Aging, frailty and complex networks.

When people age their mortality rate increases exponentially, following Gompertz's law. Even so, individuals do not die from old age. Instead, they accumulate age-related illnesses and conditions and so become increasingly vulnerable to death from various external and internal stressors. As a measure of such vulnerability, frailty can be quantified using the frailty index (FI). Larger values of the FI are strongly associated with mortality and other adverse health outcomes. This association, and the insensitivity of the FI to the particular health variables that are included in its construction, makes it a powerful, convenient, and increasingly popular integrative health measure. Still, little is known about why the FI works so well. Our group has recently developed a theoretical network model of health deficits to better understand how changes in health are captured by the FI. In our model, health-related variables are represented by the nodes of a complex network. The network has a scale-free shape or "topology": a few nodes have many connections with other nodes, whereas most nodes have few connections. These nodes can be in two states, either damaged or undamaged. Transitions between damaged and non-damaged states are governed by the stochastic environment of individual nodes. Changes in the degree of damage of connected nodes change the local environment and make further damage more likely. Our model shows how age-dependent acceleration of the FI and of mortality emerges, even without specifying an age-damage relationship or any other time-dependent parameter. We have also used our model to assess how informative individual deficits are with respect to mortality. We find that the information is larger for nodes that are well connected than for nodes that are not. The model supports the idea that aging occurs as an emergent phenomenon, and not as a result of age-specific programming. Instead, aging reflects how damage propagates through a complex network of interconnected elements.

Measuring thickness changes in thin films due to chemical reaction by monitoring the surface roughness with in situ atomic force microscopy.

Measuring the changing thickness of a thin film, without a reference, using an atomic force microscope (AFM) is problematic. Here, we report a method for measuring film thickness based on in situ monitoring of surface roughness of films as their thickness changes. For example, in situ AFM roughness measurements have been performed on alloy film electrodes on rigid substrates as they react with lithium electrochemically. The addition (or removal) of lithium to (or from) the alloy causes the latter to expand (or contract) reversibly in the direction perpendicular to the substrate and, in principle, the change in the overall height of these materials is directly proportional to the change in roughness. If the substrate on which the film is deposited is not perfectly smooth, a correction to the direct proportionality is needed and this is also discussed.

Curved tails in polymerization-based bacterial motility.

The curved actin "comet-tail" of the bacterium Listeria monocytogenes is a visually striking signature of actin polymerization-based motility. Similar actin tails are associated with Shigella flexneri, spotted-fever Rickettsiae, the Vaccinia virus, and vesicles and microspheres in related in vitro systems. We show that the torque required to produce the curvature in the tail can arise from randomly placed actin filaments pushing the bacterium or particle. We find that the curvature magnitude determines the number of actively pushing filaments, independent of viscosity and of the molecular details of force generation. The variation of the curvature with time can be used to infer the dynamics of actin filaments at the bacterial surface.

Dynamic compartmentalization of bacteria: accurate division in E. coli.

Positioning of the midcell division plane within the bacterium E. coli is controlled by the min system of proteins: MinC, MinD, and MinE. These proteins coherently oscillate from end to end of the bacterium. We present a reaction-diffusion model describing the diffusion of min proteins along the bacterium and their transfer between the cytoplasmic membrane and cytoplasm. Our model spontaneously generates protein oscillations in good agreement with experiments. We explore the oscillation stability, frequency, and wavelength as a function of protein concentration and bacterial length.

Dynamical scaling: the two-dimensional XY model following a quench.

To sensitively test scaling in the two-dimensional XY model quenched from high temperatures into the ordered phase, we study the difference between measured correlations and the (scaling) results of a Gaussian-closure approximation. We also directly compare various length scales. All of our results are consistent with dynamical scaling and an asymptotic growth law L approximately (t/ln[t/t(0)])(1/2), though with a time scale t(0) that depends on the length scale in question. We then reconstruct correlations from the minimal-energy configuration consistent with the vortex positions, and find them significantly different from the "natural" correlations - though both scale with L. This indicates that both topological (vortex) and nontopological "spin-wave" contributions to correlations are relevant arbitrarily late after the quench. We also present a consistent definition of dynamical scaling applicable more generally, and emphasize how to generalize our approach to other quenched systems where dynamical scaling is in question. Our approach directly applies to planar liquid-crystal systems.

Triangular anisotropies in driven diffusive systems: reconciliation of up and down.

Deterministic coarse-grained descriptions of driven diffusive systems (DDS) have been hampered by apparent inconsistencies with the kinetic Ising models of DDS. In the evolution towards the driven steady-state, "triangular" anisotropies in the two systems point in opposite directions with respect to the drive field. We show that this is nonuniversal behavior in the sense that the triangular anisotropy "flips" with local modifications of the Ising interactions. The sign and magnitude of the triangular anisotropy also vary with temperature. We have also flipped the anisotropy of coarse-grained models, though not yet at the latest stages of evolution. Our results illustrate the comparison of deterministic coarse-grained and stochastic Ising DDS studies to identify universal phenomena in driven systems. Coarse-grained systems are particularly attractive in terms of analysis and computational efficiency.