Hannah C. Wells

Collagen Fibril Diameter and Leather Strength

The main structural component of leather and skin is type I collagen in the form of strong fibrils. Strength is an important property of leather, and the way in which collagen contributes to the strength is not fully understood. Synchrotron-based small angle X-ray scattering (SAXS) is used to measure the collagen fibril diameter of leather from a range of animals, including sheep and cattle, that had a range of tear strengths. SAXS data were fit to a cylinder model. The collagen fibril diameter and tear strength were found to be correlated in bovine leather (r(2) = 0.59; P = 0.009), with stronger leather having thicker fibrils. There was no correlation between orientation index, i.e., fibril alignment, and fibril diameter for this data set. Ovine leather showed no correlation between tear strength and fibril diameter, nor was there a correlation across a selection of other animal leathers. The findings presented here suggest that there may be a different structural motif in skin compared with tendon, particularly ovine skin or leather, in which the diameter of the individual fibrils contributes less to strength than fibril alignment does.

Poisson's ratio of collagen fibrils measured by small angle X-ray scattering of strained bovine pericardium

Type I collagen is the main structural component of skin, tendons, and skin products, such as leather. Understanding the mechanical performance of collagen fibrils is important for understanding the mechanical performance of the tissues that they make up, while the mechanical properties of bulk tissue are well characterized, less is known about the mechanical behavior of individual collagen fibrils. In this study, bovine pericardium is subjected to strain while small angle X-ray scattering (SAXS) patterns are recorded using synchrotron radiation. The change in d-spacing, which is a measure of fibril extension, and the change in fibril diameter are determined from SAXS. The tissue is strained 0.25 (25%) with a corresponding strain in the collagen fibrils of 0.045 observed. The ratio of collagen fibril width contraction to length extension, or the Poissons ratio, is 2.1 ± 0.7 for a tissue strain from 0 to 0.25. This Poissons ratio indicates that the volume of individual collagen fibrils decreases with increasing strain, which is quite unlike most engineering materials. This high Poissons ratio of individual fibrils may contribute to high Poissons ratio observed for tissues, contributing to some of the remarkable properties of collagen-based materials.

Age Dependent Differences in Collagen Alignment of Glutaraldehyde Fixed Bovine Pericardium

Bovine pericardium is used for heart valve leaflet replacement where the strength and thinness are critical properties. Pericardium from neonatal animals (4–7 days old) is advantageously thinner and is considered as an alternative to that from adult animals. Here, the structures of adult and neonatal bovine pericardium tissues fixed with glutaraldehyde are characterized by synchrotron-based small angle X-ray scattering (SAXS) and compared with the mechanical properties of these materials. Significant differences are observed between adult and neonatal tissue. The glutaraldehyde fixed neonatal tissue has a higher modulus of elasticity (83.7 MPa) than adult pericardium (33.5 MPa) and a higher normalised ultimate tensile strength (32.9 MPa) than adult pericardium (19.1 MPa). Measured edge on to the tissue, the collagen in neonatal pericardium is significantly more aligned (orientation index (OI) 0.78) than that in adult pericardium (OI 0.62). There is no difference in the fibril diameter between neonatal and adult pericardium. It is shown that high alignment in the plane of the tissue provides the mechanism for the increased strength of the neonatal material. The superior strength of neonatal compared with adult tissue supports the use of neonatal bovine pericardium in heterografts.

Looseness in bovine leather: microstructural characterization.

BACKGROUND A substantial proportion of bovine leather production may be of poor quality, with the leather suffering from a characteristic known as looseness. This defect results in a poor visual appearance and greatly reduced value. The structural mechanism of looseness is not well understood. RESULTS Samples of loose and tight bovine leather are characterized using small-angle X-ray scattering, ultrasonic imaging, and electron microscopy. The density of fibre packing and orientation of the fibrils are analysed. Tensile strength is also measured. Loose leather is characterized by more highly aligned collagen fibrils. This results in a weaker connection between the layers. There is a looser packing of the fibres in loose leather than in tight leather, with more gaps between fibre bundles, particularly in a region in the lower grain. This region is visible with in situ ultrasonic imaging. Loose leather has a higher tensile strength than tight leather. CONCLUSION While a high degree of collagen fibril alignment is normally associated with strong leather, it has been shown that too much alignment results in loose leather. Understanding the physical basis of looseness is the first step in identifying looseness in hides and learning how to prevent looseness from developing during leather manufacture.

A small angle X-ray scattering study of the structure and development of looseness in bovine hides and leather.

BACKGROUND Some bovine hides produce poor quality leather, termed loose leather. The structural characteristics of hides and the intermediate processed stages that lead to loose leather are not well understood. In the present study, synchrotron-based small angle X-ray scattering (SAXS) is used to investigate collagen fibril orientation at the different stages of processing (i.e. from hide through to leather) that result in both tight and loose leathers. RESULTS Tight leather of a relatively isotropic texture has a lower orientation index (OI) than loose leather of a more pronounced stratified texture; conversely, tight pickled hide and wet blue have a higher OI than loose pickled hide and wet blue. There is a greater increase in OI on processing from pickled hide to dry crust (leather) for loose material. This is largely the result of a greater increase in hide thickness prior to pickling for loose hide than tight hide, followed by a greater decrease at the dry crust stage. The collagen fibrils in loose leather and wet blue more readily orient under stress than do those in tight leather. Loose leather has a more pronounced layered structure than tight leather, although this difference is not apparent from SAXS measurements of hide prior to the dry crust stage; it develops during processing. CONCLUSION The greater swelling of the loose hide during processing disrupts the structure and leads to a more layered collagen arrangement on shrinking at the final dry crust stage.

Acellular dermal matrix collagen responds to strain by intermolecular spacing contraction with fibril extension and rearrangement

Acellular dermal matrix (ADM) materials are used as scaffold materials in reconstructive surgery. The internal structural response of these materials in load-bearing clinical applications is not well understood. Bovine ADM is characterized by small-angle X-ray scattering while subjected to strain. Changes in collagen fibril orientation (O), degree of orientation as an orientation index (OI) (measured both edge-on and flat-on to the ADM), extension (from d-spacing changes) and changes to intermolecular spacing are measured as a result of the strain and stress in conjunction with mechanical measurements. As is already well established in similar systems, when strained, collagen fibrils in ADM can accommodate the strain by reorienting by up to 50° (as an average of all the fibrils). This reorientation corresponds to the OI increasing from 0.3 to 0.7. Here it is shown that concurrently, the intermolecular spacing between tropocollagen decreases by 10% from 15.8 to 14.3Å, with the fibril diameter decreasing from 400 to 375Å, and the individual fibrils extending by an average of 3.1% (D-spacing from 63.9 to 65.9nm). ADM materials can withstand large strain and high stress due to the combined mechanisms of collagen reorientation, individual fibril extension, sliding and changes in the molecular packing density.

Artificially modified collagen fibril orientation affects leather tear strength: Artificially modified collagen fibril orientation affects leather tear strength

BACKGROUND Ovine leather has around half the tear strength of bovine leather and is therefore not suitable for high-value applications such as shoes. Tear strength has been correlated with the natural collagen fibril alignment (orientation index, OI). It is hypothesized that it could be possible to artificially increase the OI of the collagen fibrils and that an artificial increase in OI could increase tear strength. RESULTS Ovine skins, after pickling and bating, were strained biaxially during chrome tanning. The strain ranged from 2 to 15% of the initial sample length, either uniformly in both directions by 10% or with 3% in one direction and 15% in the other. Once tanned, the leather tear strengths were measured and the collagen fibril orientation was measured using synchrotron-based small-angle X-ray scattering. CONCLUSION The OI increased as a result of strain during tanning from 0.48 to 0.79 (P = 0.001) measured edge-on and the thickness-normalized tear strength increased from 27 to 43 N mm-1 (P < 0.001) after leather was strained 10% in two orthogonal directions. This is evidence to support a causal relationship between high OI (measured edge-on), highly influenced by thickness, and tear strength. It also provides a method to produce stronger leather.

The influence of water, lanolin, urea, proline, paraffin and fatliquor on collagen D-spacing in leather

Water interacts with collagen to alter the structure at the fibrillar scale and therefore the mechanical properties of collagen. Humectants or moisturizers also alter the mechanical properties and fibril structure. The nature of these interactions and relationship between the different additives is not well understood. Changes in collagen D-spacing in leather were measured by synchrotron based small angle X-ray scattering in samples stored at various relative humidities and treated with lanolin, fatliquor, urea, proline or paraffin. The D-spacing increased with rising humidity and with increasing lanolin or fatliquor content, but not with treatment with urea, proline or paraffin. Strength increased with the addition of lanolin. Lanolin and fatliquor were shown to act as humectants whereas the other components did not act in this way. The Hofmeister effect is shown not to be a factor in the change in D-spacing, since samples treated with either proline or urea exhibited the same behavior. Different agents used in leather treatment and skin care function by different mechanisms, with collagen water retention being important for some additives but not others.

Effect of collagen packing and moisture content on leather stiffness

Applications for skin derived collagen materials, such as leather and acellular dermal matrices, usually require both strength and flexibility. In general, both the tensile modulus (which has an impact on flexibility) and strength are known to increase with fiber alignment, in the tensile direction, for practically all collagen-based tissues. The structural basis for flexibility in leather was investigated and the moisture content was varied. Small angle X-ray scattering was used to determine collagen fibril orientation, elongation and lateral intermolecular spacing in leather conditioned by different controlled humidity environments. Flexibility was measured by a three point bending test. Leather was prepared by tanning under biaxial loading to create leather with increased fibril alignment and thus strength, but this treatment also increased the stiffness. As collagen aligns, it not only strengthens the material but it also stiffens because tensile loading is then applied along the covalent chain of the collagen molecules, rather than at an angle to it. Here it has been shown that with higher moisture content greater flexibility of the material develops as water absorption inside collagen fibrils produces a larger lateral spacing between collagen molecules. It is suggested that water provides a lubricating effect in collagen fibrils, enabling greater freedom of movement and therefore greater flexibility. When collagen molecules align in the strain direction during tanning, leather stiffens not only by the fiber alignment itself but also because collagen molecules pack closer together, reducing the ability of the molecules to move relative to each other.

Tropical Keratopathy (Florida Spots) in Cats

The authors used microscopy and synchrotron-based small-angle X-ray scattering analysis (SAXS) to describe lesions macroscopically typical of tropical keratopathy (“Florida spots”) from 6 cats on St Kitts. Microscopically, there were varying degrees of epithelial hyperplasia and thinning of the cornea (by 4% to 18%) due to loss of corneal stroma associated with dense accumulations of collagen in the superficial stroma. The collagen fibrils in lesions were wider and had more variable diameters (39.5 ± 5.0 nm, mean ± SD) than in normal corneas (25.9 ± 3.6 nm; P < .01). There were occasional vacuoles (<1 μm) in the corneal epithelial basement membrane but no evidence of inflammation, edema, stromal neovascularization, fibrosis, acid-fast organisms, or structures suggestive of a fungal organism. SAXS analysis showed collagen fibril diameters and variation in size were greater in stroma containing the lesions compared to normal corneas (48.8 ± 4.5 nm vs 35.5 ± 2.6; P < .05). The d-spacing of collagen in the stroma of lesions and normal corneas was the same, but the average orientation index of collagen in lesions was greater (0.428 ± 0.08 vs 0.285 ± 0.03; P < .05). A survey revealed Florida spots lesions were static over time and became less obvious in only 1 of 6 affected cats adopted on St Kitts and taken to areas in the US where lesions are not reported. An anterior stromal collagen disorder with various degrees of epithelial hyperplasia is the pathologic hallmark of lesions clinically identical to Florida spots in cats from St Kitts.