Open access peer-reviewed chapter

Printing Technologies on Flexible Substrates for Printed Electronics

Written By

Sílvia Manuela Ferreira Cruz, Luís A. Rocha and Júlio C. Viana

Submitted: December 4th, 2017 Reviewed: March 2nd, 2018 Published: July 25th, 2018

DOI: 10.5772/intechopen.76161

Chapter metrics overview

3,642 Chapter Downloads

View Full Metrics


Printing technologies have been demonstrated to be highly efficient and compatible with polymeric materials (both inks and substrates) enabling a new generation of flexible electronics applications. Conductive flexible polymers are a new class of materials that are prepared for a wide range of applications, such as photovoltaic solar cells, transistors molecular devices, and sensors and actuators. There are many possible printing techniques. This chapter provides an opportunity to review the most common printing techniques used at the industrial level, the most commonly used substrates and electronic materials, giving an overall vision for a better understanding and evaluation of their different features. Several technological solutions (contact/noncontact) and its critical challenges are also presented. Inkjet Printing Technology (IPT) has been receiving a great attention and therefore higher focus is given to this technology. An overview of IPT is presented to evidence its importance and potential as a key-technology on the research field for printed electronics development, as well as on large scale industrial manufacturing. A background and a review on prior work are presented along with used materials, developed applications and potential of IPT technology. The main features of the different printing technologies, advantages and main challenges are also compared.


  • printing techniques
  • flexible polymers
  • conductive inks

1. Introduction

When an electrical device is created through a printing process, it is designated Printed Electronics (PE). Over 20 years, the manufacturing industry has been using various printing techniques to produce, e.g., antennas, sensors, membrane switches, etc. [1]. This list is 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 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 of printing being performed and on the application.

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

  • non-contact techniques (e.g., laser direct writing, aerosol printing, inkjet printing), where only the deposition material get in contact with the substrate.

Figure 1.

Printing technologies classification.

This chapter reviews simultaneously the most used techniques at the research and industrial level, substrates and electronic materials for an overall vision and a better understanding and evaluation of its different features. Their main features and examples of PE applications are discussed with greater focus on Inkjet Printing Technology.


2. Printing technologies

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.

Figure 2.

Schematic of contact printing techniques.

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].

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.

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:

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

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

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

  • shear force in the printing mechanism [17].

Widely used in magazine production, gravure printing is also highly employed for certain electronics products such as medical Electrocardiography (ECG or EKG) pads and high-volume 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

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.

Figure 3.

Major steps in soft lithography technologies.

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.

Print techniqueMechanisms and featuresChallenges
Screen printing [13, 14, 15]Most used and mature printing technique; planar or R2R system; speed and versatile.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; material wastage.
Flexography [13]High speed printing process; low-cost patterns plate; high flexibility and low pressure printing; better vertical and horizontal pattern quality compared to gravure.Halo effect (patterns with excess of ink) due to printing plate compression to the substrate, despite the low pressure applied; marbling effect; complex multi layers alignment.
Gravure printing [13, 15, 17, 24]High quality patterns in a cost-effective manner; high speed; low viscosity inks.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).
Soft lithography [20, 21, 22]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.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.

Table 1.

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

TechniqueSolution typesSolution viscosity
Print thickness (μm)Resolution (μm)Surface tension (mN/m)
Screen printingWater based, solvent based, UV or electron beam curable0.1–100.02–100
[12, 13]
FlexographyWater based, solvent based, UV curable0.01–0.10.17–830–8013.9–23
Gravure printingWater based, solvent based, UV curable0.01–1.1 [14]0.02–12
[13, 24]
Soft lithographyWater based, solvent based, UV curable~0.100.18–0.70.03–10022–80

Table 2.

Comparison between main contact printing techniques.

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 laser-induced 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 techniques:

  1. 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., Laser-induced 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, parallel to the support material.

  2. 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 and texturing of stents or other implantable biomaterials.

  3. 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 relies in its disability to create 3D structures.

Figure 4.

Schematic of non-contact printing.

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 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.

Printing techniqueMechanisms and featuresChallenges
LDW [7, 25, 26, 27]1D to 3D structures; nm to mm magnitude; no mask; three writing techniques (LDW+, LDW−, LDWM)High cost equipment; not possible to deposit organic substrates, printing only on flat substrates, parallel to the substrate
Aerosol [28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43]Complex design could be printed; complex conformal surfaces; many materials and substrates; non-planar; low-temperature processing, local sinteringDroplet 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 of the localized bonding layer
IPT [30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42]Low viscosity; deposition of many types of droplets; droplets ejection through different actuation phenomena; all type of substrates; low material wastage; environmentally friendlySlow printing speed compared to other techniques; nozzle clogging

Table 3.

Summary of non-contact printing techniques and challenges.

TechniqueSolution typesSolution viscosity (Pa.s)Thickness (μm)Resolution (μm)Surf. tension (mN/m)
LDWSolid film (donor substrate)>10ca. 0.7
AerosolSolutions and nanoparticle suspensions based on metals, alloys, ceramics, polymers, adhesives or biomaterials0.001–1>0.110–250
IPTWater based, solvent based, UV curable0.002–0.10.01–0.515–10015–35

Table 4.

Comparison between main non-contact printing techniques.

2.3. IPT mode technology systems

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).

Figure 5.

IPT mode technology systems.

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].

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].

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.

2.3.3. Main influencing factors

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.


3. Printable materials for PE

The printable materials are selected depending on the type of substrate, the type of ink, the type of printing technology and final PE application.

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 additives), in liquid or solid state, with specific proprieties adapted to the printing technology characteristic, such as viscosity, surface tension, etc., to be easily printed in a large variety of substrates. What makes conductive inks electrically conductive is the fact that it contains in the composition conductive nanoscale particles. The incorporation of conductive polymers [48], carbon (C) [49] or metallic particles (e.g., silver (Ag) [8, 36], copper (Cu) [50], and gold (Au) [51]) are the most common selections. Table 5 shows the resistivity of the bulk metal particles and the sintered metal ink form. Commonly, the metallic nanoparticles are stabilized in ink solutions by organic ligand shells, i.e., the nanoparticles are encapsulated with an organic material, called a capping agent, to form a uniform and stable dispersion, preventing particles agglomeration. This capping agent can be removed after printing through curing or sintering to allow physical contact between nanoparticles, forming continuous connectivity, i.e., a percolation path for electrical conductivity. Thus, sintering consist on welding the particles to each other below their melting point [2], and this particle welding could be achieved by exposure of the printed pattern to laser sintering [52], to microwave radiation [53], by applying an electrical voltage [54], by a chemical agent at room temperature (RT sintering) [55], or, the most conventional approach, by heating (thermal sintering) [33, 56]. In the case of thermal sintering, the temperature (typically between 100 to 400°C) must be below the softening temperature of the substrate. The presence of a few nanometers organic layer between the conductive particles is enough to block the movement of electrons from one particle to the other [33], thus reducing electrical conductivity. If this happens, the removal of this organic layer is required at high temperatures. For this reason, the sintering temperature of the nanoparticle based inks has extreme importance in plastic electronic applications, where materials, such as polyethylene terephthalate [56] and polycarbonate [56], are widely used as substrates, but have low Tg (98 and 148°C, respectively). The electrical conductivity of a printed nanoparticles based ink layer also depends on the shape and size of the nanoparticles. The amount of sintering temperature and time required depend upon how easy the organic encapsulation breaks, the particle dimensions and upon the thickness of the ink film. The smaller the particle size (2–10 nm) the lower the temperature required to sinter the particles, the short is the process and a higher electrical conductivity is achieved. Typically, the nanoparticle loading inks is higher than 20 wt%. Metal nanoparticles hold the highest electrical conductivity, although, the use of the above categorized precious metals hardly fits in the so called low cost PE.

Pure state (Ω.m)1.59 × 10−81.68 × 10−82.44 × 10−8
Printed ink (μΩ.cm)*10–505–78

Table 5.

Metal resistivity [33].

Dependent on sintering temperature and time-higher temperature.

Conductive polymers are classified into two different categories: extrinsically or intrinsically conductive polymers. The extrinsically conductive polymers normally involve a blend of conductive or nonconductive polymers, and a highly conductive additive (e.g., metallic particles) suspended in the polymer matrix [57], meaning that they are extrinsically enhanced to be conductive. Relatively to the intrinsically conductive polymer, they consist simply in a network of alternating single and double carbon bonds. It’s this alternation of bonds that produces conjugated π-bonds, resulting in an intrinsically conductive material [58]. Polyacetylene (PAc), Polyaniline (PAni), polypyrrole (PPy), polyphenylene vinylene (PPV), polythiophene (PTh) are intrinsically conductive polymers (Figure 6).

Figure 6.

Chemical structure of conductive polymers.

Within the intrinsically conductive materials, the regioregular PTh has a tremendous potential for applications in flexible organics electronics because of its low cost and specific properties, such us, solubility (thanks to the three-substituents alkyl-chain in the PTh core [59]), spectroscopic and electronic properties, low-temperature process [60], highly ordered structure and semi-crystallinity state in its solid states [61], regioregular compatibility to large-area fabrications and industrial mass production technologies. In the chemical structure of the regioregular PThs, the backbone of the polymer is formed by thiophene rings and a chemical side-chain group can be attached on each thiophene ring along the polymer (Figure 7). An end-group or a secondary copolymer chain can be added to each end of the PTh.

Figure 7.

Schematic diagram of a regioregular polythiophene based polymers.

Within the PTh and its derivatives, the poly(3-hexylthiophne) (P3HT) and poly (3,4-ethylenedioxythiophene) (PEDOT) [62] are the most well-known. The P3HT is a reference material in organic electronic, physics and chemistry to which any new p-type or donor conjugate molecule should be compared and evaluated. The PEDOT is the most widely used [63] intrinsically conductive polymer. PEDOT stands out for its high transparency [64] when deposited in thin oxidized films, high electrical conductivity [64], very high chemical stability in the oxidized state, processability and simplicity of production [65]. All these features make them suitable for several printing technologies, such as, spin coating [66], screen printing [67] and inkjet printing [68]. These unique properties make intrinsically conductive polymers excellent for various applications, such as, electrochromic devices [69], sensors [60], biosensors [68], actuators [70], capacitors [70], and photovoltaic cells [70], thin film diode [71]; organic thin film transistors (OTFTs) [72], photodiodes, Organic Field-Effect Transistor (OFETs) [52], organic light-emitting diodes (OLEDs) [73], etc., with a growing interest in PE due to its relatively low cost [74].


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.

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.

Tg (°C)155–27070–110145120–155−12580
Tm (°C)250–452115–258115–160269180
Density (g/cm3)1.36–1.431.391.20–1.221.361.031.18
Vol.Res.(Ω.cm)1.5 × 10171.0 × 10191012–10141051.2 × 10143.0 × 1014
Modulus (MPa)2.5 × 1032–4.1 × 1032.0–2.6 × 1030.1–0.5 × 10317
WorkTemp. (°C)Up to 400−50 to 150−40 to 130−45 to 200130
CTE (ppm/°C)8–2015–337520310153
Water absorption (%)1.3–3.00.4–0.60.16–0.350.3–0.4>0.10.2
Solvent resistanceGoodGoodPoorGoodPoorGood
Dimensional stabilityFairGoodFairGoodGoodGood-

Table 6.

Comparison between flexible polymeric substrates.

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


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.

Flexible substratesPrintable inksEquipment
  • Flexible substrates encapsulation

  • Cost effective barrier encapsulation material

  • Scalability to large area (e.g., OLEDs)

  • Adhesion

  • Long time reliability

  • Development of new inks formulation

  • Adhesion

  • Scalability to large area

  • Lifetime and stability

  • Appropriate, affordable

  • High volumes

  • Resolution

Table 7.

Main challenges.

5.1. Compatibility between printable material and substrate

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:

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

  • Conductive ink properties (chemical composition, rheological behavior, the rate of solvent evaporation, etc.).

  • The superficial tension (ST) of the ink and the SE of the substrate that will receive the ink, i.e., the difference between them.

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

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.

Figure 8.

Ink behavior on a substrate.

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 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].


6. Applications of IPT to flexible PE

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


7. Final remarks

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.



TSSiPRO – Technologies for Sustainable and Innovative Products, NORTE-01-0145-FEDER-000015, supported by the NORTE 2020, under the terms of Notice of Appeal No. NORTE-45-2015-2102 “Structured R&D&I Projects”, framed in the IPC/i3N Research Unit.


  1. 1. Adcock T, Fenner D. Printed Electronics Traditional Technology Addresses Today’s Smaller, Faster, Lower Cost Requirements [Internet]. 2012. Available from: [Accessed: Feb 20, 2018]
  2. 2. Nir M et al. Electrically conductive inks for inkjet printing. In: Magdassi S, editor. Handbook of Chemistry of Inkjet Inks. Jerusalem: World Scientific Publishing; 2010. pp. 225-254. DOI: 10.1142/9789812818225_0012
  3. 3. Engel J et al. Flexible multimodal tactile sensing system for object identification. In: Proceedings of the IEEE Sensors; Oct 22-25, 2006. Exco, Daegu, Korea: IEEE; 2007. pp. 563-566
  4. 4. Chiang C-C, Lin C-C K, Ju M-S. An implantable capacitive pressure sensor for biomedical applications. Sensors Actuators A: Physical. 2007;134:382-388. DOI: 10.1016/j.sna.2006.06.007
  5. 5. Cheng MY, Lin CL, Yang YJ. Tactile and shear stress sensing array using capacitive mechanisms with floating electrodes. In: Proceedings of the IEEE 23rd International Conference on Micro Electro Mechanical Systems (MEMS). Wanchai, Hong Kong, China: IEEE; Feb 2010. pp. 228-231
  6. 6. Pritchard E, Mahfouz M, Evans B, Eliza S, Haider M. Flexible capacitive sensors for high resolution pressure measurement. Sensors IEEE. 2008;1484:1484-1487. DOI: 10.1109/ICSENS.2008.4716726
  7. 7. Basiricó L. Inkjet printing of organic transistor devices [thesis]. University of Cagliari; 2012
  8. 8. Li M, Li Y-T, Li D-W, Long Y-T. Recent developments and applications of screen-printed electrodes in environmental assays-review. Analytica Chimica Acta. 2012;734:31-44. DOI: 10.1016/j.aca.2012.05.018
  9. 9. Li J, Peng T, Fang C. Screen-printable sol–gel ceramic carbon composite pH sensor with a receptor zeolite. Analytica Chimica Acta. 2002;455:53-60. DOI: 10.1016/S0003-2670(01)01540-9
  10. 10. Metters JP, Kadara RO, Banks CE. Fabrication of co-planar screen printed microband electrodes. The Analyst. 2013;138:2516-2521. DOI: 10.1039/C3AN00268C
  11. 11. Webb AJ et al. A multi-component nanocomposite screen-printed ink with non-linear touch sensitive electrical conductivity. Nanotechnology. 2013;24:165501. DOI: 10.1088/0957-4484/24/16/165501
  12. 12. Ochoteco E et al. Tactile sensors based on conductive polymers. Microsystems Technology. 2010;16:765-776. DOI: 10.1117/12.821627
  13. 13. Blayo A, Pineaux B. Printing processes and their potential for RFID printing. In: Proceedings of the 2005 Joint Conference on Smart Objects and Ambient Intelligence: Innovative Context-Aware Services: Usages and Technologies. Grenoble, France: ACM; 2005. pp. 27-30
  14. 14. Julin T. Flexo-printed piezoelectric PVDF pressure [thesis]. Tampere University of Technology; 2011
  15. 15. Clark DA, Major Trends in Gravure Printed Electronics [Internet]. 2010. Available from: [Accessed: Nov 23, 2017]
  16. 16. Sung D, Fuente VA, Subramanian V. Scaling and optimization of gravure-printed silver nanoparticle lines for printed electronics. IEEE Transactions on Components and Packaging Technologies. 2010;33:105-114. DOI: 10.1109/TCAPT.2009.2021464
  17. 17. Lee T-M et al. The effect of shear force on ink transfer in gravure offset printing. Journal of Micromechanics and Microengineering. 2010;20:125026. DOI: 10.1088/0960-1317/20/12/125026
  18. 18. Yang J et al. Organic photovoltaic modules fabricated by an industrial gravure printing proofer. Solar Energy Materials and Solar Cells. 2013;109:47-55. DOI: 10.1016/j.solmat.2012.10.018
  19. 19. Reddy A et al. Fully printed flexible humidity sensor. In: Proc. Eurosensors XXV, Sep 4-7, 2011. Athens, Greece: Procedia Engineering; 2011. pp. 120-123
  20. 20. Liu C-X, Choi J-W. Patterning conductive PDMS nanocomposite in an elastomer using microcontact printing. Journal of Micromechanics and Microengineering. 2009;19:85019. DOI: 10.1088/0960-1317/19/8/085019
  21. 21. Xia Y, Whitesides GM. Soft lithography. Annual Review of Materials Science. 1998;37:550. DOI: 0084-6600/98/0801-0153$08.00
  22. 22. Qin D, Xia Y, Whitesides GM. Soft lithography for micro- and nanoscale patterning. Nature Protocols. 2010;5:491-502. DOI: 10.1038/nprot.2009.234
  23. 23. Kawase T et al. Inkjet printing of polymer thin film transistors. Thin Solid Films. 2003;438:279-287. DOI: 10.1016/S0040-6090(03)00801-0
  24. 24. Khan S, Lorenzelli L, Dahiya R. Technologies for printing sensors and electronics over large flexible substrates: A review. IEEE Sensors Journal. 2015;15:3164-3185. DOI: 10.1109/JSEN.2014.2375203
  25. 25. Arnold CB, Piqué A. Laser direct-write processing. MRS Bulletin. 2007;32:9-15
  26. 26. Bauerle D. Handbook of Laser Processing and Chemistry. 3rd ed. Physics and Astronomy. Berlin Heidelberg: Springer-Verlag; 2000. 19p
  27. 27. Steen W, Mazumder J. Handbook of Laser Material Processing. 4th ed. London: Springer-Verlag; 2010. 558 p. DOI: 10.1007/978-1-84996-062-5
  28. 28. Christenson KK et al. Direct printing of circuit boards using Aerosol Jet. In: Technical Program and Proceedings. NIP 27 Digital Fabrication. Springfield, Va: IS&T–the Society for Imaging Science and Technology; 2011. pp. 433-436
  29. 29. Optomec [Internet]. 2018. Available from: [Accessed: Jan 2, 2018]
  30. 30. Calvert P. Inkjet printing for materials and devices. Chemistry of Materials. 2001;13:3299-3305. DOI: 10.1021/cm0101632
  31. 31. Calvert P, Yoshioka Y, Jabbour G. Inkjet printing for biomimetic and biomedical materials. In: Reis RL, Weiner S, editors. Handbook of Learning from Nature How to Design New Implantable Biomaterials. Netherlands: Kluwer Academic Publishers; 2004. pp. 169-180
  32. 32. Al-chami H. Inkjet printing of transducers [thesis]. University of British Columbia; 2010
  33. 33. Kamyshny A, Steinke J, Magdassi S. Metal-based inkjet inks for printed electronics. The Open Applied Physics Journal. 2011;4:19-36. DOI: 10.2174/1874183501104010019
  34. 34. Fuller SB, Wilhelm EJ, Jacobson JM. Ink-jet printed nanoparticle microelectromechanical systems. Journal of Microelectromechanical Systems. IEEE. 2002;11:54-60. DOI: 10.1109/84.982863
  35. 35. Perelaer J, Schubert US. Inkjet printing and alternative sintering of narrow conductive tracks on flexible substrates for plastic electronic applications. In: Turcu C, editor. Handbook of Radio Frequency Identification Fundamentals and Applications Design Methods and Solutions. Croatia: InTech; 2010. pp. 265-286. DOI: 10.5772/7983
  36. 36. Gans BJ, Duineveld PC, Schubert US. Inkjet printing of polymers: State of the art and future developments. Advanced Materials. 2004;16:203-213. DOI: 10.1002/adma.200300385
  37. 37. Hadimioglu B et al. Acoustic ink printing. In: Ultrasonics Symposium. Tucson, AZ, USA: IEEE; 1992. pp. 929-936. DOI: 10.1109/ULTSYM.1992.275823
  38. 38. Ujiie H. Handbook of Digital Printing of Textiles. 1st ed. Cambridge, London: Woodhead Publishing; 2006. 384 p
  39. 39. Paul KE, Wong WS, Ready SE, Street RA. Additive jet printing of polymer thin-film transistors. Applied Physics Letters. 2003;83:2070-2072. DOI: 10.1063/1.1609233
  40. 40. Junfeng M, Lovell MR, Mickle MH. Formulation and processing of novel conductive solution inks in continuous inkjet printing of 3-D electric circuits. IEEE Transactions on Electronics Packaging Manufacturing. 2005;28:265-273. DOI: 10.1109/TEPM.2005.852542
  41. 41. Andò B, Baglio S. Inkjet-printed sensors: A useful approach for low cost, rapid prototyping. IEEE Instrumentation & Measurement Magazine. 2011;14:36-40. DOI: 10.1109/MIM.2011.6041380
  42. 42. Caglar U. Studies of inkjet printing technology with focus on electronic materials [thesis]. Tampere University; 2009
  43. 43. Hertz [Internet]. 2015. Available from: [Accessed: Nov 6, 2018]
  44. 44. Smith L et al. Continuous ink-jet print head utilizing silicon micromachined nozzles. Sensors and Actuators A: Physoical. 1994;43:311-316. DOI: 10.1016/0924-4247(93)00707-b
  45. 45. Piqué A, Chrisey DB. Handbook of Direct-Write Technologies for Rapid Prototyping Applications: Sensors, Electronics, and Integrated Power Sources. New York: Academic Press; 2002. 726 p
  46. 46. Cabbill V. Introduction to Digital Printing Technology. Graphic Artists: Pre-Press Personnel; 1998
  47. 47. Hudd A. Inkjet printing technologies. In: Magdassi S, editor. Handbook of The Chemistry of Inkjet Inks. Singapore: World Scientific Publishing; 2010. pp. 3-18. DOI: 10.1142/9789812818225_0001
  48. 48. Lin C-T, Hsu C-H, Lee C-H, Wu W-J. Inkjet-printed organic field-effect transistor by using composite semiconductor material of carbon nanoparticles and poly(3-Hexylthiophene). Journal of Nanotechnology. 2011;2011:1-7. DOI: 10.1155/2011/142890
  49. 49. Lin C-T, Hsu C-H, Chen I-R, Lee C-H, Wu W-J. Enhancement of carrier mobility in all-inkjet-printed organic thin-film transistors using a blend of poly(3-hexylthiophene) and carbon nanoparticles. Thin Solid Films. 2011;519:8008-8012. DOI: 10.1016/j.tsf.2011.05.071
  50. 50. Haffarzadeh K, Zervos H. Conductive Ink Markets 2012-2018 Silver & Copper Inks & Pastes and Beyond. USA: IDTechEX; 2012
  51. 51. Molesa S, Redinger DR, Huang DC, Subramanian V. High-quality inkjet-printed multilevel interconnects and inductive components on plastic for ultra-low-cost RFID applications. In: Materials Research Society Symposia Proceedings. Vol. 769. Cambridge, Boston, USA: Materials Research Society; 2003. pp. 1-6
  52. 52. Ko SH et al. All-inkjet-printed flexible electronics fabrication on a polymer substrate by low-temperature high-resolution selective laser sintering of metal nanoparticles. Nanotechnology. 2007;18:345202. DOI: 10.1088/0957-4484/18/34/345202
  53. 53. Perelaer J, Gans B-J, Schubert US. Ink-jet printing and microwave sintering of conductive silver tracks. Advanced Materials. 2006;18:2101-2104. DOI: 10.1002/adma.200502422
  54. 54. Xie G, Ohashi O, Yamaguchi N, Wang A. Effect of surface oxide films on the properties of pulse electric-current sintered metal powders. Metallurgical and Materials Transactions A. 2003;34:2655-2661. DOI: 10.1007/s11661-003-0024-1
  55. 55. Wakuda D, Hatamura M, Suganuma K. Novel method for room temperature sintering of Ag nanoparticle paste in air, Chemical Physics Letters. 2007;441:305-308. DOI: 10.1016/j.cplett.2007.05.033
  56. 56. Perelaer J et al. One-step inkjet printing of conductive silver tracks on polymer substrates. Nanotechnology. 2009;20:165303. DOI: 10.1088/0957-4484/20/16/165303
  57. 57. Mcbride JW, Lam L. A review of conducting polymers in electrical contact applications. In: International Conference of Polymeric Materials in Power Engineering ICPMPE-07 I-3, University of Southampton Institutional Repository; Oct 4-6, 2007. p. 9
  58. 58. Elschner A et al. PEDOT – Principles and Applications of an Intrinsically Conductive Polymer 2010. New York: Taylor and Francis Group, CRC Press; 2011. 377 p
  59. 59. Li B et al. Inkjet printed chemical sensor array based on polythiophene conductive polymers. Sensors and Actuators B: Chemical. 2007;123:651-660. DOI: 10.1016/j.snb.2006.09.064
  60. 60. Bernardo G et al. Solid-state low-temperature extrusion of P3HT ribbons. Applied Physics A: Materials Science and Processing. 2014;117:2079-2086. DOI: 10.1007/s00339-014-8622-x
  61. 61. Chen T-A, Wu X, Rieke RD. Regiocontrolled synthesis of poly(3-alkylthiophenes) mediated by Rieke Zinc: Their characterization and solid-state properties. Journal of American Chemical Society. 1995;117:233-244. DOI: 10.1021/ja00106a027
  62. 62. Bai H, Shi G. Gas sensors based on conducting polymers. Sensors. 2007;7:267-307
  63. 63. Chen JH, Dai C-A, Chiu W-Y. Synthesis of highly conductive EDOT copolymer films via oxidative chemical in situ polymerization. Journal of Polymer Science-Part A: Polymer Chemistry. 2008;46:1662-1673. DOI: 10.1002/pola.22508
  64. 64. Kim YH, Sachse C, Machala ML, May C, Müller-Meskamp L, Leo K. Highly conductive PEDOT:PSS electrode with optimized solvent and thermal post-treatment for ITO-free organic solar cells. Advanced Functional Materials. 2011;21:1076-1081. DOI: 10.1002/adfm.201002290
  65. 65. Groenendaal JR, Jonas L, Freitag F, Pielartzik D, Reynolds H. Poly(3,4-ethylenedioxythio-phene) and its derivatives: Past, present, and future. Advanced Materials. 2000;12:481-494. DOI: 10.1002/(SICI)1521-4095(200004)12:73.3.CO;2-3
  66. 66. Greco F et al. Ultra-thin conductive free-standing PEDOT/PSS nanofilms. The Royal Society of Chemistry. 2011;7:10642-10650. DOI: 10.1039/c1sm06174g
  67. 67. Zirkl M et al. An all-printed ferroelectric active matrix sensor network based on only five functional materials forming a touchless control interface. Advanced Materials. 2011;23:2069-2074. DOI: 10.1002/adma.201100054
  68. 68. Phongphut A et al. A disposable amperometric biosensor based on inkjet-printed au/PEDOT-PSS nanocomposite for triglyceride determination. Sensors Actuators B: Chemistry. 2013;178:501. DOI: 10.1016/j.snb.2013.01.012
  69. 69. Deutschmann T, Oesterschulze E. Micro-structured electrochromic device based on poly(3,4-ethylenedioxythiophene). Journal of Micromechanics and Microengineering. 2013;23:065032. DOI: 10.1088/0960-1317/23/6/065032
  70. 70. Wang Y. Research progress on a novel conductive polymer–poly(3,4-ethylenedioxythiophene) (PEDOT). Journal of Physics Conference Series. 2009;152:12023. DOI: 10.1088/1742-6596/152/1/012023
  71. 71. Speakman SP et al. High performance organic semiconducting thin films: Ink jet printed polythiophene [rr -P3HT]. Organic Electronics. 2001;2:65-73
  72. 72. Molina-Lopez F, Briand D, Rooij NF. All additive inkjet printed humidity sensors on plastic substrate. Sensors Actuators B: Chemical. 2012;212:212-222. DOI: 10.1016/j.snb.2012.02.042
  73. 73. Xie G, et al. Fabrication and properties of an OLED-based gas sensor with poly(3-hexylthiophene) sensing film. In: IMCS 2012 – 14th Int. Meet. Chem. Sensors, AMA Services Gmbh; 2012. pp. 1130-1133
  74. 74. Kang BJ, Lee CK, Oh JH. All-inkjet-printed electrical components and circuit fabrication on a plastic substrate. Microelectronic Engineering. 2012;97:251. DOI: 10.1016/j.mee.2012.03.032
  75. 75. Caglar U et al. Analysis of mechanical performance silver inkjet-printed structures. In: 2nd IEEE Int. Nanoelectron. Conf. INEC 2008. Shanghai, China: IEEE; 2008. pp. 851-856
  76. 76. Suzuki M, Takahashi T, Aoyagi S. Flexible tactile sensor using polyurethane thin film. Micromachines. 2012;3:315. DOI: 10.3390/mi3020315
  77. 77. Cruz S, Rocha LA, Viana JC. Enhanced printability of thermoplastic polyurethane substrates by silica particles surface interactions. Applied Surface Science. 2016;360:198. DOI: 10.1016/j.apsusc.2015.10.094
  78. 78. Cruz S, Rocha LA, Viana JC. Piezo-resistive behaviour at high strain levels of PEDOT:PSS printed on a flexible polymeric substrate by a novel surface treatment. Journal of Materials Science: Materials in Electronics. 2017;28:2563-2573. DOI: 10.1007/s10854-016-5832-3
  79. 79. Siau S, Vervaet A, Van Calster A, Swennen I, Schacht E. Influence of wet chemical treatments on the evolution of epoxy polymer layer surface roughness for use as a build-up layer Appl. Surface Science. 2004;237:457. DOI: 10.1016/j.apsusc.2004.06.111
  80. 80. Arkles B, Pan Y, Kim YM. The role of polarity in the structure of silanes employed in surface modification. In: Mittal KL, editor. Handbook of Silanes and Other Coupling Agents. Vol. 5. Leiden, The Netherlands: Taylor & Francis Group; 2009. p. 14
  81. 81. Strobel M, Jones V, Lyons CS, Ulsh M, Kushner MJ, Dorai R, Branch MC. A comparison of corona-treated and flame-treated polypropylene films. Plasmas and Polymers. 2003;8:61-95
  82. 82. Morent R, Geyter N, Leys C. Effects of operating parameters on plasma-induced PET surface treatment. Nuclear Instruments and Methods in Physics Research Section B. 2008;266:3081. DOI: 10.1016/j.nimb.2008.03.166
  83. 83. McDonald JC, Whitesides GM. Poly(dimethylsiloxane) as a material for fabricating microfluidic devices. Accounts of Chemical Research. 2002;35:491. DOI: 10.1021/ar010110q
  84. 84. Lei KF, Lee K-F, Lee M-Y. Development of a flexible PDMS capacitive pressure sensor for plantar pressure measurement. Microelectronic Engineering. 2012;99:1-5. DOI: 10.1016/j.mee.2012.06.005
  85. 85. Griggs C, Sumerel J, Ph D, Industrial + Speccialty Printing [Internet]. 1899. Available from: [Accessed: Dec 21, 2017]
  86. 86. Meixner RM, Cibis D, Krueger K, Goebel H. Characterization of polymer inks for drop-on-demand printing systems. Microsystem Technologies. 2008;4:1137. DOI: 10.1007/s00542-008-0639-7
  87. 87. Kulkarni MV et al. Ink-jet printed conducting polyaniline based flexible humidity sensor. Sensors and Actuators B: Chemical. 2013;178:140-143. DOI: 10.1016/j.snb.2012.12.046
  88. 88. Matic V et al. Inkjet printed differential mode touch and humidity sensors on injection molded polymer packages. IEEE Sensors 2014 Proc. 2234.Valencia, Spain: SENSORS, IEEE; 2014. DOI: 10.1109/ICSENS.2014.6985485
  89. 89. Cruz S, Dias D, Viana J, Rocha LA. Inkjet printed pressure sensing platform for postural imbalance monitoring. IEEE Transactions on Instrumentation and Measurement. 2015;64:2813. DOI: 10.1109/TIM.2015.2433611
  90. 90. Li Y et al. An all-inkjet printed flexible capacitor for wearable applications. In: Design, Test, Integration and Packaging of MEMS/MOEMS (DTIP), 2012 Symposium. Apr 25-27, 2012. Cannes, France: IEEE, 2012. pp. 25-28
  91. 91. Someya T et al. Printed organic transistors for large-area electronics. In: 6th Int. Conf. Polymers & Adhesives in Microelectronics & Photonics, 2007. Polytronic. Japan: IEEE; 2007. pp. 6-11
  92. 92. Basiricò L et al. Inkjet printed arrays of pressure sensors based on all-organic field effect transistors. In: 32nd Annual Int. Conf. IEEE EMBS Buenos Aires, Argentina. Engineering in Medicine and Biology Society (EMBC), 2010 Annual International Conference of the IEEE. 2010. pp. 2111-2114

Written By

Sílvia Manuela Ferreira Cruz, Luís A. Rocha and Júlio C. Viana

Submitted: December 4th, 2017 Reviewed: March 2nd, 2018 Published: July 25th, 2018