A dialysis cell for microrheology was used to examine changes in local structure and rheology of block copolypeptide (BCP) hydrogels in response to imposed variations in ionic strength. The self-assembly of amphiphilic BCP molecules into porous gels is the result of a subtle balance between various attractive and repulsive intermolecular forces, one of which is the electrostatic repulsion between hydrophilic L-lysine residues. The ionic strength of the aqueous medium has therefore significant effect on the microscopic structure and mechanical properties of these hydrogels, which are critical for potential applications as tissue scaffold or in drug delivery.

For this study, the structural response of the BCP gels to controlled changes in ionic strength was characterized via particle tracking microrheology and confocal microscopy. Microrheology provided quantitative insight in the evolution of microscopic heterogeneity and mechanical properties of the sample after the addition and subsequent removal of salt in the dialysis cell. The microstructural response of the hydrogels was characterized as a function of BCP concentration, ionic strength, tracer particle size and molecular architecture. To facilitate novel transient microrheology experiments, we had to develop analytical tools, which will also be presented.

One of the key findings of this work is that diffusive exchange of ionic species leads to a distinctively different microscopic structure than convective mixing, which is typically used during sample preparation for most macroscopic characterization techniques. The dialysis cell provides unique capabilities to investigate microstructural dynamics under physiologically relevant conditions, where mass transport is generally dominated by diffusive processes.