The main objective of this research was to compare bioconjugation density in ultra-low crosslinked (ULC) microgel particles to conventional microgels. Microgels are colloidal gel particles of macromolecular networks that swell when exposed to a good solvent. Conventional microgels are synthesized with crosslinker (2-10 molar %), which increases mechanical integrity and modifies the physics of the system to behave as colloidal particles, rather than flexible polymer chains. However, in this research, microgel particles were synthesized without any added crosslinker. Thus, the name “ultra-low crosslinked”, which implies that crosslinking occurred only from physical crosslinks which emerged during the rapid precipitation polymerization process and not from chemical crosslinks from added crosslinker. It has recently been reported that ULCs behave more like highly branched macromolecules with fluid-like behavior, rather than as colloidal dispersions like conventional microgels. In addition to the bulk material properties, the surface chemistry is also expected to differ between the two types of microgels. In this work, a clear difference was observed between the two microgel systems, after biomolecules were conjugated to the surfaces of the particles. Poly <i>n</i>-isopropylacrylimde (PNIPAM) microgels were synthesized and conjugated with either albumin bovine serum (BSA) or cyanines (three different molecular weights) via zero-length crosslinking carboiimide chemistry. Potentiometric titration and NMR were used to characterize biomolecular conjugation. The results from the experiments depicted a nearly ten-fold increase in biomolecular attachment in microgels with 2 molar % BIS compared to ULCs. This data suggests that the surface chemistry of microgel particles is impacted by the concentration of chemical and physical crosslinks within the microgel particle internal structure. These results will help explain the unique phase behavior that microgels display, which will support better designed materials for future biomedical applications.

Biomolecular attachment yield analysis on ultra-low crosslinked microgels confirmed by potentiometric titration and NMR

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