Trajectories and Forces in Four-Electrode Chambers Operated in Object-Shift, Dielectrophoresis and Field-Cage Modes-Considerations from the System's Point of View.

In two previous papers, we calculated the dielectrophoresis (DEP) force and corresponding trajectories of high- and low-conductance 200-µm 2D spheres in a square 1 × 1-mm chamber with plane-versus-pointed, plane-versus-plane and pointed-versus-pointed electrode configurations by applying the law of maximum entropy production (LMEP) to the system. Here, we complete these considerations for configurations with four-pointed electrodes centered on the chamber edges. The four electrodes were operated in either object-shift mode (two adjacent electrodes opposite the other two adjacent electrodes), DEP mode (one electrode versus the other three electrodes), or field-cage mode (two electrodes on opposite edges versus the two electrodes on the other two opposite edges). As in previous work, we have assumed DC properties for the object and the external media for simplicity. Nevertheless, every possible polarization ratio of the two media can be modeled this way. The trajectories of the spherical centers and the corresponding DEP forces were calculated from the gradients of the system's total energy dissipation, described by numerically-derived conductance fields. In each of the three drive modes, very high attractive and repulsive forces were found in front of pointed electrodes for the high and low-conductance spheres, respectively. The conductance fields predict bifurcation points, watersheds, and trajectories with multiple endpoints. The high and low-conductance spheres usually follow similar trajectories, albeit with reversed orientations. In DEP drive mode, the four-point electrode chamber provides a similar area for DEP measurements as the classical plane-versus-pointed electrode chamber.

The System's Point of View Applied to Dielectrophoresis in Plate Capacitor and Pointed-versus-Pointed Electrode Chambers.

The DEP force is usually calculated from the object's point of view using the interaction of the object's induced dipole moment with the inducing field. Recently, we described the DEP behavior of high- and low-conductive 200-µm 2D spheres in a square 1 × 1-mm chamber with a plane-versus-pointed electrode configuration from the system's point of view. Here we extend our previous considerations to the plane-versus-plane and pointed-versus-pointed electrode configurations. The trajectories of the sphere center and the corresponding DEP forces were calculated from the gradient of the system's overall energy dissipation for given starting points. The dissipation's dependence on the sphere's position in the chamber is described by the numerical "conductance field", which is the DC equivalent of the capacitive charge-work field. While the plane-versus-plane electrode configuration is field-gradient free without an object, the presence of the highly or low-conductive spheres generates structures in the conductance fields, which result in very similar DEP trajectories. For both electrode configurations, the model describes trajectories with multiple endpoints, watersheds, and saddle points, very high attractive and repulsive forces in front of pointed electrodes, and the effect of mirror charges. Because the model accounts for inhomogeneous objectpolarization by inhomogeneous external fields, the approach allows the modeling of the complicated interplay of attractive and repulsive forces near electrode surfaces and chamber edges. Non-reversible DEP forces or asymmetric magnitudes for the highly and low-conductive spheres in large areas of the chamber indicate the presence of higher-order moments, mirror charges, etc.

Dielectrophoresis from the System's Point of View: A Tale of Inhomogeneous Object Polarization, Mirror Charges, High Repelling and Snap-to-Surface Forces and Complex Trajectories Featuring Bifurcation Points and Watersheds.

Microscopic objects change the apparent permittivity and conductivity of aqueous systems and thus their overall polarizability. In inhomogeneous fields, dielectrophoresis (DEP) increases the overall polarizability of the system by moving more highly polarizable objects or media to locations with a higher field. The DEP force is usually calculated from the object's point of view using the interaction of the object's induced dipole or multipole moments with the inducing field. Recently, we were able to derive the DEP force from the work required to charge suspension volumes with a single object moving in an inhomogeneous field. The capacitance of the volumes was described using Maxwell−Wagner's mixing equation. Here, we generalize this system's-point-of-view approach describing the overall polarizability of the whole DEP system as a function of the position of the object with a numerical "conductance field". As an example, we consider high- and low conductive 200 µm 2D spheres in a square 1 × 1 mm chamber with plain-versus-pointed electrode configuration. For given starting points, the trajectories of the sphere and the corresponding DEP forces were calculated from the conductance gradients. The model describes watersheds; saddle points; attractive and repulsive forces in front of the pointed electrode, increased by factors >600 compared to forces in the chamber volume where the classical dipole approach remains applicable; and DEP motions with and against the field gradient under "positive DEP" conditions. We believe that our approach can explain experimental findings such as the accumulation of viruses and proteins, where the dipole approach cannot account for sufficiently high holding forces to defeat Brownian motion.

A novel telemedicine system for monitoring congestive heart failure patients.

Congestive heart failure is a widespread cardiac disease in western countries. At present, the main measure for monitoring the level of pulmonary edema in telemedicine systems is weight, which is not a reliable indicator. The authors propose a novel bioimpedance telemedical system to monitor these patients. The system measures the resistivity of each lung using optimization methods and transmits the measurements via a modem to a call center. Preliminary results show that the measured resistivity values among healthy young patients are consistent and reproducible within 48 hours. The mean resistivity values in patients with pulmonary congestion were lower than those of the healthy patients: 887 [Omega*cm]+/-117 vs 1244 [Omega*cm]+/-87 (P<.01). The system is noninvasive, safe, and portable. It retrieves unique information correlated with the amount of fluid in the lungs and transmits the data to a medical call center in order to improve the diagnostics and treatment of congestive heart failure.

Monitoring lung resistivity changes in congestive heart failure patients using the bioimpedance technique.

The feasibility of a novel, dedicated system for monitoring lung resistivity in congestive heart failure patients, implementing a hybrid approach of the bioimpedance technique, was assessed in this preliminary study. Thirty-three healthy volunteers and 34 congestive heart failure patients were measured with the PulmoTrace system (CardioInspect, Tel Aviv University, Tel Aviv, Israel) during tidal respiration, and the ability to monitor the respective lung resistivity values was assessed. Mean left and right lung resistivity values of 1205+/-163 and 1200+/-165 ohm.cm for the control group and 888+/-193 and 943+/-187 ohm.cm for the congestive heart failure group were found, indicating a significant (p<2.10(-7)) difference between the two groups. The results of long-term monitoring of two patients during medical treatment are also shown. This hybrid approach system is believed to improve diagnostic capabilities and help physicians to better adjust medication dosage on a frequent basis.

Induced current impedance technique for monitoring brain cryosurgery in a two-dimensional model of the head.

A fast and robust finite volume solver of the two-dimensional induced current electrical impedance forward problem was developed. The numerical solver was validated by comparison with an existing analytical solution for a symmetrical geometry case, showing an accuracy of 0.07%. The solver was used to theoretically examine the sensitivity of the induced current impedance technique for the medical procedure of monitoring brain cryosurgery. The simulation was performed using a two-dimensional approximation of otherwise realistic geometry model of the head with different ice-ball sizes, simulating the expansion of the frozen lesion. The sensitivity of the scalp potential to the ice-ball size was found to be 53 x 10(-4) (relative scalp potential mm(-2)).

Dynamic cardiophysiologic variables correlate with lesion location and FIMTM in patients with ischemic stroke.

OBJECTIVE: To examine the correlation between a clinical measure of function in patients undergoing rehabilitation who had recently had an ischemic stroke and to examine cardiophysiologic measures registered during effort. DESIGN: A cohort study comprising a sample of consecutive patients, without a history of cardiac disease or rhythm disturbances, admitted for rehabilitation after an ischemic supratentorial stroke. All patients were examined for the FIM trade mark score and dynamic cardiophysiologic variables. Results were analyzed in relation to stroke location. Thirty-eight patients participated in the study. Ten patients had a superficial lesion, 20 had a deep brain lesion, and eight had no noticeable lesion by computed tomographic imaging. Function was measured with the FIM instrument 48-72 hr after admission and repeated at 1 wk before discharge. Electrocardiographic activity was recorded during physiotherapy treatment. The relationship between the RR and QT intervals in the electrocardiographic waveforms was found to estimate two cardiophysiologic variables, the constant (a) and the slope (b) values, reflecting the dynamic change of QT during physiotherapy effort. RESULTS: Only for the subgroup of patients who sustained a deep brain lesion did the motor items in the FIM instrument on admission and discharge scores have a statistically significant relationship with the slope variable (b) and an inverse statistically significant relationship with the constant variable (a). CONCLUSIONS: Deep brain infarction seems to result in a significant dysfunction of the autonomic nervous system, manifesting itself as distorted dynamic behavior of the QT interval and with impaired functional performance.

Heart rate variability parameters correlate with functional independence measures in ischemic stroke patients.

This study gives quantitative information regarding the effect of brain infarction on the regulation of the cardiovascular system. Electrocardiograms of 16 patients, ranging from 54 to 85 years old, admitted for rehabilitation after an ischemic supratentorial stroke, all without a history of cardiac disease or rhythm disturbances, were recorded during physiotherapy effort. All patients were evaluated for the functional independence measure (FIM) score 48 to 72 hours following admission and repeated at one week before discharge. The heart rate variability (HRV) parameters: standard deviation (SD) of the RR series, the power spectrum of the RR intervals for the low frequency range (LF: 0.04-0.15 Hz), and the high frequency range (HF: 0.15-0.4 Hz) were calculated. The results showed that all heart rate variability parameters had statistically significant relationship with the FIM on admission and discharge scores. In conclusion, the HRV parameters correlate with the clinical measures of function: the greater the HRV parameter the higher the FIM score. The presented technique may prove useful as a prognostic tool providing a simple way for determining functional performance of stroke patients.