New Concepts for Gelation of Alginate and its Derivatives
Raghavan, Srinivasa R
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Bioengineering applications require materials that offer tunable and precise control over material properties. In particular, hydrogels of the polysaccharide, alginate have been widely studied for applications such as drug-delivery vehicles, matrices for encapsulation of cells, and scaffolds for tissue engineering. The ability of alginate to form a physically cross-linked hydrogel under mild conditions is a key factor for many applications. Traditionally, alginate gelation has been induced by the addition of divalent ions like calcium (Ca<super>2+</super>). In this work, we explore new ways to induce gelation of alginate or its derivatives. These new routes are of interest because they can allow researchers to circumvent current limitations and moreover they can also enable new applications. Three new concepts are explored: (1) ionic gelation activated by light; (2) ionic gelation activated by an enzyme and its substrate; (3) gelation of hydrophobically modified alginate mediated by biological cells. In our first study, we demonstrate a concept for ionic gelation of alginate in response to light, which enables us to create chemically erasable and spatially selective patterns of alginate gels. We impart light responsiveness by combining alginate, an insoluble calcium vector (e.g., CaCO<sub>3</sub>) and a light responsive component, viz. a photoacid generator (PAG). Upon UV irradiation, the PAG dissociates to release H<super>+</super> ions, which react with the CaCO<sub>3</sub> to generate free Ca<super>2+</super> in-situ. In turn, the Ca<super>2+</super> ions cross-link the alginate to form a gel. We show photopatterning of alginate gels, which are used to entrap contents (e.g., microparticles) and subsequently release them by a Ca<super>2+</super> chelator. In our second study, we demonstrate enzymatic gelation of alginate. Here, we use an enzyme/substrate reaction to generate H<super>+</super> ions. The components of our system are glucose oxidase (GOx, enzyme), glucose (substrate), alginate and CaCO<sub>3</sub>. First, GOx catalyzes oxidation of glucose to generate H<super>+</super> ions. These H<super>+</super> ions solubilize CaCO<sub>3</sub> and release free Ca<super>2+</super> ions in-situ. In turn, Ca<super>2+</super> ions cross-link alginate chains into a gel. A sol-gel transition is observed only when GOx senses and catalyzes glucose. By exploiting the specificity of the enzyme for its substrate, we use this concept to build a visual test for the presence of glucose in an unknown product. In our final study, we induce gels by combining a hydrophobically modified (hm) derivative of alginate with biological cells. Gelation occurs due to hydrophobic interactions between the grafted hydrophobes and the bilayers of biological cells. The polymer chains thus get attached to the cells and bridge the cells into a three-dimensional network. This gelation can also be reversed (to release the cells) by addition of a supramolecule, α-cyclodextrin, which has a hydrophobic binding pocket that binds to the hydrophobes. Cell gelation by hm-alginate may be useful in cell culture and tissue engineering applications. As a step towards these potential applications, we show that the process of gelation by hm-alginate is benign to the cells.