Computational fluid dynamics analysis of the fluid environment of 3D printed gradient structure in interfacial tissue engineering

Mass transport properties within three-dimensional (3D) scaffold are essential for tissue regeneration, such as various fluid environmental cues influence mesenchymal stem cells differentiation. Recently, 3D printing has been emerging as a new technology for scaffold fabrication by controlling the scaffold pore geometry to affect cell growth environment. In this study, the flow field within scaffolds in a perfusion system was investigated with uniform structures, single gradient structures and complex gradient structures using computational fluid dynamics (CFD) method. The CFD results from those uniform structures indicate the fluid velocity and fluid shear stress within the scaffold structure increased as the filament diameter increasing, pore width decreasing, pore shape decreased from 90° to 15°, and layer configuration changing from lattice to stagger structure. By assembling those uniform structure as single gradient structures, it is noted that the fluid dynamic characterisation within the scaffold remains the same as the corresponding uniform structures. A complex gradient structure was designed to mimic natural osteochondral tissue by assembly the uniform structures of filament diameter, pore width, pore shape and layer configuration. The results show that the fluid velocity and fluid shear stress within the complex gradient structure distribute gradually increasing and their maximum magnitude were from 1.15 to 3.20 mm/s, and from 12 to 39 mPa, respectively. CFD technique allows the prediction of velocity and fluid shear stress within the designed 3D gradient scaffolds, which would be beneficial for the tissue scaffold development for interfacial tissue engineering in the future.The Charles M. Vest NAE Grand Challenges for Engineering International Scholarship is gratefully acknowledged. The author would like to thank Brunel Research Interdisciplinary Labs (BRIL) and Brief Award (BRIEF), and Royal Society Research Grant for supporting the research work and collaborations

Direct Ink Writing of Polycaprolactone-Based Scaffolds for Interfacial Tissue Engineering

Engineering interface tissues (e.g. cartilage-to-bone) is a complex process requiring specific designs and organisation of materials, cells and biomolecules. Polycaprolactone (PCL) is a widely applied biomaterial in tissue engineering for its mechanical properties and biocompatibility. Direct ink writing (DIW) is a promising 3D technique to fabricate personalised scaffolds in a heat-free environment by ink deposition. However, it is challenging to develop PCL-based scaffolds using DIW as PCL is water-insoluble. This study aims to develop PCL-based inks to fabricate customised scaffolds with improved properties and functionality for DIW printing. Two types of filament-based scaffolds were designed: single lay-down angle scaffolds and a complex scaffold with layers of different lay-down angles. Finite element simulation results indicate the orthotropic effect increased when the angle decreased from 90° to 15°. Gradient strain magnitudes were achieved in a complex structure with various lay-down angles, mimicking the gradient mechanical characteristics of natural tissue. PCL-based inks were formulated by blending hydrophilic polyethylene oxide (PEO), nano-hydroxyapatite (HAp) comprising acetone and dichloromethane. Ink rheology results indicate that acetone inks had more pronounced shear-thinning behaviour than the dichloromethane inks but had inferior viscosity recovery performance. The addition of PEO and HAp had improved the ink shear-thinning behaviour, viscosity recovery performance, and the scaffold surface wettability. With the varying HAp concentration (55–85%w/w) and lay-down angle, the scaffold with 65%w/w HAp and 90° lay-down angle exhibited the highest elastic modulus and yield strength. 65% w/w HAp concentration is close to the inorganic composition of natural bone tissue. Vancomycin as a model drug was embedded in the PCL/PEO/HAp scaffold. The release behaviour of vancomycin was assessed with the in vitro dissolution test and the antibacterial activity of the printed scaffold was effectively inhibited Staphylococcus aureus in the agar diffusion test. In conclusion, the combined numerical and experimental studies reported in this thesis mainly contains: (1) the use of FEM in design structures with gradient mechanical property, (2) the development of printable PCL/PEO and PCL/PEO/HAp ink formulations, and (3) the use of DIW in fabricating the predesigned scaffold. This study demonstrates that DIW can be used to fabricate scaffold with desired properties with the proper design of ink formulation and scaffold structures. The ink formulation and methodology developed in this study can be transferred to other hydrophobic biomaterials for DIW scaffold fabrication. The gradient property of the DIW tissue scaffold could be achieved by the optimal combination of ink formulations and inner structures to match with natural interfacial tissue

Composite Scaffolds with a Hydrohyapatite Spatial Gradient for Osteochondral Defect Repair

Osteochondral defects derived by traumatic injury or aging related disease are often associated with severe joint pain and progressive loss of joint functions for millions of people worldwide and represent a major challenge for the orthopedic community. Tissue engineering offers new therapeutic approach to repair the osteochondral defects, through the production of scaffolds manufactured to mimic their complex architecture, which consists of cartilage and bone layers. Composite scaffolds based on a PLLA polymeric matrix containing hydroxyapatite (HA) as a filler were prepared through a modified thermally induced phase separation (TIPS) protocol. A suspension was prepared by adding sieved HA particles to a ternary poly-L-lactic-acid (PLLA)/dioxane/water solution with a well defined solvent (dioxane) to non-solvent (water) ratio. A demixing protocol based on a well-defined temperature vs. time path was followed in order to achieve a porous structure with an architecture suitable for osteochondral defect repair. This result is obtained by imposing also a spatial gradient of hydroxyapatite on the scaffolds. Scaffolds were characterized via Scannning Electron Microscopy (SEM) and Micro-computed tomography (Micro-CT). Moreover, preliminary cell culture tests in static and dynamic conditions were successfully carried out