Figure 7. Noise spectra as a function of cell density.

The current (a) and voltage noise (b) as a function of frequency. The noise was measured unbiased using two seeding densities of glioma cells of 1 × 10⁵ cells/ml and 1 × 10⁶ cells/ml. The electrode area was 10.5 mm². Each spectrum is an average over 20 consecutive measurements of 60 s each. Each envelop represent three different measurements, each starting from a new glioma cell batch. The darker region in between is due to overlap between the envelopes. The solid line represents the baseline noise measured without cells, with cell culture medium only.