Spatially resolved mapping of hydrogel stiffness during enzymatic degradation

The atomic force microscopy force–volume technique was used to investigate the surface properties of two hydrogels during their enzymatic degradation. Agarose gels with concentrations of 0.5, 1.0 and 1.5% w/v were selected for the degradation study to show that the mechanical effects of degradation could be measured in a spatially resolved way on the hydrogel surface. The agarase enzyme was used to degrade the agarose gels. The agarose gels were found to be heterogeneous with multiple stiffness domains that were degraded at different erosion rates. It was inferred that agarose gels contained both tightly and loosely packed regions. The loosely packed regions degraded first, followed by the regions that were packed more tightly. The second hydrogels containing hyaluronic acid and gelatin with 5% w/v hyaluronic acid and 5% w/v gelatin cross-linked in equal amounts were also studied. These hybrid gels were degraded either by using hyaluronidase or collagenase type IV. With hyaluronidase, gelatin was left on the surface as a homogeneous layer. With collagenase, cross-linked hyaluronic acid remained on the surface. Post-degradation the hyaluronic acid-rich surface had a stiffness of ∼20 kPa, while the gelatin-rich surface stiffness was only ∼15 kPa.