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With the advent of the fourth industrial revolution or Industry 4.0, manufacturing processes are undergoing radical changes thanks to the adoption of enabling technologies, such as Internet of Things (IoT), Big Data, Additive Manufacturing (AM), and Cloud Computing. As such, sensors are becoming essential to obtain additional data on production, parts, equipment health, and environmental conditions. All this concerns the aim of improving many aspects of industrial processes (flexibility, efficiency, cost, etc.). Sensors need to be placed everywhere in the industrial work environment (on machines, smart devices, objects, and tools) to monitor in real-time physical quantities such as temperature, vibrations, deformations that could affect production. With its characteristics, printed electronics (PE) can allow us the production of customized sensors and systems to make “smart” conventional objects. This work aims to analyze an innovative PE technology - aerosol printing - as manufacturing technology for electronics and sensors to be integrated into objects, showing examples of sensors fabricated by PE as applications that will be adopted for smart objects and Industry 4.0.

Here, we report a preliminary analysis on the manufacturing and testing of a low-cost and eco-friendly paper-based gas sensors realized with Aerosol Jet Printing (AJP) and Flash Lamp Annealing (FLA). The hygroscopic properties of paper make it possible to detect the presence of water-soluble gases in an environment provided with a relative humidity (RH) above a given threshold $(\text{RH} > 75\%)$ . This aspect allows the usage of such technology in the realization of a future food smart packaging detecting the presence of a malevolent gas like ammonia in food. Carbon interdigitated electrodes were printed and sintered on chromatographic paper, obtaining a mean resistance value of to $(231\pm 20)\ \mathrm{k} \Omega$ and $(249\pm 28)\ \mathrm{k} \Omega$ for the left and the right electrode respectively. The sensors were tested at different relative humidity (RH) values (75, 80, 90, 100%), and once stabilized they were tested with different ammonia concentrations (3, 6, 9 and 12 ppm). A proportional decrease was evidenced in increasing RH and ammonia concentration. Considering the baseline at a constant value of $\text{RH} =75\%$ , the sensors showed a resistance decrease of 15% in presence of the lowest concentration of 3 ppm.

Smart objects are capable of measuring the surrounding environment or internal status through their sensors. A viable solution to implement sensors in objects is to print them directly on the object. Printed electronics enables customization in the design and in the choice of the materials according to the final application. Aerosol jet printing (AJP) is a technology for the fabrication of electronic components directly onto objects. In combination with AJP, passive telemetric systems can represent a valid solution to interrogate sensors, reducing the number of electronic components integrated into the object. An example of a scenario on Industry 4.0 is smart packaging, which supports the control of the production process and of the transportation chain. In this work, we propose a manufacturing process based on AJP for fabricating simultaneously the sensing coil and the tuning capacitor to be used in a passive telemetric system for the measurement of a resistive sensor at a fixed frequency. The printed devices were characterized by electrical and geometrical points of view. The printed capacitance can be tuned from 13.95 pF to 31 pF, by changing the number of depositions. Capacitors with the same characteristics differ 1.28% at maximum. For validation, printed coil and printed capacitor were then used in a telemetric system for measuring resistive loads; known commercial resistors were used and the maximum error between the nominal value and the measured value obtained by the telemetric system was lower than 6% in the range $2.2\ \mathrm{k}\Omega$ and $560\ \mathrm{k}\Omega$ .

This paper presents the development of sensors on nonconventional substrates realized through the fast emerging printing technologies. Drop-on-demand inkjet, screen printing and aerosol jet printing are the leading rapid manufacturing techniques. The functional nanomaterials in their solution form are used for patterned deposition in a single pass making the process simple and very cost-effective. The patterned deposition and deterministic placement of various functional materials ranging from conducting, semiconducting and insulators are central to the development of a wide range of sensing devices. These devices being developed on nonconventional substrates particularly made from polymeric materials are playing significant role in advancements of printed wearable sensors and transducers. Here in this paper, we summarize some of our recent developments made towards realizing sensors on multiple substrates. A particular focus is oriented towards printing technologies for developing sensors in various geometric configurations. Also, few of the challenges faced by printing technologies are highlighted.

In this work, two promising Additive Manufacturing (AM)/3D-printing technologies, enable to realize conductive elements, are analysed, and compared with respect to the realisation of RFID applications. The selected techniques are the Fused Filament Fabrication (FFF) and Aerosol Jet® Printing (AJ® P), which have been compared, for the first time, by measuring the performance of an UHF RFID tag, specifically designed and realized for the scope. Results have also been analysed against to data obtained by a reference sample realized using the well-known technique of shaping an aluminium adhesive foil with a cutting plotter. While the resulting FFF-made tag can be read up to maximum 3 meters, the AJ® P one can be read up to 7 meters. Despite having major differences in terms of performance, this work demonstrates that both the technologies are capable to realize functional RFID prototypes.

In the latest years, Industry 4.0 paradigm is leading to a reduction of industrial process tolerances, an increase in machine lifetime and an improvement of the quality of the products. The fundamental concept of this fourth industrial revolution is the data coming from the whole production process. Information is usually extracted by algorithms that work on a large amount of data that are collected on the field by a huge set of sensors. New, customized and cheap sensors that are integrated into objects and processes are required. In this work, we propose a method that allows producing a novel class of piezoelectric force sensors through an additive and novel printing method: Aerosol Jet Printing. This technology allows fabricating electronic components and sensors on a wide set of substrates also directly on the surface of the objects. Therefore, smart objects for Industry 4.0 can be designed and manufactured. With the proposed approach it was possible to fabricate both axial and shear force sensors with a d33 up to 101 $\mathrm{nC}\cdot \mathrm{N}^{-1}$ with a capacitance of 10.4 pF. The fabricated sensors are approximately $300\mu \mathrm{m}$ thick and can be shaped to fit different custom application requirements.

The use of Aerosol Jet Printing technology to fabricate frequency selective surfaces (FSS) on glass for secure WLAN building applications is presented. Aerosol Jet Printing is combined with nano particle silver inks to produce the FSS array. A square loop design in a square lattice is employed in this demonstrator. The structure operates around the 2.5 GHz frequency band commonly used in wireless communication systems and covers a wide range frequencies. A single layer array design is studied. Aerosol Jet printing is able to produce fine tracks needed for the design and provide sufficient conductivity for the filtering performance. The aim is to demonstrate a potential solution for the development of FSS for building and radio frequency (RF) shielding applications. In particular, windows can contain these printed RF structures and enhanced their RF capabilities. CST Microwave Studio was used for the simulation of the FSS. Simulations compare well with measurements.

This paper presents a high bandwidth packaging technique, utilizing printed microstrip transmission lines. Aerosol jet printing is used to fabricate in-package transmission lines, which were characterized up to 18 GHz.

This paper reports on the design and fabrication of metal-oxide gas sensors, utilizing aerosol jet printing, combined with microhotplates fabricated through a process based on the industrial PolyMUMPs technology. Fabrication is complemented with in-house post-processing steps that do not require additional alignment or dry etching equipment, which greatly reduces the fabrication complexity. Aerosol jet printing is used to deposit thin homogeneous metal-oxide films, with micron thickness. A heating temperature over 300 °C is achieved by the microhotplates, at ~24 mW of power consumption. Measurement results demonstrate that the fabricated sensors exhibit good sensitivity and feature ultra-fast response times - as low as 1.2 seconds.

Nowadays, the increasing demand to realize plastic objects and devices equipped with sensors and electronics in the field of Internet of Things (IoT) and Industry 4.0 has required new fabrication methods. The best approach is to realize the sensors directly onto the object surface, which is usually 3-dimensional (3D) or non-planar. In this way, sensors and electronics could be perfectly integrated with the objects, avoiding the use of adhesive materials. The printed electronics represents a viable solution for producing smart devices in term of costs, variety of functional materials, throughput and reliability. Among the different printing methods, the aerosol jet printing (AJP) allows producing electronics and sensors directly onto non-planar and 3D surfaces. The photonic sintering cures the deposited metallic films, reducing the process time and avoiding the overheating of substrates characterized by low thermal stability. In this way, combining with photonic sintering, the AJP can be adopted for a wide variety of substrates, like paper, plastic, fabric and tissue. In this work, a first example of strain gauge printed directly on a PVC tube with the AJP and treated with photonic sintering is presented. The silver-based sensor was designed, realized and characterized. The thickness of the silver film (8 μm in average) was measured with a profilometer. The tube was bent in a cantilever configuration up to 0.5%. The relationship between mechanical deformation and electrical resistance is linear, with a gauge factor of 1.04. The decay over time of the resistance change under constant strain is about 0.05%. The proposed method is a promising fabrication solution for smart devices in IoT and Industry 4.0 applications.

This paper presents research focused on the development of a flexible fully printed IoT antenna for a drone-deployed autonomous sensor unit optimized for IoT applications. Aerosol Jet® was chosen as manufacturing technology. DuPont™ Kapton® HN 500 polyimide foil was used as a substrate. Conductive silver nanoparticle ink Clariant Prelect TPS50 was used for printing the antenna. Antennas were prepared with the different number of printed layers. A demonstrator with the fully printed flexible antenna was fabricated by Aerosol Jet printing technology and tested.

Soft robotics is a fast growing field of engineering where there are significant opportunities to realize new forms of actuation and sensing. However, there is also a challenge to translate some of the lab-based developments into a robust industrial manufacturing process. Linked to this is also the opportunity to create new forms of functionality in soft robotic devices by employing a flexible, digital manufacturing process for their creation. Overcoming these hurdles, and unlocking the possibilities for greater functionality, is likely to be a key enabler for wider applications and adoption of soft robotics in practical applications. We present a system of computer-controlled Aerosol Jet Printing that will enable complex soft robotic structures to be manufactured. The process is demonstrated through the deposition of a carbon-based conductive ink on to elastomeric substrates for high resolution, conformal and multilayer patterning. Functional demonstrators, in the form of a dielectric elastomer actuator and strain sensor, are produced to showcase the potential of the technique.

This paper presents a research focused on a comparison of the quality and electrical parameters of the interdigital electrodes prepared by aerosol jet printing (AJP) technique and conventional lift-off technique. The Interdigital electrodes (IDE) belong among the most commonly used periodic electrode structures for a wide variety of sensors and transducers. The IDE testing sample were designed with line/space widths of 25 µm/25 µm and active area of 4.32 × 4.55 mm. The advantages of the aerosol jet printing technique over conventional techniques consist in the fact that: a) it can produce high-resolution patterns (down to a feature size of 8 µm × 8 µm), b) it is maskless additive technique, c) it is compatible with flexible or even 3D substrates and d) it is compatible with a very large range of inks. In the frame of the research Clariant nanoparticle silver ink, Fraunhofer IKTS nanoparticle gold ink were aerosol jet printed on flexible PET and Kapton® polyimide substrates. Electrical and geometrical parameters were measured for each substrate deposited by AJP technique. The measured data of all testing samples deposited by AJP were compared to the measured ones of the samples realized by the lift-off technique on the alumina substrate.

We evaluated suitability of AJP (Aerosol Jet Printing) deposited silver layer on variety of organic substrates for the most common interconnect techniques used for electronic packaging. Specifically, we checked if the AJP silver layer can be electrically interconnected by Au and Al wires bonding technique. We also evaluated suitability of AJP silver layer for surface-mount technology (SMT). We performed electrical characterization of the AJP silver layer. We realized a fully functional working prototype of Autonomous Wireless Sensor Node system using AJP silver conductive track as an electrical interconnection.