After years of not doing anything particularly special for Halloween, this year we decided to start early and actually make our own costumes. Of course our plans were way too ambitious, so despite the fact that we started in August, by Halloween morning only one of our three costumes was actually finished.

The idea: inflatable nudibranchs.  Nudibranchs (aka sea slugs) are marine invertebrates with incredible, psychedelic designs. If they look like something from a science fiction book cover, that may be because the designers of sci-fi aliens have been quietly stealing ideas from nudibranchs for decades.

I’d never made anything inflatable before, and barely ever used a sewing machine, so I learned a lot on this project. The material we used is this incredibly lightweight but sturdy stuff called 1-ounce calendered HyperD diamond ripstop nylon.  (I chose this particular kind based on one negative review where a customer had made a quilt, and complained that it was hard to stuff into a sack because it kept trapping air inside it. Which was of course exactly what we wanted it to do!)

We did a lot of experiments to figure out the mechanics of inflatable structures.  It’s pretty hard to visualize what 3D shape you’ll get from a bunch of flat parts until you’ve stitched them all together and filled it up with air. (Although I did find some interesting graphics research that solves the inverse problem!)

The trickiest part was figuring out how to attach the costume to a person’s body.  With most inflatable costumes you can buy in stores, your whole body goes inside the inflated part of the costume, with just your hands and feet sticking out of elastic cuffs. This seemed like it’d be really hot and uncomfortable over a long night of running around trick-or-treating.  So I made ours an outside-the-body design, with two belts of nylon webbing to attach to your torso, and a drawstring hood for your head. We found it wasn’t hard to rip the ripstop nylon at the point of attachment to the straps, so I reinforced those areas with a second layer, based loosely on how sails are reinforced at the clew.

One part I’m pretty happy with is the quilting of the blue fringe around the body. Without it, the body puffed up into a big potato shape. But I stitched in a pattern of alternating lines to make a little zigzag maze for the air to flow through. The result was a pretty decent match for the crinkly fringe of a real nudibranch’s foot.

The air blower is mounted in the ventral side of the body, below the waist so it would have a chance at clear airflow. And for illumination we ran two strands of LED fairy lights down the interior, from the tips of the blue cephalic tentacles down to the tail. This was okay, but not perfect: I would have preferred to light up more of the orange cerata sticking off the back. (The lighting was pretty rushed, and definitely something I’d like to do better next time…)

By now I am completely hooked on this inflatable costume idea. Which is good, because I’ve still got my own blue water dragon nudibranch costume to finish… but that’ll have to wait ’til next year!