Nanoporous elements in microfluidics for multiscale manipulation of bioparticles

Abstract

Solid materials, such as silicon, glass, and polymers, dominate as structural elements in microsystems including microfluidics. Porous elements have been limited to membranes sandwiched between microchannel layers or polymer monoliths. This paper reports the use of micropatterned carbon-nanotube forests confined inside microfluidic channels for mechanically and/or chemically capturing particles ranging over three orders of magnitude in size. Nanoparticles below the internanotube spacing (80 nm) of the forest can penetrate inside the forest and interact with the large surface area created by individual nanotubes. For larger particles (>80 nm), the ultrahigh porosity of the nanotube elements reduces the fluid boundary layer and enhances particle-structure interactions on the outer surface of the patterned nanoporous elements. Specific biomolecular recognition is demonstrated using cells (≈10 μm), bacteria (≈1 μm), and viral-sized particles (≈40 nm) using both effects. This technology can provide unprecedented control of bioseparation processes to access bioparticles of interest, opening new pathways for both research and point-of-care diagnostics.

Figure 1

Nanoporous micropatterned elements are permeable to nanometer-scale molecules and particle flow. a) Comparison between a nanoporous CNT post and a solid PDMS post. b) Confocal micrographs of 200-µm-diameter nanoporous and solid posts as a fluorescent dye solution flows through the microchannel. c) Relative intensity plots of dye infiltration with time inside nanoporous and solid posts. Intensity values were evaluated along the vertical center line across each post. d) Fluorescent QDs (10–20 nm) passing through a 200-µm-diameter nanoporous post (outlined via dotted lines) having 80-nm CNT spacing. Yellow arrows follow the passage of one QD through the post; red arrows follow another passing around the outside of the post.

Figure 2

Nanoporous Y-filter. a) Device schematic. Overall dimensions 3 mm (W)× 20 mm (L) × 100 µm (H). The inside width of the nanoporous central channel is 100 µm. Blue lines are PDMS-channel boundaries; black lines show filter structure. Circular insets show micrographs of the inlet, concentrator outlet, and waste outlet, showing selective concentration of 10-µm fluorescent beads. b) SEM image of the nanoporous filter. c) Fluorescent micrograph showing that red fluorescent BSA molecules have passed through the nanoporous Y-filter walls. d) 10-µm polymer beads (green) cannot pass through the nanoporous filter sidewalls (dotted lines) and are directed to the central channel. e) Streak images of a single 10-µm bead (green) as it enters the constricted section of the filter.

Figure 3

Nanoporous elements alter particle flow paths. a) Particle streamline tracks of 10-µm beads around a nanoporous post and a solid post, showing tightening of the particle tracks towards the element surfaces due to the nanoporous post altering the flow. b) Snapshots of single particles approaching a nanoporous circular post element and a solid post from the same start position. The particle approaching the nanoporous post eventually touches the post and the one approaching the solid post never does. c) Positional data of multiple beads as they approach both posts. All beads approaching the nanoporous post from 17 µm or less away from the center line contact the post (γ or d = 0), with most never reaching the solid post.

Figure 4

Bioparticles of three size orders are captured with functionalized nanoporous elements. a) 10-µm-size fluorescently labeled CD4 T-cells are captured on 500-µm-diameter posts. Comparison in capture between nanoporous and solid posts of identical geometry is shown. Inset boxes show capture on nonfunctionalized control devices. b) 1-µm-size Streptococcus pneumoniae capture. Nanoporous- and solid-post arrays with identical geometries are functionalized with anti-S. pneumoniae antibody and fluorescently labeled Pneumoniae S. pneumoniae samples are flowed through the device. Inset boxes show capture on nonfunctionalized control devices. c) 40-nm-size virus-like particles captured within a functionalized nanoporous filter. A mixture of nonfunctionalized 1-µm beads (green) and 40-nm avidin-coated beads (red) are flowed through nonfunctionalized and biotin-functionalized nanoporous filters, respectively. The 1-µm beads are physically trapped in front of both filters. The 40-nm beads are trapped inside the functionalized filter and the nonfunctionalized filter does not capture the 40-nm beads.