Reusable, compression-sealed fluid cells for surface mounting to

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www.rsc.org/loc | Lab on a Chip
Reusable, compression-sealed fluid cells for surface mounting to planar
substrates
Cy R. Tamanaha,* Michael P. Malito, Shawn P. Mulvaney and Lloyd J. Whitman
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Published on 27 February 2009 on http://pubs.rsc.org | doi:10.1039/B818960A
Received 27th October 2008, Accepted 27th January 2009
First published as an Advance Article on the web 27th February 2009
DOI: 10.1039/b818960a
We have developed a universal structure and mechanism for the repeatable, rapid-attachment of a fluid
cell to a planar substrate. The fluid cell and all fluidic connections are completely contained in a plastic
body such that attachment requires neither adhesives nor modification of the substrate. The geometry
of the fluid cell is defined by the active area of the planar substrate (e.g. a sensor array). All required
components have been quickly prototyped using Computer Numerical Control (CNC) machining. It is
also straight-forward to create an array of fluid cells to attach to a single substrate (e.g. a standard
microscope slide). All components are easy to assemble and can be cleaned and reused, making this
flexible approach applicable for a wide range of lab-on-a-chip applications.
Introduction
Every solid-phase bioassay requires a fluid sample, along with
any required reagents, to be delivered to a surface. A variety of
microsystems exist that deliver fluids under dynamic (often
laminar) flow across planar substrates.1–7 The continuing challenge is to integrate such fluidics, along with any required
detection technology (e.g. electrical connections, optical
windows), to substrates with sub-millimetre scale features. In
many circumstances, the fluid volumes being manipulated are
sub-microlitre, creating additional complications such as evaporation and sample loss to the channel walls. Ideally, one would
like a simple, yet flexible approach to fabricating and attaching
fluid cells to substrates that does not require adhesives and can be
routinely assembled, disassembled, cleaned, and reused.
A survey of the literature and U.S. patent database reveals
various methods to produce sealed fluid cells, chambers, or
networks of channels with micron-scale dimensions (see for
example ref. 2,4,8–10 and references therein). Many of these
existing fluid cells and microfluidic devices, however, are
encumbered by the need to fabricate and integrate multiple,
complex components or use expensive microfabrication facilities.
This in turn makes them less desirable for mass production or for
cheap disposable products, or less compatible with off-the-shelf
pumping and valving components. A common deficiency is that
their complicated construction and usage are not conducive for
routine assembly and reuse. One device that addresses this
challenge was introduced by CelTor Biosystems, Inc.11 Their
device uses a docking station that provides a mechanical force for
sealing a flat substrate (e.g. a glass slide) against a single microfluidic cell without adhesives or permanent bonding strategies.
However, the dock limits operation to a single fluid cell and
therefore only a single assay per substrate.
A more desirable solution would be a simple, reusable design
with a ‘‘press-together’’ assembly capable of multichannel operation. The design should be easy to prototype without requiring
Naval Research Laboratory, Washington, DC, 20375-5342, USA. E-mail:
cy.tamanaha@nrl.navy.mil
1468 | Lab Chip, 2009, 9, 1468–1471
modification of the substrate itself so that channel headspace can
be varied for optimum mass transfer and channel geometry
customized for different sensor or microarray layouts. Additionally, assembly of the fluid cell should occur without interfering with other components attached to the substrate, such as
wire bonds. Finally, for many applications the design should
accommodate heterogeneous assays on a solid substrate using
laminar flow and optical inspection of the assay surface.
In this report, we describe the design, fabrication, and operation of simple, reusable fluid cells with ‘‘press-together’’
assembly, where permanent bonding is not required.12 Our
approach can be easily integrated with a wide range of microdevices, from disposable assay cartridges to experimental multichannel assay platforms. It is an alternative method to achieve
the broad goal of controlling the passage of fluid over
a substrate, especially a substrate incorporating sensors or
devices for detecting components within the fluid. Possible
substrates include solid-state integrated circuit (IC) sensor chips,
glass slides, and genomic or proteomic microarrays.
Materials and methods
Design and fabrication
Our ‘‘press-together’’ assembly consists of three standard
components: a plastic support body, an elastomer gasket, and
a planar substrate. The plastic support body (e.g. acrylic, Delrin) is a monocoque construction from which the integrated
fluid cell is formed along with the structures required to maintain
compressive contact with the planar substrate (Fig. 1A, B). The
elastomer gasket functions as the side walls of the integrated fluid
cell and establishes a water tight seal against the support body
and the planar substrate. Finally, the planar substrate (upon
which the assay is performed) is typically a sensor chip, glass
slide, etc.
Our design has two key features. First, the fluid cell is incorporated completely into the solid plastic support body independent of the substrate. The geometry of the fluid cell, including the
cell height and in-plane shape, can therefore be customized in
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Fig. 1 (A) Looking at the ‘‘press-together’’ integrated flow cell through the fully assembled transparent cartridge. (B) Exploded CAD rendering of the
cartridge. The planar substrate is an IC sensor chip, the support body is a transparent plastic cartridge, and the fluidic I/O are Tygon tubes. (C)
Multichannel cartridge compatible with standard microscope slides.
each case for the desired flow conditions and the planar substrate
upon which the fluid cell will be mounted. For example, in the
case of an IC chip, considerations may include the presence of
wire bonds at the edge of the chip that must be outside the cell yet
not compressed during assembly. The second key feature is that
a reusable gasket is used to create a water-tight seal by
compression while defining the side-walls of the fluid cell.
Adhesives can alternatively be used for permanent sealing, if
desired. Because of these key features, the basic design and
manufacturing process is identical whether the fluid cell is
developed for a single microfluidic cartridge with a sensor chip or
a multi-channel flow cell system on a microscope slide.
Our approach is illustrated with the device shown in Fig. 1A,
which was designed for use with the Bead ARray Counter
(BARC) chip, a magnetoelectronic sensor array.13–15 The
primary geometric features of the fluid cell for a BARC chip are
diffuser components on opposing sides of a wide central channel.
These components were based on flow cells we have previously
modeled and designed.16 The diffusers uniformily spread the
narrow, laminar, stream emanating from the 500 mm diameter
entrance port across the 2.18 mm wide sensor array. The fluid is
then refocussed by the second diffuser and directed out of the 500
mm diameter exit port.
The protoyping process begins with the design of the cell
components in a plastic blank (Fig. 2A) using a CAD program
such as AutoDesk Inventor. Computer code is generated for
programming the CNC machine (Haas Vertical Machining
Center), at 0.5 ml tolerances, to mill out the plastic support body
leaving a recessed ledge and an exposed 50 mm tall mesa
(Fig. 2B). The recessed ledge is formed with an appropriate depth
to properly seat the planar substrate. The void around the mesa
creates space for the wirebonds to the chip. All the various fluid
cell structures described next are subsequently milled into the
mesa.
The ceiling of the fluid cell is created by milling out of the mesa
an appropriate amount of material in the shape of the desired
fluid cell (Fig. 2C). The height of the assembled fluid cell will be
the distance between this ceiling surface and the top surface of
This journal is ª The Royal Society of Chemistry 2009
Fig. 2 Starting with a plastic blank (A), a support body (B) is created
with a recessed ledge to accept a PCB board, a void to allow clearance for
wirebonds, and an exposed 50 mm tall mesa. The height and shape of the
fluid cell is defined by milling out an appropriate amount of material from
the mesa (C). Fluidic inlet and outlet ports are drilled at the opposite ends
of the cell defined in the mesa. Finally, a 750 mm wide by 500 mm deep
groove is created in the mesa that circumscribes the milled out area that
defines the cell (D). A silicone gasket is later seated in this groove.
the mesa, so this distance is carefully measured and verified with
a granite base-mounted dial indicator gage (Chicago Dial Indicator). For a BARC chip, a 100 10 mm high fluid cell is
fabricated.
Next, fluidic inlet and outlet ports are drilled at opposite ends
of the cell now defined in the mesa. External tubing (Tygon S54-HL Microbore) can later be mated to these ports. Alternately,
to move the connections away from this area, we have connected
each fluid cell to 300 mm wide by 300 mm deep extension channels
milled into the opposing side of the support body (and later
sealed with tape) (Fig. 1C).
Finally, a groove, 750 mm wide by 500 mm deep, is created in
the mesa that circumscribes the area that defines the fluid cell
(Fig. 2D). A matching gasket will be seated in this groove. The
interior edge of the groove defines the in-plane geometry of the
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Fig. 3 Gasket mold and other flow cell configuration possibilities. (A)
Gasket mold made from an aluminium block. (B) Same as Fig. 1. (C)
Instead of a groove, all material outside the defined flow cell shape is
removed down the full depth of the void. A free-standing, thick-walled,
silicone gasket fits around the sculpted mesa. (D) Double-sided acrylic
tape gasket placed over the top of the mesa for permanent bond and
water-proof seal to substrate. (E) The mesa interior is milled out to form
a trough so that silicone inserts with various flow channel geometries can
be seated securely within it. (F) Silicone insert of Fig. 3E implemented.
Because the substrate is flat (i.e. no protruding wire bonds need
accommodation), it is not necessary to create mesas, and each
grove/fluid cell is machined directly into the ledge. Hence, the
cartridge is simply a row of individually-addressable fluid cells
machined into a single support body. Fluidic connections to the
cells are provided by an array of microchannel extensions milled
into the cartridge support body. A layer of biofouling resistant
Teflon FEP tape (CS Hyde Co., Inc.) was used to enclose the
extension channels. The tape enclosure allows for rapid removal,
cleaning, and reassembly, a necessary design element for a reusable bioassay device. This cartridge is used for automated
experiments performed in an integrated fluidics platform
mounted on the stage of an upright microscope.14
A great advantage of our fluid cell design is the ease and speed
at which the design-to-prototype cycle can be accomplished,
whether it be for a single-cell cartridge, or a multichannel device.
In addition, even though tolerances are fairly tight, every
cartridge or multichannel device works—there is no trial and
error to find an operational device. A basic fluid cell design has
already been described (Fig. 3A); variations on this design that
do not require a gasket groove are illustrated in Fig. 3C–F.
Importantly, once a prototype has passed the final testing, CNC
milling could then be replaced with injection molding techniques
if mass production is desired.
Operation
fluid cell, with the interior surfaces of the gasket serving as the
side walls of the cell.
Gaskets are cast in the shape of the groove (and therefore the
cell) using an aluminium mold (Fig. 3A). For our application,
polydimethyl siloxane (PDMS) prepolymer at a 10 : 1 ratio of
base solution to curing agent was used. The vacuum-degassed
mixture is poured into the mold and tightly capped with an
acrylic sheet. The entire assembly is cured in an oven at 70 C for
1 h. Once cast, a silicone gasket is peeled from the mold and
inserted into the groove that defines the fluid cell.
To assemble the fluid cell, first a PDMS gasket is inserted;
next, a planar substrate (e.g. a BARC chip on a standard 1.6
mm thick FR4 PCB carrier board) is press-fit and secured with
screws against the recessed ledge in the support body. Note that
all components are symmetrical and self-aligning. The depth of
the ledge is set so that when the carrier board rests against the
ledge, the mounted chip compresses the PDMS gasket until the
chip surface contacts the top surface of the mesa, forms a watertight seal, and thereby sets the fluid cell height (Fig. 1B).
Using these basic manufacturing steps, we have constructed
a variety of different formats of ‘‘press-together’’ fluid cells, two
of which are illustrated here. The first cartridge format is
designed for the facile integration of fluidics with a BARC chip
(Fig. 1). The chip is secured with adhesive to a PCB carrier
board, and wirebonded to an array of pads that ultimately
contact with POGO pins from the bottom of the board. The
assembled cartridge is inserted into an integrated instrument that
contains a mechanical apparatus to complete, in one step, all
electrical and fluidics connections to the chip.
A second design we have implemented is a multichannel
cartridge for use with a standard microscope slide (Fig. 1C).14,17
1470 | Lab Chip, 2009, 9, 1468–1471
General performance
Our fluid cell design can be used under positive pressure,
however, our standard operating procedure (SOP) is to introduce
the sample and reagent solutions under negative pressure (i.e.
a pump pulls liquid through the fluidic system). The operating
characteristics of our cartridges that use the fluid extension
channels was determined using a syringe pump (New Era Pump
Systems) and a pressure gauge (OMEGA Engineering, Inc.;
gauge pressure mode) connected in-line to 0.5 mm ID PEEK
tubing with a Tee (Upchurch Scientific). The fluid cell and
cartridge can withstand a vacuum of 97.51 2.25 mm Hg,
without signs of breaching, at a volumetric flow rate of 2 mL
min1 (the maximum capability of the testing apparatus before
signs of cavitation are evident). This flow is 60 times greater than
the maximum volumetric flow rate we use in our assays (33 mL
min1).14 The dependence of the pressure (p, in mm Hg) on the set
flow rate (V, in mL min1) is linear, despite the eight 90 bends in
the milled channels, and can be expressed as, p ¼ (–50.78 mm Hg
min mL1)V + 1.80 mm Hg, with R2 ¼ 0.99. More importantly,
these measurements indicate that our fluidic system can be relied
upon for predictable control over the flow parameters that
produce the appropriate fluidic forces in our assays.14,18
As an additional test of the water-tightness of all sealed areas,
the fluid cell and cartridge have also been burst tested to
successfully pass water, without leakage, under approximately
200 kPa positive pressure. Beyond this point the Teflon FEP
tape that seals the microchannels is prone to fail at the location
where the fluid in the microchannels flows in a direction normal
towards the tape. The fluid cell, on the other hand, remains intact
without any signs of breaching. Under our SOP, the Teflon FEP
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tape used to enclose the channels has an operational life of about
one month of daily use, and can be replaced in <5 min.
fabricate and easy to assemble, clean, and reuse, making this
flexible approach applicable for a wide range of lab-on-a-chip
applications.
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Published on 27 February 2009 on http://pubs.rsc.org | doi:10.1039/B818960A
Practical applications
Our ‘‘press-together’’ designs for attaching a fluid cell to a planar
substrate grew out of a need to quickly assemble assay cartridges
for a magnetic label-based biosensor system, the compact Bead
Array Sensor System (cBASS). In this system, magnetic
microbeads are used to label biomolecules captured onto
a receptor-patterned BARC microchip.19 A critical component
of the assay is the application of controlled laminar fluidic forces
to the microbeads atop of the chip surface.14,17,18 Developing this
system requires the ability to performed many assays as efficiently as possible. Therefore, in addition to chip-based fluid
cells, we developed the multichannel cartridge (Fig. 1C) that has
identical cells, surface chemistry, and assay conditions, but uses
an inexpensive substrate (functionalized microscope slide) and
optical detection of the microbead labels. The capability to
perform multiple assays in parallel and in different fluid cell
designs has greatly accelerated the development of the cBASS
system.
The universality of our design is exemplified by its application
to surface plasmon resonance (SPR) imaging of a DNA microarray. DNA hybridization experiments performed on the
arrayed SPR substrate are highly susceptible to the uniformity of
the fluid flow across the array. The fluid cell apparatus originally
supplied with the SPR instrument has a single circular well and
inlet/outlet ports that are normal to the measurement substrate,
and creates very non-uniform flow fields. Using our fluid cell
construction strategy, we created an attachment with five
parallel, straight channels with uniform laminar flow that could
be easily attached to the SPR substrate with minimal changes to
the existing hardware. The combination of the uniform flow with
multichannel operation dramatically improved the quality and
quantity of the resulting experimental data.20
Conclusions
A simple, reusable fluid cell with a ‘‘press-together’’ design has
been described. The design incorporates the fluid cell completely
in a plastic body that can be attached directly to a planar
substrate without adhesives or modification of the substrate.
Moreover, the geometry of the fluid cell can be easily varied. It is
straight-forward to create arrays of such cells to attach to a single
substrate. Finally, the components are relatively simple to
This journal is ª The Royal Society of Chemistry 2009
Acknowledgements
This work was supported by the Office of Naval Research and
a Cooperative Research and Development Agreement with
Seahawk Biosystems, Inc. (NCRADA-NRL-04-341). We are
grateful to Dr. Dmitri Petrovykh for many helpful suggestions
during preparation of this manuscript. Authors M.P.M. and
S.P.M. are employees of Nova Research Inc., 1900 Elkins St.
Suite 230, Alexandria, VA 22308 USA.
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