A Comparative Study between Thiol-Ene and Acrylate Photocrosslinkable Hyaluronic Acid Hydrogel Inks for Digital Light Processing.

Photocrosslinkable formulations based on the radical thiol-ene reaction are considered better alternatives than methacrylated counterparts for light-based fabrication processes. This study quantifies differences between thiol-ene and methacrylated crosslinked hydrogels in terms of precursors stability, the control of the crosslinking process, and the resolution of printed features particularized for hyaluronic acid (HA) inks at concentrations relevant for bioprinting. First, the synthesis of HA functionalized with norbornene, allyl ether, or methacrylate groups with the same molecular weight and comparable degrees of functionalization is presented. The thiol-ene hydrogel precursors show storage stability over 15 months, 3.8 times higher than the methacrylated derivative. Photorheology experiments demonstrate up to 4.7-times faster photocrosslinking. Network formation in photoinitiated thiol-ene HA crosslinking allows higher temporal control than in methacrylated HA, which shows long post-illumination hardening. Using digital light processing, 4% w/v HA hydrogels crosslinked with a dithiol allowed printing of 13.5 × 4 × 1 mm layers with holes of 100 µm resolution within 2 s. This is the smallest feature size demonstrated in DLP printing with HA-based thiol-ene hydrogels. The results are important to estimate the extent to which the synthetic effort of introducing -ene functions can pay off in the printing step.