Purpose: Highly localized and limited cell death in the skin will attract a large amount of antigen presenting cells, which is beneficial to augment immunogenicity to an antigen. Millisecond-long electric pulses can cause cell death at the site of the microneedle electrodes, which can serve as “physical immune enhancer” for vaccination. A metal microneedle array vaccine delivery system is developed using millisecond-long electric pulses aiming to boost the immune response of vaccines administered via skin.
Methods: An isolated microneedle array was fabricated by assembling 10 rows of metal microneedle arrays in a 3D printed holder. Each row of microneedle array contains 9 needles with 200 µm wide, 630µm long and 50µm thick. The gap between each row of microneedle array in 3D holder was designed to be 900 µm. In this way, microneedles with opposite electric polarity (anodes and cathodes) could be positioned adjacent to each other at microneedle array. The cell viability in the skin caused by electric pulse was studied by metachromatic staining using acridine orange (AO) and ethidium bromide (EB). Confocal and multiphoton microscopy was used for visualization. The effects of voltage and pulse length on the cell viability in the skin were studied. The pH change caused by the electric pulse was also studied by a pH-sensitive dye, 8-Hydroxypyrene-1,3,6-trisulfonic acid trisodium (HPTS), in combination with confocal laser scanning microscopy and digital image analysis.
Results: The close spacing of each row of opposite electrodes provides precise localization of electric field within the skin and permits relatively low voltages to be used to generate the high electric fields needed for damaging the cells. The control group without applying electric pulses caused 1.04±0.37% damaged cells. No significant increase in cell death was observed with the increase of voltage to 100 V at 50 ms pulse length. However, a significantly increased cell death (5.04±1.07%) was observed when the voltage increased to 150V (p<0.0001). Further increasing of the voltage to 200 V resulted to 9.03±1.00% cell death in the skin. A threshold voltage was observed at 150 V, when the external electric field is above the threshold, a significant cell death was observed. Increasing pulse length from 20 ms to 146 ms at 150 V also led to a 2.6-fold increase of cell death. A pH change in tissue was observed at different electrical polarity electrode.
Conclusion: An isolated microneedle array was successfully fabricated and can be used for damaging cells at the site of the microneedle electrodes. The cell death is highly related to the voltage and pulse length applied. The recruiting of antigen presenting cells due to cell death and the effect of electric field on the immune response to vaccines are under investigation.
Mark R Prausnitz– Regents’ Professor, Georgia Institute of Technology, Atlanta, Georgia