Polyurethane shape-memory polymers for biomedical applications

Shape-memory polyurethane (SMPU) is a smart polymer material with the unique characteristic of remembering its original shape upon stimulation. Although SMPUs possess low mechanical strength and recovery stress compared to shape-memory alloys, their unique properties—such as flexibility, high shape-recovery ratio, ease of manufacture, strong resistance to organic solvents and aqueous solutions, excellent and consistent elastic properties, and biocompatibility—are promising for biomedical applications, particularly in minimally invasive surgery. Properties of SMPUs can be tailored by varying the molecular weight of polyol, hard segment content, chain extender, or even moisture. Low recovery stress, modulus and stiffness, and recovery stress in shape-memory polymers (SMPs) are the main obstacles for their wide application. Incorporation of metals, nanofillers, clays, and tubes into polyurethane matrices not only enhances the mechanical properties and recovery stress of SMPs, but also produces multifunctional composites. Shape recovery of SMPUs is normally a thermally induced process; however, it can be triggered optically, electrically, magnetically, or electromagnetically, or by water or moisture, by adding various functional fillers. SMPUs have been proposed for many biomedical applications, including SMPU actuators for treating ischemic stroke, thrombus removal activated by laser light, self-deployable SMP neuronal electrodes, vascular stents, sutures in microsurgery, ocular implants, bandages, cell manipulation, and cell therapy.