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Description  |
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INTRODUCTION
1. Technical Field
This invention relates to methods and apparatuses used for controlled
transport of liquids by capillary action and gravity, particularly the
automatic measuring and diluting of small volumes of liquids using
cartridges in which flow of sample and diluent is controlled at a junction
between capillary-flow and non-capillary-flow regions referred to herein
as a stop-flow junction.
2. Background
The phrase "stop-flow junction" was introduced to describe a control region
in a capillary passageway that is used in a number of prior inventions
arising out of the laboratories of the present invention. A stop-flow
junction is a region in a fluid track that marks the junction between an
early part of the fluid track in which sample flows by capillary action
(and optionally gravity) and a later part of the fluid track into which
sample does not normally flow until flow is initiated by some outside
force, such as an action of the user.
A stop-flow junction is not a traditional valve as it has no moving parts.
Rather, this junction relies on back pressure from the surface tension of
the liquid sample to stop flow. This back pressure can be created in a
number of ways. For example, back pressure is created when the
cross-sectional area of a liquid flowpath increases in a region in which
there is contact between the liquid and the container walls (e.g., when a
small tube enters a larger chamber or when the cross-sectional area of a
channel increases). More consistent operation of a stop-flow junction is
achieved when the increase in cross-sectional area of the flowpath is
abrupt rather than gradual, particularly when there is a break in
capillarity in the sample flowpath. In many cases, the junction will be
formed when a small-diameter capillary channel enters a larger,
non-capillary chamber. A small channel or tube can enter the larger
chamber at a right angle or at an angle other than a right angle. The
angle between the internal wall of the small tube and the surface of the
chamber in the latter case will be different at different locations around
the circumference of the junction.
In general, for small (capillary-size) junctions, the back pressure will be
largely determined by the smallest radius of curvature assumed by the
meniscus. For example, when a capillary tube with a circular crosssection
enters a larger space so that liquid bulges out into the space under
hydrostatic pressure, the meniscus will be approximately spherical, and
the back pressure (.delta.p) is given by the Young-Laplace equation:
.delta.p=2.gamma./R, were .gamma. is the surface tension of the sample
fluid and R is the radius of curvature. See, Miller and Neogi,
"Interfacial Phenomena: Equilibrium and Dynamic Effects", Marcel Dekker,
Inc., New York, 1985, and Davies and Riedeal "Interfacial Phenomena", 2nd
Ed., Academic Press, New York, 1963. If the fluid meets the surface at an
angle greater than 0.degree., this back pressure will be reduced by a
geometric term. The radius, R, will change (become smaller) as the
hydrostatic pressure increases, so that the back pressure and hydrostatic
pressure balance. As hydrostatic pressure increases, R reaches a minimum
value (maximum curvature) determined by the geometry of the device and the
contact angle. The corresponding back pressure defines the maximum
hydrostatic pressure sustainable by the stop-flow junction.
Back pressure is also created when the surface that the liquid contacts
changes to decrease adhesion between the liquid and the container wall
(for example, when an aqueous sample moves from a hydrophilic to a
hydrophobic surface). The surface properties of the various interior
surfaces of the device of the invention can and generally will be
controlled by various physical and/or chemical treatments. For a
discussion of controlling surface properties of similar devices, see
commonly assigned U.S. application Ser. No. 880,793, filed Jul. 1, 1986.
For example, plastic surfaces can be treated to increase their
hydrophilicity. Either the whole apparatus or specific parts can be
treated. Alternatively, different parts of the apparatus can be made of
different plastics. For capillary flow, contact angles of less than
90.degree. are sufficient, preferably 10.degree.-85.degree. and most
preferably 30.degree.-60.degree.. In order to provide these contact angles
for aqueous samples, the capillary surfaces will be hydrophilic (at least
to some measurable extent). For non-aqueous liquids, a hydrophobic surface
would be appropriate. By using a combination of container wall geometry
and surface wetability, a back pressure range of from 0 (no change in
cross-sectional area or surface adhesion) to 20 cm H.sub.2 O and higher
can easily be achieved with water as the liquid. When the back pressure is
0, the location in question is not a stop-flow junction. A stop-flow
junction occurs when there is sufficient back pressure to prevent the flow
of sample past a particular point in the flowpath; e.g., from the
measuring chamber to the receiving chamber of a dilution apparatus as
described herein.
When considering the amount of available back pressure for any given
design, the realities of manufacturing and of the physical world at the
microscopic level must be considered. Imperfections in the container walls
during gradual widening of chambers may cause liquid to "creep" more on
one side than another, thereby allowing the stop-flow junction to fail.
Liquid can also creep around corners when imperfections are present that
result in unbalanced forces. Unbalanced forces will also be present when
the junction is not horizontal. A horizontal junction, for example, occurs
when a vertical tube enters the top horizontal surface of a chamber. If a
horizontal tube enters a vertical wall of a container, a vertical junction
is present, and the pressure at the bottom of the stop-flow junction will
be greater than the pressure at the top of the junction, due to
hydrostatic pressure caused by the different heights of liquid.
Nonetheless, non-horizontal stop-flow junctions can be created by reducing
the diameter of the smaller channel containing liquid as it enters the
larger area, thereby reducing the difference in pressure between the upper
and lower portions of the junction, and other manufacturing imperfections
can be alleviated by quality control operations, although with increased
costs of manufacturing.
U.S. Pat. No. 4,426,451, which was developed in other laboratories,
describes a number of regions that it refers to as "meniscus control
means" for use in a device in which there is capillary flow from one
capillary zone to another. The meniscus control means described in that
patent can be used in apparatuses in which capillary/capillary
transactions and temporary stoppage of flow is desired before flow
continues into the next zone. However, the patent is not directed to
stopping flow when the second zone is not a capillary zone. In contrast to
the specific teachings of the '451 patent, which indicate that the walls
of the capillary chamber must gradually narrow and gradually expand in
order to provide for flow stop, an abrupt widening has been found to be
more effective in the practice of the present invention when the second
chamber is not a capillary space. Although it is recognized that
imperfections will exist on the molecular level, it is preferred that the
junction be as sharp as possible from a macroscopic view point,
approaching as closely as possible the ideal junction formed by the
intersection of the surface (which can be curved) forming the walls of the
measuring chamber with the surface forming the wall of the receiving
chamber surface in which the stop-flow junction is found (which can also
be curved). Maintaining a horizontal junction to avoid pressure
differentials, reducing the area of the junction, changing the surface of
the capillary so as to decrease the hydrophilic character (for aqueous
solutions), providing smooth surfaces (rough surfaces encourage creep of
liquid along the surface), and providing an abrupt change in
cross-sectional area (preferably providing an angle between intersecting
surfaces of about 90.degree. or lower) all operate to prevent creep of
liquid from one chamber to the other.
It should be recognized that flow stop can occur both stably and
metastably. A metastable flow stop is one in which flow stops on the
macroscopic level but may resume without apparent cause after a time
interval of a few seconds to a few minutes. Gradual creep of liquids along
container walls or through microscopic or submicroscopic channels
resulting from imperfections in the manufacturing process is believed to
be the mechanism by which flow starts again once it has stopped.
Additionally, vibrations (such as might be caused by persons walking near
the apparatus or starting and stopping of nearby equipment, such as
air-conditioning units) may also be sufficient to start flow in a
metastable situation. However, there is no requirement of absolute
stability in cases where an apparatus is designed for addition of a
diluent and eventual starting of flow at the stop-flow junction.
Accordingly, any flow stop which can be sustained for at least 10 seconds,
preferably at least one minute, and more preferably at least five minutes,
in sufficient for use in a diluter.
Although these prior stop-flow junctions were sufficient for most uses,
improvements in stability of the stop-flow junction against accidental
start has been desirable from the point of view of developing a commercial
apparatus. A number of factors contribute to the instability of the
junction. For example, variations in the sample physical properties (such
as density, viscosity, hematocrit, microheterogeneity, surface tension,
and contact angle with housing walls) can affect both the forward pressure
acting to favor flow and the back pressure available at the stop-flow
junction to stop flow. Density controls the hydrostatic pressure at the
junction. Surface tension and contact angle determine the pressure that
the junction can exert in opposition to flow. Viscosity determines the
rate at which the sample moves to the junction and therefore the excess
back pressure (over that necessary for an equilibrium state) required to
prevent the momentum of the sample from breaking through the junction.
Hematocrit of blood sample affects both viscosity and density.
Microheterogeneity has an impact on local properties at the junction,
which can vary significantly from the bulk properties of the sample. Other
variations include sample volume, which affects hydrostatic pressure by
varying the height of the upper sample surface above the junction; method
of sample application by different uses (or the same user at different
times); variations from lot to lot of the physical properties, such as
contact angle with a standard liquid, of the housing out of which the
diluter is made; variations in the size and shape of the junction arising
during manufacturing, such as can be caused by plastic "burrs" at corners
and edges; and local external factors, such as mechanical vibrations
caused by nearby machinery or foot travel, as well as variations in
orientation of the diluter from a horizontal operating position.
While it is possible for any of the previous diluters arising out of the
inventors' laboratory to be used despite these potential problems, such as
by designing a monitor in which the diluter will be used that is capable
of detecting when flow accidentally starts prior to the desired time,
improvement of the reliability of operation is highly desirable. For
example, few patients desire having a second finger puncture for the
purpose of obtaining a second blood sample. In other cases, the patient
may have left and no more sample may be available, thereby inconveniencing
both the patient and the physician. Thus, there remains a need for
improved stop-flow junctions having increased stability against accidental
fluid flow and for diluters that incorporate these improved features.
RELEVANT LITERATURE
West German published patent application DE3328964C1, publication date Feb.
14, 1985, describes a device for the automatic, discontinuous sampling of
fluids using a capillary tube that acts as a measuring device and which
can be either dipped into a fluid being sampled or alternatively moved
into a position from which the sample is transported with a diluent to an
analyzer by a pump or suction. U.S. Pat. No. 4,454,235 describes a
capillary tube holder for liquid transfer in immunoassays. U.S. Pat. No.
4,233,029 describes a liquid transport device formed by opposed surfaces
spaced apart a distance effective to provide capillary flow of liquid
without providing any means to control the rate of capillary flow. U.S.
Pat. Nos. 4,618,476 and 4,233,029 describe a similar capillary transport
device having speed and meniscus control means. U.S. Pat. No. 4,426,451
describes another similar capillary transport device including means for
stopping flow between two zones, flow being resumed by the application of
an externally-generated pressure. U.S. Pat. Nos. 3,811,326; 3,992,150;
4,537,747; and 4,596,780 describe various processes and devices in which a
capillary tube is used to take up a predetermined volume of the test
solution and the charged capillary is then placed in a cuvette or other
container of liquid that is used as reagent or diluent. U.S. Pat. No.
3,799,742 describes an apparatus in which a change in surface character
from hydrophilic to thereby metering the sample present. U.S. Pat. No.
5,077,017 and U.S. Pat. No. 4,868,129, both of which are assigned to the
same assignee as the present application, described a number of dilution
and mixing cartridges.
SUMMARY OF THE INVENTION
The present invention provides an improved stop-flow junction for use in,
among other potential locations, a self-contained dilution apparatus that
does not require the use of externally generated force (except gravity) to
move liquids between its various parts or to provide for reproducible
dilution of samples. The principal motive force in such devices arises
from capillarity and gravity (resulting in hydrostatic pressure), thus
giving rise to the name stop-flow junction, since a stop-flow junction
occurs at the junction of a capillary region and a region where flow does
not occur solely as a result of capillarity and gravity.
Stop-flow junctions are described herein that provide increased stability
in the "stop" state. A series of individual improvements are available in
accordance with the present invention, or all of the improvement can be
present in the same device. Specifically, the device of the invention
comprises a capillary stop-flow junction located in a housing at an end of
a capillary passageway for transporting a liquid and at the beginning of a
non-capillary chamber, in which an improvement is present which comprises:
a. means for selectively trapping a gas in said capillary passageway and
non-capillary chamber, wherein when said means for trapping is activated
and said liquid enters said capillary passageway, said gas is compressed
by said liquid as said liquid flows through said capillary channel and
stops flowing at said stop-flow junction; or
b. a stop-flow nozzle surrounding said capillary passageway and projecting
into said chamber;
c. a stop-flow junction formed from a single housing body member; or
d. a rupture junction in said capillary pathway, wherein said rupture
junction is a stop-flow junction providing less back pressure than said
capillary stop-flow junction.
One, some, or all of these improvements can be present in a single
stop-flow junction of the invention.
The improved stop-flow junctions of the invention can be used in a diluter
that, in addition to containing the improved stop-flow junctions, also
provides other advantages because of its improved design, such as
improvement in reproducibility of sample measurement and dilution control.
The improved diluter is an apparatus for automatically carrying out a
dilution of an aqueous sample with one or more aqueous diluents in a
housing, comprising in said housing:
(1) a sample application site for receiving a sample;
(2) a rupture chamber comprising a vented interior chamber;
(3) a mixing chamber comprising a vented interior chamber having a first
volume;
(4) a diluent application site for receiving a diluent;
(5) capillary flow means comprising:
(a) a central valved segment having a first and a second end;
(b) a valve located in said central valved segment;
(c) a sample segment connecting said sample application site to said first
end of said central valved segment;
(d) a rupture segment connecting said rupture chamber to said first end of
said central valved segment; and
(e) a measuring segment connected to said second end of said central valved
segment and having first and second exits, wherein said first exit
connects said measuring segment to said mixing chamber and wherein said
measuring segment has a second volume smaller than said first volume of
said mixing chamber;
(f) a first stop-flow junction located at said first exit of said measuring
segment and adapted to the surface-tension characteristics of the sample
so as to provide sufficient back pressure resulting from contact between
the sample and wall means of said housing at said first stop-flow junction
to prevent sample from flowing through said first stop-flow junction in
the absence of diluent;
(g) a second-stop flow junction located at said second exit of said
measuring segment and adapted to the surface-tension characteristics of
the sample so as to provide sufficient back pressure resulting from
contact between the sample and wall means of said housing at said second
stop flow junction to prevent sample from flowing through said second
stop-flow junction in the absence of diluent; and
(h) a third stop-flow junction located at the junction of said rupture
segment and said rupture chamber and adapted to the surface-tension
characteristics of the sample so as to provide sufficient back pressure
resulting from contact between said sample and wall means of said housing
at said third stop flow junction to prevent sample from flowing through
said third stop-flow junction in the absence of diluent, wherein said
third stop-flow junction provides less maximum-available back pressure
than said first stop-flow junction;
whereby addition of sample to said sample application site causes sample to
fill said capillary flow means; and
(6) diluent flow means connecting said diluent application site to said
second exit of said measuring segment.
BRIEF DESCRIPTION OF THE DRAWINGS
The present invention will be better understood by reference to the
following detailed description of the invention when considered in
conjunction with the attached drawings that form a part of the present
specification, wherein:
FIG. 1 is a vertical cross-section of a first embodiment of the invention
showing a vent-assisted stop-flow junction.
FIG. 2 is a vertical cross-section of a second embodiment of the invention
showing a stop-flow nozzle.
FIG. 3A is a vertical cross-section of a prior-art stop-flow junction
showing a stop-flow junction formed at the junction of two separate
housing members that have been welded together.
FIG. 3B is a vertical cross-section taken along line B--B of the embodiment
shown in 3A.
FIG. 4 is a vertical cross-section of a further embodiment of the invention
showing a through-body stop-flow junction of the invention.
FIG. 5 is a vertical cross-section of still another embodiment of the
invention showing a rupture junction in the capillary pathway that
contains a stop-flow junction that is being stabilized.
FIG. 6 is a vertical cross-section of a diluter of the invention showing a
stop-flow junction having the principal features of the stop-flow junction
embodiments of FIGS. 1, 2, 4, and 5 along with other features of the
diluter as a whole.
FIGS. 7A through 7J are a series of vertical cross-sections of the
embodiment of FIG. 6 taken at locations A--A through J--J of the
embodiment FIG. 6.
FIG. 8 in a schematic diagram of chemistry associated with a specific
analysis that can be carried out in the embodiment of FIGS. 6 and 7.
DESCRIPTION OF THE SPECIFIC EMBODIMENTS
I. New stop-flow junction
A. General background
The present invention provides an improved stop-flow junction for use in
apparatuses that require stoppage of capillary flow followed by controlled
restart of flow. Such stop-flow junctions are particularly useful in
apparatuses and methods in which small samples are automatically measured
and diluted. Such apparatuses are generally small, convenient to use, and
require no moving parts for the movement of fluid, with gravity and
capillary action being sufficient to provide all fluid motive forces
required for the sample measurement and dilution steps. Such dilution and
mixing cartridges are described in U.S. Pat. No. 4,868,129, U.S. Pat. No.
5,077,017, and U.S. Pat. No. 5,104,813. However, the apparatuses of the
present invention provide a number of improvements in stop-flow junctions
relative to those described in previous dilution and mixing apparatuses,
particularly in ease of manufacture and reliability of operation for large
numbers of diluters made from the same mold. Among the specific
improvement of the present apparatus are (1) means for selectively
trapping a gas in a capillary passageway and non-capillary chamber
adjacent to a stop-flow junction, wherein when said means for trapping is
activated and a liquid enters said capillary passageway, said gas is
compressed by said liquid as said liquid flows through said capillary
channel and stops flowing at said stop-flow junction; (2) a stop-flow
nozzle surrounding a capillary passageway and projecting into a chamber,
with the stop-flow junction being at the entrance of the capillary
passageway into the chamber; (3) a stop-flow junction formed from a single
housing body member; and (4) a rupture junction in a capillary pathway,
wherein said rupture junction is a stop-flow junction providing less
maximum available back pressure than said capillary stop-flow junction.
Each of these improvements, which can occur alone or in combination with
any other of these improvements, is discussed in detail below.
The basic features of a stop-flow junction are described in the patents and
patent applications identified above in the background section of this
application. There are two required parts to a stop-flow junction, the
first of which is a region in a fluid pathway in which fluid flow occurs
either solely under the influence of capillary action or under the
combined influence of capillary action and gravity. The junction exists at
the end of this region of free flow at a transition to a region at which
capillary flow will cease, even in the presence of a gravitationally
derived pressure arising from a liquid head above the capillary-stop
junction. Well-known examples of capillary junctions exist in familiar
devices, such as a capillary tube used for obtaining blood samples from a
finger puncture. In such a simple device, the stop-flow junction is the
end of the capillary tube, since capillary forces retain sample inside the
tube, even when the tube is oriented vertically and gravitational forces
are present on the sample. Other examples are described in the previously
discussed publications and patent applications.
B. Vent-assisted stop-flow junction
The first of the improvements that have been recognized and developed by
the current inventors is a technique (and associated apparatuses) in which
a gas (usually air from the atmosphere surrounding the apparatus in which
the stop-flow junction is located) is trapped and compressed when a liquid
enters the capillary portion of the passageway and flows through the
passageway to the stop-flow junction. The trapping must be selective since
the trapped gas will need to be vented in order for flow to continue
unimpeded to other parts of the apparatus at an appropriate time. By
properly selecting sizes of the compressed air space relative to the
gravitational and capillary forces present in the device, reliability of
flow stoppage at the stop-flow junction can be increased many fold over.
Since the volume of the trapped gas is manipulated most easily by changing
the size of the vent channels and chambers, this aspect is referred to as
a vent-assisted stop-flow junction.
The operation of a vent-assisted capillary stop-flow junction is readily
understood by reference to FIG. 1 and the mode of operation of the
apparatus shown in the figure. However, it should be recognized that this
is not the sole embodiment by which the present invention can operate and
that the embodiment shown in FIG. 1 is merely exemplary of this aspect of
the invention.
FIG. 1 is a vertical, cross-sectional schematic drawing of a dilution
apparatus having a vent-assisted stop-flow junction. The diluter shown in
FIG. 1 is similar to the single-dilution apparatus described in U.S. Pat.
No. 4,868,129 with the additional flow directing chamber of U.S. Pat. No.
5,104,813. Reference may be made to this earlier patent and patent
application for detail on the various parts of the apparatus. The present
discussion will address the vent-assisted stop-flow junction without
prolonged discussion of other aspects of the device.
Cartridge 100 contains a sample application site 110, a capillary channel
120 leading from sample application site 110 to flow directing chamber
130, capillary measuring chamber 140, mixing chamber 150, capillary
passageway 160 leading from flow directing chamber 130 to waste chamber
165, a rupturable container 175 of diluent in an internal chamber
functioning as a diluent application site 170, and a channel 180 leading
from the diluent application site to the flow directing chamber 130. All
of these parts of the apparatus have been previously described in earlier
patents and patent applications. Parts of the device relating specifically
to the vent-assisted feature include an initial capillary channel 101
leading to a relatively large interior chamber 102 referred to as a
vent-surge chamber, capillary channel 103 connecting vent-surge chamber
102 to the environment surrounding cartridge 100, where vent opening 104
exists to allow atmospheric gases to enter and leave venting channel 103
and other interior chambers of the device, and vent closure 105, which is
capable of being moved in the directions shown by the arrow to
alternatively close and open the vent at 104.
The operation of the vent-assisted stop-flow junction can readily be seen
from the following description and by reference to FIG. 1. Prior to
application of a sample to sample application site 110, vent closure 105
is moved to the left where it seals against the housing at vent 104. The
vent closure substantially seals the vent from the external environment.
Any means that accomplishes this result is satisfactory, such as providing
a flexible pad that presses against the surface of the housing at vent
exit 104; providing a close-fitting, smooth disc that contacts a
corresponding smooth surface on the housing; or any other effective means
of sealing off the internal space in the housing from the surrounding
atmosphere. The vent closure is typically operated by a monitor into which
the housing has been inserted.
After the vent is closed, sample is applied at sample application site 110.
Sample flows through capillary 120 to flow directing chamber 130 and then
into measuring chamber 140. When sample first enters measuring chamber
140, it creates a sealed interior space consisting of measuring chamber
140, mixing chamber 150, and any venting spaces. In the embodiment shown
in FIG. 1, the venting spaces consist of capillary channels 101 and 103
and vent-surge tank 102. However, this vent-surge tank is included merely
to provide an appropriate volume for the trapped air or other gas present
in the indicated chambers and is therefore optional. If measuring chamber
140, mixing chamber 150, and the vent spaces leading to vent exit 104
provide the desired compressible volume of air, no vent-surge chamber 102
is required. As sample flows down capillary measuring chamber 140, the air
trapped in the enclosed space is compressed. This compressed air will act
to oppose the forward motion of the liquid in the measuring chamber and
thus act to stabilize stop-flow junction 145 at the intersection of
measuring chamber 140 and mixing chamber 150.
Earlier applications from the laboratories of the present inventors have
described vent closur | | |
