Development of Microfluidic Sensors by Additive Manufacturing

Today, additive manufacturing (AM) technology is well-known to everybody: each of us has, at least once, heard about that and many have already seen a 3D printer at work. In last years, the cost reduction of 3D printers has meant that AM was no longer used just for rapid prototyping but, also, for the manufacturing of many end-use products. Moreover, its benefits (the material efficiency, the possibility to produce complex shapes in very fast time and at low-cost,...), attracted also the scientists, leading them to use this technology in their research. Recently, 3D-printing made its appearance in the microwave field, and the number of papers presenting devices fabricated with this technology grows every year more. It is within this scenario that my PhD thesis is contributing, being entirely dedicated to 3D-printing technology and its applications in the development of microwave devices. In particular, my work is focused on the realization of microfluidic devices based on resonant cavities. The devices were designed, fabricated, and experimentally verified to demonstrate the potential of merging the microwave field with 3D printed sensing devices. The thesis is organized in six chapters. In the first part, an introductory frame outlines the panorama in which the PhD thesis is situated, together with the state of the art of AM and microfluidics as applied to the microwave field and, eventually, the adopted technologies. The second part, the core of the thesis, deals with the fabricated microfluidic sensors, that is, devices that allow to extract liquids' dielectric properties. The retrieval of liquids properties, i.e., dielectric permittivity and loss tangent, has very important applications in chemical and biological fields. In this work, microfluidic sensors are realized through 3D-printed resonant cavities with a (3D-printed) pipe inside, where liquids under test (LUT) can be injected and their properties extracted. Sensors with two different geometries have been analyzed. The first one consists in a square Substrate Integrated Waveguide (SIW) cavity with a multi-folded pipe inside. In the second structure, instead, the high quality factor of spherical-like shapes is exploited. A pumpkin-shape cavity resonator is fabricated, with a pipe passing, straight, between the two poles. One of the main advantages of AM fabrication is the possibility of emptying both structures, so to minimize as much as possible the dielectric losses due to the substrate. Moreover, the pumpkin structure, realized with a 2 mm-thick dielectric shell, was metallized in the inner part, thanks to electroplating. This guaranteed an increase in quality factor, especially if compared with the square structure. Both these structures were tested with nine different liquids, consisting of mixtures of water and isopropanol. To extract dielectric permittivity and loss tangent of the LUTs, the shift in the resonant frequency on one hand and the change of the quality factor on the other hand, have been considered. In particular, the procedure for the extraction of the dielectric permittivity has been improved, with respect to what can be read in literature, and also a novel method for the extraction of the loss tangent is proposed. The intention to create a self-sustained device for the retrieval of LUTs properties, is then pointed out. Such an investigation has culminated in the realization of an oscillator based on the aforementioned 3D-printed resonator. The design of the oscillator was performed in such a way to obtain an output signal with a working frequency similar to the resonator one and dependent on the LUT injected in the cavity. With a spectrum analyzer, the oscillation frequency was measured in different cases, and the permittivity of the different LUTs was obtained with good accuracy.