Programmable nanomaterials for health-related flexible sensing electronics
Date of Issue2017
School of Materials Science and Engineering
Health-related flexible sensing electronics and wearable/attachable personal health monitoring systems will change the conventional diagnose method in clinical practice, and promote the revolution of next medical instruments towards portable, wearable, remote, and timely. As a crucial component, smart flexible sensing electronics can real-timely tracking physiological signals which are tightly associated with body conditions, such as heart rate, wrist pulse, body temperature, blood/intraocular pressure, and blood/sweat bio-information. Monitoring such physiological signals provide a convenient and non-invasive way for disease diagnoses and health assessments. However, the use of conventional materials and technology is yet rather limited because of their poor portability, biocompatibility, as well as high power consumption. To bring flexible electronic device towards smaller, smart, and less power consumption, as well as multi-functionality, this PhD thesis focuses on developing the novel programmable nanomaterials and new device configurations for next health-related flexible sensing electronics. Three new nanomaterials, including programmable carbon patterns, self-crosslinked carbon arrays, and ultra-thin CdS films, were developed for flexible sensing electronics. Some fundamental issues, such as growth mechanism of materials, the interface synergistic effects between each functional material for entire flexible sensing device, and the electromechanical interaction were investigated. In the first study, inspiring from the conventional photolithography process, a novel and effective method have been developed for the synthesis of high-quality micro-patterned carbon by pre-patterning pyrolyzed photoresist as the solid carbon precursors. This method can produce high crystallinity carbon patterns with a high lateral resolution of ~2 µm, which approaches to the limit resolution of the traditional near-UV photolithography (at hard contact mode) and meets the requirement of the most flexible electronics. Also, bendable electronic devices have been fabricated based on the high-quality micro-patterned carbon. The bendable carbon device can suffer the bending angle from 0o to 180o and presents distinct output response for tensile and compressive strain and high sensitivity, which makes it promise in the construction of flexible mechanical sensors. In the following works, a novel carbon nanorod array (CA) that is characterized as vertically aligned nanorods and self-crosslinked junctions was reported. Comparing with 2-dimensional (2D) networks and solid thin films, such geometries are highly resistant to crack and fragmentation under stain. In the meantime, it shows high sensitivity and good stability (~10,000 times) to detect the flexions. This CA-based flexible devices can record low-frequency vibrations (<6 Hz) and make it excellent to monitor the rest tremor of human body, which is an initial symptom of Parkinson's disease. Therefore, this self-crosslinked CA can be used to fabricate cost-effective and durable flexible sensor for early diagnosis of disease by monitoring the health-related rest tremors. Piezoelectric materials are promising in applications of accurate pressure/strain sensors. In the third study, the CdS thin films with the thickness of 2~3 nm were synthesized using chemical vapor deposition method, and demonstrated the vertical piezoelectricity d33 of CdS in atomic scale (3~5 space lattices) by single and dual-frequency resonance tracking piezoelectric force microscopy (DART-PFM). The surface potential and charge distribution were measured by SKFM. The vertical piezoelectric domains at the CdS thin films was observed, and the measured piezoelectric coefficient (d33) of 32.8 pm·V-1, which is ~3 times larger than that of the bulk CdS materials. The findings shed light on design of next-generation sensors and micro-electromechanical devices. The technology on integration of flexible sensing electronics is very important for commercialization of health monitoring system. In the fourth study, the non-standard (non-silicon technology) MEMS technology was developed for the fabrication of the integrated flexible sensing electronics. The technology is compatible to a kind of flexible substrates such as PET and PDMS. As an example, the modularized and integrated flexible sensor arrays were fabricated, which contains 9 pressure sensing units and a temperature sensing unit. The strong Van der Waals' force between two smooth PDMS substrate provides the flexible device with good stability and reproducibility. The results will pave the way for the practical application of flexible sensing devices in real-timely and simultaneously monitoring multiple physiological signals.