High cost coupled with limited supply of hard tissue substitutes make necessary the development of synthetic biomaterials, as well as economical and reproducible manufacturing techniques that can be easily scaled up. Multiple and often conflicting requirements have so far impeded the application of polymer/ceramic composites in large load-bearing defects. Although porosity is a crucial biological requirement, it has a detrimental effect on their mechanical performance. This thesis emphasizes upon methods for structuring bioceramic materials at different orders of magnitude - termed structural hierarchy. In bones, this constitutes the main mechanism contributing to their remarkable strength and fracture resistance. This can be achieved through combining AM with physical processes that cause the material to self-assemble into various configurations during or following its deposition.
The first part of this research explores the potential of Low-Temperature Deposition Modelling (LDM) in fabricating scaffolds with tailorable bimodal porosity. More specifically, LDM is treated as a hybrid technique, in which robocasting is combined with freezing-induced self-assembly. For the purposes of this study, a highly versatile and economical print-head was fabricated that can be operated at ambient temperatures down to -20oC. In addition, a highly thixotropic β-TCP paste formulation of high water content (80% v/v) was developed that takes advantage of a synergy existing between Sodium Alginate and Xanthan Gum. The findings suggest that chamber temperature and water content have the most marked effect on pore morphology. Ordered micro-architectures comprising lamellar pores with a high degree of alignment were obtained at low freezing rates (-5oC). Most importantly, this research evidences for the first time that their direction is not only dictated by toolpath geometry, but also by the ice structure in preceding layers (epitaxy).
The second part of this thesis demonstrates the combination of robocasting with a heating process to produce planar hollow-tube lattices that can be manipulated into various configurations. This technique makes use of a novel Sodium Alginate-based β-TCP slurry reinforced with cellulose nano-fibrils. When heated rapidly, it forms hollow-shell structures, which can be cross-linked in Ca2+-rich solutions to produce flexible yet strong particle-reinforced gels. Optimal results were achieved at 3 and 4% w/w alginate content, where the paste’s loss tangent reaches its maximum value.
This research demonstrates that such combinational approaches to AM can successfully produce hard tissue substitutes that exhibit multiple levels of structural hierarchy. This constitutes a first step towards achieving biomimetic bone scaffolds with superior mechanical performance
