**4. Flexible and extensible substrate for PE**

There are three types of substrates that may be used on electronic devices: glass; metal and polymers. The first two are rigid material. The glass is non flexible. The metal foil is flexible and sustain high temperature, although, is limited on the freedom of design and is high cost. Polymers composites, such as, glass-reinforced epoxy laminates with flame retardant (FR-4) have been largely used in rigid printed circuit boards (PCB). Non-reinforced polymers are flexible materials, are more economically processed, and gives greater freedom of design, providing studies with increasingly intelligent PE applications, able to be integrated in complex systems and environments [34]. Their major drawback lies on the low surface energy, which, normally requires a prior surface treatment before printing and low processing temperatures. Their selection must meet a series of physical, mechanical, chemical, thermal and optical requirements, and also important, the compatibility with the conductive inks.

properties, the process deposition rate, etc. It will be a commitment between several criteria that will allow achieve the desired PE performance, functionalities and requirements. The

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Most polymers have low surface energy (SE). The transfer and distribution of the ink on a substrate depends on the wettability and adhesion capabilities. The adhesion between two materials is the sum of a number of mechanical, physical, and chemical forces between them, at the interface, and depend on the mechanism of adhesion involved, that include mainly:

• Conductive ink properties (chemical composition, rheological behavior, the rate of solvent

• The superficial tension (ST) of the ink and the SE of the substrate that will receive the ink,

Surface wettability, spreadability and adhesion are the most important requirements in the printing process, and both are directly dependent on the fluid contact angle (**Figure 8**). When a fluid spreads evenly over the surface without the formation of droplets, the surface is said to be wettable. When a droplet is formed, the surface is said to be non-wettable, implying that cohesive forces associated with the fluid are greater than the forces associated with the interaction of the fluid with the surface. ST refers to the amount of cohesive forces between liquid molecules. The SE describes the degree of energy with which the molecules of the surface of a solid draw and allow adherence of a fluid. Often, ST and SE are interrelated, since both measure the ability of molecules to attract and to adhere to each other. In IPT, the spheroidal shape of the liquid emerging from the nozzle is defined by the ST of the liquid. The adhesion between two surfaces (ink, substrate) occurs when these droplets come into contact and develop strength in order to maintain a stable interface solid–liquid. Adhesion between a

The polymer low SE represents a great challenge in PE. In this situation, surface treatments are required to increasing the SE of the polymer, although implies an extra step in the

> • Development of new inks formulation • Adhesion

• Appropriate, affordable

• High volumes • Resolution

• Scalability to large area • Lifetime and stability

• Functional groups and their intermolecular forces present in the ink/polymer system.

• Substrate properties (chemical composition, surface topography and porosity, etc.).

main challenges are summarized in **Table 7**.

evaporation, etc.).

The

• Flexible substrates encapsulation • Cost effective barrier encapsulation

• Scalability to large area (e.g.,

material

OLEDs) • Adhesion

• Long time reliability

**Table 7.** Main challenges.

i.e., the difference between them.

**5.1. Compatibility between printable material and substrate**

solid and a liquid exists when the solid SE exceeds the liquid ST.

**Flexible substrates Printable inks Equipment**

Various types of polymers (semi-crystalline and amorphous) have been proposed as flexible substrates (e.g., polyimide [5, 12, 74], polyethylene terephthalate [11], polyethylene naphthalate [75], PVDF [14], polycarbonate [14]), and both flexible and extensible substrates (e.g., poly (PDMS) [4, 5, 75, 76], polyurethane [76], thermoplastic polyurethanes (TPU) [77, 78]), etc. **Table 6** shows the main properties of flexible polymeric substrates.


**Table 6.** Comparison between flexible polymeric substrates.

### **5. Printing technologies challenges**

Understanding the printing process and relationships between process parameters and printing quality (e.g., print resolution, uniformity and electrical conductivity of printed layer) is necessary for process optimization, as well as the suitability of the selected material in terms of adhesion and final applications; the appropriateness of the printed technology and ink properties, the process deposition rate, etc. It will be a commitment between several criteria that will allow achieve the desired PE performance, functionalities and requirements. The main challenges are summarized in **Table 7**.

#### **5.1. Compatibility between printable material and substrate**

**4. Flexible and extensible substrate for PE**

There are three types of substrates that may be used on electronic devices: glass; metal and polymers. The first two are rigid material. The glass is non flexible. The metal foil is flexible and sustain high temperature, although, is limited on the freedom of design and is high cost. Polymers composites, such as, glass-reinforced epoxy laminates with flame retardant (FR-4) have been largely used in rigid printed circuit boards (PCB). Non-reinforced polymers are flexible materials, are more economically processed, and gives greater freedom of design, providing studies with increasingly intelligent PE applications, able to be integrated in complex systems and environments [34]. Their major drawback lies on the low surface energy, which, normally requires a prior surface treatment before printing and low processing temperatures. Their selection must meet a series of physical, mechanical, chemical, thermal and

optical requirements, and also important, the compatibility with the conductive inks.

**Substrate PI PET PC PEN PDMS TPU** Tg (°C) 155–270 70–110 145 120–155 −125 80 Tm (°C) 250–452 115–258 115–160 269 – 180

Vol.Res.(Ω.cm) 1.5 × 1017 1.0 × 10<sup>19</sup> 1012–10<sup>14</sup> 10<sup>5</sup> 1.2 × 10<sup>14</sup> 3.0 × 10<sup>14</sup>

Modulus (MPa) 2.5 × 10<sup>3</sup> 2–4.1 × 10<sup>3</sup> 2.0–2.6 × 10<sup>3</sup> 0.1–0.5 × 10<sup>3</sup> 1 7 WorkTemp. (°C) Up to 400 −50 to 150 −40 to 130 – −45 to 200 130 CTE (ppm/°C) 8–20 15–33 75 20 310 153 Water absorption (%) 1.3–3.0 0.4–0.6 0.16–0.35 0.3–0.4 >0.1 0.2 Solvent resistance Good Good Poor Good Poor Good Dimensional stability Fair Good Fair Good Good Good-

Tg – glass transition temperature, Tm – melting temperature, CTE – coefficient of thermal expansion.

**Table 6** shows the main properties of flexible polymeric substrates.

**5. Printing technologies challenges**

**Table 6.** Comparison between flexible polymeric substrates.

Density (g/cm<sup>3</sup>

60 Flexible Electronics

Various types of polymers (semi-crystalline and amorphous) have been proposed as flexible substrates (e.g., polyimide [5, 12, 74], polyethylene terephthalate [11], polyethylene naphthalate [75], PVDF [14], polycarbonate [14]), and both flexible and extensible substrates (e.g., poly (PDMS) [4, 5, 75, 76], polyurethane [76], thermoplastic polyurethanes (TPU) [77, 78]), etc.

) 1.36–1.43 1.39 1.20–1.22 1.36 1.03 1.18

Understanding the printing process and relationships between process parameters and printing quality (e.g., print resolution, uniformity and electrical conductivity of printed layer) is necessary for process optimization, as well as the suitability of the selected material in terms of adhesion and final applications; the appropriateness of the printed technology and ink Most polymers have low surface energy (SE). The transfer and distribution of the ink on a substrate depends on the wettability and adhesion capabilities. The adhesion between two materials is the sum of a number of mechanical, physical, and chemical forces between them, at the interface, and depend on the mechanism of adhesion involved, that include mainly:


Surface wettability, spreadability and adhesion are the most important requirements in the printing process, and both are directly dependent on the fluid contact angle (**Figure 8**). When a fluid spreads evenly over the surface without the formation of droplets, the surface is said to be wettable. When a droplet is formed, the surface is said to be non-wettable, implying that cohesive forces associated with the fluid are greater than the forces associated with the interaction of the fluid with the surface. ST refers to the amount of cohesive forces between liquid molecules. The SE describes the degree of energy with which the molecules of the surface of a solid draw and allow adherence of a fluid. Often, ST and SE are interrelated, since both measure the ability of molecules to attract and to adhere to each other. In IPT, the spheroidal shape of the liquid emerging from the nozzle is defined by the ST of the liquid. The adhesion between two surfaces (ink, substrate) occurs when these droplets come into contact and develop strength in order to maintain a stable interface solid–liquid. Adhesion between a solid and a liquid exists when the solid SE exceeds the liquid ST.

The polymer low SE represents a great challenge in PE. In this situation, surface treatments are required to increasing the SE of the polymer, although implies an extra step in the


**Table 7.** Main challenges.

of several material layers, which consequently leads to time consumption, large material waste and high manufacturing costs, preventing the process automation to an industrial level. When the goal is large area sensing platforms, manufacture premium prices constitute a problem. In recent years, the interest for IPT to sensor fabrication has attracted attention [39, 85]. First IPT prototypes start to appear and has already been selected to step in the production of several devices, such as, integrated circuits [30, 33], transistors [32, 86], conducting polymer devices [30], structural polymers and ceramics [85], biomaterials, and printed scaffolds for growth of living tissues [30, 31]. In the field of flexible sensors, IPT it is just taking the first steps. IPT of an intrinsically conducting polymer [87] onto a flexible substrate for humidity and gas sensing applications [88] are two of many of the rapidly emerging IPT examples. Only a few examples of IPT sensors combining IPT polymer conductive ink (PEDOT:PSS and P3HT) [79, 89] or silver ink [90], printed on polymer substrate have been reported so far. Someya et al. [91] has developed flexible pressure sensors with a complex designed structure using OFET active matrices manufactured by IPT and screen printing technology. Basiricó et al. [92] have proposed a totally IPT flexible OFET assembled on plastic films as sensors for mechanical variables using a PEDOT:PSS as electrodes and a P3HT as a semiconductor. The results obtained were promising despite the lower charge carrier mobility measured. Cruz et al. [89] have developed a inkjet printed pressure sensing platform capable of measuring the central plantar pressure (CPP). The use of PEDOT:PSS for definition of the electrodes over a TPU substrate resulted in pressure sensors with higher sensitivities and better linearity. Good performance results (comparable with existing solutions) were achieved, with the particularity of offering a low-cost alternative. The printed substrate presented high flexibility, was able to follow and deform along with the substrate, without breaking or losing adhesion and its conductivity properties. The ink piezo-resistive effect and high gauge factors (>300) were demonstrated (higher than the typical value of flexible metallic strain gauges) showing the

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potential of the material to be used in several sensing applications [79].

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

**7. Final remarks**

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

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.

#### **5.2. Printable materials compatibility**

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

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