Fluidic barriers in droplet-based centrifugal microfluidics: Generation of multiple emulsions and microspheres

Droplet generation is very important in biochemical processes such as cell encapsulation, digital PCR (Polymerase Chain Reaction), and drug delivery. In the present paper, a density-based method called "fluidic barrier" is introduced to produce multiple emulsions on a centrifugal microfluidic platform or Lab-on-a-CD (LOCD). We show that the density and the viscosity of the fluids involved are important parameters for predicting the characteristics of the droplets generated with this method. Moreover, we broadened this concept by using the fluidic barriers to separate reactive chemicals. As a proof of concept, alginate and CaCl2 solutions are separated by an oil barrier to control the alginate cross-linking process. We demonstrate the continuous production of alginate microspheres and show that increasing the spinning frequency or alginate concentration leads to smaller microspheres. Furthermore, the size of the microspheres can be modulated by adjusting the size of the microchannels to fit a variety of applications such as cell encapsulation, drug delivery, or digital PCR. The concept of fluidic barriers is generic and can be expanded to include more than one barrier using different liquids with the appropriate density values.