  Thiol-ene photo-click hydrogels were prepared via step-growth polymerisation
using thiol-functionalised type-I collagen and 8-arm poly(ethylene glycol)
norbornene terminated (PEG-NB), as a potential injectable regenerative device.
Type-I collagen was thiol-functionalised by a ring opening reaction with
2-iminothiolane (2IT), whereby up to 80 Abs% functionalisation and 90 RPN%
triple helical preservation were recorded via 2,4,6-Trinitrobenzenesulfonic
acid (TNBS) colorimetric assay and circular dichroism (CD). Type, i.e.
2-Hydroxy-1-[4-(2-hydroxyethoxy)phenyl]-2-methyl-1-propanone (I2959) or lithium
phenyl-2,4,6-trimethylbenzoylphosphinate (LAP), and concentration of
photoinitiator were varied to ensure minimal photoinitiator-induced
cytotoxicity and to enable thiol-ene network formation of collagen-PEG
mixtures. The viability of G292 cells following 24 h culture in
photoinitiator-supplemented media was largely affected by the photoinitiator
concentration, with I2959-supplemented media observed to induce higher toxic
response compared to LAP-supplemented media. In line with the in vitro study,
selected photoinitiator concentrations were used to prepare thiol-ene
photo-click hydrogels. Gelation kinetics proved to be largely affected by the
specific photoinitiator, with LAP-containing thiol-ene mixtures leading to
significantly reduced complete gelation time with respect to I2959-containing
mixtures. Other than the specific photoinitiator, the photoinitiator
concentration was key to adjust hydrogel storage modulus (G'), whereby 15-fold
G' increase was observed in samples prepared with 0.5% (w/v) compared to 0.1%
(w/v) LAP. The photoinitiator-gelation-cytotoxicity relationships established
in this study will be instrumental to the design of orthogonal collagen-based
niches for regenerative medicine.
