Mechanistic insights into the directed assembly of hydrogel
blocks mediated by polyelectrolytes or microgels
In this study, we report the directed assembly of hydrogel blocks mediated by       electrostatic interactions. We compared two different assembly mechanisms, one       mediated by microgel particles and another mediated by direct interaction between       oppositely charged blocks. The system consisted of hydrogel blocks made of an       interpenetrated network of (hydroxyethyl)methacrylate-poly(ethylene       glycol)dimethacrylate (HEMA-PEGDMA) and either positively charged       polyethylenimine (PEI) or negatively charged hyaluronic acid (HA). Positively       charged hydrogel blocks were pretreated with negatively charged microgel       particles (MG) made of N-isopropylacrylamide-methacrylic acid. Both systems       (PEI/HA and PEI/MG) demonstrated spontaneous directed assembly, meaning that       positive blocks were systematically found in contact with oppositely charged       blocks. Directed assembly in water of PEI/HA blocks resulted in large and open       aggregates, while PEI/MG blocks exhibited more compact aggregates. Effects of       salt and pH were also assessed for both systems. Inhibition of blocks aggregation       was found to appear above a critical salt concentration (CSalt*) which was       significantly higher for the PEI/HA system (80 mM) compared to the PEI/MG system        (5-20 mM). The observed difference was interpreted in terms of the nanostructure        of the contact area between blocks. Blocks aggregation was also found to be       controlled by the content of negatively charged groups in the microgels as well       as the concentration of MG in the suspension (CMG) used to treat the hydrogel       block surfaces. Our results shine light on the subtle differences underlying the        adhesion mechanisms between hydrogel blocks and suggest new routes toward the       design of innovative complex soft materials.
Hydrogel block Microgels Polyelectrolytes Directed assembly Adhesion