Frequency sweep rate dependence on the dielectrophoretic response of polystyrene beads and red blood cells.

Alternating current (AC) dielectrophoresis (DEP) experiments for biological particles in microdevices are typically done at a fixed frequency. Reconstructing the DEP response curve from static frequency experiments is laborious, but essential to ascertain differences in dielectric properties of biological particles. Our lab explored the concept of sweeping the frequency as a function of time to rapidly determine the DEP response curve from fewer experiments. For the purpose of determining an ideal sweep rate, homogeneous 6.08 µm polystyrene (PS) beads were used as a model system. Translatability of the sweep rate approach to ∼7 µm red blood cells (RBC) was then verified. An Au/Ti quadrapole electrode microfluidic device was used to separately subject particles and cells to 10Vpp AC electric fields at frequencies ranging from 0.010 to 2.0 MHz over sweep rates from 0.00080 to 0.17 MHz/s. PS beads exhibited negative DEP assembly over the frequencies explored due to Maxwell-Wagner interfacial polarizations. Results demonstrate that frequency sweep rates must be slower than particle polarization timescales to achieve reliable incremental polarizations; sweep rates near 0.00080 MHz/s yielded DEP behaviors very consistent with static frequency DEP responses for both PS beads and RBCs.

Comparison of operational design criteria for subsurface flow constructed wetlands for wastewater treatment.

Many communities are investigating the use of constructed wetlands as a simple, viable alternative to poorly operating on-site wastewater treatment. Currently, the most popular design models in the U.S.A. for subsurface flow units are the US Environmental Protection Agency (EPA) and the Tennessee Valley Authority (TVA) methods. However, there has been limited data verifying if the operating performance is up to design standards. In order to address this problem, this paper documents the performance of such treatment systems over a broad range of conditions in North Alabama. The operational data is used to determine a best-fit value of reaction rate constant and proposes improved input parameters for both design models. According to this data, the average plug-flow reaction rate constant was calculated to be 0.70 d-1 for the EPA method, while the recommended organic loading criterion is 11.4 m2/kg BOD5 per day into the wetland for the TVA method.

Prolonged cancer death. A family affair.

The frequency with which families have to deal with cancer death is increasing as the population ages. The prevalence of some cancers in younger people is also increasing. In this article the authors focus on helping families deal with the anticipated cancer death of a family member. The article includes a definition of those groups of people who form families in the 1990s and examines dimensions along which families vary such as belief systems. We discuss the needs of families when a family member has a terminal cancer diagnosis and outline coping strategies that are seen in families and ways in which nurses and other health care professionals can facilitate adaptive coping.