Sensitivity Enhancement Approaches in Microwave-Microfluidic Sensors and Design of a Compact and Cost-effective RF Sensor Readout System

  • Author / Creator
    Niloofar Sharafadinzadeh
  • Microwave resonators have proved their capability as sensing devices in a wide range of applications such as lab on chip, environmental sustainability, and industrial applications, not only for analysis in the solid and liquid phase, but also recently in gaseous environments. Planar microstrip resonators made from split ring resonators (SRRs) have shown relatively sharper resonances and simultaneously increased sensitivity due to their higher coupling to the surrounding signal lines. These sensors are amenable to miniaturization, automation, mass production, and wireless interconnection due to CMOS compatibility, low cost and a facile fabrication process.
    Such microwave resonators also offer noninvasive sensing through contact-less probing, which adds to their flexibility of usage and maneuverability for in situ characterization. For certain industrial applications such as material characterization in microfluidic channels, it is crucial to use a highly sensitive device which identifies small variations in concentration. There are many approaches to increase the sensitivity of the microwave resonators. Two passive methods are proposed in this work: gap extension of the SRR, and embedding material under test (MUT) inside the substrate.
    To investigate the effect of gap extension on microwave planar resonators’ sensitivity, microstrip split rings are utilized as conventional half-wavelength resonators with sensitive spot for dielectric sensing in the gap. The sensor is configured in three different mixed electric and magnetic couplings and the sensitivities, in terms of frequency and amplitude variation, are analyzed under exposure to given materials. The passive resonators at ~2 GHz are simulated with permittivity values of samples ranging from 5 through 30. The resonator gap is engineered with an inward extension for higher sensitivity, where a uniform enhancement up to more than 20% in frequency-sensitivity is obtained to reach (change in frequency)/(change in permittivity)=6.17MHz for all coupling configurations, while the amplitude based sensitivity is preserved. An experimental application of the highly sensitive sensor is introduced in non-contact concentration measurement of glucose within wide range of 1-15 g/dL with steps of 1 g/dL.
    In this work, microwave planar sensors are discussed for liquid characterization, mainly microfluidic application. Three common sensors are implemented in complementary split ring resonator, extended gap split ring resonator, and conventional circular split ring resonator resonating at 1.7, 1.9, and 3.6 GHz, respectively. Sensitivity of material on top of the resonator is analyzed, and is modified with respect to the volume of material under test for more comprehensive evaluation of sensor’s performance. Circular resonator is found to be the optimum in terms of sample volume and offers up to 50% higher sensitivity for permittivity range from 1-30.
    This sensor is further developed into embedding the material under test inside its substrate and the sensitivity is also enhanced on average by ~ 45% for bulk material sensing of the same permittivity range. In practical verification of the embedding concept, water sample concentrated with 10%-50% methanol is placed above/inside substrate and remarkable improvement of 360% in sensitivity is observed for the latter when PTFE tubes are installed. Finally, microfluidic channels are uniquely implemented inside the substrate of circular split ring resonator and distinct signatures are detected, which proved the concept of sensitivity enhancement in resonators while minimizing the sample volume.

    In addition, the proper instrument is needed to capture the data from microwave resonance-based sensors in the field. Vector Network Analyzer (VNA) is the major equipment that is vastly used to measure and monitor the performance of RF devices. VNAs are multifunctional and very accurate tools that are used for various applications in this field. Due to their many features and capabilities, they are often bulky and expensive equipments that consist of different RF modules. This study also focuses on developing a compact, cheap, and yet accurate RF sensor read-out system to capture the amplitude of transmission response of SRRs; this novel circuitry is the first step in revolutionizing the practical use of microwave and RF sensors that focuses on miniaturization and performance of such high-demand equipment. This study focuses on sensitivity enhancement of passive SRRs, integrating them with microfluidic channels, as well as developing a read-out system to measure amplitude of transmission profile of the SRRs.

  • Subjects / Keywords
  • Graduation date
    Spring 2020
  • Type of Item
  • Degree
    Master of Science
  • DOI
  • License
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