Dimensional Characterization and Hybrid Manufacturing of Copper Parts Obtained by Atomic Diffusion Additive Manufacturing, and CNC Machining.

The combination of Atomic Diffusion Additive Manufacturing (ADAM) and traditional CNC machining allows manufacturers to leverage the advantages of both technologies in the production of functional metal parts. This study presents the methodological development of hybrid manufacturing for solid copper parts, initially produced using ADAM technology and subsequently machined using a 5-axis CNC system. The ADAM technology was dimensionally characterized by adapting and manufacturing the seven types of test artifacts standardized by ISO/ASTM 52902:2019. The results showed that slender geometries suffered warpage and detachment during sintering despite complying with the design guidelines. ADAM technology undersizes cylinders and oversizes circular holes and linear lengths. In terms of roughness, the lowest results were obtained for horizontal flat surfaces, while 15° inclined surfaces exhibited the highest roughness due to the stair-stepping effect. The dimensional deviation results for each type of geometry were used to determine the specific and global oversize factors necessary to compensate for major dimensional defects. This also involved generating appropriate over-thicknesses for subsequent CNC machining. The experimental validation of this process, conducted on a validation part, demonstrated final deviations lower than 0.5% with respect to the desired final part, affirming the feasibility of achieving copper parts with a high degree of dimensional accuracy through the hybridization of ADAM and CNC machining technologies.

Venturi Injector Optimization for Precise Powder Transport for Directed Energy Deposition Manufacturing Using the Discrete Element Method and Genetic Algorithms.

Additive manufacturing technologies such as directed energy deposition use powder as their raw material, and it must be deposited in a precise and controlled manner. Venturi injectors could be a solution for the highly precise transport of particulate material. They have been studied from different perspectives, but they are always under high-pressure conditions and mostly fed by gravity. In the present study, an optimization of the different dimensional parameters needed for the manufacturing of a Venturi injector in relation to a particle has been carried out to maximize the amount of powder capable of being sucked and transported for a specific flow in a low-pressure system with high precision in transport. For this optimization, simulations of Venturi usage were performed using the discrete element method, generating different variations proposed by a genetic algorithm based on a preliminary design of experiments. Statistical analysis was also performed to determine the most influential design variables on the objective, with these being the suction diameter (D3), the throat diameter (d2), and the nozzle diameter (d1). The optimal dimensional relationships were as follows: a D3 34 times the particle diameter, a d2 26.5 times the particle diameter, a d1 40% the d2, a contraction angle alpha of 18.73°, and an expansion angle beta of 8.28°. With these proportions, an 85% improvement in powder suction compared to the initial attempts was achieved, with a maximum 2% loss of load.

Calcium Carbonate Coating of 3D-Printed PLA Scaffolds Intended for Biomedical Applications.

The incorporation of ceramic additives is the most commonly used strategy to improve the biofunctionality of polymer-based scaffolds intended for bone regeneration. By embedding ceramic particles as a coating, the functionality improvement in the polymeric scaffolds can be concentrated on the cell-surface interface, thus creating a more favourable environment for the adhesion and proliferation of osteoblastic cells. In this work, a pressure-assisted and heat-induced method to coat polylactic acid (PLA) scaffolds with calcium carbonate (CaCO3) particles is presented for the first time. The coated scaffolds were evaluated by optical microscopy observations, a scanning electron microscopy analysis, water contact angle measurements, compression testing, and an enzymatic degradation study. The ceramic particles were evenly distributed, covered more than 60% of the surface, and represented around 7% of the coated scaffold weight. A strong bonding interface was achieved, and the thin layer of CaCO3 (~20 µm) provided a significant increase in the mechanical properties (with a compression modulus improvement up to 14%) while also enhancing the surface roughness and hydrophilicity. The results of the degradation study confirmed that the coated scaffolds were able to maintain the pH of the media during the test (~7.6 ± 0.1), in contrast to the pure PLA scaffolds, for which a value of 5.07 ± 0.1 was obtained. The ceramic-coated scaffolds developed showed potential for further evaluations in bone tissue engineering applications.

Valorization of Pennisetum setaceum: From Invasive Plant to Fiber Reinforcement of Injected Composites.

During the control campaigns of Pennisetum setaceum (invasive species widespread worldwide), the generated waste has accumulated in landfills. This study investigates its use to obtain P. setaceum fibers for their application as reinforcement of polymeric materials for injection molding, thus facilitating and promoting alternatives for the long-term sustainable management of P. setaceum. The extracted fibers were treated with alkaline, silane, acetic acid, and combined alkaline and silane treatments. Different composites with 20 and 40 wt% of fiber were extruded, and test samples were obtained by injection molding using recycled polyethylene as matrix. The composition of the fibers was determined by gravimetric methods, and contrasted with the analysis of the functional chemical groups using Fourier Transform Infrared Spectroscopy. Increases of up to 47% in the cellulose content of the treated fiber were observed. The thermal degradation was also evaluated using thermogravimetric analysis, which determined an increase in the degradation temperature, from 194 to 230 °C, after the combined alkaline-silane treatment. In order to analyze the differences in the composites, tensile, flexural, and impact properties were evaluated; in addition, differential scanning calorimetry was performed. Regarding the flexural behavior, it was possible to improve the flexural modulus up to 276% compared with that of the unreinforced polymer.

Evaluation of the Performance of Atomic Diffusion Additive Manufacturing Electrodes in Electrical Discharge Machining.

Atomic Diffusion Additive Manufacturing (ADAM) is an innovative Additive Manufacturing process that allows the manufacture of complex parts in metallic material, such as copper among others, which provides new opportunities in Rapid Tooling. This work presents the development of a copper electrode manufactured with ADAM technology for Electrical Discharge Machining (EDM) and its performance compared to a conventional electrolytic copper. Density, electrical conductivity and energy-dispersive X-ray spectroscopy were performed for an initial analysis of both ADAM and electrolytic electrodes. Previously designed EDM experiments and optimizations using genetic algorithms were carried out to establish a comparative framework for both electrodes. Subsequently, the final EDM tests were carried out to evaluate the electrode wear rate, the roughness of the workpiece and the rate of material removal for both electrodes. The EDM results show that ADAM technology enables the manufacturing of functional EDM electrodes with similar material removal rates and rough workpiece finishes to conventional electrodes, but with greater electrode wear, mainly due to internal porosity, voids and other defects observed with field emission scanning electron microscopy.

Special Issue "Design and Application of Additive Manufacturing".

Additive manufacturing (AM) is continuously improving and offering new opportunities in the manufacturing industry [...].

Comparison of CAD and Voxel-Based Modelling Methodologies for the Mechanical Simulation of Extrusion-Based 3D Printed Scaffolds.

Porous structures are of great importance in tissue engineering. Most scaffolds are 3D printed, but there is no single methodology to model these printed parts and to apply finite element analysis to estimate their mechanical behaviour. In this work, voxel-based and geometry-based modelling methodologies are defined and compared in terms of computational efficiency, dimensional accuracy, and mechanical behaviour prediction of printed parts. After comparing the volumes and dimensions of the models with the theoretical and experimental ones, they are more similar to the theoretical values because they do not take into account dimensional variations due to the printing temperature. This also affects the prediction of the mechanical behaviour, which is not accurate compared to reality, but it makes it possible to determine which geometry is stiffer. In terms of comparison of modelling methodologies, based on process efficiency, geometry-based modelling performs better for simple or larger parts, while voxel-based modelling is more advantageous for small and complex geometries.

Valorization of Invasive Plants from Macaronesia as Filler Materials in the Production of Natural Fiber Composites by Rotational Molding.

This paper compares the mechanical properties of different natural fiber composites produced by rotational molding as a way of waste valorization from campaigns to control invasive plant species in Macaronesia. Rotomolded parts produced with polymeric matrices (polyethylene) and filled with up to 20% by weight of cellulosic fibers obtained from Arundo donax L., Pennisetum setaceum, and Ricinus communis plants were characterized in terms of tensile, flexural, and impact strength. It was found that the sieving of natural fibers allowed for their introduction in higher loadings, from 10 (for un-sieved material) to 20%; fiber size greatly affected the mechanical properties of the final parts, although some combinations were proven not to reduce the mechanical properties of the neat resin. This study is a first approach to the valorization of residues obtained from periodic campaigns of the control of invasive species performed by public authorities, usually at the local level. It is important to highlight that the main objective of this research did not focus on economically profitable activity; instead, it was focused on the reduction of wastes to be disposed from ecosystem maintenance actions and the investment of potential income into preservation policies.

Experimental Analysis of the Enzymatic Degradation of Polycaprolactone: Microcrystalline Cellulose Composites and Numerical Method for the Prediction of the Degraded Geometry.

The degradation rate of polycaprolactone (PCL) is a key issue when using this material in Tissue Engineering or eco-friendly packaging sectors. Although different PCL-based composite materials have been suggested in the literature and extensively tested in terms of processability by material extrusion additive manufacturing, little attention has been paid to the influence of the fillers on the mechanical properties of the material during degradation. This work analyses the possibility of tuning the degradation rate of PCL-based filaments by the introduction of microcrystalline cellulose into the polymer matrix. The enzymatic degradation of the composite and pure PCL materials were compared in terms of mass loss, mechanical properties, morphology and infrared spectra. The results showed an increased degradation rate of the composite material due to the presence of the filler (enhanced interaction with the enzymes). Additionally, a new numerical method for the prediction of the degraded geometry was developed. The method, based on the Monte Carlo Method in an iterative process, adjusts the degradation probability according to the exposure of each discretized element to the degradation media. This probability is also amplified depending on the corresponding experimental mass loss, thus allowing a good fit to the experimental data in relatively few iterations.

Thermal and Mechanical Characterization of Banana Fiber Reinforced Composites for Its Application in Injection Molding.

Several natural materials and vegetable waste have relevant mechanical properties, mainly in its fiber format. Particularly, banana fiber (BF) provides a close behavior to the widely spread glass fibers, which places it in an advantageous position for use as a reinforcing material in plastic composites. This work characterizes the behavior of acrylonitrile butadiene styrene (ABS), high impact polystyrene (HIPS), and high density polyethylene (HDPE) reinforced with short fibers of bananas from the Canary Islands for its application in molding processes. Several thermal analyses (Thermal Gravimetric Analysis (TGA), Differential Scanning Calorimetry (DSC), and Melt Flow Index (MFI)) and mechanical tests (tensile, flexural, impact, and Dynamic Mechanical Analysis (DMA)) were carried out in composites with different percentages of banana fiber. The thermal results show that the use of banana fiber is viable as a reinforcement in composites for injection molding processes and the mechanical tests indicate an increase in stiffness and an improvement in maximum flexural stress by increasing the fiber content in composites, so the banana fiber turns out to be a natural alternative for the reinforcement of injected plastic components.

Influence of Manufacturing Parameters and Post Processing on the Electrical Conductivity of Extrusion-Based 3D Printed Nanocomposite Parts.

The influence of manufacturing parameters of filament extrusion and extrusion-based Additive Manufacturing (AM), as well as different post processing techniques, on the electrical conductivity of 3D printed parts of graphene nanoplatelets (GNP)-reinforced acrylonitrile butadiene styrene (ABS) has been analyzed. The key role of the manufacturing parameters to obtain electrically conductive filaments and 3D printed parts has been demonstrated. Results have shown that an increase in extrusion speed, as well as lower land lengths, induces higher extrudate swelling, with the consequent reduction of the electrical conductivity. Additionally, filaments with lower diameter values, which result in a higher surface-to-cross-section ratio, have considerably lower electrical conductivities. These factors tune the values of the volume and surface electrical conductivity between 10-4-100 S/m and 10-8-10-3 S/sq, respectively. The volume and surface electrical conductivity considerably diminished after 3D printing. They increased when using higher printing layer thickness and width and were ranging between 10-7-10-4 S/m and 10-8-10-5 S/sq, respectively. This is attributed to the higher cross section area of the individual printed lines. The effect of different post processing (acetone vapor polishing, plasma and neosanding, which is a novel finishing process) on 3D printed parts in morphology and surface electrical conductivity was also analyzed.

Combining transcranial direct-current stimulation with gait training in patients with neurological disorders: a systematic review.

BACKGROUND: Transcranial direct-current stimulation (tDCS) is an easy-to-apply, cheap, and safe technique capable of affecting cortical brain activity. However, its effectiveness has not been proven for many clinical applications. OBJECTIVE: The aim of this systematic review was to determine whether the effect of different strategies for gait training in patients with neurological disorders can be enhanced by the combined application of tDCS compared to sham stimulation. Additionally, we attempted to record and analyze tDCS parameters to optimize its efficacy. METHODS: A search in Pubmed, PEDro, and Cochrane databases was performed to find randomized clinical trials that combined tDCS with gait training. A chronological filter from 2010 to 2018 was applied and only studies with variables that quantified the gait function were included. RESULTS: A total of 274 studies were found, of which 25 met the inclusion criteria. Of them, 17 were rejected based on exclusion criteria. Finally, 8 trials were evaluated that included 91 subjects with stroke, 57 suffering from Parkinson's disease, and 39 with spinal cord injury. Four of the eight assessed studies did not report improved outcomes for any of its variables compared to the placebo treatment. CONCLUSIONS: There are no conclusive results that confirm that tDCS can enhance the effect of the different strategies for gait training. Further research for specific pathologies, with larger sample sizes and adequate follow-up periods, are required to optimize the existing protocols for applying tDCS.

Optimization methodology for the material assignation in bioprinted scaffolds to achieve the desired stiffness over time.

The optimum scaffold for tissue engineering must guarantee the mechanical integrity in the damaged zone and ensure an appropriate stiffness to regulate the cellular function. For this to happen, scaffolds must be designed to match the stiffness of the native tissue. Moreover, the degradation rate in the case of bioresorbable materials must also be considered to fit the tissue regeneration rate. This paper presents a methodology based on design of experiments, finite element analysis, metamodels, and genetic algorithms to optimize the assignation of material in different sections of the scaffold to obtain the desired stiffness over time and comply with the constraints needed. The method applies an initial sampling focused on a modified Latin Hypercube strategy to obtain data from the simulations. These data are used in the next stages to generate the metamodels by using kriging. The predictions of the metamodels are used by the genetic algorithms to find the best estimated solutions. Different runs of the genetic algorithm drive the sampling, improving the accuracy of the surrogate models over the optimization process. Once the accuracy of the metamodels estimates is sufficient, a final genetic algorithm is applied to obtain the optimum design. This approach guarantees a low sampling effort and convergence to carry out the optimization process. The method allows the combination of discrete and continuous design variables in the optimization problem, and it can be applied both in solid and in hierarchical-based geometries.

Experimental Analysis and Simulation of Novel Technical Textile Reinforced Composite of Banana Fibre.

The use of natural fibres allows reducing environmental impact, due to their natural renewable origin and the lower energy needed for their production and processing. This work presents the mechanical characterization of a newly developed technical textile, with banana fibre treated by enzymes, comparing experimental results with numerical simulation based on the definition of the unit cell at micromechanical level. The experimental test shows that the composite with the fabric of banana fibre presents worse mechanical behaviour than the one with commercial flax fibre. The presence of wool, necessary for producing the yarn, reduces the mechanical properties of the banana textile. The numerical simulation had an acceptable error compared with the experimental results, with a global average error of 9%, showing that the predictive modelling based on the multiscale method is suitable for the design process of this kind of composite.

In vitro model of the glial scar.

The trauma of central nervous system (CNS) can lead to glial scar, and it can limit the regeneration of neurons at the injured area, which is considered to be a major factor affecting the functional recovery of patients with CNS injury. At present, the study of the glial scar model in vitro is still limited to two-dimensional culture, and the state of the scar in vivo cannot be well mimicked. Therefore, we use a collagen gel and astrocytes to construct a three-dimensional (3D) model in vitro to mimic natural glial scar tissue. The effects of concentration changes of astrocytes on cell morphology, proliferation, and tissue performance were investigated. After 8 days of culture in vitro, the results showed that the tissue model contracted, with a measured shrinkage rate of 4.5%, and the compressive elastic modulus increased to nearly 4 times. Moreover, the astrocytes of the 3D tissue model have the ability of proliferation, hyperplasia, and formation of scar clusters. It indicates that the model we constructed has the characteristics of glial scar tissue to some extent and can provide an in vitro model for the research of glial scar and brain diseases.

Anisotropy of Photopolymer Parts Made by Digital Light Processing.

Digital light processing (DLP) is an accurate additive manufacturing (AM) technology suitable for producing micro-parts by photopolymerization. As most AM technologies, anisotropy of parts made by DLP is a key issue to deal with, taking into account that several operational factors modify this characteristic. Design for this technology and photopolymers becomes a challenge because the manufacturing process and post-processing strongly influence the mechanical properties of the part. This paper shows experimental work to demonstrate the particular behavior of parts made using DLP. Being different to any other AM technology, rules for design need to be adapted. Influence of build direction and post-curing process on final mechanical properties and anisotropy are reported and justified based on experimental data and theoretical simulation of bi-material parts formed by fully-cured resin and partially-cured resin. Three photopolymers were tested under different working conditions, concluding that post-curing can, in some cases, correct the anisotropy, mainly depending on the nature of photopolymer.

Production of Banana Fiber Yarns for Technical Textile Reinforced Composites.

Natural fibers have been used as an alternative to synthetic ones for their greener character; banana fibers have the advantage of coming from an agricultural residue. Fibers have been extracted by mechanical means from banana tree pseudostems, as a strategy to valorize banana crops residues. To increase the mechanical properties of the composite, technical textiles can be used as reinforcement, instead of short fibers. To do so, fibers must be spun and woven. The aim of this paper is to show the viability of using banana fibers to obtain a yarn suitable to be woven, after an enzymatic treatment, which is more environmentally friendly. Extracted long fibers are cut to 50 mm length and then immersed into an enzymatic bath for their refining. Conditions of enzymatic treatment have been optimized to produce a textile grade of banana fibers, which have then been characterized. The optimum treating conditions were found with the use of Biopectinase K (100% related to fiber weight) at 45 °C, pH 4.5 for 6 h, with bath renewal after three hours. The first spinning trials show that these fibers are suitable to be used for the production of yarns. The next step is the weaving process to obtain a technical fabric for composites production.

Costs of the electrochemical oxidation of wastewaters: a comparison with ozonation and Fenton oxidation processes.

In the work described here the technical and economic feasibilities of three Advanced Oxidation Processes (AOPs) have been studied: Conductive-Diamond Electrochemical Oxidation (CDEO), Ozonation and Fenton oxidation. The comparison was made by assessing the three technologies with synthetic wastewaters polluted with different types of organic compounds and also with actual wastes (from olive oil mills and from a fine-chemical manufacturing plant). All three technologies were able to treat the wastes, but very different results were obtained in terms of efficiency and mineralization. Only CDEO could achieve complete mineralization of the pollutants for all the wastes. However, the efficiencies were found to depend on the concentration of pollutant (mass transfer control of the oxidation rate). Results obtained in the oxidation with ozone (at pH 12) or by Fenton's reagent were found to depend on the nature of the pollutants, and significant concentrations of oxidation-refractory compounds were usually accumulated during the treatment. Within the discharge limits that all of the technologies can reach, the economic analysis shows that the operating cost of Fenton oxidation is lower than either CDEO or ozonation, although CD\EO can compete satisfactorily with the Fenton process in the treatment of several kinds of wastes. Likewise, the investment cost for the ozonation process seems to be higher than either CDEO or Fenton oxidation, regardless of the pollutant treated.