-Cortical rat neurons were trapped by dielectrophoresis (DEP). Experimental data were compared with theoretically deduced relationships. The neuron was represented by a single-shell model. A planar quadrupole electrode structure was used for the creation of a non-uniform field. The electrode structure was modeled as four point charges. The experimental data did almost completely fit the theoretical yieldtime relationship. The theoretical yieldamplitude relationship, however, did only apply for a restricted amount of frequencies. The experimental frequency behaviour (i.e., the DEP-spectrum) did not apply to the theory. A difference in neuronal physiological state can produce different DEP-spectra. For two frequencies (10 kHz and 14 MHz) adhesion to the substrate and outgrowth of the neurons was investigated.Key words -Cortical rat neuron, trapping, Dielectrophoresis, planar micro-electrode plate, neuronal outgrowth, adhesion I. INIRODUCTION Dielectrophoretic (DEP) trapping is used to position neurons on particular spots, like an electrode tip in a planar multi electrode array that can be used for stimulation and measurement of neuronal activity. However, a significant requirement for using trapped neurons for stimulation or measurement purposes is that they survive the DEP treatment and stay positioned.Non-uniform electric fields exert dielectrophoretic forces on polarizable particles [ 11. The time-averaged DEP force FDEp in an AC electric field is dependent on the in-phase component of the dipole moment for a particle p, which is suspended in a medium m. and can be written aswhere r is the radius of the particle, f m is the Clausius-Mosotti factor, E is the permittivity, and E is the electric field with the index rms denoting the root-mean-square value of this field.A planar quadrupole micro-electrode structure was used for the creation of a non-uniform electric field that could trap neurons in the center of the structure by negative dielectrophoretic forces. Trapping of neurons has been investigated in quantitative and qualitative terms, i.e., the number of neurons trapped in the center, and their 0-7803-6465-1/001$10.00 02000 IEEE physiological state indicated by adherence of the neurons to the substrate and dendritic outgrowth, respectively.
MATERIALS & METHODSPhotolithographic, plasma deposition and etching techniques were used to create a quadrupole micro-electrode structure on a glass plate of 5 x 5 cm. A titanium-layer was used for the adhesion of the gold-layer on the glass plate. An insulation layer, consisting of a silicon-nitride layer sandwiched between two silicon-oxide layers, was applied to avoid electrochemical processes at the electrode-culture medium interface and to reduce unnecessary heating of the medium. The 'active regions', the regions around and in between the electrode tips were kept free of this insulation layer. The tips were triangularly shaped with angles of 90" and the inter-electrode distance between two diagonally opposing tips was 100 pm. Figure 1 shows the electrode s...