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This special issue presents devices and the results of a tunable microwave microsystem associating RF circuits and microfluidic components. A channel is buried inside the substrate of a microstrip waveguide. This channel is located beneath a resonant stub. With this configuration a microfluidic passive tunable filter can be fabricated. Dielectric fluids are used to disrupt the electric field in a microstrip structure and thus modify the effective permittivity of the substrate. In this work, a notch filter is realized with an open-ended quarter-wavelength stub placed on top of a hollow SU-8 structure. This structure offers two advantages: channels can easily be fabricated and a reduction of SU8 losses. The filter shows a good performance; the initial cut-off frequency of 25 GHz shifts more than 20% when deionized water is used in fluidic channels. And the shape of RF function is kept throughout the range.

We propose a new type of on-chip micro-variable inductor fabricated by using microelectronic technology and lamination process. The proposed inductor is a dual circular coil and has an inductance of few nH. The fundamental idea is to place a liquid droplet between the metal turns of a coil in order to modify the capacitive/resistive coupling between metal tracks and hence to change the stored magnetic energy. In this study salt water has been used to fill partly or totally the channels constituting these three-dimensional coils. The numerical modeling allows us to obtain inductance values according to the liquid position, which can rise from zero to 100% of the total length of channels. The SU-8 resin was used to realize the microfluidic channels and Au as metallic tracks. The measured electrical characteristics show that these devices can be used up to 1.6 GHz frequency. The relationship between the inductance and the liquid position indicates that the tuning range of the inductance is approximately 107% (from 5.4 to 2.6 nH at a frequency of 1.6 GHz). Without liquid, the peak quality factor is 12, and the self-resonance frequency is 4 GHz; when the channels are completely full, these parameters become respectively 6 and 4.3 GHz.

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