**4.3. Irreversible electroporation**

Since equation [12] must be true irrespective of the volume, so we can write equation [12] as

This is equation of continuity, which is the principle of conservation of charge where steady

electric field pulses, then ∇.*J* =0. Now electric field is the gradient of electric potential. So

tivity of cytoplasm and external medium of the cell is higher than the cell membrane conduc‐

where acell is the outer radius of the cell, Ee is the applied electric field strength and θ is angle between field line and normal to the point of interest in the membrane which can be either

 or 1800 [82-85]. Under the ideal experimental conditions like pulse width, electric field, number of pulses, removal of external electric field for resealing of the pore membrane, pulse duration and rearrangement of the membrane protein can be preserved the cell viability. If the membrane is not spherical, then equation [14] may not be right explanation. If we consider that the cell has ellipsoidal structure, then equation [14] will not be applicable. But for any practical purpose this equation can be used to evaluate the field induced transmembrane

When a strong external electric field applied across cell and tissue, then membrane conduc‐ tance and permeability can increase significantly due to strong polarization of the cell mem‐ brane, as a result membrane can form nano scale defects (called nanopores). But when we switched off the external electric field, membrane can return from its conducting state to its normal state. This phenomenon is called reversible electric breakdown or reversible electro‐ poration [86-87]. The reversible electroporation generally involves reversible electric break‐ down (REB), which is generally a temporary high conducting state. This reversible electroporation influences both cell membrane as well as artificial planner bilayer lipid membrane. Reversible electroporation involve with rapid creation of many small pores, where membrane discharge occur before any critical pores can evolve from the small pores. To understand the method of electroporation of bilayer lipid membrane, it is necessary to use the method of voltage clamp [65,71,88] and charge relaxation [80,89] techniques, where for charge relaxation, kinetics of voltage decreases across the membrane after the application of short pulses (20 nsec to 10 µsec). It was also fact that originally membrane breakdown can occur before the start of membrane discharge. From the charge relaxation method, it used to show

 q

tivity, then ΔΨ, the field induced transmembrane potential can be written as:

y

74 Advances in Micro/Nano Electromechanical Systems and Fabrication Technologies

1.5 cos *cell e* D =

<sup>∂</sup>*<sup>t</sup>* =0 and if charges are not generated into the cell during application of

*ψ*=0, where Ψ denotes the electrical potential. If the conduc‐

¶ (13)

*a E* (14)

. 0 *<sup>J</sup> <sup>t</sup>* ¶rÑ+ =

current involve ∂*<sup>ρ</sup>*

00

potential.

Maxwell equation becomes *Δ* <sup>2</sup>

**4.2. Reversible electroporation**

In our earlier discussion of reversible electroporation, external electric field can permeabilize the cell membrane temporarily by which, the cell membrane can survive and the process known as "reversible electroporation" whereas, some of strong external electric field can cause t the cell membrane to permanently permeabilize (membrane becomes weak effect on conductance), by which the cell can die and the process is refer to as "irreversible electropo‐ ration". This irreversible electroporation was observed in early 1754 due to discharge of static electrical generator of the skin [95-96]. The main phenomenon of irreversible breakdown was stochastic quantities by which mean life time of membrane can abruptly decreased with increased of voltage. The pores of the bilayer membrane can be hydrophilic or hydrophobic [65]. For hydrophobic cases, the pores can be formed by hydrocarbon lipid tails. Whereas the inner surface of the pores can be covered by polar tails. The hydrophobic pores which can fill by water are energetically unfavorable [66] and thus should be short -lived. The formation of the pores during reversible electroporation can exist for longer periods of time due to hydro‐ philic pores. The accumulation of pores during reversible electroporation is due to membrane containing lysolecithin, which can decrease the linear tension of hydrophilic pores [97-98]. The hydrophilic pores can cause the reversible and irreversible breakdown of lipid membrane. Also every electrical field can produce the thermal effect as familiar as Joule effect is disputed, where as certain electric field is undisputed, which can provide irreversible electroporation [95]. Irreversible electroporation can affect only the membrane of living cells and spares of tissues scaffold. During irreversible electroporation, the membrane survives in two stages as (a) steady state current stage and (b) fluctuating current stage. The phenomena of irreversible electroporation can cause by charge pulse technique [80] in which membrane is charged at U=0.1 V (with pulse width 400 ns) and discharged was very slow. The large pulse of the same width, can charge the membrane towards 0.4 V, but after 300-400µs, charges can be decreased as a sigmoidal manner up to zero because of membrane rupture [78].
