Integration of Dialysis Membranes into a Poly(dimethylsiloxane) Microfluidic Chip for Isoelectric Focusing of Proteins Using Whole-Channel Imaging Detection

Detection and separation of proteins using micro/nanofluidics devices

Microfluidics is the technology or system wherein the behavior of fluids' is studied onto a miniaturized device composed of chambers and tunnels. In biological and biomedical sciences, microfluidic technology/system or device serves as an ultra-high-output approach capable of detecting and separating the biomolecules present even in trace quantities. Given the essential role of protein, the identification and quantification of proteins help understand the various living systems' biological function regulation. Microfluidics has enormous potential to enable biological investigation at the cellular and molecular level and maybe a fair substitution of the sophisticated instruments/equipment used for proteomics, genomics, and metabolomics analysis. The current advancement in microfluidic systems' development is achieving momentum and opening new avenues in developing innovative and hybrid methodologies/technologies. This chapter attempts to expound the micro/nanofluidic systems/devices for their wide-ranging application to detect and separate protein. It covers microfluidic chip electrophoresis, microchip gel electrophoresis, and nanofluidic systems as protein separation systems, while methods such as spectrophotometric, mass spectrometry, electrochemical detection, magneto-resistive sensors and dynamic light scattering (DLS) are discussed as proteins' detection system.

Protein separation under a microfluidic regime

Lab-on-a-Chip (LoC), or micro-Total Analysis Systems (μTAS), is recognized as a powerful analytical technology with high capabilities, though end-user products for protein purification are still far from being available on the market. Remarkable progress has been achieved in the separation of nucleic acids and proteins using electrophoretic microfluidic devices, while pintsize devices have been developed for protein isolation according to miniaturized chromatography principles (size, charge, affinity, etc.). In this work, we review the latest advances in the fabrication of components, detection methods and commercial implementation for the separation of biological macromolecules based on microfluidic systems, with some critical remarks on the perspectives of their future development towards standardized microfluidic systems and protocols. An outlook on the current needs and future applications is also presented.

Microchip with an open tubular immobilized ph gradient for UV whole column imaging detection

This study reports a new method for establishing an open tubular IPG in a microchip coupled with a whole column image detection (WCID) system for protein separation applications. This method allows a wider range of immobilized pH (2.6-9.5) to be established in a PDMS/quartz channel by controlling the diffusion of acidic and basic polymer solutions into the channel through well-designed channel dimensions. The developed pH gradient was experimentally validated by performing the separation of a mixture of standard pI markers. It was further validated by the separation of the hemoglobin control AFSC sample. This method is advantageous over existing IPG methods because it has a wider range of pH and maintains the open tubular feature that matches the UV WCID to improve the sensitivity.

Microfluidic two-dimensional separation of proteins combining temperature gradient focusing and sodium dodecyl sulfate-polyacrylamide gel electrophoresis

A two-dimensional separation system is presented combining scanning temperature gradient focusing (TGF) and sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis (PAGE) in a PDMS/glass microfluidic chip. Denatured proteins are first focused and separated in a 15 mm long channel via TGF with a temperature range of 16-47 °C and a pressure scanning rate of -0.5 Pa/s and then further separated via SDS-PAGE in a 25 mm long channel. A side channel is designed at the intersection between the two dimensions to continuously inject SDS into the gel, allowing SDS molecules to be compiled within the focused bands. Separation experiments are performed using several fluorescently labeled proteins with single point detection. Experimental results show a dramatic improvement in peak capacity over one-dimensional separation techniques.

Integration of lateral porous silicon membranes into planar microfluidics

In this work, we present a novel fabrication process that enables the monolithic integration of lateral porous silicon membranes into single-layer planar microchannels. This fabrication technique relies on the patterning of local electrodes to guide pore formation horizontally within the membrane and on the use of silicon-on-insulator substrates to spatially localize porous silicon within the channel depth. The feasibility of our approach is studied by current flow analysis using the finite element method and supported by creating 10 μm long mesoporous membranes within 20 μm deep microchannels. The fabricated membranes are demonstrated to be potentially useful for dead-end microfiltration by adequately retaining 300 nm diameter beads while macromolecules such as single-stranded DNA and immunoglobulin G permeate the membrane. The experimentally determined fluidic resistance is in accordance with the theoretical value expected from the estimated pore size and porosity. The work presented here is expected to greatly simplify the integration of membranes capable of size exclusion based separation into fluidic devices and opens doors to the use of porous silicon in planar lab on a chip devices.

Microchip UV absorbance detection applied to isoelectric focusing of proteins

Isoelectric focusing (IEF) is considered as an attractive separation technique for biologically amphoteric compounds (e.g., proteins and peptides) based on their isoelectric point (pI). With the advancement in micromachining technology, microchip format IEF has attracted significant attention. Both single-point and whole column imaging detection (WCID) methods have been employed for analyzing the separation performance in a microchip. WCID is more favorable than single-point detection because the latter requires the focused bands to be mobilized and thus adds more complexity to the design and operation of such microchips. Fluorescence- and UV absorbance-based WCID have been successfully adapted in glass and PDMS microchips. We have developed polydimethylsiloxane (PDMS) microchips for IEF applications where UV-WCID is employed for evaluating the separation performance. The chips are designed for use in the iCE280 analyzer (Convergent Bioscience Inc., Toronto), for capillary-based IEF where UV-WCID is employed for analyzing the separation performance. Three kinds of microchips that have been successfully developed using standard soft lithography technology are described in detail.

A new soft lithographic route for the facile fabrication of hydrophilic sandwich microchips

Manufacturing materials are an essential element for the fabrication of microfluidic chips. PDMS, the most widely used polymeric material, is associated with apparent disadvantages such as hydrophobic nature, while other materials also suffer from some limitations. In this paper, a new soft lithographic route was proposed for the facile manufacturing of hydrophilic sandwich microchips, using bisphenol A based epoxy acrylate (BABEA) as a new patterning material. The BABEA copolymers are hydrophilic, highly transparent in visible range while highly untransparent when the wavelength is less than 290 nm, and of high replication fidelity. By combining with appropriate monomers, including glycidyl methacrylate, methylmethacrylate, and acrylic acid, the copolymers contain active functional groups, which allows for easy postmodification for desirable functional units. A fabrication procedure was proposed for manufacturing hybrid quartz/BABEA copolymer/quartz microchips. In the procedure, no micromachining equipments, wet etching, or imprinting techniques were involved, making the fabrication approach applicable in ordinary chemistry laboratories. The performance of the prepared microchips was demonstrated in terms of CIEF with UV-whole channel imaging detection. The hydrophilic microchannel ensures stable focusing while the polymeric middle layer acts as a perfectly aligned optical slit for whole channel UV absorbance detection.

Label-free real-time imaging in microchip free-flow electrophoresis applying high speed deep UV fluorescence scanning

We report on label-free monitoring of microfluidic free-flow electrophoresis (μFFE) separations in real-time using a custom built high speed deep UV laser scanner. In combination with a novel layout realized in fused silica (FS) FFE chips the setup was successfully applied for continuous separations and detection of unlabeled analytes including native proteins by space-resolved intrinsic deep UV fluorescence scanning.

Quantification of pH gradients and implications in insulator-based dielectrophoresis of biomolecules

Direct current (DC) insulator-based dielectrophoretic (iDEP) microdevices have the potential to replace traditional alternating current dielectrophoretic devices for many cellular and biomolecular separation applications. The use of large DC fields suggest that electrode reactions and ion transport mechanisms can become important and impact ion distributions in the nanoliters of fluid in iDEP microchannels. This work tracked natural pH gradient formation in a 100 μm wide, 1 cm-long microchannel under applicable iDEP protein manipulation conditions. Using fluorescence microscopy with the pH-sensitive dye FITC Isomer I and the pH-insensitive dye TRITC as a reference, pH was observed to drop drastically in the microchannels within 1 min in a 3000 V/cm electric field; pH drops were observed in the range of 6-10 min within a 100 V/cm electric field and varied based on the buffer conductivity. To address concerns of dye transport impacting intensity data, electrokinetic mobilities of FITC were carefully examined and found to be (i) toward the anode and (ii) 1 to 2 orders of magnitude smaller than H⁺ transport which is responsible for pH drops from the anode toward the cathode. COMSOL simulations of ion transport showed qualitative agreement with experimental results. The results indicate that pH changes are severe enough and rapid enough to influence the net charge of a protein or cause aggregation during iDEP experiments. The results also elucidate reasonable time periods over which the phosphate buffering capacity can counter increases in H⁺ and OH⁻ for unperturbed iDEP manipulations.

Fully integrated PDMS/SU-8/quartz microfluidic chip with a novel macroporous poly dimethylsiloxane (PDMS) membrane for isoelectric focusing of proteins using whole-channel imaging detection

A fully integrated polydimethylsiloxane (PDMS)/modified PDMS membrane/SU-8/quartz hybrid chip was developed for protein separation using isoelectric focusing (IEF) mechanism coupled with whole-channel imaging detection (WCID) method. This microfluidic chip integrates three components into one single chip: (i) modified PDMS membranes for separating electrolytes in the reservoirs from the sample in the microchannel and thus reducing pressure disturbance, (ii) SU-8 optical slit to block UV light (below 300 nm) outside the channel aiming to increase detection sensitivity, and (iii) injection and discharge capillaries for continuous operation. Integration of all these components on a single chip is challenging because it requires fabrication techniques for perfect bonding between different materials and is prone to leakage and blockage. This study has addressed all the challenges and presented a fully integrated chip, which is more robust with higher sensitivity than the previously developed IEF chips. This chip was tested by performing protein and pI marker separation. The separation results obtained in this chip were compared with that obtained in commercial cartridges. Side-by-side comparison validated the developed chip and fabrication techniques.

Reagent-release capillary array-isoelectric focusing device as a rapid screening device for IEF condition optimization

This report describes the fabrication and characterization of a simple and disposable capillary isoelectric focusing (cIEF) device containing a reagent-release capillary (RRC) array and poly(dimethylsiloxane) (PDMS) platform, which allows rapid (within 10 min) screening of cIEF conditions by introducing a sample solution into plural RRCs by capillary action followed by electric field application. To prepare the RRC, covalent immobilization of poly(dimethylacrylamide) (PDMA) was conducted to suppress electro-osmotic flow (EOF), followed by physical adsorption of the mixture of carrier ampholyte (CA), surfactant, labeling reagent (LR), and other additives to the PDMA surface to construct a two-layer structure inside a square glass capillary. When the sample solution containing proteins was introduced into the RRC, physically adsorbed CA, surfactant, and LR can be dissolved and released into the sample solution. Then, complexation of LR with proteins, mixing with CA and surfactant, and exposure of the PDMA surface spontaneously occurs for the IEF experiments. Here, three different RRCs that immobilize different CAs were prepared, and simultaneous cIEF experiments involving hemoglobin AFSC mixtures for choosing the best CA demonstrated the proof of concept.

Microchip isoelectric focusing with monolithic immobilized pH gradient materials for proteins separation

Monolithic immobilized pH gradient (M-IPG) materials were prepared in microchannles by photoinitiated polymerization of acrylamide, glycidylmethacrylate and Bis, followed by the attachment of focused Ampholine onto the surface of porous monoliths via epoxide groups. With M-IPG materials as matrix, FITC-labeled ribonuclease B, myoglobin and alpha-casein were well separated by microchip isoelectric focusing (muCIEF) without carrier amphocytes (CAs) added in the buffer. Both chemical and pressure mobilization were applied to drive focused zones for LIF detection. Our experimental results showed that pressure mobilization was preferable with neglectable band broadening, and good peak shape and high detection sensitivity were obtained. All these results demonstrate that muCIEF with M-IPG materials is not only an efficient mode for protein enrichment and separation but also attractive to couple with other CE modes to achieve multi-dimensional separation or MS for further identification, without the interference of mobile CAs.

Fabrication of a hybrid PDMS/SU-8/quartz microfluidic chip for enhancing UV absorption whole-channel imaging detection sensitivity and application for isoelectric focusing of proteins

A poly(dimethylsiloxane)(PDMS)/SU-8/quartz hybrid chip was developed and applied in the isoelectric focusing (IEF) of proteins with ultraviolet (UV) absorbance-based whole-channel imaging detection (UV-WCID). Each hybrid chip was made of three layers: a PDMS flat top substrate, a bottom quartz substrate and a middle layer of SU-8 photoresist. The SU-8 serves two purposes: it contains the microchannel used for IEF separation, and acts as an optical slit that absorbs UV light below 300 nm improving detection sensitivity in WCID. The novel hybrid design demonstrates a two to three times improvement in sensitivity over a comparable PDMS/PDMS design. In addition, the hybrid chip exhibits increased heat dissipation due to the superior thermal conductivity of the bottom quartz substrate allowing for larger electric fields to be used in separations. The hybrid design with IEF-UV-WCID was successful in resolving a complicated sample, hemoglobin control, with high fidelity.