Covalent macromer photocrosslinking is a common approach for generating well-defined and mechanically robust hydrogels for use in applications ranging from 3D cell culture to drug delivery. Although in situ rheology provides a clear indicator of network formation, these techniques have limited applicability to situations where gelation occurs in geometrically confined environments and direct measurement is impossible, or when microrheological techniques are infeasible due to their reliance on specialized instruments or advanced microscopy. These shortcomings have, in part, facilitated growing interest in self-reporting smart materials that visibly signal changes in their state. Inspired by recent work implementing the tetrazole photoclick reaction for intracellular labeling, we synthesized novel PEG-tetrazole macromers which react readily under mild UV light (365 nm) with a variety of dipolarophiles (e.g. acrylates, norbornenes, maleimides) to produce autofluorescent pyrazoline adducts. Additionally, this system was redshifted by incorporation of alternative chromophoric and auxochromic moieties, allowing for long-wavelength, two-photon excitation to render complex, high-fidelity microscopic hydrogel structures. This synthetic strategy yielded a bathochromic shift of our system by greater than 40 nm, resulting in macromers that were easily activated by both 405 nm and two-photon near-IR (800 nm) light. As a photostiffening network, this material affords access to a range of irradiation-dependent storage moduli and spectroscopic properties. Moreover, this crosslinking reaction requires no radical initiator, marking potential for use in catalyst-free dynamic hydrogels, especially in sensitive cellular environments.

Redshifted tetrazole-ene PEG networks for self-reporting fluorogenic hydrogelation

ACS Fall 2021

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