Design, Optimization and Fabrication of Bio-inspired Sutures

"Nature displays many examples of materials with exceptional mechanical properties. In particular, hard biological materials such as mollusk shells, bone, and tooth enamel boast remarkable combinations of stiffness, hardness and fracture toughness that stem from complex structures of organic (biological polymers) and inorganic (minerals). As a "universal rule" in these hard materials, minerals provide stiffness and hardness, while the organic polymer rich interfaces generate nonlinear deformations and channel cracks into powerful toughening configurations. This concept can also be extended to geometrically sutured interfaces, which are common in human skulls, cephalopods or turtle shell. These sutures can arrest cracks, absorb impact energy, and provide flexibility for respiration, locomotion or growth. This thesis explores the mechanical design, optimization, fabrication and testing of two dimensional sutures with complex interlocking geometries. The emphasis is on pullout response and the effect of interlocking, which was modelled based on non-Hertzian contact solutions and kinematics. Finite element models of these sutures were also developed, which produced results in close agreement with the analytical solutions. The accuracy of these models was verified using 3D printing and mechanical testing of jigsaw interlocking sutures. The models capture the effect of tuning geometrical and interfacial properties of suture interfaces to alter the mechanical response of the material. A limiting factor in the strength of the interlocking is the fracture of the solid parts of the suture, which must be avoided. The models show that the most severe stresses in the suture arise from frictional contact stresses. Therefore, the optimum design was achieved by using low friction coefficient and relatively high locking angle. The general problem of the design and optimization of complex interlocking sutures is also considered by capturing the geometry of sutured interfaces with "line descriptors", in which thousands of sutures with different geometric properties were systematically generated and explored using analytical and finite element solutions. The results show that for all families of suture, optimum designs are achieved with low friction coefficient and relatively high locking angle, and also that sutures with multiple locking sites distributed along the pulling direction provide the best combination of material properties overall. Finally, a laser engraving method is proposed to create trenches with controlled suture patterns and depth in thin plates of aluminium oxide. These trenches guided cracks into these interfaces to implement toughening mechanisms and unusual deformation. Different kinds of interfaces were engraved: straight, transverse, sinusoidal, and jigsaw interlocking (characterized by interlocking angle) interfaces. The results show how the shape of suture interfaces can be used to tune the mechanical behavior of the material, and also show that the jigsaw interlocking interface dissipated the most energy and produced the highest toughness. The combination of models and experiments presented in this thesis provide a new framework for the geometrical design and optimization of sutured lines, for applications where the propagation of cracks and/or local compliance must be controlled."--