U.S. patent application number 10/407580 was filed with the patent office on 2003-09-04 for controlled electroporation and mass transfer across cell membranes.
This patent application is currently assigned to The Regents of the University of California. Invention is credited to Huang, Yong, Rubinsky, Boris.
Application Number | 20030166181 10/407580 |
Document ID | / |
Family ID | 23409957 |
Filed Date | 2003-09-04 |
United States Patent
Application |
20030166181 |
Kind Code |
A1 |
Rubinsky, Boris ; et
al. |
September 4, 2003 |
Controlled electroporation and mass transfer across cell
membranes
Abstract
Electroporation is performed in a controlled manner in either
individual or multiple biological cells or biological tissue by
monitoring the electrical impedance, defined herein as the ratio of
current to voltage in the electroporation cell. The impedance
detects the onset of electroporation in the biological cell(s), and
this information is used to control the intensity and duration of
the voltage to assure that electroporation has occurred without
destroying the cell(s). This is applicable to electroporation in
general. In addition, a particular method and apparatus are
disclosed in which electroporation and/or mass transfer across a
cell membrane are accomplished by securing a cell across an opening
in a barrier between two chambers such that the cell closes the
opening. The barrier is either electrically insulating, impermeable
to the solute, or both, depending on whether pore formation,
diffusive transport of the solute across the membrane, or both are
sought. Electroporation is achieved by applying a voltage between
the two chambers, and diffusive transport is achieved either by a
difference in solute concentration between the liquids surrounding
the cell and the cell interior or by a differential in
concentration between the two chambers themselves. Electric current
and diffusive transport are restricted to a flow path that passes
through the opening.
Inventors: |
Rubinsky, Boris; (Albany,
CA) ; Huang, Yong; (Albany, CA) |
Correspondence
Address: |
BOZICEVIC, FIELD & FRANCIS LLP
200 MIDDLEFIELD RD
SUITE 200
MENLO PARK
CA
94025
US
|
Assignee: |
The Regents of the University of
California
|
Family ID: |
23409957 |
Appl. No.: |
10/407580 |
Filed: |
April 3, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10407580 |
Apr 3, 2003 |
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09863117 |
May 22, 2001 |
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6562604 |
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09863117 |
May 22, 2001 |
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09358510 |
Jul 21, 1999 |
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6300108 |
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Current U.S.
Class: |
435/173.1 |
Current CPC
Class: |
C12N 15/87 20130101;
C12M 35/02 20130101; G01N 33/48728 20130101 |
Class at
Publication: |
435/173.1 |
International
Class: |
C12N 013/00 |
Claims
What is claimed is:
1. A method for performing electroporation in a biological cell in
a controlled manner, comprising: (a) placing said biological cell
in an electrically conductive medium and applying a voltage across
said medium; (b) continuously detecting the ratio of electric
current through the medium to voltage across said medium as an
indication of the degree of electroporation of said biological
cell; and (c) adjusting the magnitude of said applied voltage in
accordance with changes in the magnitude of said current-to-voltage
ratio to achieve a controlled degree of electroporation.
2. A method in accordance with claim 1 in which step (b) comprises
continuously detecting said current-to-voltage ratio as an
indication of the onset of electroporation of said biological cell,
and step (c) comprises adjusting the duration of said applied
voltage in accordance with said current-to-voltage ratio to achieve
a controlled degree of electroporation.
3. A method in accordance with claim 1 comprising placing a
plurality of said biological cells in said electrically conductive
medium, said current-to-voltage ratio thereby serving as an
indication of the degree of electroporation averaged over said
plurality of biological cells, thereby achieving a controlled
averaged degree of electroporation in said plurality of biological
cells.
4. A method in accordance with claim 1 comprising placing a single
biological cell in said electrically conductive medium, said
current-to-voltage ratio thereby serving as an indication of the
degree of electroporation of said single biological cell, thereby
achieving a controlled degree of electroporation in said single
biological cell.
5. A method in accordance with claim 1 in which said voltage is
applied between two microelectrodes positioned with said biological
cell in between.
6. A method in accordance with claim 1 in which: said voltage is
applied between two electrodes in a flow-through channel, said
electrodes positioned to apply said voltage in a direction
transverse to flow through said channel; step (a) comprises
suspending said biological cell in said medium and continuously
passing said medium through said channel; step (b) comprises
further correlating said current-to-voltage ratio with the presence
of said biological cell between said electrodes; and step (c)
comprises adjusting the magnitude of said voltage while said
biological cell is between said electrodes.
7. A method in accordance with claim 6 comprising suspending a
plurality of said biological cells in said electrically conductive
medium and continuously passing said medium through said channel
such that approximately one cell at a time passes through said
electrodes.
8. A method for performing electroporation in biological tissue in
a controlled manner, comprising: (a) placing said biological tissue
in an electrically conductive medium and applying a voltage across
said medium; (b) continuously detecting the ratio of electric
current through the medium to voltage across said medium as an
indication of the degree of electroporation in said biological
tissue; and (c) adjusting the magnitude of said applied voltage in
accordance with changes in the magnitude of said current-to-voltage
ratio to achieve a controlled degree of electroporation in said
biological tissue.
9. A method for the infusion of a biological cell with a chemical
substance by electroporation in a manner that permits detection of
the onset of and control of the electroporation, said method
comprising: (a) securing said biological cell in an electrical cell
containing a liquid with said chemical substance dissolved therein,
said electrical cell containing a barrier to electric current, said
barrier arranged such that, when a voltage is imposed across said
electrical cell, said barrier restricts electric current flow to a
flowpath passing through said biological cell while permitting
substantially no electric current to bypass said biological cell;
(b) imposing a voltage across said electrical cell and monitoring
the relative values of current passing through said cell and of
said imposed voltage as an indication of the occurrence of
electroporation in said cell.
10. A method in accordance with claim 9 in which said barrier
divides first and second electrode chambers in said electrical cell
and contains an opening smaller in width than said biological cell,
and (a) comprises securing said biological cell over an opening
such that said cell closes said opening.
11. A method in accordance with claim 10 in which said first
electrode chamber contains a first electrically conducting liquid
and said second electrode chamber contains a second electrically
conducting liquid, and said chemical substance is dissolved in only
one of said first and second electrically conducting liquids.
12. A method in accordance with claim 10 in which said first
electrode chamber contains a first electrically conducting liquid
and said second electrode chamber contains a second electrically
conducting liquid, and said chemical substance is dissolved in both
of said first and second electrically conducting liquids.
13. A method in accordance with claim 10 in which (a) is
accomplished by imposing a pressure differential across said
opening to press said biological cell against one side of said
opening.
14. A method in accordance with claim 10 in which (a) is
accomplished by a coating on an area surrounding said opening, said
coating comprised of a substance that binds to said barrier.
15. A method in accordance with claim 10 in which said first
electrode chamber is constructed and arranged to permit continuous
flow of liquid therethrough, said method further comprising
effecting continuous flow of a first electrically conducting liquid
through said first electrode chamber.
16. A method in accordance with claim 10 in which said first and
second electrode chambers are constructed and arranged to permit
continuous flow of liquid through each such chamber independently,
said method further comprising effecting continuous flow of a first
electrically conducting liquid through said first electrode chamber
and continuous flow of a second electrically conducting liquid
through said second electrode chamber.
17. A method in accordance with claim 9 comprising securing a
plurality of said biological cells in said electrical cell in which
said barrier restricts current flow to flowpaths passing through
all of said plurality of biological cells while permitting
substantially no electric current to bypass said plurality of
biological cells.
18. A method in accordance with claim 9 comprising securing only
one said biological cell in said electrical cell, and in which said
barrier restricts current flow to a flowpath through said only one
biological cell while permitting substantially no electric current
to bypass said only one biological cell.
19. A method in accordance with claim 9 in which said electric cell
is transparent, and said method further comprises observing changes
in said biological cell while said voltage is imposed.
20. A method for the passage of a chemical species across a
membrane of a biological cell in a controlled manner to achieve a
selected rate of mass transfer across said membrane or to
facilitate the determination of mass transfer properties of said
membrane, said method comprising: (a) securing said cell over an
opening in a barrier between first and second chambers, said
opening being smaller in width than said cell such that said cell
closes said opening, and (b) placing a first liquid in said first
chamber and a second liquid in said second chamber, at least one of
said first and second liquids containing a solute at a
concentration sufficiently higher than in said cell to cause said
solute to diffuse into said cell.
21. A method in accordance with claim 20 in which securing said
cell over said opening is accomplished by imposing a pressure
differential across said opening to press said cell against one
side of said opening.
22. A method in accordance with claim 20 in which securing said
cell over said opening is accomplished by a coating on an area
surrounding said opening, said coating comprised of a substance
that binds to said membrane.
23. A method in accordance with claim 20 in which said first and
second chambers are constructed and arranged to permit continuous
flow of liquid through each such chamber independently, and (b) is
accomplished by continuous flow of said first liquid through said
first chamber and continuous flow of said second liquid through
said second chamber.
24. A method in accordance with claim 20 in which the concentration
of said solute in said first liquid is sufficiently higher than in
said second liquid to cause said solute to travel from said first
liquid to said second liquid by diffusive transport, said barrier
being sufficiently impermeable to said solute and said cell being
sufficiently secured in said opening to restrict said diffusive
transport to a diffusion path through said cell.
25. A method in accordance with claim 24 in which said solute is
selected such that its mass transfer coefficient across said
membrane is known, and said method is a method for infusing said
cell with said solute and further comprises selecting the
concentration of said solute in said first liquid and the period of
time during which said diffusive transport is continued, both on
the basis of said known mass transfer coefficient, to infuse said
cell with a preselected amount of said solute.
26. A method in accordance with claim 24 further comprising
monitoring changes in said cell while said solute is diffusing into
said cell, said changes being representative of the degree of
infusion of said cell with said solute as a means of determining
the rate of mass transfer of said solute across said membrane.
27. A method in accordance with, claim 20 in which said barrier and
said chambers are enclosed by a housing that is transparent, said
method further comprising observing changes in said biological cell
while said solute diffuses into said biological cell.
28. Apparatus for the infusion of a biological cell with a chemical
substance by electroporation, said apparatus comprising: an
electric cell containing an internal support to hold a biological
cell and an internal barrier of a material substantially
impermeable to electric current, said barrier positioned to
restrict electric current flow in said electric cell to a flowpath
crossing said internal support and through any biological cell held
thereby; and means for imposing a voltage across said electric cell
and for monitoring the relative values of current and voltage as an
indication of the occurrence and degree of electroporation in any
biological cell held thereby.
29. Apparatus in accordance with claim 28 in which said barrier
divides the interior of said electric cell into first and second
electrode chambers and said internal support is an opening in said
barrier smaller in width than a biological cell.
30. Apparatus in accordance with claim 29 further comprising means
for imposing a pressure differential across said opening to lodge a
biological cell in said opening.
31. Apparatus in accordance with claim 29 in which said first
electrode chamber is a flow-through channel.
Description
[0001] This invention resides in the fields of electroporation and
mass transfer across cell membranes.
BACKGROUND OF THE INVENTION
[0002] Electroporation is a technique that is used for introducing
chemical species into biological cells, and is performed by
exposing the cells to an electric potential that traverses the cell
membrane. While its mechanism is not fully understood,
electroporation is believed to involve the breakdown of the cell
membrane lipid bilayer leading to the formation of transient or
permanent pores in the membrane that permit the chemical species to
enter the cell by diffusion. The electric potential is typically
applied in pulses, and whether the pore formation is reversible or
irreversible depends on such parameters as the amplitude, length,
shape and repetition rate of the pulses, in addition to the type
and development stage of the cell. As a method of introducing
chemical species into cells, electroporation offers numerous
advantages: it is simple to use; it can be used to treat whole
populations of cells simultaneously; it can be used to introduce
essentially any macromolecule into a cell; it can be used with a
wide variety of primary or established cell lines and is
particularly effective with certain cell lines; and it can be used
on both prokaryotic and eukaryotic cells without major
modifications or adaptations to cell type and origin.
Electroporation is currently used on cells in suspension or in
culture, as well as cells in tissues and organs.
[0003] Electroporation is currently performed by placing one or
more cells, in suspension or in tissue, between two electrodes
connected to a generator that emits pulses of a high-voltage
electric field. The pore formation, or permealization, of the
membrane occurs at the cell poles, which are the sites on the cell
membranes that directly face the electrodes and thus the sites at
which the transmembrane potential is highest. Unfortunately, the
degree of permealization occurring in electroporation varies with
the cell type and also varies among cells in a given population.
Furthermore, since the procedure is performed in large populations
of cells whose properties vary among the individual cells in the
population, the electroporation conditions can only be selected to
address the average qualities of the cell population; the procedure
as currently practiced cannot be adapted to the specific
characteristics of individual cells. Of particular concern is that
under certain conditions, electroporation can induce irreversible
pore formation and cell death. A high electric field, for example,
may thus produce an increase in transfection efficiency in one
portion of a cell population while causing cell death in another. A
further problem with known methods of electroporation is that the
efficiency of transfection by electroporation can at times be low.
In the case of DNA, for example, a large amount of DNA is needed in
the surrounding medium to achieve effective transformation of the
cell.
[0004] Many of the problems identified above are a consequence of
the fact that the process of electroporation in both individual
cells and tissues cannot be controlled in real time. There are no
means at present to ascertain in real time when a cell enters a
state of electroporation. As a result, the outcome of an
electroporation protocol can only be determined through the
eventual consequences of the mass transfer process and its effect
on the cell. These occur long after the mass transfer under
electroporation has taken place. These and other deficiencies of
current methods of electroporation are addressed by the present
invention.
[0005] Also relevant to the present invention are current
techniques for the study and control of mass transfer across cell
membranes. Knowledge of mass transfer across cell membranes in
nature, both in cells that are functioning normally and in diseased
cells, is valuable in the study of certain diseases. In addition,
the ability to modify and control mass transfer across cell
membranes is a useful tool in conducting research and therapy in
modem biotechnology and medicine. The introduction or removal of
chemical species such as DNA or proteins from the cell to control
the function, physiology, or behavior of the cell provides valuable
information regarding both normal and abnormal physiological
processes of the cell.
[0006] The most common method of effecting and studying mass
transfer across a cell membrane is to place the cell in contact
with a solution that contains the compound that is to be
transported across the membrane, either with or without
electroporation. This bulk transfer method does not permit precise
control or measurement of the mass transfer across the membrane.
The composition of the solution at specific sites is not known and
is variable. In addition, when an electric field is present, the
local field intensity will vary from one point to another.
Furthermore, the surface of the cell that is exposed to the
solution is not well defined. Cell surface areas vary among cells
in a given population, and this leads to significant differences
among the cells in the amount of mass transfer. For these reasons,
the amount of mass transfer achieved by bulk transfer processes is
not uniform among cells, and the actual amount transferred for any
particular cell cannot be determined.
[0007] Attempts made so far to overcome the limitations of bulk
transfer techniques include techniques for treating individual
cells that include either the mechanical injection (microinjection)
of chemical compounds through the cell membrane or electroporation
with microelectrodes. In injection techniques, the membrane is
penetrated with a needle to deliver a chemical agent, localizing
the application of the chemical agent to a small region close to
the point of injection. This requires manipulation of the cell with
the human hand, a technique that is difficult to perform,
labor-intensive, and not readily reproducible. Electroporation with
microelectrodes suffers these problems as well as the lack of any
means to detect the onset of electroporation in an individual cell.
These problems are likewise addressed by the present invention.
SUMMARY OF THE INVENTION
[0008] The present invention arises in part from the discovery that
the onset and extent of electroporation in a biological cell can be
correlated to changes in the electrical impedance (which term is
used herein to mean the ratio of current to voltage) of the
biological cell or of a conductive medium that includes the
biological cell. An increase in the current-to-voltage ratio across
a biological cell occurs when the cell membrane becomes permeable
due to pore formation. Likewise, a decrease in the
current-to-voltage ratio through a flowing conductive fluid occurs
when the fluid draws a biological cell into the region between the
electrodes in a flow-through electric cell. Thus, by monitoring the
impedance of the biological cell or of an electrolyte solution in
which the cell is suspended, one can detect the point in time in
which pore formation in the cell membrane occurs, as well as the
relative degree of cell membrane permeability due to the pore
formation. This information can then be used to establish that a
given cell has in fact undergone electroporation, or to control the
electroporation process by governing the selection of the voltage
magnitude. This discovery is also useful in the simultaneous
electroporation of multitudes of cells, since it provides a direct
indication of the actual occurrence of electroporation and an
indication of the degree of electroporation averaged over the
multitude. The discovery is likewise useful in the electroporation
of biological tissue (masses of biological cells with contiguous
membranes) for the same reasons.
[0009] The benefits of this process include a high level of control
over the onset and degree of electroporation, together with a more
detailed knowledge of the occurrence and degree of permeability
created in particular individual cells or cell masses. When applied
to individual cells or to a succession of individual cells, this
process assures that the individual cells are indeed rendered
permeable and are indeed transformed by the introduction of
chemical species. The process also offers the ability to increase
the efficiency of electroporation by avoiding variations in the
electrical environment that would destroy some cells while having
an insufficient effect on others.
[0010] In some of its more specific embodiments, the present
invention involves the use of an electrical cell in which a
biological cell can be placed and that contains a barrier that
directs the electric current flow and hence the ion flow through a
flow path that passes through the biological cell while permitting
substantially no electric current to bypass the biological cell. In
some of these embodiments, the invention involves the use of an
apparatus containing two liquid-retaining chambers separated by a
barrier that is substantially impermeable to an electric current.
The barrier contains an opening that is smaller than the biological
cell such that the biological cell once lodged in the opening will
plug or close the opening. To achieve electroporation, the
biological cell is secured over the opening by mechanical or
chemical means, preferably in a reversible manner so that the
biological cell can later be removed without damage to the
biological cell. Once the biological cell is secured over the
opening, a voltage is imposed between the two chambers and across
the biological cell residing in the opening. The passage of current
between the chambers is thus restricted to a path passing through
the opening and hence through the biological cell. By monitoring
the current-voltage relation in the electric cell, the onset of
electroporation is detected and the degree of pore formation is
controlled, to both assure that electroporation is occurring and to
prevent excessive pore formation and cell death. The user is thus
afforded a highly precise knowledge and control of the condition of
and the flux across the biological cell membrane.
[0011] In another series of embodiments, this invention is useful
in the diffusive transport of chemical species into or out of a
biological cell. In these embodiments, the cell is again divided
into two chambers separated by a barrier, and the biological cell
is lodged across an opening in the barrier in such a manner that
the passage of liquid around the cell from one chamber to the other
is substantially prevented. A liquid solution or the species to be
introduced into the biological cell is placed in one or both of the
chambers. The concentration of the species in the solution differs
from that in the cell (either higher or lower, depending on whether
one seeks to introduce or remove the species from the cell), or the
concentration in one chamber differs from that in the other
chamber.
[0012] In preferred methods of applying this invention to diffusive
transport, the solutions in the two chambers differ in
concentration such that the driving force for the diffusive
transport is between the two chambers themselves rather than
between the chambers and the interior of the biological cell.
Knowledge and controlled monitoring of the concentrations in each
of the two chambers on a periodic or continuous basis as the
diffusion proceeds, together with the precise knowledge of the
dimensions of the opening, enables the user to precisely observe
and control the rate and amount of the species that enters the
cell. The diffusion time can be controlled by imposing stepwise
changes in the concentrations in either or both of the chambers,
thereby imposing or removing the concentration differential. An
application of particular interest is the combination of this type
of diffusive transport of a chemical species with controlled
electroporation as described in the preceding paragraph.
[0013] Each of the various embodiments of this invention may be
used with two or more biological cells simultaneously, or cell
masses such as in tissue, rather than just one cell. The apparatus
described above can be adapted for use with two or more biological
cells by arranging the barrier such that the current or diffusive
transport will be restricted to a flow path that passes through all
of the cells while preventing bypass around the cells. A further
application of the concepts of this invention is the
electroporation of biological cells suspended in a flowing liquid.
Electrodes are placed in fixed positions in the flow channel, and a
voltage is imposed between the electrodes while current passing
between the electrodes is monitored. Biological cells entering the
region between the electrodes will lower the current, the impedance
serving as an indication of the presence of one or more cells in
the region, and optionally also as a signal to initiate the
application of a higher voltage sufficient to achieve
electroporation.
[0014] Among the advantages that this invention offers relative to
the prior art are the ability to treat cells individually and to
adapt the treatment conditions to the needs of individual cells. In
embodiments where voltage is applied, the monitoring of the
impedance affords the user knowledge of the presence or absence of
pores and shows the progress of the pore formation and whether
irreversible pore formation that might lead to cell death has
occurred. An advantage of the barrier-and-opening apparatus is its
highly efficient use of electrical energy by virtue of its
restriction of the current to a current flow path passing through
the opening. A still further advantage is the ability of the
apparatus and method to be integrated into an automated system
whereby the condition of each cell is monitored by instrumentation
and individual cells are lodged in the opening and then removed at
times governed by the monitored conditions.
[0015] These and further features, advantages and objects of the
invention will be better understood from the description that
follows.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] FIG. 1 is a cross section of a microdiffusion device useful
in the practice of the present invention for infusing a biological
cell with a chemical species without the assistance of an
electrical current to effect electroporation.
[0017] FIG. 2 is a cross section of a microelectroporation device
useful in the practice of the present invention for achieving pore
formation in a biological cell, and. optionally for infusing the
cell with a chemical species with the assistance of
electroporation.
[0018] FIG. 3a is a longitudinal cross section of an
electroporation device in accordance with this invention, designed
for a mobile suspension of biological cells. FIG. 3b is a
transverse cross section of the device shown in FIG. 3a.
[0019] FIG. 4 is a plot of current vs. voltage in a series of
electroporation experiments conducted using a microelectroporation
device of the structure similar to that of FIG. 2.
[0020] FIGS. 5a, 5b, 5c, and 5d are plots of current vs. voltage in
a further series of electroporation experiments conducted using a
microelectroporation device similar to that of FIG. 2.
DESCRIPTION OF THE INVENTION AND SPECIFIC EMBODIMENTS
[0021] While this invention extends to a variety of structures,
methods, and applications, this portion of the specification will
illustrate certain specific structures and methods in detail, from
which the concepts of the invention as a whole will become
apparent.
[0022] The first structure that will be discussed is an
electroporation cell with an internal support to hold a single
biological cell and an internal barrier that restricts the electric
current flow in the electric cell to a flow path that passes
through the biological cell. When no voltage is applied, the
structure can be used for diffusive transport alone, unassisted by
voltage-induced pore formation.
[0023] The configuration of the barrier, and the two chambers in
embodiments that include two chambers, is not critical to the
invention, and can vary widely while still serving the purposes and
advantages of the invention. Since biological cells are microscopic
in size, however, the preferred type of apparatus for the practice
of this invention in each of its various forms is one in which the
structure as a whole and/or its chambers are the size of electronic
chips, fabricated by microfabrication techniques such as those used
in electronic chip manufacture. It is further preferred that the
chambers are constructed as flow-through chambers to allow the
passage of the liquids in continuous flow, intermittent flow, or
flow at the direction of the user, and to allow changes in the
concentrations, pressure, and other conditions as needed to achieve
close control over the passage of species across the biological
cell membrane. Accordingly, a preferred structure and method of
manufacture of the apparatus are those that involve the formation
of the apparatus in layers or platelets with appropriate openings
that form flow passages when the layers or platelets are bonded
together.
[0024] Flow-through chambers offer the advantage of permitting the
successive entry and removal of individual cells so that large
numbers of cells can be treated in succession. Flow-through
chambers also permit replenishment of solute-depleted solutions so
that concentration gradients can be continuously maintained when
desired. A further function that can be served by flow-through
chambers is the increase and decrease of pressure, a function that
is useful for various purposes as described below.
[0025] The support for the biological cell in this structure can be
any structure that secures the biological cell in a fixed position
and that allows the passage of electric current. The most
convenient support is an opening in the barrier. Securement of a
biological cell over the opening serves to close, seal or plug the
opening, thereby directing the passage of electric current,
diffusive transport, or both, through the cell and eliminating or
minimizing leakage around the cell. A convenient mechanical means
of achieving this is to impose a pressure differential across the
opening in a direction that will press the cell against the
opening. The diameter of the opening will be smaller than that of
the cell, and the cell upon entering the apparatus will pass into
one of the two chambers. By increasing the pressure in the chamber
in which the cell resides, or lowering the pressure in the other
chamber, the cell will be forced against the opening, closing it
off. Once the procedure is completed, the cell is readily released
from the opening by equalizing the pressures in the two chambers or
by reversing the differential such that the higher pressure is in
the chamber other than the chamber in which the cell was
introduced. The flow of liquid in the chamber in which the cell was
introduced will then remove the cell from the opening, exposing the
opening for another cell.
[0026] An alternative method of sealing the opening with the cell
is by the use of a coating on the barrier surface, or over the rim
of the opening, of a substance that binds to the cell membrane.
Since biological cell membranes are negatively charged, the coating
may be a substance that bears a positive charge, such as
polylysine, polyarginine, or polyhistidine. The biological cell can
be directed to the opening by a pressure differential across the
opening, and held in place by the coating. One the procedure is
completed, the cell can be released from the coating by momentarily
increasing the flow rate of the liquid in the chamber on the cell
side of the opening, or by imposing a reverse pressure differential
across the opening to urge the cell away from the opening.
[0027] The size of the opening is not critical to the invention
provided that the opening exposes sufficient surface area on the
cell membrane to achieve the desired degree of either mass
transfer, the passage of an electric current, or both, within a
controllable and economically reasonable period of time. The
optimal size will thus vary with the particular cells being treated
or studied. In general, the opening is preferably circular or
approximately circular in shape, and depending on the cell size,
preferably ranges in diameter from about 1 micron to about 100
microns, more preferably from about 1 micron to about 50 microns,
and most preferably from about 2 microns to about 20 microns. The
barrier in which the hole is formed and which separates the two
chambers is preferably of a rigid dielectric material that is
impermeable to both water and solutes and that will hold a pressure
differential sufficient to secure a cell against the opening. For
devices that are manufactured by microfabrication techniques, a
convenient material for the barrier is silicon nitride. Other
materials that will serve equally well will be readily apparent to
those skilled in the art.
[0028] A further feature of preferred embodiments of this invention
is the use of apparatus made of transparent materials. This enables
the user to observe cell interiors and the processes of
microdiffusion and microelectroporation through a microscope as
they occur.
[0029] An example of a microdiffusion apparatus in accordance with
this invention for a single biological cell, for transporting
materials across the cell membrane without the application of an
electric field, is shown in FIG. 1. This components of this
apparatus, from the bottom up, are an acrylic base 11, an
intermediate silicon layer 12 (1 micron in thickness) with a
portion 13 carved out to define the lateral boundaries of the lower
of the two liquid chambers, a silicon nitride layer 14 serving as
the barrier between the two chambers, a silicon washer 15 defining
the lateral boundaries of the upper liquid chamber 16, and a glass
cover plate 17. A hole 18 in the silicon nitride barrier serves as
the opening, and a cell or contiguous cell mass such as tissue 19
is shown covering the hole. Channels extend through the acrylic
base to serve as inlet and outlet channels for the liquids that
pass through the upper and lower chambers, as shown by the arrows
in the Figure.
[0030] When the pressure in the upper chamber 16 is higher than
that in the lower chamber 13, the cell will be retained in position
over the hole, serving as a plug separating the liquids in the two
chambers from each other. When the composition of the solutions in
the two chambers differs from that of the cell interior, mass
transfer occurs across the cell membrane between the chambers and
the cell. When the composition of the solution in one chamber
differs from that in the other, mass transfer occurs through the
cell from one chamber to the other. By precisely controlling the
compositions of the solutions in the two chambers, one can
precisely control the mass transfer rate and direction within the
cell. Since the diameter of the opening 18 is known, one can
precisely determine the mass transfer that occurs through the
opening.
[0031] The numerous applications of this microdiffusion device will
be readily apparent. For example, the device can be used to infuse
a cell with a cryopreservative such as glycerol by filling the
upper chamber 16 with physiological saline and the lower chamber 13
with glycerol. When using a cell 19 for which the mass transfer
coefficient of glycerol across the cell membrane is known, one can
readily calculate the amount of glycerol that will enter the cell
and adjust the concentrations and exposure times to infuse the cell
with the amount that is known to be required for
cryopreservation.
[0032] An example of a microelectroporation apparatus in accordance
with this invention for a single biological cell, is shown in FIG.
2. The apparatus is similar in construction to the microdiffusion
apparatus of FIG. 1. Its structural components, from the bottom up,
are an acrylic base 21, a lower silicon layer 22 with a portion
carved out to define the lateral boundaries of the lower liquid
chamber 23, a silicon nitride layer 24 (1 micron in thickness)
serving as the barrier between the two chambers, an upper silicon
layer 25 defining the lateral boundaries of the upper liquid
chamber 26, and a cover consisting of an n+poly-silicon layer
(5,000 .ANG. in thickness) 27 and a silicon nitride layer (1 micron
in thickness) 28. A hole 29 in the silicon nitride barrier 24
serves as the opening, and a cell 30 (or cell mass) covers the
hole. Channels extend through the acrylic base to serve as inlets
and outlets for the liquids that pass through the upper and lower
chambers, as shown by the arrows in the Figure. A further layer of
n+poly-silicon (5,000 .ANG.) 31 resides above the acrylic base 21,
and this layer, together with n+poly-silicon layer 27 above the
upper chamber 26 serve as the two electrodes. Each electrode is
joined by electric leads to a printed circuit board 32 which
controls the voltage applied between the electrodes and measures
the current passing between them.
[0033] The microelectroporation apparatus shown in FIG. 2 can be
fabricated by conventional microfabrication techniques, typically
involving chemical vapor deposition, masking, etching and
sputtering. The operation of the apparatus will be analogous to the
operation of the microdiffusion apparatus of FIG. 1. The movement
of biological cells through the apparatus is achieved by suspending
the cells in the liquid used to fill the upper chamber, and cells
are drawn to the opening, one at a time, by imposing a pressure
differential between the chambers, which also holds a cell in place
once the cell has been drawn to the opening. A convenient method of
imposing such a pressure differential is to maintain atmospheric
pressure in the upper chamber while lowering the pressure in the
lower chamber below atmospheric by attaching a syringe to the lower
chamber and pulling on the syringe plunger. Care should be taken to
limit the pressure differential to one that will not damage the
cell.
[0034] FIGS. 3a and 3b illustrate to a different apparatus and
method within the scope of this invention. This apparatus and
method involve a fluid suspension of biological cells flowing
through a conduit or flow channel, in which the cells pass through
a region between a pair of electrodes. The longitudinal cross
section of FIG. 3a shows the walls 41 of the channel, and a
biological cell 42 passing downward through the lumen of the
channel (in the direction of the arrow). The transverse cross
section of FIG. 3b shows that the channel is rectangular in cross
section, although other cross-sectional geometries may be used.
Electrodes 43, 44 are formed as coatings on two opposing walls of
the channel. The electrodes are connected through leads to a
printed circuit board 45 which measures the impedance and controls
the voltage applied to the electrodes. The biological cell 42 is
shown passing through the region between the two electrodes.
[0035] The area of the cross section of the channel is large enough
to permit the cell to pass through essentially unimpeded by the
channel walls, and yet small enough that only one cell can pass
through the inter-electrode region at a time. In addition, each
electrode 43, 44 is either approximately equal in length or
slightly larger in length than the diameter of the biological cell,
so that the cell upon entering the region causes a significant or
measurable decrease in the current passing through the region due
to the voltage applied across electrodes. The spacing of the
electrodes, i.e., the distance between them, is likewise subject to
the same considerations. The biological cells are suspended in a
liquid solution of the species to be introduced into the cells, and
the suspension is passed through the channel. A voltage is applied
between the electrodes as suspension flows through the channel, and
the current between the electrodes (or the impedance) is monitored.
A significant drop in the current indicates the presence of a
biological cell in the inter-electrode region. Once the cell is
detected in this manner, an electroporation pulse can be applied to
the electrodes while the cell is still in the inter-electrode
region, and impedance can be observed further to detect the onset
of electroporation. The species dissolved in the liquid solution
will enter the cell as a result of the electroporation.
[0036] Variations on these structures and methods will be readily
apparent to those skilled in the art. For example, the barriers
described above can be minimized or avoided by using
microelectrodes that are the same size as or smaller than the
biological cells. Examples of such microelectrodes are carbon fiber
microelectrodes (such as ProCFE, Axon Instruments, Foster City,
Calif., USA) used in conjunction with high-graduation
micromanipulators (such as those available from Narishige MWH-3,
Tokyo, Japan). Microelectrodes can be used in place of the
electrodes shown in FIG. 2 or in place of those shown in FIGS. 3a
and 3b.
[0037] The following examples are offered for illustration, and are
not intended to impose limits on the scope of the invention.
EXAMPLE 1
[0038] A series of experiments was performed using a
microelectroporation system consisting of the microelectroporation
device described above and shown in FIG. 2, combined with flow and
pressure control units and pressure gauges for the liquids to be
circulated through the upper and lower chambers, a variable DC
power supply, a pulse generator and power amplifier for imposing
voltage pulses across the device, a digital oscilloscope for
monitoring the pulses, a fluorescent microscope, a CCD (charge
coupled device) camera, and a computer with image processing and
waveform processing software. Both chambers of the device were
filled with physiological saline and cells were introduced into the
upper chamber. Liquid motion in the top and bottom chambers was
controlled by syringes. The pressure in the upper chamber was
atmospheric while the pressure in the lower chamber was reduced
below atmospheric by pulling on the barrel of the syringe connected
to that chamber. The voltage was applied in single square pulses
ranging from zero to 120V in magnitude and from 2 microseconds to
100 milliseconds in duration. The distance between the electrodes
in the upper and lower chambers was 900 microns.
[0039] The tests in this example were performed using ND-1 human
prostate adenocarcinoma cells with a typical diameter of 20
microns. The opening in the microelectroporation device was 5
microns in diameter. A rectangular voltage pulse was applied with a
duration of 60 milliseconds, and the pulse was applied at various
amplitudes ranging from 10V to 60V in increments of 5 volts. With
each pulse, the electric current passing through the opening was
measured. Experiments were performed with the cells and were
repeated both with the opening stopped by a glass bead and with no
obstruction at all in the opening. The results in each case were
expressed as microamperes of current vs. volts of pulse amplitude
and are plotted in FIG. 4, in which the upper curve (data points
represented by x's) represents the unobstructed opening, the lower
curve (data points represented by asterisks) represents the data
taken with the glass bead residing in the opening, and the three
middle curves (open squares, open upright triangles, and open
inverted triangles) represent data taken with three different ND-1
cells residing in the opening.
[0040] The upper curve shows that the current increases in a
substantially steady manner as the voltage increases when there is
no barrier to the passage of current through the opening. The lower
curve also shows a substantially steady rise as the voltage
increases, although at a much lower level. The current values shown
in the lower curve represent stray currents through the device. The
curves of data taken with the ND-1 cells across the opening show
that at low voltages the current is close in value to that obtained
when the opening is closed by the glass bead while at high voltages
the current rises to the levels obtained with an unobstructed
opening. The transition is a sharp increase which is indicative of
the formation of pores in the cell membrane through which an
electric current can pass, i.e., the onset of electroporation. In
all three cells, the transition occurred at voltages between 30V
and 40V. In two of the three cells (open squares and open upright
triangles), the onset of electroporation occurred essentially at
the same voltage, while in the third (inverted triangles), the
onset occurred at a voltage that was lower-than the other two by
about 5V. This illustrates the value of controlling the process for
individual cells to achieve optimal results.
[0041] After the data shown in FIG. 4 was generated, the pulses
were reapplied in descending order of amplitude values, and the
resulting curves displayed hysteresis, i.e., the curves obtained
with descending amplitudes were higher in voltage than those
obtained with ascending amplitudes. This indicated that the
electroporation in these experiments was irreversible.
EXAMPLE 2
[0042] Using the same microelectroporation system used in Example
1, a series of tests were performed on rat hepatocytes (ATCC
#CRL-1439), whose typical cell diameter was 20 microns, the
microelectroporation apparatus having an opening that was 4 microns
in diameter. Here as well, rectangular voltage pulses that were 60
milliseconds in duration were used, ranging in amplitude from 10V
to 37.5V in increments of 5V in the portion from 10V to 30V and in
increments of 2.5V in the portion from 30V to 37.5V. The
experiments were performed in some cases only by increasing the
amplitudes and in others by first increasing, then decreasing the
amplitudes to evaluate reversibility. The results are plotted in
the graphs shown in FIGS. 5a, 5b, 5c, and 5d. In each case, the
upper curve (data points represented by circles) is the data taken
with neither a cell nor a glass bead residing in the opening, the
lower curve (data points represented by squares) is the data taken
with a glass bead in the opening, and the middle curve (data points
represented by triangles) is the data taken with a hepatocyte in
the opening, using different hepatocytes for each of the four
Figures.
[0043] In FIG. 5a, the amplitude was increased and not decreased,
displaying an electroporation threshold voltage of between 25V and
30V. In FIGS. 5b and 5c, the amplitude was first increased and then
decreased to produce the two middle curves. Although the ascending
and descending curves are not differentiated, they are
substantially identical in each Figure, indicating that the cell
membrane in each of these two cases resealed after each voltage
pulse and thus that the pore formation was reversible. In the test
represented by FIG. 5d, the cell disintegrated once the applied
voltage exceeded 37.5V, although this is not shown in the Figure.
It is significant to note that despite the fact that the same cell
types were used in each of FIGS. 5a, 5b, 5c, and 5d, the
electroporation threshold voltage differed among the individual
cells, although all were within the range of 20V to 35V. Adaptation
of the procedure to individual cells is readily achieved by
monitoring the current in this manner to note when the
electroporation threshold occurs. Selection of the optimal exposure
time, voltage, composition changes in the surrounding liquids, and
other parameters of the system can then be made to achieve the
desired treatment of the cell without destruction of the cell.
[0044] The methods described herein are useful tools in the
laboratory for conducting fundamental research in the
electroporation properties of biological cells, and useful tools in
industry for processing large quantities of cells in a flow-through
manner. By enabling one to observe and record the current flowing
through individual cells, one can control the amplitude and
duration of the voltage pulse to achieve optimal results. In
addition, the devices described and shown herein for use in
practicing the invention can be constructed with transparent parts
and of a size suitable for mounting on a microscope stage. This
will permit one to correlate the electrical current measurements to
visual observations and fluorescence measurements inside the cell.
The device can be used to electrically detect, through the
measurement of currents, the point in time when a cell becomes
lodged in the opening as well as the point in time when pore
formation is achieved in the cell membrane. For larger scale and
industrial applications, large numbers of microelectroporation
devices of the type described herein can be arranged in parallel.
For each cell, electrical information indicating the trapping of a
cell in the opening (such as a sharp drop in the current) can be
used to generate a signal that will initiate an electroporation
sequence, and further electrical information indicating the
completion of electroporation (such as a sharp rise in current)
will generate a signal that will release the cell (for example by
eliminating or reversing the pressure differential) and permit the
next cell to flow toward the opening.
[0045] Further implementations, applications, adaptations, and
embodiments of the concepts, features and methods described herein
that are within the scope of this invention will be readily
apparent to those skilled in the art.
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