A Computational Fluid Dynamics (CFD) Study of Material Flow in Pneumatic Microextrusion (PME) Additive Manufacturing Process

The overarching goal of this research work is to additively fabricate porous bone tissue scaffolds, incorporating patient-derived bone marrow mesenchymal stem cells (hBMSCs), for the treatment of osseous fractures, defects, and diseases. In pursuit of this goal, the objective of the work is to investigate and explain the underlying physical phenomena behind material transport and deposition in pneumatic micro-extrusion (PME) process, using a computational fluid dynamics (CFD) model. This work tests the following hypothesis: the complex multi-physics nature of the PME process is captured using a CFD model. PME is a direct-write additive manufacturing method, which has emerged as a robust, high-resolution process for the fabrication of a broad spectrum of biological tissues and organs. In the PME process, a high-pressure flow (typically, compressed air) is injected into a cartridge (barrel), which contains a bioink material, resulting in pressure-driven material deposition on a free surface via a converging micro-capillary.

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 (obtained from the manufacturer) 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 (all conservation imbalances were below a specified linearization tolerance of 10-6). 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 (supplied by an oilless, rust-free air compressor). 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. Based on the results of a CFD simulation, it was observed that when the material flow passes through the aluminum cartridge and the micro-capillary, pressure may significantly build up within the deposition head, resulting in adverse phenomena, such as uneven material deposition and consequently, dimensionally-inaccurate scaffold formation. 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.