Sai Nikhil Subraveti1,2,Srinivasa Raghavan1
University of Maryland1,Princeton University2
Sai Nikhil Subraveti1,2,Srinivasa Raghavan1
University of Maryland1,Princeton University2
Many applications of hydrogels rely on their ability to deliver encapsulated solutes such as drugs; however, small hydrophilic solutes rapidly leak out of gels by diffusion. A need exists for a way to regulate solute release out of gels - to ensure zero release until a desired time, and thereafter, for the release to be ‘switched on’ at a high rate. This should ideally be a repeatable switch, i.e., the gel should be cyclable repeatedly between the on and off states. Such perfect, cyclical on-off release of solutes is demonstrated for the first time through a ‘smart skin’ that is synthesized rapidly (in ~ 10 min) around gels. The thin (~ 50 – 100 µm) and transparent polymer skin is endowed with redox-responsive properties through the use of urethane and acrylate monomers, one of which contains a thioether group. Initially, the skin is hydrophobic and it completely prevents hydrophilic solutes from leaking out of the gel. When contacted with oxidants such as hydrogen peroxide (H<sub>2</sub>O<sub>2</sub>), the thioethers are converted to sulfoxides, making the skin hydrophilic and thereby ‘turning on’ the release of solutes. Conversely, solute release can be ‘turned off’ subsequently by adding a reducing agent such as Vitamin C that reverts the sulfoxides to thioethers and thus returns the skin to its hydrophobic state. The release rate in the ‘on’ state can be tuned via the skin thickness as well as the oxidant concentration. The ability to regulate solute delivery from gels using smart skins is likely to prove significant in areas ranging from separations to agriculture to drug delivery.