Mechanical properties of a mature biofilm from a wastewater system: from microscale to macroscale level

A fundamental understanding of biofilm mechanical stability is critical in order to describe detachment and develop biofouling control strategies. It is thus important to characterise the elastic deformation and flow behaviour of the biofilm under different modes of applied force. In this study, the mechanical properties of a mature wastewater biofilm were investigated with methods including macroscale compression and microscale indentation using atomic force microscopy (AFM). The mature biofilm was found to be mechanically isotropic at the macroscale level as its mechanical properties did not depend on the scales and modes of loading. However, the biofilm showed a tendency for mechanical inhomogeneity at the microscale level as indentation progressed deeper into the matrix. Moreover, it was observed that the adhesion force had a significant influence on the elastic properties of the biofilm at the surface, subjected to microscale tensile loading. These results are expected to inform a damage-based model for biofilm detachment.     

Molecular dynamics study on perfect and defective graphene/calcium-silicate-hydrate composites under tensile loading

ABSTRACT

Atomic models of graphene/calcium-silicate-hydrate (G/C-S-H) are constructed by embedding perfect or defective graphene in molecular structures of amorphous C-S-H. Molecular dynamics (MD) simulation is utilised to study mechanical properties of the G/C-S-H and the enhancing effect of perfect and defective graphene is compared. The effects of temperature and strain rate on perfect and defective G/C-S-H are also investigated and compared. The results from present simulations show that (i) the defective graphene has better enhancing effect in C-S-H than perfect one and it grows with the increase of defect sizes; (ii) the tensile strength of G/C-S-H decreases with the increase of temperature and the defective G/C-S-H is more susceptible to temperature than the perfect one; (iii) the ultimate strength and the failure strain increase significantly with the increase of strain rate and the effects of strain rate on perfect and defective G/C-S-H are similar. These findings provide important atomic insights for understanding the mechanical behaviours of G/C-S-H composite.     

A novel model for the molecular dynamics simulation study on mechanical properties of HMX/F2311 polymer-bonded explosive

The ‘insert’ model for β-octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX)-based polymer-bonded explosive (PBX) was proposed for finding the relation of temperatures with mechanical properties. This model was simulated by using molecular dynamics models. The elastic constants and the effective moduli were calculated with static analysis method. Cauchy pressure was also calculated. It is found that the rigidity is weakened and the ductibility is improved by adding a small amount of F₂₃₁₁ in the crystalline HMX. The rigidity is also weakened with increasing temperature. However, the ductibility of HMX/F₂₃₁₁ PBX changes as a parabola with increasing temperature duo to the enhancement of F₂₃₁₁ molecular chain movement and simultaneously the increment of high energy conformation ratio in this molecular chain, i.e. the increment of the molecular chain rigidity.     

Nonoverlapping Thalamocortical Connections to Normal and Deprived Primary Somatosensory Cortex for Similar Forelimb Receptive Fields in Chronic Spinal Cats

The fluorescent dye retrograde tracing technique, using fast blue in combination with fluorogold, was used to examine thalamocortical projections from the ventrobasal complex to primary somatosensory cortex in chronic spinal cats that sustained T12 cord transection at 2 weeks of age. Following cord transection at this age, it has been shown that forelimb afferents can excite the deprived hindlimb projection zone, in addition to the region of somatosensory cortex that they normally occupy (McKinley et al, 1987). These two regions of cortex are separated by over 10 mm, thus facilitating the determination of whether the forelimb representation in “hindlimb cortex” is derived from the sector of the ventrobasal complex of the thalamus representing the forelimb, hindlimb, or both. Injections of the two dyes into separate regions of the cortex that were excited by the same peripheral forelimb receptive fields produced single labeling of two nonoverlapping clusters of thalamic neurons. This finding suggests that the projections for these two areas are independent and distinct, and indicates that altered thalamocortical projections do not contribute the critical component underlying reorganizational changes observed at the cortical level after spinal cord transection. It is hypothesized that the degree of reorganization required to achieve the magnitude of change observed in the cortex must occur below the level of the thalamocortical relay.     

Response of Merkel cells in the palatal rugae to the continuous mechanical stimulation by palatal plate

The aim of the present study was to investigate the responses of Merkel cells that are numerous in the palatine rugae, due to the continuous mechanical stimulation exerted by the palatal plate. Forty golden hamsters were used in this experiment. The palatal plate was made of adhesive resin and it was set on the palate of the animal. To exert a continuous pressure, a 0.8?mm elevation on the internal surface of the palatal plate was created at the middle portion of the fourth palatine ruga. Thereafter, the number of Merkel cells in the mucosa was calculated by immunohistochemical observation. Morphological changes of Merkel cells were examined by electron microscopy. There was significant difference among the control and any of the treated groups on the number of CK20 positive Merkel cells (p?<?0.05) and that numbers were decreased at the sites where continuous mechanical stimulation was exerted. Degeneration of the cytoplasm mitochondria and nerve endings, and a decrease in both the number of neurosecretory granules and cytoplasmic processes were observed. Furthermore, the presence of nuclear chromatin aggregation and fragmentation was recognized. The continuous mechanical stimulation by the palatal plate affected the responses of Merkel cells and nerve endings, thus inducing a decrease in the number of Merkel cells. A portion of these changes was also associated with the expression of apoptosis.     

Biaxial mechanical properties of the inferior vena cava in C57BL/6 and CB-17 SCID/bg mice

Multiple murine models have proven useful in studying the natural history of neovessel development in the tissue engineering of vascular grafts. Nevertheless, to better understand longitudinal changes in the biomechanics of such neovessels, we must first quantify native tissue structure and properties. In this paper, we present the first biaxial mechanical data for, and nonlinear constitutive modeling of, &QJ;the inferior vena cava from two models used in tissue engineering: wild-type C57BL/6 and immunodeficient CB-17 SCID/bg mice. Results show that inferior vena cava from the latter are significantly stiffer in the circumferential direction, both materially (as assessed by a stored energy function) and structurally (as assessed by the compliance), despite a lower intramural content of fibrillar collagen and similar wall thickness. Quantifying the natural history of neovessel development in different hosts could lead to increased insight into the mechanisms by which cells fashion and maintain extracellular matrix in order to match best the host stiffness while ensuring sufficient vascular integrity.     

Programmable mechanical stimulation influences tendon homeostasis in a bioreactor system

Identification of functional programmable mechanical stimulation (PMS) on tendon not only provides the insight of the tendon homeostasis under physical/pathological condition, but also guides a better engineering strategy for tendon regeneration. The aims of the study are to design a bioreactor system with PMS to mimic the in vivo loading conditions, and to define the impact of different cyclic tensile strain on tendon. Rabbit Achilles tendons were loaded in the bioreactor with/without cyclic tensile loading (0.25 Hz for 8 h/day, 0–9% for 6 days). Tendons without loading lost its structure integrity as evidenced by disorientated collagen fiber, increased type III collagen expression, and increased cell apoptosis. Tendons with 3% of cyclic tensile loading had moderate matrix deterioration and elevated expression levels of MMP‐1, 3, and 12, whilst exceeded loading regime of 9% caused massive rupture of collagen bundle. However, 6% of cyclic tensile strain was able to maintain the structural integrity and cellular function. Our data indicated that an optimal PMS is required to maintain the tendon homeostasis and there is only a narrow range of tensile strain that can induce the anabolic action. The clinical impact of this study is that optimized eccentric training program is needed to achieve maximum beneficial effects on chronic tendinopathy management. Biotechnol. Bioeng. 2013; 110: 1495–1507. © 2012 Wiley Periodicals, Inc.     

Sequence and domain arrangements influence mechanical properties of elastin‐like polymeric elastomers

Elastin is the polymeric, extracellular matrix protein that provides properties of extensibility and elastic recoil to large arteries, lung parenchyma, and other tissues. Elastin assembles by crosslinking through lysine residues of its monomeric precursor, tropoelastin. Tropoelastin, as well as polypeptides based on tropoelastin sequences, undergo a process of self‐assembly that aligns lysine residues for crosslinking. As a result, both the full‐length monomer as well as elastin‐like polypeptides (ELPs) can be made into biomaterials whose properties resemble those of native polymeric elastin. Using both full‐length human tropoelastin (hTE) as well as ELPs, we and others have previously reported on the influence of sequence and domain arrangements on self‐assembly properties. Here we investigate the role of domain sequence and organization on the tensile mechanical properties of crosslinked biomaterials fabricated from ELP variants. In general, substitutions in ELPs involving similiar domain types (hydrophobic or crosslinking) had little effect on mechanical properties. However, modifications altering either the structure or the characteristic sequence style of these domains had significant effects on such properties. In addition, using a series of deletion and replacement constructs for full‐length hTE, we provide new insights into the role of conserved domains of tropoelastin in determining mechanical properties. © 2012 Wiley Periodicals, Inc. Biopolymers 99: 392–407, 2013.     

Carcass and meat quality traits of rabbits under heat stress

Rabbits are very sensitive to heat stress because they have difficulty eliminating excess body heat. The objective of the current study was to evaluate the effects of heat stress on slaughter weight, dressing percentage and carcass and meat quality traits of rabbits from two genetic groups. Ninety-six weaned rabbits were used: half were from the Botucatu genetic group and half were crossbreds between New Zealand White sires and Botucatu does. They were assigned to a completely randomized design in a 2 × 3 factorial arrangement (two genetic groups and three ambient temperatures: 18°C, 25°C and 30°C) and kept under controlled conditions in three environmental chambers from 5 to 10 weeks of age. Slaughter took place at 10 weeks, on 2 consecutive days. Meat quality measurements were made in the longissimus muscle. Actual average ambient temperature and relative humidity in the three chambers were 18.4°C and 63.9%, 24.4°C and 80.2% and 29.6°C and 75.9%, respectively. Purebred rabbits were heavier at slaughter and had heavier commercial and reference carcasses than crossbreds at 30°C; however, no differences between genetic groups for these traits were found at lower temperatures. No genetic group × ambient temperature interaction was detected for any other carcass or meat quality traits. The percentages of distal parts of legs, skin and carcass forepart were higher in crossbred rabbits, indicating a lower degree of maturity at slaughter in this group. The percentage of thoracic viscera was higher in the purebreds. Lightness of the longissimus muscle was higher in the purebreds, whereas redness was higher in the crossbreds. Slaughter, commercial and reference carcass weights and the percentages of thoracic viscera, liver and kidneys were negatively related with ambient temperature. Commercial and reference carcass yields, and the percentage of distal parts of legs, on the other hand, had a positive linear relationship with ambient temperature. Meat redness and yellowness diminished as ambient temperature increased, whereas cooking loss was linearly elevated with ambient temperature. Meat color traits revealed paler meat in the purebreds, but no differences in instrumental texture properties and water-holding capacity between genetic groups. Purebred rabbits were less susceptible to heat stress than the crossbreds. Heat stress resulted in lower slaughter and carcass weights and proportional reductions of organ weights, which contributed to a higher carcass yield. Moreover, it exerted a small, but negative, effect on meat quality traits.     

Instrumented Indentation Investigation on the Viscoelastic Properties of Porcine Cartilage

Articular cartilage lubricates the contact surfaces in human joints and provides a shock-absorbing effect which protects the joint under dynamic loading. However, this shock-absorbing effect is gradually reduced as the result of normal wear, tear and aging-related cartilage loss. Thus, with the increasing average human life expectancy, the issue of joint health has attracted significant interest in recent decades. In developing new materials for the repair or regeneration of damaged articular cartilage, it is essential that the difference in the mechanical properties of healthy and damaged cartilages is well-understood. In the present study, the hardness and Young＇s modulus of damaged and healthy porcine articular cartilage samples are evaluated via a quasi-static nanoindentation technique. A dynamic mechanical analysis method is then applied to determine the viscoelastic properties of the two samples. The results presented in this study provide a useful insight into the mechanical properties of articular cartilage at the mesoscale, and therefore fill an important gap in the literature.