Saltwater-Induced Rapid Gelation of Photoredox-Responsive Mucomimetic Hydrogels.

Shear-thinning hydrogels represent an important class of injectable soft materials that are often used in a wide range of biomedical applications. Creation of new shear-thinning materials often requires that factors such as viscosity, injection rate/force, and needle gauge be evaluated to achieve efficient delivery, while simultaneously protecting potentially sensitive cargo. Here, a new approach to establishing shear-thinning hydrogels is reported where a host-guest cross-linked network initially remains soluble in deionized water but is kinetically trapped as a viscous hydrogel once exposed to saltwater. The shear-thinning properties of the hydrogel is then "switched on" in response to heating or exposure to visible light. These hydrogels consist of polynorbornene-based bottlebrush copolymers with porphyrin- and oligoviologen-containing side chains that are cross-linked through the reversible formation of ß-cyclodextrin-adamantane inclusion complexes. The resultant viscous hydrogels display broad adhesive properties across polar and nonpolar substrates, mimicking that of natural mucous and thus making it easier to distribute onto a wide range of surfaces. Additional control over the hydrogel's mechanical properties (storage/loss moduli) and performance (adhesion) is achieved post-injection using a low-energy (blue light) photoinduced electron-transfer process. This work envisions these injectable copolymers and multimodal hydrogels can serve as versatile next-generation biomaterials capable of light-based mechanical manipulation post-injection.

Electrostatic loading and photoredox-based release of molecular cargo from oligoviologen-crosslinked microparticles.

Although on-demand cargo release has been demonstrated in a wide range of microparticle platforms, many existing methods lack specific loading interactions and/or undergo permanent damage to the microparticle to release the cargo. Here, we report a novel method for electrostatically loading negatively charged molecular cargo in oligoviologen-crosslinked microparticles, wherein the cargo can be released upon activation by visible light. A water-in-oil (W/O) emulsion polymerization method was used to fabricate narrowly dispersed microparticles crosslinked by a dicationic viologen-based dimer and a poly(ethylene glycol) diacrylate. A zinc-tetraphenyl porphyrin photocatalyst was also polymerized into the microparticle and used to photochemically reduce the viologen subunits to their monoradical cations through a visible-light-mediated photoredox mechanism with triethanolamine (TEOA) as a sacrificial reductant. The microparticles were characterized by microscopy methods revealing uniform, spherical microparticles 481 ± 20.9 nm in diameter. Negatively charged molecular cargo (methyl orange, MO) was electrostatically loaded into the microparticles through counteranion metathesis. Upon irradiation with blue (450 nm) light, the photo-reduced viologen crosslinker subunits lose positive charges, resulting in release of the anionic MO cargo. Controlled release of the dye, as tracked by absorption spectroscopy, was observed over time, yielding release of up to 40% of the cargo in 48 h and 60% in 120 h in single dynamic dialysis experiment. However, full release of cargo was achieved upon transferring the microparticles to a fresh TEOA solution after the initial 120 h period.

Iterative step-growth synthesis and degradation of unimolecular polyviologens under mild conditions.

An iterative step-growth addition method was used to expedite the gram-scale synthesis of main-chain polyviologens by several days, while also producing the longest main-chain polyviologen (i.e., 26 viologen subunits) reported to date. Facile degradation using inorganic and organic aqueous bases was also demonstrated for a representative oligoviologen (6V-Me·12Cl), a polyviologen (26V-Me·52Cl), and oligoviologen-crosslinked hydrogels.