Mechanical properties of pristine and nanoporous graphene

We present molecular dynamics simulations of monolayer graphene under uniaxial tensile loading. The Morse, bending angle, torsion and Lennard-Jones potential functions are adopted within the mdFOAM library in the OpenFOAM software, to describe the molecular interactions in graphene. A well-validated graphene model using these set of potentials is not yet available. In this work, we investigate the accuracy of the mechanical properties of graphene when derived using these simpler potentials, compared to the more commonly used complex potentials such as the Tersoff-Brenner and AIREBO potentials. The computational speed up of our approach, which scales O(1.5N), where N is the number of carbon atoms, enabled us to vary a larger number of system parameters, including graphene sheet orientation, size, temperature and concentration of nanopores. The resultant effect on the elastic modulus, fracture stress and fracture strain is investigated. Our simulations show that graphene is anisotropic, and its mechanical properties are dependant on the sheet size. An increase in system temperature results in a significant reduction in the fracture stress and strain. Simulations of nanoporous graphene were created by distributing vacancy defects, both randomly and uniformly, across the lattice. We find that the fracture stress decreases substantially with increasing defect density. The elastic modulus was found to be constant up to around 5% vacancy defects, and decreases for higher defect densities.     

Influence of odd and even number of Stone–Wales defects on the fracture behaviour of an armchair single-walled carbon nanotube under axial and torsional strain

Using Brenner's bond-order potential to represent the interaction of the in-plane C–C bond, an armchair (8,8) single-walled carbon nanotube is investigated by molecular dynamics simulation under axial loading and twist, both for perfect and imperfect lattices introducing an increasing number of Stone–Wales (SW) defects. The Young modulus, shear modulus, tensile strength, shear strength, ductility, stiffness and toughness are computed. All the mechanical characteristics are found to change appreciably by the inclusion of SW defects. Two distinct patterns of fracture mode are observed with odd and even numbers of defects. A clear evidence of the defect–defect interaction is observed when more than one defect is included.     

Statistical factorial analysis on the material property sensitivity of the mechanical responses of the C4-C6 under compression, anterior and posterior shear

A systematic approach using factorial analysis was conducted on the C4-C6 finite element model to analyse the influence of six spinal components (cortical shell, vertebral body, posterior elements, endplate, disc annulus and disc nucleus) on the internal stresses and external biomechanical responses under compression, anterior and posterior shear. Results indicated that the material properties variation of the disc annulus has a significant influence on both the external biomechanical responses and internal stress of the disc annulus and its neighboring hard bones. The study reveals for the first time, the significant influence of the cancellous bone under compression, while variation in the cortical shell modulus has a high influence under anterior and posterior shear. The study also reveals that the effects of interaction between two main components are insignificant.     

On the influence of point defects on the structural and electronic properties of graphene-like sheets: a molecular simulation study

The influence of vacancies and substitutional defects on the structural and electronic properties of graphene, graphene oxide, hexagonal boron nitride, and boron nitride oxide two-dimensional molecular models was studied using density functional theory (DFT) at the level of local density approximation (LDA). Bond length, dipole moment, HOMO–LUMO energy gap, and binding energy were calculated for each system with and without point defects. The results obtained indicate that the formation of a point defect does not necessary lead to structural instability; nevertheless, surface distortions and reconstruction processes were observed, mainly when a vacancy-type defect is generated. For graphene, it was found that incorporation of a point defect results in a semiconductor–semimetal transition and also increases notably its polar character. As with graphene, the formation of a point defect in a hexagonal boron nitride sheet reduces its energy gap, although its influence on the resulting dipole moment is not as dramatic as in graphene. The influence of point defects on the structural and electronic properties of graphene oxide and boron nitride oxide sheets were found to be mediated by the chemisorbed species.     

Supracellular measurement of spatially varying mechanical heterogeneities in live monolayers

The mechanical properties of tissues have profound impacts on a wide range of biological processes such as embryo development (1,2), wound healing (3, 4, 5, 6), and disease progression (7). Specifically, the spatially varying moduli of cells largely influence the local tissue deformation and intercellular interaction. Despite the importance of characterizing such a heterogeneous mechanical property, it has remained difficult to measure the supracellular modulus field in live cell layers with a high-throughput and minimal perturbation. In this work, we developed a monolayer effective modulus measurement by integrating a custom cell stretcher, light microscopy, and AI-based inference. Our approach first quantifies the heterogeneous deformation of a slightly stretched cell layer and converts the measured strain fields into an effective modulus field using an AI inference. This method allowed us to directly visualize the effective modulus distribution of thousands of cells virtually instantly. We characterized the mean value, SD, and correlation length of the effective cell modulus for epithelial cells and fibroblasts, which are in agreement with previous results. We also observed a mild correlation between cell area and stiffness in jammed epithelia, suggesting the influence of cell modulus on packing. Overall, our reported experimental platform provides a valuable alternative cell mechanics measurement tool that can be integrated with microscopy-based characterizations.     

An integrated fluid–structure interaction and thrombosis model for type B aortic dissection

False lumen thrombosis (FLT) in type B aortic dissection has been associated with the progression of dissection and treatment outcome. Existing computational models mostly assume rigid wall behavior which ignores the effect of flap motion on flow and thrombus formation within the FL. In this study, we have combined a fully coupled fluid–structure interaction (FSI) approach with a shear-driven thrombosis model described by a series of convection–diffusion reaction equations. The integrated FSI-thrombosis model has been applied to an idealized dissection geometry to investigate the interaction between vessel wall motion and growing thrombus. Our simulation results show that wall compliance and flap motion can influence the progression of FLT. The main difference between the rigid and FSI models is the continuous development of vortices near the tears caused by drastic flap motion up to 4.45 mm. Flap-induced high shear stress and shear rates around tears help to transport activated platelets further to the neighboring region, thus speeding up thrombus formation during the accelerated phase in the FSI models. Reducing flap mobility by increasing the Young’s modulus of the flap slows down the thrombus growth. Compared to the rigid model, the predicted thrombus volume is 25% larger using the FSI-thrombosis model with a relatively mobile flap. Furthermore, our FSI-thrombosis model can capture the gradual effect of thrombus growth on the flow field, leading to flow obstruction in the FL, increased blood viscosity and reduced flap motion. This model is a step closer toward simulating realistic thrombus growth in aortic dissection, by taking into account the effect of intimal flap and vessel wall motion.

     

Computational modeling of the effective Young’s modulus values of fullerene molecules: a combined molecular dynamics simulation and continuum shell model

Estimating the Young’s modulus of a structure in the nanometer size range is a difficult task. The reliable determination of this parameter is, however, important in both basic and applied research. In this study, by combining molecular dynamics (MD) simulations and continuum shell theory, we designed a new approach to determining the Young’s modulus values of different spherical fullerenes. The results indicate that the Young’s modulus values of fullerene molecules decrease nonlinearly with increasing molecule size and understandably tend to the Young’s modulus of an ideal flat graphene sheet at large molecular radii. To the best of our knowledge, this is first time that a combined atomistic–continuum method which can predict the Young’s modulus values of fullerene molecules with high precision has been reported.     

Clinical assessment of rheumatic diseases using viscoelastic parameters for synovial fluid

For the first time it is clearly exhibited that synovial fluid (SF) is thixotropic. Although no hysteresis loops were observed for SF, not even at high shear rates, thixotropy may be exhibited by measuring the rate of recovery after extensive shearing. The rebuilding of the structure in a small-amplitude oscillatory state following the high-shear-rate state reveals the thixotropic behaviour. Five different viscoelastic parameters for various synovial fluids (SF) were obtained using oscillatory rheometry. It was also shown that for SF in the low frequency range, corresponding to a knee joint almost at rest, the shear loss modulus G" is greater than the shear storage modulus G', since the system is allowed to dissipate energy at rest. However, with movement, G' increases and eventually becomes greater than G" at a characteristic frequency above which the system has insufficient time to dissipate energy and hence responds as an elastic body. This functional behaviour, characteristic for normal SF, broke down in the SF of rheumatoid arthritis. It was also absent in the SF of knee joints with meniscus lesions and ligament defects.     

The deformation of an erythrocyte under the radiation pressure by optical stretch

In this paper, the mechanical properties of erythrocytes were studied numerically based upon the mechanical model originally developed by Pamplona and Calladine (ASME J. Biomech. Eng., 115, p. 149, 1993) for liposomes. The case under study is the erythrocyte stretched by a pair of laser beams in opposite directions within buffer solutions. The study aims to elucidate the effect of radiation pressure from the optical laser because up to now little is known about its influence on the cell deformation. Following an earlier study by Guck et al. (Phys. Rev. Lett., 84, p. 5451, 2000; Biophys. J., 81, p. 767, 2001), the empirical results of the radiation pressure were introduced and imposed on the cell surface to simulate the real experimental situation. In addition, an algorithm is specially designed to implement the simulation. For better understanding of the radiation pressure on the cell deformation, a large number of simulations were conducted for different properties of cell membrane. Results are first discussed parametrically and then evaluated by comparing with the experimental data reported by Guck et al. An optimization approach through minimizing the errors between experimental and numerical data is used to determine the optimal values of membrane properties. The results showed that an average shear stiffness around 4.611x10-6 Nm(-1), when the nondimensional ratio of shear modulus to bending modulus ranges from 10 to 300. These values are in a good agreement with those reported in literature.     

Model of red blood cell membrane skeleton: electrical and mechanical properties

A theoretical membrane skeleton model of erythrocyte has been developed and successfully applied to interpret electrical and mechanical properties of the red blood cell spectrin-actin network. The model is based on the structure of the membrane skeleton that is comprised of unit cells each containing an actin protofilament and shooting forth a few spectrin heterodimers. The loose ends of the heterodimers of adjacent cells can form bonds with each other giving rise to an integrated network. The number of bonds depends on the temperature. The bond length being excessive (2.6 times the distance between the centers of adjacent cells), the bonds are flexible, and can thus be regarded as entropy springs. The advanced model has been employed to calculate the shear modulus of the membrane skeleton as well as to establish its temperature dependence. In a wide range of temperatures mu(T) is a decreasing function well fitting the experimental data. The relationship between the membrane bilayer-free size of the skeleton and the ionic strength of the solution has been derived to appear in good agreement with the results obtained previously. Experimental data combined with the advanced theory yield the average number of heterodimers per unit cell, m0, as equal to ca. 5; the spectrin heterodimer charge has been estimated.     

The shear modulus of lower-leg muscles correlates to intramuscular pressure

Shear wave elastography (SWE) is emerging as an innovative tool to evaluate muscle properties and function. It has been shown to correlate with both passive and active muscle forces, and is sensitive to physiological processes and pathological conditions. Similarly, intramuscular pressure (IMP) is an important parameter that changes with passive and active muscle contraction, body position, exercise, blood pressure, and several pathologies. Therefore, the objective of this study was to quantify the dependency of shear modulus within the lower-leg muscles on IMP in healthy individuals. Nineteen healthy individuals (age: Mean age ± SD, 23.84 ± 6.64 years) were recruited. Shear modulus was measured using ultrasound SWE on the tibialis anterior (TA) and peroneus longus (PL) muscles using pressure cuff inflation around the thigh at 40 mmHg, 80 mmHg, and 120 mmHg. Changes in IMP were verified using a catheter connected to a blood pressure monitor. It was found that IMP was correlated to TA and PL shear modulus (spearman's rank correlation = 0.99 and 0.99, respectively). Applying a gradual increase of cuff pressure from 0 to 120 mmHg increased the shear modulus of the TA and PL muscles from 15.83 (2.46) kPa to 21.88 (4.33) kPa and from 9.64 (1.97) kPa to 12.88 (5.99) kPa, respectively. These results demonstrate that changes of muscle mechanical properties are dependent on IMP. This observation is important to improve interpretation of ultrasound elastograms and to potentially use it as a biomarker for more accurate diagnosis of pathologies related to increased IMP.     

Wrinkling and thermal conductivity of one graphene sheet under shear

Tersoff-potential - based molecular dynamics method is used to simulate wrinkling deformation of one graphene sheet under shear, and the obtained deformation is compared with analytical solutions of macro-membrane. Furthermore, thermal conductivity of the wrinkled graphene at different temperatures is calculated. It is found that (1) the wrinkling deformation of graphene sheet under shear is close to the analytical solutions of macro-membrane under shear, which implies that the solutions of macro-membrane are applicable to predict the wrinkling deformation of graphene sheets under shear; (2) the more serious the wrinkling of the graphene under shear is, the stronger the phonon scattering is and, therefore, the lower the thermal conductivity of the wrinkled graphene is; (3) within the temperature range of 400–700 K, the thermal conductivity of graphene sheet decreases with increase in temperature.     

Effects of age on elastic moduli of human lungs.

The model of the lung as an elastic continuum undergoing small distortions from a uniformly inflated state has been used to describe many lung deformation problems. Lung stress-strain material properties needed for this model are described by two elastic moduli: the bulk modulus, which describes a uniform inflation, and the shear modulus, which describes an isovolume deformation. In this study we measured the bulk modulus and shear modulus of human lungs obtained at autopsy at several fixed transpulmonary pressures (Ptp). The bulk modulus was obtained from small pressure-volume perturbations on different points of the deflation pressure-volume curve. The shear modulus was obtained from indentation tests on the lung surface. The results indicated that, at a constant Ptp, both bulk and shear moduli increased with age, and the increase was greater at higher Ptp values. The micromechanical basis for these changes remains to be elucidated.     

An integrated probabilistic model for functional prediction of proteins.

We develop an integrated probabilistic model to combine protein physical interactions, genetic interactions, highly correlated gene expression networks, protein complex data, and domain structures of individual proteins to predict protein functions. The model is an extension of our previous model for protein function prediction based on Markovian random field theory. The model is flexible in that other protein pairwise relationship information and features of individual proteins can be easily incorporated. Two features distinguish the integrated approach from other available methods for protein function prediction. One is that the integrated approach uses all available sources of information with different weights for different sources of data. It is a global approach that takes the whole network into consideration. The second feature is that the posterior probability that a protein has the function of interest is assigned. The posterior probability indicates how confident we are about assigning the function to the protein. We apply our integrated approach to predict functions of yeast proteins based upon MIPS protein function classifications and upon the interaction networks based on MIPS physical and genetic interactions, gene expression profiles, tandem affinity purification (TAP) protein complex data, and protein domain information. We study the recall and precision of the integrated approach using different sources of information by the leave-one-out approach. In contrast to using MIPS physical interactions only, the integrated approach combining all of the information increases the recall from 57% to 87% when the precision is set at 57%-an increase of 30%.     

Structural, electronic, and elastic properties of K-As compounds: a first principles study

First-principle calculations are performed to investigate the structural, elastic and electronic properties of K-As compounds (KAs in NaP, LiAs and AuCu-type structures, KAs(2) in MgCu(2)-type structure, K(3)As in Na(3)As, Cu(3)P and Li(3)Bi-type structures, and K(5)As(4) in A(5)B(4)-type structure). The lattice parameters, cohesive energy, formation energy, bulk modulus, and the first derivative of bulk modulus (to fit to the Murnaghan's equation of state) of the considered structures are calculated and reasonable agreement is obtained, and the phase transition pressure is also predicted. The repeated calculations on the electronic band structures and the related partial density of states are also given. The calculated second-order elastic constants based on the stress-strain method and the other related quantities such as Young's modulus, shear modulus, Poisson's ratio, sound velocities, Debye temperature, and shear anisotropy factors for considered structures are presented, and trends are discussed.     

Adhesion Characteristics of a Novel Synthetic Polydimethylsiloxane for Bionic Adhesive Pads

Materials with appropriate adhesive properties are suitable for the fabrication of bionic adhesive pads. In this study, a novel polydimethylsiloxane （PDMS） material enhanced with two types of crosslinkers, carbon nanotubes and graphene sheets, was fabricated. The Contact Angle （CA） and cross-sectional morphology of the new material were investigated and observed using a CA meter and Scanning Electron Microscopy （SEM）, respectively. CA measurements indicate that the surface energy of the novel material is twice that of the common PDMS material. SEM observations show that carbon nanotubes and graphene sheets are well dispersed in the polymer, a feature that improves the mechanical properties of the new material. The adhesive performance of this novel composite was tested on an in-house fabricated friction machine. Results show that at a preload of only 50 mN, the adhesion of the novel PDMS material is up to -3.7 times that of common PDMS. The maximum macroscale shear strength and normal adhesion reach 4 N·cm^-2 and 1 N·cm^-2, respectively. The adhesive capability of the material is maintained even after hundreds of times of repeated use. This novel material exhibits excellent adhesion, sufficiently high elastic modulus and high repeatability at low preloads.     

Mechanical properties of hybridoma cells in batch culture

Summary Direct measurements throughout a batch culture of bursting force, bursting membrane tension, elastic area compressibility modulus, and the size of single hybridoma cells have been made by a novel micromanipulation technique. It has been found that the bursting membrane tension and compressibility modulus rise significantly in the rapid growth phase, and fall in the death phase. An approach is suggested for relating these mechanical properties to the shear sensitivity of the cells when they are exposed to shear stresses in flow fields. It is shown that reports of changes in hybridoma fragility during batch cultures, as measured using viscometers, might be explained using more fundamental micromanipulation measurements.     

Combining AFM and Acoustic Probes to Reveal Changes in the Elastic Stiffness Tensor of Living Cells

Knowledge of how the elastic stiffness of a cell affects its communication with its environment is of fundamental importance for the understanding of tissue integrity in health and disease. For stiffness measurements, it has been customary to quote a single parameter quantity, e.g., Young’s modulus, rather than the minimum of two terms of the stiffness tensor required by elasticity theory. In this study, we use two independent methods (acoustic microscopy and atomic force microscopy nanoindentation) to characterize the elastic properties of a cell and thus determine two independent elastic constants. This allows us to explore in detail how the mechanical properties of cells change in response to signaling pathways that are known to regulate the cell’s cytoskeleton. In particular, we demonstrate that altering the tensioning of actin filaments in NIH3T3 cells has a strong influence on the cell's shear modulus but leaves its bulk modulus unchanged. In contrast, altering the polymerization state of actin filaments influences bulk and shear modulus in a similar manner. In addition, we can use the data to directly determine the Poisson ratio of a cell and show that in all cases studied, it is less than, but very close to, 0.5 in value.     

Coarse-grained molecular simulation of self-assembly nanostructures of CTAB on nanoscale graphene

Coarse-grained molecular dynamics simulation has been performed to study the aggregated morphology of the cationic surfactant, cetyltrimethylammonium bromide (CTAB), adsorbed on nanoscale graphene surfaces. The CTAB surfactants can self-assemble on graphene to form various supramolecular morphologies and structures. The effect of packing density, thickness of graphene sheet and width of graphene nanoribbon on the CTAB–graphene self-assembly has been investigated. The buoyant densities of various graphene–CTAB assemblies were calculated, which increase with surfactant coverage and number of graphene layers. This result demonstrates that density gradient can be used to isolate graphenes with various layers. This simulation provides larger-scale microscopic insight into the supramolecular self-assembly nanostructures for the CTAB surfactants aggregated on graphene, which could be valuable to guide fabrication of graphene-based hybrid nanocomposites.     

The shear wave elastic modulus and the increased nuclear factor kappa B (NF-kB/p65) and cyclooxygenase-2 (COX-2) expression in the area of myofascial trigger points activated in a rat model by blunt trauma to the vastus medialis

We aimed to elucidate the increased inflammatory cytokines expression such as nuclear factor kappa B (NF-kB/p65), cyclooxygenase-2 (COX-2), and voltage-gated calcium channels (VGCC) in the area of activated myofascial trigger points (MTrPs) in a rat model by blunt trauma to the vastus medialis and to evaluate the feasibility of a quantitative analysis of muscle elastic modulus using shear wave elastography (SWE). Twelve 7-week-old male SD rats were divided into normal (NM, n = 6) and model groups (MO, n = 6). In the MO group, MTrPs were activated with a blunt strike to the left vastus medialis and subsequent eccentric exercise for 8 weeks. After 4 weeks of rest, the elastic modulus in the focal site was evaluated using SWE. Electromyography (EMG) data were collected at MTrPs and muscle tissues were evaluated for expression of nuclear factor kappa B (NF-kB/p65), cyclooxygenase-2 (COX-2) protein, and voltage-gated calcium channels (VGCC). The number of the palpable taut bands; EMG frequency and amplitude; elastic modulus values; and NFkB/p65, COX-2, and VGCC expression levels were significantly higher for the left focal area in the MO group compared to those for the NM group (p’s < 0.05). These findings suggest that elastic modulus measurement using ultrasound SWE may be effective in evaluating MPS. In addition, increased COX-2, NFkB/p65, and VGCC expression may expand the integrated hypothesis of trigger points.