The structure and performance of collagen biomaterials : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Engineering, Massey University, Palmerston North, New Zealand

Abstract

Type I collagen materials are used in a wide range of industrial applications. Some examples include leather for shoes and upholstery, acellular dermal matrix (ADM) materials for surgical applications, and bovine pericardium for the fabrication of heart valve replacements. The structure of these materials is based on a matrix of collagen fibrils, largely responsible for the physical properties and strength of the materials. How the collagen fibrils themselves contribute to the overall bulk properties of these materials is not fully understood. The first part of this work investigates a collagen structure defect in leather, known as looseness. Looseness occurs in around 5-10% of bovine leather, and is a result of the collagen fibril layers separating during processing from raw skin to leather. A greater understanding of why looseness develops in leather and a method of detecting looseness early in processing is needed to save tanners a significant amount on wasted processing time and costs. In addition, an environmentally safe method of disposing of defect and waste leather is sort after since the current method of disposing to landfill is causing environmental concern due to the possibility of chromium leaching from leather into the soil as it biodegrades. Synchrotron based small angle X-ray scattering (SAXS) revealed that loose leather has a more aligned and layered collagen fibril arrangement, meaning there is less fibril overlap, particularly in the grain-corium boundary region. This results in larger gaps in the internal structure of loose leather compared with tight. These gaps could be detected using ultrasonic imaging in partially processed pickle and wet-blue hides as well as leather. Incorporating an ultrasound system into the leather processing line could be a viable method for identifying hides deemed to develop looseness earlier in processing, and these could be diverted down a separate processing line or removed. Disposing of waste leather by first forming biochar prior to land fill proved to be an effective way of reducing chromium from leaching into the environment. XAS revealed that heating leather to temperatures above 600°C in the absence of oxygen formed a char where chromium was bound in the stable form of chromium carbide. The stability of this structure makes chromium less available to form the toxic hexavalent form in the environment and presents a possible alternative option for environmentally safe disposal of leather. The second part to this work looks at the correlation between collagen fibril structure in a range of biomaterials in relation to material strength. Leather, ADM and pericardium are three type I collagen based materials which rely on sufficient strength to carry out their industrial and medical applications. These three materials were studied to try and identify collagen fibril characteristics that relate to high material strength. SAXS on a range of leather samples from various species revealed that collagen fibril diameter had only a small influence over material strength in bovine leather, and no correlation to strength in leather from other species. Therefore it can be said that the influence of fibril orientation on leather strength takes precedence over that of fibril diameter. Fibril diameter, d-spacing and orientation were studied in pericardium using SAXS while simultaneously applying strain. It was revealed collagen materials undergo two distinct stages of deformation when strain is applied and incrementally increased. The first stage, at low strain, involves a re-orientation of fibrils to become more aligned. When strain is increased further, the fibrils themselves take up the strain, causing fibrils to stretch and decrease in diameter. The Poisson ratio of the collagen fibrils was calculated to be 2.1 ± 0.7. This high Poisson's ratio indicates the fibrils decrease in diameter at a faster rate than they elongate with strain, and as a result the volume of the fibrils decreases. This feature of collagen could help explain some of the unique behaviours and strength of collagen based materials and could be useful for optimizing industrial applications of collagen materials. ADM materials, derived from human, porcine and bovine skin was the third collagen material studied. SAXS revealed that each species of ADM material had a slightly different collagen fibril arrangement when viewing the samples perpendicular to the surface. Human ADM was highly isotropic in arrangement, porcine was largely anisotropic, and bovine was somewhere in between the two. Bovine has a more layered fibril arrangement edge on and was the strongest material, followed by human ADM, and porcine was significantly weaker. Bovine was also the most porous material of the three. The discovery of the variations in strength, porosity and fibril arrangement between the three types of ADM materials may help medical professionals select the most suitable material for specific surgical procedures and could lead to a greater number of successful surgeries taking place.