Natural templates DOI: 10.1002/smll.200600203 Hierarchically Structured Ceramics by High- Precision Nanoparticle Casting of Wood** Atul S. Deshpande, Ingo Burgert, and Oskar Paris* Nature generates highly intricate structures with exceptional mechanical properties from seemingly simple constituents. Studies on hierarchical structure and mechanical properties of biological tissues [1–4] great- ly inspire the design strat- egies of functional materi- als. [5–7] A basic strategy in de- veloping nanostructured ma- terials with controlled mor- phologies is the use of molecular moieties, assem- blies, or even lager structures as scaffolds or templates for directing structural growth. Besides various synthetic templates, [8–16] a wide range of biological templates such as DNA, [17] surface-layer pro- teins, [18] viruses, [19,20] microor- ganisms, [21,22] bioskeletons, [23] as well as silk and hair [24] have been utilized for this purpose. From a materials- science perspective, the most challenging approach is a full reproduction of complex bio- logical materials from the macroscopic tissue level down to nanometer-scale fea- tures across all levels of hier- archy. In this context, wood is a highly sophisticated and multihierarchical material and is therefore perfectly suited as a template. Besides the integral level of the entire stem, four distinct length scales of hierarchical ordering can be identified: 1) the macroscopic level of tissue structure (growth rings), 2) the microscopic level of cell structure, 3) the submicro- scopic level of cell-wall organization, and 4) the nanoscopic level of cell-wall polymer assembly. [25] Macroscopically, wood forms a honeycomb structure (Figure 1a) with seasonally alternating density (alternating cell-wall thickness), forming the well-known annual ring pattern (hierarchy level 1 = HL1). The wood cells or trache- ids (HL2; Figure 1a) typically possess a tubelike shape with a length-to-diameter ratio of about 100. Several concentric cell wall layers are distinguishable with respect to a prefer- red orientation of cellulose fibrils and the volume fractions of the biopolymers (HL3; Figure 1d). The cell-wall layers consist of parallel arrays of partly crystalline cellulose mi- crofibrils of 2–4 nm diameter embedded in an amorphous matrix of hemicelluloses and lignin (HL4; see Fig- ure 2a). [26,27] The angle of the microfibrils with respect to the cell axis – called the “microfibril angle” (MFA) – is a measure of this spatial alignment of the cellulose fibrils. This angle appears to be one of the most effective structural features in controlling mechanical properties such as the stiffness and toughness of wood. [28–30] [*] Dr. A. S. Deshpande, Dr. I. Burgert, Dr. O. Paris Department of Biomaterials Max Planck Institute of Colloids and Interfaces 14424 Potsdam (Germany) Fax: (+ 49)331-5679402 E-mail: Oskar.Paris@mpikg.mpg.de [**] We wish to thank I. Zenke for the SAXS/WAXS measurements, H. Runge and R. Pitschke for the electron microscopy investigations, M. Eder for help with wood maceration, and P. Fratzl for fruitful discussions. Financial support from the Max Planck Society is gratefully acknowledged. Figure 1. SEM images of calcined Ce 0.5 Zr 0.5 O 2 samples obtained from the templating of normal spruce wood: a, c, d) are cross sections of different magnification while b) shows a longitudinal section with bordered pits. [26] Interfaces between different wood cell-wall layers are clearly visible in (c) and (d). The middle secondary cell wall (S2) is by far the thickest layer with a small microfibril angle in normal wood, whereas the thin inner (S1) and outer (S3) secondary cell-wall layers have a large cellulose inclination angle to the axis. The S1 layer, the very thin primary cell-wall layer (P), and the interface between adja- cent cells – the so called middle lamella (ML) – are not distinguishable in the SEM images. The nearly axial striations at the lumen side of the cell wall (Figure 1c) may indicate the orientation of the cellulose fibrillar arrays in the underlying S2 layer, given that the S3 layer is very thin and the anisotropy of the composite results in pronounced lateral shrinkage perpendicular to the cell-wall axis. [38] The buckling of the cell walls compared to the original wood material can be explained by a reduction in transverse stiff- ness caused by the removal of lignin and part of the hemicelluloses. [39] 994 # 2006 Wiley-VCH Verlag GmbH &Co. KGaA, Weinheim small 2006, 2, No.8-9, 994–998 communications