This work demonstrates that hydrogen peroxide (HO) is generated in weak polyacrylamide hydrogels due to mechanochemical reactions to osmotic swelling. Hydrogels are important tools and materials for many biomedical applications, particularly for growth of stem cells. However, swollen gels are under constant tension, which makes their individual chains susceptible to mechanochemical bond breakage. In this work, an assay was developed to measure the generation of HO as a result of hydrogel swelling. Polyacrylamide hydrogels with both weak disulfide and strong PEG-diacrylate crosslinkers were synthesized and swelled. HO generation increased in the presence of weaker crosslinkers, up to 30 μM HO, whereas stronger crosslinkers reduced this to 5 μM HO. HO levels decreased when swelled in the presence of dextran to reduce osmotic stress or increased if the gels were conjugated to an acrylated surface. Finally, HO continued to form for days after the gels had reached their equilibrium sizes, independently of dissolved oxygen. The results of this work impact those working in the 3D cell culture community and demonstrate that even well-characterized systems undergo mechanochemical processes in mild environments.

This work reports the development of a mechanochemistry activated covalent conjugation (MACC) reaction that shows areas of interfacial failure in soft hydrogels. Hydrogels are prone to delamination from rigid substrates due to the competition between swelling and adhesion, which can lead to bonding failure in a mechanism similar to crack propagation in harder materials. In this work, reductive amination was shown to occur when a ketone-bearing fluorescein derivative was bonded to an amine-functionalized hydrogel, as both of these moieties were found to be necessary for covalent conjugation into the gel network. For thin, circular polyacrylamide hydrogels, wrinkle patterns and regions of subsequent delamination at the edge of the gel were found to be selectively tagged by the dye. This reaction was then used to explore the effect of gel properties on patterns of interfacial failure. As cross-linker loading increased, the propagation of the delamination front and the area fraction of delamination were both found to increase, as shown by fluorescence images of gels. Increasing the thickness of the gel increased the fraction of delaminated area but did not change its propagation toward the center of the gel. This MACC reaction shows how mechanochemical reactions can be used for fluorescence tagging without incorporating mechanophores into the polymer gel matrix.

We report the effect of incorporating conducting oligophenylenes and a cobaltocene-based redox mediator on photodriven electron transfer between thioglycolic acid capped CdS nanorods and the native nitrogenase MoFe protein by following the reduction of H+ to H2.