Abstract
Advances in bone tissue engineering and additive manufacturing (AM) have enabled the fabrication of patient-specific bone implants with a precise porous structure to allow bony ingrowth and reduce stress shielding. Porous AM titanium alloy (Ti-6Al-4V) bone-interfacing implants dominate the literature and have been translated to clinical orthopaedic applications. However, poor osseointegration and peri-prosthetic infection still remain as great challenges, which currently largely rely on autografts and antibiotics that are accompanied by respective complications. The aim of this thesis is to systematically investigate strategies for improving the biological performance of AM Ti-6Al-4V constructs for bone-interfacing implants through surface modification, biofilm inhibitor delivery and vasculature formation.
Firstly, a systematic in vitro comparative study was performed to investigate the osteogenic differentiation of human mesenchymal stromal cells (hMSCs) on substrates manufactured from Ti-6Al-4V and tantalum via commonly adopted AM techniques: selective laser melting (SLM) and electron beam melting (EBM). This study concluded that there was comparable osteogenic differentiation for hMSCs on SLM Ti-6Al-4V, EBM Ti-6Al-4V and SLM tantalum surfaces, and further surface modification was explored to improve the osteoconductive properties of SLM Ti-6Al-4V. Experimental study and Computational Fluid Dynamics (CFD) simulation were undertaken to optimize the hydrodynamics required to achieve uniform nanotube generation on Ti-6Al-4V surface following electrochemical anodization. Nanotubes on SLM Ti-6Al-4V formed a micro-/nano-topographic surface resulting in significant osteogenic differentiation enhancement of attached hMSCs. Anodized SLM Ti-6Al-4V with nanotubes were subsequently used for delivery of a novel methylthioadenosine nucleosidase inhibitor (MTANi) targeting MTAN - a key enzyme involved in bacterial biofilm formation – thereby offering simultaneous biofilm inhibition combined with micro-/nano-topography promoting osteogenesis for the SLM Ti-6Al-4V. The incorporated MTANi had no detrimental effect on the hMSCs osteogenic differentiation. Furthermore, given that angiogenesis is critical to achieving successful bone ingrowth and de novo bone formation in critical-sized defects to allow rapid fixation and osseointegration, vascularization of SLM Ti-6Al-4V scaffolds was investigated. Gelatin-based hydrogel generated via a novel visible-light crosslinking system was supportive of the survival and migration of hMSCs and human umbilical vein endothelial cells (HUVECs). Additionally, we demonstrated a proof-of-concept hybrid model showing concerted osteogenesis and vasculogenesis in the hydrogel, as well as in SLM Ti-6Al-4V and hydrogel hybrid constructs. A further translational study validated the bio-functionalization of porous regions of a full-scale AM acetabular cup with cell-laden hydrogel, demonstrating successful cell delivery and vasculature formation.
In summary, this thesis provides novel data and fundamental evidence on the osteoconductive capacity of adopted bone-interfacing implant biomaterials and state-of-the-art AM technologies. Ti-6Al-4V and tantalum manufactured by SLM and EBM are supportive for osteogenesis. Surface modification of SLM Ti-6Al-4V via anodization and MTANi delivery can be potentially addressing infection challenges while simultaneously enhancing osseointegration. Furthermore, hybrid approaches integrating hydrogel-based vascularization strategies proved to be versatile to delivery cells for mature vasculature formation of porous AM constructs. Hybrid strategies applying AM, surface modification and vascularization investigated in this thesis are anticipated to contribute to the patient-specific bone implant fabrication with matched mechanical properties, rapid bone and vasculature formation and improved osseointegration toward addressing major challenges in orthopaedics.