Evaluation of the driving pulse voltage (a) polarity (b) and duration (c) effects on the delivery/expression of the luciferase reporter gene. BALB/c mice (n = 4–8) were intradermally injected with 20 µg pVaxLuc (per site) into two sites, and injections were followed by electroporation with a BEX machine equipped with multi-needle electrodes (CUY691-5, BEX Ltd). Electroporation was initiated with a poration pulse of 400 V of 0.05 ms followed by a train of eight 50 V (a–c) or 100 V (a) pulses of the same (+/+; a–c) or opposing/alternating polarity (+/−; b,c) administered for 10 ms (Short pulses; a–c) or 50 ms (Long pulses; b,c) with 20 ms (Short pulses) or 950 ms (Long pulses) intervals. Bioluminescence was registered by in vivo imaging on day 1 and 3 after the injections. Data represent the average photon flux from all injections sites in the group (photons/s/cm2/sr), and the error bars represent the SD. All assays were performed in duplicate. *p < 0.05, **p < 0.01; p-values in the interval 0.05 to 0.1 indicate tendencies. Statistical comparisons were performed using Mann–Whitney tests.