**1. Introduction**

#### **1.1. Printing technology**

The e-market for flexible and printed large-area electronics is rapidly growing and it is expected to become a \$69 billion market in the next 10 years [1]. The growth is manly supported by the organic light-emitting diodes (OLEDs) and conductive ink industries. Nevertheless, as technology emerges from R&D, new market opportunities with huge growth potential will appear. Two potential markets are the food and health. These industries are facing a paradigm shift as society demands more regulation, quality control, and smart systems to improve life quality while being environmentally friendly and allowing continuous user interface. Printed electronics can be the key to address such demands by imparting products with solutions to acquire, store, and transfer data, communicate and carry out logic functions to take decisions, where recyclability and low cost are key vectors [2].

Printing technologies enable electronics to be readily integrated as a part of other printed media by processing them in the same press. This renders possible low cost products such as radio frequency identification (RFID), intelligent packaging, food quality control devices, or disposable diagnostic kits. Functionality and performance of printed electronics are not intended to compete with silicon-based electronics, nevertheless, mass-printing methods offers economic advantages for large-scale production of appropriate products. Printing technology is highly customizable, it is compatible with the preferable fabrication method in industry—the roll-to-roll (R2R)—does not require large vacuum chambers and has lower capital investment costs when compared with other production methods. It is estimated that a printed electronics facility will cost 100 times less than a conventional silicon electronics plant.

There are various printing techniques, such as inkjet, screen, flexography, gravure, or offset printing, and their features expand the range of applications. The selection of the printing method is dictated either by the requirements concerning printed films or the level of printing system complexity. In the field of electronics, printing techniques are used to apply coatings, to deposit precise patterning, or even to develop microstructures [3]. Inkjet printing patterns material by expelling from the nozzle one picoliter droplet of ink at a time, as the printhead moves over the substrate. It is a method suitable for low-viscosity inks (1–20 cP). Screen printing is a highly versatile technique given its simplicity and reproducibility. To print, a squeegee transfers the ink through a patterned screen onto a substrate. Gravure (or rotogravure), flexography, and offset printing use a rotary printing press. Gravure printing is the most popular process for flexible packaging manufacturing and it consists in applying ink to an engraved cylinder, which is then transferred (directly, or indirectly through a transfer roll) to the substrate. The flexographic technique prints on flexible substrates by ink transfer (with low viscosity, 50–500 cP) from a laser-etched flexible relief plate. Offset printing is the preferable method for newspaper printing. It works on the principle of oil and water repulsion. A plate is damped first in water (nonimage area) and then ink (image area); the ink adheres to the print area, then it is transferred to a rubber blanket and from it to the paper.
