**7. Final remarks**

manufacturing process, increases the time and cost of production. Adhesion-enhancing techniques such as: chemical [79] or mechanical induced roughening of the surface [77], or resorting to a primer (e.g., silane coupling agents [80]), corona discharge [81], plasma treatment [82], and flame treatment [81] are some examples. The most common techniques are plasma, flame and chemical treatment. With plasma and flame treatment, the substrate SE is changed by creating functional groups on the surface and eliminating surface contaminants. Although, the surface treatment is temporary, i.e., the treatment enhances the compatibility of the surface with the ink, but the exposure to air induces hydrophobic recovery [83]. Therefore, it is recommended to print after surface treatment. Chemical treatment is another option. The chemical treatment changes the surface characteristics (physical and chemical) by increasing the total area of interface between both layers leading to structural changes (by increasing the

interface roughness) and interactions between the fluid molecules and the substrate.

on the contained deposited drop in the determined spatial printing area.

mity of the printed pattern and the manufacturing time [32].

**6. Applications of IPT to flexible PE**

Another aspect that can pose a problem during printing is the incompatibility between different inks used in multilayered structures or between layers of the same ink, which can cause dissolution or resuspension of the previously deposited layer of ink, depriving uniform and uncontaminated layers [42]. The morphology and uniformity of the printed pattern depends

The optimization of the ink and interaction between ink and the substrate strongly affects the final resolution and constitutes a main research challenge in order to achieve repeatability of printed patterns and devices. An optimized ink formulation, according to equipment and target application, as well as the substrate treatment processes constitute the main successful factors to achieve high resolution and repeatability of the printed patterns and devices.

Equally relevant are the different post-processing treatments, such as, sintering, annealing or simply drying in air required for each ink, which defines the final morphology and unifor-

The increase of the printing resolution leads to an increased number of applications. Lee et al. [84] developed a flexible capacitive pressure sensor for plantar pressure measurement, using a flexible printed circuit film as a sensor substrate and PDMS as dielectric layer. Cheng et al. [5] developed a tactile sensor with PDMS using a highly reliable capacitive mechanism. However, the required manufacturing process involved multiple factoring steps and the use

**5.2. Printable materials compatibility**

**Figure 8.** Ink behavior on a substrate.

62 Flexible Electronics

The PE technology is not a replacement for conventional electronics, however, allows free design and unlimited applications areas. The PE benefits from new printing technologies, new material solutions, and by the combination of other manufacturing processes. The increase of research and development is reflecting a growing interest in the new generation of flexible and PE applications for, space and weight reduction. The PE had an undeniable impact on the electronic industry, economics and on the human life, revolutionizing the electronic applications, otherwise impossible to achieve with the conventional techniques and materials.

This chapter made an overview of the most important printing techniques and material solutions for the PE, with particular attention to the IPT. For the different printing technology, the process requirements, the materials and their critical limitations, highlighting their main features, were summarized. The possibility of combining technologies to overcome one technology limitation with another technology was also presented. Moreover, IPT is a promising technology which main advantages lies on its simplicity and low cost operating principle, overcoming the flaws of traditional technologies. Also, the main printing challenges are addressed, in terms of compatibility between printable material and substrate. At the end, examples of IPT flexible PE are presented. A breakthrough is expected in the next years which potentially will reduce cost with mass production applications. So far, there are other issues that need to be discussed and practical questions start to arise once the industry gets more involved and focused in developing commercial products, such us, market trend, recycling, and return of the investment.

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