Research

Our knowledge of the detailed mechanisms of the interactions between electric fields and biological systems is limited and incomplete. At the most basic level electric fields generate forces on atoms and molecules — this is the regime of nanoscale bioelectrical physics. Our goal is to identify and understand the molecular-level interactions between external electromagnetic fields and biological systems. We concentrate on the effects of very short (nanosecond), intense (megavolt-per-meter) pulsed electric fields on cells and tissues, combining experimental observations with molecular simulations. The focus of our recent work is the delineation and characterization of the biophysical mechanisms that govern electric field-driven, nondestructive perturbations of biological membranes.

Figure. Life cycle of an electropore. Only water and phosphorus atoms are shown for simplicity. Pore creation in an electric field begins with the introduction of a water defect into the bilayer interior (pore initiation), followed by the reorganization of phospholipid head groups in each leaflet around the defect (pore construction). Migration of additional water and head groups into the pore continues until an arbitrarily defined mature pore structure is formed (pore maturation).

We perform molecular dynamics (MD) simulations of electropore formation in phospholipid bilayers to understand nanoscale physics of electropermeabilization.

Experimental studies of nanosecond pulse exposure of cells

Figure. Fluorescence images of YO-PRO-1 and propidium transport into U-937 cells at 0.2, 0.4, 1, 5, 10, and 20 s after exposure to 10, 6 ns, 20 MV/m pulses delivered at 1 kHz. Circles mark the circumference of the cells, which can be seen in the transmitted light images at the left, along with + and – symbols indicating the anode and cathode directions.

Pulsed electric fields can induce transport of normally impermeant ions and small molecules across the cell membrane, a process called electroporation or electropermeabilization. This phenomenon is often studied by using microscopic imaging to monitor changes in the intracellular concentration of fluorescent dyes. Most of these studies, however, report their findings in relative or arbitrary units.

To produce truly quantitative measurements of electroporative transport we employ calibrated confocal microscopy to track YO-PRO-1 entry into cells after a single 6 ns, 20 MV/m pulse, and we correlate these observations with molecular dynamics simulations of YO-PRO-1 transport through lipid electropores. These baseline studies have been extended to quantitative observations of transport of calcein (an anionic fluorescent small molecule) and propidium (a cationic dye like YO-PRO-1 that is fluorescent only after binding to polynucleotides). Our results challenge the common assumption that small molecule transport through electropermeabilized membranes is dominated by simple diffusion through electropores.