Most of what I've done since the last blog post has involved testing different heating elements. In the process, I've built some muscles that I thought were good enough to film. They're slower than I would like (not just in the cooling phase, but also in the heating phase), but getting a proper driver circuit built might fix that. Blowing air on them helps them cool off faster, but I purposely avoided doing that so that you can see how long it takes them to relax in still air.
First of all, I'm about ready to give up on the copper magnet wire. It's simply too easy to snap – even if I could make it work, I don't want a delicate, frustrating manufacturing experience, especially since I might end up making a lot of these.
The first alternative I tried out was Beadalon 7-strand beading wire, which is made of stainless steel with a thin nylon sheath for insulation. It's highly flexible, and has a much higher resistance than the copper wire: about .30 Ω/cm, yielding a total of 14 Ω at the length I used. The same length of my copper magnet wire has a resistance of only 3.5 Ω (0.075 Ω/cm). (Don't set too much store by those numbers. My multimeter doesn't seem to be very good at reliably measuring resistances this small.) The total diameter of the wire and insulation is 0.46 mm. I made a homochiral (contracting) rod-coiled muscle using this wire and some of the 50 lb. test (711 um) nylon line, and got what I would call my first really good muscle. I was able to cycle it many times without damaging it and making the coils go flat (a problem I had previously with the muscles that used the copper magnet wire). It contracted enough to lift a suspended ferrite core about 1 cm. The main disadvantage of this wire, as compared with the copper, is that it heats up much more slowly, and I think it cools more slowly as well. This could be due to its higher resistance, which would have reduced the current flowing through the wire (I've been using about the same voltage for all my tests). Or it could be mainly due to the insulation limiting the rate of heat transfer from wire to muscle.
I also tried some tinsel wire – multiple strands of enameled copper wrapped around a fiber core – which I scavenged from the cord of an old pair of headphones. This is my favorite heating element so far. It doesn't break easily, it heats quickly, and it's less bulky than the Beadalon wire, because it's insulated with enamel rather than a nylon sheath. When it's put under tension and wound up into a muscle, the strands shift and flatten so that the wire takes up very little vertical space between the coils, and it spreads out a little to touch a greater surface area of the nylon. I made a heterochiral (expanding) rod-coiled muscle with this stuff.
In the comments of a previous blog post, bluesmokelounge suggested using a heating wire without any enamel, presumably to get quicker heat transfer. I'm a little wary of doing that. Loops of wire on adjacent muscle coils can contact each other when the muscle is fully contracted, and would short together without insulation, throwing off any calculations based on the length of the wire and possibly causing uneven heating. If the muscle were cycling continuously, this condition would be momentary, but it could be more of an issue if one wanted to hold the muscle in a contracted state for a while.
Last of all, I tried electrically heating some self-coiled muscles. Even though I like the tinsel wire best, I don't have much of it right now, so I used the stainless steel beading wire instead. My first attempt to heat a self-coiled muscle with this was a flop. I don't know if that was because the wire was too long (hence high-resistance) and I couldn't get it hot enough with the battery pack I was using, or if the large diameter of the beading wire was preventing the muscle from contracting by taking up too much space between the coils. I suspect the latter, because the very top of the muscle (which wasn't coiled as well) did try to move. So instead of coiling the wire up with the nylon monofilament, I tried wrapping a shorter length of wire around an already-formed self-coiled muscle. This wire got hotter and didn't interfere with the movement of the coils, and I was able to see some results. Using a self-coiled muscle made from my 25 lb. test (533 um) line in this configuration, I was able to lift a weight of about 55 g a millimeter or two … but it's more dramatic to watch this same muscle flex a piece of paper:
Until the next cycle,