Surface and bulk stresses drive morphological changes in fibrous microtissues

Engineered fibrous tissues consisting of cells encapsulated within collagen gels are widely used three-dimensional in vitro models of morphogenesis and wound healing. Although cell-mediated matrix remodeling that occurs within these scaffolds has been extensively studied, less is known about the mesoscale physical principles governing the dynamics of tissue shape. Here, we show both experimentally and by using computer simulations how surface contraction through the development of surface stresses (analogous to surface tension in fluids) coordinates with bulk contraction to drive shape evolution in constrained three-dimensional microtissues. We used microelectromechanical systems technology to generate arrays of fibrous microtissues and robot-assisted microsurgery to perform local incisions and implantation. We introduce a technique based on phototoxic activation of a small molecule to selectively kill cells in a spatially controlled manner. The model simulations, which reproduced the experimentally observed shape changes after surgical and photochemical operations, indicate that fitting of only bulk and surface contractile moduli is sufficient for the prediction of the equilibrium shape of the microtissues. The computational and experimental methods we have developed provide a general framework to study and predict the morphogenic states of contractile fibrous tissues under external loading at multiple length scales.Published versio

Heavy-Duty Vehicles Modeling and Factors Impacting Fuel Consumption.

A conventional heavy-duty truck PSAT model was validated and incorporated into the Powertrain System Analysis Toolkit (PSAT). The truck that was modeled was a conventional over-the-road 1996 Peterbilt tractor, equipped with a 550 hp Caterpillar 3406E non exhaust gas circulation (EGR) engine and an 18-speed Roadranger manual transmission. A vehicle model was developed, along with the model validation processes. In the engine model, an oxides of nitrogen (NOx) emissions model and a fuel rate map for the Caterpillar 3406E engine were created based on test data. In the gearbox model, a shifting strategy was specified and transmission efficiency lookup tables were developed based on the losses information gathered from the manufacturer. As the largest mechanical accessory model, an engine cooling fan model, which estimates fan power demand, was integrated into the heavy-duty truck model. Experimental test data and PSAT simulation results pertaining to engine fuel rate, engine torque, engine speed, engine power and NOx were within 5% relative error. A quantitative study was conducted by analyzing the impacts of various parameters (vehicle weights, coefficients of rolling resistance and the aerodynamic drag) on fuel consumption (FC) for the Peterbilt truck. The vehicle was simulated over five cycles which represent typical vehicle in-use behavior. Three contributions were generated. First, contour figures provided a convenient way to estimate fuel economy (FE) of the Peterbilt truck over various cycles by interpolating within the parameter values. Second, simulation results revealed that, depending on the circumstances and the cycle, it may be more cost effective to reduce one parameter value (such as coefficient of aerodynamic drag) to increase FE, or it may be more beneficial to reduce another (such as the coefficient of rolling resistance). Third, the amount of the energy consumed by auxiliary loads was found to be highly dependent upon the driving cycles. The ratios between average auxiliary power and average engine power were found to be 71.0%, 17.1%, 15.3%, 12.4% and 11.43% for creep, transient, UDDS, cruise and HHDDT_s cycles, respectively. A hybrid electric bus (HEB) also was modeled. The HEB that was modeled was a New Flyer bus with ISE hybrid system, a Cummins ISB 260H engine and a single-reduction transmission. Information and data were acquired to describe all major components of the HEB. The engine model was validated prior to modeling of the whole vehicle model. The load-following control strategy was utilized in the energy management system. Experimental data and PSAT simulated results were compared over four driving schedules, and the relative percent of errors of the FC, FE, CO2 and NOx were all within 5% except for the FE and NOx of the Manhattan cycle, which were 6.93% and 7.13%, respectively. The high fidelity of this model makes it possible to evaluate the FE and NOx emissions of series hybrid buses for subsequent PSAT users

Mechanistic origin of high retained strength in refractory BCC high entropy alloys up to 1900K

The body centered cubic (BCC) high entropy alloys MoNbTaW and MoNbTaVW show exceptional strength retention up to 1900K. The mechanistic origin of the retained strength is unknown yet is crucial for finding the best alloys across the immense space of BCC HEA compositions. Experiments on Nb-Mo, Fe-Si and Ti-Zr-Nb alloys report decreased mobility of edge dislocations, motivating a theory of strengthening of edge dislocations in BCC alloys. Unlike pure BCC metals and dilute alloys that are controlled by screw dislocation motion at low temperatures, the strength of BCC HEAs can be controlled by edge dislocations, and especially at high temperatures, due to the barriers created for edge glide through the random field of solutes. A parameter-free theory for edge motion in BCC alloys qualitatively and quantitatively captures the strength versus temperature for the MoNbTaW and MoNbTaVW alloys. A reduced analytic version of the theory then enables screening over >600,000 compositions in the Mo-Nb-Ta-V-W family, identifying promising new compositions with high retained strength and/or reduced mass density. Overall, the theory reveals an unexpected mechanism responsible for high temperature strength in BCC alloys and paves the way for theory-guided design of stronger high entropy alloys.Comment: This version corrects the theory and provides more extensive explanation

Full- & Reduced-Order State-Space Modeling of Wind Turbine Systems with Permanent-Magnet Synchronous Generator

Wind energy is an integral part of nowadays energy supply and one of the fastest growing sources of electricity in the world today. Accurate models for wind energy conversion systems (WECSs) are of key interest for the analysis and control design of present and future energy systems. Existing control-oriented WECSs models are subject to unstructured simplifications, which have not been discussed in literature so far. Thus, this technical note presents are thorough derivation of a physical state-space model for permanent magnet synchronous generator WECSs. The physical model considers all dynamic effects that significantly influence the system's power output, including the switching of the power electronics. Alternatively, the model is formulated in the $(a,b,c)$- and $(d,q)$-reference frame. Secondly, a complete control and operation management system for the wind regimes II and III and the transition between the regimes is presented. The control takes practical effects such as input saturation and integral windup into account. Thirdly, by a structured model reduction procedure, two state-space models of WECS with reduced complexity are derived: a non-switching model and a non-switching reduced-order model. The validity of the models is illustrated and compared through a numerical simulation study.Comment: 23 pages, 11 figure

Echinocyte Shapes: Bending, Stretching and Shear Determine Spicule Shape and Spacing

We study the shapes of human red blood cells using continuum mechanics. In particular, we model the crenated, echinocytic shapes and show how they may arise from a competition between the bending energy of the plasma membrane and the stretching/shear elastic energies of the membrane skeleton. In contrast to earlier work, we calculate spicule shapes exactly by solving the equations of continuum mechanics subject to appropriate boundary conditions. A simple scaling analysis of this competition reveals an elastic length which sets the length scale for the spicules and is, thus, related to the number of spicules experimentally observed on the fully developed echinocyte.Comment: Revtex, 27 pages, 8 figures; some minor change

Viscoplasticity and large-scale chain relaxation in glassy-polymeric strain hardening

A simple theory for glassy polymeric mechanical response which accounts for large scale chain relaxation is presented. It captures the crossover from perfect-plastic response to strong strain hardening as the degree of polymerization $N$ increases, without invoking entanglements. By relating hardening to interactions on the scale of monomers and chain segments, we correctly predict its magnitude. Strain activated relaxation arising from the need to maintain constant chain contour length reduces the $N$ dependence of the characteristic relaxation time by a factor $\sim \dot\epsilon N$ during active deformation at strain rate $\dot\epsilon$. This prediction is consistent with results from recent experiments and simulations, and we suggest how it may be further tested experimentally.Comment: The theoretical treatment of the mechanical response has been significantly revised, and the arguments for coherent relaxation during active deformation made more transparen