Light-Activated Ionic Gelation of Common Biopolymers

Biopolymers such as alginate and pectin are well known for their ability to undergo gelation upon addition of multivalent cations such as calcium (Ca2+). Here, we report a simple way to activate such ionic gelation by UV irradiation. Our approach involves combining an insoluble salt of the cation (e.g., calcium carbonate, CaCO3) with an aqueous solution of the polymer (e.g., alginate) along with a third component, a photo acid generator (PAG). Upon UV irradiation, the PAG dissociates to release H+ ions, which react with the CaCO3 to generate free Ca2+. In turn, the Ca2+ ions cross-link the alginate chains into a physical network, thereby resulting in a hydrogel. Dynamic Theological experiments confirm the elastic character of the alginate gel, and the gel modulus is shown to be tunable via the irradiation time as well as the PAG and alginate concentrations. The above approach is easily extended to other biopolymers such as pectin. Using this approach, a photoresponse can be imparted to conventional biopolymers without the need for any chemical modification of the molecules. Photoresponsive alginate gels may be useful in creating biomaterials or tissue mimics. As a step toward potential applications, we demonstrate the ability to photopattern a thin film of alginate gel onto a glass substrate under mild conditions.