FULL PAPER © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 6093 wileyonlinelibrary.com systems, eliminating the need for a trial and error approach. Biomineralizing organisms have evolved to form complex, hierarchical structures that are adapted to mitigate predation and environmental stresses. These biological composites, through the addition of small quantities of organic, precisely control architectures from the nano- to macro-scale, and utilize organic-mineral interfaces to achieve remarkable material properties. [5–7] Adaptations in biological composites have led to enhancements in toughness, strength and abrasion-resistance over their con- stituent components, yielding properties rivaling those of modern engineering mate- rials. [5,6,8] One such example is found in the feeding apparatus (radula) of the chiton, a marine snail that produces one of the most abrasion-resistant structures in the animal kingdom. [9,10] The chiton exhibits selective biomineralization, with shell plates con- sisting of aragonitic CaCO 3 [11,12] and radular teeth mineralized by iron oxides and iron phosphates. [13–15] The teeth of chitons are arranged in parallel rows on a tongue-like feeding organ called a radula. [8,14] The radula of Cryptochiton stelleri, the largest species of chiton ( Figure 1A), contains more than 70 rows of tricuspid teeth affixed to an organic membrane. The chiton uses these radular teeth to scrape algae from rocks (Figure 1B). The radula displays active mineralization, with the degree of mineralization increasing from a completely non-mineralized structure consisting of alpha-chitin at the posterior end to a fully mineralized tooth at the anterior end (Figure 1C). This mineralization process was shown to be guided by organic fibers within the tooth. [8] The chiton tooth is a complex hierarchically-arranged biological composite, with an elegant design spanning multiple length scales. At the macroscale, the teeth of C. stelleri consist of tri- cuspid curved structures (Figure 1D). The rasping process of the chiton has led to the designation of the concavely bent front side as the leading edge of the tooth, and the convexly bent opposing side as the trailing edge. [16] The repeated cyclic mechanical loading of chiton teeth during rasping necessi- tates that they are both wear and abrasion resistant. [10] Thus, the teeth of C. stelleri are mineralized with a shell of mag- netite, [6,8,11] displaying the highest hardness of any reported biomineral, [6] surrounding a softer core of hydrated iron phos- phate. [4,5,11,14] Initial analysis of the magnetite shell revealed rod-like elements consisting of randomly oriented crystallites, Stress and Damage Mitigation from Oriented Nanostructures within the Radular Teeth of Cryptochiton stelleri Lessa Kay Grunenfelder, Enrique Escobar de Obaldia, Qianqian Wang, Dongsheng Li, Brian Weden, Christopher Salinas, Richard Wuhrer, Pablo Zavattieri, and David Kisailus* Chiton are marine mollusks who use heavily mineralized and ultrahard teeth to feed on epilithic and endolithic algae on intertidal rocks. To fulfill this func- tion, chiton teeth must be tough and wear-resistant. Impressive mechanical properties are achieved in the chiton tooth through a hierarchically arranged composite structure consisting of a hard shell of organic-encased and highly oriented nanostructured magnetite rods that surround a soft core of organic- rich iron phosphate. Microscopic and spectroscopic analyses combined with finite element simulations are used to probe the ultrastructural features and uncover structure–mechanical property relationships in the fully mineral- ized teeth of the gumboot chiton Cryptochiton stelleri. By understanding the effects of the nanostructured architecture within the chiton tooth, abrasion- resistant materials can be developed for tooling and machining applications, as well as coatings for equipment and medical implants. DOI: 10.1002/adfm.201401091 Dr. L. K. Grunenfelder, Dr. Q. Wang, Dr. D. Li, B. Weden, C. Salinas, Prof. D. Kisailus Department of Chemical and Environmental Engineering Bourns Hall B357, Riverside, CA 92521, USA E-mail: david@engr.ucr.edu E. E. de Obaldia, Prof. P. Zavattieri School of Civil Engineering Purdue University West Lafayette, IN 47907, USA Dr. R. Wuhrer Advanced Materials Characterization Facility University of Western Sydney Penrith, NSW 2751, Australia 1. Introduction The development of wear resistant tooling and coatings is a difficult task, as wear resistance is not a material property, but rather dependent on a number of factors (e.g., hardness, micro- structure, chemical composition, geometry, loading conditions, environment, etc.), requiring a multidisciplinary approach. [1–3] Hard ceramics and ceramic-based composites are commonly used in abrasive loading applications. However, these mate- rials are problematic from an engineering perspective, as they are prone to brittle failure [4] and typically require processing at high temperatures and pressures, making microstructural con- trol difficult. Cues for the design of abrasion resistance mate- rials, however, are readily accessible in the form of biological Adv. Funct. Mater. 2014, 24, 6093–6104 www.afm-journal.de www.MaterialsViews.com