Design of multifunctional composites and their use for the 3-D printing of microsystems

Many of today\u27s high-tech products are approaching their technological limits. For example, the microelectronics community is faced with overheating devices with a demand for compact three-dimensional (3D) architectures and lower power consumption, whereas the aerospace industry is seeking lighter, stiffer and more electrically conductive materials for the creation of more energy efficient aircraft. A promising solution is to capitalize on the amazing electrical, thermal and mechanical properties of some nanoscopic materials (one billionth of a meter). However, several challenges in material processing and manufacturing must be resolved, namely exploiting these properties at the industrial scale and overcoming the current planar configuration with a truly 3D method. The nature of work presented is mainly on the development of high-performance materials for the manufacturing of microscopic or larger systems featuring multiple functionalities. On the material side, the design of polymer-based nanocomposite coatings used to protect aerospace composite structures against lightning strike will be presented. A comparative analysis between the standard copper meshes and our novel coating designs (e.g., wet chemical metallization, heterogeneous distribution of conductive fillers, hybrid fillers deposition) was performed under high current (up to 50 kA). On the manufacturing side, different 3D printing methods will be explained. These methods were used to build complex 3D shapes at the microscopic scale and above such as helical freeform microcoils, microstructured fibers and nanocomposite liquid sensors. In conclusion, we believe that the fabrication techniques presented provide an original and promising approach to resolve the aforementioned issues, thus making nanotechnology more accessible to industry, especially in aerospace, microelectronics and biomedicine

Toughening elastomers via microstructured thermoplastic fibers with sacrificial bonds and hidden lengths

Soft materials capable of large inelastic deformation play an essential role in high-performance nacre-inspired architectured materials with a combination of stiffness, strength and toughness. The rigid "building blocks" made from glass or ceramic in these architectured materials lack inelastic deformation capabilities and thus rely on the soft interface material that bonds together these building blocks to achieve large deformation and high toughness. Here, we demonstrate the concept of achieving large inelastic deformation and high energy dissipation in soft materials by embedding microstructured thermoplastic fibers with sacrificial bonds and hidden lengths in a widely used elastomer. The microstructured fibers are fabricated by harnessing the fluid-mechanical instability of a molten polycarbonate (PC) thread on a commercial 3D printer. Polydimethylsiloxane (PDMS) resin is infiltrated around the fibers, creating a soft composite after curing. The failure mechanism and damage tolerance of the composite are analyzed through fracture tests. The high energy dissipation is found to be related to the multiple fracture events of both the sacrificial bonds and elastomer matrix. Combining the microstructured fibers and straight fibers in the elastomer composite results in a ~ 17 times increase in stiffness and a ~ 7 times increase in total energy to failure compared to the neat elastomer. Our findings in applying the sacrificial bonds and hidden lengths toughening mechanism in soft materials at the microscopic scale will facilitate the development of novel bioinspired laminated composite materials with high mechanical performance

Long term availability of raw experimental data in experimental fracture mechanics

Experimental data availability is a cornerstone for reproducibility in experimental fracture mechanics, which is crucial to the scientific method. This short communication focuses on the accessibility and long term availability of raw experimental data. The corresponding authors of the eleven most cited papers, related to experimental fracture mechanics, for every year from 2000 up to 2016, were kindly asked about the availability of the raw experimental data associated with each publication. For the 187 e-mails sent: 22.46% resulted in outdated contact information, 57.75% of the authors did received our request and did not reply, and 19.79 replied to our request. The availability of data is generally low with only $11$ available data sets (5.9%). The authors identified two main issues for the lacking availability of raw experimental data. First, the ability to retrieve data is strongly attached to the the possibility to contact the corresponding author. This study suggests that institutional e-mail addresses are insufficient means for obtaining experimental data sets. Second, lack of experimental data is also due that submission and publication does not require to make the raw experimental data available. The following solutions are proposed: (1) Requirement of unique identifiers, like ORCID or ResearcherID, to detach the author(s) from their institutional e-mail address, (2) Provide DOIs, like Zenodo or Dataverse, to make raw experimental data citable, and (3) grant providing organizations should ensure that experimental data by public funded projects is available to the public

Identification of constitutive theory parameters using a tensile machine for deposited filaments of microcrystalline ink by the direct-write method

A custom-designed tensile machine is developped to characterize the 10 mechanical properties of ink micro-filaments deposited by Direct-Write method. The 11 Direct-Write method has been used for the fabrication of a wide variety of micro12 systems such as microvascular networks, chaotic mixers and laboratory on-chips. The 13 tensile machine was used to measure the induced force in ink filaments during tensile 14 and tension-relaxation tests as a function of the applied strain rate, the ink composition 15 and the filament diameter. Experimental data was fitted by a linearly viscoelastic 16 model using a data reduction procedure in order to identify the constitutive theory 17 parameters of the deposited ink filaments. The model predictions based on the defined 18 constitutive theory parameters were closed to the experimental data generated in this 19 study. Such models will be useful in the development and optimization of future 3D 20 complex structures made by direct-write method

Advances in coaxial additive manufacturing and applications

Coaxial additive manufacturing (AM) is an emerging technology involving the simultaneous deposition of two or more materials with a common longitudinal axis. It has the potential to overcome the disadvantages associated with conventional single-material AM for the production of core-shell or multi-core-shell multimaterial structures. The coaxial AM techniques can be classified into extrusion and material jetting technologies. The extrusion-based technologies rely on the co-extrusion of multiple materials through a coaxial nozzle whereas the material jetting technologies are based on the introduction of a high voltage electric field between a coaxial nozzle and a grounded collector plate. In this review, we aim to provide a comprehensive overview of multimaterial coaxial AM, including the technologies, nozzle designs, materials and applications. We highlight the advances in coaxial AM and the benefits of this novel technology in various fields. For instance, in biomedicine coaxial AM offers an exciting alternative to single-material bioprinting for the fabrication of bio-scaffolds and vascular networks as well as for tissue engineering and cell encapsulations. Coaxial AM is also a subject of growing interest in the fields of flexible sensors, e-textiles, and printed electronics. We also provide perspectives on the limitations, existing challenges, opportunities, and future directions for further development

Electric field induced alignment of multiwalled carbon nanotubes in polymers and multiscale composites

Carbon fiber reinforced polymers (CFRPs) have a highly anisotropic electrical resistivity, which limits their use in electrical applications. In this contribution, an electric field was used to align multiwalled carbon nanotubes (MWCNTs) to create preferential conductive pathways within a nanocomposite and a multiscale composite in order to reduce their resistivity. Investigation on epoxy containing MWCNTs have shown that an electric field of 40 V mm21 or higher applied for 2 h can lead to a reduction of the resistivity parallel to the field up to four orders of magnitude with only 0.01 wt-% loading. In the case of CFRPs reinforced with 0.01 and 0.1 wt-% of MWCNTs, we observed reductions of the through the thickness resistivity of 36 and 99% respectively, when an electric field of 60 V mm21 was applied for 2 h during the fabrication of the samples

Helical dielectrophoretic particle separator fabricated by conformal spindle printing

This paper reports the fabrication and testing of a helical cell separator that uses insulator-based dielectrophoresis as the driving force of its separation. The helical channel shape’s main advantage is its constant curvature radius which generates a constant electric field gradient. The presented separator was fabricated by extruding a sacrificial ink on rotating spindles using a computer- controlled robot. After being assembled, connected to the reservoir and encapsulated in epoxy resin, the ink was removed to create a helical microchannel. The resulting device was tested by circulating polystyrene microbeads of 4 and 10 μm diameter through its channel using a voltage of 900 VDC. The particles were separated with efficiencies of 94.0% and 92.5%, respectively. However, roughness in some parts of the channel and connections that had larger diameters compared to the channel created local electric field gradients which, doubtless, hindered separation. It is a promising device that could lead the way toward portable and affordable medical devices

Three-dimensional printing of multifunctional nanocomposites: Manufacturing techniques and applications

The integration of nanotechnology into three-dimensional printing (3DP) offers huge potential and opportunities for the manufacturing of 3D engineered materials exhibiting optimized properties and multifunctionality. The literature relating to different 3DP techniques used to fabricate 3D structures at the macro- and microscale made of nanocomposite materials is reviewed here. The current state-of-the-art fabrication methods, their main characteristics (e.g., resolutions, advantages, limitations), the process parameters, and materials requirements are discussed. A comprehensive review is carried out on the use of metal- and carbon-based nanomaterials incorporated into polymers or hydrogels for the manufacturing of 3D structures, mostly at the microscale, using different 3D-printing techniques. Several methods, including but not limited to micro-stereolithography, extrusion-based direct-write technologies, inkjet-printing techniques, and popular powder-bed technology, are discussed. Various examples of 3D nanocomposite macro- and microstructures manufactured using different 3D-printing technologies for a wide range of domains such as microelectromechanical systems (MEMS), lab-on-a-chip, microfluidics, engineered materials and composites, microelectronics, tissue engineering, and biosystems are reviewed. Parallel advances on materials and techniques are still required in order to employ the full potential of 3D printing of multifunctional nanocomposites

Properties of polylactide inks for solvent-cast printing of three-dimensional freeform microstructures

Solvent-cast printing is a highly versatile microfabrication technique that can be used to construct various geometries such as filaments, towers, scaffolds and freeform circular spirals by the robotic deposition of a polymer solution ink onto a moving stage. In this work, we have performed a comprehensive characterization of the solvent-cast printing process using polylactide (PLA) solutions by analyzing the flow behavior of the solutions, the solvent evaporation kinetics and the effect of process-related parameters on the crystallization of the extruded filaments. Rotational rheometry at low to moderate shear rates showed a nearly Newtonian behavior of the PLA solutions, while capillary flow analysis based on process-related data indicated shear-thinning at high shear rates. Solvent vaporization tests suggested that the internal diffusion of the solvent through the filaments controlled the solvent removal of the extrudates. Different kinds of three-dimensional (3D) structures including a layer-by-layer tower, 9-layer scaffold and freeform spiral were fabricated, and a processing map was given to show the proper ranges of process-related parameters (i.e., polymer content, applied pressure, nozzle diameter and robot velocity) for the different geometries. The results of differential scanning calorimetry revealed that slow solvent evaporation could increase the ability of PLA to complete its crystallization process during the filament drying stage. The method developed here offers a new perspective for manufacturing complex structures from polymer solutions and provide guidelines to optimize the various parameters for 3D geometry fabrication