For a successful extrusion of the ink air pressures between 20 -30 kPa were used with a tip moving speed of 12 -18 mm s -1 . The structures were deposited on 25 × 25 × 1.1 mm glass substrates (Corning 1737; DELTA Technologies substrates ® ). The glass substrates were pretreated in a piranha solution (one-part 30% H2O2; Fisher Chemicals ® ; three parts conc.