The dielectric properties of monodisperse latex particles with mean diameters in the range 5-10 wm have been studied by single-particle rotation. The surface conductance is readily deduced from the medium conductivity dependence of the rotation peak seen in the 30-1 000-kHz range. This peak satisfies the equations describing the surface-conductance-modified Maxwell-Wagner dispersion for these particles, with respect to both the optimum field frequency and the speed of rotation. The observed surface conductivities of the particles are constant over the medium conductivity range of 2-16 wS/cm. The magnitude of this conductance depended upon particle type and pretreatment, the range of values being 0.2-2.1 nS. The rotation spectra indicate the presence of an additional rotation peak at field frequencies below 1 kHz. Possible causes of this effect, such as the modification of the field seen by the particle by its own ionic atmosphere or a "negative" dielectric dispersion due to the frequency dependence of electrophoretic motion, are suggested.

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Rotation of single cells (mesophyll protoplasts o f Avena sativa) induced by a planar, homogeneous rotating field has been observed at a frequency o f 2 0 -4 0 kHz (conductivity o f the external mannitol solution 6 x 10-5 ß -1 cm-1). This variation in optimum frequency is largely due to an inverse dependence on cell radius. Rotation direction is opposite to that o f the field, and can be reversed at will by reversing the field. The maximum speed o f cell spinning was a few cycles per second (and thus always much slower than that o f the field) and was proportional to the square o f the amplitude of the field.The rotation o f a single cell in a rotating field is expected on the basis o f the dipole-dipole theory developed by Holzapfel et al., (J. Membrane Biol. 67, 1 -1 4 (1982)), for multi-cell rotation. Measurements of the dependence of optimum applied field frequency on medium conductivity indicate that the dipole is generated by interfacial (Maxwell-Wagner) polarization. The required frequency is a linear function o f the conductivity of the external solution. This relationship is used to derive a value for the specific membrane capacitance. Further applications o f this technique for cell and membrane research are discussed. Introduction Materials and MethodsRotation o f a very small percentage o f cells in a linear non-uniform field (frequency range betw een 100 Hz and 120 MHz) has been described by several authors [1][2][3][4].Zimmermann et al. [4] showed that nearly all cells of a given species if in close proxim ity rotated when exposed to a linear alternating field having a characteristic narrow frequency range. F u rth er work by this group showed that the presence o f a m ini mum of two cells in close proxim ity is necessary and presented a theory to account for this based on the interaction of two induced dipoles within the field [5]. Implicit in the theory was the local production of a rotating field by the interaction o f the two induced dipoles.Rotation o f any single cell induced by the same characteristic range of frequencies as observed by Zimmermann et al. [4] has now been observed, provided that the field itself rotates rather than alternating along one axis. The experimental cham ber consists of four elec trodes mounted on a microscope slide as shown in Fig. 1. The essentials of the technique were proven in a chamber with 8 mm electrodes; a smaller chamber with 3 mm electrodes was used when higher field strengths (greater than 37.5 V cm -1) were required. The four electrodes are driven with equal voltages of four phases spaced by 90°, which are produced by the unit shown in Fig. 2. The production o f a rotating field in this manner is discussed in Appen dix A. The unit, using sinne wave drive from a Toellner TE7702 function generator, produces up to 15 volts peak per electrode from 5 Hz to 500 kHz. The function generator was calibrated against a frequency counter (Tektronix D C 503). Mesophyll cell protoplasts o f Avena sativa were prepared as by Hampp and Ziegler [6].Observations were confi...

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Cells from three cell lines were electrorotated in media of osmotic strengths from 330 mOsm to 60 mOsm. From the field-frequency dependence of the rotation speed, the passive electrical properties of the surfaces were deduced. In all cases, the area-specific membrane capacitance (Cm) decreased with osmolality. At 280 mOsm (iso-osmotic), SP2 (mouse myeloma) and G8 (hybridoma) cells had Cm values of 1.01 +/- 0.04 microF/cm2 and 1.09 +/- 0.03 microF/cm2, respectively, whereas dispase-treated L-cells (sarcoma fibroblasts) exhibited Cm = 2.18 +/- 0.10 microF/cm2. As the osmolality was reduced, the Cm reached a well-defined minimum at 150 mOsm (SP2) or 180 mOsm (G8). Further reduction in osmolality gave a 7% increase in Cm, after which a plateau close to 0.80 microF/cm2 was reached. However, the whole-cell capacities increased about twofold from 200 mOsm to 60 mOsm. L-cells showed very little change in Cm between 280 mOsm and 150 mOsm, but below 150 mOsm the Cm decreased rapidly. The changes in Cm correlate well with the swelling of the cells assessed by means of van't Hoff plots. The apparent membrane conductance (including the effect of surface conductance) decreased with Cm, but then increased again instead of exhibiting a plateau. The rotation speed of the cells increased as the osmolality was lowered, and eventually attained almost the theoretical value. All measurements indicate that hypo-osmotically stressed cells obtain the necessary membrane area by using material from microvilli. However, below about 200 mOsm the whole-cell capacities indicate the progressive incorporation of "extra" membrane into the cell surface.

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Single particles can be manipulated by applying high frequencies to ultramicro electrode arrays fabricated on planar structures. Heat production can be reduced to the extent that intense electric fields can be applied even to unmodified cell culture media. Animal cells grow normally in the high field (up to 100 kV/m) between such continuously energized multielectrodes. As with laser tweezers [1-3], this technique can capture particles and cells in field traps, generate linear movement, and permit cell cultivation. It can also produce micropatterns of pH gradients, field-cast objects, and control cell adhesion. These microtools may be combined to develop cell separators, microsensors, and controlled-biocompatibility surfaces.