Resist-free patterning of surface architectures in polymer-based microanalytical devices.

The ability to form patterns of chemically reactive surface functionalities in microanalytical devices using a simple photopatterning approach without the need for photoresist-based methods is described. Direct UV exposure of the surfaces of poly(methyl methacrylate), PMMA, and poly(carbonate), PC, microfluidic devices through optical masks leads to the production of patterns of near monolayer quantities of surface carboxylic acid groups as determined by surface coverage, X-ray photoelectron spectroscopy, and fluorescence microscopy experiments. Formation of the reactive carboxylic acid groups without significant physical (topographical) damage to the polymer device substrates is achieved by use of low UV fluence and exposure times. Modification of the patterned, surface carboxylic acid groups with metals, thermally responsive polymers, and antibodies results in microfluidic devices possessing metallic interconnects and detection electrodes and the ability to capture intact biological cells and proteins from solution.

Photochemically patterned poly(methyl methacrylate) surfaces used in the fabrication of microanalytical devices.

We report here the photochemical surface modification of poly(methyl methacrylate), PMMA, microfluidic devices by UV light to yield pendant carboxylic acid surface moieties. Patterns of carboxylic acid sites can be formed from the micrometer to millimeter scale by exposure of PMMA through a contact mask, and the chemical patterns allow for further functionalization of PMMA microdevice surfaces to yield arrays or other structured architectures. Demonstrated here is the relationship between UV exposure time and PMMA surface wettability, topography, surface functional group density, and electroosmotic flow (EOF) of aqueous buffer solutions in microchannels made of PMMA. It is found that the water contact angle on PMMA surfaces decreases from 70 degrees to 24 degrees after exposure to UV light as the result of the formation of carboxylic acid sites. However, upon rinsing with 2-propanol, the water contact angle increases to approximately 80 degrees , and this increase is attributed to changes in surface roughness resulting from removal of low molecular weight PMMA formed from scission events. In addition, the surface roughness and surface coverage of carboxylic acid groups exhibit a characteristic trend with UV exposure time. Electroosmotic flow (EOF) in PMMA microchannels increases upon UV modification and is pH dependent. The possible photolysis mechanism for formation of carboxylic acid groups on PMMA surfaces under the conditions outlined in this work is discussed.

Cell transport via electromigration in polymer-based microfluidic devices.

Electrokinetic transport of Escherichia coli and Saccharomyces cerevisiae (baker's yeast) cells was evaluated in microfluidic devices fabricated in pristine and UV-modified poly(methyl methacrylate)(PMMA) and polycarbonate (PC). Chip-to-chip reproducibility of the cell's apparent mobilities (micro(app)) varied slightly with a RSD of approximately 10%. The highest micro(app) for baker's yeast cells was observed in UV-modified PC with 0.5 mM PBS (pH = 7.4), and the lowest was measured in pristine PMMA with 20 mM PBS (pH = 7.4). Baker's yeast in all devices migrated toward the cathode because of their smaller electrophoretic mobility compared to the EOF. In 0.5 mM and 1 mM PBS, E. coli cells migrated toward the anode in all cases, opposite to the direction of the EOF due to their larger electrophoretic mobility. E. coli cells in 20 mM PBS migrated toward the cathode, which indicated that the electrophoretic mobility of E. coli cells decreased at higher ionic strengths. Observed differential migrations of E. coli and baker's yeast cells in appropriately prepared polymer microchips were used as the basis for selective introduction into microfluidic devices of only one type of cell. As a working model, experiments were performed with E. coli and RBCs (red blood cells). RBCs migrated toward the cathode in pristine PMMA with 1 mM and 20 mM PBS (pH = 7.4), opposite to the direction of the E. coli cells. By judicious choice of the buffer concentration in which the cell suspension was prepared and the polymer material, RBCs or E. coli cells were selectively introduced into the microdevice, which was monitored via laser backscatter signals.

Solid-phase reversible immobilization in microfluidic chips for the purification of dye-labeled DNA sequencing fragments.

In this manuscript, we discuss the use of photoactivated polycarbonate (PC) for purification of dye-labeled terminator sequencing fragments using solid-phase reversible immobilization (SPRI) prior to gel electrophoretic sorting of these DNAs. An immobilization bed for the DNA purification was produced by exposing a posted microchannel to UV radiation, which induced a surface photooxidation reaction, resulting in the production of carboxylate groups. The immobilization microchannel contained microposts to increase the loading level of DNAs to improve signal intensity without the need for preconcentration. By suspending the sequencing cocktail in an immobilization buffer (TEG/ethanol), the DNA fragments demonstrated a high affinity for this carboxylated surface. The loading density of DNAs to this activated surface was found to be 3.9 pmol cm(-2). The captured DNA could be subsequently released from the surface by incubation with ddH2O. SPRI cleanup of dye-terminator sequencing fragments using the photoactivated PC chip and slab gel electrophoresis produced a read length comparable to the conventional SPRI format, which utilized carboxylated magnetic beads and a magnetic field. The read length for the PC-SPRI format was found to be 620 bases with a calling accuracy of 98.9%. The PC-SPRI cleanup format was also integrated to a capillary gel electrophoresis (CGE) system. The PC-SPRI method was shown to effectively remove excess dye terminator from the CGE tract, but yielded lower plate numbers, as compared to a direct injection method with purification accomplished off-chip. The loss in efficiency was found to result primarily from the extended injection time associated with the microchip purification method.

Microarrays assembled in microfluidic chips fabricated from poly(methyl methacrylate) for the detection of low-abundant DNA mutations.

Low-density arrays were assembled into microfluidic channels hot-embossed in poly(methyl methacrylate) (PMMA) to allow the detection of low-abundant mutations in gene fragments (K-ras) that carry point mutations with high diagnostic value for colorectal cancers. Following spotting, the chip was assembled with a cover plate and the array accessed using microfluidics in order to enhance the kinetics associated with hybridization. The array was configured with zip code sequences (24-mers) that were complementary to sequences present on the target. The hybridization targets were generated using an allele-specific ligase detection reaction (LDR), in which two primers (discriminating primer that carriers the complement base to the mutation being interrogated and a common primer) that flank the point mutation and were ligated joined together) only when the particular mutation was present in the genomic DNA. The discriminating primer contained on its 5'-end the zip code complement (directs the LDR product to the appropriate site of the array), and the common primer carried on its 3' end a fluorescent dye (near-IR dye IRD-800). The coupling chemistry (5'-amine-containing oligonucleotide tethered to PMMA surface) was optimized to maximize the loading level of the zip code oligonucleotide, improve hybridization sensitivity (detection of low-abundant mutant DNAs in high copy numbers of normal sequences), and increase the stability of the linkage chemistry to permit re-interrogation of the array. It was found that microfluidic addressing of the array reduced the hybridization time from 3 h for a conventional array to less than 1 min. In addition, the coupling chemistry allowed reuse of the array > 12 times before noticing significant loss of hybridization signal. The array was used to detect a point mutation in a K-ras oncogene at a level of 1 mutant DNA in 10,000 wild-type sequences.

Surface modification and characterization of microfabricated poly(carbonate) devices: manipulation of electroosmotic flow.

Poly(carbonate), PC, surfaces are chemically modified by treatment with sulfur trioxide gas. Sulfur trioxide gas sulfonates the aromatic rings of the poly(carbonate) surfaces, making the surfaces more hydrophilic. Sulfonation of the poly(carbonate) surface is confirmed by infrared spectroscopy. The modified polymer surfaces are found to be smoother in comparison to their unmodified counterparts, as noted by scanning force microscopy. The effects of the surface modification on electroosmotic flow are studied at a pH range of 4-10. The electroosmotic flow in sulfonated poly(carbonate) microchannels was found to be significantly higher than that in unmodified poly(carbonate) microchannels at pH values below 8.

Thin films of block copolymer blends for enhanced performance of acoustic wave-based chemical sensors.

The performance of quartz crystal oscillator-based volatile organic compound (VOC) sensors has been enhanced by using coatings made from poly(styrene-block-ethylene-co-butylene-block-styrene) block copolymers blended with resins and homopolymers. Enhanced performance is characterized by a wider operational temperature range (-10 to +50 degrees C) over which the sensors displayed, concurrently, an analyte sensitivity of >0.2 Hz/ppm toluene, minimal energy loss (resistance <120 ohms), and response times of <20 min (time required to reach 90% of full response). Atomic force microscopy images are consistent with a process in which the additive associates with the polystyrene portions of the microphase-separated block copolymer. This association reinforces the rigidity of the polystyrene network while allowing the rapid uptake of VOCs by the softer polyethylene/butylene phase.