Impedance spectroscopy of human erythrocytes: system calibration and nonlinear modeling.

We present in detail our impedance measurement method, the cell embodding technique, for human erythrocytes, and an accurate calibration procedure for a true four-electrode impedance measurement system. This technique with the calibration procedure gives reliable impedance measurements over a wide frequency range--1 Hz to 10 MHz. To achieve high sensitivity in this frequency range, we embed the cells in the pores of a Nuclepore filter. The calibration procedure assumes that the measurement system is linear, and requires measurement of three reference impedances. The reliability of this procedure is demonstrated with various RC circuits, and application of it to the bio-impedancae measurement system eliminates a quasi-dispersion in the high-frequency range, and increases the bandwidth at both the low- and high-frequency ends of the range by about a decade. We model the impedance of the cells embedded within the filter with an equivalent circuit that is consistent with the geometry and interfaces present. The experimental data are fitted to this model by means of a complex nonlinear least squares (CNLS) fit, which simultaneously fits the real and imaginary parts of the impedance with the Levenberg-Marquardt algorithm. The impedance spectra of human erythrocytes are found to display constant-phase-angle (CPA) characteristics. A CPA element is an impedance of the form Z = A/(jw) alpha, where A is a constant, j = square root -1, omega is angular frequency, and 0 < alpha < 1, and has been used to describe the ac response of the interface between the cell surface and the external electrolyte solution. Such a CPA element may be related to fractal character of the interface.

Frequency domain impedance measurements of erythrocytes. Constant phase angle impedance characteristics and a phase transition.

We report measurements of the electrical impedance of human erythrocytes in the frequency range from 1 Hz to 10 MHz, and for temperatures from 4 to 40 degrees C. In order to achieve high sensitivity in this frequency range, we embedded the cells in the pores of a filter, which constrains the current to pass through the cells in the pores. Based on the geometry of the cells embedded in the filter a circuit model is proposed for the cell-filter saline system. A constant phase angle (CPA) element, i.e., an impedance of the form Z = A/(j omega)alpha, where A is a constant, j = square root of -1, omega is angular frequency, and 0 less than alpha less than 1 has been used to describe the ac response of the interface between the cell surface and the electrolyte solution, i.e., the electrical double layer. The CPA and other elements of the circuit model are determined by a complex nonlinear least squares (CNLS) fit, which simultaneously fits the real and imaginary parts of the experimental data to the circuit model. The specific membrane capacitance is determined to be 0.901 +/- 0.036 microF/cm2, and the specific cytoplasm conductivity to be 0.413 +/- 0.031 S/m at 26 degrees C. The temperature dependence of the cytoplasm conductivity, membrane capacitance, and CPA element has been obtained. The membrane capacitance increases markedly at approximately 37 degrees C, which suggests a phase transition in the cell membrane.

Filtration sizes of human immunodeficiency virus type 1 and surrogate viruses used to test barrier materials.

Filters with well-defined holes were used to determine the effective diameters in buffer of human immunodeficiency virus type 1, herpes simplex virus type 1, and four bacteriophages (phi X174, T7, PRD1, and phi 6), which may serve as surrogate viruses for testing barrier materials. Bacteriophages phi 6 and PRD1 most closely model human immunodeficiency virus type 1 in filtration size.

A brief history of bioelectrodes.

The history of bioelectrodes is intimately associated with electrochemistry in general, through studies on electrode polarization. Electrode polarization was first recognized and studied in the early 1800s, and scientific studies in this area have continued since that time. Experimental and theoretical work on bioelectrodes, electrode polarization, and relevant electrochemistry of electrode phenomena, is traced from 1826 to Schwan's recent electrode work.

Biocompatibility considerations at stimulating electrode interfaces.

The choice of biocompatible stimulating electrodes for various biomedical applications varies with the type of electrode-tissue interface, biomolecules present, electrolyte background, preparation of electrode, interfacial potential, current density, electrode material, porosity, geometry, and inflammatory response. Illustrative examples are given to demonstrate the importance of these parameters. Topics discussed are: A) DC electrodes applied to partially keratinized epithelial membranes; B) Variation of the electrical impedance and biocompatibility of stimulating electrodes with electrode potential and surrounding pH; C) Influence of electrode geometry, porosity and pore size on biocompatibility; D) Body defense mechanisms at the sites of implantable stimulating electrodes; E) Thrombus formation at stimulating electrode interfaces and F) Sterilization of electrodes to ensure biocompatibility.

Rapid deoxygenation of red cells and hemoglobin solution using hollow capillary fibers.

A newly designed capillary deoxygenator has been constructed by using microporous polypropylene hollow fibers sealed into an airtight plexiglass housing. Oxygenated red cell suspensions and hemoglobin solutions flowing through the hollow fibers were subjected to deoxygenation with a gas mixture composed of 95 percent N2 and 5 percent CO2 passed through the housing. At a given flow rate of the oxygenated fluid, the outgoing fluid pO2 varied directly with hematocrit and inversely with the residence time. With a deoxygenator composed of 144 parallel 100-micrometers fibers with an active length of 10 cm, 2 ml of blood at 10 percent hematocrit can be converted from arterial to venous pO2 in approximately 1 min. The design of this deoxygenator provides a method for rapid deoxygenation of blood without red cell membrane damage or hemolysis.

Effects of anticoagulants on packed cell volume measurement.

A comparison was made of different types of anticoagulants on packed cell volume determinations of canine, feline, rabbit, and human blood. It was found that the use of excessive amounts of anticoagulant alters the packed cell volume when measured by the standard centrifuge method. It was also found that the results vary significantly when different anticoagulants are used. The effects are not seen when the packed cell volume is indirectly measured using an electronic cell counter. A balanced combination of ethylenediaminetetraacetic acid salts ameliorates the concentration-dependent effects.