3D PRINTING OF POLYMERIC AUXETIC STRUCTURES DISPLAYING SHAPE MEMORY BEHAVIOR

Auxetic structures display unique dimensional behaviour under stresses, characterized by a negative Poisson ratio. This behavior is due to their macro or porous structure, and allows them to be applicable in fields as different as architecture, aerospace and medicine. In addition, the 3D printing area has made an impressive progress in recent years, and has made possible the engineering of a diversity of new objects. The capability to transform 3D objects from one shape to another “on command”, by using thermoset shape memory materials, enables the generation of dynamic architectures that change over time. This research aimed at developing 3D printed auxetic structures that exhibit also shape memory behaviour, conferred to the structure by polycaprolactone dimethacrylate (PCLdMA) chains that performed as the basic printable component. Aiming at fine tuning the hydrophilicity of the resulting structure, the dimethacrylate of a polyethylene oxide-polypropylene oxide-polyethylene oxide triblock (FdMA) was co-printed with PCLdMA. Both basic building blocks were rendered photocrosslikable by reacting them with 2-Isocyanatoethyl Methacrylate (IEMA).

The effect of the PCLdMA:FdMA molar ratio was investigated and the printed construct containing an FdMA content falling in the 33%-42% range, displayed optimal auxetic behaviour. The water absorption capability of the different 3D printed polymers was studied, while the degree of crystallinity required for the PCLdMA chains to be able to perform as effective switching segments was determined by DSC and XRD analyses. The shape memory properties of the 3D printed structures exhibited R_r=91% and R_f=85% values, and their auxetic behaviour was reflected by a Poisson ratio of -0.59. SEM images revealed different pore structures, as a function of their FDMA content. The potential of these structures as stents was assessed.