**Figure 3.**

*Inkjet printing steps.*

and can be easily changed, which makes this printing technique an ideal choice for prototyping and design optimization. **Figure 3** depicts the main stages in IJP:


The IJP technology for PE applications and its technical-scientific developments have been reviewed over the last decades. Hue P. Le [44], 1999, reported the developments in the various IJP technologies, noting the significant growth rate of inkjet printers market. New ink formulations and new printhead designs were recognized as relevant for new applications. In 2010, the state-of-the-art of IJP of functional materials was reviewed by Raje and Murmu [45]. Improvements in process throughput remained the major challenge. In the same year, Derby reviewed the current understanding of the mechanisms of drop formation and the interactions between drops and the substrate, with a focus on the fabrication of structures for structural or functional materials applications [46]. Two years later, in 2012, Cummins and Desmulliez conducted a review in IJP of conductive materials [47]. IJP process, substrate properties, and types of conductive inks are the various factors that affect the quality of inkjet-printed products and their increasing relevance to the fields of electronics manufacturing, packaging, and assembly. In 2019, Nayak et al. reviewed the IJP of electronic devices, mainly addressing the fluid dynamics of inks and main properties (e.g., viscosity, surface tension, Weber number, Reynolds number, and Ohnesorge number) and their effects on defects appearance (coffee ring formation) [48]. The use of functional inks in sensors, thin-film transistors, and energy storage devices is presented. Ke Yan et al. revised the state-of-the-art related IJP strategies and functional inks for wearable electronic devices (e.g., sensors, displays, transistors, and energy storage devices) [49]. They highlighted the need of having available more intrinsically flexible and stretchable inks for avoiding, cracking, and delamination on highly flexible/stretchable substrates. Also, IJP technology development shall solve nozzle clogging issues for a more stable printing process. Kye-Si Kwon et al. reviewed piezo-driven IJP for PE. Other printing methods for high viscosity ink are also considered and compared (e.g., electrohydrodynamic jet, aerosol jet, and micro-plotter printing) [50]. There is a high demand for high-resolution printing of high viscosity inks for PE. In this case, the functionality of the device is more important than graphism perception, and the development of suitable inks for IJP remains one of the key issues. More recently, Muhammad Ali Shah reviewed the classifications and applications (textile, displays, and wearable devices) of IJP with more attention paid to piezoelectric IJP due to its higher relevance [36]. Various driving-voltage waveforms approaches are compared. Recently published studies on applications of IJP are summarized. Again, high high-viscosity IJP technologies are revised. The performance of IJP shall be improved by the development of new printheads with ink-recirculation and new techniques for printing high viscosity inks.
