U.S. patent application number 12/407664 was filed with the patent office on 2009-07-16 for methods of fabricating polymeric structures incorporating microscale fluidic elements.
This patent application is currently assigned to Caliper Life Sciences, Inc.. Invention is credited to Luc J. Bousse, Robert S. Dubrow, Khushroo Gandhi.
Application Number | 20090181228 12/407664 |
Document ID | / |
Family ID | 22115327 |
Filed Date | 2009-07-16 |
United States Patent
Application |
20090181228 |
Kind Code |
A1 |
Gandhi; Khushroo ; et
al. |
July 16, 2009 |
METHODS OF FABRICATING POLYMERIC STRUCTURES INCORPORATING
MICROSCALE FLUIDIC ELEMENTS
Abstract
The present invention provides a microfluidic device that
includes first and second polymeric substrates. The first substrate
has a higher transition temperature than the second substrate. A
first substantially planar surface of the first substrate has a
plurality of microscale channels disposed therein. A first
substantially planar surface of the second substrate is thermally
bonded to the first surface of the first substrate, neither of the
first surfaces of the first and second substrates having an
adhesive disposed thereon.
Inventors: |
Gandhi; Khushroo; (Palo
Alto, CA) ; Dubrow; Robert S.; (San Carlos, CA)
; Bousse; Luc J.; (Los Altos, CA) |
Correspondence
Address: |
CARDINAL LAW GROUP;Caliper Life Sciences, Inc.
1603 Orrington Avenue, Suite 2000
Evanston
IL
60201
US
|
Assignee: |
Caliper Life Sciences, Inc.
Mountain View
CA
|
Family ID: |
22115327 |
Appl. No.: |
12/407664 |
Filed: |
March 19, 2009 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
11460938 |
Jul 28, 2006 |
|
|
|
12407664 |
|
|
|
|
10359959 |
Feb 6, 2003 |
7138032 |
|
|
11460938 |
|
|
|
|
09590661 |
Jun 7, 2000 |
6555067 |
|
|
10359959 |
|
|
|
|
09073710 |
May 6, 1998 |
6123798 |
|
|
09590661 |
|
|
|
|
Current U.S.
Class: |
428/213 ;
428/337 |
Current CPC
Class: |
B29C 66/54 20130101;
B29C 66/712 20130101; B32B 37/0023 20130101; B29C 65/02 20130101;
B29C 66/723 20130101; Y10T 428/266 20150115; B29C 65/08 20130101;
B29C 66/73343 20130101; Y10T 428/2495 20150115; G01N 27/44791
20130101; B29L 2031/756 20130101; B01L 3/502707 20130101; B29C
66/73365 20130101; B29C 66/71 20130101; B29C 65/8253 20130101; B32B
38/06 20130101; B29C 66/73115 20130101; B29C 66/30223 20130101;
B07C 5/342 20130101; B32B 2037/0092 20130101; Y10T 156/10 20150115;
B01L 2200/12 20130101; B29C 66/8322 20130101; B32B 2310/028
20130101; B32B 2310/0843 20130101; B29C 66/71 20130101; B29K
2081/06 20130101; B29C 66/71 20130101; B29K 2069/00 20130101; B29C
66/71 20130101; B29K 2067/00 20130101; B29C 66/71 20130101; B29K
2055/02 20130101; B29C 66/71 20130101; B29K 2033/12 20130101; B29C
66/71 20130101; B29K 2027/18 20130101; B29C 66/71 20130101; B29K
2027/14 20130101; B29C 66/71 20130101; B29K 2027/06 20130101; B29C
66/71 20130101; B29K 2023/18 20130101; B29C 66/71 20130101; B29K
2023/12 20130101; B29C 66/71 20130101; B29K 2023/06 20130101 |
Class at
Publication: |
428/213 ;
428/337 |
International
Class: |
B32B 7/02 20060101
B32B007/02; B32B 5/00 20060101 B32B005/00 |
Claims
1. A microfluidic device, comprising: a first polymeric substrate
comprising a first substantially planar surface having a plurality
of microscale channels disposed therein; a second polymeric
substrate comprising a first substantially planar surface, the
first surface of the second substrate being thermally bonded to the
first surface of the first substrate, neither of the first surfaces
of the first and second substrates having an adhesive disposed
thereon, wherein the first substrate has a higher transition
temperature than the second substrate.
2. The microfluidic device of claim 1, wherein the first polymeric
substrate has a first thickness, and wherein the second polymeric
substrate has a second thickness, the first thickness being greater
than the second thickness.
3. The microfluidic device of claim 2, wherein the thickness of the
first polymeric substrate is within a range of 1.0 mm to 3.0
mm.
4. The microfluidic device of claim 1, wherein at least one of the
first and second polymeric substrates comprises a copolymer.
5. The microfluidic device of claim 1, wherein the transition
temperature of the first substrate is at least 5.degree. C. higher
than the transition temperature of the second substrate.
6. The microfluidic device of claim 1, wherein the transition
temperature of the first substrate is at least 10.degree. C. higher
than the transition temperature of the second substrate.
7. The microfluidic device of claim 1, wherein the transition
temperature of the first substrate is at least 20.degree. C. higher
than the transition temperature of the second substrate.
8. The microfluidic device of claim 1, wherein the transition
temperature of the first substrate is at least 50.degree. C. higher
than the transition temperature of the second substrate.
9. The microfluidic device of claim 1, wherein the transition
temperature of the first substrate is at least 100.degree. C.
higher than the transition temperature of the second substrate.
10. The microfluidic device of claim 1, wherein the plurality of
microscale channels in the first surface of the first substrate are
embossed in the first surface.
11. The microfluidic device of claim 1, wherein the plurality of
microscale channels in the first surface of the first substrate are
cast in the first surface.
12. The microfluidic device of claim 1, wherein the first substrate
is injection molded.
13. The microfluidic device of claim 12, wherein the plurality of
microscale channels in the first surface of the first substrate are
molded in the first surface.
14. The microfluidic device of claim 1, wherein at least one of the
plurality of channels has a cross-sectional dimension between 0.1
.mu.m and 500 .mu.m.
15. The microfluidic device of claim 1, wherein the second
substrate comprises a plurality of apertures disposed therethrough,
each aperture being in fluid communication with at least one of the
plurality of channels in the first surface of the first
substrate.
16. The microfluidic device of claim 1, wherein the first and
second substrates comprise different grades of the same
polymer.
17. The microfluidic device of claim 1, wherein the first surface
of the second substrate does not substantially project into the
plurality of channels.
18. The microfluidic device of claim 1, wherein the first surface
of the second substrate projects into the plurality of channels
less than 10% of a total cross-sectional area of an unobstructed
channel.
19. The microfluidic device of claim 1, wherein the first surface
of the second substrate projects into the plurality of channels
less than 5% of a total cross-sectional area of an unobstructed
channel.
20. The microfluidic device of claim 1, wherein the first surface
of the second substrate projects into the plurality of channels
less than 2% of a total cross-sectional area of an unobstructed
channel.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application is a continuation of U.S. patent
application Ser. No. 11/460,938, filed Jul. 28, 2006, which is a
continuation of U.S. patent application Ser. No. 10/359,959, filed
Feb. 6, 2003, now U.S. Pat. No. 7,138,032, which is a division of
U.S. patent application Ser. No. 09/590,661, filed Jun. 7, 2000,
now U.S. Pat. No. 6,555,067, which is a division of U.S. patent
application Ser. No. 09/073,710, filed May 6, 1998, now U.S. Pat.
No. 6,123,798, which are herein incorporated by reference in their
entirety.
BACKGROUND OF THE INVENTION
[0002] As with the electronics and computer industries, trends in
chemical and biochemical analysis are moving toward faster, smaller
and less expensive systems and methods for performing all types of
chemical and biochemical analyses.
[0003] The call for smaller systems and faster methods has been
answered, in part, through the development of microfluidic
technologies, which perform chemical and biochemical analyses and
syntheses in extremely small-scale integrated fluid networks. For
example, published International Patent Application No. WO 98/00231
describes microfluidic devices, systems and methods for performing
a large number of screening assays within a single microfluidic
device that is on the order of several square centimeters. Such
developments have been made possible by the development of material
transport systems that are capable of transporting and accurately
dispensing extremely small volumes of fluid or other materials. See
Published International Application No. 96/04547 to Ramsey.
[0004] By accurately controlling material transport among a number
of integrated channels and chambers, one is able to perform a large
number of different analytical and/or synthetic operations within a
single integrated device. Further, because these devices are of
such small scale, the amount of time for reactants to transport
and/or mix, is very small. This results in a substantial increase
in the throughput level of these microfluidic systems over the more
conventional bench-top systems.
[0005] By reducing the size of these microfluidic systems, one not
only gains advantages of speed, but also of cost. In particular,
these small integrated devices are typically fabricated using
readily available microfabrication technologies available from the
electronics industries which are capable of producing large numbers
of microfluidic devices from less raw materials. Despite these cost
savings, it would nonetheless be desirable to further reduce the
costs required to manufacture such microfluidic systems.
[0006] A number of reporters have described the manufacture of
microfluidic devices using polymeric substrates. See, e.g.,
Published International Patent Application No. WO 98/00231 and U.S.
Pat. No. 5,500,071. In theory, microfabrication using polymer
substrates is less expensive due to the less expensive raw
materials, and the `mass production` technologies available to
polymer fabrication and the like.
[0007] However, despite these cost advantages, a number of problems
exist with respect to the fabrication of microfluidic devices from
polymeric materials. For example, because polymeric materials are
generally flexible, a trait that is accentuated under certain
fabrication methods, e.g., thermal bonding, solvent bonding and the
like, it is difficult to accurately manufacture microscale
structural elements in such polymeric materials. In particular, the
microscale structures are easily deformed under manufacturing
conditions, either due to applied pressures or relaxation of the
polymer matrix based upon its intrinsic structural properties.
[0008] Accordingly, it would generally be desirable to have a
method of fabricating microscale devices where the structural
aspects of the device are not substantially perturbed during the
fabrication process. The present invention meets these and other
needs.
SUMMARY OF THE INVENTION
[0009] It is a general object of the present invention to provide
methods of fabricating polymeric microfluidic devices, and the
devices fabricated using these methods. In a first aspect, the
present invention provides for methods of fabricating a
microfluidic device comprising a first substrate having a first
planar surface, and a second substrate layer having a first planar
surface, wherein the first planar surface of the first substrate
comprises a plurality of microscale grooves disposed therein. The
first planar surface of the second substrate is heated
approximately to the transition temperature of the first surface of
the second substrate without heating the first surface of the first
substrate approximately to the transition temperature of the first
surface of the first substrate. The first surface of the first
substrate is then bonded to the first surface of the second
substrate.
[0010] This invention also provides methods of fabricating a
microfluidic device comprising a first substrate having a first
planar surface, and a second substrate layer having a first planar
surface wherein the first planar surface of the first substrate
comprises a plurality of microscale grooves disposed therein, and
the first planar surface of the second substrate has a lower
transition temperature than the first surface of the first
substrate. The first planar surface of the second substrate is
heated approximately to its transition temperature. The first
surface of the first substrate is then bonded to the first surface
of the second substrate.
[0011] This invention also provides methods of fabricating
microfluidic devices comprising a first substrate having a first
planar surface, and a second substrate layer having a first planar
surface, wherein the first planar surface of the second substrate
has a lower transition temperature than the first surface of the
first substrate. The first surface of the second substrate is
heated approximately to the transition temperature. The first
surface of the first substrate is bonded to the first surface of
the second substrate.
[0012] This invention also provides methods of fabricating a
microfluidic device comprising a first substrate having at least a
first surface and a second substrate having at least a first
surface, wherein at least one of the first surface of the first
substrate or the first surface of the second substrate comprises a
textured surface, and mating and bonding the first surface of the
first substrate to the first surface of the second substrate.
[0013] This invention also provides methods of fabricating a
microfluidic device comprising a first substrate having a first
planar surface, and a second substrate layer having a first planar
surface, wherein the first planar surface of the second substrate
has a lower transition temperature than the first surface of the
first substrate. The first surface of the first substrate is
thermally bonded to the first surface of the second substrate,
whereby the first surface of the second substrate does not
substantially project into the plurality of channels.
[0014] This invention also provides a microfluidic device
comprising a first polymeric substrate having at least a first
planar surface, the first planar surface comprising a plurality of
channels disposed therein. The device also includes a second
polymeric substrate layer having at least a first planar surface,
the first planar surface of the second substrate is bonded to the
first planar surface of the first substrate, and wherein the first
surface of the second substrate has a lower transition temperature
than the first surface of the first substrate.
[0015] This invention also provides a microfluidic device
comprising a first polymeric substrate comprising a first planar
surface having a plurality of microscale channels disposed therein.
The device also contains a second polymeric substrate comprising a
first planar surface, the first planar surface of the second
substrate being non-solvent bonded to the first planar surface of
the first substrate, wherein the first surface of the second
substrate does not substantially project into the plurality of
channels.
BRIEF DESCRIPTION OF THE FIGURES
[0016] FIG. 1 is a schematic illustration of a microfluidic device
incorporating a layered structure.
[0017] FIG. 2 illustrates examples of channel deformation in some
methods of fabricating layered polymeric microstructures. FIG. 2A
illustrates the extrusion of a cover layer substrate into a channel
structure fabricated into the surface of another substrate when the
two substrates are thermally bonded together using conventional
means. FIG. 2B illustrates the softening or dulling of channel
comers in a thermally bonded polymeric microfluidic device, where
the channel bearing structure is injection molded, or otherwise has
residual stresses frozen into the structure. FIG. 2C illustrates
bonded substrate layers where no channel deformation or deflection
of the upper substrate into the channel has occurred, and no
channel relaxation occurred during bonding.
[0018] FIG. 3 illustrates an example of surface texturing utilized
to fabricate layered polymeric microstructures. FIGS. 3A and 3B
illustrate the bonding layer both before and after the bonding
process, respectively.
[0019] FIG. 4 illustrates a plot of both structural deformation of
surface textures in the mating of two substrates as well as local
pressure on the raised portions of the textures over time of the
thermal bonding process.
[0020] FIG. 5 is a cross-section of two channels thermally bonded
together. FIG. 5A illustrates a channel in which an upper substrate
is protruding into the channel, whereas the channel shown in FIG.
5B is substantially clear of obstruction from the upper
substrate.
DETAILED DESCRIPTION OF THE INVENTION
I. General
[0021] As noted above, the present invention generally provides
improved methods of fabricating polymeric microfluidic devices.
Generally, these improved methods allow for the rapid fabrication
of polymeric devices that incorporate microscale fluidic
structures, whereby the fabrication process does not substantially
distort or deform such structures.
[0022] As used herein, the term "microscale" or "microfabricated"
generally refers to structural elements or features of a device
which have at least one fabricated dimension in the range of from
about 0.1 .mu.m to about 500 .mu.m. Thus, a device referred to as
being microfabricated or microscale will include at least one
structural element or feature having such a dimension. When used to
describe a fluidic element, such as a passage, chamber or conduit,
the terms "microscale," "microfabricated" or "microfluidic"
generally refer to one or more fluid passages, chambers or conduits
which have at least one internal cross-sectional dimension, e.g.,
depth, width, length, diameter, etc., that is less than 500 .mu.m,
and typically between about 0.1 .mu.m and about 500 .mu.m. In the
devices of the present invention, the microscale channels or
chambers preferably have at least one cross-sectional dimension
between about 0.1 .mu.m and 200 .mu.m, more preferably between
about 0.1 .mu.m and 100 .mu.m, and often between about 0.1 .mu.m
and 20 .mu.m. Accordingly, the microfluidic devices or systems
prepared in accordance with the present invention typically include
at least one microscale channel, usually at least two intersecting
microscale channels, and often, three or more intersecting channels
disposed within a single body structure. Channel intersections may
exist in a number of formats, including cross intersections, "T"
intersections, or any number of other structures whereby two
channels are in fluid communication.
[0023] In particularly preferred aspects, the microfluidic devices
described herein, are used in conjunction with controlled
electrokinetic material transport systems, as described in
Published International Application No. 96/04547 to Ramsey, which
is incorporated herein by reference for all purposes. Specifically,
such material transport systems are used to transport fluid and/or
other materials through the interconnected channels of the devices
in a controlled fashion.
[0024] The microfluidic devices in accordance with the present
invention include a body structure that has disposed therein, an
integrated network of microscale channels or conduits. The
different elements of the body structure may be fabricated from a
number of different separate parts to define the various channels
and/or chambers of the device. In particularly preferred aspects,
the body structure of the device is fabricated as a layered
structure. An example of a device incorporating this layered
structure is illustrated in FIG. 1. In particular, the device 10,
includes a bottom portion 12 which comprises a solid substrate that
is substantially planar in structure, and which has at least one
substantially flat upper surface 14.
[0025] The channels and/or chambers of the microfluidic device are
typically fabricated into the upper surface of the bottom substrate
or portion 12, as microscale grooves or indentations 16, using the
microfabrication techniques described herein. The top portion or
substrate 18 also comprises a first planar surface 20, and a second
surface 22 opposite the first planar surface 20. In the
microfluidic device shown in FIG. 1, the top portion of the device
optionally includes a plurality of apertures, holes or ports 24
disposed therethrough, e.g., from the first planar surface 20 to
the second surface 22 opposite the first planar surface.
[0026] The first planar surface 20 of the top substrate 18 is then
mated, e.g., placed into contact with, and bonded to the planar
surface 14 of the bottom substrate 12, covering and sealing the
grooves and/or indentations 16 in the surface of the bottom
substrate, to form the channels and/or chambers (i.e., the interior
portion) of the device at the interface of these two components. In
those embodiments utilizing incorporated reservoirs or ports, the
holes 24 in the top portion of the device are oriented such that
they are in communication with at least one of the channels and/or
chambers formed in the interior portion of the device from the
grooves or indentations in the bottom substrate. In the completed
device, these holes function as the reservoirs for facilitating
fluid or material introduction into the channels or chambers of the
interior portion of the device, as well as providing ports at which
electrodes may be placed into contact with fluids within the
device, allowing application of electric fields along the channels
of the device to control and direct fluid transport within the
device.
[0027] As noted above, at least one, and preferably both or all of
the substrate layers, e.g., as described with reference to FIG. 1,
comprise a polymeric material or substrate. In accordance with the
present invention, the polymeric substrate materials used to
fabricate the microfluidic devices described herein are typically
selected from a wide variety of different polymeric materials.
Examples of particularly useful polymer materials include, e.g.,
polymethylmethacrylate, polycarbonate, polytetrafluoroethylene,
polyvinylchloride, polydimethylsiloxane, polysulfone, polystyrene,
polymethylpentene, polypropylene, polyethylene, polyvinylidine
fluoride, and acrylonitrile-butadiene-styrene copolymer.
[0028] Because microscale fluidic structures are of such small
dimensions, e.g., channel depths typically falling in the range of
from about 1 to 50 .mu.m, even slight deformation of a channel's
structure can have seriously adverse effects on the function of the
device incorporating that channel, including partial or total
channel occlusion, formation of sharp corners in the channels along
which irregular capillary flow occurs, structural irregularities
causing disruptive flow patterns during operation, and the
like.
[0029] Unfortunately, channel distortion of the type referred to
above, is exactly the type of problems faced in fabricating
polymeric microfluidic devices. In particular, in preferred
aspects, the microfluidic devices of the present invention are
fabricated as an aggregation of different substrate layers that are
typically planar in structure. One of the layers typically includes
a series of grooves and/or depressions fabricated into its surface,
which grooves or depressions define the channels and chambers of
the ultimate microfluidic device. A second layer is overlaid and
bonded to the first layer to seal the grooves and depressions
forming the channels and chambers. Optionally, the channels and/or
chambers are defined in an intermediate layer, which defines the
sides of the channels and/or chambers. The intermediate layer is
then sandwiched and bonded between the top and bottom layers, which
form the top and bottom surfaces, respectively, of the channels
and/or chambers. The substrate layers are then bonded together
using known bonding techniques. For polymeric substrates, such
techniques include, e.g., thermal bonding, ultrasonic bonding or
welding, adhesive bonding, or solvent bonding.
[0030] In thermal bonding of solid polymeric substrates, one or
more of the substrates to be bonded is heated to the transition
temperature of the substrate surface. As used herein, the
"transition temperature" refers to the temperature at which the
polymer substrate material, which normally has a glass-like
character, undergoes the transformation from a rigid material to a
soft rubber, e.g., the melting point. In particular, as a polymer
is heated to a temperature at or just below the transition
temperature, the polymer starts to soften. In the case of
non-crystalline polymeric materials, the transition temperature is
typically referred to as the "glass transition temperature,"
typically denoted by T.sub.g. At the glass transition temperature,
the glass-like polymer begins to take on the more rubbery
character.
[0031] For non-polymeric substrates, e.g., glass, quartz, silicon
and the like, the substrate is typically sufficiently hard that
even under extremely high bonding temperatures, e.g., in excess of
500.degree. C., there is substantially no deformation of the
microscale channels between the substrates being bonded. For
polymeric substrates, however, substantial deformation can occur
during thermal bonding at substantially lower temperatures.
[0032] For example, when polymeric substrates are heated to their
transition temperature and bonded together, microscale structural
elements have a tendency to flatten under the elevated temperatures
and pressures. Similarly, otherwise flat substrate layers have a
tendency to be extruded into cavities, depressions or grooves on
the opposite substrate surface, e.g., channels and/or chambers, as
a result of their softer character and the effects of the applied
pressure. This extrusion of an upper substrate layer into a channel
or chamber creates a number of problems. For example, such
extrusion results in unknown or variable volumes for the channels
and chambers, and also results in substantial occlusion of
channels. Further, and as referenced above, this channel extrusion
can result in the generation of fluid shooters, where fluids in the
corners of channels move much faster than the remainder of the
fluid. These shooters have a tendency to travel far ahead of the
bulk fluid front in capillary filling of channels, and join
together to trap air bubbles within the channels. The presence of
such air bubbles, particularly in extremely small-scale channels
can be fatal to the proper operation of the device.
[0033] In the case of injection molded polymeric parts, additional
problems are associated with the fabrication of polymeric devices.
For example, in the injection molding process, polymeric material
injected into a mold has a tendency to align the individual polymer
molecule strands in the molded product in the direction of polymer
injection. This alignment of polymer molecules results in an
inherent or "frozen" stress in the hardened product as the polymer
strands tend toward their natural random state. This frozen stress
often results in a disproportionate shrinking of the molded part in
the length dimension of the aligned polymers, as compared to the
width, when the parts are heated to or near their transition
temperatures, e.g., for thermal bonding. This shrinking then leads
to deformation of microscale structures on the polymer part, and
even warping of the part as a whole.
[0034] FIG. 2 illustrates some examples of the types of channel
deformation that occur during these types of thermal bonding
processes for polymeric substrates. FIG. 2A illustrates the
extrusion of an upper substrate layer into a channel structure
fabricated on the lower substrate layer following thermal bonding
of the substrates. Although illustrated as a drawing, the
dimensions provided represent actual and substantial encroachment
of the upper substrate into the channel. This was a result of
heating the layers to above the transition temperature for the
material used, and applying pressure to the two substrates to
facilitate bonding. As shown, the upper layer encroaches upon the
channel structure by a significant percentage of the overall
cross-sectional area of the channel over that of the unobstructed
channel shown in FIG. 2C, resulting in reduced performance of the
device incorporating this channel, as described above.
[0035] FIG. 2B illustrates an example of thermally bonded polymeric
substrates where the lower substrate, bearing the channel structure
was injection molded, or otherwise had stresses frozen into it.
Relaxation of the polymers in the substrate when the substrate was
heated during thermal bonding resulted in a dulling of the channel
edges.
[0036] One alternative to thermal bonding is ultrasonic welding or
bonding. In these methods, a series of sharp protrusions or ridges
("energy directors") are fabricated on one of the parts to be
bonded. Under elevated pressure and high frequency vibrations,
these energy directors melt and bond with the corresponding surface
on the other substrate. Again, however, use of such methods
generally results in excessive channel distortion or irregularity,
such that such the methods are not useful in fabrication of
microscale fluidic devices. In particular, the edges of the bonded
regions resulting from these ultrasonic methods tend to be
relatively irregular in comparison with the edges of the channels.
As such, the corners at which the two substrates meet will also be
irregular, as a result of some material encroaching into the
channel, and/or openings where the bonded edge does not reach the
channel edge. These latter irregularities cause substantial
difficulty in microfluidic systems as they can give rise to fluid
"shooters" (edges of a channel at which capillary flow is faster
than capillary flow in the rest of the channel) during fluid
introduction and movement within the channel.
[0037] Another alternative to thermal bonding is the use of
adhesives to bond polymeric parts together. The use of adhesives
alleviates the problems of thermal deformation of channel
structures. However, in order to be effective in the fabrication of
microfluidic systems, adhesive must be carefully applied in order
to ensure that the channels and chambers will be entirely sealed
after bonding. Further, because microfluidic devices are generally
used in sensitive analytical operations, it is generally desirable
to avoid introducing any unwanted chemical components into the
channels and/or chambers of the device. Thus, while one must ensure
application of adequate adhesive to ensure sealing, one must avoid
getting the adhesive into the extremely small scale channels and
chambers. In addition to adverse chemical interactions, such
contaminants can potentially produce structural barriers or
occlusions which adversely affect fluid movement.
[0038] Another method of bonding polymeric substrates is through
the use of solvent bonding processes. Typically, these processes
involve the mating of two polymeric parts followed by application
of a polymer softening solvent to the space between the parts,
e.g., via capillary action. The softening and re-hardening of the
polymer interface results in a bonded part. Solvent bonding methods
are well known in the art and are described in, e.g., Plastics
Technology, Robert V. Milby (McGraw-Hill 1973), and Handbook of
Plastics Joining: A Practical Guide (Plastics Design Library,
1996), both of which are incorporated herein by reference. The same
contamination problems associated with adhesive bonding are also
present in solvent bonding methods. Further, such solvent process
typically cause at least some level of polymer softening which can
lead to adverse structural effects, e.g., as described above. In
addition, solvent bonding processes will often produce stress
cracking when used in conjunction with injection molding
processes.
II. Polymer Selection
[0039] In a first aspect, the methods of the present invention
generally address the problems typically associated with the
fabrication of microfluidic devices from polymeric substrates. In
preferred aspects, the methods described herein are directed to
thermal bonding methods of fabricating microfluidic devices.
Accordingly, the methods of the invention are generally described
with reference to the fabrication of microfluidic devices that
incorporate a layered structure. Such devices typically include a
top portion, a bottom portion, and an interior portion that is
defined by the mating of the top portion to the bottom portion.
Typically, a first substrate is provided which includes at least
one planar surface. The microscale structural elements of the
device are generally fabricated into the first surface of the first
substrate. In the case of microscale fluidic channels and/or
chambers, the structures typically are fabricated as microscale
grooves or depressions in that surface.
[0040] In addition to the channel structures of the device
fabricated into the first substrate surface, the second substrate
also typically includes a plurality of apertures disposed through
it. Each aperture is generally provided so as to be placed in fluid
communication with at least one channel that is disposed within the
interior portion of the device when the layers are bonded together.
These apertures then function as the fluid reservoirs of the
device, as well as points of access to the channel structures,
e.g., for fluid introduction, electrical sensing and controlled
electrokinetic material transport, and the like.
[0041] Fabrication of the grooves in the substrate surface is
generally carried out using known polymer fabrication methods,
e.g., injection molding, embossing, or the like. In particular,
master molds or stamps are optionally created from solid
substrates, such as glass, silicon, nickel electroforms, and the
like, using well known microfabrication techniques. These
techniques include photolithography followed by wet chemical
etching, LIGA methods, laser ablation, thin film deposition
technologies, chemical vapor deposition, and the like. These
masters are then used to injection mold, cast or emboss the channel
structures in the planar surface of the first substrate surface. In
particularly preferred aspects, the channel or chamber structures
are embossed in the planar surface of the first substrate.
[0042] By embossing the channel structures into the first
substrate, one avoids the stress relaxation problems associated
with injection molded substrates. In particular, because embossed
substrates are not flowed or injected into a mold, there is
substantially less alignment of the polymer strands from flowing of
the polymer material. Accordingly, during thermal bonding, there is
substantially less relaxation of the overall substrate when the
substrates are mated, and therefore, substantially less channel
deformation.
[0043] Typically, the grooves fabricated into the surface of the
first substrate are fabricated as a series of intersecting grooves,
to form the integrated intersecting channel structures of the
devices of the invention. The grooves are formed into channels by
mating a second substrate layer to the first, to cover and seal the
grooves and/or depressions to form the channels and/or chambers of
the device. In accordance with one aspect of the invention, the
second substrate is thermally bonded to the surface of the first
substrate over the channels. The surfaces of the two substrates are
typically planar to permit adequate contact across the surface.
[0044] In order to avoid additional distortion of channel
structures on the first substrate during the thermal bonding of the
second substrate, the first and second substrates are typically
selected to have differing transition temperatures. In particular,
the substrate that bears the microscale structures is typically
selected to have a higher transition temperature than the cover
layer that is to be bonded to it. Selection of the channel bearing
substrate to have a higher transition temperature, allows the cover
layer to be heated to its transition temperature and mated with the
channel bearing substrate, without distorting or deforming the
channel structures on the channel bearing substrate. Of course,
depending upon the desired goal, the channel bearing substrate may
be selected to have a lower transition temperature, e.g., if
substrate extrusion into the channels is the most critical, or only
actual problem to be addressed. In particularly preferred aspects,
both substrate layers are selected to minimize both channel
distortion and channel occlusion problems, by selecting substrates
that are sufficiently different in their transition temperatures to
prevent channel distortion, but sufficiently close to prevent
excessive extrusion of the upper substrate into the channel
structures. Selection of a polymer having a higher transition
temperature for the channel bearing substrate, permits the use of
injection molded parts. Specifically, because these substrates do
not need to be heated to their transition temperatures for thermal
bonding, there is less chance of the substrate relaxing, and thus,
resulting in deformation of the channels.
[0045] In preferred aspects, the transition temperature of the two
substrates are at least about 5.degree. C. apart, more preferably
at least about 10.degree. C. apart, more preferably, at least
20.degree. C. apart, often at least 50.degree. C., and in some
cases, at least 100.degree. C. apart. For example, where one
substrate (that having the lower transition temperature) has a
transition temperature of approximately 80.degree. C., the other
substrate will typically have a transition temperature of at least
85.degree. C., preferably at least 90.degree. C., more preferably
at least 100.degree. C., often at least 130.degree., and in some
cases at least 180.degree. C. Generally speaking, the transition
temperature of the substrate having the higher transition
temperature is typically at least 40.degree. C., while the
transition temperature of the substrate having the lower transition
temperature is less than 150.degree. C. Alternatively, the surface
of one substrate is heated to its transition temperature while the
surface of the other substrate is maintained at a lower
temperature. As above, the first substrate is typically heated to a
temperature at least 5.degree. C., 10.degree. C., 20.degree. C.,
50.degree. C. or even at least 100.degree. C. above the temperature
at which the other substrate is maintained.
[0046] Thus, in accordance with the methods described herein, the
planar surface of one of the substrates, typically the cover layer
substrate, is heated approximately to the surface's transition
temperature, without reaching the transition temperature of the
surface of the other substrate, e.g., the channel bearing
substrate. Typically, the entire polymeric part is fabricated from
a single polymer, and thus the transition temperature of the
surface is the same as the remainder of the substrate. However, it
will be appreciated that multilayer polymeric materials are also
envisioned in accordance with the present invention, including
polymer substrates bearing a different polymer coating.
[0047] Following the heating of the substrates to the first
transition temperature, the substrates are bonded together. In most
but not all cases, this typically involves the application of
slight pressure to the two substrates, pressing their bonding
surfaces together, to ensure adequate and complete bonding of the
parts. In those cases where a pressure is applied between the
substrates, the amount of applied pressure is typically dependent
upon the polymers and temperatures used. However, in general, the
applied pressures are generally in the range of from about 0.1
kg/cm.sup.2 to about 20 kg/cm.sup.2.
[0048] In preferred aspects, the polymeric substrate materials used
in accordance with this aspect of the invention comprise
polymethylmethacrylate (PMMA), polycarbonate,
polytetrafluoroethylene (TEFLON.TM.), polyvinylchloride (PVC),
polydimethylsiloxane (PDMS), polysulfone, polystyrene,
polymethylpentene, polypropylene, polyethylene, polyvinylidine
fluoride, ABS (acrylonitrile-butadiene-styrene copolymer), and the
like. In many aspects, the present invention utilizes those
polymers which are generally non-crystalline in structure, i.e.,
polymethylmethacrylate, polycarbonate, polyvinylchloride,
polydimethylsiloxane, polysulfone, polystyrene, polymethylpentene,
polyvinylidine fluoride, and acrylonitrile-butadiene-styrene
copolymer
[0049] In particularly preferred aspects, both substrates comprise
polymethylmethacrylate grades. In order to provide a different
transition temperature for the second substrate, or cover layer,
this substrate typically comprises an injection moldable grade of
PMMA. Examples of such polymeric materials include, e.g., Acrylite
polymers, e.g., M-30, L-40, etc., available from CYRO Industries,
for injection moldable grades of PMMA and Plexiglas V-825,
available from Atohaas, North America, for the structure bearing
substrate, which has a higher transition temperature. Typically,
adjustment of the transition temperature is accomplished through
the adjustment of the polymer composition, i.e., incorporating
different amounts of other comonomers. For example, for PMMA, lower
transition temperatures, e.g., for injection moldable grades of
PMMA, are generally achieved by incorporating other acrylic
comonomers, i.e., ethylacrylate, methylacrylate or butylacrylate
monomers, during the synthesis of the polymer. Similarly, for
polycarbonate polymers, transition temperatures are generally
adjusted by incorporation of bisphenol analogs during synthesis,
and adjusting their relative concentration. In the case of ABS
polymers, transition temperatures may be adjusted by adjusting the
relative level of the polymers in the combination, e.g.,
acrylonitrile, butadiene and styrene. Optionally, with the addition
of other additives, i.e., tackifiers, waxes and the like, one can
increase adhesive properties of substrate surfaces at or below the
transition temperature of the bulk substrate material, thereby
giving the surface of the substrate a lower effective transition
temperature or "bonding temperature."
[0050] Transition temperatures are then adjusted by adjusting the
relative percentages of these other monomers, i.e., increasing to
reduce transition temperature. Typically, these additional monomers
are present in the overall polymer at a concentration of at least
about 0.1%, 1%, 2% and often at least about 5% or 10% or even
greater, based upon the total monomer concentration used in the
polymer synthesis, depending upon the desired transition
temperature range.
[0051] Alternatively, or additionally, transition temperatures for
polymers may be adjusted by adjusting the molecular weight of the
polymers. In particular, longer and larger polymers typically have
higher transition temperatures than smaller, shorter polymers.
Thus, a substrate fabricated from a polymer having a lower average
molecular weight, has a lower transition temperature than a polymer
having a higher average molecular weight. In such cases, the
polymer having the larger average molecular weight (and higher
transition temperature) is at least about 5% larger than the
average molecular weight of the other substrate (having the lower
transition temperature), preferably, at least about 10% larger,
more preferably at least about 20% larger, 50% and often at least
100%, and in many cases, at least about 200% larger than the
polymer used to fabricate the other substrate having the lower
transition temperature.
[0052] As noted above, the methods of the present invention result
in the fabrication of microfluidic devices where the channel
structures are not substantially distorted. In addition, these
devices are characterized in that the cover layer substrate that is
bonded to the channel bearing substrate does not substantially
encroach upon, occlude or otherwise project into the channels of
the device. The phrase "does not substantially project into the
channel," as used herein, means that the cross-sectional area of
the channel structure as it is defined by the structure bearing
substrate (width of fabricated channel X depth of fabricated
channel), is not substantially blocked by extrusion of the cover
layer substrate into the channel. Such occlusion is shown in FIG.
2A. Typically, the cover layer occludes the cross-sectional area of
the channels by less than 20% of the potential channel
cross-section. In preferred aspects, the occlusion is less than 10%
of the total cross-sectional area of the channel, and more
preferably, less than 5% of the total cross-sectional area, and
still more preferably, less than 2% occlusion of the total
cross-sectional area. While solvent bonding methods are generally
capable of producing devices where the cover layer does not
substantially occlude the channels of the device, such solvent
bonding methods have a number of other disadvantages, as described
above. In the present invention, such non-occluded channels are
fabricated in non-solvent bonding and/or non-adhesive bonding
methods, e.g., bonding methods that do not utilize solvents or
adhesives, i.e., thermal bonding, ultrasonic bonding, or the
like.
III. Surface Textures
[0053] In an alternate aspect, the present invention provides
methods of fabricating microfluidic devices from polymeric
substrates by providing at least one of the substrates with a
textured surface to assist bonding. In particular, as noted above,
the use of excessive temperatures and/or excessive pressures during
thermal bonding of polymeric substrates often results in
deformation of the channel structures, and/or occlusion of the
channels by the upper substrate layers being extruded into the
channels. Like the above described aspects, the present embodiment
of the invention improves thermal bonding and other methods of
bonding polymeric substrates by reducing the temperatures and
pressures to which a substrate is exposed during the bonding
process. In accordance with this aspect of the invention, pressures
and/or temperatures for bonding are minimized by reducing the
effective surface area at which bonding occurs. In particular, the
methods of the present invention provide one or both of the
substrate layers having a textured bonding surface. By "textured
bonding surface" is meant that the surface of the substrate that
mates with and is bonded to the other substrate includes a
structural texturing, such as a series of raised ridges, pillars,
posts, or the like, that are disposed on the surface in question.
Texturing of the bonding surfaces may take on a variety of forms.
For example, the texturing optionally includes a series of parallel
raised ridges/grooves fabricated into the bonding surface. Other
textures are also useful in accordance with the present invention,
including raised ridges fabricated in a grid or diamond pattern,
raised pillars or posts fabricated in sufficiently close proximity
that upon bonding, the spaces between them will be filled in and
sealed.
[0054] In particularly preferred aspects, the surface texture is
applied to the bonding surface of the substrate bearing the channel
structures. Specifically, the microfabrication steps applied to the
manufacture of the channel structures, i.e., embossing, injection
molding, etc., can be exploited in the fabrication of the surface
texturing. In addition, in preferred aspects, the surface texture
is applied to the surface into which the channel structures are
fabricated. As such, the texture is not present within the channel
itself, e.g., as would be the case if the texturing was applied to
the cover layer substrate. The texturing may be applied uniformly
over the entire bonding surface of interest. Alternatively, the
texturing may be applied only in those areas where sealing is
desired, e.g. immediately surrounding the channels and chambers of
the device.
[0055] Because the channel structures that are defined within the
devices of the present invention have depths that typically range
from about 5 .mu.m to about 100 .mu.m, it is generally desirable to
provide surface texturing having substantially less depth. In
preferred aspects, the texturing is provided having a height (or
depth) that is from about 1% to about 50% of the channel depth, and
preferably, from about 1% to about 30% of the channel depth, and
still more preferably, between about 1% and about 10% of the
channel depth. Accordingly, while the texturing may vary depending
upon the depth of the channels of the device, the surface texturing
as described herein will typically range from about 0.1 .mu.m to
about 50 .mu.m high (or deep), and preferably, from about 0.25
.mu.m to about 30 .mu.m, and more preferably, from about 0.25 .mu.m
to about 10 .mu.m high (or deep). For channels that are on the
order of 10 to 20 .mu.m deep, surface texturing of between about
0.5 to about 2 .mu.m in depth is generally preferred.
[0056] In thermal bonding methods, the surface texturing serves to
provide localized areas at which melting and bonding occur between
substrate layers, preventing such occurrences within the channel
structures per se, and thus preventing substantial channel
distortion. In particular, because pressure between two substrates
is concentrated in the raised texture structures it requires a
lower overall substrate temperature to produce the desired bonding
between the substrates, e.g., the combined pressure and temperature
effects are concentrated at the raised ridges/structures. Further,
as the texture structures are melted and flattened during the
bonding process, the amount of surface area in contact between the
two substrates increases, thereby reducing the localized
pressure/heating effects. This increase in surface area and
effective decrease in the localized pressure creates a bonding
process that is somewhat self-regulating. In particular, after the
surface texturing is distorted or flattened enough by the heat and
pressure, the contact area between the substrates increases,
thereby effectively reducing the localized pressure, which results
in a considerable slowing of deformation. Specifically, the
constant force applied to the texture structures is dissipated over
a larger substrate surface as these textures collapse into the rest
of the substrate surface, thereby arresting the melting and bonding
process.
[0057] This self-regulating process is illustrated in FIG. 4, which
is a superimposed graph of localized pressure versus time 402 and
texture deformation versus time 404. In particular, the local
pressure at the interface of two substrates, e.g., at the top of
the texturing (ridges, posts, etc.) at the beginning of the thermal
bonding process, is spread over only the area of the interface. As
the texturing (ridges, posts, etc.) melts during the thermal
process, the area of the interface increases as the texturing
flattens out. Accordingly, the same amount of applied force is
spread over a wider area, until the texturing is nearly completely
flattened out, at which point the pressure at the interfacing
surfaces stabilizes at nor near the total applied pressure (as the
interface is substantially a single surface, thus local
pressure=total pressure).
[0058] FIG. 3A illustrates the use of surface texturing in the
bonding methods described herein. As shown, the upper
substrate/cover layer 302 is mated with the lower substrate 304
that includes a channel 306 fabricated into its surface. The upper
surface 308 of the bottom substrate 304 has provided thereon a
surface texturing that includes a plurality of raised ridges 310,
or raised posts/pillars on the bonding surface of the channel
bearing substrate. The upper substrate 302 is mated to the lower
substrate under appropriate pressure and temperature conditions. In
preferred aspects, the applied temperature is typically at or above
the transition temperature for the lower substrate, but well below
the transition temperature of the upper substrate. Under the
elevated temperature conditions, the focused pressure upon the
texturing structures 310 melts and spreads the texture structures
and bonds with the upper substrate 302. This is illustrated in FIG.
3B, where the collapsed or melted texture structures 310a form the
bond point between the two substrate layers 302 and 304. Although
preferred aspects utilize two substrates having different
transition temperatures, this is not necessarily required. In
particular, because the microstructures permit the focusing of
pressure on those texturing structures, lower pressures may be used
in the thermal bonding process. As noted previously, excessive
applied pressures are at least partially to blame for the channel
deformations described above. Therefore, by reducing the applied
pressures, one also reduces the severity of channel
deformation.
[0059] Although the surface textured methods described herein are
generally in reference to thermal bonding methods, such techniques
are also applicable to acoustic or sonic welding or bonding
techniques. In particular, the raised elements of the surface
texturing described herein, generally function in a manner similar
to energy directors in conventional acoustic welding techniques. In
use, the substrate layers are mated together and an appropriate
pressure is applied. One or both of the substrates is then
acoustically vibrated, e.g., at greater than about 10 to 20 KHz.
The vibrational friction caused at the contact point between the
two surfaces, e.g., on the texture elements or ridges, results in a
localized heating, melting and bonding of the substrate layers.
Further, as with the thermal bonding methods, once the texture
elements have completely melted or compressed into their respective
surfaces, the applied pressure is spread over the entire surface
area, and melting and bonding cease. Again, this prevents
substantial distortion of the channels. Acoustic welding methods
and systems have been described in the art, and are commercially
available, e.g., from Hermann Ultrasonics, Inc.
IV. Other Polymer Selection Criteria
[0060] In addition to selecting polymeric substrates based upon
their transition temperatures, there are also a number of other
criteria one can apply in polymer selection. For example, the
microfluidic devices of the present invention are often used in the
performance of analytical operations which employ optical detection
systems. Such devices typically include a detection window disposed
across one of the channels of the device and through which an
optical signal can pass. As such, polymeric materials that are
transparent are generally used in the fabrication of such devices.
In particularly preferred aspects, fluorescent detection systems
are utilized. This generally dictates that polymer grades be
selected that have minimal levels of background or
auto-fluorescence. Typically, auto-fluorescence is lower in certain
polymer types, e.g., polymethylmethacrylate, as well as in more
pure grades of polymers. Table 2 illustrates a comparison of the
autofluorescence of different types and grades of polymers as
compared to different types of glass.
[0061] Selection of an appropriate polymer type and grade generally
depends upon the type of detection system utilized, the wavelength
of light applied to the system, and the like. In general, however,
the background fluorescence of the polymer substrate is less than 5
times that of glass, preferably less than twice that of glass, and
more preferably, approximately the same as or less than glass, for
the desired wavelength.
[0062] In addition to detection criteria, polymer substrates are
also optionally selected for their ability to support or eliminate
electroosmotic flow. In particular, as described in U.S. Ser. No.
08/843,212 filed Apr. 14, 1997 (incorporated herein by reference
for all purposes), polymeric substrates may be selected or treated
to support a desired level of electroosmotic flow, depending upon
the application to which the device is going to be put. In
particular, some polymeric materials have a sufficiently high level
of surface charge to allow adequate electroosmotic flow in
microscale channels fabricated from those materials. Electroosmotic
flow is generally a desirable characteristic where the device is
utilized in applications that employ bulk fluid flow within the
channel networks, whereas certain other applications, e.g., nucleic
acid separations, generally seek to eliminate such flow. Again,
polymers may be selected to achieve this latter goal.
[0063] The present invention is illustrated in greater detail with
reference to the following nonlimiting examples.
EXAMPLES
Example 1
Polymer Selection
[0064] Polymers were selected based upon their clarity, low
fluorescence, processability and commercial availability. Several
polymer materials were evaluated, as set forth in Table 1,
below.
TABLE-US-00001 TABLE 1 Acrylite Acrylite Plexiglas Makrolon Lexan
M-30 L-40 VS UVT DP-1-1265 OQ1020L Property (Acrylic) (Acrylic)
(Acrylic) (Polycarb.) (Polycarb.) Transmittance 92 92 92 89 90 Haze
(%) <1 2 2 . . . . . . Melt Flow Rate 24 28 24 75 65 (g/10 min)
(all at 230.degree. C., 3.8 kg) Refract. Index 1.49 1.49 1.49 1.582
1.58 Dielectric 19.7 19.7 . . . >16 14.8-17.6 Strength (kV/mm)
Vol. Resistivity . . . . . . . . . 1.0 .times. 10.sup.16 1.0
.times. 10.sup.17 (Ohm/cm) Supplier CYRO Indust. Cyro Indust.
Atohaas, Bayer GE Plastics North Am.
Based upon the results shown in Table 1, acrylic polymers, and
particularly polymethylmethacrylate were selected as the best
polymer substrate, with polycarbonate being the next best
selection. Further tests were performed on these polymers and the
results are shown in Table 2. Polymer resins were tested using
injection molded test plates.
[0065] Fluorescence was measured using the following
conditions:
TABLE-US-00002 Excitation Wavelengths 450-480 nm Emission
Wavelengths 510-549 nm
TABLE-US-00003 TABLE 2 Fluores- Thick- cent Softening Material ness
Counts Point/T.sub.g PMMA Acrylite M-30 1.0 mm 1,720 90.degree. C.
Plexiglas UVT 1.0 mm 1,800 87.degree. C./91.degree. C. Acrylite
L-40 1.0 mm 1,100 82.degree. C. Poly- Makrolon DP1-1265 1.0 mm
12,300 144.degree. C. carbonate Lexan OQ 1020L 1.0 mm 14,800 --
Glass White Crown (Hoya) 2.8 mm 500 White Crown (Schott) 3.0 mm 400
Green Soda Lime 2.3 mm 1,080
Example 2
Thermal Bonding of Polymer Substrates
[0066] Initial bonding experiments utilized an embossed channel
plate (substrate) fabricated from Plexiglas clear 99530 (described
above). The channels had dimensions of 100 .mu.m wide and 32 .mu.m
deep. A L-40 PMMA cover plate was thermally bonded to the channel
plate at 84.degree. C., the softening point of the L-40 polymer,
and with an applied force of approximately 10 kg. Cross-sectional
examination of the bonded channel showed that while the embossed
channel plate maintained its structure, the cover plate had
deformed into the channel, as shown in FIG. 5A. The provided
dimensions are approximate. The bonding temperature was then
adjusted to 80.degree. C., and the experiment repeated. In this
latter experiment, the cross section of the bonded parts showed
that the channel had achieved a good seal, the channel was not
distorted, nor had the cover plate substantially flowed into the
channel as shown in FIG. 5B.
[0067] All publications and patent applications are herein
incorporated by reference to the same extent as if each individual
publication or patent application was specifically and individually
indicated to be incorporated by reference. Although the present
invention has been described in some detail by way of illustration
and example for purposes of clarity and understanding, it will be
apparent that certain changes and modifications may be practiced
within the scope of the appended claims.
* * * * *