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Living biological cells can be held in electric-field-induced traps produced within and slightly above a horizontal array of planar, micron-sized electrodes. The trapping is due to the balance between gravity and the levitation force produced by negative dielectrophoresis, which causes repulsion of particles from regions of high electric-field strength. Growth and division of yeast cells (Saccharomyces cerevisiae) through several generations to form a small colony can occur within each trap. The rate of growth appears to equal or exceed that in normal bulk culture: this may be a consequence of the continuous exposure to RF field strengths of 30–45 kV/m. The placement and also the culture of the cells is facilitated by the formation of a thermal convection cell above the electrode array, which occurs in response to heating of the liquid close to the electrodes. If the initially-applied voltage is higher than that which gives stable trapping, then dielectrophoretic levitation occurs within the lower part of the convection cell (“lev-vection”). This can be applied in several steps: first to concentrate cells from extremely dilute suspension, then to position them above trap positions that are optimum for observation, and finally drop them into the traps by turning down the applied voltage. |
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