Synthesis of Polyurethane Shape Memory Polymers over Nickel Titanium Wire

Abstract

In this work, the synthesis of ultra-low density Shape Memory Polymer (SMP) foams onto a nickel-titanium backbone wire is achieved, optimized, and characterized for the use in an intracranial aneurysm embolization device. The Biomedical Device Lab (BDL) is developing a new type of cerebral aneurysm embolization device that incorporates a high volume filling shape memory polymer foam onto a shape set nickel titanium (nitinol) backbone. This device seeks to improve upon current methods of endovascular treatment for intracranial aneurysms. This work was completed to streamline the current device fabrication protocol, which includes crimping the shape memory polymer over the backbone wire and fixing it in place with epoxy on the proximal and distal ends. This procedure is tedious, time consuming, and has substantial issues regarding foam slippage along the backbone wire. The key parameters for this project were to achieve substantial adhesion to the wire without disrupting the foams’ physical and chemical properties in the vicinity of the backbone wire. The synthesis of SMP foams onto a backbone wire was achieved by selecting and optimizing a coating used on the nitinol wire to ensure proper foam-wire adhesion. Foam chemistry was slightly altered to make lower viscosity solutions to enable the foaming over the necessary length of wire. The process of adhering the foam to nitinol wire during foam fabrication culminated in a formalized standard used to synthesize SMP foams onto nitinol wires. Results have shown successful adhesion and low foam densities in the vicinity of the wire. Density ratios of foam adjacent to the wire in comparison to the bulk foam are uniform, signifying little change in foam density. Pore size ratios show similar results indicating very little difference between the bulk and adhered foam. Volume recovery of the SMP foams was found to be acceptable for device incorporation. In conclusion, foam can be synthesized around appropriately coated nitinol wires without major changes to the material properties of the SMPs or to their performance.