Paper-based pump-free magnetophoresis

Abstract

Microfluidic magnetophoresis is a powerful technique that is used to separate and/or isolate cells of interest from complex matrices for analysis. However, mechanical pumps are required to drive flow, limiting portability and making translation to point-of-care (POC) settings difficult. Microfluidic paper-based analytical devices (μPADs) offer an alternative to traditional microfluidic devices that do not require external pumps to generate flow. However, μPADs are not typically used for particle analysis because most particles become trapped in the porous fiber network. Here we report the ability of newly developed fast-flow microfluidic paper-based analytical devices (ffPADs) to perform magnetophoresis. ffPADs use capillary action in a gap between stacked layers of paper and transparency sheets to drive flow at higher velocities than traditional μPADs. The multi-layer ffPADs allow particles and cells to move through the gap without being trapped in the paper layers. We first demonstrate that ffPADs enable magnetic particle separations in a μPAD with a neodymium permanent magnet and study key factors that affect performance. To demonstrate utility, E. coli was used as a model analyte and was isolated from human urine before detection with a fluorescently labeled antibody. A capture efficiency of 61.5% was then obtained of E. coli labeled magnetic beads in human urine. Future studies will look at the improvement of the capture efficiency and to make this assay completely off-chip without the need of a fluorescent label. The assay and device described here demonstrate the first example of magnetophoresis in a paper based, pump free microfluidic device.

Fig. 1

Assembly and flow characteristics of microfluidic paper-based analytical devices (μPADs) (a) Schematic of fast-flow μPAD assembly. (b) Cross-sectional view of channel of fast flow μPADs, with flow to-ward/away from the observer. (c) Plot showing flow velocities with respect to gap height.

Fig 2.

(a) Image taken of fast-flow μPAD with laminar flow. (b) Images taken of the middle channel to analyze AMI. The corresponding graphs show the effect the angle has on consistency of laminar flow by quantifying the amount of mixing of each device.

Fig 3.. Magnetophoresis in microfluidic paper-based analytical devices (μPADs)

(a) Overview of magnetophoresis concept, where the white-dashed lines are the laminar flow interface (b) 2μm magnetic particles in device (c) 8μm magnetic particles in device (d) Control image of particles following streamlines in the absence of a magnetic field. (e) 44μm magnetic particles undergoing positive magnetophoresis (f) Calibration curve of fluorescent intensity of particle capture via concentration.

Fig. 4

(a) Optimization of particle capture with respect to gap height characterized by fluorescent intensity with correlated equation showing the relationship of the velocity of the particle with respect to the fluid and magnetic velocity. (b) Actual and theoretical capture efficiency in the device with an optimized gap height.

Fig. 5

(a) CAD rendering of device design and an actual image of the fabricated μPAD. (b) Magnetic field gradient of a N52 cylindrical NdFeB magnet overlay with channel, red spheres indicate magnetic particles. Magnetic flux density estimated to be 1300 guass or 130 mT near the center of the channel. (Magnet image enlarged for viewing purpose)

Fig. 6. Detection of fluorescently labeled bacteria in μPAD magnetophoresis.

(a) Inverted image of positive magnetophoresis of E. coli complex. (b) Negative control assay (without E. coli). False color has been applied for visualization (c) Fluorescent microscope image taken of bound E. coli to bead. (d) Dose-response curve of E.coli in urine in the device