Thermosensitive materials for disposable wearable sensors

Chronic wounds are very common diseases with a prevalence of 1-2 people out of 1000 in the general population. It is estimated that approximately 1-2% of the population is affected at least once in the course of life. They are often related to comorbidities such as diabetes or venous insufficiency, and healing time can be very long, even years. Chronic wounds make patients lose quality of their life and healthcare systems to invest huge resources to tackle the problem; for example, 1.3% of the total UK health care budget is used for the treatment of this disease. The SWANi-Care project aims at developing a new negative pressure wound therapy (NPWT) device provided with sensors allowing a continuous remote monitoring of wound conditions. In this framework, the present work is focused on the development of innovative temperature sensors. Temperature is a key parameter to understand wound conditions. A normal healing process goes through an inflammatory stage during which wound temperature is increased due to an increased blood flow. A chronic wound can be stuck in this phase and a monitoring of temperature can give important information on the healing process. For a clinical use, the temperature sensor needs to fulfil specific requirements such as biocompatibility for being in contact with the wound bed, wearability, comfort and low cost, as these sensors have to be disposable. In my PhD project, I exploited carbon nanotubes, ionic liquids and reduced graphene oxide as temperature sensitive materials, both in a pristine state or in combination with a polymeric matrix to realise composites. These materials were used to fabricate thin and flexible temperature sensors on a PET film by screen printing the contact electrodes and drop casting the temperature sensitive film. Carbon nanotubes were used to obtain a nanocomposite with a polymeric matrix and a temperature sensor prototype was obtained using this material. In particular, the thermal and electrical properties of this nanocomposite were investigated and an annealing treatment was performed to improve the performances of the sensor. Ionic liquids were also tested as an alternative temperature sensitive material, both pure and mixed with polymers to obtain a composite with a good dimensional stability. The electrical properties of these materials were compared and pure ionic liquids, which showed better properties, were selected to fabricate a temperature sensor prototype. The last tested material was reduced graphene oxide. I optimized the synthesis of graphene oxide and its reduction by different reducing agents, in particular hydrazine and ascorbic acid. The composition and the electrical properties of the resulting materials were evaluated and reduced graphene oxide was used to produce a temperature sensor prototype. Sensor prototypes were tested to verify reproducibility, sensitivity and biocompatibility. At the end, reduced graphene oxide was selected as the best material to produce a temperature sensor complying with the clinical application requirements.