I think the technology to 3D print at the cellular-protein scale (microns to nanometers) is a ways off still. For anything like muscle, you'd need micromachines

I think the technology to 3D print at the cellular-protein scale (microns to nanometers) is a ways off still. For anything like muscle, you'd need micromachines (comparable to actin-myosin) linking fibers (in muscle they're about 50um in diameter and a few centimeters long) that would work in coordination using some kind of energy (in animals the fuel is ATP) source.