Responsive nanocellulose-PNIPAM millicapsules

Milli- and micro-capsules are developed to facilitate the controlled release of diverse active ingredients by passive diffusion or a triggered burst. As applications expand, capsules are required to be increasingly multi-functional, combining benefits like encapsulation, response, release, and even movement. Balancing the increasingly complex demands of capsules is a desire to minimize material usage, requiring efficient structural and chemical design. Designing multifunctional capsules with complex deformation should be possible even after minimizing the material usage through use of sparse fiber networks if the fibers are coated with responsive polymers.

Here capsules are created with a shell made from a mesh of nanoscale bacterial cellulose fibers that provide mechanical strength at very low mass levels, while a coating of thermoresponsive Poly(N-isopropylacrylamide), PNIPAM, on the fibers provides control of permeability, elastic response, and temperature response. These properties are varied by grafting different amounts of polymer using particular reaction conditions.

The addition of PNIPAM to the cellulose mesh capsule enhances its mechanical properties, enabling it to undergo large deformations and recover once stress is removed. The increased elastic response of the capsule also provides reinforcement against drying-induced capillary stresses, limiting the degree of shrinkage during dehydration. Time-lapse microscopy demonstrates thermoreversible swelling of the capsules in response to temperature change. Cycles of swelling and shrinkage drive solvent convection to and from the capsule interior, allowing exchange of contents and mixing with the bulk fluid on a time scale of seconds. Because the cellulose capsules are produced via emulsion-templated fermentation, the polymer-modified biocapsule concept introduced here presents a pathway toward the sustainable and scalable manufacture of multifunctional responsive capsules.