Cellulose paper (CF3, Cytiva) was saturated with 0.8 M (92 g/L) ammonium sulfamate (NH4SO3NH2, > 98%, Sigma) using a brush and left to dry overnight. Next, the LIG-Es were fabricated using a CO2 laser engraver (100 W, Speedy 300, Trotec) equipped with a 1.5-inch focus lens and operated using the following parameters (unless indicated otherwise): power 18 (measured value: 4.8 W), speed 5 (measured value: 16 cm/s), defocus 10 mm, laser pulsing 1000 pulses per inch (PPI), line spacing 333 lines per inch (LPI). The lasing process was performed under constant stream of nitrogen. The LIG-Es were then rinsed with deionized water, isopropanol and acetone and left to dry overnight. Optionally, the LIG-Es were treated by oxygen plasma for 90 seconds at 90 W (Atto, Diener electronic). Wax patterns were created by first printing a full layer of wax (Xerox Colorqube 8570) on a transparency (X-100, Folex), following by removal of the hydrophilic channel areas using the laser cutter (power 6, speed 1). The wax pattern was transferred onto the paper layer via lamination at a temperature of 110 °C (Photonex 325lsi, GMP). All designs (channels and electrodes) are created in Adobe Illustrator CS6.