|These nitinol springs are approximately 3cm long. They can be stretched to over 20cm long, which will deform the metal. However once they are heated to the activation temperature, they will instantly shrink back to their original length. There are two types of spring. These are activated at about 45 degrees Centigrade. Technical information - the composition of the 45°C springs is close to 55.8wt%Ni. The rest is Ti.