U.S. patent application number 14/316069 was filed with the patent office on 2015-07-16 for structures for controlling light interaction with microfluidic devices.
This patent application is currently assigned to OPKO Diagnostics, LLC. The applicant listed for this patent is OPKO Diagnostics, LLC. Invention is credited to Vincent Linder, David Steinmiller.
Application Number | 20150196908 14/316069 |
Document ID | / |
Family ID | 42199650 |
Filed Date | 2015-07-16 |
United States Patent
Application |
20150196908 |
Kind Code |
A9 |
Steinmiller; David ; et
al. |
July 16, 2015 |
STRUCTURES FOR CONTROLLING LIGHT INTERACTION WITH MICROFLUIDIC
DEVICES
Abstract
Systems and methods for improved measurement of
absorbance/transmission through fluidic systems are described.
Specifically, in one set of embodiments, optical elements are
fabricated on one side of a transparent fluidic device opposite a
series of fluidic channels. The optical elements may guide incident
light passing through the device such that most of the light is
dispersed away from specific areas of the device, such as
intervening portions between the fluidic channels. By decreasing
the amount of light incident upon these intervening portions, the
amount of noise in the detection signal can be decreased when using
certain optical detection systems.
Inventors: |
Steinmiller; David;
(Cambridge, MA) ; Linder; Vincent; (Tewksbury,
MA) |
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Applicant: |
Name |
City |
State |
Country |
Type |
OPKO Diagnostics, LLC |
Woburn |
MA |
US |
|
|
Assignee: |
OPKO Diagnostics, LLC
Woburn
MA
|
Prior
Publication: |
|
Document Identifier |
Publication Date |
|
US 20140308168 A1 |
October 16, 2014 |
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|
Family ID: |
42199650 |
Appl. No.: |
14/316069 |
Filed: |
June 26, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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13898028 |
May 20, 2013 |
8802029 |
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14316069 |
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13490055 |
Jun 6, 2012 |
8480975 |
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13898028 |
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12698451 |
Feb 2, 2010 |
8221700 |
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13490055 |
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61149253 |
Feb 2, 2009 |
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Current U.S.
Class: |
422/69 ; 422/503;
422/82.05; 422/82.09 |
Current CPC
Class: |
B01L 2300/0627 20130101;
G01N 33/53 20130101; B01L 2300/04 20130101; G01N 2333/96455
20130101; B01L 3/5027 20130101; G01N 33/57488 20130101; G01N 21/59
20130101; G01N 33/54366 20130101; G01N 33/54373 20130101; B01L
3/502715 20130101; B01L 2300/0654 20130101; G01N 33/545 20130101;
B01L 2300/0861 20130101; Y10T 436/11 20150115; G01N 21/0303
20130101; B01L 3/502707 20130101; C12Y 304/21077 20130101; B01L
2300/12 20130101; B01L 2200/12 20130101 |
International
Class: |
B01L 3/00 20060101
B01L003/00; G01N 21/59 20060101 G01N021/59; G01N 33/53 20060101
G01N033/53 |
Claims
1. A fluidic device comprising: an article comprising first and
second sides; first and second microfluidic channel segments, each
positioned at the first side of the article; and a first optical
element positioned at the second side of the article and positioned
substantially between the first and second channel segments,
wherein at least a portion of the first optical element has a
substantially triangular cross-section.
2. A fluidic device as in claim 1, wherein the first optical
element is integral to the article.
3. A fluidic device as in claim 1, wherein the first optical
element is integral to a surface of the article.
4. A fluidic device as in claim 1, wherein the first optical
element is adapted and arranged such that when the second side of
the article is exposed to light at a first intensity at an angle of
incidence between 80.degree. to 95.degree., a surface portion of
the first side of the article is not exposed to light or is exposed
to the light at a second intensity lower than the intensity of the
light at the surface portion absent the first optical element.
5. A fluidic device as in claim 1, comprising a second optical
element.
6. A fluidic device as in claim 5, wherein the second optical
element is positioned at the second side of the article, and
wherein the first or second microfluidic channel segment is
positioned between the first and second optical elements.
7. A fluidic device as in claim 5, wherein no line drawn between
any first point on or within the first optical element and any
second point on or within the second optical element intersects any
point on or within the first or second microfluidic channel
segment.
8. A fluidic device as in claim 1, wherein no line drawn between
any first point on or within the first microfluidic channel segment
and any second point on or within the second microfluidic channel
segment intersects any point on or within the first optical
element.
9. A fluidic device as in claim 1, comprising a second microfluidic
channel segment, wherein the first and second microfluidic channel
segments are sections of a microfluidic channel comprising a
meandering configuration including multiple turns, each turn of the
meandering channel being a different microfluidic channel
segment.
10. A fluidic device as in claim 1, wherein the first and/or second
microfluidic channel segment has a length of at least 1 cm.
11. A fluidic device as in claim 1, wherein the first side is a
first surface of the article.
12. A fluidic device as in claim 11, wherein the second side is a
second surface of the article.
13. A fluidic device as in claim 1, wherein the first optical
element comprises a cross-sectional dimension of at least about 50
microns and less than 1 cm.
14. A fluidic device as in claim 1, wherein the first optical
element comprises a substantially opaque material.
15. A fluidic device as in claim 1, wherein the first optical
element comprises a reflective surface.
16. A fluidic device as in claim 1, wherein the first optical
element comprises an open channel.
17. A fluidic device as in claim 1, wherein the first optical
element comprises a substantially enclosed channel.
18. A fluidic device as in claim 1, wherein the first optical
element is substantially transparent to visible light.
19. A fluidic device as in claim 1, wherein the first optical
element has a draft angle between about 12.degree. and about
60.degree..
20. A fluidic device as in claim 1, wherein the first optical
element has a draft angle between about 30.degree. and about
40.degree..
21. A fluidic device as in claim 1, wherein the article is a
single, integral piece of material without joined layers.
22. A fluidic device as in claim 1, wherein the article includes
joined layers.
23. A fluidic device as in claim 1, wherein the article is formed
of a polymeric material, wherein the polymeric material is selected
from polystyrene, cyclo-olefin-copolymer, polymethylmethacrylate,
and polycarbonate.
24. A fluidic device as in claim 1, wherein the article is made by
injection molding.
25. A fluidic device as in claim 1, wherein the first optical
element is adapted and arranged to direct at least 50% of the light
exposed to the first optical element away from the surface portion
of the first side of the article.
26. A fluidic device as in claim 1, comprising an intervening
portion positioned substantially between the first and second
microfluidic channel segments, wherein the first optical element
has a longest width that is greater than or equal to the width of
the intervening portion, but less than the combination of the
widths of the first and second microfluidic channel segments and
the width of the intervening portion.
27. A fluidic device as in claim 1, further comprising a cover
positioned over the first and second microfluidic channel segments
to substantially enclose the first and second microfluidic channel
segments.
28. A fluidic device as in claim 1, further comprising an analysis
region in fluid communication with at least the first microfluidic
channel segment.
29. A fluidic device as in claim 28, wherein the analysis region
comprises a binding partner associated with a surface of the first
microfluidic channel segment.
30. A fluidic device as in claim 29, wherein the binding partner
comprises an antibody or an antigen.
31. A fluidic device as in claim 1, further comprising a detector
associated with one or more microfluidic channel segments, wherein
the detector is adapted and arranged to determine light
transmission and/or absorbance through the one or more microfluidic
channel segments.
32. A fluidic device as in claim 1, wherein the first optical
element is adapted and arranged such that when a portion of the
article is exposed to light at a first intensity, the first optical
element redirects at least a portion of the light away from the
surface portion of the first side of the article, such that the
surface portion is exposed to the light at a second intensity at
least 25% lower than an intensity of the light at the surface
portion absent the first optical element.
33. A fluidic device as in claim 1, wherein the first optical
element is adapted and arranged such that when a portion of the
article is exposed to light at a first intensity, the first optical
element redirects at least a portion of the light away from the
surface portion of the first side of the article, such that the
surface portion is exposed to the light at a second intensity at
least 75% lower than an intensity of the light at the surface
portion absent the first optical element.
34. A fluidic device as in claim 32, wherein the second intensity
is at least 50% lower than the first intensity.
Description
RELATED APPLICATIONS
[0001] This is a continuation application of U.S. patent
application Ser. No. 13/898,028, filed May 20, 2013, and entitled
"Structures for Controlling Light Interaction with Microfluidic
Devices," which is a divisional application of U.S. patent
application Ser. No. 13/490,055, filed Jun. 6, 2012, and issued as
U.S. Pat. No. 8,480,975 on Jul. 9, 2013, and entitled "Structures
for Controlling Light Interaction with Microfluidic Devices," which
is a continuation of U.S. patent application Ser. No. 12/698,451,
filed Feb. 2, 2010 and issued as U.S. Pat. No. 8,221,700 on Jul.
17, 2012, and entitled "Structures for Controlling Light
Interaction with Microfluidic Devices," which claims priority under
35 U.S.C. .sctn.119(e) to U.S. Provisional Patent Application No.
61/149,253, filed Feb. 2, 2009, and entitled "Structures for
Controlling Light Interaction with Microfluidic Devices," each of
which is incorporated herein by reference in its entirety for all
purposes.
FIELD OF INVENTION
[0002] The present invention relates generally to microfluidic
systems, and more specifically, to systems and methods for
controlling light interaction with microfluidic devices.
BACKGROUND
[0003] Optical analysis of fluids plays an important role in fields
such as chemistry, microbiology and biochemistry. These fluids may
include liquids or gases and may provide reagents, solvents,
reactants, or rinses to chemical or biological processes. While
various microfluidic methods and devices, such as microfluidic
assays, can provide inexpensive, sensitive and accurate analytical
platforms, carrying out accurate optical measurements (e.g.,
absorbance or transmission) on a microfluidic system can be
challenging. Optical measurements of microchannels may require, for
example, time-consuming alignment procedures. In addition, optical
noise produced by light incident upon areas outside the channels
may degrade the quality of the detected signal through the
channels. Accordingly, advances in the field that could reduce
costs, simplify use, and/or improve optical detection in
microfluidic systems would be beneficial.
SUMMARY OF THE INVENTION
[0004] Systems and methods for controlling light interaction with
microfluidic devices are provided. The subject matter of the
present invention involves, in some cases, interrelated products,
alternative solutions to a particular problem, and/or a plurality
of different uses of one or more systems and/or articles.
[0005] In one set of embodiments, a series of fluidic devices are
provided. In one particular embodiment, a fluidic device comprises
an article including first and second opposing sides and first and
second microfluidic channel segments, each integral to the first
side of the article. The fluidic device also includes an
intervening portion positioned substantially between the first and
second microfluidic channel segments, and a first optical element
integral to the second side of the article and positioned
substantially between the first and second channel segments, and
opposite the intervening portion. The first optical element is
adapted and arranged such that when a portion of the article is
exposed to light at a first intensity, the first optical element
redirects at least a portion of the light away from the intervening
portion, such that the intervening portion is not exposed to the
light or is exposed to the light at a second intensity lower than
an intensity of the light at the intervening portion absent the
first optical element.
[0006] In another embodiment, a fluidic device comprises an article
comprising first and second sides, a first microfluidic channel
segment integral to the first side of the article, and first and
second optical elements, each integral to the second side of the
article, wherein the first microfluidic channel segment is
positioned substantially between the first and second optical
elements. A cover is positioned over the first microfluidic channel
segment so as to substantially enclose the first microfluidic
channel segment. Furthermore, an intervening surface portion at the
second side of the article is positioned substantially between the
first and second optical elements, the intervening surface portion
being substantially parallel to a surface portion of the cover that
substantially encloses the first microfluidic channel segment.
[0007] In another embodiment, a fluidic device comprises an article
comprising first and second sides, and first and second
microfluidic channel segments, each integral to the first side of
the article. The fluidic device also includes a first substantially
triangular optical element integral to the second side of the
article and positioned substantially between the first and second
channel segments.
[0008] In some instances, the first and/or second microfluidic
channel segments described above and herein are sections of a
microfluidic channel comprising a meandering configuration
including multiple turns, each turn of the meandering channel being
a different channel segment.
[0009] Other advantages and novel features of the present invention
will become apparent from the following detailed description of
various non-limiting embodiments of the invention when considered
in conjunction with the accompanying figures. In cases where the
present specification and a document incorporated by reference
include conflicting and/or inconsistent disclosure, the present
specification shall control. If two or more documents incorporated
by reference include conflicting and/or inconsistent disclosure
with respect to each other, then the document having the later
effective date shall control.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] Non-limiting embodiments of the present invention will be
described by way of example with reference to the accompanying
figures, which are schematic and are not intended to be drawn to
scale. In the figures, each identical or nearly identical component
illustrated is typically represented by a single numeral. For
purposes of clarity, not every component is labeled in every
figure, nor is every component of each embodiment of the invention
shown where illustration is not necessary to allow those of
ordinary skill in the art to understand the invention. In the
figures:
[0011] FIGS. 1A-1E include schematic diagrams of a device including
optical elements that can be used to control light interaction on
or within the device, according to one set of embodiments;
[0012] FIGS. 2A-2B include schematic cross-sectional diagrams of
devices showing light interactions in the devices, according to one
set of embodiments;
[0013] FIGS. 3A-3C include, according to one set of embodiments,
schematic diagrams of channel configurations in certain
devices;
[0014] FIGS. 4A-4B include cross-sectional diagrams illustrating a
fabrication process, according to one set of embodiments;
[0015] FIG. 5 includes a plot of optical density as a function of
dye concentration, according to one set of embodiments;
[0016] FIGS. 6A-6D include cross-sectional schematic diagrams and
associated plots of transmitted light as a function of detector
position, according to one set of embodiments;
[0017] FIGS. 7A-7C include cross-sectional schematic diagrams,
optical micrographs, and a plot of optical density as a function of
dye concentration, according to one set of embodiments;
[0018] FIGS. 8A-8C include schematic diagrams illustrating various
sensor layouts, according to one set of embodiments;
[0019] FIGS. 9A-9D include plots of transmitted light as a function
of sensor position, according to one set of embodiments; and
[0020] FIG. 10 includes a plot of optical density as a function of
dye concentration, according to one set of embodiments.
DETAILED DESCRIPTION
[0021] Systems and methods for improved measurement of
absorbance/transmission through fluidic systems are described.
Specifically, in one set of embodiments, optical elements are
fabricated on one side of a transparent fluidic device opposite a
series of fluidic channels. The optical elements may guide incident
light passing through the device such that most of the light is
dispersed away from specific areas of the device, such as
intervening portions between the fluidic channels. By decreasing
the amount of light incident upon these intervening portions, the
amount of noise in the detection signal can be decreased when using
certain optical detection systems. In some embodiments, the optical
elements comprise triangular grooves formed on or in a surface of
the device. The draft angle of the triangular grooves may be chosen
such that incident light normal to the surface of the device is
redirected at an angle dependent upon the indices of refraction of
the external medium (e.g., air) and the device material.
[0022] Advantageously, certain optical elements described herein
may be fabricated along with the fluidic channels of the device in
one step, thereby reducing the costs of fabrication. Furthermore,
in some cases the optical elements do not require alignment with a
detector and, therefore, facilitate assembly and/or use by an end
user. Other advantages are described in more detail below.
[0023] Additional techniques may be employed to reduce the amount
of stray light transmitted through the fluidic device. For example,
in some instances, the widths of the intervening portions between
the channel segments may be reduced. Also, the light source may be
arranged such that light is emitted only over portions of the
device that lie above the channel segments. Both of these
techniques may reduce the amount of light transmitted through the
intervening portions, thus improving the quality of the optical
image. In some embodiments, the fluidic device may include a
detector array arranged such that the areas of the array under the
channel segments are sensitive to light, while the other areas of
the array are not.
[0024] The systems and methods described herein may find
application in a variety of fields. In some cases, the systems and
methods can be used to improve the optical performance of any
microfluidic system such as, for example, microfluidic
point-of-care diagnostic platforms, microfluidic laboratory
chemical analysis systems, optical monitoring systems in cell
cultures or bio-reactors, among others. Optical measurements in
microfluidic systems may be used to monitor any suitable chemical
and/or biological reaction as it takes place, diagnostic or
otherwise. As a specific example, an optical measurement step can
be used during DNA synthesis to verify the yield of each base
addition (e.g., optical trityl monitoring) and during some forms of
PCR amplification to monitor the process.
[0025] Previous systems, such as those described in International
Patent Publication No. WO2006/113727 (International Patent
Application Serial No. PCT/US2006/014583), filed Apr. 19, 2006 and
entitled "Fluidic Structures Including Meandering and Wide
Channels," have made use of a meandering microchannel to image a
two-dimensional space. For example, a microfluidic channel may be
in the form of a tight "S" shape having multiple channel segments,
forming an area of about 2 mm, e.g., a "measurement area" including
both channel and non-channel regions. In certain embodiments, this
measurement area does not require fine alignment for optical
measurements (unlike a single straight channel) and forms a
measurement area which can be easily interrogated optically. For
instance, a detector may be positioned over all or a portion of the
measurement area made up of channel and non-channel regions. One
limitation of the use of meandering structures in the context of
transmission measurement, though, is that some of the light shining
through these measurement areas will pass through the intervening
portions between the microfluidic channel segments (that is, the
non-channel regions). This light may reach the optical detector
without reflecting changes in the optical density of the contents
of the microchannel. This "stray light" can reduce the overall
performance of the optical detection. This effect may be
particularly problematic when making measurements of channels with
high levels of optical density. A large amount of stray light on
the detector may wash out any changes in small amounts of light
passing through the microchannels.
[0026] The inventors have discovered within the context of the
invention that the amount of light that passes through an
intervening portion between microfluidic channels or channel
segments may be reduced or substantially eliminated by fabricating,
in the device, at least one optical element. The optical element
may redirect at least a portion of the light away from the
intervening portion, such that the intervening portion is not
exposed to the light or is exposed to the light at a second
intensity lower than an intensity of light to which the intervening
portion would be exposed in the absence of the optical element. The
incorporation of optical elements into microfluidic channel systems
enhances the performance of the detection system, allowing the use
of simplified optics without compromising the quality of the
optical measurements.
[0027] Furthermore, the systems and methods described herein may be
used to improve alignment in micro-scale optical detection systems.
Certain methods for optical detection/measurements in microsystems
are challenging in that they require accurate alignment of the
optics with micro-scale features (e.g., microchannels). Such
alignment can be performed manually (e.g., with a microscope and
micrometric stage) in a labor-intensive fashion, or in an automated
manner (e.g., using complex robotic positioning systems). These
techniques, however, often require a skilled and attentive operator
or expensive, delicate automation, making them suboptimal for
certain applications. The ability of the optical elements to
redirect light away from one or more intervening portions between
microfluidic channel segments may eliminate or reduce the need for
such complicated alignment procedures.
[0028] Additionally, the positioning of a detector over a
measurement area without the need for precision is an advantage,
since external (and possibly, expensive) equipment such as
microscopes, lenses, and alignment stages may not be required.
Instead, alignment can be performed by eye, or by low-cost methods
that may not require an alignment step by the user. For example, in
one embodiment, a fluidic device comprising one or more optical
elements and a measurement area including both channel and
non-channel regions can be placed in a simple holder (i.e., in a
cavity having the same shape as the fluidic device), and the
measurement area can be automatically aligned with a beam of light
of the detector.
[0029] It should be noted that the systems and methods described
herein may be used for guiding light in any suitable system
utilizing microfabricated structures, and are not limited to
microfluidic systems and/or the specific channel configurations
described herein.
[0030] Additional advantages of devices including optical elements
constructed to redirect light are described in more detail
below.
[0031] The articles, systems, and methods described herein may be
combined with those described in International Patent Publication
No. WO2005/066613 (International Patent Application Serial No.
PCT/US2004/043585), filed Dec. 20, 2004 and entitled "Assay Device
and Method"; International Patent Publication No. WO2005/072858
(International Patent Application Serial No. PCT/US2005/003514),
filed Jan. 26, 2005 and entitled "Fluid Delivery System and
Method"; International Patent Publication No. WO2006/113727
(International Patent Application Serial No. PCT/US06/14583), filed
Apr. 19, 2006 and entitled "Fluidic Structures Including Meandering
and Wide Channels"; U.S. patent application Ser. No. 12/113,503,
filed May 1, 2008 and entitled "Fluidic Connectors and Microfluidic
Systems"; U.S. patent application Ser. No. 12/196,392, filed Aug.
22, 2008, entitled "Liquid containment for integrated assays"; U.S.
patent application Ser. No. 12/428,372, filed Apr. 22, 2009,
entitled "Flow Control in Microfluidic Systems"; U.S. Patent Apl.
Ser. No. 61,263,981, filed Nov. 24, 2009, entitled "Fluid Mixing
and Delivery in Microfluidic Systems"; and U.S. patent application
Ser. No. 12/640,420 filed on Dec. 17, 2009 and entitled, "Improved
Reagent Storage in Microfluidic Systems and Related Articles and
Methods," each of which is incorporated herein by reference in its
entirety for all purposes. In addition, U.S. Provisional Patent
Application Ser. No. 61/149,253, filed Feb. 2, 2009, entitled
"Structures for Controlling Light Interaction with Microfluidic
Devices," is incorporated herein by reference in its entirety for
all purposes.
[0032] Examples of fluidic devices and methods associated therewith
are now provided.
[0033] FIGS. 1A-1E show various portions of a fluidic device
including optical elements that can be used to control light
interaction on or within the device. FIG. 1A shows a cross section
and FIG. 1B shows a perspective view of a fluidic device 10 which
includes an article 12 having a first surface 14 and a second
surface 16, as well as a first side 20 and a second side 22.
[0034] As used herein, "first and second sides" of an article
generally refers to the relative orientation of two portions of the
article. First and second sides may refer to first and second
surfaces of the article, or to a portion of the article that does
not encompass a surface, e.g., a portion of the article that is
embedded within the bulk of the article. For example, first and
second microfluidic channel segments that are said to be integral
to the first side of the article may be integral to a surface at
the first side of the article or embedded within the article at the
first side. FIG. 1A also shows the first side opposing the second
side. Two sides are said to be "opposing" when they are
substantially parallel to each other and separated by a
distance.
[0035] As shown illustratively in FIGS. 1A and 1C, first side 20
includes a plurality of channel segments (first 26, second 28, and
third 30) formed therein. A channel segment refers to a portion of
a fluidic channel that spans an entire cross-section of the channel
and has a length substantially parallel to fluid flow. A channel
segment may have any suitable length, e.g., at least 1 mm, at least
5 mm, at least 1 cm, or at least 5 cm in certain embodiments. While
three channel segments are shown in FIG. 1A, systems and methods
described herein may comprise any suitable number of channel
segments and may be configured in any suitable arrangement. For
instance, channel segments of a device may be a part of the same
fluidic channel, or may be part of separate fluidic channels that
are not in fluid communication with one another.
[0036] In some embodiments, channel segments refer to a series of
repetitive units of one or more channels; for example, each channel
of an array of channels may be a channel segment. In another
example, a channel includes a plurality of reaction areas
positioned in series, and each channel portion associated with a
distinct reaction area is a channel segment. In certain cases,
channel segments are sections of a fluidic channel having a
meandering configuration, each "turn" of the meandering channel
being a different channel segment. As used herein, a "meandering
channel" (i.e., a channel having a meandering region) includes at
least a first segment that has a flow path in a first direction and
a second segment that has a flow path in a second direction
substantially opposite (e.g., greater than 135 degrees from) the
first direction. Often, a meandering channel will include more than
two alternating channel segments that extend in opposite
directions. Examples of meandering channel regions are provided
below.
[0037] In some embodiments, the two or more channel segments of a
device are spaced apart from each other by intervening portions,
i.e., non-channel portions. For instance, first side 20 includes
intervening portions 27 and 29. An intervening portion may include
portions of a surface of an article (e.g., surface portion 14' of
FIG. 1A) and/or a portion of the article that does not encompass a
surface (e.g., portions 15 of FIG. 1A). In some embodiments, an
intervening portion has one or more dimensions (e.g., a width,
height, and/or length) of at least 0.5 mm, at least 1 mm, at least
5 mm, at least 1 cm, or at least 5 cm in certain embodiments. A
dimension of an intervention portion may, for example, define the
distance between two channel segments.
[0038] The fluidic device illustrated in FIGS. 1A and 1C also
includes a cover 31 positioned over the plurality of channel
segments. The cover may be positioned over the channel segments so
as to substantially enclose the channel segments. In some
instances, the cover may comprise a tape (e.g., flexible tape),
glass (e.g., a cover slide), rigid plastic, or any other suitable
material as described in more detail below.
[0039] In the set of embodiments illustrated in FIGS. 1A and 1C,
second side 22 includes a plurality of optical elements (first 32
and second 34) formed therein. As used herein, the term "optical
element" is used to refer to any feature formed or positioned on or
in an article or device that changes the direction (e.g., via
refraction or reflection), focus, polarization, and/or other
property of incident electromagnetic radiation relative to the
light incident upon the article or device in the absence of the
element. For example, an optical element may comprise a lens (e.g.,
concave or convex), mirror, grating, groove, or other feature
formed or positioned in or on an article. An article itself absent
a unique feature, however, would not constitute an optical element,
even though one or more properties of incident light may change
upon interaction with the article.
[0040] FIGS. 1A and 1C also show an intervening surface portion 33
between the first and second optical elements. As shown in FIG. 1A,
the intervening surface portion may be substantially parallel to
the surface portion of the cover that substantially encloses the
microfluidic channel segments. While two optical elements are shown
in FIGS. 1A and 1C, the articles described herein may comprise any
suitable number of optical elements and any suitable number of
intervening surface portions between the optical elements.
Furthermore, some articles do not include intervening surface
portions between the optical elements, e.g., a series of optical
elements may be configured to form alternating ridges and grooves
in a corrugated fashion.
[0041] The optical elements described herein may be, in some cases,
substantially transparent (e.g., to visible light, infrared
radiation, etc.). In other embodiments, optical elements may
comprise a substantially opaque material. In some cases, optical
elements may comprise one or more reflective surfaces. For example,
an optical element may comprise a channel, the walls of which are
coated with a reflective material such as a metal (e.g., Ni, Ag,
Au, Pt) or a semi-conductor (e.g., Si, glass).
[0042] An optical element may comprise a groove, which may be open
or substantially enclosed. As shown in the embodiment illustrated
in FIGS. 1A and 1C, optical elements 32 and 34 are in the form of
grooves that are substantially triangular; however, other shapes
are also possible. For instance, in other embodiments, the
cross-section of an optical element may be of any suitable shape
such as a hemisphere, square, rectangle, trapezoid, etc. Some
optical elements have a semi-spherical or semi-ovular shape. The
shape and/or angle of the groove may be selected such that incident
light normal to the surface of the device is redirected away from
the area directly below the groove. This may enhance the
probability that little or no light is incident upon unwanted areas
around the channels (e.g., intervening portions 27 and 29),
reducing the amount of noise in the detection signal. Accordingly,
an optical element may have any suitable size, configuration,
and/or shape to achieve improvements in signal to noise, as
described in more detail below.
[0043] In some cases, an optical element may be a feature that
protrudes from a surface of an article. For example, FIG. 1D
includes triangular optical elements 32 and 34 formed in the shape
of a prism. It should be understood that "triangular" optical
elements include any elements that are triangular in cross-section,
whether formed in a substrate (e.g., as in FIG. 1A) or on a
substrate (e.g., as in FIG. 1D). Other shapes that may be formed
include, for example, half-cylinders, rectangular prisms, etc.
[0044] An optical element may comprise, in some cases, one or more
fluids (e.g., a dye). For example, in one set of embodiments, the
optical element is formed as a channel (e.g., by placing a cover
over surface 16 of the article) and the channel is filled with a
light-absorbing fluid such as an opaque dye. Dyes of any suitable
concentration may be used. In some embodiments, the concentration
of the dye may be at least about 0.1 grams, at least about 0.5
grams, at least about 1 gram, at least about 5 grams, at least
about 10 grams, at least about 50 grams, or at least about 100
grams of dye material per mL of solvent (e.g., water).
[0045] FIG. 1C shows a schematic diagram of an optical device
during operation, according to one set of embodiments. During
operation, fluidic device 10 is positioned between a light source
36 and an optical detector 38 such that first side 20 (comprising
one or more channel segments) faces the detector and second side 22
(comprising one or more optical elements) faces light source 36 and
is exposed to light 42. The detector may be associated with one or
more fluidic channel segments in the fluidic device, e.g., to
determine light transmission through one or more of the channel
segments. In one set of embodiments, when a portion of the article
is exposed to light at a first intensity, the optical elements
redirect at least a portion of the light away from the intervening
portions. For example, one or more optical elements may be adapted
and arranged so as to redirect at least a portion of the light away
from a surface portion of the first side, the surface portion being
adjacent to at least one channel segment. In FIG. 1C, optical
element 32 is adapted and arranged to redirect light away from
intervening portion 27, which includes surface portion 14'.
Similarly, optical element 34 is adapted and arranged to redirect
light away from intervening portion 29, which includes surface
portion 49. Redirecting light may include, for instance, reflecting
the light (e.g., away from the article), refracting the light
(e.g., through the article in a direction away from the intervening
portion), or both.
[0046] One or more optical elements may be constructed and arranged
to redirect at least about 10%, at least about 25%, at least about
50%, at least about 75%, or at least about 90% of incident light
away from an intervening portion. As light is redirected, the
intervening portions are not exposed to the light or are exposed to
the light at a second intensity lower than an intensity of the
light at the intervening portion absent the optical elements. For
example, in some cases, at least one optical element is adapted and
arranged such that the intervening portions are exposed to the
light at a second intensity at least about 50% lower, at least
about 75% lower, or at least about 90% lower than an intensity of
the light at the intervening portion absent the optical
elements.
[0047] In some embodiments, one or more optical elements are
adapted and arranged so as to redirect at least a portion of the
light away from the center plane (e.g., 32' and 34' in FIG. 1A) of
the optical element, such that the underlying portion of the device
directly below the optical element is not exposed to light or is
exposed to light at a second intensity lower (e.g., about 25%
lower) than the intensity to which the underlying portion would be
exposed were the optical element absent. As used herein, a region
is "directly below" an object when it lies on the side of an object
opposite that which is exposed to light from the source. The region
directly below an object may span the width of the object and the
depth of the article perpendicular to the outermost surface on
either the first or second side of the object. For example, in FIG.
1C, region 52 lies directly below optical element 34.
[0048] An example of the use of optical elements to redirect light
is shown in FIG. 1C. As illustrated in FIG. 1C, portion 44 of
article 12 is exposed to light 42 from the light source. In some
cases, the light source and device are oriented such that the angle
of incidence of the light on surface 16 is between about 85.degree.
and about 95.degree., or at substantially 90.degree.. At surface
16, light that is incident on areas absent the optical elements is
transmitted into the bulk of the device without a substantial
change in direction, as indicated by arrows 42. Light incident upon
optical element 32, however, interacts at an angle substantially
different than 90.degree.. Optical element 32 redirects at least a
portion of the light away from intervening portion 27, e.g., by
reflection, refraction, and/or both. Arrows 46' represent light
that is reflected away from the optical element, while arrows 46
illustrate light that is refracted through the article, but away
from intervening portions between the fluidic channels. Thus,
intervening portion 27 is not exposed to the light, or is exposed
to the light at an intensity less than it would have been in the
absence of optical element 32. Furthermore, although not shown in
this figure, light may be absorbed by the article or redirected at
other angles, thereby reducing the amount of light detected by the
detector at intervening portion 27. Optical element 34 functions in
a similar manner, in this case redirecting light away from
intervening portion 29 and generally away from region 52 directly
below the optical element.
[0049] In some cases, one or more optical elements are adapted and
arranged to redirect at least a portion of the incident light into
one or more fluidic channels on the opposite side of the article.
For example, as shown in FIG. 1C, a portion of the light incident
upon optical element 32 is refracted through the article and
redirected into fluidic channels 26 and 28. Advantageously, this
can increase the amount of light used to interrogate a sample in
fluidic channels 26 and 28.
[0050] It should be understood that while much of the description
and figures herein describe the positioning of optical elements at
a side of an article opposite the channels, in some cases the
optical elements may positioned at the same side as the channels.
For instance, optical elements 32 and 34 of FIG. 1A-1D may be
formed in surface 14 and may optionally include a reflective
surface so as to redirect light 42' away from portion 43 of the
detector. In other cases, an article may include a combination of
optical elements formed in or on both surfaces of the article. The
geometry of the device and configuration of features may be chosen,
in some cases, such that any light passing through the bulk of the
article from a first side is redirected toward the channels on the
opposite side of the article. The design of a system with optical
elements may be undertaken with the goal of both reflecting light
and re-directing light away from the intervening portions between
channels. Without wishing to be bound by theory, the design of a
fluidic device may take into account the following:
[0051] The trajectory of refracted light is determined by Snell's
Law:
n.sub.1 sin(.beta..sub.1)=n.sub.2 sin(.beta..sub.2) [1]
where n.sub.1 and n.sub.2 are the indices of refraction of the
medium in which the light originates and is transmitted,
respectively, .beta..sub.1 is the angle between the angle of
incidence and the normal at the interface, and .beta..sub.2 is the
angle of refraction, as outlined in FIG. 1E.
[0052] Design features that may be varied to increase the amount of
light redirected away from the intervening portions include, for
example, the width of the channel (W), the pitch of the optical
elements (P1), the pitch of the channels (P2), the depth of the
channel (D), the width of the optical elements (V), the draft angle
of the optical elements (a), the thickness of the microfluidic
substrate (T), the index of refraction of microfluidic substrate
(n.sub.2), the index of refraction of external medium (n.sub.1),
and the incident angle of light on the substrate (assume
perpendicular to substrate).
[0053] For example, FIG. 2A includes device 210 comprising
substrate 211 in which channels 212, 213, and 214 and optical
elements 216, 217, 218 and 219 are formed. The thickness of the
substrate is illustrated by dimension 220 in FIG. 2A. The width of
a channel is measured as the widest cross-sectional dimension of
the channel substantially parallel to the surface in which it is
formed. For example, the width of channel 213 is indicated by
dimension 221 in FIG. 2A. The half-width of channel 214 is
indicated by dimension 222. Similarly, the width of an optical
element is also measured as the widest cross-sectional dimension of
the element substantially parallel to the surface in or on which it
is formed. For example, the width of optical element 218 is
indicated by dimension 224 in FIG. 2A. The depths of the channels,
as indicated by dimension 226 in FIG. 2A, are measured
perpendicular to the surface in which they are formed.
[0054] The pitch of two channels is measured as the distance
between a first point on a first channel and a second point on a
second channel, wherein the first and second points are located in
similar positions within their respective channels. In other words,
the pitch is equal to the width of a channel plus the gap between
that channel and the adjacent channel For example, in FIG. 2A, the
pitch of channels 213 and 214 may be measured as the distance
between similar edges of the channels, as indicated by dimension
230. In some embodiments, the pitches of all adjacent channels are
substantially constant, as indicated in FIG. 2A; however, in other
cases the pitches between channels may vary. The pitch of two
optical elements is measured in a similar manner as shown, for
example, by dimension 232 in FIG. 2A. In some embodiments, the
pitches of all adjacent optical elements may be substantially
constant, or may vary, e.g., depending the particular light
interaction desired.
[0055] To minimize stray light, improved results are obtained in
some embodiments when the pitch (P1) of the optical elements
matches the pitch of the channels (P2). The width of the optical
elements (V) may be chosen such that the area between the optical
elements (P-V) is less than the width of the channel (W). As (P-V)
decreases relative to W, the percentage of incident light that is
redirected by the optical elements increases. To increase the
amount of light redirected away from the intervening portions by
the optical elements, the thickness of the system may be set so
that light refracted by the optical elements is directed onto the
channels. Since there may be multiple channels, there may be
multiple preferred thicknesses for the system.
[0056] One may create a model to calculate preferred thicknesses by
imagining an incident light ray (e.g., perpendicular to the
article) striking the article halfway between the bottom and the
edge of the optical element (see, for example, light ray 240 in
FIG. 2A). The thickness of the article may be selected such that
this light reaches the center of a channel. To determine this
thickness, one may begin by calculating the angle of the refracted
light to the vertical within the substrate (.beta.). This angle is
a function of the angle of incidence, the draft angle of the
optical element, and the angle of refraction. Examining the
geometry, one can see:
.beta..sub.1=90 deg.-.alpha. [2]
.theta.=90 deg.-.alpha.-.beta..sub.2=.beta..sub.1-.beta..sub.2
[3]
Using Snell's Law (Equation 1), the angle of refraction
(.beta..sub.2) can be calculated as:
.beta. 2 = sin - 1 ( n 1 n 2 sin .beta. 1 ) Therefore : [ 4 ]
.theta. = 90 deg . - .alpha. - sin - 1 ( n 1 n 2 sin ( 90 deg . -
.alpha. ) ) [ 5 ] ##EQU00001##
If one were to assume a draft angle (.alpha.) of 35.3.degree., an
article refractive index (n.sub.2) of 1.57 (e.g., polystyrene), and
a refractive index of air (n.sub.1) of roughly 1.0, the internal
angle of refraction would be 23.4.degree..
[0057] Following this ray of from the center of the side of the
optical element to the center of a channel, this angle can be used
to calculate an intermediate measure of thickness (t):
tan .theta. = x t [ 6 ] ##EQU00002##
The distance (x) from the point below the incident light and the
center of a the closest channel is half the pitch minus the
distance between the bottom of the optical element and the edge
(V/4). The center of any additional channel is a multiple of the
pitch. Note that in FIG. 2A, the thickness was chosen to direct the
refracted light onto a channel two channels away (n=2) from the
closest channel below the incident light. In FIG. 2B, n=3.
x = ( P 2 - V 4 ) + nP [ 7 ] ##EQU00003##
This yields:
t = ( P 2 - V 4 ) + nP tan .theta. [ 8 ] ##EQU00004##
The total thickness of the substrate also includes the depth of the
channels and half the depth of the triangular optical elements.
Thus, a preferred thickness for a device including triangular
optical elements and multiple channel segments can be calculated
as:
T = t + D + ( V / 2 ) tan .alpha. [ 9 ] ##EQU00005##
[0058] Example 2 includes a description of experiments performed
using a device designed in this manner.
[0059] FIG. 2B includes a ray trace image generated by Mathematica
(Lenslab plug-in) of a proposed design of a system including
optical elements positioned above microfluidic channels. In FIG.
2B, device 310 comprises article 312 in which channels 314 and
optical elements 316 are formed. Light rays 318 are directed toward
surface 320 of the article, where a portion of the light is
refracted through the article. The device is constructed and
arranged such that the light is directed away from intervening
portions 322, and toward channels 314 and ultimately detector
components 324. Note that in some cases, light that is incident
upon a channel does not necessarily interact with the detector at a
position directly below the channel. For example, light rays 330
interact with channel 314' and detector component 324', which is
not directly below (indicated by region 332) channel 314'.
[0060] It should be understood that while triangular optical
elements are shown in FIGS. 1-2, a similar analysis can be
performed with devices having optical elements of other shapes and
configurations.
[0061] Light scattering or stray light may be reduced by
fabricating the walls of these optical elements to be very smooth.
In some embodiments, the root mean square (RMS) surface roughness
may be, for example, less than about 1 .mu.m. In other embodiments,
the RMS surface roughness may be less than about 0.8 .mu.m, less
than about 0.5 .mu.m, less than about 0.3 .mu.m, or less than about
0.1 .mu.m. RMS surface roughness is a term known to those skilled
in the art, and may be expressed as:
.sigma. h = [ ( z - z m ) 2 ] 1 / 2 = [ 1 A .intg. A ( z - z m ) 2
A ] 1 / 2 ##EQU00006##
where A is the surface to be examined, and |z-z.sub.m| is the local
height deviation from the mean. Substantial roughness on the
surface of an optical element may result in unwanted scattering or
redirection of light at an undesired angle.
[0062] As described herein, optical elements may have various
shapes, sizes and configurations. For example, in one set of
embodiments, the largest cross-sectional dimension of an optical
element is at least about 300 microns, 500 microns, 700 microns, 1
mm, 1.5 mm, 2 mm, or greater (typically, less than 1 cm). In some
embodiments, the largest cross-sectional dimension of an optical
element is its width. For instance, as shown in FIG. 2A, the
largest cross-sectional dimension of optical element 218 is its
width 224.
[0063] In some cases, e.g., as illustrated in FIG. 2A, at least one
optical element (e.g., optical element 218) is positioned between
first and second channel segments (e.g., segments 213 and 214,
respectively), and the optical element has a largest
cross-sectional dimension (e.g., width 224) greater than or equal
to the width of an intervening portion positioned substantially
between the first and second channel segments, but less than the
combination of the widths of the two channel segments and the width
of the intervening portion.
[0064] Optical elements may, in some cases, span at least about
50%, at least about 60%, at least about 70%, at least about 80%, or
at least about 90% of the length of one or more channel segments on
or in the article. For example, in FIG. 1B, optical elements 32 and
34 span the entire length of channel segments 26, 28, and 30.
[0065] In some fluidic devices described herein, one or more
optical elements and/or channels have a non-zero draft angle. As
known to those of ordinary skill in the art, a draft angle is the
amount of taper, e.g., for molded or cast parts, perpendicular to
the parting line. For example, as shown in FIG. 3A, a substantially
rectangular channel 110, which has walls 112-A and 112-B that are
substantially perpendicular to surface 114 (e.g., a parting line),
has a draft angle 116 of 0.degree.. The cross sections of fluidic
channels having non-zero draft angles, on the other hand, may
resemble a triangle, a parallelogram, a trapezoid, etc. For
example, as shown in the embodiment illustrated in FIG. 3B, channel
120 has a substantially triangular cross-section. Draft angle 116
is formed by the angle between a line perpendicular to surface 114
and wall 127-A of the channel, and is non-zero in this
embodiment.
[0066] The draft angle of an optical element, channel, or other
component may be, for example, between about 1.degree. and about
40.degree., between about 1.degree. and about 30.degree., between
about 1.degree. and about 20.degree., between about 1.degree. and
about 10.degree., between about 2.degree. and about 15.degree.,
between about 3.degree. and about 10.degree., or between about
3.degree. and about 8.degree.. For instance, the draft angle may be
greater than or equal to about 1.degree., 2.degree., 3.degree.,
4.degree., 5.degree., 6.degree., 7.degree., 8.degree., 9.degree.,
or 10.degree., 20.degree., 37.5.degree., or 40.degree.. In some
cases, it is desirable for optical elements or channels to have
specific draft angles so that they are compatible with a certain
detection technique.
[0067] As described herein, optical elements may be combined with a
fluidic device comprising one or more meandering channels. As shown
in a top view of the illustrative embodiment of FIG. 3C, channel
208 includes a meandering (e.g., serpentine) region having a
tightly packed channel system having a series of turns 210 and
channel segments 212 that span over a large area (A) relative to
the width of the channel. The area spanned by the meandering
channel (i.e., the area of the meandering region) is the area bound
by outermost points of the meandering channel along each axis,
shown approximately in FIG. 3C by the dashed lines. This area may
constitute a measurement area over which a detector may be
positioned, the measurement area including both channel segments
212 and intervening channel portions 220 (i.e., non-channel
segments).
[0068] FIG. 3C also shows plurality of optical elements 236
interspersed between the channel segments at the intervening
channel portions 220. One measurement area may include, for
example, greater than or equal to 3, 5, 8, 10, 15, 20, 30, 40, or
50 optical elements. The optical elements may be the same or
different from one another, and may have any suitable shape or size
as described herein. Furthermore, as illustrated, a fluidic device
may include optical elements that extend past the measurement area
and/or past the channel segments. This configuration may allow
control of light even at turns 210 of the channel segments.
[0069] As shown in FIG. 3C, the length of channel segments 212-A
and 212-C are the same. In other embodiments, however, the lengths
of the segments of the meandering channel vary within the channel.
Channel segments having different lengths may result in a
measurement area having different shapes. The meandering channel
(and the area of the channel) can be designed to have any suitable
shape, e.g., a square, rectangular, circular, oval, triangular,
spiral, or an irregular shape, since in certain cases the overall
shape does not affect the fluid flow conditions within the
channel.
[0070] In FIG. 3C, the area (A) that the meandering channel spans
is defined by the surface area given by dimension B times (x)
dimension C. Typically, the area spanned by the channel (i.e., as
viewed from above the channel, perpendicular to the direction of
fluid flow) is on the order of millimeters squared (mm.sup.2). For
instance, the area may be greater than or equal to 0.5 mm.sup.2,
greater than or equal to 1 mm.sup.2, greater than or equal to 2
mm.sup.2, greater than or equal to 5 mm.sup.2, greater than or
equal to 10 mm.sup.2, or greater than or equal to 50 mm.sup.2.
However, in other embodiments, e.g., depending on the method used
for detection, the area spanned by a meandering channel may be
between 0.25 mm.sup.2 and 0.5 mm.sup.2, or between 0.1 mm.sup.2 and
0.25 mm.sup.2. Typically, the area spanned by the meandering
channel is designed to be relatively large (e.g., on the order of
mm.sup.2) compared to conventional microfluidic systems, so that a
wide area can be used for detection and so the total amount of
signal that can be detected is increased, especially in combination
with one or more optical elements described herein.
[0071] In some embodiments, the optical elements described herein
are integral to a surface of the article. As used herein,
"integral" refers to a condition of being a single, unitary
construction, as opposed to separate parts that are connected by
other means. For instance, integral optical elements of article may
be formed in a surface of the article. Integral optical elements
may be either concave or convex relative to the surface on which
they are formed. For example, optical elements 32 and 34 in FIGS.
1A-1B are shown as concave optical elements integral to surface 16.
In some cases, such optical elements, or molds for the optical
elements, are fabricated using an photolithography process, e.g.,
as shown in FIGS. 4A-4B and as described in more detail below.
[0072] As shown in various embodiments herein, one or more optical
elements may be positioned substantially between two channel
segments and/or one or more channel segments may be positioned
substantially between two optical elements. A first object is said
to be positioned "substantially between" second and third other
objects when substantially all of the first object lies between the
center planes of the second and third objects. As used herein, a
"center plane" of an object refers to an imaginary plane that
intersects the geometric center of the cross section of the object
and is substantially perpendicular to the substrate in or on which
the object is positioned or formed. The term "geometric center" (or
"centroid") is given its normal meaning in the art. For example, in
FIG. 1A, channels 26, 28, and 30 comprise center planes 26', 28',
and 30', which intersect geometric centers 26'', 28'' and 30'',
respectively. In addition, optical elements 32 and 34 comprise
center planes 32' and 34' which intersect geometric centers 32''
and 34'', respectively. Optical element 32 is positioned
substantially between channels 26 and 28, and optical element 34 is
positioned substantially between channels 28 and 30. Channel
segment 28 is positioned substantially between optical elements 32
and 34.
[0073] In some embodiments, one or more optical elements of a
device lie on a substantially different plane than one or more
channels of the device. For example, in FIG. 1A, the plane 60
intersecting the central axes of the optical elements (positioned
at the geometric centers and extending out of and into the page)
does not intersect the plane 62 intersecting the central axes of
the channels. In some embodiments, no line drawn between any first
point on or within a first microfluidic channel segment and any
second point on or within a second microfluidic channel segment
intersects any point on or within an optical element. In some
instances, no line drawn between any first point on or within a
first optical element and any second point on or within a second
optical element intersects any point on or within a microfluidic
channel segment.
[0074] In other embodiments, however, all or a portion of an
optical element lies on the same plane as one or more channel or
channel segment. For instance, an optical element may be formed in
or on the same surface as the channels. In another example, an
optical element is formed on a side opposite a channel, but extends
such that a plane perpendicular to the surface of the article
passes through both the channel and the optical element. In some
cases, a line drawn between a first point on or within a first
channel segment and a second point on or within a second channel
segment intersects a point on or within the optical element.
[0075] Fluidic devices described herein comprising optical elements
may be optionally combined with other features (e.g., certain
detection systems, lenses, etc.) for reducing the amount of stray
light and/or for increasing the signal to noise ratio. FIGS. 5-10
show various examples of detection systems and results of
experiments performed when such systems were used in combination
with devices described herein. In some cases, however, these
features may be implemented independently of the optical elements
described herein.
[0076] In some embodiments, additional techniques may be employed
that compensate for the transmission of stray light through the
microfluidic device. For example, the size (e.g., width, surface
area, volume) of the intervening portions in the system may be
reduced, thus reducing the percentage of light incident on the
intervening portions. It should be noted that while it may not be
practical to eliminate the intervening portions between the
channels, as discussed in International Patent Publication No.
WO2006/113727, thinner intervening portions and/or wider fluidic
channels may result in less stray light transmitted and, therefore,
improved performance.
[0077] The effects of reducing the size of the intervening portions
on the amount of transmitted stray light can be evaluated by
measuring transmission or absorbance in the system when the
microchannels are filled with a perfectly absorbent fluid.
Transmission through such a system is calculated as:
Trans = I I o [ 10 ] ##EQU00007##
where I.sub.0 is the intensity of light transmitted with a
perfectly clear (index matched) fluid in the channels, and I is the
intensity of light transmitted with a perfectly absorbent fluid in
the channel.
[0078] The optical density (OD) is a measure of absorbance in such
a system, which is calculated as the negative log of
transmission:
OD=-log(Trans) [11]
A system with a minimum amount of stray light transmissions results
in a large OD. In theory, a measurement zone filled with a
perfectly absorbent fluid and with no intervening portions and no
stray light would have a transmission of 0% and very large OD. In
practice, it is difficult to completely eliminate stray light in
any system. A transmission measurement through an extremely
absorbent fluid in a microwell (no walls, or even channels) might
be 0.01%, yielding an OD of 4. In general, though, transmission
measurements below 1% can be difficult to achieve. A reasonable
range of ODs that may be achieved may be within the range of about
0 to about 2.
[0079] Assuming a perfectly absorbent fluid in the channels, the
transmission through a meandering channel region (without optical
elements to block or re-direct light) is simply a function of the
width of the intervening portions and the width of the channel. For
example, in a system with intervening portions with widths of x and
channels with widths of y, the minimum transmission would be
x/(x+y). In the case of a meandering channel with identical widths
for all intervening portions and channels, the value of x/(x+y) is
50% (yielding a maximum OD of 0.3). Similarly, a system with
channels twice the width of the intervening portions would yield a
minimum transmission of 33% (a maximum OD of 0.477). There is an
upper (and lower) range for the channel widths based on the flow
required in the system, since an increase in width of the channel
results in an increase in cross section and changes in the
properties of the channels, such as a reduction in the resistance
to flow. Likewise, there is a lower range at which intervening
portions can be reliably fabricated (e.g., depending on the
fabrication technique). Example 3 outlines a set of experiments in
which the widths of the intervening portions were varied.
[0080] In some embodiments, a detection system includes measuring
the light transmitted through the channel portions independently of
the light transmitted through the intervening portions. For
example, one may image the measurement area with a digital camera,
measure the intensity of light on the pixels that correspond to the
channels and discard the pixels corresponding to the channel walls
or the intervening portions. Optionally, lenses may be incorporated
to focus the image on the plane of the channels. Such a measurement
system could potentially deliver extremely high performance
(avoiding stray light) and yield maximum ODs greater than about 2,
e.g., OD=2-4.
[0081] However, in some cases, including a camera/imaging system
may result in a relatively high cost of the imaging device,
relatively high cost of lenses, precision required in positioning
and alignment, robustness to shock or environmental conditions, and
implementation of software to identify which pixels are to be
measured and which to be ignored. Accordingly, these factors may be
weighed with their benefits and may be suitable for certain, but
not all, applications.
[0082] In one embodiment, a relatively inexpensive and robust
imaging system was developed for a channel system utilizing a
linear image sensor. A linear image sensor is a one-dimensional
array of multiple small optical detectors which can be individually
measured. FIGS. 8A and 8B include schematic diagrams of exemplary
imaging systems used to measure transmission through meandering
channels. The optical detector 810 in FIG. 8A is a single
photodiode that may image a substantial portion of the meandering
channel. In FIG. 8B, on the other hand, the optical detector 812
comprises a linear image sensor that measures only portions of the
meandering channel. Optionally, optical components such as a
collimating lens for the light source and/or a focusing lens to
transmit the image to the linear image sensor (not shown) may be
utilized to improve imaging. FIG. 8C includes a micrograph of a
meandering channel used in one set of embodiments. A typical
measurement area for a linear image sensor is indicated as region
820. In certain devices, each optical detector of a linear image
sensor can be measured individually. Such a system can be used to
measure transmission through only a portion of a system. For
example, a linear image sensor located under a meandering channel
region as shown in FIG. 8C can be used to measure light
transmission only through the channels. To do this, only the
readings from detectors under the channels are recorded. Other
detectors positioned under non-channel portions (e.g., intervening
portions) and struck by stray light can be ignored. In this manner,
linear image sensors can be used to selectively measure light
transmission through channels in a meandering channel region,
eliminating the problem of stray light, and yielding accurate
transmission/absorbance measurements for the microfluidic
system.
[0083] Examples of linear image sensors include the Hamamatsu
S9227, a 6.4 mm long array of 512, 250-micron wide pixels at 12.5
micron spacing, the Fairchild Imaging CMOS 1421, a 14.5 mm-long
array of 2048, 7-micron wide pixels with 7 um center-to-center
spacing, and the Panavision SVI LIS-500, a 3.9 mm long array of
500, 62.5-wide pixels with 7.8 um center-to-center spacing. Example
4 outlines the use of a linear image sensor in conjunction with the
devices and methods described herein.
[0084] In some cases, the system may be designed to eliminate
potential stray light before it reaches the fluidic device. For
example, stray light may be eliminated by creating a light source
that includes a geometry that matches the pattern of channel(s),
directing light only onto the channels and away from the channel
walls or intervening portions.
[0085] A variety of determination (e.g., measuring, quantifying,
detecting, and qualifying) techniques may be used with devices
described herein. Determination techniques may include
optically-based techniques such as light transmission, light
absorbance, light scattering, light reflection as well as
luminescence techniques such as photoluminescence (e.g.,
fluorescence), chemiluminescence, bioluminescence, and/or
electrochemiluminescence. Those of ordinary skill in the art know
how to modify microfluidic devices in accordance with the
determination technique used. For instance, for devices including
chemiluminescent species used for determination, an opaque and/or
dark background may be preferred. For determination using metal
colloids, a transparent background may be preferred. Furthermore,
any suitable detector may be used with devices described herein.
For example, simplified optical detectors, as well as conventional
spectrophotometers and optical readers (e.g., 96-well plate
readers) can be used.
[0086] When more than one chemical and/or biological reaction
(e.g., a multiplex assay) is performed on a device, the signal
acquisition can be carried out by moving a detector over each
analysis region. In an alternative approach, a single detector can
detect signal(s) in each of the analysis regions simultaneously. In
another embodiment, an analyzer can include, for example, a number
of parallel optical sensors/detectors, each aligned with a analysis
region and connected to the electronics of a reader. Additional
examples of detectors and detection methods are described in more
detail in U.S. patent application Ser. No. 12/196,392, filed Aug.
22, 2008, entitled "Liquid containment for integrated assays",
which is incorporated herein by reference.
[0087] As described herein, a meandering channel of an analysis
region may be configured and arranged to align with a detector such
that upon alignment, the detector can measure a single signal
through more than one adjacent channel segments of the meandering
channel. In some embodiments, the detector is able to detect a
signal within at least a portion of the area of the meandering
channel and through more than one segments of the meandering
channel such that a first portion of the signal, measured from a
first segment of the meandering channel, is similar to a second
portion of the signal, measured from a second segment of the
meandering channel. In such embodiments, because the signal is
present as a part of more than one segment of the meandering
channel, there is no need for precise alignment between a detector
and an analysis region.
[0088] Additional examples and descriptions of detection systems
are provided in the Examples section.
[0089] In some embodiments, the fluidic devices described herein
include a reaction site in fluid communication with one or more
channels or channel segments. For example, the fluidic device may
comprise a reaction site comprising a binding partner (e.g., an
antibody, antigen, etc.) associated with a surface of a channel
segment. An entity in the fluid flowing in the channel segment may
interact (e.g., bind, chemically react, etc.) with the binding
partner, and the interaction may be optically detectable.
[0090] In one set of embodiments, a fluidic device described herein
is used for performing an immunoassay. The immunoassay may be, for
example, a direct immunoassay, a sandwich (e.g., 2-site)
immunoassay, or a competitive immunoassay, as known to those of
ordinary skill in the art. Certain devices may include a
combination of one or more such immunoassays.
[0091] In one particular embodiment, a fluidic device is used for
performing an immunoassay (e.g., for human IgG or PSA) and,
optionally, uses sliver enhancement for signal amplification. A
device described herein may have one or more similar
characteristics as those described in U.S. patent application Ser.
No. 12/113,503, filed May 1, 2008 and entitled "Fluidic Connectors
and Microfluidic Systems", which is incorporated herein by
reference. In such an immunoassay, after delivery of a sample
containing human IgG to a reaction area or analysis region, binding
between the human IgG and anti-human IgG can take place. One or
more reagents, which may be optionally stored in the device prior
to use, can then flow over this binding pair complex. One of the
stored reagents may include a solution of metal colloid (e.g., a
gold conjugated antibody) that specifically binds to the antigen to
be detected (e.g., human IgG). This metal colloid can provide a
catalytic surface for the deposition of an opaque material, such as
a layer of metal (e.g., silver), on a surface of the analysis
region. The layer of metal can be formed by using a two component
system: a metal precursor (e.g., a solution of silver salts) and a
reducing agent (e.g., hydroquinone), which can optionally be stored
in different channels prior to use.
[0092] As a positive or negative pressure differential is applied
to the system, the silver salt and hydroquinone solutions can merge
at a channel intersection, where they mix (e.g., due to diffusion)
in a channel, and then flow over the analysis region. Therefore, if
antibody-antigen binding occurs in the analysis region, the flowing
of the metal precursor solution through the region can result in
the formation of an opaque layer, such as a silver layer, due to
the presence of the catalytic metal colloid associated with the
antibody-antigen complex. The opaque layer may include a substance
that interferes with the transmittance of light at one or more
wavelengths. Any opaque layer that is formed in the microfluidic
channel can be detected optically, for example, by measuring a
reduction in light transmittance through a portion of the analysis
region (e.g., a meandering channel region) compared to a portion of
an area that does not include the antibody or antigen.
Alternatively, a signal can be obtained by measuring the variation
of light transmittance as a function of time, as the film is being
formed in a analysis region. The opaque layer may provide an
increase in assay sensitivity when compared to techniques that do
not form an opaque layer. Additionally, various amplification
chemistries that produce optical signals (e.g., absorbance,
fluorescence, glow or flash chemiluminescence,
electrochemiluminescence), electrical signals (e.g., resistance or
conductivity of metal structures created by an electroless process)
or magnetic signals (e.g., magnetic beads) can be used to allow
detection of a signal by a detector.
[0093] It should be understood that devices described herein may be
used for any suitable chemical and/or biological reaction, and may
include, for example, other solid-phase assays that involve
affinity reaction between proteins or other biomolecules (e.g.,
DNA, RNA, carbohydrates), or non-naturally occurring molecules. In
some embodiments, a chemical and/or biological reaction involves
binding. Different types of binding may take place in devices
described herein. The term "binding" refers to the interaction
between a corresponding pair of molecules that exhibit mutual
affinity or binding capacity, typically specific or non-specific
binding or interaction, including biochemical, physiological,
and/or pharmaceutical interactions. Biological binding defines a
type of interaction that occurs between pairs of molecules
including proteins, nucleic acids, glycoproteins, carbohydrates,
hormones and the like. Specific examples include antibody/antigen,
antibody/hapten, enzyme/substrate, enzyme/inhibitor,
enzyme/cofactor, binding protein/substrate, carrier
protein/substrate, lectin/carbohydrate, receptor/hormone,
receptor/effector, complementary strands of nucleic acid,
protein/nucleic acid repressor/inducer, ligand/cell surface
receptor, virus/ligand, etc. Binding may also occur between
proteins or other components and cells. In addition, devices
described herein may be used for other fluid analyses (which may or
may not involve binding and/or reactions) such as detection of
components, concentration, etc.
[0094] Non-limiting examples of analytes that can be determined
using fluidic devices described herein include specific proteins,
viruses, hormones, drugs, nucleic acids and polysaccharides;
specifically antibodies, e.g., IgD, IgG, IgM or IgA immunoglobulins
to HTLV-I, HIV, Hepatitis A, B and non A/non B, Rubella, Measles,
Human Parvovirus B19, Mumps, Malaria, Chicken Pox or Leukemia;
human and animal hormones, e.g., thyroid stimulating hormone (TSH),
thyroxine (T4), luteinizing hormone (LH), follicle-stimulating
hormones (FSH), testosterone, progesterone, human chorionic
gonadotropin, estradiol; other proteins or peptides, e.g. troponin
I, c-reactive protein, myoglobin, brain natriuretic protein,
prostate specific antigen (PSA), free-PSA, complexed-PSA, pro-PSA,
EPCA-2, PCADM-1, ABCA5, hK2, beta-MSP (PSP94), AZGP1, Annexin A3,
PSCA, PSMA, JM27, PAP; drugs, e.g., paracetamol or theophylline;
marker nucleic acids, e.g., PCA3, TMPRS-ERG; polysaccharides such
as cell surface antigens for HLA tissue typing and bacterial cell
wall material. Chemicals that may be detected include explosives
such as TNT, nerve agents, and environmentally hazardous compounds
such as polychlorinated biphenyls (PCBs), dioxins, hydrocarbons and
MTBE. Typical sample fluids include physiological fluids such as
human or animal whole blood, blood serum, blood plasma, semen,
tears, urine, sweat, saliva, cerebro-spinal fluid, vaginal
secretions; in-vitro fluids used in research or environmental
fluids such as aqueous liquids suspected of being contaminated by
the analyte. In some embodiments, one or more of the
above-mentioned reagents is stored in a channel or chamber of a
fluidic device prior to first use in order to perform a specific
test or assay.
[0095] Some embodiments of the invention are in the form of a kit
that may include, for example, a microfluidic system, a source for
promoting fluid flow (e.g., a vacuum), and/or one, several, or all
the reagents necessary to perform an analysis except for the sample
to be tested. In some embodiments, the microfluidic system of the
kit may have a configuration similar to one or more of those shown
in the figures and/or as described herein. The fluidic device of
the kit may be portable and may have dimensions suitable for use in
point-of-care settings.
[0096] The kit may include reagents and/or fluids that may be
provided in any suitable form, for example, as liquid solutions or
as dried powders. In some embodiments, a reagent is stored in the
microfluidic system prior to first use, as described in more detail
herein. When the reagents are provided as a dry powder, the reagent
may be reconstituted by the addition of a suitable solvent, which
may also be provided. In embodiments where liquid forms of the
reagent are provided, the liquid form may be concentrated or ready
to use. The fluids may be provided as specific volumes (or may
include instructions for forming solutions having a specific
volume) to be flowed in the microfluidic system.
[0097] The kit may be designed to perform a particular analysis
such as the determination of a specific disease condition. For
instance, markers (e.g., PSA) for specific diseases (e.g., prostate
cancer) may be included (e.g., stored) in a device or kit in a
fluid or dry form prior to first use of the device/kit. In order to
perform a particular analysis or test using the kit, the fluidic
device may be designed to have certain geometries, and the
particular compositions, volumes, and viscosities of fluids may be
chosen so as to provide optimal conditions for performing the
analysis in the system. For example, if a reaction to be performed
at an analysis region requires the flow of an amplification reagent
over the analysis region for a specific, pre-calculated amount of
time in order produce an optimal signal, the fluidic device may be
designed to include a channel segment having a particular
cross-sectional area and length to be used with a fluid of specific
volume and viscosity in order to regulate fluid flow in a
predetermined and pre-calculated manner. Washing solutions and
buffers may also be included. The device may optionally include one
or more reagents stored therein prior to first use. Furthermore,
the kit may include a device or component for promoting fluid flow,
such as a source of vacuum dimensioned to be connected to an
outlet. The device or component may include one or more pre-set
values so as to create a known (and optionally constant) pressure
drop between an inlet and an outlet of the fluidic device. Thus,
the kit can allow one or more reagents to flow for a known,
pre-calculated amount of time at an analysis region, or at other
regions of the system, during use. Those of ordinary skill in the
art can calculate and determine the parameters necessary to
regulate fluid flow using general knowledge in the art in
combination with the description provided herein.
[0098] A kit described herein may further include a set of
instructions for use of the kit. The instructions can define a
component of instructional utility (e.g., directions, guides,
warnings, labels, notes, FAQs ("frequently asked questions"), etc.,
and typically involve written instructions on or associated with
the components and/or with the packaging of the components for use
of the microfluidic system. Instructions can also include
instructional communications in any form (e.g., oral, electronic,
digital, optical, visual, etc.), provided in any manner such that a
user will clearly recognize that the instructions are to be
associated with the components of the kit.
[0099] In some embodiments, microfluidic systems described herein
contain stored reagents prior to first use of the device and/or
prior to introduction of a sample into the device. In some cases,
one or both of liquid and dry reagents may be stored on a single
article. Additionally or alternatively, the reagents may also be
stored in separate vessels such that a reagent is not in fluid
communication with the microfluidic system prior to first use. The
use of stored reagents can simplify use of the microfluidic system
by a user, since this minimizes the number of steps the user has to
perform in order to operate the device. This simplicity can allow
microfluidic systems described herein to be used by untrained
users, such as those in point-of-care settings, and in particular,
for devices designed to perform immunoassays. It has been
demonstrated previously that the storage of the reagents in the
form of liquid plugs separated by air gaps were stable for extended
periods of time (see, for example, International Patent Publication
No. WO2005/072858 (International Patent Application Serial No.
PCT/US2005/003514), filed Jan. 26, 2005 and entitled "Fluid
Delivery System and Method," which his incorporated herein by
reference in its entirety). Fluidic devices for storing reagents
may also include a configuration as described in U.S. patent
application Ser. No. 12/640,420 filed on Dec. 17, 2009 and
entitled, "Improved Reagent Storage in Microfluidic Systems and
Related Articles and Methods," which is incorporated herein by
reference in its entirety. In other embodiments, however,
microfluidic devices described herein do not contain stored
reagents prior to first use of the device and/or prior to
introduction of a sample into the device.
[0100] As used herein, "prior to first use" of the device means a
time or times before the device is first used by an intended user
after commercial sale. First use may include any step(s) requiring
manipulation of the device by a user. For example, first use may
involve one or more steps such as puncturing a sealed inlet to
introduce a reagent into the device, connecting two or more
channels to cause fluid communication between the channels,
preparation of the device (e.g., loading of reagents into the
device) before analysis of a sample, loading of a sample onto the
device, preparation of a sample in a region of the device,
performing a reaction with a sample, detection of a sample, etc.
First use, in this context, does not include manufacture or other
preparatory or quality control steps taken by the manufacturer of
the device. Those of ordinary skill in the art are well aware of
the meaning of first use in this context, and will be able easily
to determine whether a device of the invention has or has not
experienced first use. In one set of embodiments, devices of the
invention are disposable after first use, and it is particularly
evident when such devices are first used, because it is typically
impractical to use the devices at all after first use.
[0101] The devices described herein may comprise one or more
channels or channel segments. A "channel" or "channel portion", as
used herein, means a feature on or in an article or substrate
(e.g., formed in a surface/side of an article or substrate) that at
least partially directs the flow of a fluid. A channel, channel
portion, or channel segment, etc. can have any cross-sectional
shape (circular, oval, triangular, irregular, square or
rectangular, trapezoidal, or the like) and can be covered or
uncovered. In embodiments where it is covered, at least one portion
of the channel can have a cross-section that is substantially
enclosed, or the entire channel may be substantially enclosed along
its entire length with the exception of its inlet(s) and outlet(s).
In some cases, the inlet and/or outlet may also be enclosed or
sealed, e.g., to prevent fluids and/or other reagents from being
removed from the device (e.g., due to evaporation).
[0102] A channel, channel segment, channel portion, etc., may also
have an aspect ratio (length to average cross-sectional dimension)
of at least 2:1, more typically at least 3:1, 5:1, or 10:1 or more.
In some embodiments, one or more channels, channel segments,
channel portions, intervening channels, etc., is microfluidic.
"Microfluidic," as used herein, refers to a device, apparatus or
system including at least one fluid channel having a
cross-sectional dimension of less than 1 mm, and a ratio of length
to largest cross-sectional dimension of at least 3:1. A
"microfluidic channel" or "microfluidic channel segment" as used
herein, is a channel meeting these criteria. Though in some
embodiments, devices of the invention may be microfluidic, in
certain embodiments, the invention is not limited to microfluidic
systems and may relate to other types of fluidic systems.
Furthermore, it should be understood that all or a majority of the
channels described herein may be microfluidic in certain
embodiments. The "cross-sectional dimension" (e.g., a diameter, a
height, and/or a width) of a channel, channel segment, channel
portion, or intervening channel, etc. is measured perpendicular to
the direction of fluid flow. In one set of embodiments, the maximum
cross-sectional dimension of one or more channels or channel
segments containing embodiments described herein are less than
about 750 microns, less than about 500 microns, less than about 300
microns, less than about 200 microns, less than about 100 microns,
less than about 50 microns, less than about 25 microns, less than
about 10 microns, or less than about 5 microns. In some cases, at
least two cross-sectional dimensions (e.g., a height and a width)
of a channel, channel segment, or channel portion have one or more
of the dimensions listed above (e.g., a width of less than 500
microns and a height of less than 200 microns).
[0103] One or more channels or channel segments described herein
may have any suitable length. In some cases, the channels or
channel segments may be at least about 1 mm long, at least about 2
mm long, at least about 5 mm long, at least about 10 mm long, at
least about 20 mm long, at least about 50 mm long, or longer.
[0104] The channels or channel segments may also be spaced any
suitable distance apart from each other. For example, in some
cases, the width of one or more intervening portions between
channels or channel segments may be less than about 5 mm, less than
about 2 mm, less than about 1 mm, less than about 500 microns, less
than about 300 microns, less than about 200 microns, less than
about 100 microns, less than about 50 microns, less than about 25
microns, less than about 10 microns, less than about 5 microns, or
less. In certain embodiments, channel segments may be separated by
a distance of less than 0.01 times, less than 0.1 times, less than
0.25 times, less than 0.5 times, less than 1 times, less than 2
times, less than 5 times, or less than 10 times the average largest
width of the channel segment.
[0105] The channels or channel segments may also be oriented in any
suitable manner. In some instances, all channels or channel
segments are spaced a substantially equal distance from each other
(i.e., the widths of the intervening portions are all substantially
the same). The channels or channel segments may also be oriented
such that two or more (e.g., all) are substantially parallel to
each other.
[0106] In some cases the dimensions of a channel may be chosen such
that fluid is able to freely flow through the article or substrate.
The dimensions of the channel may also be chosen, for example, to
allow a certain volumetric or linear flowrate of fluid in the
channel. Of course, the number of channels and the shape of the
channels can be varied by any method known to those of ordinary
skill in the art. In some cases, more than one channel or capillary
may be used.
[0107] In some embodiments described herein, microfluidic systems
include only a single interconnected channel with, for example,
less than 5, 4, 3, 2, or 1 channel intersection(s) when in use. A
layout based on a single channel with minimal or no intersections
may be reliable because there is only one possible flow path for
any fluid to travel across the microfluidic chip.
[0108] A microfluidic system described herein may have any suitable
volume for carrying out a chemical and/or biological reaction or
other process. The entire volume of a microfluidic system includes,
for example, any reagent storage areas, reaction areas, liquid
containment regions, waste areas, as well as any fluid connectors,
and microfluidic channels associated therewith. In some
embodiments, small amounts of reagents and samples are used and the
entire volume of the microfluidic system is, for example, less than
10 milliliters, less than 5 milliliters, less than 1 milliliter,
less than 500 microliters, less than 250 microliters, less than 100
microliters, less than 50 microliters, less than 25 microliters,
less than 10 microliters, less than 5 microliters, or less than 1
microliter.
[0109] A fluidic device and/or an article described herein may be
portable and, in some embodiments, handheld. The length and/or
width of the device and/or article may be, for example, less than
or equal to 20 cm, 15 cm, 10 cm, 8 cm, 6 cm, or 5 cm. The thickness
of the device and/or article may be, for example, less than or
equal to 5 cm, 3 cm, 2 cm, 1 cm, 8 mm, 5 mm, 3 mm, 2 mm, or 1 mm.
Advantageously, portable devices may be suitable for use in
point-of-care settings.
[0110] All or a portion of a fluidic device such as an article or a
cover can be fabricated of any suitable material. For example,
articles that include channels may be formed of a suitable for
forming a microchannel. Non-limiting examples of materials include
polymers (e.g., polyethylene, polystyrene, polymethylmethacrylate,
polycarbonate, poly(dimethylsiloxane), PTFE, PET, and a
cyclo-olefin copolymer), glass, quartz, and silicon. The article
and/or cover may be hard or flexible. Those of ordinary skill in
the art can readily select a suitable material based upon e.g., its
rigidity, its inertness to (e.g., freedom from degradation by) a
fluid to be passed through it, its robustness at a temperature at
which a particular device is to be used, its transparency/opacity
to light (e.g., in the ultraviolet and visible regions), and/or the
method used to fabricate features in the material. For instance,
for injection molded or other extruded articles, the material used
may include a thermoplastic (e.g., polypropylene, polycarbonate,
chlorotrifluoroethylene, acrylonitrile-butadiene-styrene, nylon 6),
an elastomer (e.g., polyisoprene, isobutene-isoprene, nitrile,
neoprene, ethylene-propylene, hypalon, silicone), a thermoset
(e.g., epoxy, unsaturated polyesters, phenolics), or combinations
thereof. In some embodiments, the material and dimensions (e.g.,
thickness) of an article and/or cover are chosen such that it is
substantially impermeable to water vapor. For instance, a fluidic
device designed to store one or more fluids therein prior to first
use may include a cover comprising a material known to provide a
high vapor bather, such as metal foil, certain polymers, certain
ceramics and combinations thereof. In other cases, the material is
chosen based at least in part on the shape and/or configuration of
the device. For instance, certain materials can be used to form
planar devices whereas other materials are more suitable for
forming devices that are curved or irregularly shaped.
[0111] In some instances, a fluidic device is formed of a
combination of two or more materials, such as the ones listed
above. For instance, the channels of the device may be formed in a
first material (e.g., poly(dimethylsiloxane)), and a cover that is
formed in a second material (e.g., polystyrene) may be used to seal
the channels. In another embodiment, a first set of channels is
formed in a first article comprising a first material and a second
set of channels is formed in a second article comprising a second
material. In yet another embodiment, channels of the device may be
formed in polystyrene or other polymers (e.g., by injection
molding) and a biocompatible tape may be used to seal the channels.
The biocompatible tape may include a material known to improve
vapor barrier properties (e.g., metal foil, polymers or other
materials known to have high vapor barriers). A variety of methods
can be used to seal a microfluidic channel or portions of a
channel, or to join multiple layers of a device, including but not
limited to, the use of adhesives (such as acrylic or silicone based
adhesives), use adhesive tapes, gluing, bonding, lamination of
materials, or by mechanical methods (e.g., clamping).
[0112] Sealing a channel and/or any inlets and outlets may protect
and retain any gases, liquids, and/or dry reagents that may be
stored within a channel. In addition or alternatively to one or
more covers described herein, in certain embodiments, a fluid
having low volatility, such as an oil or glycol may be placed in
the end of a tube to help prevent evaporation and/or movement of
other fluids contained therein.
[0113] Devices comprising optical elements and channels (e.g.,
microchannels) described herein may be fabricated using a variety
of techniques. For example, the devices described herein may be
formed using injection molding, hot embossing, or other plastic
engineering techniques. The devices may also be manufactured using
traditional machining techniques. In some cases, the devices may be
fabricated by producing a mold and transferring the features of the
mold to a hardenable polymer (e.g., to PDMS). Molds may be
fabricated by, for example, etching features into a silicon wafer
(e.g., via an anisotropic KOH etch) and transferring the features
onto a hardenable material (e.g., SU-8) which may then serve as a
mold. In some cases, the microfluidic devices described herein
include an article that is a single, integral piece of material
without joined layers.
[0114] In one set of embodiments, purely photolithographic
techniques are used to fabricated the channels and optical elements
in a polymer. FIGS. 4A-4B illustrate a fabrication process that may
be used to produce triangular optical elements in photoresist. In
FIG. 4A, a layer of photoresist 410 overlies substrate 412.
Photomask 414, comprising UV-transparent feature 416, is exposed to
ultraviolet light 418. The ultraviolet light is directed at an
angle 420 from the normal of the photomask. The development of the
photoresist layer results in the formation of a triangular feature
430, as shown in FIG. 4B. This technique may produce features
having smooth surfaces. In addition, the technique may be used to
fabricate features with a relatively wide range of draft angles
(e.g., from about 0.degree. to about 20.degree.). Such methods are
known to those of ordinary skill in the art.
[0115] The manufacturing processes used to produce devices by
injection molding (or other plastic engineering techniques, such as
hot embossing), often require molds having non-zero draft angles on
some or all of the features to be replicated in plastic. As
discussed above, a draft angle is the amount of taper for molded or
cast parts perpendicular to the parting line (a square channel with
walls perpendicular to the floor having a draft angle of zero
degrees). A non-zero draft angle is often necessary to allow
demolding of the replica from the molding tool.
[0116] The fabrication of elements with non-zero draft angles is
challenging. For instance, for microfluidic structures (e.g.,
channels) having various depths, the corresponding mold must have
features with multiple heights in addition to non-zero draft
angles. These types of molds can be challenging to fabricate on the
microscale, as molding microchannels in plastic with constrictions
in draft angle, depth, as well as in width is not trivial.
[0117] In fact, few techniques can yield the appropriate shapes for
a mold having non-zero draft angles. To widen the breadth of
technologies able to produce the appropriate shapes, an indirect
route to the fabrication of the mold can be chosen. For instance,
the to channels themselves can be created in various materials, by
various techniques to produce a master. The negative shape of the
master is then obtained (e.g., by electrodeposition), resulting in
a mold for injection molding. The techniques capable of yielding a
master with non-zero draft angles and various depths include: (1)
milling with one or more trapezoidal-shaped bits, (2)
photolithographic techniques in combination with thick
photosensitive polymers, for instance photosensitive glass or
photoresist like SU8, in combination with a back-side exposure or a
top-side exposure with light with a non-normal angle. An example of
the use of non-normal top-side exposure with photosensitive glass
to produce features with non-zero draft angles is described in U.S.
Pat. No. 4,444,616. The preparation of multiple depths can be
achieved by multiple photolithographic exposures onto multiple
layers of photosensitive material. (3) KOH etching on silicon
substrates can also produce non-zero draft angles, according to the
crystalline planes of the silicon. (4) Alternative to straight
draft angles, channels having rounded side-walls can also produce
suitable master for molds. Such rounded side-walls can be achieved
by isotropic etching onto planar surface (e.g., HF etching on Pyrex
wafers), or by reflowing structures photoresist by heat treatment.
(5) Deep Reactive Ion Etching (DRIE) can also produce non-zero
degree draft angles under certain parameters.
[0118] The following examples are intended to illustrate certain
embodiments of the present invention, but are not to be construed
as limiting and do not exemplify the full scope of the
invention.
Example 1
Fabrication of Microfluidic Channels
[0119] A method for fabricating a microfluidic channel system is
described.
[0120] Channel systems, such as the ones shown in FIGS. 1A and 1B,
were designed with a computer-aided design (CAD) program. The
microfluidic devices were formed in poly(dimethylsiloxane) Sylgard
184 (PDMS, Dow Corning, Ellsworth, Germantown, Wis.) by rapid
prototyping using masters made in SU8 photoresist (MicroChem,
Newton, Mass.). The masters were produced on a silicon wafer and
were used to replicate the negative pattern in PDMS. The masters
contained two levels of SU8, one level with a thickness (height) of
.about.70 .mu.m defining the channels in the immunoassay area, and
a second thickness (height) of .about.360 .mu.m defining the
reagent storage and waste areas. Another master was designed with
channel having a thickness (height) of 33 .mu.m. The to masters
were silanized with
(tridecafluoro-1,1,2,2-tetrahydrooctyl)trichlorosilane (ABC-R,
Germany). PDMS was mixed according to the manufacturer's
instructions and poured onto the masters. After polymerization (4
hours, 65.degree. C.), the PDMS replica was peeled off the masters
and access ports were punched out of the PDMS using stainless steel
tubing with sharpened edges (1.5 mm in diameter). To complete the
fluidic network, a flat substrate such as a glass slide, silicon
wafer, polystyrene surface, flat slab of PDMS, or an adhesive tape
was used as a cover and placed against the PDMS surface. The cover
was held in place either by van der Waals forces, or fixed to the
microfluidic device using an adhesive.
[0121] In other embodiments, the microfluidic channels were made in
polystyrene or other thermoplastics by injection molding. This
method is known to those of ordinary skill in the art. The volume
of an injection molding cavity can be defined by a bottom surface
and a top surface separated by a hollow frame which determines the
thickness of the molded article. For an article including channel
features and or other microscale elements on two opposing sides of
the article, the bottom and top surfaces of the molding cavity may
include raised features that create the channel features on either
side of the article. For an article including channel features on
only one side of the article, only the top or bottom surface of the
molding cavity includes such features. Thru-holes that pass through
the entire thickness of the article can be produced by pins
traversing the cavity, embedded in one or more surfaces of the
cavity and contacting the other side. For instance, the pins may
extend from only the top surface, only the bottom surface, or from
both the top and bottom surfaces.
Example 2
Performance of a System Comprising Triangular Optical Elements
[0122] This example describes the transmission profiles of systems
employing a meandering channel, one with triangular optical
elements (grooves) and another without. An article was fabricated
in polystyrene with identical systems of fluidic channels on one
side. Some of these channels included triangular optical elements
between the channels on the other side (shielded channels). Other
channels did not include triangular optical elements between them
(normal/standard channels with no shielding). The channels were 160
microns in width. Intervening portions between the channels were 60
microns in width. The article thickness was designed using the
model described above. Triangular optical to elements were also
designed as described in the model above with an angle of
35.3.degree., a width of 160 microns, and a pitch of 220 microns.
Optical measurements were performed using a single collimated LED
light source and a single photodiode detector.
[0123] Measurements were performed with an approximate
index-matching liquid in the channels (water) and with a
concentrated absorbing dye (Methylene Blue, 20 mg/ml in water).
Using water in the "normal" channel (channel without optical
elements) as the baseline, the following transmission measurements
were made:
TABLE-US-00001 Transmission OD Water in Normal Channel 100% 0.00
Dye in Normal Channel 27% 0.56 Water Shielded Channel 26% 0.58 Dye
in Shielded Channel 1% 1.98
Assuming a perfectly absorbing dye, the transmission through normal
channels should be 27%, since the channel walls make up
60/(60+160)=27% of the area of the measurement zone. Experimental
results confirmed this prediction. Note that the range of ODs
provided by a non-shielded channel of these dimensions would be 0
to 0.56. In shielded channels with dye, only 1% of the incident
light was transmitted. The triangular optical element was designed
to either block light that would be transmitted through the
intervening portions or directed the light into the channels. The
dye in the channels absorbed most, if not all, the light striking
the channels.
[0124] With water in the shielded channels, 26% of the light
incident on the measurement zone was transmitted. With a width 60
microns and a pitch of 220 microns, the triangular optical elements
blocked 73% of the top surface of the measurement area. The
remaining 27% of the area was positioned directly above channels.
Since these channels were filled with index matching liquid, it was
assumed that they transmitted all of the light striking them. A
total transmission of 26% indicated that, in this particular
experiment, significantly more of the light incident on the optical
elements was reflected out of the system than was directed to the
channels.
[0125] To understand the measurement range of the shielded
channels, a comparison was made between the intensity of light
transmitted through the shielded channels with dye and the
intensity of light transmitted through the shielded channels with
water. Using the to shielded channels with water as a baseline, the
transmission with dye was 4%. This indicated that the range of ODs
provided by the shielded system with channels of these dimensions
would be 0 to 1.40. This represents a significant improvement over
the normal configuration. FIG. 5 presents a comparison of ODs
measured in shielded meandering channels and ODs measured in
unshielded meandering channels. In this example, erioglaucine dye
was used. As can be seen, the shielding delivered a larger dynamic
range of ODs corresponding to superior performance.
[0126] A more detailed comparison of transmitted light can be
obtained using the linear image sensor system described above. FIG.
6A includes a schematic diagram outlining light transmission
through a microfluidic meandering channel measurement zone without
optical elements. In this set of experiments, the channels were
filled with dark dye (10 mg/mL eriogalucine dye). A collimated
light source was used to shine incident light onto the meandering
channel measurement zone. A focusing lens and a linear image sensor
was used to detect light through the measurement zone. The light
incident upon the channels was absorbed by the dye, while the light
incident between the channels passed through the article. FIG. 6B
is a plot of the transmitted light as a function of position across
the measurement zone. The peaks in FIG. 6B indicate the presence of
a large amount of stray light between the channels.
[0127] FIG. 6C includes a schematic diagram outlining light
transmission through a microfluidic meandering channel measurement
zone with optical elements. As in the previous set of experiments,
the channels were filled with dye, which absorbed light incident
upon the channels. FIG. 6D, like FIG. 6B, includes a plot of
transmitted light as a function of position across the measurement
zone. However, in this instance, the peaks corresponding to the
positions between the channels have been reduced dramatically,
meaning stray light between the channel reduced due to the presence
of the optical elements. Due to the shielding provided by the
optical elements, when they are employed, a single photosensor may
provide nearly equivalent optical performance compared to a more
complex linear image sensor. This shows that simplified optical
systems can be used in combination with fluidic devices described
herein.
Example 3
Reducing the Width of Intervening Portions
[0128] In this example, several samples with various widths of
intervening portions were to fabricated and tested. FIG. 7A
includes a micrograph and a schematic illustration of a device
comprising 120-micron-wide optical elements spaced 100 microns
apart. In FIG. 7B, the optical elements are spaced only 30 microns
apart. The channels were filled with dye (Erioglaucine) at various
concentrations, and measurements of transmissions through the
meandering channel region were taken. FIG. 7C includes a plot of
the net OD as a function of the dye concentration for several
devices including varied inter-element spacings. As can be seen
from the plot, the OD increases with an increase in dye
concentration and a decrease in inter-element spacing. Table 1
summarizes the theoretical maximum projected optical density
(minimum transmission) and actual optical performance of these
systems.
TABLE-US-00002 TABLE 1 Predicted and measured maximum optical
densities for various devices. Channel Width of width Intervening
Portions Predicted Max OD Measured Max OD 120 .mu.m 50 .mu.m 0.53
0.49 120 .mu.m 60 .mu.m 0.48 0.43 120 .mu.m 70 .mu.m 0.43 0.38 120
.mu.m 80 .mu.m 0.40 0.35 120 .mu.m 90 .mu.m 0.37 0.32 120 .mu.m 100
.mu.m 0.34 0.30
Example 4
Use of Linear Image Sensors
[0129] This example describes the use of a linear image sensor in
conjunction with the systems and methods described herein.
[0130] A linear image sensor was positioned underneath a meandering
channel as shown in FIG. 8B such that detection elements were
positioned below the surface including the channel. A focusing lens
was mounted between the sensor and the meandering channel so that a
focused image of the channel was projected onto the sensor surface.
Collimated light was used to illuminate the meandering channel. In
an alternative experimental setup, the linear image sensor was
placed immediately underneath the meandering channel (i.e., within
less than 0.5 mm), alleviating the need for a lens to be placed
between the meandering channel and the surface of the optical
detector.
[0131] Measurements of the system were performed with various
fluids in the channel including index-matching liquid, dye diluted
in water, and concentrated dye. FIGS. 9A-9D include plots of
transmitted light as a function of position along the linear image
sensor for various dye concentrations. In FIG. 9A, a low dye
concentration (0.05 mg/mL erioglaucine) was used in the channel.
FIGS. 9B-9D show dye concentrations of 0.4 mg/mL, 1.6 mg/mL, and 50
mg/mL respectively. Less light was transmitted through the channels
(i.e., absorbance increased) as the dye concentration increased. A
software program was written to identify which pixels corresponded
to positions within the channel. Selecting only these pixels,
transmission was calculated as:
T = Intensity of light detected with target liquid channel
Intensity of light detected with index matching liquid in channel [
12 ] ##EQU00008##
A total transmission value was calculated by averaging the
measurements from all the identified channel pixels.
[0132] Various concentrations of dyes were imaged in the channels
(corresponding to various levels of absorption in the channels).
Transmissions were calculated using the method explained above and
converted into ODs. FIG. 10 includes plots of OD as a function of
dye concentration when using a single photodetector (measuring the
light traveling throughout the channels and between the channels)
and when using a linear image sensor (discriminating pixels). The
linear image sensor delivered a larger dynamic range of ODs
corresponding to superior performance.
[0133] While several embodiments of the present invention have been
described and illustrated herein, those of ordinary skill in the
art will readily envision a variety of other means and/or
structures for performing the functions and/or obtaining the
results and/or one or more of the advantages described herein, and
each of such variations and/or modifications is deemed to be within
the scope of the present invention. More generally, those skilled
in the art will readily appreciate that all parameters, dimensions,
materials, and configurations described herein are meant to be
exemplary and that the actual parameters, dimensions, materials,
and/or configurations will depend upon the specific application or
applications for which the teachings of the present invention
is/are used. Those skilled in the art will recognize, or be able to
ascertain using no more than routine experimentation, many
equivalents to the specific embodiments of the invention described
herein. It is, therefore, to be understood that the foregoing
embodiments are presented by way of example only and that, within
the scope of the appended claims and equivalents thereto, the
invention may be practiced otherwise than as specifically described
and claimed. The present invention is directed to each individual
feature, system, article, material, kit, and/or method described
herein. In addition, any combination of two or more such features,
systems, articles, materials, kits, and/or methods, if such
features, systems, articles, materials, kits, and/or methods are
not mutually inconsistent, is included within the scope of the
present invention.
[0134] All definitions, as defined and used herein, should be
understood to control over dictionary definitions, definitions in
documents incorporated by reference, and/or ordinary meanings of
the defined terms.
[0135] The indefinite articles "a" and "an," as used herein in the
specification and in the claims, unless clearly indicated to the
contrary, should be understood to mean "at least one."
[0136] It should also be understood that, unless clearly indicated
to the contrary, in any methods claimed herein that include more
than one step or act, the order of the steps or acts of the method
is not necessarily limited to the order in which the steps or acts
of the method are recited.
[0137] In the claims, as well as in the specification above, all
transitional phrases such as "comprising," "including," "carrying,"
"having," "containing," "involving," "holding," "composed of," and
the like are to be understood to be open-ended, i.e., to mean
including but not limited to. Only the transitional phrases
"consisting of" and "consisting essentially of" shall be closed or
semi-closed transitional phrases, respectively, as set forth in the
United States Patent Office Manual of Patent Examining Procedures,
Section 2111.03.
* * * * *