Osmotic-Stress-Mediated Control of Membrane Permeability of Polymeric Microcapsules

Microcapsules with regular pore size can provide size-selective permeation, which is promising for immunoisolation of cells, protection of enzymes or catalysts, and development of capsule-type sensors. However, conventional approaches have limited biocompatibility or poor dispersion stability of encapsulants. Here, we suggest a simple yet pragmatic method to produce semipermeable microcapsules using osmotic stress. With a capillary microfluidic device, monodisperse microcapsules with ultrathin polymer membranes are prepared by double-emulsion templating. The microcapsules are subjected to a hypotonic condition, by which water is pumped in imposing a tensile stress on the membrane. The osmotic stress initiates cracks at weak spots. As cracks propagate, the pressure gradually reduces as ions diffuse through them, finally resulting in a finite width of cracks. The final width can be controlled from 5 to 10 nm using an initial osmotic pressure of 230 to 690 kPa, enabling fine adjustment of the cutoff threshold of permeation. This osmotic-pressure-mediated control is highly compatible with delicate biological molecules and colloidal dispersions as no etching chemicals are required to form pores. Taking advantage of this method, we demonstrate a capsule-type molecular sensor based on surface-enhanced Raman scattering that obviates pretreatment of samples because the membrane allows the entrance of small target molecules while blocking the large adhesive proteins