A review on recyclable printed electronics: Fabrication methods, inks, substrates, applications and environmental impacts

Innovations in industrial automation, information and communication technology 1 (ICT), renewable energy, monitoring and sensing fields have been paving the way for smart devices, 2 which can acquire and convey information to the internet, in every aspect of our lives. Since 3 there is ever-increasing demand for large yet affordable production volumes for such devices, 4 printed electronics has been attracting great attention in both industrial and academic research. 5 In order to understand the potential and future prospects of the printed electronics, the present 6 paper summarizes the basic principles and conventional approaches while providing the recent 7 progresses in the fabrication and material technologies, applications and environmental impacts. 8


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Printed electronics has a great potential to offer biodegradable and recyclable solu-12 tions, which is a way forward to minimize the electronic waste (e-waste) caused by the 13 ever-increasing number of disposable electronic devices [1,2]. In case of additive manu-14 facturing (AM) of these devices, e.g. by conventional and the state-of-the-art printing   In the work by Rivadeneyra et al., it was concluded that the sensitivity of printed 177 temperature sensors could be increased by altering the order of fabrication steps [45]. 178 Higher sensitivity values were obtained by following a sequence of first printing and 179 drying the silver electrodes, then printing and drying PEDOT:PSS and finally, sintering 180 the sensor. In addition, sensitivity of these sensor modules were also enhanced by a 181 factor of 2.2 as a result of increase in electrode spacing from 150 µm to 200 µm. 182 Strain gauge sensors, as being another common sensor application, are used across 183 numerous industries for monitoring strains caused by external forces and/or moments. 184 As seen in Figure 5, such sensors are used to monitor curing and exerted pressure during  printed TFTs, which is a step forward to fully recyclable electronics [65][66][67]. 208 In addition to low power TFTs, multiple transistors have been recently combined 209 with larger electronic circuits by Matsui  RFID is a technology that automates the process of extracting data from RFID tags 232 or smart labels and identifying them [74,75]. As the name suggests, RFID is used to 233 transfer data wirelessly via RF transmissions. As schematized in Figure 6, a typical RFID 234 system consists of three parts: RFID reader, RFID tag or smart label categorized as active        The technologies used for printing electronic components are well-known in the 317 graphic arts, and some example options are gravure printing, flexography, offset printing, 318 screen printing, and inkjet printing as depicted in Figure 11.  The key properties and parameters of the different printing techniques are summa-345 rized in Table 2. Flexographic printing creates a thin printed layer with a feature size        The main advantage of inkjet printing is high print quality (resolution 2880 dpi), but  should not be lower than 1 mm, must be also maintained for the accuracy. Exceeding   . In addition to these commonly used metals, gold is another easy to 548 prepare metal as an ink, environmentally stable and requires relatively low sintering 549 temperatures in order to function as a conductor; however, the price of gold is high.

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Aluminium has a high tendency for oxidation and is more chemically active compared

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In Table 3,   Ink properties such as viscosity, surface tension, particle size and solid content have 663 large impact on the printed electronics. To elaborate, the viscosity of an ink specifies 664 the resistance against the flow at a specific shear rate. As listed in Table 4

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There is a tension at the surface between a liquid and a gas, because of the asym-672 metric attractive force between the molecules, this phenomenon is called surface tension.

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The functionality of metal particle inks improve with decreasing particle size [115].

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Decreasing the particle size increases the surface area and increases the amount of 679 stabilizing agents required. Additionally, smaller particle size in the ink cause a high 680 surface to volume ratio, which leads to the requirement of lower sintering temperatures.  Table 4.  properties of the conductive ink [130].

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The surface roughness can be analysed by studying the surface topography and the 704 cross-section of a sample. An atomic force microscope (AFM) or scanning electron micro-705 scope (SEM) can be used to analyse the surface topography [142]. SEM can additionally 706 be used to study the cross-section of a sample after cutting the sample using a focused 707 ion beam (FIB) instrument. Other topographic measurement devices can also be used to 708 study the surface roughness. The porosity can be examined using a print penetration 709 test or a mercury porosimeter [130]. ink laterally, which on the other hand allows the printing of more accurate patterns.

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The differences in the resistivity of silver nanoparticle inks on glass and PI substrates 857 using different sintering methods are compiled in Table 6. In addition to the sintering 858 method, substrate and ink, the resistivity depends on the silver particle size, other 859 materials used in the ink and the printed pattern.  Figure 12. Holistic overview for life-cycle of a printed electronics device. Operational energy is the energy required to use the device while embodied energy refers to the consumed energy by the processes associated with the production and end of life.
In order to address the environmental impacts of the printed electronics, compre-862 hensive assessment throughout the device life, known as life cycle assessment (LCA), is 863 a must. As holistically schematized in Figure 12, LCA of printed electronics provides rel-