DESIGN AND TENSILE TESTING OF 3D PRINTED MULTI-MATERIAL AND BIO-INSPIRED SPECIMENS THAT MIMIC THE MECHANICAL BEHAVIOR OF SOFT TISSUES

In this Thesis work, the 3D printing PolyJet technology was used to manufacture dog-bone-shaped multi-material specimens with bio-inspired pattern-embedded. The specimens were provided by Stratasys Ltd Company. This research work aimed to imitate and characterize the tendon and ligament behavior when these are subjected to uniaxial tensile strength while optimizing at the same time the tensile mechanical response of the multi-material specimen. The specimens were printed using the Stratasys J750 Digital Anatomy PrinterTM (DAP). The Agilus30 TM rubber-like material in different combinations of hardness Shore A scale (A50, A60, and A70) assigned to each pattern component was selected. First, specimens were produced with preliminary patterns based on regular fiber shapes (single and helical configuration). Subsequently, a Finite Element Analysis was conducted to evaluate the proper arrangement of the pattern within the specimen. Finally, four pattern typologies were designed using a biomimetic approach, basing on the hierarchical arrangement and the irregular shape of the collagen fibers of the tendon on different scale sizes and the hierarchical structure of concentric layers of the bamboo fiber. To investigate the influence of the pattern on the mechanical properties of the printed parts, the mechanical tensile response of the bio-inspired specimens was compared to the homogeneous materials (Agilus30TM A50, A60, A70, and TissueMatrix). This Thesis work concluded that the inclusion of hard fibers (Shore A70) featuring non-uniform morphology within a soft matrix (Shore A50) improved the tensile mechanical response in terms of stiffness. The opposite configuration (Fiber A50 and matrix A60) resulted in a remarkable decrease in all mechanical parameters. A comparison with the native soft tissues was performed by matching the stress-strain relationship of the best and worst individuated multi-material specimen with the stress-strain behavior of a healthy and tendinopathic native tendon.