USE OF PHOTOPATTERNED NANOPOROUS POLYMER MONOLITHS AS PASSIVE MIXERS TO ENHANCE MIXING EFFICIENCY FOR ON-CHIP

LABELING REACTIONS D.A. Mair1, E. Geiger1,2, T. Schwei2,3, T. Dinio2,3, J. Fréchet2,4, F. Svec2,4

1Fluigence, LLC, USA, 2Molecular Foundry, Lawrence Berkeley National Laboratory, USA, 3Department of Chemical Engineering, University of California, Berkeley, USA, 4Department of Chemistry, University of California, Berkeley, USA

ABSTRACT

A passive micromixer was prepared by photopatterning a periodic arrangement of nanoporous polymer monolith (nPPM) structures directly within the channel of a plastic microfluidic chip. By optimizing the composition of the polymerization solution and irradiation time we demonstrate the ability to fabricate nPPM in regular 100 μm segments. By monitoring laser-induced fluorescence (LIF) of reaction products we found this photopatterned array to have the highest mixing efficiency when compared to an equivalent length of continuous segment plug of nPPM and an open channel. These results indicate that the regularly spaced open areas between the plugs of nPPM enhances mixing efficiency. KEYWORDS: porous polymer monolith, passive micromixer, photopattern, chip INTRODUCTION

Efficient mixing is an important component of micro total analytical systems that rely on chemical reactions such as DNA sequencing, cell lysis and protein analysis. However, laminar flow in these devices causes mixing to be diffusion-limited and slow. In order to increase the extent of reaction for on-chip fluorescent labeling of proteins, a passive mixer was prepared by using UV light to photopattern a periodic arrangement of nanoporous polymer monolith (nPPM) structures directly within the channel of a plastic microfluidic chip. By optimizing the composition of the polymerization solution and irradiation time we demonstrate the ability to pattern nPPM in regular 100 μm segments. The mixing efficiency of this patterned array was evaluated by monitoring LIF intensity of the reaction product of fluorescamine and lysine introduced into a tee-junction (Figure 1B).

Figure 1. Tee-junction microfluidic chip manufactured by plastic injection molding (A). Schematic for labeling lysine with fluorescamine in a tee-junction microfluidic chip and detection of reaction products by laser-induced fluorescence (B).

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Twelfth International Conference on Miniaturized Systems for Chemistry and Life SciencesOctober 12 - 16, 2008, San Diego, California, USA

EXPERIMENTAL Chips featuring a fully integrated fluidic interconnect were manufactured by

injection molding using highly UV transparent cyclo-olefin copolymer (Figure 1A) [1]. Prior to photopatterning nPPM within the device, the surface chemistry of the channel wall was modified to ensure a covalent linkage between monolith and chip. This was achieved by filling the channels with a photografting solution consisting of photoinitiator, monomer and crosslinker followed by irradiation with UV light. The nPPMs were then photopatterned by filling the channels with a polymerization solution consisting of monomer, crosslinker, porogen and photoinitiator followed by UV irradiation through a microfabricated quartz mask. The mixing efficiency of three monolith configurations was evaluated by LIF measurements of reaction products 1 cm after the tee-junction. Fluorescamine, which fluoresces only upon reaction with amines, was used to fluorescently label lysine.

RESULTS AND DISCUSSION

In the photografting procedure used to modify the channel surface, polymerization reactions are initiated from the channel wall resulting in a thin polymer skin containing a multiplicity of pendant vinyl groups. These polymerizable vinyl functionalities are then incorporated into the nPPM during its in situ preparation. As illustrated in Figure 2, omission of this surface modification results in an undesirable void at the monolith-wall interface.

We investigated the effect of polymerization solution composition and UV

exposure time on the photopatterning resolution of nPPM. Initial attempts to optimize the photopatterning resolution of a polymerization solution containing 16 wt% crosslinker by varying the exposure time, resulted in poor photopatterning resolution. Irradiation times required for complete polymerization also resulted in poorly defined segments of monolith.

On the other hand, polymerization solutions with greater crosslinker content (28 wt%) were consistently photopatterned with higher resolution due to favorable reaction kinetics that required shorter irradiation times. As expected, excessive exposure times yielded a continuous plug of monolith instead of individual segments and short irradiation times led to poorly crosslinked monoliths that were flushed from the channel during subsequent rinsing. Optimization of the irradiation time for this polymerization solution facilitated photopatterning of nPPM in regular and well-defined 100 μm segments, the highest resolution known to date for this material.

Figure 2. SEM images of channel cross-sections with monolith in unmodified (A, B) and photografted (C, D) channel walls.

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Twelfth International Conference on Miniaturized Systems for Chemistry and Life SciencesOctober 12 - 16, 2008, San Diego, California, USA

To demonstrate the functional advantage of a photopatterned nPPM as a passive mixer, the concentration of fluorescent reaction products was measured by LIF 1 cm after introduction in a tee-junction for three channel configurations. As indicated in Figure 4A, more fluorescamine reacted with lysine after the photopatterned monolith compared to both the open channel and continuous monolith. A fluorescence ratio for each channel configuration was calculated by dividing the average peak fluorescence intensity by that of the open channel (Figure 4B). By this normalization technique the amount of fluorescently labeled lysine produced after a patterned monolith was 22% more than an open channel (ratio of unity). These results show that the regularly spaced open areas between the nPPM plugs created by photopatterning are directly responsible for the improvement in mixing efficiency.

CONCLUSIONS NPPM was photopatterned in well-defined 100 μm segments. The periodic

arrangement of monolith separated by open areas showed improved mixing efficiency over both an open channel and continuous monolith of equal length. ACKNOWLEDGEMENTS

Support of this research was provided by a grant of the National Institute of Biomedical Imaging and Bioengineering, National Institutes of Health (EB-006133). REFERENCES [1] “Injection molded microfluidic chips featuring integrated interconnects,” D.A.

Mair, E.J. Geiger, A.P. Pisano, J.M. Fréchet, F. Svec. Lab Chip, 6, 1346 (2006).

Figure 4. LIF scan across channel (A) and ratio of fluorescence (B) for open channel, continuous monolith and a patterned monolith 1 cm from the tee-junction using image software.

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Figure 3. SEM images of photopatterned nPPM in a periodic arrangement (A). An individual plug of monolith demonstrating 100 μm patterning resolution (B).

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Twelfth International Conference on Miniaturized Systems for Chemistry and Life SciencesOctober 12 - 16, 2008, San Diego, California, USA