U.S. patent application number 10/369120 was filed with the patent office on 2004-11-25 for method and device for sampling and analyzing interstitial fluid and whole blood samples.
Invention is credited to Chambers, Garry, Chatelier, Ron, Hodges, Alastair.
Application Number | 20040236250 10/369120 |
Document ID | / |
Family ID | 24137712 |
Filed Date | 2004-11-25 |
United States Patent
Application |
20040236250 |
Kind Code |
A1 |
Hodges, Alastair ; et
al. |
November 25, 2004 |
Method and device for sampling and analyzing interstitial fluid and
whole blood samples
Abstract
The invention disclosed in this application is a method and
device for combining the sampling and analyzing of sub-dermal fluid
samples, e.g., interstitial fluid or whole blood, in a device
suitable for hospital bedside and home use. It is applicable to any
analyte that exists in a usefully representative concentration in
the fluid, and is especially suited to the monitoring of
glucose.
Inventors: |
Hodges, Alastair; (San
Diego, CA) ; Chatelier, Ron; (San Diego, CA) ;
Chambers, Garry; (San Diego, CA) |
Correspondence
Address: |
Johnson and Johnson
International Patent Law Division
Attention Bernard E Shay
P O Box 1222
New Brunswick
NJ
08903
US
|
Family ID: |
24137712 |
Appl. No.: |
10/369120 |
Filed: |
February 13, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10369120 |
Feb 13, 2003 |
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10166487 |
Jun 10, 2002 |
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10166487 |
Jun 10, 2002 |
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09536235 |
Mar 27, 2000 |
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6612111 |
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Current U.S.
Class: |
600/583 ;
600/345; 600/584; 73/864.01 |
Current CPC
Class: |
A61B 5/1519 20130101;
A61B 2010/008 20130101; A61B 5/150503 20130101; A61B 5/150755
20130101; A61B 5/14514 20130101; A61B 5/14546 20130101; A61B
5/14865 20130101; A61B 5/150221 20130101; A61B 5/15105 20130101;
A61B 5/15117 20130101; A61B 5/15113 20130101; A61B 5/15142
20130101; A61B 5/150389 20130101; A61B 5/150358 20130101; A61B
5/14532 20130101; A61B 2562/0295 20130101; A61B 5/150213 20130101;
A61B 5/150022 20130101 |
Class at
Publication: |
600/583 ;
600/584; 073/864.01; 600/345 |
International
Class: |
A61B 005/00 |
Claims
What is claimed is:
1. A fluid sampling device, the device comprising a body, the body
comprising: a dermal layer penetration probe having a penetrating
end and a communicating end, the penetration probe having a volume;
and an analysis chamber having a proximal end and a distal end, the
analysis chamber having a volume, wherein the volume of the
penetration probe is greater than the volume of the analysis
chamber, and wherein the penetration probe is in fluid
communication with the analysis chamber such that fluid can flow
from the penetration probe to the analysis chamber.
2. The device of claim 1, wherein the penetration probe is capable
of exerting a first capillary force and the analysis chamber is
capable of exerting a second capillary force, and wherein a
differential in capillary force exists between the first capillary
force and the second capillary force.
3. The device of claim 2, wherein the second capillary force is
greater than the first capillary force.
4. The device of claim 3, wherein an interior surface of the
penetration probe comprises a first penetration probe wall and a
second penetration probe wall, wherein the first penetration probe
wall and the second penetration probe wall are spaced apart at a
first distance to define a penetration probe height, and wherein an
interior surface of the analysis chamber comprises a first analysis
chamber wall and a second analysis chamber wall, wherein the first
analysis chamber wall and the second analysis chamber wall are
spaced apart at a second distance to define an analysis chamber
height, wherein the analysis chamber height is less than the
penetration probe height, and wherein the differential in capillary
force derives at least in part from a difference between the
penetration probe height and the analysis chamber height.
5. The device of claim 3, wherein at least one of the penetration
probe and the analysis chamber comprises a substance capable of
enhancing or diminishing a capillary force.
6. The device of claim 5, wherein the substance is selected from
the group consisting of a polymer, a resin, a powder, a mesh, a
fibrous material, a crystalline material, a porous material, and a
combination thereof.
7. The device of claim 6, wherein the substance is selected from
the group consisting of polyethylene glycol, polyvinylpyrrolidone,
a surfactant, a hydrophilic block copolymer, and
polyvinylacetate.
8. The device of claim 1, wherein the penetration probe comprises a
first penetration probe wall and a second penetration probe wall
and wherein the analysis chamber comprises a first analysis chamber
wall and a second analysis chamber wall, and wherein the distance
between the first penetration probe wall and the second penetration
probe wall is greater than the distance between the first analysis
chamber wall and the second analysis chamber wall.
9. The device of claim 1, wherein the penetration probe comprises a
component selected from the group consisting of a needle, a lancet,
a tube, a channel, and a solid protrusion.
10. The device of claim 1, wherein the device has a proximal edge,
the proximal edge comprising a recess, wherein the penetration
probe is positioned within the recess.
11. The device of claim 10, wherein the recess is configured to
substantially align with a shape of a selected dermal surface.
12. The device of claim 1, further comprising a releasable
actuator, wherein the actuator is capable of supplying a force
sufficient to cause the penetration probe to penetrate a dermal
layer.
13. The device of claim 12, wherein the actuator is external to the
body, and wherein upon release the actuator propels the body to the
dermal layer.
14. The device of claim 12, wherein the actuator is integral with
the body.
15. The device of claim 14, wherein upon release the actuator
propels the penetration probe toward the dermal layer.
16. The device of claim 1, wherein the analysis chamber comprises
an electrochemical cell, the cell comprising a working electrode
and a counter/reference electrode.
17. The device of claim 1, further comprising an interface for
communication with a meter.
18. The device of claim 17, wherein the interface communicates a
voltage or a current.
19. The device of claim 1, wherein the analysis chamber comprises a
hollow electrochemical cell, the hollow electrochemical cell
comprising a working electrode, a counter or reference electrode,
and an opening for admitting an analyte to the cell, the working
electrode being spaced from the counter or reference electrode by a
distance of less than 500 .mu.m.
20. The device of claim 19, wherein the penetration probe comprises
a component selected from the group consisting of a needle, a
lancet, a tube, a channel, and a solid protrusion.
21. The device of claim 19, wherein the penetration probe is
capable of exerting a first capillary force and the analysis
chamber is capable of exerting a second capillary force and wherein
a differential exists between the first capillary force and the
second capillary force.
22. The device of claim 21, wherein the second capillary force is
greater than the first capillary force.
23. The device of claim 1, wherein a distal end of the penetration
probe is interfaced with the proximal end of the analysis
chamber.
24. The device of claim 1, wherein a distal end of the penetration
probe is integrated with the proximal end of the analysis
chamber.
25. A fluid sampling device comprising a body, the body comprising
a dermal layer penetration probe having a penetrating end and a
communicating end; an analysis chamber having a proximal end and a
distal end, the analysis chamber having a volume, wherein the
analysis chamber comprises a hollow electrochemical cell, the
hollow electrochemical cell comprising a working electrode, a
counter or reference electrode, and an opening for admitting an
analyte to the cell, the working electrode being spaced from the
counter or reference electrode by a distance of less than 500
.mu.m; and a pre-chamber having a proximal end and a distal end,
the pre-chamber having a volume, wherein the pre-chamber is
interposed between the penetration probe and the analysis chamber
such that the proximal end of the pre-chamber is adjacent the
communicating end of the penetration probe and the distal end of
the pre-chamber is adjacent the proximal end of the analysis
chamber, wherein the volume of the pre-chamber is greater than the
volume of the analysis chamber, and wherein the penetration probe
is in fluid communication with the analysis chamber such that fluid
can flow from the penetration probe to the analysis chamber.
26. The device of claim 25, wherein the penetration probe comprises
a component selected from the group consisting of a needle, a
lancet, a tube, a channel, and a solid protrusion.
27. The device of claim 25, wherein the pre-chamber is capable of
exerting a first capillary force and the analysis chamber is
capable of exerting a second capillary force and wherein a
differential in capillary force exists between the first capillary
force and the second capillary force.
28. The device of claim 27, wherein the second capillary force is
greater than the first capillary force.
29. The device of claim 25, wherein the distal end of the
pre-chamber is interfaced with the proximal end of the analysis
chamber.
30. The device of claim 25, wherein the distal end of the
pre-chamber is integrated with the proximal end of the analysis
chamber.
31. A method for measuring a quantity of an analyte in a fluid
sample, the method comprising the steps of: providing a fluid
sampling device, the sampling device comprising: a dermal layer
penetration probe, having a penetrating end and a communicating
end; an analysis chamber having a proximal end and a distal end,
the analysis chamber having a volume, wherein the penetration probe
is in fluid communication with the analysis chamber such that a
fluid sample can flow from the penetration probe to the analysis
chamber; and a pre-chamber having a proximal end and a distal end,
the pre-chamber having a volume, wherein the pre-chamber is
interposed between the penetration probe and the analysis chamber
such that the proximal end of the pre-chamber is adjacent the
communicating end of the penetration probe and the distal end of
the pre-chamber is adjacent the proximal end of the analysis
chamber, and wherein the volume of the pre-chamber is greater than
the volume of the analysis chamber; penetrating a dermal layer with
the penetration probe; substantially filling the analysis chamber
with the fluid sample by allowing the sample to flow from the
penetration probe to the analysis chamber; and measuring a quantity
of an analyte in the fluid sample.
32. The method of claim 31, wherein the sample is selected from the
group consisting of interstitial fluid and whole blood.
33. The method of claim 31, wherein the analyte is selected from
the group consisting of an ion, an element, a sugar, an alcohol, a
hormone, a protein, an enzyme, a cofactor, a nucleic acid sequence,
a lipid, a pharmaceutical, and a drug.
34. The method of claim 31, wherein the analyte is selected from
the group consisting of potassium ion, ethanol, cholesterol,
glucose, and lactate.
35. The method of claim 31, wherein a flow of fluid sample to the
analysis chamber is driven by a driving force, wherein the driving
force comprises a force selected from the group consisting of a
capillary force and a pressure differential.
36. The method of claim 31, wherein the pre-chamber is capable of
exerting a first capillary force and the analysis chamber is
capable of exerting a second capillary force and wherein a
differential in capillary force exists between the first capillary
force and the second capillary force.
37. The method of claim 31, wherein the second capillary force is
greater than the first capillary force.
38. The method of claim 31, wherein an interior surface of the
pre-chamber comprises a first pre-chamber wall and a second
pre-chamber wall, wherein the first pre-chamber wall and the second
pre-chamber wall are spaced apart at a first distance to define a
pre-chamber height, and wherein an interior surface of the analysis
chamber comprises a first analysis chamber wall and a second
analysis chamber wall spaced apart at a second distance to define
an analysis chamber height, wherein the analysis chamber height is
less than the pre-chamber height, wherein the pre-chamber is
capable of exerting a first capillary force and the analysis
chamber is capable of exerting a second capillary force, and
wherein a differential in the first capillary force and the second
capillary force derives at least in part from a difference between
the pre-chamber height and the analysis chamber height.
39. The method of claim 31, wherein at least one of the pre-chamber
and the analysis chamber comprises a substance capable of enhancing
or diminishing a capillary force.
40. The method of claim 39, wherein the substance is selected from
the group consisting of a polymer, a resin, a powder, a mesh, a
fibrous material, a crystalline material, a porous material, and a
combination thereof.
41. The method of claim 39, wherein the substance is selected from
the group consisting of polyethylene glycol, polyvinyl pyrrolidone,
a surfactant, a hydrophilic block copolymer, and polyacrylic
acid.
42. The method of claim 35, wherein the pressure differential
comprises a positive pressure applied to the analysis chamber.
43. The method of claim 35, wherein the pressure differential
comprises a negative pressure applied from the analysis
chamber.
44. The method of claim 31, wherein the analysis chamber comprises
a hollow electrochemical cell, the hollow electrochemical cell
comprising a working electrode, a counter or reference electrode,
and an opening for admitting an analyte to the cell, the working
electrode being spaced from the counter or reference electrode by a
distance of less than 500 .mu.m.
45. The method of claim 44, wherein the penetration probe comprises
a component selected from the group consisting of a needle, a
lancet, a tube, a channel, and a solid protrusion.
46. The method of claim 44, wherein the pre-chamber is capable of
exerting a first capillary force and the analysis chamber is
capable of exerting a second capillary force and wherein a
differential in capillary force exists between the first capillary
force and the second capillary force.
47. The method of claim 44, wherein the second capillary force is
greater than the first capillary force.
48. A method for measuring a quantity of an analyte in a fluid
sample, the method comprising the steps of: providing a fluid
sampling device, the device comprising: a dermal layer penetration
probe having a penetrating end and a communicating end, the
penetration probe having a volume; an analysis chamber having a
proximal and distal end, the analysis chamber having a volume,
wherein the volume of the penetration probe is greater than the
volume of the analysis chamber, wherein the penetration probe is in
fluid communication with the analysis chamber such that a fluid
sample can flow from the penetration probe to the analysis chamber;
penetrating a dermal layer with the penetration probe;
substantially filling the analysis chamber with a fluid sample by
allowing the sample to flow from the penetration probe to the
analysis chamber; and measuring a quantity of an analyte in the
fluid sample.
49. The method of claim 48, wherein the sample is selected from the
group consisting of interstitial fluid and whole blood.
50. The method of claim 48, wherein the analyte is selected from
the group consisting of an ion, an element, a sugar, an alcohol, a
hormone, a protein, an enzyme, a cofactor, a nucleic acid sequence,
a lipid, a pharmaceutical, and a drug.
51. The method of claim 48, wherein the analyte is selected from
the group consisting of potassium ion, ethanol, cholesterol,
glucose, and lactate.
52. The method of claim 48, wherein a flow of sample to the
analysis chamber is driven by a driving force, wherein the driving
force comprises a force selected from the group consisting of a
capillary force and a pressure differential.
53. The method of claim 48, wherein the penetration probe is
capable of exerting a first capillary force and the analysis
chamber is capable of exerting a second capillary force and wherein
a differential in capillary force exists between the first
capillary force and the second capillary force.
54. The method of claim 48, wherein the second capillary force is
greater than the first capillary force.
55. The method of claim 48, wherein an interior surface of the
penetration probe comprises a first penetration probe wall and a
second penetration probe wall, wherein the first penetration probe
wall and the second penetration probe wall are spaced apart at a
first distance to define a penetration probe height, and wherein an
interior surface of the analysis chamber comprises a first analysis
chamber wall and a second analysis chamber wall, wherein the first
analysis chamber wall and the second analysis chamber wall are
spaced apart at a second distance to define an analysis chamber
height, wherein the height of the analysis chamber is less than the
height of the penetration probe, wherein the penetration probe is
capable of exerting a first capillary force and the analysis
chamber is capable of exerting a second capillary force and wherein
a differential in capillary force exists between the first
capillary force and the second capillary force, and wherein the
differential capillary force derives at least in part from a
difference between the penetration probe height and the analysis
chamber height.
56. The method of claim 48, wherein at least one of the penetration
probe and the analysis chamber comprises a substance capable of
enhancing or diminishing a capillary force.
57. The method of claim 56, wherein the substance is selected from
the group consisting of a polymer, a resin, a powder, a mesh, a
fibrous material, a crystalline material, a porous material, and a
combination thereof.
58. The method of claim 56, wherein the substance is selected from
the group consisting of polyethylene glycol, polyvinyl pyrrolidone,
a surfactant, a hydrophilic block copolymer, and polyacrylic
acid.
59. The method of claim 52, wherein the pressure differential
comprises a positive pressure applied to the analysis chamber.
60. The method of claim 52, wherein the pressure differential
comprises a negative pressure applied from the analysis
chamber.
61. The method of claim 48, wherein the analysis chamber comprises
a hollow electrochemical cell, the hollow electrochemical cell
comprising a working electrode, a counter or reference electrode,
and an opening for admitting an analyte to the cell, the working
electrode being spaced from the counter or reference electrode by a
distance of less than 500 .mu.m.
62. The method of claim 61, wherein the penetration probe comprises
a component selected from the group consisting of a needle, a
lancet, a tube, a channel, and a solid protrusion.
63. The method of claim 61, wherein the penetration probe is
capable of exerting a first capillary force and the analysis
chamber is capable of exerting a second capillary force and wherein
a differential in capillary force exists between the first
capillary force and the second capillary force.
64. The method of claim 61, wherein the second capillary force is
greater than the first capillary force.
Description
RELATED APPLICATIONS
[0001] This application is a continuation of Application No.
10/166,487, filed Jun. 10, 2002, which is a continuation of
Application No. 09/536,235, filed Mar. 27, 2000.
FIELD OF THE INVENTION
[0002] The present invention relates to a method and device for
combining the sampling and analyzing of interstitial fluid or whole
blood samples which is suitable for hospital bedside and home
use.
BACKGROUND OF THE INVENTION
[0003] The management of many medical conditions requires the
measurement and monitoring of a variety of analytes in bodily
fluid. Historically, the measurement of analytes in blood has
required an invasive technique, such as a venipuncture or finger
puncture, to obtain blood for sampling purposes. An example of an
analyte which is routinely tested by obtaining a blood sample
through an invasive technique is glucose. In order to control their
condition, diabetics must monitor their glucose levels on a regular
basis. Invasive techniques used to obtain a blood sample for
analysis have the disadvantage of being painful, which can reduce
patient compliance in regular monitoring. Repeated testing, e.g.,
on a fingertip, can result in scar tissue build-up which makes
obtaining a sample in that region more difficult. Moreover,
invasive sampling procedures pose a risk of infection or disease
transmission.
[0004] An alternative is to sample interstitial fluid rather than
whole blood. Interstitial fluid is the fluid that fills the space
between the connective tissue and cells of the dermal layer of the
skin. An application where interstitial fluid has been shown to be
an appropriate sampling substitute for plasma or whole blood is in
the measurement of glucose concentration (J. Lab. Clin. Med. 1997,
130, 436-41).
[0005] In the patents U.S. Pat. No. 5,879,367, U.S. Pat. No.
5,879,310, U.S. Pat. No. 5,820,570 and U.S. Pat. No. 5,582,184 are
disclosed methods of sampling using a fine needle in conjunction
with a device to limit the penetration depth to obtain small
volumes of interstitial fluid for the purpose of glucose
monitoring. However, there is no method disclosed for analyzing the
drawn samples that is suitable for home use or hospital bedside
use.
SUMMARY OF THE INVENTION
[0006] It is desirable to be able to measure the concentration of
analytes in humans or other animals without having to draw a blood
sample by conventional methods. It is further desirable to be able
to do so with an inexpensive disposable device that is simple
enough for home or hospital bedside use.
[0007] The invention provides a suitable alternative to
conventional sampling devices and methods that is less invasive
than traditional whole blood sampling techniques and that requires
a considerably smaller sample volume than is required in the
conventional venipuncture or finger puncture sampling methods.
Because of the smaller sample volume required, a smaller wound is
necessary to obtain the sample. In the conventional finger stick
method, a drop of blood is formed on the tip of a finger, then the
sensor sample entrance is wetted with the drop. Because the sample
comes into contact with the skin surface, contamination of the
sample by material on the skin surface is possible. The devices and
methods disclosed herein do not require forming a blood drop on the
surface of the skin, and therefore have less risk of sample
contamination.
[0008] In one embodiment of the present invention, a fluid sampling
device is provided which includes a body, the body including a
dermal layer penetration probe having a penetrating end and a
communicating end, and an analysis chamber having a proximal and
distal end, the analysis chamber having a volume, wherein the
penetration probe is in fluid communication with the analysis
chamber such that fluid can flow from the penetration probe toward
the analysis chamber. The analysis chamber can have at least one
flexible wall which can be compressed to reduce the volume of the
analysis chamber. The penetration probe can include, for example, a
needle, a lancet, a tube, a channel, or a solid protrusion and can
be constructed of a material such as carbon fiber, boron fiber,
plastic, metal, glass, ceramic, a composite material, mixtures
thereof, and combinations thereof. The penetration probe can
include two sheets of material in substantial registration, having
a protrusion on each sheet, wherein the sheets are spaced apart
such that liquid can be drawn between the sheets by capillary
action. The two sheets of material can extend into the device so as
to form a pre-chamber. The penetration probe can be positioned
within a recess in the proximal end of the device, and the recess
can be configured to substantially align with a shape of a selected
dermal surface.
[0009] In a further embodiment, the device can further include a
pre-chamber having a volume and a first and second end, wherein the
pre-chamber is interposed between the penetration probe and the
analysis chamber such that the first end of the pre-chamber is
adjacent the communicating end of the penetration probe and the
second end of the pre-chamber is adjacent the proximal end of the
analysis chamber. The volume of the pre-chamber can be greater than
or equal to the volume of the analysis chamber. The pre-chamber can
have at least one flexible wall that can be compressed to reduce
the volume of the pre-chamber. The pre-chamber can also include a
valve at the first end capable of substantially sealing the
pre-chamber from the penetration probe.
[0010] In another embodiment, the device further includes a
compressible bladder in communication with the analysis chamber,
the compressible bladder being capable of applying a positive or a
negative pressure to the analysis chamber.
[0011] In yet another embodiment, the pre-chamber and the analysis
chamber can be capable of exerting different capillary forces. The
capillary force exerted by the analysis chamber can be greater than
the capillary force exerted by the pre-chamber. The differential
capillary force can be derived, at least in part, from a difference
between the pre-chamber height and the analysis chamber height. In
this embodiment, the interior surface of the pre-chamber can
include at least first and second pre-chamber walls spaced apart at
a first distance to define a pre-chamber height, and the interior
surface of the analysis chamber can include at least first and
second analysis chamber walls spaced apart at a second distance to
define an analysis chamber height, wherein the height of the
analysis chamber is less than the height of the pre-chamber.
[0012] In yet another further embodiment, at least one of the
chambers can include a substance capable of enhancing or
diminishing the capillary force exerted by the chamber. The
substance can include, for example, a polymer, a resin, a powder, a
mesh, a fibrous material, a crystalline material, or a porous
material. Suitable substances include polyethylene glycol,
polyvinylpyrrolidone, a surfactant, a hydrophilic block copolymer,
and polyvinylacetate.
[0013] In a further embodiment, the device further includes a
releasable actuator capable of supplying a force sufficient to
cause the penetration probe to penetrate a dermal layer. The
actuator can be external to or integral with the body, and upon
release propels the body toward the dermal layer.
[0014] In a further embodiment, the analysis chamber can include an
electrochemical cell including a working electrode and a
counter/reference electrode and an interface for communication with
a meter, wherein the interface communicates a voltage or a
current.
[0015] In yet another embodiment of the present invention, a method
for determining a presence or an absence of an analyte in a fluid
sample is provided including the steps of providing a fluid
sampling device as described above; penetrating a dermal layer with
the penetration probe; substantially filling the analysis chamber
with a fluid sample by allowing the sample to flow from the
penetration probe toward the analysis chamber; and detecting a
presence or an absence of the analyte within the analysis chamber.
The sample can include, for example, interstitial fluid and whole
blood. A qualitative or quantitative measurement of a
characteristic of the sample can be obtained in the detecting step.
The characteristic of the sample can include, for example, a
reaction product of the analyte, such as a color indicator, an
electric current, an electric potential, an acid, a base, a reduced
species, a precipitate, and a gas. The analyte can include, for
example, an ion such as potassium, an element, a sugar, an alcohol
such as ethanol, a hormone, a protein, an enzyme, a cofactor, a
nucleic acid sequence, a lipid, a pharmaceutical, and a drug.
Cholesterol and lactate are examples of substances that can be
analyzed.
[0016] In a further embodiment, the flow of sample toward the
analysis chamber can be driven by a driving force, e.g., capillary
force or a pressure differential. Where the analysis chamber has a
flexible wall, the wall can be compressed to reduce the volume of
the analysis chamber prior to penetrating the dermal, then the
compression released to form a partial vacuum in the analysis
chamber. Where the fluid sampling device further includes a
compressible bladder, the bladder can be compressed to reduce its
volume, then after penetration of the dermal layer the compression
can be released to form a partial vacuum in the compressible
bladder and analysis chamber.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] FIG. 1 shows a top view (not to scale) of one embodiment of
a sampling device illustrating an arrangement of the penetration
probe, pre-chamber, and analysis chamber.
[0018] FIG. 2 shows a cross section (not to scale) along the line
A-A' of FIG. 1.
[0019] FIG. 3 shows a top view (not to scale) of one embodiment of
a sampling device illustrating an arrangement of the penetration
probe, pre-chamber, and analysis chamber wherein the proximal edge
of the device forms a recess.
[0020] FIG. 4 shows a top view (not to scale) of one embodiment of
a sampling device illustrating an arrangement of the penetration
probe, pre-chamber, and analysis chamber.
[0021] FIG. 5 shows a cross section (not to scale) along the line
B-B' of FIG. 4.
[0022] FIGS. 6a and 6b (not to scale) depict an embodiment of the
invention wherein the device is loaded in a releasable actuator to
facilitate penetration of a dermal layer by the penetration probe.
FIG. 6a depicts the device loaded in the actuator, wherein the
actuator is in the cocked position, ready to be triggered. FIG. 6b
depicts the device and actuator after triggering.
[0023] FIG. 7 is a schematic drawing (not to scale) of a first
embodiment according to the invention shown in side elevation.
[0024] FIG. 8 shows the embodiment of FIG. 7 in plan, viewed from
above.
[0025] FIG. 9 shows the embodiment of FIG. 7 in plan, viewed from
below.
[0026] FIG. 10 shows the embodiment of FIG. 7 viewed in end
elevation.
[0027] FIG. 11 is a schematic drawing (not to scale) of a second
embodiment according to the invention in side elevation.
[0028] FIG. 12 shows the embodiment of FIG. 11 in plan, viewed from
above.
[0029] FIG. 13 is a schematic drawing (not to scale) of a third
embodiment according to the invention, in side elevation.
[0030] FIG. 14 shows the embodiment of FIG. 13 in plan, viewed from
above.
[0031] FIG. 15 is a schematic drawing (not to scale) according to
the invention in plan view, viewed from above.
[0032] FIG. 16 shows the embodiment of FIG. 15 in end
elevation.
[0033] FIG. 17 shows the embodiment of FIG. 15 in side
elevation.
[0034] FIG. 18 shows a schematic drawing (not to scale) of a hollow
cell embodiment according to the invention, viewed in cross
section.
[0035] FIG. 19 is a graph showing a plot of current (ordinate axis)
versus time (co-ordinate axis) during conduct of a method according
to the invention.
[0036] FIG. 20 is a further graph of use in explaining the method
of the invention.
[0037] In FIGS. 11 to 12, components corresponding in function to
components of the embodiment of FIGS. 7 to 10 are identified by
identical numerals or indicia.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0038] Introduction
[0039] The following description and examples illustrate various
embodiments of the present invention in detail. Those of skill in
the art will recognize that there are numerous variations and
modifications of this invention that are encompassed by its scope.
Accordingly, the description of a preferred embodiment should not
be deemed to limit the scope of the present invention. Methods and
devices for optimizing sampling of fluid samples are discussed
further in copending U.S. patent application Ser. No. 09/536,234,
filed on Mar. 27, 2000, entitled "METHOD OF PREVENTING SHORT
SAMPLING OF A CAPILLARY OR WICKING FILL DEVICE," which is
incorporated herein by reference in its entirety.
[0040] The invention disclosed in this application is a method and
device for combining the sampling and analyzing of a fluid sample
from sub-dermal tissue in a device suitable for hospital bedside
and home use. The fluid sample can comprise, but is not limited to,
interstitial fluid or whole blood samples obtained from an animal.
Any fluid sample obtained from sub-dermal tissue of a plant or an
animal can sampled and analyzed, thus the invention has broad
application in the fields of human medicine, veterinary medicine,
and horticultural science. The device and method are applicable to
any analyte that exists in a usefully representative concentration
in the fluid sample. For clarity, the present disclosure will
discuss the application to glucose monitoring. However, it is to be
understood that the invention is not limited to the monitoring of
glucose, and that other analytes, as discussed below, can also be
measured.
[0041] The method utilizes an integrated sampling and analyzing
device 10 incorporating a penetration probe 12 capable of
penetrating a patient's dermal layers to extract an interstitial
fluid or whole blood sample, and a method for transferring the
sample from the penetration probe 12 to the analysis chamber 20. In
one embodiment, the device 12 can be a one-shot disposable device
which can be inserted into a meter which communicates with the
analysis chamber 20 to perform the analysis of the sample and
present and optionally store the result.
[0042] In the device 10, a penetration probe 12 for penetrating the
subject's dermal layers to collect an interstitial fluid or whole
blood sample is integrated with an analysis chamber 20. A property
of sampling interstitial fluid is that it can take from several to
tens of seconds to collect sufficient sample to analyze. This is
often not desirable for an analysis chamber 20 wherein the analyte
undergoes a reaction as part of the analysis process, as it can be
difficult to obtain an accurate start time for the test as well as
achieve an even reacting reagent distribution in the sample. In a
second aspect of the current invention a method is disclosed for
collecting the sample in a pre-chamber 14 and, when full,
transferring the sample quickly to an analysis chamber 20.
[0043] In this disclosure, unless a different meaning is clear from
the context of its usage, "proximal" refers to a region or
structure of the device situated toward or adjacent to the dermal
surface to be penetrated, and "distal" refers a region or structure
of the device situated toward the opposite (non-proximal) end of
the device. For example, the penetration probe 12 is at the
proximal end of the device.
[0044] The Penetration Probe
[0045] The penetration probe 12 can be any device capable of
penetrating the patient's dermal layers to the desired extent and
capable of transporting a sample to a pre-chamber 14 or analysis
chamber 20. The penetration probe 12 comprises two ends, as
illustrated in FIG. 1. The penetrating end 11 of the penetration
probe 12 is the end inserted into the dermal layer. The
communicating end 13 of the penetration probe 12 is the end which
is in communication with either the pre-chamber 14 or the analysis
chamber 20.
[0046] One or more protrusions 12 with at least one sharp edge or
point are suitable as the penetration probe 12. The penetration
probe 12 can be fabricated from materials including plastic, metal,
glass, ceramic, a composite material (e.g., a composite of ceramic
and metal particles), or mixtures and combinations of these
materials. The penetration probe 12 can be in the form of a solid
protrusion, a needle, a lancet, a tube or a channel. The channel
can optionally be open along one or more of its elongated sides. As
illustrated in FIG. 2, a preferred embodiment of the penetration
probe 12 is two sheets 30 of material formed so as to have a
sharply pointed protrusion 12 on each sheet 30 in substantial
registration, with the sheets 30 spaced apart such that liquid can
be drawn between the sheets 30 by capillary action. In a
particularly preferred embodiment, the two sheets 30 of material
extend to and overlap with the analysis chamber 20 to form a
pre-chamber 14 for sample collection.
[0047] When interstitial fluid is sampled, the penetration depth
can be controlled by limiting the length the penetration probe 12
protrudes from the proximal surface 34 of the sampling device 10 to
less than the thickness of the dermal layer. In a preferred
embodiment, the length of the protrusion 12 will be less than 2 to
3 mm, more preferably about 1.5 mm. After penetration to a suitable
depth corresponding to the length of the protrusion 12, contact
between the surface of the dermal layer and the surface 34 of the
analyzing device prevents further penetration. For other uses, such
as in sampling interstitial fluid from regions having a thick
dermal layer, or for veterinary uses, it can be desirable for the
length of the protrusion 12 to be greater than 3 mm. Accordingly,
the invention contemplates protrusions 12 of any length, wherein
the length is sufficient to sample interstitial fluid. When whole
blood is sampled, a slightly longer penetration probe 12 should be
used, i.e., one having a length greater than 2 to 3 mm.
[0048] The diameter or width of the penetration probe 12 depends
upon the design of the penetration probe 12. Suitable diameters or
widths are those which provide sufficient sample flow. In the case
of a protrusion 12 forming a sharp edge or point, or a tube or
channel, the minimum diameter or width is typically greater than
about 10 .mu.m. When the penetrating means 12 comprises two sheets
30 in substantial registration, each having a sharply pointed
protrusion 12, the two protrusions 12 are typically spaced from 1
mm to 10 .mu.m apart.
[0049] The penetration probe 12 can be located on any suitable part
of the test strip 10, i.e., an edge 34, a corner 42, or one of the
flat surfaces 44. Protection can be provided to the penetration
probe 12 by locating it within a recess formed in the distal edge
34 of the test strip 10, as shown in FIG. 3, or in a depression on
the surface 44 of the test strip 10. In a preferred embodiment, the
recess in the distal edge 34 of the test strip 10 can be configured
to substantially align with the shape of a selected dermal surface,
e.g., a fingertip. However, the recess can be configured in other
suitable shapes, e.g., a square recess, a V-shaped recess, a curved
recess, a polygonal recess, and the like. In a preferred
embodiment, the penetration probe 12 does not protrude past the
proximal-most portion of the proximal edge 34 or surface 44 of the
device 10, but when pressed against the skin, the skin deforms into
the recess and is punctured by the penetration probe 12. Such an
arrangement aids sampling by compressing the area of the skin
around the sampling point. The penetration probe 12 can form an
integral part of another component of the test strip 10, e.g., a
side of the pre-chamber 54, as shown in FIG. 2. Alternatively, the
penetration probe 12 can comprise a separate part which is attached
to or incorporated into the test strip 10 by any suitable means,
e.g., adhesive, thermal bonding, interlocking parts, pressure, and
the like. The penetration probe 12 can be retractable or
non-retractable.
[0050] Penetration itself can be accomplished by any suitable
means, including inserting the penetration device 12 manually or by
means of a releasable actuator 84 such as, for example, a
spring-loaded mechanism 84 as depicted in FIGS. 6a and 6b. Such a
spring-loaded mechanism 84 incorporates a spring 86 which is
compressed and held in place by a trigger 88 which can release the
force compressing the spring 86 when the triggering mechanism is
activated. The trigger 88 can be activated manually, or the device
84 can incorporate a pressure sensor which indicates that
sufficient pressure has been applied to obtain the sample, thereby
activating the trigger 88. In one embodiment, the distal end of the
device 10 is placed in the spring-loaded mechanism 84 such that
when the force compressing the spring 86 is released by activating
the trigger 88, force is transferred to the device 10, which is
ejected from the mechanism 84, thereby inserting the penetrating
probe 12 into the dermal layer.
[0051] Any suitable body part can be used for sampling. In a
preferred embodiment, the sampling area is one which does not have
a high density of nerve endings, e.g., the forearm. Typically, 5 to
15 seconds is required to obtain sufficient sample. Application of
pressure to the sampling area can be needed to extract interstitial
fluid or whole blood. To facilitate the appropriate amount of
pressure being applied, a pressure sensor can be incorporated into
the device 10 which indicates when sufficient pressure has been
applied. Sample acquisition time can be improved by applying
increased pressure to the area surrounding the direct sampling
area. Some of the factors that can affect interstitial fluid or
whole blood sample acquisition include the patient's age, skin
thickness, temperature, and hydration. The amount of interstitial
or whole blood sample collected for testing can preferably be about
0.02 .mu.l or greater, more preferably 0.1 .mu.l or greater, and
most preferably about 0.5 .mu.l or greater.
[0052] In one preferred embodiment, the device 10 can be inserted
into a meter prior to sample acquisition. In such an embodiment,
the meter serves multiple functions, including supporting the
device 10, providing an automated means of initiating sample
acquisition, and indicating when sample acquisition is
complete.
[0053] Transfer of Sample from Penetration probe to Analysis
Chamber
[0054] In a preferred embodiment of the sampling device 10, the
device comprises two parts--the penetration probe 12 and an
analysis chamber 20. In another preferred embodiment, illustrated
in FIGS. 1 and 2, the device 10 comprises the penetration probe 12
and a pre-chamber 14. The pre-chamber 14 can then be integrated
with or can be interfaced to the analysis chamber 20.
[0055] In a further embodiment, the analysis chamber 20 is
integrated with or can be interfaced to a means for facilitating
filling of the analysis chamber 20. This means can comprise a
collapsible or compressible bladder 22, as shown in FIGS. 3 and 4,
which can be used to apply a positive or negative pressure (i.e.,
partial vacuum) to the analysis chamber 20. The compressible
bladder 22 can comprise any chamber with flexible walls that can be
compressed to reduce the volume of the chamber. When the force
compressing the compressible bladder 22 is released, a partial
vacuum is formed which draws sample into the analysis chamber 20.
In a preferred embodiment, the volume of the compressible bladder
22 is sufficiently large so that when the bladder 22 is
substantially fully compressed, the reduction in volume of the
bladder 22 is larger than or equal to the total volume of the
analysis chamber 20, thereby ensuring that the analysis chamber 20
is substantially filled. However, a compressible bladder 22 with a
smaller volume than the analysis chamber 20 can also be effective
in assisting the filling of the analysis chamber 20.
[0056] Alternatively, the analysis chamber 20 itself can be
collapsible or compressible. In such an embodiment, a piston or
other compressing agent, such as a patient's or clinician's
fingers, can first compress then release the analysis chamber 20,
thereby forming a partial vacuum. When the compressing force is
released, the partial vacuum causes the sample to flow from the
penetration probe toward the analysis chamber.
[0057] Pre-chamber
[0058] In a preferred embodiment, as illustrated in FIGS. 1 and 2,
a pre-chamber 14 is provided in the integrated sampling and testing
device 10 for accumulation and storage of the collected sample
prior to its being transferred to the analysis chamber 20. A
pre-chamber 14 is useful when using an analysis method which
requires that the sample fill the analysis chamber 20 in a short
period of time to return accurate results, i.e., a time shorter
than that required to draw sufficient sample from the dermal layer.
In a preferred embodiment, the volume of the pre-chamber 14 is
larger than that of the analysis chamber 20, thus ensuring that
once the pre-chamber 14 is filled, sufficient sample has been
collected to completely fill the analysis chamber 20.
[0059] In a preferred embodiment, as illustrated in FIGS. 1 and 2,
the penetration probe 12 opens into the pre-chamber 14 at a first
end, and at the second end the pre-chamber 14 opens to the analysis
chamber 20. The pre-chamber 14 can be free of reagents or other
substances, or can optionally contain one or more substances to
enhance or diminish the capillary force exerted by the walls of the
pre-chamber 14 or to pre-treat the sample prior to analysis. These
substances can include, for example, polymers, resins, powders,
meshes, fibrous materials, crystalline materials, porous materials,
or a mixture or combination thereof. To facilitate effective
filling of the analysis chamber 20, a preferred embodiment utilizes
a pre-chamber 14 and analysis chamber 20 of different heights, as
shown in FIG. 2. Where the analysis chamber 20 is formed so that
its height (typically referring to the smallest chamber dimension)
is smaller than the height of the pre-chamber 14, a capillary force
is generated that is capable of drawing fluid out of the
pre-chamber 14 and into the analysis chamber 20. A first air vent
64 can be formed at the end 70 of the analysis chamber 20 opposite
the opening 62 to the pre-chamber 14, facilitating the filling of
the analysis chamber 20 by allowing air to be displaced from the
analysis chamber 20 as sample enters. Optionally, a second vent 74
can be formed opening into the pre-chamber 14 at the substantially
opposite end 60 of the pre-chamber 14 to where the penetration
probe 12 opens into the pre-chamber 14. This vent 74 provides air
to the pre-chamber 14 to replace the sample as it is transferred
from the pre-chamber 14 to the analysis chamber 20. The vent 74 can
be placed in any suitable position on the test strip 10. In a
preferred embodiment, the vent 74 incorporates a sharp corner,
e.g., at a 90.degree. angle, which functions as a "capillary stop"
to prevent sample from exiting the device 10 through the vent
74.
[0060] In another embodiment, the pre-chamber 14 consists of a
tube, or other shaped chamber, with flexible walls, attached to the
penetration probe 12. In this embodiment, the pre-chamber 14 is
either permanently fixed to the analysis chamber 20 or is placed
next to and aligned with a port to the analysis chamber 20. Such
alignment can occur during use by suitable placement in an external
device such as the measurement meter.
[0061] In one aspect of this embodiment, the pre-chamber 14 further
comprises a valve, defined as a device to control the flow of fluid
sample between the penetration probe 12 and the pre-chamber 14. The
valve can comprise one or more rollers, pistons, or squeezing
devices capable of simultaneously closing off the first end 60 of
the pre-chamber 14, and compressing the pre-chamber 14 such that
the fluid in the pre-chamber 14 is forced towards the second end 62
of the pre-chamber 14 and subsequently into the analysis chamber
20.
[0062] Alternatively, the analysis chamber 20 consists of a tube,
or other shaped chamber, with flexible walls, attached to the
penetration probe 12. In one aspect of this embodiment, the
analysis chamber 20, prior to penetration, is compressed by one or
more rollers, pistons, or other squeezing devices. After the
penetration probe 12 is inserted, the compression is released,
forming a vacuum which pulls sample into the analysis chamber 20.
In such an embodiment, the pre-chamber 14 can not be necessary if
sufficient vacuum is generated for rapid sample acquisition. In
such an embodiment, the device 10 can not require a vent 64, 74 if
such would interfere with forming a vacuum.
[0063] In another embodiment, illustrated in FIGS. 3 and 4, a
pre-chamber 14 of suitable size is formed which opens to the
penetration probe 12 on one end 60 and to the analysis chamber 20
on the other end 62. The end 70 of the analysis chamber 20 opposite
to that opening to the pre-chamber 14 opens to a compressible
bladder 22. The bladder 22 can be formed separately and attached to
the end 70 of the analysis chamber 20. Alternatively, it can be
formed by removing a section on the middle laminate 82 in the test
strip 10, similar to those described in WO97/00441 (incorporated
herein by reference in its entirety), as illustrated in FIGS. 3 and
4.
[0064] In use, the bladder 22 in the strip 10 is compressed by
suitable means prior to the penetration probe 12 being inserted
into the patient. Insertion of the penetration probe 12 can be
confirmed by use of a sensor, such as a pressure sensor, or the
patient can confirm that the penetration probe 12 is inserted
either visually or by touch. In the latter case, the patient
sensing can signal the meter, such as by pushing a button. At this
point, the means compressing the bladder 22 is withdrawn to a
halfway position to draw sample into the pre-chamber 14. When the
pre-chamber 14 is full, as indicated by a suitable sensor, the
meter indicates to the patient to withdraw the penetration probe
12. The compressing means then moves to its fully withdrawn
position and so draws the sample from the pre-chamber 14 into the
analysis chamber 20. In the case where the initial suction from the
bladder 22 causes the sample to be accumulated with sufficient
speed, the pre-chamber 14 can be dispensed with and the bladder 22
used to draw sample through the penetration probe 12 directly into
the analysis chamber 20. A vent 64, 74 which would interfere with
forming a vacuum need not be incorporated into the device in some
embodiments.
[0065] Analysis Chamber
[0066] In a preferred embodiment, the analysis chamber 20 is
contained in an analyzing device 10 comprising a disposable
analysis strip similar to that disclosed in WO97/00441. The
analysis strip of WO97/00441 contains a biosensor for determining
the concentration of an analyte in a carrier, e.g., the
concentration of glucose in a fluid sample. The electrochemical
analysis cell 20 in this strip has an effective volume of 1.5 .mu.l
or less, and can comprise a porous membrane, a working electrode on
one side of the membrane, and a counter/reference electrode on the
other side. In a preferred embodiment, an analysis cell 20 having
an effective volume of about 0.02 .mu.l or greater is used. More
preferably, the cell 20 has a volume ranging from about 0.1 .mu.l
to about 0.5 .mu.l.
[0067] In one aspect of this embodiment, the penetration probe 12
is a small needle integrated into the analysis strip 10 by being
inserted through a wall of the analysis chamber 20 such that one
end of the needle 12 opens into the strip analysis chamber 20. In
using a device 10 having this arrangement to obtain and analyze a
sample of interstitial fluid, the needle 12 is inserted into the
patient's dermal layer and sample is drawn into the needle 12 via
capillary action. The sample is then transferred from the needle 12
into the analysis chamber 20 by capillary action whereupon the
sample is analyzed. An opening 64 in the analysis chamber 20 to
atmosphere, remote from the point where the needle 12 opens into
the chamber, acts as a vent 64 to allow the escape of displaced air
as the analysis chamber 20 fills with sample. Analysis devices of
the type disclosed in WO97/00441 are particularly suited for use
with this arrangement because of their ability to utilize the very
small volumes of sample typically available with interstitial fluid
sampling.
[0068] The analysis chamber 20 can contain one or more substances
to enhance or diminish the capillary force exerted by the walls of
analysis chamber 20. Such materials can include polymers, resins,
powders, meshes, fibrous materials, crystalline materials, porous
materials, or a mixture or combination thereof, as can also be used
in the pre-chamber, discussed above. For example, the walls 24 of
the analysis chamber 20 can be coated with a hydrophilic material
to encourage the flow of fluid sample into the analysis chamber.
Suitable hydrophilic materials include polyethylene glycol,
polyvinylpyrrolidone, a surfactant, a hydrophilic block copolymer,
and polyacrylic acid. The analysis chamber 20 can also contain
reagents capable of reacting with the analyte or other substances
present in the sample. Such other substances can include substances
which interfere in determining the presence or absence of the
analyte. In such cases, the reagent will react with the substance
so that it no longer interferes with the analysis.
[0069] Any analyte present in a fluid sample in a detectable amount
can be analyzed using the device 10. A typical analytes can
include, but is not limited to, an ion, an element, a sugar, an
alcohol, a hormone, a protein, an enzyme, a cofactor, a nucleic
acid sequence, a lipid, and a drug. In a preferred embodiment,
glucose is the analyte to be tested. Typical analytes could
include, but are not limited to, ethanol, potassium ion,
pharmaceuticals, drugs, cholesterol, and lactate.
[0070] The presence or absence of the analyte can be determined
directly. Alternatively, the analyte can be determined by reacting
the analyte with one or more reagents present in the analysis
chamber. The product of that reaction, indicative of the presence
or absence of the analyte, would then be detected. Suitable
reaction products include, but are not limited to, a color
indicator, an electric current, an electric potential, an acid, a
base, a precipitate, or a gas.
[0071] Any suitable analytical method can be used for determining
the presence or absence of the analyte or a reaction product of the
analyte. Suitable analytical methods include, but are not limited
to, electrochemical methods, photoabsorption detection methods,
photoemission detection methods, and the measurement of magnetic
susceptibility. In the case of a reaction product having a
different color than the analyte, or the formation of a precipitate
or a gas, a visual determination can be a suitable method for
determining the presence or absence of the analyte.
[0072] With reference to FIGS. 7 to 10 there is shown a first
embodiment of apparatus of the invention, in this case a biosensor
for determining glucose in blood. The embodiment comprises a thin
strip membrane 1 having upper and lower surfaces 2, 3 and having a
cell zone 4 defined between a working electrode 5 disposed on upper
surface 2 and a counter electrode 6 disposed on lower surface 3.
The membrane thickness is selected so that the electrodes are
separated by a distance "I" which is sufficiently close that the
products of electrochemical reaction at the counter electrode
migrate to the working electrode during the time of the test and a
steady state diffusion profile is substantially achieved.
Typically, "I" will be less than 500 .mu.m. A sample deposition or
"target" area 7 defined on upper surface 2 of membrane 1 is spaced
at a distance greater than the membrane thickness from cell zone 4.
Membrane 1 has a diffusion zone 8 extending between target area 7
and cell zone 4. A suitable reagent including a redox mediator "M",
an enzyme "E" and a pH buffer "B" are contained within cell zone 4
of the membrane and/or between cell zone 4 and target area 7. The
reagent may also include stabilisers and the like.
[0073] In some cases it is preferable to locate the enzyme and
mediator and/or the buffer in different zones of the membrane. For
example the mediator may be initially located within
electrochemical cell zone 4 while the enzyme may be situated below
target area 7 or in diffusion zone 8.
[0074] Haemoglobin releases oxygen at low pH's, but at higher pH's
it binds oxygen very firmly. Oxygen acts as a redox mediator for
glucose oxidase dehydroienase (GOD). In a glucose sensor this
competes with the redox mediator leading to low estimates of
glucose concentration. Therefore if desired a first pH buffer can
be contained in the vicinity of target area 7 to raise the pH to
such a level that all the oxygen is bound to haemoglobin. Such a pH
would be non-optimal for GOD/glucose kinetics and would
consequently be detrimental to the speed and sensitivity of the
test.
[0075] In a preferred embodiment of the invention a second pH
buffer is contained as a reagent in the vicinity of the working
electrode to restore the pH to kinetically optimal levels.
[0076] The use of a second buffer does not cause oxygen to be
released from the haemoglobin as the haemoglobin is contained
within the blood cells which are retained near blood target area 7
or are retarded in diffusion in comparison with the plasma and
therefore not influenced by the second buffer. In this manner
oxygen interference may be greatly reduced or eliminated.
[0077] In use of the sensor a drop of blood containing a
concentration of glucose to be determined is placed on target zone
7. The blood components wick towards cell zone 4, the plasma
component diffusing more rapidly than red blood cells so that a
plasma front reaches cell zone 4 in advance of blood cells.
[0078] When the plasma wicks into contact with the reagent, the
reagent is dissolved and a reaction occurs that oxidises the
analyte and reduces the mediator. After allowing a predetermined
time to complete this reaction an electric potential difference is
applied between the working electrode and the counter electrode.
The potential of the working electrode is kept sufficiently anodic
such that the rate of electrooxidation of the reduced form of the
mediator at the working electrode is determined by the rate of
diffusion of the reduced form of the mediator to the working
electrode, and not by the rate of electron transfer across the
electrode/solution interface.
[0079] In addition the concentration of the oxidised form of the
mediator at the counter electrode is maintained at a level
sufficient to ensure that when a current flows in the
electrochemical cell the potential of the counter electrode, and
thus also the potential of the working electrode, is not shifted so
far in the cathodic direction that the potential of the working
electrode is no longer in the diffusion controlled region. That is
to say, the concentration of the oxidized form at the counter
electrode must be sufficient to maintain diffusion controlled
electrooxidation of the reduced form of the mediator at the working
electrode.
[0080] The behavior of a thin layer cell is such that if both
oxidised and reduced forms of the redox couple are present,
eventually a steady state concentration profile is established
across the cell. This results in a steady state current. It has
been found that by comparing a measure of the steady state current
with the rate at which the current varies in the current transient
before the steady state is achieved, the diffusion coefficient of
the redox mediator can be measured as well as its
concentration.
[0081] More specifically, by solving the diffusion equations for
this situation it can be shown that over a restricted time range a
plot of ln(i/i-1) vs. time (measured in seconds) is linear and has
a slope (denoted by S) which is equal to -4.pi..sup.2D/1.sup.2,
where "i" is the current at time "t", "V" is the steady state
current, "D" is the diffusion coefficient in cm.sup.2/sec, "1" is
the distance between the electrodes in cm and ".pi." is
approximately 3.14159. The concentration of reduced mediator
present when the potential was applied between the electrodes is
given by 2.pi..sup.2i /FA1S, where "T" is Faraday's constant, "A"
is the working electrode area and the other symbols are as given
above. As this later formula uses S it includes the measured value
of the diffusion coefficient.
[0082] Since I is a constant for a given cell, measurement of i as
a function of time and i enable the value of the diffusion
coefficient of the redox mediator to be calculated and the
concentration of the analyte to be determined.
[0083] Moreover the determination of analyte concentration
compensates for any variation to the diffusion coefficient of the
species which is electrooxidised or electroreduced at the working
electrode. Changes in the value of the diffusion coefficient may
occur as a result of changes in the temperature and viscosity of
the solution or variation of the membrane permeability. Other
adjustments to the measured value of the concentration may be
necessary to account for other factors such as changes to the cell
geometry, changes to the enzyme chemistry or other factors which
may effect the measured concentration. If the measurement is made
on plasma substantially free of haematocrit (which if present
causes variation in the diffusion coefficient of the redox
mediator) the accuracy of the method is further improved.
[0084] Each of electrodes 5, 6 has a predefined area. In the
embodiments of FIGS. 7 to 10 cell zone 4 is defined by edges 9, 10,
11 of the membrane which correspond with edges of electrodes 5, 6
and by leading (with respect to target area 7) edges 12, 13 of the
electrodes. In the present example the electrodes are about 600
angstrom thick and are from 1 to 5 mm wide.
[0085] Optionally, both sides of the membrane are covered with the
exception of the target area 7 by laminating layers 14 (omitted
from plan views) which serves to prevent evaporation of water from
the sample and to provide mechanical robustness to the apparatus.
Evaporation of water is undesirable as it concentrates the sample,
allows the electrodes to dry out, and allows the solution to cool,
affecting the diffusion coefficient and slowing the enzyme
kinetics, although diffusion coefficient can be estimated as
above.
[0086] A second embodiment according to the invention, shown in
FIGS. 11 and 12, differs from the first embodiment by inclusion of
a second working electrode 25 and counter/reference electrode 26
defining a second cell zone 24 therebetween. These electrodes are
also spaced apart by less than 500 .mu.m in the present example.
Second electrodes 25, 26 are situated intermediate cell zone 4 and
target area 7. In this embodiment the redox mediator is contained
in the membrane below or adjacent to target area 7 or intermediate
target area 7 and first cell zone 4. The enzyme is contained in the
membrane in the first cell zone 4 and second cell zone 24. The
enzyme does not extend into second cell 24. In this case when blood
is added to the target area, it dissolves the redox mediator. This
wicks along the membrane so that second electrochemical cell 24
contains redox mediator analyte and serum including
electrochemically interfering substances. First electrochemical
cell receives mediator, analyte, serum containing electrochemically
interfering substances, and enzyme.
[0087] Potential is now applied between both working electrodes and
the counter electrode or electrodes but the change in current with
time is measured separately for each pair. This allows the
determination of the concentration of reduced mediator in the
absence of analyte plus the concentration of electrochemically
interfering substances in the second electrochemical cell and the
concentration of these plus analyte in the first electrochemical
cell. Subtraction of the one value from the other gives the
absolute concentration of analyte.
[0088] The same benefit is achieved by a different geometry in the
embodiment of FIGS. 13 and 14 in which the second working electrode
and second counter/reference electrode define the second cell 24 on
the side of target area 7 remote from first electrochemical cell 4.
In this case the enzyme may be contained in the membrane strip
between the target area and cell 1. The redox mediator may be in
the vicinity of the target area or between the target area and each
cell. The diffusion coefficient of mediator is lowered by
undissolved enzyme and the arrangement of FIGS. 13 and 14 has the
advantage of keeping enzyme out of the thin layer cells and
allowing a faster test (as the steady state current is reached more
quickly). Furthermore the diffusion constant of redox mediator is
then the same in both thin layer cells allowing more accurate
subtraction of interference.
[0089] Although the embodiments of FIGS. 7 to 14 are unitary
sensors, it will be understood that a plurality of sensors may be
formed on a single membrane as shown in the embodiment of FIGS. 15
to 17. In this case the electrodes of one sensor are conductively
connected to those of an adjacent sensor. Sensors may be used
successively and severed from the strip after use.
[0090] In the embodiment of FIGS. 15 to 17 electrode dimensions are
defined in the diffusion direction (indicated by arrow) by the
width of the electrode in that direction.
[0091] The effective dimension of the electrode in a direction
transverse to diffusion direction is defined between compressed
volumes 16 of the membrane in a manner more fully described in
co-pending Application PCT/AU96/00210. For clarity optional
laminated layer 14 of FIG. 7 has been omitted from FIGS. 15 to
17.
[0092] In the embodiment of FIG. 18 there is shown a hollow cell
according to the invention wherein the electrodes 5, 6 are
supported by spaced apart polymer walls 30 to define a hollow cell.
An opening 31 is provided on one side of the cell whereby a sample
can be admitted into cavity 32. In this embodiment a membrane is
not used. As in previous embodiments, the electrodes are spaced
apart by less than 500 .mu.m, preferably 20-400 .mu.m and more
preferably 20-200 .mu.m. Desirably the effective cell volume is 1.5
microlitres or less.
[0093] It will be understood that the method of the invention may
be performed with a cell constructed in accord with co-pending
application PCT/AU95/00207 or cells of other known design, provided
these are modified to provide a sufficiently small distance between
electrode faces.
[0094] The method of the invention will now be further exemplified
with reference to FIGS. 19 and 20.
[0095] A membrane 130 microns thick was coated on both sides with a
layer of Platinum 60 nanometers thick. An area of 12.6 sq. mm was
defined by compressing the membrane. 1.5 microlitres of a solution
containing 0.2 Molar potassium ferricyanide and 1% by weight
glucose oxidase dehydrotenase was added to the defined area of the
membrane and the water allowed to evaporate.
[0096] The platinum layers were then connected to a potentiostat to
be used as the working and counter/reference electrodes. 3
microlitres of an aqueous solution containing 5 millimolar
D-glucose and 0.9 wt % NaCl was dropped on to the defined area of
the membrane. After an elapse of 20 seconds a voltage of 300
millivolts was applied between the working and counter/reference
electrodes and the current recorded for a further 30 seconds at
intervals of 0.1 seconds.
[0097] FIG. 19 is a graph of current versus time based on the above
measurements.
[0098] Using a value of the steady state current of 26.9 microamps
the function ln(i/26.9-1) was computed and plotted versus time. The
slope of the graph (FIG. 20) is -0.342 which corresponds to a
diffusion coefficient of 1.5.times.10.sup.6 cm.sup.2 per second and
a corrected glucose concentration (subtracting background
ferrocyanide) of 5.0 millimolar.
[0099] The steady state current is one in which no further
significant current change occurs during the test. As will be
understood by those skilled in the art, a minimum current may be
reached after which there may be a drift due to factors such as
lateral diffusion, evaporation, interfering electrochemical
reactions or the like. However, in practice it is not difficult to
estimate the "steady state" current (i). One method for doing so
involves approximating an initial value for i. Using the fit of the
i versus t data to the theoretical curve a better estimate of i is
then obtained. This is repeated reiteratively until the measured
value and approximated value converge to within an acceptable
difference, thus yielding an estimated i.
[0100] In practice, the measurements of current i at time t are
made between a minimum time t min and a maximum time t max after
the potential is applied. The minimum and maximum time are
determined by the applicability of the equations and can readily be
determined by experiment of a routine nature. If desired the test
may be repeated by switching off the voltage and allowing the
concentration profiles of the redox species to return towards their
initial states.
[0101] It is to be understood that the analysis of the current v.
time curve to obtain values of the Diffusion Coefficient and/or
concentration is not limited to the method given above but could
also be achieved by other methods.
[0102] For instance, the early part of the current v. time curve
could be analysed by the Cottrell equation to obtain a value of
D.sup.1/2.times.Co (Co=Concentration of analyte) and the steady
state current analysed to obtain a value of D.times.Co. These two
values can then be compared to obtain D and C separately.
[0103] It will be understood that in practice of the invention an
electrical signal is issued by the apparatus which is indicative of
change in current with time. The signal may bean analogue or
digital signal or may be a series of signals issued at
predetermined time intervals. These signals may be processed by
means of a microprocessor or other conventional circuit to perform
the required calculations in accordance with stored algorithms to
yield an output signal indicative of the diffusion coefficient,
analyte concentration, haematocrit concentration or the like
respectively. One or more such output signals may be displayed by
means of an analogue or digital display.
[0104] It is also possible by suitable cell design to operate the
cell as a depletion cell measuring the current required to deplete
the mediator. For example in the embodiment of FIG. 5 the method of
the invention may be performed using electrodes 5, 6, which are
spaced apart by less than 500 .mu.m. An amperometric or
voltammetric depletion measurement may be made using electrodes 5
and 26 which are spaced apart more than 500 .mu.m and such that
there is no interference between the redox species being
amperometrically determined at electrodes 5, 26.
[0105] The depletion measurement may be made prior to, during or
subsequent to, the measurement of diffusion coefficient by the
method of the invention. This enables a substantial improvement in
accuracy and reproducibility to be obtained.
[0106] In the embodiments described the membrane is preferably an
asymmetric porous membrane of the kind described in Patent No.
4,629,563 and 4,774,039. However symmetrical porous membranes may
be employed. The membrane may be in the form of a sheet, tube,
hollow fibre or other suitable form.
[0107] If the membrane is asymmetric the target area is preferably
on the more open side of the asymmetric membrane. The uncompressed
membrane desirably has a thickness of from 20 to 500 .mu.m. The
minimum thickness is selected having regard to speed, sensitivity,
accuracy and cost. If desired a gel may be employed to separate
haematocrit from GOD. The gel may be present between the electrodes
and/or in the space between the sample application area and the
electrodes.
[0108] The working electrode is of any suitable metal for example
gold, silver, platinum, palladium, iridium, lead, a suitable alloy.
The working electrode may be preformed or formed in situ by any
suitable method for example sputtering, evaporation under partial
vacuum, by electrodeless plating, electroplating, or the like.
Suitable non-metal conductors may also be used for electrode
construction. For example, conducting polymers such as
poly(pyrrole), poly(aniline), porphyrin "wires", poly(isoprene) and
poly (cis-butadiene) doped with iodine and "ladder polymers". Other
non-metal electrodes may be graphite or carbon mixed with a binder,
or a carbon filled plastic.
[0109] Inorganic electrodes such as In.sub.2O.sub.3 or SnO.sub.2
may also be used. The counter/reference electrode may for example
be of similar construction to the working electrode. Nickel
hydroxide or a silver halide may also be used to form the
counter/reference electrode.
[0110] Silver chloride may be employed but it will be understood
that chloridisation may not be necessary and silver may be used if
sufficient chloride ions are present in the blood sample. Although
in the embodiments described the working electrode is shown on the
upper surface of the biosensor and the counter/reference electrode
is on the lower surface, these may be reversed.
[0111] It is preferable that the working electrode and counter (or
counter/reference) electrodes are of substantially the same
effective geometric area.
[0112] If a separate reference and counter electrode are employed,
they may be of similar construction. The reference electrode can be
in any suitable location.
[0113] It will be understood that the features of one embodiment
herein described may be combined with those of another. The
invention is not limited to use with any particular combination of
enzyme and mediator and combinations such as are described in
EP0351892 or elsewhere may be employed. The system may be used to
determine analytes other than glucose (for example, cholesterol) by
suitable adaptation of reagents and by appropriate membrane
selection. The system may also be adapted for use with media other
than blood. For example the method may be employed to determine the
concentration of contaminants such as chlorine, iron, lead,
cadmium, copper, etc., in water.
[0114] Although the cells herein described have generally planar
and parallel electrodes it will be understood that other
configurations may be employed, for example one electrode could be
a rod or needle and the other a concentric sleeve.
[0115] Display/Storage of Measurement Data
[0116] In a preferred embodiment, an analysis strip as described
above or another embodiment of the sampling device 10 is integrated
with a measuring device, e.g., a meter, which can display, store or
record test data, optionally in computer-readable format. In such
an embodiment, the test strip 10 comprises an interface for
communicating with the meter, e.g., conductive leads from the
electrodes of the electrochemical cell 20. In the case of obtaining
an electrochemical measurement, the interface communicates a
voltage or a current to the electrochemical cell 20.
[0117] The above description discloses several methods and
materials of the present invention. This invention is susceptible
to modifications in the methods and materials, as well as
alterations in the fabrication methods and equipment. Such
modifications will become apparent to those skilled in the art from
a consideration of this disclosure or practice of the invention
disclosed herein. Consequently, it is not intended that this
invention be limited to the specific embodiments disclosed herein,
but that it cover all modifications and alternatives coming within
the true scope and spirit of the invention as embodied in the
attached claims.
* * * * *