The rapid expansion of the Internet of Things (IoT) and its integration across various industries is contributing to the significant increase in electronic waste (e-waste), posing serious environmental and sustainability challenges. This thesis addresses the pressing need for eco-friendly IoT sensing solutions by developing transient sensors designed for smart packaging applications, with a particular focus on the cold chain supply, where accurate temperature and humidi-ty monitoring is crucial for preserving perishable goods.
The research begins with the validation of biodegradable materials for capacitive and chipless sensing devices intended for continuous environmental monitoring. As substrates, shellac resin is promising due to its low sensitivity to moisture, while a novel cellulose-PHBH composite polymer is introduced for its compatibility and stability in printed radio-frequency (RF) applications. Sustainable production of these sensors is achieved by the use of ecoresorbable materials and additive manufacturing techniques, which are more energy-efficient and generate less material waste compared to traditional clean room processes, while also offering high scalability.
We successfully demonstrate the effectiveness of resistive, capacitive, and resonant transducers made of car-bon and zinc on various biodegradable substrates. Carbon-based resistive temperature sensors exhibit a linear sensitiv-ity of 5300 ppm/K over a temperature range of 15°C to 35°C. For humidity monitoring, interdigitated electrode (IDE) capacitors coated with egg albumen achieve a capacitance change of 1.1%/%RH within a relative humidity range of 20% to 80%. A RF printed zinc microstrip line on paper, protected by beeswax, operating between 1-3 GHz, shows a linear resonance shift of -1.3 MHz/°C for temperatures between -10°C and 35°C, and a sensitivity of up to 8 MHz/%RH for humidity sensing using konjac glucomannan. The incorporation of a natural hydrophobic beeswax encapsulation en-hances the stability and lifespan of the sensors by protecting the zinc transducer from oxidation, enabling reliable sim-ultaneous monitoring of temperature and humidity.
Additionally, we introduces bio-based phase-change materials (PCMs) such as olive oil, jojoba oil, and coconut oil for temperature-threshold detection, targeting specific temperature crossings at 8°C, 15°C, and 25°C, respectively. These PCMs are integrated into both capacitive and wireless near-field communication (NFC) devices. Sensor design parameters are optimized through simulations and experimental validations, focusing on the capacitive behaviour of the PCMs during melting, with a novel capillary structure ensuring reliable operation at inclined angles. Furthermore, an ecoresorbable chipless NFC tag made from a zinc transducer on a PHBH-cellulose substrate, resonating at 1.8 GHz, demonstrates a significant irreversible resonance shift of over 30 MHz after PCM melting when temperature thresholds are exceeded.
The transient nature of these sensors is validated through lab-based composting tests, confirming disintegration within one to three months and ensuring environmentally responsible disposal.
In conclusion, this thesis advances the development and manufacturing of innovative transient environmental sensors, for the IoT applications. By utilizing transient materials and additive manufacturing techniques, this research paves the way for future advancements in sustainable electronics.