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This article studies the size effect on the elastic behavior of solid and hollow polymer nanofibers (e.g., electrospun nanofibers) subjected to uniaxial tension. A one-dimensional nonlinear elastic tension model is proposed that takes into account the coupling effect of fiber elastic deformation and surface tension. The fiber axial force-displacement and stress-strain relations are obtained in explicit forms. It is shown that, at nanoscale, fiber radius has appreciable effect on the elastic response of polymer nanofibers. With consideration of the fiber radial effect, it is shown that the actual contribution of surface energy of the solid polymer fibers to the axial tensile force is πr0γ rather than 2πr0γ (where r0 is the fiber radius after deformation and γ is the surface tension), as commonly used in literature. Compared to solid polymer fibers, the tensile behavior of hollow polymer nanofibers appears more complex with greater axial stiffening effect depending upon the combination effect of the fiber exterior and interior radii and the material properties. The results presented in this study can be utilized for data reduction of the nanoscale tension tests of polymer nanofibers and the analysis and design of nanofiber devices. [ABSTRACT FROM AUTHOR]

Surface effect on the longitudinal and flexural wave propagation in straight nanofibers/nanowires is studied within the framework of continuum mechanics. Modified Euler-Bernoulli and Rayleigh-Love rod equations and modified Timoshenko beam equation were developed to take into account the surface effect. It was found that surface tension/stress plays appreciable influence on the wave propagation in nanofibers/nanowires at very small diameters. Wave dispersion relations were presented in closed form, and numerical calculations were performed to show the variation of the phase velocity with the wave number at varying surface property and fiber diameter. The given results can be used as the theoretical basis of dynamic characterization of nanofibers/nanowires and nanodevice design and analysis. [ABSTRACT FROM AUTHOR]

We study the guided self-assembly of symmetric/asymmetric diblock copolymer (BCP) films on heterogeneous substrates with chemically patterned surface by using a coarse-grained phase-separation model. During the procedure, the free energy employed for the BCP films was modeled by the Ginzburg-Landau free energy with nonlocal interaction, and the flat, chemically patterned surface was considered as a heterogeneous surface with short-range interaction with the BCP molecules. The resulting Cahn-Hilliard equation was solved by means of an efficient semi-implicit Fourier-spectral algorithm. Effects of pattern scale, surface chemical potential, and BCP asymmetry on the self-assembly process were explored in detail and compared with those without chemically patterned substrate surfaces. It was found that the morphology of both symmetric and asymmetric BCP films is strongly influenced by the commensurability between the unconstrained natural period λ* of the bulk BCP and the artificial pattern period. Simulation shows that patterned surface with period close to λ* leads to highly ordered morphology after self-assembly for both symmetric and asymmetric BCP films, and it also dramatically accelerates the guided self-assembly process. The present simulation is in a very good agreement with the recent experimental observation in BCP nanolithography. Finally, the present study also expects an innovative nanomanufacturing method to produce highly ordered nanodots based on the guided self-assembly of asymmetric BCP films on chemically patterned substrates. [ABSTRACT FROM AUTHOR]

Due to their unique characteristics such as multifold change of volume in response to minute change in the environment, resemblance of soft biological tissues, ability to operate in wet environments, and chemical tailorability, stimuli-responsive gels represent a versatile and very promising class of materials for sensors, muscle-type actuators, biomedical applications, and autonomous intelligent structures. Success of these materials in practical applications largely depends on their ability to fulfill application-specific mechanical requirements. This article provides an overview of recent application-driven development of covalent polymer gels with special emphasis on the relevant mechanical factors and properties. A short account of mechanisms of gel swelling and mechanical characteristics of importance to stimuli-responsive gels is presented. The review highlights major barriers for wider application of these materials and discusses latest advances and potential future directions toward overcoming these barriers, including interpenetrating networks, homogeneous networks, nanocomposites, and nanofilamentary gels. [ABSTRACT FROM AUTHOR]

The biomechanics of large- and medium-sized arteries influence the pathophysiology of arterial disease and the response to therapeutic interventions. However, a comprehensive comparative analysis of human arterial biaxial mechanical properties has not yet been reported. Planar biaxial extension was used to establish the passive mechanical properties of human thoracic (TA, $$n=8$$ ) and abdominal (AA, $$n=7$$ ) aorta, common carotid (CCA, $$n=21$$ ), subclavian (SA, $$n=12$$ ), renal (RA, $$n=13$$ ) and common iliac (CIA, $$n=16$$ ) arteries from 11 deceased subjects ( $$54\pm 21$$ years old). Histological evaluation determined the structure of each specimen. Experimental data were used to determine constitutive parameters for a structurally motivated nonlinear anisotropic constitutive model. All arteries demonstrated appreciable anisotropy and large nonlinear deformations. Most CCA, SA, TA, AA and CIA specimens were stiffer longitudinally, while most RAs were stiffer circumferentially. A switch in anisotropy was occasionally demonstrated for all arteries. The CCA was the most compliant, least anisotropic and least frequently diseased of all arteries, while the CIA and AA were the stiffest and the most diseased. The severity of atherosclerosis correlated with age, but was not affected by laterality. Elastin fibers in the aorta, SA and CCA were uniformly and mostly circumferentially distributed throughout the media, while in the RA and CIA, elastin was primarily axially aligned and concentrated in the external elastic lamina. Constitutive modeling provided good fits to the experimental data for most arteries. Biomechanical and architectural features of major arteries differ depending on location and functional environment. A better understanding of localized arterial mechanical properties may support the development of site-specific treatment modalities for arterial disease. [ABSTRACT FROM AUTHOR]

Abstract: Significant reinforcing effects that are often observed in polymer nanoparticulate composites are usually attributed to strong interfacial interactions over extended interfaces in these systems. Here, we study linear low density polyethylene (LLDPE) reinforced with 1–4% fumed silica nanoparticles. Nanocomposite modulus, evaluated as a function of filler volume fraction, significantly exceeds classical micromechanics predictions. Possible reasons for the observed discrepancy are evaluated experimentally and theoretically. It is concluded that primary nanoparticle aggregation rather than polymer–nanoparticle interaction at the interface is mainly responsible for the observed reinforcement effect. A simple micromechanics-informed model of a composite with primary particle aggregates is presented based on the model of secondary aggregation developed earlier. The model is shown capable of predicting nanocomposites behavior by introducing a single new structural parameter with a straightforward physical interpretation. As nanoparticles are prone to agglomerate, their primary or secondary aggregates may be present in many nanocomposite systems and the aggregation state and its effects need to be thoroughly evaluated, along with the classical interfacial interactions. The described reinforcing mechanism may be responsible for other anomalous property changes in nanoparticulate composites reported in the literature. [Copyright &y& Elsevier]

Background: The mechanical environment and properties of the carotid artery play an important role in the formation and progression of atherosclerosis in the carotid bifurcation. The purpose of this work was to measure and compare the range and variation of circumferential stress and tangent elastic moduli in the human common (CCA), external (ECA), and internal (ICA) carotid arteries over the cardiac cycle in vivo. Methods: Measurements were performed in the surgically exposed proximal cervical CCA, distal ECA, and distal ICA of normotensive patients (n = 16) undergoing carotid endarterectomy. All measurements were completed in vivo over the cardiac cycle in the repaired carotid bifurcation after the atherosclerotic plaque was successfully removed. B-mode Duplex ultrasonography was used for measurement of arterial diameter and wall thickness, and an angiocatheter placed in the CCA was used for concurrent measurement of blood pressure. A semiautomatic segmentation algorithm was used to track changes in arterial diameter and wall thickness in response to blood pressure. These measurements were then used to calculate the variation of circumferential (hoop) stresses, tangent elastic moduli (the slope of the stress-strain curve at specified stresses), and strain-induced stiffness of the arterial wall (stiffening in response to the increase of intraluminal blood pressure) for each patient. Results: The diameter and wall thickness of the segments (CCA, ECA, and ICA) of the carotid bifurcation were found to decrease and strain-induced stiffness to increase from proximal CCA to distal ECA and ICA. The circumferential stress from end-diastole (minimum pressure) to peak-systole (maximum pressure) varied nonlinearly from 25 ± 7 to 63 ± 23 kPa (CCA), from 22 ± 7 to 57 ± 19 kPa (ECA), and from 28 ± 8 to 67 ± 23 kPa (ICA). Tangent elastic moduli also varied nonlinearly from end-diastole to peak-systole as follows: from 0.40 ± 0.25 to 1.50 ± 2.05 MPa (CCA), from 0.49 ± 0.34 to 1.14 ± 0.52 MPa (ECA), and from 0.68 ± 0.31 to 1.51 ± 0.69 MPa (ICA). The strain-induced stiffness of CCA and ECA increased more than 3-fold and the stiffness of ICA increased more than 2.5-fold at peak-systole compared with end-diastole. Conclusions: The in vivo mechanical behavior of the three segments of the carotid bifurcation was qualitatively similar, but quantitatively different. All three arteries—CCA, ECA and ICA—exhibited nonlinear variations of circumferential stress and tangent elastic moduli within the normal pressure range. The variability in the properties of the three segments of the carotid bifurcation indicates a need for development of carotid models that match the in vivo properties of the carotid segments. Finally, the observed nonlinear behavior of the artery points to the need for future vascular mechanical studies to evaluate the mechanical factors of the arterial wall over the entire cardiac cycle. [Copyright &y& Elsevier]

Abstract: Pristine and Co-doped In2O3 nanowires were synthesized via electrospinning with subsequent calcination. Scanning electron microscopy, X-ray diffraction, and energy-dispersive X-ray spectroscopy were used to characterize nanowire morphology and structure. Ethanol sensing performance analyzed in the range of temperatures and concentrations showed that Co-doped In2O3 nanowires exhibited significantly enhanced sensitivity and rate of performance with the response and recovery times of 2s and 3s, respectively. Combined with excellent selectivity and linearity, these properties make the fabricated nanowires a good candidate for practical ethanol sensing. Further performance improvements are possible with utilization of nanofiber continuity intrinsic of the used top-down nanowire nanomanufacturing process. [Copyright &y& Elsevier]

Abstract: This paper considers the explicit solutions of free-edge stresses near circumferential cracks in surface coatings of circular torsion bars and their application in determining the progressive cracking density in the coating layers. The problem was formulated within the framework of linear elastic fracture mechanics (LEFM). The free-edge stresses near crack tip and the shear stresses in the cross-section of the torsion bar were approached in explicit forms based on the variational principle of complementary strain energy. Criterion for progressive cracking in the coating layer was established in sense of strain energy conservation, and the crack density is thereby estimated. Effects of external torque, aspect ratio, and elastic properties on the density of progressive cracking were examined numerically. The present study shows that, in the sense of inducing a given crack density, compliant coating layer with lower modulus has much higher critical torque than that of a stiffer one with the same geometries and substrate material, i.e., compliant coating layer has greater cracking tolerance. Meanwhile, the study also indicates that thicker surface coating layer is more pliant to cracking than the thinner ones. The present model can be used for analyzing the damage mechanism and cracking tolerance of surface coatings of torsion shafts and for data reduction of torsional fracture test of brittle surface coatings, etc. [Copyright &y& Elsevier]

Abstract: Dispersion relation of antiplane surface acoustic waves (SAWs) propagating in elastic half-space coated with an anisotropic laminate was determined explicitly by means of Stroh’s formalism within the subsonic range, where is no energy leakage into the substrate. During the procedure, the governing dynamic equation in each anisotropic layer and the displacement–traction continuity at interfaces were exactly satisfied by Stroh’s functions. Explicit algebraic equation was derived for determining the dispersion relation. As an example, the wave number vs. phase velocity diagram for steel half-space coated with a graphite-fiber/epoxy laminate with a lay-up was demonstrated. The given method can be used for the study of SAW properties of anisotropic coating systems and non-destructive evaluation based on surface/guided wave methods. [Copyright &y& Elsevier]

Abstract: Probabilistic edge-delamination strength of a thermosetting polymer composite was studied experimentally in this work. During the procedure, by means of edge-delamination tensile test, the graphite–fiber/epoxy laminate made of unidirectional Toray P7051S-20Q-1000 prepregs with an optimized lay-up of [122/–122/02]S was used for examining its probabilistic distributions of the edge-delamination onset stress and the ultimate tensile strength. Acoustic emission (AE) equipment and optical microscope were utilized to capture the edge-delamination initiation and characterize the failure modes during delaminating. Fractographical analysis based on scanning electron microscope (SEM) was conducted for the characterization of the microscopic failure mechanisms of the composite laminate during edge delaminating. Probabilistic models were introduced for data reduction. Test results show that the edge-delamination onset stress and the ultimate tensile strength of the laminate tested satisfy two two-parameter Weibull distributions very well. AE signals show the main failure mode during edge delaminating is interlaminar shear failure, corresponding to that predicted by numerical simulation and confirmed by SEM fractographical results in this study. [Copyright &y& Elsevier]

Explicit elastic solutions are given for an edge-cracked orthotropic bimaterial butt joint under antiplane singularity of a screw dislocation and a line-force. During the procedure, conformal mapping and known dislocation solution are utilized for constructing the fundamental solution to the present problem. Stress intensity factor (SIF) and energy release rate (ERR) of the edge-cracked butt joint are given in closed-form. In limiting cases, results provided in this work cover those in literature. [ABSTRACT FROM AUTHOR]

This paper is concerned with the interaction between an interfacial edge-crack and a screw dislocation under out-of-plane mechanical and in-plane electric loading in a piezoelectric bimaterial strip. In addition to a discontinuous electric potential across the slip plane, the dislocation is subjected to a line-force and a line-charge at the core. Under the framework of linear piezoelectricity, the out-of-plane displacement and in-plane electric potentials are constructed in closed-form by means of conformal mapping technique and the known solution for screw dislocation in cracked piezoelectric bimaterial. The intensity factors (IFs) and energy release rate (ERR) are derived explicitly. [ABSTRACT FROM AUTHOR]

This paper is concerned with the electroelastic potentials and the fracture parameters of a twin-edge-cracked piezoelectric bimaterial strip with a screw dislocation. By means of conformal mapping technique and the known dislocation solution, the antiplane displacement and inplane electric potentials are obtained in closed-form. The intensity factors and the energy release rate are extracted explicitly. In some limiting cases, the present solutions cover those in the literature. [Copyright &y& Elsevier]

Polymer composites have high potential for applications in repair of aerospace structures. Adhesively bonded composite patches minimize balance and clearance problems on control surfaces and can be readily formed to complex aircraft contours. Delamination in composites can be detrimental to joint performance and durability and should be accounted for in joint design. In this work, a simple engineering model is developed for cracked adhesive joints with arbitrary orthotropic laminated adherends. Joints with unidirectional and cross-ply adherends are analyzed and compared as an example. Variations of the strain energy release rates with the crack location and size are calculated. It is shown that joints with the cross-ply adherends have higher energy release rates than the joints with the unidirectional adherends. Experimental observations of the delamination growth are also performed and found to corroborate the theoretical predictions. [Copyright &y& Elsevier]

A closed-form solution is obtained for the problem of a mode-III interfacial edge crack between two bonded semi-infinite dissimilar elastic strips. A general out-of-plane displacement potential for the crack interacting with a screw dislocation or a line force is constructed using conformal mapping technique and existing dislocation solutions. Based on this displacement potential, the stress intensity factor (SIF, KIII) and the energy release rate (ERR, GIII) for the interfacial edge crack are obtained explicitly. It is shown that, in the limiting special cases, the obtained results coincide with the results available in the literature. The present solution can be used as the Green’s function to analyze interfacial edge cracks subjected to arbitrary anti-plane loadings. As an example, a formula is derived correcting the beam theory used in evaluation of SIF (KIII) and ERR (GIII) of bimaterials in the double cantilever beam (DCB) test configuration. [Copyright &y& Elsevier]

Describes the development of the probabilistic model for damage evolution analysis of composite laminates subjected to random loading. Probabilistic variation functions for mesovolumes stiffness; Calculations of the deterioration of ply stiffness; Effects of the mean level and stochastic deviation of loading on damage evolution and time to failure.

Nanotubes of carbon and other materials are arguably the most fascinating materials playing an important role in nanotechnology today. Advanced continuous fibers produced a revolution in the field of structural materials and composites in the last few decades as a result of their high strength, stiffness, and continuity, which, in turn, meant processing and alignment that were economically feasible. Electrospinning technology enables production of continuous polymer nanofibers from polymer solutions or melts in high electric fields. The charged jet is elongated and accelerated by the electric field, undergoes a variety of instabilities, dries, and is deposited on a substrate as a random nanofiber mat.

The dual nano–macro nature of continuous nanofibers holds great promise in bridging the unique nanoscale properties of DNA to macroscopic devices. However, due to the limitations of existing manufacturing methods, controlled fabrication of macroscopically long DNA nanofibers has not been achieved so far. This article presents optimized electrospinning of substrate‐free and uniform DNA nanofibers with macroscopic lengths and diameters ranging from ultrafine (<50 nm) to submicron scale. Mechanical tests of individual DNA nanofibers show highly nonlinear deformation in tension that is accompanied by unexpectedly high true strength (>1 GPa) and toughness (>200 MPa) values for the thinnest tested nanofibers. Molecular characterization with X‐ray diffraction and polarized Raman spectroscopy indicates that the high mechanical properties and their unusual size effects in DNA nanofibers originate from the high molecular alignment that is preserved with the increase in diameter. These results show that electrospun DNA nanofibers can overcome the weaknesses of DNA nanostructures manufactured by other top‐down and bottom‐up methods and that they offer novel capabilities for the DNA‐based nanomaterials. This article reports manufacturing of DNA nanofibers with macroscopic lengths and controlled diameters ranging from ultrafine (<50 nm) to submicron scale. It is also shown for the first time that individual DNA nanofibers exhibit highly nonlinear deformation in tension that is accompanied by unexpectedly high true strength (>1 GPa) and toughness (>200 MPa) values. [ABSTRACT FROM AUTHOR]

The article focuses on structural nanocomposites reinforcement. It relates that reinforcement of nanocomposites in microscopic structures or critical volumes of nanomaterials enable near-term application that can drive longer-term research. It mentions that most research on structural nanocomposites relied on ultrastrong nanoreinforcement such as single-walled carbon nanotubes. It is noted that tailorability and controlled anisotropy are other useful special features of nanocomposites which is valuable in nanoreinforcement.

A response by Yuris Dzenis to a letter to the editor about his article "Structural Nanocomposites" in the January 25, 2008 issue is presented.

Stimuli-responsive polymerized hydrogels represent a promising class of materials for biomedical and other applications such as muscle-type sensors/actuators due to substantial change in dimensions or shape in response to minute change in their environment, resemblance of soft biological tissues, ability to operate in wet environments, and chemical and physical tailorability. However, despite extensive fundamental research, slow response rate and poor mechanical robustness and durability of gels remain the major unresolved barriers for wider application of these materials. Nanostructured responsive polymer gels based on continuous electrospun nanofilaments are expected to have fast response and enhanced strength, modulus, and toughness, thus resolving both main current shortcomings. In this article, poly(vinyl alcohol)-poly(acrylic acid) and Dextran-poly(acrylic acid) pH-responsive nanofilamentary gels were prepared and compared to bulk gels. In addition to fast response rate, the nanofilamentary gels demonstrated unusual mechanical properties such as cross-over of load vs. length curves obtained at different pH and anisotropy of response. [ABSTRACT FROM AUTHOR]

Understanding the atomistic mechanisms of carbon structure formation during templated multi-step carbonization is very important for further optimization of carbon fiber mechanical properties. Here with use of reactive force field molecular dynamics we have elucidated the mechanism driving double-walled carbon nanotube- and graphite nanoparticle-based in situ templating of polyacrylonitrile derived fibers. Depending on carbonization temperature, the mechanism involves either physisorption (physical templating) or chemisorption (chemical templating) of the fiber medium to the template surface. In either case, strong interaction between template and medium leads to the production of aligned structures that are more robust for nanotubes than graphite. We provide a unique analysis of atomistic simulations that enables quantitative comparison of templating results with the relevant electron diffraction data. [ABSTRACT FROM AUTHOR]

Carotid endarterectomy has a long history in stroke prevention, yet controversy remains concerning optimal techniques. Two methods frequently used are endarterectomy with patch angioplasty (CEAP) and eversion endarterectomy (CEE). The objective of this study was to compare hemodynamics-related stress and strain distributions between arteries repaired using CEAP and CEE. Mathematical models were based on in vivo three-dimensional arterial geometry, pulsatile velocity profiles, and intraluminal pressure inputs obtained from 16 patients with carotid artery disease. These data were combined with experimentally derived nonlinear, anisotropic carotid artery mechanical properties to create fluid-structure interaction models of CEAP and CEE. These models were then used to calculate hemodynamic parameters thought to promote recurrent disease and restenosis. Combining calculations of stress and strain into a composite risk index, called the integral abnormality factor, allowed for an overall comparison between CEAP and CEE. CEE demonstrated lower mechanical stresses in the arterial wall, whereas CEAP straightened the artery and caused high stress and strain concentrations at the suture-artery interface. CEAP produced a larger continuous region of oscillatory, low-shear, vortical flow in the carotid bulb. There was a more than two-fold difference in the integral abnormality factor, favoring CEE. In conclusion, in a realistically simulated carotid artery, fluid-structure interaction modeling demonstrated CEE to produce less mechanical wall stress and improved flow patterns compared with CEAP. Clinical validation with larger numbers of individual patients will ultimately be required to support modeling approaches to help predict arterial disease progression and comparative effectiveness of reconstruction methods and devices. [ABSTRACT FROM AUTHOR]

Background: The role of carotid artery stenting (CAS) for the treatment of carotid artery disease continues to evolve, despite higher stroke and restenosis risks for CAS compared with conventional open endarterectomy. Understanding the effects of CAS on arterial geometry, which strongly influence hemodynamics and wall mechanics, can assist in better stratifying the inherent risk of CAS to individual patients. Study Design: Fifteen consecutive patients undergoing CAS had pre- and post-stenting CT angiograms. These images were used to reconstruct the 3-dimensional geometries of the bilateral carotid arteries from their origin to the skull base. Quantitative assessment of the carotid bifurcation angle, cross-sectional area, tortuosity and artery length, were compared pre- and post-stenting. Plaque volume and calcification were also measured. Mathematical models were devised to determine the mechanisms of CAS-induced geometric changes, and their mechanical and hemodynamic significances. Results: Major and moderate changes in arterial tortuosity and elongation were seen in 5 (33%) patients. Characteristics most associated with the development of CAS-induced geometric changes were stenoses located in the internal carotid artery distal to the carotid bulb, circumferential distribution of plaque, and plaque calcification. Modeling did not demonstrate substantial alterations in wall shear stress due to geometric changes, but did show considerable increases in arterial wall axial stress. Conclusions: Cartoid artery stenting can produce geometric changes to the artery that promote favorable conditions for complications and recurrent disease. Patients with circumferential, highly calcified plaques that are located relatively distal in the internal carotid artery are most likely to have post-stenting geometric changes. [Copyright &y& Elsevier]

Abstract: Man-made high-performance fibers produced a revolution in structural materials in the twentieth century. Fiber properties are known to increase with the decrease of their diameters. Ultrafine sub-micron and nanometer-sized fibers attract rapidly growing interest due to their unique advantages for diverse applications. Electrospinning is a straightforward low-cost top-down nanomanufacturing technique producing continuous nanofibers. Electrospinning allows direct fabrication of macroscopic nanofilamentary assemblies for structural and functional applications. Recently, high-performance polyimide nanofibers have been produced by electrospinning of polyamic acid precursors followed by thermal curing. However, curing resulted in fusion of nanofilaments at contacts. While fusion might be beneficial for some applications, it usually results in deterioration of the ordered molecular structure and mechanical properties of fibers in bundles and tows. In this report, we investigated the effect of residual solvent dimethylacetamide (DMAc) on the fusion of nanofibers in electrospun nanofiber bundles during thermal curing of polyimide. Feasibility of removal of the residual DMAc by aging in vacuum and by washing with other solvents was explored. The effect of an additive, dodecylethyldimethylammonium chloride (DEDAC) on fusion was investigated. It was found that the removal of residual DMAc by aging in vacuum reduced the nanofilament fusion during imidization. The presence of DEDAC in the absence of DMAc was shown to further reduce the fusion. DEDAC may represent a new class of additives with a potential to preserve nanostructure of polyamic acid precursors during thermal conversion to polyimides. The results of this report can be used in the development of ultrahigh-performance nanofilamentary fibers. [Copyright &y& Elsevier]

Abstract: Interest in generating energy from biomass has grown tremendously in recent years. Corn stover is an agricultural by-product, which is abundant in quantity. Gasification and pyrolysis are efficient methods of harnessing energy efficiently from corn stover. The performances of mathematical models to predict the product gas quality rely on characterization of feed materials and the reaction kinetics of their thermal degradation. The objective of this research was to determine selected physical and chemical properties of corn stover related to thermochemical conversion. Thermogravimetric analyses were performed at heating rates of 10, 30, and 50°Cmin−1 in nitrogen (inert) and air (oxidizing) atmospheres. The parameters of the reaction kinetics were obtained and compared with other biomass. The weight losses of corn stover in both inert and oxidizing atmospheres were found to occur in three stages. [Copyright &y& Elsevier]

Solvent evaporation plays a critical role in nanofiber formation in electrospinning. Here, we present a nonlinear mass diffusion-transfer model describing the drying process in dilute polymer solution jets. The model is used to predict transient solvent concentration profiles in polyacrylonitrile/N,N-dimethylformamide (PAN/DMF) jets with the initial radii ranging from 50 μm down to 100 nm. Numerical simulations demonstrate high transient inhomogeneity of solvent concentration over the jet cross-section in microscopic jets. The degree of inhomogeneity decreases for finer, submicron jets. The simulated jet drying time decreases rapidly with the decreasing initial jet radius, from seconds for microjets to single milliseconds for nanojets. The results demonstrate the need for further improved coupled multiphysics models of electrospinning jets. [ABSTRACT FROM AUTHOR]

While electrospun nanofibers have demonstrated the potential for novel tissue engineering scaffolds, very little is known about the molecular mechanism of how cells sense and adapt to nanofibers. Here, we revealed the role of focal adhesion kinase (FAK), one of the key molecular sensors in the focal adhesion complex, in regulating mesenchymal stem cell (MSC) shaping on nanofibers. We produced uniaxially aligned and randomly distributed nanofibers from poly( l -lactic acid) to have the same diameters (about 130 nm) and evaluated MSC behavior on these nanofibers comparing with that on flat PLLA control. C3H10T1/2 murine MSCs exhibited upregulations in FAK expression and phosphorylation (pY397) on nanofibrous cultures as assessed by immunoblotting, and this trend was even greater on aligned nanofibers. MSCs showed significantly elongated and well-spread morphologies on aligned and random nanofibers, respectively. In the presence of FAK silencing via small hairpin RNA (shRNA), cell elongation length in the aligned nanofiber direction (cell major axis length) was significantly decreased, while cells still showed preferred orientation along the aligned nanofibers. On random nanofibers, MSCs with FAK-shRNA showed impaired cell spreading resulting in smaller cell area and higher circularity. Our study provides new data on how MSCs shape their morphologies on aligned and random nanofibrous cultures potentially via FAK-mediated mechanism. [ABSTRACT FROM AUTHOR]

Continuous carbon nanofibers (CNF) present an attractive building block for a variety of multifunctional materials and devices. However, the carbonization of poly(acrylonitrile) (PAN) precursors usually results in CNFs with poor graphitic structure and, consequently, modest/non-optimized properties. This paper reports that the graphitic structure of CNFs can be improved with an addition of a small amount of graphene oxide into PAN prior to processing. Continuous CNFs with 1.4 wt% of graphene oxide nanoparticles are prepared from PAN solutions by electrospinning, stabilized, and carbonized at 800 °C, 1200 °C, and 1850 °C. While the as-prepared graphene oxide-filled PAN nanofibers exhibit a considerable reduction in polymer crystallinity, Raman analysis of the carbonized nanofibers shows that both templating with graphene oxide and increasing the carbonization temperature significantly improve the graphitic order in CNFs. The effect of graphene oxide is more significant at higher carbonization temperatures. Selected area electron diffraction analysis of individual nanofibers reveals increased graphitic order and preferred orientation both in the vicinity of visible graphene oxide nanoparticles and in the regions where nanoparticles were not visible. These results indicate a possibility of global templating in CNFs, where the addition of a small amount of graphene oxide nanoparticles can template the formation of good, preferentially oriented graphitic crystallites in CNFs, leading to improved structure and mechanical and transport properties. [ABSTRACT FROM AUTHOR]

High failure rates of femoropopliteal artery reconstruction are commonly attributed to complex 3D arterial deformations that occur with limb movement. The purpose of this study was to develop a method for accurate assessment of these deformations. Custom-made stainless-steel markers were deployed into 5 in situ cadaveric femoropopliteal arteries using fluoroscopy. Thin-section CT images were acquired with each limb in the straight and acutely bent states. Image segmentation and 3D reconstruction allowed comparison of the relative locations of each intra-arterial marker position for determination of the artery's bending, torsion and axial compression. After imaging, each artery was excised for histological analysis using Verhoeff-Van Gieson staining. Femoropopliteal arteries deformed non-uniformly with highly localized deformations in the proximal superficial femoral artery, and between the adductor hiatus and distal popliteal artery. The largest bending (11 + 3-6 +1 mm radius of curvature), twisting (28 + 9-77 + 27°/cm) and axial compression (19 + 10-30 + 8%) were registered at the adductor hiatus and the below knee popliteal artery. These deformations were 3.7, 19 and 2.5 fold more severe than values currently reported in the literature. Histology demonstrated a distinct sub-adventitial layer of longitudinally oriented elastin fibers with intimal thickening in the segments with the largest deformations. This endovascular intra-arterial marker technique can quantify the non- uniform 3D deformations of the femoropopliteal artery during knee flexion without disturbing surrounding structures. We demonstrate that 3D arterial bending, torsion and compression in the flexed lower limb are highly localized and are substantially more severe than previously reported. [ABSTRACT FROM AUTHOR]

: Tensile tests were performed on carbon nanofibers in situ a transmission electron microscope (TEM) using a microelectromechanical system (MEMS) tensile testing device. The carbon nanofibers tested in this study were produced via the electrospinning of polyacrylonitrile (PAN) into fibers, which are subsequently stabilized in an oxygen environment at 270°C and carbonized in nitrogen at 800°C. To investigate the relationship between the fiber molecular structure, diameter, and mechanical properties, nanofibers with diameters ranging from ∼100 to 300nm were mounted onto a MEMS device using nanomanipulation inside the chamber of a Scanning Electron Microscope, and subsequently tested in tension in situ a TEM. The results show the dependence of strength and modulus on diameter, with a maximum modulus of 262GPa and strength of 7.3GPa measured for a 108nm diameter fiber. In particular, through TEM evaluation of the structure of each individual nanofiber immediately prior to testing, we elucidate a dependence of mechanical properties on the molecular orientation of the graphitic structure: the strength and stiffness of the fibers increases with a higher degree of orientation of the 002 graphitic planes along the fiber axis, which coincides with decreasing fiber diameter. [ABSTRACT FROM AUTHOR]

Accurate characterization of carotid artery geometry is vital to our understanding of the pathogenesis of atherosclerosis. Three-dimensional computer reconstructions based on medical imaging are now ubiquitous; however, mean carotid artery geometry has not yet been comprehensively characterized. The goal of this work was to build and study such geometry based on data from 16 male patients with severe carotid artery disease. Results of computerized tomography angiography were used to analyze the cross-sectional images implementing a semiautomated segmentation algorithm. Extracted data were used to reconstruct the mean three-dimensional geometry and to determine average values and variability of bifurcation and planarity angles, diameters an cross-sectional areas. Contrary to simplified carotid geometry typically depicted and used, our mean artery was tortuous exhibiting nonplanarity and complex curvature and torsion variations. The bifurcation angle was 36 deg ± 11 deg if measured using arterial centerlines and 15 deg ± 14 deg if measured between the walls of the carotid bifurcation branches. The average planarity angle was 11 deg ± 10 deg. Both bifurcation and planarity angles were substantially smaller than values reported in most studies. Cross sections were elliptical, with an average ratio of semimajor to semiminor axes of 1.2. The cross-sectional area increased two-fold in the bulb compared to the proximal common, but then decreased 1.5-fold for the combined area of distal internal and external carotid artery. Inter-patient variability was substantial especially in the bulb region; however, some common geometrical features were observed in most patients. Obtained quantitative data on the mean carotid artery geometry and its variability among patients with severe carotid artery disease can be used by biomedical engineers and biomechanics vascular modelers in their studies of carotid pathophysiology, and by endovascular device and materials manufacturers interested in the mean geometrical features of the artery to target the broad patient population. [ABSTRACT FROM AUTHOR]

Patch angioplasty is the most common technique used for the performance of carotid end-arterectomy. A large number of patching materials are available for use while new materials are being continuously developed. Surprisingly little is known about the mechanical properties of these materials and how these properties compare with those of the carotid artery wall. Mismatch of the mechanical properties can produce mechanical and hemodynamic effects that may compromise the long-term patency of the endarterectomized arterial segment. The aim of this paper was to systematically evaluate and compare the biaxial mechanical behavior of the most commonly used patching materials. We compared PTFE (n = 1), Dacron (n = 2), bovine pericardium (n = 10), autogenous greater saphenous vein (n = 10), and autogenous external jugular vein (n = 9) with the wall of the common carotid artery (n = 18). All patching materials were found to be significantly stiffer than the carotid wall in both the longitudinal and circumferential directions. Synthetic patches demonstrated the most mismatch in stiffness values and vein patches the least mismatch in stiffness values compared to those of the native carotid artery. All biological materials, including the carotid artery, demonstrated substantial nonlinearity, anisotropy, and variability; however, the behavior of biological and biologically-derived patches was both qualitatively and quantitatively different from the behavior of the carotid wall. The majority of carotid arteries tested were stiffer in the circumferential direction, while the opposite anisotropy was observed for all types of vein patches and bovine pericardium. The rates of increase in the nonlinear stiffness over the physiological stress range were also different for the carotid and patching materials. Several carotid wall samples exhibited reverse anisotropy compared to the average behavior of the carotid tissue. A similar characteristic was observed for two of 19 vein patches. The obtained results quantify, for the first time, significant mechanical dissimilarity of the currently available patching materials and the carotid artery. The results can be used as guidance for designing more efficient patches with mechanical properties resembling those of the carotid wall. The presented systematic comparative mechanical analysis of the existing patching materials provides valuable information for patch selection in the daily practice of carotid surgery and can he used in future clinical studies comparing the efficacy of different patches in the performance of carotid endarterectomx. [ABSTRACT FROM AUTHOR]

Fueled in part by governmental regulatory requirements and in part because of science's own interest, public engagements are used to provide input about the policy, ethical, legal, social, and other impacts of science and technology. While public engagements show promise for ensuring that public values are incorporated into science and technology policy, current models of public engagement are disconnected from empirical evidence, are too general to guide decisions about public engagement in specific contexts or for specific purposes, and fail to adequately explain why public engagement outcomes differ across studies. In this paper we briefly review prior models, as well as an approach to building new models that we are currently employing in our research. We describe preliminary results from an experimental study conducted using our approach, which varies the cognitive goals emphasized by an engagement concerning nanotechnology, and the social context for giving input. Results suggest that variations in these factors do matter: Compared to individual input, group discussion of input questions resulted in less-focused, less argumentative, and more active and self-regulated forms of engagement; and emphasis on cognitive learning goals did appear to support learning to a greater extent than emphasis on critical thinking goals. We conclude that future experimental research is warranted and necessary to populate a social science of public engagement with models that answer questions of which public engagements work under what conditions, for what purposes, and why. [ABSTRACT FROM AUTHOR]

Introduction: The hemodynamic effects of carotid artery patching are not well known. Our objective was to develop a fluid-solid finite element model of the endarterectomized and patched carotid artery. Methods: Hyperelastic materials parameters were determined from studies of 8 cadaveric carotids. Blood flow characteristics were based on intraoperative data from a patient undergoing endarterectomy. Wall shear stress, cyclic strain and effective stress were computed as hemodynamic parameters with known association with endothelial injury, neointimal hyperplasia abd atherogenesis. Results: Low wall shear stress, high cyclic strain and high effective stress were identified diffusely in the carotid bulb, at the margins around the patch and in the flow divider. Conclusion: Endarterectomy and Polytetrafluoroethylene patching produce considerable abnormalities in the hemodynamics of the repaired carotid. Advanced mechanical modeling can be used to evaluate different carotid revascularization approaches to obtain optimized biomechanical and hemodynamic results for the care of patients with carotid bifurcation disease. [ABSTRACT FROM AUTHOR]

A novel method to perform nanoscale mechanical characterization of highly deformable nanofibers has been developed. A microelectromechanical system (MEMS) test platform with an on-chip leaf-spring load cell that was tuned with the aid of a focused ion beam was built for fiber gripping and force measurement and it was actuated with an external piezoelectric transducer. Submicron scale tensile tests were performed in ambient conditions under an optical microscope. Engineering stresses and strains were obtained directly from images of the MEMS platform, by extracting the relative rigid body displacements of the device components by digital image correlation. The accuracy in determining displacements by this optical method was shown to be better than 50 nm. In the application of this method, the mechanical behavior of electrospun polyacrylonitrite nanofibers with diameters ranging from 300 to 600 nm was investigated. The stress-strain curves demonstrated an apparent elastic-perfectly plastic behavior with elastic modulus of 7.6±1.5 GPa and large irreversible strains that exceeded 220%. The large fiber stretch ratios were the result of a cascade of periodic necks that formed during cold drawing of the nanofibers. [ABSTRACT FROM AUTHOR]

The mechanical deformation of 12 μm long electrospun polyacrylonitrile (PAN) nanofibers with diameters of 300–600 nm was investigated. The nanofibers were subjected to cold drawing in atmospheric conditions and at strain rates between 10-2 and 10-4 s-1. The ultimate strain of the PAN nanofibers was 60%–130% varying monotonically with the strain rate. On the contrary, the fiber tensile strength, ranging between 30 and 130 MPa, varied nonmonotonically with the slowest drawing rate resulting in the largest ductilities and fiber strengths. At the two faster rates, the large fiber ductilities originated in the formation of a cascade of ripples (necks), while at the slowest strain rate, the nanofibers deformed homogeneously allowing for the largest engineering strengths and extension ratios. [ABSTRACT FROM AUTHOR]

The authors report a technique for measuring Young’s modulus of a single electrospun nanofiber using the vibrations of two microcantilevers coupled with the nanofiber. The modulus is calculated from the resonant frequency shift resulting from the nanofiber. Polyacrylonitrile nanofibers (200 nm diameter) were collected during electrospinning and wrapped on two similar microcantilevers causing a shift in first resonance from 10.0 to 19.4 kHz. Finite element analysis was used to analyze the frequency shift using images from a scanning electron microscope giving a modulus of the as-spun polyacrylonitrile nanofiber of 26.8 GPa. [ABSTRACT FROM AUTHOR]

Ultrafine diameter fibers of polyacrylonitrile (PAN), obtained from electrospinning, have huge potential for structural applications since they exhibit an unusual combination of strength and toughness. However, the difficulty to characterize their supramolecular architecture limits their production at the industrial scale. In this work, the glass transition of electrospun nanofiber mats of PAN was investigated by means of thermal analysis techniques. Modulated temperature differential scanning calorimetry (MT–DSC) and dielectric relaxation spectroscopy (DRS) were used, and relaxation parameters characteristic of the glass transition were obtained. Reduction in average fiber diameter resulted in broadening of the glass transition and a shift of its midpoint to higher temperatures as observed by MT–DSC, revealing additional level of constraints in the amorphous phase. The DRS curves, obtained above the calorimetric signature of the glass transition, superimpose independently on the fiber diameter. This result, which contrasts with MT–DSC observations, shows that the constraint of mobility evidenced at the glass transition, is suppressed when driving the fiber mat to higher temperatures. The dielectric strength increases with temperature, revealing an increase in the density of dipoles participating to the relaxation dynamics. This result, commonly attributed to the progressive mobilization of initially constrained amorphous phase, supports the hypothesis that electrospinning process induces higher level of polymer chain orientation at small fiber diameters, which fades away when crossing the glass transition. The orientation impacts the temperature dependence of the relaxation time close to the glass transition, as it shows higher deviation from Arrhenius behavior with the decrease of the fiber diameter. This leads to an increase of the fragility index which comes in opposition to the decrease in the cooperativity length, estimated from the temperature fluctuation approach of the cooperative rearranging region (CRR) concept. To explain this result, both volume and thermal contributions of the fragility index have been calculated, and a strong increase in the thermal contribution has been observed for the most oriented material. This result is interpreted as a signature of an increase in the polymer chain rigidity. Image 1 • Fiber mats of polyacrylonitrile were obtained from electrospinning. • Relaxation dynamics were investigated from thermal analysis. • Cooperativity decreases when reducing the average fiber diameter. • In contrast the thermal contribution to fragility increases. • Both changes were attributed to the orientation of polymer chains. [ABSTRACT FROM AUTHOR]

Graphene oxide sheets get anisotropically crumpled and oriented by radial and shear forces in fast‐thinning electrospun jets. On page 5763, Y. Dzenis and co‐workers demonstrate that a small fraction of these nanoparticles in polyacrylonitrile precursor nanofibers can produce global improvements of the graphitic order and orientation in the resulting carbon nanofilaments. Such a templating methodology utilizing low‐cost constituents, efficient top‐down nanomanufacturing, and relatively low carbonization temperatures may lead to inexpensive continuous carbon nanofibers with improved mechanical and transport properties. Artwork by Joel Brehm. [ABSTRACT FROM AUTHOR]