MEAM Ph.D. Thesis Defense: “Design, Characterization, and Fabrication of Low-Cost, Passive, and Biodegradable Sensors For Precision Agriculture”

With the global population projected to reach 9.1 billion people by 2050 there is a need to develop highly efficient agricultural systems that maximize crop yield. Precision Agriculture (PA) systems enabled by the Internet of Things (IoT) offer a potential solution through improvements in labor, resource, and time efficiency to improve agricultural output. PA systems enable this by providing a detailed characterization of field environment (e.g., soil moisture, pH, temperature, etc.) so that these resources can be properly deployed spatially and temporally. To realize these systems, sensors that give information about the state of the field are required. However, for the technology to be scalable and practically implemented, these sensors must balance performance and cost. These requirements limit the materials and methods that can be used to develop the technology, including many that are common in modern sensor development. Additionally, the challenge of biocompatibility and biodegradability must be addressed.

In this work, a passive RF sensing system is presented for the detection of soil moisture. First, a fabrication process for a fully biodegradable cellulose nanofibril (CNF) based composite substrate is presented. By using small quantities of CNF, we are able to planarize the surface of a pulp-based cardstock paper to achieve smooth surfaces that are suitable for the fabrication of electrical structures. Next, the hygroscopic properties of cellulose are leveraged to develop a capacitive sensor that utilizes the substrate as the sensing mechanism. By utilizing screen printing, we repeatably produce capacitive structures with a high degree of fidelity. We demonstrate the ability to detect both humidity and soil moisture over a wide range and show the ability of the sensor to operate within the 902 – 928 MHz band. Additionally, sensor cycling and repeatability is demonstrated, a key requirement for in-field application.

Finally, integration and packaging of the sensor is investigated. Capacitive structures are integrated into a wired PCB system and are shown to exhibit good performance. Natural wax-based packagings are also explored, with mechanical characterization of various wax mixtures conducted to choose a suitable candidate. We show that the selected wax mixture can protect devices in a soil environment while still enabling capacitive sensing, demonstrating the ability for natural waxes to be used as a suitable packaging material for biodegradable sensors.