Jie Qi, from the High-Low Tech Group at MIT Media Lab, gave a short intro into the world of electronic popables and Shape-Memory Alloys (SMAs) – an integration of paper, electronics, mechanics and computation. I call it electronic origami. Many cool ideas are possible from this mix of paper circuits:

• Pop-up Books
• Paper Mixtapes
• Paper dj/vj GIF mixer
• Circuit Graffiti
• Actuated Fashions/Textiles
• Actuated Living Architectures

Jie Qi offered many helpful tips for working with these responsive materials like memory metals(nitinol/flexinol wire), , 3M copper tape, fabric tape(expensive but more flexible). Sites like robotshop and her MakeZine blogpost are also excellent resources. The trick is to find the “sweet spot” between calculating for the correct resistance and finding the just-right combination of wire gauge to paper weight. Higher heat, thicker gauge wire is stronger but requires more power(resistance) and smaller gauge is weaker but easier to work with (i guess she means, easier to power). She suggested we start off with 0.006 HT wire to play with. You can’t solder directly to nitinol/flexinol so she suggested wrapping leads to a crimp tube and soldering to it, instead.

0.006
R = 1.3 Ω/ in
current = 0.4 amp
wall wart 5v (cellphone charges work well)

V = current * resistance
5v = 0.4 a * r = 12.5Ω
12.5 ohm = 1.3Ω/in * Length = 9.6 in

According to Jie Qi:

The hardest part about using muscle wire is controlling the amount of current running through the wire. You want to give it enough for a dramatic effect, but not so much current that the wire burns out (and stops contracting). Flexinol wire has a consistent resistance per length and an optimum current as specified in the flexinol technical data.

One simple technique is to look at the target current from the data sheet and then use Ohm’s law (voltage = current x resistance) to calculate the length of wire that is needed to maintain this amount of current based on the power supply you have. Since these wires generally require hundreds of milliamps, I recommend getting a strong lithium-ion battery or use a wall power supply. For example, if I were using the 0.006″ diameter wire, which needs 0.400 Amps, and I have a 5V power supply, I would need a total resistance of 5/0.4 = 12.5 ohms. Since the resistance of this particular wire is 1.3 ohms/in, I would need 12.5/1.3 = 9.5 inches.

In general, always test your mechanism using low power and turn on the wire in short intervals. If you see a jerking motion, chances are the wire has gotten too much power and might in fact have burned out. If you have an Arduino, you can hook up the wire using a transistor and PWM the power. Start with a low duty cycle and work your way up until you get a strong enough movement. Note that once the metal is getting enough power to change shape, adding more power won’t make the movement more dramatic, it will only make the shape change more quickly.

Then you get to do fun stuff like ticklish plants or animated vines.