Selective Laser Melting of Al-Based Matrix Composites with Al2O3 Reinforcement: Features and Advantages

Aluminum matrix composites (AMC) are of great interest and importance as high-performance materials with enhanced mechanical properties. Al₂O₃ is a commonly used reinforcement in AMCs fabricated by means of various technological methods, including casting and sintering. Selective laser melting (SLM) is a suitable modern method of the fabrication of net-shape fully dense parts from AMC with alumina. The main results, achievements, and difficulties of SLM applied to AMCs with alumina are discussed in this review and compared with conventional methods. It was shown that the initial powder preparation, namely the particle size distribution, sphericity, and thorough mixing, affected the final microstructure and properties of SLMed materials drastically. The distribution of reinforcing particles tends to consolidate the near-melting pool-edges process because of pushing by the liquid–solid interface during the solidification process that is a common problem of various fabrication methods. The achievement of an homogeneous distribution was shown to be possible through both the thorough mixing of the initial powders and the precise optimization of SLM parameters. The strength of the AMCs fabricated by the SLM method was relatively low compared with materials produced by conventional methods, while for superior relative densities of more than 99%, hardness and tribological properties were obtained, making SLM a promising method for the Al-based matrix composites with Al₂O₃.     

Assessing the estrogenic activity of chemicals present in resin based dental composites and in leachates of commercially available composites using the ERα-CALUX bioassay

ObjectiveThe biocompatibility of resin based dental composites has not yet been fully characterized even though certain monomers used in these composites are synthesized from Bisphenol A (BPA), a well-known estrogenic endocrine disruptor. As a result, they show structural relationship to BPA and can contain it as an impurity. Therefore, the estrogenic activity of 9 monomers, 2 photoinitiators, one photostabilizer and leachates of 4 commercially available composites was determined.MethodsThe ERα-CALUX bioassay was used to determine both agonistic and antagonistic estrogenic activities of the pure compounds (BPA, BisDMA, BisGMA, BisEMA(3), BisEMA(6), BisEMA(10), TEGDMA, TCD-DI-HEA, BADGE, UDMA, HMBP, DMPA, CQ) and the leachates of cured composite disks. The leachates of 4 commercially available composites (Solitaire 2, Ceram.x Spectra ST, G-ænial Posterior and Filtek Supreme XTE) in water and 0.1 M NaOH (pH = 13, ‘worst-case scenario’) were tested for estrogenic activity (pooled leachates from 10 cured composite disks).ResultsAgonistic estrogenic activity was found for the monomer BisDMA, the photostabilizer HMBP and photoinitiator DMPA. All leachates from the 4 tested composites showed significant agonistic estrogenic activity higher than the DMSO control, and the highest activity (potency and efficacy) was found for Solitaire 2, followed by Ceram.x Spectra ST. Furthermore, antagonistic estrogenic activity was found in the leachates from G-ænial Posterior.SignificanceThese results show that significant estrogenic activity was found in all leachates of the cured composite disks, and that this estrogenicity is most likely due to a mixture effect of multiple estrogenic compounds (including BPA, HMBP and DMPA). This indicates that further research into the endocrine activity of all the compounds that are present in these composites (even at low quantities) and their possible mixture effect is warranted to guarantee their safe use.     

A Computational Geometry Generation Method for Creating 3D Printed Composites and Porous Structures

A computational method for generating porous materials and composite structures was developed and implemented. The method is based on using 3D Voronoi cells to partition a defined space into segments. The topology of the segments can be controlled by controlling the Voronoi cell set. The geometries can be realized by additive manufacturing methods, and materials can be assigned to each segment. The geometries are generated and processed virtually. The macroscopic mechanical properties of the resulting structures can be tuned by controlling microstructural features. The method is implemented in generating porous and composite structures using polymer filaments i.e., polylactic acid (PLA), thermoplastic polyurethane (TPU) and nylon. The geometries are realized using commercially available double nozzle fusion deposition modelling (FDM) equipment. The compressive properties of the generated porous and composite configurations are tested quasi statically. The structures are either porous of a single material or composites of two materials that are geometrically intertwined. The method is used to produce and explore promising material combinations that could otherwise be difficult to mix. It is potentially applicable with a variety of additive manufacturing methods, size scales, and materials for a range of potential applications.     

Differences in the intensity of light-induced fluorescence emitted by resin composites

BackgroundThe aims of this study were to compare the intensities of fluorescence emitted by different resin composites as detected using quantitative light-induced fluorescence (QLF) technology, and to compare the fluorescence intensity contrast with the color contrast between a restored composite and the adjacent region of the tooth.MethodsSix brands of light-cured resin composites (shade A2) were investigated. The composites were used to prepare composite discs, and fill holes that had been prepared in extracted human teeth. White-light and fluorescence images of all specimens were obtained using a fluorescence camera based on QLF technology (QLF-D) and converted into 8-bit grayscale images. The fluorescence intensity of the discs as well as the fluorescence intensity contrast and the color contrast between the composite restoration and adjacent tooth region were calculated as grayscale levels.ResultsThe grayscale levels for the composite discs differed significantly with the brand (p < 0.001): DenFil (10.84 ± 0.35, mean ± SD), Filtek Z350 (58.28 ± 1.37), Premisa (156.94 ± 1.58), Grandio (177.20 ± 0.81), Charisma (207.05 ± 0.77), and Gradia direct posterior (211.52 ± 1.66). The difference in grayscale levels between a resin restoration and the adjacent tooth was significantly greater in fluorescence images for each brand than in white-light images, except for the Filtek Z350 (p < 0.05). However, the Filtek Z350 restoration was distinguishable from the adjacent tooth in a fluorescence image.ConclusionsThe intensities of fluorescence detected from the resin composites varied. The differences between the composite and adjacent tooth were greater for the fluorescence intensity contrast than for the colors observed in the white-light images.     

自体同源骨移植治疗根尖周病骨缺损的初步研究

本研究的目的是了解自体同源骨移植治疗根尖周病骨缺损的效果。18例患者在行根尖刮治术同时接受自体同源骨移植,自体同源骨来自上颌梨状孔边缘骨。随访结果表明:术后局部反应较轻,骨缺损修复较快。术后6个月根尖肉芽肿的治愈率为100％;根尖囊肿的治愈率为92.31％。提示自体同源骨移植是根周手术较理想的辅助治疗方法。     

One-year clinical evaluation of a flowable resin liner associated with a microhybrid resin in noncarious cervical lesions

The aim of this study was to compare the clinical performance over 1 year of a microhybrid composite resin for class V restorations both lined and not lined with a flowable composite resin. Nineteen patients having at least two pairs of cervical noncarious lesions under occlusion were enrolled in this study. A total of 38 restorations were placed, half for each group (Single-Bond + Filtek-Flow + Filtek Z250, and Single-Bond + Filtek Z250). Two calibrated operators placed all restorations according to the manufacturers instructions. Two other independent examiners evaluated the restorations at baseline and after a 12-month period according to the USPHS criteria and modified criteria for color match. The classic alpha score was divided into A1 for not detectable and A2 for slightly discernible filling. Statistical analysis was conducted using Fishers exact test and McNemars test (P=0.05). One restoration was lost after 12 months for each group (retention rate 95% for each group). After 12 months, 18 restorations showed a trend towards dark yellowing (color match A2). The use of Filtek Flow as a liner under Filtek Z250 restorations did not improve the clinical performance of class V restorations after 6 and 12 months of evaluation.     

Marginal quality of flowable 4-mm base vs. conventionally layered resin composite

Objective

This study evaluated marginal integrity of bonded posterior resin composite fillings to enamel and dentine with and without 4 mm flowable base, before and after thermo-mechanical loading (TML).

Methods

80 MOD cavities with one proximal box beneath the CEJ were prepared in extracted human third molars. Direct resin composite restorations (SDR with CeramX mono, Tetric EvoCeram, Filtek Supreme XT, and Venus Diamond or the respective resin composites alone) were bonded with system immanent adhesives XP Bond, Xeno V, Syntac, Adper Prompt L-Pop, and iBond self-etch. Before and after thermomechanical loading (100,000 × 50 N, 2500 thermocycles between 5 and 55 °C), marginal gaps were analysed using SEM of epoxy resin replicas. Results were analysed with Kruskal-Wallis and Mann-Whitney U-tests (p < 0.05). After thermomechanical loading, specimens were cut longitudinally in order to investigate internal dentine adaptation by epoxy replicas under a SEM (200× magnification).

Results

In enamel, high percentages of gap-free margins were initially identified for all adhesives. After TML, etch-and-rinse adhesives performed better than self-etch adhesives (p < 0.05). Also in dentine, initially high percentages of gap-free margins were found for all adhesives. After TML, etch-and-rinse adhesives again performed better than self-etch adhesives for both marginal and internal adaptation (p < 0.05). The presence of a 4 mm layer of SDR had no negative influence on results in any group (p > 0.05).

Conclusions

SDR as 4 mm bulk fill dentine replacement showed an good performance with the material combinations under investigation.     

两种材料桩钉对牙根应力分布影响的三维光弹应力分析

目的：研究不同弹性模量材料桩钉对牙根应力分布的影响。方法：根据加载方式和桩钉材料将模型分为四组，用三维光弹应力分析法分析FRC桩钉和金属桩钉在两种加载(垂直和斜向)方式下牙根内应力分布情况。结果：桩钉材料对牙根应力分布有明显的影响，弹性模量高的合金桩钉组在两种加载方式下在桩钉末端牙体组织内均有较大的应力集中，而模量较低的FRC桩钉组牙根应力分布比较均匀。结论：不同弹性模量材料桩钉对牙根应力分布有明显的影响，弹性模量与牙体组织接近的FRC桩钉有利于保护牙根防止折裂。