Have a look at my project on Instructables here: https://www.instructables.com/id/3D-Printing-High-Strength-Bio-composites/

Using cotton thread and a modified 3D printer nozzle of my own design I created high-strength composite parts with a flexural modulus (stiffness) over 14 times greater than unreinforced nylon with my low cost Prusa I3 printer.
By coaxially extruding a continuous thread of cotton through a 0.7mm nozzle with nylon, much stronger parts can be produced - similar to the method used by Markforged composite 3D printers. However extensive cooling and far slower print speeds (≈ 6mm/s) have to be used to prevent the cotton being pulled out of the composite matrix. Printing with fine copper wire was also tried with possible future uses for 3D printing electronics e.g. NFC / RFID antenna, electromagnets and motors.