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.

Signaling MoleculeFmoc-Phe -pH-Responsive Proteins Reporter E. coli Scheme 1 . Biofabrication of multifunctional soft matter. a) Schematic illustrating the coupling of triggered self-assembly (i.e., electrodeposition) and enzymatic-assembly. b) Schematic illustrating that conjugated enzymes (Pfs and LuxS) allow for in situ generation of a bacterial signaling molecule (AI-2) that is detected by entrapped reporter cells.

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Hydrophobically modified chitosan (hmC) is a self-assembling polymer that has attracted recent attention for many applications, including as a hemostatic agent. One limitation with chitosan and its derivatives like hmC is that these polymers are soluble in water only under acidic conditions (because the pKa of chitosan is about 6.5), which could be undesirable for biomedical applications. To circumvent this limitation, we have synthesized a derivative of a C12-tailed hmC that is soluble in water at neutral pH. This water-soluble hmC (ws-hmC) is obtained by grafting O-carboxymethyl groups onto some of the primary hydroxyls on hmC. The solubility of ws-hmC at neutral pH is shown to be the result of a net anionic character for the polymer due to ionization of the carboxymethyl groups (in comparison, hmC is cationic). We also demonstrate that ws-hmC retains the self-assembling properties of hmC. Specifically, ws-hmC is able to induce gelation at neutral pH in dispersions of anionic surfactant vesicles as well as polymethylmethacrylate latex nanoparticles. Gelation is attributed to hydrophobic interactions between the hydrophobes on ws-hmC with vesicle bilayers and nanoparticle surfaces. In each case, gelation can be reversed by the addition of α-cyclodextrin, a supramolecule with a hydrophobic cavity that sequesters the hydrophobes on the polymer.

Vesicles are self-assembled nanocontainers that are used for the controlled release of cosmetics, drugs, and proteins. Researchers have been seeking to create photoresponsive vesicles that could enable the triggered release of encapsulated molecules with accurate spatial resolution. While several photoresponsive vesicle formulations have been reported, these systems are rather complex as they rely on special light-sensitive amphiphiles that require synthesis. In this study, we report a new class of photoresponsive vesicles based on two inexpensive and commercially available amphiphiles. Specifically, we employ p-octyloxydiphenyliodonium hexafluoroantimonate (ODPI), a cationic amphiphile that finds use as a photoinitiator, and a common anionic surfactant, sodium dodecylbenzenesulfonate (SDBS). Mixtures of ODPI and SDBS form "catanionic" vesicles at certain molar ratios due to ionic interactions between the cationic and anionic headgroups. When irradiated with ultraviolet (UV) light, ODPI loses its charge and, in turn, the vesicles are converted into micelles due to the loss of ionic interactions. In addition, a mixture of these photoresponsive vesicles and a hydrophobically modified biopolymer gives a photoresponsive vesicle-gel. The vesicle-gel is formed because hydrophobes on the polymer insert into vesicle bilayers and thus induce a three-dimensional network of vesicles connected by polymer chains.Upon UV irradiation, the network is disrupted because of the conversion of vesicles to micelles, with the polymer hydrophobes getting sequestered within the micelles. As a result, the gel is converted to a sol, which manifests as a 40 000-fold light-induced drop in sample viscosity.