Impedance measurements: Impedance measurements were performed using an MTZ-35 impedance/gain-phase analyzer (Bio-Logic). Mats were placed on finger electrodes with an interelectrode distance of 1 mm and the electrodes were contacted using a probe station micromanipulator. A 50 mV AC bias was applied during the measurements without DC bias. A frequency range of 10 MHz to 10 Hz was used.