Advances in Science and Technology Vol. 81

Abstract: MEMS (MicroElectroMechanical-Systems) technology applied to the field of Radio Frequency systems (i.e. RF-MEMS) has emerged in the last 10-15 years as a valuable and viable solution to manufacture low-cost and very high-performance passive components, like variable capacitors, inductors and micro-relays, as well as complex networks, like tunable filters, reconfigurable impedance matching networks and phase shifters, and so on. The availability of such components and their integration within RF systems (e.g. radio transceivers, radars, satellites, etc.) enables boosting the characteristics and performance of telecommunication systems, addressing for instance a significant increase of their reconfigurability. The benefits resulting from the employment of RF-MEMS technology are paramount, being some of them the reduction of hardware redundancy and power consumption, along with the operability of the same RF system according to multiple standards. After framing more in detail the whole context of RF MEMS technology, this paper will provide a brief introduction on a typical RF-MEMS technology platform. Subsequently, some relevant examples of lumped RF MEMS passive elements and complex reconfigurable networks will be reported along with their measured RF performance and characteristics.

Abstract: This paper discusses use of MEMS technologies in radio frequency (RF) frontend. First, configuration of RF front-end in current wireless communication systems is surveyed, and research trends of the flexible RF front-end and software defined radio (SDR) are discussed. Second, various RF tunable filters are introduced, and we discuss how high performances are expected by the use of tunable RF surface and bulk acoustic wave (SAW/BAW) filters provided that above mentioned key technologies are developed. Finally, our attempts for realization of tunable RF SAW/BAW filters are introduced.

Abstract: The lab-on-a-chip (LOC) technology was expected to influence our every day live in a similarly fundamental way as integrated circuits have. Unfortunately, this demand has not been met yet. The cause therefore lies in the complexity of microelectromechanical systems (MEMS), which form the base of the current LOC technology. We present a new concept of LOC which are based on fluidic microchemomechanical systems (μCMS). During the fabrication process, these μCMS are preprogrammed by monolithic integration of special active components. These active components are holding chemical energy that can be transformed at least once into mechanical energy and thus provide a timed and quantitative exactly defined fluidic function. With our simple and inexpensive fabrication method combined with the above mentioned advantages of the invented μCMS, new and better LOC technology can be developed.

Abstract: Over the last ten years, microfluidic technologies have gained considerable importance. However, realising highly integrated microsystems is a major challenge, which so far has only been solved insufficiently. Here, we present an innovative approach to fabricate low-cost, integrable mi- crofluidic platforms. As active elements, photopolymerised hydrogels based on Poly(N-isopropylacrylamide) (PNIPAAm) are introduced. PNIPAAm is temperature-sensitive. Heated in water above its lower crit- ical solution temperature (LCST), it reversibly changes from a swollen to a shrunken state (volume change in the order of 90%) and can, via an electrothermic interface, be employed as electrothermally switchable actuator. Varying specific parameters in the swelling agent, for example varying its alco- hol concentration, can shift the LCST. So not only micropumps or microvalves, but also valves with an appointed threshold value, so-called chemostats or chemical transistors, can be realised. Using the example of a microchip performing enzymatic endpoint analyses, we investigate characteristic be- haviour of active elements based on PNIPAAM and show the ability of integrating different fluidic operations like fluid transportation, metering, valving and mixing into one fully polymeric microchip.

Abstract: In this work the effect of air plasmas on wettability of Polydimethylsiloxane (PDMS) and polyethylene terephthalate (PET) was studied. These polymers are widely used materials in the fabrication of microfluidic devices. The microfluidic system fabricated from native PET and PDMS requires active pumping mechanism, due to a low hydrophilic surface behavior. To render hydrophilic and increase the capillary flow into the device, plasma treatments can be used. Air plasma treatment is an interesting technology for microfluidic fields due to simplicity of use and low cost. This study describes the effect of the working plasma pressure on wettability of polymers. The polymers were treated by RF plasma and the wettability was studied by means of sessile contact angle. The results established that the air plasma can increase the wettability of both polymers. Moreover we demonstrated that by optimizing the working pressure a superhydrophilic surface (with a contact angle less than 5°) can be obtained. The findings suggest that air plasma treatments are a suitable technology to enhance polymers surface wetting performance for microfluidic devices.

Abstract: In the present work, we have experimentally investigated the stability and hysteresis behaviors of CuO-water nanofluid when submitted to a repeated heating and cooling process. Data has shown that for a low particle volume concentration, 1.6% in particular, the hysteresis phenomenon did not occur for the temperature range considered. For a higher particle concentration, 5% in particular, the hysteresis behaviour was clearly observed when fluid temperature exceeded 52°C approximately. Beyond this critical temperature, the nanofluid viscosity has increased, and such an increase even continued with a decrease of temperature during the cooling phase. Subsequent measured viscosity and observations in laboratory after the first occurrence of the hysteresis phenomenon have confirmed that the alterations on the particle suspension and on the nanofluid stability appear indeed permanent. Such alterations were found to worsen with further heating/cooling cycles.