**5.1. Wax printing technology**

Among the different patterning techniques, wax printing represents one of the most promising technology for paper patterning, given its simple and fast (5–10 min) fabrication process. The printing process uses a solid wax printer, in which the ink is supplied as solid wax and then melted, before being ejected from the print head, and once it rests on the paper surface it immediately solidifies. Since the wax remains only on the paper surface, the printed paper is then processed on a hot plate (120–140°C, 1–5 min; depending on the type of wax and paper) to allow the wax to diffuse vertically through the entire paper thickness, hence creating the hydrophobic barriers to confine the fluids (**Figure 12**). The wax is formulated from a nontoxic resin-based polymer, composed of a mixture of hydrophobic carbamates, hydrocarbons (e.g., paraffin), and dyes.

**Figure 12.** Schematic of the lab-on-paper technology process with wax printing: in the first step, a Xerox ColorQube printer, with solid ink (wax) cartridges, is used to print the sensor layout. The paper sheet is then processed in a hot plate (140°C, 2 min) for melting, in the second step. Digitalized images of the paper front view and the optical micrographs of the cross-section, before and after wax diffusion, show the behavior of the channel when a red dye solution is used and highlight the effect of the hydrophobic barriers. Adapted from Marques et al. [81].
