From Jie Qi's projects page:

Shape memory alloys (SMAs) are metals that change shape when heated up. They are wonderful actuators in that they are light, silent and can be "turned on" by simply running current through. The shape that they change to can also be set, though this process is a bit more tricky. Flexinol is a particular brand of nitinol, which is an SMA made of nickel and titanium, and is pre-set to contract about 10% of its original length when heated.

In my projects, I generally used the 0.006" to 0.01" diameter, High-Temp wires.

Since Flexinol draws a lot of current (about 300mA for the diameters I used), you need a strong power supply like a wall supply or a good lithium-ion battery. I've used from 3.7V up to 6V (any more and my Flexinol wires would start overheating). To turn the Flexinol on, I would simply short the ends of the wire to the power. For digital control, I used a standard MOSFET circuit which is a digital switch that can be turned on and off using a microcontroller... (cont'd)