(718e) Dynamically Responsive Microcapsules from Microfluidic Complex Emulsion Drop Templating

Authors: 
Werner, J. G., Harvard University
Nawar, S., Harvard University
Weitz, D. A., Harvard University
Wu, Z., Harvard
Water-cored microcapsules with shell membranes that undergo triggered and reversible property changes such as hydrophobic-to-hydrophilic transitions are fabricated from microfluidic double emulsion drop templating. Microcapsules are widely employed to protect and release sensitive cargo at predetermined trigger-events and rates, yet their functionality is one-directional with single-use applicability, because common release mechanisms involve degradation or destruction of the protective shell. Microcapsules that reversibly respond to external stimuli by changing the shell membrane’s property without structural degradation could turn permeability on and off repeatedly upon changing trigger events. The encapsulating shell in such systems essentially acts as an active gate-keeper, regulating diffusion in and out of the aqueous core compartment in response to changes in its environment. The challenge to fabricate aqueous microcapsules with shells whose permeability can be reversibly altered without sacrificial templates has hampered the development of such dynamic microcapsule systems. In particular, the water solubility of commonly employed functional monomeric and polymeric precursors prohibits the fabrication of water-cored double emulsion drops for capsule templating. To date, functional microcapsules that interact reversibly with aqueous triggers employ sacrificial templates such as oil drops or solid spheres as a place-holder for the aqueous core, making the direct incorporation of aqueous cargo in the core impossible. Here we describe the development of a number of polymer chemistries that allow for the synthesis of trigger-responsive hydrogel shells in complex emulsion drops for dynamically responsive microcapsules. We employ microfluidic fabrication of double emulsion drops with hydrophobic monomers as shells to synthesize trigger-responsive hydrogel membranes directly around water drops. The permeability and molecular weight cut-off for molecules that are able to diffuse through the shell is dynamically and actively tunable with external triggers such as pH and ionic species. The trigger responsive microcapsules are distinctly different from microgels, as the properties in microcapsules are dictated only by the state of the shell membrane that can make up less than 10% of the microparticle. Hence, the microcapsules exhibit a unique response to triggered swelling of the hydrogel membrane, during which the surface of the capsule increases much faster than the water diffusion into the core can accommodate, causing significant buckling of the shell. Furthermore, we demonstrate that the release from these dynamic microcapsule systems can be controllably activated and deactivated, enabling self-adjusting on-off release profiles. Due to their capability of repeated cycling between their permeable and impermeable states, the reversibly responsive microcapsules can capture, trap, and release substances from their environment with molecular selectivity.