Functional polymer nanocomposites for wearable sensors

<p dir="ltr">Stretchable sensors, based on elastic polymer nanocomposites, have recently garnered significant attention due to their potential in various applications, such as personalized healthcare devices, human-machine interfaces, soft robotics, and electronic skin. However, the widespread use of these flexible and stretchable strain sensors still faces great challenges as there is a trade-off between sensing range and sensitivity. That is, sensors showing high sensitivity usually exhibit low stretchability while highly stretchable sensors display low sensitivity. Therefore, it is crucial to develop sensors that possess both high sensitivity and high working range. Additionally, most of the reported sensors are made of non-degradable polymers combined with conductive materials and their rapid growth concerns associated with electronic pollution. To address these challenges, this thesis starts with new strategies to achieve both high stretchability and high sensitivity via engineering the materials and design of the sensors. It was found that by combining gold (Au) and one-dimensional carbon nanofibers (CNFs) as active sensing materials deposited on an elastic PDMS substrate, stretchability of the sensors could be significantly enhanced, by up to 225% strain. In contrast, sensors based on only Au thin films failed electrically at a strain (~ 4.5%). Introducing one-dimensional CNFs significantly increase the workable strain range by bridging and deflecting the microcracks formed in the Au thin film during stretching. This thesis then investigated the design and fabrication of biodegradable, stretchable, soft polymer thin films based on natural polymers (i.e., sodium carboxymethyl cellulose and starch), which were then used as the substrate for developing sensors. This was an effort to address the above-mentioned issue of electronic wastes by seeking biodegradable and sustainable alternatives to synthetic polymers like styrene-ethylene-butylene-styrene, polydimethylsiloxane (PDMS), and polyamide. By adding plasticizers like glycerol and tuning the composition of the otherwise brittle sodium carboxymethyl cellulose and starch thin polymers, the film stretchability improved by up to 330%. Moreover, a low hysteresis was noted due to the formation of inter- or intramolecular hydrogen bonds. Sensors made by depositing conductive hybrids of carbon nanofibers and poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) thin film onto these thin polymer films show excellent sensing performance with a respectable sensitivity up to 3.9. By incorporating these sustainable polymer substrates, this thesis seeks to mitigate the environmental impact of electronic waste while advancing the field of flexible electronics. The findings of this research are expected to contribute to the development of next-generation healthcare technologies, providing more sustainable solutions for healthcare.</p>