Metal oxide based wearable electronic devices and sensors

The disparity in the physical properties of skin and electronics poses significant challenges to researchers for the development of next-generation wearable electronics. The combination of nanostructured functional materials with flexible and stretchable polymeric substrates leading to skin-like electronics, epidermal electronics and lab on skin is a common strategy towards these challenges. Though organic semiconducting materials are the natural choice in wearable electronics field, their poor electrical mobility and chemical instability limit their efficiency and long-time operational stability. On the other hand, functional metal oxides and inorganic materials are rich in electronic properties and chemical stability and are also more amenable to doping based control and batch processing. They can be classified into room temperature and high temperature processing materials. As soft polymeric substrate cannot withstand high temperature processing, the most promising binary and complex oxide systems are usually ruled out in wearable platforms. However, they can be effectively used in the form of soft-hard integration in skin-like electronics for mimicking skin-functionalities and monitoring remote sensing and communication. The work presented in this doctoral dissertation focuses on room temperature deposited SrTiO3 based flexible and stretchable resistive random-access memories (ReRAM) and high temperature annealed VO2 based skin-like somatosensory system and skin-mounted battery less, wireless smoke/vape sensor. A stretchable non-volatile resistive memory is a fundamental element in realizing complex neuromorphic computing and compact logic application adaptable to wearable electronics. A room temperature deposited SrTiO3-x (STO) based resistive memory on stretchable polydimethylsiloxane (PDMS) substrate has been developed. The fabricated devices exhibit state of art memory performance under uniaxial strain. In addition, STO based multifunctional flexible memristor is fabricated on polyimide (PI) substrate with the variation of top and bottom electrodes in metal-insulator-metal (MIM) structure. By utilizing different bottom electrodes of various work functions while the top electrode is fixed, differential Volta potential are induced in STO, to induce bipolar (BP) or complementary (CS) switching whenever required. By using a combination of stretchable pressure sensors, phase-change VO2 thin films and STO based resistive switching memory elements, the critical somatosensory system of skin is demonstrated. The ability to detect and respond to pressure, temperature, and pain stimuli above a threshold with real-life performance characteristics are demonstrated with explanation of underlying mechanisms. Finally, a battery free, wireless, skin mounted nicotine sensor is introduced; where VO2 is exploited as sensor and a light weight (~0.1 g) near-field communication (NFC) interface enables both wireless power delivery and data transmission to miniature electronic devices such as mobile phone, smart watch or tablet. The sensors can remotely sense the presence of nicotine of conventional cigarette smoke and vape (electronic cigarette smoke).