Although remarkable success has been achieved to mimic the mechanically excellent structure of nacre in laboratory-scale models, it remains difficult to foresee mainstream applications due to time-consuming sequential depositions or energy-intensive processes. Here, we introduce a surprisingly simple and rapid methodology for large-area, lightweight, and thick nacre-mimetic films and laminates with superior material properties. Nanoclay sheets with soft polymer coatings are used as ideal building blocks with intrinsic hard/soft character. They are forced to rapidly self-assemble into aligned nacre-mimetic films via paper-making, doctor-blading or simple painting, giving rise to strong and thick films with tensile modulus of 45 GPa and strength of 250 MPa, that is, partly exceeding nacre. The concepts are environmentally friendly, energy-efficient, and economic and are ready for scale-up via continuous roll-to-roll processes. Excellent gas barrier properties, optical translucency, and extraordinary shape-persistent fire-resistance are demonstrated. We foresee advanced large-scale biomimetic materials, relevant for lightweight sustainable construction and energy-efficient transportation.

Biological materials fascinate us with their ability to withstand extreme mechanical forces under complex conditions. Their excellent performance originates from a multilevel hierarchical structure; understanding these structures is pursued in structural biology and biomechanics research. A common feature in many biological materials with superior mechanical properties is the combination and ordered arrangement of hard and soft building blocks. [1][2][3] Therein, the hard matter serves as the load bearing and reinforcing part, whereas energy can be dissipated into the soft segments. Many of these materials combine good toughness with admirable strength and stiffness. For instance, in nacre, the layered arrangement of platelet-shaped CaCO 3 crystals and proteins into a brick and mortar structure leads to a synergistic performance with respect to the mechanical properties.[5] The Youngs modulus and stress at break can reach 40-70 GPa and 80-135 MPa, respectively. [6][7][8] The material is remarkably tough under wet conditions. Dynamic processes, such as sacrificial (dynamic) bonds and hidden length scales contribute significantly to toughness improvements or the ability of a material to undergo self-healing. Recently, it was shown that infiltration of metal ions drastically increase the toughness of silk dragline or increase stiffness and strength in layer-by-layer (LbL) materials. [9,10] Moreover, modeling by Fratzl and co-workers showed how randomly distributed multivalent binding sites in layered materials can lead to sacrificial bonds and provide shear deformability and larger deformations similar to that found in natural materials.[12] Thus, ionic bonding is a promising tool for tailoring the mechanical properties of biological or biomimetic systems, and to access important features such as sacrificial bonds and hidden length scales.Considering the lightweight character of the mechanically strong and tough biomaterials, a large-scale preparation of biomimetic materials is of preeminent importance for future construction and coating applications. However, this is a major scientific challenge. Various efforts have been undertaken to mimic the layered hard/soft composite structure of nacre by synthetic means. Nacre mimics can be obtained by several sequential approaches, such as layer-by-layer (LbL) [13][14][15][16] and other multilayer deposition strategies, [17] icetemplating and sintering of ceramics, [18,19] uncontrolled cocasting of polymer/clay mixtures, [20][21][22] or processes at interfaces. [23][24][25] Unfortunately, most of the approaches are limited to the structural characterization of the materials at very small scales, and often there have been challenges in even producing large enough specimens for mechanical characterization beyond nanointendation. Using LbL [26] deposition of polymers and nanoclay, the maximum stiffness and strength could even exceed those of natural nacre, [13][14][15][16] thus demonstrating how valuable such layered polymer/clay structures can be. Toughness could be increased...

Several key enzymes in lignin biosynthesis of Populus have been down-regulated by transgenic approaches to investigate their role in wood lignification and to explore their potential for lignin modification. Cinnamate 4-hydroxylase is an enzyme in the early phenylpropanoid pathway that has not yet been functionally analyzed in Populus . This study shows that down-regulation of cinnamate 4-hydroxylase reduced Klason lignin content by 30% with no significant change in syringyl to guaiacyl ratio. The lignin reduction resulted in ultrastructural differences of the wood and a 10% decrease in wood density. Mechanical properties investigated by tensile tests and dynamic mechanical analysis showed a decrease in stiffness, which could be explained by the lower density. The study demonstrates that a large modification in lignin content only has minor influences on tensile properties of wood in its axial direction and highlights the usefulness of wood modified beyond its natural variation by transgene technology in exploring the impact of wood biopolymer composition and ultrastructure on its material properties.

In 1628, the Swedish warship Vasa capsized on her maiden voyage and sank in the Stockholm harbor. The ship was recovered in 1961 and, after polyethylene glycol (PEG) impregnation, it was displayed in the Vasa museum. Chemical investigations of the Vasa were undertaken in 2000, and extensive holocellulose degradation was reported at numerous locations in the hull. We have now studied the longitudinal tensile strength of Vasa oak as a function of distance from the surface. The PEG-content, wood density, and cellulose microfibril angle were determined. The molar mass distribution of holocellulose was determined as well as the acid and iron content. A good correlation was found between the tensile strength of the Vasa oak and the average molecular weight of the holocellulose, where the load-bearing cellulose microfibril is the critical constituent. The mean tensile strength is reduced by approximately 40%, and the most affected areas show a reduction of up to 80%. A methodology is developed where variations in density, cellulose microfibril angle, and PEG content are taken into account, so that cell wall effects can be evaluated in wood samples with different rate of impregnation and morphologies.

To prevent deformation and cracking of waterlogged archaeological wood, polyethylene glycol (PEG) as a bulk impregnation agent is commonly applied. PEG maintains the wood in a swollen state during drying. However, swelling of wood can reduce its mechanical properties. In this study, the cellular structure of oak and cell wall swelling was characterized by scanning electron microscopy (SEM) of transverse cross-sections, and the microfibril angle of oak fibers was determined by wide angle X-ray scattering (WAXS). Samples of recent European oak (Quercus robur L) impregnated with PEG (molecular weight of 600) were tested in axial tension and radial compression. Mechanical tests showed that axial tensile modulus and strength were only slightly affected by PEG, whereas radial compressive modulus and yield strength were reduced by up to 50%. This behavior can be explained by the microstructure and deformation mechanisms of the material. Microfibril angles in tensile test samples were close to zero. This implies tensile loading of cellulose microfibrils within the fiber cell walls without almost any shear in the adjacent amorphous matrix. These results are important because they can help separate the impact of PEG on mechanical properties from that of chemical degradation in archaeological artifacts, which display only small to moderate biological degradation.

Polarisation Fourier transform infra-red (FTIR) microspectroscopy was used to characterize the organisation and orientation of wood polymers in normal wood and tension wood from hybrid aspen (Populus tremula × Populus tremuloides). It is shown that both xylan and lignin in normal wood are highly oriented in the fibre wall. Their orientation is parallel with the cellulose microfibrils and hence in the direction of the fibre axis. In tension wood a similar orientation of lignin was found. However, in tension wood absorption peaks normally assigned to xylan exhibited a 90° change in the orientation dependence of the vibrations as compared with normal wood. The molecular origin of these vibrations are not known, but they are abundant enough to mask the orientation dependence of the xylan signal from the S₂ layer in tension wood and could possibly come from other pentose sugars present in, or associated with, the gelatinous layer of tension wood fibres.

The impact of drying on the structure of the never-dried hardwood cell wall was studied at nanometer level by means of wide- and small-angle X-ray scattering (WAXS, SAXS), and at micrometer level by X-ray microtomography (μCT). Never-dried silver birch, European aspen and hybrid aspen samples were measured by WAXS in situ during drying in air. The samples included juvenile and mature wood, as well as normal and tension wood to allow comparison of the effects of different matrix compositions and microfibril angles. The deformations of cellulose crystallites and amorphous components of the cell wall were detected as changes in the cellulose reflections 200 and 004 and amorphous halo in the WAXS patterns. Especially, the width of the reflection 004, corresponding to the cellulose chain direction, increased due to drying in all the samples, indicating an increase of strain and disorder of the chains. Also, the cellulose unit cell shrank 0.2–0.3% during drying in this direction in all the samples except in hybrid aspen tension wood. According to the SAXS results of silver birch, the distance between micro-fibrils decreased during drying. It was detected by μCT that the mean cross-sectional maximum width of the parenchymatous rays decreased from that of never-dried to air-dried birch by roughly 16%.

The degradation of oak wood of the historical warship Vasa was studied, focusing on cellular structure by X-ray microtomography (mCT) and on the nanostructure of the cell wall by wide-and small-angle X-ray scattering (WAXS, SAXS). Solid samples wpolyethylene glycol (PEG)-, impregnated and PEG-extractedx were submitted to X-ray analysis and the results compared to those of recent oak. The cellular structure of the Vasa oak was surprisingly well preserved at the micrometer level, according to the mCT images. As revealed by WAXS, the fraction of crystalline cellulose was lower in the Vasa samples compared with recent oak, but the average length and width of cellulose crystallites (25"2 nm and 3.0"0.1 nm, respectively), and the mean microfibril angles (4-98), showed no significant differences. Accordingly, the crystalline parts of cellulose microfibrils are well preserved in the Vasa oak. The SAXS results indicated a declined shortrange order between the cellulose microfibrils and a higher porosity of the Vasa oak compared with recent oak, which may be explained by modification of the hemicelluloselignin matrix.