TWI is working to develop novel techniques to join thin nitinol (NiTi) wires for medical applications. We have conducted an experimental study into various micro welding methods, with mixed results. Preliminary experiments found failures in welded specimens joined using resistance discharge and arc percussive welding; however, laser micro welding was found to be a consistently successful joining method.
Unique properties; unique challenges
NiTi, an equiatomic nickel-titanium alloy, is widely used in the medical industry for its unique properties. It is a shape-memory alloy, whose superelasticity makes it an ideal material for cardiovascular implants, such as heart valves, stents and stent grafts. NiTi is usually used in combination with biopolymers: NiTi provides strength and ductility, while the biopolymer contributes physio-chemical resistance and promotes lightness.
Although NiTi’s unique properties make it ideal for medical applications, they also present complications in the manufacturing of cardiovascular implants and devices. Most medical devices are currently obtained from laser-cutting of NiTi tubes, or from assemblies of NiTi elements (e.g. wires) joined at specific locations by crimping or soldering. NiTi metal stents are hand-stitched to the fabric as shown in Figure 1.