Forces acting on biological cells in external electrical fields

Biological cells stressed by external electric fields undergo mechanical deformation and remodeling caused by electro-mechanical Maxwell stresses. These stresses appear at the internal and external membrane interfaces due to differences in dielectric properties of the biological membrane and the surrounding and internal fluids. Electro-mechanical loading of the cells results in the reversible or irreversible development of pores in the membrane. This could cause disruption in normal functioning of the cell and even leads to cell death. Such a phenomenon has several practical applications including pulsed electric field (PEF) processing of fluids. In the PEF process liquids and pumpable products are subjected to short high electric field pulses that lead to non-thermal inactivation of bacteria. In the present paper the dynamics of electromechanical loading of spherical cells in external pulsed electric fields has been considered. The cell was modeled as a simple insulating dielectric shell containing a conducting fluid. Cells were suspended in a second conducting fluid. For a single spherical cell electrical fields and electro-mechanical forces have been calculated both analytically and using finite-element software. The transient electrical fields induced in a cell membrane subjected to a high- field step pulse are increased due to Maxwell-Wagner polarization effects at the membrane/conductive fluid interfaces. The electro-mechanical forces acting on the cell also grow during this transient polarization process, the duration depending on the conductivities and permittivities of the inter-cellular and extra-cellular fluids. As a result the electro-mechanical stresses in the membrane reach a maximal value up to three orders of magnitude higher than the initial stress. This maximum value occurred in a time comparable to the Maxwell-Wagner relaxation time of the conducting fluid. In the case of two spherical cells located near to each other the finite difference software was used to calculate the induced stresses. It was shown that the field and electro-mechanical stresses in the two-cell model were practically identical to the single sphere model.