>Can you make digital negs using sheets of acetate or overhead transparencies,
>or do you need special materials?
Yes, you can, but the quality of the output depends in part on the printer.
For example, I can't get a dense black with my aging inkjet. Kodak makes
transparency sheets esp. for inkjet printers -- the sheet has a slightly
rough surface to hold the ink better. (Although I get thin negatives with
my printer, I've discovered it's fun to work into them with ink and make
something different from the original intention.)
I haven't used a laser printer for digital negs, but others have with success.