Probing the Interstitial Pore Structure of Packed Granular Microgels

At relatively low polymer concentrations, packs of highly swollen granular hydrogel particles, commonly called microgels, undergo a jamming-like transition. Under small applied stresses, these packed microgels behave like soft elastic solids but will flow like viscous fluids when the applied stress exceeds the yield stress. By tuning the rheological properties, support baths of packed microgels have been used to 3D-print soft materials in their fluid phase. Recently, this 3D-printing approach has been extended to 3D-bioprinting by swelling the microgel particles in cell growth media. However, long-term assays in this 3D-environment require the rapid exchange of cell growth media to provide fresh nutrients and the removal of cellular waste. Understanding the relationship between the interstitial pore structure of packed microgels that controls the permeability and the rheological properties that enable embedded 3D-bioprinting will further enhance the ability to systematically study cell behavior in a 3D-environment. Here, we investigate the interstitial pore structure of uncharged, polyacrylamide microgels with increasing cross-linker and polymer concentrations through a combination of cryo-SEM and single particle tracking. In addition, we compare the changes in the pore structure to the rheological behavior of the packed microgels. We believe that these studies will guide the development of microgels as sacrificial support materials for embedded 3D-bioprinting applications.