Multiscale Characterization of the Mechanical Properties of Fibrin and Polyethylene Glycol (PEG) Hydrogels for Tissue Engineering Applications

Shear rheology and atomic force microscopy (AFM) are used to characterize the stiffness of hydrogels in tissue engineering applications, with several studies reporting differences of several orders of magnitude in the elastic moduli determined by these two methods. This work compares the elastic properties of soft fibrin and polyethylene glycol (PEG) hydrogels used for stem cell applications, determined by AFM indentation with different probe sizes (from nano- to micrometer) to shear rheometry data. For all hydrogels, AFM nanoscale probing consistently yields higher elastic modulus (E) values and variability than micrometer-probe indentation, while the shear modulus (G) values determined are the lowest. Colloidal probe AFM results are closer to rheology data for the stiffest samples, where E/G ratios converge to the theoretical Trouton ratio of 3. The results suggest that high polymer concentration hydrogels are better described by the affine elastic network theory, whereas low polymer concentration hydrogels deviate significantly from the Trouton ratio. Thus, for soft hydrogels relevant for stem cell culture, the assumption E = 3G is often invalid and care should be taken when comparing data from studies where different characterization methods are used in order to discern the impact of material properties on cell behavior.