Increasing the permittivity and conductivity of cellular electromanipulation media

Suspended particles, including biological cells, can be moved and/or rotated by application of alternating, rotating or traveling electric fields. The magnitude of the induced force or torque generally increases with the permittivity of the suspension medium. The required field strength can therefore be minimised by increasing the medium permittivity: in order to maintain compatibility with biological cells, this can be done by use of solutions of amino acids, peptides, proteins, or other Zwitterions. A further important point is that electric-field work with biological cells often suffers from problems caused by attraction of the cells to the electrodes, where the intense local fields may damage them. This may be avoided by reversing the direction of the induced force, which requires that the medium be made more polarisable than the particles. This is facilitated not only by increase of the permittivity of the medium, but more easily by use of media of high conductivities, even as high as those of normal tissue-culture media. Depending upon the permittivity and conductivity, it appears possible to separate cells of different types by means of their field-induced movements. The high power dissipation that occurs when intense fields are applied to conductive media does not give rise to a large temperature increase if the field-treated volume is reduced sufficiently, and if the thermal resistance of the surroundings is low enough. These conditions can be met in microstructures fabricated using semiconductor technology, and cell growth is possible even during long-term application of intense high-frequency fields