To resolve the dilemma of cell clogging or solution parasitics encountered by Coulter counters and to evolve a general-purpose electrical detection technique, we used broadband microwave measurements to overcome electrode polarization, ac dielectrophoresis to precisely place cells between narrowly spaced electrodes, and relatively wide microfluidic channels to prevent cell clogging. This unique combination of approaches resulted in reproducible sensing of single Jurkat and HEK cells, both live and dead, of different cultures at different times.

The aim of this paper is to propose a new method for the better assessment of cytoplasm conductivity, which is critical to the development of electroporation protocols as well as insight into fundamental mechanisms underlying electroporation. For this goal, we propose to use nanosecond electrical pulses to bypass the complication of membrane polarization and a single cell to avoid the complication of the application of the “mixing formulas.” Further, by suspending the cell in a low-conductivity medium, it is possible to force most of the sensing current through the cytoplasm for a more direct assessment of its conductivity. For proof of principle, the proposed technique was successfully demonstrated on a Jurkat cell by comparing the measured and modeled currents. The cytoplasm conductivity was best assessed at 0.32 S/m and it is in line with the literature. The cytoplasm conductivity plays a key role in the understanding of the basis mechanism of the electroporation phenomenon, and in particular, a large error in the cytoplasm conductivity determination could result in a correspondingly large error in predicting electroporation. Methods for a good estimation of such parameter become fundamental.

This paper compares the characteristics of an RF microelectromechanical systems (MEMS) capacitive switch with a molybdenum membrane versus that of a switch with similar construction but with an aluminum membrane. In comparison, the molybdenum switch exhibits a significantly reduced sensitivity to ambient temperature change so that its pull-in voltage varies by less than 0.035 V/°C. In addition, large-signal RF performance of the switches was compared under both continuous wave and pulse conditions. The results show that under large RF signals, the self-biasing effect is exacerbated by the self-heating effect and the self-heating effect is in turn amplified by nonuniform current and temperature distributions on the membrane. Measurements of both molybdenum and aluminum switches demonstrate a hot-switched power-handling capacity of approximately 600 mW. Since aluminum has been used as a membrane material for over a decade while molybdenum is new, the above results indicate that molybdenum is a promising membrane material for RF MEMS capacitive switches.

In order to evaluate the importance of stress relaxation on device performance of capacitive RF MEMS switches, stress relaxation has been measured in 1.2-mum-thick Au films using a membrane bulge technique. When the residual stress in the films is small, the stress relaxation is fully recoverable and is well described by linear anelasticity (viscoelasticity) theory. A 27% reduction in the effective elastic modulus occurs over a three-day period under constant strain conditions at room temperature. The time dependence of the relaxation can be represented by a series of time constants with values extending from seconds to days. Linear superposition of the anelastic response can be used to accurately predict the stress under any time dependence of the strain. The prediction is accurate even during cyclic loading and unloading, and even when the strain is cycled at rates that are fast compared with any of the relaxation times. The restoring force available to open a capacitive RF MEMS switch is modeled for two different switch designs. The restoring force is shown to drop by approximately 7% or 20% at room temperature for the two cases presented.

A novel technique is used to distinguish the charging of the surface from that of the bulk of the dielectrics of different types of RF MEMS capacitive switches under different electric fields and humidity levels. In general, bulk charging dominates in dry air, while surface charging increases linearly with increasing humidity. Under comparable electric fields and humidity levels, switches made of silicon dioxide are less susceptible to surface charging than switches made of silicon nitride. These quantitative results not only underscore the importance of packaging the switches in a dry ambient atmosphere, but also validate the novel technique for evaluating the effectiveness of dielectric preparation and packaging.