Ultra-thin IDE Pulse Wave Sensor

The monitoring of vital signs is used to determine human health status. Healthcare monitoring devices are usually attached to the human skin to obtain information about the human body. However, the main inconvenience of using conventional electronic devices is the mechanical mismatch between the devices and the skin. This issue can lead to measurement errors, and patient comfort can be affected negatively when these devices are used continuously. Therefore, it is needed to develop skin-conformal electronic devices to overcome these drawbacks. This thesis explores the fabrication process of ultrathin interdigitated pulse wave sensors based on the piezoelectric effect. The aim of this research is to demonstrate that printed electronics technologies are an excellent alternative to fabricate low-cost skin-conformal sensors. First, this thesis explores the theoretical background of piezoelectricity, flexible and ultrathin piezoelectric pressure sensors, and printed electronics technologies. Then, the fabrication process is analyzed. The sensor is fabricated onto a Parylene-C substrate using the piezoelectric polymer P(VDF-TrF) and the conductive polymer PEDOT:PSS. Preliminary experiments are done to determine substrate wettability and to characterize the electrical properties of the conductive ink. A substrate surface treatment is used to modify the wetting properties of the substrate. The effect of the surface treatment exposure time is evaluated by measuring the width of printed lines. The experiment results are used to evaluate the sensor structure printing process. IDE structure is fabricated by inkjet printing, and the piezoelectric layer is screen printed on top of the electrodes. Electrical properties and piezoelectric sensitivity of the final samples are characterized. The results of this research show that the ink and substrate properties have an impact on the performance of the printed structures. The surface energy of the substrate is modified to improve its wettability. Thus, UV/O₃ surface treatment can be used to make Parylene-C hydro-philic. Furthermore, the IDE structure can be fabricated by inkjet printing technology. However, the coffeering effect is observed in narrow PEDOT:PSS inkjet printed lines (i.e. IDE fingers). This may have an impact on the conductivity of the lines due to the non-uniform distribution of the material. On the other hand, the validation of the piezoelectric sensitivity characterization suggests that the poling process has to be improved to guarantee the operation of the device as a piezoelectric sensor. The results of this research validate that ultrathin sensors can be fabricated using printed electronics technologies. The overall thickness of the sensors is below 6 µm. In conclusion, further research has to be done to activate properly the piezoelectric properties of the P(VDF-TrFE) material in this sensor configuration