You've most likely seen vacuum tubes set into bread boards as a standard prop for indicating either high science or the mad scientist. Since their introduction in 1904, vacuum tubes have been an essential element for the creation of electronic components. However, in recent years they have been replaced in many areas by semiconductor technology. This is not universally true though and the airwaves are getting jammed with the large amount of traffic. Therefore, vacuum electronic devices are looking to utilize much higher frequencies and therefore are required to be both smaller in size and be created with an incredibly high degree of precision. Therefore, DARPA is investigating the possibilities offered for the creation of these devices through 3D printing and has started a funding initiative for researchers to contribute to this area of growing interest in engineering knowledge. Check it out here: http://3dprint.com/89780/darpa-vacuum-tube-research/