"Characterization of Suspended Micro - Channel Resonators"

The Suspended Microchannel Resonators (SMRs) are recently attracting great attention in the biosensing field, and in particular in the density and mass monitoring of cells, bacteria growth and particles loading. One of the main drawback in the implementation of this platform in lab-on-chip devices is represented by the time consuming and expensive fabrication approach used to create the SMR itself. In fact, in order to realized a buried channel in a suspended structure complex fabrication methods are needed, which requires several photo lithographic, depositions and etching process. In this work it has proposed an innovative method of fabrication based on the femtosecond laser writing. In particular a glass SMR has been fabricated optimizing a two steps fabrication process, which includes the patterning of the buried channel and of the suspended beam followed by a chemical etching of the exposed substrates. The strength of this approach is represented by the possibility to define and release the buried channel just simply patterning the geometries with the laser that can be focused at any depth inside the silica glass. Furthermore, considering the simplicity of this method a new SMRs design was optimized. This is characterized by the presence of specific obstacles in the middle of the buried channel that in principles, could be used to reduce the liquid flow velocity. This means to have the possibility to increase the time of permanence of cells, bacteria and particles in the beam region, which gives the higher amplitude signal. In this perspective, the main purpose of this thesis work was to understand how the mechanical behavior of SMRs could be affected by the presence of this constriction geometries build inside the hollow channel. In order to do that some fundamental aspect should be take into account: first the simulation of the mechanical behavior of the SMRs with the constriction; second, the experimental confirm of this behavior; and third the mechanical performance of the SMRs containing the constriction have to be compared with SMRs characterized by the same dimensions but without constriction. Furthermore, the effective ability of the new SMR design in distinguish liquid with different density must be verify. Remarkable correlation exist between the characterized resonance frequencies that were obtained from the experimental measurement and the values obtained from modeling the suspended microchannel resonators using Comsol. In fact, the variation percentage of these values is less than 4%. In total four suspended microchannel resonators were model and characterized in this study, characterized by four different constriction geometries. The four suspended micro channel resonators were model using the Multiphysics Comsol applying the solid mechanics suite. The first flexural eigenfrequencies for the four resonators were theoretical computed to be 1.0334 MHz, 1.0336 MHz, 1.0905 MHz and 0.4938 MHz respectively. The model did not only provide the first flexural mode but also model generated the second, third flexural mode as well as the first torsional mode for each suspended micro channel resonator. The final stage of the simulation was to perform the above simulation filling the micro channel with liquids and theoretical computing the eigenfrequency. From the experimental point of view, a mechanical characterization was performed by the means of a Laser Doppler Vibrometer. The main properties of the SMRs were extracted from this measurements and Q factor, Responsivity and Frequency stability have been defined for each SMRs. The presence of the constriction in the middle of the buried channel seems does not affect in a negative way the Responsivity and the Frequency stability of the SMRs. On the contrary, the Q factor shows a worsening for the SMRs containing the constriction respect to the SMRs without it. Finally, in order to evaluate the effective capability of the SMRs (fabricated with the new design) three different liquids were made to flow through the suspended micro channel resonator and the experimental sensitivity with the minimum detectable mass were computed. The experimental results are in agreement with what was observed from the simulation, and the minimum detectable mass obtained (in the range of hundreds of femtograms) reveal that this platforms can be competitive with the well know SMRs already presented in the literature.