All-natural bioinspired nanolignocellulose-derived bulk engineering materials with excellent mechanical properties and environmental stability

The construction of sustainable biomimetic lightweight material with excellent mechanical properties is a kernel part of global high-performance organic − inorganic composite engineering material. However, the intrinsic contradiction between specific engineering material performances, such as strength and toughness. Therefore, making the materials achieve these performance targets simultaneously becomes a challenge. Herein, inspired by microstructures of ordered multilayered nacre, we prepared bioinspired laminated bulk artificial wooden engineering materials (AWEM) by a scalable mechanical/chemical mineralization and directional deforming assembly method. The resulting AWEM has a large-size lightweight, high specific strength, high toughness, durable dimensional stability, and excellent resistance to liquids, including acid/alkali solutions and boiling water. This design strategy allows the mass production of lightweight engineering materials with high strength-to-weight ratios. Simultaneously, it opens the way for the design of additional biomimetic materials based on nanolignocellulose. Excellent mechanical properties and environmental stability of AWEM


Introduction
Engineered materials are prevalent in almost all fields, such as metal, ceramics, and polymers (Guan et al. 2020b). But the fact is that structural engineering materials require them to be both strong and tough (Ritchie 2011). However, in most materials, the properties of strength and toughness are always mutually exclusive. Simultaneously, artificial materials either have limited mechanical properties or are nonrenewable, or complex manufacturing processes lead to high costs (Guan et al. 2020a;Hillmyer 2017;Mohanty et al. 2018;Zhu et al. 2016). For example, synthesizing high-strength structural materials requires extreme conditions such as high pressure and high temperature (Chen et al. 2015;McMillan 2002), resulting in potential safety and colossal energy consumption. Simultaneously, the manufacturing process usually emits large amounts of exhaust gas, which is harmful to the environment. Currently, many researchers are constantly pursuing more substantial and stiffer materials, which are useless without suitable fracture resistance. Therefore, it is important that sustainable, low-cost designs of high-performance structural materials can be mass-produced by simple processes, but this has remained a challenge to date.
Lignocellulose, an earth-abundant renewable, sustainable, biodegradable, and environmentally friendly resource, mainly exists in wood, grass, straw, and their solid waste materials (Chen et al. 2016(Chen et al. , 2018aFang et al. 2020;Khan et al. 2018;Ngo et al. 2018;Wang et al. 2021;Yang et al. 2020). It mainly composes of cellulose, hemicellulose, and lignin. Lignocellulose, hemicellulose, and other polysaccharides are usually bound together to form a robust polymer architecture (Chen 2014). The specific strength of lignocellulose bulk is 1.0-5.1 GPa/(g/cm 3 ) (Dufresne 2013), which is superior to most reported engineering materials, including titanium alloys (Chen et al. 2021). These highly desirable features indicate the potential for building high-performance engineering materials from all-green bio-based lignocellulose. However, the unique properties of lignocellulose-based composites, including low density, high strength, high stiffness, and easily adjustable surface, making lignocellulose construction blocks for designing and synthesizing high-specific strength material. The conventional methods have been improved to some extent, but are still insufficient for practical engineering applications. Example, Chen et al. (Chen et al. 2018b) designed artificial wooden composites with 1.8-4.4 times higher specific strength than lignocellulose-based materials through the mechanochemical process and flow-directed assembly followed by hot-pressing (Chen et al. 2018b). Meanwhile, microfibers obtained from wood cellulose nanofibrils through the flow-assisted organization have Young's modulus and tensile strength of 86 GPa and 1.57 GPa, respectively (Mittal et al. 2018). Such materials have surpassed most known natural or synthetic biopolymers, as well as some metals and alloys. However, sustainable structural materials constructed from biological resources are highly susceptible to water and moisture absorption, mold, and decay due to many hydroxyl groups. In particular, the stability of structural materials applied in harsh environments, such as water, acids, and bases, is worth considering (Pan et al. 2021). Incredibly, natural organisms have evolved by selection over billions of years to combine a minimal number of components into exceptional biological structural materials (Barthelat et al. 2016;Eder et al. 2018;Guan et al. 2020a;Huang et al. 2019). For example, the shell of the Chrysomallon squamiferum can withstand severe corrosion by predators and dissolve the protection of the marine environment, such as brackish water, cold water, and low pH (Yao et al. 2010). In addition, multiscale hierarchy of natural pearl layers and the multi-scale exogenous toughening mechanism can balance the conflict between strength and toughness. Currently, many successful cases of biomimetic design for improved mechanical properties have used this promising strategy to enhance the performance of existing engineering materials (Bouville et al. 2014;Chen et al. 2020;Guan et al. 2020a;Mao et al. 2016). It combines a material with the mutually exclusive properties of strength and toughness of all-natural raw materials under moderate conditions (Yao et al. 2014). Therefore, high-performance block building materials constructed with biomimetic design can promote the development of lignocellulose.
Herein, we demonstrate a simple biomimetic design approach-directed deformation assembly, that can process natural bulk lignocellulose into a largescale structural material. Due to the high-density reversible interaction network between nanofibers, AWEM has high specific strength of 162.24 MPa/(g/ cm 3 ), fracture toughness of ~ 7.7 MPa m 0.5 , and low water absorption swelling (5.26%). In particular, the mechanical properties of AWEM remain stable in a series of harsh environments, including acid (HCl) and alkalies (NaOH) solutions, such as Coca-Cola, vinegar, tea, and boiling water. It exhibits excellent properties compared to typical polymers, metals, and ceramics. Our research highlights the possibility of exploiting the designability of soft polymers in structural biomass material systems, overcoming the conflict between strength and toughness. This design strategy opens the way for designing durable and versatile biomimetic structural materials for practical applications, expanding the range of applications for various agricultural and forestry residues.

Preparation of AWEM
A flocculating suspension of pure lignocellulose mixed with 10 wt% CaCO 3 , 2.5 M NaOH, and 0.4 M Na 2 SO 3 was subjected to mechanical/chemical treatment in a dissociator (speed 2880r/mins, the distance between grinding discs 0.1 mm) for 6 h. Specifically, pure lignocellulose was dried and added to the beaker. 10 wt% CaCO 3 , 2.5 M NaOH, 0.4 M Na 2 SO 3 , and pure lignocellulose (5 wt.%) was added to the deionized water. Meanwhile, the lignocellulose was softened during the soaking process, and water molecules entered the amorphous zone of the pure lignocellulose. Attraction occurred with the polar hydroxyl groups in the molecular structure of the cellulose and hemicellulose, which increased the distance between the molecular chains of the lignocellulose, and caused swelling. After sufficient expansion, the flocculating suspension was subjected to mechanical/chemical treatment in a dissociator (speed 2880 r/min, distance between grinding discs 0.1 mm) for 6 h. After pulverization, the flocculating suspension was heated to 90 ℃ for 0.5 h to allow the curled and deformed lignocellulose to stretch during the pressing process and increase the contact area. After separating and refining, the flocculated suspension was heated to 90 ℃ for 0.5 h to allow the crimped lignocellulose to be stretched and the contact area increased during the pressing process. Then, the suspension was transferred into 2 L of simulated seawater solution containing a pH value of 8.1 (NaCl (62 mmol/L), Na 2 SO 4 (8 mmol/L), NaHCO 3 (4 mmol/L), KCl (2.4 mmol/L), CaCl 2 (2.5 mmol/L), MgCl 2 ·6H 2 O (1.6 mmol/L), and 500 mL of deionized water prepared to simulate the process of nacre biomineralization in seawater), and allowing the aragonite crystals to grow to form lignocellulosic building blocks at 50 °C for 24 h through a hydrothermal mineralization method. The mixed hydrogel was crosslinked by spraying CaCl 2 (150 ml, 0.5 mol/L) to form three-dimensional nanolignocellulose hydrid hydrogels and cut into thin slices, which were then oxidized with polyacrylic acid pairs for 6 h. Next, the hydrogel slices were then stacked layer-bylayer and cross-linked, and pressed at a pressure of 1 MPa for 1 day, and finally pressed at 100 °C under a pressure of 10 MPa until it was utterly drying and solidification to obtain densified all-natural bio-structured material (Fig. 1).
Characterization AWEM dimensions and shapes were recorded on an optical (ECLIPSE 80i, Nikon, Tokyo, Japan) and the microtopography was investigated by scanning electron microscopy (SEM, Quanta 200, FEI) with an EDX system (EDS QUANTAX, Bruker XFlash6, Germany) at an accelerating voltage of 10 kV and a working distance of roughly 8 mm. The X-ray diffraction analyses (XRD) of the AWEM were recorded Fig. 1 Schematic illustration of directional deforming assembly method to manufacture bioinspired AWEM on a Rigaku D/MAX 2200 diffractometer equipped with Cu Kα radiation (λ = 1.5418 Å) at 40 kV, 40 mA, with a scale range of 10° to 60° (scan rate of 4°/min). FTIR spectra (400-4,000 cm −1 ) were measured on a Magna-IR 560 ESP spectrometer in the range of 400-4000 cm −1 with a resolution of 4 cm −1 . The element compositions and main groups at the surface and chemical components of pure lignocellulose material and AWEM were carried out by XPS (XPS, Thermo ESCALAB 250XI, USA). Mechanical properties of AWEM samples were measured by a mechanical testing machine of Shenzhen New Sansi 50 kN microcomputer controlled electronic capability testing machine with 2 kN load at a rate of 1 mm/min for three-point bending tests and 1 μm/s for the single-edge notched bend. Threepoint flexural tests were carried out based on the ASTM D-790 standard with a span-to-depth ratio of 16:1. Three-point bending tests and single-edge notched bend were conducted for these AWEM samples with a spacing of 16 mm between the two bottom rollers. And a 300 μm diamonds-saw blade was used to cut the AWEM specimen to about half its width, and then the notch was cut by repeated light sliding of the razor blade. The density of the AWEM was calculated using the equation density = mass/ volume. The water absorption thickness expansion rate, internal bonding strength, and surface bonding strength of the samples were measured by GB17657-2013. Each value recorded represented the average of five samples. The specific fracture toughness and strength were calculated by dividing by the density. Water contact angle (WCA) was measured with 5 μL of droplets of ultrapure water on the surface of the AWEM specimen, and the droplets were observed at room temperature using a contact angle tester (WCA 20, Dataphysics, Germany). Determination of WCA after water droplets stay at different positions on each AWEM surface for 5 s. The average of five measurements at different places of each AWEM is applied to calculate the final WCA. In addition, all mechanical property data are the results of 5 specimens prepared under the same conditions and tested 5 times according to GBT11718-2021, 1-5 (pure lignocellulosic nano blocks) and 1-5 (AWEM), respectively.

Calculation of crystallinity
When assessing the crystallinity of wood cellulose materials by XRD diffraction, coherent interference produces peaks only in the crystalline region. In contrast, the non-crystalline region does not have peaks due to diffuse reflections. Therefore, the relative crystallinity of cellulose is calculated based on the ratio of the peak area to the total area. To describe quantitatively the proportion of crystalline and non-crystalline regions of cellulose (Tribulová et al. 2019;Xia et al. 2022). The calculation formulas of crystallinity are as follows: where CrI is the percentage of relative crystallinity diffraction, I 200 and I 110 are maximum diffraction intensities for cellulose I (200) and II (110). I am is the diffraction intensity of the amorphous cellulose (2θ = 18° for cellulose I, and 2θ = 15° for cellulose II).

Calculation of fracture toughness
The fracture toughness was calculated by a singleedge notched bend test, and crack initiation (K IC ) (Bouville et al. 2014;Mao et al. 2016) of AWEM was calculated by where P IC is the maximum load for the single-edge notched bend test, S is the span, B is the thickness of the AWEM specimen, W is the width, and a is the length of the pre-crack. The function ƒ(a/W) is given by (2) CelluloseII:CrI= The maximum fracture toughness (K JC ) is determined by where J el represents the elastic component of J-integral, J pl is the plastic element of J-integral, and E′ is given by where E represents the elastic modulus and ν the Poisson's ratio.
The elastic component J el was based on linear elastic fracture mechanics.
The plastic component J pl was calculated by where A pl is the area of the plastic region under the load − displacement curve and (W-a) is the remaining crack ligament.

Results and discussion
Morphological characterization of AWEM Although the initial improvement was obtained by densification of the dissociated pure lignocellulose, there are still many defects, such as the presence of many pores between the elongated slender layered branches of pure nanolignocellulosic components,  Fig. 2a-c. Therefore, multiple architectures of biomimetic pearls are feasible in terms of architecture and performance tuning. Based on this, we prepared oriented components of all-natural material-based multiple hierarchically arranged structures into high-performance AWEMs. As shown in Fig. 2d, the hybrid crosslinked 3D nanolignocellulose composite exhibit a laminated architecture. This is due to compression, shear, and friction processes leading to the dissociation of more fibers or micro/nanofibers, which increase the contact area of fibers and facilitate crosslinking. This nanolignocellulose polymer can be crosslinked into a hydrogel. It can be pressed into fine and homogeneous structural materials, as shown in Fig. 2e. The resulting AWEM exhibit a low density of approximately 0.87 g/cm 3 , which has a similar laminated architecture to that of natural nacre (Fig. 2f). In addition, Fig. 2g-l shows the EDS elemental mapping images of the AWEM composite. The main element of the AWEM composite is C, O, Na, Cl, and Ca, and the distribution of Ca is the same as that of C and O.

Characterization of AWEM
As shown in Fig. 3a, the positions of the 101 and 002 diffraction characteristic peaks were still 16° and 22.5° compared to the XRD patterns of pure nanolignocellulose and AWEM (Tribulová et al. 2019;Xia et al. 2022). According to the calculation, however, the crystallinity of AWEM was higher than that of pure nanolignocellulose. The destruction of the amorphous zone mainly causes an increase in crystallinity. This is mainly due to the hydrolysis of water molecules into the amorphous area of cellulose under exciter, high temperature, and high-pressure conditions during the whole process of dissociative preparation of wood chip fibers. This reduces the polymerization degree and increases the crystallinity. This change in crystallinity confirmed the self-bonding mechanism of AWEM from another aspect. Concurrently, FTIR spectra of pure nanolignocellulose and AWEM determine the shift of pure lignocellulose throughout the whole processing, as shown in Fig. 3b. As a result, the maximum absorption hydroxyl peak of AWEM shifted from 3351 to 3342 cm -1 , indicating that hydrogen groups can be formed during the compaction control. Asymmetric C-H stretching vibrations of methyl, methylene, and methylene groups of lignin and hemicellulose were observed at 2920 cm -1 . The C-H stretching vibration peaks at 2910 and 2804 cm -1 corresponded to the characteristic peaks of cellulose. 1600-1429 cm -1 is the absorption peak of the lignin aromatic ring, and 1594 cm -1 is the telescopic vibration of the C = C characteristic peak of the benzene ring backbone. Furthermore, absorption peaks appear at 1466, 877, and 711 cm -1 , corresponding to the antisymmetric stretching vibration of the a b  C-O bond, the absorption peak of the deformation vibration outside the CO 2− 3 -plane, and the absorption peak of the deformation vibration inside the O-C-O in-plane. This is mainly due to the presence of CaCO 3 .
The XPS broad scan spectra of the surface atomic composition and chemical bonding of pure nanolignocellulose and AWEM are shown in Fig. 4a. The binding energy sharp peaks of C 1 s, O 1 s, and Ca 2p in the AWEM were 285.17 eV, 530.11 eV, and 368.8 eV, respectively. Figure 4b shows that the high-resolution XPS spectra of Ca 2p from AWEM, and sharp two peaks at 348.4 eV were Ca 2p 3/2 , and 351.9 eV was due to Ca 2p 1/2 . This further confirms the existence of calcium carbonate in AWEM. To reveal the chemical constructions of C1s, the high-resolution C1s peaks of AWEM were divided into three peaks, and the suitable binding energies are 284.66, 283.67, and 281.64 eV, respectively, corresponding to C = O/O-C-O, C-OH/C-O-C, and C-C/C-H) bonds, as shown in Fig. 4c, d. In contrast, pure nanolignocellulose bulk has a more straightforward O 1 s peak shape (Fig. 4e) and binding energy of 529.9 eV. However, it becomes complicated in AWEM (Fig. 4f). The fitting peak at about 528.52 eV is attributed to the oxygen of the OH group, which indicates that AWEM is obtained from hydroxyl bonds on the surface of pure nanolignocellulose.
For the atomic content and the C/O ratio, it is using peak splitting and calculations, which showed that the result is listed in Table 1. The elemental C/O value is 1.89. It is mainly due to the addition of NaOH during a b c d Fig. 5 Mechanical properties of pure nanolignocellulose and AWEM. a Stress-strain curves of pure nanolignocellulose and AWEM. b Comparison of flexural strength and stiffness of pure nanolignocellulose and AWEM. c Rising crack-extension resistance curves to evaluate the steady-state fracture toughness of pure nanolignocellulose and AWEM. d Fracture toughness for crack initiation (K IC ) and steady-state (K JC ) of pure nanolignocellulose and AWEM soaking and swelling, and the lower the lignin content of the lignocellulose during thermal dissociation, the smaller the C/O value. Moreover, the increase in Ca content was mainly due to the purification and biomineralization of lignocellulose during pretreatment and dissociation. Given this, the pressure-controlled process generates a large amount of vapor, and the self-bonding mechanism of AWEM exposed here seems to be more complex.

Mechanical strength of AWEM
The flexural strength and modulus of pure lignocellulose bulk can only reach ~ 32.44 MPa and ~ 3076 MPa, respectively, as shown in Fig. 5a-b. This is because only hydrogen bonds exist in the pure nanolignocellulose bulk, and the interfacial interaction between nanolignocellulose is poor. In addition to hydrogen bonds, the carboxyl groups on the surface of AWEM were crosslinked by Ca 2+ , forming a robust ionic bond network. This further enhanced the interaction between nanolignocellulose. Furthermore, the polyacrylic acid oxidized nanolignocellulose hydrogels were ordered to create a solid brick-and-mortar architecture by the directional assembly. The resulting AWEM prepared has a high flexural strength of 126.55 MPa and a high Young's modulus of 9153 MPa, which was more than four times that of pure lignocellulose bulk (Fig. 5a, b and Table 2). Currently, a longterm challenge in the design of engineering materials is the mutual balance between strength and toughness (Chen et al. 2018a, b;Guan et al. 2020a;Ritchie 2011). As a result, the strength, and toughness of AWEM are enhanced by the highly ordered laminated architecture, as shown in Fig. 5c, d, and Table 2. The fracture toughness (K IC ) of AWEM is ~ 5.11 MPa m 0.5 , which was much higher than that of pure nanolignocellulose bulk (1.42 MPa m 0.5 ), natural Anodonta woodiana nacre (~ 2.1 MPa m 0.5 ) and Pinctada margaritifera nacre (~ 4.0 MPa m 0.5 ) (Guan et al. 2020a). This excellent fracture toughness confirms our AWEM to resist crack initiation effectively. Although K IC can visually and effectively evaluate the resistance to crack initiation, it cannot assess the energy dissipation during crack extension propagation due to multiple extrinsic  (Bouville et al. 2014;Munch et al. 2008).
Wettability assessment of AWEM using contact angle measurement In addition to the superior mechanical properties of AWEM, wettability is also an important indicator. Therefore, a contact angle tester was used to measure the WCA values on the specimen surface versus the wetting time to analyze their surface wettability. Figure 6 shows the WCA values of the surface of pure nanolignocellulose bulk and AWEM at different wetting times. The resulting initial WCA and final stable WCA of the AWEM surface are greater than those of the pure nanolignocellulose bulk. Its maximum WCA can reach 120° and remains > 80° after 10 min of wetting. This is due to the high-water absorption thickness swelling rate of pure nanolignocellulose (higher than GB/T11718-2021 values) and hydrophilicity lead to strong water absorption. AWEM maintains a smaller water absorption thickness swelling rate (< 5%) and a higher WCA. This indicates that the AWEM matrix component polymers bond the pure nanolignocellulose interfaces to form a stable laminated architecture. This not only provides strong and tough (damage-tolerant), but also enhances water repellency and dimensional stability.
Water permeability and environmental stability evaluation of AWEM Meanwhile, morphological changes of the typical liquid droplets can be used to demonstrate the wettability of the AWEM surface. The water droplets exhibited hydrophilicity and slowly unfolded on the surface of pure nanolignocellulose bulk. The boiling water droplets (full red marks) flattened out upon contact with them (Fig. 7a). The water droplets on the surface of AWEM exhibit a spherical appearance morphology in both water and boiling water. These water droplets did not undergo osmotic diffusion on the AWEM surface, but only a slight reduction in volume (Fig. 7b). It indicates the hydrophobic characteristics of the specimens, which is the same as the results in Fig. 6. Simultaneously, the dried AWEM specimens remained repulsive to hydrochloric acid, sodium hydroxide, tea, water, boiling water, and cola and vinegar droplets after 2 h of treatment in behavior of the surface of AWEM after abrasion. K-p Optical images of the AWEM immersed in different PH solutions, including HCl, NaOH, Coca-Cola, vinegar, green tea, and boiling water for 24 h boiling water for (Fig. 7c). After 30 days of storage in a daily environment, the surface of AWEM specimens remained hydrophobic in all directions (Fig. 7d-e). It also remains repellent to droplets, such as hydrochloric acid (Fig. 7d), sodium hydroxide (Fig. 7f), tea (Fig. 7g), Coca-Cola (Fig. 7h), vinegar (Fig. 7i), and boiling water (Fig. 7j). The results indicate that the AWEM specimens had liquid resistance stability. As expected, the AWEM prepared exhibits good configuration and excellent mechanical stability when immersed for 24 h in several typical harsh environments ( Fig. 7k-p), including 1 mol/L HCl solution (Fig. 7k), 1 mol/L NaOH solution (Fig. 7l), Coca-Cole (Fig. 7m), vinegar (Fig. 7n), tea (Fig. 7o), boiling water (Fig. 7p) and 80% relative humidity (RH), respectively. As a result, the AWEM specimens immersed with different solutions maintained their original conformation with insignificant topographical changes (Fig. 7k-p). In addition, mechanical tests demonstrated that the flexural strength (specific strength) and MOE of AWEM specimens decreased slightly but remained at high values ( Fig. 8a-b). Notably, the AWEM bulk prepared shows much better mechanical stability than the pure nanolignocellulosic bulk. The experimental results demonstrate the remarkable environmental stability of the AWEM prepared, which is elusive for many previously reported pearl-like composites.

Conclusions
In summary, we have developed a lightweight and high-strength engineering material with a simple and effective mechanochemical dissociation process and directed deformation assembly followed by pressing control. The obtained AWEM exhibits low density (0.87 g/cm3), high flexural strength (126.55 MPa), high fracture toughness (9153 MPa), high flexural modulus (5.11 MPa m0.5), and high internal bonding strength (0.93 MPa) and water absorption thickness expansion rate (7.98%). Based on the resulting laminar architecture makes it superior to natural structural material, biomimetic structural material, engineering structural material, and metallic alloys. Owing to the combination of ordered laminated microstructure and hydrogen bonding, AWEM exhibits excellent hydrophobicity and dimensional stability (low water absorption rate and thickness swelling rate). Its mechanical performances can remain stable in some harsh environments, including 1 M HCl solution, 1 M NaOH solution, Coca-Cole, Tea, boiling water, and 80% relative humidity (RH) for 24 h. Globally, the methods described in this work will encourage the development of entirely green and sustainable biomaterials with lightweight, excellent mechanical behaviors, and extraordinary dimensional stability. b a Fig. 8 Impressive mechanical stability of AWEM in some harsh environments. Specific strength and MOE, and its retention of the AWEM, respectively, after being immersed in