Use of photopatterned porous polymer monoliths as passive micromixers to enhance mixing efficiency for on-chip labeling reactions

Micromixer with Fine-Tuned Mathematical Spiral Structures

Micromixers with the microchannel structure can enable rapid and efficient mixing of multiple types of fluids on a microfluidic chip. Herein, we report the mixing performance of three passive micromixers based on the different mathematical spiral structures. We study the fluid flow characteristics of Archimedes spiral, Fermat spiral, and hyperbolic spiral structures with various channel widths and Reynolds number (Re) ranging from 0 to 10 via numerical simulation and visualization experiments. In addition, we analyze the mechanism of streamlines and Dean vortices at different cross sections during fluid flows. As the fluid flows in the Fermat spiral channel, the centrifugal force induces the Dean vortex to form a chaotic advection, enhancing the fluid mixing performance. By integrating the Fermat spiral channel into a microfluidic chip, we successfully detect acute myocardial infarction (AMI) marker with the double-antibody sandwich method and reduce the detection time to 10 min. This method has a low reagent consumption and a high reaction efficiency and demonstrates great potential in point-of-care testing (POCT).

In situ photo-patterning of pressure-resistant hydrogel membranes with controlled permeabilities in PEGDA microfluidic channels

We report the fabrication of highly permeable membranes in poly(ethylene glycol) diacrylate (PEGDA) channels, for investigating ultra- or micro-filtration, at the microfluidic scale. More precisely, we used a maskless UV projection setup to photo-pattern PEG-based hydrogel membranes on a large scale (mm-cm), and with a spatial resolution of a few microns. We show that these membranes can withstand trans-membrane pressure drops of up to 7 bar without any leakage, thanks to the strong anchoring of the hydrogel to the channel walls. We also report in situ measurements of the Darcy permeability of these membranes, as a function of the deposited energy during photo-polymerization, and their formulation composition. We show that the use of PEG chains as porogens, as proposed in [Lee et al., Biomacromolecules, 2010, 11, 3316], significantly increases the porosity of the hydrogels, up to Darcy permeabilities of about 1.5 × 10-16 m2, while maintaining the strong mechanical stability of the membranes. We finally illustrate the opportunities offered by this technique, by investigating frontal filtration of colloidal dispersions in a straight microfluidic channel.

Solid supports for extraction and preconcentration of proteins and peptides in microfluidic devices: A review

Determination of proteins and peptides is among the main challenges of today's bioanalytical chemistry. The application of microchip technology in this field is an exhaustively developed concept that aims to create integrated and fully automated analytical devices able to quantify or detect one or several proteins from a complex matrix. Selective extraction and preconcentration of targeted proteins and peptides especially from biological fluids is of the highest importance for a successful realization of these microsystems. Incorporation of solid structures or supports is a convenient solution employed to face these demands. This review presents a critical view on the latest achievements in sample processing techniques for protein determination using solid supports in microfluidics. The study covers the period from 2006 to 2015 and focuses mainly on the strategies based on microbeads, monolithic materials and membranes. Less common approaches are also briefly discussed. The reviewed literature suggests future trends which are discussed in the concluding remarks.

Integrated on-chip mass spectrometry reaction monitoring in microfluidic devices containing porous polymer monolithic columns

Chip-based microfluidics enable the seamless integration of different functions into single devices. Here, we present microfluidic chips containing porous polymer monolithic columns as a means to facilitate chemical transformations as well as both downstream chromatographic separation and mass spectrometric analysis. Rapid liquid phase lithography prototyping creates the multifunctional device economically.

Advances in monoliths and related porous materials for microfluidics

In recent years, the use of monolithic porous polymers has seen significant growth. These materials present a highly useful support for various analytical and biochemical applications. Since their introduction, various approaches have been introduced to produce monoliths in a broad range of materials. Simple preparation has enabled their easy implementation in microchannels, extending the range of applications where microfluidics can be successfully utilized. This review summarizes progress regarding monoliths and related porous materials in the field of microfluidics between 2010 and 2015. Recent developments in monolith preparation, solid-phase extraction, separations, and catalysis are critically discussed. Finally, a brief overview of the use of these porous materials for analysis of subcellular and larger structures is given.

New organic–inorganic hybrid material based on a poly(amic acid) oligomer: a promising opportunity to obtain microfluidic devices by a photolithographic process

Miniaturized total analysis systems are becoming a powerful tool for analytical and bioanalytical applications.

High-performance liquid chromatography on glass chips using precisely defined porous polymer monoliths as particle retaining elements

A stable and permanent integration of miniature packed bed separation columns into microfluidic systems is a major issue in nano liquid chromatography. Various approaches like differently shaped retaining elements or the use of key stone effect have been investigated. We show a flexible integration of miniature packed bed separation columns into microfluidic chips utilising common HPLC material achieved by laser-assisted generation of narrow, photopolymerised frits. The generated retaining elements serve as an in- and outlet frits for the columns. An optimised pre-polymeric solution, consisting of butyl acrylates and a porogen, allows a precise fabrication of frit-type structures with lengths of less than 100 m and the capability to withstand common slurry packing pressures of more than 250 bar. The separation of seven polycyclic aromatic hydrocarbons by pressure-driven, reversed-phase chromatography proves the high quality of the created chromatographic column inside a glass chip. Plate heights down to 2.9 were achieved and extremely fast separations with sub-second peak widths were performed in isocratic and gradient elution modes on very short columns (≤ 25 mm).

Solventless fabrication of porous-on-porous materials

Here we fabricate patterned porous polymer membranes on porous substrates by a combination of physical masking and chemical vapor deposition. This all-dry technique eliminates solvent-related issues and allows for the fabrication of hierarchical porous-on-porous structures with a wide range of chemical compositions and shapes. The porous polymer membranes are made by operating at unconventional processing conditions to simultaneously deposit and polymerize monomer. The solid monomer serves as a porogen and creates microstructures around which polymer forms. Membranes with thicknesses ranging from a few hundred micrometers to a millimeter are fabricated on porous paper substrates. The resolution of the patterning process and the structure of the resulting membranes are analyzed as a function of the deposition time. It was found that the patterned membranes exhibit a tapered structure and the dimensions are in good agreement with the dimensions of the mask. One potential application of these patterned polymer membranes is demonstrated for the selective separation of analytes for diagnostic applications on paper-based microfluidic devices. The ability to pattern porous-on-porous structures can be useful for the development of hierarchical membranes for water purification and gas separation, and for sensing, patterned tissue scaffolding, and other lab-on-a-chip applications.

Microfluidic devices with permeable polymer barriers for capture and transport of biomolecules and cells

We report a method for fabricating permeable polymer microstructure barriers in polydimethylsiloxane (PDMS) microfluidic devices and the use of the devices to capture and transport DNA and cells. The polymer microstructure in a desired location in a fluidic channel is formed in situ by the polymerization of acrylamide and polyethylene diacrylate cross-linker (PEG-DA) monomer in a solution which is trapped in the location using a pair of PDMS valves. The porous polymer microstructure provides a mechanical barrier to convective fluid flow in the channel or between two microfluidic chambers while it still conducts ions or small charged species under an electric field, allowing for the rapid capture and transport of biomolecules and cells by electrophoresis. We have demonstrated the application of the devices for the rapid capture and efficient release of bacteriophage λ genomic DNA, solution exchange and for the transport and capture of HeLa cells. Our devices will enable the multi-step processing of biomolecules and cells or individual cells within a single microfluidic chamber.

Microfluidic filtration system to isolate extracellular vesicles from blood

Extracellular vesicles are released by various cell types, particularly tumor cells, and may be potential targets for blood-based cancer diagnosis. However, studies performed on blood-borne vesicles to date have been limited by lack of effective, standardized purification strategies. Using in situ prepared nanoporous membranes, we present a simple strategy employing a microfluidic filtration system to isolate vesicles from whole blood samples. This method can be applied to purify nano-sized particles from blood allowing isolation of intact extracellular vesicles, avoiding the need for laborious and potentially damaging centrifugation steps or overly specific antibody-based affinity purification. Porous polymer monoliths were integrated as membranes into poly(methyl methacrylate) microfluidic chips by benchtop UV photopolymerization through a mask, allowing precise positioning of membrane elements while preserving simplicity of device preparation. Pore size could be manipulated by changing the ratio of porogenic solvent to prepolymer solution, and was tuned to a size proper for extraction of vesicles. Using the membrane as a size exclusion filter, we separated vesicles from cells and large debris by injecting whole blood under pressure through the microfluidic device. To enhance isolation purity, DC electrophoresis was employed as an alternative driving force to propel particles across the filter and increase the separation efficiency of vesicles from proteins. From the whole blood of melanoma-grown mice, we isolated extracellular vesicles and performed RT-PCR to verify their contents of RNA. Melan A mRNA derived from melanoma tumor cells were found enriched in filtered samples, confirming the recovery of vesicles via their cargo. This filtration system can be incorporated into other on-chip processes enabling integrated sample preparation for the downstream analysis of blood-based extracellular vesicles.

Thirty-minute total synthesis of microfluidic systems and functionalized porous elements via "living" radical photo-polymerization

A "living" radical photo-polymerization (LRPP) technique is used to rapidly fabricate microfluidic channels and micro-patterned porous polymer monoliths. Surface-initiated LRPP is then used to functionalize porous elements in a robust one-step surface modification process. Assay-ready platforms can be fully realized in less than 30 minutes. An application relevant to clinical diagnostics is presented.

A novel surface modification technique for forming porous polymer monoliths in poly(dimethylsiloxane)

A new method of surface modification is described for enabling the in situ formation of homogenous porous polymer monoliths (PPMs) within poly(dimethylsiloxane) (PDMS) microfluidic channels that uses 365 nm UV illumination for polymerization. Porous polymer monolith formation in PDMS can be challenging because PDMS readily absorbs the monomers and solvents, changing the final monolith morphology, and because PDMS absorbs oxygen, which inhibits free-radical polymerization. The new approach is based on sequentially absorbing a non-hydrogen-abstracting photoinitiator and the monomers methyl methacrylate and ethylene diacrylate within the walls of the microchannel, and then polymerizing the surface treatment polymer within the PDMS, entangled with it but not covalently bound. Four different monolith compositions were tested, all of which yielded monoliths that were securely anchored and could withstand pressures exceeding the bonding strength of PDMS (40 psi) without dislodging. One was a recipe that was optimized to give a larger average pore size, required for low back pressure. This monolith was used to concentrate and subsequently mechanical lyse B lymphocytes.

Microfluidic chips for point-of-care immunodiagnostics

We might be at the turning point where research in microfluidics undertaken in academia and industrial research laboratories, and substantially sponsored by public grants, may provide a range of portable and networked diagnostic devices. In this Progress Report, an overview on microfluidic devices that may become the next generation of point-of-care (POC) diagnostics is provided. First, we describe gaps and opportunities in medical diagnostics and how microfluidics can address these gaps using the example of immunodiagnostics. Next, we conceptualize how different technologies are converging into working microfluidic POC diagnostics devices. Technologies are explained from the perspective of sample interaction with components of a device. Specifically, we detail materials, surface treatment, sample processing, microfluidic elements (such as valves, pumps, and mixers), receptors, and analytes in the light of various biosensing concepts. Finally, we discuss the integration of components into accurate and reliable devices.

System Integration - A Major Step toward Lab on a Chip

Microfluidics holds great promise to revolutionize various areas of biological engineering, such as single cell analysis, environmental monitoring, regenerative medicine, and point-of-care diagnostics. Despite the fact that intensive efforts have been devoted into the field in the past decades, microfluidics has not yet been adopted widely. It is increasingly realized that an effective system integration strategy that is low cost and broadly applicable to various biological engineering situations is required to fully realize the potential of microfluidics. In this article, we review several promising system integration approaches for microfluidics and discuss their advantages, limitations, and applications. Future advancements of these microfluidic strategies will lead toward translational lab-on-a-chip systems for a wide spectrum of biological engineering applications.

Photopatterned materials in bioanalytical microfluidic technology

Microfluidic technologies are playing an increasingly important role in biological inquiry. Sophisticated approaches to the microanalysis of biological specimens rely, in part, on the fine fluid and material control offered by microtechnology, as well as a sufficient capacity for systems integration. A suite of techniques that utilize photopatterning of polymers on fluidic surfaces, within fluidic volumes, and as primary device structures underpins recent technological innovation in bioanalysis. Well-characterized photopatterning approaches enable previously fabricated or commercially fabricated devices to be customized by the user in a straight-forward manner, making the tools accessible to laboratories that do not focus on microfabrication technology innovation. In this review of recent advances, we summarize reported microfluidic devices with photopatterned structures and regions as platforms for a diverse set of biological measurements and assays.

Integrated Multi-process Microfluidic Systems for Automating Analysis

Microfluidic technologies have been applied extensively in rapid sample analysis. Some current challenges for standard microfluidic systems are relatively high detection limits, and reduced resolving power and peak capacity compared to conventional approaches. The integration of multiple functions and components onto a single platform can overcome these separation and detection limitations of microfluidics. Multiplexed systems can greatly increase peak capacity in multidimensional separations and can increase sample throughput by analyzing many samples simultaneously. On-chip sample preparation, including labeling, preconcentration, cleanup and amplification, can all serve to speed up and automate processes in integrated microfluidic systems. This paper summarizes advances in integrated multi-process microfluidic systems for automated analysis, their benefits and areas for needed improvement.

Advances in the preparation of porous polymer monoliths in capillaries and microfluidic chips with focus on morphological aspects

Porous polymer monoliths have emerged as unique materials for many applications, including liquid-chromatographic analyses at an unrivaled speed, solid-phase extraction, and enzyme immobilization in capillary and microfluidic chip format. This article reviews the state of the art in the preparation of monoliths in narrow-bore capillaries and microfluidic chips and their miniaturization under conditions of spatial confinement. New developments in their preparation mainly using free radical polymerization techniques with a focus on morphological aspects in view of homogeneous porous materials are described. The suitability of monoliths for analysis of both large and small molecules is also discussed.

Holographic fabrication of three-dimensional nanostructures for microfluidic passive mixing

In this study, we incorporated mixing units of three-dimensional (3D) interconnected pore network inside microfluidic channels by combining single prism holographic lithography and photolithography. 3D pore network structures were generated by the interference of four laser beams generated by a truncated triangular pyramidal prism. The levelling between the 3D porous structures and the channel walls was greatly improved by employing supercritical drying, which induced negligible internal capillary stresses and reduced substantially anisotropic volume shrinkage of 3D structures. Also, complete sealing of the microfluidic chips was achieved by attaching flexible PDMS cover substrates. Overall mixing performance of the systems with completely sealed mixing units was 84% greater than that obtained without such mixers. Splitting and recombination of flows in the 3D interconnected pore structures enhanced the mixing efficiency by decreasing the diffusion path and increasing the surface contact between two liquid streams. Because the flow splitting and recombination was developed through the 3D interconnected pore network, high mixing efficiency (>0.60) was achieved at low Reynolds numbers (Re < 0.05) and Péclet numbers in the regime of Pe < 1.4 x 10(3).