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Powder bed fusion (PBF) is one of seven different classes of additive manufacturing (AM) technologies identified by ASTM and ISO. In polymer PBF, an infra-red energy source ...selectively fuses powder particles layer-by-layer into a three-dimensional structure. This enables the production of parts without the use of a mold, which is useful for prototyping and low-volume production. The early research in polymer PBF has focused largely on exploring the expanded design space afforded by the technology and modeling the heat transfer during fabrication. These are aspects that emphasize the manufacturing process and resultant quality in a material agnostic manner. Early investigations into structure-process-property relationships focused on the industrially dominant polyamide family. Only recently has research been conducted towards expanding the PBF material portfolio beyond nylon-12 and its composites. Guiding this research is the knowledge gained from studying the behavior of polyamides in PBF, which resulted in pervasive guidelines for material screening and process parameter development in polymer PBF, including the concepts of a “stable sintering region” and utilizing the “energy melt ratio” to set machine parameters. However, these guidelines are largely empirical and disproportionately focus on process parameter effects on the mechanical properties of the printed parts, instead of the intrinsic polymer properties and first principles of polymer science and engineering.
This review categorically compiles the PBF AM literature by the three process sub-functions: powder recoating, energy input, and coalescence and cooling. The literature outlining the governing physics, structure-property-processing relationships enabling printing, and the process-structure-property relationships enabling targeted final part properties are discussed within each sub-function. Establishing these polymer-manufacturing relationships, both for printability and for final part property prediction, is important to aid in the identification and adaptation of existing polymers, and development of novel polymers, for PBF AM.
Abstract This review highlights the synthesis, properties, and advanced applications of synthetic and natural polymers 3D printed using stereolithography for soft tissue engineering applications. ...Soft tissue scaffolds are of great interest due to the number of musculoskeletal, cardiovascular, and connective tissue injuries and replacements humans face each year. Accurately replacing or repairing these tissues is challenging due to the variation in size, shape, and strength of different types of soft tissue. With advancing processing techniques such as stereolithography, control of scaffold resolution down to the μm scale is achievable along with the ability to customize each fabricated scaffold to match the targeted replacement tissue. Matching the advanced manufacturing technique to polymer properties as well as maintaining the proper chemical, biological, and mechanical properties for tissue replacement is extremely challenging. This review discusses the design of polymers with tailored structure, architecture, and functionality for stereolithography, while maintaining chemical, biological, and mechanical properties to mimic a broad range of soft tissue types.
Toward Recyclable Thermosets Long, Timothy E.
Science (American Association for the Advancement of Science),
05/2014, Letnik:
344, Številka:
6185
Journal Article
Recenzirano
Advances in synthesis are leading to thermoset plastics that can be converted to the starting monomers.
Also see Report by
García
et al.
Recycling codes on plastic food and beverage packaging serve ...to guide consumers' daily decisions about the disposal of used packaging. However, technological obstacles remain for the recycling of more sophisticated polymeric packaging, ranging from multilayered food packaging to composite polymeric materials for electronic packaging. Moreover, many electronic devices contain heat-resistant, chemically stable polymers called thermosets that are not amenable to conventional collecting and recycling. On page 732 of this issue, García
et al.
(
1
) report a crucial step toward recyclable thermosets with the synthesis of ductile, insulating, temperature-resistant, and chemically inert thermosets that can be returned to their monomeric state through a pH trigger.
Systemic insecticides are applied to plants using a wide variety of methods, ranging from foliar sprays to seed treatments and soil drenches. Neonicotinoids and fipronil are among the most widely ...used pesticides in the world. Their popularity is largely due to their high toxicity to invertebrates, the ease and flexibility with which they can be applied, their long persistence, and their systemic nature, which ensures that they spread to all parts of the target crop. However, these properties also increase the probability of environmental contamination and exposure of nontarget organisms. Environmental contamination occurs via a number of routes including dust generated during drilling of dressed seeds, contamination and accumulation in arable soils and soil water, runoff into waterways, and uptake of pesticides by nontarget plants via their roots or dust deposition on leaves. Persistence in soils, waterways, and nontarget plants is variable but can be prolonged; for example, the half-lives of neonicotinoids in soils can exceed 1,000 days, so they can accumulate when used repeatedly. Similarly, they can persist in woody plants for periods exceeding 1 year. Breakdown results in toxic metabolites, though concentrations of these in the environment are rarely measured. Overall, there is strong evidence that soils, waterways, and plants in agricultural environments and neighboring areas are contaminated with variable levels of neonicotinoids or fipronil mixtures and their metabolites (soil, parts per billion (ppb)-parts per million (ppm) range; water, parts per trillion (ppt)-ppb range; and plants, ppb-ppm range). This provides multiple routes for chronic (and acute in some cases) exposure of nontarget animals. For example, pollinators are exposed through direct contact with dust during drilling; consumption of pollen, nectar, or guttation drops from seed-treated crops, water, and consumption of contaminated pollen and nectar from wild flowers and trees growing near-treated crops. Studies of food stores in honeybee colonies from across the globe demonstrate that colonies are routinely and chronically exposed to neonicotinoids, fipronil, and their metabolites (generally in the 1–100 ppb range), mixed with other pesticides some of which are known to act synergistically with neonicotinoids. Other nontarget organisms, particularly those inhabiting soils, aquatic habitats, or herbivorous insects feeding on noncrop plants in farmland, will also inevitably receive exposure, although data are generally lacking for these groups. We summarize the current state of knowledge regarding the environmental fate of these compounds by outlining what is known about the chemical properties of these compounds, and placing these properties in the context of modern agricultural practices.
Nurturing a global choir of perspective Long, Timothy E.
Polymer international,
February 2023, 2023-02-00, 20230201, Letnik:
72, Številka:
2
Journal Article
The imidazole ring is ubiquitous in nature and imidazole functionality plays a critical role in many structures within the human body, notably as histamine and histadine. Imidazoles offer many ...biophysical interactions including their ability to hydrogen bond with drugs and proteins. In contrast, imidazolium salts have lost their strong hydrogen-bonding ability through alkylation of both nitrogens, but they are able to aggregate electrostatically. Imidazolium salts are used to extract metal ions from aqueous solutions, dissolve carbohydrates, create polyelectrolyte brushes on surfaces, coat metal nanoparticles, provide antimicrobial action, and create oriented liquid crystals. Bioactive applications include imidazolium hydrogels, antiarrhythmics, and anti-metastic agents. This review will describe the synthesis and design of imidazole derivatives and imidazolium-containing polymers as bioactive materials. Imidazole-based polymers readily associate with biological molecules through hydrogen-bonding, and imidazolium analogs offer electrostatic interactions, aggregation, and self-assembly. Design of novel imidazole- and imidazolium-based macromolecules remains as an exciting and emerging field.
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Redox transitions of uranium from U(VI) to U(IV) in low-temperature sediments govern the mobility of uranium in the environment and the accumulation of uranium in ore bodies, and inform our ...understanding of Earth’s geochemical history. The molecular-scale mechanistic pathways of these transitions determine the U(IV) products formed, thus influencing uranium isotope fractionation, reoxidation, and transport in sediments. Studies that improve our understanding of these pathways have the potential to substantially advance process understanding across a number of earth sciences disciplines. Detailed mechanistic information regarding uranium redox transitions in field sediments is largely nonexistent, owing to the difficulty of directly observing molecular-scale processes in the subsurface and the compositional/physical complexity of subsurface systems. Here, we present results from an in situ study of uranium redox transitions occurring in aquifer sediments under sulfate-reducing conditions. Based on molecular-scale spectroscopic, pore-scale geochemical, and macroscale aqueous evidence, we propose a biotic–abiotic transition pathway in which biomass-hosted mackinawite (FeS) is an electron source to reduce U(VI) to U(IV), which subsequently reacts with biomass to produce monomeric U(IV) species. A species resembling nanoscale uraninite is also present, implying the operation of at least two redox transition pathways. The presence of multiple pathways in low-temperature sediments unifies apparently contrasting prior observations and helps to explain sustained uranium reduction under disparate biogeochemical conditions. These findings have direct implications for our understanding of uranium bioremediation, ore formation, and global geochemical processes.
High‐performance, all‐aromatic, insoluble, engineering thermoplastic polyimides, such as pyromellitic dianhydride and 4,4′‐oxydianiline (PMDA–ODA) (Kapton), exhibit exceptional thermal stability (up ...to ≈600 °C) and mechanical properties (Young's modulus exceeding 2 GPa). However, their thermal resistance, which is a consequence of the all‐aromatic molecular structure, prohibits processing using conventional techniques. Previous reports describe an energy‐intensive sintering technique as an alternative technique for processing polyimides with limited resolution and part fidelity. This study demonstrates the unprecedented 3D printing of PMDA–ODA using mask‐projection stereolithography, and the preparation of high‐resolution 3D structures without sacrificing bulk material properties. Synthesis of a soluble precursor polymer containing photo‐crosslinkable acrylate groups enables light‐induced, chemical crosslinking for spatial control in the gel state. Postprinting thermal treatment transforms the crosslinked precursor polymer to PMDA–ODA. The dimensional shrinkage is isotropic, and postprocessing preserves geometric integrity. Furthermore, large‐area mask‐projection scanning stereolithography demonstrates the scalability of 3D structures. These unique high‐performance 3D structures offer potential in fields ranging from water filtration and gas separation to automotive and aerospace technologies.
High‐performance, thermoplastic, fully aromatic polyimide structures are 3D printed for the first time using mask‐projection stereolithography (MPSL). Synthesis of soluble, photo‐crosslinkable polyamic diacrylate ester enables MPSL upon dissolution in a solvent. The 3D organogels with micrometer‐scale resolution undergo conversion to a 3D polyimide on thermal imidization without loss in part fidelity. Mechanically robust, high‐performance, fully aromatic 3D polyimide is obtained.
Abstract Background Major depressive disorder is associated with disturbed circadian rhythms. To investigate the causal relationship between mood disorders and circadian clock disruption, previous ...studies in animal models have employed light-dark manipulations, global mutations of clock genes, or brain area lesions. However, light can impact mood by non-circadian mechanisms, clock genes have pleiotropic, clock-independent functions, and brain lesions not only disrupt cellular circadian rhythms but also destroy cells and eliminate important neuronal connections, including light reception pathways. Thus, a definitive causal role for functioning circadian clocks in mood regulation has not been established. Methods In this study, we stereotaxically injected viral vectors encoding shRNA to knock down expression of the essential clock gene Bmal1 into the brain’s master circadian pacemaker, the suprachiasmatic nucleus (SCN). Results In these SCN-specific Bmal1 -knockdown (SCN- Bmal1 -KD) mice, circadian rhythms are greatly attenuated in the SCN, while the mice are maintained in a standard light/dark cycle, SCN neurons remain intact, and neuronal connections are undisturbed, including photic inputs. In the learned helplessness paradigm, these mice are slower to escape, even before exposure to inescapable stress. They also spend more time immobile in the tail suspension test and less time in the lighted section of a light/dark box. SCN- Bmal1 -KD mice also show greater weight gain, an abnormal circadian pattern of corticosterone, and an attenuated increase of corticosterone in response to stress. Conclusion Thus, disrupting SCN circadian rhythms is sufficient to cause helplessness, behavioral despair, and anxiety-like behavior in mice, establishing SCN- Bmal1 -KD mice as a new animal model of depression.
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