Biomedical Engineering Reference
In-Depth Information
techniques, such as cryosurgery (Davalos et al., 2005).
Thus, for certain medical applications IRE alone could be
used as an effective technique for tissue ablation without
the use of cytotoxic drugs like in chemotherapy (Davalos
et al., 2005).
hydrophobic pores (Weaver, 2000). The basic idea is
that a circular region of membrane is replaced with
a pore. As primary pores appear in the membrane, its
resistance drops, and the voltages within the system
redistribute on a time scale governed by the in-
stantaneous values of the various conductivities and ca-
pacitance (Weaver, 2000). Both experiment and theory
show that the membrane capacitance change is small
(Chernomordik et al., 1982; Freeman et al., 1994), so
that the main electrical result is drastically decreased
barrier resistance. Overall, bilayer membrane electro-
poration results in dynamic, non-linear changes as a het-
erogeneous pore population evolves rapidly in response
to the local value of the transmembrane voltage
U
mlocal
along the surface of a cell membrane (Weaver, 2000). At
the time of maximummembrane conductance, pores are
nevertheless widely separated, occupying only about
0.1% of the electroporated membrane area (Hibino
et al., 1991; Freeman et al., 1994). In this sense, elec-
troporation is catalytic (Weaver, 1994). Not only is there
the possibility of binding and lateral diffusion to the
other side of the membrane as pores form and then
vanish, but there is a tremendous increases in rate (of
transport) due to small entities (pores) that occupy
a small fraction of the membrane (Weaver, 2000).
Due to the changes in the cell membrane resistance
during electroporation the technique can also be con-
trolled and monitored with EIT IRE.
Electrical properties of tissue during
electroporation
The electrical properties of any material, including
biological tissue, can be broadly separated into two
categories: conducting and insulating. In a conductor,
the electric charges move freely in response to the
application of an electric field, whereas in an insulator
(dielectric), the charges are fixed and not free to
move.
If a conductor is placed in an electric field, charges will
move within the conductor until the interior field is zero.
In the case of an insulator, no free charges exist, so net
migration of charge does not occur. In polar materials,
however, the positive and negative charge centers in the
molecules do not coincide. An electric dipole moment,
p
,
is said to exist. An applied field,
E
0
, tends to orient the
dipoles and produces a field inside the dielectric,
E
p
,
which opposes the applied field. This process is called
polarization. Most materials contain a combination of
orientable dipoles and relatively free charges so that the
electric field is reduced in any material relative to its
free-space value. The net field inside the material,
E
,is
then
Single cell microelectroporation technology
There are different techniques to overcome the cell
membrane barrier and introduce exogenous imperme-
able compounds, such as dyes, DNA, proteins and
amino acids into the cell. Some of the methods include
lipofection, fusion of cationic liposome, electro-
poration, microinjection, optoporation, electroinjection
and biolistics. Electroporation has the advantage of
being a non-contact method for transient permeabili-
zation of cells (Olofsson et al., 2003). In contrast to
microinjection techniques for single cells and single
nuclei (Capecchi, 1980), the electroporation technique
can be applied to biological containers of sub-femto-
liter volumes, that are less than a few micrometers in
diameter. Also, it can be extremely fast and well timed
(Hibino et al., 1991; Kinosita et al., 1988), which is of
importance
E ¼ E
0
E
p
(4.1.13)
The net field is lowered by a significant amount rela-
tive to the applied field if the material is an insulator and
is essentially zero for a good conductor. This reduction is
characterized by a factor 3
r
, which is called the relative
permittivity or dielectric constant, according to
E ¼
E
0
3
r
(4.1.14)
Biological systems are electrically heterogeneous (Gift
and Weaver, 1995). Application of an electric field pulse
results in rapid polarization changes that can deform
mechanically unconstrained cell membranes (e.g.,
suspended vesicles and cells) followed by ionic charge
redistribution governed by electrolyte conductivities and
distributed capacitance (Weaver, 2000; Ivorra and
Rubinsky, 2007). For most cells and tissues the latter
charging times are of order s
CHG
z 10
-6
seconds. Thus, if
U
m
is to exceed 0.5-1 V, much larger pulses must be used
if the pulse is significantly shorter than s
CHG
(Weaver,
2000).
Electroporation is hypothesized to involve in-
homogeneous nucleation of primary, hydrophilic
pores based on transitions from much more numerous
in studying
fast
reaction phenomena
(Ryttsen et al., 2000).
In addition to bulk electroporation methods, in-
strumentation has been developed that can be used for
electroporation of a small number of cells in suspension
(Kinosita and Tsong, 1979; Chang, 1989; Marszalek et al.,
1997), and for a small number of adherent cells grown on
a substratum (Zheng and Chang, 1991; Teruel and
Meyer, 1997). These electroporation devices create
