In a method for printing a three dimensional structure, a continuous length of fiber that includes, interior to a surface of the fiber, a plurality of different materials arranged as an in-fiber functional domain, with at least two electrical conductors disposed in the functional domain in electrical contact with at least one functional domain material, is dispensed through a single heated nozzle. After sections of the length of fiber are dispensed from the heated nozzle, the sections are fused together in an arrangement of a three dimensional structure. The structure can thereby include a continuous length of fiber of least three different materials arranged as an in-fiber functional device, with the continuous length of fiber disposed as a plurality of fiber sections that are each in a state of material fusion with another fiber section in a spatial arrangement of the structure.