Collagen Fibril Orientation in Ovine and Bovine Leather Affects Strength: A Small Angle X-ray Scattering (SAXS) Study

Abstract

There is a large difference in strength between ovine and bovine leather. The structure and arrangement of fibrous collagen in leather and the relationship between collagen structure and leather strength has until now been poorly understood. Synchrotron based SAXS is used to characterize the fibrous collagen structure in a series of ovine and bovine leathers and to relate it to tear strength. SAXS gives quantitative information on the amount of fibrous collagen, the orientation (direction and spread) of the collagen microfibrils, and the d-spacing of the collagen. The amount of collagen varies through the thickness of the leather from the grain to the corium, with a greater concentration of crystalline collagen measured toward the corium side. The orientation index (OI) is correlated strongly with strength in ovine leather and between ovine and bovine leathers. Stronger leather has the fibrils arranged mostly parallel to the plane of the leather surface (high OI), while weaker leather has more out-of-plane fibrils (low OI). With the measurement taken parallel to the animal's backbone, weak (19.9 N/mm) ovine leather has an OI of 0.422 (0.033), stronger (39.5 N/mm) ovine leather has an OI of 0.452 (0.033), and bovine leather with a strength of (61.5 N/mm) has an OI of 0.493 (0.016). The d-spacing profile through leather thickness also varies according to leather strength, with little variation being detected in weak ovine leather (average=64.3 (0.5) nm), but with strong ovine leather and bovine leather (which is even stronger) exhibiting a dip in d-spacing (from 64.5 nm at the edges dropping to 62 nm in the center). This work provides a clear understanding of a nanostructural characteristic of ovine and bovine leather that leads to differences in strength.

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

“…For collagen in bovine pericardium, at low strain, the Poisson's ratio appears to have a very high value (15)(16)(17)(18)(19)(20)(21)(22)(23)(24)(25)(26)(27), but for strain above 0.09, the Poisson's ratio is in the range 2.1-2.8. For the total strain (from 0 to 0.25), the change in d-spacing and diameter gives 0 ¼ 2.1 6 0.7 (these values of 0 can be calculated from Fig.…”

“…where OA (orientation angle) is the minimum azimuthal angle range that contains 50% of the fibrils, based on the method of Sacks for light scattering 25 but converted to an index, 26 using the spread in azimuthal angle of one or more d-spacing diffraction peaks. The OI is used to give a measure of the spread of fibril orientation (an OI of 1 indicates the fibrils are parallel to each other; 0 indicates the fibrils are randomly oriented).…”

Age Dependent Differences in Collagen Alignment of Glutaraldehyde Fixed Bovine Pericardium

“…Ovine and bovine leather show a similar flat profile of OI with skin depth [15, 16]. The flat profile between the two halves of the pericardium is somewhat surprising here as the histology shows two distinct layers.…”

“…OI is defined as (90° − OA)/90°, where OA is the minimum azimuthal angle range that contains 50% of the microfibrils. This was based on the method of Sacks for light scattering [10] but converted to an index [15], using the spread in azimuthal angle of, typically, the sixth order peak at approximately 0.055–0.059 Å −1 . This peak was selected as it is one of the most intense diffraction peaks.…”

“…The orientation of collagen measured edge on (alignment in plane) has been shown in bovine and ovine skin to be correlated with strength [15, 16]. This correlation extends across a range of mammal species with a strength range of over a factor of five [17].…”