Local structure of directed networks

Previous work on undirected small-world networks established the paradigm that locally structured networks tend to have high density of short loops. On the other hand, many realistic networks are directed. Here we investigate the local organization of directed networks and find, surprisingly, that real networks often have very few short loops as compared to random models. We develop a theory and derive conditions for determining if a given network has more or less loops than its randomized counterpart. These findings carry broad implications for structural and dynamical processes sustained by directed networks

Stress-induced stabilization of pyrolyzed polyacrylonitrile and carbon nanotubes electrospun fibers

The unique properties of graphitic carbons have gained widespread attention towards their development and application. Carbon materials can be synthesized by thermal decomposition and, more specifically, carbon pyrolysis from polymer precursors. The paper shows the pyrolysis process of polyacrylonitrile (PAN) in the presence of multi-walled carbon nanotubes (MWCNTs) according to different manufacturing process conditions. The electrospinning process of the PAN-MWCNTs solution on multi-plates collectors was firstly analyzed. The morphology and the particles arrangement of the electrospun fibers was studied under scanning and transmission electron microscopes. Moreover, the composite fibrous mats were characterized by RAMAN spectroscopy to identify the effects of a mechanical tension application during the thermal stabilization phase performed before the pyrolysis treatment to obtain carbon fibers from the precursor polymer. The results show that the graphitization of the pyrolyzed fibers is enhanced by the combination of MWCNTs and a mechanical stress applied during the thermal treatment

Hybrid multi-layered scaffolds produced via grain extrusion and electrospinning for 3D cell culture tests

Purpose: The purpose of this paper is to focus on the production of scaffolds with specific morphology and mechanical behavior to satisfy specific requirements regarding their stiffness, biological interactions and surface structure that can promote cell-cell and cell-matrix interactions though proper porosity, pore size and interconnectivity. Design/methodology/approach: This case study was focused on the production of multi-layered hybrid scaffolds made of polycaprolactone and consisting in supporting grids obtained by Material Extrusion (ME) alternated with electrospun layers. An open source 3D printer was utilized, with a grain extrusion head that allows the production and distribution of strands on the plate according to the designed geometry. Square grid samples were observed under optical microscope showing a good interconnectivity and spatial distribution of the pores, while scanning electron microscope analysis was used to study the electrospun mats morphology. Findings: A good adhesion between the ME and electrospinning layers was achieved by compression under specific thermomechanical conditions obtaining a hybrid three-dimensional scaffold. The mechanical performances of the scaffolds have been analyzed by compression tests, and the biological characterization was carried out by seeding two different cells phenotypes on each side of the substrates. Originality/value: The structure of the multi-layered scaffolds demonstrated to play an important role in promoting cell attachment and proliferation in a 3D culture formation. It is expected that this design will improve the performances of osteochondral scaffolds with a strong influence on the required formation of an interface tissue and structure that need to be rebuilt

micro structuring of titanium collectors by laser ablation technique a promising approach to produce micro patterned scaffolds for tissue engineering applications

Abstract Multi-scale micro-structured scaffolds can sustain attachment and orientation of different cells phenotypes. An innovative use of laser ablation technique to build micro-structured titanium surfaces to be used as collectors in both electrophoretic deposition and electrospinning processes was investigated. To produce micro-patterned scaffolds, a negative replica patterning was exploited by designing specific patterns to be laser ablated on titanium plates. This method allows the deposition of the scaffolds on the mold, thus reproducing the micro-features on the scaffold surface. The titanium surface morphology depending on ablation parameters was studied and the capability of the process in replicating the micro-pattern was characterized

Production of carbonized micro-patterns by photolithography and pyrolysis

The preparation of carbon micro-patterns is reported in this paper. Different carbon micro-patterns were created using photolithography of the epoxy-based negative photoresist SU-8. Photoresist patterns were optimized in terms of resolution and aspect ratio and subsequently subjected to pyrolysis to obtain carbonized and conductive 3D structures. The latter step requires the optimization of the resist cross-linking time as well as the temperature and time of the resist post-bake. This step is crucial in order to avoid any severe modification of the geometry of the patterns produced during the actual pyrolysis. By observing optical and scanning electron microscope images, the morphology of the structures before and after pyrolysis was studied and the same patterns were also characterized by a laser probe profilometer. Finally, the thus obtained carbon patterns on Si wafers were used to carry out cell culture tests with Neural Stem Cells (NSC). The adhesion and the arrangement of the stem cells were analyzed to verify the ability of the patterned substrates to guide the orientation and, therefore, the differentiation of the cells

Number of loops of size h in growing scale-free networks

The hierarchical structure of scale-free networks has been investigated focusing on the scaling of the number $N_h(t)$ of loops of size h as a function of the system size. In particular we have found the analytic expression for the scaling of $N_h(t)$ in the Barab\'asi-Albert (BA) scale-free network. We have performed numerical simulations on the scaling law for $N_h(t)$ in the BA network and in other growing scale free networks, such as the bosonic network (BN) and the aging nodes (AN) network. We show that in the bosonic network and in the aging node network the phase transitions in the topology of the network are accompained by a change in the scaling of the number of loops with the system size.Comment: 4 pages, 3 figure

Multi-layered Scaffolds Production via Fused Deposition Modeling (FDM) Using an Open Source 3D Printer: Process Parameters Optimization for Dimensional Accuracy and Design Reproducibility

Abstract One of the most applied strategies in tissue engineering consists in the development of 3D porous scaffolds with similar composition to the specific tissue. In fact, the microstructure of the scaffolds influences the final structure of the in growing tissue. In this study, multi-layered PCL scaffolds were produced with modified Fab@home FDM printer in order to analyze the influence of the extrusion technology (filament or powder extrusion head) and of the process parameters on the deposited material. In particular, dimensions and uniformity of both deposited filament and grid of the scaffolds were analyzed to understand the influence of the process parameters so as to optimize the FDM production technology

Statistical Mechanics of the Chinese Restaurant Process: lack of self-averaging, anomalous finite-size effects and condensation

The Pitman-Yor, or Chinese Restaurant Process, is a stochastic process that generates distributions following a power-law with exponents lower than two, as found in a numerous physical, biological, technological and social systems. We discuss its rich behavior with the tools and viewpoint of statistical mechanics. We show that this process invariably gives rise to a condensation, i.e. a distribution dominated by a finite number of classes. We also evaluate thoroughly the finite-size effects, finding that the lack of stationary state and self-averaging of the process creates realization-dependent cutoffs and behavior of the distributions with no equivalent in other statistical mechanical models.Comment: (5pages, 1 figure

The entropy of randomized network ensembles

Randomized network ensembles are the null models of real networks and are extensivelly used to compare a real system to a null hypothesis. In this paper we study network ensembles with the same degree distribution, the same degree-correlations or the same community structure of any given real network. We characterize these randomized network ensembles by their entropy, i.e. the normalized logarithm of the total number of networks which are part of these ensembles. We estimate the entropy of randomized ensembles starting from a large set of real directed and undirected networks. We propose entropy as an indicator to assess the role of each structural feature in a given real network.We observe that the ensembles with fixed scale-free degree distribution have smaller entropy than the ensembles with homogeneous degree distribution indicating a higher level of order in scale-free networks.Comment: (6 pages,1 figure,2 tables

Production and relevance of cosmogenic radionuclides in NaI(Tl) crystals

The cosmogenic production of long-lived radioactive isotopes in materials is an hazard for experiments demanding ultra-low background conditions. Although NaI(Tl) scintillators have been used in this context for a long time, very few activation data were available. We present results from two 12.5 kg NaI(Tl) detectors, developed within the ANAIS project and installed at the Canfranc Underground Laboratory. The prompt data taking starting made possible a reliable quantification of production of some I, Te and Na isotopes with half-lives larger than ten days. Initial activities underground were measured and then production rates at sea level were estimated following the history of detectors; a comparison of these rates with calculations using typical cosmic neutron flux at sea level and a selected description of excitation functions was also carried out. After including the contribution from the identified cosmogenic products in the detector background model, we found that the presence of 3H in the crystal bulk would help to fit much better our background model and experimental data. We have analyzed the cosmogenic production of 3H in NaI, and although precise quantification has not been attempted, we can conclude that it could imply a very relevant contribution to the total background below 15 keV in NaI detectors.Comment: Proceedings of the Low Radioactivity Techniques 2015 workshop, March 2015, Seattle (US