Investigation of Material Transport Through Triply Periodic Minimal Surface (TPMS) Bone Scaffolds Using Computational Fluid Dynamics (CFD)
Authors: Coburn Brandon , Salary Roozbeh “Ross” 
Published: August 2024

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.