CHARACTERIZATION AND FABRICATION OF
HIGH-DENSITY, ON-DEMAND, REUSABLE, INPLANE POLYMER INTERCONNECTS TOWARDSSTANDARDIZED MICROFLUIDIC PACKAGING
1Ronalee Lo1 and Ellis Meng1

Department of Biomedical Engineering, University of Southern California, USAABSTRACT
The first on-demand, high-density, reusable, in-plane interconnects that can beintegrated with microfluidic systems are presented. Interconnects are arrayed forhigh-density microfluidic packaging. The self-sealing nature and mechanical robustness of in-plane connections allow fluidic connections to be repeatedly broken andre-established [1]. Mechanical characterized through insertion/removal force measurements, finite-element-modeling (FEM) and real-time photoelastic stress distribution studies is presented.
KEYWORDS: Packaging, interconnect, world-to-chip, microfluidics
INTRODUCTION
The full potential of micro total analysis systems (µTAS) and lab-on-a-chip(LOC) microfluidic devices is limited by the lack of standardized and reliable packaging between the micro and macro worlds. Current micro-to-macro world connection methods are custom-made solutions which require adhesives, complicated fabrication steps, precise alignment, and extensive manual labor.
We present reusable and arrayed microfluidic interconnects created using standard batch microfabrication techniques and integrated in-plane with microfluidic devices (Fig. 1). A “pin-and-socket” approach was used where commercially-available
non-coring needles (33G) puncture polydimethylsiloxane (PDMS) septa to establish
access to the microfluidic system. First, a surface micromachined channel was created in either SU-8 [1] or Parylene (Fig. 2A-C). SU-8 was spun and patterned to create mechanical anchors and filled with PDMS to create the septa (Fig. 2D, E). The
system was capped with a glass plate to enclose the microfluidic system (Fig. 2F).
Interconnects consist of an anchor and septum at each inlet/outlet (Fig. 3A). Needle arrays held fixed in a polymer housing are manually inserted through the septa(Fig. 3B). The in-plane design utilizes a standard center-to-center spacing between
needles (2.54mm and 1mm).

Figure 1. Microfluidic device with integrated interconnect. Fluid is introduced
into the microchannel via a needle array which pierces the setpa.

Twelfth International Conference on Miniaturized Systems for Chemistry and Life Sciences
October 12 - 16, 2008, San Diego, California, USA
978-0-9798064-1-4/µTAS2008/$20©2008CBMS709

Figure 2. Fabrication steps for integrated interconnect.

Figure 3. Exploded Solidworks illustration of a) a multiple interconnect assembly. b) Close-up image of an assembled multiple interconnect setup.
THEORY AND EXPERIMENTAL
Insertion force, finsertion, is the force necessary to puncture the PDMS septum witha needle array; it is composed of three main components [2]:(1)
f insertion = f stiffness + f cutting + f friction
The stiffness force (fstiffness) is due to material deformation pre-puncture. Postpuncture, the needle experiences a combination of cutting force (fcutting), the force required to push through the material, and frictional force (ffriction). The force necessary
to puncture the septum (PDMS slab measuring 2 ± 0.1mm thick) with a single 33G
needle and needle arrays (4 or 8 33G needles, 2.54mm spacing), was measured.
The stress distribution within the septum during insertion was modeled using
FEM analysis of constrained PDMS. The FEM results were validated with photoelastic imaging. Stress in PDMS results in a visible phase difference (Δ) and is proportional to principle stresses (σx, σy), wavelength (λ), stress-optical coefficient (C),and material thickness (h) [3]:2πh(2)Δ=C (σ − σ )λxy

RESULTS AND DISCUSSION
The various forces components for single needle and needle array implementation (4 and 8 needles) were measured (Table 1). Stiffness force of the needle arrays
could not be measured due to equipment resolution and lack of simultaneous tip contact with the PDMS surface. In general, forces scaled with the needle number, effectively limiting the maximum interconnect density due to mechanical robustness andthe range of practical forces needed to manually insert a dense needle array.
FEM of the stress distribution during needle insertion of single and arrayed interconnects was performed using measured forces (Fig. 4). Photoelastic imaging
confirmed FEM results. The maximum stress in the FEM analysis is approximately
22% of the tensile strength of PDMS, indicating no critical failure locations. Fur-Twelfth International Conference on Miniaturized Systems for Chemistry and Life SciencesOctober 12 - 16, 2008, San Diego, California, USA710

thermore, the compliant PDMS septa localizes stresses around the needle and thus
minimizes stress transmission to the anchors.

Table 1. Measured forces (mean ± SE, n=4) for needle insertion into and removalfrom a PDMS slab.
Force [N]Single Needle (33G)

Stiffness
Max Insertion
Cutting
Frictional
Removal-0.18 ± 0.016-0.87 ± 0.022-0.22 ± 0.053-0.65 ± 0.069
0.60 ± 0.029

Needle Array(Four 33G Needles)
N/A-3.30 ± 0.050-0.36 ± 0.099-2.94 ± 0.056
2.65 ± 0.066

Needle Array(Eight 33G Needles)
N/A-5.58 ± 0.171-0.85 ± 0.023-4.73 ± 0.190
4.54 ± 0.150

Figure 4. Stress distribution in PDMS septa during needle array insertion using
FEM analysis: a) partially inserted and b) through. c) Polarized stress imaging setup and d, e) photoelastic imaging of stresses in PDMS.
CONCLUSIONS
The fabrication and characterization of a novel on-demand method for integrating robust and high-density interconnects to microfluidic systems is presented. Bothmechanical modeling and experimental validation were performed. These interconnects offer a practical solution for standardizing microfluidic connections.
ACKNOWLEDGEMENTS
This work was funded in part by an NSF CAREER grant (EEC-0547544). Theauthors would like to thank Dr. Donghai Zhu, Dr. James C. Eckert, Dr. David A.
Mann, Mr. Merrill Roragen, and members of the Biomedical Microsystem Lab at theUniversity of Southern California.
REFERENCES[1] R. Lo and E. Meng, "Integrated and reusable in-plane microfluidic interconnects," Sensors and Actuators B: Chemical, vol. 132, pp. 531-539, (2008).
[2] C. Simone and A. M. Okamura, "Modeling of needle insertion forces for robotassisted percutaneous therapy," (2002).
[3] S. Timoshenko and J. N. Goodier, Theory of Elasticity, 3rd ed. New York:
McGraw-Hill, (1970).
Twelfth International Conference on Miniaturized Systems for Chemistry and Life Sciences
October 12 - 16, 2008, San Diego, California, USA711