U.S. patent application number 11/027469 was filed with the patent office on 2006-04-06 for deformable polymer membranes.
Invention is credited to Narayan Sundararajan.
Application Number | 20060073035 11/027469 |
Document ID | / |
Family ID | 36125737 |
Filed Date | 2006-04-06 |
United States Patent
Application |
20060073035 |
Kind Code |
A1 |
Sundararajan; Narayan |
April 6, 2006 |
Deformable polymer membranes
Abstract
Embodiments of the present invention provide microfluidic
devices containing deformable polymer membranes. The devices can be
fabricated from a single polymeric block. Actuation of the
membranes within the device allows the fluid contained within a
microfluidic channel to be manipulated. Exemplary microfluidic
devices, such as, peristaltic pumps, sample sorters, and mixers are
described.
Inventors: |
Sundararajan; Narayan; (San
Francisco, CA) |
Correspondence
Address: |
BLAKELY SOKOLOFF TAYLOR & ZAFMAN
12400 WILSHIRE BOULEVARD
SEVENTH FLOOR
LOS ANGELES
CA
90025-1030
US
|
Family ID: |
36125737 |
Appl. No.: |
11/027469 |
Filed: |
December 30, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60615525 |
Sep 30, 2004 |
|
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Current U.S.
Class: |
417/412 |
Current CPC
Class: |
F04B 43/043
20130101 |
Class at
Publication: |
417/412 |
International
Class: |
F04B 43/00 20060101
F04B043/00 |
Claims
1) A microfluidic device comprising a housing formed from a unitary
section of polymer; at least one microfluidic channel formed in the
unitary section of polymer; at least two operating channels formed
in the unitary section of polymer that are each operably connected
to the microfluidic channel by a deformable membrane formed in the
unitary section of polymer and separating an operating channel from
the microfluidic channel wherein the at least two operating
channels are located on opposite sides of the microfluidic inlet
channel; and a substrate to which the housing is attached.
2) The microfluidic device of claim 1 wherein the device comprises
at least four operating channels and at least four membranes.
3) The microfluidic device of claim 1 wherein the device comprises
six operating channels and six membranes and wherein three of the
six membranes are disposed on one side of the microfluidic channel
and three of the six membranes are disposed on an opposite side of
the microfluidic channel and wherein the edge-to-edge distance
between the three membranes on a side of the microfluidic channel
is about 100 .mu.m or less.
4) The microfluidic device of claim 3 wherein three operating
channels disposed on one side of the microfluidic channel are
directly across from the three operating channels on the opposite
side of the microfluidic channel.
5) The microfluidic device of claim 3 wherein the three operating
channels disposed on one side of the microfluidic channel are
staggered relative to the three operating channels on the opposite
side of the microfluidic channel.
6) The microfluidic device of claim 1 wherein the polymer is
poly(dimethyl siloxane).
7) The microfluidic device of claim 1 also including mechanism to
actuate the membranes comprising at least one valve operably
coupled to at least one valve drive that is operably coupled to a
computer capable of generating an actuation pattern.
8) The microfluidic device of claim 1 wherein the operating
channels also comprise a piezoelectric material for actuating the
operating channels.
9) A microfluidic device comprising, a housing formed from a
unitary section of polymer; an inlet microfluidic channel formed in
the unitary section of polymer having a branched end comprising two
microfluidic outlet channels; least two microfluidic hydrodynamic
focusing channels formed in the unitary section of polymer to
convey focusing flows into the inlet microfluidic channel; at least
two operating channels formed in the unitary section of polymer and
operably connected to the microfluidic inlet channel by a
deformable membrane that separates an operating channel from the
microfluidic inlet channel wherein the at least two operating
channels are located on opposite sides of the microfluidic inlet
channel; and a substrate to which the housing is attached.
10) The microfluidic device of claim 9 also including a UV-vis,
fluorescence, or Raman detector positioned to interrogate a
hydrodynamically focused fluid flow.
11) The microfluidic device of claim 10 also including mechanism to
actuate the membranes comprising a valve that is controlled by a
valve drive and a computer generating an actuation signal in
response to a signal from the detector operably coupled to the
valve drive.
12) The microfluidic device of claim 9 wherein the polymer is
poly(dimethyl siloxane).
13) The microfluidic device of claim 9 wherein the operating
channels also comprise a piezoelectric material for actuating the
operating channels.
14) A microfluidic device comprising, a housing formed from a
unitary section of polymer; an inlet microfluidic channel formed in
the unitary section of polymer having a branched end comprising two
microfluidic outlet channels; at least two microfluidic
hydrodynamic focusing channels formed in the unitary section of
polymer to convey focusing flows into the inlet microfluidic
channel; at least four operating channels formed in the unitary
section of polymer and operably connected to the microfluidic
outlet channels by a deformable membrane that separates an
operating channel from the microfluidic outlet channel wherein two
pairs of operating channels are located on opposite sides of a
microfluidic outlet channel; and a substrate to which the housing
is attached.
15) The microfluidic device of claim 14 also including a UV-vis,
fluorescence, or Raman detector positioned to interrogate a
hydrodynamically focused flow.
16) The microfluidic device of claim 15 also including mechanism to
actuate the membranes including a valve that is controlled by a
valve drive and a computer generating an actuation signal in
response to a signal from the detector operably coupled to the
valve drive.
17) The microfluidic device of claim 14 wherein the polymer is
poly(dimethyl siloxane).
18) The microfluidic device of claim 14 wherein the substrate is
selected from the group consisting of poly(dimethyl siloxane),
glass, silicon, polystyrene, polyethylene, silicon nitride.
19) A microfluidic device comprising, a housing formed from a
unitary section of polymer; an inlet microfluidic channel formed in
the unitary section of polymer having branched end comprising two
microfluidic outlet channels; at least four operating channels
formed in the unitary section of polymer and operably connected to
the microfluidic outlet channels by a deformable membrane that
separates an operating channel from the microfluidic outlet channel
and wherein two pairs of operating channels are located on opposite
sides of a microfluidic outlet channel; and a substrate to which
the housing is attached.
20) The device of claim 19 wherein the two microfluidic outlet
channels rejoin to form a single microfluidic outlet channel.
21) The microfluidic device of claim 19 wherein the polymer is
poly(dimethyl siloxane).
22) The microfluidic device of claim 19 wherein the substrate is
selected from the group consisting of poly(dimethyl siloxane),
glass, silicon, polystyrene, polyethylene, silicon nitride.
23) The microfluidic device of claim 19 wherein the operating
channels also comprise a piezoelectric material for actuating the
operating channels.
24) A microfluidic device comprising, a housing formed from a
unitary section of polymer; two inlet microfluidic channels to
convey two solutions that join to form a single inlet microfluidic
channel formed in the unitary section of polymer; at least four
operating channels formed in the unitary section of polymer and
operably connected to the single microfluidic inlet channel by a
deformable membrane that separates an operating channel from the
microfluidic inlet channel and wherein two pairs of operating
channels are located on opposite sides of a microfluidic inlet
channel; a substrate to which the housing is attached.
25) The microfluidic device of claim 24 wherein the two operating
channels disposed on one side of the microfluidic channel are
directly across from the three operating channels on the opposite
side of the microfluidic channel.
26) The microfluidic device of claim 24 wherein the two operating
channels disposed on one side of the microfluidic channel are
staggered relative to the two operating channels on the opposite
side of the microfluidic channel.
27) The microfluidic device of claim 24 wherein the polymer is
poly(dimethyl siloxane).
28) A microfluidic device comprising, a housing formed from a
unitary section of polymer; an inlet microfluidic channel formed in
the unitary section of polymer to convey a sample fluid; a carrier
microfluidic channel formed in the unitary section of polymer
joined at one end to the inlet microfluidic channel; a second inlet
microfluidic channel formed in the unitary section of polymer to
convey a carrier fluid that is joined to the carrier microfluidic
channel at the other end of the carrier microfluidic channel; an
operating channel formed in the unitary section of polymer operably
connected to the inlet microfluidic channel by a deformable
membrane that separates the operating channel from the microfluidic
inlet channel and wherein the operating channel is located opposite
the carrier microfluidic channel; and a substrate to which the
housing is attached.
29) The microfluidic device of claim 28 wherein the polymer is
poly(dimethyl siloxane).
30) The microfluidic device of claim 28 wherein the substrate is
selected from the group consisting of poly(dimethyl siloxane),
glass, silicon, polystyrene, polyethylene, silicon nitride.
31) The microfluidic device of claim 28 wherein the operating
channel also comprises a piezoelectric material for actuating the
operating channel.
32) A method of pumping a fluid in a microfluidic channel
comprising, providing a housing formed from a unitary section of
polymer having a microfluidic channel formed within the housing
that has two sides and at least two polymer membranes formed in at
least one of the sides of the channel and operating channels formed
within the housing to allow for the actuation of the membranes;
flowing a liquid through the microfluidic channel; and actuating
one or more membranes to cause a change in the flow characteristics
of the liquid.
33) The method of claim 32 wherein the actuation of the membrane
occurs pneumatically, hydraulically, piezoelectrically, or
thermopneumatically.
34) The method of claim 32 wherein the housing is formed from
poly(dimethyl siloxane).
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] The present application claims the benefit under 35 U.S.C.
.sctn. 119(e) of U.S. Provisional Patent Application No.
60/615,525, filed Sep. 30, 2004, the disclosure of which is
incorporated herein by reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] Embodiments of the present invention relate generally to
microfluidic operations and deformable polymer membranes.
[0004] 2. Background Information
[0005] Microfluidic components for performing a variety of
operations are integral parts of micro-total analysis system
applications. For example, cell sorters have become a vital
component in micro total analysis systems aiming to investigate
biological events at the single cell level. However it has not been
easy to integrate different microfluidic components together into a
single chip. This has been due to the different and sometimes
difficult fabrication requirements for each of the microfluidic
components. For example, pumping in micro total analysis is
generally achieved using external devices such as syringes or
peristaltic pumps or using voltages across the channels generating
electrokinetic or electroosmotic flow.
[0006] Essential processes such as bonding, aligning, clamping and
interconnections for realizing a micro total analysis system
generally cause significant device failure rates. Making components
from the same basic unit and material facilitates the integration
of operations and components. For example, polymers such as
poly(dimethyl siloxane) (PDMS) can be used to fabricate various
components in microfluidic devices. In addition, easy fabrication
processes and simplicity of the device greatly help in integration
of these components into a single device.
BRIEF DESCRIPTION OF THE FIGURES
[0007] FIGS. 1A and 1B provide schematics (a top view and a side
view, respectively) of a deformable membrane operably connected to
a microfluidic channel.
[0008] FIGS. 2A and 2B show exemplary designs for peristaltic
pumps.
[0009] FIGS. 3A, 3B, and 3C graph measurements of fluid flow rate
versus the frequency of pressure applied to the operating channels
for several peristaltic pump configurations.
[0010] FIGS. 4A and 4B show exemplary designs for microfluidic
sorting devices.
[0011] FIG. 5 demonstrates sorting action in an exemplary
microfluidic sorting device.
[0012] FIG. 6 shows results of measurements to determine the
fidelity of an exemplary sorter device as a function of the
actuation times of the deformable membranes.
[0013] FIGS. 7A and 7B show an exemplary design for a microfluidic
mixer. FIG. 7C shows an idealized depiction of the distribution of
the liquids in the microfluidic channels before and after mixing in
the device of FIGS. 7A and 7B.
[0014] FIGS. 8A, 8B, and 8C show data from video microscopy of
microfluidic channels containing fluorescent microbeads passing
through an exemplary active mixer unit.
[0015] FIGS. 9A, 9B, and 9C show an exemplary design for a device
that can be used to mix fluids within a microfluidic channel.
[0016] FIGS. 10A and 10B show exemplary designs for devices that
can be used to break and coalesce droplets of fluid.
[0017] FIG. 11 shows an exemplary design for a device that can be
used to introduce precise amounts of sample into a carrier
fluid.
[0018] FIG. 12 shows the operation of an exemplary device for
dispensing a substance into a microfluidic channel.
[0019] FIGS. 13A, 13B, and 13C show exemplary devices for
piezoelectric membrane actuation.
[0020] FIG. 14 shows a diagram of a system used for deflecting and
pulsing membranes.
[0021] FIG. 15 shows a method for fabricating a microfluidic device
using single layer soft lithography.
DETAILED DESCRIPTION OF THE INVENTION
[0022] Embodiments of the present invention provide deformable
polymer membranes as active components of a microfluidic system.
The deformable membranes perform functions associated with the
manipulation of liquids in a microfluidic channel. Because the
polymer membranes are disposed in the same polymer layer as the
active microfluidic channel, manufacture of the microfluidic device
is simplified. Although deformable membranes have been exemplified
using PDMS, the present invention is not so limited as other
elasomeric polymers can be used to fabricate membranes. Using a
deformable membrane unit such as that shown schematically in FIGS.
1A and 1, consisting of an active microfluidic channel, an
operating channel, and a membrane separating the channels,
microfluidic components functioning, for example, as pumps,
sorters, and mixers can be designed and fabricated.
[0023] Referring to FIG. 1, a basic deformable membrane as part of
a microfluidic device is illustrated. FIG. 1A provides a top-down
view and FIG. 1B provides a side-view of the same section of a
microfluidic device. A microfluidic channel 10 is formed in a solid
polymer block 20. An operating channel 30 is operably connected to
a membrane 40 that is formed by the intersection of the
microfluidic channel 10 and the operating channel 30 within the
polymer block 20. As used herein, the term operating channels
refers to channels that are operably connected to the deformable
membrane to allow for deformation, actuation, and or pulsing of the
membrane. The polymer block housing 20 is attached to a substrate
50. Actuation, deflection, or pulsing of the membrane 40 causes a
change in the flow characteristics of the fluid contained within
the microchannel 10. By placing the membrane in the same polymer
layer as the microfluidic channel, device fabrication is
facilitated.
[0024] In embodiments of the present invention, peristaltic pumping
of fluids within a microchannel is effectuated using deformable
membranes and operating channels that are disposed in the same
polymer layer as the active microfluidic channel. The deformable
membrane unit can be actuated, for example, by pressurizing the
operating channels with a gas or liquid. Peristaltic pumps were
realized by placing multiple deformable membrane units (a membrane
unit is a pair of membranes disposed on opposite sides of a
microfluidic channel) in series along a microfluidic channel.
Referring now to FIG. 2, two different designs were built to
compare pumping efficiency as a function of the placement of
deformable membranes along a microfluidic flow channel 60. In one
example, a symmetric parallel design had membranes 70 and operating
channels 80 placed symmetrically on opposite sides of the
microfluidic channel 60, as shown schematically in FIG. 2A. In
another example, an asymmetric alternating design had membranes 70
and operating channels 80 placed asymmetrically on each side of the
fluid channel 60, as shown schematically in FIG. 2B. In the example
shown in FIG. 2B, deformable membrane units 70 were staggered by 50
.mu.m on opposite sides of the active microfluidic channel 60.
Alternative dimensions are possible. The membranes on a side of the
channel can be separated from each other, for example, by distances
of about 200 .mu.m to about 50 .mu.m. Pumping was visualized using
a diluted solution of 1 .mu.m fluorescent poly(styrene) beads in
water. Three different phase angles of actuation for the membranes,
60.degree., 90.degree., and 120.degree. (corresponding to actuation
patterns of (100, 110, 111, 011, 001, 000), (100, 110, 011, 001),
and (101, 100, 110, 010, 011, 001), where 1 indicates the membrane
is actuated (distended into the microfluidic channel) and 0
indicates the membrane is not actuated), were tested and it was
found that for these designs, the 60.degree. phase angle of
actuation provided the fastest flow rate for both exemplary
designs.
[0025] Several different parameters, including the external
regulated pressure, frequency of actuation, microfluidic channel
width, membrane thickness, channel height, and gap between air
channels, were tested. Typical operating channel width was 100
.mu.m. Flow rates were calculated by measuring the time taken for
fluorescent beads to traverse through a 2.7 mm long serpentine
channel. FIG. 3A shows the frequency dependence of flow rate for an
exemplary parallel membrane device design (as diagrammed in FIG.
2A) for different external applied pressures. The microfluidic
device in this example had a membrane thickness of 20 .mu.m, a
microfluidic channel width of 20 .mu.m, a channel height of 100
.mu.m, and an operating channel gap of 50 .mu.m. As can be seen
from the graph in FIG. 3A, the flow rate increases to a maximum at
about 30 Hz and then drops down rapidly as frequency of actuation
increases for all external pressures applied. It is believed that
these results can be attributed to the spring force effect of the
membrane in which, after a certain frequency, the membrane does not
revert back to its original position thereby reducing the volume
displacement of the fluid achieved. Also, as the external pressure
applied is increased, the maximum flow rate obtained increases. It
is believed that the increased external pressure applied to the
membrane increases the deflection of the PDMS membrane thereby
increasing the volume of the fluid displaced.
[0026] FIG. 3B shows the flow rate dependence at different
frequencies of actuation for two exemplary parallel design devices
(as diagrammed in FIG. 2A) that had microfluidic channel widths of
20 .mu.m and 30 .mu.m. The devices had membrane thicknesses of 20
.mu.m, channel heights of 100 .mu.m, and operating channel gaps of
50 .mu.m. The pressure applied to the operating channels was 50
psi. As seen from the graph in FIG. 3B, the trend of the flow rate
dependence on the frequency of actuation is the same while the
maximum flow rate obtained using the 30 .mu.m width channel is
higher than that of the 20 .mu.m.
[0027] The results acquired from two exemplary designs for
deformable membrane unit placement in a peristaltic pump (as shown
in FIGS. 2A and B) are shown in FIG. 3C. In both cases, the
dimensions of both the microfluidic and the operating channels were
the same and the pressure applied was 30 psi. The membrane
thickness was 20 .mu.m, the microfluidic channel width was 30
.mu.m, the microfluidic channel height was 100 .mu.m, and the
operating channel gap was 50 .mu.m. As seen from the graph in FIG.
3C, the alternating design provided about twice the maximum flow
rate of the parallel design. It was also found that the alternating
design example prototype was only better at the higher pressure of
30 psi while the parallel design performed slightly better than the
alternating design at pressures of 10 and 20 psi. It is believed
that this observed enhancement can be attributed to the fact that
in the alternating design, the deflection of the membrane is higher
because it is not as constrained by the membrane on the other side
of the microfluidic channel.
[0028] By controlling the various parameters of actuation and
dimensions of the components of the basic deformable membrane unit,
it is possible to control the flow velocities and rates. In
general, channel aspect ratios of about 1:2 to about 1:10 (width to
height) and widths of about 10 to about 100 .mu.m have been used in
embodiments of the present invention. Additionally, in general,
average membrane thicknesses of about 5 to about 50 .mu.m and
distances between membranes located on a side of a channel of about
50 to about 200 .mu.m can be used in embodiments of the present
invention. The height and the width of the membranes are typically
determined by the dimensions of the intersection of the
microchannels that form the membranes which in turn are
user-defined variables.
[0029] Referring now to FIG. 4, the placement of operating channels
in several exemplary microfluidic sorting devices is diagrammed. In
this example, deformable membranes 90 were placed along the main
inlet microfluidic channel 100 or along each of the branch outlet
microfluidic channels 110. The sample flow was hydrodynamically
focused in the main microfluidic channel by using sheath flows from
intersecting sheath microfluidic channels 120 on either side of the
sample solution inlet channel 100. Two exemplary designs are shown:
in FIG. 4A deformable membranes 90 are placed alongside the main
inlet microfluidic channel 100, and in FIG. 4B deformable membranes
90 are placed alongside each of the branch microfluidic channels
110 (the branch channels are labeled "outlet to bin 1" and "outlet
to bin 2" in FIG. 4B). In one embodiment, the membrane units 90
were activated by increasing air pressure in the operating channel
130 and causing the membrane 90 to deflect into the microfluidic
channel 100 or 110. A diluted sample solution of 6 .mu.m
fluorescent poly(styrene) beads was hydrodynamically focused using
branch sheath flows of DI water from either side of the main
channel. To sort particles contained in a flow in the main
microfluidic channel in an exemplary device having deformable
membrane units placed alongside the main microfluidic channel,
either the left or the right membrane is deflected into the channel
to guide the microfluidic stream into the left or the right outlet
channel, respectively. It was found that placement of the
deformable membrane unit in the main microfluidic channel far from
the Y-branch results in poorer sorting fidelity because of recovery
of the laminar streams before reaching the Y-branch. Additionally,
a sorting device may also optionally comprise a device for
interrogating the sample stream and providing input to the switcher
that activates the pressure in the operating channels. For example,
the device for interrogating may be a UV-vis, fluorescence, or
Raman detector that detects the presence of a cell, a virus, a
bacterium, a label molecule, or a nanoparticle. When the detector
detects a species of interest, it communicates to the switcher to
direct the species into a selected outlet channel. For example,
when the light intensity is above a certain threshold from a CCD
camera used to detect a nanoparticle, the above-threshold signal
can be converted using an algorithm to provide a voltage to the
solenoid valves and cause a switcher to activate pressure in the
operating channels.
[0030] In an exemplary design, the deformable membrane units were
placed in the branch channels that when actuated would increase the
resistance to flow in the respective branch thereby diverting the
direction of flow of the sample to the other branch. FIG. 5 shows 6
.mu.m beads sorted by using the deformable membrane units placed in
a branch channel. The exemplary device pictured in FIG. 5 had a
microfluidic channel width of 100 .mu.m, a channel height of 100
.mu.m, a membrane thickness of 20 .mu.m. In the left-hand picture
(FIG. 5A), actuation of the deformable membranes in the right
branch outlet directs the 6 .mu.m bead to the left branch outlet.
In the right-hand picture (FIG. 5B), actuation of the deformable
membranes in the left branch outlet directs the 6 .mu.m bead to the
right branch outlet. This exemplary device design worked with
approximately 100% fidelity for hydrodynamically focused beads,
that is to say, approximately 100% of the beads went to the right
branch when the deformable membrane on the left branch was actuated
and vice versa.
[0031] Referring now to FIG. 6, the time of actuation of the
deformable membrane units along each of the branch channels for the
device shown in FIG. 5 was varied and the number of beads that
flowed into each of the branches in a minute of hydrodynamically
focused sample flow was counted. FIG. 6 shows that the sorting
ratios at different ratios of times of actuation for the deformable
membrane units. The actuation times of the deformable membrane
units in the right and left branch channels were varied and the
number of beads flowing through each branch was measured using
video microscopy. Data was also collected when the deformable
membrane units were both open and both actuated. Even when both the
deformable membrane units were actuated, there was flow of bead
solution into both branches as the deformable membranes in this
exemplary device did not act as a binary valve.
[0032] FIGS. 7A and 7B show an exemplary design for a single mixer
segment of an active microfluidic mixer having deformable membrane
units. FIG. 7B is a close-up of the deformable membranes 140 of the
active mixer shown in FIG. 7A. In this example, two microfluidic
inlet channels 150 carrying solutions for mixing intersect and form
a single inlet channel 160 which then has two microfluidic channels
170 branching from the inlet channel 160 each having a deformable
membrane unit (a pair of deformable membranes 140). Liquids from
microfluidic inlet channels 1 and 2 flow into the main channel 160
and the flow is then branched again into two microfluidic channels
170 each having a deformable membrane unit with operating channels
180 that can be, for example, pneumatically controlled. The
branches are then combined again to complete the first mixer
segment and the outlet channel 190 can be connected to more mixer
segments. These units, when actuated, increase the resistance to
flow in the branches thereby introducing plugs as shown in FIG. 7C.
FIG. 7C shows the concept of an active mixer using the deformable
membrane unit with an idealized depiction of the distribution of
the liquids along the microfluidic channels. The labels, A, B, C,
and D above the boxes in FIG. 7C correspond to the labels in FIG.
7A indicating different segments of the microfluidic mixing device.
Consider two liquids 1 (grey) and 2 (white) flowing from
microfluidic input channels 1 and 2. The flow will be laminar in
the main channel and liquid 1 will flow to the upper branch and
liquid 2 through the lower branch. Mixing is achieved by actuating
the deformable membrane unit in each branch in an alternating
fashion at different frequencies. Due to the increase in resistance
to flow when the deformable membrane unit is actuated in the upper
branch, the liquids 1 and 2 flow predominantly through the lower
branch, and similarly through the upper branch when the lower
branch deformable membrane unit is actuated. Thus alternating plugs
of liquid 1 and liquids 1 and 2 as shown in FIG. 7C flow through
the upper branch while alternating plugs of liquid 2 and liquids 1
and 2 flow through the lower branch. When the flow from the two
branches combine (as shown in FIG. 7C, box D), the diffusion length
required for mixing of the two liquids is greatly reduced.
[0033] FIG. 8 shows qualitative results of mixing that were
obtained in an exemplary mixer of FIG. 7A in which liquid 1 is a
diluted solution of 1 .mu.m fluorescent poly(styrene) beads in DI
water and liquid 2 is DI water. In FIG. 8A, video microscopy of the
main channel shows the distribution of the fluorescent beads after
the first mixer segment before and after actuation of the
deformable membrane units. The bead solution is limited to the left
side of the main channel due to laminar flow before actuation, but
after actuation, the bead solution is more uniformly distributed in
the main channel. FIG. 8B shows the temporal average of the
fluorescent intensity of each point within the channel before and
after actuation of the deformable membrane units. In this Figure,
180 and 280 frames were averaged respectively from the video
sequences before and after. FIG. 8C is a graphical representation
of the data shown in FIGS. 8A and B, in which the y-axis is
intensity in arbitrary units plotted across the channel before and
after mixing. The deformable membrane units were actuated at a
frequency of 1.66 Hz in an alternating fashion and video microscopy
of the main channel after the first and the second segments was
performed. As can be seen from FIG. 8, the bead solution showed
mixing with the DI water.
[0034] Referring now to FIG. 9, a device for inducing and enhancing
mixing between solutions is diagrammed. FIG. 9A provides a top view
of the device, FIG. 9B provides a side view of a slice through the
indicated region, and FIG. 9C provides a side view showing the
microfluidic channel 230 and membranes 250. In this embodiment, two
or more inlet microfluidic channels 200 formed in a unitary section
of polymer 210 form a junction 220 at which they merge into a
single inlet microfluidic channel 230. The angle at which the two
inlet channels 200 meet in the diagram is arbitrary and any angle
up to and including about 180.degree. can be used. Operating
channels 240 abut deformable membranes 250 formed from the polymer
block 210. The polymer block 210 is attached to a substrate 260.
Actuation of the membranes 250 causes the solutions to mix. A
device such as this is useful, for example, for breaking up laminar
flows of solutions. Although four deflectable membranes are
pictured, a device could contain more or less than four membranes.
Additionally, the membranes may also be staggered relative to a
membrane on the opposite side of a channel. The frequencies of
actuation of the membranes is dictated in part by the flow rates
for the solutions and the desired state of the resulting
solution.
[0035] Referring now to FIG. 10A, an exemplary device is pictured
that can be used to break and coalesce droplets formed from the
mixing of immiscible fluids. In this embodiment, two inlet
microfluidic channels 270 form a junction 280 at which they merge
into a single microfluidic channel 290. The angle at which the two
inlet channels 270 meet in the diagram is arbitrary and any angle
up to and including about 180.degree. can be used. Operating
channels 300 abut membranes 310 formed in a unitary section of
polymer. Actuation of the membranes 310 causes the droplets that
are formed from the meeting of the immiscible fluids in the
microfluidic channel 290 to be mixed through the breaking and
coalescing of the droplets. Although four deflectable membranes are
pictured, such a device could contain more or less than four
membranes. Additionally, the membranes may also be staggered
relative to the membrane on the opposite side of a channel. Such a
device is useful, for example, for facilitating the reaction
between species contained in immiscible liquids. In an additional
embodiment shown in FIG. 10B, two or more junctions 280 of two
microfluidic inlet channels 270 are merged to form an additional
junction 320 leading into a microfluidic channel 330 containing
membranes 340 operably connected to operating channels 350 and
capable of being deformed into the microfluidic channel 330.
Although four deflectable membranes are pictured, such a device
could contain more or less than four membranes. Additionally, the
membranes may also be staggered relative to the membrane on the
opposite side of a channel. Such a device is useful, for example,
for mixing droplets formed in the two or more junctions of the
microfluidic channels 280. The droplets may contain, for example,
two different reactive species and the mixing of the droplets
facilitates the reaction between the two different species.
[0036] Referring now to FIG. 11, a schematic shows a device that
can be used for the introduction of precise amounts of a first
fluid into a second fluid. Additionally, this device can be used
for precise droplet formation via emulsion by using immiscible
liquids as the sample and carrier fluids. In FIG. 11, a
microfluidic channel 360 is provided for a first fluid, such as,
for example, a sample stream, and a membrane 370 is operably
connected to an operating channel 380. A second microfluidic
channel 390 is located between the first microfluidic channel 360
and a third microfluidic channel 400 in which a second fluid can
flow. In operation, deflection of the membrane 370 delivers a plug
of the sample stream through the connecting channel 390 and into a
carrier fluid contained in microfluidic channel 400. In general,
the dimension labeled T.sub.IC can range from about 10 to about 200
.mu.m and the dimension labeled L.sub.IC can range from about 5 to
about 100 .mu.m. The dimension T.sub.IC, for example, can be used
to determine the drop size.
[0037] FIG. 12 shows an exemplary device for controllable release
of a fluid into a microchannel. In FIG. 12, a microfluidic channel
410 is flanked by deformable membranes 420 that separate operating
channels 430 from microfluidic channel 410. The number of operating
channels chosen depends on the number of reagents or fluids that
are desired to be added to the microfluidic channel and one, two,
three, four, and more are possible. There can be only one operating
channel 430 and membrane 420, or as many as desired. In operation,
a first liquid is pumped into an operating channel with increasing
pressure until the membrane 420 separating the operating channel
430 from the microfluidic channel 410 is ruptured and releases the
first liquid into the microfluidic channel 410. Introduction of
reagents can be controlled and sequenced by controlling the
membrane thickness and the pumping of the various fluids in the
operating channels. Higher pressures and thinner membranes will
allow the membranes to rupture more quickly. The microfluidic
channel can be a reaction vessel for performing a variety of
chemical and biochemical reactions. In the case of a PDMS housing
bonded reversibly to a substrate, the opening of the membrane
occurs at lower pressures, since delamination or peeling of the
membrane can occur at the substrate-PDMS interface. Additionally,
in operation the membrane separating the operating channel
enclosing the first fluid can be ruptured through increasing the
flow rate of a second fluid in the microfluidic channel. The
increased flow rate increases pressure within the microfluidic
channel and results in membrane rupture and introduction of the
first fluid into the second fluid.
[0038] The micro-fluidic channels represent micro-sized fluid
passages that may have a cross-sectional dimensions, channel width,
channel height, channel diameter, etc. that may be not greater than
approximately one millimeter (mm, one-thousandth of a meter, also
1000 .mu.m). In various embodiments the cross-sectional dimension
may be not greater than approximately 500 micrometers (.mu.m, one
millionth of a meter), 200 .mu.m, 100 .mu.m, 50 .mu.m, or 10 .mu.m.
The invention is not limited to any known minimum cross-sectional
dimension for the channels. In various embodiments the
cross-sectional dimension may be greater than approximately 0.001
.mu.m (1 nm), greater than approximately 0.01 .mu.m (10 nm), or
greater than approximately 0.1 .mu.m (100 nm). The optimal
dimension of the channel may depend upon the characteristics of the
fluids and/or particles to be conveyed therein. An exemplary
micro-fluidic channel which may be used for one or more of an
inlet, outlet, or focusing channel, may comprise a rectangular
channel having a channel width of approximately 100 .mu.m and a
channel height of approximately 50 .mu.m. The rectangular shape and
specific dimensions are not required. These miniaturized channels
are often useful for handling small sized samples and allow many
channels to be constructed in a small substrate, although this is
not a requirement. There is no known minimum or maximum length for
the channels. Commonly the channel lengths are at least several
times their width and not more than several centimeters.
[0039] PDMS may offer certain advantages such as compatibility with
biological materials and chemicals and transparency to facilitate
alignment, although the use of PDMS is not required and other
materials may optionally be employed for forming the housing
containing the membranes and microchannels. Any machinable,
etchable, reformable, moldable, stampable, embossable, or castable
elastomeric material (a material that is capable of deforming when
pressure is applied and returning to its original shape when
pressure is removed) may potentially be used. In general, there are
a wide variety of formulations for elastomeric polymers, and a
choice of materials may be based upon considerations such as
elasticity, gas and/or liquid permeability, cost of fabrication,
and/or temperature stability. Suitable polymers include among
others, polyurethanes, silicones, polybutadiene, polyisobutylene,
polyisoprene, elastomeric formulations of polyvinylchloride,
polycarbonate, polymethylmethacrylate, polytetrafluoroethylene
(Teflon.RTM.), and combinations of these materials. It may be
appropriate to form focusing devices of polymers because these
materials are inexpensive and may be injection molded, hot
embossed, and cast.
[0040] In general, almost any non-absorbent material capable of
presenting a smooth surface can be used to form the substrate.
Possible substrates that could be used include glass; silicon;
polymers, such as for example, PDMS, polystyrene, and polyethylene;
silicon nitride; silicon dioxide; and metals, such as for example,
gold, aluminum, and the like. The housing in which the channels and
the membranes are formed may be reversibly or irreversibly attached
to the substrate. For example, a PDMS housing can be reversibly
attached to, for example, a PDMS or a glass surface through van der
Waals forces. Additionally, adhesives such as silicone adhesives
and epoxies can be used to bond the housing to the substrate.
Choice of method of bonding is dependent in part on the materials
chosen for the housing and the substrate, the desired user-chosen
operating pressure ranges, and functional compatibility with
operating fluids chosen for a particular application and can be
effectuated according to well-known methods in the art.
Additionally, PDMS, for example, can be oxidatively sealed to, for
example, PDMS, silicon, polystyrene, polyethylene, silicon nitride,
or glass by exposing the surfaces to be bonded to an air plasma and
bringing the surfaces into contact within about a minute after
oxidation.
[0041] The invention is generally not limited to any known process
flow. Suitable process flows may comprise an aqueous, organic, or
biological solution. The process flow may contain a species of
interest. The species of interest may comprise a biological
material, such as a cell, organelle, liposome, biological molecule
or macromolecule, enzyme, protein, protein derivative, protein
fragment, polypeptide, nucleic acid, DNA, RNA, nucleic acid
derivative, biological molecule tagged with a particle,
fluorescently labeled biological molecule, charged species, or
charged protein. Additionally, a process flow may contain reagents
for chemical reactions and the products of chemical reactions.
[0042] In general, the deformable membranes can be actuated
(deflected) pneumatically, hydraulically, piezoelectrically,
thermopneumatically, and magnetically. Pneumatic and hydraulic
actuation can be accomplished by pumping a gas or liquid,
respectively, into an operating channel. Typically, the gas or
liquid can be supplied and vented through a valve that is
controlled by a valve drive and a computer generating a programmed
actuation pattern that is converted into a control signal.
Piezoelectric actuation can be accomplished using, for example, the
devices shown in FIG. 13. In FIG. 13, piezoelectric disk 431 is
mounted with either a support plate 432 or a support structure 433
above an actuation reservoir 439. An operating channel 434 is
separated from microfluidic channel 435 by deformable membrane 436.
Polymer housing 437 is attached to substrate 438. FIG. 13C provides
a top down view of the device shown in FIG. 13A. Deformation of the
piezoelectric disk 431 into the operating channel 434 causes the
membrane 436 to actuate. Piezoelectric disks are commercially
available from, for example, Piezo Systems, Inc (Cambridge,
Mass.).
EXAMPLES
[0043] Precursors for poly(dimethyl siloxane), Sylgard A and B were
obtained from Dow Corning Inc. 1 and 6 .mu.m YG fluorescent
poly(styrene) beads used to visualize flow were obtained from
Polysciences Inc. SU-2035 Photoresist was obtained from Microchem
Corp.
[0044] An actuation system consisting of hardware and software
components was constructed for pneumatically controlling the
operating channels. Referring to FIG. 14, the actuation system
consisted of a control computer 440 generating a programmed
actuating pattern that is converted into a control signal through a
digital output board (NI MIO-16XE-10, National Instruments) 450.
The control signal operates the valve drive (NI SCCDO01, National
Instruments) 460 that converts the control signals into the
appropriate power leveled operating power patterns for switching
the solenoid valves (LHDA1223111H, Lee company) 470. Regulated
external gas pressures (10-30 psi) were provided to the normally
closed port of the manifold on which the solenoid valves were
mounted allowing the operating channels to be pressurized or
vented.
[0045] The valve drives are enclosed in the signal conditioning box
(NI SCC2345, National Instruments) having two RJ45 connectors, two
sets of banana connectors and four LEDs. Two sets of banana
connectors are to provide the external power which then is
converted into the pulsing power by valve drives. There are eight
valve drives and each set of banana connector is connected to four
valve drives so that enough external power is supplied. Two 12 V
power supplies are connected to the banana connectors. The role of
valve drive is to turn on and off the external power for solenoid
valves so that it generates the patterned pulsing power with
particular frequencies.
[0046] The application for the actuation system was written in C
language. In order to increase the response time to maximum,
Graphic User Interface (GUI) was not implemented. Actuation
patterns for performing synchronized actuation of the different
deformable membrane units were implemented in the software
depending on the microfluidic operations. Video microscopy was done
using a Canon Digital camera ZV20 that captures the video via an
S-video port from the Hamamatsu color CCD camera mounted onto an
inverted fluorescence microscope.
[0047] FIG. 15 provides a general outline for microfluidic chip
fabrication using standard single layer soft lithography. In FIG.
8, a photoresist on silicon master is prepared using standard
photolithography using a thick SU-8 photoresist spun at thickness
of 100 .mu.m. This is followed by micromolding with PDMS after
which the PDMS mold is peeled off the master and bonded to, for
example, a glass or PDMS substrate. Other methods for forming
microfludic structures are known and the invention is not limited
to a particular method of forming the structures.
[0048] Designs of the micro fluidic channels to be fabricated were
drawn to scale using L-Edit (Tanner Research) and chrome masks were
printed using a Micronics laser writer at Stanford nanofabrication
facility.
[0049] SU-8 2035 photoresist was spun onto 4'' silicon wafers at
2000 rpm for 30 sec. The wafers were then baked at 65.degree. C.
for 6 min. and at 95.degree. C. for 20 min. The wafers are then
exposed using UV light (365 nm) at a dose of .about.400
mJ/cm.sup.2. The exposed wafers were then baked at 65.degree. C.
for 1 min and at 95.degree. C. for 5 min. After post-exposure bake,
the wafers were immersed in SU-8 developer for .about.10 min. to
develop the unexposed regions. The SU-8 photoresist on the wafer
was then silanized for 1 hr by placing the wafers in close
proximity with a few drops of trimethylchlorosilane in a vacuum
desiccator. The silanized photoresist on the wafer was used as the
master for subsequent micromolding experiments.
[0050] Ten parts by weight of Sylgard A were added to 1 part by
weight of Sylgard B, mixed thoroughly and degassed to remove any
air bubbles to form the PDMS precursor. PDMS precursor was poured
onto the silanized master and then cured at 65.degree. C. for 1 hr.
The cured PDMS was peeled off the master and holes were punched for
reservoirs. In order to irreversibly seal the PDMS to a glass
cover, the PDMS and the glass cover were placed in a plasma cleaner
and treated with plasma (100 W) generated from ambient air for 1
min. and brought into conformal contact within 30 sec.
[0051] In the examples shown liquids were flowed through
microfluidic channels using gravity. Other methods are also
possible, including, for example, pumps and syringes.
* * * * *