Hierarchical porous structures have been gathering interest in different fields owing to their unique properties associated with their multi-scale features. The observation of natural materials brought new insights into the functionality of cellular materials and inspired new processes to produce synthetic hierarchical structures. Such hierarchical cellular materials have shown significant potential in many applications, as filtering, tissue engineering and drug delivery. Polymers in particular gathered a burgeoning interest thanks to their ease of processing, which allows to produce structures at high strength/density ratio and high surface area with defined porosity.
In the present study, it is proposed to develop novel technologies to manufacture polymer cellular structures. From the application of Supercritical Carbon Dioxide Foaming (ScCO2) to Fused Deposition Modelling / Fused Filament Fabrication (FDM / FFF), an extrusion based additive manufacturing technology, hierarchical porous structures were created. The material transformation phenomena and the processing window of this homothetic foaming processed were studied. The fine tuning of the foaming parameters allowed creating a micro cellular porosity with-in a 3D printed structure, without modifying the 3D topology. This allowed producing a wide range of cellular struc-tures with controlled multi scale porosity and stiffness reduced up to hundred times the stiffness of the 3D printed cellular structures.
Homothetic foaming was then applied to biopolymers in order to create gradient hierarchical porous structures. In particular, articular cartilage and bone (osteochondral) defects were targeted. Cartilage repair is a challenging clinical problem because large defects do not regenerate. Tissue engineering offers a solution by implanting a material, defined as scaffold, loaded with cells to induce a regeneration in the tissue that otherwise would not occur. Multi material Poly(lactide-co-caprolactone) â Poly(lactide-BTCP) cellular structures were successfully processed into a scaffold able to replicate the complex gradient mechanical properties of the osteochondral tissue. In particular, the processing windows to process multi material 3D printed and foamed structures was established. The mechanical properties under compression of these scaffolds were compared to the values measured by nanoindentation on human articular cartilage, showing a good correlation between scaffold and target application.
Furthermore, from the developed knowledge, a novel additive manufacturing method is proposed from the inte-gration of FDM/FFF and ScCO2 into a single process, named 3D Foam Printing. 3D Foam Printing was applied to process structures with hollow filaments or filaments with radial porosity with live porosity control during the process. The influence of processing parameters on foam morphology was investigated. Different cellular structures achievable by tuning the printing temperature and speed were described for different biomaterials.
The processes described in this work allowed to mimic the complex mechanical properties of the osteochondral tissue, a natural hierarchical material. However, they demonstrated to be relevant to a wide range of materials, as polymers, blends and composites. This is particularly true for 3D Foam Printing, which could positively influence many engineering applications, as aerospace, medicine and energy, where tuneable cellular polymers are highly demanded.