Reversible addition‐fragmentation chain‐transfer (RAFT) polymerization is a valuable tool for synthesizing macromolecules with controlled topologies and diverse chemical functionalities. However, the ...application of RAFT polymerization to additive‐manufacturing processes has been prevented due to the slow polymerization rates of typical systems. In this work, we developed and optimized a rapid visible (green) light mediated RAFT polymerization process and applied it to an open‐air 3D printing system. The reaction components are non‐toxic, metal free and environmentally friendly, which tailors these systems toward biomaterial fabrication. The inclusion of RAFT agent in the photosensitive resin provided control over the mechanical properties of 3D printed materials and allowed these materials to be post‐functionalized after 3D printing. Additionally, photoinduced spatiotemporal control of the network structure provided a one‐pass approach to 4D printed materials. This RAFT‐mediated 3D and 4D printing process should provide access to a range of new functional and stimuli‐responsive materials.
RAFT 3D and 4D printing is achieved using an eco‐friendly resin under mild conditions. The resulting materials show RAFT‐dependent mechanical properties and can be post‐functionalized after fabrication. Solvation induced actuation of photocontrolled 3D printed hydrogels demonstrates a 4D printing system.
The type of load application on layered concrete structure affects on the final mechanical properties of specimens.
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•Using geopolymers in additive processes resulted in adequate ...flexural strength.•Inclusion fibers involved negative effects on the bond strength between layers.•Reducing time gaps between subsequent layers enhanced flexural strength results.•Increasing thicknesses of objects’ layers improved the flexural strength result.
This paper focuses on inspecting the structural buildability of layered objects. Simulating extrusion method process was used to investigate three geopolymer mixes, three-time gaps, and two layering patterns through 18 layered samples. This paper also evaluates effects of the layering process on hardened properties of build-up materials through 9 standard specimens. The used materials were Gladstone fly ash, sand, 8 M sodium hydroxide solution, and D-grade sodium silicate. The weight ratio of sodium silicate to sodium hydroxide was 1, and activators to fly ash was 0.26. Mix 1, mix 2, and mix 3 contained 0%, 1% steel fibers, and 0.5% polypropylene fibers respectively. The structural buildability of layered samples and the influence of the layering process were assessed in terms of flexural strength through 3-point bending tests. Flexural strength results indicated that the layering process has negative impact on the mechanical strength of build-up materials. Also, mix 2 resulted in the highest flexural strength values in standard specimens and layered samples. However, the most bond separation issues between additive layers were achieved with layered samples produced with mix 2. Most layered samples produced with minimum time gaps recorded the highest flexural strength results.
Ordered meso/macroporous metal oxides have gained increasing attention in heterogeneous catalysis arising from their large surface areas and pore volumes, elevated catalytic activity and good thermal ...stability. Compared to nonporous metal oxides, their most prominent feature is the ability to interact with molecules not only at their exterior surface but also within the large interior surface of the material. The past decade has witnessed substantial advances in the synthesis of new porous metal oxides with ordered structures for use in a wide range of applications. By recalling some of the classical fundamentals of porous materials, this review examines the recent developments in ordered meso- and macro-porous metal oxide catalysts for heterogeneous catalysis. Additionally, we outline the current challenges in the field of nanoparticle-based catalysis, including the role played by the morphology (size, shape, and porosity) of ordered meso/macroporous metal oxides, and provide a perspective on the need for further advances in porous materials so that their contribution to heterogeneous catalysis can continue to expand.
The present review article highlights the preparation, characterization, properties, and recent developments in porous metal oxide catalysts for heterogeneous catalysis.
Extrusion printing processes allow for manufacturing complex shapes in a relatively cheap way with low-cost machines. The present study analyzes the effect of printing parameters on dimensional ...error, roughness, and porosity of printed PLA parts obtained with grid structure. Parts are obtained by means of the fused filament fabrication (FFF) process. Four variables are chosen: Layer height, temperature, speed, and flow rate. A two-level full factorial design with a central point is used to define the experimental tests. Dimensional error and porosity are measured with a profile projector, while roughness is measured with a contact roughness meter. Mathematical regression models are found for each response, and multi-objective optimization is carried out by means of the desirability function. Dimensional error and roughness depend mainly on layer height and flow rate, while porosity depends on layer height and printing speed. Multi-objective optimization shows that recommended values for the variables are layer height 0.05 mm, temperature 195 ºC, speed 50 mm/min, and flow rate 0.93, when dimensional error and roughness are to be minimized, and porosity requires a target value of 60%. The present study will help to select appropriate printing parameters for printing porous structures such as those found in prostheses, by means of extrusion processes.
3D printing has changed the fabrication of advanced materials as it can provide customized and on‐demand 3D networks. However, 3D printing of polymer materials with the capacity to be transformed ...after printing remains a great challenge for engineers, material, and polymer scientists. Radical polymerization has been conventionally used in photopolymerization‐based 3D printing, as in the broader context of crosslinked polymer networks. Although this reaction pathway has shown great promise, it offers limited control over chain growth, chain architecture, and thus the final properties of the polymer networks. More fundamentally, radical polymerization produces dead polymer chains incapable of postpolymerization transformations. Alternatively, the application of reversible deactivation radical polymerization (RDRP) to polymer networks allows the tuning of network homogeneity and more importantly, enables the production of advanced materials containing dormant reactivatable species that can be used for subsequent processes in a postsynthetic stage. Consequently, the opportunities that (photoactivated) RDRP‐based networks offer have been leveraged through the novel concepts of structurally tailored and engineered macromolecular gels, living additive manufacturing and photoexpandable/transformable‐polymer networks. Herein, the advantages of RDRP‐based networks over irreversibly formed conventional networks are discussed.
The application of reversible deactivation radical polymerization (RDRP) to polymer networks and 3D printing allows the tuning of network homogeneity and more importantly, enables the production/manufacturing of advanced materials containing dormant reactivatable species that can be used to change the material's properties in a postsynthetic/printing stage, a characteristic that is not achievable using conventional techniques.
Degenerative spine surgeries often require postoperative immobilization or reduced mobility, predisposing patients to the formation of thrombosis and higher risk of thromboembolic complications. ...Despite the significance of this issue, there remains a lack of consensus on the optimal anticoagulant agent for postoperative thromboprophylaxis in spinal stenosis and degenerative spine surgeries. Low molecular weight heparins and direct Xa inhibitors represent two anticoagulant groups with high chemoprophylactic potential.
This study included a prospective cohort of patients undergoing posterior decompressive surgery with or without instrumentation for degenerative spine disease and/or spinal stenosis. Patients receiving postoperative prophylactic Enoxaparin and Apixaban were selected to evaluate the rate of complications, as assessed by Clavien-Dindo classification, thromboembolic events, and 30-day mortality, readmission, and reoperation rate between the two anticoagulants.
130 patients were included in the analysis. 65 patients received Apixaban and Enoxaparin in each group. Mean age of the participants was 57.6±11.0. 83.1% underwent laminectomy and posterior spinal fusion, while 22 patients underwent decompressive surgery only. The incidence of venous thromboembolism (P-value=0.403), deep vein thrombosis (p-value=0.999), hematoma formation (p-value=0.403), surgical site infection (p-value=0.901), readmission (p-value=0.545), reoperation (p=0.510), mortality (p=0.648), and complications rate (p-value=0.232) were not statistically different between Enoxaparin and Apixaban.
Both Apixaban and Enoxaparin may be viable options for postoperative thromboprophylaxis in spine surgeries with comparable efficacy and safety profile. Future research endeavors should investigate the efficacy of these agents in comparison to placebo in a randomized setting.
•The use of Apixaban and Enoxaparin led to a similar rate of complications.•The rate of mortality was comparable between the anticoagulants.•The efficacy of two anticoagulants for thromboprophylaxis was not different.
3D printed scaffolds can be used, for example, in medical applications for simulating body tissues or for manufacturing prostheses. However, it is difficult to print porous structures of specific ...porosity and pore size values with fused deposition modelling (FDM) technology. The present paper provides a methodology to design porous structures to be printed. First, a model is defined with some theoretical parallel planes, which are bounded within a geometrical figure, for example a disk. Each plane has randomly distributed points on it. Then, the points are joined with lines. Finally, the lines are given a certain volume and the structure is obtained. The porosity of the structure depends on three geometrical variables: the distance between parallel layers, the number of columns on each layer and the radius of the columns. In order to obtain mathematical models to relate the variables with three responses, the porosity, the mean of pore diameter and the variance of pore diameter of the structures, design of experiments with three-level factorial analysis was used. Finally, multiobjective optimization was carried out by means of the desirability function method. In order to favour fixation of the structures by osseointegration, porosity range between 0.5 and 0.75, mean of pore size between 0.1 and 0.3 mm, and variance of pore size between 0.000 and 0.010 mm² were selected. Results showed that the optimal solution consists of a structure with a height between layers of 0.72 mm, 3.65 points per mm² and a radius of 0.15 mm. It was observed that, given fixed height and radius values, the three responses decrease with the number of points per surface unit. The increase of the radius of the columns implies the decrease of the porosity and of the mean of pore size. The decrease of the height between layers leads to a sharper decrease of both the porosity and the mean of pore size. In order to compare calculated and experimental values, scaffolds were printed in polylactic acid (PLA) with FDM technology. Porosity and pore size were measured with X-ray tomography. Average value of measured porosity was 0.594, while calculated porosity was 0.537. Average value of measured mean of pore size was 0.372 mm, while calculated value was 0.434 mm. Average value of variance of pore size was 0.048 mm², higher than the calculated one of 0.008 mm². In addition, both round and elongated pores were observed in the printed structures. The current methodology allows designing structures with different requirements for porosity and pore size. In addition, it can be applied to other responses. It will be very useful in medical applications such as the simulation of body tissues or the manufacture of prostheses.
Research into 3D printing using reversible addition–fragmentation chain transfer (RAFT) polymerization has garnered interest since it was first reported in 2019. This technique was initially ...developed to expand the scope of light-based 3D printing technologies by producing materials that can be modified postprinting, termed “living” 3D printing. The livingness can be achieved by incorporating reactivatable RAFT functionalities within the polymer networks, enabling 3D materials to be modified after printing. As the field of RAFT-mediated 3D printing has progressed, further studies have revealed its applications in advanced materials. These include spatially resolved surface functionalization and patterning, self-healing, welding, and nano- and microscale structuring of 3D polymers. Additionally, RAFT-mediated 3D printing enables the production of scaffolds with controlled interconnected channel-pore architecture, suitable for customized drug delivery. This Perspective provides a review of the chemical mechanisms employed in RAFT-mediated 3D printing and highlights the advanced materials manufactured through this technology. Potential research directions in this field are also discussed and organized for future investigation.
This article attempts to answer the question of why Iran is reluctant to discuss its missile program. Unlike other studies that focus on the importance of Iran s missile program in providing ...deterrence for the country and establishing a balance of military power in the region, or that view the missile program as one of dozens of post-revolutionary contentious issues between Iran and the United States, this article looks into Iran's ontological security. The paper primarily argues that the missile program has become a source of pride for Iranians, inextricably linked to their identity. As a result, the Iranian authorities face two challenges when it comes to sitting at the negotiation table with their Western counterparts: deep mistrust of the West, and the ensuing sense of shame over any deal on the missile issue. Thus, Iranian officials opted to preserve the identity components of the program, return to normal and daily routines of life, insist on the missile program's continuation despite sanctions and threats, and emphasize the dignity and honor of having a missile program. The article empirically demonstrates how states can overcome feelings of shame and mistrust. It also theoretically proves that when physical security conflicts with ontological security, governments prefer the former over the latter, based on the history of Iran s nuclear negotiations. They appeal to create new narratives to justify changing their previous policies.
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