Microfluidic droplet encapsulation
Microfluidic devices and methods for the encapsulation of particles within liquid droplets are disclosed. The new methods and devices form 1-100 picoliter-size monodisperse droplets containing the particles, such as single cells, encapsulated in individual liquid droplets. The particles can be encapsulated in droplets of a fluid by passing a fluid containing the particles through a high aspect-ratio microchannel to order the particles in the fluid, followed by forming the fluid into droplets. The resulting fraction of the liquid droplets with a single particle (e.g., a cell) is higher than the corresponding fraction of single-particle liquid droplets predicted by Poisson statistics.
1. A method of encapsulating particles in liquid droplets, the method comprising
passing a plurality of particles in a fluid medium through a channel having a minimum cross-sectional dimension D, wherein the largest particle in the plurality has a maximum cross-sectional dimension that is at least about 0.1D;
ordering the plurality of particles into one or two ordered streams of particles in the fluid medium within the channel by flowing the fluid medium through the channel at a flow rate having a particle Reynolds Number (Rp) of about 2.9 and a Reynolds Number (Rc) of about 90, wherein ordering of particles occurs within a distance of about 1.0 cm along the channel; and
forming the fluid medium into a plurality of droplets outside the channel, under conditions wherein the proportion of the plurality of droplets containing only one particle is at least about 80% of the total number of droplets formed outside the channel.
2. The method of claim 1 , wherein the particles are of substantially the same size and the largest particle in the plurality has a maximum cross-sectional dimension that is about 0.1 D to 0.4 D.
3. The method of claim 1 , wherein the particles are cells.
4. The method of claim 1 , wherein forming the fluid medium into a plurality of droplets comprises contacting the fluid medium with a second medium immiscible in the first medium to form the droplets in the second medium.
5. The method of claim 4 , wherein the first medium is an aqueous medium containing particles comprising cells.
6. The method of claim 5 , wherein the second medium is an oil.
7. The method of claim 6 , wherein the fluid medium is passed through the channel at a rate of at least about 10 microliters per minute, and the flow rate of the oil is maintained at a velocity effective to generate droplets with a frequency not less than the frequency with which particles leave the channel.
8. The method of claim 4 , wherein the fluid medium is contacted at a transverse angle with the second medium, the flow rate of the second medium being greater than the flow rate of the fluid medium exiting the channel.
9. The method of claim 1 , wherein the fluid is passed through the channel at a rate of at least about 10 microliters/minute.
10. The method of claim 1 , wherein the fluid passes through a curved channel portion having a Dean number of up to about 30.
11. The method of claim 1 , wherein forming the fluid medium into the plurality of droplets comprises passing the particles from the channel through a nozzle to form the plurality of droplets outside the nozzle and the channel.
12. The method of claim 1 , wherein the rate of droplet formation has a frequency that is not less than the frequency with which the particles in the fluid medium leave the channel.
13. The method of claim 1 , wherein the average volume of one of the plurality of droplets is about 1-100 pL.
14. The method of claim 1 , wherein the droplets are formed at a rate of about 1-20 kHz.
15. The method of claim 1 , wherein
R
p
=
R
c
a
2
D
h
2
,
R
c
=
U
m
D
h
v
,
D h is a hydraulic diameter of the channel, a is a particle diameter, and v is a kinematic viscosity of the fluid medium.
16. The method of claim 1 , wherein the proportion of droplets containing only one particle is at least about 90% of the total number of droplets formed outside the channel.
17. The method of claim 1 , wherein the proportion of droplets containing zero particles is less than about 17% of the total number of droplets formed outside the channel.