Subscribe to our newsletter to receive the latest news and events from TWI:

Subscribe >
Skip to content

Joining techniques for cardiovascular device

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.

Objectives

The project’s objectives were as follows:

  • Develop novel joining techniques to provide increased mechanical strength and sutureless technology with polymer-to-NiTi bonding.
  • Methodically study the different joining techniques available for medical devices and optimise the techniques to achieve an improved joint between metal frame and polymer.
  • Consider how an improved joining method may enable growth in the use of NiTi in the medical industry, resulting in improvements in the safety and durability of implants.
Figure 1 (a) Stent-graft; (b) NiTi wire hand-sewn to the fabric (courtesy of Vascutek and TWI Ltd)
Figure 1 (a) Stent-graft; (b) NiTi wire hand-sewn to the fabric (courtesy of Vascutek and TWI Ltd)

Programme of work

This study was broken into three main phases: joining biopolymer to biopolymer, NiTi to NiTi and NiTi to biopolymer. When joining biopolymer to biopolymer, we tried laser transmission, ultrasonic welding and hot tool welding. We explored each technique in detail to understand the effect of the processes on the shape memory effect and superelastic properties of NiTi.

When joining NiTi to NiTi we experimented with resistance discharge, arc percussive and laser micro welding techniques. Laser micro welding was the most successful.

We then conducted further analysis of the laser micro welded NiTi, comparing the mechanical strength of the welded wires against the as-received wires using tensile testing, and examining the micro-structure and microhardness of welded specimens. We used scanning electron microscopy to understand the surface morphology of the welded region, heat-affected zone and parent material, and determined transformation temperatures for the austenitic, martensitic and rhombohedral structure phases using differential scanning calorimetry.

The next step

Future work includes the following:

  • Optimising laser welding technique to improve the weld quality.
  • Comparing current techniques with welding processes.
  • Durability testing of the fatigue properties of the welded wires.
  • Other techniques to be studied such as adhesive bonding.
  • NiTi–NiTi bonding using medical-grade adhesives.
  • Phase three of the experiment: joining NiTi to biopolymers.

Find out more

To find out more about our work on this project, please email contactus@twi.co.uk

Avatar Farshad Salamat-Zadeh Medical Sector Manager – Adhesives, Composites and Sealants

For more information please email:


contactus@twi.co.uk