IOP PUBLISHING

JOURNAL OF MICROMECHANICS AND MICROENGINEERING

doi:10.1088/0960-1317/21/5/054021

J. Micromech. Microeng. 21 (2011) 054021 (13pp)

Reusable, adhesiveless and arrayedin-plane microfluidic interconnects
R Lo1 and E Meng
Department of Biomedical Engineering, University of Southern California, 149 Commonwealth Drive,
Menlo Park, CA 94025, USA
E-mail: ellis.meng@usc.edu

Received 10 September 2010
Published 28 April 2011
Online at stacks.iop.org/JMM/21/054021
Abstract
A reusable, arrayed interconnect capable of providing multiple simultaneous connections toand from a microfluidic device in an in-plane manner without the use of adhesives ispresented. This method uses a ‘pin-and-socket’ design in which an SU-8 anchor houses
multiple polydimethysiloxane septa (the socket) that each receive a syringe needle (the pin). A
needle array containing multiple commercially available 33G (203 µm outer diameter) needles(up to eight) spaced either 2.54 or 1 mm (center-to-center) pierces the septa to access the
microfluidic device interior. Finite element modeling and photoelastic stress experiments wereused to determine the stress distribution during needle insertion; these results guided the SU-8
septa housing and septa design. The impact of needle diameter, needle tip style, insertion rate
and number of needles on pre-puncture, post-puncture and removal forces was characterized.
Pressurized connections to SU-8 channel systems withstood up to 62 kPa of pressurized waterand maintained 25 kPa of pressurized water for over 24 h. The successful integration andfunctionality of the interconnect design with surface micromachined Parylene C
microchannels was verified using Rhodamine B dye. Dual septa systems to access a single
microchannel were demonstrated. Arrayed interconnects were compatible with integrated
microfluidic systems featuring electrochemical sensors and actuators.
(Some figures in this article are in colour only in the electronic version)

1. Introductionof microfluidics research. The lack of reliable microfluidic
connector options severely limits practical implementation
and adoption of many microfluidic technologies. Standardized
macro-to-micro fluidic interconnects, that are batch fabricated,have high assembly yield, have a small interface footprint
relative to the overall device layout, and are compatible with
commonly used materials and device layouts are still underdevelopment.
Fluidic interconnects, to date, are predominantly custom
solutions and typically their design process commences only
after the design and fabrication of the microfluidic device
are complete. Few commercially available solutions exist
and even so, these interconnects are large relative to theoverall device size. Thus, the overall device footprint must
often be increased to accommodate the large interconnects.
Current fluidic interconnect methods may require precisionalignment, adhesives to secure the interconnect, additional
fabrication steps and extensive manual assembly [3, 4]. Most

Micro total-analysis systems (µTAS) and lab-on-a-chip (LOC)
devices, which integrate microfluidic components such as
microchambers, microchannels, pumps and valves, enableimproved chemical and biological analyses. Miniaturization
offers unique advantages such as laminar flow, reduction
in reagent/sample volumes, higher throughput and shorter
analysis time. Furthermore, devices with precisely controlled
dimensions can be batch fabricated. Given these advantages,
microfluidics enjoy widespread popularity in applications suchas chemical synthesis, genetic analysis, drug screening andsingle cell/molecule analysis [1, 2].
Microfluidic devices require a reliable means to introduce
and access the fluids or biological materials to and frommicroscopic channels. Reliable microfluidic connectors, orinterconnects, however, remain a challenge despite decades
1

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0960-1317/11/054021+13$33.00

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J. Micromech. Microeng. 21 (2011) 054021

R Lo and E Meng

(b)

(a)

(c)

Figure 2. Schematic indicating key features of our interconnect
technology and microfluidic system. Here, interconnects withsurface micromachined Parylene channels are shown. Needle guides
align the needle arrays with the septa. Optional features include
sideports and interdigitated electrodes for electrolysis actuation orelectrochemical sensing.

Figure 1. (a) Side view showing concept of single or arrayedmicrofluidic interconnects. A needle pierces septum to access a
microchamber and microchannel in both SU-8 and Parylene C
microfluidic devices. (b) Top view schematic showing a needle
array piercing septa to allow multiple macro-to-micro connections
to be established in an SU-8 microfluidic device. (c) Photograph ofan arrayed interconnect mated with an SU-8 microfluidic system.

SU-8 microchannels and surface micromachined Parylene
microfluidic devices.

interconnects are oriented perpendicularly relative to thedirection of fluid flow (out-of-plane) in the device which
may result in increased dead volume or trapped air pockets[5–13].
Interconnects assembled using adhesives lackrobustness and mechanical strength due to the limited bondcontact area. Clogs resulting from adhesive flow may also
occur and reduce overall yield. Most interconnects areintended for single application/use and cannot be removed
without damage to the device, such as the creation of unwanted
leakage paths [5, 7–11, 13–16]. Establishing microfluidic
connections to surface micromachined microchannels isparticularly difficult. Connections are made either by directlyinterfacing to the channel from the top side or by etchingaccess ports for backside connection. In either case, adhesiveor clamped connections require precision alignment and may
damage delicate components.
We previously presented an in-plane interconnect usinga ‘pin-and-socket’ approach in which a single commerciallyavailable non-coring needle (33G) accessed a microfluidic
device by puncturing a polydimethysiloxane (PDMS) septum(figure 1(a)) [17]. The septa shape and location were definedusing an SU-8 anchor patterned on a glass substrate. Thisapproach was only demonstrated in simple, single channel
SU-8 microfluidic devices.
Practical application of this interconnection approach tobroad microfluidic technologies requires a robust method forestablishing multiple fluidic interconnections at once. Thus, anarrayed interconnection method for simultaneous connections
to multi-channel systems is presented here (figures 1(b) and(c)) [18]. Several arrayed interconnects with different spacing(2.54 and 1 mm) and septa shape (oval, rectangular andoverlapping-ovals) were fabricated and evaluated based on
our original concept. Design modifications were confirmed
through simulations and photoelastic stress measurements.
Needle array insertion forces were experimentally determined.
Furthermore, robust fluidic connections were demonstrated
with these interconnects under pressurized conditions in both

2. Device design
2.1. Overall design
The arrayed interconnect uses the pin-in-socket approach,similar to that of the single interconnect [17]. Based on ourearlier work, interconnect design was improved to increase
the packing density. The high-density design allows multiple
needles to simultaneously access corresponding microfluidic
channels via their integrated septa without the use of adhesives.
Septa were fabricated by filling an SU-8 housing with PDMS
prepolymer that was subsequently cured. Then, an arrayof non-coring needles was inserted through the septa toestablish a fluidic connection. PDMS was selected as thesepta material for its compliant and resealing properties. The
septa formed seals around the needles following insertionand to themselves following removal, preventing unwanted
fluid leakage. Several versions of the arrayed interconnect
were fabricated to demonstrate the versatility of the technique.
Differing septa shapes (oval, oval overlap, rectangular) were
investigated; the septa shape used determined the septa
spacing (2.54 or 1 mm center-to-center spacing). Additionalfeatures unique to the arrayed design were integrated including(1) merged septa for higher-density interconnects, (2) dual
septa access to a single channel in the form of a primary
access port and sideport, (3) needle guides to align theneedle to the septum centers, and (4) lithographically-definedSU-8 or surface micromachined Parylene microchannels(figure 2).
Electrochemical transducers (interdigitatedelectrodes) were also integrated into the microchambers andmicrochannels.
Oval, overlapped and rectangular septa housing shapes
were chosen for their ability to retain PDMS without additionalanchoring features (figure 3). We previously presented therelationship between septa design and pull-out force [17].
The theoretical equations and experimental results verified
that pull-out force increased linearly with the contact surface
2

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are compatible with our septa interconnect technology, can beused. The channel dimensions and Parylene thickness must be
carefully selected to prevent the channel from collapsing when
the sacrificial photoresist within the channel is removed [20].
The channel width is obtained from the critical length (lcrit )for preventing the collapse and stiction of cantilevers. It isgoverned by the Young’s modulus of the channel material (E),the channel wall thickness (t), gravity (g), the surface tension
between an air–liquid interface (γ la ) and the contact angle ofthe material (θ c ) [21]:"!
3Et 3 g 2.
(1)
lcrit =
16γla cos θcFigure 3. Septa configurations used in the arrayed interconnect
designs.

area between the septa and needle. Septa shape did notaffect the pull-out force; however, designs which included
a septa locking feature (e.g. a shape which prevented thesepta from being dislodged from the housing during needle
insertion/removal) were preferred.
The oval septa design contains several distinct septa
on 2.54 mm center-to-center spacing. Septa housings wereindividually filled with PDMS. Overlapped and rectangularsepta feature denser interconnect packing (1 mm spacing)and simultaneous filling of multiple septa housings as they
are fluidically connected. The thickness of the SU-8 septa
housing was determined by the practical fabrication limits
for thick planar SU-8 layers (time and process complexity)
and the needle outer diameter. For 33G non-coring needles(203 µm OD), a 300 µm thick layer of SU-8 was used.
Septa spacing, and consequently, the density of theconnections were determined by the dimensions of the
microfluidic channels and integrated components, as well as
the septum shape. Two spacing arrangements were considered.
A 2.54 mm center-to-center spacing design utilized separated
oval septa. This spacing was chosen to emulate the standard
spacing found in electrical pin packages. Denser interconnectswith 1 mm center-to-center spacing were fabricated using
overlapping oval septa and connected rectangular septa.
Sideports, as well as converging microchannel designs, thatallow multiple fluids to be introduced into a single input orchannel, respectively, were also investigated.
The angle of needle insertion is important for properalignment and to prevent interconnect failure. Misalignment
may cause the needle to veer off-center during insertion andbecome lodged against the SU-8 anchor walls. Misalignment
in our previous interconnects resulted in blockage of theneedle lumen and fluid path [17]. This improved interconnect
incorporates needle alignment structures in the SU-8 housing,which direct the needle through the center of the septum.
The microchannels in the arrayed interconnect design
were fabricated using SU-8 or Parylene. The previously
presented single interconnect design featured an SU-8 channel
that was fabricated simultaneously with the SU-8 septa
housing using the same photomask. However, the height of
the SU-8 microchannel was determined by the thickness of
the SU-8 layer; furthermore, the width of the microchannel
was also limited based on the manufacturer’s recommended
aspect ratio for SU-8 structures (10:1 height to width ratio,[19]). Therefore, for a 300 µm thick SU-8 layer, a minimum
channel width of 30 µm was used. For applications requiring
channels with dimensions independent of those of the SU8 housing, surface micromachined Parylene channels, which

Yao et al determined a maximum distance of 150 µm between
supporting structures in a 4.5 µm thick Parylene beam. In achannel, the channel walls serve as the supporting structure;
therefore, a channel width of 100 µm and a channel height of
4 µm were chosen, eliminating the need for support
posts within the channel [20]. Both narrower and wider
Parylene channels are also possible with proper design.
This microchannel is significantly thinner than the SU-8
microchannel; the Parylene microchannel has 22.5 times lessvolume than a SU-8 microchannel of equivalent length.
Microfluidic
deviceswith
integrated
arrayedinterconnects with different septa shapes (oval, oval
overlap or rectangular), number of septa (4 or 8), septa
spacing (1 or 2.54 mm), microchannel material (SU-8 orParylene) and optional sideports were designed and fabricated.
Some devices also incorporated converging channels andmetal structures to demonstrate the versatility of thisinterconnect technique. A representative arrayed interconnect
with Parylene microchannels, oval overlap septa with eight
inputs and outputs, sideports and needle guides is shown infigure 4.

3. Fabrication
3.1. SU-8 microchannel without metal
The fabrication process for arrayed interconnects with SU8 microchannels and no metal features is identical to that
previously reported for the single interconnect [17]. Insummary, an SU-8 anchor was fabricated using a soda lime
wafer substrate (Silicon Quest International, Santa Clara, CA).
A 2 µm layer of Parylene (Specialty Coating Systems, Inc.,
Indianapolis, IN) was vapor deposited onto the adhesion
promoter treated-wafer (A-174, Specialty Coating Systems,
Indianapolis, IN). A 300 µm layer of SU-8 (SU-8 2100,
Microchem Corp., Newton, MA) was spin coated onto the
Parylene using a two-step process. A controlled soft bakeusing a programmable hotplate was performed following
application of each SU-8 layer. The SU-8 was patterned(600 mJ cm−2 ), hardbaked and developed. Septa were createdby filling the SU-8 anchor with PDMS. A water masking
technique, which was previously demonstrated, was used
to prevent the PDMS from spreading outside the septa area
[17, 22]. A half-cured and uncured layer of PDMS was used
to secure a glass cap to seal the microfluidic system [23].
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Figure 4. Fabricated arrayed interconnect with Parylene microchannels and sideports. Salient features of the arrayed microfluidic systemwith integrated interconnects are highlighted. External access provided via needles inserted through the septa is not shown in thesephotographs.

3.2. SU-8 microchannel with metal
Incorporating metal patterns, such as resistive elements
and interdigitated electrodes, into the microfluidic device
design required additional fabrication steps prior to Parylene
deposition. A 4 µm layer of AZ 4400 (4 krpm, 40 s) was spun
onto the wafer surface. The liftoff photoresist pattern was
exposed (240 mJ cm−2 ) and developed. Then the wafer was
descummed in oxygen plasma (30 s, 60 W, 100 mTorr) and alayer of Ti/Pt (300 Å/1000 Å) was electron beam evaporated
onto the substrate. Liftoff in acetone revealed the metal
patterns. Then a 2 µm layer of Parylene was deposited onto
the wafer. A masking layer of photoresist (AZ 4400, 4 µm,
4 krpm, 40 s) was spun and patterned. Parylene was etched inoxygen plasma (150 W, 100 mTorr, 10 min) to remove open
the contact pads and electrodes. SU-8 was applied as directedin the fabrication steps listed above.

Figure 5. Process steps to fabricate and package an arrayedinterconnect with a surface micromachined Parylene microchannel.

3.3. Parylene microchannel with metal

sacrificial photoresist can be accelerated using an undiluted
photoresist developer. The fabrication process for Parylene
microchannels without metal is shown in figure 5.

Arrayed interconnect with Parylene microchannels started
with Parylene (2 µm) coated soda lime wafers. A photoresist
etch mask (AZ 4400, 4 krpm, 30 s) was spin coated andpatterned. The Parylene layer was etched using oxygen
plasma to create contact pad openings. Lithography andmetal liftoff were performed as previously mentioned to lay
down interdigitated electrochemical electrodes. Sacrificial
photoresist (AZ 4400, 4 krpm, 30 s) was applied and patterned
to define channels. A second layer of Parylene (4 µm)
was deposited to form the remaining three walls of the
microchannel. The wafer was then baked in vacuum at
160 ◦ C for 40 h to anneal the two layers of Parylene [24].
Next, a 10 µm photoresist etch mask (AZ 4620, 2 krpm,
45 s) was spin coated and patterned to define the channel
openings. The Parylene was removed using oxygen plasma.
Finally, the SU-8 layer, septa formation and device packaging
were completed using the same process steps as described
for the SU-8 microchannel design. The sacrificial photoresist
defining the Parylene microchannels was dissolved in a room
temperature isopropyl alcohol (IPA) bath to prevent adverse
interactions to SU-8 structures following exposure to acetone(e.g. SU-8 delamination and cracking). Removal of the

3.4. Needle array
The needles were housed in channels (343 µm in diameter)
spaced 1 or 2.54 mm apart, center-to-center. Two different
types of needle arrays were fabricated: shared input(figures 6(a)–(c)) and separated inputs (figures 6(d) and (e)).
The shared input needle array was fabricated by drilling
channels (#80 drill bit) partially through a Plexiglas block.
A larger diameter channel (#60 drill bit) was drilled throughthe side of the block that intersected all the smaller channels.
A 10–32 threaded hole allowed connection of a liquid orgas source to the needle array with conventional fittings.
Commercially available 33G non-coring (i.e. beveled tip)needles (21033A Point Style 2, Hamilton Company, Reno,
NV) were carefully placed in each of the #80 holes and affixedusing epoxy. In this configuration, pressurized media wereapplied to all the needles simultaneously; however, needle
arrays with separate fluidic access to each needle were alsofabricated.
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(a)

(b)

(c)

(d)

(e)

Figure 6. Photographs of needle arrays that provide shared or separate access to microchannels: (a) four shared needles with 1 mm spacing,(b) four shared needles with 2.54 mm spacing, (c) eight shared needles with 1 mm spacing, (d) four separate needles with 1 mm spacing, and(e) eight separate needles with 2.54 mm spacing. The scale bar represents 10 mm.

A custom-made Plexiglas mold was used to fabricate aneedle array with individual needle access. The needles wereplaced into grooves in the mold that controlled spacing andalignment. Casting acrylic was poured into the mold to form amonolithic block with embedded needles. Silicone tubes wereattached to each needle.

dynamic interaction which is difficult to approximate in themodel. The septa faces not in contact with the SU-8 housing,
substrate or packing (e.g. faces through which the needle canenter or exit) were not constrained. The insertion force atthe needle tip and friction force along the needle shaft wereapplied using force data obtained experimentally with a Bose
3100 ElectroForce mechanical fatigue test instrument. The
modeling results were also compared to photoelastic stressimages.

4. Experimental methods
4.1. FEM analysis of stress distribution

4.2. Photoelastic stress

Finite element analysis and modeling estimated the stress on
the septa as the needles were inserted. Only the oval overlap
septa were modeled; the simple design and ease of fabrication
make it a likely candidate for future use. The single septum
case had been previously studied for the single interconnect
design [25]. Additionally, the oval overlapped model wasused because multiple septa were merged into a single PDMS
slab; therefore, any insertion stress affecting neighboring
septum/needle pairs will be apparent in the model. The stress
distribution in the PDMS septa set a practical limit to theachievable needle density by dictating the minimum spacing
between the needle path and SU-8 housing. A suggested
design rule is a minimum distance between the needle shaft
and the SU-8 housing equaling twice the length required todissipate to 36.8% (1/e) of the maximum stress value.
The static stress distribution within the septa was modeledat three needle insertion points: (1) as needles touched thesurface of the septa, (2) after the needles pierced the septa andare partially inserted, and (3) after needles were fully inserted.
Septa surface area adjacent to the housing (e.g. SU-8 anchor,
device substrate and device cap) was held constant. The
bonding strength between the PDMS and the SU-8 housingor the Parylene coated substrate does not preclude the PDMS
from delaminating from either surface; however, debonding
between the PDMS and SU-8 or substrate boundaries is a

PDMS is a photoelastic material; stresses in the PDMS during
needle array insertion and removal can be visualized using
polarized light. The principal stresses (σ x , σ y ) can be visuallyobserved as a phase difference ($) that is dependent on the
wavelength (λ), the material stress-optical coefficient (C), andmaterial thickness (h) [26]:2π hC(σx − σy ).
(2)λPDMS slabs were pierced using a single needle andneedle array to visualize resulting stresses. Each slab wasplaced between two polarizing plates and illuminated witha broadband light source positioned below the stack. The
polarizing plates were rotated to an orientation which provided
the greatest visible contrast in stressed versus non-stressed
areas. Low stress areas appeared as a white haze where higher
stress areas exhibited a rainbow effect. Photoelastic images
were taken during initial needle puncture and partial needle
insertion into the PDMS sample to compare with FEM results.
$=

4.3. Insertion test
The force required to insert and remove a single needle orneedle array into a PDMS slab (thickness 2 ± 0.1 mm) was
5

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measured using a Bose 3100 ElectroForce mechanical fatigue
test instrument and custom-made laser-cut Plexiglas jigs. The
PDMS slab was mounted between two clamped Plexiglas
plates. The plates had through holes (1, 4 or 8) spaced 2.54 mm
apart which guided the corresponding needle arrangements
through the PDMS slab. The jig was attached to a load cellat the base of the Bose instrument. Needles were lowered at
a constant rate and the force required to puncture the PDMS
sample (insertion force) and to remove the needles from thesample (pull-out force) was recorded.
Insertion force is a combination of pre-puncture and postpuncture forces. The pre-puncture force, or stiffness force(f stiffness ), is attributed to material deformation. Post-puncture
force is a combination of the force required to push a needle
through the PDMS slab (f cutting ) and the friction force (f friction )between the needle shaft and the PDMS [27, 28]. Thus, theinsertion force is expressed as
finsertion = fstiffness + fcutting + ffriction .

Failure pressures were obtained for all three septa designs(oval, oval overlap and rectangular) in the four-microchannel
configuration. The design with the greatest failure pressure
was compared to that of the eight-microchannel device withthe same septa design. Needles were inserted into the input
and output septa. Dyed DI H2 O was first introduced into thesystem to ensure that the microchannel was open and free from
obstructions. Then the output needles were removed. Pressure
was applied in increments of 0.5 psi (3.45 kPa) with a 5 minhold period to allow system equilibration. The failure pressure
was recorded when water leakage was observed.
Prolonged pressure was applied to a device to ensure thatthe arrayed interconnect can withstand pressure application
for an extended period of time. The applied pressure was atleast 50% of the average maximum pressures of all the tested
interconnects. The pressure was applied for 24 h and observed
to ensure no leakage during the entire period.

(3)

The force required to remove the needle (pull-out force) is acombination of frictional and debonding forces [29]:fpull−out = fdebonding + ffriction .

4.5. Parylene microchannel(4)Surface micromachined Parylene microchannels with PDMS
septa were also demonstrated. These may be used insteadof SU-8 microchannels when finer dimensional control ofchannels or a Parylene surface is required. Also, complex andhighly integrated microfluidic systems are possible only using
Parylene surface micromachining technology. Microchannel
patency was verified using capillary wicking of diluted
Rhodamine B dye into the microchannel opening prior toforming the PDMS septa. Flow of pressurized Rhodamine
B introduced through the inserted needles was monitored by
optical microscopy.

A discussion of the pull-out force was previously presented
for a single interconnect case [17]. In summary, Gent and Liumodeled debonding between a fiber (i.e. the needle) embedded
in a matrix (i.e. the septa) using a modified theory based on the
Griffith fracture energy criterion [29]. The debonding force
is a function of the cross-sectional area of the matrix, fiber
radius, the Young’s modulus of the matrix and the adhesive
fracture energy between the matrix and the fiber. The frictional
force varies linearly with respect to the coefficient of friction
between the fiber and the matrix, the compressive stress andthe contact area between the fiber and the matrix.
The relationship between insertion force and the numberof needles (1, 4 or 8), needle type (C: coring, or NC: noncoring), needle gauge (33, 30 or 27G), and insertion rate (0.5
or 1 mm s−1 ) was determined. All needles were obtained
from Hamilton Company (Reno, NV). Additionally, needle
insertion was completed multiple times on a single sample toexamine magnitude of the insertion and removal forces witheach subsequent use. The two needle tip types used in thisexperiment are classified as coring (C) or non-coring (NC).
The coring needle has a blunt tip which cores a cylindricalsection from the material as it is inserted whereas the noncoring needle has a beveled tip and displaces material as theneedle is pushed through.
The insertion rates were selected based on the limitationsof the Bose 3100 ElectroForce instrument. The maximum
stroke distance of the displacement motor is 4 mm. A 1 mm
s−1 rate modeled a practical insertion rate while maintaininga high sample rate resolution from the Bose instrument. A
second rate, which was half of the base rate value, was selected
for comparison.

4.6. Electrolysis pressure generation
Internal pressure was generated using the interdigitatedelectrodes. The water within the device was converted
to hydrogen and oxygen gas following current application(0.3 mA) to the electrolysis electrodes. The internal pressure
was monitored (ASDX 015D44R, Honeywell International,
Morristown, NJ). A baseline reading of atmospheric pressure
was obtained prior to each test. Current was applied to thedevice until the entire device interior was voided of any visible
water (approximately 100 s).
4.7. Sideport functionality
The dual septa feature with one primary septum and one
sideport was demonstrated. 33G non-coring needles wereinserted through the septa of the sideport and main channel.
A 30G non-coring needle was inserted into the output port
to minimize any fluidic resistance at the output. Syringes
containing either deionized water (DI H2 O) or dyed DI H2 O
were connected to each input needle. A syringe pump wasused to deliver a constant steady flow to both needles.

4.4. Pressure test
Pressurized DI H2 O was applied to assembled SU-8
microfluidic systems to determine failure pressures and modes.
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(b)

(c )

Figure 7. FEM images of stress distribution within septa during needle insertion, (a) pre-puncture, (b) partial puncture, and (c) complete
insertion.

5. Results
5.1. FEM analysis of stress distribution
Finite element modeling of both a single septum andmultiple septa designs showed that the stress concentration
remained localized around the needle. A radial stresspattern was observed at the needle tip pre-puncture andbecame asymmetric during insertion, matching the beveled
tip of the needle. After insertion, the stress was uniformly
distributed along the needle shaft and extended to neighboring
septa (figure 7). The maximum stress was observed at theneedle/septa interface and decayed exponentially away fromthe needle. The distance over which the stress decayed to1.35 × 105 N mm−2 (36.8% of the maximum stress, 3.63 ×
105 N mm−2 ) was approximately 60 µm.
The FEM results modeled the stress distribution for ideal
conditions where (1) all needles are aligned within the septa,(2) needles do not deviate from a straight path, (3) needles
maintain a uniform distance from one another, (4) all needles
are of the same length, (5) all needle tips (e.g. the bevels) areorientated in the same direction, and (6) needle insertion force
is only directed parallel to the needle shaft. The resultant stress
distribution identified the minimum dimensions necessary forthe interconnect design to prevent unwanted stress interference
on the SU-8 septa housing or adjacent septum/needle pairs.
The data suggest a minimum 240 µm spacing between adjacent
33G needles, indicating that tighter septa packing and thus
denser microfluidic interconnects are possible compared towhat is demonstrated here.

Figure 8. Photoelastic stress in PDMS from needle insertion for asingle needle (18G) and needle array (four 27G). The arrows
indicate visualized photoelastic stress during pre-puncture (whenthe needle tip touches the PDMS surface) and partial insertion(when the needle has been inserted partially through the PDMS).

shaft did not overlap with that of a neighboring needle.
This suggests that stresses from each needle/septum pair do
not affect neighboring needle/septum pairs. These results
are similar to the FEM prediction for stress distribution(figure 8); however, a direct comparison cannot be made dueto the difference in needle size and variation in needle bevel
orientation in hand-made needle arrays.
5.3. Insertion test
Stiffness, insertion (post-puncture insertion force is acombination of friction and cutting forces) and removal
forces were identified for several needle arrangements(table 1). Figure 9 shows a representative graph of the force
measurement for a needle array during insertion and removal.
The stiffness force was measured by identifying a slight
dip in force between the needle touching the PDMS surface(figure 9(a)) and full needle puncture (figure 9(b)). The postpuncture force (cutting and frictional forces) was measured infigure 9(b) and the frictional force was measured in figure 9(c).
The cutting force was calculated by subtracting the frictional
force value (figure 9(c)) from the post-puncture force. The
maximum pull-out force was measured in figure 9(e). Previous
work demonstrated that puncturing the same location multiple
times decreased sealing ability of the material, comparedto random insertion locations [30].
Therefore, needle

5.2. Photoelastic stress
Larger diameter needles (single needle: 18G, needle array:27G) were necessary to enhance the visible photoelastic stressduring needle insertion. Photoelastic stress using 33G needles
was not visible with the limited resolution of our imaging
equipment. From the FEM analysis using a 33G needle,
the visible stress within the PDMS extended approximately
100 µm from the needle shaft; the recording camera did nothave sufficient resolution to detect this distance.
Stress concentrations were visible along the needle shaft
and as a plume at the needle tip during insertion. Photoelastic
visualization of insertion of a 2.54 mm spaced needle arraydemonstrated that the stress distribution from one needle
7J. Micromech. Microeng. 21 (2011) 054021

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Table 1. Summary of relationship between insertion and removal forces and the needle type (coring (C) versus non-coring (NC)), needle
gauge (27G or 33G), number of needles (1, 4 or 8) and rate of insertion (0.5 or 1 mm s−1 ) (mean ± SE, n = 4).
Needlegauge(OD (µm))

Needle
point
type

No of
needles

Insertionrate(mm s−1 )No ofinsertions

Stiffnessforce (N)Insertion
force (N)Frictionforce (N)Cutting
force (N)Removalforce (N)27G (406)
33G (203)
33G (203)
27G (406)
33G (203)
33G (203)
33G (203)
27G (406)
33G (203)
33G (203)
27G (406)
33G (203)
33G (203)
33G (203)

C
NC
C
NC
NC
NC
NC
C
NC
C
NC
NC
NC
NC

1
1
1
4
4
4
8
1
1
1
4
4
4
8

1
1
1
1
1
0.5
1
1
1
1
1
1
0.5
1

1
1
1
1
1
1
1
9
10
9
10
10
10
10

1.87 ± 0.14
0.18 ± 0.02
0.83 ± 0.03
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A

1.67 ± 0.16
0.87 ± 0.02
1.2 ± 0.01
5.41 ± 0.34
3.3 ± 0.05
3.48 ± 0.08
5.72 ± 0.15
1.25 ± 0.09
0.61 ± 0.01
0.66 ± 0.01
4.67 ± 0.21
2.45 ± 0.08
2.71 ± 0.07
4.42 ± 0.15

1.3 ± 0.11
0.74 ± 0.02
0.83 ± 0.02
5.24 ± 0.49
2.81 ± 0.06
3 ± 0.05
4.88 ± 0.16
1.25 ± 0.09
0.61 ± 0.01
0.66 ± 0.01
4.67 ± 0.21
2.42 ± 0.07
2.7 ± 0.07
4.42 ± 0.13

0.37 ± 0.07
0.14 ± 0.04
0.37 ± 0.02
0.18 ± 0.11
0.49 ± 0.04
0.49 ± 0.05
0.84 ± 0.02
0±0
0±0
0±0
0±0
0.03 ± 0.03
0.01 ± 00±01.4 ± 0.2
0.6 ± 0.03
0.89 ± 0.02
2.53 ± 0.17
2.65 ± 0.07
2.7 ± 0.1
4.6 ± 0.15
1.11 ± 0.02
0.54 ± 0.03
0.71 ± 0.02
2.64 ± 0.2
2.43 ± 0.09
2.49 ± 0.02
4.37 ± 0.12

(a)

(b)

(c)

(d )

(e)

(f )

Figure 10. Relationship of post-puncture insertion forces (friction
and cutting forces) and removal forces with respect to the number of
insertion needles.

Figure 9. Representative results from insertion force tests:(a) needle touches the surface of the PDMS sample, (b) needle
pierces the PDMS sample (combination of stiffness and puncture
forces), (c) needle moves through PDMS (friction force), (d) needle
stops moving and material relaxes, (e) needle removal from PDMS
(max removal force), and (f ) needle fully removed. Inset:
corresponding needle displacement over time.

overall device robustness as related to the magnitude of force
necessary to insert the needle array.
Stiffness force increased with the needle diameter (i.e.
decreasing needle gauge). As expected, coring needles
were associated with larger stiffness force due to the blunt
profile compared to the beveled profile for non-coringneedles. Stiffness force could not be determined for needle
arrays because the needle tips were not perfectly aligned
due to limitations in manual fabrication of the arrays.
Therefore, each tip punctured the PDMS sample at slightly
different times and a reliable stiffness force could not be
determined.
The insertion force, the combination of friction andcutting forces, was also expected to increase with surface
area. As surface area increases, frictional force should likewise
increase. Frictional force in a 33G coring needle was larger
than that for the non-coring case due to the area contributed byinsertion and removal cycles were performed at least ninetimes at the same location in a PDMS slab to determine
the effect of multiple punctures on insertion and removal
forces.
As predicted from insertion force equations [29], theforces for the 33G single-, four- and eight-needle array, on
average, varied linearly with the number of needles (figure 10).
This result is also in agreement with the results presented by
Okamura et al where the forces of insertion in soft tissue were
presented [28]. Practically speaking, the maximum numberof needles that can be simultaneously inserted is limited by8

J. Micromech. Microeng. 21 (2011) 054021

R Lo and E Meng

Table 2. Summary of failure pressure and failure locations for all septa designs. The arrows indicate failure points.
Channelmaterial
SU-8

Septa type

No of
septa

Failurepressure(kPa)

Oval Overlap

4

3.79
15.86

SU-8

Oval

4

2.054

Failure location
Insertion site, delamination at the septa/substrateinterface
Delamination at the septa/substrate interface andSU-8/substrate interface

67.57

Edge input port (delamination at the septa/substrateinterface)
Edge input port (delamination at the septa/substrateinterface)
Output port

26.2

SU-8

Rectangular

4

36.54

Delamination at the substrate/SU-8 interface

SU-8

Rectangular

8

8.96

Insertion site, delamination at the septa/substrateinterface

the lumen of the coring needle which was also in contact with
the PDMS. Coring needles were also associated with higher
cutting force in general due to their blunt profile compared tothe beveled tip of the non-coring needle.
Different insertion rates (1 and 0.5 mm s−1 ) exhibitedslight differences in the frictional forces. The lower rate (0.5mm s−1 ) had a small increase in insertion and removal forces;
this may be caused by a change of dynamic frictional force
between the needle and PDMS. As expected, the cutting force
was not affected.
In all cases, multiple insertions reduced the insertion andremoval forces, and therefore affected the sealing capabilityof the reusable arrayed interconnect. We previously presented
information on the sealing capability of a PDMS slab that was
punctured multiple times at the same location. As the numberof punctures increased, the pressure at which leakage occurred
through the puncture site (leakage pressure) decreased. A
decrease in sealing ability was attributed to the damage of
the PDMS from the repeated insertion and removal damage.
However, the relative change in leakage pressure between
each additional puncture and removal event decreased witheach insertion/ removal event; this suggested a limit to thedamage inflicted. The pull-out force of the first puncture
for the four-needle array (33G, non-coring), normalized withrespect to contact surface area between the needle and septa,
was 0.52 N mm−2 and decreased to 0.48 N mm−2 for the tenthremoval event. This result was similar to removal forces ofcomparable designs of other published reusable connectors,
which ranged from 0.08 to 0.95 N mm−2 for the firstremoval event to 0.02 to 0.22 N mm−2 for the tenth removal[8, 13, 14, 17].
5.4. Pressure test
The leakage pressure associated with each of the three septa
designs was determined (table 2). Several factors wereidentified to explain the wide range of failure pressures for each
setpa design. The lower failure pressures for the oval overlap
design were attributed to septa delamination from the substrate.
During needle array insertion, slight adjustments to align oneof the needles induced torque on the other adjacent needles
and the septa resulting in delamination of the PDMS fromthe Parylene-coated substrate. The Parylene coating mitigatedthermally induced stress between the SU-8 and glass but did
not adhere well to PDMS [30]. Future designs will include anetch step to remove the excess Parylene to allow a direct bond
between the PDMS septa and glass substrate.
The distinct septa in the oval design minimized couplingof torque to adjacent needles. Instead, leakage pressure
was determined by the weakest needle/septum interface.
Following failure of one pair (at 2 kPa), the septum was
resealed using additional PDMS and repressurized. The
9J. Micromech. Microeng. 21 (2011) 054021

R Lo and E Meng

same input location failed again at 26 kPa, whereas the otherthree input locations did not fail. Again, needle alignment
affects sealing. The needle guides mitigate the possibility of
misalignment of the needle in the horizontal plane; however,small changes in the needle insertion angle translate into
increasing angular misalignment the further the needle moves
into the septum. If the needle was inserted at a large angle
relative to the substrate surface, as opposed to parallel to thesubstrate surface, then the needle may veer off course and comeinto contact with the substrate. In this case, the needle is nolonger sealed around its circumference by PDMS throughout
the entire length of the septum and causes the PDMS topartially delaminate from the substrate near the contact site.
This ‘tenting’ effect creates a potential leakage path along
the needle shaft. The third failure point following input resealing was at 67 kPa and located at the output septum. The
same factors apply for both input and output interconnects.
However, this result indicated that the input septa which did
not fail at 2 and 26 kPa of applied pressure were well aligned
and tightly sealed.
The robustness of the rectangular septa interconnect wasalso highly dependent on needle alignment. The SU-8
anchoring posts in this design resulted in a narrow needle
path through the septa area. Slight needle misalignment
may result in needle tips lodging in the SU-8 posts or, with
sufficient insertion force, dislodging of SU-8 posts from thesubstrate. The four septa rectangular design failed at thedevice edge. This failure was not related to the interconnect
portion of the device, instead indicated poor local adhesion
between the SU-8 and Parylene or the SU-8 and substrate.
The difference in failure pressure between the four and eight
septa arrays was again attributed to manual needle alignment;
fewer needles were easier to align and had better performance.
The additional force necessary to push eight needles into thesepta caused the needles within the array to buckle. The
buckling resulted in redirection of the inserting force as torquewhich in turn caused delamination between the septa and the
Parylene. Additionally, several SU-8 posts were dislodged
from the substrate, creating additional areas where the septa
and substrate were not in contact.
Extended application (>24 h) of pressurized water wasapplied to the oval septa design, the most robust design basedon the leakage pressure results. The oval design survived
continuous application of pressure, 25 kPa, without any visible
leakage. This result demonstrated the interconnect’s ability to
be used in applications which require survival in extended
pressurized conditions.

Figure 11. Time lapse images of pressurized Rhodamine B(∼0.5 psi, 3.45 kPa) moving through a half-packaged microfluidic
device with surface micromachined Parylene microchannels.
Waterfront is outlined to help identify the liquid/air interface (scalebars represent 1 mm).

Complete removal of the sacrificial photoresist wasverified optically by capillary wicking of a drop of Rhodamine
B introduced at the channel opening. A half-packaged
microfluidic system (inlets only) with Parylene microchannels
was also tested. A needle array was inserted into the input septa
and Rhodamine B was manually introduced using a syringe(∼0.5 psi or ∼3.45 kPa of pressure was applied) (figure 11).
5.6. Electrolysis pressure generation
The flexibility of this interconnect approach and compatibility
with microfluidic systems incorporating different features
were demonstrated by the inclusion of electrochemicalelectrodes within interfaced microchannels. Electrolysis
actuation was performed and the resulting pressure generated
was monitored (figure 12). The electrolysis pressure that canbe generated depends on the electrolysis variables (e.g. current,electrode design, electrolyte), the internal volume of the device
and volume of air already present in the device or testing setup.
The generated pressure within the device was well below the
failure pressure of the septa (table 2). Additionally, when
the current was removed from the electrodes, recombinationof the hydrogen and oxygen gases was observed. Complete
recombination of generated gas occurred within 1 h and noleakage was observed.

5.5. Parylene microchannel
Removing the sacrificial photoresist using IPA tookapproximately 28 days.
Acetone is incompatible withphotoresist removal in the presence of SU-8 due todelamination [31, 32]. However, dissolution of the photoresist
can be accelerated by heating the IPA bath (∼50 ◦ C); fulldissolution can be achieved in ∼20 days. Later, it wasdetermined that the undiluted photoresist developer (AZ 351)
achieved full dissolution within 24 h with no observabledelamination of the SU-8 structures.

5.7. Sideport functionality
Sideport functionality was verified by introducing clear liquid
stream through the sideport and a dyed liquid stream in the
primary access port. Both streams were injected at a rate of
500 µL min−1 . Due to laminar flow characteristics inmicrofluidic devices, distinct and adjacent streams were10

J. Micromech. Microeng. 21 (2011) 054021

R Lo and E Meng

recommended. The rectangular septa design, the one having
the smallest spacing between the needle and SU-8 anchor,
had a septum width of 500 µm. Insertion of a 33G needle
with an outer diameter of 203 µm left approximately 150 µm
of PDMS between the needle exterior and SU-8 anchor
components. Denser rectangular septa are possible; however,additional space between the needle wall and SU-8 anchor
may be prudent to account for any deviations in the needle
path (i.e. needle misalignment) from the septa center.
As shown in the insertion and removal force tests, force
scaled linearly with the number of needles and decreased forneedles with (1) smaller diameters and (2) non-coring tips. The
number of needles in an array can be maximized by using thesmallest diameter non-coring needles possible. However, theneedles must possess adequate rigidity to penetrate the PDMS
septa without buckling which may lead to needle misalignment
and interconnect failure.
Needle misalignment, leading to septa/substratedelamination, was the main cause of failure. Needle guides in
the SU-8 housing helped ensure that the needles were inserted
at the center of the septa; however, the needle guides only
provided lateral alignment and were not sufficient to keep
the needles parallel to the substrate. Furthermore, Okamura
et al demonstrated that needle bending is more prevalent forbeveled tip needles [33]. Beveled needles are necessary tominimize insertion and removal forces, therefore, maximizing
the reusability of this interconnect. Additionally, commercial
bevel-tipped needles are readily available. Therefore, othermethods to improve needle alignment need to be implemented.
Needle misalignment can be mitigated by improving
needle stiffness; however, trade-offs need to be considered.
The needles used to establish the microfluidic connections
were 2.54 cm in length and the septa were 4–4.5 mm inlength. The needle length can be shortened to better matchthe septa length; however, the needle length determines the
maximum length of the septa which is linked to needle removal
force. Removal force is an indicator of the maximum leakagepressure the septa can withstand. A smaller needle gauge(i.e. larger diameter) may be used. Larger diameter needles
require greater insertion force and greater septa thicknesses
to accommodate the increased diameter. Finally, the needle
guides can be altered (e.g. lengthened) to help establish
and maintain the needle position and absorb any bucklingeffect, but lengthening the needle guides increases the device
footprint. Thus, many tradeoffs exist and must be carefully
considered when designing interconnects. Also, greater
control of septa and needle alignment is possible with precisioncommercial manufacturing and may mitigate some of thefactors observed here.
Septa delamination from the Parylene or the SU-8 was also
a common failure mode. Bonding between the septa to the SU8 anchor and substrate package can be improved. First, theParylene can be removed using oxygen plasma from the area
outlined by the SU-8 anchor prior to adding the SU-8 layer.
PDMS has a higher bond strength to glass than Parylene. The
SU-8 sidewalls can also be roughened to increase the contact
surface area.
The sideport allowed the injection of two distinct fluids
into a single microchannel. However, the sideport feature

Figure 12. Internal pressure change following electrolysis
actuation. Pressure increases when current (0.3 mA) is applied tothe interdigitated electrodes, and pressure decreases when current isturned off and the oxygen and hydrogen gas recombine into water.

(a)

(b)

Figure 13. Time lapse images of the sideport function: (a) dyedwater introduced in the main septum and un-dyed water through thesideport, (b) close-up image of the laminar flow within the
microchannel.

observed in the channel (figure 13). More sideports for
multiple inputs into a single microchannel can be added but at
the expense of chip real estate.

6. Discussion
The arrayed septa and needle design allows for multiple
simultaneous macro-to-micro connections to be established ina microfluidic device. PDMS is an excellent choice as a septa
material. The compliant nature of PDMS is beneficial for threereasons: (1) it allows the septa to deform around the needle,
providing a reliable and robust seal suitable for multiple uses;(2) the septa material displaced during insertion returns to theoriginal state after the needles are removed, sealing accessto the device interior; and (3) it allows densely packed andproperly aligned needles to be inserted simultaneously without
insertion stresses interfering with neighboring needles.
FEM results showed that needle insertion induced stresswithin the septa dissipates exponentially with respect todistance from the needle shaft. The stress decayed to 36.8%of the maximum value within 60 µm. To prevent the stressfrom extending to the SU-8 anchor, a minimum distance
between the needle shaft and SU-8 housing of 120 µm is
11

J. Micromech. Microeng. 21 (2011) 054021

R Lo and E Meng

can be extended to allow multiple inputs into the same
microchannel, thus providing a modular design which uses
none, some, or all of the sideports. Sideports can also beplaced perpendicular to the microchannel; these sideports
would allow samples to be drawn from any location along
the fluidic stream, or for the introduction of sensors into themicrochannel.

[4] Lo R and Meng E 2009 Macro-to-micro fluidic interfacing
Lab-on-a-Chip Technology: vol 1. Fabrication andMicrofluidics ed K E Herold and A Rasooly (Norfolk, UK:
Caister Academic Press) chapter 22 pp 353–70[5] Anderson J R, Chiu D T, Jackman R J, Cherniavskaya O,
McDonald J C, Wu H K, Whitesides S H andWhitesides G M 2000 Fabrication of topologically complex
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prototyping Anal. Chem. 72 3158–64[6] Bhagat A A S, Jothimuthu P, Pais A and Papautsky I 2007
Re-usable quick-release interconnect for characterization of
microfluidic systems J. Micromech. Microeng. 17 42–9
[7] Christensen A M, Chang-Yen D A and Gale B K 2005
Characterization of interconnects used in PDMS
microfluidic systems J. Micromech. Microeng. 15 928–34[8] Li S F and Chen S C 2003 Polydimethylsioxane fluidic
interconnects for microfluidic systems IEEE Trans. Adv.
Packag. 26 242–7[9] Meng E, Wu S and Tai Y-C 2001 Silicon couplers for
microfluidic applications Fresenius’ J. Anal. Chem.
371 270–5[10] Murphy E R, Inoue T, Sahoo H R, Zaborenko N and
Jensen K F 2007 Solder-based chip-to-tube andchip-to-chip packaging for microfluidic devices Lab Chip
7 1309–14[11] Pattekar A V and Kothare M V 2003 Novel microfluidic
interconnectors for high temperature and pressure
applications J. Micromech. Microeng. 13 337–45[12] Yang Z and Maeda R 2003 Socket with built-in valves for theinterconnection of microfluidic chips to macro constituents
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microfluidic interconnects Sensors Actuators B 132 531–9[18] Lo R and Meng E 2008 Characterization and fabrication ofhigh-density, on-demand, reusable, in-plane polymer
interconnects towards standardized microfluidic packaging
12th Int. Conf. on Miniaturized Systems for Chemistry andLife Sciences (µTAS) pp 709–11[19] MicroChem Permanent Epoxy Negative
Photoresist-Processing Guidelines for: SU-8 2100 and
SU-8 2150 www.microchem.com[20] Yao T J, Yang X and Tai Y C 2002 BrF3 dry releasetechnology for large freestanding parylene microstructuresand electrostatic actuators Sensors Actuators A 97–8 771–5[21] Tas N, Sonnenberg T, Jansen H, Legtenberg R
and Elwenspoek M 1996 Stiction in surface
micromachining J. Micromech. Microeng. 6 385–97[22] Li P Y, Shih J, Lo R, Saati S, Agrawal R, Humayun M S,
Tai Y C and Meng E 2008 An electrochemical intraoculardrug delivery device Sensors Actuators A 143 41–8
[23] Lo R and Meng E 2007 A reusable in-plane polymer
integrated microfluidic interconnect Solid-State Sensors,
Actuators and Microsystems Conf. pp 2067–70[24] Rodger D C et al 2008 Flexible parylene-based multielectrodearray technology for high-density neural stimulation andrecording Sensors Actuators B 132 449–60

7. Conclusion
An arrayed interconnect design capable of rapid and multiple
simultaneous connections without the use of adhesives toa microfluidic device is presented. The interconnects arereusable and were proven to allow at least ten complete
uses where connections were established, broken and reestablished. Interconnect design parameters were explored.
33G non-coring needles provided adequate strength for
insertion while minimizing SU-8 septa housing thickness. The
linear relationship between insertion and removal forces andthe number of needles was theoretically and experimentally
verified. While the failure pressure of the arrayed interconnect
was limited by the weakest point, up to 62 kPa of pressure was
supported. Also, interconnects were able to maintain 25 kPa ofpressure for over 24 h. Connector spacings of 1 and 2.54 mm
were fabricated; however, the FEM analysis of stress
distribution shows that 33G needles can be spaced as close as
443 µm, center-to-center. Functionality of additional features
such as Parylene microchannels, needle guides, sideports for
combining two fluids and electrolysis structures was shown.
The arrayed interconnect method is advantageous overcurrent methods because it (1) can be reused if necessary,(2) has a horizontal orientation, which increases the connection
robustness without the need for adhesives and does notinterfere with visual or microscope observations, and (3)establishes multiple connections simultaneously, reducing thetime. Additionally, it is suitable for a standard method for
microfluidic connections, thereby simplifying microfluidic
packages.

Acknowledgments
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 Dr Donghai Zhu and other members of the
Biomedical Microsystems Lab at the University of Southern
California for their contributions to this work.

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