**2. Printing technologies**

continuously increasing. Today users' demands (for lower cost, flexible and smarter products) are a decisive factor for the selection of PE fabrication technologies, therefore, contributing to novel and better products. The interest on flexible electronic systems to be used, for example, on non-planar surfaces grew tremendously in recent years, [2] in areas such as aerospace and automotive, [3] biomedical, [4] robotics, [5] and health applications [6]. This is possible thanks to the combination of different polymeric materials (compared to traditional silicon substrates) with new coating and printing techniques able to work at temperatures compatible with the polymeric substrate, or even the manufacturing of non-planar surfaces otherwise impossible with old-fashioned fabrication techniques. The use of flexible polymers has many advantages compared to traditional hard substrates including: higher contact area, capability to fold/roll, lightweight, etc., therefore, they have a key role in the development of new conductive circuits. Thanks to better and flexible materials combined with PE, commercial applications diversity will continue to emerge. According to Markets and Markets latest report, the progress of flexible applications based on PE market will worth \$12.1B by 2022. According to Electronics.ca Publications, Printed organic & flexible electronics market will be worth over \$73B by 2027. Each technology is selected according to the type of electronic components or devices (e.g., small, thin, lightweight, flexible, inexpensive and disposable, etc.), the production cost and volume. The essential aspects for the success of any type of PE device is the processability, performance and long-term reliability [1] of the materials used [7]. The pastes, inks or coatings can be based both on organic and inorganic materials [7]. Inorganic inks normally contain metallic (e.g., copper, gold, silver, aluminum) nanoparticles dispersed in a retaining matrix and they are used, for example, in the fabrication of passive components and transistor electrodes [7]. Organic inks are based on organic materials, such as polymers (conductors, semiconductors and dielectrics). The inks based on high conductive polymers are employed in batteries, electromagnetic shields, capacitors, resistors and inductors, sensors, etc., while inks based on organic semiconductors are employed as active layers of active devices such as, Organic PhotoDiodes (OPDs), Organic Light Emitting Diodes (OLEDs), Organic Field-Effect Transistors (OFETs), organic solar cells (OSC), sensors, etc. [7]. Due to the wide range of printing technologies, the materials must meet certain requirements depending on the type

PE technologies can be divided in contact and non-contact techniques as shown on **Figure 1**:

• contact techniques (e.g., screen printing, flexography, gravure printing and soft lithogra-

• non-contact techniques (e.g., laser direct writing, aerosol printing, inkjet printing), where

phy), in which the printing plate is in direct contact with the substrate;

only the deposition material get in contact with the substrate.

of printing being performed and on the application.

48 Flexible Electronics

**Figure 1.** Printing technologies classification.

### **2.1. Contact printing technologies**

The contact printing technologies are the predominant printing processes in the current days. They involve high material waste and limitations around the resolution and range of the materials used (substrates, inks, solvents). Main contact printing technologies are described in the following.

#### *2.1.1. Screen printing*

Screen printing (SP) is a mature printing technique that may be performed in a planar system or in a roll-to-roll (R2R) process (**Figure 2**). The planar system uses a SP mesh, which is in direct contact with the substrate; the blade moves, distributes the ink and helps filling the mesh. The ink passes into the standard image in the mesh to the substrate and defines the final image. The substrates could be epidermis [8], paper, glass, metal [9], ceramic, [10] wood, textiles [10], polymers [10]. Webb et al. [11] describes a SP functional ink, comprising a combination of semiconducting acicular particles, electrically insulating nanoparticles and a base polymer ink, that exhibits pronounced pressure sensitive electrical properties for applications in sensing and touch sensitive surfaces. understanding the

In the R2R process, the squeegee is replaced by a roller and the ink and the blade are placed inside. The blade forces the ink through the mesh. The process is continuous, contrary to the planar system, allowing high speeds production, although rotary setup is expensive and hard to clean.

SP is a technology that has been often used for PE [12]. This technique produces large waste of production material (including the ink). The biggest limitation is reflected in the level of resolution. Also, the planar system speed is low in comparison to other conventional printing processes [13].

**Figure 2.** Schematic of contact printing techniques.

#### *2.1.2. Flexography*

The flexography is a R2R direct printing technology, where the final pattern stands out from the ink transfer. A ceramic anilox roller, covered with micro-cavities on its surface, allows the collection of ink, and then is transferred to the printing plate cylinder (**Figure 2**). A closed chamber supplies the ink to the anilox roller [13]. A doctor blade removes excess ink from the cylinder and prevents the output from the ink supply chamber. The printing plate continuously rotates in contact with the substrate ensuring a continuous high speed printing process.

than 20 μm size is required for electronic structures [15], and the parasitic capacitances are to be avoid; the proper layer deposition alignment, e.g., in electronic applications, repeating conductive film deposition is sometimes required in order to reduce sheet resistance. When it comes to R2R techniques (e.g., screen-printing, flexography and gravure printing) another level of complexity is added to these technologies. Also, frequent replacements of the gravure

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Soft lithography technology encompasses several printing techniques (**Figure 3**), such as microcontact printing (μCP) [20], replica molding (REM), micro transference molding (μTM), micromolding in capillaries (MIMIC), and solvent assisted micromolding (SAMIM) [21]. It provides a convenient, effective method for the manufacturing of high quality micro- and nanostructured systems [22]. In this set of technologies, an elastomeric (commonly of poly (dimethylsiloxane) (PDMS)) stamp or mold with patterned relief structures on its surface is used to transfer patterns and structures with feature sizes ranging from 30 nm to 100 μm [21]. Usually the master is prepared using either e-beam or photolithography. From this master, several stamps can be molded. The material of interest is deposited on the stamp and transferred on the substrate. However, soft lithography does not offer better economic advantages when compared to R2R printing techniques due to the rapid throughput [7]. The fabrication includes several manufacture steps with the involvement of photolithographic technology [5, 23]. Other challenges rely on a proper adjustment of the surface energies of substrates and inks for efficient transfer to the substrate to be printed, on common swelling of transferring materials, resulting on increased features size.

**Tables 1** and **2** summarize qualitatively the mechanisms, the process requirements, material and critical limitations of the contact printing technologies, highlighting their main features. These tables also provide the possibility of merging the different techniques in order to com-

bine technologies to overcome one technology limitation with another technology.

cylinders are needed, which adds a maintenance cost.

*2.1.5. Comparing contact printing technologies*

**Figure 3.** Major steps in soft lithography technologies.

*2.1.4. Soft lithography*

However, situations such as the Halo effect (patterns with excess of ink) occur due to the compression between the printing plate and the substrate, despite the low pressure applied. This leads to limitations on image size stability and resolution [13]. This technology is commonly used for the fabrication of on-label battery testers, drug delivery patches, printed batteries and other e-label applications [1]. Julin used flexography to produce flexible piezoelectric pressure sensors [14]. They investigated the suitability of flexography printing and new electrode materials in their manufacture, developing a flexo-printed piezoelectric PolyVinyliDene Fluoride (PVDF) pressure sensor. Although the sheet resistance of the fabricated samples presented high values and a lot of variability, the devices showed a non-uniform structure and some difficulties were reported on achieving a uniform pressure sensor.

#### *2.1.3. Gravure printing*

The gravure printing technology is the reverse process of flexography, where the image to be printed is negative (**Figure 2**). The ink is received directly by the ink supplier container or by an additional roller to the gravure plate, where the pattern image is located. A flexible metal blade removes the excess ink. The ink is transferred through capillary action from the small engraved cavities on the cylinder surface to the substrate. This technology is capable of producing high quality patterns in a cost-effective manner and is suitable for printing with inks of low viscosity, and high manufacturing speeds (up to 0.1 m/s [15]) can be achieved. A careful optimization of the process and of the materials is important because the final print quality is highly dependent upon:


Widely used in magazine production, gravure printing is also highly employed for certain electronics products such as medical Electrocardiography (ECG or EKG) pads and highvolume Radio-Frequency Identification devices (RFID) [1], Thin-Film-Transistors (TFT) [16], solar cells [18] and sensors [19]. However, this process presents two main limitations: the printing image is built from separate cells, and when printing a straight line, a jagged line is observed [13], which represents a major obstacle when high resolution is need, e.g., less than 20 μm size is required for electronic structures [15], and the parasitic capacitances are to be avoid; the proper layer deposition alignment, e.g., in electronic applications, repeating conductive film deposition is sometimes required in order to reduce sheet resistance. When it comes to R2R techniques (e.g., screen-printing, flexography and gravure printing) another level of complexity is added to these technologies. Also, frequent replacements of the gravure cylinders are needed, which adds a maintenance cost.

### *2.1.4. Soft lithography*

*2.1.2. Flexography*

50 Flexible Electronics

*2.1.3. Gravure printing*

important;

quality is highly dependent upon:

The flexography is a R2R direct printing technology, where the final pattern stands out from the ink transfer. A ceramic anilox roller, covered with micro-cavities on its surface, allows the collection of ink, and then is transferred to the printing plate cylinder (**Figure 2**). A closed chamber supplies the ink to the anilox roller [13]. A doctor blade removes excess ink from the cylinder and prevents the output from the ink supply chamber. The printing plate continuously rotates in contact with the substrate ensuring a continuous high speed printing process. However, situations such as the Halo effect (patterns with excess of ink) occur due to the compression between the printing plate and the substrate, despite the low pressure applied. This leads to limitations on image size stability and resolution [13]. This technology is commonly used for the fabrication of on-label battery testers, drug delivery patches, printed batteries and other e-label applications [1]. Julin used flexography to produce flexible piezoelectric pressure sensors [14]. They investigated the suitability of flexography printing and new electrode materials in their manufacture, developing a flexo-printed piezoelectric PolyVinyliDene Fluoride (PVDF) pressure sensor. Although the sheet resistance of the fabricated samples presented high values and a lot of variability, the devices showed a non-uniform structure and

The gravure printing technology is the reverse process of flexography, where the image to be printed is negative (**Figure 2**). The ink is received directly by the ink supplier container or by an additional roller to the gravure plate, where the pattern image is located. A flexible metal blade removes the excess ink. The ink is transferred through capillary action from the small engraved cavities on the cylinder surface to the substrate. This technology is capable of producing high quality patterns in a cost-effective manner and is suitable for printing with inks of low viscosity, and high manufacturing speeds (up to 0.1 m/s [15]) can be achieved. A careful optimization of the process and of the materials is important because the final print

• inks properties, i.e., its rheological behavior (viscosity), solvent evaporation rate and curing;

• feature dimensions on gravure cylinder for proper cell emptying capability [16] are very

Widely used in magazine production, gravure printing is also highly employed for certain electronics products such as medical Electrocardiography (ECG or EKG) pads and highvolume Radio-Frequency Identification devices (RFID) [1], Thin-Film-Transistors (TFT) [16], solar cells [18] and sensors [19]. However, this process presents two main limitations: the printing image is built from separate cells, and when printing a straight line, a jagged line is observed [13], which represents a major obstacle when high resolution is need, e.g., less

some difficulties were reported on achieving a uniform pressure sensor.

• proper cell spacing (1.06–1.4 μm) for print quality [16, 17];

• shear force in the printing mechanism [17].

Soft lithography technology encompasses several printing techniques (**Figure 3**), such as microcontact printing (μCP) [20], replica molding (REM), micro transference molding (μTM), micromolding in capillaries (MIMIC), and solvent assisted micromolding (SAMIM) [21]. It provides a convenient, effective method for the manufacturing of high quality micro- and nanostructured systems [22]. In this set of technologies, an elastomeric (commonly of poly (dimethylsiloxane) (PDMS)) stamp or mold with patterned relief structures on its surface is used to transfer patterns and structures with feature sizes ranging from 30 nm to 100 μm [21]. Usually the master is prepared using either e-beam or photolithography. From this master, several stamps can be molded. The material of interest is deposited on the stamp and transferred on the substrate. However, soft lithography does not offer better economic advantages when compared to R2R printing techniques due to the rapid throughput [7]. The fabrication includes several manufacture steps with the involvement of photolithographic technology [5, 23]. Other challenges rely on a proper adjustment of the surface energies of substrates and inks for efficient transfer to the substrate to be printed, on common swelling of transferring materials, resulting on increased features size.

#### *2.1.5. Comparing contact printing technologies*

**Tables 1** and **2** summarize qualitatively the mechanisms, the process requirements, material and critical limitations of the contact printing technologies, highlighting their main features. These tables also provide the possibility of merging the different techniques in order to combine technologies to overcome one technology limitation with another technology.

**Figure 3.** Major steps in soft lithography technologies.

#### **2.2. Non-contact printing technologies**

Compared to the contact printing technologies, the non-contact has the advantage of the substrate only getting in contact with the deposition material. This lowers the risks of contamination, of damaging the substrate and the patterns alignment is more accurate. This last issue is an indispensable functionality to pattern multilayered devices. For non-contact printing techniques there is no need for physical mask of the to-be-printed image, only requiring a digital image, simplifying the switching process without no additional cost. However, the non-contact technologies also stumble upon some difficulties when completing multilayered devices [23] are needed. They work with all kinds of substrates, such as, wood, glass, metals and most interesting, rubbers, polymers, which require low processing temperatures, and risk to be damaged and deformed when subjected to thermal stresses and high temperature processes. Main non-contact printing techniques are described in the following.

#### *2.2.1. Laser direct writing*

Laser direct-writing (LDW) techniques enable the realization of 1D to 3D structures by laserinduced deposition of metals, semiconductors, polymers and ceramics, without using masks and without physical contact between a tool or nozzle and the substrate material. Operated by a computer, the laser pulses are manipulated to control the composition, structure, and properties of individual three-dimensional volumes of materials, across length-scales spanning six orders of magnitude from nanometers to millimeters [25]. The ability to process complex or delicate material systems and the achieved resolutions enable LDW to fabricate structures that are not possible to generate using other techniques. Within LDW, there are three writing

**viscosity (Pa.s)**

**Print thickness (μm)**

[12, 13]

[13, 24]

0.01–0.1 0.17–8 30–80 13.9–23

~0.10 0.18–0.7 0.03–100 22–80

LDW

0.1–10 0.02–100

0.01–1.1 [14] 0.02–12

**Resolution (μm)**

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**Surface tension (mN/m)**

53

**i.** LDW addition (LDW+) technique, where the material can be deposited from gaseous, liquid and solid precursors (e.g., Laser Chemical Vapor Deposition (LCVD)) or by transfer, by laser beam, from an optically transparent support onto a parallel substrate (e.g., Laserinduced forward transfer (LIFT) [7], **Figure 4**). These techniques entail high cost due to the sophisticated equipment (e.g., reaction chamber associated with vacuum equipment); it does not allow to deposit organic substrates; and it can only print on flat substrates,

**ii.** LDW subtraction (LDW-) technique, where the material is removed by ablation (e.g., photochemical, photothermal, or photophysical ablation [26], laser scribing, cutting, drilling, or etching [27]). An industrial application example is the high-resolution manufacturing

**iii.** LDW Modification (LDWM) technique, where the material is modified thermally or chemically [25] (e.g., Laser-Enhanced Electroless Plating, LEEP). The substrate is submerged in a chemical solution that contains the metallic ions required for the deposition. A laser beam is responsible for local temperature rise, decomposing the liquid and leading to the deposition of a metallic layer on the substrate surface. The main disadvantage

Aerosol jet printing (**Figure 4**), also known as Maskless Mesoscale Materials Deposition (M3D) is another material deposition technology for printed electronics [28] developed by Optomec [29]. The ink (solutions and nanoparticle suspensions based on metals, alloys, ceramics, polymers, adhesives or biomaterials) is placed into an atomizer where it aerosolizes in liquid

techniques:

parallel to the support material.

**Technique Solution types Solution** 

UV or electron beam curable

Screen printing Water based, solvent based,

Flexography Water based, solvent based, UV curable

Gravure printing Water based, solvent based, UV curable

Soft lithography Water based, solvent based, UV curable

**Table 2.** Comparison between main contact printing techniques.

and texturing of stents or other implantable biomaterials.

relies in its disability to create 3D structures.

*2.2.2. Aerosol jet printing*


**Table 1.** Summary of contact printing techniques: mechanisms, features and main challenges.


**Table 2.** Comparison between main contact printing techniques.

**2.2. Non-contact printing technologies**

52 Flexible Electronics

*2.2.1. Laser direct writing*

there

Screen printing [13–15]

Gravure printing [13, 15, 17, 24]

Soft lithography [20–22]

Compared to the contact printing technologies, the non-contact has the advantage of the substrate only getting in contact with the deposition material. This lowers the risks of contamination, of damaging the substrate and the patterns alignment is more accurate. This last issue is an indispensable functionality to pattern multilayered devices. For non-contact printing techniques there is no need for physical mask of the to-be-printed image, only requiring a digital image, simplifying the switching process without no additional cost. However, the non-contact technologies also stumble upon some difficulties when completing multilayered devices [23] are needed. They work with all kinds of substrates, such as, wood, glass, metals and most interesting, rubbers, polymers, which require low processing temperatures, and risk to be damaged and deformed when subjected to thermal stresses and high temperature

Laser direct-writing (LDW) techniques enable the realization of 1D to 3D structures by laserinduced deposition of metals, semiconductors, polymers and ceramics, without using masks and without physical contact between a tool or nozzle and the substrate material. Operated by a computer, the laser pulses are manipulated to control the composition, structure, and properties of individual three-dimensional volumes of materials, across length-scales spanning six orders of magnitude from nanometers to millimeters [25]. The ability to process complex

> Hard to clean; solvents deteriorate mask patterns; high resolution of uniform line patterns are not possible under 30 μm; Unfeasible use of low viscosity inks to prevent spreading and bleed out;

> Halo effect (patterns with excess of ink) due to printing plate compression to the substrate, despite the low pressure applied; marbling effect;

Cylinder life and high cost; demanding and careful optimization of the process (several variables) influence final print quality representing a major obstacle where high resolution is required (e.g., PE).

Proper adjustment of the surface energies for efficient transfer to the substrate to be printed; common swelling of transferring materials, resulting an increased features size; pattern reproduction and resolution is a challenge due to

used forces on stamp; costly solution.

complex multi layers alignment.

material wastage.

processes. Main non-contact printing techniques are described in the following.

**Print technique Mechanisms and features Challenges**

Flexography [13] High speed printing process; low-cost

gravure.

Most used and mature printing technique; planar or R2R system; speed and versatile.

patterns plate; high flexibility and low pressure printing; better vertical and horizontal pattern quality compared to

High quality patterns in a cost-effective manner; high speed; low viscosity inks.

Encompasses several printing techniques (μCP, REM, μTM, MIMIC, SAMIM); fabrication of micro-and nanostructures of high quality; convenient, effective method; mostly used by the biological science area.

**Table 1.** Summary of contact printing techniques: mechanisms, features and main challenges.

or delicate material systems and the achieved resolutions enable LDW to fabricate structures that are not possible to generate using other techniques. Within LDW, there are three writing techniques:


#### *2.2.2. Aerosol jet printing*

Aerosol jet printing (**Figure 4**), also known as Maskless Mesoscale Materials Deposition (M3D) is another material deposition technology for printed electronics [28] developed by Optomec [29]. The ink (solutions and nanoparticle suspensions based on metals, alloys, ceramics, polymers, adhesives or biomaterials) is placed into an atomizer where it aerosolizes in liquid

of living tissues [30], as well as for building 3D electric circuits [40], MEMS [34], and sensors [37]. No special processing conditions are needed. IPT stands out for being a one-step process, with a simple operating principle, reduced number of manufacturing steps, with the possibility of using low cost raw materials [41]. Thickness around nanometer range is easily achieved by increasing the electric field value along with the distance between the print head and substrate. The used inks have a particular set of physical specifications in particular its viscosity, the superficial tension [36], and the amount of humectant (10–20%) [42]. Sometimes, modifications on the ink viscosity, concentration and solvent system are necessary for proper droplet injection without blocking the nozzle. Although the low process velocity and possible clogging of the nozzles, presenting a challenge to the industrial production, IPT becomes ideal for laboratory research providing innovative fabrication, high quality and low cost productions.

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**Tables 3** and **4** summarize the mechanisms, the process requirements, material and critical

The IPT can operate in two different modes: Continuous InkJet (CIJ) and Drop-On-Demand (DoD) [36]. The method for controlling the droplet movement is quite different between the

In the CIJ system, the ejection of the droplet is continuous in all nozzles of the printer. In the traditional CIJ, a piezoelectric transducer is coupled to the print head to provide a periodic excitation [38]. After leaving the nozzle, an electric field determines and controls the trajectory

substrate

clogging

of the localized bonding layer

High cost equipment; not possible to deposit organic substrates, printing only on flat substrates, parallel to the

Droplet carrier creates a cloud of powder in surrounding printed area; sheath gas creates a localized crystallization/ solidification phase at the trace pattern reducing the quality

Slow printing speed compared to other techniques; nozzle

of the droplet to the desired position on the substrate (**Figure 5**).

**Mechanisms and features Challenges**

1D to 3D structures; nm to mm magnitude; no mask; three writing techniques (LDW+, LDW−, LDWM)

Complex design could be printed; complex conformal surfaces; many materials and substrates; non-planar; low-temperature processing, local

of droplets; droplets ejection through different actuation phenomena; all type of substrates; low material wastage;

**Table 3.** Summary of non-contact printing techniques and challenges.

IPT [30–42] Low viscosity; deposition of many types

environmentally friendly

limitations of the non-contact printing technologies, highlighting their main features.

*2.2.4. Comparing non-contact printing techniques*

**2.3. IPT mode technology systems**

*2.3.1. Continuous inkjet (CIJ) mode*

sintering

two systems.

**Printing technique**

LDW [7, 25–27]

Aerosol [28–43]

**Figure 4.** Schematic of non-contact printing.

particles of diameter between 20 nm and 5 μm, depending on the ink viscosity. Then, the ink is transported into the deposition head by a nitrogen flow, the aerosol being focused by jet stream onto the substrate. As being a low-temperature process, many materials and substrates can be handled by Aerosol Jet printing. The technique is also scalable to support high volume production needs. It is suitable for non-planar capability and complex designs could be printed (e.g., displays, thin film transistors, TFT, and solar cells) [29]. Complex conformal surfaces (3D printed electronics) are also possible, thanks to the ability to control the position in z-direction of the writing head over the substrate. This technique also stumbles upon some difficulties. The droplet carrier creates a cloud of powder in the surroundings of the print area. The sheath gas creates a localized crystallization/solidification phase at the trace pattern, reducing the localized bonding layer quality.

#### *2.2.3. Inkjet printing*

Inkjet printing is new technology with a grown interest from the scientific community and is considered to be in an early stage of development [2]. In Inkjet printing technology (IPT), a content stored in a digital format is transferred by a direct deposition (from small openings in print-heads, without the use of masks and without contact between the print-head and the substrate) of droplet fluid or powder, proteins or minerals [30, 31], conductive polymers [32], nanoparticles [33, 34] and a wide range of materials (e.g., bioactive fluids, which cannot tolerate exposure to photolithography and etching chemicals present in conventional techniques [32]). Under the print-head ejection, the gravity force, and air resistance, the ink is project into a specified position of the substrate creating the printing patterns (**Figure 4**).

In the case of fluids, it dries through the evaporation of the solvent, by chemical changes (e.g., cross-linking of polymers) or crystallization. Eventually, a post-processing treatment is required, as thermal annealing or sintering [35]. When compared to other deposition methods, IPT is adaptable for patterning on a high variety (rigid or flexible, smooth or rough surfaces [2, 36]) of substrates (glass, plastic [36], paper [37], textile [38], etc.), with low consumption of raw materials [36] and low levels of waste production harmful to the environment [2]. IPT is intended for a wide range of applications: transducers [32], transistors [39], structural polymers and ceramics [30], biomimetic and biomedical materials [31], printed scaffolds for growth of living tissues [30], as well as for building 3D electric circuits [40], MEMS [34], and sensors [37]. No special processing conditions are needed. IPT stands out for being a one-step process, with a simple operating principle, reduced number of manufacturing steps, with the possibility of using low cost raw materials [41]. Thickness around nanometer range is easily achieved by increasing the electric field value along with the distance between the print head and substrate. The used inks have a particular set of physical specifications in particular its viscosity, the superficial tension [36], and the amount of humectant (10–20%) [42]. Sometimes, modifications on the ink viscosity, concentration and solvent system are necessary for proper droplet injection without blocking the nozzle. Although the low process velocity and possible clogging of the nozzles, presenting a challenge to the industrial production, IPT becomes ideal for laboratory research providing innovative fabrication, high quality and low cost productions.

#### *2.2.4. Comparing non-contact printing techniques*

**Tables 3** and **4** summarize the mechanisms, the process requirements, material and critical limitations of the non-contact printing technologies, highlighting their main features.

#### **2.3. IPT mode technology systems**

particles of diameter between 20 nm and 5 μm, depending on the ink viscosity. Then, the ink is transported into the deposition head by a nitrogen flow, the aerosol being focused by jet stream onto the substrate. As being a low-temperature process, many materials and substrates can be handled by Aerosol Jet printing. The technique is also scalable to support high volume production needs. It is suitable for non-planar capability and complex designs could be printed (e.g., displays, thin film transistors, TFT, and solar cells) [29]. Complex conformal surfaces (3D printed electronics) are also possible, thanks to the ability to control the position in z-direction of the writing head over the substrate. This technique also stumbles upon some difficulties. The droplet carrier creates a cloud of powder in the surroundings of the print area. The sheath gas creates a localized crystallization/solidification phase at the trace pattern,

Inkjet printing is new technology with a grown interest from the scientific community and is considered to be in an early stage of development [2]. In Inkjet printing technology (IPT), a content stored in a digital format is transferred by a direct deposition (from small openings in print-heads, without the use of masks and without contact between the print-head and the substrate) of droplet fluid or powder, proteins or minerals [30, 31], conductive polymers [32], nanoparticles [33, 34] and a wide range of materials (e.g., bioactive fluids, which cannot tolerate exposure to photolithography and etching chemicals present in conventional techniques [32]). Under the print-head ejection, the gravity force, and air resistance, the ink is project into

In the case of fluids, it dries through the evaporation of the solvent, by chemical changes (e.g., cross-linking of polymers) or crystallization. Eventually, a post-processing treatment is required, as thermal annealing or sintering [35]. When compared to other deposition methods, IPT is adaptable for patterning on a high variety (rigid or flexible, smooth or rough surfaces [2, 36]) of substrates (glass, plastic [36], paper [37], textile [38], etc.), with low consumption of raw materials [36] and low levels of waste production harmful to the environment [2]. IPT is intended for a wide range of applications: transducers [32], transistors [39], structural polymers and ceramics [30], biomimetic and biomedical materials [31], printed scaffolds for growth

a specified position of the substrate creating the printing patterns (**Figure 4**).

reducing the localized bonding layer quality.

**Figure 4.** Schematic of non-contact printing.

54 Flexible Electronics

*2.2.3. Inkjet printing*

The IPT can operate in two different modes: Continuous InkJet (CIJ) and Drop-On-Demand (DoD) [36]. The method for controlling the droplet movement is quite different between the two systems.

#### *2.3.1. Continuous inkjet (CIJ) mode*

In the CIJ system, the ejection of the droplet is continuous in all nozzles of the printer. In the traditional CIJ, a piezoelectric transducer is coupled to the print head to provide a periodic excitation [38]. After leaving the nozzle, an electric field determines and controls the trajectory of the droplet to the desired position on the substrate (**Figure 5**).


**Table 3.** Summary of non-contact printing techniques and challenges.


**Table 4.** Comparison between main non-contact printing techniques.

Within this technology, the droplets can be diverted by binary or multiple deflection systems. On the binary systems, the droplets are directed to a single pixel location on the substrate or to the gutter, for later recycling of the ink. In the multiple deflection system, the droplets are charged and deflected to the substrate at different levels, this way creating multiple pixels. Hertz et al. [43] used the binary CIJ and developed a method consisting in the formation of a layer of irregularly droplets of ink size. In the Hertz method, the droplets are dispersed in a straight line to a gutter so as to converge into the recirculation system. This method also introduced a new procedure and methodology relatively to the use of volatile solvents that allows a quick drying of the ink and the adhesion to the substrate materials. The CIJ system benefits from the ability to combine the printing speed (on the order of 25 m/s) with the possibility of achieving extended distances and the ability to divert droplets independent of gravity [44].

*2.3.3. Main influencing factors*

**Figure 5.** IPT mode technology systems.

ability of the printed layers over the long term.

type of printing technology and final PE application.

**3. Printable materials for PE**

The control of ink drop, the print-head temperature, the sintering or cure of the ink, and the printing control of each layer are key parameters to ensure the quality of a multilayer printed structure. Also important is to evaluate the properties of the substrate, such as, the service temperature, its barrier properties against humidity, electrical, optical, mechanical and chemical properties. Equally important is consider the receptivity of the ink by the substrate or with previously printed layers, in the case where a different ink has been used. The droplet size can vary depending on the interactions between the ink and the substrate. The droplet size sets the width of the printed line, establishing the pattern space and the electric design limits, and defines the final specifications of the printed pattern and application system (e.g., resolution, bandwidth in the case of a PE). Thereby, during the manufacturing step, the printed pattern characteristics are dependent on the materials and their interaction (i.e., the properties of the ink must be chosen in advance to understand its behavior during and after the printing process over a given substrate). Sintering and cure of conductive materials are essential because it defines its chemical, electrical and physical performance and the reli-

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The printable materials are selected depending on the type of substrate, the type of ink, the

The conductive inks are gathering increasingly attention over the past two decades, and are revolutionizing the industry. Elected due to their attributes, such as, conductivity, suitability for printing substrates, its processing simplicity and mechanical flexibility, but also due to its ability to assign new properties, capabilities and complex functionalities. These emerging inks are penetrating the market with an opportunity to reach \$400 m by 2027, according to IDTechEX report "Condutive Ink Market 2017-2027". A large variety of materials, organic and inorganic, conductors and semiconductors, have been explored for electronics applications. The most common types of inks are water, oil or solvents based. The general form of the ink consist of a mixture of compounds (pigments or dyes, resins, solvents, fillers, humectant and

CIJ technology is typically used for large industrial productions of bar codes and labels of food products or medicines. This process can be comparatively fast, with the advantage of circumscribing large printing areas with a single pass and its printing heads have a long duration. The droplet size can reach values such as 20 μm, with a standard size of 150 μm [45]. However, in the manufacture of electronic products, the CIJ produces droplets of inadequate resolution due to the long distance between the print-head and the substrate [13]. Other less positive factors are the high cost of initial investment in such equipment, the lower resolution compared to some DoD systems, the need to use low viscosity electrolyte inks (in the range of 3–6 mPa.s), resulting in some final ink waste [46]. Other constructed <sup>a</sup>

#### *2.3.2. Drop-on-demand (DoD) mode*

In the DoD system, the print-head ejects a single droplet only when activated (**Figure 5**). The printer is based on several injector nozzles in the print-head and, at each pulse, the droplets are ejected in parallel to each other. The image is constructed from successive pulses, which largely differentiates from CIJ. The DoD is a high speed method, of high scalability that uses high frequency multiple nozzles. The method that is used to generate these pulses defines the subcategories of the primary DoD, namely: the acoustic, the electrostatic, the thermal, the piezoelectric, and an additional method, sometimes controversial, the MEMS [47] method. This last method is more related to the fabrication process, since the drop generation is based on thermal or piezo print-heads.

Printing Technologies on Flexible Substrates for Printed Electronics http://dx.doi.org/10.5772/intechopen.76161 57

**Figure 5.** IPT mode technology systems.

#### *2.3.3. Main influencing factors*

Within this technology, the droplets can be diverted by binary or multiple deflection systems. On the binary systems, the droplets are directed to a single pixel location on the substrate or to the gutter, for later recycling of the ink. In the multiple deflection system, the droplets are charged and deflected to the substrate at different levels, this way creating multiple pixels. Hertz et al. [43] used the binary CIJ and developed a method consisting in the formation of a layer of irregularly droplets of ink size. In the Hertz method, the droplets are dispersed in a straight line to a gutter so as to converge into the recirculation system. This method also introduced a new procedure and methodology relatively to the use of volatile solvents that allows a quick drying of the ink and the adhesion to the substrate materials. The CIJ system benefits from the ability to combine the printing speed (on the order of 25 m/s) with the possibility of achieving extended distances and the ability to divert droplets independent of gravity [44].

**viscosity (Pa.s)**

LDW Solid film (donor substrate) – >10 ca. 0.7 –

**Thickness (μm)**

0.001–1 >0.1 10–250 –

0.002–0.1 0.01–0.5 15–100 15–35

**Resolution (μm)**

**Surf. tension (mN/m)**

CIJ technology is typically used for large industrial productions of bar codes and labels of food products or medicines. This process can be comparatively fast, with the advantage of circumscribing large printing areas with a single pass and its printing heads have a long duration. The droplet size can reach values such as 20 μm, with a standard size of 150 μm [45]. However, in the manufacture of electronic products, the CIJ produces droplets of inadequate resolution due to the long distance between the print-head and the substrate [13]. Other less positive factors are the high cost of initial investment in such equipment, the lower resolution compared to some DoD systems, the need to use low viscosity electrolyte inks (in the range of

In the DoD system, the print-head ejects a single droplet only when activated (**Figure 5**). The printer is based on several injector nozzles in the print-head and, at each pulse, the droplets are ejected in parallel to each other. The image is constructed from successive pulses, which largely differentiates from CIJ. The DoD is a high speed method, of high scalability that uses high frequency multiple nozzles. The method that is used to generate these pulses defines the subcategories of the primary DoD, namely: the acoustic, the electrostatic, the thermal, the piezoelectric, and an additional method, sometimes controversial, the MEMS [47] method. This last method is more related to the fabrication process, since the drop generation is based

3–6 mPa.s), resulting in some final ink waste [46].

**Technique Solution types Solution** 

suspensions based on metals, alloys, ceramics, polymers, adhesives or

**Table 4.** Comparison between main non-contact printing techniques.

Aerosol Solutions and nanoparticle

56 Flexible Electronics

biomaterials

IPT Water based, solvent based, UV curable

*2.3.2. Drop-on-demand (DoD) mode*

on thermal or piezo print-heads.

The control of ink drop, the print-head temperature, the sintering or cure of the ink, and the printing control of each layer are key parameters to ensure the quality of a multilayer printed structure. Also important is to evaluate the properties of the substrate, such as, the service temperature, its barrier properties against humidity, electrical, optical, mechanical and chemical properties. Equally important is consider the receptivity of the ink by the substrate or with previously printed layers, in the case where a different ink has been used. The droplet size can vary depending on the interactions between the ink and the substrate. The droplet size sets the width of the printed line, establishing the pattern space and the electric design limits, and defines the final specifications of the printed pattern and application system (e.g., resolution, bandwidth in the case of a PE). Thereby, during the manufacturing step, the printed pattern characteristics are dependent on the materials and their interaction (i.e., the properties of the ink must be chosen in advance to understand its behavior during and after the printing process over a given substrate). Sintering and cure of conductive materials are essential because it defines its chemical, electrical and physical performance and the reliability of the printed layers over the long term.
