Nanostructure and Nanomechanics of Collagen Self-Assemblies

Abstract

This thesis outlines the development and application of methods for characterization of nanostructured biomaterials, specifically, collagen. Collagen is the most abundant protein in the human body and plays an important structural role. Therefore, research on the structural and mechanical properties of this protein is beneficial for disease treatment and health improvement, including the development of new materials for bioengineering. These newly developed techniques and methods demonstrated their merit in this research. They can potentially find the way to applications in broader areas, e.g. bio-engineering, medical science, nano technology and industry. A novel method called minimum indentation, which I developed, extracts the mechanical properties of superficial layers or nanometer scale objects in a sample with high precision. This is a true surface measurement with a detection depth of less than 10 nanometers. During sample testing, an atomic force microscope (AFM) tip jumps to contact with the sample surface when the tip-sample attractive force gradient increases and exceeds the cantilever spring constant. The jump-to-contact distance is determined by the sample mechanical properties and tip-sample surface adhesion. Hence proper interpretation of the jump-to-contact phenomenon yields sample surface mechanical properties. I present different models to suit for hydrophilic and hydrophobic surfaces. The minimum indentation method requires a different treatment in the presence of strong capillary effects. I develop this in the context of a study of segment-longspacing collagen crystallites (SLS). A combination of morphological and nanomechanical data yields a more complete picture of SLS nanostructure, including proposed growth mechanism and internal structure. Nanoidentation and persistence length are different methods to access complementary mechanical information. I apply them to investigate the fibrillogenosis of type I and type II collagens, demonstrating distinct growth stages and structure transition phenomena. Type I collagen fibrils are much longer than type II collagen fibrils and present different internal structures, with consequently different mechanical properties.