in mineral bridges [9]. In addition, given that the mineral bridge is a one-dimensional nanomaterial, its elastic modulus may approach its theoretical value [28]. Therefore, the energy required for a crack to break through a forest of mineral bridges is higher than through a glass. The existence of the random distribution of mineral bridges, which strengthen and toughen the microregions of ceramics, fluctuates the loads. As shown in Fig. 4b, the MBC and NMBC samples reach their maximum loads at displacements of 0.62 and 0.87 mm, respectively. In addition, the corresponding accumulated energy consumed by the MBC (22.92 × 10−3 J) is slightly higher than that of NMBC (20.22 × 10−3 J). Before displacement of 0.63 mm, the cumulative energy absorbed by the NMBC is higher than that of MBC. However, as the displacement increases, the energy absorbed by the MBC is gradually higher than that of NMBC, and the energy consumed by the MBC sample at 2 mm (126.14 × 10−3 J) is 2.54 times higher than that of NMBC (49.60 × 10−3 J). The results show that the NMBC sample is accompanied by trabecula
rupture and absorbs a large amount of energy in the elastic deformation stage, whereas the MBC sample has good stiffness in the elastic deformation stage, and the trabecula is mainly fractured in the dense strain stage. The energy absorption of MBC at this stage is much higher than that of NMBC. The above results indicate the toughening and strengthening effects of mineral bridges on MBC. According to the inset in Fig. 4a, the crack propagation is carried out in the grain boundary glass phase, indicating that the intergranular fracture is the main type of crack propagation. Therefore, the influence of the presence or absence of the mineral bridge on the stress distribution and maximum stress value of the grain boundary glass is worthy of discussion. The model of grain boundary unit with the mineral bridge is shown in Fig. 5a. One Al2O3 grain is fixed, and the other is subjected to a tensile force of 100 N. The maximum tensile stress produced during loading is used to evaluate the mechanical properties of brittle materials of the glass because they exhibit tensile capacity lower than their compression capacity [21,29]. Figs. 6a and b show the normal stress (Y-axis) distributions of grain boundary of pure glass phase and glass phase with mineral bridges, respectively. In the glass phase without mineral bridge, the high stress areas shown in red are distributed on the glass/Al2O3 interface. Obvious blue low stress areas are observed at both ends of the glass. The difference between the maximum stress and the minimum stress is 20.95 MPa. After the introduction of the mineral bridge, the high stress areas shown in red are not only distributed on the glass/Al2O3 interface but also on the glass/mineral bridge interface. In addition, the large blue
Fig. 5. (a) Three-dimensional model of grain boundary with glass and mineral bridges.
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Fig. 6. (a) and (b) are normal stress (Y-axis) distributions of grain boundary glass with and without of mineral bridges, respectively. The red triangle points to the high stress areas and the blue triangle to the low stress areas.
area becomes sharply small, and the difference between the maximum stress and the minimum stress is up to 21.13 MPa. This result indicates the uniform stress distribution in MBC. The CTS in the pure glass phase is 54.78 MPa, whereas in the glass phase containing the mineral bridges, the CTS is reduced, and the stress is 46.10 MPa. This value is 15.85% lower than that of the pure glass phase. If inherent cracks are not considered, the strengths of glass and glass/Al2O3 interface are set as constants. Once the CTS is greater than the strengths of glass or glass/Al2O3 interface, large CTS means easy generation of cracks. When inherent cracks are considered, the strength of grain boundary, the size, number and shape of crack are fixed. The following Orowan's formulas are applicable [30]: pffiffiffiffiffiffiffiffi σ A ¼ 2σ c=a; (1)where σA is the stress on the crack tip; σ is the external stress; c is the major axis length of elliptical crack, and a is the atomic pitch. Notably, σA increases with the rise of σ, providing easy propagation of the crack. In general, the decreased stress will be transferred from Al 2O 3 grain to glass and glass/Al 2O 3 interface as the mineral bridges grow, and small CTS on the grain boundary signifies difficulty in generating and propagating crack. This condition causes the external load for the MBC fracture to be greater than that of NMBC. Thus, MBC with mineral bridges have more excellent compressive properties than NMBC.
Weibull plot curves are shown in Fig. 7a. The Weibull modulus of the MBC (7.85) is 2.70 times higher than that of the NMBC (2.91), indicating that MBC with mineral bridges has high reliability. This result is attributed to the strengthening of the grain boundary by the mineral bridge, increasing the threshold of the force causing crack expansion. Comparing with the reported porosity and compressive Weibull modulus (CWM) of man-made porous materials (Fig. 7b), although NMBC has only stereom bionics, its CWM (2.91) is still higher than that of SC (2.0) and calcium phosphate cements (1.6 and 2.7) with low porosity. Notably, except for the D16.2 sample with a low porosity of 40.8% and an excellent CWM of 8.67, the CWM of other reported materials is lower than 6, and the porosity is less than 60%. Meanwhile, MBC has the largest porosity of 88.16% but with a good Weibull modulus of 7.85. This finding indicates that MBC is a high porosity material but with significant reliability. In materials with porosity of up to 88%, the effect of mineral bridge on strengthening and improving reliability of ceramics are still evident. Therefore, to further improve the strength and reliability of the ceramic is very feasible to bionic the mineral bridge in a low-porosity ceramic material. Moreover, the structure of mineral bridge forest (MBF) on the grain surface may be important in many fields. In the composite, the MBF on composite substrate can increase the contact area and form mechanical chelating, beneficial to strengthen the composite. If used for the catalytic carrier, MBF can provide a large surface area for adsorption of catalytic
Fig. 7. (a) Weibull plot of MBC and NMBC. (a) Comparison of porosity and compressive Weibull modulus of several man-made porous materials. ■-Hydroxyapatite scaffolds fabricated with 16.2 or 5.96 μm PMMA as a pore former [31]; ●-Porous ceramic Raschig rings fabricated by three different green processes of uniaxial pressing (UP), extrusion (E) and slip casting (SC), correspondingly [32]; ▼-Glass-ceramic foam scaffolds [33]; ►-Calcium phosphate cement with different porosities [34]. The red arrow points to the mineral bridge, and yellow arrow denotes the crack deflection.
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substances, improving the catalytic efficiency. For the sound absorbing material, MBF tends to improve the friction and the viscous effects of sound waves, thereby improving sound absorption. These outlooks shows a good application prospect for the ceramic combined with sea urchin stereom structure and nacre mineral bridge structure. 4. Conclusions A new ceramic (MBC) simultaneously possessing the sea urchin stereom structure and nacre mineral bridge structure has been successfully biomimetic-fabricated with porosity, pore size and mineral bridge diameter of 88.16%, 284.65 μm and 30–230 nm, respectively. The microstructure, compressive strength, porosity and Weibull modulus of MBC are compared with the biomimetic ceramics with single stereom structure (NMBC). The compressive strength (2.32 MPa) and Weibull modulus (7.85) of MBC with many mineral bridges on the grain boundary are 2.73 and 2.70 times higher than those of the NMBC, respectively, although their porosity and bulk density are almost the same. The Al2TiO5 mineral bridge has the crack deflection mechanism characterised by SEM, and has the mechanisms of homogenising stress and reducing the maximum stress on the grain boundary calculated by the finite element method, thereby improving the mechanical properties and reliability of the porous ceramic. These biomimetic ceramics may have a good application prospect in composite ceramic substrate, catalyst carrier and sound absorbing material. Moreover, multibiological-inspired multiscale bionics provides novel ideas for the design and synthesis of structural biomimetic materials. CRediT authorship contribution statement Hui Yu: Formal analysis, Investigation, Writing - original draft, Software, Methodology. Jianlin Li: Methodology, Conceptualization, Writing - review & editing. Jianbao Li: Conceptualization, Resources, Supervision, Funding acquisition. Xue Hou: Validation, Investigation. Shuaifeng Chen: Validation, Data curation. Haotian Yang: Data curation, Formal analysis. CRediT authorship contribution statement Hui Yu: Formal analysis, Investigation, Writing - original draft, Software, Methodology. Jianlin Li: Methodology, Conceptualization, Writing - review & editing. Jianbao Li: Conceptualization, Resources, Supervision, Funding acquisition. Yongjun Chen: Funding acquisition, Resources. Xue Hou: Validation, Investigation. Shuaifeng Chen: Validation, Data curation. Haotian Yang: Data curation, Formal analysis. Declaration of Competing Interest The authors declare that they have no conflict of interest. Acknowledgements The work is supported by the National Natural Science Foundation of China (NO. 51662006). Ethical approval All procedures performed in studies involving animals were in accordance with the ethical standards of the institution or practice at which the studies were conducted. References [1] P.Y. Chen, Y. Po, J. Mckittrick, M.A. Meyers, Biological materials: functional adaptations and bioinspired designs, Prog. Mater. Sci. 57 (2012) 1492–1704, https://doi. org/10.1016/j.pmatsci.2012.03.001.
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