Influence of interfacial interactions on deformation mechanism and interface viscosity in α-chitin–calcite interfaces
Graphical abstract
Introduction
Interfaces significantly dominate mechanical properties of biomaterials [1]. Biomaterials such as marine crustacean exoskeleton, bone, and nacre are composed of a hierarchy of interfaces with an organic phase (e.g. chitin (CHI) or tropocollagen (TC)) and a mineral phase (e.g. calcite (CAL) or hydroxyapatite (HAP)) arranged hierarchically at the structural length scale of nm.
Structural studies of such biological materials have shown that at mesoscale (∼100 nm to few μm), the mineral crystals are preferentially aligned along the length of organic phase polypeptide molecules in a hierarchical (e.g. staggered or Bouligand pattern) arrangement as shown in Fig. 1. Studies also show that such staggered hierarchical structure leads to a unique combination of toughness and strength properties. Interfaces play a significant role in the stress transfer resulting in improved stiffness and strength of such material systems. However, how exactly the change in interfacial chemical configuration leads to change in interface mechanical properties need further investigation. Current study is focusing on the calcite–chitin interactions, which widely exist in exoskeleton materials [2] but have not been studied in detail before. With this view the present work focuses on quantifying interface stress as a function of interface chemical changes (water, organic molecules) in selected chitin–calcite (CHI–CAL) interfaces using classical non-equilibrium molecular dynamics (NEMD) simulations and steered molecular dynamics (SMD) simulations. One important step in understanding the relationship between interfaces and material properties is the knowledge of the fundamental mechanisms guiding stress transfer across the organic–inorganic interfaces. Therefore, this work characterizes the interaction between organic–inorganic interfaces as a function of mechanical straining. The present work is based on the use of NEMD and SMD simulations and therefore does not consider the effect of factors such as quasi-static loading conditions, structural gradients, presence of other organics and hydration, longer length-scales, etc that cannot be addressed using molecular simulations due to fundamental limitations of such. However, the present work presents mechanistic insights behind interface behavior that are not accessible using experiments. The presented results should be considered in light of such interactions.
Experimental techniques such as electron microscopy, X-ray diffraction, indentation, and tensile (or compressive) testing have previously been performed to characterize the structural and mechanical properties (including hardness, modulus, elastic–plastic deformation, etc.) of chitin based materials such as cuticles, exoskeletons from prawn, crab and lobsters (Guille et al. [3], [4], Roer and Dillaman [5], Raabe et al. [6], [7], [8], Al-Sawalmih et al. [2], Joffe et al. [9], Vincent and Wegst [10], Sachs et al. [11], [12], Fabritius et al. [13]). Previous analytical work includes studies based on the tension shear chain model developed by Gao and coworkers [14], [15]. Atomistic simulations have been widely used to investigate the mechanical behavior of biomaterials, especially bone, with the goal of understanding the role of organic phases (i.e. TC in bone) and inorganic phases (i.e. HAP in bone) [16], [17], [18], [19]. However, atomistic studies on chitin based materials have been limited. Jin et al. [20] studied mechanical properties of the chitin–protein interface by performing equilibrium molecular dynamics (EMD) simulations. Nikolov et al. [21], [22] reported the stiffness tensor of single crystalline chitin by performing ab initio simulations. They predicted effective properties of chitin–protein fibers with a mean-field homogenization method. Molecular mechanics-based studies on the interfacial interactions in a multiphase composite system have also been performed using SMD to study the interfaces in collagen based materials by peeling off collagen filaments from the HAP substrates [23], [24], [25].
Despite such significant advances mentioned above, molecular-simulation-based studies aimed at explicitly calculating interface stress in biological and polymeric systems have been limited. Lindal and Edholm [26] introduced a computational method to decompose the net surface tension in polymeric membrane systems simulated using NEMD. A viscoplastic model has also been used by Frankland and Harik [27] to characterize the sliding process between carbon nanotubes and epoxy based polymer matrix. Buehler [28] studied the homogeneous shear and slip pulses process between collagen fibrils using MD simulations based on a one-dimensional model of fracture. Dubey and Tomar [29] performed 3-D ab initio MD (AIMD) simulations to understand atomic interactions in TC–HAP interfaces under tensile loading. However, AIMD simulations to date have limited length scales. The framework developed in this work to understand interface deformation mechanics using SMD is based on a combination of frameworks reported by Katti and coworkers [23], [24], [25], Lindal and Edholm [26], and Frankland and Harik [27]. Similar molecular mechanics framework has been incorporated with the continuum micromechanics to top-down access to interface viscosity of bone material systems in the earlier studies [30], [31], [32], [33]. NEMD simulations to evaluate interface strength are based on the pressure profile calculation method, [26]. The shear interactions between the inorganic (CAL) and organic (CHI) phases are characterized using SMD simulations performed to initiate the interfacial sliding process. A viscoplastic model is then used to calculate the shear yield stress and shear viscosity of the interfacial material systems.
Section snippets
Method and framework
All NEMD and SMD supercells analyzed in the present work had organic fibers (CHI molecules) aligned parallel to the surface of inorganic crystals (CAL molecules).
The c-axis of the trigonal CAL crystals is along the z-axis in the system. According to available previous studies [34], [35], [36], this configuration corresponds to the simplified unit cell structure shown in Fig. 2. 3-D NEMD and SMD simulations were performed on the interfaces modeled by embedding the organic molecular systems (i.e.
Interface effect on the overall mechanical deformation behavior of the analyzed CHI–CAL system
It has been shown that the failure of organic–inorganic bio-interfaces is a result of combined tensile and shear loading [17]. Visual deformation analyses have revealed that calcite and chitin portions individually develop different levels of individual strains corresponding to the uniformly imposed external strain. These factors together with the observation of ductile failure combined with visual deformation analyses, led us to believe that shear stress plays an important role in the failure
Conclusions
Chitin-based interfacial supercells were analyzed for their mechanical properties using two different variants of classical MD simulation method. The plastic shear deformation behavior was characterized with a well known visco-plastic interfacial sliding model. The conclusions are summarized below.
- (1)
Inorganic phases in the material systems carry the uniaxial tensile loading while the organic phases carry primarily the shear loading.
- (2)
Organic interfacial systems exhibit plastic shear deformation,
Disclosures
There is no conflict of interest with any party reported in this work or otherwise.
Acknowledgement
This work was supported by National Science Foundation Grant CMMI1131112.
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