Electrokinetically operated microfluidic devices for integrated immunoaffinity monolith extraction and electrophoretic separation of preterm birth biomarkers

3D printed microfluidic devices for integrated solid-phase extraction and microchip electrophoresis of preterm birth biomarkers

BACKGROUND

Preterm birth (PTB) is a leading cause of neonatal mortality, such that the need for a rapid and accurate assessment for PTB risk is critical. Here, we developed a 3D printed microfluidic system that integrated solid-phase extraction (SPE) and microchip electrophoresis (μCE) of PTB biomarkers, enabling the combination of biomarker enrichment and labeling with μCE separation and fluorescence detection.

RESULTS

Reversed-phase SPE monoliths were photopolymerized in 3D printed devices. Microvalves in the device directed sample between the SPE monolith and the injection cross-channel in the serpentine μCE channel. Successful on-chip preconcentration, labeling and μCE separation of four PTB-related polypeptides were demonstrated in these integrated microfluidic devices. We further show the ability of these devices to handle complex sample matrices through the successful analysis of labeled PTB biomarkers spiked into maternal blood serum. The detection limit was 7 nM for the PTB biomarker, corticotropin releasing factor, in 3D printed SPE-μCE integrated devices.

SIGNIFICANCE

This work represents the first successful demonstration of integration of SPE and μCE separation of disease-linked biomarkers in 3D printed microfluidic devices. These studies open up promising possibilities for rapid bioanalysis of medically relevant analytes.

High-performance microchip electrophoresis separations of preterm birth biomarkers using 3D printed microfluidic devices

We employed digital light processing-stereolithography 3D printing to create microfluidic devices with different designs for microchip electrophoresis (µCE). Short or long straight channel, and two- or four-turn serpentine channel microfluidic devices with separation channel lengths of 1.3, 3.1, 3.0, and 4.7 cm, respectively, all with a cross injector design, were fabricated. We measured current as a function of time and voltage to determine a separation time window and conditions for the onset of Joule heating in these designs. Separations in these devices were evaluated by performing µCE and measuring theoretical plate counts for electric field strengths near and above the onset of Joule heating, with fluorescently labeled glycine and phenylalanine as model analytes. We further demonstrated µCE of peptides and proteins related to preterm birth risk, showing increased peak capacity and resolution compared to previous results with 3D printed microdevices. These results mark an important step forward in the use of 3D printed microfluidic devices for rapid bioanalysis by µCE.

Past, current, and future roles of 3D printing in the development of capillary electrophoresis systems

3D printing, an additive manufacturing technology, has made significant inroads into improving systems for bioanalysis in recent years. This approach is particularly powerful due to the ease and flexibility in rapidly creating novel and complex designs for analytical applications. As such, 3D printing offers an emerging technology for creating systems for electrophoretic analysis. Here, we review 3D printing work on improving and miniaturizing capillary electrophoresis (CE), emphasizing publications from 2019‒2022. We describe enabling uses of 3D printing in interfacing upstream sample preparation or downstream detection with CE. Recent developments in miniaturized CE enabled by 3D printing are also elaborated, including key areas where 3D printing could further improve over the current state-of-the-art. Lastly, we highlight promising future trends for using 3D printing in miniaturizing CE and the significant potential for innovative advancements. 3D printing is poised to play a key role in moving forward miniaturized CE in the coming years.

3D printed microfluidic device for automated, pressure-driven, valve-injected microchip electrophoresis of preterm birth biomarkers

A 3D printed, automated, pressure-driven injection microfluidic system for microchip electrophoresis (µCE) of preterm birth (PTB)-related peptides and proteins has been developed. Functional microvalves were formed, either with a membrane thickness of 5 µm and a layer exposure time of 450 ms or with a membrane thickness of 10 µm and layer exposure times of 300-350 ms. These valves allowed for control of fluid flow in device microchannels during sample injection for µCE separation. Device design and µCE conditions using fluorescently labeled amino acids were optimized. A sample injection time of 0.5 s and a separation voltage of 450 V (460 V/cm) yielded the best separation efficiency and resolution. We demonstrated the first µCE separation with pressure-driven injection in a 3D printed microfluidic device using fluorescently labeled PTB biomarkers and 532 nm laser excitation. Detection limits for two PTB biomarkers, peptide 1 and peptide 2, for an injection time of 1.5 s were 400 pM and 15 nM, respectively, and the linear detection range for peptide 2 was 50-400 nM. This 3D printed microfluidic system holds promise for future integration of on-chip sample preparation processes with µCE, offering promising possibilities for PTB risk assessment.

MIMAS: microfluidic platform in tandem with MALDI mass spectrometry for protein quantification from small cell ensembles

Understanding cell-to-cell variation at the molecular level provides relevant information about biological phenomena and is critical for clinical and biological research. Proteins carry important information not available from single-cell genomics and transcriptomics studies; however, due to the minute amount of proteins in single cells and the complexity of the proteome, quantitative protein analysis at the single-cell level remains challenging. Here, we report an integrated microfluidic platform in tandem with matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF-MS) for the detection and quantification of targeted proteins from small cell ensembles (> 10 cells). All necessary steps for the assay are integrated on-chip including cell lysis, protein immunocapture, tryptic digestion, and co-crystallization with the matrix solution for MALDI-MS analysis. We demonstrate that our approach is suitable for protein quantification by assessing the apoptotic protein Bcl-2 released from MCF-7 breast cancer cells, ranging from 26 to 223 cells lysed on-chip (8.75 nL wells). A limit of detection (LOD) of 11.22 nM was determined, equivalent to 5.91 × 10⁷ protein molecules per well. Additionally, the microfluidic platform design was further improved, establishing the successful quantification of Bcl-2 protein from MCF-7 cell ensembles ranging from 8 to 19 cells in 4 nL wells. The LOD in the smaller well designs for Bcl-2 resulted in 14.85 nM, equivalent to 3.57 × 10⁷ protein molecules per well. This work shows the capability of our approach to quantitatively assess proteins from cell lysate on the MIMAS platform for the first time. These results demonstrate our approach constitutes a promising tool for quantitative targeted protein analysis from small cell ensembles down to single cells, with the capability for multiplexing through parallelization and automation.

Immunoaffinity monoliths for multiplexed extraction of preterm birth biomarkers from human blood serum in 3D printed microfluidic devices

In an effort to develop biomarker-based diagnostics for preterm birth (PTB) risk, we created 3D printed microfluidic devices with multiplexed immunoaffinity monoliths to selectively extract multiple PTB biomarkers. The equilibrium dissociation constant for each monoclonal antibody toward its target PTB biomarker was determined. We confirmed the covalent attachment of three different individual antibodies to affinity monoliths using fluorescence imaging. Three different PTB biomarkers were successfully extracted from human blood serum using their respective single-antibody columns. Selective binding of each antibody toward its target biomarker was observed. Finally, we extracted and eluted three PTB biomarkers from depleted human blood serum in multiplexed immunoaffinity columns in 3D printed microfluidic devices. This is the first demonstration of multiplexed immunoaffinity extraction of PTB biomarkers in 3D printed microfluidic devices.

Multilabel hybridization probes for sequence-specific detection of sepsis-related drug resistance genes in plasmids

Emerging antimicrobial drug resistance is increasing the complexity involved in treating critical conditions such as bacterial induced sepsis. Methods for diagnosing specific drug resistance tend to be rapid or sensitive, but not both. Detection methods like sequence-specific single-molecule analysis could address this concern if they could be adapted to work on smaller targets similar to those produced in traditional clinical situations. In this work we demonstrate that a 120 bp double stranded polynucleotide with an overhanging single stranded 25 bp probe sequence can be created by immobilizing DNA with a biotin/streptavidin magnetic bead system, labeling with SYBR Gold, and rinsing the excess away while the probe retains multiple fluorophores. These probes with multiple fluorophores can then be used to label a bacterial plasmid target in a sequence-specific manner. These probes enabled the detection of 1 pM plasmid samples containing a portion of an antibiotic resistance gene sequence. This system shows the possibility of improving capture and fluorescence labeling of small nucleic acid fragments, generating lower limits of detection for clinically relevant samples while maintaining rapid processing times.

Application of Microfluidics in Immunoassay: Recent Advancements

In recent years, point-of-care testing has played an important role in immunoassay, biochemical analysis, and molecular diagnosis, especially in low-resource settings. Among various point-of-care-testing platforms, microfluidic chips have many outstanding advantages. Microfluidic chip applies the technology of miniaturizing conventional laboratory which enables the whole biochemical process including reagent loading, reaction, separation, and detection on the microchip. As a result, microfluidic platform has become a hotspot of research in the fields of food safety, health care, and environmental monitoring in the past few decades. Here, the state-of-the-art application of microfluidics in immunoassay in the past decade will be reviewed. According to different driving forces of fluid, microfluidic platform is divided into two parts: passive manipulation and active manipulation. In passive manipulation, we focus on the capillary-driven microfluidics, while in active manipulation, we introduce pressure microfluidics, centrifugal microfluidics, electric microfluidics, optofluidics, magnetic microfluidics, and digital microfluidics. Additionally, within the introduction of each platform, innovation of the methods used and their corresponding performance improvement will be discussed. Ultimately, the shortcomings of different platforms and approaches for improvement will be proposed.

Microfluidics for Peptidomics, Proteomics, and Cell Analysis

Microfluidics is the advanced microtechnology of fluid manipulation in channels with at least one dimension in the range of 1-100 microns. Microfluidic technology offers a growing number of tools for manipulating small volumes of fluid to control chemical, biological, and physical processes relevant to separation, analysis, and detection. Currently, microfluidic devices play an important role in many biological, chemical, physical, biotechnological and engineering applications. There are numerous ways to fabricate the necessary microchannels and integrate them into microfluidic platforms. In peptidomics and proteomics, microfluidics is often used in combination with mass spectrometric (MS) analysis. This review provides an overview of using microfluidic systems for peptidomics, proteomics and cell analysis. The application of microfluidics in combination with MS detection and other novel techniques to answer clinical questions is also discussed in the context of disease diagnosis and therapy. Recent developments and applications of capillary and microchip (electro)separation methods in proteomic and peptidomic analysis are summarized. The state of the art of microchip platforms for cell sorting and single-cell analysis is also discussed. Advances in detection methods are reported, and new applications in proteomics and peptidomics, quality control of peptide and protein pharmaceuticals, analysis of proteins and peptides in biomatrices and determination of their physicochemical parameters are highlighted.

3D Printed Microfluidic Devices for Solid-Phase Extraction and On-Chip Fluorescent Labeling of Preterm Birth Risk Biomarkers

Solid-phase extraction (SPE) is a general preconcentration method for sample preparation that can be performed on a variety of specimens. The miniaturization of SPE within a 3D printed microfluidic device further allows for fast and simple extraction of analytes while also enabling integration of SPE with other sample preparation and separation methods. Here, we present the development and application of a reversed-phase lauryl methacrylate-based monolith, formed in 3D printed microfluidic devices, which can selectively retain peptides and proteins. The effectiveness of these SPE monoliths and 3D printed microfluidic devices was tested using a panel of nine preterm birth biomarkers of varying hydrophobicities and ranging in mass from 2 to 470 kDa. The biomarkers were selectively retained, fluorescently labeled, and eluted separately from the excess fluorescent label in 3D printed microfluidic systems. These are the first results demonstrating microfluidic analysis processes on a complete panel of preterm birth biomarkers, an important step toward developing a miniaturized, fully integrated analysis system.

Recent Advances and Applications of Microfluidic Capillary Electrophoresis: A Comprehensive Review (2017-Mid 2019)

Microfluidic capillary electrophoresis (MCE) is the novel technique resulted from the CE mininaturization as planar separation and analysis device. This review presents and discusses various application fields of this advanced technology published in the period 2017 till mid-2019 in eight different sections including clinical, biological, single cell analysis, environmental, pharmaceuticals, food analysis, forensic and ion analysis. The need for miniaturization of CE and the consequence advantages achieved are also discussed including high-throughput, miniaturized detection, effective separation, portability and the need for micro- or even nano-volume of samples. Comprehensive tables for the MCE applications in the different studied fields are provided. Also, figure comparing the number of the published papers applying MCE in the eight discussed fields within the studied period is included. The future investigation should put into consideration the possibility of replacing conventional CE with the MCE after proper validation. Suitable validation parameters with their suitable accepted ranges should be tailored for analysis methods utilizing such unique technique (MCE).

Single droplet detection of immune checkpoints on a multiplexed electrohydrodynamic biosensor

Monitoring soluble immune checkpoints in circulating fluids has the potential for minimally-invasive diagnostics and personalised therapy in precision medicine. Yet, the sensitive detection of multiple immune checkpoints from small volumes of liquid biopsy samples is challenging. In this study, we develop a multiplexed immune checkpoint biosensor (MICB) for parallel detection of soluble immune checkpoints PD-1, PD-L1, and LAG-3. MICB integrates a microfluidic sandwich immunoassay using engineered single chain variable fragments and alternating current electrohydrodynamic in situ nanofluidic mixing for promoting biosensor-target interaction and reducing non-specific non-target binding. MICB provides advantages of simultaneous analysis of up to 28 samples in <2 h, requires as little as a single sample drop (i.e., 20 μL) per target immune checkpoint, and applies high-affinity yeast cell-derived single chain variable fragments as a cost-effective alternative to monoclonal antibodies. We investigate the assay performance of MICB and demonstrate its capability for accurate immune checkpoint detection in simulated patient serum samples at clinically-relevant levels. MICB provides a dynamic range of 5 to 200 pg mL-1 for PD-1 and PD-L1, and 50 to 1000 pg mL-1 for LAG-3 with a coefficient of variation <13.8%. Sensitive immune checkpoint detection was achieved with limits of detection values of 5 pg mL-1 for PD-1, 5 pg mL-1 for PD-L1, and 50 pg mL-1 for LAG-3. The multiplexing capability, sensitivity, and relative assay simplicity of MICB make it capable of serving as a bioanalytical tool for immune checkpoint therapy monitoring.

Rapid affinity chromatographic isolation method for LDL in human plasma by immobilized chondroitin-6-sulfate and anti-apoB-100 antibody monolithic disks in tandem

Low-density lipoprotein (LDL) is considered the major risk factor for the development of atherosclerotic cardiovascular diseases (ASCVDs). A novel and rapid method for the isolation of LDL from human plasma was developed utilising affinity chromatography with monolithic stationary supports. The isolation method consisted of two polymeric monolithic disk columns, one immobilized with chondroitin-6-sulfate (C6S) and the other with apolipoprotein B-100 monoclonal antibody (anti-apoB-100 mAb). The first disk with C6S was targeted to remove chylomicrons, very-low-density lipoprotein (VLDL) particles, and their remnants including intermediate-density lipoprotein (IDL) particles, thus allowing the remaining major lipoprotein species, i.e. LDL, lipoprotein(a) (Lp(a)), and high-density lipoprotein (HDL) to flow to the anti-apoB-100 disk. The second disk captured LDL particles via the anti-apoB-100 mAb attached on the disk surface in a highly specific manner, permitting the selective LDL isolation. The success of LDL isolation was confirmed by different techniques including quartz crystal microbalance. In addition, the method developed gave comparable results with ultracentrifugation, conventionally used as a standard method. The reliable results achieved together with a short isolation time (less than 30 min) suggest the method to be suitable for clinically relevant LDL functional assays.

3D Printed Microfluidic Devices for Microchip Electrophoresis of Preterm Birth Biomarkers

This work demonstrates for the first time the creation of microchip electrophoresis devices with ∼50 μm cross-sectional dimensions by stereolithographic 3D printing and their application in the analysis of medically significant biomarkers related to risk for preterm birth (PTB). We determined that device current was linear with applied potential up to 800 V (620 V/cm). We optimized device and separation conditions using fluorescently labeled amino acids as a model system and compared the performance in our 3D printed microfluidic devices to that in other device materials commonly used for microchip electrophoresis analysis. We demonstrated for the first time microchip electrophoresis in a 3D printed device of three PTB biomarkers, including peptides and a protein, with suitable separation characteristics. Limits of detection for microchip electrophoresis in 3D printed microfluidic devices were also determined for PTB biomarkers to be in the high picomolar to low nanomolar range.

Device Fabrication and Fluorescent Labeling of Preterm Birth Biomarkers for Microchip Electrophoresis

An unmet need exists for a clinical diagnostic to determine preterm birth (PTB) risk. Such an assessment is possible with high sensitivity and specificity using a panel of nine biomarkers. An integrated microfluidic analysis system for these biomarkers is being developed which includes microchip electrophoresis (μCE) separation. A t-shaped microchip device can be used to test the μCE portion of this integrated system to find appropriate separation conditions. These t-shaped microchips can be fabricated using photolithographically patterned Si templates and hot embossing. PTB biomarkers can be fluorescently labeled using an amine-reactive dye prior to μCE. The μCE conditions established using this t-shaped device should be useful in developing a complete integrated microfluidic system for PTB risk assessment.

3D printed microfluidic devices with immunoaffinity monoliths for extraction of preterm birth biomarkers

Preterm birth (PTB) is defined as birth before the 37th week of pregnancy and results in 15 million early deliveries worldwide every year. Presently, there is no clinical test to determine PTB risk; however, a panel of nine biomarkers found in maternal blood serum has predictive power for a subsequent PTB. A significant step in creating a clinical diagnostic for PTB is designing an automated method to extract and purify these biomarkers from blood serum. Here, microfluidic devices with 45 μm × 50 μm cross-section channels were 3D printed with a built-in polymerization window to allow a glycidyl methacrylate monolith to be site-specifically polymerized within the channel. This monolith was then used as a solid support to attach antibodies for PTB biomarker extraction. Using these functionalized monoliths, it was possible to selectively extract a PTB biomarker, ferritin, from buffer and a human blood serum matrix. This is the first demonstration of monolith formation in a 3D printed microfluidic device for immunoaffinity extraction. Notably, this work is a crucial first step toward developing a 3D printed microfluidic clinical diagnostic for PTB risk.

Droplet Microfluidics in Thermoplastics: Device Fabrication, Droplet Generation, and Content Manipulation using Integrated Electric and Magnetic Fields

We have developed droplet microfluidic devices in thermoplastics and demonstrated the integration of key functional components that not only facilitate droplet generation, but also include electric field-assisted reagent injection, droplet splitting, and magnetic field-assisted bead extraction. We manufactured devices in poly(methyl methacrylate) and cyclic olefin polymer using a hot-embossing procedure employing silicon masters fabricated via photolithography and deep reactive ion etching techniques. Device characterization showed robust fabrication with uniform feature transfer and good embossing yield. Channel modification with heptadecafluoro-1,1,2,2-tetrahydrodecyltrichlorosilane increased device hydrophobicity, allowing stable generation of 330-pL aqueous droplets using T-junction configuration. Picoinjector and K-channel motifs were also both successfully integrated into the thermoplastic devices, allowing for robust control over electric field-assisted reagent injection, as well as droplet splitting with the K-channel. A magnetic field was also introduced to the K-channel geometry to allow for selective concentration of magnetic beads while decanting waste volume through droplet splitting. To show the ability to link multiple, modular features in a single thermoplastic device, we integrated droplet generation, reagent injection, and magnetic field-assisted droplet splitting on a single device, realizing a magnetic bead washing scheme to selectively exchange the fluid composition around the magnetic particles, analogous to the washing steps in many common biochemical assays. Finally, integrated devices were used to perform a proof-of-concept in-droplet β-galactosidase enzymatic assay combining enzyme-magnetic bead containing droplet generation, resorufin-β-D-galactopyranoside substrate injection, enzyme-substrate reaction, and enzyme-magnetic bead washing. By integrating multiple droplet operations and actuation forces we have demonstrated the potential of thermoplastic droplet microfluidic devices for complex (bio)chemical analysis, and we envision a path toward mass fabrication of droplet microfluidic devices for a range of (bio)chemical applications.

Advancement of analytical modes in a multichannel, microfluidic droplet-based sample chopper employing phase-locked detection

In this work, we expand upon our recently developed droplet-based sample chopping concepts by introducing a multiplexed fluidic micro-chopper device (μChopper). Six aqueous input channels were integrated with a single oil input, and each of these seven channels was controlled by a pneumatic valve for automated sampling through software control. This improved design, while maintaining high precision in valve-based droplet generation at bandwidths of 0.03 to 0.05 Hz, enabled a variety of analytical modes to be employed on-chip compared to previous devices limited to sample/reference alternations. The device was analytically validated for real-time, continuous calibration with a single sample and five standards; multiplexed analysis during calibration using a mixed mode; and standard addition through spiking of six sample droplets with varying amounts of standard. Finally, the standard addition mode was applied to protein quantification in human serum samples using on-chip, homogeneous fluorescence immunoassays. Ultimately, with only ~1.2 μL of total analyzed solution volume- representing 100-fold and 75-fold reductions in reagent and serum volumes, respectively-we were able to generate full, six-point standard addition curves in only 1.5 min, and results correlated well with those from standard plate-reader equipment. This work thus exploited microfluidic valves for both their automation and droplet phase-locking capabilities, resulting in a micro-analytical tool capable of complex analytical interrogation modes on sub-microliter sample volumes while also leveraging drastic noise rejection via lock-in detection. The multichannel μChopper device should prove particularly useful in analyzing precious biological samples or for dynamic analyses at small volume scales.

Microchip electrophoresis separation of a panel of preterm birth biomarkers

Preterm birth (PTB) is responsible for over one million infant deaths annually worldwide. Often, the first and only indication of PTB risk is the onset of early labor. Thus, there is an urgent need for an early PTB risk diagnostic that is inexpensive, reliable, and robust. Here, we describe the development of a microchip electrophoresis (μCE) method for separating a mixture of six PTB protein and peptide biomarkers present in maternal blood serum. μCE devices were photografted with a poly(ethylene glycol) diacrylate surface coating to regulate EOF and reduce nonspecific analyte adsorption. Separation conditions including buffer pH, buffer concentration, and applied electric field were varied to improve biomarker peak resolution while minimizing deleterious effects like Joule heating. In this way, it was possible to separate six PTB biomarkers, the first μCE separation of this biomarker panel. LODs were also measured for each of the six PTB biomarkers. In the future, this μCE separation can be integrated with upstream maternal blood serum sample preparation steps to yield a complete PTB risk diagnosis microdevice.