Wearable Carbon Nanotube Sensors: Fabrication and Applications for Bio/Chemical Sensors
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With the advancement of micro/nanotechnology, wearable device technology is rapidly changing human lifestyle. Wearable devices armed with high-performance sensors potentially offer real-time health monitoring of human body conditions and produce massive database correlating physiological parameters with diseases, health, and behavior. However, most wearable devices do not offer wearer’s comfort because the manufacturing methods are based on a stiff silicon substrate. Among nanomaterials, carbon nanotubes (CNTs) are emerging as electronic or sensing materials on a flexible substrate. For such devices, CNTs need to be patterned on a flexible substrate like polyethylene terephthalate (PET) film. Inkjet printing is one of the major patterning methods, but the pattern is discrete because of droplet-based printing. Also, inkjet printing is limited by the ink properties, including surface tension and viscosity. Fountain pens can be used to print continuous lines but potentially damage the substrate. The contact printing methods may not be suitable to print multiple functional layers because the pen nib induces damage to existing layers. In the dissertation, nano ink bridge-induced capillary pen printing is proposed as a novel method for continuous line printing of carbon nanotubes. Firstly, the control parameters of the noncontact capillary method are studied in terms of line width, edge roughness, and sheet resistance for uniform printing. Nanoink liquid bridge forms between the tip of the stylographic pen and substrate by capillary action. The printed pattern is characterized in the contexts of nano ink bridge formation between pen nib and substrate. Ink properties, printing temperature, printing speed, and contact angles are studied to find optimal printing conditions. The nano ink bridge-induced printing allows multiples layers of nanomaterials without damaging existing layers. This printing method facilitates the fabrication of low-cost wearable sensors on a flexible substrate. As printing applications, gas and pH sensors are demonstrated using the carbon nanotube pattern and chemical doping by the capillary pen printing method. For biosensor fabrication, a point-of-care (POC) platform for tuberculosis screening is presented using a carbon nanotube film. Tuberculosis, caused by Mycobacterium tuberculosis (MTB), is one of the serious infectious diseases worldwide. Various methods are available for TB diagnoses, such as a Ziehl-Neelsen (ZN) method for microscopic detection, immunoassays for antigen detection, and polymerase chain reaction (PCR) for DNA or RNA detection. For a highly sensitive and specific screening tool, nanomaterials have been persistently investigated for infectious disease diagnosis. Resistive single-walled carbon nanotube (SWCNT) sensors have shown potential for rapid TB screening. However, hydrogen bonding on SWCNTs interferes the resistance change due to target binding. In this dissertation, a resistive SWCNT biosensor is fabricated on a flexible film (PET) for low-cost TB screening. Silver electrodes are stamped as probing electrodes for SWCNTs. The sensing mechanism of SWCNTs, coupled with silver electrodes, is investigated in conjunction with hydrogen desorption. The sensitivity and specificity are characterized by MTB and surface antigen (MPT64) in physiological buffer. Subsequently, the sensor is characterized by tongue swab samples spiked MTB and MPT64. Simple resistive measurement is conducted before and after immunocomplex formation for detecting targets. The presented biosensor will offer a stepping stone for an inexpensive and versatile POC platform for rapid TB screening. In summary, the critical challenges for SWCNT-based wearable sensors are addressed in terms of scalable fabrication and hydrogen bonding. The printing physics is investigated for nano ink-bridge induced printing of SWCNTs. A rapid TB screening sensor is developed with the investigation of hydrogen adsorption and desorption effects on SWCNTs. A thin, flexible sensing platform will facilitate the scalable fabrication of bio- and chemical sensors with low cost.
- Mechanical engineering