Nonlinear thermal transport and negative differential thermal conductance in graphene nanoribbons

We employ classical molecular dynamics to study the nonlinear thermal transport in graphene nanoribbons (GNRs). For GNRs under large temperature biases beyond linear response regime, we have observed the onset of negative differential thermal conductance (NDTC). NDTC is tunable by varying the manner of applying the temperature biases. NDTC is reduced and eventually disappears when the length of the GNR increases. We have also observed NDTC in triangular GNRs, where NDTC exists only when the heat current is from the narrower to the wider end. These effects may be useful in nanoscale thermal managements and thermal signal processing utilizing GNRs.Comment: 5 pages, 4 figure

Tunable thermal rectification in graphene nanoribbons through defect engineering: A molecular dynamics study

Using non-equilibrium molecular dynamics, we show that asymmetrically defected graphene nanoribbons (GNR) are promising thermal rectifiers. The optimum conditions for thermal rectification (TR) include low temperature, high temperature bias, similar to 1% concentration of single-vacancy or substitutional silicon defects, and a moderate partition of the pristine and defected regions. TR ratio of similar to 80% is found in a 14-nm long and 4-nm wide GNR at a temperature of 200 K and bias of 90 K, where heat conduction is in the ballistic regime since the bulk effective phonon mean-free-path is around 775 nm. As the GNR length increases towards the diffusive regime, the TR ratio decreases and eventually stabilizes at a length-independent value of about 3%-5%. This work extends defect engineering to 2D materials for achieving TR. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.3703756

Size Effect in Non-equilibrium Molecular Dynamics

Direct method is commonly used to compute the thermal conductivity of a nanoscale material after molecular dynamics simulation. Direct method simply applies Fourier\u27s Law to get the value of thermal conductivity, which requires heat flux, cross sectional area and temperature gradient. A typical structure includes one heat source, one heat sink and a device region between them. Although it is usually assumed that the temperature gradient is a constant through the entire device region, the temperature profile is not linear for a material in nanoscale because phonon mean free path is comparable to the size of the whole system. Furthermore, bath length and device length can have influence on temperature profile. In this project, two methods of temperature gradient computing and the size effect of each method are discussed. Method 1 uses the center region of the device to get temperature gradient and method 2 uses the temperature difference between hot bath and cold bath divided by the device length as temperature gradient. The thermal conductivity computed from Green-Kubo method is used as a standard to test the two calculation methods and the size effect. Argon with atomic weight 40 is used as the nanoscale material because of its moderate phonon mean free path. Result shows that both method 1 and method 2 can compute the bulk-limit thermal conductivity but the necessary size conditions are different. Method 1 requires a long device and method 2 requires a long bath region

Serum metabolomic analysis of the effect of exercise on nonalcoholic fatty liver disease

Objective: Exercise benefits people with nonalcoholic fatty liver disease (NAFLD). The aim of this study was to identify a panel of biomarkers and to provide the possible mechanism for the effect of exercise on NAFLD patients via an untargeted mass spectrometry-based serum metabolomics study. Methods: NAFLD patients were classified randomly into a control group (n = 74) and a 6-month vigorous exercise (n = 68) group. Differences in serum metabolic profiles were analyzed using untargeted ultra-performance liquid chromatography coupled with quadrupole time-of-flight mass spectrometry (UPLC-Q-TOF/MS) technology. Principal component analysis (PCA) and orthogonal partial least squares discriminant analysis (OPLS-DA) were used to validate the differences between these two groups, and altered metabolites were obtained by ANOVA (fold change >2, P < 0.05) and identified with the online database Metlin and an in-house database. Results: Metabolic profiling and multiple statistical analyses of the serum samples indicated significant differences between the NAFLD patients in the control and the 6-month vigorous exercise groups. Finally, 36 metabolites were identified between the control vs exercise groups. These metabolites were mainly associated with glycerophospholipid- and sphingolipid-related pathways. Conclusion: Our study demonstrates that glycerophospholipid and sphingolip id alterations may contribute to the mechanism underlying the effect of exercise on NAFLD patients. A LC-MS-based metabolomics approach has a potential value for screening exercise-induced biomarkers

Single-cell transcriptomics of hepatic stellate cells uncover crucial pathways and key regulators involved in non-alcoholic steatohepatitis

Background: Fibrosis is an important pathological process in the development of non-alcoholic steatohepatitis (NASH), and the activation of hepatic stellate cell (HSC) is a central event in liver fibrosis. However, the transcriptomic change of activated HSCs (aHSCs) and resting HSCs (rHSCs) in NASH patients has not been assessed. This study aimed to identify transcriptomic signature of HSCs during the development of NASH and the underlying key functional pathways. Methods: NASH-associated transcriptomic change of HSCs was defined by single-cell RNA-sequencing (scRNA-seq) analysis, and those top upregulated genes were identified as NASH-associated transcriptomic signatures. Those functional pathways involved in the NASH-associated transcriptomic change of aHSCs were explored by weighted gene co-expression network analysis (WGCNA) and functional enrichment analyses. Key regulators were explored by upstream regulator analysis and transcription factor enrichment analysis. Results: scRNA-seq analysis identified numerous differentially expressed genes in both rHSCs and aHSCs between NASH patients and healthy controls. Both scRNA-seq analysis and in-vivo experiments showed the existence of rHSCs (mainly expressing a-SMA) in the normal liver and the increased aHSCs (mainly expressing collagen 1) in the fibrosis liver tissues. NASH-associated transcriptomic signature of rHSC (NASHrHSCsignature) and NASH-associated transcriptomic signature of aHSC (NASHaHSCsignature) were identified. WGCNA revealed the main pathways correlated with the transcriptomic change of aHSCs. Several key upstream regulators and transcription factors for determining the functional change of aHSCs in NASH were identified. Conclusion: This study developed a useful transcriptomic signature with the potential in assessing fibrosis severity in the development of NASH. This study also identified the main pathways in the activation of HSCs during the development of NASH

Activation of Piezo1 or TRPV2 channels inhibits human ureteral contractions via NO release from the mucosa

We aimed to investigate the expression and motor modulatory roles of several mechano-sensitive channels (MSCs) in human ureter. Human proximal ureters were obtained from eighty patients subjected to nephrectomy. Expression of MSCs at mRNA, protein and functional levels were examined. Contractions of longitudinal ureter strips were recorded in organ bath. A fluorescent probe Diaminofluoresceins was used to measure nitric oxide (NO). RT-PCR analyses revealed predominant expression of Piezo1 and TRPV2 mRNA in intact ureter and mucosa. Immunofluorescence assays indicate proteins of MSCs (Piezo1/Piezo2, TRPV2 and TRPV4) were mainly distributed in the urothelium. Ca2+ imaging confirmed functional expression of TRPV2, TRPV4 and Piezo1 in cultured urothelial cells. Specific agonists of Piezo1 (Yoda1, 3–300 μM) and TRPV2 (cannabidiol, 3–300 μM) attenuated the frequency of ureteral contractions in a dose-dependent manner while the TRPV4 agonist GSK1016790A (100 nM–1 μM) exerted no effect. The inhibitory effects of Piezo1 and TRPV2 agonists were significantly blocked by the selective antagonists (Dooku 1 for Piezo1, Tranilast for TRPV2), removal of the mucosa, and pretreatment with NO synthase inhibitor L-NAME (10 μM). Yoda1 (30 μM) and cannabidiol (50 μM) increased production of NO in cultured urothelial cells. Our results suggest that activation of Piezo1 or TRPV2 evokes NO production and release from mucosa that may mediate mechanical stimulus-induced reduction of ureter contractions. Our findings support the idea that targeting Piezo1 and TRPV2 channels may be a promising pharmacological strategy for ureter stone passage or colic pain relief

Independent Association of Serum Fibroblast Growth Factor 21 Levels With Impaired Liver Enzymes in Hyperthyroid Patients

Fibroblast growth factor 21 (FGF21) is identified as a potential biomarker for liver diseases. However, information is limited regarding serum FGF21 and impaired liver function in hyperthyroidism. We aim to determine the potential association of serum FGF21 levels with impaired liver enzymes in hyperthyroid patients. In this case-control study, 105 normal subjects and 122 overt hyperthyroid patients were included. Among them, 41 hyperthyroid patients who obtained euthyroid status after thionamide treatment received second visit. Serum FGF21 levels were determined using the ELISA method. Compared to the normal subjects, patients with hyperthyroidism had significantly elevated serum liver enzymes, including alanine transaminase (ALT) (p &lt; 0.001), aspartate aminotransferase (AST) (p &lt; 0.001) levels, as well as FGF21 levels (p &lt; 0.001). Further analysis showed serum FGF21 (p &lt; 0.05), as well as thyroid hormone (TH) free T3 (p &lt; 0.05), free T4 (p &lt; 0.05) levels were higher in hyperthyroid patients with impaired liver enzymes than in those with normal liver enzymes. After reversal of hyperthyroid state, elevated serum FGF21 levels in hyperthyroid patients declined significantly (p &lt; 0.001), with a concomitant decrease in serum ALT (p &lt; 0.001), AST (p &lt; 0.001) levels. Correlation analysis showed close correlation between FGF21 and ALT (p &lt; 0.002), AST (p &lt; 0.012), free T3 (p &lt; 0.001), free T4 (p &lt; 0.001). Further logistic regression analysis revealed FGF21 is significantly associated with elevated ALT [Odds Ratio, OR 1.79, (95% confidence interval, CI), (1.30–2.47), P &lt; 0.001], AST [1.59 (1.07–2.34), p &lt; 0.020]. After adjustment of potential confounders, the association between FGF21 and elevated ALT remained significant [1.42 (1.01–1.99), p &lt; 0.043]. In conclusion, serum FGF21 is independently associated with impaired liver enzymes in hyperthyroid patients

Two-temperature nonequilibrium molecular dynamics simulation of thermal transport across metal-nonmetal interfaces

We have used a two-temperature nonequilibrium molecular dynamics method for predicting interfacial thermal resistance across metal-nonmetal interfaces. This method is an extension of the conventional nonequilibrium molecular dynamics for the dielectric-dielectric interface, where a temperature bias is imposed and the heat current is derived. We have included the electron degree of freedom for the interfacial thermal transport problem by treating the electron-phonon coupling with the two-temperature model. The method is demonstrated on two model systems, that is, silicon-copper interface and carbon-nanotube-copper interface. Temperature nonequilibrium between electrons and phonons in the metal side is quantitatively predicted, and a temperature drop across the interface is observed. The results agree with experimental data better than those obtained from conventional nonequilibrium molecular dynamics simulations where only phonons are considered. Our approach is capable of taking into account both the electron and lattice degrees of freedom in a single molecular dynamics simulation and is a generally useful tool for modeling interfacial thermal transport across metal-nonmetal interfaces