Mechanical characterization of collagen at small length scales using computational and experimental methods

Abstract

Collagen is the most abundant proteins in soft and hard tissues of the mammals, and collagen of several types form different hierarchy levels of these tissues by assembling at various length scales. Due to lack of availability of complete information on the mechanical characteristics of the individual collagen molecule as well as the assemblies of collagen molecules, there is presently a dearth of knowledge pertaining to tissue mechanics and disease progression which is primarily controlled by the structure of tissues at molecular length scales. These tissues are heterogeneous, non-linear, hydrated, viscoelastic, and their biochemical characterization is possible through presently available instrumentation. Since hard and soft tissues are load-bearing, the characterization of biochemical properties alone is insufficient for assessing the healthy state and functionality of the tissues. Therefore, the information on the mechanical characteristics of collagen at small length scales is very important. Recent developments in the computing power and the availability of advanced instrumentation have facilitated the mechanical characterization of collagen at small length scales using computational and experimental methods. Atomistic modeling is a very useful computational tool which is capable of determining the mechanical characteristics of different materials at small length scales. However, due to unavailability of all force-field parameters for various biomolecules and lack of sufficient computing power, detailed information regarding the mechanical characteristics of collagen at small length scales could not be determined through atomistic modeling. The advent of nanoindentation technique has provided a method for estimating quasistatic and time-dependent mechanical properties of a material, on a size scale compatible with tissue dimensions in the controlled physiological environments. However, nanoindentation theory and instrumentation were originally developed for hard, elastic-plastic materials, and a limited nanoindentation work has been performed in the domain of biological materials. Though most nanoindentation studies of biological materials have adopted analytical and experimental methods originally derived for elastic-plastic materials, the use of these methods for biological materials should only be made by understanding the methodology adopted for the estimation of material property from Load-Displacement (L-D) data as well as various factors effecting the nanoindentation measurement. The goal of the thesis is to characterize the mechanical properties of collagen at small length scales using computational and experimental methods. In the present work, atomistic modeling through molecular dynamics (MD) method has been used to investigate the mechanical behaviour of collagen-like-peptides. The mechanical behavior of individual collagen molecule as well as assemblies of collagen molecules is quantified by measuring the strains along longitudinal and transverse directions when they are subjected to various hydrostatic pressure magnitudes. The measurements of compressibility, bulk modulus, linear compressibility and linear elastic modulus for individual collagen molecule as well as assemblies of collagen molecules (in the form of micro-fibrils) have been made by using atomistic model and incorporating continuum model using linear elasticity theory. These measurements showed that individual collagen molecule as well as assemblies of collagen molecules have almost similar mechanical response, and they behave as transversely isotropic materials, and show higher stiffness at larger strain magnitudes. For experimental work, the assemblies of collagen molecules have been obtained from demineralization of cortical region of the bovine bone. Here, a well defined protocol has been used to prepare samples surface with roughness (RMS) of the order of few nanometers with the use of other complementary techniques like Atomic force microscope (AFM), Scanning electron microscope (SEM), Energy dispersive X-ray spectroscopy (EDX). Instrumented indentation has been used to determine the mechanical characteristics of the assemblies of collagen molecules as well as their dependence on loading rate, by performing quasistatic and dynamic nanoindentation respectively. The modulus magnitudes of assemblies of collagen molecules quantified through dynamic indentation also highlighted that the mechanical response of the assemblies of collagen molecules is dependent on the loading rate. The modulus magnitudes for collagen at small length scales obtained by using both computational and experimental approach showed a good agreement.