There’s an interesting proposal to produce “hollow filament”.
filament2
Filament2 Secures Patent for Coaxial Extrusion System Enabling New FFF Materials
Filament2 has secured a patent for their highly unusual coaxial extrusion system.
Filament2 Reinvents Paste 3D Printing with Simple FFF Upgrades
One of the more interesting technologies we saw at Formnext 2024 was from Filament2.