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
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Date
2016
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Massey University
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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.
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Keywords
Leather, Collagen, Structure, Biomedical materials, Hazardous wastes