Photo-crosslinkable Poly(aspartic acid) for Light-based additive Manufacturing: Chain-growth versus Step-growth crosslinking

Crosslinked poly(aspartic acid) (pAsp) hydrogels have been evaluated in various applications benefitting from their biocompatibility and biodegradability. Several crosslinking mechanisms for pAsp derivatives have been investigated, yet research focusing on functionalization of pAsp with photo-crosslinkable moieties is scarce. However, the latter would be beneficial for processing of pAsp through light-based additive manufacturing techniques. pAsp was functionalized comparing two types of photo-crosslinkable moieties (i.e. norbornene versus methacrylate), resulting in a thiol-ene step-growth crosslinking mechanism and a chain-growth mechanism, respectively. The influence of the crosslinking mechanism on the photo-crosslinking kinetics, mechanical properties and biocompatibility of the hydrogels was studied. Hydrogels based on norbornene-modified pAsp with Li-TPO-L photo-initiator and a thiol-based crosslinker showed fast crosslinking kinetics and a high swelling ratio, along with a relatively low storage modulus of 29.4 +/- 1.3 kPa. Methacrylate-modified pAsp formulations with Li-TPO-L crosslinked slower and exhibited a lower swelling ratio, yet a higher storage modulus (135.1 +/- 4.7 kPa). Both hydrogel materials were non-cytotoxic to cells growing in their vicinity. The applicability of the hydrogels to serve as materials for digital light processing (DLP) and two-photon polymerization (2PP) was elucidated. Both materials were processable via DLP and 2PP, offering possibilities towards processing of these materials into constructs serving biomedical applications.