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Deterministic bead-in-droplet ejection utilizing an integrated plug-in bead dispenser for single bead – based applications

View Article: PubMed Central - PubMed

ABSTRACT

This paper presents a deterministic bead-in-droplet ejection (BIDE) technique that regulates the precise distribution of microbeads in an ejected droplet. The deterministic BIDE was realized through the effective integration of a microfluidic single-particle handling technique with a liquid dispensing system. The integrated bead dispenser facilitates the transfer of the desired number of beads into a dispensing volume and the on-demand ejection of bead-encapsulated droplets. Single bead–encapsulated droplets were ejected every 3 s without any failure. Multiple-bead dispensing with deterministic control of the number of beads was demonstrated to emphasize the originality and quality of the proposed dispensing technique. The dispenser was mounted using a plug-socket type connection, and the dispensing process was completely automated using a programmed sequence without any microscopic observation. To demonstrate a potential application of the technique, bead-based streptavidin–biotin binding assay in an evaporating droplet was conducted using ultralow numbers of beads. The results evidenced the number of beads in the droplet crucially influences the reliability of the assay. Therefore, the proposed deterministic bead-in-droplet technology can be utilized to deliver desired beads onto a reaction site, particularly to reliably and efficiently enrich and detect target biomolecules.

No MeSH data available.


Time-lapse images for each mode.(a) Trapping mode during the ON state of the pneumatic valve; (b) releasing mode during the OFF state of the pneumatic valve; (c) loading mode to transfer a bead onto the loading site defined by the micropillars; (d) dispensing mode to eject a bead-encapsulated droplet. (a–c) were captured at 4,000 fps, and (d) was captured at 10,000 fps. All scale bars are 100 μm.
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f2: Time-lapse images for each mode.(a) Trapping mode during the ON state of the pneumatic valve; (b) releasing mode during the OFF state of the pneumatic valve; (c) loading mode to transfer a bead onto the loading site defined by the micropillars; (d) dispensing mode to eject a bead-encapsulated droplet. (a–c) were captured at 4,000 fps, and (d) was captured at 10,000 fps. All scale bars are 100 μm.

Mentions: The trapping, releasing, loading, and dispensing modes were observed through in-situ monitoring using a high-speed camera (Fig. 2). In the trapping mode, the beads introduced from the inlet reservoir were aligned using micropillars to allow them to flow into the trapping stream26 (Fig. 2a). These pillars can effectively align the beads without additional force (e.g., sheath flow and magnetic phoresis27), thereby simplifying dispenser design and operation. A single bead was successfully trapped by utilizing dynamic changes in hydraulic resistance and trapping space, as described in our previous work20. Once the trap is filled with a bead, the hydraulic resistance of the pneumatic valve part increases, reducing the width of trapping stream. Sufficient reduction of the trapping stream can prevent subsequent beads from entering the trap. The trapping condition of a single bead was experimentally characterized as described in following the ‘optimization for trapping and releasing single beads’ section. In the releasing mode, the pneumatic valve was switched OFF for 15 ms to release the trapped bead (Fig. 2b), after which it was immediately turned ON again, because a longer OFF time could induce the additional inflow of beads from the main channel. The released bead flowing through a sinuous channel was loaded onto the loading site defined by the micropillars, which are 20 μm apart, a spacing smaller than the bead diameter (25 μm; Fig. 2c). The bead travelled from the pneumatic valve to the loading site in a few dozen milliseconds. After loading a bead onto the loading site, a bead-encapsulated droplet was formed at the nozzle by positively pressurizing the dispensing reservoir, which is negatively pressurized during the normal state (Fig. 2d). To transfer sufficient energy for forming a droplet against the capillary force, a positive pressure of 80 kPa was applied to the dispensing reservoir for 30 ms. The bead on the loading site rushed toward the nozzle because the sinuous channel was designed to have sufficient hydraulic resistance to backflow. Thus, a bead-encapsulated droplet was finally obtained through these sequential modes. We name this technology “deterministic bead–in-droplet ejection (BIDE).”


Deterministic bead-in-droplet ejection utilizing an integrated plug-in bead dispenser for single bead – based applications
Time-lapse images for each mode.(a) Trapping mode during the ON state of the pneumatic valve; (b) releasing mode during the OFF state of the pneumatic valve; (c) loading mode to transfer a bead onto the loading site defined by the micropillars; (d) dispensing mode to eject a bead-encapsulated droplet. (a–c) were captured at 4,000 fps, and (d) was captured at 10,000 fps. All scale bars are 100 μm.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC5385560&req=5

f2: Time-lapse images for each mode.(a) Trapping mode during the ON state of the pneumatic valve; (b) releasing mode during the OFF state of the pneumatic valve; (c) loading mode to transfer a bead onto the loading site defined by the micropillars; (d) dispensing mode to eject a bead-encapsulated droplet. (a–c) were captured at 4,000 fps, and (d) was captured at 10,000 fps. All scale bars are 100 μm.
Mentions: The trapping, releasing, loading, and dispensing modes were observed through in-situ monitoring using a high-speed camera (Fig. 2). In the trapping mode, the beads introduced from the inlet reservoir were aligned using micropillars to allow them to flow into the trapping stream26 (Fig. 2a). These pillars can effectively align the beads without additional force (e.g., sheath flow and magnetic phoresis27), thereby simplifying dispenser design and operation. A single bead was successfully trapped by utilizing dynamic changes in hydraulic resistance and trapping space, as described in our previous work20. Once the trap is filled with a bead, the hydraulic resistance of the pneumatic valve part increases, reducing the width of trapping stream. Sufficient reduction of the trapping stream can prevent subsequent beads from entering the trap. The trapping condition of a single bead was experimentally characterized as described in following the ‘optimization for trapping and releasing single beads’ section. In the releasing mode, the pneumatic valve was switched OFF for 15 ms to release the trapped bead (Fig. 2b), after which it was immediately turned ON again, because a longer OFF time could induce the additional inflow of beads from the main channel. The released bead flowing through a sinuous channel was loaded onto the loading site defined by the micropillars, which are 20 μm apart, a spacing smaller than the bead diameter (25 μm; Fig. 2c). The bead travelled from the pneumatic valve to the loading site in a few dozen milliseconds. After loading a bead onto the loading site, a bead-encapsulated droplet was formed at the nozzle by positively pressurizing the dispensing reservoir, which is negatively pressurized during the normal state (Fig. 2d). To transfer sufficient energy for forming a droplet against the capillary force, a positive pressure of 80 kPa was applied to the dispensing reservoir for 30 ms. The bead on the loading site rushed toward the nozzle because the sinuous channel was designed to have sufficient hydraulic resistance to backflow. Thus, a bead-encapsulated droplet was finally obtained through these sequential modes. We name this technology “deterministic bead–in-droplet ejection (BIDE).”

View Article: PubMed Central - PubMed

ABSTRACT

This paper presents a deterministic bead-in-droplet ejection (BIDE) technique that regulates the precise distribution of microbeads in an ejected droplet. The deterministic BIDE was realized through the effective integration of a microfluidic single-particle handling technique with a liquid dispensing system. The integrated bead dispenser facilitates the transfer of the desired number of beads into a dispensing volume and the on-demand ejection of bead-encapsulated droplets. Single bead–encapsulated droplets were ejected every 3 s without any failure. Multiple-bead dispensing with deterministic control of the number of beads was demonstrated to emphasize the originality and quality of the proposed dispensing technique. The dispenser was mounted using a plug-socket type connection, and the dispensing process was completely automated using a programmed sequence without any microscopic observation. To demonstrate a potential application of the technique, bead-based streptavidin–biotin binding assay in an evaporating droplet was conducted using ultralow numbers of beads. The results evidenced the number of beads in the droplet crucially influences the reliability of the assay. Therefore, the proposed deterministic bead-in-droplet technology can be utilized to deliver desired beads onto a reaction site, particularly to reliably and efficiently enrich and detect target biomolecules.

No MeSH data available.