In precision agriculture, information technology is used to improve farm management practices. Thereby, productivity can be increased and challenges with overfertilization and water consumption can be addressed. This requires low-power and wireless underground sensor nodes for monitoring the physical, chemical and biological soil parameters at the position of the plant roots. Three ESP32-based nodes with these capabilities have been designed to measure soil moisture and temperature. A system has been developed to collect the measurement data from the sensor nodes with a drone and forward the data to a ground station, using the LoRa transmission standard. In the investigations of the deployed system, an increase in the communication range between the sensor node and the ground station, from 300 m to 1000 m by using a drone, was demonstrated. Further, the decrease in the signal strength with the increasing sensor node depth and flight height of the drone was characterized. The maximum readout distance of 550 m between the sensor node and drone was determined. From this, it was estimated that the system enables the readout of the sensor nodes distributed over an area of 470 hectares. Additionally, analysis showed that the antenna orientation at the sensor node and the drone influenced the signal strength distribution around the node due to the antenna radiation pattern. The reproducibility of the LoRa signal strength measurements was demonstrated to support the validity of the results presented. It is concluded that the system design is suitable for collecting the data of distributed sensor nodes in agriculture.

This study describes a procedure for preparing microfluidic chips made from polydimethylsiloxane (PDMS). It uses a UV ozone cleaner for surface activation instead of the more common oxygen plasma treatment. Although this process has some limitations compared to oxygen plasma bonding, it can be used when a low-cost procedure for a small number of microfluidic chips is required or when an oxygen plasma etcher is unavailable. The challenges of this process arise from the slight hardening of the PDMS surface when it is activated for 70 minutes, which is necessary for reliable bonding. It is demonstrated that damage resulting from this hardening in conjunction with careless handling of the microfluidic chip is mitigated by incorporating predetermined break structures and using tubes with an outer diameter that is smaller than the inlets. Additionally, pre-polymerized PDMS glue and PDMS seals are suggested to ensure that the tubes are properly sealed. To demonstrate the concept, two microfluidic structures were prepared and tested, achieving flow rates of at least 170 µL/min at ±180 mbar. Bonding remains stable up to a pressure of approximately 0.5 bar.

This study describes a procedure for preparing microfluidic chips made from polydimethylsiloxane (PDMS) layers. It uses an Ossila UV ozone cleaner for surface activation instead of the more common oxygen plasma treatment. The process is limited to PDMS@PDMS bonding, and it can be used when a low-cost procedure for a small number of microfluidic chips is required or when an oxygen plasma etcher is unavailable. The challenges of this process arise from the slight hardening of the PDMS surface when it is activated for at least 70 min, which is necessary for reliable bonding. Activation of the surface is confirmed by contact angle measurements. In a static simulation and lab tests, it is demonstrated that damage resulting from this hardening in conjunction with careless handling of the microfluidic chip is mitigated by incorporating predetermined break structures and using tubes with an outer diameter that is smaller than the inlets. Additionally, pre-polymerized PDMS glue and PDMS seals are suggested to ensure that the tubes are properly sealed. A burst test in air was performed to demonstrate the stability of the bonding and gluing. IR and XPS measurements were performed to investigate surface and bulk changes due to the treatment. To demonstrate the concept, six microfluidic chips were prepared and tested, achieving flow rates of at least 170 µL/min at ±180 mbar. Bonding remains stable up to a pressure of approximately 428 ± 60 mbar.