The Food and Agriculture Organization (FAO) has forecasted that food productivity must be enhanced by 60% by 2050 to meet the demand for 10 billion people worldwide. [1] Climate change, harsh environments, and infertile lands have challenged agricultural production. Moreover, abiotic factors, including temperature and humidity, play significant roles in the growing conditions of plants in greenhouses and open farms. [2,3] Therefore, incorporating smart sensor technology that monitors and controls critical environmental factors such as temperature and humidity is becoming essential in the agricultural industry. [4][5][6][7] Moreover, the need to measure temperature and humidity under different circumstances has driven the development of various device structure designs, materials exploration, and cost-effective fabrication methods. [8,9] Additively manufactured electronics, or so-called printed electronics, have grown significantly in recent decades due to their fast design to manufacturing cycle, low cost, mechanical flexibility, and feasibility for rapid production. [10][11][12][13] Such printing technologies include aerosol-jet printing, ink-jet printing, drop-on-demand printing, gravure printing, and screen printing. [14][15][16][17][18][19][20][21] These integration possibilities have rapidly developed flexible hybrid electronics and sensors. However, due to the liquid-based nature of such methods and their incompatibility with biodegradable papers, most of the prints are done on non-biodegradable polymer-based substrates. Thus, creating significant disposal and e-waste issues after their service. To bypass such limitations, there are some reports on using waxed or laminated paper. [22] However, such treatments diminish their biodegradability and affordability.
Paper comprises cellulose fibers with a porous network that is abundantly available, cheap, and lightweight. [23,24] It offers other advantages, such as a mature manufacturing process, mechanical flexibility, biocompatibility, environmental friendliness, and nontoxicity, over its counterparts, polyethylene terephthalate (PET) and polyimide (PI). [25,26] The biodegradability of paper enables sensors printed on paper to be easily recycled or disposed of, as they can be broken down more easily than nonbiodegradable substrates.
A capacitance-based relative humidity sensor includes an interdigitated structure with a paper substrate serving as a dielectric. [27][28][29] Thus, the paper absorbs water vapor to achieve equilibrium with the external environment. The overall capacitance of the interdigitated electrode is determined by the electrostatic contribution of the neighboring electrode and changes when the absorption of water molecules alters the dielectric properties of the substrate. [30][31][32] The dielectric constant is lower in dry conditions compared to wet conditions, resulting in an increase in capacitance as water molecules are absorbed by the paper. This change in capacitance can be measured to reflect the relative humidity of the surrounding environment. The temperature sensing mechanism of the resistance-based temperature sensor relies on the increase of the resistivity of the metallic conductor with an increase in temperature. [33,34] This study demonstrated the design, fabrication methods, and characterization of capacitive and resistive humidity and temperature sensors, respectively. We dry-printed Ag patterns on various papers using the dry ANM method, creating capacitive and resistive structures for sensing temperature and humidity. [35][36][37][38] Four different grades of paper of varying solubility and thickness (24-lb, 5-Pt fast water-soluble, 32-lb, and cleanroom paper) were used for printing the sensors. The sensing properties of the samples were characterized by monitoring the changes in capacitance and resistance when subjected to varying temperature and humidity levels.
Sensor Fabrication:
The samples were printed using our dry additive nanomanufacturing (ANM) method, as shown in Figure 1a. It consists of a nanoparticle generation chamber, a solid silver target, an ablation laser for nanoparticle generation, and a second sintering laser for sintering the landed nanoparticles through a nozzle on the substrate, placed on an XY-positioning stage. The ablation laser interaction with the target caused the materials to be ablated, forming a laser plasma plume. [39] The interaction of these ablated species with background argon gas resulted in condensation and the formation of nanoparticles. [40] An argon gas flow transported a stream of nanoparticles toward the nozzle for printing and sintering the nanoparticles into desired patterns. The nozzle created a nanoparticle flow from the tip of the nozzle to the surface of the paper substrate. The momentum of the nanoparticles during the flow caused them to collide with the surface of the paper, and at the same time, the sintering laser beam was directed onto the substate's surface, where the nanoparticles landed. The controlled interaction of the sintering laser with the nanoparticles led to localized heating and sintering of the nanoparticles without burning the paper. The programmable XY stage controlled the position and created specific patterns on papers with varying thicknesses and solubilities. Our previous work thoroughly discussed the dry ANM printing technique. [41,42] This paper extends our earlier research by investigating the printing, fabrication, and characterization of paper-based sensors, such as humidity and temperature sensing. The capacitive humidity sensor consisted of 42 fingers, each with a line width of ~125 um and a 2.3 cm length, with a gap of ~140 um between the fingers. The overall size of the sensor was 2.4 cm × 2.7 cm. A photograph of the capacitive humidity sensor printed on 32-lb paper is shown in Figure 1b. The resistive temperature sensor consisted of a conductive line of silver particles arranged to maximize the length of the line. The photograph of the fabricated resistive temperature sensor on a clean room paper is shown in Figure 1c. The sensor consisted of printed silver lines, each of ~155 um wide and 2.6 cm long, with an overall dimension of 2.7 cm × 3.3 cm. The gap between the printed silver lines was ~800 um. Figure 1d,e shows the 10x magnification optical image of a printed line of the capacitive humidity sensor and resistive temperature sensor. Figure 1f displays a 100x magnified SEM image of the printed silver line, highlighting the enhanced contrast between the printed silver and the substrate. It is evident from the SEM in Figure 1g that the silver lines are nicely coated on the paper and well connected to each other, exhibiting good conductivity. Additionally, the thin layer of nanoparticles did not obscure the paper's surface texture and remained clearly visible.
Measurement Set-up: A schematic illustration of the measurement setup is shown in Figure 2. The setup comprised a custom-built acrylic chamber to maintain constant humidity. Two hot plates, one outside the chamber for boiling water and another placed inside the chamber (TLBZK-100), capable of reaching up to 400 ℃ for sensor testing, were used. The measurement setup included a test head, a 16194A high-temperature test fixture, an Agilent 4287A RF LCR meter, a DOQAUS digital hygrometer, and a variable-area flow meter. Water vapor, produced by boiling water on a hot plate outside, was introduced into the chamber through controlled airflow regulated by a variable flow meter to achieve and maintain the desired humidity. The relative humidity and temperature inside the chamber were monitored using the DOQAUS digital hygrometer. Furthermore, the TLBZK-100 heater was utilized to set the desired temperature. Capacitance and resistance were measured using an Agilent 4287A RF LCR meter with a built-in PC and digital display. The test head was utilized for mounting the 16194A test fixture, facilitating the connection of conducting wires from the sensor electrodes. This setup enabled the sensor to be connected to the LCR meter for electrical characterization. The fabricated sensors were positioned in the test chamber, and two wires were employed to connect with two electrodes of the interdigitated capacitor on one side. On the other hand, these wires were connected to the test fixture of the LCR meter. Measurements were conducted at varying test frequencies of 1 MHz and 10 MHz, with each measurement test triggered manually.
The effectiveness of a paper-based capacitive humidity sensor hinged on the absorption and desorption of water molecules within its fiber network. [29] As the interdigitated humidity sensor was exposed to different humidity levels, the printed sensor absorbed and desorbed varying amounts of water molecules from the surrounding ambient. As the humidity rises, the dielectric constant increases, which directly increases the capacitance of the interdigitated capacitor. Conversely, as the humidity decreases, the dielectric constant decreases, resulting in a reduction of capacitance. The experimental arrangement and equipment used to measure these effects are depicted in Figure 2.
The capacitive relative humidity measurement was conducted cyclically to access the sensor hysteresis. Figure 3. illustrates the changes in sensor capacitance with respect to 20%─ 90% relative humidity for both increasing and decreasing humidity levels. At higher RH, the sensor absorbed more water vapor. When RH was reversed to the initial low level of 20 %, the sensor took longer to desorb water and reach equilibrium. The absorption process was relatively rapid. These cycles displayed a rise in forward and a decrease in reverse RH for the single measurement cycle on different paper substrates. The fitted characteristic curves are displayed in the figures. Dynamic measurement to assess the performance of the capacitance-based humidity sensor was conducted by varying relative humidity (RH) from 20%─ 90% in 10% RH steps. All data were taken in laboratory conditions at room temperature of 22±2 ℃. Figure 3a,c,d show that the capacitance increased from 24.9 pF to 47.3 pF for 24-lb, 28.5 pF to 57.1 pF for 32-lb, and 27.6 pF to 68.7 pF for the humidity sensor printed on cleanroom paper substrate. From the Figures 3, it can be noted that the capacitance increased with increasing RH, which is in accordance with previously reported literature. [43,44] The capacitance increased for sensors printed on 24-lb, 32lb, and clean room paper. The sensor printed on fast water-soluble paper initially exhibited a nearly linear increase in capacitance from 24.5 pF to 60.2 pF as RH increased from 20% to 70%, as shown in Figure 3c. However, the capacitance dropped to 36.9 pF at 90% RH. This behavior was attributed to the degradation of the paper substrate and, hence, discontinuity of the printed metal electrodes on fast-soluble paper at very high humidity levels.
Illustrates that the clean room paper exhibited the most significant change in capacitance from 27.63 pF to 67.71 pF at 1 MHz test frequency. An overall percentage change in capacitance of 89.7%, 100%, and 148.6% was observed for 24-lb, 32-lb, and cleanroom paper, respectively, as compared to base capacitance when RH was varied from 20% to 90% at 1MHz test frequency. Similarly, Figure 4b. depicts the normalized capacitance for the humidity sensor printed on the four different types of paper substrate when the relative humidity was increased. We calculated the normalized value by using (Csen -Cmin) / (Cmax-Cmin). [45] The capacitive sensitivity (SC) of the humidity sensor is displayed in Figure 4c. The capacitive sensitivity of the humidity sensor was calculated by using the formula SC = (ΔC / C0) × 100. [45] SC describes the percentage change in capacitance exhibited by the sensor in response to a change in relative humidity. The humidity sensor printed on 24-lb, 32-lb, and cleanroom paper showed a sensitivity of 89%, 100%, and 148.6%, respectively, when Rh was increased from 20% to 90%. Sensor printed on fast soluble paper showed the highest sensitivity of 153% at 70% RH. However, the sample deteriorated at higher relative humidity due to the rapid absorption of water molecules. Similarly, the percentage change in capacitance was more pronounced at 1 MHz (148.9%) than at 10 MHz (70.2%) for the sensor on cleanroom paper. The capacitive sensitivity of 100% and 148.6% at 1 MHz test frequency was more significant than that of 56.3% and 69.9% noted at 10 MHz, respectively, for sensors printed on 32-lb and cleanroom paper substrate. This suggests that the capacitors have a low-frequency response behavior, as expected. This is in agreement with previously reported results and matches our expectations. [25,46,47] Printed temperature sensor performance in a humid environment
The stable operation of printed sensors under diverse environmental conditions is essential for their integration into field applications. Relative humidity and temperature are some of the standard variable environmental parameters, especially in agricultural applications. Electrical measurements were performed to evaluate the effect of applied heat on the printed temperature sensors in a controlled environment. The resistance-based temperature sensor operates by detecting the rise in resistivity of the metal conductor as the temperature and humidity are increased. This occurs because the increase in temperature causes more phonon vibrations, which in turn impede the movement of free electrons. [33] Also, variation in temperature affects the fibers and their binding. [48] These changes impact the surface roughness and dimensional changes, leading to cracks and disconnections in the printed tracks, affecting conductivity. Figure 6 illustrates the resistive response of the printed temperature sensor on a cleanroom paper substrate with temperatures varying from 25 ℃ to 50 ℃ at different humidity levels from 10% RH to 70%. As the temperature was increased from 20 ℃ to 50 ℃, it was observed that the resistance of the sensor increased from ~402 Ω to ~428 Ω at 10% relative humidity level, as shown in Figure 6a. This is in agreement with the temperature-dependent electrical resistivity of metal, where generally, the resistance of metal increases with temperature and is also known as positive temperature coefficient behavior. [49] For 30%, 50%, and 70% RH, the resistance increased from ~181 Ω to 193.4 Ω, 180 Ω to 189 Ω, and from 169 Ω to 179 Ω when increasing the temperature from 25 ℃ to 50 ℃. The resistance curves are linear, and negligible hysteresis is observed in the full cycle, as shown in Figure 6a,b,c,d. Overall, there was a change of about 4%, 6.8%, 4.9%, and 5.9% in resistance with increasing the temperature from 25 ℃ to 50 ℃ at 10%, 30%, 50%, and 70% relative humidity, respectively. Figure 8. Illustrating the water solubility of the printed pattern. Images of a printed sample on a fast water-soluble paper after 20 seconds (a) and 2 hours (b) immersion in water. Images of printed sample on a 32-lb paper after one day (c) and two months (d) immersion in water.
To evaluate the longevity of the sensors on different samples, sensors were printed on fast-soluble and 32-lb papers and immersed in water. The fast water-soluble paper was quickly dissolved, as shown in Figure 8a, and the paper substrate was completely disintegrated after two hours, as illustrated in Figure 8b. The silver remained in the water. Reports have suggested that silver nanoparticles may not be harmful to crops, and rather, they may improve plant growth and crop productivity. [50] Figure 8c,d. shows a sensor pattern printed on 32-lb paper, dissolved on day one and after two months. It can be observed that both the printed silver pattern and the paper substrate are undamaged after two months, showing that the printed sensor on the 32-lb paper substrate was reliable.
In this study, interdigitated capacitance-based humidity sensors and resistance-based temperature sensors were printed on various uncoated biocompatible paper substrates using the dry ANM process for various applications, including smart agricultural fields. The capacitive response of the interdigitated humidity sensors was investigated across a relative humidity range of 20% to 90% for samples printed on 24-lb, fast water-soluble paper, 32-lb, and clean room paper. The humidity sensor printed on cleanroom paper exhibited the most significant change in capacitance, from 27.6 pF to 67.1 pF at 1 MHz test frequency, followed by 32-lb, and 24-lb paper substrate. An overall percentage change in capacitance of 87.7%, 100%, and 148.7% was observed compared to base capacitance when changing the relative humidity from 20% to 90%. The sensors demonstrated reliable and sensitive responses to changes in temperature and humidity levels. Our findings highlight the importance of substrate selection, with cleanroom paper showing superior performance in terms of capacitance change for RH sensing. The resistance-based temperature sensor response highlights the relationship between temperature and sensor resistance. Our printed temperature showed a sensitivity of 6.8% and 5.8% at 30% and 70% RH, respectively. In conclusion, these sensors showed great promise for biodegradable and eco-friendly monitoring of environmental conditions with implications for agricultural applications.
The materials used in the experiment include a 1-inch solid silver target (99.99% pure, from Kurt J Lesker), standard printing paper (24-lb and 32-lb Hammer mill premium printing paper), as the substrate and 5pt fast water-soluble paper was obtained from SamrtSolve.
The SEM and BSE image was obtained using a ZEISS EVO-10 scanning electron microscope equipped with a BSD detector.
The printed silver lines of the printed sensors were optically imaged using a Keyence VHX-6000 series microscope at 10x magnification.
A DOQAUS digital hygrometer with a 5-second response time was used to measure humidity and temperature within the chamber environment. TLBZK-100 heater with a digital display was utilized to set the desired temperature for testing the sensor.
Measurement parameters like capacitance and resistance were measured using an Agilent 4287A RF LCR meter with a built-in PC and digital display. The LCR meter comes with a test head and 16194A test fixture. The 16194A test fixture was mounted on the test head, facilitating the connection of conducting wires from the sensor electrodes. The sensors underwent measurement at 1 MHz and 10 MHz test frequency.