The impact of individual retirement accounts on savings

Bills to expand individual retirement accounts have been introduced in both houses of Congress this year. While proponents argue that these accounts can help reverse the nation's declining saving rate, recent economic research suggests that the effect of the accounts on savings is in fact quite small.Individual retirement accounts ; Saving and investment

A Field Theoretic Description of Non-Equilibrium Chemical Work Relations

We develop a field theoretic description of non-equilibrium chemical work relations,generalizing the well-known Jarzynski equality using Doi-Peliti field theory. The Jarzynski equality relates the average non-equilibrium work performed on a system to the equilibrium free energy. We consider classical particles undergoing detailed balanced diffusion and chemical reactions in a local potential. The particles are coupled to a chemostat, which is a reservoir of particles, and also a thermal reservoir.Work protocols are imposed by varying the local potential, which drives the system out of equilibrium. We derive the Jarzynski relation in both the Doi representation and in the Doi-Peliti field theory. The Doi representation is a rewriting of the dynamics in terms of creation and annihilation operators, and Doi-Peliti Field theory is an extension of the Doi representation that is convenient for going to the spatial continuum limit. The Jarzynski equality is recovered in the Doi representation due to conditions set by detailed balance. Work relations, in the field theory, appear simply as a result of a gauge-like transformation combined with time reversal. We present the derivation with a one-dimensional system on a lattice and two species of particles but it can be generalized to multiple dimensions with N species of particles. We expect this formalism to be useful in describing spatial chemical reaction networks,for example, sodium-potassium pumps which are distributed along a cell membrane and consume ATP

Mechanics of Helical and Fabric-Like Mesostructures from Polymer-Nanoparticle Hybrids

Hierarchical structures developed from nanoscale building blocks offer an excellent opportunity to control properties on all length scales, from the molecular level up to the macroscale. Many beautiful examples in Nature have demonstrated the significance of controlling geometry and mechanics on small length scales to control function on an organism-level, shown by the strength of bones, the toughness of a mollusk\u27s shell, or the gecko\u27s ability to climb walls. Inspired by stunning examples in both Nature and common man-made materials and structures, we assemble polymers and inorganic nanoparticles (NPs) with well-defined surface chemistry into long ribbons and fabric-like networks with unprecedented length scales. In particular, we focus on the geometry and mechanics of these structures when released from their underlying substrate, as well as the fabrication methods to create such structures. This thesis describes four concepts in detail: (1) the development of an evaporative assembly method used to prepare polymer and NP mesoscale structures, referred to as flexible blade flow coating, (2) the spontaneous formation of helical ribbons, driven by a 2-phase elastocapillary balance between surface tension and elasticity of an asymmetric geometry, (3) the mechanical stretching properties of NP-based helical ribbons, and (4) the deformation, shape and fluid-structure interactions of small, flexible, polymeric microhelices in viscous flow. We first describe flexible blade flow coating, a technique that enables the fabrication of polymer, NP and hybrid mesostructures spanning several length scales. By controlling the fabrication parameters, a wide range of materials can be used to create a wide range of geometries, such as ribbons and fabrics. This method relies upon controlled evaporation of a dilute solution confined between a thin polymer film and a flat substrate. By taking advantage of crosslinkable ligand chemistry and the use of a water-soluble sacrificial layer, the structures are liberated from their substrates, affording robust structures floating at the air-liquid interface or fully submerged. When fully submerged in a fluid with sufficient interfacial tension, like water, we discovered that ribbons spontaneously formed helices. By starting from a general expression that balances the elastic bending energy and surface tension of an asymmetric cross-sectional geometry, we determined that this helical formation is due to the asymmetric reduction of surface area upon bending (serving to lower the system energy). This leads ribbons to bend into helices with a preferred radius governed by both the modulus and interfacial tension, as well as ribbon thickness (R ~ Et2/γ). This universal, geometry-based mechanism provides a new opportunity to create helices from any class of material, which is demonstrated by implementing metallic, ceramic and magnetic NPs, as well as homopolymers. Upon understanding the mechanics of helical formation, we examined the mechanical properties of NP-based helical ribbons. Through the use of a custom-designed mechanical measurement tool, which is capable of measuring ~nN forces over displacements of 100s of microns or greater, we experimentally measured the force-displacement relationship of these helices. We show that this curve can be predicted through the elastic energy and surface-driven helical shape. Our experiments revealed massive stretchability, where helices are able stretch to their fully straightened contour length, as high as 23 times their original length. At low strains, the helices display stiffness values similar to single polymer chains or biological helices (~10-6 N/m), and when fully stretched, display properties similar to synthetic polymer nanofibers. Motivated by small, flexible helices in fluids found in Nature, like swimming bacterial flagella, we expand our studies to examine single helices in viscous fluid flow. We fixed one end of a helix while leaving the other free, placed it into a microchannel, and applied a controlled fluid flow rate. Using PMMA as our model polymeric material, we found that the axial deformation is well-described by a nonlinear helix of finite extensibility, defined by a balance between the viscous and elastic forces. From our experiments, we describe the pitch distribution of a deformed helix in flow, as well as calculate a frictional coefficient for the helical geometry. At high flow rates, we qualitatively observed a global-to-local helical shape instability. Finally, we extend the study to show preliminary results on the deformation of NP-based helices

Hypocretin-1 receptors regulate the reinforcing and reward-enhancing effects of cocaine: pharmacological and behavioral genetics evidence.

Considerable evidence suggests that transmission at hypocretin-1 (orexin-1) receptors (Hcrt-R1) plays an important role in the reinstatement of extinguished cocaine-seeking behaviors in rodents. However, far less is known about the role for hypocretin transmission in regulating ongoing cocaine-taking behavior. Here, we investigated the effects of the selective Hcrt-R1 antagonist SB-334867 on cocaine intake, as measured by intravenous (IV) cocaine self-administration in rats. The stimulatory effects of cocaine on brain reward systems contribute to the establishment and maintenance of cocaine-taking behaviors. Therefore, we also assessed the effects of SB-334867 on the reward-enhancing properties of cocaine, as measured by cocaine-induced lowering of intracranial self-stimulation (ICSS) thresholds. Finally, to definitively establish a role for Hcrt-R1 in regulating cocaine intake, we assessed IV cocaine self-administration in Hcrt-R1 knockout mice. We found that SB-334867 (1-4 mg/kg) dose-dependently decreased cocaine (0.5 mg/kg/infusion) self-administration in rats but did not alter responding for food rewards under the same schedule of reinforcement. This suggests that SB-334867 decreased cocaine reinforcement without negatively impacting operant performance. SB-334867 (1-4 mg/kg) also dose-dependently attenuated the stimulatory effects of cocaine (10 mg/kg) on brain reward systems, as measured by reversal of cocaine-induced lowering of ICSS thresholds in rats. Finally, we found that Hcrt-R1 knockout mice self-administered far less cocaine than wildtype mice across the entire dose-response function. These data demonstrate that Hcrt-R1 play an important role in regulating the reinforcing and reward-enhancing properties of cocaine and suggest that hypocretin transmission is likely essential for establishing and maintaining the cocaine habit in human addicts

When the going gets rough – studying the effect of surface roughness on the adhesive abilities of tree frogs

Tree frogs need to adhere to surfaces of various roughnesses in their natural habitats; these include bark, leaves and rocks. Rough surfaces can alter the effectiveness of their toe pads, due to factors such as a change of real contact area and abrasion of the pad epithelium. Here, we tested the effect of surface roughness on the attachment abilities of the tree frog Litoria caerulea. This was done by testing shear and adhesive forces on artificial surfaces with controlled roughness, both on single toe pads and whole animal scales. It was shown that frogs can stick 2–3 times better on small scale roughnesses (3–6 µm asperities), producing higher adhesive and frictional forces, but relatively poorly on the larger scale roughnesses tested (58.5–562.5 µm asperities). Our experiments suggested that, on such surfaces, the pads secrete insufficient fluid to fill the space under the pad, leaving air pockets that would significantly reduce the Laplace pressure component of capillarity. Therefore, we measured how well the adhesive toe pad would conform to spherical asperities of known sizes using interference reflection microscopy. Based on experiments where the conformation of the pad to individual asperities was examined microscopically, our calculations indicate that the pad epithelium has a low elastic modulus, making it highly deformable