Geometrical Optimization of Printed Interdigitated Electrode Sensors to Improve Soil Moisture Sensitivity

Printing electronics allows us to fabricate low-cost, simple sensor networks. Distributed soil moisture sensors in agriculture industry will result in better water management practices; however, price and reliability do not allow for ubiquitous availability. Here, we demonstrate the fabrication of low-cost, flexible, interdigitated electrode (IDE) soil moisture sensors with the electrode widths of 537, 877, 1055, 1204, and
$1916~\mu \text{m}$
on polyimide (PI) substrates using a silver nanoparticle (Ag NP) ink. The sensors were tested in sandy textured soil with a volumetric water content ranging from 0.05 to 0.25 m3/m3. Analytically, the sensitivities of all the sensors are expected to be equal, as the proposed mechanisms dictate that the capacitance should depend linearly on the dielectric constant of the moist soil. However, we found the sensitivity for the widest IDE, which measured
$1916~\mu \text{m}$
to be 442 pF/H2O volumetric percentage, whereas the sensitivity for the sensor with 537-
$\mu \text{m}$
-wide electrodes to be only 221 pF/H2O volumetric percentage. We hypothesized that the increase in sensitivity can be directly related to the increase in the capacitive fringing field height, which results in increased sensing volume for the widest IDE. The increased sensing volume leads to increased water molecule polarization, increasing the overall capacitance change with respect to changing water content. To validate the hypothesis, we conducted electric fringing field experiments to observe the trend between the maximum fringing field height and the electrode width of an IDE. Results confirmed that increased sensing volume was caused by the increased fringing field height of the electrode in an IDE.