Wednesday, March 19, 2014

Nylon Fishing Line Artificial Muscles II

It's time for some updates on the artificial muscles made from nylon fishing line. In case you missed the last blog post, I'm trying to replicate the research results described in this news article: I've successfully demonstrated Phase II of the project: activating a muscle with heat produced by electricity running through a wire. First, though, there's some unfinished business from Phase I to talk about.

Some More Thoughts on Coiling

There are a couple of basic ways to coil the nylon filaments. The first is to twist the filament as tightly as possible without causing it to form secondary coils, then wind it around a rod. I'm going to refer to the results of this technique as “rod-coiled” muscles in the future. The second is to keep twisting the filament until secondary coils form all along its length, without any need for a rod in the center; I'll call these “self-coiled” muscles. I was having trouble making self-coiled muscles as of my last post. Rather than forming secondary coils, the filament wanted to bunch sideways, forming whiskers that extended horizontally away from the line.

Self-coiled filament close-up. Left to right: 711 um filament with wire, 533 um filament, 381 um filament.
I blamed my cheap nylon line at first, but since getting some better types, I've discovered that they all have a tendency to form whiskers. A commenter on the previous blog post tipped me off that the researchers loaded their lines with 17 MPa of tension during coiling. I decided to try adjusting the weights on my lines to provide that level of tension, and got much better results. The filaments do not form whiskers when kept sufficiently tense during the coiling process. The following are the three types of nylon monofilament I am working with now:

Cousin Clear Monofilament, test strength 8 lb., estimated diameter 0.015 in. (381 um). I had to guess the diameter, since it's not advertised on the package.
Zebco Omniflex, test strength 25 lb., diameter .021 in. (533 um).
Trilene Big Game, test strength 50 lb., diameter .028 in. (711 um). This stuff claims to have “Extreme Fighting PowerTM,” and it's bright green, lest you forget how Extreme it is.

Filament Weighting Calculations
Test (lb)
Diameter (um)
Cross-Sectional Area (mm2)
Force (N)
Mass (g)
6.99 oz
13.6 oz
1.52 lb

You might think that it's safe to exceed the weight needed for 17 MPa of tension, so long as you don't go over the line's test rating. But don't do that. The coiling process puts additional stress on the line, and I've found that it can snap, even when loaded with much less than its test weight. The thinner the line, the more tightly you need to adhere to the calculated ideal weight. When using the 711 um line, I had good success with weights from 1.5 up to 2 lb., and did not try anything heavier. For the 533 um line, 8 oz. is too little (resulting in whisker formation), but 1 lb. is too much (causing the line to snap). A weight of about 12 oz. works well, however. It was difficult to find the right weight for the 381 um line – I don't know if this is because of the smaller margin of error for this very thin filament, or because this particular line is intended only for stringing beads and is not of very high quality. It might also be thinner than I thought it to be. The weight I used when I finally coiled a good muscle with this filament was probably close to 6 oz. (I don't have a scale yet. Sorry. I am also stubbornly using imperial units because most of my items with known weights are labeled with those.)

The self-coiled muscles again, with a penny and a cat for scale.
The ability to make the self-coiled muscles opens up new possibilities. They have a larger spring constant than the rod-coiled muscles, and can therefore support and lift heavier loads. However, I suspect that there is a trade-off between the maximum contraction force and the maximum contraction length. The loops of the self-coiled muscles are already very close together, so there just isn't anywhere they can go. The loops of the rod-coiled muscles spread farther apart under load, and contract dramatically when heat is applied. For this reason, I recommend making a rod-coiled muscle for your very first tests … it's just easier to see whether it's doing something when you heat it. Also, you can only get heterochiral muscles that expand when heated by using the rod-coiling method, as far as I know.

Electrical Heating of Muscles

Perhaps the most convenient way to supply the temperature change needed to activate the muscles is by electrically heating them. I made several muscles that were supplied with a resistive heating element, in the form of a piece of magnet wire twisted into the muscle. “Magnet wire” is very thin copper wire with a coating of insulating enamel. I salvaged mine from the coil of an old electromagnet, so I don't know its exact thickness. For my most successful experiment, I made a homochiral rod-coiled muscle from the 711 um line, with a single magnet wire wrapped around the line. (I tied the nylon line and the wire together at the ends, so that wire and line were twisted together during the coiling process. Then I wrapped the two around the rod and annealed the muscle in my toaster oven.) I loaded the muscle with a pair of ferrite cores, weighing (I estimate) about 30 g. I was able to see it contract and lift the cores a millimeter or two when I connected the wire to a 6 V battery pack, and relax again when the electrical current was removed.  Not much, but it proves the concept.

Ideally, the muscles would be run by a controller which would first provide a burst of high current for quick heating, then back it off to avoid exceeding the maximum temperature of the muscle. A small amount of current could then be maintained to hold the muscle in its contracted position, if desired. My “quick and dirty” version was to simply hook the battery pack to the wire and disconnect it as soon as I saw the muscle start to move. Given the length of wire I was using (about 60 cm) and a voltage drop of ~6 V, the amount of heat produced is enough to permanently deform the muscles and eventually melt them in half, if the current is left connected too long. So one must be very careful and have quick reaction time if working with this much voltage. My muscle would heat up enough to show a response within a few seconds, and relax within maybe 15 s or so.

Besides the ease of overheating, the other major annoyance was working with the magnet wire itself. It breaks very easily when it experiences strain as a result of being coiled up with the muscle. I don't think I made a single 711 um muscle without having the wire break – for my successful tests, I either manually wrapped some wire around the muscle after it was coiled, or pulled one of the broken ends out of the middle of the muscle and stripped it so I could get a connection point.

To Do List (Stay Tuned)

Come up with a way to make muscles reliably without constant wire breakage. I plan to investigate some other possible resistive heating elements, besides the single strand of magnet wire.

Calculate the heat dissipated by the wire, and find maximum “safe” voltages for muscles as a function of their parameters.

Use a microcontroller and driver circuit to control a muscle.

Experiment with a variety of muscles and get more details on lifting power, maximum compression or extension, time to contract/relax, etc.

Until the next cycle,

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