applied sciences Article Microstructure and Mechanical Properties of the Dactylopodites of the Chinese Mitten Crab (Eriocheir sinensis) Ying Wang 1, Xiujuan Li 1,* ID , Jianqiao Li 1 and Feng Qiu 2 ID 1 Key Laboratory of Bionic Engineering, Ministry of Education, Jilin University, Changchun 130025, China; [email protected] (Y.W.); [email protected] (J.L.) 2 Key Laboratory of Automobile Materials, Ministry of Education, Department of Materials Science and Engineering, Jilin University, Changchun 130025, China; [email protected] * Correspondence: [email protected]; Tel.:+86-431-8509-5760 Received: 1 April 2018; Accepted: 21 April 2018; Published: 26 April 2018 Abstract: The dactylopodites of the Chinese mitten crab (Eriocheir sinensis) have evolved extraordinary resistance to wear and impact loading after direct contact with rough surfaces or clashing with hard materials. In this study, the microstructure, components, and mechanical properties of the dactylopodites of the Chinese mitten crab were investigated. Images from a scanning electron microscope show that the dactylopodites’ exoskeleton was multilayered, with an epicuticle, exocuticle, and endocuticle. Cross sections and longitudinal sections of the endocuticle revealed a Bouligand structure, which contributes to the dactylopodites’ mechanical properties. The main organic constituents of the exoskeleton were chitin and protein, and the major inorganic compound was CaCO3, crystallized as calcite. Dry and wet dactylopodites were brittle and ductile, respectively, characteristics that are closely related to their mechanical structure and composition. The findings of this study can be a reference for the bionic design of strong and durable structural materials. Keywords: dactylopodite; microstructure; composition; mechanical properties 1. Introduction After hundreds of millions of years of evolution, the functional properties of parts of organisms tend to have optimal structural and material characteristics, as well as excellent adaptability and longevity [1,2]. The strong exoskeletons of decapod crustaceans such as crabs and lobsters are an adaption to harsh environments [3] and play important roles in supporting the body, resisting impacts from external forces, providing protection from external erosion, predators, and disease, and preventing the evaporation of water [4–6]. The Chinese mitten crab (Eriocheir sinensis) is a widespread freshwater crustacean. It has an impressive ability to move on hard and deformable terrain, such as mire. Due to their unique strong shell and means of locomotion, Chinese mitten crabs are able to adapt to a variety of complex terrains, a characteristic that has attracted widespread attention from researchers [7–10]. Most studies of crustaceans’ hard shells have focused on pincers and other parts [3,8], whereas studies of pereiopods have focused mostly on gait, kinematics, and energy conversion [11,12]. However, in adaptation to complex terrains, in addition to the Chinese mitten crabs’ means of locomotion, their dactylopodites, which come into contact with the terrain during movement, also play an important role [13]. As the end parts of the pereiopods, dactylopodites are in direct contact with rough ground and clash with hard materials such as gravel. They possess excellent mechanical properties and unique biological characteristics, such as shape, structure, and composition. Studies on the dactylopodites of the Chinese mitten crab are scarce. Appl. Sci. 2018, 8, 674; doi:10.3390/app8050674 www.mdpi.com/journal/applsci Appl. Sci. 2018, 8, x FOR PEER REVIEW 2 of 12 Appl. Sci. 2018, 8, 674 2 of 13 unique biological characteristics, such as shape, structure, and composition. Studies on the dactylopodites of the Chinese mitten crab are scarce. ThisThis paper paper presents presents an an analysis analysis of of the the shape, shape, structure, structure, components, components, and and mechanical mechanical properties properties of theof dactylopoditesthe dactylopodites of Chinese of Chinese mitten crabs,mitten helping crabs, ushelping to further us to understand further understand the excellent the adaptability excellent andadaptability athletic ability and athletic of these ability crustaceans. of these Thecrustaceans. research willThe provideresearch newwill bionicprovide design new bionic methods design for strongmethod ands for durable strong structural and durable materials, structural which materials, would be which applied would to the be development applied to the and development preparation ofand walking preparation mechanism of walking design with mechanism optimal performance, design with optimization optimal performance, of environmental optimization adaptation, of andenvironmental minimum energy adaptation, consumption and minimum on soft energy road terrain. consumption on soft road terrain. 2.2. ExperimentalExperimental MethodsMethods 2.1.2.1.Preparation Preparation ofof Dactylopodite Dactylopodite Samples Samples from from Chinese Chinese Mitten Mitten Crabs Crabs ChineseChinese mittenmitten crabscrabs (Figure(Figure1 )1) were were purchased purchased from from an an aquatic aquatic products products market market in in Changchun, Changchun, China.China. SevenSeven crabscrabs werewere selected,selected, ofof whichwhich fourfour werewere malemale (two(two werewere larger,larger, twotwo werewere smaller)smaller) andand × threethree were were female. female. TableTable1 1shows shows the the crabs’ crabs’ shell shell sizes sizes (length (length ×width) width) and and their their weights. weights. Studies Studies havehave shownshown thatthat thethe secondsecond andand thirdthird pairspairs ofof paraeiopodsparaeiopods areare importantimportant forfor movement;movement; therefore,therefore, thethe secondsecond andand thirdthird paraeiopods paraeiopods werewere selectedselected asas thethe subjects subjects of of study. study. TheThe dactylopoditesdactylopodites werewere removedremoved from from the the propodite–dactylopodite propodite–dactylopodite joints joints (as (as shown shown in in Figure Figure1) 1) for for experimental experimental use. use. FigureFigure 1. 1.Chinese Chinese mitten mitten crab. crab. TableTable 1. 1.Parameters Parameters of of Chinese Chinese mitten mitten crab crab specimens. specimens. Dimension Specimen Dimension Weight (g) Specimen Length (mm) Width (mm) Weight (g) 1 (male) Length71.21 (mm) Width64.03 (mm) 167 2 (male)1 (male) 68.42 71.21 64.0362.38 167 174 3 (male)2 (male) 57.46 68.42 62.3852.73 174 92 3 (male) 57.46 52.73 92 4 (male) 55.74 51.61 81 4 (male) 55.74 51.61 81 5 (female)5 (female) 54.81 54.81 51.32 77 77 6 (female)6 (female) 56.12 56.12 51.18 75 75 7 (female)7 (female) 51.50 51.50 49.22 63 63 MeanMean ± SD± SD59.32 59.32 ± 7.44± 7.44 54.64 ± ± 5.96 104.14104.14± 46.18 ± 46.18 2.2.2.2.Biological Biological CharacteristicsCharacteristics ofof Chinese Chinese Mitten Mitten Crabs Crabs 2.2.1.2.2.1.Microstructure Microstructure of of the the Dactylopodites Dactylopodites ToTo obtain obtain high-quality high-quality microstructure microstructure images, images, the surfacethe surface of the of specimens the specimens to be observed to be observed should beshould flat, and be flat, the structureand the structure should be should clear. Whenbe clear. samples When weresamples taken, were a Cartesian taken, a Cartesian coordinate coordinate grid was establishedgrid was established with the dactylopodite with the dactylopodite as the origin as (Figure the origin2). The (Figure dactylopodites 2). The dactylopodites were divided intowere fivedivided segments into five along segments the x-axis along with the a surgical x-axis with blade, a surgical and the filmblade, and and muscles the film inside and weremuscles removed. inside Thewere segments removed. were The marked segments 1, 2, were 3, 4, andmarked 5 in order1, 2, 3, of 4 decreasing, and 5 in cross-sectionalorder of decreasing diameter, cross left-sectional to dry naturally,diameter, and left thento dry sprayed naturally, gold. a Thend then structures sprayed of the gold. cross The sections structures were of observed the cross with sections a scanning were Appl. Sci. 2018, 8, x FOR PEER REVIEW 3 of 12 Appl. Sci. 2018, 8, 674 3 of 13 observed with a scanning electron microscope (SEM) (Zeiss EVO 18, Cambridge, England). When electronobserving microscope the longitudinal (SEM) (Zeiss section EVO 18,structure, Cambridge, the UK).segments When were observing cut thealong longitudinal the axis sectionof the structure,dactylopod theite. segments were cut along the axis of the dactylopodite. FigureFigure 2.2. CoordinateCoordinate gridgrid withwith ChineseChinese mittenmitten crabcrab dactylopodites.dactylopodites. 2.2.2.2.2.2. CompositionComposition ofof thethe DactylopoditesDactylopodites X-rayX-ray diffractiondiffraction (XRD)(XRD) (Model(Model D/MaxD/Max 2500PC2500PC Rigaku,Rigaku, Tokyo,Tokyo, Japan)Japan) andand FourierFourier transformtransform infraredinfrared (FTIR)(FTIR) analyses were were conducted conducted to to identify identify the the components components of the of theChinese Chinese mitten mitten crab crabdactylopodites dactylopodites.. SamplesSamples forfor XRDXRD werewere preparedprepared byby breakingbreaking fragmentsfragments fromfrom thethe dactylopodites, dactylopodites, removingremoving thethe musclemuscle tissue,tissue, dryingdrying thethe fragmentsfragments naturally,naturally, andand crushingcrushing andand grindinggrinding themthem intointo aa powderpowder inin aa mortarmortar untiluntil thethe particleparticle sizesize