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Correlation of Structure, Composition and Local Mechanical Properties in the Dorsal Carapace of the Edible Crab Cancer Pagurus

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Correlation of Structure, Composition and Local Mechanical Properties in the Dorsal Carapace of the Edible Crab Cancer Pagurus 766 Z. Kristallogr. 2012, 227, 766–776 / DOI 10.1524/zkri.2012.1532 # by Oldenbourg Wissenschaftsverlag, Mu¨nchen Correlation of structure, composition and local mechanical properties in the dorsal carapace of the edible crab Cancer pagurus Helge-Otto Fabritius*,I, Eva Simone KarstenI, Keerthika BalasundaramI, Sabine HildII, Katja HuemerII and Dierk RaabeI I Max-Planck-Institut fu¨r Eisenforschung GmbH, Department Microstructure Physics and Alloy Design, 40237 Du¨sseldorf, Germany II Johannes Kepler University Linz (JKU), Department of Polymer Science, 4040 Linz, Austria Received February 29, 2012; accepted June 26, 2012 Published online: August 27, 2012 Crustacean cuticle / Structure-property relations / Introduction Biomineralization In recent years, biological hard tissues have received in- Abstract. The exoskeleton of crustaceans is formed by creasing attention by materials science as sources of in- the cuticle, a chitin-protein-based nano-composite with hier- spiration and templates for the development of novel mate- archical organization over at least eight levels. On the mo- rials with tailored properties that can perform new, lecular level, it consists of chitin associated with proteins unusual or multiple functions. The body of crustaceans is, forming fibres, which are organized in the form of twisted like that of all other Arthropoda, covered by a cuticle plywood. On the higher levels, the twisted plywood orga- which is produced by the outer epithelium and serves as nization is modified and forms skeletal elements with ela- an exoskeleton (Hadley, 1986). The crustacean cuticle can borate functions. The load-bearing parts of crustacean cuti- be regarded as an organic/inorganic composite based on a cle are reinforced with both crystalline and amorphous matrix formed by the structural biopolymer chitin asso- biominerals. During evolution, all parts of the exoskeleton ciated with proteins and a mineral phase that consists were optimized to fulfill different functions according to mainly of calcium carbonates and phosphates. Structurally, different ecophysiological strains faced by the animals. the cuticle is hierarchically organized over at least eight This is achieved by modifications in microstructure and levels from the molecular scale to the fully differentiated chemical composition. In order to understand the relation- structure (Fabritius et al., 2009). Molecules of the sugar ship between structure, composition, mechanical properties N-acetylglucosamine polymerize to form anti-parallel and function we structurally characterized cuticle from the chains of a-chitin. Crystallized chitin chains (Carlstro¨m, dorsal carapace of the edible crab Cancer pagurus using 1957) coated with a protein matrix form about 5–7 nm light and scanning electron microscopy (SEM). The local thick nanofibrils. The nanofibrils further assemble to chit- chemical composition was investigated using energy dis- in-protein fibres with diameters of 50–250 nm. These fi- persive X-ray spectroscopy (EDX) and confocal m-Raman bres arrange with parallel long axes to form horizontal spectroscopy. Nanoindentation tests were performed to planes. In stacks of these planes, the fibre orientation study the resulting local mechanical properties. The results changes helically and generates a twisted plywood struc- show local differences in structure on several levels of the ture (Bouligand, 1972) that forms the three main layers of structural hierarchy in combination with a very heteroge- the cuticle, the exocuticle, endocuticle and membraneous neous mineralization. The distal exocuticle is mineralized layer. Together with the external thin epicuticle, they form with calcite, followed by a layer containing a magnesium, the material that constitutes skeletal elements which can phosphate and carbonate rich phase and ACC in the prox- have different functions (Roer and Dillaman, 1984). The imal part. The endocuticle contains magnesian calcite and physical properties of cuticle are adapted to these func- ACC in special regions below the exocuticle. Structure tions. This is achieved by altering structure and composi- and mineral phase are reflected in the local stiffness and tion at different hierarchical levels utilizing the morpholo- hardness of the respective cuticle regions. The heterogene- gical and genetic prerequisites available to the organism. ity of structural organization and mechanical properties To understand the underlying design principles, it is neces- suggests remarkable consequences for the mechanical be- sary to study the relationship between structure, composi- haviour of the bulk material. tion and the resulting properties. The crustacean cuticle has been shown to possess excellent mechanical properties (Sachs et al., 2006a; Nikolov et al., 2010; 2011). The me- chanical response of the bulk material to external loads is the integral sum of the mechanical properties and behav- iour of the individual composite constituents as well as * Correspondence author (e-mail: [email protected]) their hierarchical structural arrangement on different length Cancer pagurus dorsal carapace cuticle 767 scales (Fabritius et al., 2009). In addition to the structural cuticle surface to expose the cross section and the trans- hierarchy, the mechanical properties are also influenced by verse section in different depths. For SEM, the samples the organization of the pore canal system (Sachs et al., were mounted on standard aluminium holders, rotary sha- 2008) and type and grade of mineralization (Sachs et al., dowed with a 3 to 4 nm thick platinum layer (Gatan PECS 2006b). The cuticle of crustaceans has been shown to con- 682) and viewed in a Zeiss Gemini 1540XB dual beam tain different types of minerals like magnesium calcite, microscope at an acceleration voltage of 5 kV using a amorphous calcium carbonate (ACC) and also amorphous small aperture (30 mm) and an in-lens detector. Where ne- calcium phosphate (ACP). They can occur together in the cessary, samples were etched superficially using aqueous same layers but also be restricted to specific regions of the EDTA solution (0.15M, 15 min) followed by a quick wash cuticle (Levi-Kalisman et al., 2002; Al-Sawalmih et al., in H2O bidest and 100% methanol for 1 s each. To obtain 2008; Hild et al., 2008). A number of recent studies have images at higher magnification, a set of samples was criti- correlated the anisotropy in structure, mineral content and cal point dried (Baltec CPD 030), rotary shadowed with composition of cuticle from different crustacean species 2 nm of platinum (Balzers BAF 300) and viewed in a Hi- with evolutionary adaptations to different habitats and de- tachi S-5200 FESEM at an acceleration voltage of 2 kV. fence strategies that impose different mechanical require- For LOM, EDX and m-Raman spectroscopy, air dried ments (Boßelmann et al., 2007; Neues et al., 2007; Hild samples were glued to standard aluminium stubs or, for et al., 2009). However, in contrast to other mineralized nanoindentation, to round magnetic steel discs with the biological materials like bone (Gupta et al., 2006) little is surface of interest oriented parallel to the support. Subse- known about the influence of the local structure and quently, these surfaces were polished to the desired depth mineralization differences on mechanical properties like using an ultramicrotome (Power-Tome PT-XL, Labtec) stiffness and hardness. This is especially true for the lower equipped with a diamond knife (Diatome). The feed rate hierarchical levels like the different layers constituting the was successively reduced from 1 mm to 10 nm to obtain a cuticle. A previous study using nanoindentation on air- low surface roughness. Light optical micrographs were re- dried cuticle of the lobster Homarus americanus has corded in bright field mode on a Leica DM 400M micro- shown that the elastic properties are mainly affected by scope. the grade of mineralization (Sachs et al., 2006b). In terms of structural organization, the cuticle forming Energy-dispersive X-ray spectroscopy the dorsal carapace of the large decapod crustacean Cancer pagurus, the edible crab, has been shown to be remark- For EDX analysis, polished samples were coated with 3 nm ably complex by a number of early investigations (Drach, of platinum to ensure conductivity. Elemental maps were 1939; Hegdahl et al., 1975a–c). In an attempt to analyse recorded using the EDAX system (PV7716/08 ME) at- the relationships of structure, mineralization and mechani- tached to the SEM (Zeiss Gemini 1540XB). The post-pro- cal properties in the individual layers of the dorsal cara- cessing of the maps including background correction was pace of C. pagurus, we combined light optical microscopy performed using the Genesis software package (EDAX). (LOM), scanning electron microscopy (SEM), energy- dispersive X-ray spectroscopy (EDX), scanning confocal Scanning confocal m-Raman spectroscopy m-Raman spectroscopic imaging (SCm-RSI) and nanoin- dentation testing. The results show that the two mechani- Raman spectral images were recorded on microtome po- cally relevant layers exo- and endocuticle differ both in lished cross sections of carapace cuticle with a confocal the structural organization and the type of mineralization, Raman microscope (WITec Alpha300 R) using a Nd-YAG which is reflected in the mechanical properties. Addition- laser (wavelength 532 nm, 17 mW) and a Nikon objective ally, the complex structure and local gradients in composi- lens (100Â,Na¼ 0,925). In the areas of interest, Raman tion
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