Mechanical behaviour of 3D printed scaffolds for bone tissue engineering

Abstract

Bone is the second most transplanted tissue in the world1. The current solution for bone grafts is autologous grafts in 80% of cases. As the tissue is harvested from the patient, there is limited availability. In up to 30% of cases, complications appear, like donor site morbidity or rejection. One answer to this problem is to create synthetic scaffold material with bone attributes. It should combine porosity, biocompatibility, stiffness, and strength similar to the surrounding tissues to avoid stress shielding. Here, the focus will be on fused filament fabrication (FFF). In this process, thermoplastic is extruded in a layer-by-layer manner creating complex architectures. FFF gradient scaffolds are known to improve cell seeding, help the differentiation of stem cells and allow the creation nutrient transport network. However, the impact of this complex architecture modifies the load paths inside the scaffold and the deformation mode, creating regions with different mechanical behaviours. These different regions are interesting to influence cell differentiation through mechanical stimulation. These parameters have not yet been studied, and this work will explore how the diameter of the filament, the spacing, the layer height and the offset between the filaments will impact the mechanical behaviour of FFF gradient scaffolds.
Micromechanical modelling is used on the different regions of a gradient scaffold to predict the mode and magnitude of microstructural deformation under compressive loading and the corresponding bulk mechanical response and properties. Predictions are compared to experimental data to validate the models and elucidate the key parameters for the design of bone scaffolds.
A systematic softening of the low porosity scaffolds compared to the models was observed. This phenomenon was attributed to the contact area between the filament which is overlooked in the micromechanical models. Even though the models are not yet able to capture the complexity of this architecture, two significant parameters were isolated. The first one is the porosity of the scaffold; it depends on the diameter, the layer height and the spacing. The second one is the contact area between the filaments; it is related to the diameter, the layer height, the position of the filaments and the material used. The contact area has been studied under the name of interlayer adhesion in general FFF. However, no studies have explored this parameter in the scaffold architecture studied. Therefore, the impact of the printing parameters on the contact area was of great interest, identifying the printing speed and the layer height as defining factors for the contact area. By modifying the diameter of the filaments, the spacing of the filaments, the layer height, the offset between the filaments, and the printing speed it is possible to fine tune the mechanical properties of FFF scaffolds for tissue engineering.

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Identifiers

ORCID for Cesar Faurat-Narkisian: orcid.org/0000-0002-0149-9451

ORCID for Andrew Hamilton: orcid.org/0000-0003-4627-849X

ORCID for Martin Browne: orcid.org/0000-0001-5184-050X

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