I've used 0,25 mm Kynar electrical wire.

• Must be printed at 0,1 mm layer height to preserve wire channel size.
• Set a pause at 1,80 mm layer.
• Make a 0,8 mm loop at both ends of the wire. Wrapping once or twice around small tweezers should be enough.
• Put the wire during the pause with the ends inside the hole. Make sure it doesn't protrude the sides of the hole.
• Resume the print and the wire will be closed inside.