Extensive studies have been carried out on charged droplets,which are charged through induced or contact charging under a DC electric field.Although the transportation of bio-samples by charged droplets on EWOD and ECOD has been examined [1,2],it is still an issue that whether a contact-charged droplet can be a reliable bio-sample carrier in long working time with mass of charging/discharging processes.Therefore,in this study,live cell transport by contact-charged droplets is demonstrated experimentally,and over 90% cell viability can be obtained after thousands charging/discharging processes.Fig.1(a) shows the schematic plot of our experiment setup.A deionized water droplet is placed between two parallel electrode plates,one is connected to a power supply and the other is grounded.Under applied DC fields,the electrical charging of the droplet occurs and is followed by different droplet behaviors,which can be classified into four categories shown in Fig.1(b).Considering the applicability,only the stable back and forth droplet motion will be discussed.To study the charge exchange between the droplet and the electrodes during their contact,an oscilloscope in parallel with a resistor is connected to the ground electrode to record the potential variation.Figure 2(a) shows the measured electrical signal when a 2 μl droplet is driven by a 4.21 kV/cm electric field.The voltage peaks in this figure are due to the charge exchange from the positive electrode to the ground electrode delivered by the traveling droplet.The electrical charging can be obtained by integrating the current,which is the measured potential divided by the resistance,over time (see Fig 2(b)).As can be observed from Fig.2(b),the experimental results for a water droplet are a little lower than the theoretical prediction for a perfectly conducting sphere.In addition,the overall trend of the experimental data is not linear in contrast to the linear trend of the theoretical data.The reason cause this is believed to be the electrostriction effect of droplets at large electric fields.Figure 3(a) shows the moving frequency of a 1 μl cell solution droplet (160×104 Jurkat cells/ml) in comparison with that of a 1 μl DI water droplet.For the same driving electric field,we found that the moving frequency of cell solution droplet is slightly higher than that of DI water droplet.Figure 3(b) shows the average viability of cells after the cell solution droplet contacts with the electrodes for different numbers of times under 3.5 kV/cm driving electric field.The cell viability is still over 95% after 300 charge-discharge cycles (300 s working time),and is still very high even after 3000 periodic contacts with the electrodes.The high cell viabilities provide powerful evidence that the electrical charging of a droplet barely influences the interior biological samples over a long working period.