In nature, various soft materials have a water-rich core covered by a hydrophobic layer, i.e., a membrane or skin. These ‘smart’ or ‘adaptive’ membranes regulate the molecules that can enter or leave the core. Membranes enclose cells (microscale structures) as well as vesicles in the cells (nanoscale structures). At the macroscale, fruits and vegetables, as well as the human body are covered with skins. Currently, researchers are developing many soft, aqueous materials across length scales, including hydrogels, capsules, and vesicles, which are being used in areas such as drug delivery, pharmaceutics, and cosmetics. In this study, we will explore the synthesis of ‘smart’ skins or membranes around these structures with a goal of regulating solute release.

In our first study, we put forward a simple technique for synthesizing a hydrophobic skin to cover any hydrogel. An analogy is made to the peelable skin around fruits and vegetables. To make the skin, we employ an inside-out polymerization, where one component of the polymerization (the initiator) is present only in the gel core while other components (the monomers) are present only in the external medium. The thin polymeric skin (~ 10 to 100 µm in thickness) grows outward from the core in a few minutes. We show that the skin prevents the gel from swelling in water and also from drying in air. Hydrophilic solutes are completely prevented from leaking out into the external solution, while harmful microbes are prevented by the skin from attacking the gels. The properties of the skin are tunable, including its thickness and its mechanical properties. A polyurethane skin is elastomeric, transparent, and peelable from the core hydrogel. Conversely, other skins can be hard and brittle (glass-like).

Next, we alter the recipe for the skin around hydrogels by incorporating redox-responsive monomers. In the presence of an oxidizing agent, the initially hydrophobic skin becomes hydrophilic, thereby ‘turning on’ the release of solutes out of the gel. The release rate of various solutes can be easily controlled by changing the parameters such as solute loading, skin thickness as well as the concentration of oxidative species in the external medium. Conversely, solute release can also be ‘turned off’ at a later time by adding a reducing agent that reverts the skin to its hydrophobic state. Thus, our smart skin enables the on-off release of solutes out of a gel, and this concept is likely to be useful in many applications.

Lastly, we turn our attention to smaller nanocontainers, i.e., vesicles. We have come up with a way to make the membranes of vesicles responsive to multiple stimuli such as reactive oxygen species (ROS), temperature and ultraviolet (UV) light. The vesicle membrane is formed by a combination of cationic and anionic surfactant molecules, and the stimuli alter the geometry of these molecules. In turn, the vesicles are converted into micelles, resulting in the burst release of solutes out of the vesicle core. High-energy radiation used in cancer treatment is known to generate ROS – so, one application of these ‘smart’ vesicles could be in the radiation-induced burst-release of chemotherapeutic drugs, which could increase the effectiveness of cancer treatment.