Psuedo-affine behaviour of collagen fibres during the uniaxial deformation of leather

Chromium tanned bovine leather has been dried under uniaxial strain and its collagen fibre distribution examined using high-angle X-ray diffraction. Microstructural modelling of the fibre kinematics showed that under large-strain deformation (30%) the fibres behave in a psuedo-affine manner. At decreasingly lower strains the fibre re-orientation is seen to progressively deviate from the pseudo-affine prediction; an observation which can be understood in terms of a combination of fibre-fibre adhesion and changes in fibre substructure. The material was also subjected to mechanical testing and tensile data is presented here which indicates how fibre orientation affects tensile modulus. This work represents the first quantitative microstructural model for collagen fibre distribution in strained leather.     

Engineering extruded collagen fibers for biomedical applications

Extruded collagen fibers constitute a promising biomimetic scaffold for tissue engineering applications. In this study, we compared the structural, thermal, and mechanical properties of fibers produced from either NaCl or poly(ethylene glycol) with a number-average molecular weight of 8000 (PEG 8K), the only two coagents that have been used in the fabrication process. As novel, we report the fabrication of fibers with properties similar to native or synthetic fibers using other coagents. NaCl derived fibers were characterized by higher thermal stability (p < 0.026), stress (p < 0.001), and modulus (p < 0.0025) values than PEG 8K, whereas the latter yielded more extendable fibers (p < 0.012). Poly(ethylene glycol)s with number-average molecular weights of 200 and 1000 produced fibers with similar mechanical properties (p > 0.05) that were thinner (p < 0.033), stiffer (p < 0.022), and less extendable (p < 0.0002) than those of PEG 8K. Poly(vinyl alcohol) (PVA) with a number-average average molecular weight of 9–10,000 and PEG 8K yielded fibers with similar diameters and stress-at-break values (p > 0.05); however, the poly(ethylene glycol) derived fibers were more extendable (p < 0.0003), whereas the PVA fibers were stiffer (p < 0.029). Gum-arabic- and soluble-starch-derived fibers were of similar tensile strength, extendibility, and stiffness (p > 0.05). In this in vitro study, the thickest (p < 0.011) and the weakest (p < 0.0066) fibers were produced in the presence of sodium sulfate. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

The influence of a natural cross-linking agent (Myrica rubra) on the properties of extruded collagen fibres for tissue engineering applications

Extruded collagen fibres have been shown to be a competitive biomaterial for both soft and hard tissue repair. The natural cross-linking pathway of collagen does not occur in vitro and consequently reconstituted forms of collagen lack sufficient strength. Numerous cross-linking approaches have been investigated through the years, but still there is no ideal method accepted. The use of plant extracts to cross-link collagen scaffolds has been advocated due to superior mechanical properties. As first herein we investigate the stabilisation effect of Myrica rubra on extruded collagen fibres. Fibres treated with M. rubra exhibited higher denaturation temperature (p < 0.005) and lower enthalpy of denaturation (p < 0.034) than formaldehyde of glutaraldehyde. Uniaxial tensile tests of wet tested fibres revealed j-shape curves similar to those of native tissues. Thin fibres exhibited high stress/low strain graphs, whilst thick fibres yielded low stress/high strain graphs. Cross-linking reduced significantly the fibre diameter (p < 0.005) and increased significantly the stress (p < 0.004) and force (p < 0.001) at break and the modulus at 2.0% strain (p < 0.003). An inverse relationship between stress at break and fibre diameter was observed for every treatment. Overall, our findings demonstrate the potential of M. rubra in stabilisation of collagen-based materials for tissue engineering applications.     

Differences in strength between the grain and corium layers of leather

The tearing resistance of the two principal strata of leather (the grain and the corium layers) has been assessed via the measurement of tearing energy and notch sensitivity. Observations of the distribution of strain around notches and the form of the tear tips are also reported. It was found that the grain layer had only 20% of the tearing energy possessed by the corium. In addition, the strength of the grain was considerably more sensitive to the presence of sharp notches. It is suggested that these differences in strength between the two strata of leather are associated with the greater ability of the corium layer's fibre structure to impede propagating tears by means of tear tip blunting and fibre pull out.     

PREDICTING LEATHER HANDLE LIKE AN EXPERT BY ARTIFICIAL NEURAL NETWORKS

This study developed an artificial neural network model to predict the subjective assessment of leather handle by an expert using its measurable physical characteristics. A statistical method was applied to prune the inputs of the network and the “error band” conception was proposed during training.     

The effects of the biaxial stretching of leather on fibre orientation and tensile modulus

Leather has been subjected to different degrees of equal biaxial strain (up to 20%) during drying and its tensile modulus has been measured when dry. The collagen fibre orientation distribution in the dried leather has been assessed using wide angle X-ray diffraction. It was found that drying under biaxial strain caused the tensile modulus to increase markedly (by up to 400% at 20% biaxial strains) but with a dependence on the angle of test axis in relation to the principal axes of biaxial strain. The fibre orientation distribution in planes parallel to the surface was affected less by biaxial strain than in planes perpendicular to the surface and it is concluded that the latter type of fibre reorientation is the main factor responsible for the observed increases in tensile modulus.     

Compliances and failure modes of oriented chain-extended polyethylene

The quantitative dependence of Young's modulus and fracture behaviour on the angle between the tensile axis and the orientation direction has been determined for oriented polyethylene with lamellae of chain-extended dimensions. The form of the modulus dependence was similar to that for the majority of oriented polymers which have been studied, but the fracture behaviour was atypical, ductility only being obtained when the test axis was parallel to the orientation direction, with brittle fracture occurring in all other directions for all except very high molecular mass materials.The behaviour of this material in compression alongc has also been studied; in this case kink bands andc-axis cleavage were observed. It is shown that kink band formation may be explained solely by uniformc axis shear deformation of the lamellar crystals.     

On the plastic deformation of chain extended polyethylene

Deformation of the lamellar microstructure (average lamellar thickness 500 nm) of a high molecular weight chain-extended polyethylene after drawing five-fold has been investigated using a variety of microscopic and other techniques. As the interlammelar regions in this material are expected to be qualitatively similar to those in chain-folded polyethylene, the former may be considered as a large scale copy of the latter. It is shown that: the great majority of lamellae survive the draw with their thickness along c only slightly reduced although 20% appear to become disrupted; many lamellae undergo a high shear parallel to c, although not always a uniform one; lamellar rotation and interlammelar shear probably also occur as may the formation of voids and/or crazes; drawing does not involve melting.