The necessity for microchannel wall coatings in capillary and chip-based electrophoretic analysis of biomolecules is well understood. The regulation or elimination of EOF and the prevention of analyte adsorption is essential for the rapid, efficient separation of proteins and DNA within microchannels. Microchannel wall coatings and other wall modifications are especially critical for protein separations, both in fused-silica capillaries, and in glass or polymeric microfluidic devices. In this review, we present a discussion of recent advances in microchannel wall coatings of three major classes--covalently linked polymeric coatings, physically adsorbed polymeric coatings, and small molecule additives. We also briefly review modifications useful for polymeric microfluidic devices. Within each category of wall coatings, we discuss those used to eliminate EOF, to tune EOF, to prevent analyte adsorption, or to perform multiple functions. The knowledgeable application of the most promising recent developments in this area will allow for the separation of complex protein mixtures and for the development of novel microchannel wall modifications.

We review the wide range of polymeric materials that have been employed for DNA sequencing separations by capillary electrophoresis. Intensive research in the area has converged in showing that highly entangled solutions of hydrophilic, high molar mass polymers are required to achieve high DNA separation efficiency and long read length, system attributes that are particularly important for genomic sequencing. The extent of DNA-polymer interactions, as well as the robustness of the entangled polymer network, greatly influence the performance of a given polymer matrix for DNA separation. Further fundamental research in the field of polymer physics and chemistry is needed to elucidate the specific mechanisms by which DNA is separated in dynamic, uncross-linked polymer networks.

Polymers and hydrogels that swell or shrink in response to environmental stimuli such as changes in temperature, pH, or ionic strength are of interest as switchable materials for applications in biotechnology. In this paper, we show that thermoresponsive polymers offer some particular advantages as entangled matrices for DNA sequencing by capillary and microchip electrophoresis. Matrices based on conventional water-soluble polymers demand a compromise in their design for microchannel electrophoresis: whereas highly entangled solutions of high molar mass polymers provide optimal sequencing performance, their highly viscous solutions require application of high pressures to be loaded into electrophoresis microchannels. Here, we demonstrate the reproducible synthesis, precise characterization, and excellent DNA sequencing performance of high molar mass, thermoresponsive polymer matrices that exhibit a reversible, temperature-controlled "viscosity switch" from high-viscosity solutions at 25 degrees C to low-viscosity, microphase-separated colloidal dispersions at a chosen, elevated temperature. The viscosity switch decouples matrix loading and sieving properties, enabling acceleration of microchannel flow by 3 orders of magnitude. DNA sequencing separations yielding read lengths of 463 bases of contiguous sequence in 78 min with 97% base-calling accuracy can be achieved in these matrices. Switchable matrices will be particularly applicable to microfluidic devices with dynamic temperature control, which are likely to provide the next major leap in the efficiency of high-throughput DNA analysis.

To elucidate the impact of matrix chemical and physical properties on DNA sequencing separations by capillary electrophoresis (CE), we have synthesized, characterized and tested a controlled set of different polymer formulations for this application. Homopolymers of acrylamide and N,N-dimethylacrylamide (DMA) and copolymers of DMA and N,N-diethylacrylamide (DEA) were synthesized by free radical polymerization and purified. Polymer molar mass distributions were characterized by tandem gel permeation chromatography - laser light scattering. Polymers with different chemical compositions and similar molar mass distributions were selected and employed at the same concentration so that the variables of comparison between them were hydrophobicity and average coil size in aqueous solution. We find that the low-shear viscosities of 7% w/v polymer solutions decrease by orders of magnitude with increasing polymer hydrophobicity, while hydrophilic polymers exhibit more pronounced reductions in viscosity with increased shear. The performance of the different matrices for DNA sequencing was compared with the same sample under identical CE conditions. The longest read length was produced with linear polyacrylamide (LPA) while linear poly-N,N-dimethylacrylamide (PDMA) gave approximately 100 fewer readable bases. Read lengths with DMA/DEA copolymers were lower, and decreased with increasing DEA content. This study highlights the importance of polymer hydrophilicity for high-performance DNA sequencing matrices, through the formation of robust, highly-entangled polymer networks and the minimization of hydrophobic interactions between polymers and fluorescently-labeled DNA molecules. However, the results also show that more hydrophobic matrices offer much lower viscosities, enabling easier microchannel loading at low applied pressures.

We present the polymer poly-N-hydroxyethylacrylamide (PHEA) (polyDuramide) as a novel, hydrophilic, adsorbed capillary coating for electrophoretic protein analysis. Preparation of the PHEA coating requires a simple and fast (30 min) protocol that can be easily automated in capillary electrophoresis instruments. Over the pH range of 3-8.4, the PHEA coating is shown to reduce electroosmotic flow (EOF) by about 2 orders of magnitude compared to the bare silica capillary. In a systematic comparative study, the adsorbed PHEA coating exhibited minimal interactions with both acidic and basic proteins, providing efficient protein separations with excellent reproducibility on par with a covalent polyacrylamide coating. Hydrophobic interactions between proteins and a relatively hydrophobic poly-N,N-dimethylacrylamide (PDMA) adsorbed coating, on the other hand, adversely affected separation reproducibility and efficiency. Under both acidic and basic buffer conditions, the adsorbed PHEA coating produced an EOF suppression performance comparable to that of covalent polyacrylamide coating and superior to that of adsorbed PDMA coating. The protein separation performance in PHEA-coated capillaries was retained for 275 consecutive protein separation runs at pH 8.4, and for more than 800 runs at pH 4.4. The unique and novel combination of hydrophilicity and adsorptive coating ability of PHEA makes it a suitable wall coating for automated microscale analysis of proteins by capillary array systems.