Fibril Rupture Research Articles

Assessing collagen fibrils molecular damage after a single stretch-release cycle.

Mechanical testing of connective tissues such as tendons and ligaments can lead to collagen denaturation even in the absence of macroscale damage. The following tensile loading protocols, ramp loading to failure, overloading and release, cyclic overloading and cyclic fatigue loading, all yield molecular damage in rat or bovine tendons. Single collagen fibrils extracted from the positional common digital extensor tendon of the forelimb also show molecular damage after tensile loading to failure. Using fibrils from the same source we assess changes to the molecular and supramolecular structure after tensile stress relaxation at strains between 4 and 22% followed by release. We observe no broken fibril and no significant change in D-band spacing. However, we observe significant binding of a fluorescent collagen hybridizing peptide to the fibrils indicating that collagen denaturation occurs in a strain dependent way for relaxation times between 1 s and 1500 s. We also show that peptide binding is associated with a decrease of the cross-sectional area of the fibrils providing an estimate of the dry volume loss due to molecular denaturation as well as an estimate of the mechanical energy density required, 25-110 MJ m-3. In summary we show that collagen molecular damage can occur in the absence of fibril failure and without visible changes to the supramolecular structure.

Morphology-properties relationship of gas plasma treated hydrophobic meso-porous membranes and their improved performance for desalination by membrane distillation

The impact on performance of the surface energy and roughness of membrane materials used for direct contact membrane distillation are critical but yet poorly investigated parameters. The capacity to alter the wettability of highly hydrophobic materials such as poly(tetra-fluoro-ethylene) (PTFE) by gas plasma treatments is reported in this paper. An equally important contribution from this investigation arises from illustrating how vaporized material from the treated sample participates after a short while in the composition of the plasma and fundamentally changes the result of surface chemistry processes. The water contact angle across the hydrophobic membranes is generally controlled by varying the plasma gas conditions, such as the plasma power, chamber pressure and irradiation duration. Changes to surface porosity and roughness of the bulk material as well as the surface chemistry, through specific and partial de-fluorination of the surface were detected and systematically studied by Fourier transform infra-red analysis and scanning electron microscopy. It was found that the rupture of fibrils, formed during membrane processing by thermal-stretching, led to the formation of a denser surface composed of nodules similar to these naturally acting as bridging points across the membrane material between fibrils. This structural change has a profound and impart a permanent effect on the permeation across the modified membranes, which was found to be enhanced by up to 10% for long plasma exposures while the selectivity of the membranes was found to remain unaffected by the treatment at a level higher than 99.99%. This is the first time that an investigation demonstrates how the permeation characteristics of these membranes is directly related to data from spectral, morphological and surface charge analyses, which provide new insights on the impact of plasma treatments on both, the surface charge and roughness, of PTFE porous materials.

High strain‐rate response of spiropyran mechanophores in PMMA

ABSTRACTWe report the high strain‐rate response of a spiropyran (SP) mechanophore in poly(methylmethacrylate). Previous work on this system has demonstrated a reversible bond scission in the SP under local tensile force, converting it to a fluorescent merocyanine form. A Hopkinson bar was used to apply fast compressive loads at rates from 102 to 104 s−1, resulting in significant activation of the SP near fracture surfaces. However, comparison with a similar thermochromic SP reveals that much of the observed activation likely arises from thermal effects during high‐rate fracture. These results show the importance of a thermally active control system in distinguishing mechanochromic response during high‐rate loading. Microscale fluorescence mapping of the fracture surfaces using a confocal Raman microspectrometer suggests that some distinct mechanical activation may be occurring in craze‐like regions during fibril rupture. The thermal response of the SP is useful in its own right for characterizing plastic heating regions during dynamic fracture. Published 2014. J. Polym. Sci., Part B: Polym. Phys. 2014, 52, 1347–1356

Elasticity, microstructure and thermal stability of foliage and fruit fibres from four tropical crops

A comparative analysis of the elasticity, microstructure and thermal stability of fibres (thickness ranging from 43.4 to 189.4 µm) isolated from pineapple leaves (PALF), coconut coir (COIR), banana leaf-stem (BAN) and oil palm empty fruit bunch (OPEFB) reported in this study. Statistical analysis of the mechanical properties derived from tensile test to rupture reveals significant differences (P<0.05) in the fibre strength (σ), stiffness (E) and extensibility (parameterized by the strain to rupture, ɛ). It is observed that COIR fibres yield the smallest strength, σ (=99.8±22.5 MPa), and stiffness, E (= 0.5±0.1 GPa), while PALF fibres yield the largest σ (=639.5±301.6 MPa) and E (=7.1±3.1 GPa); PALF fibres exhibit the smallest ɛ (=0.11±0.03) but OPEFB fibres yield the largest ɛ (=2.0±1.3). From scanning electron micrographs, it is observed that cellulose fibril rupture predominates in OPEFB, COIR and BAN fibres; a large proportion of the cellulose fibrils fail by pullout in PALF fibres. Thermogravimetric analysis reveals that all fibres are thermally stable up to 250 °C; the fibre residue ranges from 30 to 80 wt% after heating to 500 °C. In particular, BAN experiences the highest weight loss and PALF experiences the lowest weight loss. The findings lend to a simple approach for determining the performance of the composites by assessing the type of natural fibres for reinforcing polymeric matrices.

Branching toughens fibrous networks

Fibrous collagenous networks are not only stiff but also tough, due to their complex microstructures. This stiff yet tough behavior is desirable for both medical and military applications but it is difficult to reproduce in engineering materials. While the nonlinear hyperelastic behavior of fibrous networks has been extensively studied, the understanding of toughness is still incomplete. Here, we identify a microstructure mimicking the branched bundles of a natural type I collagen network, in which partially cross-linked long fibers give rise to novel combinations of stiffness and toughness. Finite element analysis shows that the stiffness of fully cross-linked fibrous networks is amplified by increasing the fibril length and cross-link density. However, a trade-off of such stiff networks is reduced toughness. By having partially cross-linked networks with long fibrils, the networks have comparable stiffness and improved toughness as compared to the fully cross-linked networks. Further, the partially cross-linked networks avoid the formation of kinks, which cause fibril rupture during deformation. As a result, the branching allows the networks to have stiff yet tough behavior.

Structure-Property Relationship of Burn Collagen Reinforcing Musculo-Skeletal Tissues

An investigation has been conducted on burn ligaments, addressing the specific conditions in burn arising from dehydration and heating, and how these affect the structure-property relationship of collagen for reinforcing the tissue. Collagen fibres were isolated from a sheep’s anterior cruciate ligament, i.e. our model for this study, and divided into six groups. The first group was designated as control; the second (D) group was dehydrated without exposure to elevated temperature. The remaining (DH) groups were dehydrated followed by heating at 120oC for 30 minutes, 2, 4 and 24 hours, respectively. Tensile test to rupture was carried out to derive the fibre modulus of elasticity (E), maximum stress (σ), strain at maximum stress (ε) and strain energy density to maximum stress (u). Electron micrographs of the ruptured ends reveal a mixed mode of fibril pull-out and rupture: fibril pull-out dominates in the control group but fibril rupture dominates increasingly in the other groups, i.e. with increasing exposure time to elevated temperature. Apart from ε, there is significant increase in E, σ and u in the D and DH groups with respect to the control group but there is no evidence of variation among the D and DH groups. The results of this study implicate (1) the removal of water in the hydrated proteoglycan-rich matrix, leading to shrinkages at micrometer length-scale during dehydration, and (2) the alteration of the collagen organisation arising from the underlying changes in the crystallinity and denaturation during heating, on the mechanical properties of burn collagen fibres.

Optical trapping with high forces reveals unexpected behaviors of prion fibrils

Amyloid fibrils are important in diverse cellular functions, feature in many human diseases and have potential applications in nanotechnology. Here we describe methods that combine optical trapping and fluorescent imaging to characterize the forces that govern the integrity of amyloid fibrils formed by a yeast prion protein. A crucial advance was to use the self-templating properties of amyloidogenic proteins to tether prion fibrils, enabling their manipulation in the optical trap. At normal pulling forces the fibrils were impervious to disruption. At much higher forces (up to 250 pN), discontinuities occurred in force-extension traces before fibril rupture. Experiments with selective amyloid-disrupting agents and mutations demonstrated that such discontinuities were caused by the unfolding of individual subdomains. Thus, our results reveal unusually strong noncovalent intermolecular contacts that maintain fibril integrity even when individual monomers partially unfold and extend fibril length.

Effects of frozen storage temperature on the elasticity of tendons from a small murine model

The basic mechanism of reinforcement in tendons addresses the transfer of stress, generated by the deforming proteoglycan (PG)-rich matrix, to the collagen fibrils. Regulating this mechanism involves the interactions of PGs on the fibril with those in the surrounding matrix and between PGs on adjacent fibrils. This understanding is key to establishing new insights on the biomechanics of tendon in various research domains. However, the experimental designs in many studies often involved long sample preparation time. To minimise biological degradation the tendons are usually stored by freezing. Here, we have investigated the effects of commonly used frozen storage temperatures on the mechanical properties of tendons from the tail of a murine model (C57BL6 mouse). Fresh (unfrozen) and thawed samples, frozen at temperatures of −20°C and −80°C, respectively, were stretched to rupture. Freezing at −20°C revealed no effect on the maximum stress (σ), stiffness (E), the corresponding strain (ε) at σ and strain energy densities up to ε (u) and from ε until complete rupture (up). On the other hand, freezing at −80°C led to higher σ, E and u; ε and up were unaffected. The results implicate changes in the long-range order of radially packed collagen molecules in fibrils, resulting in fibril rupture at higher stresses, and changes to the composition of extrafibrillar matrix, resulting in an increase in the interaction energy between fibrils via collagen-bound PGs.

Papain‐gel degrades intact nonmineralized type I collagen fibrils

Papain-gel has been utilized as a chemo-mechanical material for caries removal due to its ability to preserve underlying sound dentin. However, little is known about the effect of the papain enzyme on intact type I collagen fibrils that compose the dentin matrix. Here we sought to define structural changes that occur in intact type I collagen fibrils after an enzymatic treatment with a papain-gel. Intact and nonmineralized type I collagen fibrils from rat tail were obtained and treated with a papain-gel (Papacarie) for 30 s, rinsed with water and imaged using an atomic force microscope (AFM). Additionally, polished healthy dentin specimens were also treated using the same protocol described above and had their elastic modulus (E) and hardness (H) measured by means of AFM-based nanoindentation. AFM images showed that the papain-gel induced partial degradation of the fibrils surface, yet no rupture of fibrils was noticed. The distinction between gap and overlap zones of fibrils vanished in most regions after treatment, and overlap zones appeared to be generally more affected. Mechanical data suggested a gradual decrease in E and H after treatments. A significant two-fold drop from the values of normal dentin (E=20+/-1.9, H=0.8+/-0.08 GPa) was found after four applications (E=9.7+/-3.2, H=0.24+/-0.1 GPa) (P<0.001), which may be attributed to the degradation of proteoglycans of the matrix. In summary, this study provided novel evidence that intact nonmineralized type I collagen fibrils are partially degraded by a papain-gel.

The mobility of the amorphous phase in polyethylene as a determining factor for slow crack growth

Polyethylene (PE) pipes generally exhibit a limited lifetime, which is considerably shorter than their chemical degradation period. Slow crack growth failure occurs when pipes are used in long-distance water or gas distribution though being exposed to a pressure lower than the corresponding yield stress. This slow crack growth failure is characterized by localized craze growth and craze fibril rupture. In the literature, the lifetime of PE pipes is often considered as being determined by the density of tie chains connecting adjacent crystalline lamellae. But this consideration cannot explain the excellent durability of the recent bimodal grade PE for pipe application. We show in this paper the importance of the craze fibril length as the determining factor for the pipe lifetime. The conclusions are drawn from stress analysis. It is found that longer craze fibrils sustain lower stress and are deformed to a lesser degree. The mobility of the amorphous phase is found to control the amount of material that can be "sucked" in by the craze fibrils and thus the length of the craze fibrils. The mobility of the amorphous phase can be monitored by dynamic mechanical analysis measurements. Excellent agreement between the mobility thus derived and lifetimes of PE materials as derived from FNCT (full notch creep test) is given, thus providing an effective means to estimate the lifetime of PE pipes by considering well-defined physical properties.

Effects of sorbed water on crack propagation in poly(methyl methacrylate) under static tensile stress

Effects of sorbed water on crack propagation in poly(methyl methacrylate) (PMMA) under static tensile stress have been investigated. The specimens were kept for more than two years in temperature and humidity-controlled conditions. Sorbed water of less than 0.40 wt% scarcely affected K Ith value (threshold stress intensity factor for crack propagation), however, K Ith value for the specimens containing water of more than 0.40 wt% increased with the amount of sorbed water. K Ith values related to the balance among the radius of the curvature of crack tip, crazing stress and craze fibril rupture stress, which are functions of the amount of sorbed water. At a crack propagation rate of more than 1 × 10−7 m/s, the slopes of K-da/dt curves for the specimens containing water less than 0.40 wt% were gentle, however, that for the specimens containing more than 0.40 wt% was steep; and unstably fractured. It was found that the gentler slopes for the specimens containing little sorbed water may be caused by craze-shear controlled crack propagation mechanisms, while the steeper slopes for the other specimens may be caused by a craze controlled crack propagation mechanism.

The Development of Transfer Films in Ultra-High Molecular Weight Polyethylene/Stainless Steel Oscillatory Sliding

The friction and wear behavior of a polymer/metal oscillatory sliding system is largely dictated by the film of polymer which may form on the metallic counterface. This research utilizes scanning electron microscopy as the principal tool for examination of post-test frictional surfaces in an effort to identify the processes of film formation in an ultra-high molecular weight polyethylene pin/440C stainless steel counterface oscillatory pair. Strong adhesive junction formation at contacts leads to fibril drawing upon continued motion. Eventual fibril rupture deposits polyethylene residues upon the counterface and these residues are cumulatively smeared by the pin into a film. An increase in temperature from 15 to 30 degrees C aids the smearing process, forming a smoother film which reduces wear rate by limiting polymer/metal interaction. Comparative tests with polytetrafluoroethylene support earlier conclusions that film peeling due to weaker adhesion results in higher wear rates for PTFE than UHMWPE.

Static fatigue fracture of polyethylene: A morphological analysis

AbstractThe fracture properties of a number of high‐density polyethylenes were evaluated and related to their morphology. The resistance to initiation of fracture is related to the resistance to voiding of the materials and the rate of coalescence of voids. The propensity of the materials to void has been shown to be related to the size of the amorphous region. The resistance to propagation of the fracture is also related to the above factors as well as the ease of stretching and rupture of fibrils. The ease of fibril formation has been shown to be related to the degree of perfection of the crystallites.