Flat Flow Profiles Achieved with Microfluidics Generated by Redox-Magnetohydrodynamics

Characterization of Nanoparticles in Diverse Mixtures Using Localized Surface Plasmon Resonance and Nanoparticle Tracking by Dark-Field Microscopy with Redox Magnetohydrodynamics Microfluidics

Redox magnetohydrodynamics (RMHD) microfluidics is coupled with dark-field microscopy (DFM) to offer high-throughput single-nanoparticle (NP) differentiation in situ and operando in a flowing mixture by localized surface plasmon resonance (LSPR) and tracking of NPs. The color of the scattered light allows visualization of the NPs below the diffraction limit. Their Brownian motion in 1-D superimposed on and perpendicular to the RMHD trajectory yields their diffusion coefficients. LSPR and diffusion coefficients provide two orthogonal modalities for characterization where each depends on a particle’s material composition, shape, size, and interactions with the surrounding medium. RMHD coupled with DFM was demonstrated on a mixture of 82 ± 9 nm silver and 140 ± 10 nm gold-coated silica nanospheres. The two populations of NPs in the mixture were identified by blue/green and orange/red LSPR and their scattering intensity, respectively, and their sizes were further evaluated based on their diffusion coefficients. RMHD microfluidics facilitates high-throughput analysis by moving the sample solution across the wide field of view absent of physical vibrations within the experimental cell. The well-controlled pumping allows for a continuous, reversible, and uniform flow for precise and simultaneous NP tracking of the Brownian motion. Additionally, the amounts of nanomaterials required for the analysis are minimized due to the elimination of an inlet and outlet. Several hundred individual NPs were differentiated from each other in the mixture flowing in forward and reverse directions. The ability to immediately reverse the flow direction also facilitates re-analysis of the NPs, enabling more precise sizing.

Flattening of Diluted Species Profile via Passive Geometry in a Microfluidic Device

In recent years, microfluidic devices have become an important tool for use in lab-on-a-chip processes, including drug screening and delivery, bio-chemical reactions, sample preparation and analysis, chemotaxis, and separations. In many such processes, a flat cross-sectional concentration profile with uniform flow velocity across the channel is desired to achieve controlled and precise solute transport. This is often accommodated by the use of electroosmotic flow, however, it is not an ideal for many applications, particularly biomicrofluidics. Meanwhile, pressure-driven systems generally exhibit a parabolic cross-sectional concentration profile through a channel. We draw inspiration from finite element fluid dynamics simulations to design and fabricate a practical solution to achieving a flat solute concentration profile in a two-dimensional (2D) microfluidic channel. The channel possesses geometric features to passively flatten the solute profile before entering the defined region of interest in the microfluidic channel. An obviously flat solute profile across the channel is demonstrated in both simulation and experiment. This technology readily lends itself to many microfluidic applications which require controlled solute transport in pressure driven systems.

Combining magnetic forces for contactless manipulation of fluids in microelectrode-microfluidic systems

A novel method to drive and manipulate fluid in a contactless way in a microelectrode-microfluidic system is demonstrated by combining the Lorentz and magnetic field gradient forces. The method is based on the redox-reaction [Fe(CN)₆]^3-/[Fe(CN)₆]^4- performed in a magnetic field oriented perpendicular to the ionic current that crosses the gap between two arrays of oppositely polarized microelectrodes, generating a magnetohydrodynamic flow. Additionally, a movable magnetized CoFe micro-strip is placed at different positions beneath the gap. In this region, the magnetic flux density is changed locally and a strong magnetic field gradient is formed. The redox-reaction changes the magnetic susceptibility of the electrolyte near the electrodes, and the resulting magnetic field gradient exerts a force on the fluid, which leads to a deflection of the Lorentz force-driven main flow. Particle Image Velocity measurements and numerical simulations demonstrate that by combining the two magnetic forces, the flow is not only redirected, but also a local change of concentration of paramagnetic species is realized.

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.

Redox-Magnetohydrodynamically Controlled Fluid Flow with Poly(3,4-ethylenedioxythiophene) Coupled to an Epitaxial Light Sheet Confocal Microscope for Image Cytometry Applications

We present the merging of two technologies to perform continuous high-resolution fluorescence imaging of cellular suspensions in a deep microfluidics chamber with no moving parts. An epitaxial light sheet confocal microscope (e-LSCM) was used to image suspensions enabled by fluid transport via redox-magnetohydrodynamics (R-MHD). The e-LSCM features a linear solid state sensor, oriented perpendicular to the direction of flow, that can bin the emission across different numbers of pixels, yielding electronically adjustable optical sectioning. This, in addition to intensity thresholding, defines the axial resolution, which was validated with an optical phantom of polystyrene microspheres suspended in agarose. The linear fluid speed within the microfluidics chamber was uniform (0.16-2.9%) across the 0.5-1.0 mm lateral field of view (dependent upon the chosen magnification) with continuous acquisition. Also, the camera's linear exposure periods were controlled to ensure an accurate image aspect ratio across this span. Poly(3,4-ethylenedioxythiophene) (PEDOT) was electrodeposited as an immobilized redox film on electrodes of a chip for R-MHD, and the fluid flow was calibrated to specific linear speeds as a function of applied current. Images of leukocytes stained with acridine orange, a fluorescent, amphipathic vital dye that intercalates DNA, were acquired in the R-MHD microfluidics chamber with the e-LSCM to demonstrate imaging of biological samples. The combination of these technologies provides a miniaturizable platform for large sample volumes and high-throughput, image-based analysis without the requirement of moving parts, enabling development of robust, point-of-care image cytometry.

Redox-magnetohydrodynamics, flat flow profile-guided enzyme assay detection: toward multiple, parallel analyses

A proof-of-concept superparamagnetic microbead-enzyme complex was integrated with microfluidics pumped by redox-magneto-hydrodynamics (MHD) to take advantage of the magnet (0.56 T) beneath the chip and the uniform flat flow profile, as a first step toward developing multiple, parallel chemical analyses on a chip without the need for independent channels. The superparamagnetic beads were derivatized with alkaline phosphatase (a common enzyme label for biochemical assays) and magnetically immobilized at three different locations on the chip with one directly on the path to the detector and the other two locations adjacent to, but off the path, by a distance >5 times the detector diameter. Electroactive p-aminophenol, enzymatically generated at the bead-enzyme complex from its electroinactive precursor p-aminophenyl phosphate in a solution containing a redox species [Ru(NH3)6](3+/2+) for pumping and Tris buffer, was transported by redox-MHD and detected with square wave voltammetry at a 312 μm diameter gold microdisk stationed 2 mm downstream from the bead-complex on the flow path. Oppositely biased pumping electrodes, consisting of 2.5 cm long gold bands and separated by 5.6 mm, flanked the active flow region containing the bead-enzyme complex and detection site. The signal from adjacent paths was only 20% of that for the direct path and ≤8% when pumping electrodes were inactive.