Molecular Level Interfacial Mechanics in Biomaterials

A current important area of focus in the development of composite materials is structural composites based on biomimetic principles. An important aspect to focus on is the chemomechanics of the organic-inorganic interfaces and its correlation with overall mechanical behavior. This understanding is vital for selecting appropriate constituents, their size scales and their relative arrangements, which in turn is governed by the functional requirements of the composite materials. For example, 3 dimensional explicit atomistic failure analyses of model Tropocollagen (TC)-Hydroxyapatite (HAP) interfacial biomaterial (similar to material found in bone tissues) performed at the nanoscopic length scale have pointed out that maximizing the contact area between the TC and HAP phases result in higher interfacial strength as well as higher fracture strength. Analyses have also shown that high toughness and strain hardening behavior of such biomaterial is due to reconstitution of columbic interactions between TC and HAP surfaces during interfacial sliding due to mechanical deformation. In this work a review of recent interfacial biomaterial interfacial mechanics work is provided. Focus is on developing an understanding that could lead to a significant development in bioinspired composite materials for aerospace applications. I. Introduction significant focus of the current materials development and characterization is on developing bioinspired materials. Such materials are expected to follow the form and functionality of materials available in nature. At the same time such materials are expected to perform significantly better than the currently available nonbioinspired materials. One area of focus in this respect is the development of structural composites based on biomimetic principles. Such bioinspired structural composites are expected to have improved structural strength as well as improved fracture resistance. A binding principle underlying the composition of such composites is a combination of an organic phase with an inorganic phase. Interfacial interactions determined by the chemomechanics of the organic-inorganic interfaces affect overall mechanical behavior of such materials. An understanding of such interactions is vital for selecting appropriate constituents, their size scales, and their relative arrangements, which in turn is governed by the functional requirements of the composite materials. The concept of biomimetic structural materials is inherently linked with the concept of hierarchy. It can be argued that all materials are hierarchically structured as the change in length scale affects the mechanisms of deformation and damage. However, in biological materials this hierarchical organization is inherent to the design. Unlike engineered synthetic materials, the structural design and material property in biological materials are intricately coupled with each other for carrying out required functions optimally. The most prominent feature of such hierarchical design is the organic-inorganic interfacial interaction and the resulting biomaterial mechanics. To understand the importance of such interfacial interactions, next sections describe work done in the two most widely studied biological materials, bone and nacre.