There were two main tips that the manufacturers gave for using their ink. They were to make sure to store the ink in a cool, dark place and to try to print in a humid environment. The ink was always stored in a cool, dark place so that was not an issue. It made sense to me that the moisture from my fingerprint could be enough to trigger the ink and that the lab was to dry by itself.
I first created a test circuit in Fritzing that involves an ATtiny blinking an LED. The entire circuit would be powered by a 9V battery. The circuit and print mask can be seen below (Fig 1 & 2). I then continued to print out this test circuit with the transparency and printer undergoing different conditions. I began by applying water to the transparencies. This was supposed to replicate the moisture from the fingerprint. After many tests ranging from "misting" the transparencies to "drenching" the transparencies with water from a spray bottle, none of the tested circuits were conductive.
|Fig 1. Circuit Design|
|Fig 2. Printed Circuit Design|
I then had the idea that it was a combination of the water and pressure from my fingerprint. I first misted some water on the transparency and put on a pair of latex gloves. I then proceeded to press down my thumb multiple times across the transparency. This did not yield a conductive circuit. I then believed that there was too much water on the transparency and gently sprayed the water onto the fingers of the gloves first. I then applied pressure to the substrate again. This also did not yield conductivity.
It was then suspected that it was a different substance from the fingerprint oils then water that was causing it to activate. Not having very many options to test with however, it was resorted to doing a quick test with unscented hairspray and isopropal rubbing alcohol. These also did not yield any promising results.
Thankfully, today was a very humid day in South Bend with a humidity of 93%. The lab seemed to be too dry of an environment, so the printing station was set up outside (Fig 3). After running multiple tests, the outdoor humidity had no positive effect on the circuits. I even brought the printer into a small bathroom and hoped the steam from a hot shower would provide the appropriate humidity needed for a conductive circuit. That failed as well.
|Fig 3. Outdoors, High-Humidity Set Up|