Microfluidic Generation of Anisotropic Capsules
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Anisotropic particles are those having different properties (physical or chemical) along different directions; in contrast, isotropic particles are identical in all directions. The technical importance of anisotropic structures relies on the fact that a unit with two different characteristics can also exhibit two different functionalities. Such particles have attracted attention due to their potential applications in microrobotics, sensor technologies, drug delivery, and as components in optical devices. In this study, we focus on using microfluidic devices to create anisotropic microcapsules with advanced functionalities. Microfluidic platforms enable the generation of uniform liquid droplets, and we configure these platforms such that the droplets are converted into capsules, i.e., particles with a liquid core and a solid shell. Three different platforms are described, which each generate a unique type of anisotropic capsule.
In our first study, we describe the microfluidic assembly of Janus-like dimer capsules by the fusion of individual capsules with distinct properties. Microscale aqueous droplets bearing the biopolymer chitosan are generated in situ within a chip and, as they travel downstream, pairs of droplets are made to undergo controlled crosslinking and coalescence (due to a channel expansion) to form stable dimers. These dimers are very much like Janus particles: the size, shape, and functionality of each individual lobe within the dimer can be precisely controlled. To illustrate the diverse functionalities possible, we have prepared dimers wherein one lobe encapsulates paramagnetic nanoparticles. The resulting dimers undergo controlled rotation in an external rotating magnetic field, much like a magnetic stir bar.
In our second study, we describe a new way to create patchy spherical particles. Here, we generate droplets of a chitosan solution containing nanoparticles with an iron (Fe) core and a platinum (Pt) shell. The collected droplets are placed on top of a neodymium magnet to draw the Fe-Pt nanoparticles to their bottom side. The droplets are then crosslinked to convert them into capsules, with the nanoparticles localized on one end as a “patch”. The resulting capsules possess both magnetic and catalytic properties. When the capsules are placed in a solution of hydrogen peroxide (H2O2), the H2O2 is catalytically decomposed by the Pt to generate oxygen bubbles, which cause the capsule to move. Thus, our patchy capsules can act as “micromotors” and their motion can also be controlled by an external magnet.
In our final study, we employ a pulsed-air microfluidic droplet generator to create multi-compartment polymer capsules. These are capsules that have smaller capsules within them. Our technique uses no oil, and is thus very compatible with biological payloads such as proteins or cells. We can also place different payloads within each individual compartment. To demonstrate the unique capabilities of this setup, we have encapsulated different kinds of bacteria in different compartments. Furthermore, we show that the two bacteria engage in bacterial cross-talk through small molecules known as autoinducers. Specifically, one bacteria produces autoinducer 2 (AI2), which then diffuses across its compartment into the adjacent one, where the second bacteria imbibes the AI2 and in turn, produces a fluorescence response.