Nacre, or mother of pearl as it is more commonly known, is a biomaterial derived from the inner part of the common mollusk shell. The material has high strength and toughness and is lightweight. The major reason nacre has such excellent properties can be attributed to its highly organized hierarchical structure as well as the self-sustaining properties of its organic matrix. These factors make nacre abalone an ideal biomaterial, whose properties will be useful in developing biomimetic technologies [1
Nacre is composed of inorganic as well as organic layers. The inorganic layer is made up of calcium carbonate (CaCO
), referred to as aragonite tablets, which are arranged in parallel to each other and in an overlapping pattern, which from the mesoscale appears as a columnar stack [2
]. The aragonite tablets have high stiffness and low toughness [7
]. However, the disadvantage of merely considering aragonite alone or as dry nacre, as it is referred to, is that such material fails in a brittle fashion [2
]. Interspersed between the organic and inorganic layer is the intra-lamellar protein chitin, which from a nanoscale view, is visibly entwined within and around the aragonite platelets [2
]. The organic layer is composed mainly of various proteins and glycoproteins that are randomly interspersed. This organic layer is, in part, responsible for generating the inorganic aragonite structure into the overlapping stacks via mineral bridges and growth enhancers (as well as through inhibition) [15
]. This layer also possesses elastic properties that, when impinged upon, distributes the deformation along the membrane and reinforces the entirety of nacre in resisting crack formation [6
]. Although nacre is composed of only 5% by organic matrix weight (the remaining 95% being that of the inorganic matrix), it follows that the organic matrix must be in part responsible for nacre’s unique strength, flexibility, and toughness, and, thus, deserves further attention [18
Although the mechanical properties of the organic matrix are not fully understood, the basic structure and protein-specific properties are well-known. Several proteins make up the organic matrix of nacre abalone: Perlucin, Lustrin A, Perlwapin, Perlinhibin, and Perlustrin. Perlinhibin and Perlwapin are both inhibitory proteins, whose role is mainly to control unrestrained biomineralization within nacre. It is considered that they are playing a minor role with regards to the specific mechanical properties sought in this study [20
]. Furthermore, there are only trace amounts of Perlinhibin per shell, supporting its role in contributing to nacre’s overall mechanical strength being minor [4
]. The effects of Perlustrin are not unlike the process of bone growth. However, this is also a minor component of the organic matrix, as it is only approximately 10
]. Lustrin A is an abundant multi-domain protein, which consists of both proline- and cysteine-rich domains [23
]. Additionally, Lustrin A has the largest discrete structural domain of 275 residues, of which 250 are either glycine (G) or serine (S). Shen et al. [24
] and Zhang et al. [15
] suggested that this additional domain has a high elastic property, which could prevent the fracture and separation within the mineral phase. Although Perlucin’s function is not yet fully understood, it also has a clear role in helping with biomineralization as well as being a promoter of calcium carbonate nucleation [22
]. Therefore, it is clear that the organic matrix of nacre has two main proteineous components, Perlucin and Lustrin A, which are responsible for its pertinent mechanical properties. In the present work, a combination of both components was used in the MD simulations of the organic matrix.
MD simulations are capable of gaining insight into the deformation mechanisms from an atomistic-scale. Moreover, influences of the mechanical response of the organic matrix in biological minerals [19
], as well as the role of the minerals to the proteins, have been investigated [27
] using MD simulations. Furthermore, Atomic Force Microscopy (AFM) and inverse FE simulations were used to determine the elastic modulus of the organic matrix [28
]. In the literature, due to various experimental techniques and computational methods, shear modulus of the organic matrix of nacre varies between 150 Mpa and 15 GPa [1
]. Therefore, these studies evidently estimated a wide range of values of shear modulus.
In the past few decades, numerous studies have been done, including experimental, theoretical, and computational approaches, to unveil the mystery of the extraordinary mechanical properties of nacre. Special attention was given to the FE simulation techniques that measure the mechanical properties, as well as structural feature effects, such as the organic matrix properties [32
], tablet interlocking [33
], and surface waviness [11
]. Furthermore, there have been many attempts made to replicate the structural organization of nacre in artificial composites [36
]. The percentage of each of the constituents plays a significant role in determining the overall behavior of the nacre. A recent study on the interface strength of such composite materials has been performed to obtain optimal overall mechanical properties [40
In this work, the shear modulus was computed using the organic matrix between two slabs of aragonite tablets. The mechanical properties obtained from the proposed molecular-based models can be used in the continuum models, which do not require any empirical parameters. In the continuum models of nacre, the percentages of the constituents are varied while keeping an intact brick–mortar structural arrangement. Furthermore, finite element (FE) simulations were conducted to study the macroscale properties, and comparisons were made with previous works in the literature. This work will contribute towards an improved understanding of nacre mechanics during deformation. The quantitative analysis of the viscoelastic properties of the organic matrix presents an essential step towards the development of multiscale modeling approaches of nacre.
4. Conclusions and Future Work
The organic matrix of nacre abalone has two main constituents, Lustrin A and Perlucin. Here, it was found that the mechanical response is independent of the constituents themselves as well as the size of the organic matrix. The shear modulus of the organic matrix was obtained by performing the shearing of a bundle of organic polypeptide chains using MD simulations. The shear modulus value was computed to be in the range of 1.25–1.45 GPa. It is an essential mechanical property that can be used to perform complex FE simulations of nacre models, which may not be otherwise feasible in the laboratory-setting.
Moreover, the results obtained from the FE simulations of nacre were found to be in good agreement with the experiment. By varying the volume fractions of the constituents of the nacre, it was found that a higher toughness in the synthetic nacre-like composite can be achieved by increasing the volume fraction of the organic matrix. However, it would come at the cost of a decrease in yield strength. It was also found that comparatively lower elastic modulus of the organic matrix would result in higher optimum toughness of the nacre-like composite for 10–20% of the volume fraction of the organic matrix. For a high value of elastic modulus of the organic matrix (>11 GPa), the toughness value is expected not to vary much in the 0–30% volume fraction range of the organic matrix. Therefore, if a composite with high toughness is to be designed, an organic matrix of low elastic modulus, as well as an optimum volume fraction, must be selected.
This study is an advancement towards a physical structure-based model for nacre. The final aim of the ongoing effort in the field of nacre mechanics is to link the nanoscale structure mechanics of nacre to its macroscopic properties. A correct model of the nacre structural architecture and the use of best-estimated properties gives us valuable insights in order to design new nacre-inspired high-performance materials. As the current model does not reflect the plastic regime of nacre, improvements can be made by incorporating more features such as nano-asperities and the mineral bridges in the model. This can be achieved by stochastically placing the mineral bridges and nano-asperities in the inter-lamellar layers. Hence, a better model will help us to establish a more reliable relationship between the properties of the constituents of nacre to the macro properties of the composite, such as toughness and strength.