How do they join the bottoms of the bottles? They seem to be prejoined in the dome timelapse.

Good question! Had to look it up in the paper, and it turns out they use wood screws.

Either regular ones using a long screwdriver through one of the bottle's mouths, or more specialized double-ended screws (with two points) that let you grab the bottles and counter-rotate them to drive in the screw in both bottoms.

Neat, I hadn't thought the bottoms of PET bottles had enough material to make this work, I would have expected bolts with nuts, i.e. more metal-to-metal in the fasteners.

Woodscrews. See the image on page 7, column 1.

These would be cool to sell on etsy.

I'm guessing you could get the connectors mass produced pretty easily?

Is there anywhere to get empty botttles in bulk?

I have an idea that I haven't tried yet. 3D print hollow joints that, for example, connect wood panels into a book case. Then fill the hollow spaces in the 3D printed joints with concrete.

That has potential, though it may be hard to find a shape that's open enough to get concrete packed into, while still wrapping around the panels effectively (and you'd want to use a concrete with fibers mixed in to help with shear[1]). It might be possible to use epoxy resin instead.

1: https://en.wikipedia.org/wiki/Fiber-reinforced_concrete

I would love to make a few of these connectors on my laser cutter. Seems like I will have to look carefully for source files...

This is incredible. Make sure to watch the video to get a sense of just how rigid these things are!

That is sweet