Thin Bilayer Research Articles

Numerical simulation of shock initiation of Ni/Al multilayered composites

The initiation of chemical reaction in cold-rolled Ni/Al multilayered composites by shock compression is investigated numerically. A simplified approach is adopted that exploits the disparity between the reaction and shock loading timescales. The impact of shock compression is modeled using CTH simulations that yield pressure, strain, and temperature distributions within the composites due to the shock propagation. The resulting temperature distribution is then used as initial condition to simulate the evolution of the subsequent shock-induced mixing and chemical reaction. To this end, a reduced reaction model is used that expresses the local atomic mixing and heat release rates in terms of an evolution equation for a dimensionless time scale reflecting the age of the mixed layer. The computations are used to assess the effect of bilayer thickness on the reaction, as well as the impact of shock velocity and orientation with respect to the layering. Computed results indicate that initiation and evolution of the reaction are substantially affected by both the shock velocity and the bilayer thickness. In particular, at low impact velocity, Ni/Al multilayered composites with thick bilayers react completely in 100 ms while at high impact velocity and thin bilayers, reaction time was less than 100 μs. Quantitative trends for the dependence of the reaction time on the shock velocity are also determined, for different bilayer thickness and shock orientation.

Membrane Properties Affect Opening Behaviors of the Bacterial Mechanosensitive Channel MscL: Molecular Dynamics Study

Mechanosensitive channel MscL is constituted of homopentamer of a subunit with two transmembrane inner (TM1) and outer (TM2) helices and its 3D structure of the closed state has been resolved. TM1s line an ion permeable pore and cross each other near cytoplasmic side, forming the most constricted part of the pore called gate. TM2s face lipids and some amino acids in TM2 act as a tension sensor. MscL is activated by sensing membrane tension and the major issue of MscL is to understand the gating mechanism driven by membrane tension. Previous studies propose that MscL embedded in thin membrane can activate with a lower threshold than that embedded in thick membrane. However, it remains unclear how MscL activation (opening of the gate) depends on membrane thickness in detail. In this study, we performed molecular dynamics (MD) simulations of MscL embedded in four types of lipid bilayer with different membrane thickness (DLPC, DMPC, DPPC and DSPC) to get insight into the dependency at an atomic level. As a result, it was shown also in our results that MscL in a thin lipid bilayer DLPC opened more widely than that embedded in a thick lipid bilayer DSPC. Furthremore, it was found that the thinner membrane tended to make the transmembrane helices of MscL tilt more largely. In order to check MscL-lipid interactions, we calculated the interaction energy between MscL and lipids and found that the interaction energy between Phe93 and lipids, located at the cytoplasmic side, was smaller as the membrane was thinner. This seems to be due to a hydrophobic mismatch between MscL and lipids, which affect the tilting of transmembrane helices followed by expanding of the gate during opening.

Atomistic Simulations of Pore Formation and Closure in Lipid Bilayers

Cellular membranes separate distinct aqueous compartments, but can be breached by transient hydrophilic pores. A large energetic cost prevents pore formation, which is largely dependent on the composition and structure of the lipid bilayer. The softness of bilayers and the disordered structure of pores make their characterization difficult. We use molecular-dynamics simulations with atomistic detail to study the thermodynamics, kinetics, and mechanism of pore formation and closure in DLPC, DMPC, and DPPC bilayers, with pore formation free energies of 17, 45, and 78 kJ/mol, respectively. By using atomistic computer simulations, we are able to determine not only the free energy for pore formation, but also the enthalpy and entropy, which yields what is believed to be significant new insights in the molecular driving forces behind membrane defects. The free energy cost for pore formation is due to a large unfavorable entropic contribution and a favorable change in enthalpy. Changes in hydrogen bonding patterns occur, with increased lipid-water interactions, and fewer water-water hydrogen bonds, but the total number of overall hydrogen bonds is constant. Equilibrium pore formation is directly observed in the thin DLPC lipid bilayer. Multiple long timescale simulations of pore closure are used to predict pore lifetimes. Our results are important for biological applications, including the activity of antimicrobial peptides and a better understanding of membrane protein folding, and improve our understanding of the fundamental physicochemical nature of membranes.

Kidney regeneration by non‐platelet RNA‐containing particle‐derived cells

We found a group of non-platelet RNA-containing particles (NPRCPs) in human umbilical cord blood. These particles can aggregate, fuse and become non-nucleated cells when cocultured with nucleated cells in vitro. The non-nucleated cells further differentiate into nucleated cells expressing octamer binding transcription factor 4 (OCT4). The NPRCPs are approximately 1-5 μm in diameter, have a thin bilayer membrane, contain short RNAs and microRNAs and express OCT4, sex-determining region Y 2 (SOX2) and DEAD box polypeptide 4 (DDX4). To confirm the function of NPRCPs in vivo, we examined the effects of tail vein-injected green fluorescent protein (GFP)-labelled NPRCPs on mouse kidneys damaged by prior ischaemia and reperfusion from Day 1 to Week 6. Within 1 day of injection of NPRCPs, immunofluorescence and immunohistochemistry revealed a large number of extravasated NPRCPs in the renal calyces, damaged glomeruli and duct tubules. During the course of regeneration, NPRCPs fused into large, non-nucleated cellular structures that further became large nucleated cells to regenerate multicellular kidney tubules. In addition, many NPRCPs became tiny nucleated cellular structures that further differentiated into interstitial cells in connective tissue. The extravasated NPRCPs also arranged themselves into non-cell glomerular structures before further regenerating into nucleated cells of the glomerulus. In conclusion, the results demonstrate that, via different patterns of differentiation, NPRCP-derived cells can regenerate mouse kidney tissue damaged by ischaemia.

In situ electron microscopy investigations of solid-state synthesis in Al/Au thin bilayer films

In situ transmission electron microscopy investigations of solid-state synthesis in Al/Au thin bilayer films are conducted. The samples are heated in the column of a transmission electron microscope. The heating temperature is changed from room temperature to 300°C with a heating rate of up to 120°C min−1. It is found that solid-phase synthesis starts at ≈100°C. At 140 ± 5°C, two crystal phases, Al2Au (Fm3m) and AlAu2 (I4/mmm), are simultaneously observed, while at 235 ± 5°C and higher (up to 300°C) only Al2Au phase is detected.

Pre‐stem cell formation by non‐plateletRNA‐containing particle fusion

1. We found a group of non-platelet RNA-containing particles (NPRCP) in human umbilical cord blood. To understand the origin, characterization and differentiation of NPRCP, we examined cord blood-isolated NPRCP in vitro.2. The NPRCP range in size from < 1 to 5 μm, have a thin bilayer membrane and various morphological features, contain short RNA and microRNA and express octamer-binding transcription factor 4 (OCT4), sex-determining region Y 2 (SOX2) and DEAD box polypeptide 4 (DDX4). On coculture with nucleated cells from umbilical cord blood, NPRCP fuse to small, active, non-nucleated cells called ‘particle fusion-derived non-nucleated cells’ (PFDNC). The PFDNC are approximately 8 μm in diameter and are characterized by their twisting movement in culture plates. They can easily move into and out of nucleated cells and finally differentiate into mesenchymal-like cells. In addition, the larger non-nucleated cellular structures that are derived from the aggregation and fusion of multiple NPRCP can further differentiate into large stem cells that also release OCT4- and SOX2-positive non-nucleated small cells.3. Our data provide strong evidence that NPRCP can fuse into PFDNC, which further differentiate into mesenchymal-like cells. Multiple NPRCP also fuse into other types of large stem cells. We believe that stem cells are derived from NPRCP fusion. There is considerable potential for the use of NPRCP in clinical therapy.

Chemotactic Behavior of Catalytic Motors in Microfluidic Channels

Chemotactic Behavior of Catalytic Motors in Microfluidic Channels

Fast Ion Surface Energy Loss and Straggling in the Surface Wake Fields

We have measured the stopping powers and straggling of fast, highly ionized atoms passing through thin bilayer targets made up of metals and insulators. We were surprised to find that the energy losses as well as the straggling depend on the ordering of the target and have small but significantly different values on bilayer reversal. We ascribe this newly found difference in energy loss to the surface energy loss field effect due to the differing surface wake fields as the beam exits the target in the two cases. This finding is validated with experiments using several different projectiles, velocities, and bilayer targets. Both partners of the diatomic molecular ions also display similar results. A comparison of the energy loss results with those of previous theoretical predictions for the surface wake potential for fast ions in solids supports the existence of a self-wake.

Liquid Scaffolds From A 3-D Printer

Scientists trying to engineer tissue typically start with biodegradable solid or gel scaffolds and then seed living cells onto them. But having greater control over cell spreading and tissue growth would be a big plus for researchers. A scaffold made of liquid compartments could provide that versatility. A method for fabricating such frameworks has been reported by a team led by Hagan Bayley of Oxford University (Science, DOI: 10.1126/science.1229495). To create liquid scaffolds, the researchers custom-built a three-dimensional printer—a device that usually constructs solid objects layer by layer—to squirt tiny liquid droplets from its nozzles. When the machine prints lipid-coated water droplets onto a platform submerged in an oil bath, the 50-µm-diameter droplets adhere to one another. Oil-water repulsion partly drives the interaction. “Instead of fusing to form a larger droplet, the tiny droplets ‘kiss’ and form a very thin bilayer interface” because of their lipid coatings, says former Oxford ...

Formation of supersaturated Au–Ni nanoparticles via dewetting of an Au/Ni bilayer

Supersaturated Au–Ni solid-solution nanoparticles are formed via dewetting of a nanometer Au/Ni thin bilayer using rapid thermal treatment. Annealing at temperatures well below the melting temperatures but above the miscibility gap induces the dewetting and involves the formation of Au–Ni solid-solutions. When cooling down to room temperature, particles formed are usually expected to decompose. When this is kinetically hindered, the supersaturated solution remains in dewetted state. These supersaturated particles are found to lose the ferromagnetic properties of Ni.

An X-ray fluorescence method for determining the mass absorption coefficient in thin-film V/Ge and Cr/Ge bilayer systems

A new approach has been proposed for determining the mass absorption coefficient in the X-ray fluorescence characterization of V/Ge and Cr/Ge thin bilayer structures using simple-to-prepare unified layers grown by depositing vanadium or chromium on a polymer film. We have calculated correction coefficients that take into account the absorption of the primary X-ray tube radiation and that of the analytical line of an element from the lower layer in the upper layer.

Mutual influence of elements in x-ray fluorescence analysis of thin bilayer Ni/Ge-systems

A new approach to the determination of the layer thickness in x-ray fluorescence analysis of bilayer Ni/Ge-systems was proposed. The approach was based on accounting for the degree of attenuation in the upper layer of both the primary x-ray tube radiation and the analytical line of the lower-layer element and the fluorescence intensity amplification of the upper layer by radiation from lower-layer atoms.

Control of Morphology in Pattern Directed Dewetting of a Thin Polymer Bilayer

We report the dewetting of a thin polymer bilayer on a low surface energy topographically patterned substrate with grating geometry. The bilayer, comprising of a polystyrene (PS) top and poly(methyl methacrylate) (PMMA) bottom layer was prepared by direct sequential spin coating on the patterned substrate, using mutually exclusive solvents. Depending on the coating conditions, three distinct initial morphologies of the as coated bilayer is possible: type 1, a discontinuous bottom layer under a discontinuous top layer, resulting in polymer threads confined within the substrate grooves; type 2, discontinuous threads of bottom layer polymer (PMMA) confined within the substrate grooves under a continuous top layer; type 3, continuous bottom and top layers. Our experiments reveal that the initial morphology of the film, particularly, that of the bottom layer significantly influences the final dewetted patterns. For example, in a type 1 or type 2 bilayer the morphology depends significantly on the relative widths of the PMMA threads (LT–PMMA) and that of the substrate grooves (LP). In case LT–PMMA < LP, the bottom PMMA layer disintegrates into isolated droplets aligned along substrate grooves, irrespective of the thickness or morphology of the top PS layer. On the other hand, the overall morphology of the dewetted film is rather strongly influenced by the thickness of the PS layer and the configuration of the bilayer. In case the PMMA threads span the entire width of the substrate grooves (LT–PMMA = LP), the droplet formation is suppressed in favor of an intact PMMA thread, with periodic undulations, submerged under either an undulating thread or an intact layer of PS. In case of a type 3 bilayer, the continuous PMMA bottom layer in most cases ruptures over the substrate stripes, where it is thinnest. This result in the top PS layer coming in direct contact with the substrate and subsequently rupture over the same locations, resulting in core shell threads localized over the substrate grooves. In case of a type 3 bilayer with an ultrathin top film, the two layers rupture simultaneously at different locations and subsequent dewetting results in an exotic structure comprising alternating array of PS droplets and undulating PMMA threads. For a thicker bottom layer, the PMMA film is seen to remain intact, over which the PS film dewets, forming undulating threads. We also construct a morphology phase diagram that depicts the influence of the individual layers on the final dewetted morphology.

Electric field induced instabilities of thin leaky bilayers: Pathways to unique morphologies and miniaturization

Charge leakage of the weakly conducting liquid layers in a thin bilayer can engender interesting interfacial instabilities when exposed to an external electrostatic field. A general linear stability analysis including the full descriptions of the Maxwell stresses uncovers the key short to long-wave features of the instabilities of the bilayers composed of purely dielectric films, leaky dielectric films, and a combination of leaky and dielectric films. The study highlights that for the leaky bilayers the additional electrostatic stress due to the presence of free charges at the interface(s) can significantly reduce the length scale to enforce pattern miniaturization. Unlike a purely dielectric bilayer where the dielectric-contrast across the interfaces dictates the direction of the interfacial deformations, for leaky bilayers the nature of the charge (positive or negative) at the interface can also contribute to the deformation towards or away from the electrodes (anode or cathode). Nonlinear simulations uncover that the interfaces can develop unique morphologies when the spatiotemporal variation of the attractive or repulsive force at the charged interface act together or against the electrical stress due to the induced charge separation across the interface. Exploiting these features a host of periodic interfacial patterns such as core-shell columns, a hole encapsulated by a column, a bundle of columns embedded inside a single column, a collection of holes embedded under a column, and "caged" columns are obtained, which are rather difficult to assemble using other conventional patterning techniques. The results reported can be of importance in the diverse areas of micro/nanotechnology.

The Structure of Cross‐β Tapes and Tubes Formed by an Octapeptide, αSβ1

Elaborate morphology: The αSβ1 peptide, a fragment of α-synuclein, assembles into flat tapes consisting of a peptide bilayer, which can be modeled based on the cross-β structure found in amyloid proteins. The tapes are stabilized by hydrogen bonding, whilst the amphiphilic nature of the peptide results in the thin bilayer structure. To further stabilize the structure, these tapes may twist to form helical tapes, which subsequently close into nanotubes.

Altering Hydrophobic Sequence Lengths Shows That Hydrophobic Mismatch Controls Affinity for Ordered Lipid Domains (Rafts) in the Multitransmembrane Strand Protein Perfringolysin O

The hypothesis that mismatch between transmembrane (TM) length and bilayer width controls TM protein affinity for ordered lipid domains (rafts) was tested using perfringolysin O (PFO), a pore-forming cholesterol-dependent cytolysin. PFO forms a multimeric barrel with many TM segments. The properties of PFO mutants with lengthened or shortened TM segments were compared with that of PFO with wild type TM sequences. Both mutant and wild type length PFO exhibited cholesterol-dependent membrane insertion. Maximal PFO-induced pore formation occurred in vesicles with wider bilayers for lengthened TM segments and in thinner bilayers for shortened TM segments. In diC(18:0) phosphatidylcholine (PC)/diC(14:1) PC/cholesterol vesicles, which form ordered domains with a relatively thick bilayer and disordered domains with a relatively thin bilayer, affinity for ordered domains was greatest with lengthened TM segments and least with shortened TM segments as judged by FRET. Similar results were observed by microscopy in giant vesicles containing sphingomyelin in place of diC(18:0) PC. In contrast, in diC(16:0) PC/diC(14:0) PC/diC(20:1) PC/cholesterol vesicles, which should form ordered domains with a relatively thin bilayer and disordered domains with a relatively thick bilayer, relative affinity for ordered domains was greatest with shortened TM segments and least with lengthened TM segments. The inability of multi-TM segment proteins (unlike single TM segment proteins) to adapt to mismatch by tilting may explain the sensitivity of raft affinity to mismatch. The difference in width sensitivity for single and multi-TM helix proteins may link raft affinity to multimeric state and thus control the assembly of multimeric TM complexes in rafts.

Electric field induced pearling instability in cylindrical vesicles

A theoretical and experimental study on the effect of an electric field on a cylindrical vesicle is presented. Experiments show that a cylindrical vesicle when subjected to an axial electric field, displays an axisymmetric pearling instability (the Rayleigh–Plateau instability) beyond a threshold electric field. Unlike fluid jets, in which electric fields are known to stabilize the Rayleigh–Plateau instability, cylindrical vesicles exhibit dual effects of electric field. The tension required to induce the instability is produced by the electric field. At higher values of field strength however, a stabilizing action of the electric field is seen. This renders the fastest growing wavenumber, km, independent of the electric field at high electric field strength. Theoretical results predict, albeit qualitatively, a weak dependence of km on the electric field. It also explains the substantially higher experimental values of km (around 0.64–1.0) observed in vesicles as compared to km = 0.56 for fluid cylinders. Theory and experiments show that the instability is induced using a much lower value of pulsed DC field as compared to an AC field. At long times, a pearled state is observed, with the pearls separated by short cylindrical nanotubules. The pearled structure is converted into crowded, taut spherical globules, which are connected by very thin short bilayer cylinders when the field is switched off, before they eventually disappear and the original cylindrical vesicle is recovered. All of the morphological changes in cylindrical vesicles in electric fields are completely reversible. This study demonstrates the dominance of tension and stretching at short and long timescales respectively.

Fabrication Parameter Optimization for a Multilayer Photovoltaic Cell Based on the Heterojunction: Zinc(II)-Meso-Tetrakis(4-Bromophenyl) Porphyrins/Fullerenes

The photoelectric properties of multilayer organic photovoltaic cells (OPV cells) were studied. The active organic layers consisted of a planar heterojunction between a layer of Meso-Tetrakis(4-BromoPhenyl) Zinc(II) Porphyrin (BrPhPZn) as electron donor (ED) and a layer fullerene molecules. The devices were fabricated in a high vacuum by thermal sublimation, a technique that allows multilayer devices realization easily by successive depositions, and it does not require solvents, achieving purer films with reproducible characteristics. Taking into account that the anodic contact, a key factor for cell efficiency, is favored by the inclusion of a thin anodic buffer layer (ABL), the effect on the yield after including one or two (ABL): MoO3 or MoO3-CuI layers was studied. The cell which has the best photovoltaic characteristics has a BrPhPZn (ED) thickness of only 12.5 nm. This small thickness is related with the low conductivity of this organic molecule. On the other hand, including a thin MoO3-CuI bilayer increased, such device’s efficiency in a 200%, with regard to a cell without ABL, getting for one cell ITO/MoO3-CuI/BrPhPZn/C60/Alq3/Al, with a 1.03% yield.

Membrane Curvature Elastic Stress Strongly Modulates Metarhodopsin II Formation

We studied the influence of curvature elastic stress in lipid monolayers induced by hydrophobic mismatch between lipid bilayer thickness and the hydrophobic length of rhodopsin transmembrane helices on the metarhodopsin I (MI)/metarhodopsin II (MII) equilibrium. Experiments were conducted at the low rhodopsin/lipid ratio of 1/1,000 to suppress rhodopsin oligomerization. Elastic stress was generated by reconstituting dark-adapted rhodopsin into a series of phosphatidylcholine (PC) bilayers with acyl chains of 14-20 carbons in length and cholesterol content varied from 0 to 30 mol %. 2H NMR was used to monitor the adjustment of the length of hydrocarbon chains to the protein and to characterize monolayer curvature at the protein-lipid interface. We found that the average length of hydrophobic regions on rhodopsin transmembrane helices is 2.6 ± 0.1 nm. Thinner bilayers stretch and bend with negative monolayer curvature to match the length of hydrophobic helices, while thicker bilayers get thinner with positive monolayer curvature near the protein. Shifts in the MI/MII equilibrium depended on the sign of monolayer curvature: negative curvature favored MI while positive curvature favored MII. The results suggest that MII formation generates negative curvature in monolayers near the protein, thus raising curvature stress for thin bilayers but releasing stress for thicker bilayers. Addition of cholesterol increases bilayer hydrophobic thickness, but otherwise showed identical behavior: bilayers with a hydrophobic thickness less than 2.6 nm favored MI while thicker bilayers favored MII. In case of severe hydrophobic mismatch between rhodopsin and bilayers, the behavior was complicated by rhodopsin oligomerization that seems to favor MI. The sensitivity of the MI/MII equilibrium to negative curvature elastic stress was observed earlier in experiments with phosphatidylethanolamines by us and the Brown laboratory. Negative curvature stress upon MII formation is critical for understanding shifts in the MI/MII equilibrium.

Both Detergent Effects Upon Domain Size and Transmembrane Protein Length Effects Upon Domain Binding Suggest that Hydrophobic Mismatch can Control the Properties of Ordered Membrane Domains (“Rafts”)

Hydrophobic mismatch between co-existing domains potentially plays an important role in determining domain size and domain interaction with transmembrane proteins. We recently found that in bilayers with the potential to form co-existing Ld and Lo domains, the detergent Triton X-100 enlarges ordered domain size without perturbing ordered domain formation. This can be explained if the effect of detergent is greatest at the boundary between Lo and Ld domains, which would be the case if the main effect of detergent bound to the membranes is to alter the difference between Lo and Ld bilayer width. In other studies, we found that for a multi-transmembrane segment protein, perfringolysin O (PFO), altering the length of transmembrane sequences controls affinity of the protein for membranes domains in a fashion that is dependent upon domain bilayer width. In bilayers with co-existing Ld and Lo domains, PFO with shortened transmembrane segments preferred to partition into the membrane domains with a thin bilayer width, while PFO with lengthened transmembrane segments preferred to partition into domains forming a wider bilayer. PFO with intermediate length, wild type, transmembrane segments exhibited intermediate behavior. This relative bilayer width preference was the same whether the Lo domains were the thinner or wider membrane domains. The effect of transmembrane length upon domain localization was observed both in vesicles that have domains large enough to see by light microscopy and in vesicles with sub-microscopic domains, in which domain affinity was assayed with FRET. Thus, for both of detergent effects and protein-domain association experiments it is likely that hydrophobic mismatch is a key parameter.