People so rarely use light string accompaniment as the soundtracks for their 3D printing projects these days, but I think this video deserves it. It depicts a new, syringe-based metal printing system made by researchers at NC State. It uses gallium and indium which oxidizes in air, thereby sealing the droplets together. While you won't be building any 3D guns with this process – it's far too slow and the dots are microscopic – you can create self-healing wires and stretchable circuits.
“We've found that a liquid metal alloy of gallium and indium reacts to the oxygen in the air at room temperature to form a ‘skin' that allows the liquid metal structures to retain their shapes," wrote Dr. Michael Dickey.
They have also created a system to inject liquid metal into a mold and then dissolve the mold away, leaving only the metal. Because the droplets bind and not “heap,” the system allows you to build 3D objects without resulting in a little pool of metal.
You could also feasibly use this process alongside standard plastic 3D printing systems to create leads that would snake their way through the inside of an object. It's non-toxic – so presumably you can eat a lot of it – but it's still quite expensive so don't expect to build any stainless steel rats any time soon.