Step 2: Problem: current inkjet printers are too high resolution!
We've spent a good amount of time looking at inkjet print heads under the microscope. The silver strip you see at the bottom of an ink cartridge is called the nozzle plate. It's just a sliver of stainless steel that has precision nozzles pierced through it. In this black ink cartridge, the nozzles are arranged in four rows, of which you can see two in the first image above. The nozzles are actually numbered, from 1 to 416. 416 nozzles in a print head that's about 1/3 of an inch comes out to 1200 dots per inch.
Now, 1/1200th of an inch means a spacing of 21 microns. Also, the nozzles themselves are actually around 23 microns in diameter. That's on the order of the size of a eukaryotic cell - uh-oh! We should still be able to print much smaller E. coli cells (~1 µm diameter) with this print head, and possible yeast cells (~10 µm diameter).
But wait - there's more!
When you peel away the metal nozzle plate, you get to the actual print head, a marvel of silicon engineering that incorporates both the liquid handling, the microscopic heaters that vaporize part of the ink in a thermal inkjet printer, and a bunch of integrated electronics. Excellent targets for an amateur microscopist!
The last image above has us a bit worried, though. There seems to be a filter integrated into the silicon, sitting between the ink reservoir and the print head itself! From the microscope picture, we estimate the holes in the filter to be about 3 micron, which may be getting too small even for E. coli cells. Doh!
In addition, if you use a commercial inkjet printer, you'd have to take out the paper path to be able to print on anything else (like an agar plate!). Plus, if you stick with the existing printer drivers you have very little control over exactly what the print head does. You could try adapting Linux drivers to do what you want, but that turns into a project in itself.
So perhaps a clean break is better - let's just build our own printing platform that we will have full control over!