On the tear resistance of skin.

Tear resistance is of vital importance in the various functions of skin, especially protection from predatorial attack. Here, we mechanistically quantify the extreme tear resistance of skin and identify the underlying structural features, which lead to its sophisticated failure mechanisms. We explain why it is virtually impossible to propagate a tear in rabbit skin, chosen as a model material for the dermis of vertebrates. We express the deformation in terms of four mechanisms of collagen fibril activity in skin under tensile loading that virtually eliminate the possibility of tearing in pre-notched samples: fibril straightening, fibril reorientation towards the tensile direction, elastic stretching and interfibrillar sliding, all of which contribute to the redistribution of the stresses at the notch tip

Probing the Mechanisms of Fibril Formation Using Lattice Models

Using exhaustive Monte Carlo simulations we study the kinetics and mechanism of fibril formation using lattice models as a function of temperature and the number of chains. While these models are, at best, caricatures of peptides, we show that a number of generic features thought to govern fibril assembly are present in the toy model. The monomer, which contains eight beads made from three letters (hydrophobic, polar, and charged), adopts a compact conformation in the native state. The kinetics of fibril assembly occurs in three distinct stages. In each stage there is a cascade of events that transforms the monomers and oligomers to ordered structures. In the first "burst" stage highly mobile oligomers of varying sizes form. The conversion to the aggregation-prone conformation occurs within the oligomers during the second stage. As time progresses, a dominant cluster emerges that contains a majority of the chains. In the final stage, the aggregation-prone conformation particles serve as a template onto which smaller oligomers or monomers can dock and undergo conversion to fibril structures. The overall time for growth in the latter stages is well described by the Lifshitz-Slyazov growth kinetics for crystallization from super-saturated solutions.Comment: 27 pages, 6 figure

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

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

Second-harmonic generation microscopy analysis reveals proteoglycan decorin is necessary for proper collagen organization in prostate.

Collagen remodeling occurs in many prostate pathologies; however, the underlying structural architecture in both normal and diseased prostatic tissues is largely unexplored. Here, we use second-harmonic generation (SHG) microscopy to specifically probe the role of the proteoglycan decorin (Dcn) on collagen assembly in a wild type (wt) and Dcn null mouse (Dcn  -    /    -  ). Dcn is required for proper organization of collagen fibrils as it regulates size by forming an arch-like structure at the end of the fibril. We have utilized SHG metrics based on emission directionality (forward-backward ratio) and relative conversion efficiency, which are both related to the SHG coherence length, and found more disordered fibril organization in the Dcn  -    /    -  . We have also used image analysis readouts based on entropy, multifractal dimension, and wavelet transforms to compare the collagen fibril/fiber architecture in the two models, where all these showed that the Dcn  -    /    -   prostate comprised smaller and more disorganized collagen structures. All these SHG metrics are consistent with decreased SHG phase matching in the Dcn  -    /    -   and are further consistent with ultrastructural analysis of collagen in this model in other tissues, which show a more random distribution of fibril sizes and their packing into fibers. As Dcn is a known tumor suppressor, this work forms the basis for future studies of collagen remodeling in both malignant and benign prostate disease

Polymorphic Aβ42 fibrils adopt similar secondary structure but differ in cross-strand side chain stacking interactions within the same β-sheet.

Formation of polymorphic amyloid fibrils is a common feature in neurodegenerative diseases involving protein aggregation. In Alzheimer's disease, different fibril structures may be associated with different clinical sub-types. Structural basis of fibril polymorphism is thus important for understanding the role of amyloid fibrils in the pathogenesis and progression of these diseases. Here we studied two types of Aβ42 fibrils prepared under quiescent and agitated conditions. Quiescent Aβ42 fibrils adopt a long and twisted morphology, while agitated fibrils are short and straight, forming large bundles via lateral association. EPR studies of these two types of Aβ42 fibrils show that the secondary structure is similar in both fibril polymorphs. At the same time, agitated Aβ42 fibrils show stronger interactions between spin labels across the full range of the Aβ42 sequence, suggesting a more tightly packed structure. Our data suggest that cross-strand side chain packing interactions within the same β-sheet may play a critical role in the formation of polymorphic fibrils

The biomechanics of amnion rupture: an X-ray diffraction study

Pre-term birth is the leading cause of perinatal and neonatal mortality, 40% of which are attributed to the pre-term premature rupture of amnion. Rupture of amnion is thought to be associated with a corresponding decrease in the extracellular collagen content and/or increase in collagenase activity. However, there is very little information concerning the detailed organisation of fibrillar collagen in amnion and how this might influence rupture. Here we identify a loss of lattice like arrangement in collagen organisation from areas near to the rupture site, and present a 9% increase in fibril spacing and a 50% decrease in fibrillar organisation using quantitative measurements gained by transmission electron microscopy and the novel application of synchrotron X-ray diffraction. These data provide an accurate insight into the biomechanical process of amnion rupture and highlight X-ray diffraction as a new and powerful tool in our understanding of this process