The Engineering
Behind EPULSUS®

Transmembrane potential and the charging time constant

The cell membrane is an electrical capacitor: a lipid bilayer ~5 nm thick with a dielectric constant ε ≈ 2, surrounded by conductive cytoplasm and extracellular medium. At rest, the natural transmembrane potential (TMP) is approximately −70 mV. When an external electric field E is applied, the induced TMP across a spherical cell of radius r is described by the Schwan equation:

For a typical mammalian cell (r ≈ 10 μm), τ is of order 100–500 ns. This charging time constant is critical: it defines the regime in which the pulse operates.

• τ_pulse ≫ τ_cell (μs–ms range)

  The plasma membrane fully charges. TMP builds up to 1.5·E·r at the poles. Above a critical threshold (~0.2–1 V), hydrophilic pores nucleate in the lipid bilayer. Reversible if the dose is controlled; irreversible (IRE) above a lethal threshold.

• τ_pulse ≪ τ_cell (ns range — nsPEF)

  The plasma membrane cannot fully charge. The field penetrates into the cytoplasm and drives TMP across intracellular membranes (ER, mitochondria, nuclear envelope). This is supra-electroporation — simultaneous nanoporation of all membranes at much higher field strengths (10–300 kV/cm).

The pulse strength–duration space

Every electroporation protocol occupies a point in the two-dimensional space defined by electric field strength E and pulse duration τ. The accessible biological effects are strongly determined by this location:

The dashed vertical line marks the approximate membrane charging time constant τ_cell. To the right: classical electroporation of the plasma membrane. To the left: nsPEF, where intracellular targets dominate. Each EPULSUS generator is shown by its pulse-duration range (horizontal extent) and the field strengths typically accessed in practice (vertical extent depends on chamber gap and electrode geometry).

nsPEF — supra-electroporation and intracellular effects

When pulse duration falls below τ_cell, the plasma membrane acts as a high-pass filter — the field propagates through it and charges intracellular membranes directly. The result is supra-electroporation: the simultaneous formation of large numbers of nanopores (<1 nm) in every membrane compartment, at field strengths of 10–300 kV/cm.

The biological cascade is distinct from classical electroporation: calcium release from the ER, collapse of the mitochondrial membrane potential ΔΨ_m, caspase activation and programmed apoptosis — without the necrotic, inflammatory response associated with thermal methods. At sub-lethal doses, nsPEF modulates intracellular signalling without membrane disruption. The FBML1-5, with sub-50 ns rise times, accesses this regime.