Light-Activated Ionic Gelation of Common Biopolymers

Abstract

Biopolymers such as alginate and pectin are well known for their ability to undergo gelation upon addition of multivalent cations such as calcium (Ca(2+)). 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, CaCO(3)) with an aqueous solution of the polymer (e.g., alginate) along with a third component, a photoacid generator (PAG). Upon UV irradiation, the PAG dissociates to release H(+) ions, which react with the CaCO(3) to generate free Ca(2+). In turn, the Ca(2+) ions cross-link the alginate chains into a physical network, thereby resulting in a hydrogel. Dynamic rheological 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.

Structured Macroporous Hydrogels: Progress, Challenges, and Opportunities

“…[154,155] Thus, while UV photopoly merization is certainly a viable option for avoiding solvent use is currently used as a water-based structuring chemistry (with notable successes [94,156] ), its limitations should also be recognized. A range of physical mechanisms including hydrophobic self-association, [157] thermogelation, [158,159] charge complexation, [160,161] stereocomplexation, [162] and host-guest interactions [163] have been reported to enable in situ gelation in water. However, such interactions typically lead (at least alone) to weaker gels that are less predictably stable in a range of potential environments, particularly problematic in separation-based applications in which a range of different conditions may be used during a single separation process.…”

Photoresponsive self-assemblies based on fatty acids

“…[9][10][11] The photolysis of the PAG in aqueous solution results in the generation of an acid and causes a pH decrease. The photolysis of PAG has already been used to produce photogelling colloidal dispersions, 9,10,12 to induce the hydrogelation of dipeptide 11 and to produce photoresponsive polyacrylamide hydrogel. 13 Here, we report how the UV light can control the structural fatty acid self-assemblies transitions in solution by using a PAG as photoresponsive component, which tunes the ratio between the protonated/deprotonated states of the carboxylic acid.…”

pH-responsive fatty acid self-assembly transition induced by UV light

“…The rate of pH decrease is controlled by the rate of UV light absorption by the fatty acid dispersion [25]. Once the UV light is absorbed, the PAG photolyzes in nanoseconds [26][27][28][29].…”

“…After UV irradiation, fatty acid molecules coexisted in two forms, the protonated and the deprotonated forms. UV irradiation is known to induce PAG dissociation and the release of protons into solution [20,21,25,29]. The protons react with the deprotonated fatty acid headgroup to form carboxylic acid.…”