Free-energy-based framework for early forecasting of stem cell differentiation.

Commitment of stem cells to different lineages is inherently stochastic but regulated by a range of environmental bio/chemo/mechanical cues. Here, we develop an integrated stochastic modelling framework for predicting the differentiation of hMSCs in response to a range of environmental cues, including sizes of adhesive islands, stiffness of substrates and treatment with ROCK inhibitors in both growth and mixed media. The statistical framework analyses the fluctuations of cell morphologies over approximately a 24 h period after seeding the cells in the specific environment and uses the cytoskeletal free-energy distribution to forecast the lineage the hMSCs will commit to. The cytoskeletal free energy which succinctly parametrizes the biochemical state of the cell is shown to capture hMSC commitment over a range of environments while simple morphological factors such as cell shape, tractions on their own are unable to correlate with lineages hMSCs adopt.

Tunable Energy Absorption Characteristics of Architected Honeycombs Enabled via Additive Manufacturing.

Tailoring of material architectures in three-dimensions enabled by additive manufacturing (AM) offers the potential to realize bulk materials with unprecedented properties optimized for location-specific structural and/or functional requirements. Here we report tunable energy absorption characteristics of architected honeycombs enabled via material jetting AM. We realize spatially tailored 3D printed honeycombs (guided by FE studies) by varying the cell wall thickness gradient and evaluate experimentally and numerically the energy absorption characteristics. The measured response of architected honeycombs characterized by local buckling (wrinkling) and progressive failure reveals over 110% increase in specific energy absorption (SEA) with a concomitant energy absorption efficiency of 65%. Design maps are presented that demarcate the regime over which geometric tailoring mitigates deleterious global buckling and collapse. Our analysis indicates that an energy absorption efficiency as high as 90% can be achieved for architected honeycombs, whereas the efficiency of competing microarchitected metamaterials rarely exceeds 50%. The tailoring strategy introduced here is easily realizable in a broad array of AM techniques, making it a viable candidate for developing practical mechanical metamaterials.

The homeostatic ensemble for cells.

Cells are quintessential examples of out-of-equilibrium systems, but they maintain a homeostatic state over a timescale of hours to days. As a consequence, the statistics of all observables is remarkably consistent. Here, we develop a statistical mechanics framework for living cells by including the homeostatic constraint that exists over the interphase period of the cell cycle. The consequence is the introduction of the concept of a homeostatic ensemble and an associated homeostatic temperature, along with a formalism for the (dynamic) homeostatic equilibrium that intervenes to allow living cells to evade thermodynamic decay. As a first application, the framework is shown to accurately predict the observed effect of the mechanical environment on the in vitro response of smooth muscle cells. This includes predictions that both the mean values and diversity/variability in the measured values of observables such as cell area, shape and tractions decrease with decreasing stiffness of the environment. Thus, we argue that the observed variabilities are inherent to the entropic nature of the homeostatic equilibrium of cells and not a result of in vitro experimental errors.

Analysis of electro-permeation of hydrogen in metallic alloys.

A reaction-diffusion type modelling framework is presented to analyse both electro-permeation (EP) and thermal desorption spectrometry (TDS) measurements of hydrogen in metallic alloys. It is assumed that the kinetics of hydrogen motion is governed by diffusion through the lattice, along with trapping/detrapping at specific sites such as dislocations, grain boundaries, etc. It is shown that the trapping and detrapping rates are typically much faster than the diffusion rate, and consequently a simplification of the governing equations suffices such that local equilibrium exists between lattice and trapped hydrogen. Using this local equilibrium assumption, we then present an asymptotic analysis of the governing kinetic equation for the EP test. This asymptotic analysis reveals that four regimes of behaviour exist, ranging from negligible trapping to the complete filling of deep traps. The analysis suggests that EP tests should be so-arranged that three regimes of behaviour are spanned, in order to extract the relevant material properties associated with hydrogen transport. The numerical solutions presented in this study support the asymptotic analysis. The hydrogen kinetics framework is also deployed to analyse both EP and TDS tests on the same martensitic steel. The EP measurements all lie in regime I and are thus insufficient to uniquely determine both the trap density and binding energy. Reasonable agreement is obtained between measurements and numerical predictions of TDS tests using parameters estimated from the EP tests. Further improvements in measurements are required to confirm the fidelity of this modelling approach.This article is part of the themed issue 'The challenges of hydrogen and metals'.

Discrete dislocation plasticity analysis of loading rate-dependent static friction.

From a microscopic point of view, the frictional force associated with the relative sliding of rough surfaces originates from deformation of the material in contact, by adhesion in the contact interface or both. We know that plastic deformation at the size scale of micrometres is not only dependent on the size of the contact, but also on the rate of deformation. Moreover, depending on its physical origin, adhesion can also be size and rate dependent, albeit different from plasticity. We present a two-dimensional model that incorporates both discrete dislocation plasticity inside a face-centred cubic crystal and adhesion in the interface to understand the rate dependence of friction caused by micrometre-size asperities. The friction strength is the outcome of the competition between adhesion and discrete dislocation plasticity. As a function of contact size, the friction strength contains two plateaus: at small contact length [Formula: see text], the onset of sliding is fully controlled by adhesion while for large contact length [Formula: see text], the friction strength approaches the size-independent plastic shear yield strength. The transition regime at intermediate contact size is a result of partial de-cohesion and size-dependent dislocation plasticity, and is determined by dislocation properties, interfacial properties as well as by the loading rate.

Prediction of failure in notched carbon-fibre-reinforced-polymer laminates under multi-axial loading.

A damage-based finite-element model is used to predict the fracture behaviour of centre-notched quasi-isotropic carbon-fibre-reinforced-polymer laminates under multi-axial loading. Damage within each ply is associated with fibre tension, fibre compression, matrix tension and matrix compression. Inter-ply delamination is modelled by cohesive interfaces using a traction-separation law. Failure envelopes for a notch and a circular hole are predicted for in-plane multi-axial loading and are in good agreement with the observed failure envelopes from a parallel experimental study. The ply-by-ply (and inter-ply) damage evolution and the critical mechanisms of ultimate failure also agree with the observed damage evolution. It is demonstrated that accurate predictions of notched compressive strength are obtained upon employing the band broadening stress for microbuckling, highlighting the importance of this damage mode in compression. This article is part of the themed issue 'Multiscale modelling of the structural integrity of composite materials'.

A thermodynamically motivated model for stress-fiber reorganization.

We present a model for stress-fiber reorganization and the associated contractility that includes both the kinetics of stress-fiber formation and dissociation as well as the kinetics of stress-fiber remodeling. These kinetics are motivated by considering the enthalpies of the actin/myosin functional units that constitute the stress fibers. The stress, strain and strain rate dependence of the stress-fiber dynamics are natural outcomes of the approach. The model is presented in a general 3D framework and includes the transport of the unbound stress-fiber proteins. Predictions of the model for a range of cyclic loadings are illustrated to rationalize hitherto apparently contrasting observations. These observations include: (1) For strain amplitudes around 10 % and cyclic frequencies of about 1 Hz, stress fibers align perpendicular to the straining direction in cells subjected to cyclic straining on a 2D substrate while the stress fibers align parallel with the straining direction in cells constrained in a 3D tissue. (2) At lower applied cyclic frequencies, stress fibers in cells on 2D substrates display no sensitivity to symmetric applied strain versus time waveforms but realign in response to applied loadings with a fast lengthening rate and slow shortening. (3) At very low applied cyclic frequencies (on the order of mHz) with symmetric strain versus time waveforms, cells on 2D substrates orient perpendicular to the direction of cyclic straining above a critical strain amplitude.

Simulation of the cytoskeletal response of cells on grooved or patterned substrates.

We analyse the response of osteoblasts on grooved substrates via a model that accounts for the cooperative feedback between intracellular signalling, focal adhesion development and stress fibre contractility. The grooved substrate is modelled as a pattern of alternating strips on which the cell can adhere and strips on which adhesion is inhibited. The coupled modelling scheme is shown to capture some key experimental observations including (i) the observation that osteoblasts orient themselves randomly on substrates with groove pitches less than about 150 nm but they align themselves with the direction of the grooves on substrates with larger pitches and (ii) actin fibres bridge over the grooves on substrates with groove pitches less than about 150 nm but form a network of fibres aligned with the ridges, with nearly no fibres across the grooves, for substrates with groove pitches greater than about 300 nm. Using the model, we demonstrate that the degree of bridging of the stress fibres across the grooves, and consequently the cell orientation, is governed by the diffusion of signalling proteins activated at the focal adhesion sites on the ridges. For large groove pitches, the signalling proteins are dephosphorylated before they can reach the regions of the cell above the grooves and hence stress fibres cannot form in those parts of the cell. On the other hand, the stress fibre activation signal diffuses to a reasonably spatially homogeneous level on substrates with small groove pitches and hence stable stress fibres develop across the grooves in these cases. The model thus rationalizes the responsiveness of osteoblasts to the topography of substrates based on the complex feedback involving focal adhesion formation on the ridges, the triggering of signalling pathways by these adhesions and the activation of stress fibre networks by these signals.

Study of pulmonary (lung) functioning of commercial wheat grinding machine operators in India using spirometric testing.

In ancient times, in India each household had a chakki to mill the wheat. There was no concept of getting wheat grounded from outside. With the fast changing lifestyle, this tradition has almost disappeared now. Today every city of India has numerous commercial wheat grinding machine shops located at various places. Other than states like Punjab, this occupation is mostly unorganized and little has been done to look into the welfare and health of these machine operators. Like most of the occupations, this occupation of wheat grinding also has several occupational hazards and injuries associated with it. The most obvious of them all in this case is due to the continuous exposure of the operators to the rising dust wheat particles, commonly called atta). This perpetual inhaling ofpollutants exposes the operators to risks of pulmonary malfunctioning. Therefore, this paper attempts to determine the pulmonary functioning of the commercial wheat grinding machine operators in India using spirometric testing. On the basis of the anthropometric data, effort has been done to develop the expected lung performance. Further, the actual lung performance of the operators is measured and compared with expected performance.

Simulation of the contractile response of cells on an array of micro-posts.

A bio-chemo-mechanical model has been used to predict the contractile responses of smooth cells on a bed of micro-posts. Predictions obtained for smooth muscle cells reveal that, by converging onto a single set of parameters, the model captures all of the following responses in a self-consistent manner: (i) the scaling of the force exerted by the cells with the number of posts; (ii) actin distributions within the cells, including the rings of actin around the micro-posts; (iii) the curvature of the cell boundaries between the posts; and (iv) the higher post forces towards the cell periphery. Similar correspondences between predictions and measurements have been demonstrated for fibroblasts and mesenchymal stem cells once the maximum stress exerted by the stress fibre bundles has been recalibrated. Consistent with measurements, the model predicts that the forces exerted by the cells will increase with both increasing post stiffness and cell area (or equivalently, post spacing). In conjunction with previous assessments, these findings suggest that this framework represents an important step towards a complete model for the coupled bio-chemo-mechanical responses of cells.

The effect of shape on the adhesion of fibrillar surfaces.

Experimental data have demonstrated that mushroom-shaped fibrils adhere much better to smooth substrates than punch-shaped fibrils. We present a model that suggests that detachment processes for such fibrils are controlled by defects in the contact area that are confined to its outer edge. Stress analysis of the adhered fibril, carried out for both punch and mushroom shapes with and without friction, suggests that defects near the edge of the adhesion area are much more damaging to the pull-off strength in the case of the punch than for the mushroom. The simulations show that the punch has a higher driving force for extension of small edge defects compared with the mushroom adhesion. The ratio of the pull-off force for the mushroom to that of the punch can be predicted from these simulations to be much greater than 20 in the friction-free case, similar to the experimental value. In the case of sticking friction, a ratio of 14 can be deduced. Our analysis also offers a possible explanation for the evolution of asymmetric mushroom shapes (spatulae) in the adhesion organ of geckos.

Environment-friendly cycle time optimization and quality improvisation using Six Sigma.

Healthy environment in any organization can make a difference in improving productivity and quality with low defect, lack of concentration, willingness to work, minimum accidental problems etc. Six Sigma is one of the more recent quality improvement initiatives to gain popularity and acceptance in many industries across the globe. It is an alternative to TQM to obtain minimum manufacturing defect, cycle time reduction, cost reduction, inventory reduction etc. Its use is increasingly widespread in many industries, in both manufacturing and service industries with many proponents of the approach claiming that it has developed beyond a quality control approach into a broader process improvement concept.

Mechanisms of N-acetylcysteine-driven enhancement of MK886-induced apoptosis.

N-Acetylcysteine (NAC), besides being a precursor of glutathione, has an array of other effects including an ability to scavenge free radicals, modulate gene expression and signal transduction pathways, and regulate cell survival and apoptosis. At concentrations lower than 20 mmol/L, NAC is nontoxic to cultured cells and can protect against apoptosis induced by a number of agents. A few recent reports, however, have indicated that NAC can also increase apoptosis. MK886, a 5-lipoxygenase activating protein (FLAP) inhibitor, induces apoptosis in many cell lines by an unknown mechanism that is independent of FLAP and lipoxygenase activity but is possibly related to effects on kinases such as Akt. In Jurkat T lymphocytes, NAC pretreatment (10 mmol/L) enhanced MK886-induced apoptosis by 2.4-fold. Following NAC-MK886 treatment, there was a significant increase in caspase-3 activity, and a decrease in mitochondrial transmembrane potential compared to MK886 alone. However, the extent of cytochrome c release was comparable between MK886 alone and MK886-NAC treatments. The enhancement of MK886-induced apoptosis by 10 mmol/L NAC appears to be partly related to a decrease in pH caused by this concentration of NAC, because an acidic environment favors activation of effector caspases and triggering of mitochondrial apoptosis. However, because neutralized NAC also enhanced apoptosis (1.6-fold), a direct role for NAC in augmenting the apoptotic pathways initiated by MK886 is suggested.

Coronary artery anomalies diagnosed by magnetic resonance angiography.

Coronary artery anomalies are uncommon entities that may be associated with sudden death. Because of its 2-D projection imaging nature, conventional X-ray coronary angiography may not accurately delineate the origins and course of aberrant coronary arteries with respect to the great vessels. Non-invasive, cross-sectional imaging techniques such as coronary CT angiography and magnetic resonance angiography are increasingly used in clinical practice to diagnose coronary artery anomalies. Although this study reviews coronary artery anatomy and selected anomalies as seen with true fast imaging with steady-state precession magnetic resonance angiography, the information provided is equally applicable to electrocardiogram-gated coronary CT angiography.

3D magnetization-prepared true-FISP: a new technique for imaging coronary arteries.

The purpose of this work was to develop an ECG-triggered, segmented 3D true-FISP (fast imaging with steady-state precession) technique to improve the signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) of breath-hold coronary artery imaging. The major task was to optimize an appropriate magnetization preparation scheme to permit saturation of the epicardial fat signal. An alpha/2 preparation pulse was used to speed up the approach to steady-state following a frequency-selective fat-saturation pulse in each heartbeat. The application of dummy cycles was found to reduce the oscillation of the magnetization during data acquisition. The fat saturation and magnetization preparation scheme was validated with simulations and phantom studies. Volunteer studies demonstrated substantially increased SNR (55%) and CNR (178%) for coronary arteries compared to FLASH (fast low-angle shot) with the same imaging time. In conclusion, true-FISP is a promising technique for coronary artery imaging.

Assessment of regional differences in myocardial blood flow using T2-weighted 3D BOLD imaging.

The feasibility of detecting regional differences in myocardial blood flow based on the blood oxygen level-dependent (BOLD) effect was evaluated in vivo in dogs (N = 9) using a 3D T2-prepared segmented gradient-echo sequence at 1.5 T. Regional differences in myocardial blood flow were created by administering adenosine through a catheter placed in the left circumflex coronary artery (LCX). The difference in the R2 (1/T2) relaxation rate between the left ventricular myocardial region supplied by the LCX and regions supplied by the left anterior descending coronary artery (LAD) or septal artery during adenosine administration was correlated to the corresponding regional myocardial blood flow difference determined using fluorescent microspheres. A correlation coefficient of 0.80 was found between the MR BOLD measurements and the myocardial flow assessment. Our results show that the sequence used in this study allows fast 3D BOLD imaging of the heart, and is a promising technique for detecting regional myocardial perfusion differences.

Coronary artery imaging using contrast-enhanced 3D segmented EPI.

The purpose of the work was to evaluate the effectiveness of extracellular contrast media in improving MR coronary angiography using breath-hold segmented echo-planar imaging (SEPI). Two protocols were designed to optimize the inversion recovery-prepared contrast-enhanced SEPI method. In 15 healthy volunteers, significant improvements in signal-to-noise ratio (SNR), contrast-to-noise ratio (CNR), vessel sharpness, and length of visualization were observed post-contrast. The method with two targeted scans to cover the left and right arteries, respectively, following separate 20-mL contrast injections, was found to yield thinner slices and longer right coronary artery (RCA) visualization than a single scan following a 40-mL contrast injection without compromising SNR and CNR. In conclusion, extracellular contrast media substantially improves the delineation of coronary arteries with SEPI. J. Magn. Reson. Imaging 2001;13:676-681.

Coronary arteries: magnetization-prepared contrast-enhanced three-dimensional volume-targeted breath-hold MR angiography.

A volume-targeted contrast agent-enhanced breath-hold coronary magnetic resonance angiographic technique was optimized and evaluated in 16 volunteers. Substantial increases in coronary signal-to-noise ratio, contrast-to-noise ratio, lengths of depiction, and vessel sharpness were observed on enhanced images. The imaging approach with two 20-mL injections of contrast agent covers the left and right coronary arteries in two breath holds and is a promising method for coronary imaging.

Characterization of lidocaine metabolism by rat nasal microsomes: implications for nasal drug delivery.

Lidocaine has been recently approved for use as an intranasal spray in the treatment of migraine. In this study, we investigated the metabolism of lidocaine to its primary metabolite monoethylglycine xylidide (MEGX), by rat nasal olfactory and respiratory microsomes. The metabolic parameters were compared with metabolism employing rat and human hepatic microsomes. The olfactory and respiratory microsomes both exhibited considerable activity for conversion of lidocaine to MEGX in comparison with the activity in the hepatic tissues. The rat olfactory microsomes had a markedly higher affinity than the rat hepatic or respiratory microsomes. However, the turnover rate was only about one-half that of rat liver. Employing Western immunoblotting we investigated the presence of cytochrome P450s (CYPs) 1A2, 3A2, 2B1 and 2C11 in rat nasal tissues; these isozymes are known to partcipated in the metabolism of lidocaine in rat liver. These isozymes were found to be present in significant amounts in both the nasal olfactory and respiratory tissue; this is the first known report of the presence of CYP2C11 in nasal mucosae. Our studies underscore the importance of CYP-mediated drug metabolism in nasal tissues. The effect of this 'nasal first-pass' should be weighed carefully while considering the fate and the bioavailability of drugs delivered via the intranasal route.