Microcapsules with selective permeability
Microcapsules, whose membranes are regularly perforated, show selective permeability of encapsulated materials depending on their size; encapsulants smaller than the pore diameter penetrate through the membrane, while larger ones remain inside. This semi-permeability is a useful property for stable encapsulation of large materials without loss, while allowing penetration of small molecules. In addition, control of pore density and size enables controlled release of active materials. Therefore, microcapsules with perforated membranes are very useful for delivery of drugs, cosmetics, and nutrients and immune-isolation of living cells. Therefore, efficient encapsulation of active materials and formation of robust microcapsules with tailored membrane morphology remains an important yet unmet need.
We have developed a new microfluidic emulsification technology to make double-emulsion drops with an ultra-thin middle layer. Emulsification of biphasic core-sheath flows into continuous phase enables the production of such an ultra-thin membrane. The stability of double-emulsion drops can be highly enhanced by making the middle layer very thin, resulting in strong confinement of the innermost drops by lubrication resistance.
By combining this microfluidic technology with the controlled anchoring of colloids at interface, we have fabricated well-defined pores, which connect the internal and continuous spaces of microcapsules. The surfaces of the colloids are chemically modified to expose part of the colloids in both the inner and outer interfaces, and the ultra-thin middle phase is solidified. Subsequent selective removal of colloidal templates from a solidified membrane results in perforated membranes which show size-selective permeability. For example, microcapsules templated by 1 micrometer colloids allow a diffusion of 100 nm particles (green) through holes, while they do not allow a diffusion of 1 micrometer particles (red).