Inkjet-Printed Multisensor Platform on Flexible Substrates for Environmental Monitoring

The development of low-cost smart labels with sensing capabilities is raising a high interest among the scientific community due to the increasing need for monitoring ambient conditions in the fields of wearable electronics and logistics. Printed electronics gathers all the requirements to enable the development of cost-effective smart systems, the reason why the framework of this thesis, the EU project FlexSmell, targets the development of a printed radio frequency identification (RFID) smart label with sensing capabilities for perishable goods monitoring. In that respect, I present in this thesis the design, fabrication and characterization of different chemical gas, humidity and temperature microsensors fabricated on plastic foil by means of additive methods compatible with large-area and large-scale production. The high potential of integrating these microsensors together was demonstrated by inkjet printing a multisensing platform able to detect relevant parameters for perishable goods monitoring, such as temperature, relative humidity (R.H.) and presence of diverse chemical gases. On one hand the use of cost-effective polymeric substrates brings novel functionalities to the system, namely flexibility and light weight. On the other hand, owing to their simplicity and additive character, printed techniques can potentially lead to a reduction of the fabrication cost of smart labels by minimizing the number of processing steps, the usage of raw material and the need of expensive clean-room infrastructure. Inkjet printing is especially appealing for research among other printing techniques because it permits quick prototyping due to its digital character (no need of a mask) and enables local functionalization of the different sensors in the platform. The issue of the typical low resolution of inkjet printing was addressed in this work by proposing suitable sensors architecture and optimizing the fabrication processes of inkjet printing silver nanoparticles, plating on plastic foil and inkjet-printed polymers. The combination of the optimal process and materials with the investigation of different sensor architectures permitted the development, for the first time, of fully printed capacitive R.H. sensors with high performance and reduced footprint. The sensors developed in this thesis were based on the absorption of the analyte in the sensing layer, which modified its relative permittivity and thickness. Cellulose acetate butyrate (CAB) has been inkjet-printed to perform as R.H. sensing layer in this work. Several theoretical models have been proposed and employed to understand the working principle of the different developed transducers and sensing layers, facilitating their optimization. Since the employed polymeric foils provided the system with mechanical flexibility, a new theoretical model to predict the effect of bending in sensors based on interdigitated electrodes (IDE) has been thoroughly described. The first developed sensor was based on IDE to facilitate the interaction