Contents lists available at ScienceDirect Journal of the Mechanical Behavior of Biomedical Materials journal homepage: www.elsevier.com/locate/jmbbm A comparative study of piscine defense: The scales of Arapaima gigas, Latimeria chalumnae and Atractosteus spatula Vincent R. Sherman a , Haocheng Quan a , Wen Yang b , Robert O. Ritchie c , Marc A. Meyers a,d, ⁎ a Department of Mechanical and Aerospace Engineering, Materials Science and Engineering Program, University of California San Diego, La Jolla, CA 92093, USA b Department of Materials, ETH Zurich, 8093 Zurich, Switzerland c Department of Materials Science and Engineering, University of California Berkeley, CA 94720, USA d Department of Nanoengineering, University of California San Diego, La Jolla, CA 92093, USA ARTICLE INFO Keywords: Scales Bioinspiration Bouligand Alligator gar Coelacanth Arapaima ABSTRACT We compare the characteristics of the armored scales of three large fish, namely the Arapaima gigas (arapaima), Latimeria chalumnae (coelacanth), and Atractosteus spatula (alligator gar), with specific focus on their unique structure-mechanical property relationships and their specialized ability to provide protection from predatory pressures, with the ultimate goal of providing bio-inspiration for manmade materials. The arapaima has flexible and overlapping cycloid scales which consist of a tough Bouligand-type arrangement of collagen layers in the base and a hard external mineralized surface, protecting it from piranha, a predator with extremely sharp teeth. The coelacanth has overlapping elasmoid scales that consist of adjacent Bouligand-type pairs, forming a double-twisted Bouligand-type structure. The collagenous layers are connected by collagen fibril struts which significantly contribute to the energy dissipation, so that the scales have the capability to defend from predators such as sharks. The alligator gar has inflexible articulating ganoid scales made of a hard and highly mineralized enamel-like outer surface and a tough dentine-like bony base, which resist powerful bite forces of self-predation and attack by alligators. The structural differences between the three scales correspond with the attack of their predators, and show refined mechanisms which may be imitated and incorporated into superior bioinspired and biomimetic designs that are specialized to resist specific modes of predation. 1. Lessons from natural dermal armors Nature has produced an extraordinary number of unique and specialized materials over hundreds of millions and even billions of years of evolution. For thousands of years natural designs have provided inspiration for manmade structures, such as ancient armors. However, it is only in recent times that humans have come to realize that studying, understanding, and mimicking these materials may serve as an important route for the design and development of new specialized synthetic materials. Despite being comprised of only a limited palette of constituents with relatively modest mechanical properties, biological materials can exhibit remarkable combinations of strength, toughness and reliability that are crafted through ingenious designs involving hierarchical assemblies and gradients in composi- tion, structure and properties. This has stimulated many studies throughout the world to seek to understand biological materials and the mechanisms that are responsible for their functions, e.g., Sacks and Sun (2003), Meyers et al. (2008), Ji and Gao, 2010 and Chen et al. (2012). As the principles underlying the properties of biological materials become clarified, they can be applied to the development of new materials. Two recent examples include a bioinspired glass, produced by Chintapalli et al. (2014), which mimics natural designs to display exceptional toughness, and freeze-cast bioinspired aniso- tropic ceramic scaffolds produced by Porter et al. (2012) as a refine- ment of a synthesis method developed by Deville et al. (2006) and Munch et al. (2008). Unfortunately, there are not too many current examples of successful bioinspired structural materials and processing them can be extremely complex (Wegst et al., 2015). However, advancements in manufacturing are opening new and exciting oppor- tunities, and the development of a bioinspired, synthetic flexible armor is a goal worth pursuing. With regards to natural dermal armor, fish scales are a common example and have been the subject of much research, particularly over the past decade. They are an intriguing topic because they have provided effective protection to fish for eons; some armored fish have existed prior to the dinosaurs, which came into existence 225 million http://dx.doi.org/10.1016/j.jmbbm.2016.10.001 Received 18 May 2016; Received in revised form 1 October 2016; Accepted 3 October 2016 ⁎ Corresponding author at: Materials Science and Engineering Program, University of California, San Diego, CA 92093, USA. E-mail address: mameyers@ucsd.edu (M.A. Meyers). Journal of the mechanical behavior of biomedical materials 73 (2017) 1–16 Available online 04 October 2016 1751-6161/ © 2016 Elsevier Ltd. All rights reserved. MARK