Physical modelling of electroporation in close cell-to-cell proximity environments

3D-printing enabled micro-assembly of a microfluidic electroporation system for 3D tissue engineering

Electro-transfection is an essential workhorse tool for regulating cellular responses and engineering cellular materials in tissue engineering. However, most of the existing approaches are only focused on cell suspensions in vitro, which fails to mimic an in vivo tissue microenvironment regarding the 3D electric field distribution and mass transport in a biological matrix. However, building a 3D electro-transfection system that is compatible with 3D cell culture for mimicking the in vivo tissue microenvironment is challenging, due to the substantial difficulties in control of the 3D electric field distribution as well as the cellular growth. To address such challenges, we introduce a novel 3D micro-assembly strategy assisted by 3D printing, which enables the molding of 3D microstructures as LEGO® parts from 3D-printed molds. The molded PDMS LEGO® bricks are then assembled into a 3D-cell culture chamber interconnected with vertical and horizontal perfusion microchannels as a 3D channel network. Such a 3D perfusion microchannel network is unattainable by direct 3D printing or other microfabrication approaches, which can facilitate the highly-efficient exchange of nutrition and waste for 3D cell growth. Four flat electrodes are mounted into the 3D culture chamber via a 3D-printed holder and controlled by a programmable power sequencer for multi-directional electric frequency scanning (3D μ-electro-transfection). This multi-directional scanning not only can create transient pores all over the cell membrane, but also can generate local oscillation for enhancing mass transport and improving cell transfection efficiency. As a proof-of-concept, we electro-delivered the pAcGFP1-C1 vector to 3D cultured HeLa cells within peptide hydrogel scaffolding. The expressed GFP level from transfected HeLa cells reflects the transfection efficiency. We found two key parameters including electric field strength and plasmid concentration playing more important roles than the pulse duration and duty cycles. The results showed an effective transfection efficiency of ∼15% with ∼85% cell viability, which is 3-fold higher compared to that of the conventional benchtop 3D cell electro-transfection. This 3D μ-electrotransfection system was further used for genetically editing 3D-cultured Hek-293 cells via direct delivery of CRISPR/Cas9 plasmid which showed successful transfection with GFP expressed in the cytoplasm as the reporter. The 3D-printing enabled micro-assembly allows facile creation of a novel 3D culture system for electro-transfection, which can be employed for versatile gene delivery and cellular engineering, as well as building in vivo like tissue models for fundamentally studying cellular regulation mechanisms at the molecular level.

Cell-cell proximity effects in multi-cell electroporation

We report a fundamental study of how the electropermeabilization of a cell is affected by nearby cells. Previous researchers studying electroporation of dense suspensions of cells have observed, both theoretically and experimentally, that such samples cannot be treated simply as collections of independent cells. However, the complexity of those systems makes quantitative modeling difficult. We studied the change in the minimum applied electric field, the threshold field, required to affect electropermeabilization of a cell due to the presence of a second cell. Experimentally, we used optical tweezers to accurately position two cells in a custom fluidic electroporation device and measured the threshold field for electropermeabilization. We also captured video of the process. In parallel, finite element simulations of the electrostatic potential distributions in our systems were generated using the 3-layer model and the contact resistance methods. Reasonably good agreement with measurements was found assuming a model in which changes in a cell's threshold field were predicted from the calculated changes in the maximum voltage across the cell's membrane induced by the presence of a second cell. The threshold field required to electroporate a cell is changed ∼5%-10% by a nearby, nearly touching second cell. Cells aligned parallel to the porating field shield one another. Those oriented perpendicular to the field enhance the applied field's effect. In addition, we found that the dynamics of the electropermeabilization process are important in explaining observations for even our simple two-cell system.