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Thermal sintering has traditionally been the most popular sintering method to enhance conductivity after the printing process in the manufacturing of printed electronics. Nevertheless, in recent years, there has been a growing interest in electrical sintering as an alternative method to overcome some of the limitations of thermal curing. This paper makes a comparative study of both sintering methods in terms of surface morphology, electrical dc conductance, and radiofrequency performance for different applied voltage waveforms. To this end, microstrip transmission lines have been inkjet-printed using nanoparticle-based silver ink on flexible polyimide substrate. The traces have been tested under different sintering conditions, achieving electrical sintering resistivity values only 2.3 times higher than that of bulk silver. This implies a 62% reduction in comparison with the best resistivity value achieved using thermal sintering in our samples. The main novelty of this contribution lies in the analysis of RF behavior as a function of electrical sintering conditions. Lower resistivities have been achieved with slower voltage ramps or allowing higher density current during sintering. It has also been proved that electrically sintered lines have similar RF performance than high-temperature thermally sintered lines in terms of insertion losses, regardless of their very different surface topology. Therefore, we can take advantage of the benefits that electrical sintering offers over thermal sintering regarding significant shorter sintering times maintaining suitable RF performance., This work was supported in part by the Spanish Ministry of Economics and Competitiveness under Grant CTQ2016-78754-C2-1-R.

The astonishing boom of radio-frequency identification (RFID) technology is stimulating plenty of new RFID-based industrial applications. Consequently, in the very near future, an almost unlimited number of RFID tags could be embedded into manufactured goods of various shapes, assets, and machineries to enable their communication abilities. As a result, prototyping techniques of RFID tags on flexible substrates are becoming more crucial. In this article, four different techniques suitable for prototyping flexible tags are briefly explained and tested from many points of view: ease of use, processing time, cost, tag sensitivity, radiation pattern, impedance, and robustness of the realized prototype. Characterization methods and experimental setups are presented, and two tag layouts, one commercial and one appositely designed, are used to compare the different techniques.

An interdigitated capacitive structure whose fingers have branches across them is described in this article. By changing the traditional interdigitated electrodes (IDE), the area where the sensor is defined can be reduced by a third, while its capacitance and sensitivity are maintained. This enhancement with respect to the traditional one is obtained thanks to the contribution to the capacitance not only in the finger length direction (unidimensional) but also in the finger width dimension (2-D). In order to test the sensitivity enhancement of this device in comparison with the conventional IDE, two version of this structure and the classical IDE have been manufactured by inkjet printing on flexible foil and characterized as relative humidity (RH) sensor. For this purpose, a substrate whose electrical permittivity changes with moisture content has been selected. Their capacitances have been measured as a function of RH in a wide range of frequencies at different temperatures, obtaining comparable results as a function of the RH and temperature. However, in the case of the sensor with closer branches, the response changes above 50%RH, showing a significantly higher sensitivity due to the fact that electrodes are much closer than in the other structures. The accumulation of water molecules between consecutive fingers occurs at lower values of moisture content than in the other layouts, resulting in higher capacitance values due to the higher electrical permittivity of water.

This paper describes printed resonant structures on a flexible substrate for humidity detection. The devices consist of a screen-printed spiral inductor working as radio frequency identification (RFID) antenna and two different inkjet-printed planar capacitive structures forming LC resonators. The sensitive material is directly the chosen substrate: a polyimide whose electrical permittivity is dependent on the moisture content. Therefore, no extra sensitive layer is required to functionalize these structures. In particular, two different approaches can be followed to use it. On one hand, the resonant structure can be used as a threshold tag including an RFID chip. The tags will be detected or not by an RFID reader depending on the humidity. On the other hand, these structures can be directly used as quantitative humidity sensors where the moisture content is associated with the resonant frequency of the structure.

Cantilevers are a widely used structure in electronics, covering many applications from switches to different kinds of sensors. The majority of them have been manufactured with complimentary metal–oxide–semiconductor (CMOS) technologies, but the increasing demand on printed electronics also motivates its development with printing techniques. Here, we present a cantilever beam fabricated by printing techniques with a novel manufacturing process that simplifies the fabrication procedure in terms of cost and time. The sacrificial layer is a poly(methyl methacrylate) film, which is not placed between the substrate and the beam, but, as a peculiarity of this process, on top of the beam; that is to say, the sacrificial layer is used as a sacrificial substrate. Another challenge faced in this paper is the use of a plastic foil as the substrate of this structure. In addition, we show experimental results of the physical and electrical characterization of these devices, which are in a reasonable good agreement with simulations by a finite element modeling tool. We tested the deflection of these cantilevers at different values of acceleration and frequency to show the efficiency of the innovative process. [2014-0073]

The design of optical sensor arrays encompasses tasks such as the acquisition of sensor optical parameters, as for example color features, and the calibration of a multivariate method to model the array global behavior. Different techniques have been used to model the sensor array responses, such as multivariate regression or neural networks, although they show certain limitations. The former methods require either high amount of computer memory or speed so therefore they are not suitable for implementation in portable electronic devices with low resources, while the later is a black box whose operation cannot be easily modeled and explained for industrial development validation. This work addresses these problems and proposes an expert system to overcome the previous drawbacks. The approach makes an accurate pH prediction, and it comprises a balance between memory and microprocessor speed for its integration within embedded systems with low memory and chip resources. In addition, it is also able to provide a high expressive explanation of why such prediction was made, so that the industrial validation is easier than using other proposals such as neural networks.

In this paper, we present a hybrid sensor for simultaneous vapor determination of relative humidity and toluene concentration. This sensor has been manufactured by two printing techniques: inkjet printing to define the electrodes and screen printing to deposit a sensitive composite. The structure is based on three meandered electrodes, acting the central one as common of both sensitive parts of the sensor. Half of this device is able to measure toluene concentration by changes in the resistance of a graphite-polystyrene mixture, whereas the other half determines the moisture content through variations in the electrical permittivity of the flexible substrate. The response to toluene shows a quadratic response with a linear coefficient of 0.3 $\text{m}\Omega \cdot [{T}_{\mathrm {ppm}}]^{-1}$ and a quadratic part of $5\times 10^{-8}\,\,\Omega \cdot [{T}_{\mathrm {ppm}}]^{-2}$ . With respect to the humidity determination, a sensitivity of 11 fF/%RH has been achieved at 100 kHz and 5.5 fF/%RH at 1 MHz. No dependence with toluene has been found in the capacitive element, being possible the simultaneous detection of both magnitudes. The same working principle would be applicable to sense other gas species by only changing the resistive composite.