Step 3: Use an InkShield as print head
So, it looks like we may not be able to use the latest 1200dpi generation of inkjet printers to print anything larger than yeast cells. And we may not even be able to print yeast or even E. coli cells with the inkjet cartridge on this particular printer!
Other groups using inkjet printers typically use ancient HP printers that only do 300dpi, with nozzle sizes presumably around 80 micron or so, which should be plenty large to print human cells. For example, this group is using an HP DeskJet 500, a model that dates all the way back to 1990 - good luck still finding one of those!
Now, Parallax used to have an inkjet printer development kit available to drive a 96dpi HP 51604A cartridge, but both the cartridge and Parallax kit went out of production a few year back.
Luckily for us, Nicholas Lewis recognized the need for a DIYable inkjet platform, and started a kickstarter campaign to build Inkshield: An Open Source Inkjet Shield for Arduino. (Back in stock by March - see the comments below!) InkShield is designed around the HP C6602 inkjet cartridge, a specialty cartridge with only 12 nozzles at 96 dpi, designed to print labels on things like cabling (or... inkjet cartridges!). 96 dpi equals a dot pitch of 265 micron. As you can see in the last image above, the actual nozzles are only about 1/3 of the distance between dots, or about 85 micron - just perfect for our purposes!
The InkShield is designed to be driven from an Arduino, but does need a higher voltage than the 5V the Arduino gets when it is powered from USB. So you'll need to supply a 9-12V input via an optional power connector on the InkShield, or via the power connector on the Arduino.