Multimode microwave sensors for microdroplet and single-cell detection

Abstract

A novel detection mechanism which can reveal both morphological and electrical properties of analytes are needed for Lab-on-a-Chip applications. Herein, a label-free, real-time and non-contact detection paradigm in microwave domain is constructed, by using rst and second electromagnetic modes of a microwave resonator. As the resonator, microstrip line is chosen since it o ers accessible boundary conditions. In order to deliver the analytes into the sensing region, micro uidic channels are fabricated. As a proof-of-concept, while the microstrip line resonator's rst and second modes are tracked simultaneously, analytes are delivered through the microchannel embedded underneath the signal layer. As the analytes, water microdroplets in oil, cervical (HeLa) and breast (MDA-MB-157) cancer cells in their appropriate medium are used and detected; their position and electrical volume informations are obtained and compared. Allan Deviation of the measurement is smaller than 2 10􀀀8 for both modes, and due to analyte properties in microwave domain, such as the great permittivity di erence between the biological analyte and medium, detection is possible. In order to test the accuracy of nding the position of the analyte, two di erent microchannel geometries are designed. The rst geometry is based on a zigzag channel, where microstrip line crosses the channel at 6 di erent locations. Secondly, a branched channel is designed, to send the microdroplets at four di erent locations. This delivery mechanism is mostly based on the hydraulic resistance: Each droplet chooses its path by the hydraulic resistance that is caused by the previous droplet. Hence, microdroplets are distributed to four channels. Due to the mode shape of the speci c mode, when analyte passing through these regions, it induces di erent frequency shifts. For these applications, micro uidic part is fabricated by conventional soft lithography methods, and the material for the microchannels is PDMS. Electrical volume of the droplets is also obtained. After the usage of prototypes for position and electrical volume calculations, second generation devices are fabricated. Compared to the PDMS-based devices, these devices o er rapid and low-cost prototyping. Additionally, Kapton is chosen for the dielectric material, and it has some material-wise bene ts, such as the tangent loss level. Analytes are delivered through the sensing region by capillary tubings, hence soft lithography steps can be eliminated. Due to the equipment limitations, only the rst mode is tracked with these novel devices, while another type of breast cancer cells (SK-BR-3) are delivered through the sensing region. Signal-to-noise ratio, when compared to the PDMS based devices, is improved. In this work, the rst two modes of microstrip line resonators are used. However, by using higher order modes, more properties about the particles as skewness, geometrical volume, orientation, and composition can be obtained. These informations can be used to construct a global image of the analyte; rather than a pixel by pixel image. Additionally, ow cytometry applications, detection of Circulating Tumour Cells and applications for long term cultivation on chip (also known as Organ-on-Chip platforms) can be achieved.