Network Of Bundles Research Articles

Simulation of Proton Exchange Membrane Durability Under Fuel Cell Vehicle Operation – a Fundamental Study

Proton exchange membrane fuel cells (PEMFCs) have been proven to be a promising candidate to replace combustion engines due to their zero-carbon emission and high power densities. Despite the recent success in PEMFC commercialization, a number of challenges such as high cost and difficulty in lifetime estimation still hinder their further development. PEMFC durability tests require a long time to complete; therefore, durability predicting models are increasingly important as a supporting tool for further development and implementation.Fuel cell membranes undergo a variety of dynamic conditions during regular operation such as varying temperature, humidity, current density, and cell potential. The cyclic variations of humidity and temperature (hygrothermal variations) during dynamic operation lead to swelling and contraction of the membrane. The fluctuating stress caused by the continuous expansion and contraction of the membrane when confined within the cell leads to mechanical membrane degradation. The recurring swelling and contraction of the membrane which stem from water content changes in the membrane cause fatigue and creep and ultimately the formation of pinholes, cracks, and tears [1-2].Chemical membrane degradation occurs when radicals such as hydroxyl and hydroperoxyl are formed and attack the membrane and is escalated by high operating cell potentials. Chemical degradation results in decay in the ionomer chemical structure, membrane thinning, increased gas crossover, and potential electrode shorting [3]. The outcomes of such degradation are aggravated in the presence of mechanical degradation [4]. Hence, the study of combined chemo-mechanical membrane degradation is crucial for overall fuel cell membrane durability [5].The objective of the present work is to establish a predictive membrane lifetime simulation tool designed to represent chemo-mechanical membrane degradation under various operating conditions for automotive fuel cell applications. To this end, a statistical model is developed based on the membrane fibrillar morphology presented by El Hannach et al. [6-7]. A network of fibre bundles is generated to represent the membrane structure where any bundle is assumed as the primary building block of the membrane [8-10]. Each bundle contains an aggregate of backbone chains with typical mechanical and chemical properties. Mechanical and chemical degradation rates are separately studied and coupled in the model. The thermally activated breaking rate of each fibre is calculated to evaluate the mechanical degradation rate at any given time. In order to calibrate the mechanical degradation rate in the model, the mechanical degradation of a reinforced membrane under cross-pressure accelerated mechanical stress tests ( p-AMST) [11] is investigated to obtain the membrane fatigue lifetime for a variety of cross pressures and temperatures. Then, the genetic algorithm is successfully applied to optimize the experimental parameters considering the least squares method. Furthermore, accelerated membrane durability tests (AMDT) carried out by Macauley et al. [12] are considered to calibrate the chemical degradation rate parameter in the model. Finally, the fully calibrated model is used to estimate the membrane lifetime under realistic fuel cell vehicle operating conditions (e.g. drive cycles).Fuel cell components experience different degradation mechanisms. In this regard, this modelling framework on the membrane lifetime can be used in conjunction with other methodologies on the cathode catalyst lifetime such as [13-14] to scrutinize the key factors and their impacts on fuel cell durability under similar operating conditions. Acknowledgements This research was supported by the Natural Sciences and Engineering Research Council of Canada, Canada Research Chairs, and Simon Fraser University Community Trust Endowment Fund.

Direct Writing of Aligned Carbon Nanotubes across a Trench.

Aligned and suspended carbon nanotubes can outperform randomly oriented networks in electronic biosensing and thin-film electronics. However, carbon nanotubes tend to bundle and form random networks. Here, we show that carbon nanotubes spontaneously align in an ammonium deoxycholate surfactant gel even under low shear forces, allowing direct writing and printing of nanotubes into electrically conducting wires and aligned thin layers across trenches. To demonstrate its application potential, we directly printed arrays of disposable electrical biosensors, which show femtomolar sensitivity in the detection of DNA and SARS-CoV-2 RNA.

Anatomical variations in morphology of right and left brachial plexus among black African population; a cadaveric study in western Kenya

Background: Brachial plexus (Bp) is a network of bundles of nerves of spinal cord around neck and axilla in the body formed by union of anterior primary rami of C5 to C8, and T1. Technical expertise required for clinical and surgical procedures remains challenging for the beginners. Variations may be due to trauma, irradiation, neoplasm, infection, and autoimmune inflammatory disease. The purpose of this study was to determine the difference in variation of right and left Bp of the study population. Methods: Cross sectional descriptive study was used in Maseno, Uzima and Masinde Muliro Universities anatomy laboratory to get sample size of 35male and 35 female cadavers. Chi- square test was set at a p= 0.05. Ethical consideration was observed. Results: Statistically significant difference (p=0.009) between the right and the left upper limb in the branching of musculocutaneous nerve was recorded. A total of 17.8% variations in origin and segments of Bp, majority 9.9% were from left while 7.9% from right. Highest frequency of variation occurred at the prefixed root of Bp with 8.7% occurring on left upper limb and 12.13% on right upper limb. Right upper limbs had highest incidence of variations in branching patterns of Bp at 43% whilst left had incidence of 23.5%. Highest frequency of main terminal variation was of musculocutaneous nerve at 18.6% with 13.6% occurring on right side and 5% on left side. Conclusion: Knowledge of variations is essential in evaluation of unexplained sensory and motor loss after trauma and surgical interventions to the upper limb.

Tension formation of a reconstructed nonmuscle sarcomere.

The interactions between actin and nonmuscle myosin 2 enable tension dependent processes in cells such as migration and adhesion. Although cortical actomyosin networks have been described as mostly disordered, several recent super resolution light microscopy studies have shown evidence for sarcomeric arrangement of actin and myosin in various cell types. Despite their resemblance to muscle sarcomeres where bipolar myosin filaments contract antiparallel actin fibers to bring about shortening, these structures are not as easily amenable to mechanical studies due to their subcellular scale and localization. To address this gap, we reconstituted a synthetic sarcomere using laser guided micropatterning of the actin elongator formin to polymerize antiparallel actin bundles. By adding nonmuscle 2 paralogs, the tension formation and shortening of these arrangements was observed using TIRF microscopy and quantified by tracking fluorescently labeled myosin filaments. Additionally, a FRET based force-sensor was added to the myosin tail and formin, elucidating the tension formation in actin and myosin filaments. Both actin and myosin displayed features resembling isometric tension like force generation within minutes after the start of the experiment, while shortening and processive motility of myosin could be observed more than 1 hour later. This allows us to propose a mechanism of tension formation inside cells, where isometric tension of 2 pNs is mediated by sliding individual actin filaments within a network of bundles, pushing against their bundling and independently of their length. This force generation mechanism at the myosin filament level would enable cells to mediate tension and contractility over the entire cortex, while deformations occur much slower, such as during the formation of a cytokinetic ring.

Organization and dynamics of cross-linked actin filaments in confined environments

The organization of the actin cytoskeleton is impacted by the interplay between physical confinement, features of cross-linking proteins, and deformations of semiflexible actin filaments. Some cross-linking proteins preferentially bind filaments in parallel, although others bind more indiscriminately. However, a quantitative understanding of how the mode of binding influences the assembly of actin networks in confined environments is lacking. Here we employ coarse-grained computer simulations to study the dynamics and organization of semiflexible actin filaments in confined regions upon the addition of cross-linkers. We characterize how the emergent behavior is influenced by the system shape, the number and type of cross-linking proteins, and the length of filaments. Structures include isolated clusters of filaments, highly connected filament bundles, and networks of interconnected bundles and loops. Elongation of one dimension of the system promotes the formation of long bundles that align with the elongated axis. Dynamics are governed by rapid cross-linking into aggregates, followed by a slower change in their shape and connectivity. Cross-linking decreases the average bending energy of short or sparsely connected filaments by suppressing shape fluctuations. However, it increases the average bending energy in highly connected networks because filament bundles become deformed, and small numbers of filaments exhibit long-lived, highly unfavorable configurations. Indiscriminate cross-linking promotes the formation of high-energy configurations due to the increased likelihood of unfavorable, difficult-to-relax configurations at early times. Taken together, this work demonstrates physical mechanisms by which cross-linker binding and physical confinement impact the emergent behavior of actin networks, which is relevant both in cells and in synthetic environments.

Oxygen Adsorption and Desorption Kinetics in CuO Nanowire Bundle Networks: Implications for MOx-Based Gas Sensors

Copper oxide (CuO) nanostructures have demonstrated a good chemiresistive response for the sensing of various gases. The precise sensing mechanisms of this material and, in particular, the kinetics of the response to gases and the sensor recovery remain much less investigated. For metal oxide (MOx) chemiresistive gas sensors, oxygen adsorption and desorption reactions on a material surface play a major role in the sensor kinetics, as most other gases react with oxygen on the sensor surface in order to provide the sensing signal, and nanoscale materials are known to achieve faster response. Here we investigate the oxygen adsorption and desorption kinetics on CuO nanowire bundle (NWB) networks by analyzing the electrical response and recovery curves of sensing devices exposed to oxygen. The observation of multiple time constants for the response and recovery curves led us to discuss the various potential mechanisms related to surface reaction kinetics and diffusion of oxygen species. By comparing the reaction rate constants and diffusion constants extracted from the obtained time constants, we conclude that the diffusion of oxygen defects into the material might be the main reason for the observed large time constants. Surprisingly, we observe that the sensor devices based on as-deposited materials show 4–16 times faster response and recovery as compared to devices after being annealed, depending on the temperature. By comparing this result with detailed characterization using scanning electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy, we discuss the role of the junctions between nanostructures in the network and of the hydroxylated CuO surface to explain this result. Our investigation on the kinetics of adsorption and desorption of oxygen of CuO NWB provides a direct understanding of the general response of the sensor and will be useful in order to further understand and optimize these materials for the sensing of various gases.

Functional Remodeling of the Contractile Smooth Muscle Cell Cortex, a Provocative Concept, Supported by Direct Visualization of Cortical Remodeling.

Simple SummaryAs a key element of the smooth muscle cell contractile apparatus, the actin cytoskeleton participates in the development of force by acting as a molecular track for the myosin cross bridge motor. At the same time, the actin cytoskeleton must transmit the force developed during contraction to the extracellular matrix and, thus, to neighboring cells. This propagation of force to the cell periphery and beyond is initiated in part on specifically localized cellular cortical actin filaments also involved in mechano-chemical transduction. During the contractile process itself and in response to extracellular structural and chemical alterations, the smooth muscle actin cytoskeletal remodels. This indicates that the cytoskeleton is a dynamic cellular organelle that adapts to the changes in cell shape and chemical cues. Current evidence connecting contractile function and mechano-transduction mechanisms to the plasticity of the vascular smooth muscle actin cytoskeleton is reviewed; we then describe new evidence for cytoskeletal remodeling in vascular smooth muscle cells. Here, using immunoelectron microscopy, we visualize the actin binding proteins filamin A, zyxin and talin in these cells and show that they participate in the cortical cell cytoskeletal alteration, thus supporting the premise that smooth muscle cell remodeling occurs during contraction.Considerable controversy has surrounded the functional anatomy of the cytoskeleton of the contractile vascular smooth muscle cell. Recent studies have suggested a dynamic nature of the cortical cytoskeleton of these cells, but direct proof has been lacking. Here, we review past studies in this area suggesting a plasticity of smooth muscle cells. We also present images testing these suggestions by using the technique of immunoelectron microscopy of metal replicas to directly visualize the cortical actin cytoskeleton of the contractile smooth muscle cell along with interactions by representative cytoskeletal binding proteins. We find the cortical cytoskeletal matrix to be a branched, interconnected network of linear actin bundles. Here, the focal adhesion proteins talin and zyxin were localized with nanometer accuracy. Talin is reported in past studies to span the integrin–cytoplasm distance in fibroblasts and zyxin is known to be an adaptor protein between alpha-actinin and VASP. In response to activation of signal transduction with the alpha-agonist phenylephrine, we found that no movement of talin was detectable but that the zyxin-zyxin spacing was statistically significantly decreased in the smooth muscle cells examined. Contractile smooth muscle is often assumed to have a fixed cytoskeletal structure. Thus, the results included here are important in that they directly support the concept at the electron microscopic level that the focal adhesion of the contractile smooth muscle cell has a dynamic nature and that the protein–protein interfaces showing plasticity are protein-specific.

Three‐Level Hierarchical 3D Network Formation and Structure Elucidation of Wet Hydrogel of Tunable‐High‐Strength Nanocomposites

AbstractThe structural characterization of wet gel state hydrogels at multi‐length scales ranging from nanometers to micrometers is very challenging and rarely reported. Herein, tunable high‐strength nanocomposite hydrogels based on cellulose nanofibers with various cross linkers are synthesized and characterized. Synchrotron tomography and small‐angle X‐ray scattering methods are combined to quantitatively elucidate 3D hierarchically wet gel state network structures, providing an understanding on the structure–properties relationship of hydrogels. The hierarchical 3D network of wet gel demonstrates three levels of structures: 1) the bundle aggregated by cellulose nanofibers; 2) physically crosslinked bundle networks; and 3) fractal‐structure agglomerates composed of aggregated network domains. The finding of fractal agglomerate structures tuned by crosslinker characteristics provides fundamental knowledge to explain a variation of several orders of magnitude of mechanical properties and reveals different formation mechanisms, opening a new and flexible prospect for wet gel state material design and processing control.

A fast computational model for the electrophysiology of the whole human heart

In this study we present a novel computational model for unprecedented simulations of the whole cardiac electrophysiology. According to the heterogeneous electrophysiologic properties of the heart, the whole cardiac geometry is decomposed into a set of coupled conductive media having different topology and electrical conductivities: (i) a network of slender bundles comprising a fast conduction atrial network, the AV–node and the ventricular bundles; (ii) the Purkinje network; and (iii) the atrial and ventricular myocardium. The propagation of the action potential in these conductive media is governed by the bidomain/monodomain equations, which are discretized in space using an in–house finite volume method and coupled to three different cell models, the Courtemanche model [1] for the atrial myocytes, the Stewart model [2] for the Purkinje Network and the ten Tusscher–Panfilov model [3] for the ventricular myocytes. The developed numerical model correctly reproduces the cardiac electrophysiology of the whole human heart in healthy and pathologic conditions and it can be tailored to study and optimize resynchronization therapies or invasive surgical procedures. Importantly, the whole solver is GPU–accelerated using CUDA Fortran providing an unprecedented speedup, thus opening the way for systematic parametric studies and uncertainty quantification analyses.

Interplay between Brownian motion and cross-linking controls bundling dynamics in actin networks

Morphology changes in cross-linked actin networks are important in cell motility, division, and cargo transport. Here, we study the transition from a weakly cross-linked network of actin filaments to a heavily cross-linked network of actin bundles through microscopic Brownian dynamics simulations. We show that this transition occurs in two stages: first, a composite bundle network of small and highly aligned bundles evolves from cross-linking of individual filaments and, second, small bundles coalesce into the clustered bundle state. We demonstrate that Brownian motion speeds up the first stage of this process at a faster rate than the second. We quantify the time to reach the composite bundle state and show that it strongly increases as the mesh size increases only when the concentration of cross-links is small and that it remains roughly constant if we decrease the relative ratio of cross-linkers as we increase the actin concentration. Finally, we examine the dependence of the bundling timescale on filament length, finding that shorter filaments bundle faster because they diffuse faster.

Unfolding the mechanical properties of buckypaper composites: nano- to macro-scale coupled atomistic-continuum simulations

Carbon-based nanostructures are receiving increasing attention over the past two decades due to their unprecedented multi-functional features. However, the macro-scale structural applications of these nanostructures have not yet come to full fruition due to the involvement of complex multi-scale computations and manufacturing. Recently, the research community has started investigating buckypaper, which can be described as a sheet or membrane developed using a network of bundles of single-wall carbon nanotubes, multi-wall carbon nanotubes, or a mixture of both. This article aims to focus on the computational bridging of different length scales involving six levels in the range of nano- to macro-scale behaviour concerning buckypaper composites. The sequential derivatives of carbon at six levels, as analyzed in this paper, involve graphene, CNT, CNT bundle, buckypaper, and buckypaper composite automotive components. Here, we adopt a coupled atomistic-continuum modelling approach for the multi-level simulations. Graphene, CNTs, and CNT bundles are modelled using atomistic simulations, while the buckypaper and its composites are modelled using equivalent beam representations for the bundles and continuum solid representation for resin. At the macro-scale, an industry-relevant multi-material composite automotive component has been investigated, wherein the buckypaper is proposed to be embedded involving sheet moulding compound and carbon prepreg. The current simulations have led to the determination of mechanical properties at each level of the carbon-based materials and their mutual dependence. The numerical results demonstrate that a buckypaper composite can enhance the natural frequency and stiffness up to 25 and 37% with respect to conventional monolithic metallic designs, while reducing the weight by 57%. Such outcomes lead to the realization that carbon-based nanostructural derivative in the form of buckypaper can significantly improve the mechanical properties of advanced lightweight structural components as reinforcements for the next generation of aerospace and automotive structures.Graphical abstract

Microtubule organization changes severely after mannitol and n-butanol treatments inducing microspore embryogenesis in bread wheat

BackgroundA mannitol stress treatment and a subsequent application of n-butanol, known as a microtubule-disrupting agent, enhance microspore embryogenesis (ME) induction and plant regeneration in bread wheat. To characterize changes in cortical (CMT) and endoplasmic (EMT) microtubules organization and dynamics, associated with ME induction treatments, immunocytochemistry studies complemented by confocal laser scanning microscopy (CLSM) were accomplished. This technique has allowed us to perform advanced 3- and 4D studies of MT architecture. The degree of MT fragmentation was examined by the relative fluorescence intensity quantification.ResultsIn uni-nucleated mannitol-treated microspores, severe CMT and EMT fragmentation occurs, although a complex network of short EMT bundles protected the nucleus. Additional treatment with n-butanol resulted in further depolymerization of both CMT and EMT, simultaneously with the formation of MT aggregates in the perinuclear region. Some aggregates resembled a preprophase band. In addition, a portion of the microspores progressed to the first mitotic division during the treatments. Bi-nucleate pollen-like structures showed a high MT depolymerization after mannitol treatment and numerous EMT bundles around the vegetative and generative nuclei after n-butanol. Interestingly, bi-nucleate symmetric structures showed prominent stabilization of EMT.ConclusionsFragmentation and stabilization of microtubules induced by mannitol- and n-butanol lead to new configurations essential for the induction of microspore embryogenesis in bread wheat. These results provide robust insight into MT dynamics during EM induction and open avenues to address newly targeted treatments to induce ME in recalcitrant species.

Diagnosis and Management of Complex Reentrant Arrhythmias Involving the His-Purkinje System.

The His-Purkinje system is a network of bundles and fibres comprised of specialised cells that allow for coordinated, synchronous activation of the ventricles. Although the histology and physiology of the His-Purkinje system have been studied for more than a century, its role in ventricular arrhythmias has recently been discovered with the ongoing elucidation of the mechanisms leading to both benign and life-threatening arrhythmias. Studies of Purkinje-cell electrophysiology show multiple mechanisms responsible for ventricular arrhythmias, including enhanced automaticity, triggered activity and reentry. The variation in functional properties of Purkinje cells in different areas of the His-Purkinje system underlie the propensity for reentry within Purkinje fibres in structurally normal and abnormal hearts. Catheter ablation is an effective therapy in nearly all forms of reentrant arrhythmias involving Purkinje tissue. However, identifying those at risk of developing fascicular arrhythmias is not yet possible. Future research is needed to understand the precise molecular and functional changes resulting in these arrhythmias.

Motor-Driven Restructuring of Cytoskeleton Composites Leads to Tunable Time-Varying Elasticity.

The composite cytoskeleton, comprising interacting networks of semiflexible actin and rigid microtubules, generates forces and restructures by using motor proteins such as myosins to enable key processes including cell motility and mitosis. Yet, how motor-driven activity alters the mechanics of cytoskeleton composites remains an open challenge. Here, we perform optical tweezers microrheology and confocal imaging of composites with varying actin-tubulin molar percentages (25-75, 50-50, and 75-25), driven by light-activated myosin II motors, to show that motor activity increases the elastic plateau modulus by over 2 orders of magnitude by active restructuring of both actin and microtubules that persists for hours after motor activation has ceased. Nonlinear microrheology measurements show that motor-driven restructuring increases the force response and stiffness and suppresses actin bending. The 50-50 composite exhibits the most dramatic mechanical response to motor activity due to the synergistic effects of added stiffness from the microtubules and sufficient motor substrate for pronounced activity.

Digital Literacy and Utilization of ICT Resources for Teaching and Learning amongst Secondary School Teachers in Anambra State, Nigeria: Implications amidst Covid-19 Pandemic

The importance of digital learning was made abundantly clear when schools across the world transited from the traditional classroom learning to on-line learning as a result of the Covid-19 pandemic. The pandemic showed the world that we were not as prepared for digital learning as we thought. Teachers were not familiar with the issues brought on by digital learning and many students did not know how to properly employ available technology . This research, assess the level of digital literacy and use of ICT resources by secondary school teachers in Awka South, Anambra State. Specifically, the objectives of the study were to find out if the teachers in the secondary schools in Awka South LGA are digital literate, the extent of teacher’s awareness of ICT resources, find out if the teachers utilize ICT resources in curriculum delivery and examine the various problems encountered by the teachers in the use of e-learning resources, and as well explore what problems this poses to teaching and learning in the time of Covid-19. The study adopted a survey method. A total of two hundred and forty (240) senior class teachers were selected from nineteen (19) public secondary schools in Awka South. A formal questionnaire was constructed and used to collect data from a target group of 240 respondents as adopted. Descriptive analytical techniques such as frequency, percentage, mean and standard deviation were applied to analyze and interpret the data. The outcomes of such analysis are presented on tables. The result of the study showed that a number of the teachers are not digitally literate and are not aware of ICT and e-learning resource, which led to the disruption of teaching and learning during the Covid-19 pandemic. A major factor which contributed to inadequate awareness of ICT among teachers is lack of teacher’s professional development on ICT awareness and limited support and motivation from school leaders and the government. The few who are digital literate and could access ICT resources are confronted with problems such as lack of internet facilities, poor electricity, poor network, poor management and high cost of ICT facilities and internet data bundles. The study recommended provision of ICT facilities in secondary schools, the installation of internet facilities in secondary schools, subsidized data bundles, provision of e-libraries and creation of awareness about e-learning resources and compulsory ICT and Digital literacy training for teachers. Key words : ICT Resources, Digital Literacy, Awareness, Utilization, Secondary School Teachers. DOI: 10.7176/JEP/12-21-08 Publication date: July 31 st 2021

Event-Chain Monte-Carlo Simulations of Dense Soft Matter Systems

We discuss the rejection-free event-chain Monte-Carlo algorithm and several applications to dense soft matter systems. Event-chain Monte-Carlo is an alternative to standard local Markov-chain Monte-Carlo schemes, which are based on detailed balance, for example the well-known Metropolis-Hastings algorithm. Event-chain Monte-Carlo is a Markov chain Monte-Carlo scheme that uses so-called lifting moves to achieve global balance without rejections (maximal global balance). It has been originally developed for hard sphere systems but is applicable to many soft matter systems and particularly suited for dense soft matter systems with hard core interactions, where it gives significant performance gains compared to a local Monte-Carlo simulation. The algorithm can be generalized to deal with soft interactions and with three-particle interactions, as they naturally arise, for example, in bead-spring models of polymers with bending rigidity. We present results for polymer melts, where the event-chain algorithm can be used for an efficient initialization. We then move on to large systems of semiflexible polymers that form bundles by attractive interactions and can serve as model systems for actin filaments in the cytoskeleton. The event chain algorithm shows that these systems form networks of bundles which coarsen similar to a foam. Finally, we present results on liquid crystal systems, where the event-chain algorithm can equilibrate large systems containing additional colloidal disks very efficiently, which reveals the parallel chaining of disks.

Recapitulating Actin Module Organization in the Drosophila Oocyte Reveals New Roles for Bristle-Actin-Modulating Proteins.

The generation of F-actin bundles is controlled by the action of actin-binding proteins. In Drosophila bristle development, two major actin-bundling proteins—Forked and Fascin—were identified, but still the molecular mechanism by which these actin-bundling proteins and other proteins generate bristle actin bundles is unknown. In this study, we developed a technique that allows recapitulation of bristle actin module organization using the Drosophila ovary by a combination of confocal microscopy, super-resolution structured illumination microscopy, and correlative light and electron microscope analysis. Since Forked generated a distinct ectopic network of actin bundles in the oocyte, the additive effect of two other actin-associated proteins, namely, Fascin and Javelin (Jv), was studied. We found that co-expression of Fascin and Forked demonstrated that the number of actin filaments within the actin bundles dramatically increased, and in their geometric organization, they resembled bristle-like actin bundles. On the other hand, co-expression of Jv with Forked increased the length and density of the actin bundles. When all three proteins co-expressed, the actin bundles were longer and denser, and contained a high number of actin filaments in the bundle. Thus, our results demonstrate that the Drosophila oocyte could serve as a test tube for actin bundle analysis.

Effects of the nanotube length and network morphology on the deformation mechanisms and mechanical properties of cross-linked carbon nanotube films

The effects of the carbon nanotube (CNT) length and material structure on the mechanical properties of free-standing thin CNT films with continuous networks of bundles of nanotubes and covalent cross-links are studied in large-scale simulations. The simulations are performed based on a dynamic mesoscopic model that accounts for stretching and bending of CNTs, van der Waals interaction between nanotubes, and inter-tube cross-links. It is found that the tensile modulus and strength of the CNT films strongly increase with increasing CNT length, but the effect of the nanotube length is altered by the cross-link density. The mutual effect of the nanotube length and cross-link density on the modulus and strength is primarily determined by a single parameter that is equal to the average number of cross-links per nanotube. The modulus and strength, as functions of this parameter, follow the power-type scaling laws with strongly different exponents. The film elongation at the maximum stress is dominated by the value of the cross-link density. The dispersion of nanotubes without formation of thick bundles results in a few-fold increase in the modulus and strength. The variation of the film properties is explained by the effects of the CNT length, cross-link density, and network morphology on the network connectivity. The in-plane compression results in the collective bending of nanotubes and folding of the whole film with only minor irreversible changes in the film structure. Depending on the CNT length, the reliefs of the folded films vary from a complex two-dimensional landscape to a quasi-one-dimensional wavy surface.

Quantification of the mesh structure of bundled actin filaments

Biopolymer networks are essential for a wide variety of cellular functions. The biopolymer actin is known to self-assemble into a variety of spatial structures in response to physiological and physical mechanisms. So far, the mechanics of networks of single actin filaments and bundles has previously been described. However, the spatial structure of actin bundles remains poorly understood. Here, we investigate this question by bundling actin filaments with systematically varied concentrations of known physical bundling agents (MgCl2 and PEG) and physiological bundling agents (α-actinin and fascin). We image bundled actin networks with confocal microscopy and perform analysis to describe their mesh size and the nearest-distance distribution, which we call "mesh structure". We find that the mesh size ξ scales universally with actin concentration as ξ ∼ [actin]-1/2. However, the dependence of ξ on the concentration of the bundling agent depends on the agent used. Finally, we find that nearest-distance distributions are best fit by Weibull and Gamma distributions. A complete understanding of the mesh structure of biopolymer networks leads to a more mechanistic understanding of the structure of the cytoskeleton, and can be exploited to design filters with variable porosity for microfluidic devices.

Confinement Geometry Tunes Fascin-Actin Bundle Structures and Consequently the Shape of a Lipid Bilayer Vesicle.

Depending on the physical and biochemical properties of actin-binding proteins, actin networks form different types of membrane protrusions at the cell periphery. Actin crosslinkers, which facilitate the interaction of actin filaments with one another, are pivotal in determining the mechanical properties and protrusive behavior of actin networks. Short crosslinkers such as fascin bundle F-actin to form rigid spiky filopodial protrusions. By encapsulation of fascin and actin in giant unilamellar vesicles (GUVs), we show that fascin-actin bundles cause various GUV shape changes by forming bundle networks or straight single bundles depending on GUV size and fascin concentration. We also show that the presence of a long crosslinker, α-actinin, impacts fascin-induced GUV shape changes and significantly impairs the formation of filopodia-like protrusions. Actin bundle-induced GUV shape changes are confirmed by light-induced disassembly of actin bundles leading to the reversal of GUV shape. Our study contributes to advancing the design of shape-changing minimal cells for better characterization of the interaction between lipid bilayer membranes and actin cytoskeleton.