Rebound Rate Research Articles

Spray-based 3D concrete printing parameter design model: Actionable insight for high printing quality

A spray-based 3D concrete printing parameter design model to design and optimize printable concrete is proposed. The effects of different relatable printing parameters, such as nozzle travel speed (V), spray nozzle diameter (A), and spray standoff distance (D) on spray-based 3D printability and printing quality are investigated systematically. The hydration mechanism of spray-based 3D printed concrete is then explored in terms of mechanical performance, layer moisture content, micromorphology, and pore structure. The experimental results show that the printing quality of the printed concrete can be effectively improved by optimizing the relatable printing parameters. Based on the three key printing performance indexes of layer thickness, surface flatness, and rebound rate, the spray-based 3D concrete printing parameters design model β = f(V,A,D) is proposed. In the high-precision zone of β [0.7,1], the printing parameter V70A8D60 (β = 0.71) is selected to further verify the effectiveness of the model. It shows that the spray-based 3D printing process with appropriate printing parameters ensures the excellent hydration process of the printed concrete. Compared with those of the cast counterpart, the flexural, compressive, and splitting strengths of spray-based 3D printed V70A8D60 specimens increase by 5.67–14.48 %, 15.79–29.79 %, and 19.95–37.27 %, respectively. Further, high-pressure spraying in the spray-based 3D printing process optimizes the pore structure of the printed concrete with a 69.4 % decrease in total porosity. An “Olympic rings” convex structure with different accuracy grade parameters in the β model is designed and fabricated to demonstrate the feasibility and practical applicability of the currently proposed parametric design model.

SHOTCRETES AND MORTARS FOR 3D PRINTERS USING CHINESE INDUSTRIAL WASTE

Currently, ordinary Portland cement is the main cementing material for shotcrete in tunnels in my country. Part of the use of single-doped silica fume or fly ash, however, there are few cementitious materials that use double-doped or multi-doped mineral admixtures. Due to the single composition of the cementitious material, poor gradation, and low quality, the concrete has poor compactness, insufficient viscosity, and large rebound. The use of "multiple" mineral admixtures can better solve the problem of high rebound rate of shotcrete in tunnels. Among them, mineral admixtures include mineral powder, fly ash, silica fume and limestone powder. The cementitious material in concrete can be single-mixed, double-mixed or multi-mixed with mineral admixtures. In this way, the heat of hydration when the concrete sets and hardens can be reduced, reduce the cracking of concrete, improve the compactness and viscosity of concrete and reduce the rebound rate of concrete.
 The aim of research: the overall idea and technical measures to solve the high rebound rate of shotcrete or 3D printer mix and the high amount of cementitious materials using Chinese industrial waste.
 Scientific novelty of research: optimal compositions using Chinese industrial waste for wet shotcrete, optimal compositions using Chinese industrial waste for 3D construction printers, basic properties of wet shotcrete composites with using Chinese industrial waste, basic properties of 3D construction printer composites with using Chinese industrial waste.

Wet continuous mixing technology and extrusion rheological properties of carbon black/natural rubber composites

AbstractIn order to solve the problems of dust, low mixing efficiency, and poor quality stability in dry mixing and continuous mixing, this study combined the flocculation extrusion technology with continuous mixing technology of carbon black/additives/natural rubber composites. The flocculation of latex was achieved by carbon black itself and the shearing of the twin screw. The continuous mixing was carried out in a double‐rotor continuous mixer, which was composed of functional elements such as the exhaust section and extrusion section. The results showed that the technology promoted the formation of the filler‐rubber network, the sensitivity of viscosity to temperature was weakened, and the rubber was closer to a Newtonian fluid, with less extrusion swell phenomenon and no obvious spiral distortion. Compared with the dry mixing, the tensile strength, rebound rate, and abrasion resistance of the rubber composites prepared by this technology were increased by 11%, 31%, and 9%, respectively, providing theoretical and technical guidance for the green production and continuous production of high‐performance natural rubber composites.Highlights The process is achieved by a twin‐screw extruder and a twin‐rotor continuous mixer. The process promotes the formation of the filler‐rubber network. The sensitivity of viscosity to temperature is weakened, with no spiral distortion. The tensile strength, rebound rate, and abrasion resistance are increased by 11%, 31%, and 9%.

SARS-CoV-2 Rebound With and Without Use of COVID-19 Oral Antivirals.

Early treatment with a first-line therapy (nirmatrelvir/ritonavir [Paxlovid] or remdesivir) or second-line therapy (molnupiravir) prevents hospitalization and death among patients with mild-to-moderate COVID-19 who are at risk for severe disease and is recommended by the National Institutes of Health COVID-19 Treatment Guidelines. On May 25, 2023, the Food and Drug Administration approved nirmatrelvir/ritonavir for treatment of adults at high risk for severe disease. Although antiviral therapies are widely available, they are underutilized, possibly because of reports of SARS-CoV-2 rebound after treatment. To enhance current understanding of rebound, CDC reviewed SARS-CoV-2 rebound studies published during February 1, 2020- November 29, 2023. Overall, seven of 23 studies that met inclusion criteria, one randomized trial and six observational studies, compared rebound for persons who received antiviral treatment with that for persons who did not receive antiviral treatment. In four studies, including the randomized trial, no statistically significant difference in rebound rates was identified among persons receiving treatment and those not receiving treatment. Depending on the definition used, the prevalence of rebound varied. No hospitalizations or deaths were reported among outpatients who experienced rebound, because COVID-19 signs and symptoms were mild. Persons receiving antiviral treatment might be at higher risk for rebound compared with persons not receiving treatment because of host factors or treatment-induced viral suppression early in the course of illness. The potential for rebound should not deter clinicians from prescribing lifesaving antiviral treatments when indicated to prevent morbidity and mortality from COVID-19.

419. Viral SARS-CoV-2 rebound rates in linked commercial pharmacy-based SARS-CoV-2 RT-qPCR testing and healthcare claims

Abstract Background Viral SARS-CoV-2 RNA rebound has been described as a positive viral antigen or PCR test that follows a negative test. Although early descriptions of this phenomenon were linked to nirmatrelvir/ritonavir (NMV-r) treatment, subsequent studies have shown that viral rebound occurs both in untreated patients and patients treated with other antiviral therapies and is not associated with progression to severe disease. The rate of viral SARS-CoV-2 rebound has not been well characterized in large cohorts outside of clinical trials. Schematic representation of study population (Upper) High frequency testing and (Lower) High frequency testing with viral RNA rebound, along with relevant clinical information indicated by dashed lines. Each circle represents a SARS-CoV-2 RT-qPCR test with the darker shade indicating the minimum required tests to meet the definition and the lighter shade to reflect the possibility of >3 tests. The index date is the first positive test within 28-days. Methods In this large retrospective US-based cohort study, RT-qPCR SARS-CoV-2 test data were linked to healthcare claims to calculate the viral rebound rate in treated and untreated individuals, among subjects tested at least 3 times during a SARS-CoV-2 infection. This cohort of 30,646 unique patients includes SARS-CoV-2 vaccinated patients, patients infected during the Omicron era, and high risk patients (those with underlying medical conditions associated with a higher risk for severe SARS-CoV-2 infection). Results The observed rate of viral SARS-CoV-2 RNA rebound was approximately 3.5% in NMV-r treated infections compared to 1.5% in untreated infections during the Omicron era. Viral rebound in vaccinated, high-risk, or older patients occurred at comparable rates to the overall cohort. Viral rebounds to high RNA levels in NMV-r treated infections were rare (8% of viral rebounds) and comparable in frequency to those in untreated infections (11% of viral rebounds) during the Omicron era. Viral RNA rebound rates Viral RNA rebound rates and characteristics of the patient cohort, stratified by risk group, Omicron era, and NMV-r treatment. Untreated groups exclude patients who received NMV-r or a different SARS-CoV-2 treatment 28 days post-index date. Conclusion This study demonstrates that SARS-CoV-2 viral RNA rebound is rare and occurs in both treated and untreated SARS-CoV-2 infections, with rates comparable to those seen in the EPIC-HR clinical trial for NMV-r. Even when viral rebound occurs, only a small minority of cases rebound to a high viral RNA level and rarely result in progression to severe disease. Disclosures Shishi Luo, PhD, Helix: Employee Lisa M. McEwen, PhD, Helix: Employee Scott P. Kelly, PhD, Pfizer: Employee|Pfizer: Stocks/Bonds Magnus Isaksson, PhD, Helix: Employee Sarah Murphy, n/a, Helix: Former employee Simon White, n/a, Helix: Employee JT McCrone, PhD, Helix: Employment Geraint Levan, n/a, Helix: Employee Mary Lynn Baniecki, PhD, Pfizer Inc: Employee|Pfizer Inc: Stocks/Bonds Candace Bramson, MD, Pfizer Inc: Employee|Pfizer Inc: Stocks/Bonds Heather Rubino, PhD, MS, Pfizer: Stocks/Bonds Victoria Hendrick, BSc, Pfizer: Employee|Pfizer: Stocks/Bonds Holly Soares, PhD, Pfizer: Stocks/Bonds Jennifer Hammond, PhD, Pfizer: Employee|Pfizer: Stocks/Bonds

Aging characteristics of drip irrigation belt in Xinjiang cotton fields and their effects on its recovery and recycling

The old drip irrigation belts in Xinjiang cotton fields are prone to damage and have low recovering efficiency when being mechanically recovered, as well as poor mechanical performance and short service life of the old materials used in the production of various new products. Therefore, experiments were carried out from the macroscopic mechanical properties and microscopic chemical composition changes of the old drip irrigation belt to explore how the changes in its mechanical and chemical properties affect the recovery and recycling of the old drip irrigation belt. Mechanical properties of the old drip irrigation belts were tested by statics and dynamics test methods. The experiment results of indicators about mechanical properties showed that the tensile strength, elongation at break, natural rebound rate, impact strength and other mechanical parameters of the old drip irrigation belt samples were significantly lower than the corresponding values of the new ones. Which will affect the tensile capacity of the drip irrigation belt when it is recovered in the field. Furthermore, X-ray energy spectrum and infrared spectrum methods were used to analyse the chemical composition changes of the old drip irrigation belt. The determination results of X-ray energy spectrum showed that the oxygen content of the new drip irrigation belt samples was 1.73%, while that of old drip irrigation belt samples reached 12.15% ~ 15.27%. Then, the infrared spectrum experiment results showed that there were significantly more carbon groups in the old drip irrigation belt samples than that in the new ones. In addition, the correlation between the mechanical properties and oxidation degree of drip irrigation belt samples was analyzed, results showed that there was a significant negative correlation between them. This study results can provide basic data and theoretical guidance for the research and development of drip irrigation belt recovery equipment in Xinjiang cotton field, the research of rapid detection method of drip irrigation belt aging, the manufacture of anti-aging drip irrigation belt and the cyclic utilization of old materials.

Preparation of toughened thermal insulating polylactic acid/thermoplastic polyester elastomer (TPEE) microcellular foam using in situ fibrillation and multi-hydrazide nucleating agent self-assembly

Traditional foamed plastics can no longer meet the requirements of modern industries. High-performance biodegradable thermal-insulation materials are required for the food transportation and packaging industry. In this study, we introduced the synergistic self-assembly effect of nanoelastic (TPEE) fibers and TMC-300 (sebacic acid diphenyl hydrazide) through an in situ fibrosis to control the crystal morphology and phase structure. This effect improves the melt viscoelasticity of branched polylactic acid (CBPLA) and improves the foaming performance of PLA. The sea-island structure of PLA composites was transformed into an imperfect bi-continuous structure and its toughness was improved without the loss of PLA stiffness. Further, the toughness of the CBPLA/TMC/TPEE composite was improved by 76%, the thermal stability was significantly improved, and there was no effect on the tensile strength. CBPLA/TMC/TPEE microcellular foams were successfully prepared using supercritical CO2 in an intermittent high-pressure foaming autoclave with average cell diameter of 4 μm. The cell density of CBPLA foam was increased by 3 orders of magnitude compared to that of pure PLA foam, average cell diameter successfully reduce from 34 μm to 4 μm. The thermal conductivity of air in the micron-sized cells was reduced significantly, and therefore, the microcellular foam showed excellent thermal insulation performance with a thermal conductivity as low as 25.2 mW/mK. The TPEE/TMC elastic self-assembled fiber network dispersed on the cell wall cooperatively supported the PLA cell wall, which significantly improved the compressive resistance of PLA foams; the compression rebound rate of the foam was approximately 34%. The results indicate that CBPLA/TMC/TPEE microcellular foam materials are potential candidates for high-performance thermal applications because of their excellent thermal stability, toughness, and thermal insulation performance.

Enhanced Dielectric and Mechanical Performances of the (La + Nb) Codoped TiO2/Silicone Rubber Composites

Silicone rubber (SR) has gained great interest because of its fast deformation response, low modulus, high rebound rate, and wide temperature application range. However, their development is hampered by the low dielectric constant. Adding dielectric ceramics may be a solution. Herein, (La + Nb) codoped TiO2/SR composites are fabricated and their properties are systematically tested. By adding codoped TiO2, the performances of the composites are enhanced. High permittivity (3.63) and low loss tangent (7.07 × 10−4) are gained in the SR composites with 30 wt% codoped TiO2. The evenly dispersed codoped TiO2 and the enhanced interfacial polarization effect can be responsible for this. The composites also exhibit enhanced tensile strength (565 kPa) and elongation (259%). In addition, the SR composites are thermally stable up to 415 °C. These superior properties can improve the electrodeformation performance of SR composites, which has great potential in the fields of flexible actuators, bionic flexible robots, next‐generation intelligent medical devices, and green energy harvesting.

Rheological analysis of wet-mix shotcrete with various viscosity-modifiying admixtures

Shotcrete must be suitable for the process of transport (pumping) and application (spraying) on site. Therefore, it is necessary to obtain a suitable consistency and rheology for its placement. This paper evaluates the influence of various viscosity modifying admixtures (VMAs) on the rheology and thixotropy for application in wet-mix shotcrete. Six VMAs were used and classified into three groups, according to their composition: additives based on silica, phyllosilicates, and polymeric additives. These rheology modifiers were studied in depth in mortars, and the values of the yield stress (τo) and plastic viscosity (μ) of the material were obtained, as well as the thixotropy (hysteresis areas), which represents the energy required for the structural recovery of the fluid. To obtain these parameters, the fluid was tested under dynamic state with a laboratory rheometer, applying a ramp of shear rates. Additionally, the consistency in mortars was determined by obtaining the flow table diameter on the flow table test. This evaluation was carried out in mortars with different contents of superplasticizer (SP). All this information made it possible to evaluate the influence of SP in combination with each of the VMAs, obtaining a workability box, determining the hysteresis areas and verifying which combinations obtained a rheological behaviour superior to the control mix (without VMAs). The results described were correlated with the rebound index obtained in the shotcrete mixes performed on-site. The thixotropy in mortars and the values of the rebound rate on site led to the most accurate correlation that allowed the selection of the most effective VMAs for use in shotcrete. Lastly, both combined results (laboratory and on-site) permitted a process that facilitated analysis of the wet-mix shotcrete to be designed and optimised.

Preparation and Characterization of a Biobased Temporary Plugging Material: Self-Healing and Degradable.

Due to the low success rate in high-temperature gas well drilling or workover operations, a new type of biobased temporary sealing material was synthesized using vanillin, succinic anhydride (SA), cellulose, and acetylacetone zinc hydrate (AZH) as catalysts. Infrared analysis proved the synthesis of vanillin-derived epoxy and the formation of a vitrimer with hydroxyl ester. The results of thermodynamic properties tests show that it has excellent thermal stability with a high glass transition temperature (Tg). The dynamic mechanical test results show that the characteristic relaxation time of the material at 120 °C is 3500 s and the self-healing rate can reach 61% within 20 min. The test results of mechanical properties show that under 10 MPa pressure, it has good elastic deformation performance, the rebound rate is more than 36%, and the crushing rate is less than 17%. The degradation performance results show that the decomposition increases to 80% under 120 °C and 1.2% NaOH. The comprehensive performance evaluation results show that the new temporary plugging material has good compatibility, and its plugging performance is better than those of a gel, composite material, and shape memory polymer. The maximum fracture plugging capacity is 5 × 4 mm, with a pressure up to 10 MPa.

Enhancing Starch-Based Packaging Materials: Optimization of Plasticizers and Process Parameters.

In order to actively promote green production and address these concerns, there is an urgent need for new packaging materials to replace traditional plastic products. Starch-based packaging materials, composed of starch, fiber, and plasticizers, offer a degradable and environmentally friendly alternative. However, there are challenges related to the high crystallinity and poor compatibility between thermoplastic starch and fibers, resulting in decreased mechanical properties. To address these challenges, a novel approach combining plasticizer optimization and response surface method (RSM) optimization has been proposed to enhance the mechanical properties of starch-based packaging materials. This method leverages the advantages of composite plasticizers and process parameters. Scanning electron microscopy and X-ray crystallography results demonstrate that the composite plasticizer effectively disrupts the hydrogen bonding and granule morphology of starch, leading to a significant reduction in crystallinity. Fourier transform infrared spectroscopy results show that an addition of glycerol and D-fructose to the starch can form new hydrogen bonds between them, resulting in an enhanced plasticizing effect. The optimal process parameters are determined using the RSM, resulting in a forming temperature of 198 °C, a forming time of 5.4 min, and an AC content of 0.84 g. Compared with the non-optimized values, the tensile strength increases by 12.2% and the rebound rate increases by 8.1%.

Vertical compressive bearing performance and optimization design method of large-diameter manually-excavated rock-socketed cast-in-place piles

To study the vertical compressive bearing characteristics of large-diameter rock-socketed cast-in-place piles, eight manually-excavated rock-socketed cast-in-place piles were subjected to vertical compressive on-site load and pile stress tests. The test results showed that the load–displacement (Q-s) curves of the eight test piles were all slow-varying, and the settlement of the piles was less than 11 mm, which met the minimum engineering requirements. The unloading rebound rate was between 55 and 75%, and the elastic working properties of the piles were apparent. The pile axial force gradually decreased with depth, and the slope of the axial force distribution curve reached a minimum in the moderately weathered muddy siltstone layer while the pile side friction resistance reached its maximum value. Pile end friction increases with the increase of load. But the pile end resistance was inversely proportional to the single pile length-to-diameter (L/D) ratio and the depth of rock embedment for the pile. The percentage of pile side friction resistance under maximum load was 86%, indicating that these were characteristic friction piles. Based on the test results and the current Chinese code, the friction coefficient of the pile side soil layer η and the total resistance coefficient of the rock-socketed section ζ were introduced. A revision to the calculation equation for the vertical bearing capacity of the rock-socketed cast-in-place pile in the code was proposed, together with an optimization design method for large-diameter rock-socketed cast-in-place piles.

Development of a new type sprayed high ductility concrete (SHDC) and uniaxial compression test of rock-SHDC combined body

The application prospect of sprayed high ductility concrete (SHDC) as a tunnel and coal mine roadway supporting material is extensive. In this paper, modified desulfurization gypsum, manufactured sand, PVA-PP hybrid fiber, expansion agent, and shrinkage reducer were used as the primary raw materials to solve the environmental and cost problems caused by traditional SHDC, and an orthogonal experiment with four factors and three-level designs was conducted. The SHDC mix proportion in the orthogonal experiment was optimized on the basis of entropy weight method to optimize the grey correlation analysis. Then, the specimens were prepared by spraying to verify their performance. Finally, a low-cost, high-performance, and green SHDC was designed. The rebound rate of the vault was 17%, and the full face was 11%. The 28d compressive strength was 32.18 MPa, the ultimate tensile strain was 1.19%, and the autogenous shrinkage was 518.79με. In addition, the microstructure of SHDC and the synergistic mechanism of hybrid fibers were analyzed by optical microscopy, SEM, XRD, and TGA. On the basis of the engineering background of sprayed concrete, the rock-SHDC combined body was processed and manufactured, and uniaxial compression test was carried out. The DIC technology and surface-bonded strain gauge were used to explore the deformation and crack propagation characteristics of the composite, the energy evolution, and the impact tendency. When multiple cracks in the rock mass propagated to SHDC, convergence occurred, and 2–3 cracks converged into 1 crack. The rock monomer showed weak impact tendency, whereas the combined body and SHDC monomer showed non-impact tendency. So, SHDC can effectively limit rock deformation and maintain good integrity.

Permafrost saline water and Early to mid-Holocene permafrost aggradation in Svalbard

Abstract. Deglaciation in Svalbard was followed by seawater ingression and deposition of marine (deltaic) sediments in fjord valleys, while elastic rebound resulted in fast land uplift and the exposure of these sediments to the atmosphere, whereby the formation of epigenetic permafrost was initiated. This was then followed by the accumulation of aeolian sediments, with syngenetic permafrost formation. Permafrost was studied in the eastern Adventdalen valley, Svalbard, 3–4 km from the maximum up-valley reach of post-deglaciation seawater ingression, and its ground ice was analysed for its chemistry. While ground ice in the syngenetic part is basically fresh, the epigenetic part has a frozen freshwater–saline water interface (FSI), with chloride concentrations increasing from the top of the epigenetic part (at 5.5 m depth) to about 15 % that of seawater at 11 m depth. We applied a one-dimensional freezing model to examine the rate of top-down permafrost formation, which could be accommodated by the observed frozen FSI. The model examined permafrost development under different scenarios of mean average air temperature, water freezing temperature and degree of pore-water freezing. We found that even at the relatively high air temperatures of the Early to mid-Holocene, permafrost could aggrade quite fast down to 20 to 37 m (the whole sediment fill of 25 m at this location) within 200 years. This, in turn, allowed freezing and preservation of the freshwater–saline water interface despite the relatively fast rebound rate, which apparently resulted in an increase in topographic gradients toward the sea. The permafrost aggradation rate could also be enhanced due to non-complete pore-water freezing. We conclude that freezing must have started immediately after the exposure of the marine sediment to atmospheric conditions.

Does the Sleeper Plate Application for Temporary Epiphysiodesis Make Life Easier or Complicated? Increased Risk of Tethering.

The present study aims to investigate the frequency of recurrence and tethering effect after only metaphyseal screw removal (sleeper plate technique) compared with the conventional complete plate removal in the treatment of lower extremity deformities with guided growth surgery. Seventy-two patients (107 limbs) treated by an 8-plate hemiepiphysiodesis technique around the knee joint were evaluated. After the desired correction, only metaphyseal screw was removed (sleeper plate group) in 35 limbs (25 patients), whereas both screws and plate were removed (plate removal group) in 72 limbs (47 patients). An increase of 5 degrees or more in joint orientation angles in the direction of the initial deformity was considered as recurrence. The rate of rebound, tethering, and maintenance of correction in groups was analyzed at the latest follow-up (mean of 49mo). The mean age of the patients was 97 months (range: 80 to 129mo) at the time of index surgery. After a mean of 49 months (range: 16 to 86), 17 (48.5%) limbs maintained the desired stable correction in the sleeper plate group compared with 59 stable limbs (72.2%) in the plate removal group ( P <0.001). There was no statistically significant difference regarding recurrence between the sleeper plate group and the plate removal group (34.3% vs. 27.8%, respectively) ( P =0.216). Reinsertion of the metaphyseal screw was possible 8/12 limbs, and the remaining 4 limbs underwent further surgeries. There were 6 limbs (17.3%) of tethering in the sleeper plate group, and 4/6 limbs required further corrective surgeries. The remaining 2 limbs with slight tethering did not require further surgeries. Removing only metaphyseal screw increases the risk of tethering. In addition, reinsertion of the screw may not be possible in all cases due to bony growth, and further corrective surgeries may be necessary. Close follow-up is required if the sleeper plate technique is to be applied. Level III.

Three-dimensional printed 316L stainless steel cardiovascular stent's electrolytic polishing and its mechanical properties

The interventional therapy of vascular stent implantation is a popular treatment method for cardiovascular stenosis and blockage. However, traditional stent manufacturing methods such as laser cutting are complex and cannot easily manufacture complex structures such as bifurcated stents, while three-dimensional (3D) printing technology provides a new method for manufacturing stents with complex structure and personalized designs. In this paper, a cardiovascular stent was designed, and printed using selective laser melting technology and 316L stainless steel powder of 0-10 µm size. Electrolytic polishing was performed to improve the surface quality of the printed vascular stent, and the expansion behavior of the polished stent was assessed by balloon inflation. The results showed that the newly designed cardiovascular stent could be manufactured by 3D printing technology. Electrolytic polishing removed the attached powder and reduced the surface roughness Ra from 1.36 µm to 0.82 µm. The axial shortening rate of the polished bracket was 4.23% when the outside diameter was expanded from 2.42 mm to 3.63 mm under the pressure of the balloon, and the radial rebound rate was 2.48% after unloading. The radial force of polished stent was 8.32 N. The 3D printed vascular stent can remove the surface powder through electrolytic polishing to improve the surface quality, and show good dilatation performance and radial support performance, which provides a reference for the practical application of 3D printed vascular stent.

Trajectories of COVID-19: A longitudinal analysis of many nations and subnational regions.

The COVID-19 pandemic is the first to be rapidly and sequentially measured by nation-wide PCR community testing for the presence of the viral RNA at a global scale. We take advantage of the novel "natural experiment" where diverse nations and major subnational regions implemented various policies including social distancing and vaccination at different times with different levels of stringency and adherence. Initially, case numbers expand exponentially with doubling times of ~1-2 weeks. In the nations where interventions were not implemented or perhaps lees effectual, case numbers increased exponentially but then stabilized around 102-to-103 new infections (per km2 built-up area per day). Dynamics under effective interventions were perturbed and infections decayed to low levels. They rebounded concomitantly with the lifting of social distancing policies or pharmaceutical efficacy decline, converging on a stable equilibrium setpoint. Here we deploy a mathematical model which captures this V-shape behavior, incorporating a direct measure of intervention efficacy. Importantly, it allows the derivation of a maximal estimate for the basic reproductive number Ro (mean 1.6-1.8). We were able to test this approach by comparing the approximated "herd immunity" to the vaccination coverage observed that corresponded to rapid declines in community infections during 2021. The estimates reported here agree with the observed phenomena. Moreover, the decay (0.4-0.5) and rebound rates (0.2-0.3) were similar throughout the pandemic and among all the nations and regions studied. Finally, a longitudinal analysis comparing multiple national and regional results provides insights on the underlying epidemiology of SARS-CoV-2 and intervention efficacy, as well as evidence for the existence of an endemic steady state of COVID-19.

Targeting senescent dermal fibroblasts responsible for hyperactive melanocytes in melasma

Targeting senescent dermal fibroblasts responsible for hyperactive melanocytes in melasma

One-year clinical observation of the effect of internal bleaching on pulpless discolored teeth.

This study aimed to observe the color rebound and rebound rates of non-pulp discolored teeth within 1 year after routine internal bleaching to guide clinical practice and prompt prognosis. In this work, the efficacy of bleaching was observed in 20 patients. The color of discolored teeth was measured by using a computerized colorimeter before bleaching; immediately after bleaching; and at the 1st, 3rd, 6th, 9th, and 12th months after bleaching. The L*, a*, and b* values of the color of cervical, mesial, and incisal parts of the teeth were obtained, and the color change amounts ΔE*, ΔL*, Δa*, and Δb* were calculated. The overall rebound rate (P*) and the color rebound velocity (V*) were also analyzed over time. In 20 patients following treatment, the average ΔE* of tooth color change was 14.99. After bleaching, the neck and middle of the teeth ΔE* and ΔL* decreased in the 1st, 3rd, 6th, 9th, and 12th months, and the differences were statistically significant. Meanwhile, from the 9th month after bleaching, the rebound speed was lower than that in the 1st month, and the difference was statistically significant. The incisal end of the tooth ΔE* and ΔL* decreased in the 6th, 9th, and 12th months after bleaching, and the differences were statistically significant. No significant difference was found in the rebound speed between time points. However, this rate settled after the 9th month, with an average color rebound rate of 30.11% in 20 patients. The results indicated that internal bleaching could cause a noticeable color change on pulpless teeth. The color rebound after bleaching was mainly caused by lightness (L*), which gradually decreased with time, and it was slightly related to a* and b*. The color of the teeth after internal bleaching rebounded to a certain extent with time, but the color rebound speed became stable from the 9th month. Clinically, secondary internal bleaching can be considered at this time according to whether the colors of the affected tooth and the adjacent tooth are coordinated and depending on the patient's needs.

Development of sprayable ultra-high ductility magnesium phosphate cement-based composites based on the rheological properties

To develop a sprayable ultra-high ductility magnesium phosphate cement-based composite (UHDMC), this study explored the effects of starch ether content, water–solid ratio, powder-binder ratio, borax content (B/M), fly ash (FA) substitution and volume content of polyvinyl alcohol (PVA) fiber on the shootability and rheological properties of magnesium phosphate cement-based materials. The correlation between shootability factors (build-up thickness and rebound rate) and rheological parameters (yield stress and plastic viscosity) was established, and the optimum mixture proportion suitable for the sprayed UHDMC was selected and then analyzed by the mechanical properties and pore structure between the cast and sprayed specimens. The results showed that the optimum mixture proportion of the sprayed UHDMC was as follows: starch ether content of 0.15%, water–solid ratio of 0.14, powder-binder ratio of 0.3, borax content (B/M) of 8%, FA substitution of 40%, and the PVA fiber volume content of 2.0%. The yield stress and plastic viscosity were both positively correlated with the build-up thickness, and negatively correlated with rebound rate. To some extent, rheological parameters can be used to characterize the shootability factors, in which the best shootability was shown when the ranges of yield stress and plastic viscosity were 91.64–118.77 Pa and 4.85–7.17 Pa·s, respectively. The compressive strength and ultimate tensile stress of the sprayed UHDMC were lower than that of the cast UHDMC, while the ultimate tensile strain of the sprayed UHDMC could still reach above3.6%, presenting good performance to meet the requirement of above 3% strain of high ductility cement-based materials. Moreover, the analysis of pore structure verified that the capillary pores and macropores contents were mainly affected the strength and tensile strain, respectively.