2,225 research outputs found
Understanding the relevance of national culture in international business research: a quantitative analysis
This review is a comprehensive quantitative analysis of the International Business literature whose focus is on national culture. The analysis relies on a broad range of bibliometric techniques as productivity rankings, citation analysis (individual and cumulative), study of collaborative research patterns, and analysis of the knowledge base. It provides insights on (I) faculty and institutional research productivity and performance; (II) articles, institutions, and scholarsâ influence in the contents of the field and its research agenda; and (III) national and international collaborative research trends. The study also explores the body of literature that has exerted the greatest impact on the researched set of selected articles.info:eu-repo/semantics/publishedVersio
Microfluidic smartphone quantitation of <i>Escherichia coli</i> in synthetic urine
In spite of the clinical need, there is a major gap in rapid diagnostics for identification and quantitation of E. coli and other pathogens, also regarded as the biggest bottleneck in the fight against the spread of antimicrobial resistant bacterial strains. This study reports for the first time an optical, smartphone-based microfluidic fluorescence sandwich immunoassay capable of quantifying E. coli in buffer and synthetic urine in less than 25âŻmin without sample preparation nor concentration. A limit of detection (LoD) up to 240âŻCFU/mL, comensurate with cut-off for UTIs (103-105âŻCFUs/mL) was achieved. Replicas of full response curves performed with 100-107âŻCFUs/mL of E. coli K12 in synthetic urine yielded recovery values in the range 80-120%, assay reproducibility below 30% and precision below 20%, therefore similar to high-performance automated immunoassays. The unrivalled LoD was mainly linked to the 'open fluidics' nature of the 10-bore microfluidic strips used that enabled passing a large volume of sample through the microcapillaries coated with capture antibody. The new smartphone based test has the potential of being as a rapid, point-of-care test for rule-in of E. coli infections that are responsible for around 80% of UTIs, helping to stop the over-prescription of antibiotics and the monitoring of patients with other symptomatic communicable diseases caused by E. coli at global scale.</p
How dense can one pack spheres of arbitrary size distribution?
We present the first systematic algorithm to estimate the maximum packing
density of spheres when the grain sizes are drawn from an arbitrary size
distribution. With an Apollonian filling rule, we implement our technique for
disks in 2d and spheres in 3d. As expected, the densest packing is achieved
with power-law size distributions. We also test the method on homogeneous and
on empirical real distributions, and we propose a scheme to obtain
experimentally accessible distributions of grain sizes with low porosity. Our
method should be helpful in the development of ultra-strong ceramics and high
performance concrete.Comment: 5 pages, 5 figure
Fast prototyping using 3D printed templates and flexible fluoropolymer microcapillary films offers enhanced micromixing in immobilised (bio)catalytic reactions
Microreactors offer large surface-area-to-volume (SAV) ratios and short diffusion distances, yet, even at sub-millimetre space, there remains a level of mass transfer control of reactions. This can be minimised using passive micromixers currently requiring access to advanced microfabrication techniques. We report the fast fabrication and in-depth micromixing characterisation of inexpensive non-linear microstructure prototypes using, for the first time, a flexible fluoropolymer microcapillary film (MCF) re-shaped, post-extrusion, with 3D printed templates offering in-flow enhancement of mass transfer in immobilised (bio)catalytic reactions not previously studied for this type of micro-engineered material. The versatile âpush-and-clickâ 3D printed templates allow one-step production of multi-bored, non-invasive micromixers with simple architectures, namely âsquareâ, âzigzagâ and âwavyâ geometries without the limitations of conventional microfluidic devices. The passive micromixers were numerically evaluated using Computational Fluid Dynamics (CFDs) and extensively validated experimentally using novel Residence time distribution (RTD) data which assessed the role of Reynold numbers (0.6 â 60) and molecular diffusion coefficients (10â6 â10â11 m2/s), providing significant in-depth understanding of fluid flow distribution. By evaluating the in-flow oxidative coupling reaction of o-phenylenediamine (OPD, a chromogenic substrate) to 2,3-diaminophenazine (DAP) with an immobilised enzyme, horseradish peroxidase (HRP), we demonstrated the reaction rates in the âsquareâ and âzigzagâ (sharp bends) were improved by âŒ43 and âŒ46% respectively, compared to straight microcapillaries. This is linked to enhanced radial fluid movement. The proposed prototypes can be readily tailored for facilitated fabrication of practical high-performance microreactors suited for heterogeneous assays or in-flow (bio)catalytic reactions in non-microdevice dedicated labs.</p
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Antibody Surface Coverage Drives Matrix Interference in Microfluidic Capillary Immunoassays
The performance of biosensors is often optimised in buffers, which brings inconsistencies during applications with biological samples. Current strategies for minimising sample (matrix) interference are complex to automate and miniaturise, involving e.g. sample dilution or recovery of serum/plasma. This study shows the first systematic study using hundreds of actual microfluidic immunoassay fluoropolymer strips to understand matrix interference in microflow systems. As many interfering factors are assay-specific, we have explored matrix interference for a range of enzymatic immunoassays, including a direct mIgG/anti-mIgG, a sandwich cancer biomarker PSA and a sandwich inflammatory cytokine IL-1ÎČ. Serum matrix interference was significantly affected by capillary antibody surface coverage, suggesting for the first time the main cause of serum matrix effect is low-affinity serum components (e.g. auto-antibodies) competing with high-affinity antigen for the immobilised antibody. Additional experiments carried out with different capillary diameters confirmed the importance of antibody surface coverage in managing matrix interference. Building on these findings we propose a novel analytical approach where antibody surface coverage and sample incubation times are key for eliminating and/or minimising serum matrix interference, consisting in bioassay optimization carried out in serum instead of buffer, without compromising the performance of the bioassay nor adding extra cost nor steps. This will help establishing a new route towards faster development of modern point-of-care tests and effective biosensors development
Sensitive optical detection of clinically relevant biomarkers in affordable microfluidic devices: Overcoming substrate diffusion limitations
One of the biggest challenges in miniaturization of optical immunoassays is the short light path distance of microchannels/microcapillaries. Protein biomarkers are often presented in circulating blood in the picomolar-femtomolar range, requiring exceptional levels of sensitivity that cannot be met with traditional chromogenic substrates and without sophisticated, bulky detection systems. This study discloses an effective strategy for increasing the sensitivity and shorten the total test time for sandwich ELISAs in microfluidic devices optically interrogated, based on enhancing enzymatic amplification. We found that activity of Horseradish Peroxidase (HRP) in mesofluidic systems is highly limited by diffusion, therefore increasing the concentration of enzymatic substrate in these systems does not translate into an enhancement in enzymatic conversation. The opposite happens in microfluidic systems due to short diffusion distances, however increased concentration of the second enzymatic substrate, hydrogen peroxide (H 2O 2), leads to enzyme inhibition as herein reported. Consequently, we found that the molar ratio of o-phenylenediamine (OPD) to hydrogen peroxide from commercially substrate formulations is not suitable for miniaturized systems. Sandwich ELISA quantitation of a cancer biomarker PSA and human cytokine IL-1ÎČ in fluoropolymer microfluidic strips revealed over one order of magnitude increase in sensitivity and 10-fold decrease in incubation time by simply changing the molar ratio of OPD:H 2O 2 from 1:3 to 1:1 and increasing OPD concentration from 1 to 4 mg/ml. This enhancement in enzymatic amplification offers finally the sensitivity required for optical interrogation of novel portable and affordable microfluidic devices with inexpensive and ubiquitous smartphones and flatbed scanners. </p
WZW branes and gerbes
We reconsider the role that bundle gerbes play in the formulation of the WZW
model on closed and open surfaces. In particular, we show how an analysis of
bundle gerbes on groups covered by SU(N) permits to determine the spectrum of
symmetric branes in the boundary version of the WZW model with such groups as
the target. We also describe a simple relation between the open string
amplitudes in the WZW models based on simply connected groups and in their
simple-current orbifolds.Comment: latex, 4 figures incorporate
Adipose tissue derived stem cells secretome: soluble factors and their roles in regenerative medicine
Stem cells have been long looked at as possible therapeutic vehicles for different health related problems. Among the different
existing stem cell populations, Adipose derived Stem Cells (ASCs) have been gathering attention in the last 10 years. When compared to
other stem cells populations and sources, ASCs can be easily isolated while providing higher yields upon the processing of adipose tissue.
Similar to other stem cell populations, it was initially thought that the main potential of ASCs for regenerative medicine approaches was
intimately related to their differentiation capability. Although this is true, there has been an increasing body of literature describing the
trophic effects of ASCs on the protection, survival and differentiation of a variety of endogenous cells/tissues. Moreover, they have also
shown to possess an immunomodulatory character. This effect is closely related to the ASCsâ secretome and the soluble factors found
within it. Molecules such as hepatocyte growth factor (HGF), granulocyte and macrophage colony stimulating factors, interleukins (ILs)
6, 7, 8 and 11, tumor necrosis factor-α (TNF-α), vascular endothelial growth factor (VEGF), brain derived neurotrophic factor (BDNF),
nerve growth factor (NGF), adipokines and others have been identified within the ASCsâ secretome. Due to its importance regarding future
applications for the field of regenerative medicine, we aim, in the present review, to make a comprehensive analysis of the literature
relating to the ASCsâ secretome and its relevance to the immune and central nervous system, vascularization and cardiac regeneration.
The concluding section will highlight some of the major challenges that remain before ASCs can be used for future clinical applications
Angiogenic potential of gellan gum-based hydrogels for application in nucleus pulposus regeneration : in vivo study
Hydrogels for nucleus pulposus (NP) regeneration should be able to comprise a nonangiogenic or even antiangiogenic
feature. Gellan gum (GG)âbased hydrogels have been reported to possess adequate properties for being
used as NP substitutes in acellular and cellular strategies, due to its ability to support cell encapsulation, adequate
mechanical properties, and noncytotoxicity. In this study, the angiogenic response of GG-based hydrogels was
investigated by performing the chorioallantoic membrane assay. The convergence of macroscopic blood vessels
toward the GG, ionic-crosslinked methacrylated GG (iGG-MA), and photo-crosslinked methacrylated GG (phGGMA)
hydrogel discs was quantified. Gelatin sponge (GSp) and filter paper (FP) alone and with vascular endothelial
growth factor were used as controls of angiogenesis. The images obtained were digitally processed and analyzed
by three independent observers. The macroscopic blood vessel quantification demonstrated that the GG-based
hydrogels are not angiogenic as compared with FP controls. No statistical differences between the GG-based
hydrogels tested in respect to its angiogenic ability were observed. Hematoxylin and eosin staining and SNA-lectin
immunohistochemistry assay indicated that the iGG-MA and phGG-MA hydrogels do not allow the ingrowth of
chick endothelial cells, following 4 days of implantation. On the contrary, GG, GSp, and FP controls allowed cell
infiltration. The histological data also indicated that the GG-based hydrogels do not elicit any acute inflammatory
response. The results showed that the GG, iGG-MA, and phGG-MA hydrogels present different permeability to
cells but functioned as a physical barrier for vascular invasion. These hydrogels present promising and tunable
properties for being used as NP substitutes in the treatment of degenerative intervertebral disc.The authors thank the funds provided by Portuguese Foundation for Science and Technology (FCT) through POCTI and FEDER programs (SFRH/BI/33503/2008). This work was also carried out with the support of the European Union funded Collaborative Project Disc Regeneration (NMP3-LA-2008-213904)
Structural and longitudinal analysis of the knowledge base on spin-off research
Following the dynamism in spin-off research, in this study we conduct a structural and longitudinal bibliometric analysis of a sample of 812 articles on spin-offs published in 234 journals included in the ISI Web of Knowledge over a period of three decades. The analyses do not seek to establish a new conceptualization but rather to reveal the intellectual structure of the field and how it has evolved, and the profile of the knowledge network established in the three perspectives: corporate, academic and entrepreneurial spin-offs. The diversity involved in the three streams of spin-off research signals substantial differences. Theoretically, transaction costs, agency and the resource-based view have remained a foundation of spin-off research, albeit that research has been driven more by the phenomena than by developing the theory. The more traditional focus on corporate spin-offs was followed by emphasis on academic spin-offs and more recently on entrepreneurial spin-offs. This shift has been accompanied by a more business/management theoretical orientation, replacing a more financial and taxation-based perspective underlying corporate spin-offs. This study systematizes the existing stock of knowledge and raises avenues for additional inquiry.info:eu-repo/semantics/publishedVersio
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