The effect of surface roughness of scaffolds as synthetic bone on blood flow within them

Nowadays, many people suffer from bone loss due to accidents or serious diseases such as cancer. Generally, three main resources to help these losses are listed as; a bone from another part of the patient's body (autograft), donated bone (allograft), animal bone tissue (xenograft) or synthetic bone (graft). The latter substitutes to mimic the structure of the bone consist of porous structures that are called scaffolds. The scaffolds properties are investigated from different perspectives, such as their behavior under mechanical loads and biological characterization, such as fluid flow dynamics within their pores. In terms of scaffolds mechanical properties, parameters such as strain and yield strength under loads are important and their biological characterization mainly is determined by permeability and fluid flow induced wall shear stress (WSS) the parameters contribute in cellular activities in scaffolds. This study consists of two sections that consider these characterizations of scaffolds. In the first part, among factors affecting these properties of scaffolds, porosity and architecture were investigated using finite element method. So, three different scaffolds with Lattice-based (Kay), square pore (Kag) and gyroid (Gyr) architectures were developed at six porosity (65, 70, 75, 80, 85 and 90%). The scaffold deformation and yield strength (according to von-Mises criterion) under a static load were calculated using structural finite element approach. The results showed a decrease in the mechanical property of the models with their porosity increasing. However, the Kay scaffold models possessed a higher modulus of elasticity and yield strength than the other two scaffolds. The pressure drop, velocity, and the fluid flow induced wall shear stress within scaffolds were analyzed using computational fluid dynamics (CFD). Given the pressure drop from CFD analysis and scaffolds geometric parameters, the permeability of the scaffolds is calculated using Darcy's law. While porosity increased, all three scaffolds showed an increase in permeability and decrease in WSS. Among three scaffolds, the lattice-based models exhibited higher permeability and lower WSS than the other models. According to the elasticity modulus and permeability results, scaffolds which met the cancellous bone properties were determined. Then, considering the homogeneous distribution of WSS on the scaffolds walls, the best model of the study was selected. CFD analyses were performed using water and blood as work fluids. As water is a Newtonian fluid a constant viscosity assigned to related models and due to the non-Newtonian behavior of blood its viscosity was modeled by the Power-Law approach. In terms of fluid type, scaffolds showed about five times higher permeability for water than the blood. In contrast, the WSS obtained for the blood fluid was about six times larger than the water. In the second section of this work, the effect of surface roughness on fluid flow dynamics of scaffolds was modeled exploiting a scaffold in three different pore sizes (300, 600 and 900 μm) with a square pore at a constant porosity (65%). Six levels (0, 4, 8, 12, 16 and 20 μm) of roughness were assigned to the walls of these scaffolds. The results showed that the effect of pore size on the permeability of scaffolds was more pronounced than the roughness effect. However, the effect of roughness in the scaffolds with the small pore sizes was found non-negligible. Moreover, the effects of surface roughness on the scaffolds WSS with 600 and 900 μm pores was higher than scaffolds with 300 μm of pore size. This result shows that the surface quality that is rising from production has a significant role in the fluid flow behavior within scaffolds (permeability and WSS). 2017, 184 pages Keywords: Bone graft, Scaffolds, CFD analysis, Surface roughness, non-Newtonian fluids