A simple guideline for designing droplet microfluidic chips to achieve an improved single (bio)particle encapsulation rate using a stratified flow-assisted particle ordering method

Abstract

Encapsulation of a single (bio)particle into individual droplets (referred to as single encapsulation) presents tremendous potential for precise biological and chemical reactions at the single (bio)particle level. Previously demonstrated successful strategies often rely on the use of high flow rates, gel, or viscoelastic materials for initial cell ordering prior to encapsulation into droplets, which could potentially challenge the system's operation. We propose to enhance the single encapsulation rate by using a stratified flow structure to focus and pre-order the (bio)particles before encapsulation. The stratified flow structure is formed using two simple aqueous Newtonian fluids with a viscosity contrast, which together serve as the dispersed phase. The single encapsulation rate is influenced by many parameters, including fluid viscosity contrast, geometric conditions, flow conditions and flow rate ratios, and dimensionless numbers such as the capillary number. This study focuses on investigating the influences of these parameters on the focused stream of the stratified flow, which is key for single encapsulation. The results allow the proposal of a simple guideline that can be adopted to design droplet microfluidic chips with an improved single encapsulation rate demanded by a wide range of applications. The guideline was validated by performing the single encapsulation of mouse embryonic stem cells suspended in a gelatin-methacryloyl solution in individual droplets of phosphate buffer saline, achieving a single encapsulation efficiency of up to 70%.

FIG. 1.

(a) A range of hydrodynamic focusing widths suitable for achieving single encapsulation. The inner aqueous fluid is a solution with 50% glycerol, the outer fluid is DI water, and the continuous phase fluid is silicon oil 20cSt; (b) the geometrical of the two crosses in series; and (c) the CAD design of the microfluidic chip.

FIG. 2.

Normalized focusing width $(W_f^{\ast })$ vs the flow rate ratio between the two dispersed phases, corresponding to the total dispersed phase (fixed condition: $\varphi \cong 1.4;Ca\cong 1\times {10}^{-3}$). Scale bar is 100 μm.

FIG. 3.

The formation of $L_S^{\ast }$depending on the viscosity contrast between DP1 and DP2 and the flow rate ratio between them (fixed condition: $\varphi \cong 1.4;Ca\cong 1\times {10}^{-3}$). The scale bar is 100 μm.

FIG. 4.

Stability of a normalized focusing width vs a variation in the flow rate ratio between the total dispersed phase and the continuous phase (fixed condition: $\lambda \cong 0.8;\eta =5;Ca\cong 1\times {10}^{-3}$).

FIG. 5.

Normalized droplet volume vs a variation in the flow rate ratio between the total dispersed phase and the continuous phase, at different flow rate ratios between the DP1 and DP2 (fixed condition: $\eta =2$). The scale bar is 100 μm.

FIG. 6.

Unstable focusing tip during the filling stage in the case without an orifice. Experimental conditions: $Ca=0.005$, $\lambda =0.85$, $\varphi \cong 0.4,\mathrm{and}\eta =5$. The scale bar is 50 μm.

FIG. 7.

(Left) Schematic of single stem cell encapsulation. (Right) Experiment showing single stem cell within droplets under the conditions: $Ca=0.002;\varphi =1.4;\eta \cong 2;\mathrm{and}\lambda =0.7$. Multimedia available online.