Pneumatic micro-extrusion (PME) is a direct-write additive manufacturing process, which has emerged as a robust, high-resolution method for the fabrication of a broad spectrum of biological tissues and organs. PME allows for non-contact multi-material deposition of functional inks for tissue engineering applications. In spite of the advantages and engendered potential applications, the PME process is inherently complex, governed by not only complex physical phenomena, but also material-process interactions. Consequently, investigation of the influence of PME process parameters as well as the underlying physical phenomena behind material transport and deposition in PME would be inevitably a need. The overarching goal of this research work is to fabricate biocompatible, porous bone tissue scaffolds for the treatment of osseous fractures, defects, and diseases. In pursuit of this goal, the objectives of the work are: (i) to investigate the influence of seven consequential scaffold design factors and PME process parameters on the mechanical properties of fabricated bone tissue scaffolds; (ii) to explore the underlying dynamics behind material transport in the PME process, using a 3D computational fluid dynamics (CFD) model. To investigate the effects of the design and process parameters, a series of experiments were designed and conducted. Layer height was identified as the most significant factor in this study. An increase in the layer height led to less overlap between subsequent layers, which allowed for more shrinkage and ultimately a reduction in scaffold diameter. In addition, print speed appeared as an influential factor in this study. An increase in the print speed resulted in a decline in linear mass density and thus in the extent of fusion between subsequent deposited layers. Besides, it was observed that there was a strong correlation between deposition mass and compression modulus. Overall, the results of this study pave the way for future investigation of PME-deposited PCL scaffolds with optimal functional and medical properties for incorporation of stem cells toward the treatment of osseous fractures and defects.
Pneumatic micro-extrusion (PME) is a high-resolution direct-write additive manufacturing process, which has emerged as the process of choice for tissue engineering and biofabrication of a broad spectrum of organs and tissues (e.g., bone, aortic valve, blood vessels, human ear, and nose). Despite the advantages and host of biomedical applications engendered by the PME process — including, for example, (i) accommodation of a wide range of material viscosity (enabled via thermopneumatic material deposition), (ii) large build volume and standoff distance for tissue engineering, (iii) in situ UV curing, and (iv) high-resolution multimaterial deposition — there are intrinsically complex design, material, and process factors as well as interactions, which influence the functional properties of PME-fabricated tissues and organs. Consequently, investigation of the impact and interaction of each factor aligned with establishment of a physics-based, optimal material deposition regime is inevitably a burgeoning need. In this study, using the Taguchi design, the influence of four significant factors, i.e., layer height, infill density, infill pattern, and print speed, is investigated on the compression properties as well as the dimensional accuracy of polycaprolactone (PCL) bone scaffolds, fabricated using the PME process. Furthermore, a 3D, transient two-phase flow CFD model is forwarded with the aim to observe the flow of material within the deposition head as well as the micro-capillary (nozzle). The results of this study pave the way for further investigation of the bio-functional properties of bone scaffolds, e.g., biodegradation, cell proliferation and growth rate.
The objective of this study is to investigate the underlying physical phenomena behind material transport in pneumatic micro-extrusion (PME) process, using a computational fluid dynamics (CFD) model. The geometry of the PME deposition head assembly (including a micro-capillary having a diameter of 200 μm) was set up in the ANSYS-Fluent environment, based on a patented design in addition to direct measurements of the dimensions of the assembly. Subsequently, the geometry was meshed using tetrahedron cells. Besides, five layers of inflation were defined with the aim to obtain an accurate solution near all wall boundaries. The transient, pressure-based Navier-Stokes algorithm (based on absolute velocity formulation) was the mathematical model of choice, used to obtain transient solutions. To account for the effects of compressibility as well as viscose heating, the energy equation (in addition to the continuity and momentum equations) was utilized in the CFD model. Furthermore, the explicit volume of fluid model (composed of two Eulerian phases) and the laminar viscose model were used to collectively establish a viscose two-phase flow model for the molten polymer (PCL) deposition in the PME process. Pressure-velocity coupling was implemented using the semi-implicit method for pressure linked equations (SIMPLE). Finally, experimental sensor data was used with the aim to: (i) define the boundary conditions (as follows), and (ii) validate the CFD model. In this study, PCL powder was loaded into the cartridge, maintained at 120 °C, defined as the temperature of all stationery walls (with no slip condition). Pressure inlet was the type of boundary defined for the high-pressure gas flow in the PME process, set at 550 kPa. The laminar molten PCL flow was deposited on a glass substrate, steadily and uniformly kept at 45 °C, defined as the temperature of the substrate wall, moving with a speed of 0.35 mm/s. Overall, the results of this study pave the way for better understanding of the causal phenomena behind material transport and deposition in the PME process toward fabrication of bone tissue scaffolds with optimal functional properties.
Pneumatic micro-extrusion (PME), a direct-write additive manufacturing process, has emerged as a high-resolution method for the fabrication of a broad range of biological tissues and organs. However, the PME process is intrinsically complex, governed by complex physical phenomena. Hence, investigation of the effects of consequential parameters would be an inevitable need. The goal of this research work is to fabricate biocompatible, porous bone tissue scaffolds for the treatment of osseous fractures, defects, and eventually diseases. In pursuit of this goal, the objective of this study is to investigate the influence of material deposition factors — i.e., (i) deposition head temperature, (ii) flow pressure, and (iii) infill pattern — on the mechanical performance of PME-fabricated bone scaffolds. It was observed that the deposition head temperature as well as the flow pressure significantly affected scaffold diameter (unlike scaffold height). In addition, material deposition rate increased significantly as a result of an increase in the deposition temperature; this phenomenon stems from a reduction in Polycaprolactone (PCL) viscosity. Furthermore, there was a direct correlation between the amount of deposited mass and scaffold stiffness. Overall, the results of this study pave the way for future investigation of PME-deposited PCL scaffolds with optimal functional properties for incorporation of stem cells toward the treatment of osseous fractures and defects.
Pneumatic micro-extrusion (PME) is a direct-write additive manufacturing process, which has emerged as a robust, high-resolution method for the fabrication of a broad spectrum of biological tissues and organs. In the PME process, a high-pressure flow is injected into a cartridge, which contains a bioink material, resulting in pressure-driven material deposition on a free surface via a converging conical micro-capillary. In this study, PCL powder was loaded into the cartridge, maintained at 120 °C. The flow pressure was set to 550 kPa. Laminar molten PCL flow was deposited on a glass surface (steadily and uniformly kept at 45 °C), using a 200 μm nozzle. A porous, cylindrical scaffold was designed (honeycomb-filled), having a diameter and height of 10 mm and 3 mm, respectively. To investigate the effects of the design and process parameters, a series of experiments were designed and conducted where print speed was varied at four levels in the range of 0.30–0.45 mm/s with 0.05 mm/s increments. In addition, similarly, layer height and layer width were changed at four levels in the range of 125–200 μm with 25μm increments. Finally, infill density was set at four levels in the range of 0.20–0.35 with 5% increments. As a result, 16 experimental runs were characterized, each replicated four times. Of each of the PME-fabricated samples, an image was acquired (both horizontally and vertically) using a high-resolution CCD camera. Illumination was provided by an LED ring light (being of a brightness in the range of 30,000–40,000 Lux as well as a color temperature of 6000 K). Subsequently, the acquired images were analyzed using in-house digital image processing algorithms, forwarded with the aim to characterize both the diameter and the height of the fabricated bone scaffolds. The veracity of the image-based measurements was corroborated, using offline caliper measurements. Furthermore, the compression properties of the fabricated bone scaffolds were measured using a compression testing machine; the samples were subjected to a compression load, applied with a velocity of 0.08 mm/s. Overall, the results of this study pave the way for future investigation of PME-deposited PCL scaffolds with optimal mechanical and morphological properties for incorporation of hBMSCs toward the treatment of osseous fractures and defects.
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