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 Poisson's ratio, is 2.1 ± 0.7 for a tissue strain from 0 to 0.25. This Poisson's ratio indicates that the volume of individual collagen fibrils decreases with increasing strain, which is quite unlike most engineering materials. This high Poisson's ratio of individual fibrils may contribute to high Poisson's ratio observed for tissues, contributing to some of the remarkable properties of collagen-based materials.
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.
Strength is an important characteristic of acellular dermal matrix (ADM) materials used for surgical scaffolds. Strength depends on the material's structure, which may vary with the source from which the product is produced, including species and animal age. Here, variations in the physical properties and structures of ADM materials from three species are investigated: bovine (fetal and neonatal), porcine, and human materials. Thickness normalized, the bovine materials have a similar strength (tear strength of 75−124 N/m) to the human material (79 N/m), and these are both stronger than the porcine material (43 N/m). Thickness-normalized tensile strengths were similar among all species (18−34 N/mm 2 for bovine although higher in fetal material, 18 N/mm 2 for human and 21 N/mm 2 for porcine). Structure is investigated with synchrotron-based small-angle X-ray scattering (SAXS) for collagen fibril orientation index (OI) and scanning electron microscopy (SEM). SEM reveals a more open structure in bovine ADM than in the porcine and human material. A correlation is found between OI and thickness-normalized tear strength in neonatal bovine material measured with the X-rays edge-on to the sample, but this relationship does not extend across species. The collagen fibril arrangement, viewed perpendicular to the surface, varies between species, with the human material having a unimodal distribution and rather isotropic (OI 0.08), the porcine being strongly bimodal and rather highly oriented (OI 0.61), the neonatal bovine between these two extremes with a bimodal distribution tending toward isotropic (OI 0.14−0.21) and the fetal bovine material being bimodal and less isotropic than neonatal (OI 0.24). The OI varies less through the thickness of the porcine and human materials than through the bovine materials. The similarities and differences in structure may inform the suitability of these materials for particular surgical applications.
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. Synchrotronbased 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.
Scaffold biomaterials are typically applied surgically as reinforcement for weakened or damaged tissue, acting as substrates on which healing tissue can grow. Natural extracellular matrix (ECM) materials consisting mainly of collagen are often used for this purpose, but are anisotropic. Ovine forestomach matrix (OFM) ECM was exposed to increasing strain and synchrotron-based SAXS diffraction patterns and revealed that the collagen fibrils within underwent changes in orientation, orientation index (a measure of isotropy), and extension. Response to the strain depended on the direction the collagen fibrils were oriented. When the ECM was stretched in the direction of collagen fibril orientation, the fibrils become more oriented and begin to take up the strain immediately (as shown by the increased d-spacing). Stretch applied perpendicular to dominant fibril direction caused the fibrils to initially become less oriented as they were pulled away from the original direction, and less force was initially transmitted along the length of the fibrils (i.e., the d-spacing changed less). SAXS analysis of OFM and the starting raw tissue showed there is no difference in the structural arrangement of the collagen fibrils. Understanding the directional structural response of these materials under strain may influence how surgeons select and place the materials in use.
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