Experimental and Computational Micromechanical Investigations for Developing Mechanically Competent 3D Printable Nanocomposite Biomaterials

Abstract

The development of mechanically competent biomaterials that can also facilitate new bone formation is a major challenge in the field of bone repair, especially critically-sized bone defect reconstruction. 3D printed synthetic bone grafts are a possible alternative to conventional bone-substitutes if engineered to provide appropriate structure with adequate mechanical properties and biocompatibility. Nanocomposites may provide better mechanical properties and cell-material interactions compared to other classes of biomaterials due to their nano-scale reinforcements with advantageous surface-to-volume ratio and surface chemistry. In the context of 3D printed bone grafts, nanohydroxyapatite (nHA)-based biopolymer nanocomposites are attractive. The biopolymer matrix component represents the organic phase of natural bone, while nHA, the primary constituent of bone mineral, represents the mineral phase. The mechanical properties of these 3D printable biopolymer nanocomposites can be enhanced through different strategies such as optimizing the processing and manufacturing techniques, biopolymer matrix selection and optimization, surface functionalization of nanoparticles and improved dispersion and interfacial bonding between matrix and nanoparticle, all of which are attributed to improving the mechanical behavior at the micron and sub-micron scales. The focus of the current thesis was to conduct experimental and computational micromechanical investigations toward the development of mechanically competent 3D printable nanocomposite biomaterials (with a focus on strength). The objective was to enhance the mechanical properties of these nanocomposites at the nano-to-micro scales. To achieve this goal, various strategies were employed, including optimizing material design and fabrication techniques. Using a custom-designed direct ink writing (DIW) 3D printer, shear stress was varied during extrusion to force the nHA nanoparticles and/or polymer chains into alignment. The high shear extrusion led to improved mechanical properties through the formation of polymer chain crystallites and nanoconfined thermoset crystallinity phenomena, while no nHA alignment was observed. Another approach focused on optimizing the composition of the matrix by additional functionalization of the biopolymer matrix as well as heat-treating the nHA particles to remove absorbed moisture to improve the dispersion. The high shear extruded functionalized biopolymer nanocomposite system demonstrated Young’s modulus of 6.6 GPa and ultimate tensile strength of 75.5 MPa, providing a strength safety factor of approximately 1.5 against the minimum required standard mechanical properties for bone cement according to ISO standard 5833. It also improved the shape-holding properties of ink, making it suitable for printing objects using the DIW printing system. As part of the effort to enhance the mechanical properties, a novel processing technique and surface functionalization vii were introduced to achieve a homogeneous dispersion of nHA particles and enhance the interfacial bonding between nHA and the biopolymer matrix. This new approach significantly improved the nHA dispersion with few sub-micron scale agglomerations detected. Additionally, interfacial bonding was improved. Interestingly, the enhanced dispersion and interface properties resulted in greater ductility. Finally, an elastoplastic micromechanical computational model as a design tool was developed and biopolymer matrix characterization experiments were conducted to calibrate the elastoplastic material model used for predicting the elastoplastic behaviour of the biopolymer matrix. The model was developed to provide a design tool for virtual investigations of the effect of nHA distribution and orientation on the elastoplastic properties of the nanocomposite systems reported in this thesis. The model was validated using data from tensile tests conducted on these nanocomposite systems. The findings of the current thesis may pave the way for the fabrication of mechanically competent 3D printable nanocomposite grafts for use in bone reconstruction technology.