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. 2012 Jun;6(2):24103-2410310.
doi: 10.1063/1.3699972. Epub 2012 Apr 3.

Novel on-demand droplet generation for selective fluid sample extraction

Robert Lin  1 , Jeffery S Fisher, Melinda G Simon, Abraham P Lee
Affiliations

Novel on-demand droplet generation for selective fluid sample extraction

Robert Lin et al. Biomicrofluidics. 2012 Jun.

Abstract

A novel microfluidic device enabling selective generation of droplets and encapsulation of targets is presented. Unlike conventional methods, the presented mechanism generates droplets with unique selectivity by utilizing a K-junction design. The K-junction is a modified version of the classic T-junction with an added leg that serves as the exit channel for waste. The dispersed phase fluid enters from one diagonal of the K and exits the other diagonal while the continuous phase travels in the straight leg of the K. The intersection forms an interface that allows the dispersed phase to be controllably injected through actuation of an elastomer membrane located above the inlet channel near the interface. We have characterized two critical components in controlling the droplet size-membrane actuation pressure and timing as well as identified the region of fluid in which the droplet will be formed. This scheme will have applications in fluid sampling processes and selective encapsulation of materials. Selective encapsulation of a single cell from the dispersed phase fluid is demonstrated as an example of functionality of this design.

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Figures

Figure 1
Figure 1
Schematic of the on-demand droplet generation device. Blue lines indicate the dispersed phase channel, the yellow line represents the continuous phase channel, and the green line is the control channel. Red square denotes the droplet generation region of the device. Inset: The droplet generation area enlarged. The light blue arrow shows the direction of the dispersed phase flow. The red arrow denotes the ground outlet flow direction. The yellow arrow indicates the flow direction of the continuous phase and the flow direction of the generated droplet. Chamber in the fluidic line is visible beneath the green actuation line chamber.
Figure 2
Figure 2
Photograph and schematic of the microfluidic device. (A) Photograph of assembled microfluidic device filled with dye: fluidic channel—red, control channel—blue. (B) Exploded schematic of the device showing the three layers. Layer i—thick top PDMS layer containing the control channel. Layer ii—thin PDMS layer containing the fluidic channels. Layer iii—bottom glass substrate. The solid circles indicate the locations of the connective via.
Figure 3
Figure 3
Time sequence of a single droplet being generated with this mechanism. Blue line indicates the initial flow direction of the dispersed phase; red is the waste outlet; and yellow is the continuous phase and flow direction of the generated droplet. The actuation membrane is actuated at T0. (A) Prior to membrane actuation the fluid flows towards the waste outlet. (B) and (C) As the membrane is actuated, the dispersed phase is displaced into the main fluidic channel. (D)-(F) The fluid volume is sheared by the continuous phase into a droplet.
Figure 4
Figure 4
Visualization of membrane deformation in the actuation chamber. (A) The fluidic channel is filled with dye and appears to be dark. (B) The membrane is actuated and the dye inside the fluidic chamber is displaced. (C) Intensity measured along the dotted yellow line as indicated in A and B. The brighter the pixel, the less the presence of dye. The profile of the plot serves as an analog to the shape of the membrane.
Figure 5
Figure 5
Droplet size is measured with TAct = 100 ms while changing the actuation pressure of the membrane. The size increases but reaches a plateau after approximately 13 psi. Above 13 psi, further increase in membrane actuation pressure does not increase droplet size, n = 5.
Figure 6
Figure 6
Droplet size is measured with PAct = 10 psi, while varying the actuation time of the membrane. As TAct increases the droplet size increases but reaches a plateau at around 100 ms. After 100 ms, further increase in membrane actuation time does not significantly increase droplet sizes, n = 5.
Figure 7
Figure 7
(A) and (B) are examples of analyzed images from bead encapsulation experiments. Green circles mark beads that were encapsulated in generated droplet and red circles indicate beads that were not encapsulated. (C) Analysis result of 47 beads represented as colored regions. Green region area marks the encapsulation zone and red region indicates fluid volumes that were not encapsulated.
Figure 8
Figure 8
Photograph showing the extent of maximum deformation of dispersed phase into the main fluidic channel. Red lines indicate the paths of 3 beads that were not encapsulated into the formed droplet. Green lines represent the paths of beads that were encapsulated when a droplet is generated. The starting location of the beads is marked with open circles.
Figure 9
Figure 9
Image sequence of a single cell encapsulated by the droplet generation system. The cell is indicated by yellow arrow and the encapsulation zone is denoted by the dotted green box. (A) Cell moving towards the droplet generation junction. (B) Cell inside the encapsulation zone (green dotted region) the moment the membrane is actuated. (C) Cell position at maximum droplet deformation. The cell travels a path almost identical to that of bead 4 in Figure 7. (D) Cell encapsulated inside the generated droplet.
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