7 research outputs found
Enhancement of surface characteristics of direct metal laser sintered stainless steel 316L by shot peening
No full textInfluence of pulsed current GTAW-WAAM process parameters on the single layer bead geometry and multi bead multi-layer deposition of a nickel-based superalloy
No full textWire + arc additive manufacturing (WAAM) is a state-of-the-art and highly efficient technique utilized to produce near net-shaped products on a large scale, employing a layer-by-layer approach. This research used pulsed current mode welding for single-layer bead-on-plate experiments to optimise the process parameters for WAAM of Hastelloy C-276. The effect of process parameters including pulsed current, pulsed frequency, and pulse duty cycle was systematically investigated on the weld appearance, depth of penetration, layer width, and layer height. The optimisation of single layer experimental runs was conducted using Box-Behnken designs (BBD) and the response surface method to construct several regression models. An analysis of variance was employed to validate the accuracy of both the measured and generated models. The BBD results indicate that interactions have a more significant impact on the peak current parameter than the resulting impact of pulse duty cycle and frequency. Validation tests were performed on the model with the optimal process variables that were identified, and its mechanical and metallurgical properties were analysed. Macrostructure and microstructural analysis of the single layer showed that the specified process parameters led to acceptable base metal fusion and bead is free from cracking. There was a considerable decrease in elemental segregation while using the pulse mode technique. Finer grain structure and reduced microsegregation enhance the hardness. Further residual stress (RS) at weld bead and base plate was 213 MPa and −240 MPa. Nonhomogeneous heat transfer during welding affects RS compressive and tensile characteristics. WAAM printing quality requires precise control of LH, LW, and DOP. This research aimed to propose suitable parameter values for manufacturing WAAM component for usage in chemical processing, nuclear, marine, and industrial settings by using unique pulsing features. </p
Fusion-Based Additive Manufacturing of Hastelloy C-Series: A Comparative Study on Microstructure, Mechanical Properties, and Residual Stress
No full textThe fusion-based single pulsed gas metal arc welding (SP-GMAW) additive manufacturing (AM) process has attracted considerable attention because of its high production efficiency, elevated deposition rates, and near-net-shape capabilities. The present study presents a comparative investigation of Hastelloy C-276 and Hastelloy C-22 thin-walled components fabricated using a SP-GMAW based AM process. This study thoroughly investigates the microstructure, material properties, and residual stress of the components. The microstructures in various regions comprise dendrite structure in Hastelloy C-276 and C-22 superalloys. The Scanning Electron Microscopy and Energy Dispersive Spectroscopy (SEM/EDS) analysis revealed a discrepancy in elemental composition between C-276 and C-22 materials. Additionally, the average grain size in the top, middle, and bottom portions of C-22 are m, m, and m while C-276 has m, m, and m, respectively. Compared to the build direction, the travel direction has a higher mean microhardness. Hastelloy C-22 achieves a maximum hardness of 320 HV, while Hastelloy C-276 has a hardness of 286 HV. The highest recorded tensile strength for Hastelloy C-22 was MPa, whereas Hastelloy C-276 displayed a tensile strength of MPa in the upper regions along the travel direction. According to the stress distribution, the as-fabricated specimens of Hastelloy C-276 and C-22 are mostly impacted by tensile residual stress. Research on Hastelloy C series alloy comparisons and single pulsed GMAW-based WAAM technologies is limited and progressing. The comparative results of this research will be significant in the chemical-based, nuclear energy, maritime, and manufacturing industries
Turnover and function of DNA methylation at transcription factor binding sites
No full textCell type identity is largely determined by regulatory networks consistent of various
transcription factors. Transcription factor activity requires interaction with DNA and thus
critically depends on the accessibility of binding motifs. Growing evidence suggests that
interactions between transcription factors and DNA are modulated by distinct chromatin
modifications which in turn are influenced by transcription factors. Thus, ultimately
transcriptional output is a product of intimate interactions between DNA, transcription
factors and chromatin modifications. While recent studies support a model in which DNA
sequence in collaboration with transcription factors can autonomously determine
chromatin states, exact relationship between all these components is not well
understood.
Full genome single basepair resolution mammalian methylomes (Hodges et al, 2011;
Stadler et al, 2011) demonstrated a correlation between transcription factor occupancy
and hypomethylation at distal regulatory regions. Importantly, these low methylated
states critically depend on the presence of transcription factors. Here we analyzed how
DNA binding factors impact DNA methylation. Using chromatin immunoprecipitation
followed by bisulfite sequencing, we show that CTCF bound molecules can vary in their
methylation levels at such low methylated regions (LMRs). This observation suggests
that no tight link exists between DNA binding of transcription factors and unmethylated
state. While cytosines which are highly occupied by CTCF indeed are fully devoid of
methylation, cytosines within sites of low occupancy display heterogeneous methylation
levels. Moreover, at these sites CTCF occupancy correlates with the likelihood of being
demethylated. 5-hydroxymethylcytosine (5hmC) is a putative intermediate of active
demethylation. In support of a dynamic model of interaction between transcription factors
and DNA methylation, we found that 5hmC is highly enriched at cell type specific and
constitutive LMRs in embryonic stem cells and upon their neuronal differentiation.
Furthermore, regions with hydroxymethylation changes between these cell types are
enriched for cell type specific LMRs. This suggests a participation of transcription factor
mediated oxidative demethylation in reprogramming of distal regulatory elements.
Knockout of CTCF is lethal for embryonic stem cells. Therefore, in order to test the
relationship between transcription factor binding and hydroxymethylation we chose an
embryonic stem (ES) cell line with genetic deletion of REST, another factor previously
shown to be involved in formation of low methylated states. Indeed, deletion of REST
decreased 5-hydroxymethylcytosine levels while concomitantly increasing methylation
levels at its binding sites within the analyzed LMRs. These results indicate that
transcription factor mediated turnover of DNA methylation acts in maintenance and
reprogramming of distal regulatory regions.
To test whether the observed turnover is selective for active regulatory regions, we
decided to delete the two de novo DNA methyltransferases DNMT3A and DNMT3B in
embryonic stem cells. Surprisingly, using this approach we detected loss of methylation
at both, low and fully methylated regions. In order to compare the turnover kinetics
between different segment subtypes, we collected DNA from ES cells at various time
points after DNMT3A/B deletion. This indeed revealed an accelerated turnover at low
methylated regions. On average full demethylation was achieved after eight days,
suggesting that binding of transcription factors can induce rapid changes in DNA
methylation.
In summary, this study supports a model in which methylation at distal regulatory regions
is maintained and reprogrammed by a transcription factor mediated turnover. We
furthermore provide evidence that this turnover depends on TET proteins for
demethylation and on DNMT3A/B for remethylation. Quantification suggests that while
DNA methylation turnover is present throughout the genome it is accelerated at active
distal regulatory elements
Non-coding RNA and transcriptional regulation in CD4 T cell lineages
No full textCD4 T cell lineage choice epitomises the ability of the immune system to become tailored to a specific threat and provides a framework for understanding the mechanisms behind cell specification. The differentiation of T effectors from naïve cells gives rise to pro-inflammatory lineages including T helper 1 (Th1) and Th2 and anti-inflammatory regulatory T cells (Treg). An additional lineage of Treg also exits the thymus in parallel to naïve T cells and together these Treg are required for prevention of autoimmunity. These T cell lineages are distinct in terms of their cytokine production and functional effects but also through their differences in gene expression and its regulation, which are orchestrated by the presence of lineage-specifying transcription factors specific for each lineage. In addition, post-translational modification of histones also provide insights into this transcriptional regulation and more recently the pervasive and tissue-specific transcription of multiple classes of RNA species without protein coding capacity, non-coding RNA (ncRNA), has been found to play a role in cell differentiation and function. In this thesis I identify several ncRNAs with differential expression different T cell lineages. This includes ncRNAs upregulated Treg compared to T responders. The characterisation of these, including their expression in the autoimmune context of systemic lupus erythematosus (SLE), is presented and their possible biological functions are examined. The relevance of histone modifications for influencing Treg identity in SLE is also investigated. An additional class of ncRNAs that originate from gene enhancer regions, eRNA, is also investigated in the context of Th1 versus Th2 lineage choice. This enhancer transcription is increased genome-wide in Th1 cells at enhancers with high density T-bet binding in, termed ‘super-enhancers’. The functional relevance of these eRNAs, including at the super-enhancer upstream of the Th1 signature cytokine gene, IFNG, is also investigated in knockdown experiments
Vasopressor use after noncardiac surgery: an international observational study
No full textBackground: Hypotension after major noncardiac surgery is associated with increased morbidity, mortality, and costs, and is often treated with postoperative vasopressor infusions. The frequency of administration in the postoperative period is unknown. Methods: This international prospective cohort study was conducted between October 2020 and October 2023. At each hospital, adults undergoing noncardiac surgery were enrolled into two cohorts: all consecutive patients for 1 week (Cohort A) and an additional sample of up to 30 consecutive patients administered postoperative vasopressor infusions within 1 yr (Cohort B). The primary outcome (Cohort A) was the incidence of postoperative vasopressor infusions, defined as any continuous infusion of vasopressors. Secondary outcomes included in-hospital mortality, organ dysfunction, length of hospital stay, and complications associated with postoperative vasopressor infusions (both cohorts). Results: In total, 25 675 participants were enrolled from 228 hospitals across 42 countries. In Cohort A, 770/19 768 (3.9%) participants received postoperative vasopressor infusions, with vasopressor use ranging between 0% and 18% across hospitals (median odds ratio: 2.30 [credible interval 1.96–2.73]). This variability did not alter after adjustment for case-mix and procedural characteristics. For both cohorts, postoperative vasopressor infusions were associated with higher (15.5%) in-hospital mortality, higher rates of organ failure, and longer hospital stay. Conclusions: Administration of postoperative vasopressors after noncardiac surgery varied across hospitals and was associated with worse outcomes. Variable practice across hospitals could not be explained by differences in case-mix. Clinical trial registration: https://clinicaltrials.gov/study/NCT03805230, ESAIC tracking ID: ESAIC_CTN_SQUEEZE
