Theoretical demonstration of density-based magnetic levitation.

<p><b>(a)</b> Representative microspheres are suspended in a paramagnetic medium for two cases: when the microspheres are more dense than the suspending medium (left) and when the microspheres are less dense than the suspending medium (right). The magnetic force (F_m), buoyancy or corrected gravitational force (F_g’), and drag force (F_d) acts on microspheres, causing them to approach equilibrium (purple dotted line). F_m exerts a force on the microspheres in the direction of the centerline between the two magnets (orange dotted line) and changes in magnitude depending on the microsphere’s location in the magnetic field. F_g’ exerts either a downward force (in the case of microspheres which are denser than the suspending medium, left) or an upward force (in the case of microspheres which are less dense than the suspending medium, right). F_d acts on the object in a direction opposite the direction of motion until the bead reaches a line of equilibrium below the centerline (as in the case of denser objects, left) or above the centerline (as in the case of less dense objects, right). At equilibrium, F_m and F_g’ are equal and opposite and F_d has zero magnitude. <b>(b)</b> The magnetic field in the cross-section between the magnets demonstrated by the magnetic forces exerted on an object in the field; the forces have directionality toward the centerline between the two magnets and magnitude greatest near the magnets’ surfaces and approaching zero at the centerline. <b>(c)</b> Contour plot of magnitude of the magnetic field strength in the cross-sectional area at z = 0, the center between the two magnets. The magnitude of the magnetic field is constrained between 0 T and 0.4 T. <b>(d-f)</b> Contour plots showing the magnitude of the magnetic field at the back surface of the smart-phone in the <b>(d)</b> x-direction, <b>(e)</b> y-direction, and <b>(f)</b> z-direction. <b>(g)</b> Representative images of polymer microspheres in a 50 mM gadolinium solution levitating and focusing to an equilibrium height over 120 seconds.</p

Experimental demonstration of magnetic levitation of polymer microspheres and quantification of levitation height.

<p><b>(a)</b> Time-dependent focusing of 10 μm polystyrene microspheres in 30 mM and 100 mM gadolinium solution. Images shown at 0, 15 and 75 seconds after being placed in the magnetic field demonstrate time-dependent focusing of microspheres influenced by the gadolinium concentration. Graphs show the top (blue) and bottom (red) confinement limits of the microspheres over time in 30 mM and 100 mM gadolinium paramagnetic mediums. <b>(b)</b> Quantification of equilibrium time (red) and equilibrium height (blue) for several concentrations of gadolinium. <b>(c)</b> Time-dependent focusing of microspheres with different diameters (5.35 and 20 μm) in 50 mM gadolinium solution. Images shown at 0, 60, and 120 seconds after insertion into the magnetic field demonstrate time-dependent focusing of microspheres influenced by the microsphere size. Graphs show the width of microsphere confinement over time for 6 different trials with exponential decay approximations. <b>(d)</b> Quantification of equilibrium time (red) and equilibrium height (blue) as a function of microsphere size, demonstrating that increasing size decreases the time to equilibrium, but the size has no statistically significant effect on levitation height. <b>(e)</b> Calibration of levitation height to density using eight density standard microspheres. Five different gadolinium concentrations are used to demonstrate the ability to obtain greater resolution at lower gadolinium concentrations. <b>(f)</b> Images representing different levitation heights of three different density standard microspheres in two different concentrations of paramagnetic medium. Scale bars are 100 μm.</p

Optical quantification of density-based magnetic levitation.

<p><b>(a)</b> Quantification of image distortion (blue), background illumination (magenta), and microsphere sharpness (red) along the horizontal field of view. Microsphere sharpness is shown as the line of best fit for the data shown in (b). <b>(b)</b> Data points representing sharpness of microspheres located at different distances from the center of the field of view. The line of best fit approximates the decrease in image sharpness as the distance from the centerline increases. Red data points represent microspheres located to the left of the centerline and blue data points represent microspheres located to the right of the centerline. <b>(c)</b> 210 μm and <b>(d)</b> 5.25 μm diameter microspheres demonstrating qualitatively the optical resolution of the platform.</p

Continuous-Ink, Multiplexed Pen-Plotter Approach for Low-Cost, High-Throughput Fabrication of Paper-Based Microfluidics

There is an unmet need for high-throughput fabrication techniques for paper-based microanalytical devices, especially in limited resource areas. Fabrication of these devices requires precise and repeatable deposition of hydrophobic materials in a defined pattern to delineate the hydrophilic reaction zones. In this study, we demonstrated a cost- and time-effective method for high-throughput, easily accessible fabrication of paper-based microfluidics using a desktop pen plotter integrated with a custom-designed multipen holder. This approach enabled simultaneous printing with multiple printing heads and, thus, multiplexed fabrication. Moreover, we proposed an ink supply system connected to commercial technical pens to allow continuous flow of the ink, thereby increasing the printing capacity of the system. We tested the use of either hot- or cold-laminating layers to improve (i) the durability, stability, and mechanical strength of the paper-based devices and (ii) the seal on the back face of the chromatography paper to prevent wetting of the sample beyond the hydrophilic testing region. To demonstrate a potential application of the paper-based microfluidic devices fabricated by the proposed method, colorimetric urine assays were implemented and tested: nitrite, urobilinogen, protein, blood, and pH

Nanoplasmonic Quantitative Detection of Intact Viruses from Unprocessed Whole Blood

Infectious diseases such as HIV and hepatitis B pose an omnipresent threat to global health. Reliable, fast, accurate, and sensitive platforms that can be deployed at the point-of-care (POC) in multiple settings, such as airports and offices, for detection of infectious pathogens are essential for the management of epidemics and possible biological attacks. To the best of our knowledge, no viral load technology adaptable to the POC settings exists today due to critical technical and biological challenges. Here, we present for the first time a broadly applicable technology for quantitative, nanoplasmonic-based intact virus detection at clinically relevant concentrations. The sensing platform is based on unique nanoplasmonic properties of nanoparticles utilizing immobilized antibodies to selectively capture rapidly evolving viral subtypes. We demonstrate the capture, detection, and quantification of multiple HIV subtypes (A, B, C, D, E, G, and subtype panel) with high repeatability, sensitivity, and specificity down to 98 ± 39 copies/mL (<i>i.e</i>., HIV subtype D) using spiked whole blood samples and clinical discarded HIV-infected patient whole blood samples validated by the gold standard, <i>i</i>.<i>e</i>., RT-qPCR. This platform technology offers an assay time of 1 h and 10 min (1 h for capture, 10 min for detection and data analysis). The presented platform is also able to capture intact viruses at high efficiency using immuno-surface chemistry approaches directly from whole blood samples without any sample preprocessing steps such as spin-down or sorting. Evidence is presented showing the system to be accurate, repeatable, and reliable. Additionally, the presented platform technology can be broadly adapted to detect other pathogens having reasonably well-described biomarkers by adapting the surface chemistry. Thus, this broadly applicable detection platform holds great promise to be implemented at POC settings, hospitals, and primary care settings