The microfluidic synthesis of composite hollow microfibers for K⁺-responsive controlled release based on a host-guest...

A novel type of composite hollow microfiber with K⁺-responsive controlled-release characteristics based on a host-guest system is prepared by embedding K⁺-responsive poly(N-isopropylacrylamide-co-acryloylamidobenzo-15-crown-5) (P(NIPAM-co-AAB15C5)) microspheres in the wall of poly(lactic-co-glycolic acid) (PLGA) microfibers as "micro-valves" using a controllable microfluidic approach. By adjusting the volume change of microspheres in response to the environmental K⁺ concentration, the release rate of the encapsulated drug molecules from the composite hollow microfibers can be flexibly regulated owing to the change in the interspace size between the microfiber wall and microspheres. When the environmental K⁺ concentration is increased, due to the formation of stable 2 : 1 "sandwich-type" host-guest complexes of 15-crown-5 units and K⁺ ions, P(NIPAM-co-AAB15C5) microspheres change from a swollen state to a shrunken state. Thus, the interspace size becomes larger, resulting in a rapid increase in the release rate of encapsulated drugs. When the ambient K⁺ concentration is decreased, the interspace size becomes smaller due to isothermal swelling of microspheres caused by the decreased amount of host-guest complexes, resulting in a decrease in the release rate. The K⁺-responsive drug release behaviors are reversible. This kind of K⁺-responsive hollow microfiber with K⁺-concentration-dependent controlled-release properties provides a new mode in the design of more rational drug delivery systems, which are highly attractive for biomedical applications.