T. Müller

Dielectric spectroscopy of single human erythrocytes at physiological ionic strength: dispersion of the cytoplasm.

Usually dielectrophoretic and electrorotation measurements are carried out at low ionic strength to reduce electrolysis and heat production. Such problems are minimized in microelectrode chambers. In a planar ultramicroelectrode chamber fabricated by semiconductor technology, we were able to measure the dielectric properties of human red blood cells in the frequency range from 2 kHz to 200 MHz up to physiological ion concentrations. At low ionic strength, red cells exhibit a typical electrorotation spectrum with an antifield rotation peak at low frequencies and a cofield rotation peak at higher ones. With increasing medium conductivity, both electrorotational peaks shift toward higher frequencies. The cofield peak becomes antifield for conductivities higher than 0.5 S/m. Because the polarizability of the external medium at these ionic strengths becomes similar to that of the cytoplasm, properties can be measured more sensitively. The critical dielectrophoretic frequencies were also determined. From our measurements, in the wide conductivity range from 2 mS/m to 1.5 S/m we propose a single-shell erythrocyte model. This pictures the cell as an oblate spheroid with a long semiaxis of 3.3 microns and an axial ratio of 1:2. Its membrane exhibits a capacitance of 0.997 x 10(-2) F/m2 and a specific conductance of 480 S/m2. The cytoplasmic parameters, a conductivity of 0.4 S/m at a dielectric constant of 212, disperse around 15 MHz to become 0.535 S/m and 50, respectively. We attribute this cytoplasmic dispersion to hemoglobin and cytoplasmic ion properties. In electrorotation measurements at about 60 MHz, an unexpectedly low rotation speed was observed. Around 180 MHz, the speed increased dramatically. By analysis of the electric chamber circuit properties, we were able to show that these effects are not due to cell polarization but are instead caused by a dramatic increase in the chamber field strength around 180 MHz. Although the chamber exhibits a resonance around 180 MHz, the harmonic content of the square-topped driving signals generates distortions of electrorotational spectra at far lower frequencies. Possible technological applications of chamber resonances are mentioned.

Linear motion of dielectric particles and living cells in microfabricated structures induced by traveling electric fields

Arrangements of microelectrodes as obtained by a microfabrication technique are found to be well suited for a linear transfer of microscopic particles such as biological cells and other objects of microscopic dimensions. The conditions for an effective manipulation of the particles are electrode geometries which correspond to the dimensions of the particle and adapted electrical excitation of the electrodes (traveling high-frequency waves). The motion of particles was found to be a super-position of dielectrophoresis and charge relaxation processes as they are dominant, e.g. in dielectric induction motors. Microparticle velocities of some hundreds mu /s could be achieved by applying phase-shifted rectangular pulses with amplitudes between 5 and 15 volts.<<ETX>>

Differences in the rotation spectra of mouse oocytes and zygotes

Rotation spectra of mouse oocytes, zygotes and embryos in the two-cell stage under the influence of high-frequency rotating fields were studied. The characteristic frequency (fc1) of cells isolated from superovulated + mated mice is different from that of oocytes. This was attributed to an increase in the membrane resistance and, less probably, to a change in the zona pellucida conductivity. The rotation spectra can be used to differentiate between non-fertilized and fertilized eggs. A theoretical interpretation of the measured spectra and simulation of the changes caused by fertilization is given.

Dielectric induction micromotors: Field levitation and torque-frequency characteristics

Abstract The characteristic motor operation of miniaturized dielectric induction motors is discussed with respect to microfabrication in semiconductor technology. The influence of a fluid filling the interspace between rotor and stator is explained. Electric field traps can be produced by applying a.c. and also rotating electric fields of high frequencies using appropriate electrode geometries. At certain frequencies, negative dielectrophoresis can be used for self-centring of a spherical or disk-shaped rotor in field traps. It is discussed how the resistance to abrasion can be increased by avoiding mechanical bearing elements. Remarks on the design of induction motors are added.

Dielectric single particle spectroscopy for measurement of dispersion

Measuring the frequency-dependent behaviour of single particles or biological cells in inhomogeneous and/or rotating electric fields is a sensitive method for characterising their dielectric properties. This technique is able to detect broad dispersion in the megahertz range of homogeneous artificial Sephadex G15 spheres. Recent progress has opened up the possibility of carrying out dielectric spectroscopy in cell culture media. Dielectrophoretic and electrorotational spectra of different cells in media of varying conductivity can only be explained by the introduction of dispersive cell compartments. The cytoplasm of animal cells typically exhibits a broad dispersion around 15 MHz and there is evidence for membrane dispersion around 50 MHz.

Reversible and irreversible rotating field-induced membrane modifications

We analyse the charge distribution as submitted by additionally induced transmembrane potentials in rotating electric fields. In contrast to d.c. and a.c. fields, in rotating fields the induced peak potential differences across the membrane systems are phase shifted with respect both to each other and to the external field vector. These phase differences are strongly frequency-dependent but were also influenced by the electrical properties of both the cell and the surrounding medium. We extend our investigation up to the non-linear field strength range of electrorotation and found reversible and irreversible changes in the rotational behavior of several cells. The most convenient variables for describing non-linear electrorotation are the characteristic frequency (fc1) and the corresponding angular velocity (omega c) of the cells. With increasing field strength the observed rotational behavior becomes more and more irreversible and finally rupture of the membrane occurs.

Handling and investigation of adherently growing cells and viruses of medical relevance in three-dimensional micro-structures

The investigation of animal cells in microstructures fabricated by semiconductor technology is a new field of research and has found its first biotechnological and medical uses. This paper contains selected examples of three-dimensional structures to handle and characterise adherently growing cells, to determine their adhesion, surface migration and to cultivate them under strong electric field. Two new principles, (i) the use of electronic resonances to increase polarisation forces and (ii) the computer-based investigation of cell rotation/cell dielectrophoresis by photoelement signal analysis, are explained. Additionally, some results dealing with the trapping and concentration of submicron particles such as viruses in extremely miniaturised electric field cages are given. Perspectives for such semiconductor micro-devices are outlined.

Fluid-filled dielectric induction micromotor with Al-SiO/sub 2/ rotor

Results obtained with a dielectric induction micromotor with an Al-SiO/sub 2/ rotor are presented. The rotor and gold electrode stator, fabricated separately in IC processes, are immersed in a fluid (water, ethanol or glycerine). The aluminium rotors, 2- mu m thick and 100-200 mu m in diameter, are covered on all surfaces by a SiO/sub 2/ layer. The motor was operated at field frequencies between 0.3 kHz and 30 MHz and applied voltages varying between 5 and 110 V. At maximum voltage, a rotation speed of 16000 rpm was measured. The calculated value of torque was approximately 3*10/sup -13/ Nm.<<ETX>>