pH-responsive jello: gelatin gels containing fatty acid vesicles.

We describe a new way to impart pH-responsive properties to gels of biopolymers such as gelatin. This approach involves the embedding of pH-sensitive nanosized vesicles within the gel. The vesicles employed here are those of sodium oleate (NaOA), a fatty-acid-based amphiphile with a single C18 tail. In aqueous solution, NaOA undergoes a transition from vesicles at a pH approximately 8 to micelles at a pH higher than approximately 10. Here, we combine NaOA and gelatin at pH 8.3 to create a vesicle-loaded gel and then bring the gel in contact with a pH 10 buffer solution. As the buffer diffuses into the gel, the vesicles within the gel get transformed into micelles. Accordingly, a vesicle-micelle front moves through the gel, and this can be visually identified by the difference in turbidity between the two regions. Vesicle disruption can also be done in a spatially selective manner to create micelle-rich domains within a vesicle-loaded gel. A possible application of the above approach is in the area of pH-dependent controlled release. A vesicle-to-micelle transition releases hydrophilic solutes encapsulated within the vesicles into the bulk gel, and in turn these solutes can rapidly diffuse out of the gel into the external bath. Experiments with calcein dye confirm this concept and show that we can indeed use the pH in the bath to tune the release rate of solutes from vesicle-loaded gels.