Nanomechanical-testing of individual of collagen fibrils from an osteogenesis imperfecta mouse model

Abstract

Collagen is one of the most abundant proteins and constitutes 25% of the total protein mass of the human body. In general, collagen is the building block of most, if not all, biological tissues and contributes to a large extend to the mechanical properties of these tissues. Bone, skin, tendon, ligaments, blood vessels or the cornea of the eye consist mainly of collagen and thereby credit their mechanical properties to collagen. The versatility of the mechanical properties of the different connective tissues is attributed by their hierarchical architecture and the composition of the basic building blocks. Each tissue adapts to their demand. This is achieved through the combination of collagen with different elements. For example, in bone, collagen is combined with minerals which results in increased fracture resistance. In tendons, collagen is assembled with elastin and thereby a high resilience and toughness is achieved. Due to the fact that collagen provides the structural stability, collagen is one of the most interesting and important macromolecules within the human body. Collageneous tissues, for example tendons and ligaments, possess a hierarchical architecture. At the lowest length scale level, collagen molecules consist of three alpha-polypeptide chains. In tendons, collagen molecules assemble into fibrils which further assemble into fibres, the fibres create fascicles which then constitute the tendon. Mechanical tests of these tissues have been the focus of a number of studies. However, only a few studies generated data on the length scale of fibril, which can be accomplished via atomic force microscopy indentation and tensile tests. This project focuses on mechanical tests of individual collagen fibrils. Two types of mechanical tests were performed; nano-indentation and nano-tensile tests. Both tests were established with an atomic force microscope (AFM). The main aim of this thesis was to improve and optimize an existing protocol for tensile testing, which enabled fracture tests of individual collagen fibrils. The sample preparation for tensile testing was also optimized, resulting in shorter preparation times. In the next step the improved tests where applied to individual collagen fibrils. The second part of the thesis was to employ these tests and examine the influence of structural alterations at the molecular level of collagen on the mechanical properties at the collagen fibril level, by examining two different collagen types. Collagen fibrils from mice with osteogenesis imperfecta (OI) and wild type (WT) mice. OI originates from mutations within the collagen coding genes and results in an alteration of the collagen molecule. To study the effects of OI a osteogenesis imperfecta mouse (OIM) model was used. The collagen fibrils were obtained from mouse-tail tendons of two different, one OIM and one WT, five month old female mice. During this thesis two different tensile tests were performed. The first test engaged with elongations in the physiological strain range up to 10 %, whereas in the second test the fibrils were elongated until fracture. Nano-scale indentation tests gave information about the radial mechanical properties of the fibrils. Through the combination of the tests different parameters where obtained, such as the radial and longitudinal elastic modulus, ultimate strength, ultimate strain and the energy dissipation. All the values were statistically analyzed and the results are visualized. Further comparisons between the generated values and literature were performed.