There’s an interesting proposal to produce “hollow filament”.
hollow
Hollow Spiral Lattice Design Marries Thermal And Mechanical Performance
Researchers propose a hollow spiral lattice architecture that improves both heat transfer and load bearing, a combination AM operators need.
“Blow Extrusion” Nozzle Promises Lightweight and Efficient 3D Prints
I saw a new 3D print nozzle technology that can print tubular extrusions.