Shape memory alloys debut in textiles
Shape memory alloys (SMAs) have been incorporated into textiles for the first time, say researchers participating in a pan-European project called Avalon.
They have developed seven textile-containing prototypes that incorporate nickel–titanium (Nitinol) wire – including a motorcycle helmet, a stent graft for treating vascular diseases and an orthopaedic support bandage.
‘Shape memory alloys are known as materials for high-tech applications, but techniques for application in textile production were lacking,’ says Professor Sven-Volker Rehm of DITF Denkendorf, Germany’s largest textile research institute.
The aim is to move European textile manufacturers towards technical textiles to decrease reliance on the clothing industry, which is increasingly sourcing cheaper materials from the Far East. ‘This will help create added-value textiles,’ adds Rehm.
In the case of the helmet, Nitinol and Aramid fibre are joined together using an automated technique called warp knitting. A high-energy collision forces the material to change between two different sold states, giving rise to energy dissipation that improves impact resistance.
Each prototype combines traditional textiles – such as polyester or Lycra – with Nitinol SMA. The hybrid textiles are made using traditional processes such as weaving, knitting and braiding on existing machinery, but the equipment needs slight modification, because the hybrid textiles must be processed more slowly.
Furthermore, the programmed shape, to which the SMAs revert to, is set by heating the material to 400ºC. One challenge for researchers was reducing this level for hybrid materials. 'At these temperatures, most conventional textiles will burn,’ notes Rehm.
The Institute of Physics at the Academy of Sciences of the Czech Republic has developed a heat treatment technique for SMAs that works below 200ºC, but has not released the details because it is patent pending.
The specific temperature at which the shape memory effect then occurs must also be carefully chosen for each application. For example, the support bandage would change shape at body temperature and the Nitinol formulation needs to take account of this. Supplier Saes Getters, headquartered in Milan, Italy, delivers this by varying the ratio composition of the alloy and by heat treatment.
Avalon’s Technical Coordinator, Alessandra Monero of Italian company D’Appolonia, says beyond this, there are no particular problems of compatibility between the yarn and the Nitinol wire. The wires used by the project partners range in thicknesses from 50-500µm. Monero adds that some of the products could make it to market within a year.