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Pneumatic micro-extrusion (PME) presents inherent challenges in achieving precise control over material deposition due to the small scale and high viscosity of the material flow. Traditional experimental methods often fall short in capturing the complex mechanisms, interactions, and dynamics governing material transport and deposition in PME, highlighting the necessity for advanced computational approaches to unveil these intricate physical phenomena. The primary objective of this study is to develop a robust computational framework for simulating material transport and deposition in PME, offering detailed insights into the fluid dynamics, flow complexities, and deposition characteristics intrinsic to the PME process. This involves systematically investigating the influence of key process parameters, such as material properties, print speed, and flow pressure, on deposition dynamics. Three-dimensional (3D) computational fluid dynamics (CFD) modeling was employed using ANSYS Fluent, with boundary conditions defined to replicate pneumatic extrusion on a moving substrate. The CFD simulations captured three distinct deposition regimes: (i) under-extrusion, (ii) normal extrusion, and (iii) over-extrusion. Results show that decreasing print speed at constant inlet velocity produced over-extrusion, with increased bead height and width due to material overflow. At normal extrusion, bead geometry remained stable, while under-extrusion reduced bead width and ultimately led to discontinuities when flow stresses exceeded cohesive strength. Notably, bead width decreased significantly between 10 mm/s and 15 mm/s, but showed little difference between 15 mm/s and 20 mm/s. Instead, the higher speed produced discontinuities consistent with experimental observations. Centerline velocity profiles revealed that the flow inside the cartridge was very slow at approximately 0.058 mm/s, whereas the velocity increased sharply within the nozzle throat, reaching nearly 12.88 mm/s. These predictions were further supported by validation experiments, where the measured nozzle velocity of 12.95 mm/s closely matched the CFD-simulated value of 12.88 mm/s, demonstrating strong agreement between simulation and experiment. Additionally, pressure decreased slightly with increasing print speed due to reduced backflow, while nozzle velocity and wall shear stress remained unchanged under fixed inlet velocity conditions. Overall, the outcomes of this study are expected to inform parameter optimization strategies and enhance PME process efficiency, providing critical insights into how variations in PME process dynamics influence print morphology and quality, advancing the path toward optimized fabrication of scaffolds for tissue engineering applications. 

Globally, around 2.2 million bone graft procedures are performed annually, with costs reaching approximately $664 million as of 2021. The number of surgeries to repair bone defects is projected to increase by about 13 % each year. However, traditional bone grafts often carry risks such as donor site morbidity and limited availability, driving the need for innovative solutions. This study explores the fabrication of biodegradable bone tissue scaffolds inspired by the nuclear pasta theory using extrusion-based Fused Deposition Modeling (FDM). The nuclear pasta theory, which describes complex geometrical formations within neutron stars, serves as a novel source of inspiration for designing scaffolds with enhanced mechanical properties and optimized porosity. Two bio-based, biodegradable polymers, Luminy LX175 and ecoPLAS, were used to fabricate scaffolds via an in-house filament extrusion process utilizing the Filabot EX6 system. The extrusion parameters were optimized to achieve a consistent filament diameter of 1.75 mm suitable for 3D printing on a Creality K1C printer. Seven scaffold designs were developed, including five based on Triply Periodic Minimal Surfaces (TPMS) and two inspired by nuclear pasta configurations, namely “lasagna” and a hybrid “lasagna-spaghetti” structure. The scaffolds were evaluated for their mechanical properties using uniaxial compression testing. Results showed that TPMS-inspired designs generally achieved a favorable balance between porosity and mechanical strength, while the nuclear pasta-inspired designs exhibited unique anisotropic and isotropic compression characteristics. The study concluded that nuclear pasta-inspired scaffold architectures exhibit unique mechanical properties and porosity characteristics, emphasizing their potential for future optimization in bone tissue engineering applications. Additionally, these structures can be further reinforced through material modifications or hybrid scaffold designs to enhance their load-bearing capabilities. This work demonstrates the potential of using bio-inspired designs in conjunction with sustainable, biodegradable materials for bone tissue engineering. Future research will focus on optimizing co-extrusion techniques and exploring composite materials to further enhance scaffold properties for clinical applications. 

Bone scaffolds are essential in regenerative medicine treatments for bone defects, fractures, and disease. Despite the popularity in bone scaffold research, many challenges still remain including mechanical strength. This study focuses on compression analysis of 10 novel bone scaffold designs with each design created using Rhino 7 with the Grasshopper extension. This coding software took existing scaffold design equations and converted them into 3D models utilizing code based on previous studies. The equations were created by combining and manipulating popular equations used for bone scaffold fabrication. The scaffold models were 3D printed using SimuBone, a PLA biomaterial known for its bone-like properties and printability. The results concluded that Design 6 had the highest compression modulus and mass/density, while Design 9 had a moderate compression modulus and mass/density. Design 8 had the lowest compression modulus and Design 2 had the lowest mass/density. Additionally, Design 6 exhibits the highest stiffness but increased weight, and Design 8 performs the worst in these categories. Therefore, Design 2 was the most optimal for balancing stiffness, mass, and density. The evolution of failure between all 10 designs was also analyzed. This concluded that Design 9 and Design 6 had the highest strength with minimal collapse. Design 8 had the lowest strength with little to no collapse, while Design 2 had medium compression strength with significant collapse. Although Design 2 was found to have significant collapse, it is still considered the most optimal scaffold within this study due to having the best overall mass/density ratio and stiffness modulus with a moderate compression strength. 

Aerosol jet printing (AJP) is a direct-write additive manufacturing technique used for producing high-resolution electronic components such as sensors, capacitors, and optoelectronic devices. The increasing adoption of AJP is attributed to its ability to precisely deposit conductive inks, such as silver nanoparticle-based inks, onto both rigid and flexible substrates. The AJP system comprises three core components: (i) the atomizer, (ii) the virtual impactor (VI), and (iii) the deposition head. The VI, situated between the atomizer and the deposition head, plays a critical role by separating aerosol particles based on their size. This aerodynamic separation ensures that only appropriately sized particles continue to the deposition head, directly influencing print resolution and quality. Despite the advantages of AJP, challenges remain related to process efficiency, repeatability, and print fidelity influenced by the VI. This research work contributes to addressing these issues by establishing a computational fluid dynamics (CFD) model to analyze the internal flow dynamics of the VI. The geometry of the VI, modeled in ANSYS Fluent using design data provided by Optomec (a manufacturer of AJP systems), includes the housing, stem, impactor, collector, and exhaust outlet. In this study, a zone-adapted mesh structure was generated to discretize the internal flow domain. The boundary conditions of the CFD model were set based on experimental observations. Pressure-based CFD formulation using Navier-Stokes equations was utilized to simulate incompressible, turbulent flows under steady-state conditions. The aim of this study is to investigate the effects of several design parameters on VI performance, including: (i) impactor-to-collector diameter ratio (IDtCDR), (ii) number of aerodynamic transport channels (pores), (iii) pore diameter, (iv) impactor length, and (v) collector length. The results of this study revealed that flow behavior in the virtual impactor (VI) is highly sensitive to geometric parameters, particularly the impactor-to-collector diameter ratio (IDtCDR), impactor length (IL), and collector length (CL). An IDtCDR of 0.5 results in backflow, low pressure, and a very high level of turbulence near the collector nozzle, while IDtCDR=1.0 (i.e., when the impactor and collector have equal diameters) provides uniform flow and optimal exhaust velocity. Increasing impactor length as well as collector length raises overall turbulence and pressure. In contrast, variations in the number and diameter of aerodynamic transport channels (ATC) have minimal influence on turbulence or pressure. Overall, this study provides new insights into the influence of geometric design on flow characteristics within the VI and establishes a foundation for optimizing AJP systems. By understanding how design parameters affect flow velocity, pressure distribution, and turbulence behavior, this work supports the advancement of consistent, high-performance AJP processes for the precise fabrication of next-generation electronic devices. 

The patient-specific treatment of bone fractures using porous osteoconductive scaffolds has faced significant clinical challenges due to insufficient mechanical strength and bioactivity. These properties are essential for osteogenesis, bone bridging, and bone regeneration. Therefore, it is crucial to develop and characterize biocompatible, biodegradable, and mechanically robust scaffolds for effective bone regeneration. The objective of this study is to systematically investigate the mechanical performance of SimuBone, a medical-grade biocompatible and biodegradable material, using 10 distinct triply periodic minimal surface (TPMS) designs with various internal structures. To assess the material’s tensile properties, tensile structures based on ASTM D638-14 (Design IV) were fabricated, while standard torsion structures were designed and fabricated to evaluate torsional properties. Additionally, this work examined the compressive properties of the 10 TPMS scaffold designs, parametrically designed in the Rhinoceros 3D environment and subsequently fabricated using fused deposition modeling (FDM) additive manufacturing. The FDM fabrication process utilized a microcapillary nozzle (heated to 240 °C) with a diameter of 400 µm and a print speed of 10 mm/s, depositing material on a heated surface maintained at 60 °C. It was observed that SimuBone had a shear modulus of 714.79 ± 11.97 MPa as well as an average yield strength of 44 ± 1.31 MPa. Scaffolds fabricated with horizontal material deposition exhibited the highest tensile modulus (5404.20 ± 192.30 MPa), making them ideal for load-bearing applications. Also, scaffolds with large voids required thicker walls to prevent collapse. The P.W. Hybrid scaffold design demonstrated high vertical stiffness but moderate horizontal stiffness, indicating anisotropic mechanical behavior. The Neovius scaffold design balanced mechanical stiffness and porosity, making it a promising candidate for bone tissue engineering. Overall, the outcomes of this study pave the way for the design and fabrication of scaffolds with optimal properties for the treatment of bone fractures. 

Abstract Aerosol jet printing (AJP) is a direct-write additive manufacturing technique used to fabricate electronics, such as sensors, capacitors, and optoelectronic devices. It has gained significant attention in being able to utilize aerodynamic principles to deposit conductive inks (such as silver nanoparticle-based inks) onto rigid and flexible substrates. The aerosol jet printing system consists of three main components to execute the printing process: (i) the pneumatic atomizer, (ii) the virtual impactor, and (iii) the deposition head. The virtual impactor (VI) lies between the pneumatic atomizer and the deposition head, accepting the accelerated flow of differently sized aerosol particles from the pneumatic atomizer while acting as an “aerodynamic separator.” With the challenges associated with efficiency as well as resulting quality of the AJP process, the virtual impactor presents a unique opportunity to gain a deeper understanding of the component itself, aerosol particle flow behavior, and how it contributes to overall printing inefficiencies, poor repeatability, and resulting print quality. Broadly, this effort enables the expedited adoption of AJP in the electronics industry and beyond large scales. The challenges mentioned are addressed in this work by conducting a computational fluid dynamics (CFD) study of the virtual impactor to visualize fluid transportation and deposition under specific conditions. The objective of this study is to observe and characterize a single-phase, compressible, turbulent flow through the virtual impactor in AJP. The virtual impactor geometry is modeled in the ANSYS FLUENT environment based on the design by Optomec. The virtual impactor is assembled using a housing, collector, jet, stem, O-rings, and a retaining nut. Subsequently, a mesh structure is generated to discretize the flow domain. In addition, material properties, boundary conditions, and the relevant governing equations (based on the Navier–Stokes equations) are utilized to, ultimately, generate an accurate steady-state solution. The fluid flow is examined with respect to mass flow rates set at boundary conditions. The aerosol particles' interactions with the inner walls of the virtual impactor are observed. Particularly, an insight into the characteristics of aerosol particles entering the virtual impactor and their transition into a smoother flow before entering the deposition head is gained. Furthermore, the analysis provides an opportunity to observe fluid flow separation based on the design of the virtual impactor, one of its main functions in the AJP process. This exposes probable causes for inaccurate print quality, flow blockages, inconsistent outputs, process instability, and other material transport inefficiencies. Overall, this research work lays the foundation for improvements in the knowledge and performance of aerosol jet printing's virtual impactor toward optimal fabrication of printed electronics. 

Abstract Cell-laden, scaffold-based tissue engineering methods have been successfully utilized for the treatment of bone fractures and diseases, caused by factors such as trauma, tumors, congenital anomalies, and aging. In such methods, the rate of scaffold biodegradation, transport of nutrients and growth factors, as well as removal of cell metabolic wastes at the site of injury are critical fluid-dynamics factors, affecting cell proliferation and ultimately tissue regeneration. Therefore, there is a critical need to identify the underlying material transport mechanisms and factors associated with cell-seeded, scaffold-based bone tissue engineering. The overarching goal of this study is to contribute to patient-specific, clinical treatment of bone pathology. The overall objective of the work is to establish computational fluid dynamics (CFD) models: (i) to identify the consequential mechanisms behind internal and external material transport through/over porous bone scaffolds designed based on the principles of triply periodic minimal surfaces (TPMS) and (ii) to identify TPMS designs with optimal geometry and flow characteristics for the treatment of bone fractures in clinical practice. In this study, advanced CFD models were established based on ten TPMS scaffold designs for (i) single-unit internal flow analysis, (ii) single-unit external flow analysis, and (iii) cubic, full-scaffold external flow analysis, where the geometry of each design was parametrically created. The influence of several design parameters, such as surface representation iteration, wall thickness, and pore size on geometry accuracy as well as computation time, was investigated in order to obtain computationally efficient and accurate CFD models. The fluid properties (such as density and dynamic viscosity) as well as the boundary conditions (such as no-slip condition, inlet flow velocity, and pressure outlet) of the CFD models were set based on clinical/research values reported in the literature, according to the fundamentals of internal and external Newtonian flow modeling. The main fluid characteristics influential in bone regeneration, including flow velocity, flow pressure, and wall shear stress (WSS), were analyzed to observe material transport internally through and externally over the TPMS scaffold designs. Regarding the single-unit internal flow analysis, it was observed that P.W. Hybrid and Neovius designs had the highest level of not only flow pressure but also WSS. This can be attributed to their relatively flat surfaces when compared to the rest of the TPMS designs. Schwarz primitive (P) appeared to have the lowest level of flow pressure and WSS (desirable for development of bone tissues) due to its relatively open channels allowing for more effortless fluid transport. An analysis of streamline velocity exhibited an increase in velocity togther with a depiction of potential turbulent motion along the curved sections of the TPMS designs. Regarding the single-unit external flow analysis, it was observed that Neovius and Diamond yielded the highest level of flow pressure and WSS, respectively, while Schwarz primitive (P) similarly had a relatively low level of flow pressure and WSS suitable for bone regeneration. Besides, pressure buildup was observed within the inner channels of almost all the TPMS designs due to flow resistance and the intrinsic interaction between the fluid flow and the scaffold walls. Regarding the cubic (full-scaffold) external flow analysis, the Diamond and Schwarz gyroid (G) designs appeared to have a relatively high level of both flow pressure and WSS, while Schwarz primitive (P) similarly yielded a low level of flow pressure and WSS. Overall, the outcomes of this study pave the way for optimal design and fabrication of complex, bone-like tissues with desired material transport properties for cell-laden, scaffold-based treatment of bone fractures. 

Abstract In tissue engineering, once a scaffold has completed mechanical property testing, it must then undergo biological characterization which determines if the scaffold is capable of supporting cell viability. To perform biological tests, cells must be seeded onto a scaffold with the help of bioreactors, the four main types being: (i) rotating wall, (ii) spinner flask, (iii) compression, and (iv) perfusion bioreactor. In perfusion bioreactors, a consistent flow of material is introduced (using a pump) into the inlet of the bioreactor chamber where multiple scaffolds of a disc geometry are located. However, the intrinsic, complex interaction between the scaffolds and material flow as it goes through the bioreactor chamber affects the viability of the seeded stem cells. Therefore, there is a need to identify consequential fluid dynamics phenomena governing the material flow in a perfusion bioreactor. In this study, using a CFD model, the effects of critical scaffold parameters (such as the number of scaffolds, scaffold diameter, scaffold thickness, and number of pores) on the main flow properties (i.e., flow pressure, wall shear stress, and streamline velocity) influential in cell proliferation and bone development will be investigated. It was observed that increasing the number of pores, in addition to decreasing the pore diameter had an adverse effect on the maximum forces occurring on the scaffold. In addition, changing the overall scaffold diameter did not appear to have as much as an effect as the other parameters. Furthermore, it was observed that a decrease in porosity would lead to an increase in wall shear stress and consequently in cell death. Overall, the outcomes of this study pave the way for optimal design, fabrication, and preparation of cell-laden bone scaffolds for treatment of bone fractures in clinical settings. 

Abstract Aerosol jet printing (AJP) is a direct-write additive manufacturing technique used to fabricate electronics, such as sensors, capacitors, and optoelectronic devices. It has gained significant attention in being able to utilize aerodynamic principles to deposit conductive inks (such as silver nanoparticle-based inks) onto rigid and flexible substrates. The aerosol jet printing system consists of three main components to execute the printing process: (i) the pneumatic atomizer, (ii) the virtual impactor, and (iii) the deposition head. The virtual impactor (VI) lies between the pneumatic atomizer and the deposition head, accepting the accelerated flow of differently sized aerosol particles from the pneumatic atomizer, while acting as an “aerodynamic separator.” With the challenges associated with the efficiency as well as resulting quality of the AJP process, the virtual impactor presents a unique opportunity to gain a deeper understanding of the component itself, aerosol particle flow behavior, and how it contributes to overall printing inefficiencies, poor repeatability, and resulting print quality. Broadly, this effort enables the expedited adoption of AJP in the electronics industry and beyond at large scales. The challenges mentioned are addressed in this work by conducting a computational fluid dynamics (CFD) study of the virtual impactor to visualize fluid transportation and deposition under specific conditions. The objective of this study is to observe and characterize a single-phase, compressible, turbulent flow through the virtual impactor in AJP. The virtual impactor geometry is modeled in the ANSYS-Fluent environment based on the design by Optomec. The virtual impactor is assembled using a housing, collector, jet, stem, O-rings and a retaining nut. Subsequently, a mesh structure is generated to discretize the flow domain. In addition, material properties, boundary conditions, and the relevant governing equations (based on the Navier-Stokes equations) are utilized to, ultimately, generate an accurate steady-state solution. The fluid flow is examined with respect to mass flow rates set at boundary conditions. The aerosol particles’ interactions with the inner walls of the virtual impactor are observed. Particularly, an insight into the characteristics of aerosol particles entering the virtual impactor and their transition into a smoother flow before entering the deposition head is gained. Furthermore, the analysis provides an opportunity to observe fluid flow separation based on the design of the virtual impactor, one of its main functions in the AJP process. This exposes probable causes for inaccurate print quality, flow blockages, inconsistent outputs, process instability, and other material transport inefficiencies. Overall, this research work lays the foundation for improvements in the knowledge and performance of aerosol jet printing’s virtual impactor toward optimal fabrication of printed electronics. 

Abstract Cell-laden, scaffold-based tissue engineering methods have been successfully utilized for the treatment of bone fractures and diseases, caused by factors such as trauma, tumors, congenital anomalies, and aging. In such methods, the rate of scaffold biodegradation, transport of nutrients and growth factors, as well as removal of cell metabolic wastes at the site of injury are critical fluid-dynamics factors, affecting cell proliferation and ultimately tissue regeneration. Therefore, there is a critical need to identify the underlying material transport mechanisms and factors associated with cell-seeded, scaffold-based bone tissue engineering. The overarching goal of this study is to contribute to patient-specific, clinical treatment of bone pathology. The overall objective of the work is to establish computational fluid dynamics (CFD) models to identify: (i) the consequential mechanisms behind internal and external material transport through/over porous bone scaffolds and (ii) optimal triply periodic minimal surface (TPMS) scaffold designs toward cell-laden bone fracture treatment. In this study, 10 internal-flow and 10 external-flow CFD models were established using ANSYS, correspondingly based on 10 single-unit TPMS bone scaffold designs, where the geometry of each design was parametrically created using Rhinoceros 3D software. The influence of several design parameters, such as surface representation iteration, merged toggle iso value, and wall thickness, on geometry accuracy as well as computational time, was investigated in order to obtain computationally efficient and accurate CFD models. The fluid properties (such as density and dynamic viscosity) as well as the boundary conditions (such as no-slip condition, inlet flow velocity, and pressure outlet) of the CFD models were set based on clinical/research values reported in the literature as well as according to the fundamentals of internal/external Newtonian flow modeling. Several fluid characteristics, including flow velocity, flow pressure, and wall shear stress, were analyzed to observe material transport internally through and externally over the TPMS scaffold designs. Regarding the internal flow CFD modeling, it was observed that “P.W. Hybrid” (i.e., Design #7) had the highest-pressure output, with “Neovius” (i.e., Design #1) following second to it. These two designs have a relatively flatter surface area. In addition, “Schwarz P” (i.e., Design #2) was the lowest pressure output of all 10 TPMS designs. “Neovius” and “Schwarz P” had the highest and lowest values of wall shear stress. Besides, the velocity streamlines analysis showed an increase in velocity along the curved sections of the scaffolds’ geometry. Regarding the external flow CFD modeling, it was observed that “Neovius” yielded the highest-pressure output within the inlet section, which contains the area of the highest-pressure location. Furthermore, “Diamond” (i.e., Design #8) displayed having the highest values of wall shear stress due to the results of fluid interaction that accrues with complex curved structures. Also, when we look at designs like “Schwarz G”, the depiction of turbulent motion can be seen along the internal curved sections of the structure. As the external velocity streamlines decrease within the inner channels of the designs, this will lead to an increased pressure buildup due to the intrinsic interactions between the fluid with the walls. Overall, the outcomes of this study pave the way for optimal design and fabrication of complex, bone-like tissues with desired material transport properties for cell-laden, scaffold-based treatment of bone fractures.