UC Berkeley

The combination of microfabrication and microfluidics has enabled a variety of opportunities in making new tools for biological and diagnostic applications. For example, microdroplets-based systems have attracted lots of attentions in recent years due to potential advantages in controlled environments with fast reaction time, high-throughput and low noises. This work presents a number of advanced microfluidic systems in process, control and manipulation of microdroplets, including finger-powered pumps to generate microdroplets, continuous-flow rupture reactors for the rupture and content retrieval of microdroplets, and magnetic microcapsules for drug delivery applications.

Prototype `finger-powered' pumping systems have been designed and constructed and integrated with passive fluidic diodes to pump microfluidics, including the formation of microdroplets. No electrical power is needed for pumping by using a human finger as the actuation force to generate pressure heads. Both multilayer soft lithography and injection molding processes have been successfully utilized to make the pumping systems. Experimental results revealed that the pressure head generated from a human finger could be tuned based on the geometric characteristics of the system, with a maximum observed pressure of 7.6±0.1 kPa. In addition to the delivery of multiple, distinct fluids into microfluidic channels, the finger-powered pumping system is also employed to achieve rapid formation of both water-in-oil droplets (106.9±4.3 μm in diameter) and oil-in-water droplets (75.3±12.6 μm in diameter), as well as the encapsulation of endothelial cells in microdroplets without using any external or electrical controllers.

To advance the technology of microdroplets in microfluidic systems, the technique to rupture microdroplets via the continuous-flow micropost array railing systems has been developed. The key step is to transport water-in-oil microdroplets with surfactant into the pure oil microchannel to wash away the surfactant and allow the washed microdroplets to transport to the next water microchannel and rupture at the oil-water interface boundary. Microdroplets-based nanoparticle synthesis systems have been fabricated to demonstrate synthesis and retrieval of iron oxide nanoparticles without the need of an external centrifuge machine. In a second demonstration, a rapid solution alteration system for the bead-in-droplet microreactors has been demonstrated via the continuous flow micropost array railing technique. The prototype system has accomplished: (i) the retrieval of microbeads in water-in-oil droplets by the 'rupture' of the droplets, (ii) transfer of the released microbeads into a second solution, and (iii) the formation of new water-in-oil droplets containing the original microbeads and a different, second droplet solution. In these experiments, a total of four different microdroplets generation systems have been fabricated and different designs and operation conditions result in different sizes of microdroplets, including 41.1 μm for the basic microdroplets rupture demonstration, 67.5 μm for nanoparticle synthesis experiments, 61.1 μm in the original solution, and 38.6 μm for the new solution in the bead-in- droplets alternation experiments.

In the last example, a new class of magnetic microcapsules with aqueous core and polymer shell containing magnetic nanoparticles has been demonstrated for possible drug delivery applications. The combination of multi-layer flow-focusing methodology and an optofluidic polymerization process is employed to form double emulsions of water-in-photocurable polymer microdroplets. A subsequently polymerization process cure the magnetic polymer shells and encapsulates drug materials in the core. Experimentally, remote manipulations of the magnetic microcapsules by applying an external magnetic field have been achieved. As such, the proposed microcapsules have the potential to overcome a number of hurdles associated with current state-of-art technologies: (1) magnetic shells can be guided by DC magnetic field for location control; (2) magnetic particles can be heated by AC magnetic field to break or change the porosity of the shells for active drug release control; and (3) encapsulated microdroplets can prevent the possible degradation and contamination of the drug materials during the transportation processes.