Analyzing Methods For Nanoindentation And Nanomechanical Properties Of The Cuticle Of Dung Beetle Copris Ochus Motschulsky

More and more attentions have been paid into the micro-or nano-mechanical properties of outer and inner of the biomaterials with the rapid development of bionics, especially the biomaterials and biomimetic materials. So, the investigation into the micro-or nano-mechanical properties of biomaterial has been the requirements of the development of materials science, bioscience, and their interdisciplinary field -bionic science and technology. Biomechanics is a part of quantified physiology and, in biomechanics; the engineering theory and method are applied in biosystems. The biological configurations and morphologies, the functions of organs and microstructure of materials and tissues are considered mainly and then the logical physical models are constituted. The mechanical properties of tissues or materials are measured and their constitutive relationships are established. The differential equation or integral equation describing the constitutive relationship of biomaterials can be deduced. Linked with the working conditions of an organ, the true boundary conditions can be acquired and the boundary-value problems are solved by using analytical and/or numerical method. The rationality and validity of a solution can be verified through physiological experiments. The results of biomechanics investigation of biomaterials should be useful and important to practical applications in engineering, especially in the development of biomimetic materials. Lamellicornia beetles have been extensively studied in both entomology and bionics. The cuticle surfaces of such organs as clypeus, pronotum and elytra of all beetles have non-smooth (textured) geometry and hydrophobic function. Those characters are in favor of anti-adhesion of the beetles against soil and water, such as dung beetles and have been applied to design agricultural soil-engaging components. The cuticle of Lamellicornia beetles is in the micro-or nano-scale along the vertical direction. So, its size effect is an important problem in research of its mechanical properties. It is difficult for conventionally measuring techniques and testing apparatuses for macromechanical properties of materials to directly measure the mechanical properties of such biomaterials as beetle cuticle. The appearance of nanoindenter solves the measuring problem of mechanical properties of biomaterials in nano-scale. Nanoindentation technique has provided a powerful tool for examining the nano-indentation properties of the biological surface materials including Lamellicornia beetles in micro-or nano-scale. Different structures of the cuticle materials of Lamellicornia beetles should present different properties. There will be an intrinsic relationship among nanoindentation, material structure and organ functions. The nanoindentation properties of the laminative composite structural biomaterial need to be systemically studied. In this work, the dung beetle Copris ochus Motschulsky was studied. Considering the constitutional features of the cuticle of the dung beetle, which mainly consists of the composite material of chitin-protein, the ICP-ACES instrument was used for determining metal elements and their contents and components. It was found that the cuticle of the dung beetle of almost all organs contains such metal elements as iron (Fe), zinc (Zn) and manganese (Mn) and non-metal element phosphorus (P). The concentration of Fe, Zn and P were found the much higher in the clypeus cuticle than that in the leg cuticle, while, the concentration of Mn was approximately equal to each other. FTIR (Fourier transform infrared spectrum) analysis of the metal ionic complexes demonstrated that the adsorption of these metal ions was mainly due to chemisorptions. The component of chitin in the cuticle was discussed in the present work. In order to examine the effects of water content on the nanoindentation properties of the cuticle, the relation of the loss of water of the cuticle sample with its duration in air at room temperature after cutting from living body was determined. It was found that the mass of sample decreased by 8% after the duration of 1 hour and was towards about invariable mass value after duration of 25 hours. These will contribute to comparing investigation of the cuticle in vivo with in vitro. The creeping law and relaxation of the dung beetle's cuticle happening in nanoindentation testing procedure were investigated in detail. It was demonstrated that there existed the creep and relaxation phenomenon of the cuticle material during indenting. A minus load found appearing near the end of unloading segment of the force-displacement curve and reached 2 μN when the tip apart from thedung beetle's cuticle sample, showing that the holding time and the loading rate for nanoindentation tests are important factors impacting the accuracy of such indentation property as the reduced modulus ( E r) and the harness ( H ) of the dung beetle cuticle. Under a constant load, the impact of the creeping phenomenon on the measuring values of E r and H can be reduced considerably with prolong loading time and holding time. The length of the loading time will have little impact on the measuring results as long as holding time enough. The creeping law of the cuticle during the holding procedure at the maximum load can be regressed by a general logarithmic equation. The threshold values of the holding time and loading rate of the dung beetle cuticle were acquired based on the nanoindentation experiments. Under load of 500 μN , there existed a threshold holding time and threshold loading rate of 20 sand 53 μN s, respectively. The elastic modulus and nano-hardness of the several organs'cuticle of the dung beetle were systematically examined. Nanomechanical properties of the several organs'cuticle were compared and preliminary analyses of that mechanism were conducted from view of life habit and behavior of the male and female dung beetles. The result showed that the elastic modulus of the cuticle was in between the elastic modulus of protein and the elastic modulus of chitin, but the mechanical property of the cuticle was better than that of protein and chitin, because the cuticle can avoid both brittle failure like some harder brittle materials and premature yield damage like some softer materials. It was obvious that the nanomechanical properties of the cuticle were closely related with both its material nature and its structure. The distribution and changes of the nanomechanical properties of the dung beetle's cuticle across section were examined. The results showed that different cross-linking level and hydrate conditions of the material in different layers led to a larger elastic modulus and a larger nano-hardness of the outer cuticle than that of inner cuticle. The elastic modulus and nano-hardness of the pronotum cuticle of the female dung beetle were larger than those of the male one. The thickness of the clypeus cuticle changes from 0.1mm to 0.2mm from the tip to the head horn. The elastic modulus of the clypeus material increased from the tip of clypeus to near head horn, but the nano-hardness had little variation. From the inner to the outer of the cuticle, the elastic modulus and nano-hardness of the cuticle of the claw tipwere very high. The elastic modulus and nano-hardness of the outer layer of the cuticle were 342% and 587% of those of the inner layer of the cuticle, respectively. The results would provide better clue to design biomimetically nano-multilayered composite materials. The nanoindentation properties of dung beetle Copris ochus Motschulsky were compared to two different species of beetles, Geotrupes stercorarius Linnaeus and Holotrichia sichotana Brenske. It was found from the comparative analysis that the different properties of materials of the three beetles'cuticle have been evolved successfully because of their differences in living environment and life habit. The nanoindentation properties of the fumer cuticle of the hindleg of dung beetle Copris ochus Motschulsky were compared to two different species of animals'biomaterials, cattle horn and mollusc shell. It was demonstrated that the difference of the nanoindentation properties of the three biomaterials was closely related to their respective structure features. The stress-strain relationship of the cuticle materials of varied organs of the dung beetle was investigated. It was found that the stress decreased with the testing time, suggesting that the interacting force between tip and sample material changed towards slowly and then reached to a homeostasis state. As a result, the strain changed continuously until an equilibrium point due to the viscoelastic phenomena. The value of stress were higher and hardly changed when the holding time was increased from 10 to 100 sec and decreased considerably toward a slow change when the holding time was over 130 sec. The change of stress versus time was weak and the value of stress was affected by the holding time. Under the varied holding time, the stress–strain curve changed toward an identical property. The more the large holding time was, the more flat the stress-strain curve was. The viscoelastic properties of the femur cuticle of foreleg of the dung beetle were systematically analyzed. The results indicated that creep exponent of the material was arranged in between 12.75 and 16.33 for varied holding time. A unidimensional constitutive equation was established for the femur cuticle of foreleg of the dung beetle based on its analysis of viscoelastic properties. The Burgers model and Fung-Kelvin viscoelastic model were used for analyzing its relaxation property. It was found by fitting experimental results that the viscoelastic property of the fumer cuticle was closed to theoretical expression of Fung-Kelvin viscoelastic model. Then, a differential equation expressing the viscoelastic model was established.