(122b) Hydrogels Equipped with an ‘On-Off Switch’ for Solute Release

Many applications of hydrogels rely on their ability to deliver encapsulated solutes such as drugs or dyes to an external medium. However, a problem in many such applications is that the moment the gel is placed in contact with water, the solutes rapidly leak out by diffusion. Ideally, there should be a way to regulate the release of solutes – i.e., to ensure zero release until a desired time, and thereafter, for the release to be ‘switched on’. Moreover, it would be even better if this was a repeatable switch, i.e., if the gel could be cycled many times between the on and off states. Such a perfect, cyclical on-off release of contents from hydrogels has never been achieved to our knowledge.

We address this challenge by taking inspiration from structures found in nature such as fruits and vegetables, which have a hydrophobic skin that envelops and protects their water-rich core. We have found a way to synthesize a soft hydrophobic skin around any gel. The initial skin completely prevents hydrophilic solutes from leaking out of the gel into the external solution. The skin is also equipped with redox-responsive properties. That is, in the presence of oxidants, the skin becomes hydrophilic, thereby ‘turning on’ the release of solutes out of the gel. Conversely, solute release can be ‘turned off’ at any time by adding a reducing agent that reverts the skin to its hydrophobic state. Thus, our smart skin enables regulated (on-off) release of solutes out of a gel, and this concept is likely to be useful in many applications.