  Covalent functionalisation of collagen has been shown to be a promising
strategy to adjust the mechanical properties of highly swollen collagen
hydrogels. At the same time, secondary interactions between for example, amino
acidic terminations or introduced functional groups also play an important role
and are often challenging to predict and control. To explore this challenge,
4-vinylbenzyl chloride (4VBC) and methacrylic anhydride (MA) were reacted with
type I collagen, and the swelling and rheological properties of resulting photo
activated hydrogel systems investigated. 4VBC-based hydrogels showed
significantly increased swelling ratio, in light of the lower degree of
collagen functionalisation, with respect to methacrylated collagen networks,
whilst rheological storage moduli were found to be comparable between the two
systems. To explore the role of benzyl groups in the mechanical properties of
the 4VBC-based collagen system, model chemical force microscopy (CFM) was
carried out in aqueous environment with an aromatised probe against an
aromatised gold coated glass slide. A marked increase in adhesion force (F:
0.11 +/- 0.01 nN) was measured between aromatised samples, compared to the
adhesion force observed between the non-modified probe and a glass substrate
(F: 2.64 +/- 1.82 nN). These results suggest the formation of additional and
reversible {\pi}-{\pi} stacking interactions in aromatic 4VBC-based networks
and explain the remarkable rheological properties of this system in comparison
to MA based hydrogels.
