Available online at www.sciencedirect.com

Sensors and Actuators B 132 (2008) 531–539

Integrated and reusable in-plane microfluidic interconnects
Ronalee Lo, Ellis Meng ∗Biomedical Microsystems Laboratory, Department of Biomedical Engineering, University of Southern California,
1042 Downy Way, DRB-140, Los Angeles, CA 90089-1111, United States
Available online 22 November 2007

Abstract
A concept for reusable, in-plane polymer microfluidic interconnects that are directly integrated into microfluidic systems is reported. Convenient,
on-demand world-to-chip interfaces are established by utilizing a pin-and-socket approach in which commercially available non-coring syringe
needles are manually inserted through predefined polydimethylsiloxane (PDMS) septa integrated at the inlets/outlets of microchannels. Receptacles
for the septa were batch-fabricated in a single layer of SU-8 that also formed microchannels in the microfluidic system. Due to the self-sealing
nature of PDMS, the septa were used repeatedly and withstood multiple needle insertions and removals without leakage. Needle pull-out force wasmodeled as debonding in a fiber/matrix system and evaluated experimentally both in single and multiple pull-out studies. Results were comparedwith other published interconnects of similar construction. Interconnects were also evaluated under pressurized conditions and tested to failure.
The simple format of these interconnects is readily adapted to many microfluidic fabrication processes and thus can be easily incorporated into
existing microfluidic systems.
© 2007 Elsevier B.V. All rights reserved.
Keywords: Microfluidics; Packaging; Interconnects; World-to-chip interface

1. Introduction
Microfluidics, and in particular micro total analysis systems (␮TAS) and lab-on-a-chip (LOC), have many features
advantageous for advanced chemical and biological analyses.
The improved performance achieved in part through systemminiaturization, laminar flow, high throughput, reduced sample consumption, and shorter analysis time has enabled new
diagnostic tools and resulted in the application of these technologies to areas such as chemical synthesis, genetic analysis,
drug screening, and even single cell/molecule analysis [1,2]. To
realize the full potential of microfluidics, a suitable packaging
technology to reliably couple micromachined channels to themacro-world is required.
The micro-to-macro fluidic interface, or interconnect, is usually a custom designed solution and assembled one at a time.
This assembly process is time consuming and can require precision alignment or complicated fabrication steps. The lack of abatch-fabricated approach to interconnects greatly complicatesthe packaging of the microfluidic system and is an obstacle to thewidespread commercialization of these microfluidic systems.

∗ Corresponding author. Tel.: +1 213 740 6952; fax: +1 213 821 3897.

E-mail address: ellis.meng@usc.edu (E. Meng).
0925-4005/$ – see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.snb.2007.11.024

Also, many interconnects have out-of-plane interfaces [3–11] inwhich the tubing and interconnect are connected perpendicularly
with respect to the substrate surface. Out-of-plane interconnects
can obstruct optical viewing of the microfluidic system andinterfere with operation of microscope objectives. To accommodate microscope operation, interconnects are placed further apartwhich increases dead volume and overall system size. Robust
mechanical connections in an out-of-plane approach are difficult
to achieve due to limited contact area between the device andtubing. Adhesives can reinforce the connection [3,9,11], but are
difficult to control; device yield is reduced when adhesive bleeds
beyond the application area and clogs the microfluidic system.
When a removable connection or modular approach is desired as
opposed to a permanent connection, adhesives cannot be used.
Most existing interconnects cannot be removed from the system without leaving a permanent fluidic breach in the system[3,4,6,8,9,11–13]. Thus, an alternative approach is required.
A novel interconnect is investigated to address the current
challenges in microfluidics packaging. The interconnect design
follows the plug-in format (pin-and-socket) found in microelectronics. In this case, the “pin” is a commercially-available, small
diameter needle while the “socket” corresponds to an integrated
polydimethylsiloxane (PDMS) septum contained within a SU-8
housing located at the inlet and/or outlet of a microfluidic system (Fig. 1). The SU-8 housing also serves as a needle insertion

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Fig. 1. Microfluidic system with integrated circular interconnects. 33-gauge
non-coring needles were inserted into the input and output septums. Rhodamine
was introduced into the system to demonstrate system functionality. PDMS
septum is outlined to indicate its location.

guide and forms the microchannel between the inlet and outlet.
The microfluidic system can be accessed by piercing a needle
through the septum. By using a non-coring syringe needle, fluidic connections may be repeatedly established via septa; once
the needle is removed, the PDMS septum reseals preserving the
integrity of the microfluidic system and any contained fluids[14].
The in-plane layout of the interconnect, in which the interconnect is oriented in the same plane as the substrate, enables robust
microfluidic connections by increasing the contact area between
the macro-world needle and the microchannels in the chip.
Connections are established on-demand and without the needfor adhesives, precision alignment, thru-wafer drilling/etching,or other complex post-processing and assembly steps. Furthermore, the interconnect does not impede microscope observationof the system. This microfluidic interconnect approach incorporates packaging in the design and layout of the microfluidic
system and can be batch-fabricated. These interconnects areeasily adapted to other microfluidic system designs by simply
adding a single mask to an existing fabrication process to create
SU-8 anchors for the septums.
2. Theory
A needle inserted through a PDMS septum can be modeled
as a stiff fiber embedded in a soft matrix. Total pull-out force,the force required to completely remove the needle from the
PDMS septum, can be used as a measure of the strength of
the interface between the needle and the septum. Debonding inthis model system can be achieved by the application of tensile force, torque, or a compressive force, however, only thetension-induced debonding is considered here. Pull-out forces
for fiber/matrix joints subjected to tension have been explainedby Gent and Liu using a modified theory based on the Griffith fracture energy criterion for debonding that also accounts
for the work associated with frictional sliding at the debonded
interface [15]. The strength of adhesion is associated with theenergy criterion for fracture and can be expressed as:
F02 = 4πArEm Ga(1)

where F0 is the adhesion force, A is the cross-sectional area ofthe matrix, r is the fiber radius, Em is the Young’s modulus of
the matrix material, and Ga is the adhesive fracture energy. The
frictional pull-out force, Ff , is expressed as:
Ff = 2πrμpX(2)where r is the radius of the fiber; μ is the coefficient of friction;p is the compressive stress; and X is the contact length between
the fiber and the matrix. This result assumes that the coefficientof friction is constant. If there is a 2% or greater pressure at
the fiber tip compared to the Young’s modulus of the matrix,
then the product of the coefficient of friction and compressive
stress can be represented by a constant value, k = μp. Under thiscondition, the coefficient of friction is independent of pressure
and the overall force expression becomes:
Ff = 2πrkX(3)The total pull-out force, F, is a combination of the frictional
and adhesion forces:
F = Ff + F0 = 2πrkX + F0

(4)

and yields a linear relationship between the pull-out force andthe fiber/matrix contact length. The strength of adhesion Ga isfound from Eq. (1) by extrapolating the pull-out force F to thecase where X = 0 to obtain the intercept F0 .
3. Design and fabrication
The pin-and-socket interconnect consists of a PDMS septum
housed in a SU-8 anchor and a non-coring needle that punctures the septum to establish the macro-micro interface. The
PDMS septum size and shape is defined by the layout of theSU-8 anchor. Anchor thickness is dictated by the desired flow
resistance (determined by the outer diameter (OD) of the needle) and practical limits of fabricating thick film SU-8 structures(including film uniformity and process time). In this work, theSU-8 structure height was selected to be 100–200 ␮m greaterthan the OD of the needle. For a 33-gauge needle (OD 203 ␮m)
the SU-8 layer was 300 ␮m thick, for a 30-gauge needle (OD
305 ␮m) the SU-8 layer was 500 ␮m thick.
The SU-8 anchors mechanically interlock with the PDMS
septum preventing septum damage or removal during needle
insertion and removal. Three anchor shapes (square, circular,
and barbed) were fabricated and examined (Fig. 2). The lengthof the septum also determines the contact length between theneedle and PDMS. Thus, there exists a design trade-off between
interconnect strength and footprint.
The SU-8 anchor was fabricated on a glass slide substrate.
For devices in which electrical traces were required, Parylene
C was deposited between the glass substrate and the SU-8
to provide electrical isolation, but was also found to alleviate the thermal stress-induced delamination of the SU-8 [16].
The difference in the coefficient of thermal expansion of theglass slide (8.9 × 10−6 /◦ C) compared to SU-8 (5.2 × 10−5/◦ C)
can result in delamination of SU-8 structures during
processing.
Interconnects were fabricated at the inlet and outlet of a single SU-8 microchannel. The interconnect was also integrated
into a simple microfluidic system that contained a microchannel,
microchambers, electrolysis pump, and resistive sensors without
the need for any additional masks (Fig. 3).

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533

Fig. 2. (A) Image of an assembled circle septum interconnect. (B) Top view of three different septum connector shapes (circle, square, barbed) that were designed
and integrated into the test microfluidic system. Needle, PDMS septum, SU-8 housing, and microchannel in the designs are indicated. (C) Side view of the needle
piercing the PDMS septum. Image is not drawn to scale.

3.1. System fabrication
Microfluidic systems each containing one channel andintegrated interconnects at both the inlet and outlet werebatch-fabricated using conventional micromachining techniques(Fig. 4). First, the substrate, either a 76 mm (3 in.) soda lime
wafer (Silicon Quest International, Santa Clara, CA) or soda
lime slide substrate (75 mm × 50 mm, Corning Glass Works,
Corning, NY), was spin coated with AZ 4400 photoresist (AZElectronic Materials, Branchburg, NJ) (4 krpm, 40 s, 4 ␮m)(Fig. 4A). After exposure, a liftoff mask was produced. Ti/Pt(200 Å/2000 Å) (International Advanced Materials, Spring Valley, NY) was e-beam evaporated and defined using standard

liftoff processes by removing the photoresist layer in acetone,
isopropyl alcohol, and deionized water (Fig. 4B). The resistive
thermal sensors and electrolysis electrodes were formed withthis process. Parylene C (Specialty Coating Systems, Inc., Indianapolis, IN) was vapor deposited (2 ␮m thick) to electricallyisolate the metal traces (Fig. 4C). A 4 ␮m layer AZ 4400 was
spin coated (4 krpm, 40 s), patterned, and used as an etch maskfor oxygen plasma removal of Parylene C by reactive ion etching
to reveal the contact pads (Fig. 4D).
Next, a 300 ␮m layer of SU-8 2100 (MicroChem Corp., Newton, MA) was obtained by using a two-step spin coating process
to provide better thickness uniformity across the substrate. The
first coat was spun at 1.5 krpm (approximately 200 ␮m thick)Fig. 3. Integrated interconnect with microfluidic system. System contains SU-8 layer which defines the septum housing, microchambers, and microchannel.
Electrolysis structure and flow sensors are fabricated with a Ti/Pt metal layer. PDMS septum and glass cover plate are not present.

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Fig. 5. Edge view of the needle insertion location. The 33-gauge non-coringneedle pierces the PDMS septum through the edge of the system, creating an
in-plane connection.

Fig. 4. Simplified fabrication process for the microfluidic chip with integrated interconnect. Cross-section views are through the PDMS septum andmicrochamber with interdigitated electrodes for an electrolysis pump.

followed by a second planarization coating spun at 3 krpm (foran additional 100 ␮m) (Fig. 4E). After each spin coat, the applied
SU-8 layer rested at room temperature for 3 h to improve planarization. Then, the layers were each softbaked at 90 ◦ C; thefirst layer softbaked for 90 min and the second for 3 h. Baking
steps were all performed on a programmable hotplate (Dataplate
Series 730, Barnstead International, Debuque, IA) set to ramp at
3 ◦ C/min. The lower softbake temperature was selected to avoid
thermal degradation of the underlying Parylene C. Substrates
were allowed to slowly cool to room temperature after eachbake step to avoid thermal stress cracks in the SU-8. The SU-8
was patterned (600 mJ/cm2 ), post-exposure baked for 30 min at
90 ◦ C, and then developed using SU-8 developer (MicroChem
Corp., Newton, MA) (Fig. 4F). A final hardbake step was performed at 90 ◦ C for 30 min.
3.2. Septum formation
The SU-8 layer includes the microchannel which is terminated on either end by a region reserved for the PDMS septum(Sylgard 184, Dow Corning, Midland, MI). A PDMS septum islocated at both the input and output and allows rapid access tothe sealed microchannel (Figs. 1 and 2). Mechanical interlocking structures patterned in the SU-8 layer secure the PDMS septa

to the substrate during both needle insertion and removal. Thesemechanical anchors were filled with PDMS as follows (Fig. 4G).
First, the regions adjacent to the septum (microchambers andmicrochannel) were filled with deionized water injected with asyringe. Water serves as a convenient mask that prevents PDMS
prepolymer from flowing into water-masked areas. PDMS is oilbased and is thus immiscible in water, allowing water to serve
as a mold for casting PDMS [17]. PDMS also has a low surface
energy and its hydrophobic property further enhances the water
masking technique. A similar water masking technique has beenpreviously demonstrated [18,19]. PDMS was mixed in a 10:1
elastomer-to-curing agent ratio (AR-250 Hybrid Mixer, Thinky
Corp., Tokyo, Japan) to obtain the prepolymer and then precisely
introduced into the septum area through a 20-gauge needle andsyringe. PDMS prepolymer was degassed in an evacuated chamber to remove trapped bubbles and then partially-cured (65 ◦ C
for 20 min) in order to facilitate the final adhesion of the capping
layer.
To complete the microfluidic system, the top of the
microchannel was formed by capping the SU-8 channel
and PDMS septum with a soda lime glass slide (75 mm ×
50 mm × 1 mm, Corning Glass Works, Corning, NY) (Fig. 4H).
First, the glass slide was diced to obtain 10 mm × 50 mm pieces
to match the die size. To join the substrate and cover, PDMS wasused as an adhesive sealant. A thin, partially-cured PDMS membrane (∼300–400 ␮m thick, 65 ◦ C for 20 min) was placed on topof the substrate chip to prevent excess PDMS from entering the
microchannel below. Uncured PDMS prepolymer was spread on
top of the partially-cured PDMS membrane as an adhesive layer
and the glass cover was placed on top of the microchannel assembly. This prepolymer layer further planarizes any remaining
thickness non-uniformity in the SU-8 layer. The entire structure
was left at room temperature for 24 h to allow the PDMS to curecompletely or rapidly cured at 70 ◦ C for 1 h. Finally, a 33-gauge
stainless steel non-coring syringe needle (Hamilton Company,
Reno, NV) was manually inserted into the interconnect (Fig. 5).
4. Experimental methods
To optimize the performance of this new microfluidic
interconnect method, needle tip types were evaluated. The performance of the interconnects was then evaluated using a needle

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535

Fig. 6. (A) Comparison of 30-gauge coring and non-coring needle tip shapes. (B) Top and (C) cross-section views of needle tracks through PDMS membranes areshown.

with the most suitable tip type. The robustness and reusabilityof the needle-septum connection was evaluated by measuring
the pull-out forces associated with needle removal. Leakage was
examined in both short- and long-term studies under pressurized
operation.

was calculated from the final pull-out weight and recorded. The
same interconnect was used repeatedly in up to 8 trials following
the same procedure described above.

4.1. Coring versus non-coring needle tip typeThe operating range of each of the three interconnect designs
was determined for both pressurized dyed water and nitrogengas. Non-coring needles were inserted through the input andoutput septums. Dyed water was introduced from the input,
through the microchannel, and to the output needle to ensure
a continuous fluidic pathway between the input and output. The
output needle was blocked (P-656 luer lock assembly and P-770
plug, Upchurch Scientific, Oak Harbor, WA) and the input needle was attached to a custom pressure testing system (Fig. 8).

Access to a microchannel was gained by piercing a needle
through the PDMS septum such that the lumen of the needle
established a continuous fluidic path to the microchannel. Stainless steel coring and non-coring needles (Hamilton Company,
Reno, NV) (30-gauge) were examined to determine the needle
tip type that could maximize the lifetime of the interconnect [14].
PDMS membranes were casted and cured (1.85 mm thick). A
coring or non-coring needle was inserted through each membrane and then removed. The membrane was then dissected
through the needle puncture site and optically imaged (Fig. 6).

4.3. Maximum operating pressure

4.2. Pull-out force and reusability
The reusability of the interconnects was assessed by determining the pull-out forces required to remove the needle after
multiple insertions into the same septum. For these experiments,
a 33-gauge non-coring needle was used and the three-septum
shapes were each evaluated. The microfluidic system was
mounted such that the microchannel was oriented perpendicular to the ground by using a custom laser machined test setup(Mini/Helix 800, Epilog, Golden, CO) (Fig. 7). The needle wasinserted through a hole in the base of the test fixture and thenthrough the PDMS septum to gain access to the microchannel.
Pull-out forces were applied to the needle by gradually increasing the brass weights contained in a bag attached to the needle.
The bag was preloaded with a 50 g brass weight. Weights werethen added incrementally with a 10 min pause between weight
additions to allow the system to equilibrate. The smallest weightused was 1 g which corresponds to a force resolution of 9.8 mN.
The total weight attached to the needle was recorded after theneedle was pulled out of the septum. The resulting pull-out force

Fig. 7. Pull-out test setup. Connector is held perpendicular to the ground byplacing the microfluidic device in the Plexiglas test fixture. Weights are added
to a container attached to the luer lock portion of the needle. Pull-out force isdetermined by multiplying gravity by the combined mass of the weights, needle,
and container. Image is not drawn to scale.

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interconnect started to leak and the location of the leakage wererecorded.
4.4. Prolonged pressure operation
Long-term operation of the interconnects under pressurizedwater was also evaluated. The same custom pressure testing system was used (Fig. 9). An interconnect was pressurized at 36 kPa
and monitored over a 24 h period. This pressure value is half of
the average value of the maximum operating pressures for water.
5. Results and discussion
5.1. Coring versus non-coring
Fig. 8. Test setup for leakage pressure test and prolonged pressure test usingpressurized water. Output needle is blocked using an Upchurch plug.

30-gauge (OD 305 ␮m) coring and non-coring needles werecompared to determine which needle would facilitate interconnect reusability. The non-coring needle displaced the PDMS
material as it moved through the PDMS slab. The displaced
material resealed behind the needle as the needle was removed(Fig. 6B–C). The coring needle removed material as it was
pushed into the PDMS; a cylindrical pathway was observed
when the needle was removed (Fig. 6B–C). The permanent pathway becomes a potential leakage site, which limits interconnect
reusability. After inserting the needle, the cylindrical PDMS coreremained inside the shank of the needle and effectively clogged
the fluid path.
5.2. Pull-out force and reusability

Fig. 9. Test setup for leakage pressure test using pressurized N2 . Leakage isvisualized by N2 bubbles escaping from the submerged the microfluidic chip.

Pressure was slowly increased with a 10 min pause between
pressure increments to allow the system to equilibrate. Leakage was determined by directly noting the leakage location
when using dyed water. The interconnect assembly was placedon an absorbent cloth to facilitate leakage observation. Whenusing pressurized nitrogen gas, the assembly was submerged indeioinized water and the escape of nitrogen bubbles was used
to determine leakage failure (Fig. 9). The pressure at which the

The pull-out forces for each septum shape were obtained
and for a single pull-out trial, theoretical maximum (1.19 N)
and minimum (0.86 N) pull-out force for interconnects wereobtained from (4) using the values in Table 1. The wide range intheoretical values is attributed to the variation published values
for the Young’s modulus of PDMS [15,20]; differences in material properties may arise due to varying processing conditions(e.g. temperature and time for PDMS curing). The pull-out force
for the first removal with respect to contact length and contact
area for the circle, square, and barbed interconnect was compared to other pull-out values for published connectors (Fig. 10
and Table 2) [9,13,21]. As predicted in the simple fiber/matrix

Fig. 10. Comparison of the first pull-out force with respect to (A) contact length and (B) contact area for our interconnects (circle, square, barbed) and that of otherpublished interconnects.

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Table 1
Values used to calculate theoretical pull-out force

537

Table 3Results from leakage pressure tests

Variable

Value

Em
Ga
r
μp
A

360–870 kPa
180–240 J/m2
101.5 ␮m
0.1 kPa
2.1 mm2

model, pull-out force increased with interconnect contact lengthand contact area. It is important to note that Gent and Liu modeled pull-out of stainless steel fibers from PDMS that wereembedded into the material before curing. The model may notadequately account for the increase in compressive force experienced by the needle as it is inserted into the PDMS septum or
the irregular path the needle may take. The first pull-out force
value for the square, circular, and barbed interconnects may varyfrom the model due to the inability to precisely determine thecontact length and area. The flexible, small diameter needle wasmanually inserted and the angle and precise location of needle
insertion varied between interconnects. It is possible that someinterconnects may have been inserted near or at the glass or SU8 interfaces. Thus, an incomplete PDMS-to-needle seal would
result in which the PDMS would compress the needle against
the hard surface instead of completely surrounding it. Deviations
from the linear behavior predicted by the model may further be
attributed to the differences in operating principle between eachinterconnect system and the fiber/matrix materials used.
The pull-out force data for multiple trials (up to 8 pull-outs)
were compared to the performance of other published microfluidic interconnects (Fig. 11A) [9,13,21]. The measured pull-out
force was consistent over 8 pull-outs and in some cases, exhibited a slight decrease after the first pull-out. Again, pull-out force
was dependent on the contact area between the interconnect fiberand matrix. The pull-out force was normalized with respect tocontact area in order to better compare the pull-out force ofthe existing interconnects (Fig. 11B). Most of the normalized
pull-out forces fell within the range of 0.1–0.5 N/mm2 .
Each of the mechanical anchoring schemes for the interconnects (square, circular, barbed) prevented the septum fromPort type

Test medium

Leakagepressure (kPa)

Leakage location

Circle

Water

36.27

Square

Water

20.82

BarbedCircle

Water
N2

51.37
60.4

SquareBarbed

N2
N2

25.72
15.72

Between glass and paryleneinterface
Between glass and paryleneinterface
Needle insertion at output
Between glass and paryleneinterface
Needle insertion at output
Between glass and paryleneinterfacedislodging from the SU-8 housing. Though all of the interconnect shapes survived multiple uses, the circular shaped anchor
may be the most desirable structure for two reasons. First, thecircular shape does not contain as many corners that may allow
for stress concentration in the SU-8 layer. Secondly, the smooth,circular septa did not contain sharp corners and were easier to fillwith PDMS. In the square and barbed septum cavities, the PDMS
prepolymer required manual spreading to ensure complete fillingof the cavity corners.
5.3. Pressurized testing
The leakage test identified the maximum operating pressure
as well as the failure mode in each case (Table 3). The maximum operating pressure was 51 kPa for water operation (barbed
interconnect) and 60.4 kPa for N2 operation (circle interconnect). Two failure modes were present: (1) the needle insertion
point and (2) at the glass–parylene interface. The first failuremode suggests the presence of a leakage path along the needle
insertion path or a compromise of the seal between the needle
and the PDMS septum (Fig. 12).
5.4. Prolonged pressure operation
The prolonged pressure test demonstrated that the connector
can maintain pressurized water at a pressure of 36.2 kPa for overTable 2Comparison of existing interconnects
Connector

Diameter (mm)Contact length (mm)Contact area (mm2 )Fiber material

Matrix material

Circle 1
Circle 2
Square 1
Square 2
Barbed 1
Barbed 2
Chiou 1
Chiou 2
Chiou 3
Yao 1
Yao 2
Li 1
Li 2

0.203
0.203
0.203
0.203
0.203
0.203
0.361
0.361
0.361
0.500
0.400
1.020
0.840

7.010
9.400
7.240
10.5408.030
8.960
14.990
9.960
6.260
0.250
0.250
3.000
3.000

4.471
5.995
4.617
6.722
5.121
5.714
17.00011.296
7.100
0.393
0.314
9.613
7.917

Stainless SteelStainless SteelStainless SteelStainless SteelStainless SteelStainless Steel
Fused Silica
Fused Silica
Fused Silica
Glass
Glass
Teflon (PTFE)
Glass

Sylgard 184
Sylgard 184
Sylgard 184
Sylgard 184
Sylgard 184
Sylgard 184
Sylgard 184
Sylgard 184
Sylgard 184
MRTV1 American Safety Technologies, Inc.
MRTV1 American Safety Technologies, Inc.
Sylgard 184
Sylgard 184

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Fig. 11. (A) Comparison of the pull-out force for our interconnects (circle, square, barbed) compared to other published connectors. Pull-out force varies oversubsequent pull-outs and is dependent on contact area. (B) Comparison of normalized pull-out force with respect to contact area.

Fig. 12. Interconnect failure at the PDMS septum and stainless steel needle interface. (A) Water surrounds the needle shaft as PDMS is debonded from the needle
and (B) seeps from the needle insertion point.

24 h. No measurable leakage was observed over this period oftime.
5.5. Failure modes
Needle insertion angle through the input and output septums
was observed to be a critical factor in interconnect failure. Whenthe needle was inserted at a downward angle relative to the glass
substrate, the tip of the needle would penetrate into the parylene
layer and could potentially become lodged at the glass/paryleneinterface. When a test medium, such as dyed deionized water,
was introduced through the needle, the fluid was observed seeping into the interface (Fig. 13). The parylene delaminated fromthe glass substrate and the interconnect failed. Subsequent fabrication runs were modified to include an adhesion promotion

Fig. 13. Interconnect failure due to Parylene C delamination. Dyed water canbe seen spreading between the Parylene C and substrate layers.

step with silane-based A-174 prior to parylene deposition toprevent parylene delamination from the glass substrate. In theseinterconnects, if the needle was inserted at an angle, the needle
tip would scratch the parylene but the film would not readilydelaminate from the glass surface. Conversely, another failuremode occurred when the needle was inserted at an upward angle
relative to the glass substrate. The needle could become lodged
in the PDMS/glass interface where its circumference was notcompletely sealed by PDMS. When the needle was inserted
immediately adjacent to the glass cap, the needle was not surrounded by PDMS. The small gaps between the needle shaft andthe displaced PDMS form low resistance leakage paths (Fig. 12).
Misalignment of the needle may occur as the needle penetrates the PDMS septum at an angle parallel to the plane of thesubstrate. In this case, the needle trajectory would cause the needle tip to collide with the surrounding SU-8 housing and thus
block the lumen of the needle. The needle could be removed andreinserted; however, it was demonstrated that repeated insertions
were associated with decreasing needle pull-out forces. Thus,
careful alignment of the needle to the microchamber is necessary to avoid degradations in interconnect strength related tomultiple misaligned needle insertions.
A future refinement to this interconnect would be to include
a needle insertion guide, either external or integrated, to prevent
failure modes resulting from the needle penetrating through the
PDMS septum boundaries and into interfaces. The needle guide
would direct the needle to the center of the PDMS septum andprevent the needle from being inserted at an angle. For highdensity microfluidic connections, multiple interconnects can be

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positioned in a linear array at the edges of a microfluidic chip.
This interconnect format would be analogous to ribbon cables
and sockets on jumper pins commonly found in the microelectronics industry.
6. Conclusion
Practical in-plane integrated interconnects that can be batchfabricated with microfluidics have been demonstrated. Needle
pull-out force was modeled as debonding in which both frictional
and adhesion forces were accounted for. In this model, pull-out
force scales with needle/septum contact length and this trend
was confirmed in experiments and by comparisons with otherpublished interconnects. The reusability of the interconnects wasdemonstrated in multiple pull-out trials in which the pull-out
force remained constant over 8 trials. In experiments which theinterconnects were subjected to incrementally increasing gas orwater pressure, the Parylene C-to-glass interface adhesion was
found to be critical in preventing leakage. Also needle insertion
angle and alignment affected the robustness of this interconnect
scheme. No leakage was observed under prolonged exposure topressurized water.
Acknowledgement
This work was funded in part by the Engineering ResearchCenters Program of the National Science Foundation (NSF)
under Award Number EEC-0310723 and an NSF CAREER
grant under Award Number EEC-0547544. The authors would
like to thank Mr. Merrill Roragen for fabrication assistance, Dr.
Tuan Hoang for valuable discussions, and other members of
the Biomedical Microsystem Lab at the University of Southern
California for their contributions to this work.
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Ronalee Lo received her BS degree in engineering from Harvey Mudd College
in 2001. She worked at a Washington, D.C. based Biotechnology consulting firm,
BioStar, for one year prior to enrolling at the University of Southern California
to pursue a PhD. She received a MS in Biomedical Engineering Medical Devices
and Diagnostics from the University of Southern California in 2004. Ronalee
is currently a member of the Biomedical Microsystems Laboratory where hercurrent research focuses are on MEMS drug delivery devices and microfluidic
interconnects.
Ellis Meng received her BS degree in engineering and applied science from the
California Institute of Technology in 1997. She pursued her graduate studies
in electrical engineering and received her MS in 1998 and PhD in 2003 at thesame institution. She is now an assistant professor of Biomedical Engineering
at the University of Southern California. In the National Science Foundation
Biomimetic MicroElectronic Systems Engineering Research Center (BMES
ERC) she is a Thrust Leader for Interface Technology and the Associate Director of Education and Student Diversity. She is a member of Tau Beta Pi, theInstitute of Electrical and Electronics Engineers (IEEE), the American Societyof Mechanical Engineers (ASME), the Society of Women Engineers (SWE),
and the Biomedical Engineering Society (BMES). Dr. Meng is a recipient of
the NSF CAREER award and was recently appointed the Viterbi Early Career
Chair in the Viterbi School of Engineering.