**2. Synergy of printed electronics**

on a blend of regioregular poly (3-hexylthiophene-2,5-diyl) and [6,6]-phenyl C61 butyric acid methyl ester (PCBM). It demonstrated the feasibility of spray deposition as a method for printed organic electronics especially since all layers were successively deposited by the same technology. **Figure 5** summarizes the work displaying the setup used, the final device as well

**Figure 6.** Schematic depictions of common 3D processing techniques of (a) Selective Laser Sintering and (b)

**Figure 5.** Summarizing figure displaying spray deposition setup used for the fabrication of fully-sprayed, flexible OPDs,

the final device as well as key electrical characteristics (adapted from [24] with authorization).

3D printing is a term used to encompass additive manufacturing techniques that aim at fabricating solid objects through the deposition of material. Although only recently having gained broad awareness, due to the surge in available privately affordable devices, 3D printing patents date back to 1986 [25]. Used mostly for rapid prototyping and in research the main fabrication was limited to static objects with limited mechanical features. With the increase research of nano-based materials, 3D printing has moved beyond creating scaffolds for electrical devices with the integration of electrical characteristic being embedded into the 3D structure [26–28].

as key electrical performance.

Stereolithography and Fused Deposition Modeling.

**1.6. 3D printing**

98 Flexible Electronics

After reviewing the main fabrication techniques for printed electronics, it is clear that each of them have potential to define different electronic components. For example, antennas require thicker metallic layers in order to enhance their performance. Although the desired thickness could be achieved with any of the described technologies, the most efficient one is screen printing because we can achieve tens of μm selecting a coarse mesh with only one turn. The same thickness would need 20 layers in case of using inkjet printing, resulting in a very time consuming process because it is needed to wait until the layer is dried and the area to cover is quite large. In the case of spray deposition, also the stack of several layers would be mandatory. This technique is much faster than inkjet printing but the pattern definition is poorer in comparison to screen printing.

Another example is the definition of interdigitated electrodes (IDE) for capacitive structures. If the IDE is going to be used to build a capacitive sensor, the distance among consecutive fingers determines the sensitivity of the sensors: the closer they are placed, the higher its sensitivity can be. In this sense, the resolution of the printing technique limits the performance of the device.

Therefore, each technique can be more suitable for different electronic components, as highlighted in the previous examples. As the ultimate goal is to fabricate a fully working and efficient electronic system, it is mandatory to properly combine manufacturing techniques to fulfill it. In the last years, some authors have worked in this director, using different printed techniques to design electronic circuits on flexible substrates.

Some authors have already illustrated how the fabrication of each component of a final device or system should be carefully selected in order to optimize its performance. In particular, Salmerón et al. described two ultrahigh frequency (UHF) radio frequency identification (RFID) tags with sensing capabilities [29]. As demonstrated before [30], they used screen printing to define the antenna in order to achieve a better performance in the RF link. The sensing capabilities comes from the substrate (a polyimide), whose electrical permittivity changes with the moisture content. To exploit this feature, they printed an array of planar capacitive structures. In one of the tags, the electrodes were fabricated with inkjet printing, whereas screen printing was employed in the other design. These tags include an UHF RFID chip with an in-built temperature chip and a sensor frontend capable of measuring capacitive sensors. Although both tags covered a 30% of relative humidity (RH) and showed very low thermal drifts (below 0.05%RH/°C), their features were different. In the case of the inkjetted capacitive array, the sensitivity achieved was 100 fF/RH%. Whereas in the case of the screen printed ones, the sensitivity was about the half (54 fF/RH%). The tag with the inkjetted sensor exhibited a higher performance than the screen printed one in terms of area saving and higher humidity sensitivity. The screen printed sensors approach can be fabricated with just one step but a much bigger area was needed (see **Figure 7a**).

Another device manufactured with two printing techniques was a hybrid sensor for simultaneous vapor determination of RH and toluene concentration [31]. They exploited two strategies to provide with sensing capabilities their device. The RH was measured again by changed in the selected substrate, whereas the toluene concentration was measured by a sensing layer deposited on top of the electrodes. The chosen electrode layout is shown in **Figure 8**, assuring that there was no electrical path between the capacitive electrode and the resistive electrode. Although other electrode configurations present higher, they do not allow the integration of more functionalities in the same side of the device with virtually no interference between them [32]. Inkjet printing was selected to define the electrodes, whereas screen printing was used to deposit the sensitive resistive composite. Inkjet printing allows better resolution, and

therefore, electrodes can be placed closer, increasing the sensitivity, especially of the capacitive part. But the viscosity of the resistive composite sensitive to toluene was too high to be deposited by inkjet printing, and therefore they employed screen printing to define this layer

**Figure 9.** Left: schematics of the two fabricated sensors: (a) electrodes on top of CNT film; (b) CNT film on top of the electrodes.

concentrations. Images adapted from [33] with authorization.

(b)

Technological Integration in Printed Electronics http://dx.doi.org/10.5772/intechopen.76520 101

**Figure 8.** (a) Layout of the novel designed hybrid capacitive-resistive sensor indicating the notation of the dimensions. R corresponds to resistive terminal, C for capacitive sensor, and H corresponds to the common terminal. (b) Image of the hybrid sensor. The resistive element is shown in the upper part of the device, defined between the terminals R and H. The capacitive element is measured between the terminals H and C. (c) Experimental relative humidity and temperature sensitivities as a function of frequency, measuring between H and C. (d) Resistance vs. toluene concentration, measuring

between H and R. Images adapted from [31] and [32] with authorization.

Another example of mixing PE techniques was illustrated by Abdelhalim et al. [33]. They showed a fully printed ammonia sensor on a flexible substrate. The electrodes were fabricated

on top of the electrodes.

Right: normalized response under different values of NH<sup>3</sup>

**Figure 7.** Left: (a) RFID tag with inkjetted serpentine sensors and (b) RFID tag with interdigitated sensors by screen printing. Right: (a) ADC counts of tag with inkjetted serpentine sensor and (b) ADC counts of screen printed tag with IDE sensors. Images adapted from [29] with authorization.

Some authors have already illustrated how the fabrication of each component of a final device or system should be carefully selected in order to optimize its performance. In particular, Salmerón et al. described two ultrahigh frequency (UHF) radio frequency identification (RFID) tags with sensing capabilities [29]. As demonstrated before [30], they used screen printing to define the antenna in order to achieve a better performance in the RF link. The sensing capabilities comes from the substrate (a polyimide), whose electrical permittivity changes with the moisture content. To exploit this feature, they printed an array of planar capacitive structures. In one of the tags, the electrodes were fabricated with inkjet printing, whereas screen printing was employed in the other design. These tags include an UHF RFID chip with an in-built temperature chip and a sensor frontend capable of measuring capacitive sensors. Although both tags covered a 30% of relative humidity (RH) and showed very low thermal drifts (below 0.05%RH/°C), their features were different. In the case of the inkjetted capacitive array, the sensitivity achieved was 100 fF/RH%. Whereas in the case of the screen printed ones, the sensitivity was about the half (54 fF/RH%). The tag with the inkjetted sensor exhibited a higher performance than the screen printed one in terms of area saving and higher humidity sensitivity. The screen printed sensors approach can be fabricated with just one step

Another device manufactured with two printing techniques was a hybrid sensor for simultaneous vapor determination of RH and toluene concentration [31]. They exploited two strategies to provide with sensing capabilities their device. The RH was measured again by changed in the selected substrate, whereas the toluene concentration was measured by a sensing layer deposited on top of the electrodes. The chosen electrode layout is shown in **Figure 8**, assuring that there was no electrical path between the capacitive electrode and the resistive electrode. Although other electrode configurations present higher, they do not allow the integration of more functionalities in the same side of the device with virtually no interference between them [32]. Inkjet printing was selected to define the electrodes, whereas screen printing was used to deposit the sensitive resistive composite. Inkjet printing allows better resolution, and

**Figure 7.** Left: (a) RFID tag with inkjetted serpentine sensors and (b) RFID tag with interdigitated sensors by screen printing. Right: (a) ADC counts of tag with inkjetted serpentine sensor and (b) ADC counts of screen printed tag with

but a much bigger area was needed (see **Figure 7a**).

100 Flexible Electronics

IDE sensors. Images adapted from [29] with authorization.

**Figure 8.** (a) Layout of the novel designed hybrid capacitive-resistive sensor indicating the notation of the dimensions. R corresponds to resistive terminal, C for capacitive sensor, and H corresponds to the common terminal. (b) Image of the hybrid sensor. The resistive element is shown in the upper part of the device, defined between the terminals R and H. The capacitive element is measured between the terminals H and C. (c) Experimental relative humidity and temperature sensitivities as a function of frequency, measuring between H and C. (d) Resistance vs. toluene concentration, measuring between H and R. Images adapted from [31] and [32] with authorization.

**Figure 9.** Left: schematics of the two fabricated sensors: (a) electrodes on top of CNT film; (b) CNT film on top of the electrodes. Right: normalized response under different values of NH<sup>3</sup> concentrations. Images adapted from [33] with authorization.

therefore, electrodes can be placed closer, increasing the sensitivity, especially of the capacitive part. But the viscosity of the resistive composite sensitive to toluene was too high to be deposited by inkjet printing, and therefore they employed screen printing to define this layer on top of the electrodes.

Another example of mixing PE techniques was illustrated by Abdelhalim et al. [33]. They showed a fully printed ammonia sensor on a flexible substrate. The electrodes were fabricated by inkjet printing of silver nanoparticles whereas the sensitive film was made of carbon nanotubes (CNT) deposited by spray by an air atomizing nozzle. Two different approaches were followed to manufacture the sensors: first, spraying the CNT solution on top of the electrodes (**Figure 9a**) (conventional approach) and second, printing the electrodes on top of the sprayed CNT film (**Figure 9b**). In the case of conventional approach, the resistance values were one order of magnitude higher than in the case inverted method (printing the electrodes on top of the sensing layer). This can be explained by the fact that the thickness of the silver layer and the mean value of the CNT are about the same value (~450 nm), therefore, the establishment of an electrical path between the CNT is more difficult in the classical approach than in the inverted one.

The results obtained for NH<sup>3</sup> sensing showed a good performance in terms of sensitivity and time response to the test gas, with performance comparable with that obtained with evaporated metal electrodes and conventional approach [34].

As apparent from what we have described so far, the integration of different printing techniques can bring to innumerable benefits, and lets the designer exploit the advantages of each method. Conventional printing, however, constrains the degrees of geometrical freedom two, limiting the possibilities of integration to an *in plane* approach. It is, nevertheless, possible to push the boundaries of technological integration, embedding 2D thin film devices into 3D structures, in order to create *electronic objects,* more than just electronic devices. One of the possible approaches for the obtainment of such structures is 3D printing of scaffolds, in which discrete electronics are cast and embedded. The 3D printed object and the electronics can be designed separately and the only major concern would be the attachment and interconnection of the devices in the 3D housing. Opposed to this approach, Falco et al. demonstrated the facile integration of conformal organic electronics devices in 3D printed structures, where the device is directly fabricated on or in the printed object [35]. The 3D printing technique of choice was FDM, because of its low cost, ease of use and high customizability. The major caveat, though, is the elevated RMS roughness (tens of microns) of the objects produced with means of this method. As the typical active layers employed in organic and printed electronics have thickness of few hundreds of nanometers, it was necessary to also develop a high throughput, sustainable and easy-to-integrate planarization process.

In this work the authors exploit spray deposition not only for the definition of the active materials (i.e. conductive polymers, silver nanowires, carbon nanotubes), but also to achieve the required planarization **Figure 10**.

could be measured on the as-printed ones, and comparable to the reference material on glass. Finally, they show an application of 3D print and spray concept, by designing a semitransparent 3D printed heating chamber, which represents the first example of a fully-printed and cost

**Figure 11.** Left: image of the tag with all its elements labeled; right: cross-sectional view of the OPD. An inkjet-printed

Ag line modified with spray-coated PEI. Both images adapted from [38] with authorization.

**Figure 10.** Proposed schematic for multi-technology integration. The substrate is printed with FDM printing technology (a). Spray deposition—or other printing techniques—is employed to deposit a functional layer (b), and (c) the upper stacks of the 3D printed object are realized on top of the spray deposited thin film. Image adapted from [35] with authorization.

Technological Integration in Printed Electronics http://dx.doi.org/10.5772/intechopen.76520 103

One last example of combination of PE techniques is a printed passive radiofrequency identification (RFID) tag in the UHF band for light and temperature monitoring [38]. In this case, the antenna and interconnects were realized with silver nanoparticles via inkjet printing. Temperature measurements came from an in-built sensor in the silicon RFID chip whereas the light monitoring was performed by a sprayed photodetector. This work showed for the first time the feasibility of the embedment of large-scale organic photodetectors onto inkjet-printed RFID tags. To succeed in the fabrication process, it was necessary to spray polyethylenimine (PEI) thin layer on top of the inkjetted silver electrode to obtain a working photodiode. In this case, they claimed that the antenna was done by inkjet printing instead of screen printing

effective functional object.

The spray-planarization is obtained dissolving the 3D printable material itself in an organic solvent, and spraying it on the rough as-printed structure in a wet deposition regime. In this manner, it is possible to re-utilize some wastes of the 3D printing process (material employed to prime the nozzle or to print the outskirts of the pieces), while preparing the structure for successive thin film deposition. Their results outperform alternative methods presented in previous works, and yield RMS roughness of the substrate lower by one order of magnitude with respect to literature [36, 37]. In order to test the viability of this method as preparation tool for printed electronics, the authors deposited and characterized thin films of a conducting polymer (PEDOT:PSS), silver nanowires and carbon nanotubes. Remarkably, the sheet resistance of the layers deposited on planarized samples were up to ten times lower than what

by inkjet printing of silver nanoparticles whereas the sensitive film was made of carbon nanotubes (CNT) deposited by spray by an air atomizing nozzle. Two different approaches were followed to manufacture the sensors: first, spraying the CNT solution on top of the electrodes (**Figure 9a**) (conventional approach) and second, printing the electrodes on top of the sprayed CNT film (**Figure 9b**). In the case of conventional approach, the resistance values were one order of magnitude higher than in the case inverted method (printing the electrodes on top of the sensing layer). This can be explained by the fact that the thickness of the silver layer and the mean value of the CNT are about the same value (~450 nm), therefore, the establishment of an electrical path between the CNT is more difficult in the classical approach than in the

time response to the test gas, with performance comparable with that obtained with evapo-

As apparent from what we have described so far, the integration of different printing techniques can bring to innumerable benefits, and lets the designer exploit the advantages of each method. Conventional printing, however, constrains the degrees of geometrical freedom two, limiting the possibilities of integration to an *in plane* approach. It is, nevertheless, possible to push the boundaries of technological integration, embedding 2D thin film devices into 3D structures, in order to create *electronic objects,* more than just electronic devices. One of the possible approaches for the obtainment of such structures is 3D printing of scaffolds, in which discrete electronics are cast and embedded. The 3D printed object and the electronics can be designed separately and the only major concern would be the attachment and interconnection of the devices in the 3D housing. Opposed to this approach, Falco et al. demonstrated the facile integration of conformal organic electronics devices in 3D printed structures, where the device is directly fabricated on or in the printed object [35]. The 3D printing technique of choice was FDM, because of its low cost, ease of use and high customizability. The major caveat, though, is the elevated RMS roughness (tens of microns) of the objects produced with means of this method. As the typical active layers employed in organic and printed electronics have thickness of few hundreds of nanometers, it was necessary to also develop a high

In this work the authors exploit spray deposition not only for the definition of the active materials (i.e. conductive polymers, silver nanowires, carbon nanotubes), but also to achieve the

The spray-planarization is obtained dissolving the 3D printable material itself in an organic solvent, and spraying it on the rough as-printed structure in a wet deposition regime. In this manner, it is possible to re-utilize some wastes of the 3D printing process (material employed to prime the nozzle or to print the outskirts of the pieces), while preparing the structure for successive thin film deposition. Their results outperform alternative methods presented in previous works, and yield RMS roughness of the substrate lower by one order of magnitude with respect to literature [36, 37]. In order to test the viability of this method as preparation tool for printed electronics, the authors deposited and characterized thin films of a conducting polymer (PEDOT:PSS), silver nanowires and carbon nanotubes. Remarkably, the sheet resistance of the layers deposited on planarized samples were up to ten times lower than what

sensing showed a good performance in terms of sensitivity and

inverted one.

102 Flexible Electronics

The results obtained for NH<sup>3</sup>

required planarization **Figure 10**.

rated metal electrodes and conventional approach [34].

throughput, sustainable and easy-to-integrate planarization process.

**Figure 10.** Proposed schematic for multi-technology integration. The substrate is printed with FDM printing technology (a). Spray deposition—or other printing techniques—is employed to deposit a functional layer (b), and (c) the upper stacks of the 3D printed object are realized on top of the spray deposited thin film. Image adapted from [35] with authorization.

**Figure 11.** Left: image of the tag with all its elements labeled; right: cross-sectional view of the OPD. An inkjet-printed Ag line modified with spray-coated PEI. Both images adapted from [38] with authorization.

could be measured on the as-printed ones, and comparable to the reference material on glass. Finally, they show an application of 3D print and spray concept, by designing a semitransparent 3D printed heating chamber, which represents the first example of a fully-printed and cost effective functional object.

One last example of combination of PE techniques is a printed passive radiofrequency identification (RFID) tag in the UHF band for light and temperature monitoring [38]. In this case, the antenna and interconnects were realized with silver nanoparticles via inkjet printing. Temperature measurements came from an in-built sensor in the silicon RFID chip whereas the light monitoring was performed by a sprayed photodetector. This work showed for the first time the feasibility of the embedment of large-scale organic photodetectors onto inkjet-printed RFID tags. To succeed in the fabrication process, it was necessary to spray polyethylenimine (PEI) thin layer on top of the inkjetted silver electrode to obtain a working photodiode. In this case, they claimed that the antenna was done by inkjet printing instead of screen printing for simplicity of the process but in order to obtain an antenna with larger read range, screen printing would be the optimal process **Figure 11**.

connected chips: a semipassive RFID chip, used to log eventual intrusions in carton packages. The system includes a resistive sensing network, an active microcontroller used to record and log the sensor data, powered up by a flexible battery, and a passive communication system. The connecting lines were ink-jet printed and screen-printed, while the hybrid interconnections were obtained with means of a commercial ACA, as shown in **Figure 13**. An extensive study on the stability and reliability of similar solutions has been presented, already in 2014, by Happonen et al. [42], who thoroughly investigated the resiliency of conducting adhesives employed for the connection of separate flexible foils. They explored different bonding solutions and performed a live measurement of the DC 4-wires resistance during several thermal and bending cycles. Interestingly, they show that the stability of hybrid interconnections is enhanced by the presence of supportive, non-conductive adhesives. These hybrid structures can undergo more than 1000 thermal cycles (0–100°C and back to 0°C in 1 h) and bending cycles (with bending radius down to 20 mm). The number of samples and the statistical analyses behind this analysis are solid and prove how flexible to flexible interconnects can be sufficiently reliable for consumer electronics applications. In spite of such promising results,

Technological Integration in Printed Electronics http://dx.doi.org/10.5772/intechopen.76520 105

An altogether different approach, however, could reduce the problematics of interconnection technology to their minimum terms. The newest research in flexible electronics, in fact, shows how it is possible to develop complex and fully functional circuits, with the employment of metal-oxide n-type and carbon based p-type semiconductors. Complete circuital systems, which are inherently flexible, would restrict the need for interconnects to very few and controllable points. These points can be designed to be in positions subject to minimal mechanical stress, hence reducing the probability of failure. Most of the effort in this direction has been put in the realization of only n-type semiconducting circuits, given the superior stability of metal oxides with respect to carbon based materials [8]. Although limited by the high power consumption of unipolar circuits, these studies show the avenue to follow to reach flexible and integrated electronics with the minimal interconnection technology. A remarkable work in this context is the one presented by Hung et al. [43], where an ultra-low power RFID tag is developed on plastic foil. All the components of the tag, including logic gates, decoders,

**Figure 12.** (a) Cracks left on the paper coating, on the printed lines and at the soldering, indicated by arrows, for a 0805 packaging component (b) similar issues for QFP and SOP packages, showing that the problematic is insensitive of

packaging type and, up to a certain extent, size insensitive. Image adapted from [39] with authorization.

however, the interconnections remain a significant point of failure.

Therefore, the manufacturing choice will depend on the restrictions of each application, in terms of printing technology availability, performance, area and materials and processes compatibility.
