After figuring out how to drive the printhead — 5 to 6 μs pulses of 18 volts through a ULN2803 Darlington array driver chip seemed to do the trick — he mounted everything to the gantry of an ...

Note the dual vertical assemblies – one carrying the extruder, the other carrying an inkjet print head. The hack is simple, consisting of a second vertical frame added on to the printer.