<b>Click Crosslinked PEG Hydrogels with Tunable Degradation</b>

<p dir="ltr">Poly(ethylene glycol) (PEG) hydrogels crosslinked by orthogonal chemistries have emerged as an ideal platform for tissue engineering and drug delivery applications due to their rapid crosslinking kinetics and excellent biocompatibility. Norbornene-functionalized PEG is routinely synthesized through Steglich esterification using 5-norbornene-2-carboxylic acid with hydroxyl-terminated PEG (PEGNB). A new synthesis method replaced the pungent 5-norbornene 2-carboxylic acid with odorless carbic anhydride (PEGNB_CA). Hydrogels crosslinked by the new macromer, PEGNB_CA, experienced accelerated hydrolytic degradation compared to the PEGNB counterpart. Through modularly blending PEGNB_CA with PEGNB along with different functionality of thiol crosslinkers, the rate of hydrolytic degradation could be tuned with complete degradation times ranging from a few days up to a year. The tunability in degradation allowed for multi-modal release of various model cargo with controlled kinetics. PEGNB can also be crosslinked via iEDDA click reaction with tetrazine-bearing crosslinkers; however, these hydrogels exhibited unprecedented hydrolytic stability owing to the supramolecular interactions between norbornene-tetrazine complexes. In contrast, PEGNB_CA crosslinked with tetrazine bearing PEG (PEGTz) exhibited fast hydrolytic degradation, while methyltetrazine functionalized PEG (PEGmTz) exhibited slow hydrolysis kinetics. Through dual functionalized methyltetrazine/tetrazine PEG crosslinkers at different ratios, the rate of hydrolytic degradation of iEDDA click crosslinked PEG-based hydrogels could be tuned. Through secondary amide conjugation onto the carboxylate group of PEGNB_CA (PEGNB-X, where X is an amide group), hydrogels crosslinked via thiol-norbornene click reaction underwent rapid ester hydrolysis with complete degradation occurring within hours. In contrast, iEDDA click crosslinked PEGNB-X macromers initially degraded from ester hydrolysis and formed stable supramolecular interactions between tetrazine-norbornene complexes. A fully aqueous synthesis of norbornene functionalized PEG was developed using amine terminated PEG reacted with carbic anhydride under basic conditions. The macromer, PEGaNB_CA, was capable of being crosslinked via iEDDA click reaction and thiol-norbornene click chemistries and exhibited hydrolytic stability owing to the amide bond. Additionally, PEGaNB_CAhydrogels could be 3D printed using a digital light processing printer and was capable of being photodegraded through secondary light exposure at 365 or 405 nm wavelength light in the presence of the photoinitiator LAP.</p>