Multi-sensor epidermal electronics for vital signs monitoring: application to core body temperature and respiratory function

Monitoring of key vital signs is crucial to provide a valuable insight into a person’s health, to make clinical deterioration preventable by a timely intervention. However, it makes use of sophisticated equipment, uncomfortable for the patients, and mainly limited to in-hospital use. Wearable devices, that are intended to be worn and carried around with limited restraints, can overcome these issues, although still with some limitations in terms of lightweight and power requirement. Accordingly, focusing on Core Body Temperature (CBT) and Respiratory Rate (RR), this doctoral thesis investigates three different techniques that can ensure the collection of sensed data directly from the surface of the skin by soft, compact, and flexible devices that are fully-passive (i.e., no battery to provide power for operation) and can communicate wirelessly with remote processing hubs. Although passive solutions could also be implemented for wearables, epidermal devices are investigated here as enablers of a seamless monitoring of the target vital signs. Moreover, as an additional improvement to broaden the health indicators that can be monitored with just a simple device, the proposed devices become multi-sensor by embedding an increased number of sensors, i.e., at least two for RR and at least four for CBT. The first technological method explored in this thesis exploits Radio Frequency waves to passively harvest energy from the surroundings and then wirelessly transmit the sensed data back to the reader by the Ultra-High Frequency (UHF) Radio Frequency Identification (RFID) protocol. It led to the development of a first-of-a-kind core body temperature thermometer that innovatively combines the Dual-Heat-Flux model for on-skin thermometry with epidermal UHF RFID antennas for data sensing and transmission. Similarly, a novel compact nasal breath device has been developed that, by doubling the embedded sensors, provides the novel possibility to track nasal breathing by two distinct measurement channels, each dedicated to one nostril. To conceive and design such multi-chip RFID devices embedding auto-tuning microchips, a theoretical formulation was missing and it has been accordingly derived by taking into account the entangled coupling that occurs within the close antennas and auto-tuning chips. The second and third techniques explored in this thesis were the colorimetric method using Liquid Crystals temperature sensors and the calorimetric method for flow monitoring, that respectively provide an alternative approach to design core thermometers and breath sensors. Overall, the electromagnetic approach exploiting RF waves to sense and transmit data seems the most suitable for the considered applications, and therefore, the most significant innovations are brought in this field, both in terms of designs, materials, and manufacturing procedures, and, above all, by providing the RFID community with a novel theoretical formulation that enables the estimation of the electromagnetic performance of any coupled multi-chip auto-tuning system, not necessarily epidermal.