Jie Qi at the High-Low Tech lab at MIT’s Media Lab, posted a HOWTO on nitinol and origami. In the HOWTO she mentions that you can’t solder the nitinol directly, and so you’ll have to have create a soldering pad for it. (She used a craft crimp bead.) Another tip she gives is the need to preheat the nitinol by running a 9 volt charge through it for five seconds. When the wire relaxes, it will become be longer than it was originally, and so you’ll need to retention the wire. Last of all, she warns against keeping the wire energized too long, lest your “burn out” the nitinol. In another project, Qi mentions she used 0.006 inch flexinol for the origami, but used 0.01 inch for the more rapid vine/snake project.
I have had a fairly long interest in synthetic plants and was thinking if nitinol could be used to in a heliostat or some sort of dinural deployable structure, but I never knew the reaction time of nitinol. Seeing it used understanding what voltages are required was helpful. (Poking around just now, I also ended up finding a handy nitinol wire width-voltage-time-force table.) While I doubt that I will ever actually build whatever vague idea idea I have for synthetic plant, I’ve come to conclusion that nitinol perhaps isn’t the best choice of materials if you want it to hold position for any considerable length of time (or at least not without some sort of mechanical latching).
Ying Liu, et al. from Michael Dickey’s Lab at North Carolina State demonstrate how simply using an inkjet printer to put black lines on shrinky-dinks, one can transform them into a heat activated self-folding material. (Paper.)
The idea is very simple. When a light is shone on the taped up shrinky-dink, the black tape absorbs more energy, and thus the area under the tape heats up faster than the uncovered material. Since shrinky-dinks contract when heated, a hinge is created. By varying the width of the tape, the rate of closure and the resultant angle can be controlled.
MIT has published a paper entitled Programmable Matter by Folding (full article) that describes paper that can fold itself into a variety of shapes. The paper is covered by is divided into triangular sections that are joined by a network of thin nitinol actuators that contract under voltage. At the center of each section is a magnet that is used to retain the paper’s shape.
While I’m sure MIT had bigger plans for this tech (Well if it was the Media Lab, perhaps not.), I immediately thought that this was the perfect thing for synthetic plants. I’ve been thinking about how nitinol wires, or at least something like them, could deform a paper but thought that the being able to compresses only about 4% was a problem. When I first saw this video, I thought they were using something else besides nitinol, but they’re not. The trick they they used to get 180 degree bending is folding and annealing the 100μm foil so that the nitinol will remember the folded shape. Once it cooled, the foil is manually flattened, and then reheating the foil with electrical current will cause it return to the folded shape.