Hierarchical Cell Structure Research Articles

Anisotropic compression response of functionally gradient syntactic foam comprising of fly ash cenosphere and polyurethane

A novel functionally gradient (FG) syntactic foam with hierarchical cell structure was prepared by incorporating fly ash cenosphere of different size distribution into rigid polyurethane foam (PUR). The mechanical anisotropic behavior was experimentally studied by axial and traverse compression, and the influence of density gradient on failure mechanism was discussed through digital image correlation (DIC) technique. Results shown that the density graded (GD) syntactic foam produced stepwise stress strain response under axial compression, but failed by global shear pattern under traverse compression. The plateau stress of layered density (LD) syntactic foam gradually increased under both compression modes. The mechanical anisotropic property of FG syntactic foam was much smaller than that of functionally gradient aluminum foam. Failure mechanism of FG syntactic foam under axial compression was mainly controlled by the compression strain band generated in low density layers, and the deformation band successively extended to high density region. Under traverse compression, the material interfaces in LD foam significantly affected the propagation of shear deformation bands generated in high density layers, producing more short-range interpenetrating shear bands. The layered density gradation shown great potential in decreasing the anisotropic characteristics of syntactic foams. These findings can provide a valuable guide for designing and optimizing the impact resistance of functionally gradient foams.

Transparent wood composite prepared from two commercially important tropical timber species

Transparent wood (TW) has garnered significant global attention due to its unique properties. In this study, TW composites were fabricated using two timber species of different density classes: Ailanthus triphysa (common name: Ailanthus wood) and Hevea brasiliensis (common name: Rubberwood). Sodium hydroxide (NaOH) and Hydrogen peroxide-based alkali method was used to modify the lignin in these veneer samples, producing a white cellulose template with a fully intact hierarchical cell structure. Subsequently, a cost-effective thermosetting unsaturated polyester resin (UPR) was infiltrated into the redesigned framework and polymerized to create rigid nanostructured transparent composites. High optical haze (of 94% and 89%) and favourable light transmittance of 59 and 55 percent were exhibited by the UPR-TW composites made from rubberwood and ailanthus wood, respectively. TW was characterised using Scanning electron microscopy and Fourier-transform infrared spectroscopy. The mechanical properties of TW were measured and compared with those of natural wood and pure-polymer. Furthermore, the anisotropic light diffusion behaviour displayed by TW in accordance with the fibre orientation indicates the utility of material as a potential light shaping device. Therefore, a cost-effective and commercially viable strategy to fabricate multipurpose TW composites using a combination of lesser-known timber species (LKTS) and UPR resin was successfully demonstrated.

Fast Multipole Boundary Element Method for Aerodynamic Sound Field Analysis Based on Lighthill’s Equation

The primary disadvantage of the aerodynamic sound field analysis based on the Lighthill’s equation using the boundary element method (BEM) is the computational time; this is mainly because contributions from numerous aerodynamic sound sources are computed at all boundary element nodes and sound-receiving points. This study proposes a fast method for computing source contributions based on the fast multipole method (FMM). Along with the fast multipole BEM, which is already in practical use as a fast BEM, the analysis is substantially accelerated. The use of a common hierarchical cell structure for grouping boundary element nodes, sound-receiving points and aerodynamic sound sources, enables coefficients of the FMM to be reused, thereby further accelerating the analysis. To deal with the increasing hierarchical level, a wideband FMM is applied. In the sound radiation analysis of a quadrupole source located in a free field, the accuracy is validated. Sound radiation from a cylinder located in a flow is analyzed as a practical problem; the accuracy and numerical settings are discussed. Finally, the proposed method is applied to a problem with more than 0.4 million degrees-of-freedom and more than 3 million aerodynamic sound sources to demonstrate its applicability to large-scale problems.

Ultrafast Self-propelling directionally water transporting wood via cell wall reshaping for water manipulation

Water-transport based on biomimetic self-propelled directional water transport materials have attracted wide attention but are limited by low transport speed, costly and complicated preparation, and scaling difficulty. By retaining native cellulose microfibril structure within wood cell walls and their controlled shrinkage, we constructed a self-propelling directionally water transporting wood (SDTW) by reshaping the cell wall to generate robust capillary forces due to aligned longitudinal hierarchical wood cell structures. Hierarchical structures containing directional parallel macro- and micro-sized ridge-groove structures and creating nano-voids are fabricated. They are further intactly fixed after reaction with maleic anhydride to improve the dimensional stability and environmental durability without changing the hydrophilicity on surface. Resulting SDTW exhibited an ultrafast water transport speed of 200.4 mm/s (∼260 % of Nepenthes alata showing a fast speed of 78 mm/s as a native structure) and water absorbing rate of 1.15 × 105 L/m2/h (over 4200-fold higher than that of natural wood). With these ultrafast water transport characters, SDTW was used as water manipulating systems for fast bottom to up transportation of liquid water at a rate of 193.5 mm/s, and as high fog harvesting system with an efficiency of 10.6 g/cm2/h. With its facility, durability, scalability, and sustainability, the SDTW demonstrates a new class of sustainable wood-based materials for practical water manipulation applications.

Bio-Inspired Hierarchical Carbon Nanotube Yarn with Ester Bond Cross-Linkages towards High Conductivity for Multifunctional Applications

The cross-linked hierarchical structure in biological systems provides insight into the development of innovative material structures. Specifically, the sarcoplasmic reticulum muscle is able to transmit electrical impulses in skeletal muscle due to its cross-linked hierarchical tubular cell structure. Inspired by the cross-linked tubular cell structure, we designed and built chemical cross-links between the carbon nanotubes within the carbon nanotube yarn (CNT yarn) structure by an esterification reaction. Consequently, compared with the pristine CNT yarn, its electrical conductivity dramatically enhanced 348%, from 557 S/cm to 1950 S/cm. Furthermore, when applied with three voltages, the electro-thermal temperature of esterified CNT yarn reached 261 °C, much higher than that of pristine CNT yarn (175 °C). In addition, the esterified CNT yarn exhibits a linear and stable piezo-resistive response, with a 158% enhanced gauge factor (the ratio of electrical resistance changing to strain change ~1.9). The superconductivity, flexibility, and stable sensitivity of the esterified flexible CNT yarn demonstrate its great potential in the applications of intelligent devices, smart clothing, or other advanced composites.

Polyamide foams prepared by solution foaming approach and their adsorption property towards bisphenol A

Solution foaming technique was adopted to fabricate polyamide-610 (PA610) foams, in which anhydrous formic acid and sodium carbonate were selected as the acid/base for performing neutralization reaction to in situ produce CO 2 working as foaming agent. The major affecting parameters were investigated to adjust the cellular structure and diameter distribution, such as controlling the polyamide concentration, the sodium carbonate concentration and mixing rate (the rate to add carbonate solution into polyamide-610 solution) to tune the generation rate of CO 2 . The best foaming parameters were determined as 16 wt% PA concentration and 7 wt% sodium carbonate concentration, while the mixing rate was 1 mL/min. The resulting foams were subjected to diverse characterizations. SEM micrographs show that the foams have hierarchical open cell structures. The foaming process has effects on crystallization of the foams, further affecting foams’ thermal performance. The adsorption capacities of the foams towards bisphenol A (BPA) were tested, and the maximum adsorption quantity can be up to 94.84 mg/g. The adsorption isotherm fits better with Freundlich isotherm, indicating multilayer adsorption of the foams towards BPA. The adsorption kinetic follows the pseudo-second order model. After 3 adsorption-desorption cycles, the foams still have medium adsorption level towards BPA. • PA610 foams were prepared by solution foaming based on acid/base reaction. • The foaming process has advantages, particular the mild preparation conditions. • Cellular structure was easily adjusted by tuning experimental parameters. • The foams have good adsorption capacity towards bisphenol A.

Modelling the dynamic compressive response of syntactic foam with hierarchical cell structure

Rate-dependent mechanical response of fly ash cenospheres polyurethane syntactic foams (CPSFs) with hierarchical cell structure was examined through dynamic compression tests. Quasi-static compression strength and specific energy absorption varied in the range of 9.5–29.5 MPa and 4.8–13.0 MJ/m3 within the density of 0.5–0.7 g/cm3. The thermal conductivity coefficient of CPSFs increased from 0.08 W/m/K to 0.13 W/m/K with density, while the specific heat capacity varied in a narrow range of 0.9–1.07 J/g/K. At high strain rates the strength and plateau stress of CPSFs with density of 0.6 g/cm3 increased 25%–39% and 13%–43%. The proposed foam is comparative potential in civil engineering according to its thermal insulation performance and excellent shock dissipation properties. Due to complicated microstructure, CPSFs exhibited different strain rate sensitive behaviors in the initial collapse stage and the stable collapse stage. A constitutive relation was developed to describe dynamic stress-strain response which includes the elastic stage, the initial collapse stage characterized by nonlinear stress increasing and decreasing process, the platform stage and the densification stage. The predictions shown good accordance with the experimental results in wide strain rate range.

Origin, Stemness, Marker and Signaling Pathway of Oral Cancer Stem Cell

Cancer constitutes of complex heterogeneous organ-like structures with a hierarchical cell structure, and only minor phenotypic subpopulations with stem-like properties have a dual capacity to indefinitely self-renew and generate all heterogeneous cell phenotypes consisting of bulk tumor cells. Cancer stem cells (CSC) has similar properties to ordinary stem cells. It is understood that CSC is responsible for the recurrence of metastasis and drug resistance. Thus, control of CSC can provide successful therapy intervention that inhibits cancer growth and aggressive behavior. Conventional cancer therapy is realized to be insufficient for oral cancer therapy. Meanwhile, accurate targeting of OCSC has proved to be a significant challenge due to the commonality of many markers between OCSC and healthy cells. This article discusses the current understanding of oral CSC, with focus on origin, stemness, marker and signalling pathway.Keywords: oral cancer stem cell, CSC, marker, origin, stemness, therapy

Characterization of a new natural cellulosic fiber extracted from Derris scandens stem

The present study aims to identify a potential substitute for the harmful synthetic fibers in the field of polymer composites. With this objective, a comprehensive characterization of Derris scandens stem fibers (DSSFs) was carried out. The presence of high strength gelatinous fibers with a traditional hierarchical cell structure was found in the anatomical study. The chemical compositional analysis estimated the cellulose, hemicellulose, and lignin contents of 63.3 wt%, 11.6 wt%, and 15.3 wt%, respectively. Further analysis with XRD confirmed the presence of crystalline cellulose having a size of 11.92 nm with a crystallinity index of 58.15%. SEM and AFM studies show that these fibers are porous, and the average roughness is 105.95 nm. Single fiber tensile tests revealed that the DSSFs exhibited the mean Young's modulus and tensile strength of 13.54 GPa and 633.87 MPa respectively. Furthermore, the extracted fibers were found to be thermally stable up to 230 °C, as confirmed by thermogravimetric analysis. The fibers extracted from the stem of medicinal plant Derris scandens have the properties comparable to that of existing natural fibers, thus, suggesting it to use as a highly promising reinforcing agent alternative to synthetic fibers in polymer matrix composites.

A Machine Learning Approach for Handover in LTE Networks with Signal Obstructions

Legacy cell-deployment strategies have been adapted to fulfill the increasing demand for wireless broadband internet access. One of them, the Hierarchical Cell Structure (HCS), that is already in use in LTE-A and it is considered essential for the 5G, consists of the deployment of several types of small cells under the umbrella of macrocells, creating overlaid coverages. Due to their low power and bellow-rooftop-level, sometimes indoor base stations, the small cells are severely affected by the surrounding obstacles, making the perceived Quality of Service (QoS) of the users subject to fast variations, thus rendering ineffective the classical approaches to mobility management, that are unable to predict those severe fading situations (coverage holes). Considering the amount of available information on the network performance and the evolution of real-time processing capabilities, the enhancement of LTE functionalities (such as the handover) by means of machine learning algorithms became possible. This work proposes and evaluates the performance of a machine learning based approach to handover in scenarios with the presence of signal-blocking obstacles. We use the ns-3 simulation for our proof of concept simulations. Our machines learn from experience and they are, therefore, able to choose the eNB that will most likely offer the user the highest long term QoS after the handover procedure, even in severe propagation conditions. The proposed schemes substantially improve the users' QoS in certain circumstances.

Theoretical analyses of mechanical performance of higher-order vertex-based hierarchical square cell structure subjected to compression

Hierarchical structure has gained great popularity in recent years, due to its significant promotion on structural strength and stiffness. In this study, the concept of high-order vertex-based hierarchy structure (VHS) was incorporated based on the new-rising multi-cell square tubes by replacing every vertex with scaled itself, layer by layer. The theoretical solution of VHS structure subjected to static compression has been conducted, based on the classical Super Folding Element. Mean crushing force ( MCF, P ) and energy absorption E have been calculated theoretically. The innovative finding lies that the relationship of the mean load between case lever i to lever i+1 has been determined. The relationship of the mean load between case lever i to lever i+1 mainly related to the cell number and length scale. All these achievements shield a light in aid to design novel thin-walled and light-weight energy absorption device.

Joint Load Control and Energy Sharing for Autonomous Operation of 5G Mobile Networks in Micro-Grids

In this paper, we focus on the design of energy self-sustainable mobile networks by enabling intelligent energy management that allows the base stations to mostly operate off-grid by using renewable energies. Many papers are available in the literature on this problem, and however, we are approaching this issue from a different angle. In fact, we advocate for future mobile networks with a hierarchical cell structure and powered by energy harvesting hardware. Base stations within the same geographical area are grouped in a micro-grid and operate almost autonomously from the power grid. To achieve this goal, we target the design of the optimal traffic and computational load control method with energy sharing within the micro-grid. We solve the optimization problem using a graph-based method, and we demonstrate, via software simulations, that a combination of load control plus energy sharing represents a viable and economically convenient solution for enabling energy self-sustainability of mobile networks grouped in micro-grids.

Comprehensive characterization of natural cellulosic fiber from Coccinia grandis stem

The anatomical, physico-chemical, mechanical, thermal and surface characteristics of Coccinia grandis fiber (CGF) were studied for a potential substitute to the harmful synthetic fibers. The anatomical analysis of Coccinia grandis stem reveals the presence of high strength xylem fibers. Polarized light microscopy and scanning electron microscopy of CGF shows a hierarchical cell structure composed of a primary and a secondary cell wall, cell lumen and the middle lamellae. The average cross-sectional area and density of the CGF were 0.0111 mm2 and 1.5175 ± 0.005 g/cm3 respectively. The x-ray diffraction and Fourier transform infrared analyses of the fiber indicates the presence of cellulose Iβ with a crystallinity index of 46.09%. The mean Young’s modulus and tensile strength of the CGF were 124 GPa and 775 MPa respectively, which is sufficient enough for the reinforcement in polymer composites. The thermogravimetric analysis confirms the thermal stability of CGF up to 250 °C, which is well within the polymerization process temperature.

Improved Applied Quality of Fast-Growing Poplar Derived by in-situ Formation of Itaconic Acid-Silica Sol Hybrid Polymer within Wood Hierarchical Cell Structure

Poplar wood (Populus euramericana cv. “I-214”) was impregnated by pulse dipping at 0.7 MPa to 0.8 MPa for 30 min with a catalyst Ln∼SO42−/TiO2–SiO2, itaconic acid, and silica sol. Then, the modifier was cured within the wood micropores during in situ polymerization via kiln drying. The treated wood exhibited increased mechanical strength and decreased hygroscopicity. The modified samples were also characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), and scanning electron microscopy (SEM). The results from the FTIR analysis indicated that the itaconic acid and SiO2 polymerized with the active groups of the wood cell wall. The TGA revealed that the crosslinking reaction between the modifier and wood enhanced the thermal stability of the composite. Lastly, the SEM results indicated the presence of the good interfacial adhesion in the wood modifier between the wood fibers and polymer.

Changing organizational structures of jihadist networks in the Netherlands

This paper uses Social Network Analysis to study and compare the organizational structures and division of roles of three jihadist networks in the Netherlands. It uses unique longitudinal Dutch police data covering the 2000–2013 period. This study demonstrates how the organizational structures transform from a hierarchical cell-structure with a clear division of labor to horizontal and dense networks with less clear orientation on tasks. The core member types in the jihadist movement transform from international jihad veterans with clear leadership skills to homegrown radicals with less status and often a lack of expertise.

Microscopical, physico-chemical, mineralogical, and mechanical characterization of Sansevieria zeylanica fibers as potential reinforcement of composite structures

The aim of this article is to examine for the first time the morphological, physical, chemical, mechanical, and thermal properties of a new kind of fibers, extracted from the leaves of a plant of the Asparagaceae family, to make it possible to use them as potential reinforcement for composite structures. The fibers were extracted from the leaves of Sansevieria zeylanica by decortication process. The presence of mechanical fibers and ribbon fibers were identified through the anatomy of Sansevieria zeylanica leaves. The hierarchical cell structure of these fibers was analyzed through polarized optical microscopy and scanning electron microscopy. It consists of primary cell wall, secondary cell wall, fiber lumen, and middle lamellae. The chemical composition of the natural fibers, in terms of cellulose 76.12%, hemicelluloses 9.32%, lignin 4.28%, and ash content 1.36%, was analyzed by using standard test methods and compared with other natural fibers. The fiber density and fineness were found to be 0.945 ± 0.004 g/cm3 and 8.35 tex, respectively. The thermal behavior of the fiber was investigated through thermogravimetric analysis/differential thermogravimetric analysis. The initial degradation temperature of the cellulose component is 304℃. The results obtained through Fourier transform infrared spectroscopy and X-ray diffraction showed the presence of cellulose with the crystallinity index of 66.67%. Finally, single fiber tensile tests have been performed to assess the mechanical properties. Tensile test of Sansevieria zeylanica fibers showed the tensile strength of 359 MPa and Youngs modulus of 8 GPa.

State-of-the-Art Business Performance Evaluation of the Advanced Wireless Heterogeneous Networks to be Deployed for the “TERA Age”

This article originally presents a conceptual model to assess the business performance of the future advanced wireless and mobile heterogeneous network aiming to preserve enormous broadband demand of the so called "TERA age" in which a monthly usage will be expressed in terabytes and average data rates of 1.0 Gbps per single user. During the assessment from techno-economic perspective, in addition to the level of the user demand, we also take into account the wall penetration losses, density of existing/new base station sites and the recent improvements in cost of the hierarchical cell structures. We consider a spectral efficiency relevant for the forthcoming advanced radio access technologies like: 4G LTE-Advanced or LTE-A (Long Term Evolution Release 10 and beyond), 5G cellular systems deployed in the millimeter-wave (mmW) bands and high-capacity wireless local area network (WLAN) standards like IEEE 802.11ac/ad. In order to compare the achievable profit margins of various network architectures, through extensive network dimensioning for the dense-urban environment and considering the total network cost in present value, we determine the cost-efficient capacity expansion strategies for the mobile network operators by means of proper relation of the production cost per transferred data to the revenues. The findings show that the future micro base stations equipped with 5G mmW radio access technology and WLAN equipped with IEEE 802.11ac access points, can be a guarantee for the business sustainability of outdoor operations. For the highly demanding office environments the most cost-efficient solutions are the pico 5G mmW systems placed inside the buildings together with the IEEE 802.11ad enabled access points. The infrastructure cost is most elastic to a decrease of the unit cost per macro and micro base stations from one side and the capacity and coverage increase of the 5G and WLAN systems form other side. In the cases of the capacity over-provisioning, we determine the principles to provide guaranteed quality of service in terms of data rates to particular number of users.

Cost-effectiveness assessment of 5G systems with cooperative radio resource sharing

By use of techno-economic analysis of heterogeneous hierarchical cell structures and spectral efficiencies of the forthcoming advanced radio access technologies, this paper proposes various cost-efficient capacity enlargement strategies evaluated through the level of the production cost per transferred data unit and achievable profit margins. For the purpose of maximizing the aggregate performance (capacity or profit), we also assess the cooperative manners of radio resource sharing between mobile network operators, especially in the cases of capacity over-provisioning, when we also determine the principles to provide guaranteed data rates to a particular number of users. The results show that, for heavily loaded office environments, the future 5G pico base stations could be a preferable deployment solution. Also, we confirm that the radio resource management method with dynamic resource allocation can significantly improve the capacity of two comparably loaded operators which share the resources and aim to increase their cost effectiveness.

Interference Mitigation for Broadcast in Hierarchical Cell Structure Networks: Transmission Strategy and Area Spectral Efficiency

In this paper, a hierarchical cell structure (HCS) is considered, where an access point (AP) broadcasts to local nodes (LNs) over orthogonal frequency subbands within a local cell located in a macrocell. Since the local cell shares the spectrum licensed to the macrocell, a given LN is interfered with by the macrocell user (MU)'s transmissions over the same subband. To improve the performance of the AP's broadcast service, a novel transmission strategy is proposed to mitigate the interference from the MU to the LN while achieving diversity gain. For the purpose of performance evaluation, the ergodic capacity of the proposed scheme is quantified, and the corresponding closed-form expression is obtained. By comparing with the traditional transmission scheme, which suffers from MU's interference, illustrative numerical results substantiate that the proposed scheme achieves better performance than the traditional scheme as the MU-LN mean channel power gain is larger than half of the AP-LN mean channel power gain. Subsequently, we develop an optimized network design by maximizing the area spectral efficiency (ASE) of the AP's broadcast in the local cell.

Hybrid wood materials with magnetic anisotropy dictated by the hierarchical cell structure.

Anisotropic and hierarchical structures are bound in nature and highly desired in engineered materials, due to their outstanding functions and performance. Mimicking such natural features with synthetic materials and methods has been a highly active area of research in the last decades. Unlike these methods, we use the native biomaterial wood, with its intrinsic anisotropy and hierarchy as a directional scaffold for the incorporation of magnetic nanoparticles inside the wood material. Nanocrystalline iron oxide particles were synthesized in situ via coprecipitation of ferric and ferrous ions within the interconnected pore network of bulk wood. Imaging with low-vacuum and cryogenic electron microscopy as well as spectral Raman mapping revealed layered nanosize particles firmly attached to the inner surface of the wood cell walls. The mineralogy of iron oxide was identified by XRD powder diffraction and Raman spectroscopy as a mixture of the spinel phases magnetite and maghemite. The intrinsic structural architecture of native wood entails a three-dimensional assembly of the colloidal iron oxide which results in direction-dependent magnetic features of the wood-mineral hybrid material. This superinduced magnetic anisotropy, as quantified by direction-dependent magnetic hysteresis loops and low-field susceptibility tensors, allows for directional lift, drag, alignment, (re)orientation, and actuation, and opens up novel applications of the natural resource wood.