This ink is an ordinary ink, the only difference compared to the ink in your pen is that it contains particles of conductive elements, such as silver powder or flakes, or graphite particles.
These particles are what give the ink the ability to transmit current when energy is applied.
Ink is found in two phases, liquid and solid. When it is deposited on the material it is first in a liquid state, where the conductive particles float in the form of spheres allowing the electrons to jump. Once the ink has dried it solidifies, making the connections between the conductive particles rigid so that the current can flow.