Effects of Fibril Morphology and Interfacial Interactions on the Behavior of Polymer-Grafted Cellulose Nanofibril Reinforced Thermoplastic Composites

Mechanically refined cellulose nanofibrils (CNFs) promise to be a high-volume, sustainable, nanoscale reinforcement for thermoplastic composites. They are currently held back by poor interfacial interactions with composite matrices, energy intensive drying, and drying induced fibril aggregation. In this dissertation, we explored how a grafting-through polymerization scheme modified the surface of CNFs with a wide variety of commodity polymers and overcame many of these technical challenges. The first phase of the research was concerned with characterizing the unique morphology of these CNFs as a function of refinement energy. This characterization was employed to understand how the materials’ morphologies affected their interfacial interactions with porous substrates. In this work, optical, scanning electron, and atomic force microscopy were used to characterize the materials and mechanical testing was used to assess their interfacial interactions with porous model substrates. The second phase of the research explored how the grafting-through polymerization of commodity monomers occurred in the presence of methacrylated CNFs. Infrared spectroscopy measurements were used to explore the degree of grafting and microscopic analyses were employed to understand how these modifications affected the materials’ suspension morphology. The final phase of the research looked at the modifications’ effects on drying behavior, surface energetics, and reinforcement ability in poly(lactic acid) (PLA). Scanning electron microscopy and inverse gas chromatography provided insights into how the grafted-polymer modifications improved the fibrillar morphology of spray-dried CNFs and increased their interfacial adhesion to PLA. Tensile testing and rheological characterization of composites made from these spray dried materials revealed their improved dispersion and network formation in the PLA matrix. Scale up of bench scale reactions to the pilot scale are demonstrated and 3D printing trials were conducted. Dramatic improvements in mechanical properties were seen for 3D printed samples modified with poly(N-isopropyl acrylamide). These improvements in mechanical properties were explored by dynamic mechanical analysis and tensile testing, revealing the effects of fibril alignment during printing

Fabrication and Mechanical Properties of Micro-Architectured 3D Scaffolds

Freeze casting is a physical process for the fabrication of porous anisotropic materials. In this method, an aqueous slurry of ceramic particles is frozen directionally, creating lamellar columns of ice that push particles between the growing crystals. Then, the frozen material is lyophilized to remove the ice and sintered to densify the ceramic. Scaffolds made by freeze casting often have significant strength in the solidification direction, while they lack sufficient strength in the transverse direction. To enhance strength in the transverse direction, magnetite particles are added to a slurry of paramagnetic particles, and an external magnetic field is applied during solidification. Interactions between the magnetite and paramagnetic particles compete with thermal and viscous forces, resulting in different colloidal behaviors. Under relatively weak magnetic fields, the particles are attracted to one another, forming aligned chains that are trapped by the ice front and result in bridges spanning the lamellar walls. When interactions between magnetite and paramagnetic particles are strong, the alignment of magnetite also results in alignment of the paramagnetic particles. Under stronger magnetic fields, however, a gradient magnetic force attracts particles toward the field’s poles, creating biphasic regions of iron-rich and iron-poor microstructures. To further investigate the relationship between microstructure and mechanical properties observed, 3D printed scaffolds mimicking patterns observed in magnetic freeze casting were designed and fabricated for comparison. The 3D printed scaffolds were tested in compression in three orthogonal directions. To compare their performance, permutated radar charts were used to simultaneously analyze the strength, toughness, resilience, elastic modulus and strain to failure across each orthogonal direction

3D biofabrication for tubular tissue engineering

The therapeutic replacement of diseased tubular tissue is hindered by the availability and suitability of current donor, autologous and synthetically derived protheses. Artificially created, tissue engineered, constructs have the potential to alleviate these concerns with reduced auto-immune response, high anatomical accuracy, long term patency and growth potential. The advent of 3D bio-printing technology has further supplemented the technological toolbox, opening up new biofabrication research opportunities and expanding the therapeutic potential of the field. In this review, we highlight the challenges facing those seeking to create artificial tubular tissue with its associated complex macro and microscopic architecture. Current biofabrication approaches, including 3D printing techniques, are reviewed and future directions suggested

Robocasting of advanced ceramics: ink optimization and protocol to predict the printing parameters - A review

Direct-Ink-Writing (or robocasting) is a subset of extrusion-based additive manufacturing techniques that has grown significantly in recent years to design simple to complex ceramic structures. Robocasting, relies on the use of high-concentration powder pastes, also known as inks. A successful optimization of ink rheology and formulation constitutes the major key factor to ensure printability for the fabrication of self-supporting ceramic structures with a very precise dimensional resolution. However, to date achieving a real balance between a comprehensive optimization of ink rheology and the determination of a relevant protocol to predict the printing parameters for a given ink is still relatively scarce and has been not yet standardized in the literature. The current review reports, in its first part, a detailed survey of recent studies on how ink constituents and composition affect the direct-ink-writing of ceramic parts, taking into account innovative ceramic-based-inks formulations and processing techniques. Precisely, the review elaborates the major factors influencing on ink rheology and printability, specifically binder type, particle physical features (size, morphology and density) and ceramic feedstock content. In the second part, this review suggests a standardized guideline to effectively adapt a suitable setting of the printing parameters, such as printing speed and pressure, printing substrate, strut spacing, layer height, nozzle diameter in function of ink intrinsic rheology

Open source multi-Head 3D printer for polymer-metal composite component manufacturing

As low-cost desktop 3D printing is now dominated by free and open source self-replicating rapid prototype (RepRap) derivatives, there is an intense interest in extending the scope of potential applications to manufacturing. This study describes a manufacturing technology that enables a constrained set of polymer-metal composite components. This paper provides (1) free and open source hardware and (2) software for printing systems that achieves metal wire embedment into a polymer matrix 3D-printed part via a novel weaving and wrapping method using (3) OpenSCAD and parametric coding for customized g-code commands. Composite parts are evaluated from the technical viability of manufacturing and quality. The results show that utilizing a multi-polymer head system for multi-component manufacturing reduces manufacturing time and reduces the embodied energy of manufacturing. Finally, it is concluded that an open source software and hardware tool chain can provide low-cost industrial manufacturing of complex metal-polymer composite-based products

Utilizing RepRap Style 3D Printers for the Manufacturing of Composite Heat Exchangers

The low cost 3D printing market is currently dominated by the application of RepRap (self-replicating rapid-prototyper) variants. Presented in this document are practical utilizations of RepRap technology. Developed are innovative processes to manufacture composite materials systems for thermal management solutions. First, a laser polymer welder system is validated by quantifying maximum peak load and weld width of linear low density polyethylene (LLDPE) lap welds as a function of linear energy density. The development of practical engineering data, in this application, is critical to producing mechanically durable welds. Developed laser and printer parameter sets allow for manufacturing of LLDPE multi-layered heat exchangers Second, newly introduced metal-polymer composite materials (e.g. copper-PLA, bronze-PLA, iron-PLA and stainless steel-PLA) were shown to influence the thermal conductivity (W/m·K) of the composite matrix. Increased volume percentage of metallic constituent was shown to increase thermal conductivity. Air void fraction, a resultant of the manufacturing process, reduced the bulk composite 3D printed component. No significant effects were realized dependent upon the metallic constituent morphology (i.e. flake-like vs. spherical). Third, development and fabrication of a large format multi-head RepRap 3D printer displays the ability of large-scale manufacturing potential. Energy efficiencies are realized upon utilization of all hot-ends (i.e. the embodied energy of each printer movement (X, Y and Z)) and are simultaneously shown at each hot-end. Furthermore, multi-head format printers are proven to develop composite components. Utilizing a novel weaving and layering method 1000-series aluminum wire is embedded into a polyethylene terephthalate glycol modified (PETG) matrix. Parametric customized gcode commands allow for innovative manufacturing. In total, laser parameter development, material characterization, custom machine fabrication and printing process development are quantified. The three presented projects demonstrate the engineering advancement of RepRap technology in application to thermal management solutions and composite material development

Clay Minerals as Bioink Ingredients for 3D Printing and 3D Bioprinting: Application in Tissue Engineering and Regenerative Medicine

The adaptation and progress of 3D printing technology toward 3D bioprinting (specifically adapted to biomedical purposes) has opened the door to a world of new opportunities and possibilities in tissue engineering and regenerative medicine. In this regard, 3D bioprinting allows for the production of tailor-made constructs and organs as well as the production of custom implants and medical devices. As it is a growing field of study, currently, the attention is heeded on the optimization and improvement of the mechanical and biological properties of the so-called bioinks/biomaterial inks. One of the strategies proposed is the use of inorganic ingredients (clays, hydroxyapatite, graphene, carbon nanotubes and other silicate nanoparticles). Clays have proven to be useful as rheological and mechanical reinforcement in a wide range of fields, from the building industry to pharmacy. Moreover, they are naturally occurring materials with recognized biocompatibility and bioactivity, revealing them as optimal candidates for this cutting-edge technology. This review deals with the use of clays (both natural and synthetic) for tissue engineering and regenerative medicine through 3D printing and bioprinting. Despite the limited number of studies, it is possible to conclude that clays play a fundamental role in the formulation and optimization of bioinks and biomaterial inks since they are able to improve their rheology and mechanical properties, thus improving printability and construct resistance. Additionally, they have also proven to be exceptionally functional ingredients (enhancing cellular proliferation, adhesion, differentiation and alignment), controlling biodegradation and carrying/releasing actives with tissue regeneration therapeutic activities.This research was funded by the BASQUE COUNTRY GOVERNMENT/EUSKO JAURLARITZA (Department of Education, University and Research, Consolidated Groups IT907-16). Authors S.R.-A. and M.S.-R. thank the BASQUE COUNTRY GOVERNMENT for the granted fellowship (PRE_2020_2_0143) and the UNIVERSITY OF THE BASQUE COUNTRY/EUSKAL HERRIKO UNIBERTSITATEA (UPV/EHU) for the granted pre-doctoral fellowship (PIF17/79), respectively