Paper‐Based Printed Antenna: Investigation of Process‐Induced and Climatic‐Induced Performance Variability

Printing technologies have emerged as a viable method for the fabrication of various electronic components, including sensors, actuators, energy harvesters, thin‐film transistors and circuits, as well as antennas. However, printing processes have limitations in terms of surface roughness and thickness. Printing conductive structures on novel substrates, such as cellulose‐based sustainable paper, also leads to further challenges linked to the high surface porosity and ink carrier absorption. Herein, the variability of paper‐based printed antenna performance due to different printing processes, ink carrier absorption, and temperature is investigated. The resonance frequency and gain of different printed antennas (e.g., screen, inkjet, and dispense‐printed) are compared in terms of surface roughness, thickness, and resonance frequency. Screen‐printed antennas show better performance compared to other printed antennas. The results show that the resonance frequency of antenna shifts 20, 30, and 50 MHz for screen printed, dispense printed, and inkjet printed respectively, from the nominal 2.6 GHz. In the case of the inkjet‐printed antenna, a clear effect of skin depth is observed, due to the 0.91  μm$\text{μm}$ thickness. Furthermore, it is demonstrated that the permittivity/dielectric constant of the paper substrate is significantly influenced by ink carrier absorption and temperature variance.


Introduction
Printing technologies have been known and used since the first millennium A.D and have been evolving continuously from the first printed book "Diamond Sutra 868 A.D" to the first printed transistor in 2004. [1][2][3] Due to the recent rapid development of a wide range of customizable printable functional inks, the interest in printed electronics (PE) has further increased. PE has gained considerable interest in the last decade, and the estimated growth of its market share is expected to be 77.3 billion USD by 2029. [4] The main driving force behind this exponential growth is due to the emergence of the internet of things (IoT), for which the development of low-cost sensors and devices is inevitable. Printed sensors such as capacitive sensors, electrochemical sensors, and printed radio frequency identification (RFID) tags are among the major beneficiaries of the printing technologies.
As compared to the standard subtractive process used in the integrated circuit (IC) industry (which requires removal of the unwanted residues on the substrate), printing is an additive process. The additive nature of printing leads to a significant reduction of the chemical usage, which results in minimization of the negative environmental impacts of the manufacturing process. [5][6][7] Printing technologies are reliable in terms of scalability, cost-effectiveness, efficiency, and high throughput, so their widespread adoption is essential for low-cost electronics devices, particularly for printed RFID antennas. Currently several different printing techniques are used for the fabrication of electronic devices, such as printed diodes, [8,9] transistors, [2,10] sensors, [11,12] and antennas. [13,14] Another significant advantage of printing techniques is the ability to print on environmentally friendly cellulose-based paper substrates. In comparison to traditional PE substrates like polyethylene terephthalate (PET) and polyimide (PI), the paper substrate has a number of advantages, including affordability, foldability, and biodegradability. [15] These distinguishing characteristics, combined with additive manufacturing, opened up new research avenues for the fabrication of green electronics, with the goal of reducing e-waste and meeting the UN 2030 Sustainable Development Goals (SDGs). [13,16] Cellulose fibers are important component in paper fabrication and are derived primarily from plant-based derivatives. Depending on the application, the thickness, size, and weight of commercially available papers vary. However, when it comes to manufacturing, they are divided into two categories: coated paper and uncoated paper. Uncoated paper is primarily made of cellulose pulp, whereas coated paper has a matte (clay composed of CaCO 3 ) coating layer. For the deposition of conducive/functional inks, coated paper is typically used because of its low surface roughness and porosity. [17] Typically, the conductive/functional ink consists of primarily two main ingredients: the loaded particles (functional materials such as silver, carbon, and silicon nanoparticles) and the carrier fluid (known as a solvent, e.g., water, glycerol, triethylene glycol). After printing, the carrier fluid must be removed while the loaded particles must remain on the substrate surface, making the printed surface conductive. It is possible to print conductive ink on uncoated paper as well; however, because of the high surface porosity (caused by the cellulose fiber network), ink-loaded particles can permeate the paper and render the printed structures nonconductive. The coating fills the surface pores of the paper improving the surface roughness and preventing the permeation of loaded particles. For these reasons, when depositing conductive/functional inks, coated paper is preferred.
Despite the overwhelming economic benefits of printing and the environmentally friendly nature of paper as a substrate, there are some limitations of each printing technique and the use of paper as a substrate. For instance, inkjet-printed layer thickness is in nanometer range, screen printing requires a mask, and the dispense-printed structure surface roughness is slightly higher. For each printing technique, the required different ink viscosity, substrate thickness, surface roughness, and achievable print resolution are extensively reported. [18] Aside from printing process variation, the major disadvantage of paper substrates is surface roughness, porosity, and structural deformation caused by humidity, which adds additional uncertainty to the final printed device. While thickness and surface roughness are important parameters to consider in the fabrication of high-frequency electronics circuits and antennas due to effective skin depth, they become even more significant in the microwave and beyondfrequency spectrum. [19][20][21] For this reason, the antenna performance becomes dependent on the fabrication techniques.
Different printing techniques have been utilized to develop RF systems and antennas, including screen printing, [22] dispense printing, [23] and inkjet printing, [19,24] and also printed antenna on paper substrate has been reported. [25][26][27] However, no systemic studies have been carried out on paper-based printed antenna to investigate the printing process-induced performance variability. Apart from printing process variation, the paper substrate permittivity is important and alerted permittivity (due to ink carrier absorption and climatic variance) can significantly affect the performance of antenna. The substrate permittivity is not an important parameter for low-frequency electronics like printed electrochemical sensors [28] and for DC passive components like printed resistors, actuators, [29] and printed hearts. [30] However, for high-frequency electronics components such as antennas and filters, permittivity is a significantly (apart from surface roughness) important parameter. Even a minor change in permittivity can induce a significant shift in the resonance frequency.
In this work, a microstrip patch antenna has been designed in the long-term evolution (LTE) band 2.6 GHz (from 2500 to 2690 MHz), one of the global frequency bands for wireless data transmission and IoT application. The designed antenna has been printed on a paper substrate, using three different printing techniques: screen, inkjet, and dispense printing. The results of all printed antennas have been compared in terms of printed structure thickness, surface roughness, and resonance frequency, to evaluate the antenna performance variability due to each printing process. Furthermore, the best-performing printed antenna (i.e., screen printed) has been investigated further to assess the variability of antenna performance because of altered permittivity due to the ink carrier absorption by the paper substrate and temperature variation. To the best of our knowledge, the altered permittivity of the paper substrate caused by ink carrier absorption has not been reported in recent literature.

Results and Discussion
The antenna design, fabrication using various printing techniques, and characterization are described in the following sections. First, the paper substrate was characterized to understand the design parameters for the antenna simulation and its feasibility to be used as substrate for printing. Afterward, the antenna simulation was performed to define the antenna dimension and the optimized antenna was printed using three different printing techniques, namely, inkjet, screen, and dispense printing. Furthermore, all printed antennas were evaluated in terms of surface roughness, thickness, and reflection coefficient (S 11 ). Finally, the altered permittivity of the substrate caused by ink carrier absorption and environmental temperature was investigated.

Paper Substrate Characterization
A structural schema of the commercially available matte-coated paper used in this work is shown in Figure 1a. Fourier-transform infrared spectroscopy (FTIR) was used to investigate its exact chemical fingerprint and to determine the presence of cellulose and matte coating. Figure 1b shows the FTIR absorption spectrum of both matte-coated and uncoated paper. The former was confirmed with the 1383, 870, 712 cm À1 peaks representing the presence of CaCO 3 on coated paper, [31] while 3332, 1314, and 1022 cm À1 peaks can be attributed to the presence of cellulose, detected on the uncoated area of the paper, similarly to the findings of Aydemir et al. [32] Paper is not a typical substrate for printed antennas; for this reason, it is difficult to identify a standard value of permittivity ε r that can be used to design an antenna. Moreover, such value is influenced by both the porosity and the concentration of cellulose fibers in the paper, [33] which is usually unknown information. For this reason, the electrical properties of the substrate were evaluated, in terms of capacitance C P and resistance R p , using an impedance analyzer in the frequency range of 1-15 MHz. The C p and R p values were extracted by the instrument based on a parallel equivalent circuit model, [34] depicted in the inset of Figure 2a. The impedance analyzer derives the values of C P and R p from the admittance (Y ), which is the reciprocal of the impedance (Z ). [34] As shown in Figure 2a, as expected, the capacitance C P decreased with increasing frequency, because the capacitive reactance decreases as frequency increases, showing a typical capacitance frequency-dependent behavior. [35] Due to the presence of moisture and impurities (increasing the conductivity in the paper substrate), a significant decrease in www.advancedsciencenews.com www.aem-journal.com the R p value was also observed, by increasing frequency. [36] Afterward, using the parallel plate capacitor equation, the value of the frequency-dependent permittivity (ε r ) of the paper was extracted, as illustrated in inset Figure 2b, to observe the permittivity trend in the MHz frequencies range. The measured ε r ranged from approximately from 2.5 to 2.2 in a frequency range from 1 to 15 MHz, which is in agreement with literature. [13] However, the calculated permittivity showed frequency-dependent behavior, as depicted in Figure 2b. Such a frequency-dependent permittivity decreasing trend is even more visible in the GHz frequency range, which is the intended working frequency of the proposed device. This behavior can be attributed to the  www.advancedsciencenews.com www.aem-journal.com charge-space polarization effect [37] and it needs to be taken into account especially when designing printed antenna on complex substrates such as paper. For this reason, ε r in GHz frequency range was calculated using a ring resonator, by fitting the measured and simulated resonance frequency of the resonator (as shown in Figure S1a,b, Supporting Information, as reported before in other studies). [38,39] The calculated value of ε r was 1.6, as also reported in the literature. [40] This was confirmed by both the simulated and the measured results of the fabricated copper tape antenna (as shown in the Experimental Section and Figure 3b). In order to evaluate the potential structural deformation of the paper substrate caused by the ink carrier during the printing process, its absorption behavior was examined. This test is especially relevant for printed antennas, where even a minor structural deformation can increase the fabrication tolerance due to changes in antenna dimensions, thereby affecting the desired resonance frequency. For this reason, the compatibility of the carrier solvent should be considered when printing on paperbased substrates due to possible structural deformation caused by the carrier solvent. Here, the organic solvent triethylene glycol (TEG), a widely used carrier for functional ink formulations (especially for inkjet-printed inks), was dropped on top of the paper substrate. In Figure 2c, the absorption state of the 2 μL TEG microdroplet is depicted at four different time intervals, indicating good absorption behavior of the paper substrate. The droplet contact angle was 25 ∘ immediately after touching the substrate surface and became 0 ∘ after complete absorption, which occurred in a 4 min time span, as shown in Figure 2d. This fast absorption facilitates the ink drying process preventing the ink from spreading on the substrate, one of the aspects which limits the resolution of the printing techniques. [41] Moreover, upon absorption of TEG, no visible structural deformation was observed, confirming the ink compatibility with the substrate. Instead, as shown in the Figure S2, Supporting Information, water-based ink induced structural deformation in the paper. Indeed, in the presence of moisture, the cellulose fibers swell up (because the hydroxyl group in cellulose fiber attracts water molecules and forms a strong hydrogen bond) and the fiber network slides over each other, causing the substrate to slightly deform or expand in the vertical direction. Based on this test, TEG-based carrier silver ink was selected for inkjet printing, because inkjet-printed ink consists of 80% carrier and 20% solid-loaded particles.

Evaluation of Printed Antenna Using Multiple Printing Processes
The realization of the printed antenna was carried out in three stages. First, the microstrip patch antenna was designed and optimized using ANSYS HFSS simulation software. Afterward, a reference antenna was realized using copper tape (having predefined surface roughness, thickness, and sheet resistance) on the paper substrate (as depicted in Figure 4a) in order to validate the simulated design (as explained in Section 4). Finally, after validating the copper tape antenna design (as shown in Figure 3b), the optimized antenna prototype design was fabricated using three different printing techniques, namely, screen, inkjet, and dispense printing. The resulting printed antennas are shown in Figure 4b,c,d.
As shown in Figure 4e, the printed antenna resonance frequency changes in function of the employed printing technique. Specifically, the resonance frequencies shifted toward lower frequencies from the nominal 2.6 GHz of 20, 30, and 50 MHz for screen printed, dispense printed, and inkjet printed, respectively. Similar behavior of printed antennas on PET substrate was also reported in another study. [42] However, the use of a paper substrate adds a larger degree of variability compared to PET, mostly due to the substrate surface roughness and the ink carrier absorption of the paper, as discussed in Section 2.3. Since the printing technique determines the thickness, surface roughness, edge, and ink broadening over the nominal dimension of the printed design, the performance of a physical antenna depends on its fabrication method, regardless of the antenna dimension or the employed substrate.
As shown in Figure 5, the printed structure morphology using different printing techniques varied significantly, from 0.6 to 5.5 μm peak height of the surface morphology (based on line roughness profile R a ). The roughness and thickness of the www.advancedsciencenews.com www.aem-journal.com printed structure are important parameters of a printed antenna, which greatly influence the performance of the antenna in terms of radiation efficiency, gain, and the desired resonance frequency. This is particularly evident at high frequencies (30-300 GHz), when the wavelength falls in a range comparable to the value of surace roughness; the ohmic losses of the conductor increase, significantly affecting the electrical properties of the conductor and altering the effective conductivity of the printed conductive structure. [43,44] The variation in surface roughness observed in this work can be attributed to factors related to each printing technique, specifically to the ink particle size and the roughness profile of the substrate. In the case of the screen-printed structure, the surface roughness is mainly influenced by the particle size of the ink, which is around 4 -6 μm and has a thickness of 1 μm as evaluated by atomic force microscopy (AFM) (see Figure S3a-d, Supporting Information). The particle size of inkjet-printed ink is around 50 nm, which is significantly lower than screen-printed ink particle size, for this reason; the inkjet-printed structure almost replicates the surface roughness of the substrate. For this reason, the surface roughness of inkjet-printed structure becomes dependent on the surface morphology of the substrate. Finally, in the case of the dispense-printed structure, the surface roughness is significantly influenced by the printing mechanism. In fact, the dispense printer prints the structure line by line also called infill lines (as shown in the Figure S4b, Supporting Information). The thickness of infill lines is around 6-7 μm, which is the main contributor to the surface roughness profile of the final printed structure, clearly visible in Figure 5 and in Figure S5a-d, Supporting Information.
Moreover, both surface roughness and thickness are correlated with the skin depth. This parameter directly influences the skin effect, defined as the tendency of alternating current (AC) (combined with frequency dependency) to concentrate near the "skin" (surface) of a conductor, described by the following equation.
where δ is skin depth, ρ is resistivity, f 0 is signal frequency, μ r is relative permeability, and μ o is the permeability of the free space. [45] As clearly shown in Equation (1), the skin depth of a conductor depends on the frequency. As the frequency increases, the current distribution moves toward the surface, consequently decreasing the required skin depth for the current, as shown in Table S1, Supporting Information, where the calculated skin depth for silver at different frequencies is depicted. The skin depth of a conductor depends also on the surface roughness of the conductive path; when the roughness of a conductor becomes close to the skin depth, the ohmic losses of a transmission line increase; this effect has been explained through empirical formulas in other studies. [46,47] For this reason, the minimum thickness of the antenna should be at least two times greater than the skin depth and the surface roughness value should be lower than the skin depth at the desired operating frequency.  www.advancedsciencenews.com www.aem-journal.com Table 1 depicts the comparison of all fabricated antennas profile in terms of thickness, surface roughness, and sheet resistance. Surface roughness parameters in terms of S a (arithmetic mean of the surface) and S t (maximum peak-to-valley height) were calculated based on the 3D surface profile (complete surface 3d profile is shown in Figure S5a-d, Supporting Information) obtained by an optical profilometer, which uses white light and phase-shifting interferometry. S a and S t were calculated based on a 30 000 μm 2 scan area for all samples.
The inkjet-printed antenna shows the minimum surface roughness (S a ) of 0.68 μm and thickness (0.9 μm, almost equivalent to the skin depth at 2.6 GhZ) and the highest sheet resistance of 250 mΩ. Due to this, the antenna is characterized by a consistent ohmic loss rather than radiation losses, which ultimately affects the antenna radiation efficiency. The dispense-printed antenna structure has the lowest sheet resistance but has the highest surface roughness compared to other printed antennas, due to infill pattern lines. [48] However, because of the greater thickness than the effective skin depth (10.1 and 1.2 μm), the performance of dispense-printed antenna is better than the inkjet-printed antenna in terms of measured return loss (S 11 ), À13 and À6 dB, respectively. The screen-printed antenna surface roughness and thickness both fall in the acceptable range of the skin depth condition; for this reason, the measured (S 11 ) is À18 dB at the resonance frequency, which is better than inkjet, dispense, and even copper antennas. The poor performance of a copper tape antenna as compared the screen-printed antenna can be attributed to weak adhesion and air bubbles between the antenna radiating element and the substrate.

Altered Permittivity Due to Ink Carrier
The paper substrate's absorption behavior (as demonstrated in Section 2.1) facilitates the ink absorption process. However, the carrier solvent remains inside the cellulose microfiber capillaries, making it difficult to evaporate from the substrate at low temperatures (maximum stable temperature for paper is 150°C). [49] The ink solvent permittivity  is typically higher than that of the paper substrate (1. 6-4.2). For this reason, if the carrier solvent remains inside the substrate, as illustrated in Figure 6a, it can alter  Figure 6. a) Schematic illustration of altered permittivity, when the ink is cured below the boiling point of ink solvent, the ink solvent will remain inside the substrate and will alter the effective permittivity. b) The measured reflection coefficient of the screen-printed antenna at cured 100 and 140°C temperatures. c) The measured reflection coefficient S 11 before and after TEG layer of the copper tap antenna. d) FTIR spectra before and after coating TEG layer on the matte-coated paper substrate (the substrate was cured at 140°C for 30 min).
www.advancedsciencenews.com www.aem-journal.com the overall substrate permittivity, potentially causing a resonance shift. This effect is particularly significant when the carrier solvent possesses high permittivity and boiling point. To test this hypothesis and observe the antenna resonant shift due to unevaporated ink solvent, a screen-printed antenna was cured at two different temperatures, 100 and 140°C for 20 min. Such values were chosen because they are respectively below and above the carrier solvent boiling point (the recommended curing temperature from the ink manufacturer is 120°C). The measured return loss S 11 of the antenna (measured at room temperature %22°C) for each curing temperature was compared with a copper tape reference antenna, as shown in Figure 6b. As expected, in the case of the low curing temperature, the resonance frequency shifted to lower frequencies (2.52 GHz), indicating an increase in the substrate's permittivity value due to the presence of the solvent of the ink. Meanwhile, the antenna cured at 140°showed a resonance frequency (2.58 GHz) close to the reference copper antenna (2.6 GHz)).
When the boiling point of the ink carrier solvent is lower than the temperature range supported by the substrate, the carrier can be completely evaporated by increasing the temperature slightly above the boiling point of the solvent. However, in the case of inkjet printing, the ink solvent (TEG) is characterized by a high boiling point which negatively affects its evaporation process, especially when printed on a paper substrate. In fact, TEG has a boiling point of 285°C, while the matte-coated substrate's maximum stable temperature limit is 140°C. To test the effect of the presence of TEG in the cellulose microfiber, four layers of TEG were inkjet printed on a matte-coated paper substrate and then heated at 140°C for 30 min. Afterwards, the return loss S 11 of a copper-tape patch antenna made with TEG-coated and uncounted paper was measured. As shown in Figure 6c, the resonance frequency of the coated paper shifts toward lower frequencies due to the higher permittivity of TEG (ε r ¼ 26). To prove the presence of the solvent also after the curing, FTIR analysis was performed on both TEG-coated and uncoated paper. Figure 6d represents the FTIR spectrum of TEG-coated paper cured at 140°C, showing strong absorption peaks at 3425, 1352, and 1105 which correspond to the presence of TEG. [50] As was observed, both the use of ink with a high-boiling-point (and higher permittivity) solvent and the absorption characteristics of the substrate have a significant impact on the performance of the antenna in terms of resonance shift and increased fabrication tolerance. Such drawbacks can be compensated by shortening the antenna length, because the wavelength of EM wave decreases in materials having high value of permittivity. [51] A similar effect of resonance shift due to the TEG-based solvent ink has been observed in the case of inkjet-printed antenna, as shown in Figure 4e, where the antenna resonance showed a 50 MHz frequency shift from the nominal 2.6 GHz, which is higher as compared to dispense and screen-printed antennas.
While in this paper it is presented as a drawback, it is essential to highlight that this absorption behavior can be exploited for improving the substrate ε r value. In fact, a controlled coating of the substrate with an organic solvent having a higher permittivity value, such as TEG or glycerol, can be used for the sake of antenna miniaturization by increasing the substrate permittivity (as shown in Figure S6, Supporting Information). [52]

Altered Permittivity Due to Climatic Variability
Screen-printed antennas showed better performance as compared to other printed antennas. Therefore, this antenna was selected to carry out further tests in a climatic chamber, to evaluate the possible environmental conditions interference on its performance. The resonance frequency was measured while the temperature was swept from 10 to 70°C at constant humidity (i.e., 30% RH). As shown in Figure 7, as the temperature was increased, the resonance frequency changed, moving from 2.58 GHz at 10°C to 2.43 GHz at 70°C. The antenna resonance shift due to change in humidity is shown in Figure S7, Supporting Information.
A similar temperature resonance shift has been reported in the recent literature and has been exploited for temperaturesensing applications, as shown in Table 2. In our case, the patch antenna showed a linear behavior over a temperature range of 10-70°C, with a sensitivity of 22 MHz/10°C. This resonance shift is larger as compared to commercially available substrates such as FR4, printed circuit board (PCB) Rogers, and Alumina, and it can be attributed to the presence of paper substrate, making it ideal for sensing applications.
The reason for this shift can be attributed to a change in antenna length ΔL or a change in permittivity Δε r , as shown in Equation (3) and (4). By changing the temperature, the www.advancedsciencenews.com www.aem-journal.com coefficient of thermal expansion (CTE) can induce a change; however, the CTE for the paper substrate is small (around 44.6 ppm°C À1 ) and is strongly anisotropic (it expands on the vertical axis) and mainly influenced by a change in moisture contents in the substrate. [53,54] Since the humidity was kept constant, the shift in the resonance frequency can be mainly attributed to Δε r , which is due to the increased mobility of cellulose dipole chain. [55] 3. Conclusion The variability of paper-based printed antenna performance due to the printing process, altered permittivity due to ink carrier absorption, and temperature have been investigated in this work. The paper substrate was first characterized to better understand its composition (cellulose pulp coated with CaCO 3 ), permittivity value for antenna simulation, and suitability for printing conductive structures. The antenna simulation in Ansys HFSS was then carried out, and the optimized antenna was printed using three different printing techniques: inkjet, screen, and dispense. The surface roughness, thickness, and reflection coefficient of each printed antenna were evaluated and discussed in detail. As compared to all other printed antennas, screen-printed antenna showed better performance (-18 dB). Furthermore, the substrate's altered permittivity as a result of ink carrier absorption and temperature was thoroughly investigated and discussed. Especially, when designing antennas and high-frequency electronics circuits for printing processes, it is essential to take into account the altered permittivity caused by the ink carrier and temperature, in the higher-frequency range (5 G) because even small changes in permittivity can induce a significant shift in the resonance frequency.

Experimental Section
Antenna Design and Simulation: The geometrical dimension of the antenna was calculated using the following equations. [56] W ¼ c where W = width of the microstrip patch antenna, L = length of the microstrip patch antenna, ε r = dielectric constant of the substrate, ε eff = the effective relative permittivity, f 0 = resonance frequency, and c = light speed in free space (3 Â 10 8 m s À1 ). In order to obtain an initial approximate geometric dimension of the antenna, calculations were carried out by defining f 0 ¼ 2.6 and ε r ¼ 1.6. However, theoretical calculations are not sufficient to fine tune the resonance frequency of the antenna at the desired one (2.6 GHz) because there are several parameters, such as the surface roughness and thickness of the antenna radiating element that are not taken into account in Equation (2)- (4), which can influence the final resonance frequency of the antenna. Thus, further optimization of the antenna was carried out by simulating it in Ansys HFSS. In order to quickly verify the results, a copper-taped antenna was used as a reference. After several iterations (by adjusting the width and feeding gap of the antenna for better impedance matching), the antenna dimension was optimized (as shown in Figure 3a, and the simulation and measured results were found to be in good agreement. The simulated resonance frequency was 2.6 GHz, while the measured resonance frequency was 2.61 GHz, as shown in Figure 3b. Materials: 123 AE 2 μm-thick matte coated cellulose-based paper (matte-coated fedrigoni-paper) was used as a substrate. For the sake of rapid prototyping, a vinyl cutter (Plotter da taglio Roland, Camm-1 GS-24) was used to cut the copper tape (Alomejor Copper Tape, thickness = 10 μm) and vinyl sheets (for screen mask) according to the antenna dimension. Silver nanoparticle (AgNP) ink (Silverjet DGP-40LT-15C) and silver microparticle paste (LOCTITE EDAG PF 410 E&C) were used for inkjet, screen, and dispense printing, respectively. The concentration of AgNPs in the inkjet printer ink was 35 wt%, which is highly concentrated for the for 20 μm printer head (as shown in Figure S8a,b, Supporting Information), which could potentially clog the printer head because of the high concentration of AgNPs. [57] For this reason, an extra volume of solvent (triethylene glycol [TEG] monomethyl ether, purchased from Sigma-Aldrich) was added to the ink, in order to reduce the concentration of loading particles to 15 wt%, to make it suitable for inkjet printing.
Inkjet Printing: Commercially available drop-on-demand piezoelectric inkjet printer, Epson stylus office bx300f (shown in Figure S8b, Supporting Information, and refillable ink cartridges, Hemei T0715 T071, were used in this work. The printer had a resolution of 1200 Â 2400 dpi and a droplet volume of 3 pl (in high-resolution mode). [58] The designed antenna was printed using an open-source vector graphic software (INKSCAPE), through the default Epson driver. Four silver layers were printed per each sample in order to achieve a good conductivity (less than 1 Ω). Printing more than four layers would have increased the conductivity of the printed structure; however, it would also increase the fabrication tolerance due to misalignment and ink spreading.
Dispense Printing: A commercial dispenser printer, (Voltera V-one) ( Figure S4a, Supporting Information), was used. During printing, the paper substrate was attached to the surface of FR4 rigid sheets (1.5 mm) using Kapton tape, to prevent the paper from bending. The antenna design was generated using Eagle PCB design software. Printing optimization was performed by adjusting the amount of extruded ink and piston speed using a sample test pattern (zigzag lines). [8] Screen Printing: A semiautomatic screen printing machine, C290 Aurel automation S.P.A., equipped with a polymeric steel mesh with 120 mesh count per centimeter was used. The screen-mask design was realized using a vinyl stencil placed over the printing mesh, as shown in Figure  S9a-d, Supporting Information.
Ink Curing: The screen, dispense, and inkjet-printed antennas were cured in a thermal oven at 14°C for 30 min. In addition, the inkjet-printed antenna was selectively laser sintered, to further reduce the sheet resistance of the inkjet silver ink, using an optimized laser power (i.e., 15% power, max. power 60 w). The optimization of the laser parameters was carried out using test patterns (inkjet-printed lines, 5 Â 30 mm). www.advancedsciencenews.com www.aem-journal.com The sheet resistance optimization graph is shown in Figure S3a,b, Supporting Information; the optimized sheet resistance was 0.25-0.35 Ω sq À1 (using laser power 15% power, Max power 60 w). The AFM images of thermal and laser-sintered AgNPs are shown in Figure S3a, Supporting Information. Characterization Morphological and Chemical Characterization: The surface morphology was characterized using an atomic force microscope (Core AFM, Nanosurf ) and an optical profilometer (ProFilm3D Filmetrics). FTIR was performed using FTIR Spectrometer INVENIO (Bruker).
Electrical Characterization: An impedance analyzer (Keysight e4990a) was used to measure the capacitance of the parallel plate paper-based capacitor. The contact angle measurements were performed using a USB digital microscope and ImageJ software. The sheet resistance measurements were performed using a four-point probe setup (Ossila 4-probe). The antenna reflection coefficient (S 11 ) was measured using the Keysight E5061B Vector Network Analyzer, in single-port mode. The frequency range was set from 1 to 3 GHz. Keysight ECal N7550A electronic calibration module was used for port calibration.
Climatic Chamber Characterization: The measurements of antenna S 11 under different environmental conditions (temperature and relative humidity) were performed in a climatic chamber (Espec SH262). The temperature ranged from 10 to 70°C, while the relative humidity was kept constant (i.e., 30%), and the measurement setup is shown in Figure S10, Supporting Information.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.